GEOLOGY OF AUSTRALIAN AND PAPUA NEW GUINEAN MINERAL DEPOSITS Monograph 22
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GEOLOGY OF AUSTRALIAN AND PAPUA NEW GUINEAN MINERAL DEPOSITS Monograph 22
i
Cover photograph: Aerial view of Main-Barton open pit, Jundee gold mine, WA, viewed from the east, June 1997. The pit is approximately 1.4 km long and 50 m deep. Other Jundee and Nimary pits in the background. Courtesy of Great Central Mines Limited.
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GEOLOGY OF AUSTRALIAN AND PAPUA NEW GUINEAN MINERAL DEPOSITS Monograph 22
Edited by D A Berkman and D H Mackenzie
Published by THE AUSTRALASIAN INSTITUTE OF MINING AND METALLURGY Level 3, 15-31 Pelham Street, Carlton Victoria Australia 3053
iii
© The Australasian Institute of Mining and Metallurgy 1998
The Institute is not responsible as a body for the facts and opinions advanced in any of its publications.
ISBN 1 875776 53 2
Desktop published by: Katrina Fogg, Penelope Griffiths and Angela Spry for The Australasian Institute of Mining and Metallurgy
Printed by: RossCo Print Factory 4/188 Plenty Road Preston South Vic 3072
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Dedication This volume is dedicated to geologists, past and present, who have contributed so much to the knowledge and understanding of the mineral wealth of Australia and Papua New Guinea.
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Committees MONOGRAPH 22 COMMITTEE S E Close (Chairman) D A Berkman P F Griffiths D H Mackenzie H M Tutt
EXPERT COMMITTEE S E Close (Chairman) D E Clarke K E Fletcher I G Gould L de Graaf G R T Hudson W R H Ramsay L C Ranford
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Acknowledgements The assistance rendered by the following organisations and their staffs in the production of this monograph is gratefully acknowledged: The advice and assistance of the following individuals are also acknowledged with thanks:
Australian Geological Survey Organisation
Northern Territory Department of Mines and Energy
Australian Mineral Industries Research Association Limited
Papua New Guinea - Department of Mining and Petroleum
Bureau of Resource Sciences
Queensland Department of Minerals and Energy
Mineral Resources Tasmania
Tenement Administration Services
Mines and Energy SA
Victorian Department of Natural Resources and Environment
New South Wales Department of Mineral Resources
Western Australian Department of Minerals and Energy
David Blake
Russell Harris
Cathy Brown
Ian Hodgson
Tom Dickson
Kerry O’Sullivan
Brian Elliott
Sandy Paine
Stewart Girvan
Ric Rogerson
Graham Hancock
Len Skotsch
Colour Plates Colour plates have been subscribed for individually and these contributions are acknowledged with thanks. The subscribers and page numbers are listed below: Eagle Mining Corporation Placer Granny Smith Pty Limited Delta Gold NL WMC Resources Limited Outokumpu Mining Australia Hamersley Iron Pty Limited Hancock Prospecting Pty Limited Acacia Resources Limited Otter Gold Mines Limited Nord Pacific Limited Peak Gold Mines
93 180, 183 and 185 203, 204 and 205 234 368 and 369 375, 376, 377 and 378 381, 382, 383 and 384 420 and 421 443 593, 597 and 598 609, 610 and 611
Battle Mountain (Australia) Inc
686 and 687
MIM Holdings Limited
702 and 704
Ross Mining NL
712
Rio Tinto Exploration Pty Limited
827, 828, 829 and 830
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Sponsorship This monograph was made possible by the provision of loans from the companies and from Branches of The AusIMM listed below. The Australasian Institute of Mining and Metallurgy sincerely appreciates their support.
Sydney Branch, The AusIMM Newcrest Mining Limited North Limited Acacia Resources Limited Delta Gold NL Homestake Gold of Australia Limited Pasminco Limited Placer Pacific Limited Plutonic Resources Limited North Queensland Branch, The AusIMM Southern Queensland Branch, The AusIMM Aberfoyle Limited Australian Resources Limited Great Central Mines Limited Melbourne Branch, The AusIMM Kalgoorlie Branch, The AusIMM Nuigini Mining Limited Central Victorian Branch, The AusIMM
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Foreword It is a great pleasure to introduce this monograph. For the fifth time in almost 50 years the geology of the mineral deposits of the region is described in detail, under the auspices of The Australasian Institute of Mining and Metallurgy (The AusIMM). While the papers in this volume present the latest information, together with the previous volumes they continue a worthy tradition and provide a superb historical record, for the benefit of present and future exploration and mining professionals. This monograph spans the last decade and focusses on the discoveries and major changes in geological knowledge and interpretation in that time. Due to the large number of discoveries, the volume includes most of the more significant deposits of minerals, but excludes coal and petroleum. It clearly demonstrates the continuing achievements and dedication of geologists and their colleagues, as well as the success of companies in developing and operating mines. The coverage of a wide range of commodities and the geographic spread throughout Australia and Papua New Guinea well illustrates the variety and adaptability of the industry. The number of papers on gold deposits, especially in Western Australia, emphasises the major exploration and mining focus in that region. In the last decade Australian gold production has lifted from about 110 t in 1987 to an estimated 310 t in 1997, with three-quarters of current Australian gold production mined in Western Australia. The volume is primarily concerned with geological aspects, rather than financial and economic impacts. However, the overview papers clearly demonstrate the mineral industry’s importance and its significant contribution to the economies of both Australia and Papua New Guinea. The industry’s role as a major exporter and its ranking in world terms are highlighted. Also briefly described are the many changes which have occurred in the last decade and the wide range of issues that have been, and still are being, addressed. It is clear that a continuing and vibrant exploration and mining industry is essential for the well being of the region and its peoples. The monograph marks a major change in organisation and production. It is the product of a small, dedicated and efficient team. I pay particular tribute to the Editors, Don Berkman and David Mackenzie, who drew on their experience of Monograph 14, in undertaking their daunting task. Helen Tutt was invaluable as Project Coordinator, with the assistance of the staff of The AusIMM Publications Department. I take responsibility for the planning and direction of the project. We appreciated the support of the Expert Committee and were pleased that we did not need to call unduly upon their expertise. This is the first AusIMM publication to be issued simultaneously in both hardcopy and CD ROM format, which it is believed will be welcomed by many users. Funding for the project has been provided by loans from companies and individual AusIMM Branches. The loans will be repaid from sales proceeds. The generosity of these sponsors is much appreciated, especially that of The AusIMM Sydney Branch, the major lender. This monograph, as with its predecessors, would not have been possible without the enthusiasm and goodwill of the authors and the companies they represent. The support of many in the geological community is gratefully acknowledged. S E Close Chairman of Committees
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Editors’ Preface This is the fifth comprehensive compilation of ore deposit geology produced by The Institute, and the first wholly prepared for publication by The Institute employees and consultants. The series began in 1953, with ‘Geology of Australian Ore Deposits’ launched at the Fifth Empire Mining and Metallurgical Congress. The objective of that volume, as stated in the editor’s preface, was ‘. . . to present a geological account of mineralisation in Australia, with emphasis on the factors that have controlled ore deposition . . . The book includes descriptions of all the chief mines now working, and many of the lesser mines of geological interest’. The same objective was applied to the monographs published in 1965, 1975 and 1990, and to this publication.
D A Berkman.
The editors were fortunate in learning their early geological practice under C L Knight (the editor of the 1975 four-volume set), and in assisting F E Hughes in the preparation of the fourth member of the series, the successful two-volume Monograph 14. The present volume is a companion to Monograph 14 and closely follows its style and format. Monograph 22 records the geological setting of the large number of ore deposits found since 1989, when Monograph 14 was completed. Unlike the earlier volumes it contains a minimum of information on regional geology, as we consider that this is adequately covered in Monograph 14. It begins with two review papers, on the mineral industries of Australia and Papua New Guinea, which itemise the economic, legal and technical climate in the context of the deposits described. Another general paper details the Code of public reporting of resources and reserves of the Joint Ore Reserves Committee (JORC), which has been used throughout the volume and is becoming internationally accepted as a standard. The fourth general paper examines current research on exploration techniques and ore deposit geology. Papers on deposits were selected on the basis that they described a deposit, or group of deposits, with a total resource value of $50 million or more, equivalent to at least 100 000 ounces of contained gold, discovered and/or developed since about 1988. Smaller deposits with unique or outstanding geological features were also included as were papers which significantly updated the geological knowledge of older mines and earlier known deposits.
D H Mackenzie.
The descriptions of the deposits are arranged by location, starting in WA, and then across each State of Australia, ending in Papua New Guinea. They are grouped by principal commodity in each State, and then in sequence from north to south. The deposit sites are shown in the end paper maps
The type of deposits described provides an indication of the current focus of the Australian mineral industry. As 69 of the 120 ‘deposit’ papers document orebodies or mineral fields in which gold is the principal or only commercial component, we may conclude that gold mining continues its 20 year dominance of the industry. Several new gold deposits are described, in the Plutonic and Yandal greenstone belts of WA, at Lake Cowal in NSW and at Dead Bullock Soak, NT, and discoveries are also documented in old fields as at Cadia, NSW, Kanowna Belle, WA and Hamata, PNG. Major copper and lead-zinc deposits described include discoveries in the Mount Isa-Cloncurry region such as the blind deposits at Ernest Henry and Cannington and the largely concealed orebody at Century. Additions to resources at old mines at Rosebery and Renison, Tas, Mount Morgan and Gunpowder, Qld and at Norseman and Bulletin, WA, are discussed. New topics include some recently discovered and some revitalised nickel sulphide deposits in the Norseman–Wiluna belt of WA, some lateritic nickel deposits made viable by changes in ore processing methods, and silver-rich deposits at Nimbus, WA, and Bowdens, NSW. The geology of the exciting discovery of coarser-grained heavy mineral deposits in the Murray Basin is documented. The long list of discoveries, often of blind or concealed orebodies, demonstrates that industry growth by exploration is still within our grasp.
xiii
Editorial changes to the papers submitted have, as far as possible, been limited to matters of clarity, seeking to retain the individual author’s style. The editors have, in line with current practice, listed references in full, and not in the timeconsuming abbreviated CASSI system. Much of the editing has been involved with achieving the format and punctuation standards set by The Institute, which generally conform with the AGPS ‘Style Manual’. The editors are conscious that the contract editing system used for the monograph has required authors to make many minor ‘format type’ changes to several drafts of their papers. We thank to authors for their forbearance in accommodating these ‘strictures of the editors’, as one author saw them, and hope they are pleased with the result. We acknowledge assistance from the Australian Geological Survey Organisation (AGSO), who checked that the stratigraphic terms in the monograph conform to the Australian Register of Stratigraphic Nomenclature, and from the Bureau of Resource Sciences, who checked the locations of deposits for the end paper location maps. Most importantly, we extend our gratitude to the hundreds of geoscientists who have found time to record the geology of the ore deposits on which they work, and to whom this publication is dedicated. Monograph 22 owes much to the commitment of Sandra Close who gave it her support from initiation to completion. D A Berkman and D H Mackenzie Joint Editors
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Project Coordinator’s Preface Geology of Australian and Papua New Guinean Mineral Deposits heralds a new era in the publication of The AusIMM Monograph Series. The new methodology involves a small Committee, including a Project Coordinator. It has proved most effective and economical. The Monograph 22 Committee was established early in 1996, under the direction of Sandra Close, (Chairman of The AusIMM Publications Committee) with the Joint Editors, The AusIMM Publications Manager and the Project Coordinator. Overall parameters and criteria for papers were determined and a detailed Guide to Authors and Standard Operating Procedures were developed, to ensure a streamlined process. Over 300 companies were then invited to submit papers if their deposits met the criteria. The invitation was repeated in The AusIMM Bulletin and The Australian Geologist, to ensure that the volume’s coverage was comprehensive. The experience and knowledge the Joint Editors, Don Berkman and David Mackenzie, brought to the project was extremely valuable. Their commitment during the three stage editorial process has enhanced the quality and consistency of the monograph. Penelope Griffiths, The AusIMM Publications Manager provided expertise on publishing and related matters throughout the project. Pamela Bell’s methodical approach established clear and effective procedures for the control and flow of papers through the editorial process. Katrina Fogg and Angie Spry assisted with desktop publishing. I would like to express my appreciation to all those who contributed to the production of this volume, especially the authors of the individual papers whose enthusiasm and cooperation made my task much easier. I have valued Sandra Close’s patient guidance. My role has been both challenging and rewarding and I have enjoyed the opportunity to contribute to this monograph. H M Tutt Project Coordinator
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Contents REVIEW PAPERS The Australian mineral industry, 1986–1996
L C Ranford, D J Perkin and W A Preston
3
Papua New Guinea’s mineral industry, 1986–1996
R Rogerson
33
The JORC Code, 1987–1997
P R Stephenson and N Miskelly
45
Australian mineral exploration research
J Cucuzza and A D T Goode
53
WESTERN AUSTRALIA Mineral deposits of the Padbury, Bryah and Yerrida basins
F Pirajno and W A Preston
63
Plutonic gold deposit
N M Vickery, P M Buckley and R J Kellett
71
Gold deposits of the Peak Hill area
M A Harper, M G Hills, J I Renton and S E Thornett
81
Nimary gold deposits
A P Byass and D R Maclean
89
Jundee gold deposit
G N Phillips, J R Vearncombe and R Murphy
97
Bulletin gold deposit
S C Chanter, P Eilu, M E Erickson, G F P Jones and E Mikucki
105
Some gold deposits of the Bluebird, Nannine and Cuddingwarra goldfields, Murchison district
N J Winnall, T J Hibberd, D S Thynne and E Wahdan
111
Omega gold deposit, Gidgee
D I Ross and D W Smith
119
Kingfisher gold deposit, Gidgee
N J Hazard
123
Bronzewing gold deposit
G N Phillips, J R Vearncombe, I Blucher and D Rak
127
Mount McClure gold deposits
J L Harris
137
Tuckabianna gold deposits
M E Smith
149
New Holland, New Holland South and Genesis gold deposits, Lawlers
N A Inwood
155
Agnew gold deposits
J Broome, T Journeaux, C Simpson, N Dodunski, J Hosken, C De-Vitry and L Pilapil
161
xvii
Deflector gold-copper deposit
P Hayden and G Steemson
167
Tarmoola gold deposit
M C Fairclough and J C Brown
173
Sunrise-Cleo gold deposit
P G N Newton, D Gibbs, A Grove, C M Jones and A W Ryall
179
Lights of Israel gold deposit, Davyhurst
R M Joyce, W K Woodhouse and C H Young
187
Mount Dimer gold deposits
J R McIntyre and A Czerw
191
Broads Dam gold deposits
M J Glasson, R G Henderson and M Tin
197
Kanowna Belle gold deposit
T S Beckett, G J Fahey, P W Sage and G M Wilson
201
Kundana gold deposits
J R Lea
207
Geko gold deposit
G R Hemming
211
Centurion gold deposit, Binduli
M E Ivey, M J Fowler, P G Gent and A J Barker
215
Jubilee gold deposit, Kambalda
I K Copeland and the Geological Staff of New Hampton Goldfields NL
219
Randalls gold deposits
P G N Newton, B Smith, C Bolger and R Holmes
225
Revenge gold deposit, Kambalda
P T Nguyen, J S Donaldson and S G Ellery
233
Nelson’s Fleet gold deposit, St Ives
M Kriewaldt
239
Kambalda-St Ives gold deposits
R B Watchorn
243
Yilgarn Star gold deposit
R A Crookes and D Dunnet
255
Two Boys gold deposit, Higginsville
S H Shedden
261
Norseman gold deposits
N R Archer and B J Turner
265
Nimbus silver-zinc deposit
I R Mulholland, A Cowden, I P Hay, J C Ion and A L Greenaway
273
Weld Range platinum group element deposit
J Parks
279
Panorama zinc-copper deposits
P Morant
287
Magellan lead deposit
B M McQuitty and D J Pascoe
293
Honeymoon Well nickel deposits
M J Gole, D L Andrews, G J Drew and M Woodhouse
297
Mount Keith nickel deposit
S Hopf and D L Head
307
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Rocky’s Reward nickel deposit
C De-Vitry, J W Libby and P J Langworthy
315
Perseverance nickel deposit
J W Libby, P R Stockman, K M Cervoj, M R K Muir, M Whittle and P J Langworthy
321
Murrin Murrin nickel-cobalt deposits
V W Fazakerley and R Monti
329
Cawse nickel-cobalt deposit
K Hellsten, C R Lewis and S Denn
335
Silver Swan, Cygnet and Black Swan nickel deposits
J D Hicks and G D Balfe
339
Kambalda nickel deposits
W E Stone and E E Masterman
347
Maggie Hays nickel deposit
P S Buck, S A Vallance, C S Perring, R E Hill and S J Barnes
357
Forrestania nickel deposits
K M Frost, M Woodhouse and J T Pitkäjärvi
365
The Y2–3 and Y10 iron ore deposits, Yarrie
P J Waters
371
Brockman No. 2 detritals (B2D) iron ore deposit
D M McKenna and R A Harmsworth
375
Hope Downs iron ore deposits
R D Paquay and P K Ness
381
Speewah fluorite deposit
K A Rogers
387
SOUTH AUSTRALIA Perseverance gold deposit, Tarcoola
F J Hughes
395
Iron ore deposits of the northern Gawler Craton
B J Morris, M B Davies and A W Newton
401
NORTHERN TERRITORY Brocks Creek gold deposits, Pine Creek
G C Miller, C M Kirk, G Hamilton and J R Horsburgh
409
Union Reefs gold deposit
P G N Newton, C Switzer, J Hill, G Tangney and R Belcher
417
Mount Todd gold deposits
W R Ormsby, K L Olzard, D J Whitworth, T A Fuller and J E Orton
427
Gold Creek gold deposit
I J Morrison and J A Treacy
433
White Devil gold deposit
C A Bosel and G P Caia
439
Gold deposits of the Tanami Corridor
A Tunks and S Marsh
443
Dead Bullock Soak gold deposits
M E H Smith, D R Lovett, P I Pring and B G Sando
449
Merlin diamondiferous kimberlite pipes
D C Lee, T H Reddicliffe, B H Scott Smith, W R Taylor and L M Ward
461
xix
TASMANIA Tasmania gold deposit, Beaconsfield
P B Hills
467
Henty gold deposit
T Callaghan, S Dunham and W Edgar
473
Rosebery lead-zinc-gold-silver-copper deposit
M V Berry, P W Edwards, H T Georgi, C C Graves, C W A Carnie, R J Fare, C T Hale, S W Helm, D J Hobby and R D Willis
481
Renison Bell tin deposit
B M McQuitty, R H Roberts, P A Kitto and C J Cannard
487
VICTORIA Victorian gold province
G N Phillips and M J Hughes
495
Fosterville gold deposits
N Zurkic
507
Williams United gold deposit, Bendigo
G J McDermott and P W Quigley
511
Bailieston gold deposit
R S Sebek
517
Deborah line of reef gold deposits, Bendigo
D G Turnbull and G J McDermott
521
Eaglehawk-Linscotts reef gold deposits, Maldon
G B Ebsworth, J de Vickerod Krokowski and J Fothergill
527
Stawell gold deposits
D C Fredericksen and M Gane
535
Ballarat gold deposits
D H Taylor
543
NEW SOUTH WALES Timbarra gold deposits
R Mustard, R Nielsen and P A Ruxton
551
Mount Terrible gold deposits
G S Teale
561
McKinnons gold deposit, Cobar
S M Elliott, A Bywater and C Johnston
567
Browns Creek gold-copper deposit
C Wilkins and G Smart
575
Endeavour 42 (E42) gold deposit, Lake Cowal
P McInnes, I Miles, D Radclyffe and M Brooker
581
Elura zinc-lead-silver deposit, Cobar
A E Webster and C Lutherborrow
587
Girilambone district copper deposits
J M Fogarty
593
CSA copper-lead-zinc deposit, Cobar
B L Shi and G C Reed
601
Peak gold-copper-lead-zinc-silver deposit, Cobar
W G Cook, J A Pocock and C L Stegman
609
xx
Potosi zinc-lead-silver deposit, Broken Hill
R Morland and P R Leevers
615
Broken Hill lead-zinc-silver deposit
R Morland and A E Webster
619
Bowdens silver-lead-zinc deposit, Mudgee
I J Pringle and J Elliot
627
Lewis Ponds gold-silver-copper-lead-zinc deposits
R I Valliant and R M D Meares
635
Cadia gold-copper deposit
Newcrest Mining Staff
641
Heavy mineral sand deposits, central Murray Basin
A J Mason, M Teakle and P A Blampain
647
Hillview vermiculite deposit
A R Martin
651
Thuddungra magnesite deposits
V A Diemar
655
QUEENSLAND Atric gold deposit
J S Birch
663
Anastasia gold deposit
J E Nethery
669
Mount Wright gold deposit
K J Harvey
675
Ravenswood gold deposits
D Collett, C Green, D McIntosh and I Stockton
679
Vera North and Nancy gold deposits, Pajingo
D R Richards, G J Elliott and B H Jones
685
Wirralie gold deposit
M J Seed and P A Ruxton
691
Yandan gold deposit
P A Ruxton and M J Seed
695
Tick Hill gold deposit
P J Forrestal, P J Pearson, T Coughlin and C J Schubert
699
Belyando gold deposit
R Mustard
707
Mount Morgan gold-copper deposits
P R Messenger, A Taube, S D Golding and J S Hartley
715
Red Dome and Mungana gold-silver-copper-lead-zinc deposits
J E Nethery and M J Barr
723
Century zinc-lead-silver deposit
G C Broadbent and A E Waltho
729
Surveyor 1 copper-lead-zinc-silver-gold deposit
P S Rea and R J Close
737
Gunpowder copper deposits
S M Richardson and A D Moy
743
Grevillea zinc-lead-silver deposit
D R Jenkins, J P Laurie and S D Beams
753
Ernest Henry copper-gold deposit
A J Ryan
759
xxi
Greenmount copper-cobalt-gold deposit
G D Hodgson
769
Mount Elliott copper-gold deposit
D B Fortowski and S J A McCracken
775
Cannington silver-lead-zinc deposit
A Bailey
783
Osborne copper-gold deposit
N D Adshead, P Voulgaris and V N Muscio
793
Brolga nickel-cobalt deposit
J M Parianos, N F Morwood and J Cook
801
Westmoreland uranium deposits
G M Rheinberger, C Hallenstein and C L Stegman
807
Kunwarara magnesite deposit
D Milburn and S Wilcock
815
PAPUA NEW GUINEA Mount Sinivit gold deposits
I D Lindley
821
Wafi copper-gold deposit
D Tau-Loi and R L Andrew
827
Hamata gold deposit
K P Denwer and B A Mowat
833
Tolukuma gold-silver deposit
D G Semple, G J Corbett and T M Leach
837
Mount Bini copper-gold deposit
M A Dugmore and P W Leaman
843
Gameta gold deposit
K G Chapple and S Ibil
849
Nena copper-gold deposit
A L Bainbridge, S P Hitchman and G J DeRoss
855
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Ranford, L C, Perkin, D J and Preston, W A, 1998. The Australian mineral industry, 1986–1996, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 3–32 (The Australasian Institute of Mining and Metallurgy: Melbourne).
The Australian mineral industry, 1986–1996 1
2
by L C Ranford , D J Perkin and W A Preston INTRODUCTION This paper provides a broad overview of the Australian mineral industry for the decade July 1986 to June 1996. It describes the context in which it has operated, a summary of the more significant technological developments, an overview of the mineral discoveries and some thoughts on the future issues, opportunities and challenges for the industry during the next decade. The first section of the paper, entitled The Australian Mineral Industry: 1.
reflects the importance of the mineral industry to the Australian economy;
2.
provides an overview of Australia’s mineral production and trade;
3.
identifies the trends in mineral exploration expenditure in Australia and the level of overseas exploration by Australian companies; and
4.
provides a broad overview of the changes that have occurred to Australia’s mineral resource inventory over the decade and the costs of additions to resources.
The second section, Issues and Trends, 1986–1996, examines the national and international economic, social and political issues and developments that have impacted on the Australian mineral industry during the last decade. It also considers the developments and trends that have occurred within the mineral industry as a consequence of external factors and technological developments. The third section, Significant Mineral Discoveries and Developments, focuses on the mineral deposits that are described in this publication. It considers the evidence for new mineral deposit styles and provinces, the methods of discovery of the deposits and their geological and geographical spread. The final section, Future Issues, Opportunities and Challenges, briefly considers the various international and national issues expected to influence developments in the mineral industry over the next decade. It summarises the authors’ views on the scientific and technical needs and opportunities to be addressed if the industry is to maintain and increase its contribution to the welfare of all Australians.
1.
Director General, Western Australian Department of Minerals and Energy, 100 Plain Street, East Perth WA 6004.
2.
Principal Geologist, Bureau of Resource Sciences, PO Box E11, Kingston ACT 2604.
3.
Project Manager, Western Australian Department of Resources Development, 170 St George’s Terrace, Perth WA 6000.
Geology of Australian and Papua New Guinean Mineral Deposits
3
The information compiled to prepare this paper indicates that Australia has a vibrant, internationally competitive mineral industry which is attracting about 19% of the funds allocated internationally for mineral exploration. A very high proportion of these funds has been directed towards the search for gold over the last decade, and this seems likely to continue if the gold price and discovery costs remain at the levels experienced over the last few years. There seems little doubt that the world demand for minerals will increase, particularly in the Asian region. However, Australian producers will face strong international competition, especially from the developing countries eager to gain the economic benefits that flow from resource development. It appears that unless there is a satisfactory resolution of the land access problems associated with native title and environmental issues, many Australian companies will continue to move their activities offshore and some foreign investors are likely to go elsewhere. Australia has the technological skills, the prospectivity and the legal and administrative framework necessary to maintain and improve its international position in the world mineral industry. The major hurdles over the next decade are likely to be: 1.
national land access issues related to native title and environmental concerns which could seriously discourage greenfield exploration investment and hence constrain the future growth of the mineral industry; and
2.
international decisions on greenhouse gas emissions which could have serious impacts on the coal sector and on growth in energy intensive processing of some minerals mined in Australia.
THE AUSTRALIAN MINERAL INDUSTRY IMPORTANCE The mineral industry continues to be a major pillar of the Australian economy. In 1995–1996 mining, smelting and refining, including oil and gas, contributed 6% of GDP, compared with rural 4%, manufacturing 14%, with ‘other and services’ making up the remaining 76%. In 1996 the industry employed around 84 000 people directly, and indirectly another 300 000. The sector provided about 60% of Australia’s commodity exports, worth $A34 billion, 45% of all merchandise exports and 35% of all exports, ie total of all goods, including services, exported (ABARE, 1996, 1997). In 1996 the Australian mineral industry produced and exported a wider range of mineral commodities, and in a number of cases in greater quantities, than any other country in the world. Today Australia is as much dependent for its high standard of living on minerals as it was in the latter part of the
3
L C RANFORD, D J PERKIN and W A PRESTON
1800s when Australia was the envy of the world with its high per capita income based on gold production.
Mineral production
TABLE 1 Production of selected mineral commodities, Australia - 1986 and 1996.
The value of Australia’s mineral production has increased by 71% from 1986 to 1996, from $19.7 billion to $33.8 billion, which translates into a 6% annual increase in constant dollar terms over the decade (Fig 1).
Units
The quantity of Australian mineral production increased by an average of 56% on a non-weighted average basis of 20 principal commodities since 1986 (Table 1). The reason for the less than commensurate increase in the value of mineral production in real terms over the decade was the significant decline in metal and mineral prices in constant dollar terms. The commodities whose prices declined most in real Australian dollar terms were coking coal which fell by 38%, steaming coal 31%, gold 43%, nickel 30%, iron ore 28%, zinc 27%, rutile 25% and alumina 24%. The decreases in the real price of zircon concentrates by 13%, lead 9%, and copper 6% were relatively small by comparison. There were production increases for every commodity except tungsten during the decade. The commodities to show the largest increases over the period were gold whose output almost quadrupled, copper which more than doubled, ilmenite which increased by 64%, iron ore 57%, zinc 50%, raw black coal 48%, nickel 47%, diamond 44%, bauxite 33%, manganese ore 28%, uranium 19% and lead 17%. Production of tin, zircon concentrates and silver increased by less than 5% over the decade.
Mineral exports Australia is one of the worlds’ leading mineral exporting nations. Many of the emerging nations as well as Japan and Europe have come to rely on Australia for raw and partly processed mineral products. From 1985–1986 to 1995–1996
Commodity
1986
1996
% Increase(b)
32.384
43.063
33
0.170
0.252
48
37.607
53.600
43
0.525
111
Mt
Bauxite
Bt
Coal, black, raw
Mt
Coal, brown
Mt
Copper (a)
0.248
Mc
Diamonds
29.232
41.993
44
t
Gold (a)
75.079
288.880
285
Bt
Iron Ore
0.094
0.147
57
Mt
Lead (a)
0.448
0.522
17
Mt
Manganese ore
1.649
2.109
28
kt
Nickel (a)
0.077
0.113
47
Petroleum ML
Crude oil and condensate
29.764
31.579
6
Mm3
Natural gas (sales)
14.869
29.799
100
ML
LPG
3.929
3.718
-5
kt
Silver
1.023
1.020
0
kt
Tin (a)
8.515
8.828
4
Mt
Ilemite conc
1.238
2.028
64
Mt
Rutile conc, syn Rutile
0.230
0.634
176
Kt
Uranium (U3O8)
4.899
5.831
19
Mt
Zinc (a)
0.712
1.071
50
Mt
Zircon conc
0.452
0.462
2
(a) Total metallic content of minerals produced (b) Average percentage increase of the 20 commodities considered on a non-weight basis equal to 56% Source: ABARE (1997)
FIG 1 - Value of Australian mineral and petroleum production, investment and exports - 1985–86 to 1995–96 (in constant 1995–96 dollars). Source: ABARE (1997).
4
Geology of Australian and Papua New Guinean Mineral Deposits
THE AUSTRALIAN MINERAL INDUSTRY, 1986–1996
the value of Australia’s mineral exports increased from $16.3 billion to $34.1 billion. Some of the largest increases were recorded by gold, which jumped by five times to a value of $5.6 billion, iron and steel which went up more than ten times to $1.5 billion, copper which increased almost four times to $928 million and zircon which trebled to $223 million (Table 2). The value of exports of coking coal rose by 51% to $4.7 billion, steaming coal increased by 35% to $3.0 billion, and bauxite, alumina and aluminium ingot metal exports combined almost doubled to $5.2 billion. Overall, the value of iron ore, iron and steel and ferroalloys exports more than doubled to $4.5 billion. The value of nickel exports almost trebled to $1.2 billion, as did crude oil exports which reached $1.7 billion in 1995–1996. Zinc exports increased in value by 67% to $816 million over the decade. In quantity terms, mineral and energy exports increased by over 50% for the important bulk commodities like iron ore, coal, and alumina, aluminium ingots and bauxite. However, iron and steel exports increased sixfold to 3.1 Mt and exports of refined gold increased by over six times to reach 341 t in 1995–1996. Crude oil exports more than doubled over the period, and tonnages of tin, uranium and zinc exports increased by around a third. Mineral sands was the only major commodity which did not show significant growth over the decade, reflecting in part the switch to further processing for some of the feedstock previously exported. However, synthetic rutile and titanium dioxide pigment exports, not listed in Table 2, rose more than threefold since 1988–1989 to 304 000 t and 127 000 t respectively in 1995–1996.
Investment in the mining sector Although commanding a high profile in the perceptions of the public, over the decade new capital expenditure on mineral (including petroleum) projects has not attracted more than 20% of total Australian investment in any year. In fact it is only in the last five years that this 20% level has been approached and maintained. In the last half of the 1980s it was 13–15% (Fig 2).
FIG 2 - Mining as a percentage of capital investment in Australia 1986 to 1996.
The Australian Bureau of Agricultural and Resource Economics reported that investment in the mining and petroleum sector was around $7.2 billion in 1995–1996
TABLE 2 Exports of selected commodities from Australia - 1985–86 and 1995–96. Commodity
Quantity Units
1985–1986
Value ($M)
1995–1996
% Increase
1985–1986**
1995–1996
% Increase
4(e)
2(e)
-63
242(e)
78
kt
7 687
10 984
43
1 427
2 717
90
Aluminium (ingot metal)
kt
579
1 043
80
975
2 381
144
Black coal - coking - steaming
Mt Mt
49 43
77 61
58 41
3133(e) 2234(e)
4 746 3 014
51 35
Bauxite
Mt
Alumina
-68
Copper
kt
125
250
100
248
928
274
Gold, refined
t
56
341
509
972
5 607
477
Iron and steel - Iron ore - Iron and steel - Ferroalloys
Mt kt kt
79.5 522 85
126 3 130 156
58 500 84
1 939 129 36
2 863 1 510 104
48 1 071 189
Lead
kt
412
460
11.7
361
456
26
Mineral sands - Ilmenite concentrate - Rutile concentrate - Zircon concentrate
kt kt kt
1 034 230 446
1 192 203 450
15 -12 1
48 116 74
110 137 223
129 18 201
Nickel
kt
na
na
na
438
1 157
164
Petroleum - Crude oil - LPG
ML ML
4 402 1 495
10 899 1 513
148 1.2
667 298
1 675 196
151 -34
Tin
t
7 512
10 812
44
58
72
24
Uranium
t
3 533
4 483
27
373
242
-35
Zinc
kt
662
965
46
489
816
67
**Calendar 1986 (e) BRS estimates Abbreviations t = tonne; L = litre; kt = 103t; Mt = 106t; ML = 106L na = not applicable Source: ABARE (1997)
Geology of Australian and Papua New Guinean Mineral Deposits
5
L C RANFORD, D J PERKIN and W A PRESTON
(ABARE, 1996) and the industry survey for the Minerals Council of Australia indicated that this could approach $8 billion in 1996–1997 (Minerals Council of Australia, 1996). Whereas increases in mining investment were above 30% per year in the mid 1980s, growth ceased by the late 1980s, followed by a major but short-lived resurgence at the start of the 1990s. Since then the annual increase in capital expenditure has averaged around 12%, in line with growth of total new capital investment in Australia (Fig 3). The Minerals Council survey suggests that smelting and refining investment is a particularly strong area of growth at present. FIG 4 - Funds employed and profit pre-tax and before interest for mining in Australia - 1986 to 1996.
smelting and refining end of the industry are significantly higher than in mining (which includes the cost of exploration).
FIG 3 - New capital investment in Australia - 1986 to 1996.
The proliferation of, and emphasis on, gold projects has possibly given an over-inflated perception of investment in the mining sector over the decade. Gold projects tend to be relatively low cost in overall investment terms and only very large gold projects may individually require as much as $100 million investment. However, coal and base metal projects, new iron ore mines, a number of nickel laterite projects in Western Australia, aluminium at Boyne Island and alumina expansions and various gas projects and expansions each require investment in the range $400 to $2000 million. The number of projects currently under construction, plus many at an advanced stage of evaluation, suggests that investment will continue to be at a high level in Australia, barring a dramatic drop in world commodity prices.
Profitability of the Australian mining industry Profitability can be analysed in a number of ways, and results in different sectors of the industry may be markedly different from the aspect of a shareholder, a lending institution or government. To gauge the performance of, or return from, the minerals industry in total, it is appropriate to assess the net profits on funds employed, before taxes and interest charges on borrowings are deducted. In this respect overall industry profits have been relatively low for the last five years, compared to the late 1980s and early 1990s (Fig 4). Return averaged between 12 and 13% over the decade and in 1994–1995 was less than 9%. Such figures are significantly lower than returns in most other sectors of the economy. Despite low profitability, industry balance sheets have generally improved, with large increases in sales volumes and continued reductions in real unit production costs as productivity has improved due to industrial reform. Although company returns vary greatly, as do returns in the different commodity sectors, it is generally the case that returns in the
6
Over the decade the return on investor funds has averaged just under 12%. This, however, is carried by the last three years of the 1980s in which net profit return on average shareholders funds climbed rapidly to reach 23% in 1989–1990. In the two subsequent years there was a massive drop to 12% and then 8%, before levelling out and then plummeting again in 1994–1995 to 5.3%, before reviving to 8.8% in the last year (1995–1996). The results of the last six years are far from satisfactory and are unlikely to attract new investors to mining. Total borrowings in mid 1996 were nearly $8.6 billion which, on assets valued at over $52 billion, is small compared to historical percentages. Debt to equity ratios have progressively decreased over the last five years from 33:67 to 22:78 for the last two years (Fig 5). This contrasts with the 55% debt recorded ten years ago (1986–1987) when borrowings were around $13 billion.
Government revenue from mining The various levels of government - Commonwealth, State and local - obtain significant revenue from the mining industry. Some is collected for services provided by governments and some is by taxes and royalties. Annual industry surveys for the Mining Council of Australia suggest that the average annual government revenue from mining has been over $5 billion for the decade (Fig 6). Royalty figures collected by the Australian Bureau of Statistics, which include petroleum, suggest that these figures may in fact be $1.5 billion higher. About half of the $5 billion is provided by company taxes and royalties and the other half is more or less evenly split between employee income tax and government-provided services. Some charges for services are considered to be excessive by industry and are viewed as de facto taxes. The combination of direct taxes such as company income tax, resources taxes, licence fees and royalties, and indirect taxes like land charges or rates, payroll tax, fringe benefits tax and fuel excise have fluctuated between 40 and 60% of the total industry pre-tax profits in recent years, although 70% of profits was collected five to ten years ago (Fig 7). In more recent years, the figures have been within the range 42–50% and represent a very significant impost on an industry whose returns to shareholders are low. In terms of the distribution of industry revenue, direct and indirect charges to government range between 8 and 12% and
Geology of Australian and Papua New Guinean Mineral Deposits
THE AUSTRALIAN MINERAL INDUSTRY, 1986–1996
FIG 5 - Debt to equity ratio for mining investment in Australia, 1987 to 1996. Source: Minerals Council of Australia (1997).
FIG 6 - Government revenue from mining in Australia, 1987 to 1996. Source: Minerals Council of Australia (1997).
Geology of Australian and Papua New Guinean Mineral Deposits
7
L C RANFORD, D J PERKIN and W A PRESTON
FIG 7 - Government taxes and royalties as a percentage of profit: mining sector in Australia, 1987 to 1996. Source: Minerals Council of Australia (1997).
have averaged around 10% over the decade. The most recently available figure, for 1995–1996, gives a tax and royalty take of $2.4 billion, split 52% as company income tax, 27% as resource taxes, licence fees and royalties and 21% in indirect taxes. Although the income tax figure fluctuates widely, being based on profits, the aggregate of the other taxes and royalties is more consistent as there is a significant proportion of gross revenuebased and specific rate assessments.
Employment and investment multipliers Mining is a capital, rather than labour intensive industry. Direct employment in the industry is estimated by the Australian Bureau of Statistics to be 84 000 in 1997 or only 1% of the total workforce. Approximately three-quarters of the workforce is engaged in mining and exploration and one quarter in smelting and refining. Over the last decade direct employment in mining has dropped from over 100 000 or 1.4% of the Australian workforce as significant productivity improvements have been achieved and large scale, highly mechanised bulk mining activities have become more prevalent. The distribution of employment in the different sectors is 38% or 32 000 persons in metalliferous mining, 25% or 21 000 in coal, 21% or 17 000 in services to mining, 12% or 10 000 in other mining and only 5% or 4000 in oil and gas. Such figures indicate the variation in automation and mechanisation in the different sectors.
Multiplier Mining
Metallic minerals
4.1
Coal, oil and gas
3.6
Services to mining
3.2
Mineral Processing Basic metal products
4.7
Chemical, petroleum and coal products 4.3 Significant multipliers in mining have also been identified for output and income figures by the same study. The study suggests that, in Western Australia, every $1 of output from mining generates approximately the same in output in the nonmining industries, ie, a multiplier of 2. In the service area a multiplier of 3 has been estimated. With respect to income, the multipliers are even higher with around 3 in metallic mining and services to mining, and around 3.5 in mineral processing. In coal, oil and gas the figure is slightly lower at 2.2. Therefore, although overall employment in mining is low, the very significant flow-on effects to other sectors of industry in terms of employment, income and output are much higher than in most other industry sectors.
MINERAL PRODUCTION AND TRADE General
Despite low direct employment figures, multipliers related to mining provide very significant increases in employment. A study by the Economic Research Centre of the University of Western Australia indicates that multipliers in mining are in the range 3.2–4.1 and in mineral processing 4.3–4.7 (Clements and Qiang, 1996).
Australia is a world class producer of many mineral commodities. It is self sufficient in most primary metallic products and many industrial minerals. Its production surplus has enabled it to grow into a major world source of many mineral commodities and it has developed a reputation, particularly in more recent times, as a reliable supplier of high quality products.
This compares with an average estimate of 2.6 across all industries. Individual sector figures are shown below:
Much of the industry is well established and has taken advantage of market expansion opportunities in most sectors
8
Geology of Australian and Papua New Guinean Mineral Deposits
THE AUSTRALIAN MINERAL INDUSTRY, 1986–1996
over the last decade. There has been growth in many of its world ranking mineral outputs, including alumina, zinc, copper, coal, diamonds, titanium mineral products, iron ore, salt, tantalum and lithium, and most spectacularly gold, and to a lesser degree nickel. Levels of smelting and/or refining vary for different commodities, but a significant amount of primary product is exported as ore or concentrate. Major process developments are evident in such sectors as nickel and titanium minerals, and further processing projects for iron ore are either committed or at an advanced stage of assessment.
Some alumina is smelted to aluminium in Qld, Vic, Tas and NSW. With 1.3 Mtpa aluminium output Australia ranks fourth in the world (6% in total) and with almost 1 Mtpa exported Australia is the third largest trader behind Russia and Canada. Aluminium production in Australia has remained relatively constant for about eight years.
Base metals (copper, lead, zinc)
Alumina and aluminium
Australia holds the premier world position in mine production of lead with 17% of world total, is second in zinc at 14% and ninth in copper at 4%. Whereas NW Qld, centred on Mount Isa, provides the long term production base for zinc and copper and Broken Hill in NSW provides the base for zinc and lead, there are a number of other centres in the NT, NSW, WA, Qld, SA and Tas which contribute to the overall output. The development and expansion of the polymetallic Olympic Dam mine in SA has substantially increased copper output and the Carpentaria region in the NW Qld–NT border area is progressively increasing its output. This area will undoubtedly provide the long term focus of base metal production on the basis of discoveries discussed later.
About 10% of Australian bauxite production is exported, but the rest is converted to alumina in integrated operations, in WA, Qld and the NT. Australia is the world’s largest alumina producer and exporter, with one-third of total world output, and around 80% of Australian production is exported.
Although output fluctuates considerably from year to year with variations in the market, the general trend has been growth in mine copper production of the order of 50% and zinc of 30% over the decade. Lead production has increased only marginally over the period and has in fact tended to decline through the 1990s after peaking at the start of the decade.
Over the decade growth in Australian alumina capacity has been around, but slightly below, that of expansion in world production (45%). There are plans for further growth, for which the timing is dependent on market demand.
A moderate proportion of base metal output is processed to either smelted or refined product. About a third of the contained lead, half of the contained copper and nearly threequarters of the zinc are exported as concentrates.
The list of mineral imports to Australia is relatively short, with significant members being potassium fertilisers, phosphate rock, sulphur and the platinum group metals. Imports of processed products such as ferroalloys, speciality steel product and cut-gem diamonds highlight some of the downstream processing deficiencies in Australia. Comments on the individual commodities are set out below and percentages of world production are shown in Fig 8.
FIG 8 - Australia’s mineral production in a world context, at 1 January 1996. Source: ABARE (1997).
Geology of Australian and Papua New Guinean Mineral Deposits
9
L C RANFORD, D J PERKIN and W A PRESTON
Most Australian production is exported and Australia plays a significant role in primary concentrate and refined products trade. In terms of primary concentrates Australia ranks first in the world in zinc and third in lead. Australia is the leading exporter of smelted and/or refined lead products and is the second largest exporter of smelted and/or refined products of zinc.
Coal Australia’s coal production from mines in Qld and NSW makes it one of the world’s major sources of high quality coking coal and the world’s largest seaborne exporter of black coal. Between 65 and 70% of production of black coal is exported and the ratio of coking to steaming coal exports is about 55:45. Black coal production has grown around 30% over the decade, with expansion particularly noticeable over the last two years. Production is currently nearly 200 Mtpa of saleable product of which about 140 Mt is exported. In addition to the domestically consumed black coal, mainly steaming coal, a further 50 Mt of brown coal is produced for domestic power generation.
Diamonds Australia’s diamond output of around 40 M carats per year is based on the single operation at Argyle in the Kimberley region of WA. It provides around one-third of the world’s natural diamond output, although a large portion is of industrial quality, resulting in only 4 to 5% of the world total in value terms. Although Australia supplies over one-third of the world’s gem and cheap gem output and almost double that of any other producer, an overwhelming proportion is at the cheaper gem end of the scale. The continuance of diamond production from Australia beyond the middle of the next decade is, at this stage, dependent on the decision to develop the mine at Argyle at depth, possibly by underground mining. There are no commitments to development of other Australian diamond prospects as yet.
Gold Australia has experienced exponential growth in gold production through the 1980s, mainly centred on WA, but also in Qld. During the decade output increased by a factor of 3.5 times, although plateauing at around 250 t of fine gold in the first half of the 1990s. Australia has improved its ranking from fifth to third largest world primary gold producer during the decade, and now supplies about 11% of world mine output. All gold, apart from that contained in base metal concentrate shipments, is refined in Australia and over 80% of refined output is exported. Australian refineries also refine a small quantity of imported bullion. It appears that the current high production level can be sustained, but major discoveries will be necessary to cause any significant increase in output.
Heavy mineral sands The heavy mineral sands industry, now commonly described as the titanium minerals industry, has experienced fluctuating market conditions over the decade. Long overdue improvements from the mid 1980s during a three year period provided significant growth in the industry and, most particularly, the establishment and expansion of capacity for production of synthetic rutile and titanium dioxide pigment in
10
WA. A further, shortlived boom was experienced at the beginning of the 1990s leading to further mine expansion and process development for added value product. The industry continued to expand subsequently, despite less favourable conditions. Whereas ilmenite production has increased by 40% over the decade, concentrate exports have only grown marginally as most of the expanded output has been converted to synthetic rutile in WA. Through the 1990s synthetic rutile production has doubled and although a significant amount of this is exported, some is used as a feedstock to a modernised and expanded pigment sector. Pigment output has grown by 50% since the beginning of the decade and plans are in place for significant further expansion. Around 70% of pigment production is exported. Australia remains the world’s leading supplier of titanium minerals (ilmenite 25% and rutile 50%), although major expansion of titanium slag production in South Africa is providing strong competition in the market place. There has been little growth in natural rutile output in Australia over the last decade as a result of minimal growth in eastern Australian production. The ilmenite based output, including synthetic rutile from WA, has been the growth area throughout the period. Zircon is a by- or co-product of titanium minerals production and Australia has accounted for about 50% of world output over a long period. Production declined significantly in the early 1990s, but has recovered to 450 000–500 000 tpa in recent years. Most Australian zircon production is exported and extreme volatility in price has been a feature of the market. Monazite sales have progressively declined from around 18 000 t in the mid 1980s to zero by 1995. Competition from other rare earth minerals, especially from China, has been partly instrumental in the decline. However, the cessation of shipments to the French company, Rhone Poulenc, because of waste disposal problems in France, finally led to a complete stop of production. A second attempt to establish a monazite cracking plant in WA is at an advanced stage of approval and commitment.
Iron ore Australia is the world’s leading exporter of iron ore, providing 30% of traded product. Brazil and Australia have dominated the world market for nearly 30 years, supplying about 60% of world trade. Current Australian iron ore shipments of 130 Mtpa are mainly from the Pilbara region of WA, with a small amount from Tas. Approximately 90% of production is exported. Some domestic production is shipped from the Pilbara to eastern states’ iron and steelmaking facilities and locally produced ore is also used for iron and steel making in SA. There has been about 20% growth in iron ore production since the beginning of the 1990s, and all of this growth has come from the Pilbara. In addition, over the last four years there has been increased interest in iron and steel making projects in WA because of the availability of gas supplies at significantly lower prices. The level of processing of iron ore in Australia has been low to date, at about 8.5 Mtpa of steel production, but the potential for iron making developments, through direct reduction technology, is high.
Geology of Australian and Papua New Guinean Mineral Deposits
THE AUSTRALIAN MINERAL INDUSTRY, 1986–1996
Australia is expected to maintain and has the potential to increase its importance in the world iron ore industry, and could become a significant supplier of direct reduced iron to the world market in the medium term.
Manganese ore Australia ranks fifth in the world as a manganese ore producer. Production is mainly from Groote Eylandt in the NT, although in recent years this has been supplemented by production from smaller producers from the East Pilbara region of WA. With production of 2.0–2.2 Mtpa Australia supplies about 10% of world demand. Most ore is exported, although small quantities of manganese-based ferroalloys have been produced in Australia.
Nickel Nickel production, centred in WA, has expanded significantly over the last four years, based on expansions of existing mines and new sulphide ore developments. These expansions have overshadowed the demise of the Greenvale lateritic mine in central Qld. The net nickel output from mining operations has grown by 30% over the last decade, and has exceeded 100 000 tpa of contained nickel since 1995. Lateritic nickel projects at an advanced stage of evaluation or committed to development could result in substantial growth in nickel output in the next few years, and advance Australia’s position from third at present to second ranking in terms of world production. Australia currently produces 10% of world mine output and most is destined for the world market either as concentrate, matte or metal. About 75% of concentrate output is processed to matte or other intermediate product and a large and increasing proportion of this is taken through to refined nickel product.
Salt Salt production is largely from solar evaporation projects on the NW coast of WA. Growth in output has been about 30% over the last decade and is currently 8.7 Mtpa. In terms of world production this is relatively low, at 4% of total. However, it is significant in terms of seaborne trade, where, along with Mexico, Australia dominates the Pacific-based trade where much of the world’s petrochemical industry growth is focussed.
Uranium Despite limitations on production as a result of the Commonwealth Government’s ‘three mines’ policy between 1983 and 1996, Australia has retained its position as the second-ranked world producer, behind Canada, with about 16% of world output. This ranking is more a result of a decline in world production by about 30% throughout the 1990s, than any increase in Australian production. Through the early 1990s Australian output dropped to 2200 t of U308 compared with 4300–4500 tpa through the 1980s. In the last couple of years production has increased to over 5000 tpa U308, due to expansion of the Olympic Dam mine in SA. Future uranium production from Australia could increase significantly, given market opportunities. Expansion at Olympic Dam, the development of Jabiluka in the NT, and of Kintyre in WA plus a number of other projects could provide the increased output.
Geology of Australian and Papua New Guinean Mineral Deposits
Other minerals Australia is a world ranking producer of a number of other minerals. It is the world’s leading producer of tantalite and the lithium-based mineral spodumene, although in terms of lithium content Australia produces considerably less than China. A new plant to produce lithium carbonate in WA will provide a more desirable and competitive lithium product. Aggressive and innovative marketing of tantalite and spodumene over the last decade has resulted in Australia achieving its current position in world markets. Australia has long been a significant tin producer. However the long depression in the market has resulted in little expansion, and tin is now merely a by-product of tantalite production in WA. Australia ranks seventh in world tin production with just over 4% of world output. Australia is a major supplier of silver with 7–9% of world output. Most is recovered as by-product and co-product from base metal mines and as a by-product of gold operations. Despite the exponential growth in gold production there has been no corresponding growth in silver production over the last ten years. Of the industrial minerals, Australia is now a significant producer of magnesite and silica sand. In gemstones Australian opal and sapphire are of world significance and repute.
MINERAL EXPLORATION During the last decade private expenditure on exploration for minerals other than petroleum remained at a relatively high level, in real terms, although it fluctuated in a band between $700 million and $1 billion, as measured in 1996 constant dollars (Fig 9, Table 3). The peak achieved in 1988 was largely due to an increase in risk capital raised prior to the October 1987 crash, as a response to a wave of optimism about commodity growth worldwide which flowed through to investment in exploration and mining companies in Australia. In 1988, >72% of exploration expenditure was for gold. Gold has remained the dominant commodity sought with exploration expenditure consistently accounting for more than 50% of total mineral exploration expenditure. Gold exploration declined in 1996 for the first time since 1992 but was still about 57% of the total. Base metal exploration expenditure, the next largest component overall, has shown a fairly consistent increase in its share from 10 to 20% in the period 1986 to 1988 to around 25 to 30% of the total in the last few years. Diamond exploration expenditure and coal exploration expenditure have remained at around $30 to $50 million per year in 1996 $A over the decade and each represents about 5% of the total. Although diamond exploration has been focussed on frontier, greenfields activity, coal exploration has been concentrated in defined production areas. Iron ore exploration expenditure peaked at about $40 million in 1996 $A terms in 1992 but has fallen substantially since. Similarly, exploration expenditure for heavy mineral sands has fallen from $23 million in 1990, in 1996 A$, to about $9 million in 1996. The largest falls in exploration expenditure over the decade have been for uranium and tin-tungsten, decreasing from about $81 million and $12 million to $7 million and less than $1 million respectively in 1996 dollars. In the case of uranium,
11
L C RANFORD, D J PERKIN and W A PRESTON
FIG 9 - Exploration expenditure by commodity in Australia, 1986 to 1996 (in 1995–96 dollars). Source: ABS, adjusted to 1995-96 using ABARE data.
TABLE 3 Mineral exploration expenditure in Australia, 1986 to 1996 (constant 1996 dollars). Year
Gold
1986
346.3
Copper, lead, zinc, silver, nickel, cobalt 127.7
Diamond
37.3
Coal
Iron ore
52.0
18.7
Mineral sands 9.5
Tin, tungsten 12.4
Uranium Construction materials 81.1
2.7
Other
Total
26.0
713.5
1987
527.4
113.5
25.4
54.0
17.1
10.8
4.4
32.4
4.9
31.9
821.6
1988
799.0
115.4
33.4
37.8
16.2
17.2
3.9
31.9
1.2
46.9
1 103.1
1989
574.9
128.7
41.8
37.2
9.0
21.1
1.8
36.3
1.4
41.9
894.1
1990
405.0
140.0
44.1
39.6
13.4
22.9
3.3
22.3
3.3
27.1
720.9
1991
339.1
193.7
44.0
26.0
12.6
21.6
2.7
14.4
3.0
20.9
678.3
1992
336.9
160.2
39.5
30.4
40.8
15.4
1.9
14.2
1.8
26.9
667.8
1993
350.6
200.9
41.7
26.5
26.3
10.7
0.5
9.6
1.2
24.0
691.9
1994
488.0
206.1
63.1
29.8
20.2
9.1
1.2
8.2
1.4
25.3
852.6
1995
578.1
209.6
50.5
39.6
12.6
6.0
1.1
8.1
0.9
24.5
931.3
1996
547.1
251.8
52.9
52.7
14.1
9.3
0.6
7.2
0.7
23.8
960.2
Source: ABS (1997)
this represents a decrease from 11% of total expenditure in 1986 to less than 1% in 1996. This is attributed to lower spot prices for uranium and the former Commonwealth Government’s three mine policy for uranium. The decline in exploration expenditure for tin and tungsten is closely linked to the fall in tin and tungsten prices in real terms. This decline is considered to be due in part to the collapse of the International Tin Council in the early 1980s and to high production, particularly from China and Brazil, despite subdued demand for both metals. The last several years have seen a noticeable slowing down in the rate of growth of Australian exploration expenditure in real terms and this is a cause for concern. Unless a high and growing level of investment in exploration is maintained a decrease in the rate of mineral deposit discoveries and resource development can be expected to follow over the next 5 to 10 years.
12
OVERSEAS EXPLORATION BY AUSTRALIAN COMPANIES As part of the annual survey of the minerals industry, the Minerals Council of Australia (MCA) has provided information on overseas exploration activity since 1986. This has been compiled by Coopers and Lybrand, based on information supplied to them in confidence by the respondents (Minerals Council of Australia, 1996). The respondent companies constitute the major portion of the industry by aggregate size, ranging from the largest companies through medium to some small exploration ventures although numerically speaking, the companies responding probably constitute less than 10% of all companies exploring in Australia. Omissions from the survey include most of the smaller mining and exploration companies, some overseas controlled companies and a proportion of some joint venture operations.
Geology of Australian and Papua New Guinean Mineral Deposits
THE AUSTRALIAN MINERAL INDUSTRY, 1986–1996
FIG 10 - Estimates of exploration expenditure by Australian companies in various overseas countries or regions (in constant 1996 $A). Source: Minerals Council of Australia (1997).
To enable a comparison with previous surveys on exploration expenditure by the MCA, returns from those respondents which have participated in the survey over a period of years are separately reported as a ‘constant group’, and these data are discussed below. The increase in overseas exploration activities has been particularly marked since 1993 (Fig 10), when expenditure rose from the 30% of constant group total level it had broadly maintained since 1986, rising steeply over three years to reach 41% of total group at $319 million in 1996. In terms of trends in commodity sought over the period, overseas spending on gold and platinum exploration accounted for just over 40% of the total overseas spending by respondents in 1996, well down on the 79% recorded in 1989. In 1988 base metal exploration expenditure overseas was around 10% of the total, rising to 35% by 1993 and remaining at about 30% in 1996. Base metals are the major commodities sought worldwide. Although exploration expenditure for diamonds was the third largest at about 16% of the overseas total in 1996 and represented a fall from its peak of 28% in 1995, its proportion up to and including 1993 was less than 10% of the total spent by the constant group of exploration and mining companies. Australian exploration expenditure in Canada and USA has remained at around 30% of the MCA overseas total since 1988. In 1996 North America accounted for around $100 million, having grown rapidly in 1993–1994. The proportion of exploration spending in Asia and South America has doubled over the last six years and now each represents about 25% of the total. A significant proportion of this is in ‘grassroots’ exploration. The share of expenditure in Papua New Guinea fell from 16% in 1992 to 4% of the total in 1996, a pattern also followed by expenditure in Africa which fell from a peak of around 14% in 1994 and 1995 to 7% in 1996. To date there has been limited spending in countries formerly part of the Commonwealth of Independent States (CIS) and Eastern Europe, even though these areas have attracted significant interest by Australian companies.
Geology of Australian and Papua New Guinean Mineral Deposits
RESOURCES General Based on current knowledge, Australia is undoubtedly one of the world’s richest nations in terms of mineral resources. However, this may change in the future as countries with a range of under-explored geological environments come under the scrutiny of the international exploration industry in the face of growing world demand for minerals. Some of the reasons for Australia’s large stocks of known mineral resources include a large land mass, a stable political system and a history of sustained modern mineral exploration since 1950. This is built on gold and base metal mines and mineral provinces discovered in the preceding 100 years, particularly during the period 1851 to 1900. It is also based on a natural endowment comprising an enviable and complete, but only partially exposed, geological framework comprising rocks from every period of the earth’s history. The rocks host representatives of each period’s characteristic mineral deposit types. Australia’s geological setting, beginning with the Archaean of WA and evolving eastward in a series of successively younger Proterozoic and Phanerozoic basins, fold belts and metamorphosed cratons or blocks, provides the foundation for Australia’s petroleum and mineral resources.
World context In terms of world ranking, Australia has the world’s largest known economic demonstrated resources (EDR) for eight major tradeable mineral commodities - lead, zinc, silver, ilmenite, rutile, zircon, uranium and gem or near gem diamond (Table 4). Australia is ranked in the top three countries in the world for resources of bauxite, copper, gold, iron ore, manganese ore, tantalum and industrial diamonds. In addition, Australia’s stocks of EDR are within the top six countries worldwide for an additional 13 vitally important commodities bauxite, black coal, brown coal, cobalt, copper, gold, iron ore, lithium, manganese ore, nickel, rare earths, tantalum and industrial diamonds. Australia also has almost all of the world’s opal and a significant share of the sapphire resources. Australia’s only apparent mineral deficiencies are
13
L C RANFORD, D J PERKIN and W A PRESTON
TABLE 4 Australia’s identified resources and world economic demonstrated resources of major minerals, 1995. AUSTRALIA 1995 IDENTIFIED RESOURCES
DEMONSTRATED
WORLD 1995
INFERRED
Commodity
Units
Economic
Bauxite
(Mt)
2540
5245
2134
Black coal (recoverable)
(Gt)
49
6
Brown coal (recoverable)
(Gt)
41
(kt Cr)
-
Cobalt
(kt Co)
274
Copper
(Mt Cu)
24
Diamonds - gem and cheap gem - industrial
(106c) (106c)
101 128
Gold
(t Au) (Gt)
Lead Lithium
Chromium
Iron ore
Magnesite
AUSTRALIA’S SHARE OF WORLD ECONOMIC RESOURCES
Subeconomic Undifferentiated
Economic* demonstrated resources
%
Ranking
20 000 (a)
13
3
very large
708
7
6
3
166
313
<1
3
263.3
1 623.8
3 700 000
na
na
330
228
4100
7
4
16.3
11.9
327
7
3
141.3 176.4
41.1 61
300(b) 608(a)
34 21
1(g) 2
4263
1148
1378
45 000
9
3
17.8
14.2
17.2
153
12
2
(Mt Pb)
18.2
13.7
18
67
27
1
(Kt Li)
152
3
7
2100(f)
7
4
(Mt contained Mg)
70
85
66
2570
3
4
8(e)
3
Manganese ore
(Mt)
121.2
194.1
166.6
1885(a)
Mineral sands -Ilmenite -Rutile -Zircon
(Mt) (Mt) (Mt)
135.8 15.0 22.5
67.3 33.6 24.4
99.1 26.3 20.9
575(a) 42(a) 62(a)
24 36 36
1 1 1
Molybdenum
(kt Mo)
-
7.9
832.6
5500
<1
na
Nickel
(Mt Ni)
3.7
6.4
4.4
49
8
5
-
2 095
1947
11 000
<1
na
17.2
23.8
81.3
56 000
<1
low
1
14.1
4
100
1
4
Phosphate rock PGM (Pt, Pd, Os, Ir, Ru, Rh) Rare earths - REO and Y2O3
(Mt) (t metal) (Mt)
Silver
(kt Ag)
41.5
20.9
26.1
291.5
14
1
Tantalum
(kt Ta)
6.2
5.6
65.1
23.5
26
2
Tin
(kt Sn)
136.2
189.0
344.5
7400
2
9
Tungsten
(kt W)
1
62.1
180.1
2100
<1
low
Uranium(c)
(kt U)
629
77
194
2116(d)
30
1
(Mt Zn)
38.8
24.9
22.3
166
23
1
Zinc
Abbreviations: t = tonne; c = carat; kt = 103t; Mt = 106t; Gt = 109t; na = not applicable * Based largely on “Mineral Commodity Summaries” for 1995 (USBM 1996) (a) Adjusted by BRS; (b) 1994 data; (c) BRS scheme for classifying uranium resources; (d) Source OECD/NEA and IAEA (1995); (e) Based on contained manganese content; (f) Excludes Russia, China, Portugal, Namibia; (g) BRS estimate.
molybdenum, platinum group elements, chromium, native sulphur and some potassium and sodium salts.
Changes in Australia’s resource inventory An inventory of Australia’s mineral resources is compiled by the Bureau of Resource Sciences (BRS) and updated each year (BRS, 1996). An analysis of those data shows that, despite consistent or expanding annual production, Australia’s EDR for most mineral commodities has been increasing as a result of continuing successful exploration (Table 5). The exceptions are tin, manganese and diamonds where the EDR has fallen as a result of price decreases and lack of exploration in the case of tin, reassessment in the face of price and cost changes in the
14
case of manganese and depletion of resources concomitant with lack of exploration success in the case of diamonds. For all other commodities resource levels have either remained static or increased over the period. The most noteworthy increase in EDR over the period was for gold which went up sixfold in gross terms. This increase reflects the successes resulting from the high proportion (greater than 50%) of all exploration expenditure which was directed towards the search for gold between the end of 1986 and the start of 1997. However, because of the corresponding sharp increases in annual gold production the resource to production ratio for gold remained about the same at approximately 14 years, over the last decade.
Geology of Australian and Papua New Guinean Mineral Deposits
THE AUSTRALIAN MINERAL INDUSTRY, 1986–1996
TABLE 5 Australia’s economic demonstrated resources (EDR) and the resources (EDR) to production ratios, end of 1986 and end of 1996. Commodity and unit of quantity Bauxite (Mt)
EDR EDR Net increase end 1986 end 1996 EDR 1986-1996
Mine production end 1986 to 1996 (a)
Gross addition EDR/Production EDR/Production to EDR (b) ratio at end 1986 ratio at end 1996 (years) (years)
2 854
3 024
170
399.7
569.7
88
70
55
68
13
2.099
15.1
323
270
41.9
46
4.1
0.4873
4.6
1 114
858
16.00
23.60
7.6
3.6835
11.3
64
45
386
175
-211
380.1
169.1
13
4
Gold (t)
1 026
4 454
3 428
2 233.3
5 661.3
14
15
Ilmenite (Mt)
44.10
135.00
90.9
17.01
107.9
36
67
Iron ore (Gt)
15.85
17.8
1.95
1.181
3.1
169
121
Lead (Mt)
Black coal (Gt) Brown coal (Gt) Copper (Mt) Diamond (Mc)
15.80
18.70
2.9
5.5569
8.5
35
36
Manganese ore (Mt) (c)
192
118
-74
19.357
-54.6
116
56
Nickel (Mt)
1.10
6.40
5.3
0.8241758
6.1
14
57
Rutile (Mt)
9.00
14.90
5.9
2.14
8
39
24
Silver (kt)
32.50
43.30
10.8
10.95
21.8
32
42
Tin (kt) (c)
249
119.5
-129.5
75
-54.5
29
14
Uranium (kt)
462
622
160
33.994
194
94
107
Zinc (Mt)
24.90
39.90
15
10.8023
25.8
35
37
Zircon (Mt)
12.90
21.40
8.5
4.473
13
29
46
(a) Includes allowance for recovery losses; does not include 1986 data. (b) Sum of net increase in EDR and mine production; does not include 1986 data. (c) Large decreases in EDR from 1987 to 1996 resulted from reclassification of some resources during the period, mainly due to price falls. Source: BRS (1996).
Other large net and gross increases in EDR over the period have been for nickel which registered a sixfold increase, heavy mineral sands (ilmenite, rutile and zircon) which averaged a doubling in EDR, and zinc-(lead)-silver which also almost doubled. Substantial increases in net EDR were also recorded for copper at 48% and uranium at 35%. There were modest net increases in EDR for the bulk commodities over the period, including bauxite 6%, black coal 24%, brown coal 10% and iron ore 12%.
Average cost of additions to resources Based on the increase in EDR for gold each year when compared with annual gold exploration expenditure, there is no evidence that the discovery cost per ounce of new gold discovered is increasing. To the end of 1996 the overall trend line slopes downward over time, although in the last year there was an apparent upturn in discovery cost, to around $35 per new ounce. Over the decade the average discovery cost was $28 in constant 1996 dollars per ounce of gold discovered. The gross additions to EDR of base metals was about 52 Mt of contained metal. Over the decade this comprised 11 Mt of newly discovered contained copper, 9 Mt of lead, 26 Mt of zinc and 6 Mt of nickel, at an average gross discovery cost of $36 per tonne of contained metal. Compared with the previous decade there is no evidence that the cost of base metal discovery is rising. Base metal exploration expenditure, at an average of about 20% of total expenditure, has resulted in an average increase of 5.2 Mt of contained metal each year over the decade.
Geology of Australian and Papua New Guinean Mineral Deposits
ISSUES AND TRENDS, 1986–1996 The Australian mineral industry is very largely oriented towards the export market and has become a major world supplier of raw and processed mineral products. Thus it is susceptible to national and international developments which impact on its ability to compete in world markets. As the local industry has expanded and become more sophisticated it has also been more willing to look outside Australia for opportunities, and this is reflected in the growth in overseas exploration expenditure in recent years. The more important social, economic and political developments that have impacted on the Australian industry over the last decade are discussed below.
INTERNATIONAL ISSUES AND TRENDS International factors which have set the context for the industry over the decade include regional changes in demand for minerals, world commodity price trends and variations in currency exchange rates. Other factors which may have affected international investors’ attitudes include political and economic developments in the old Soviet block, China, India, Africa, Asia–Pacific and South and Central America. In addition, we have had the social and political impact of the United Nations sponsored report on sustainable development and the International Convention on climate changes related to global warming and the greenhouse effect. The changes in the old Soviet block, China, India and Africa have had relatively little impact on mineral investment and
15
L C RANFORD, D J PERKIN and W A PRESTON
development in Australia. Similarly the effects of the international push for sustainable development and measures to reduce greenhouse gas emissions have yet to have any discernible affect on the local industry. However, the political and economic developments in the Asia–Pacific region and Central and South America have already had an impact on investment decisions by the mining sector.
Demand for minerals Population growth is one factor in the demand equation and, over the decade world population has increased from about 4.8 billion to nearly 5.8 billion. Around 50% of this growth has occurred in the Asian region, of particular relevance to the Australian mineral industry. Another influence on demand for minerals is world economic growth, and the data suggest an average annual growth rate of slightly less than 4% over the decade. The OECD countries, which have a major influence on aggregate demand for minerals and hence prices, have only averaged 2.5% growth in GDP over the period. However Asia, which is the main focus of Australia’s mineral and energy exports, has experienced growth rates more than three times that of the OECD countries. Waring, Hogan and Haine (1997) point out that East Asia, excluding Japan, has doubled its share of world consumption of a range of minerals and fuels during the last decade (Fig 11).
Fig 12 - Linear price trend for selected commodities, 1986 to 1996, in 1996 dollars based on daily prices compiled in WA Department of Minerals and Energy.
Virtually all mineral commodity prices have shown a downward trend in real terms over the decade despite increased consumption over the period (Fig 12) and this has forced substantial lowering of unit costs of production throughout the Australian industry to ensure survival in the face of strong international competition. Although the Australian mineral industry is a major supplier of minerals to world markets, it is not generally in a position to dictate prices, and it faces strong competition from alternative sources for all of its products.
Changing pattern of world exploration and development expenditure The downward trend in real prices of mineral commodities reflects the increasing efficiency in exploration, mining and processing of minerals and the increased competition from countries which have more recently recognised the benefits that may be derived from the development of their mineral resources. It also reflects the increasing contribution to supply from recycling.
Fig 11 - East Asia’s share of world consumption of selected fuels and minerals. Source: ABARE (1997).
Mineral price and currency trends Mineral commodity price trends and currency exchange rates are important in determining the investment climate affecting both mineral exploration and development. The currency exchange rate between the $A and $US is particularly important to the Australian mineral industry because most commodities are traded in $US. Although the relationship between the two currencies shows considerable volatility with monthly average values for the $A between $US0.59 in July 1986 and $US0.89 in January 1989, there is no discernible trend in the relative values over the decade.
16
The changing pattern of world mineral exploration, with significant increases in expenditure in South America and the Asia–Pacific region at the expense of North America, is indicative of the changes that have occurred over the last few years (Fig 13). Data collected by Metals Economics Group (1996) indicate that world exploration expenditure increased by 170% between 1992 and 1996 and that the proportion of expenditure in South American, Pacific and Asian countries almost doubled, from 22% to 40%, while the proportion spent in North America almost halved, from 39% to 22%. The data also show that Australia was able to maintain its share at around 19% of the world total over that period. Social and political changes that have occurred around the world over the decade have been instrumental in driving the changed pattern of exploration and development. Although there have been substantial impediments and restrictions placed in the way of explorers and miners in many industrialised countries, there have been substantial improvements in the political environment and administrative regimes in a number of developing nations which have long been recognised as prospective for minerals. Being largely unexplored, the perception of prospectivity and possibility of a large ‘bonanza’ greenfields discovery remains an attraction, at least in the initial years after entry.
Geology of Australian and Papua New Guinean Mineral Deposits
THE AUSTRALIAN MINERAL INDUSTRY, 1986–1996
FIG 13 - Estimate of world exploration expenditure by region or country - 1986 to 1996 (in constant 1995–96 $US). Source: estimates by BRS, 1986-1991; largely MEG data 1992-1996.
Australia has managed to maintain its position in the international scene because of its combination of prospectivity, political support and sound legislative and administrative framework. However, over the last couple of years concerns over access to land related to native title and environmental issues have reached the point where it is most unlikely that Australia will be able to hold its current proportion of world exploration expenditure. In addition, as about 70% of Australian exploration expenditure has in recent years been directed towards the search for gold, any significant sustained reduction in the price of gold is likely to have a relatively larger impact on investment in Australia than in most other countries.
NATIONAL ISSUES AND CHANGES Over the decade there have been important changes to social attitudes, economic policy and law within Australia which have affected the local mineral industry. Some of the more important changes are mentioned below.
Increasing concern about the environment There has been increasing community interest in, and concern about, the natural environment over the decade. This is of particular importance to the mineral industry because it needs access to land to identify new mineral deposits, and because the public has a relatively poor opinion of the industry based on its past environmental performance. Unrehabilitated mine sites remind the public of the industry’s past practices and raise doubts about its ability to protect the environment at current and proposed operations. Local interest in environmental matters has been heightened during the decade by international developments such as the report entitled Our Common Future (the Brundtland report) by the United Nations-sponsored World Commission on Environment and Development (The Commission for the Future, 1990) and the Convention on Climate Change, which was signed by Australia in June 1992 and ratified in December 1992.
Geology of Australian and Papua New Guinean Mineral Deposits
The Brundtland report The report proposed the concept of sustainable development which is about ‘meeting the needs of the present without compromising the ability of future generations to meet their own needs’. As pointed out by Ranford (1991) the Brundtland Commission did not stress the need for less development, but rather the need for economic development that will better serve the environment. He suggested it was unfortunate that the Commonwealth Government (1990) chose to give such strong emphasis to the ecological aspects with its use of the term Ecologically Sustainable Development and that the Commonwealth’s choice of terminology had served to foster and entrench differences of opinion on the compatibility and interaction between economic and environmental objectives. The resolution of these differences remains a major challenge to the long-term viability of the Australian mineral industry.
Concerns about greenhouse gas emissions The impact of the International Convention on Climate Change has been a focus of attention on greenhouse gas emissions. Prior to ratification of the Framework Convention on Climate Change, the Commonwealth Government released a National Greenhouse Response Strategy (NGRS) that was endorsed by all State Governments in December 1992. The NGRS sought improved efficiency in the energy sector by measures such as: 1.
reforming the structure of the electricity sector;
2.
pricing energy to better reflect economic, social and environmental costs;
3.
promoting co-generation and renewable energy projects; and
4.
promoting energy efficiency in the transport, industrial and residential or commercial sector.
The International Energy Agency (IEA) reviewed Australia’s energy policies in 1996 (IEA, 1997). It acknowledged that the Commonwealth Government had taken positive steps to reduce the rate of increase in emissions but pointed out that energy-related CO2 emissions and emissions
17
L C RANFORD, D J PERKIN and W A PRESTON
per capita in Australia are both high with regard to the OECD average and that emissions are expected to increase substantially by the year 2000. The IEA recommended that the Commonwealth Government undertake further measures designed to abate emissions, increase energy efficiency and encourage the States to set mandatory standards for air pollutant emissions drawing on international experience. The actions taken to date to reduce greenhouse gas emissions in Australia do not appear to have had any significant impact on the development of the mineral sector. However, there is a potential conflict emerging between the political objective to increase the level of processing of minerals mined in Australia and the national commitment to reduce greenhouse gas emissions.
Taxation During the last decade there have been changes to the Australian taxation regime which have impacted on the mineral industry. The more important changes were: 1.
reduction in company income tax from 49 to 36%;
2.
introduction, from 1 July 1987, of an imputation system whereby Australian tax paid at the corporate level is imputed to shareholders who receive ‘franked’ dividends;
3.
removal of the exemption from tax of income derived from gold mining from 1 January 1991;
4.
phasing out between 1996 and 2001 of the exemption from income tax of a prospector’s income derived from the sale of rights to mine for gold;
5.
allowance, as a deduction against assessable foreign income, of expenditure on exploration carried out overseas as from August 1990;
6.
introduction in July 1986 of a fringe benefits tax (FBT) which must be paid by employers on the value of certain fringe benefits provided to their employees;
7.
acceptance of successful cash bids for exploration permits and production licences, paid after June 1993, as allowable capital expenditure for taxation purposes;
8.
allowance as a deduction for taxation purposes of expenditure incurred after June 1991 on site rehabilitation on sites used for mining, petroleum production, quarrying and exploration activities; and
9.
reduction in taxation allowance against research and development (R&D) from 150% to 125%.
Economic and industrial relations policy The broad thrust of economic policy in Australia in recent years has been one of internationalisation and improved competitiveness. The increased internationalisation of the Australian economy has been complemented by an emphasis on the opportunities for increased trade with the rapidly expanding economies in the Asia–Pacific Region. Improved competitiveness has been pursued by means of deregulation of the Australian financial system including abolition of exchange controls, progressive reductions in tariff protection, structural reform of labour markets and wage fixing procedures, and improved efficiency in transport, communications and public utilities. As the mineral industry is internationally focussed, the issue of competitiveness is of utmost importance and hence the broad thrust of economic and industrial relations policy has been advantageous to the industry. Over the decade Australian industry has benefited from increased competition between banks and other financial institutions, and from increased activities of foreign banks in Australia. It has also benefited from increased flexibility in the labour market, reductions in real aviation, electricity, telephone and postal charges, and elements of reform on the waterfront. There has been a reduction in interest rates over the decade and they are now less than half their level at the beginning of the period (Fig 14). Similarly, inflation rates have halved over the decade. There have also been significant reforms in industrial relations, and the advent of enterprise and workplace agreements replacing the traditional award arrangements in many operations. The new agreements allow for the removal of the distinction between staff and award employees and for employment conditions that better meet the requirements of the employees and the enterprises.
Many of the changes have been beneficial to the mineral industry. However, the removal of the tax exemptions on income derived from gold mining and prospectors’ income derived from the sale of rights to mine for gold have decreased the attractiveness of the gold sector to investors. In addition the reduction in the taxation allowance for R&D is likely to reduce the level of R&D activity in the mineral sector and the FBT arrangements have certainly made it more attractive to establish fly-in, fly-out operations than to build new towns and associated infrastructure in remote areas. In 1996, the Commonwealth Government proposed amendments to the Diesel Fuel Rebate Scheme with a view to removing the rebate of excise on diesel fuel used for off-road purposes in mining operations. Following strong protests by industry and State Governments, the Commonwealth decided to amend its proposal to deny the rebate to unintended recipients and clarify eligibility criteria. However, the amended proposal was still unacceptable to some State Governments and some sections of industry and there was strong pressure for further changes to reduce the impact on the offshore petroleum industry, the onshore petroleum and mineral exploration industry and on mineral processing activities. Agreement was subsequently reached to restrict eligibility to core activities and the amendments were legislated in 1997.
Uranium policy FIG 14 - Bank interest and CPI rates, 1985–86 to 1995–96. Source: ABARE, Australian commodity statistics, 1996.
18
At the beginning of the decade Australia was bound by the socalled three mines policy introduced by the Hawke Labor Government in 1983.
Geology of Australian and Papua New Guinean Mineral Deposits
THE AUSTRALIAN MINERAL INDUSTRY, 1986–1996
There were a number of attempts to change the policy over the decade but it remained unchanged until 1996 when a Liberal–National Party coalition came to power. Under the coalition’s policy, uranium mining will be treated in the same way as all other forms of mining, except that export controls have been retained as a means of enforcing the Government’s nuclear non-proliferation objectives. As a result, a number of Australia’s undeveloped uranium deposits were being reevaluated in 1997 in the context of improving world uranium market opportunities.
Native title issues On 3 June 1992 a majority of the High Court of Australia decided in Mabo (No 2) that the Meriam people were entitled to the possession, occupation, use and enjoyment of (most of) the land of the Murray Islands in the Torres Strait. In reaching that conclusion the majority of the Court held that the common law of Australia recognises a form of native land title, and indicated that the principles applied to the mainland as well as the Murray Islands. The Court also held that such a native land title exists in accordance with the laws and customs of indigenous people where: 1.
those people have maintained their connection with the land; and
2.
their title has not been extinguished by the Acts of Imperial, Colonial, State, Territory or Commonwealth governments.
On 24 December 1993, the Commonwealth Native Title Act 1993 came into operation. In summary, this Act: 1.
recognised native title rights and set out some basic principles concerning native title in Australia;
2.
provided for the validation of past acts which may be invalid because of the existence of native title;
3.
provided for a future regime in which native title rights will be protected and conditions imposed on acts affecting native title land and waters;
4.
provided a process by which native title rights can be established and compensation determined; and
5.
provided a process by which determination can be made as to whether future grants can be made or acts done over native title land and waters.
An inevitable consequence of the common law recognition of native title and the passage of the Native Title Act was an increase in uncertainty about land access. This was recognised by the Industry Commission (1996) which pointed out that the longer the uncertainties about native title persist, the greater will be the costs to the mining industry. The Commission stated that much of the uncertainty reflects a lack of clarity about the property rights in native title and that the unresolved status of pastoral leases was a major concern. The High Court added to the uncertainty when on 23 December 1996 it decided by a majority of four to three that pastoral leases in Queensland do not confer rights of exclusive possession on the lessees and do not necessarily extinguish any native title rights that may exist on those areas (the ‘Wik’ decision). Prior to the Wik decision, the Commonwealth Government had prepared draft amendments to the Native Title Act to overcome difficulties experienced with the legislation since its introduction. These amendments were detailed by the Prime
Geology of Australian and Papua New Guinean Mineral Deposits
Minister in October 1996 and included the following proposed changes: 1.
native title claims to be subject to a threshold test to determine their legitimacy;
2.
once-only right-to-negotiate requirement to apply to mining companies and developers;
3.
exploration and prospecting titles to be excluded from right-to-negotiate obligations; and
4.
measures to be introduced to reduce the possibility of multiple land claims.
Following the Wik decision, the Commonwealth deferred action on the proposed amendments to the Native Title Act. Extensive consultations with interested parties began, in an attempt to get agreement on the additional changes necessary to resolve the very considerable difficulties flowing from that decision. At the end of 1996 there were 451 native title claims in Australia registered by the National Native Title Tribunal. Fifteen of these claims were under consideration by the Federal Court. Only one claim appeared to have been resolved since December 1993 and at the end of 1996 that agreement was awaiting determination by the Federal Court. About 72% of Western Australia, the premier mining State, was covered by registered native title claims at the end of 1996. It became demonstrably clear to all governments in Australia that the existing Native Title Act processes are not a practical way of dealing with native title rights in Australia. On 8 May 1997, the Prime Minister released a 10 point plan to address the uncertainties associated with native title and the Native Title Amendment Bill 1997 was introduced into the House of Representatives on 4 September 1997 with a view to passage by the end of 1997. As a consequence of the court decisions and the current native title legislation, the mining and petroleum industries have become the prime targets for Aboriginal people in their efforts to exercise their recently recognised rights and achieve compensation for what they see as the past injustices of European settlement of Australia.
Mining legislation During the decade many Australian jurisdictions have significantly changed their mining legislation. Completely new legislation has been proclaimed by the Commonwealth (Offshore Minerals Legislation), Queensland (Mineral Resources Act 1989), Victoria (Mineral Resources Development Act 1990), New South Wales (Mining Act 1992), Western Australia (Mine Safety and Inspection Act 1995) and Tasmania (Mineral Resources Development Act 1995). The new legislation has in some cases replaced legislation developed in the last century or in the early 1900s and in each case has been developed in consultation with industry to reflect current operating requirements. The changes to legislation have led to an improved operating environment for mineral explorers and miners. Australian mining legislation is highly regarded internationally and has been accepted as a model by a number of developing countries.
Regional geoscience mapping Following industry pressure in the mid 1980s governments in Australia have come to appreciate the benefits to the industry and the nation arising from allocating higher levels of resources
19
L C RANFORD, D J PERKIN and W A PRESTON
to regional geoscience mapping. This change in attitude was reflected in the Commonwealth-State Geoscience Mapping Accord whereby it was agreed in 1989 that the Commonwealth would match the funding by the States or Territory of geoscientific mapping. In practice the accord has not worked as planned because there has been a substantial increase in State allocations to this activity and the Commonwealth contribution has been static or decreased. However, it has provided a framework for consultation and coordination of Commonwealth and State geoscience mapping programs and ensured that there has not been an unnecessary duplication of effort. There is little doubt that the increased investment in geoscience mapping has provided a stimulus to investment in mineral exploration in Australia and led to some significant discoveries and developments.
Occupational health and safety (OH&S) There has been a substantial improvement in industry performance in terms of OH&S in the mineral sector over the decade and a major change in the philosophy behind safety management. Legislation has been introduced in most jurisdictions and there has been a major shift from a prescriptive regulatory approach to one based on systems audit emphasising the duty of care by all parties and the ultimate responsibility of the employer to provide a safe working environment. International best practice in the industry recognises the economic benefits of providing a safe working environment and gives safety matters the highest priority in the corporate agenda. In general it appears that the new approach to safety has been accepted and embraced by the larger mineral companies operating in Australia. However, management in many of the small and medium sized companies has yet to come to grips with its responsibilities under the new legislation.
TRENDS AND DEVELOPMENTS WITHIN THE MINERAL INDUSTRY There have been significant changes within the Australian mineral industry over the decade. Changes have taken place in the mode of operations and in exploration and mining techniques, and there has been increased emphasis on added value processing. The decade has also seen increased internationalisation of the Australian mining sector. Some of the more important changes and trends are outlined below.
Changes to mode of operations During the decade, there have been a number of significant changes to the basic mode of mining operations in Australia. There has been a major move towards contracting out of major segments of each operation and the subsequent development of partnering arrangements with key contractors to ensure stability and continuity within mining projects. There has also been an increase in the number of fly-in, fly-out operations. Fly-in, fly-out arrangements have been facilitated by changes to mining regulations which permit greater flexibility, changes to industrial relations provisions, increased availability of air transport at reasonable prices and fringe benefit tax provisions which discourage companies from providing subsidised housing and town facilities.
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The changes have meant that some operations have reduced contact or interaction with the local communities, and staff tend to change employment more often as their home base is not a consideration in the decision to move from one operation to another. In addition the greater reliance on contractors and increased mobility of staff has resulted in reduced levels of ‘inhouse’ skills and significant loss of ‘corporate memory’ at some operations. While these changes have resulted in improvements in productivity, they have provided a new set of challenges for management and those responsible for safety and environmental regulation.
Exploration and mining techniques Although geological models of different styles of mineralisation continue to form the basis of exploration, various styles new to Australia have been identified over the last decade in base metal, polymetallic and heavy mineral sand deposits. Deep weathering profiles throughout Australia have long created a challenge to mineral search. Significant developments in understanding of the regolith have provided insights into the underlying mineral potential. This has taken the form of changes to geochemical exploration methods, including greater use of trace element associations, and developments in processing of satellite imagery and airborne geophysical survey data and other techniques. Extensive shallow non-core drilling, to sample through and below the regolith, is now common at an early exploration stage. Computer modelling of orebodies has significantly advanced understanding of mineralisation and, with geostatistics, provided a platform to develop efficient mine planning procedures allowing fuller exploitation of many deposits. This can facilitate more sophisticated, selective mining but the general trend has been towards large scale, bulk mining of lower grade orebodies by open pit methods utilising sophisticated scheduling and grade control methods. This is particularly so in the gold, nickel and heavy mineral sands industries.
Mineral processing Adoption of carbon in pulp (CIP) and carbon in leach (CIL) techniques in the early 1980s revolutionised the exploitability of low grade, near surface oxide gold deposits and has enabled Australia to become a major world gold producer, with exponential growth through the latter 1980s and into the 1990s. With increasing depth of mining to the base of the zone of oxidation, a new challenge is being experienced in mineral processing and this is being met by the processing of sulphide ores by biological leaching ahead of CIP processing. Although developed for gold operations, the process is also being assessed for the treatment of nickel sulphides. Traditional smelting and refining methods still make up the bulk of base metal and nickel processing, but oxide mineralisation and very low grade ores are increasingly being treated by new developments in heap leach, solvent extraction and electro-winning techniques. Large-scale heap leaching has become a common feature at many large gold mines for the treatment of low grade material. Developments in pressure leaching techniques have provided an opportunity for exploitation of large lateritic
Geology of Australian and Papua New Guinean Mineral Deposits
THE AUSTRALIAN MINERAL INDUSTRY, 1986–1996
nickel-cobalt deposits at what are expected to be very competitive costs. Although developments in mining, geological modelling and primary processing have been instrumental in the rapid growth of the industry over the decade, developments in added-value processing have, until recently, been less obvious. However, there are now some indications of higher investment growth in smelting and refining and some added-value processing is being implemented and/or considered. At the smaller scale there are developments designed to produce lithium carbonate, fused materials and rare earth nitrates, and at the other end of the investment spectrum are ironmaking projects including a plus $1.5 billion direct reduction plant and a plus $300 million trial HI-smelt plant. In addition it seems probable that the combination of low priced natural gas and iron ore could result in other direct reduced iron (DRI) projects and integrated ‘minimill’ steel developments.
SIGNIFICANT MINERAL DISCOVERIES AND DEVELOPMENTS OVERVIEW The location of the Australian deposits described in this monograph, many of them discoveries over the last decade, is shown on the map on the inside front cover.
‘Brownfields’ versus ‘greenfields’ exploration
Mineral deposit discoveries over the decade have been dominated by gold, as might be expected given the high proportion of gold exploration expenditure (60%) over the period. Of about 200 significant discoveries since the end of 1986, about 60% are gold deposits. Base metal discoveries are the next largest group constituting about 25% of the listed discoveries, made up of copper 10%, zinc-lead 10%, and nickel 5%, resulting from base metals’ 20% share of exploration expenditure on average over the period. The remaining discoveries (15%) are a mixture of significant deposits of iron ore, heavy mineral sands, diamonds, manganese ore and others.
Brownfields discoveries are those made virtually in the shadow of an old headframe or an extension or repetition of known orebody whereas greenfield discoveries are those where no defined deposits are known within the general area.
Compared with the previous decade, this last decade has seen comparatively more base metal (including nickel) deposits discovered, about the same high proportion of gold deposits and far fewer uranium and tin deposits and relatively fewer heavy mineral sand deposits.
Gold exploration has dominated the mineral exploration scene in Australia for over ten years. The emphasis on the exploitation of low grade near-surface, previously uneconomic deposits has resulted in a re-examination of all the goldfields identified in Australia over the last 150 years. With the need to sustain ore supplies to CIP plants constructed to process 3–4 year life orebodies, there has been significant exploration for gold in brownfields areas. Similarly there has been a high level of base metal exploration and/or evaluation projects in the vicinity of known mineral deposits. Most of the greenfields exploration over the decade has been focussed on the search for diamonds and base metals and the majority of this activity has been undertaken by major companies.
Internationalisation of the Australian mining sector The success and expansion of Australian mining over the last ten to fifteen years has seen major growth in a whole raft of medium and small scale producers and an increase in stature on the international scene of many of the major companies. This, with opening of ‘virgin’ areas overseas and increasingly difficult land access issues in Australia, has resulted in increased participation by Australian companies in overseas projects. Although SE Asia, especially the island nations, has been the dominant focus, there has also been significant interest in projects in North and South America. Some 16% of all current mining projects recorded by Australian Stock Exchange–listed companies are outside Australia. Many of these projects are of a grassroots nature and some of the bigger Australian companies now allocate up to 40% of their exploration and evaluation expenditure to overseas projects. On the other side of the equation, around one-third of mineral production revenue in Australia is earned by foreign-owned companies and foreign investment is growing to meet the large sums needed for major project developments. This is likely to continue as there is insufficient money available in Australia for such developments.
Geology of Australian and Papua New Guinean Mineral Deposits
Most of the discoveries over the decade have been gold deposits within the highly prospective Western Australian Archaean Yilgarn Craton. Apart from gold in the Archaean of WA, perhaps the most significant region for new mineral discoveries since the beginning of 1987 has been the Carpentaria region, Qld-NT comprising the Mount Isa–Century*–McArthur River province to the west and the Ernest Henry*–Cloncurry–Cannington*– Osborne* province to the east. Not only have very substantial new resources been outlined in this region but several significant new deposit types have been identified and these have added to the range of mineral commodities known in the region. The discovery of the Cannington* deposit has established a benchmark for large tonnage and stratiform Proterozoic silverrich lead-zinc deposits associated with magnetite in the eastern region. Similarly, discovery of the Proterozoic stratabound Eloise, Osborne*, Ernest Henry* and Starra-Selwyn-Mount Elliott* copper-gold deposits associated with magnetite, and the lower grade Greenmount* copper-gold deposit provide evidence of the impressive mineral fecundity of the eastern region. The western part of the Carpentaria region, north from Mount Isa, is remarkable for the crop of shale-hosted stratiform lead-zinc-silver deposits discovered in the decade. These include the large Century* deposit, but also Grevillea*, Walford Creek and George Fisher (Hilton North) and the higher grade portions of the very large McArthur River deposit. Other exciting discoveries during the decade include the Challenger gold deposit in the Gawler Craton, SA, the Merlin * cluster of diamond-bearing kimberlite pipes in the McArthur Basin, NT, the Cadia-Cadia East*-Ridgeway porphyry goldcopper belt in central western NSW, the Tritton* style massive sulphide copper deposits in Ordovician sediments and mafic volcanic rocks in western NSW, new occurrences of Broken Hill* type mineralisation in the Broken Hill Block, the wholly * signifies deposits described in this publication.
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oxidised Magellan* lead deposit in the Glengarry Basin, WA, and coarse-grained heavy mineral sand deposits of the Murray Basin* of western NSW and the Ouyen area in Vic. Significant lateritic nickel deposits have been delineated at Cawse*, Bulong and Murrin Murrin* in WA and it is clear that there is the potential for many more large tonnage lateritic nickel deposits formed over the Archaean of WA.
Mode of discovery Of the nearly 200 significant new mineral deposits reported in this publication and discovered since 1986, about half had either an outcrop or subcrop surface expression or a distinct, relatively shallow geochemical soil anomaly which led directly to discovery of the deposit by deeper drilling. The remaining 40 to 45% of the discoveries were essentially blind deposits discovered as a result of extensive drilling programs. The proportion of blind orebodies discovered in the last decade was significantly greater than in the previous decade. About 55 to 60% of discoveries since the end of 1986 have been in greenfields areas and brownfields discoveries make up about 20% of the deposits discovered. The discoveries near known deposits or prospects are more difficult to classify and these discoveries, which constitute about 40% of all deposits found during the period, overlap into the brownfields discovery group. It is difficult to assign the relative importance of geology, geochemistry, or geophysics to any discovery with any degree of certainty. However it would appear that, over the last decade, just under 10% of discoveries were made by drilling geophysical (predominantly magnetic) anomalies, whereas about 60% of deposits were found by drilling geochemical anomalies. Many of the mainly geophysical and geochemical discoveries also had a strong geological input and discoveries with a predominant geological component appear to account for at least 40% of the deposits found. An increasing proportion of discoveries is being made in areas with considerable superficial cover and hence exploration is relying increasingly on extensive scout drilling programs to identify geochemical anomalies. Deeper drilling is then used to seek orebodies where significant geochemical anomalies have been detected.
Deposit types Most of the discoveries made over the decade are shear-hosted Archaean gold deposits, including about half of all of the gold deposits. Proterozoic shear, BIF or sediment hosted gold deposits made up about 15% of the deposits discovered over the period. About 30% of the gold discoveries in the decade have been porphyry-related skarn or shear gold deposit types, a larger percentage than in the previous decade. Palaeozoic epithermal or breccia pipe–related deposit discoveries accounted for about 10% of the total of gold deposits found and this is consistent with estimates of discoveries in the previous decade.
DISCOVERIES AND DEVELOPMENTS Gold - Archaean The basis of the 1980s gold boom in the Yilgarn Craton of WA has often been reported as a reappraisal of old, worked-out areas after CIP and/or CIL technology allowed the near-surface
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oxidised ores to be profitably treated. Although this may have been largely true in the early part of the decade, it has not been the case in the late 1980s and in the 1990s. Two greenstone belts which had few old workings have proved to be the most productive in respect of discoveries and development. These are the Yandal belt, in the NE of the Yilgarn Craton and the Marymia Dome (Plutonic* deposits), on the northern extension of the Craton. In the mid 1980s similarities to typical Archaean gold terrains were recognised in the Yandal greenstone belt, with regional structural settings related to granitoid margins, ironrich mafic host rock sequences and certain alteration features. These similarities led to a focus of interest in the belt. However, poorly exposed greenstones with thick accumulations of overlying transported material and deep weathering have made exploration difficult despite availability of excellent aeromagnetic maps. Major advances in the understanding of the regolith were important and anomalies from bulk leach (cyanide) extractable gold (BLEG) soil geochemical sampling have largely been responsible for rotary air blast (RAB) drilling targets and follow up systematic drilling identifying the Mount McClure* deposits between 1987 and 1990, Bronzewing in 1992 and Nimary *-Jundee* in 1993. Three Yandal belt discoveries, namely Jundee*, Bronzewing* and Mount McClure*, have combined resources amounting to nearly 8.5 Moz of contained gold. Perhaps it has been the discovery of ‘blind’ orebodies, despite their deep cover of transported material to 80 m thick, that has been the major exploration success throughout the Yilgarn greenstone belts over the decade. Determination of a favourable structural setting for Archaean gold mineralisation, primarily from aeromagnetic survey data and field mapping, has provided a regional basis for exploration. Lag sampling, soil geochemical surveys and overall regolith interpretation, followed by reconnaissance or grid RAB drilling, has been the common sequence of exploration. Examples discussed in this publication are the gold deposits in the Agnew–Lawlers greenstone belt, the Sunrise-Cleo* deposit of the Laverton belt and the Broads Dam*, Kundana* and Geko* deposits to the north and NW of Kalgoorlie. The Sunrise-Cleo deposit is perhaps atypical in that it appears to be controlled by shallow thrusted duplexes, as opposed to the steeply dipping shears commonly associated with many Archaean gold deposits. Structural interpretation of complex shear trends using geophysical, especially magnetic survey data, has led to a number of important discoveries below salt lakes and thick Tertiary cover in the SE part of the Yilgarn Craton. The Revenge* orebody between St Ives and Kambalda is described as the first blind deposit discovered by conceptual geophysical modelling in the craton. This has been followed by other discoveries at Nelsons Fleet*, near Revenge, and at Harlequin* near Norseman. Some deposits described in this publication are adjacent to long established and recent production centres. Probably the most notable is Kanowna Belle*, NE of Kalgoorlie, where historic deep lead operations ignored the potential of underlying bedrock until 1987. Kanowna Belle was found by systematic grid drilling in 1990, below 40 m of gold-depleted cover. Some 4 Moz of gold resource has been established. In the Murchison Province of the NW Yilgarn, operations had continued for a number of years around Meekatharra before the nearby South Junction * deposit was identified in mid 1989,
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THE AUSTRALIAN MINERAL INDUSTRY, 1986–1996
following up water bore anomalies, and Bluebird East* was discovered after following-up mineralised quartz float with systematic RAB drilling in 1992, in an area adjacent to the depleted pit. Whereas iron-rich tholeiitic basalts and differentiated dolerites are the prime host rocks for Archaean gold deposits in the Eastern Goldfields Province of the Yilgarn, the importance of other hosts and associations such as felsic porphyries and granite contacts is increasingly being recognised. Examples of recent discoveries are Kanowna Belle* , Granny Smith and Nelsons Fleet* and in the Murchison region, Cuddingwarra*, Bluebird* and some of the Tuckabianna* deposits. BIF-hosted deposits are more common in the Southern Cross and Murchison provinces than elsewhere with examples at Omega*, Tuckabianna* and Nannine*. The development of biological leaching processes has led to increased exploration and evaluation of primary refractory orebodies below depleted and depleting pits. Wiluna is the prime example, where all production is now from underground, eg the Bulletin* orebody. Others include Youanmi, Bronzewing*, Kanowna Belle* and Yilgarn Star*.
Gold - Proterozoic Shear and/or sediment hosted gold deposits of Palaeoproterozoic age are becoming increasingly important as new discoveries continue to be made in Early Proterozoic provinces in Australia. In the Pine Creek Geosyncline, discoveries within preexisting goldfields include shear-hosted, quartz vein and/or stockwork gold deposits like Mount Todd* and Union Reefs*, whereas the Brocks Creek* and Gold Creek* discoveries near old mines have been characterised as comprising either structurally controlled, concordant, quartz stockworks or veinsets or gold within stratabound silicate-sulphide facies BIF within Palaeoproterozoic sediments. In The Granites area, shear-hosted gold deposits occur with or without incidental iron oxide in Palaeoproterozoic metasediments. Situated 40 km west of The Granites, the Dead Bullock Soak* deposits are structurally controlled BIF and/or chert-hosted stratabound gold deposits in iron-rich metasediments and the associated near-conformable subadjacent dolerite sills. Most gold occurs within the quartzchlorite veins within iron-rich pelite although disseminated amphibole-hosted and sulphide-hosted mineralisation is still important. There is a close positive correlation between arsenopyrite and gold abundance and higher grade shoots are closely associated with parasitic folding or structural thickening. In the Jims Find-Dogbolter*-Redback* area of the Tanami mine corridor, transgressive shear-hosted gold deposits of Palaeoproterozoic age are associated with an intercalated sequence of mafic lavas and turbiditic sandstone, siltstone and carbonaceous shale. They are considered to be slightly younger than the host sediments of The Granites style of mineralisation (Dead Bullock Soak* deposits). In the Tanami corridor the gold occurs in transgressive quartz-carbonate-sulphide veins and as fine disseminations in sulphides within the wall rock. The Tanami corridor sequence is thought to be possibly time equivalent to rocks of the Leichardt River fault trough (NW Qld) and related to a widespread rifting event post-dating the Barramundi Orogeny.
Geology of Australian and Papua New Guinean Mineral Deposits
Palaeoproterozoic sequences in the Padbury, Bryah and Yerrida basins* of WA (formerly the Glengarry Sub-basin of the Nabberu Basin) have also been found to host significant shear-related gold deposits.
Gold-copper - Proterozoic These are an important class of gold and base metal–enriched deposits some of which have been newly recognised in the Australian Palaeoproterozoic over the last decade or so. The deposits occur sub-adjacent to probable Archaean domes or similar source rocks in NT, Qld, and WA. The Proterozoic iron oxide copper-gold family of mineral deposits includes the Palaeoproterozoic (ca 1825 Myr) Tennant Creek copper-gold ores as well as the younger (ca 1650 Myr) deposits around Cloncurry in NW Qld. The White Devil* gold deposit is hosted by turbiditic Warramunga Group sediments and is an iron-rich, copperpoor, Palaeoproterozoic gold-bismuth deposit similar in many respects to other deposits in the Tennant Creek field. The structurally controlled, epigenetic deposit results from a two stage mineralising process involving formation of ironstone bodies at the time of deformation which are then in some parts later mineralised by gold±bismuth-bearing fluids. The copper-gold discoveries of the Eastern Fold Belt in the Cloncurry area of NW Qld include the magnetite-bearing Mount Elliott*-Ernest Henry*-Osborne deposits*. To the west the Tick Hill* gold deposit, which is also associated with ironrich sediments, occurs in the Mary Kathleen Fold Belt and may well belong in this group of deposits.
Lead-zinc-silver-gold Exploration over the past decade has revealed new areas within the Archaean of WA which contain significant VMS-related zinc-copper deposits (the Panorama* group in the Pilbara and the Nimbus* silver-lead-zinc deposit near Kalgoorlie). Exploration led to the discovery of outcropping gossans in each case. The mineralising event at Panorama is known to have occurred about 3240 Myr ago and they are among the oldest known VMS deposits in the world. Significant new Palaeoproterozoic stratiform lead-zinc massive sulphide deposits of the Mount Isa–Hilton type, ranging from those containing substantial beds of massive pyrite (ie high iron sulphide deposits like Grevillea *) to the large world-class zinc-rich but iron sulphide–poor Century* deposit illustrate the importance of this province. The large magnetite-associated Cannington* lead-zinc deposit carrying particularly high silver values is a major recent discovery in the Eastern Fold Belt and has further emphasised the demonstrable additional potential of the entire Mount Isa Inlier. Drilling around and beneath known early Palaeozoic VMS lead-zinc-copper deposits has also resulted in significant increases in base metal resources due to the discovery in many cases of additional lenses associated with the known deposits at Rosebery*, Tas, Surveyor *, Qld and Lewis Ponds*, NSW. Of particular interest in this regard is the discovery of the Henty* deposit, south of Rosebery, Tas. This is a high grade gold deposit spatially associated with small VMS massive sulphide lenses within or adjacent to two major intersecting fault zones.
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Nickel Exploration over the decade has defined a number of nickel sulphide deposits and at least three lateritic nickel deposits associated with the greenstone belts of the Yilgarn Craton in WA. In addition, the Brolga* laterite deposit 45 km NNW of Rockhampton in Qld was defined and mined for treatment at the Yabulu refinery near Townsville between 1993 and 1995. New nickel sulphide deposits described include the Honeymoon Well* deposits some 45 km south of Wiluna, the Rocky’s Reward * deposit some 15 km north of Leinster, the Silver Swan-Cygnet-Black Swan* group some 43 km NNE of Kalgoorlie, and the Kambalda* district group including the Helmut, Mariners, Blair and Coronet deposits. In addition papers are included on the Mount Keith* deposits, the Perseverance* (Agnew) deposit, and the Forrestania* group of deposits. Nickel sulphide mineralisation was first discovered in these areas in the 1960s and 1970s but more detailed investigations based on new interpretations of the geology in recent years have added substantially to reserves and resources. Of the deposits described mining has occurred during the last decade at Mount Keith*, Rocky’s Reward*, Perseverance*, Silver Swan *, the Kambalda* district group (Helmut, Mariners, Blair and Carnilya Hill) and the Forrestania* group (Cosmic Boy and Flying Fox). Geological studies over the last decade have indicated that the nickel sulphides were most likely erupted with ultramafic volcanic rocks and have been subsequently remobilised to varying degrees during tectonism, resulting in a variety of different structural and lithological settings for the massive sulphide deposits in particular. Lateritic nickel deposits formed on Archaean ultramafic rocks in the eastern Goldfields of WA at Cawse*, Bulong and Murrin Murrin* are currently being prepared for production. Lateritic nickel-cobalt mineralisation has been known near each of these deposits for about 25 years but it was not until the early 1990s that they attracted attention as potentially commercial projects. The higher grade Cawse* deposit was discovered during an exploration program designed to find gold-bearing palaeochannels. Unlike the other deposits which contain nickel in silicates, the nickel at Cawse is in limonitic clay. The impetus for development of these lateritic projects has arisen from the availability of natural gas in the region following the completion of the Goldfields gas pipeline, and the availability of sulphuric acid following the construction of the large acid plant at Western Mining Corporation’s nickel smelter between Kalgoorlie and Kambalda. By-product cobalt will be an important factor in determining the economic viability of each of the projects.
Iron ore The decade has been one of evaluation and development in the iron ore industry rather than discovery. The search for premium quality, high grade, low phosphorus ores of the Brockman Iron Formation in the Hamersley Basin of Western Australia continues to consume a significant proportion of the exploration dollars, but, to date, without success. Developments have partly replaced depleting orebodies, have partly expanded WA’s share of the export market and provided a diversity of product to the portfolios of existing producers. Over the decade known deposits have been
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developed at Channar, Jimblebar, Newman Orebodies 23 and 25 which contain high grade Brockman Iron Formation ore, and Marandoo, the first major Marra Mamba Iron Formation ore development, all in the Hamersley Basin sequence. In addition mining of Tertiary pisolite ore, known as channel iron deposits (CID), has been developed at Mesa J at Deepdale and at the BHP’s Marillana Creek at Yandicoogina. Development is now proceeding at Hamersley Iron’s Junction deposit at Yandicoogina. As part of a replacement program of the mines near Mount Goldsworthy, developments at Nimingarra and Yarrie* in the Archaean greenstones in the Pilbara have taken place. It should be noted that in addition to Archaean enriched ores, at Yarrie, the Y10* deposit consists of a Palaeoproterozoic hematite conglomerate, unconformably overlying and derived from the Archaean banded iron formation (BIF). The Koolyanobbing deposit in the Yilgarn Archaean BIF has also been redeveloped during this period, and other enriched and primary BIF deposits in the Yilgarn (Tallering Peak and Mount Gibson) are currently being reappraised as part of integrated iron and/or steelmaking projects. Intense evaluation of Marra Mamba type deposits is currently taking place in the Hamersley Basin at the West Angelas, Mining Area C and Hope Downs*. Generally as part of iron and steelmaking projects, primary, low grade BIF is being considered (probably for the first time in Australia) as high purity feedstock for direct reduction processes. Evaluation of primary BIF deposits is proceeding in the Archaean (Pilbara and Yilgarn) cratons and in the Hamersley Basin (Brockman Iron Formation), WA and in the northern part of the Gawler Craton*, SA. During the late 1970s and early 1980s testing of small detrital iron deposits was successfully undertaken to provide lump ore supplement to blended products. Targets of broad topographic features and gravity anomalies, and follow-up scout drilling, have led to the discovery of a number of moderate-sized detrital deposits adjacent to Brockman outcrop in the Hamersley Basin over the last decade. The deposits may form as scree slopes, outwash plains and channel fill and provide important high quality lump ore supplement to meet contracted requirements. The Brockman No 2 Detritals (B2D) * deposit is a significant example of this type.
Heavy mineral sands The heavy mineral sand deposits of Australia were described in some detail in an earlier monograph (Baxter, 1990; Shepherd, 1990; Masters, 1990; Wallis and Oakes, 1990; Williams, 1990). Since that time there have been substantial additions to resources in the SW of WA and more recently a number of significant new discoveries in the Murray Basin* in northwestern Vic and southwestern NSW. To date in WA most of the production of heavy mineral sands has come from well-defined strand lines of Plio–Pleistocene or Recent age. The deposits mined were high grade, linear and relatively near surface. Some more recent discoveries appear to be in new geological settings including some deposits possibly in older sediments of the Perth Basin overlying basement rocks of the Leeuwin Complex. Some of the new deposits appear to be broader, significantly deeper and include less oxidised minerals. Beenup is the major deposit of this type to be identified to date and it has now been developed for commercial production.
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THE AUSTRALIAN MINERAL INDUSTRY, 1986–1996
Recent discoveries of heavy mineral sand deposits in the Mallee area in north western Vic have led to applications for large tracts of land in Vic and across the Murray River in NSW. Preliminary results indicate that unlike the earlier WIM discoveries of fine grained mineralisation in the Murray Basin, the latest discoveries are amenable to conventional mineral sand separation techniques. The Murray Basin is now being hailed as having the potential to be a major source of titanium and zircon for the next 30 years or more.
To the mineral explorer or developer there is a strategic attraction in being one of the first to assess a relatively unexplored region. Furthermore a number of governments have been keen to attract investment, and have made aggressive efforts to provide incentive packages including legislative and fiscal reforms. International organisations, such as UNDP and World Bank, have also played a part in a number of countries by providing funding for the establishment of new legislative regimes.
FUTURE ISSUES, OPPORTUNITIES AND CHALLENGES
In some countries, the reconciliation of the governments’ and investors’ objectives has not been successful and there are some indications that the diversion of exploration and development funds from the established mining regions to these countries may be short lived. It seems likely that, as exploration and evaluation proceed, companies will experience increasing difficulties in some developing countries because of cultural and sovereign protection issues. This could lead to some return of investors over the next decade to areas with more stable legislative and administrative systems and a history of successful investment in high risk mining activities.
The Australian mineral industry has the resource base to maintain and expand its international role in the coming decade. However, there are a number of issues to be addressed and challenges to be met if it is to achieve its full potential. The major issues for the industry in the coming decade are summarised below. The international issues are dealt with first, followed by the national issues and finally there are brief comments on the major scientific and technical challenges which, if answered, could provide our industry with the competitive edge necessary to succeed.
INTERNATIONAL ISSUES Economic growth and regional demand for minerals There seems little doubt that population growth will continue at about current levels for the next decade or so and it is expected that a major part of this growth will occur in the Asian region immediately to the north of Australia. Economic growth is also expected to continue with rates in the Asian region expected to average about two or three times that of the OECD countries. The expected growth in population and economic activity will be accompanied by increasing consumption of minerals irrespective of any increased efficiency of use. Furthermore, we can expect very high rates of growth in consumption to continue in the Asian region. The expected pattern of demand for minerals in the Asian region will give Australian producers a competitive advantage especially for relatively large-volume, low-cost commodities. However, there will be strong competition from resource producers in some of the developing countries and there is likely to be a continuation in the downward trend in real prices for most commodities in the foreseeable future. The challenge for Australian companies will be to contain costs and maintain competitiveness in the international market-place. Unless we are competitive in terms of costs, infrastructure and legislative support, Australia will not attract the continuing high level of investment funding necessary to find and develop our resources.
Competition for exploration and development investment The natural progression from less developed to developed nation status has gained impetus over the last 15 years or so in many regions of the world, most notably South America, Africa and SE Asia. Additionally, political stabilisation in Asia, Eastern Europe and the former Soviet block and the changes towards market economies, have resulted in opening up of new frontiers for mineral explorers and developers. At the same time the traditional mining areas such as Australia, Canada and the USA have, on the other hand, suffered from increasing land access difficulties.
Geology of Australian and Papua New Guinean Mineral Deposits
Greenhouse gas emissions and global warming The United Nations Framework Convention on Climate Change (FCCC) came into force in March 1994. The first conference of parties to the Convention was held in Berlin in 1995, where there was agreement on a mandate for further negotiations aimed at setting greenhouse gas emission reduction objectives for developed countries for the period beyond 2000. The issue of greenhouse gas emissions and especially the prospect of mandatory targets is likely to present a major challenge to the government and the mineral industry in Australia in the next decade. Australia is a relatively efficient user of energy but our per capita emissions of greenhouse gases are high in world terms and will increase considerably if plans to increase the level of mineral processing in Australia come to fruition. Studies by ABARE (Woffenden et al, 1997) have shown how unevenly the costs of emission abatement would fall on different countries. The ABARE simulations show that by 2020 the welfare loss for the average Australian arising from a uniform reduction in emissions would be 5.5 times the loss of the average European and twice that of the average American. Australia’s per capita emissions are high because of our large reliance on coal to generate power and the high energy intensity of our industry. If Australia is unable to convince the FCCC of the merits of its suggested approach of ‘equitable burden-sharing’ in the application of emission targets, many large energy-intensive mineral developments in Australia will be at risk. In addition the imposition of emission targets will have the effect of reducing the demand for and price of coal which is Australia’s major commodity export.
NATIONAL ISSUES The most important national issues for the Australian mineral industry in the next decade are likely to be native title and environmental protection. Other significant issues will be the international competitiveness of our financial and legislative framework, the costs and benefits of promoting and encouraging value adding to our raw mineral products, the
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L C RANFORD, D J PERKIN and W A PRESTON
adequacy of our education, training and research schemes to meet the industry’s needs, and industry’s ability to maintain and increase the mineral reserves on which the industry is based. Many of these issues will be influenced by the public’s perception of the industry and its contribution to their welfare.
The challenge for industry will be to show that such a system can operate without significant impact on environmental values and that defining all the ‘no go’ areas is unnecessary and undesirable in the national interest.
The industry is global in its scope and investment decisions are based on perceptions of the relative competitiveness of the alternatives. The consequences of not meeting this challenge will be reduced investment in exploration and mining, a gradual run down in the industry and a substantial decrease in economic welfare for all Australians.
Financial and legislative framework
Native title issues
There have been substantial moves towards internationalisation of the Australian economy over the last decade and some improvements in taxation. The challenge in the next decade will be to achieve the further changes necessary to maintain our competitiveness, as other countries with equal or better prospectivity match our past advantages in terms of stability and security of tenure. Industry has pointed to the need for tax reform and the need to streamline administrative procedures by eliminating unnecessary duplication of regulation by the Commonwealth Government and the State Governments. The Commonwealth Government has foreshadowed action on both fronts, but has not yet specified measures it will take. In the absence of appropriate action, these issues are likely to become more important as international competition increases throughout the forthcoming decade.
The flow-on effects of the High Court’s Mabo decision of June 1992 and the Wik decision of December 1996 will be a major issue and challenge to governments in Australia during the next decade. The issue and the various government decisions to deal with the matter will also present a very real challenge to the mineral industry. The industry seems likely to bear the brunt of the attempts that will be made by Aboriginal people to establish an economic base for themselves in Australian society. This is because mining operations must be sited where the orebody occurs and the industry is perceived as having the potential to create great wealth for those involved. The challenge for all concerned will be to develop arrangements and agreements which take account of the rights and objectives of all parties involved, in a manner which is conducive to continued investment in exploration and mining.
Environmental concerns Although issues associated with native title will provide a major challenge to governments in Australia over the next decade, environmental issues are likely to be the number one concern of the public in relation to mining. The results of the recent opinion survey conducted by CRA (1996) suggest that this may already be the case. Respondents to the CRA survey indicated that education was the highest priority challenge facing the Australian community and that protection of the environment was next. They also indicated that the five most important environmental responsibilities of companies were: 1.
taking great care when conducting exploration and mining in sensitive areas;
2.
sound rehabilitation of areas mined;
3.
taking responsibility for environmental impacts caused by the company;
4.
minimising waste and emissions; and
5.
raising environmental awareness among employers and communities.
Few people would disagree that it is important to preserve indigenous flora and fauna, maintain biodiversity and protect unique and special land forms. The best means of meeting these conservation objectives while maximising economic welfare is through multiple and sequential land use and this approach is currently espoused by the Commonwealth Government and some State Governments. A multiple land use approach allows governments the freedom to set aside large areas to be managed for conservation objectives in the knowledge that they can be assessed and evaluated for other land uses without prejudice as to the eventual use of any particular part of any area.
26
The international competitiveness of the Australian mineral industry will continue to be an important issue during the next decade. The financial and legislative framework for the industry will be a significant element in determining our competitiveness.
Value adding in the mineral sector Further processing of mineral commodities has been and still is supported by governments in Australia as a means of increasing employment and reducing our current account deficit. However, some economists have pointed out that the fundamental policy objective of maximising community welfare is more likely to be achieved by maximising value added per person employed, rather than per unit of any commodity (White, 1992). There is probably general agreement that profitable industrial activities which add value to commodities are desirable. However, there is some concern about policies that require government intervention to promote greater value adding other than action to remove impediments. The mining sector already produces the highest level of value added per person employed and, although it is relatively capital intensive, the sector produces nearly as much output per unit of capital as the manufacturing sector and more than some other sectors such as finance, recreation, transport and communication, utilities and agriculture. There may be ongoing debate about the community benefits to be derived from government intervention to promote value adding to our mineral commodities. However, there is broad support for reforms designed to achieve greater economic efficiency in the provision of basic services including energy, water and transport. It is generally accepted that such improvements will provide the stimulus necessary to encourage further domestic processing of some mineral commodities. Considering the magnitude of Australian exports of mineral commodities, the apparent opportunities that exist for value adding, the energy intensive nature of most mineral processing and the pressure to reduce greenhouse gas emissions, this topic is likely to be a subject of considerable debate in industry and government over the next decade.
Geology of Australian and Papua New Guinean Mineral Deposits
THE AUSTRALIAN MINERAL INDUSTRY, 1986–1996
Education and training The adequacy of education and training systems to meet the industry’s future needs has been a topic of discussion throughout the last decade and this issue is also likely to be prominent in the next decade. In 1986 John Udd delivered a plenary address to the 13th Congress of the Council of Mining and Metallurgical Institutions on this subject suggesting that, in the 21st century, a minerals industry professional will require:
1.
a solid technical knowledge base in the area of specialisation;
2.
knowledge of occupational health and safety, environmental and cultural legislation and of expectations of the broader community with respect to the industry;
3.
a basic understanding of economic analysis and how business decisions are made; and the ability to operate in a diverse range of mineral operations and countries.
1.
education in information systems and improved communication skills;
4.
2.
education in management and the psychology of management;
3.
greater emphasis on studies of fundamental principles (basic applied science); and
The task force also mentioned acceptance of continuous learning and other skills including management skills and communications skills.
4.
continuing education or re-education.
Udd argued there had been too much emphasis on ‘training’ rather than ‘education’ and that what was needed were professionals educated to understand the philosophy of problem definition and solving, scientific methods, and basic principles. More recently Yearley (1995) summed up his views to students at the Colorado School of Mines with the following comments: ‘You may think you are going to be a mining engineer or a mineral economist. But to be successful you will also need to be an ecologist, a lobbyist, an advocate, a public affairs person, an educator, a psychologist capable of dealing with peers and a sociologist capable of understanding social movements.’ Yearley suggested that the future of mining in the United States is dependent on the nation’s ability to produce a new breed of professionals who are not only knowledgeable about mining but are knowledgeable, caring and concerned about the attitudes, culture and communities in which the industry operates. Willson (1995) has suggested that a decade or so ago a good education in mining engineering, geology or metallurgy, coupled with a willingness to work hard, was all that was needed for a successful career in the mining industry. He suggested that in the future a mining career will involve negotiating through a maze of laws on everything from tenure to taxation, respecting the rights and cultures of local inhabitants and land owners, and protecting the natural environment. Addressing the topic of ‘Major Challenges for Mining and Mineral Processing at the End of the 20th Century’, Rohde (1995) contended that the major challenges will be the effects of the collapse of the socialist ideology, advances in global communication and the protection of the environment. To deal with these challenges Rohde suggested that mining industry professionals must be educated with a broad technical, economic and philosophical background to enable them to follow rapid advances in knowledge, and trained to think and behave globally, with a command of foreign languages and a knowledge of the spiritual and cultural values of the countries in which they work. A recent discussion paper published by the Chamber of Minerals and Energy of Western Australia (1996) considered issues related to the development of the next generation of minerals industry specialist professionals. The industry task force which prepared the paper concluded that the graduate of the future should have:
Geology of Australian and Papua New Guinean Mineral Deposits
The above discussion paper attracted considerable interest from industry and the universities throughout Australia and as a result the Minerals Council of Australia established a National Tertiary Education Taskforce (NTET) in late 1996 for The Development of World Class Education Institutions for a World Class Mining Industry. The NTET is to prepare a series of discussion papers during 1997 leading to the formulation of a strategic plan for industry to achieve its educational requirements. From the comments offered by a wide range of people involved in the minerals industry, it is clear that the demands on the future industry professional will be quite different from those of the past. However, as in the past, the mix of skills and knowledge required will change with career progression, and because of the accelerating rate of change, the need for continuing education will be greater than ever before.
Improved coordination and focus of R&D There has been a significant change in the minerals industry R&D environment in Australia over the last decade and the rate of change has accelerated in the last five years (Napier-Munn, 1996). Minerals industry R&D is funded by and/or performed by a large number of Commonwealth, State, industry and tertiary sector institutions and much of the change has been driven by changes to the mechanisms used to fund R&D in Government institutions. Some Commonwealth agencies such as CSIRO and AGSO have had to obtain 30% of their funding from industry sources and there has also been a requirement for industry involvement and support in the Cooperative Research Centres (CRCs) funded by the Commonwealth Government. There now appears to be general consensus that the most effective and focussed research relevant to industry needs will result from cooperative arrangements that involve the range of parties who will feel some commitment to its outcomes. Furthermore, as pointed out in the ANZMEC paper on Gaps in Research in the Minerals Sector released in 1993, many of the gaps identified in Australia’s minerals R&D effort have the potential to be met within the CRC arrangements. The ANZMEC study revealed that industry was, in general, of the view that commercial imperatives have ensured that the applied R&D required by individual firms is being done and that government should have no input into determining research requirements. The paper indicates that there was a strong view across industry that there should be an increased contribution by government in the pre-competitive stage of research with additional funding to provide an internationally strong technology base. Industry made the point that there was an urgent need for government to direct additional funds to
27
L C RANFORD, D J PERKIN and W A PRESTON
universities to train quality graduates and to finance mapping programs by geological surveys. Companies which produce products for the mining industry also indicated that there is a need for substantial additional funding for development and commercialisation of the products of Australian research.
Recent research conducted in the United States indicates that the agenda for discussion of the gold mining industry in that country is determined by its critics (Bateman, 1997) and the same would appear to be so in Australia. The critics of the mining industry commonly have genuine concerns and fears but poor understanding of the industry and its likely impacts.
The challenge in the coming decade will be to obtain maximum value for the Australian mineral industry from the limited research funding that will be available. It appears that further moves to create cooperative and collaborative arrangements between the various government and industry institutions will be essential to meet industry’s research needs and to provide the quality of graduates necessary to maintain Australia’s international competitiveness.
The indications are that with growing public interest in environmental protection and conservation, the mining industry will have to mount a major public relations effort in the next decade or face the prospect of reduced access to land and additional stringent, mandatory restrictions and performance criteria. This will probably be one of the challenges for the industry and its representative bodies during the next decade.
Ore reserves on which to base our industry
SCIENTIFIC AND TECHNICAL NEEDS AND OPPORTUNITIES
Australia is fortunate to have the reserves and identified resources necessary to ensure that we can be a major world supplier of minerals such as iron ore, bauxite, heavy mineral sands, nickel, coal, uranium, lead, silver and zinc, manganese, tantalum and lithium well into the next century. However, our reserves of gold, diamonds and copper are not adequate to guarantee our long term future as a major supplier to world markets. The challenge over the next decade will be to attract the investment necessary to identify and delineate the reserves of those commodities necessary to maintain current production levels, and establish the production basis for higher levels, and for an even more diverse range of minerals in the longer term.
Public perceptions of mining It is widely accepted throughout the mineral industry that public perceptions of the industry are not particularly favourable and that this is an important issue with implications for the political process. The industry is not a major employer of labour and although it is indirectly responsible for many jobs, this relationship is not generally appreciated by the community. Many mining operations are based in remote locations and our largely urban-based population is not very familiar with the way the business is conducted. The industry, through its various industry associations, has made efforts over the last decade to explain its mode of operations and its contribution to society. Surveys conducted by these organisations indicate that some progress is being made to improve the industry’s image. Programs designed to educate teachers about the industry and involve students in projects on aspects of the petroleum and mineral industries appear to have been particularly successful in helping to change attitudes. A comprehensive community survey was conducted by the CSIRO in the early 1990s to determine the attitudes to mining in Western Australia (Syme, 1992). Of the respondents who indicated an interest in mining-related issues, the most commonly nominated were environmental matters and the next most common were to do with industry operations and management. The concerns about operations and management were related to foreign ownership, standard of management and safety issues. The mining industry was seen as being rich, dirty, large, multinational and aggressive. It was rated as being similar to agriculture and tourism in terms of desirability and reliability and was rated as being more environmentally responsible than agriculture, the chemical industry and forestry but less so than tourism.
28
Mineral exploration In the next decade mineral explorers will be asked to discover and delineate ore reserves to replace those currently being mined and additional reserves to serve as a basis for an expanded industry. It seems likely that the long term trend of decreased real prices for minerals will continue and explorers will be expected to find more reserves not only at lower cost but exploitable at a lower unit cost and with minimum impact on the environment. The challenge will be to develop exploration approaches and techniques which enable us to identify, delineate and evaluate targets more quickly, effectively, efficiently and with minimal disturbance to the natural environment. In those areas which have already been subject to modern exploration techniques the challenge will be to develop and test targets which have little or no obvious surface expression and which may be at considerable depth beneath the earth’s surface (Cucuzza and Goode, this publication). The challenges and opportunities for explorers can be considered in terms of the sciences and technologies involved as outlined below. However, successes are more likely to result from the integration of a number of these various facets of the exploration process. The challenges and opportunities for explorers can be considered in terms of geology, geophysics, geochemistry, spatial data management and analysis, and sampling technology.
Geological challenges and opportunities An improved understanding of regional geology and the geological controls and processes involved in the formation of ore deposits offers the potential for better area selection at an early stage, the development of conceptual targets for testing and the basis for more efficient delineation and evaluation of mineral discoveries. As noted by McKelvey (1996) the geological map will remain the fundamental research tool in enhancing our ability to discover mineral deposits. The first stage of exploration in any area has generally been focussed on the obvious surface expression of ore deposits such as gossans and direct geochemical evidence. Later phases of exploration require increasingly sophisticated three dimensional understanding of the regional geology and the processes involved in ore formation.
Geology of Australian and Papua New Guinean Mineral Deposits
THE AUSTRALIAN MINERAL INDUSTRY, 1986–1996
The challenge for Australian geologists in the next decade will be to ensure that there is continuous improvement in the understanding of the regional geological framework of the Australian continent, that they stay abreast of world developments in the understanding of the processes and controls of ore formation, and that they provide development and training of economic geologists in analytical and conceptual skills to interpret the vast amount of data that will become available.
Geophysical challenges and opportunities Geophysics, and in particular airborne magnetic surveys, has been a major contributor over the last decade to the improved understanding of regional geology in areas of sparse exposure or with thin surficial cover. Regional airborne magnetic surveys have resulted in improvements in the efficiency of geological mapping and the quality of the product. This has in turn led to improved target identification and assessment. Gee (1997) has suggested that, despite undoubted future improvements in this technique, there are apparent limitations to the future value of its use in 3-D modelling. He believes that airborne gravity offers the greatest potential for an advance in exploration geophysics in the coming decades. He also expects great improvements in electrical methods such as airborne EM and IP which could not only assist in 3-D geological understanding but also directly identify anomalies reflecting the presence of ore deposits.
processing software to facilitate interpretation and the development of improved knowledge of the geochemical background so that anomalies can be identified and interpreted.
Geographic information systems The need to handle and interpret vast and increasing amounts of data in a 3-D context is common to all aspects of mineral exploration and this will undoubtedly be a focus of research and development. The expansion during the next decade of geographic information systems (GIS) which will allow the integration and interrogation of a range of spatial data sets and the development of software to allow the 3-D visualisation of geological, geophysical and chemical data could provide a significant competitive advantage to explorers operating in areas at the mature stage of investigation and data acquisition.
Drilling and sampling technology
The challenge will be to develop gravity and electrical methods that can rapidly cover large areas at relatively low cost while maintaining and or improving the quality and depth penetration of the existing ground based systems. We also require more sophisticated computer software to enable us to interpret these data and look through conductive overburden.
The science and technology referred to above is all about identifying exploration targets. The other element of exploration is the testing or sampling of the identified target. This sampling, usually by drilling, accounts for between onethird and two-thirds of the total cost of exploration. It presents particular challenges and opportunities for the application of new technology. We have seen continuous improvement of drilling technology in recent years and the development of some down-hole techniques which enable greater use of the access created. However, there appear to be opportunities for completely new approaches to enable rapid sampling from considerable depth, at relatively low cost and with minimal ground disturbance. Such a technique could revolutionise the exploration process and facilitate the rapid and economical investigation of a whole range of targets at greater depths than is currently practical.
Geochemical challenges and opportunities
Mineral extraction and processing
Geochemistry has been a major contributor to the discovery and delineation of new ore deposits over the last decade. Etheridge and Henley (1997) consider that geochemistry has been the principal method used in the exploration for gold over the last 20 years. This probably means that it has been the principal scientific method used in all mineral exploration because gold exploration has accounted for between 50 and 80% of the total mineral exploration effort throughout the period.
Gould (1993) pointed out that future mining companies will need to be technically innovative if they are to bring new mineral deposits into production at competitive costs. He suggested that the areas requiring technical advancement include: 1.
in-situ leaching;
2.
fine and ultrafine grinding;
3.
fine particle beneficiation;
The success of geochemical surveys is partly a result of improved analytical techniques which allow accurate, low cost analyses at increasingly low detection limits for a wide range of elements. In addition there have been dramatic improvements in our understanding of the dissolution, migration and concentration of elements in the weathered zone near the earth’s surface.
4.
heap leach, bio-oxidation;
5.
bacterial pretreatment and recovery of non-sulphide metals;
6.
enhanced coal preparation and dewatering;
7.
solution of various environmental problems;
8.
sophisticated ore-sorting methods; and
There is no doubt that the next decade will see significant improvements in the analysis of rock and soil samples to lower detection limits at reduced costs. There will also be a significant increase in regional geochemical data sets and 3-D geochemical models of a range of different types of economic ore deposits and their expression in the zone of weathering. It would seem, however, that the major challenge will be to convert this vast amount of geochemical data into information of value to the mineral explorer. Gee (1997) has suggested that this challenge will lead to the development of robust data bases for storage and manipulation, the development of image
9.
mine-site smelting.
Geology of Australian and Papua New Guinean Mineral Deposits
Gould is of the opinion that over many years, a large portion of Australian R&D has been devoted to achieving technical breakthroughs. This, he believes, has led to a few large step changes in quality, cost or delivery spread over a long period with few incremental improvements in between. He says that business is now focusing more on the Japanese Kaizen approach aimed at bringing about smaller step improvements on a regular basis and that the client-driven approach now in vogue will push public and university research increasingly in that direction.
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L C RANFORD, D J PERKIN and W A PRESTON
On the other hand, Lawrence (1996) has stated that the breakthroughs in mineral extraction technology over the last 30 years have resulted from a series of evolutionary changes rather than revolutionary changes. He has suggested that the drivers of change in the future are likely to be the ability of technology to: 1.
reduce costs by improving processes or eliminating steps;
2.
reduce occupational risk; and
3.
reduce environmental risk or consequence.
He has also suggested that evaluation of the foreseeable technological improvements over the next decade indicates that there will be strong pressure to develop in situ leaching processes, the use of direct electrowinning and the recovery of by-products from wastes. Lawrence believes that the combination of pressures for cost reduction and improved safety will lead to developments in continuous rock excavation, in-mine processing and increased use of automation and robotics. In 1992 Torlach pointed to the challenge of managing risk in the mining industry with increased mechanisation and application of higher technology. He has pointed out improvements which could initiate improved safety such as: 1.
improved explosives technology;
2.
mining methods which break away from the drill, blast and muck cycle;
3.
advances in geomechanics which might allow improved prediction of ground stability;
4.
improvements in communication technology which will allow improved emergency control and general efficiency;
5.
improved equipment to detect noxious gases and other harmful atmospheric contaminants; and
6.
improved process control and monitoring systems.
In summary it seems certain that over the next decade the mining industry will face the challenge of containing and lowering costs and improving its environmental and safety performance, while it deals with orebodies of lower grade, at greater depth and with increased metallurgical problems. In the face of these challenges there will be opportunities for: 1.
significant developments in geotechnical understanding and prediction;
2.
improved computer modelling for mine planning;
3.
improved explosives technology and performance resulting in reduced dilution and improved grade control;
4.
greater mechanisation of mining processes with increasing use of robots;
5.
improved metallurgical extraction processes with reduced energy consumption and less environmental impact; and
6.
improved communication and information systems to enable central monitoring and control of all facets of the extraction process.
To address these challenges and take advantage of the opportunities Australia will need an education system and research facilities capable of attracting and developing the necessary core of highly skilled specialists.
30
ACKNOWLEDGEMENTS The authors are indebted to staff of the Policy and Planning Division of the WA Department of Minerals and Energy who assisted with the compilation of economic data, the preparation of the tables and figures and the typing of the manuscript. Particular thanks are due to J Hughes, E Sisti, G Hartley and M Bryan.
REFERENCES ABARE, 1996. Australian Commodity Statistics, 1996 (Australian Bureau of Agricultural and Resource Economics: Canberra). ABARE, 1997. Australian Mineral Statistics, December quarter 1996 (Australian Bureau of Agricultural and Resource Economics: Canberra). ABS, 1987 to 1997. Actual and Expected Private Mineral Exploration, Australia, ABS Catalogue No 8412.0 (Australian Bureau of Statistics: Canberra). ANZMEC, 1993. Gaps in research in the minerals sector - a background paper, Commonwealth Department of Primary Industries and Energy, Canberra (unpublished). Bateman, P, 1997. Do public perceptions matter? in Proceedings of the 1997 Australian Gold Conference, pp 267–277 (Chamber of Minerals and Energy of WA: Perth). Baxter, J L, 1990. Heavy mineral sand deposits of Western Australia, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1587–1590 (The Australasian Institute of Mining and Metallurgy: Melbourne). BRS, 1996. Australia’s Identified Mineral Resources, 1996 (Bureau of Resource Sciences: Canberra). Chamber of Minerals and Energy, 1996. Western Australia Minerals Industry Tertiary Education Taskforce - Discussion, Chamber of Minerals and Energy, Perth. Clements, K W and Qiang, Ye, 1996. Multiplier Effects of the Western Australian Mining and Mineral Processing Industries, Economic Research Centre, Results Statement No 1 (unpublished), The University of Western Australia, Perth. Commonwealth Government, 1990. Ecologically Sustainable Development - A Commonwealth Discussion Paper (Australian Government Publishing Service: Canberra). CRA, 1996. Issues and Expectations - Results of CRA’s 1996 Community Issues and Priorities Survey (CRA Limited: Melbourne). Etheridge, M E and Henley, R, 1997. Managing risk in gold exploration - effective use of modern geoscience technology, in Proceedings of the 1997 Australian Gold Conference, pp 197–209 (Chamber of Minerals and Energy of WA: Perth). Gee, R D, 1997. Trends in exploration: the next 10 years, in Proceedings of a Seminar entitled Directions in Exploration (Key Centre, Department of Geology and Geophysics, The University of Western Australia: Perth). Gould, I, 1993. Research needs for the mining industry, Mining Review, June 1993, pp 37–44. Industry Commission, 1996. Implications for Australia of Firms Locating Offshore, Report No 53 (Australian Government Publishing Service: Canberra). International Energy Agency, 1997. Energy Policies of IEA Countries - Australia 1997 Review (OECD: Paris). Lawrence, I R, 1996. Mineral processing in the 21st century, in Proceedings of 1996 Mining Technology Conference, Fremantle, 10–11 September 1996, pp 126–149 (Cooperative Research Centre for Mining Technology and Equipment: Brisbane). Masters, B K, 1990. Heavy mineral deposits in the Yoganup Formation, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1587–1590 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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THE AUSTRALIAN MINERAL INDUSTRY, 1986–1996
McKelvey, G E, 1996. Frontiers in exploration research - philosophical overview, Nonrenewable Resources, 5(2): 85–90. Metals Economic Group, 1996. Corporate Exploration Strategies: A Worldwide Analysis (Halifax: Canada). Minerals Council of Australia, 1997. Minerals Industry ‘96 - Survey Report (Minerals Council of Australia: Canberra). Napier-Munn, T J, 1996. Living with change - a university view of the minerals industry research in the 1990s, The AusIMM Proceedings, 301(1):47–52. Ranford, L C, 1991. Sustainable development, resource management and land access, in Mineral Exploration in an Environmentally Conscious Society, Australian Institute of Geoscientists Bulletin No 11, pp 23–31 (Geological Society of Australia Inc, and Australian Institute of Geoscientists: Sydney). Rohde, P, 1995. Major changes for mining and mineral processing at the end of the Twentieth Century: A European view, in View from the Helm - Mineral Industries in Transition (Ed: J E Tilton), pp 46–69 (Mining Journal Books Limited: London). Shepherd, M S, 1990. Eneabba heavy mineral sand placers, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1587–1590 (The Australasian Institute of Mining and Metallurgy: Melbourne). Syme, G, 1992. Report on a major survey of community perceptions of the industry, in Proceedings Minerals Outlook 1992, pp 13–21 (Chamber of Mines and Energy Western Australia Inc: Perth). The Commission for the Future, 1990. Our Common Future (Oxford University Press: Melbourne). Torlach, J M, 1992. The future workplace - implications for occupational health and safety, Department of Minerals and Energy report, Western Australia, December 1992 (unpublished).
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Udd, J E, 1986. Some thoughts on mineral industry education for the 21st Century, in Proceedings of the 13th CMMI Congress, Vol 6 (Ed: J T Woodcock), pp 69–71 (The Australasian Institute of Mining and Metallurgy: Melbourne). Wallis, D S and Oakes, G M, 1990. Heavy mineral sands in Eastern Australia, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1587–1590 (The Australasian Institute of Mining and Metallurgy: Melbourne). Waring, T, Hogan, J and Haine, I, 1997. Australia and the global supply of minerals and energy - an overview, in Proceedings of the National Agriculture and Resources Outlook Conference 1997, 3:3–20 (ABARE: Canberra). White, G, 1992. The economics of value adding: myths and reality, in Proceedings of The Future of Australia’s Mining and Energy Industries 1992, pp 93–101 (Economic Research Centre, The University of Western Australia: Perth). Williams, V A, 1990. WIM 150 detrital heavy mineral deposit, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1587–1590 (The Australasian Institute of Mining and Metallurgy: Melbourne). Willson, J M, 1995. The role of mining in the evolving world, in View from the Helm - Mineral Industries in Transition (Ed: J E Tilton), pp 70–77 (Mining Journal Books Limited: London). Woffenden, K, Penn, J, Sheales, T and Fisher, B, 1997. Commodities sector outlook and issues to 2001–02, in Proceedings of the National Agriculture and Resources Outlook Conference 1997, 1: 3–35 (ABARE: Canberra). Yearley, D C, 1995. Forces of change: finding a middle ground for the mining industry and the environment, in View from the Helm Mineral Industries in Transition (Ed: J E Tilton), pp 78–88 (Mining Journal Books Limited: London).
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Rogerson, R, 1998. Papua New Guinea’s mineral industry, 1986–1996, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 33–44 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Papua New Guinea’s mineral industry, 1986–1996 by R Rogerson
1
INTRODUCTION
PNG’S MINERAL SECTOR ENVIRONMENT
Papua New Guinea’s mineral industry has grown significantly since 1986 when only two large mines, Panguna and Ok Tedi, and only one small scale mine at Wau were operating. Although the Panguna mine on Bougainville was forced to close in 1989, production from new world-class mines at Misima and Porgera have maintained, and the new Lihir mine will continue, Papua New Guinea’s share of world mine gold production at about 2%. In addition, several modern, small to medium scale mines have started production, and more are on the drawing board. However, Papua New Guinea (PNG) remains critically dependent on gold and copper for its mineral wealth, with only the potential development of the Ramu nickel-cobalt deposit being capable of increasing this narrow commodity base in the medium term. Fortunately, PNG became a petroleum exporter in 1992 when the Kutubu field in the Southern Highlands Province began producing over 100 000 barrels of oil per day and thereby reduced PNG’s reliance on the mining industry as the source of much of its foreign exchange.
PNG’s population of approximately 4 million is largely rural based, where land ownership remains customary, with complex usage rights being distributed amongst clan members. Alienated land, largely concentrated in towns and cities, amounts to only about 3% of the total land area of 463 000 km2. A large percentage of the population is under 18 years of age and high population densities, particularly in the Highlands of the mainland and on some islands, cause conflict between competing land users. The Mining Act 1992 requires effective consultation between land owners and mineral tenement applicants prior to the grant by the State of what amounts to a form of non-traditional use of land.
Growth in the mineral industry has been against a background of increasing calls from land owners for more direct tangible benefits and more compensation for alleged environmental impacts from mineral development, and a declining government presence in rural areas where most new mines are located. Perceptions arising out of sometimes vocal calls for more benefits and compensation by land owners, Government reactions to such calls, and reporting of mining issues in the local and overseas media, have led to PNG being ranked the least attractive country for mineral investment out of the 15 listed in EFIC’s 1997 risk survey (Australia’s Mining Monthly, February 1997).
Political power in PNG is vested in the unicameral Parliament, based in the capital Port Moresby, that consists of 109 members elected by citizens every five years. Twenty provincial governments, with powers delegated by the National Parliament, consist of ex-officio representatives of local governments, ex-officio National Parliament members with electorates in the province, and several appointed members. Elections for rural and urban local government councils are held every five years at the same time as national elections. Regulation of the mineral industry is the responsibility of the National Parliament. The Mining Act 1992, Mining (Safety) Act, taxation and environmental legislation, and the Organic Law on Provincial and Local Level Governments are the principal legislative instruments impacting on the industry. Government policy and the Mining Act 1992 empower the State to purchase up to 30% beneficial interest in large scale mineral projects.
Despite this poor ranking, PNG continues to attract exploration funds because of its prospectivity and the demonstrable success of previous and new investors who continue to discover, develop and produce minerals within the existing socio-political, legal and fiscal environment.
From the grant of political independence in 1975 until 1994, successive PNG governments maintained what was referred to as the ‘hard kina policy’ whereby the nation’s currency was fixed by reference to a basket of currencies. Since late 1994 a managed float of the currency has resulted in an effective 35% devaluation against the Australian dollar. In early 1997, the kina (K) was valued at approximately $A0.92.
This review of the PNG mineral industry follows a summary by Welsh (1990) and draws upon a more recent one by Hancock (1996). Readers are referred to these authors for additional information, including Welsh’s potted history of the mining industry in PNG. Figure 1 shows the location of mineral projects in PNG.
Formation of the PNG Chamber of Mines and Petroleum in 1986 has facilitated an increasing level of dialogue between industry and government. The Chamber also promotes community awareness of mineral and petroleum resource issues by production of booklets, pamphlets, media releases and by the convening of conferences and seminars.
MINERAL PRODUCTION 1.
Formerly Director, Mining Division, PNG Department of Mining and Petroleum; now Project Manager, Mineral Resources Group, Geological Survey of Western Australia, 100 Plain Street, East Perth WA 6004.
Geology of Australian and Papua New Guinean Mineral Deposits
Annual metal production statistics for the period 1986–1996 are shown in Tables 1 and 2, and Fig 2 summarises gold production. Table 1 demonstrates that PNG has maintained its
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FIG 1 - Map showing mines, major projects and location of seafloor sulphide deposits.
TABLE 1 PNG’s share of world gold mine production, 1986–1996. Year
Western world mine production
Other countries (t)
Total (t)
PNG share of world production (%)
1986
1296
340
1636
2.25
1987
1384
350
1734
2.02
1988
1551
357
1908
2.03
1989
1683
380
2063
1.73
1990
1755
378
2133
1.77
1991
1790
370
2160
2.81
1992
1872
360
2232
3.14
1993
1904
385
2289
2.64
1994
1898
379
2277
2.62
1995
1891
378
2269
2.36
1996
1959
386
2345
2.20
Other countries: Soviet Union (former), Russia, Uzbekistan, other CIS, China, North Korea, Mongolia. Statistics from Murray et al (1997)
share of world gold mine production during the period within a band ranging from a low of 1.73% in 1989 to a maximum of 3.14% in 1992.
LARGE SCALE MINING Copper production has remained relatively constant at approximately 200 000 tpa during the period, despite the closure of the Panguna mine in 1989. The marked rise in copper production at Ok Tedi in 1989 followed closure of the gold-
34
only processing circuit in 1988 allowing additional copper concentrate production. PNG gold production was dominated by Ok Tedi, Misima and Porgera after the closure of Panguna. Ok Tedi remains a very large gold mine, producing between 18.7 t in 1986 and 10.49 t in 1992. Gold production from the low grade Misima deposit has averaged a little over 10 tpa since full production was attained in 1990. Throughput has increased to maintain the level of metal production, but harder ore and declining grades are beginning to take their toll. Discovery of ore in surrounding exploration tenements is required urgently if mining on Misima is to continue for more than a few years. Porgera has been the engine room of PNG gold production since its commissioning in 1990. Initial development of bonanza grades in the Zone 7 underground orebody resulted in maximum production of 46 t in 1992. Open pit and underground production totalled 26.6 t in 1996 after a major increase in throughput was achieved by an expansion of milling and pressure oxidation facilities to cope with higher volumes of lower grade ore from the open pit. Ore reserves were recalculated in 1995 as a result of better understanding of the mineralisation system obtained by drilling and development of both underground and open pit workings. Total Proved and Probable Reserves were estimated to be 65.4 Mt of ore at 4.5 g/t gold containing 9.4 Moz of gold. Total Measured, Indicated and Inferred Resources were estimated to be 108.0 Mt at 3.8 g/t gold containing 13.2 Moz of gold. The Porgera Joint Venture is actively exploring for additional ore within the Porgera Special Mining Lease and in surrounding exploration tenements. Early results are encouraging and will hopefully ensure that Porgera continues to produce gold well into the second decade of the next century.
Geology of Australian and Papua New Guinean Mineral Deposits
PAPUA NEW GUINEA’S MINERAL INDUSTRY 1986–1996
TABLE 2 PNG’s metal production, 1986–96. Year
Copper (t) Panguna
Ok Tedi
Gold (t) Total
Panguna
Ok Tedi
Misima
Porgera
Tolukuma
Wapolu
Artisan
Total
1986
178 593
0
178 593
16.367
18.706
1.8
36.873
1987
178 211
39 488
217 699
15.088
18.16
1.8
35.048
1988
165 957
52 677
218 634
13.862
18.042
1989
68 717
135 309
204 026
6.977
15.954
4.873
170 210
170 210
13.801
9.865
1990
8.27
6.8
38.704
7.8
35.604
5.8
37.736
1991
204 459
204 459
11.067
10.05
37.825
1.8
60.742
1992
193 359
193 359
10.494
11.642
46.191
1.8
70.127
1993
203 184
203 184
12.255
10.458
35.977
1.8
60.490
1994
206 329
206 329
15.137
10.492
32.122
1.8
59.551
1995
212 737
212 737
14.996
10.302
26.403
1.8
53.501
1996
186 715
186 715
13.115
8.065
26.587
1.8
51.635
1.852
0.216
Statistics from quarterly bulletins of the Mining Division, PNG Department of Mining and Petroleum. Figures for artisan production are estimated from a number of sources.
(Samuel and Sie, 1991). This was the first modern small scale open pit mine in PNG and demonstrated the potential for developing small orebodies using portable mills. Reserves prior to mining were 260 000 t at 3.63 g/t. In the last few years, PNG has witnessed the revival of its potential for small–medium scale mining with commissioning of the small but high grade Tolukuma* gold mine and prospects of establishing gold mines in the D’Entrecasteaux Islands, Woodlark Island, Mount Sinivit*on New Britain, and on Simberi Island near Lihir. Tolukuma has lived up to predictions of its richness by producing 1.85 t of gold in its first year of production (1996).
FIG 2 - PNG gold production, 1986–1996. Figures supplied by PNG Department of Mining and Petroleum.
Average PNG gold production costs are very low by world standards. Murray et al (1996) list 1995 production costs for major gold producing countries that demonstrate that PNG’s average cash costs and total costs per ounce are the lowest at $US169 and $US246 respectively. Thus despite relatively high exploration costs in PNG’s rugged, tropical environment with little transport infrastructure, large scale gold mining can be extremely profitable at prevailing gold prices.
SMALL–MEDIUM SCALE MINING The closure of New Guinea Goldfields’ Golden Ridges mine in 1990 marked the end of 60 years of open cut lode gold production at Wau. However in the same year, Edie Creek Mining began operations at Edie Creek, producing alluvial gold and lode gold from epithermal quartz stringers (Neale, 1994). The operation produced about 80 kg of gold in 1994. Niugini Mining’s Mount Victor gold mine in the mainland Highlands commenced operations in November 1987 and ceased mining in January 1990 after exhausting ore reserves
Geology of Australian and Papua New Guinean Mineral Deposits
Mining at Tolukuma*, which began in December 1995, is totally helicopter-supported as the deposit is located in rugged country 100 km north of Port Moresby. Mining was initially by open cut, but production has now commenced from an adit driven under the open cut. At present, head grades are approximately 15 g/t gold and 50 g/t silver with monthly production of approximately 160 kg of gold and 300 kg of silver from a 100 000 tpa conventional gravity and CIL plant. Considerable potential exists for increasing reserves as parts of the vein system have not been drill tested. Unfortunately, Union Mining’s Wapolu gold mine on Fergusson Island closed in early 1997 after only a little over one year of production. Based on a largely oxide mining reserve of 2 Mt at 2.4 g/t gold, the shallow open pit mine was expected to have a life of four years. PNG’s industrial minerals and construction materials sector is poorly developed, largely reflecting the underdeveloped domestic manufacturing and contracting sector. Lime produced domestically for use in the mining industry represents the only industrial mineral production. Construction materials are abundant in most parts of the country and are quarried for local road material and other construction purposes. There is potential for development of a dimension stone industry based on limestone and granitic rocks near the port of Lae, but the enterprise would be new to PNG and would initially require overseas capital and expertise.
*Deposits described in this publication are identified by an asterisk.
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TABLE 3 Value of PNG’s mineral production, 1986–96 (million kina). Gold
Silver
Total metals
Crude oil
Total PNG exports
Metals/ total exports (%)
Crude oil/ total exports (%)
Mineral plus oil/ total exports (%)
Year
Copper
1986
156.0
398.4
6.7
561.1
975.7
57.51
57.51
1987
281.9
422.9
10.1
714.9
1123.2
63.65
63.65
1988
446.9
405.1
9.5
861.5
1256.1
68.59
68.59
1989
344.9
316.9
14.3
676.1
1111.6
60.82
60.82 67.49
1990
349.2
393.2
15.1
757.5
1122.4
67.49
1991
323.8
666.9
14.6
1005.3
1390.5
72.30
1992
313.5
745.9
10.7
1070.1
1862.6
57.45
301.4
72.30 16.18
73.63
1993
256.3
681.6
12.1
950.0
817.8
2527.3
37.59
32.36
69.95
1994
367.4
702.3
10.3
1080.0
702.7
2662.0
40.57
26.40
66.97
1995
754.5
840.1
13.1
1607.7
827.7
3399.8
47.29
24.35
71.63
1996
387.0
773.6
10.1
1170.7
1073.9
3296.0
35.52
32.58
68.10
Statistics from quarterly bulletins of the Mining Division, PNG Department of Mining and Petroleum.
VALUE OF PNG’S MINERAL INDUSTRY Any analysis of the PNG economy reveals the dramatic impact of the resources sector, solely dominated by the minerals industry until 1991, and then by minerals and petroleum since 1992. Table 3 demonstrates the importance of the resources sector which has consistently contributed between 60 and 70% of total export value over the period 1987–1996. From a low of 57.5% in 1986, minerals attained a high of 73.6% of total PNG export value in 1992 when production of gold from Porgera neared its maximum. Effects of the forced closure of the Panguna copper–gold mine in 1989 on both national copper and gold production and export value statistics are partly masked by the 1989 commissioning of the Porgera gold mine and the massive increase in copper production at Ok Tedi in 1989. The drop in mineral production as a proportion of total exports since 1992 has been against a background of petroleum exports from the Kutubu oil field that began in 1992. With the marked exception of 1995 when copper prices were high, the annual monetary value of mineral production has averaged a little over one billion kina since 1991. A considerable range of benefits arises from the exploration for and development of mineral resources. Even during the exploration phase, mineral explorers are substantial employers of local rural labour and usually provide some limited medical facilities if a semi-permanent exploration camp is established in an area. The National Government, the provincial government host of the mine, and local land owners gain direct and indirect economic benefits from mineral production. Royalty payments totalling 2% of production are split between land owners and the host provincial government under agreements specific to each project. Royalty payments totalling about K23 million were made to provincial governments and land owners in 1996, and from start up in 1990 to January 1996, the Porgera project paid a total of K29 million in royalties to land owners and provincial government authorities. Income tax on the profits of mining companies and service companies as well as from PAYE tax of employees in the industry are collected by the
36
National Government. As part of the Government’s budgetary process, a portion of these incomes is remitted to provinces, with a larger portion to provinces hosting major resource developments. Land owners and provincial governments have purchased equity in mines and will gain further benefits from dividends. The National Government owns 51% of Orogen Minerals Limited, the corporate vehicle holding State equity in mineral and petroleum projects, and can expect to gain significant dividends from its equity in major projects. Land owners also receive compensation payments for use and damage to their land and benefit by having improved transport, medical and educational services. Unskilled and some semi-skilled employment is offered as a priority to land owners and provincial labour pools and provincial-based groups are encouraged by developers and the government to form service and supply companies to support mining projects. During the last ten years there has been growth in the number and sophistication of PNG-based companies servicing the mineral industry. PNG professionals now occupy a considerable proportion of geological, metallurgical, engineering and environmental positions as well as some legal and accounting positions within mining, exploration and service companies. Although no statistics are available on the multiplier effect of the industry in PNG, it is probably smaller than the figures quoted for the industry in Australia because of the significant value of supplies and services provided by organisations based outside the country. In areas with relatively restricted, easily developed alluvial gold resources such as the Wau–Bulolo area, artisan-scale, usually unmechanised alluvial gold mining is an important rural income supplement. Although some individuals and family groups may derive all of their income from gold mining, it is more common for the level of gold mining activity to be seasonal, being used to supplement income from cash cropping. At a production level of approximately 1.8 t and an average price of $US350 per ounce (approximately K480), the
Geology of Australian and Papua New Guinean Mineral Deposits
PAPUA NEW GUINEA’S MINERAL INDUSTRY 1986–1996
annual income to PNG from small scale alluvial gold mining was about K28 million in 1994 and 1995. This is only a little less than the annual income from cocoa exports or about half the export income from the copra industry.
CHANGES IN THE LEGAL AND POLICY FRAMEWORK OF THE INDUSTRY The period since 1986 has been marked by evolution of mineral sector policy rather than abrupt change. With the development of the major mines at Misima and Porgera, the smaller Mount Victor, Edie Creek, Wapolu and Tolukuma mines, and several major petroleum projects, governments and the bureaucracy have become more adept at considering the issues involved, and have also refined the administrative and financial systems by which the country approves and benefits from natural resource development. Four policy changes may be distinguished: the requirement that a ‘development forum’ be conducted prior to approval of a major development; simplification and modernisation of mining law and introduction of an alluvial mining lease; clarification of the compensation package for land owners directly impacted by mining development; and establishment and partial privatisation of the State’s mineral equity vehicle, Orogen Minerals Limited. Conduct of a ‘development forum’ began with the Porgera project in an attempt to consult all of the individuals and groups with an interest in the project. The forum, usually hosted by the Minister for Mining and Petroleum, involves land owners, land owner companies and associations, the provincial government, the National Government and the developers. The 1992 Mining Act encoded the conduct of a ‘development forum’ which was used for all major projects, including petroleum projects, since 1989. Passing of the Mining Act by Parliament in August 1992 gave PNG a modern mining code and at the same time simplified administration of the industry. More than 30 types of tenement under the repealed act were replaced by four primary tenements (exploration licence, mining lease, special mining lease, and alluvial mining lease) and two ancillary tenements (lease for mining purposes and mining easement). The new Act introduced the alluvial mining lease, which is restricted to PNG citizens, to foster mining of alluvial deposits (mainly gold). Individual alluvial mining leases cannot exceed 5 ha in area, and are limited in depth, but can be granted over another tenement if the primary tenement holder does not object. The new Act also entrenches minimum expenditure guidelines for exploration licences. Except for the grant of an alluvial mining lease, exclusive occupancy of exploration and production tenements for mineral sector purposes was introduced for the first time in PNG. Since development of the Panguna mine on Bougainville in 1972, there has been an evolution in the benefit package obtained by traditional land owners and provincial governments. A small proportion of the then royalty of 1.25% and compensation at agreed rates was obtained by land owners of the Panguna and Ok Tedi special mining leases, though compensation was also paid to other land owners outside the Special Mining Lease who were impacted by mining-related activities. State equity in these early projects was held by the National Government.
Geology of Australian and Papua New Guinean Mineral Deposits
However, with the development of the Misima and Porgera mines, new forms of benefits were obtained by land owners and provincial governments who were by then demonstrating more awareness of the potential rewards from mining and the techniques of successful negotiation. The passing of the Organic Law on Provincial and Local Level Governments in 1995 provided a legal framework for a number of policy changes that had modified the benefit package over a number of years. A range of indirect benefits and cash benefits derived from development of mineral resources now flows to both land owners and provincial governments. Benefits include increased infrastructure spending in the province funded by royalties, grants from the National Government, and from the innovative ‘tax credit scheme’ whereby mining companies construct roads, schools, sporting and medical facilities, normally funded directly by government, to a value not exceeding 2% of their assessable income for taxation purposes in a given year. Land owners now receive an agreed share of the 2% ad valorem royalty, and will receive 5% ‘free equity’ in new mining projects, in addition to at times substantial compensation packages. Provincial Governments receive the remainder of the royalty. 1996 saw the successful float of 49% of the shares in Orogen Minerals Limited, the PNG Government’s vehicle for holding State equity in large mining and petroleum projects. Orogen Minerals announced a K16.4 million profit after tax and before abnormals for the first eight months to 31 December 1996, exceeding the prospectus forecast of K12.6 million. On the first day of trading, the 157 million shares offered reached a price of $A2.13, valuing the company at approximately $A670 million. Assets of Orogen Minerals include 6.8% of Lihir, 15% of Porgera and 20% of Misima, in addition to 15.75% of the Kutubu petroleum production project and 20.5% of the Gobe oil development project.
MINERAL EXPLORATION ACTIVITY PATTERNS Mineral exploration activity in PNG remains focussed on gold and copper, with the notable exception of large expenditure on prefeasibility studies of the Ramu nickel-cobalt deposit. Figure 3 shows that total expenditure on mineral exploration was approximately K40 million in 1996, which is a little higher than the comparable figure for 1986. Exploration expenditure peaked in 1988, but declined steadily in the early 1990s as the effects of the 1987 equity market crisis were felt. The City Resources group of companies, which had significant tenement holdings in PNG, was a conspicuous casualty of the 1987 crisis. Grass roots exploration expenditure, a critical measure of the sustainability of the mineral industry in PNG, also peaked in 1988, reaching a nadir of less than K10 million in 1993. A brief recovery in 1994 to K17 million was followed by a steady decline to about K12 million in 1996. Most of this expenditure would have been on gold exploration. Figures released by the PNG Department of Mining and Petroleum (Mining Division Quarterly Bulletin, 4th Quarter 1996) reveal a corresponding decline in the number of stream sediment samples collected by the exploration industry from 3394 in 1994 to 1706 in 1996. Expenditure on advanced projects such as Lihir, Nena, and Ramu accounts for much of the difference between grass roots and total exploration expenditure.
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geochemistry and stable isotope ratios obtained from rock, mineral and fluid inclusions have yielded sophisticated models with considerable predictive capabilities for epithermal systems and associated mineralisation. Painful lessons have been learned by exploration companies that have neglected the critical importance of effective relations with land owners in PNG. The last ten years has seen the growth of exploration service companies with expertise in this field, and mining projects usually employ an in-house team to manage land owner relations. Customary ownership of most land has meant that exploration companies operating in PNG have gained valuable generic skills useful in dealing with native title holders in other jurisdictions, including Australia.
SIGNIFICANT DISCOVERIES SINCE 1986
FIG 3 - Exploration statistics, 1986–1996. Figures supplied by PNG Department of Mining and Petroleum.
The number of granted exploration licences reached a maximum of 220 in 1988, declining gradually to 93 in 1993. From 1994 to the end of 1996 the number of licences has remained relatively constant at approximately 107. This number of licences may well have been higher but for the effects of Government fiscal restraint on the Department of Mining and Petroleum (DMP) since 1995 following the devaluation of the kina. Long delays in the grant of exploration licences have occurred because the DMP did not have sufficient funds to conduct the Mining Warden’s hearings required for the grant of an exploration licence in a timely manner. Towards the end of 1996, there were signs of increasing exploration activity following successful fund raising on the Vancouver Stock Exchange by a number of companies with interests in exploration licences in PNG. Together with a slightly improved DMP budgetary allocation for the conduct of Mining Warden’s hearings, additional Canadian equity should see a slow down and perhaps a reversal of the trend of PNG receiving a declining share of world exploration expenditure.
EXPLORATION TECHNOLOGY Considerable advances have been made over the last ten years in exploration technology applicable to wet tropical areas with rugged terrain. In remote sensing, the availability of spectral data from satellites and improved aeromagnetic data and imagery have assisted exploration. In PNG, much use has been made of airborne synthetic radar imagery (SAR) for structural geology interpretation. SAR has the advantage of imaging the land surface below jungle canopy. Satellite-based surveying techniques now permit the collection of spatially accurate field data and samples at an early stage of exploration. Bulk cyanide leach and partial extraction geochemical techniques for drainage sediment samples now complement traditional strong acid leach techniques. Perhaps the greatest advances have been made in the understanding and modelling of epithermal systems using a variety of field-based and laboratory techniques. Alteration studies using mineragraphy,
38
The period 1986–1996 has witnessed a number of new mineral discoveries resulting from new exploration strategies in PNG. Significant discoveries include Wafi porphyry gold-copper*, Hamata gold*, Mount Kare gold, Tolukuma gold*, Mount Bini copper-gold* and seafloor massive sulphides in the Solomon and Bismarck Seas. Table 4 lists resources for some of these deposits. Wafi*, only 60 km from Lae, is one of the deposits, including Tolukuma, Mount Bini and Hamata, discovered in the last ten years on the Papuan Peninsula. All are hosted by the Cretaceous to Oligocene Owen Stanley Metamorphics, or by intrusions into the metamorphics, but vary in age from 14 Myr for Wafi, 4.8 Myr for Tolukuma (Langmead and McLeod, 1991) to 4.42 Myr for Mount Bini (Leaman, 1996). Corbett (1994) noted that all of these deposits are on or adjacent to major, deep rooted, NE-trending transfer structures that localised intrusion centres. Although surface gold mineralisation was discovered at Wafi by CRA Exploration Pty Ltd (CRAE) in the early 1980s, it was not until 1989 that high grade epithermal gold mineralisation was discovered, and still later in 1990 that several deep drill holes intersected high grade, porphyryhosted copper mineralisation (Erceg et al, 1991). Tau-Loi and Andrew (this publication) conclude that the high-sulphur epithermal event overprints the only slightly older porphyry event. The deposit, hosted by the high pressure facies Owen Stanley Metamorphics, forms part of a large mineralisation system that will take many more years to evaluate. An Indicated plus Inferred Resource of 100 Mt at 1.3% copper and 0.6 g/t gold at a 0.5% copper cutoff has been estimated for the Wafi deposit by Tau-Loi and Andrew. Tolukuma* is an epithermal vein-type gold deposit hosted by metasediments of the Owen Stanley Metamorphics at their boundary with a downfaulted graben containing Pliocene volcanolithic conglomerates and minor basalt. Newmont identified the deposit during regional grass roots exploration in 1986 and disposed of the property to Dome Resources in 1993. Corbett, Semple and Leach (1994) and Semple, Corbett and Leach (this publication) suggest that a number of features of the deposit are indicative of a porphyry copper association, including diatreme breccias and localised microdiorite intrusions. Bonanza gold and silver grades occur at Tolukuma which has an Inferred Resource of 1.48 Mt at 13.77 g/t gold using a 4 g/t gold cutoff (Langmead and McLeod, 1991) and considerable silver credits. Exploration is continuing and considerable potential exists to increase the resource in the Tolukuma vein system which is over 1 km long.
Geology of Australian and Papua New Guinean Mineral Deposits
PAPUA NEW GUINEA’S MINERAL INDUSTRY 1986–1996
TABLE 4 PNG’s mineral resources. Deposit
Deposit type
Grade
Resource (Mt)
Comments
References
Panguna
Porphyry Cu–Au
496
0.42% Cu 0.55 g/t Au
Reserves (1989)
DMP figures
Ok Tedi
Porphyry Cu–Au system including skarns
440
0.87% Cu 0.90 g/t Au
Reserves (end 1995)
DMP figures
Misima
Lode gold
36
1.1 g/t Au 9.72 g/t Ag
Reserves (end 1995)
DMP figures
Porgera
Intrusive-hosted Au including lode gold
76.4
4.5 g/t Au
Proved plus Probable Reserves (end 1996)
DMP figures
Lihir
Epithermal Au
104
4.37 g/t Au
Mining reserve end 1994 (2 g/t cutoff)
DMP figures
Wapolu
Epithermal Au
2.185
Proved 1.725 at 1.725 g/t Au Probable 0.275 at 2.4 g/t Au Indicated 0.185 at 2.6 g/t Au
Reserves at 1.2 g/t cutoff
DMP figures
Tolukuma*
Lode Au
0.43
Zone C: 18.1 g/t Au, 46 g/t Ag
Measured Resource
0.27
Zone A: 26.5 g/t Au
Indicated Resource
Semple, Corbett and Leach (this publication)
Nena*
High sulphidation Cu–Au
51
Sulphide: 2.2% Cu, 0.6 g/t Au
18
Oxide: 1.4 g/t Au
Measured plus Indicated Resource at 0.5% Cu cutoff
Bainbridge, Hitchman and DeRoss (this publication)
Frieda
Porphyry Cu–Au
866 (total)
Horse: 115 Mt at 0.5% Cu, 0.3 g/t Au Ivaal: 371 Mt at 0.6% Cu, 0.3 g/t Au Koki: 260 Mt at 0.4% Cu, 0.2 g/t Au Ok Nerenere: 60 Mt at 0.5% Cu, 0.2 g/t Au Ekwai: 60 Mt at 0.5% Cu, 0.3 g/t Au
Inferred plus Indicated Resources in named parts of porphyry system
DMP figures
Ramu
Lateritic Ni–Co
100 (total)
Limonite: 40 Mt at 0.9% Ni, 0.1% Co Saprolite: 15 Mt at 1.0% Ni, 0.1% Co Rocky saprolite: 45 Mt at 1.0% Ni
Inferred Resource at 0.5% Ni cutoff
DMP figures
Wafi*
Porphyry Cu–Au system Epithermal Au
100
1.3% Cu, 0.6 g/t Au
18
2.6 g/t
Indicated plus Inferred Resource at 0.5% Cu cutoff
Tau-Loi and Andrew (this publication)
Hidden Valley
Lode Au
48.6
Hidden Valley: 33.9 Mt at 2.28 g/t Au, 40 g/t Ag Kaveroi Creek: 14.7 Mt at 2.06 g/t Au, 42.2 g/t Ag
Inferred Resources
Gold Gazette November 18, 1996
Simberi
Epithermal Au–Ag
4.1
Oxide: 1.6 g/t Au
Mining reserve
Hancock (1996)
Mount Sinivit* (Wild Dog deposit)
Lode Au
0.306
Oxide: 4.0g/t Au
Probable Reserve at 0.5 g/t Au cutoff
Lindley (this publication)
0.2
Sulphide: 9.43 g/t Au
Laloki
Sediment-hosted massive sulphides
0.36
4.2% Cu, 3.7 g/t Au, 2.5% Zn, 11.4 g/t Ag
Indicated Resource
DMP figures
Mount Bini*
Porphyry Cu–Au
85
0.4% Cu, 0.6 g/t Au
Inferred Resource at 0.3% copper equivalent cutoff
Dugmore and Leaman (this publication)
Hamata*
Mesothermal Au veins
9.2
3.1 g/t Au
Inferred Resource at 0.5% cutoff
Denwer and Mowat (this publication)
Gameta*
Epithermal Au
1.89
2.6 g/t Au
Inferred Resource at 1 g/t cutoff
Chapple and Ibil (this publication)
Woodlark
Epithermal Au–Ag
2.55
3.7 g/t Au
Inferred resource at 1 g/t Au cutoff (several prospects)
Corbett et al (1994)
DMP figures are from the Quarterly Bulletin of the Mining Division, PNG Department of Mining and Petroleum.
The Hamata* gold deposit (Wells and Young, 1991; Denwer and Mowat, this publication), 12 km SW of Wau, was identified by RGC Exploration during a regional stream sediment sampling program in 1987. Mesothermal gold mineralisation is related to a series of stacked pyrite and quartz-pyrite veins associated with northeasterly-trending shear zones within the
Geology of Australian and Papua New Guinean Mineral Deposits
Middle Miocene Morobe Granodiorite, though the mineralisation event is likely to be Late Miocene to Pliocene in age and related to mineralisation in the nearby Hidden Valley deposit (Pascoe, 1991). An Inferrred Resource of 9.2 Mt at 3.1 g/t has been estimated for Hamata.
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R ROGERSON
Porphyry-style copper–gold mineralisation at Mount Bini * (Leaman, 1996) was discovered by BHP Minerals in 1992 as a result of a regional stream sediment survey. Located about 50 km NE of Port Moresby, the deposit is hosted by a quartz diorite porphyry (‘Mount Bini diorite’) that has intruded the Owen Stanley Metamorphics. The approximately 650 by 300 m area of porphyry-style mineralisation is within a 15–20 km wide northeasterly trending, down-faulted structural corridor. Mineralisation is Early Pliocene in age (Leaman, 1996) and is probably related to the same volcano-intrusive event that gave rise to mineralisation at Wau (including Hidden Valley and Hamata), and Tolukuma. A zone of epithermal quartz veins west of Mount Bini post-dates the main porphyry system. A subeconomic Inferred Resource of 85 Mt at 0.4% copper and 0.6 g/t gold was estimated for the deposit, which remains open at depth (Leaman, 1996). Perhaps the best known mineral discovery since 1986 was the Mount Kare deposits, at 2800 m elevation about 18 km SW of Porgera (Fig 1). A program of drainage geochemical sampling carried out in early 1986, and follow-up exploration in 1987 and 1988 led to delineation by CRAE of an alluvial gold resource that was thought to have been sourced from a nearby hard-rock deposit (Bartram, Heape and Caithness, 1991). During early 1988 an uncontrolled gold rush started, with up to 7000 individuals flocking to the area to win alluvial gold from alluvials and palaeochannels. Typical gold rush conditions existed at the site (Ryan, 1991) and it has been estimated that up to 15 t of alluvial gold was won using non-mechanised techniques until the main rush ended in 1990 with the grant of the Special Mining Lease to Mount Kare Alluvial Mining Pty Ltd (MKAM) for development of the alluvial resource only. MKAM was owned 51% by CRA and 49% by Kare Puga Development Corporation, an umbrella company representing the interests of land owners of the area. Hard rock exploration rights remained with CRA. Mechanised production of alluvial gold by MKAM began in 1990, but was plagued by technical and security problems until early 1993 when mining ceased. A few individual miners using gold pans remain in the area which continues to produce up to 10 kg of gold per month (Department of Mining and Petroleum, Mining Division Quarterly Bulletin, 4th Quarter 1996). Litigation concerning the hard rock mineral rights at Mount Kare began in 1994 and was resolved in 1996 by the PNG Supreme Court awarding Matu Mining, a wholly owned subsidiary of Carpenter Pacific Resources, the exploration licence covering the Mount Kare area. At the end of 1996 attempts were also being made to restart mechanised alluvial mining at Mount Kare. Hard rock exploration by CRAE at Mount Kare prior to January 1992 identified hydrothermal mineralisation associated with Early Tertiary sedimentary rocks that had been intruded by irregular microdiorite to doleritic dykes and stocks. Gold mineralisation occurs predominantly within the sedimentary rocks at the contacts with the intrusives (Bartram, Heape and Caithness, 1991). Extensive fracture-fill supergene gold was observed in trenches and drill pads but no primary gold was observed in drill core. The discovery of quartzroscoelite mineralisation in a fault zone at Mount Kare led CRAE to believe that the Porgera-type mineralisation model might be applicable. With the resolution of hard rock mining rights at Mount Kare, Carpenter Pacific Resources and its Canadian joint
40
venture partner, Madison Enterprises Corporation, recommenced exploration in 1996. Results to date (Gold Gazette Australia, 1996) are extremely encouraging and further drilling and an airborne magnetic and radiometric survey will be carried out. Intensive exploration programs by Highlands Pacific Limited in the Frieda River copper-gold district and at the Ramu cobalt-nickel deposits have resulted in prefeasibility studies on these two areas, originally discovered by Government geologists in the 1960s. Highlands Pacific have focussed on the Nena* deposit, a high grade copper-gold sulphide system with an oxidised gold-rich blanket, about 10 km from the large Frieda porphyry coppergold deposits. Recent resource estimates for Nena suggest a Measured plus Indicated Resource of 51 Mt of sulphide mineralisation grading 2.2% copper and 0.6 g/t gold using a 0.5% copper cutoff, and approximately 18 Mt of oxide material grading 1.4 g/t gold using a 0.6 g/t gold cutoff. A full feasibility study has been commenced, with provision of power being a significant factor in the viability of the project. Further studies will be carried out on the porphyry copper-gold system as part of the feasibility study. Technological advances in metallurgy prompted Highlands Pacific to undertake a vigorous exploration program at the Ramu lateritic nickel-cobalt deposits and to complete a prefeasibility study. Results of the study were encouraging and Highlands Pacific have moved to full feasibility appraisal. The project has a likely capital cost of $US750 million at estimated production rates of 33 000 t of nickel and 2800 t of cobalt per year. Macmin and Union Mining have had considerable success in their intensive gold exploration activities in the D’Entrecasteaux Islands, the same island group that hosts the Wapolu gold deposit. Promising prospects within high level epithermal systems related to faults bounding actively rising core complexes have been defined, including Imwauna and Weioko (Sehulea) on Normanby Island and Gameta* on Fergusson Island about 40 km east of Wapolu. Union Mining have estimated an Inferred Resource of 1.89 Mt at 2.6 g/t gold (at a 1.0 g/t Au cutoff) for Gameta on the basis of a vigorous exploration program involving 105 RC drill holes. The mineralisation defined to date at Gameta is localised within ultramafic and basement metamorphic rocks on and just below a dome-bounding fault. Considerable potential exists for increasing the resource which is open in most directions. The number of mineral occurrences discovered at this early stage of exploration suggests that the D’Entrecasteaux Islands constitute a new mineral district and that several deposits could be developed in the coming years. A natural advantage of the district is that almost all known deposits occur at or near the coast. Although it is a well established fact that the PNG mainland is a gold-rich geological province, exploration of contiguous seafloors in the last 10 years has greatly extended the area of auriferous anomalism. Three locations with gold-rich massive sulphide mineralisation (Fig 1), two in the Bismarck Sea (Manus Basin) and one in the Woodlark Sea, have been discovered recently as a result of marine geoscience cruises (Binns, 1994). The Manus Basin is a Pliocene to Recent back arc basin which opened by oblique extension north of the New Britain
Geology of Australian and Papua New Guinean Mineral Deposits
PAPUA NEW GUINEA’S MINERAL INDUSTRY 1986–1996
Trench where northward-directed subduction of the Solomon Sea is taking place. The Vienna Woods hydrothermal field, discovered by Both et al (1986), is at 2500 m water depth in the axial rift valley of a northeasterly-trending spreading centre (Tufar, 1990). Individual active and inactive mineral occurrences, generally circular or elliptical in shape, are hosted by basalt. Sulphide chimneys to 20 m high and fragments of broken chimneys dominate most deposits. The principal minerals, sphalerite, wurtzite [(Zn,Fe)S], pyrite, marcasite and chalcopyrite occur in a gangue of opaline silica, barite, anhydrite and native sulphur. Average metal values for 71 massive sulphide samples from Vienna Woods (Binns, 1994) were 1.3% copper, 20% zinc, 0.6% lead and 370 ppm silver, and average gold grade for 15 samples was 15 ppm. Unlike the basalt-hosted Vienna Woods deposits, host rocks of the PACMANUS hydrothermal field are associated with dacite to rhyodacitic lava domes and breccias and andesites (Binns, 1994) at 1650 m water depth. Numerous hydrothermal mineral occurrences scattered along a linear zone 8 km long have been localised by crosscutting fractures parallel to regional transform faults. Individual deposits consist of low mounds of massive sulphides with massive sulphide chimneys to 5 m high. Average compositions of massive sulphide samples reveal higher copper than Vienna Woods, but similar zinc, lead, silver and gold abundances (Binns, 1994). Seafloor spreading in the Woodlark Basin began in latest Miocene to earliest Pliocene times and has progressed westward into continental crust of the D’Entrecasteaux Islands which are marked by extensional tectonics including active core complexes (Davies and Warren, 1988; Hill, Baldwin and Lister, 1992). Binns et al (1987) located hydrothermal crusts on the Franklin seamount adjacent to the tip of the spreading centre. Massive sulphides do not appear to be well developed on the Franklin seamount, but chimneys in its crater floor carry high silver (130–550 ppm ) and gold (4–21 ppm) values.
ISSUES, OPPORTUNITIES AND CHALLENGES INTERNATIONAL PNG is facing increasing international competition from countries modifying their regulatory regimes in order to attract additional investment in their minerals industries. So far PNG has resisted calls for an easing of its fiscal regime for mining, and in 1995, announced an increase in royalty from 1.25% to 2% and introduced the concept of ‘free’ equity for land owners at major new mining projects. There is little doubt that grass roots exploration expenditure in PNG has declined (Fig 3), but the lack of funding for holding of Mining Warden’s hearings by the DMP has retarded the rate of licence grants since 1994. To what extent this lack of funding has contributed to the decline in grass roots mineral exploration expenditure is hard to quantify. PNG has maintained a remarkably stable fiscal regime for mining since political independence in 1975, with a generation of its political and bureaucratic leaders being brought up in a minerals environment dominated by large mines. The only significant change in fiscal regime was a reduction in the corporate tax rate for medium and small scale mining from 35 to 25% and the increase in royalty from 1.25 to 2%. PNG’s reluctance to modify its regulatory regime, and particularly its fiscal regime, is perhaps related to the demonstrated success of the country’s mining industry, and also the view that mineral industry investment, and the easing and tightening fiscal regimes by individual countries, both tend to be cyclical.
Geology of Australian and Papua New Guinean Mineral Deposits
A potential threat to the sustainability of PNG’s mineral industry is that by missing out on the current wave of exploration investment sweeping some countries, PNG will not discover new mines at the rate required to replace reserves mined from existing operations. The long lead times from discovery of a mineral occurrence to its possible development as a major mine are not appreciated by many PNG leaders and senior bureaucrats. Reliance on gold and copper production is potentially a major risk for PNG’s economic stability unless additional mineral commodities are discovered and developed. Although PNG-based miners are low cash cost producers and some have hedged future gold production, this protection will not last if gold prices do not improve in the medium term. Development of the Ramu nickel-cobalt mine by Highlands Pacific would broaden PNG’s mineral commodity base, but the project faces considerable competition for markets from low cost producers such as Murrin Murrin (WA) and Voisey’s Bay (Canada) which will come on stream in the next few years. The small goldcopper-zinc resource at Laloki, 15 km from Port Moresby (Shedden, 1990), is the only other significant deposit of base metals other than copper and nickel in PNG.
NATIONAL The main issues impinging on the sustainable growth of the minerals industry in PNG relate to environmental impact, effective regulation, and equitable distribution of benefits among stakeholders in the industry. During 1995 and 1996, environmental issues related to mining in PNG and in particular Ok Tedi, were brought sharply into focus by litigation brought against BHP, the largest shareholder in Ok Tedi Mining Limited (the PNG-based owner of the Ok Tedi mine) in the Melbourne Supreme Court. By claiming damages for alleged negligence arising out of release of tailings into the Ok Tedi and Fly river systems, villagers living downstream of the Ok Tedi mine internationalised a PNG-based dispute, providing a precedent for other groups to claim headline-generating financial compensation amounts for alleged environmental impacts related to mines with significant or controlling overseas equity. In the event, the Ok Tedi compensation issue was settled out of court, but the dispute accelerated a trend towards adoption of developed country environmental standards by Australian-based mining companies operating in developing countries. The human environment encroaches far more on to the minerals industry in PNG than it does in Australia, and conflict between mining and other land uses are common, but mostly minor. Although often minor, disputes are widely reported in the media. Because mines and advanced mineral exploration activities are in rural areas, often poorly served by governmentfunded infrastructure, villagers tend to view the company as a de facto government that should provide and maintain basic road, health and education facilities. Naturally, companies wish to maintain cordial working relations with the land owners of their tenement and those directly impacted by their operations. Although the resulting dependence on the company could be viewed as a positive feature, two unfortunate corollaries arise: increased demands for more or grandiose public facilities, and potential abrogation of the Government role of providing services to rural areas.
41
R ROGERSON
Lack of Government funding for regulators of the mineral industry, including the DMP and the Department of Environment and Conservation, poses a serious risk to the sustainability of the industry. To some extent the successful partial privatisation of Orogen Minerals poses both a threat and an opportunity. On the one hand, effective corporate governance by Orogen Minerals should assist PNG to oversee its mineral industry and maximise returns. However, effective regulation of the industry relies heavily on well resourced departments of mining and petroleum, and environment and conservation, neither of which are effectively funded or staffed at present. However, the PNG Government still has to address the issue of potential conflict between its dual roles of regulator and mineral investor. At present it appears that the investor role is in the ascendancy. The distribution of benefits from mineral development has always been an issue in PNG, and was highlighted by the Bougainville secessionist movement. The PNG Government has attempted since major mining on Bougainville first started in 1972, and with subsequent major projects, to achieve an equitable share of benefits between land owners of the project site, those impacted by the project, the province and the nation. Getting the balance right has been a difficult task, and to some extent has evolved over the last 20 years. At the macro level, the Government has agonised over the level of equity it should acquire in mining projects. The Mining Act 1992 permits the Government to purchase equity at cost in projects, but it is mainly policy that limits such equity to 30%. In 1993 the Government wrestled with its joint venture partners to increase its Porgera equity from 15% to 25%, and there were calls in 1994 for the Government to acquire 50% equity in the Lihir project. The last modification was the introduction of the concept of ‘free’ equity for land owners of a project area, implemented for the first time with the Gobe oil development. Free equity, the ‘tax credit scheme’, an agreed split of all royalties between local land owners and provincial government, plus special grants for development of infrastructure in the province, suggest that the province and land owners are receiving a reasonable share of benefits from mineral developments. Land owners and provincial authorities tend to view mining as an ephemeral economic activity and call for benefits to be maximised during the mine life. They are also concerned about the maintenance of expensive infrastructure by a provincial government with diminished cash flow after mining ceases. Calls for more benefits are particularly vocal the closer one gets to mining centres. Management of the response to these calls is a responsibility of Government alone and is another compelling reason for adequate resourcing of Government departments such as the DMP. PNG’s mining industry is marked at present by the imbalance between the number of large mines (Ok Tedi, Porgera, Misima, Lihir) and the relative paucity of small–medium scale mines. Earlier than planned closure of just one large mine could have profound detrimental effects on Government revenue and land owner expectations. Development of a mineral industry based on a range of commodities with a few large mines and a relatively stable and moderate number of small–medium scale, short-life mines would reduce PNG’s dependence on large scale mines and smooth Government cash flows.
42
Development of a more broadly-based minerals industry would foster growth of a strong, PNG-based service sector and training facilities. Though PNG’s ability to service the mineral industry is restricted, development of more smaller mines, which could be increasingly serviced by PNG-based contractors, would increase the economic multiplier for the minerals industry within PNG and more effectively integrate the industry with other economic activities in the country. In this regard, it is pleasing to see the increased exploration activities by a number of small to medium scale mine developers in PNG over the last few years.
SUMMARY PNG’s mineral industry has matured significantly over the ten years from 1986, when the country only exported concentrate containing copper and gold from the Panguna and Ok Tedi mines. At the end of 1996, production from the Ok Tedi, Misima and Porgera mines and from the small Tolukuma mine provide PNG with the bulk of its foreign exchange; income from taxation, royalties and dividends; employment for its citizens; and development of spin-off businesses including a gold refinery and a developing service sector. Exploration, the Government approval processes leading to development, and eventual production will probably continue to take place against a background of an ongoing, at times vigorous debate within PNG concerning the level and distribution of benefits arising from mining. With the planned commencement of gold production from Lihir in June 1997, and possible development of small to medium scale mines at Mount Sinivit (Wild Dog), Woodlark, the D’Entrecasteaux Isalnds and Simberi in the near future, PNG’s future as a mineral producer seems assured as long as commodity prices do not collapse and the pragmatism that has so far marked political decision making in the country is maintained.
ACKNOWLEDGEMENTS The assistance of former colleagues in Papua New Guinea in providing information for this review or discussing issues is deeply acknowledged. G E Hancock (Director, Mining Division, PNG Department of Mining and Petroleum) is particularly thanked for reviewing an earlier draft of the manuscript. The paper is published with the permission of the Director, Geological Survey of Western Australia.
REFERENCES Bartram, J A, Heape, J M T and Caithness, S J, 1991. The Mount Kare gold project, in Proceedings of the PNG Geology, Exploration and Mining Conference 1991 (Ed: R Rogerson), pp 96–100 (The Australasian Institute of Mining and Metallurgy: Melbourne). Binns, R A, 1994. Submarine deposits of base and precious metals in Papua New Guinea, in Proceedings of the PNG Geology, Exploration and Mining Conference 1994 (Ed: R Rogerson), pp 71–83 (The Australasian Institute of Mining and Metallurgy: Melbourne). Binns, R A, Scott, S D and PACLARK Participants, 1987. Western Woodlark Basin: Potential analogue setting for volcanogenic massive sulfide deposits, in Proceedings of the Pacific Rim Congress 87, pp 531–535 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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PAPUA NEW GUINEA’S MINERAL INDUSTRY 1986–1996
Both, R, Crook, K, Taylor, B, Brogan, S, Chappell, B, Frankel, E, Liu, L, Sinton, J and Tiffin, D, 1986. Hydrothermal chimneys and associated fauna in the Manus back-arc basin, Papua New Guinea, EOS, American Geophysical Union Transactions, 67:489–491.
Murray, S, Klapwijk, P, le Roux, H and Walker, P, 1997. Gold 1997 (Gold Fields Mineral Services Ltd: London).
Corbett, G J, 1994. Regional structural control of selected Cu/Au occurrences in Papua New Guinea, in Proceedings of the PNG Geology, Exploration and Mining Conference 1994 (Ed: R Rogerson), pp 57–70 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Neale, T I, 1994. Mining at Edie Creek — past and present, in Proceedings of the PNG Geology, Exploration and Mining Conference 1994 (Ed: R Rogerson), pp 255–263 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Corbett, G J, Semple, D G and Leach, T M, 1994. The Tolukuma Au/Ag vein system, Papua New Guinea, in Proceedings of the PNG Geology, Exploration and Mining Conference 1994 (Ed: R Rogerson), pp 230–238 (The Australasian Institute of Mining and Metallurgy: Melbourne). Davies, H L and Warren, R G, 1988. Origin of eclogite-bearing, domed, layered metamorphic complexes (‘core complexes’) in the D’Entrecasteaux Islands, Papua New Guinea, Tectonics, 7(1):1–21. Erceg, M M, Craighead, G A, Halfpenny, R and Lewis, P J, 1991. The exploration history, geology and metallurgy of a high sulphidation epithermal gold deposit at Wafi River, Papua New Guinea, in Proceedings of the PNG Geology, Exploration and Mining Conference 1991 (Ed: R Rogerson), pp 58–65 (The Australasian Institute of Mining and Metallurgy: Melbourne). Hancock, G E, 1996. Mining and petroleum in Papua New Guinea: Achievements and prospects, Australian Journal of Mining, November/December 1996:40–51. Hill, E J, Baldwin, S L and Lister, G S, 1992. Unroofing of active metamorphic core complexes in the D’Entrecasteaux Islands, Papua New Guinea, Geology, 20:907–910. Langmead, R P and McLeod, R L, 1991. Characteristics of the Tolukuma Au–Ag deposit, in Proceedings of the PNG Geology, Exploration and Mining Conference 1991 (Ed: R Rogerson), pp 77–81 (The Australasian Institute of Mining and Metallurgy: Melbourne). Leaman, P W, 1996. The Mt Bini porphyry copper–gold deposit and its tectonic setting, Papua New Guinea, in Proceedings of the Porphyry Related Copper & Gold Deposits of the Asia Pacific Region Conference, Cairns 1996, pp 13.1–13.10 (Australian Mineral Foundation: Adelaide).
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Murray, S, Klapwijk, P, Sutton-Pratt, A and Walker, P, 1996. Gold 1996 (Gold Fields Mineral Services Ltd: London).
Pascoe, G J, 1991. Hidden Valley gold project development summary, 1987–1991, in Proceedings of the PNG Geology, Exploration and Mining Conference 1991 (Ed: R Rogerson), pp 69–76 (The Australasian Institute of Mining and Metallurgy: Melbourne). Ryan, P, 1991. Black Bonanza: A Landslide of Gold (Hyland House: Melbourne). Samuel, K and Sie, A, 1991. The Mount Victor gold mine, Eastern Highlands Province, Papua New Guinea, in Proceedings of the PNG Geology, Exploration and Mining Conference 1991 (Ed: R Rogerson), pp 125–131 (The Australasian Institute of Mining and Metallurgy: Melbourne). Shedden, S H, 1990. Astrolabe mineral field, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1707–1708 (The Australasian Institute of Mining and Metallurgy: Melbourne). Tufar, W, 1990. Modern hydrothermal activity, formation of complex massive sulfide deposits and associated vent communities in the Manus back-arc basin (Bismarck Sea, Papua New Guinea), Österrreichische Geologische Gesellschaft Mitteilungen, 82:183–210. Wells, K and Young, D J, 1991. Geology and exploration of the Hamata deposit, in Proceedings of the PNG Geology, Exploration and Mining Conference 1991 (Ed: R Rogerson), pp 66–68 (The Australasian Institute of Mining and Metallurgy: Melbourne). Welsh, T C, 1990. The mineral industry in Papua New Guinea, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1681–1688 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Stephenson, P R and Miskelly, N, 1998. The JORC Code, 1987–1997, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 45–52 (The Australasian Institute of Mining and Metallurgy: Melbourne).
The JORC Code, 1987–1997 1
by P R Stephenson and N Miskelly
2
INTRODUCTION Most professionals in the mining and exploration industry and the mining investment community are now familiar with the Australasian Code for Reporting of Identified Mineral Resources and Ore Reserves (the JORC Code). However, in the mid to late 1980s, when the predecessor of this Monograph was being prepared (Hughes, 1990), standards of ore reserve classification and public reporting in Australia were inconsistent and of mixed quality. The introduction of the JORC Code in 1989 and its almost immediate adoption in full by the Australian Stock Exchange (ASX) rapidly brought about substantial improvements in the standard of public reporting by mining and exploration companies and fundamentally changed the environment surrounding this critical activity. The Australasian Institute of Mining and Metallurgy (The AusIMM) has had a long history of involvement with ore reserve estimation and classification, going back to The AusIMM President’s Presidential address in 1904 on the subject of prospectuses (Danvers Power, 1905). In 1953, The AusIMM set up a committee to examine the ‘Nomenclature of Classification of Ore Reserves’. However the response from industry to its recommendations was disappointing and no further substantial developments occurred in this field until the formation, in 1971, of the Joint Ore Reserves Committee (JORC), then a joint committee of The AusIMM and the Australian Mining Industry Council (AMIC). This paper examines how the JORC Code has changed the practice of Resource and Reserve classification and reporting in Australia since 1989, how the Code works, and what the future is likely to hold for public reporting standards here and overseas. A draft of this paper has been reviewed by JORC, now a joint committee of The AusIMM, Australian Institute of Geoscientists (AIG) and Minerals Council of Australia (MCA), but the opinions and views expressed herein are those of the authors and are not necessarily those of JORC.
RESOURCE-RESERVE ESTIMATION AND REPORTING IN THE 1980s By the mid 1980s, JORC had been in existence for approximately 15 years. JORC arose from the ashes of the so-called ‘nickel’ or ‘Poseidon’ boom on Australian sharemarkets in the late 1960s and early 1970s. It was formed by the Australasian mining industry in 1971 in response to pressure from its constituents 1.
Principal, P R Stephenson Pty Ltd, Consulting Geologists, PO Box 2271, Gosford, NSW 2250; Secretary, JORC.
2.
Associate Director - Resources, Paul Morgan Securities Pty Ltd, Level 12, 4 O’Connell Street, Sydney, NSW 2000; Chairman, JORC.
Geology of Australian and Papua New Guinean Mineral Deposits
and from government and regulatory authorities to establish an acceptable set of reporting standards covering ‘reserves’ and exploration results (Stephenson and Glasson, 1992). The first JORC report was released in 1972 (JORC, 1972) and was endorsed without change three years later (JORC, 1975). It established some important concepts which have been carried through in all subsequent JORC documents: 1.
that reports on ore or mineralisation should be standardised as far as possible;
2.
that public reporting of ore or mineralisation is solely the responsibility of a company’s Board of Directors;
3.
that such public reporting should be based on work by a Competent Person, as defined; and
4.
that reports on mineralisation up to and including the first ore reserve statement should include basic data such as the method of sampling and the distribution, dimension, assays and relative location of all relevant samples.
In its recommendations on ore reserve terminology, JORC recognised the diversity of opinion within the mining industry at the time and proposed that classification should be in accordance with one of four systems: 1.
Proved-Probable-Possible with definitions closely following those recommended by the Committee of the Society of Economic Geologists in 1956;
2.
Measured-Indicated-Inferred with definitions closely following those adopted by the United States Bureau of Mines in 1943;
3.
unclassified, ie quoting solely as ‘reserves’; or
4.
traditional, such as a company using its own wellestablished system.
In an amendment to its official listing requirements as at March 1973, the Australian Associated Stock Exchange (later to become the ASX) adopted a number of these recommendations, including the Proved-Probable-Possible classification, the concept of a Competent Person (although the words Competent Person were not used) and the requirements concerning pre-ore reserves reporting. Despite these developments, improvement in public reporting of ore reserves and mineralisation was not a rapid process. In a study of reporting practice, Miskelly (1981) concluded that, with few exceptions, major Australian Stock Exchange-listed mining companies had a very poor standard of reporting of ore reserves. JORC (1981) revised the 1975 report and recommended: 1.
adoption of the Proved-Probable-Possible system as the only classification system and phasing out of other terminology as soon as possible;
2.
that an ore reserve statement should specify whether figures given refer to in situ reserves or to recoverable reserves; and
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P R STEPHENSON and N MISKELLY
3.
that ore reserve figures should be rounded so as to reflect the degree of confidence in the estimate.
These early JORC reports had the status of guidelines only and were not binding in a regulatory sense. Through peer pressure and recognition of the good sense of the principles, most mining and exploration companies eventually adopted the standards recommended, although, by the mid 1980s, there was still some diversity of classification systems and public reporting standards. In particular, a separate subdivision of mineral resources had not yet been developed, and companies did not always make it clear whether the ore reserves reported were in situ or recoverable by mining. In the early 1980s, apart from the activities of JORC, two important developments occurred in the field of ore reserve classification and reporting which were to have major impacts on the eventual JORC Code: 1.
2.
The US Geological Survey (USGS) and US Bureau of Mines (USBM) released ‘Principles of a Resource/Reserve Classification for Minerals’ (USGS and USBM, 1980) in which, for the first time, a clear division between resources, representing in situ material, and reserves, representing economically extractable material, was presented. This key development was to form an important basis for the JORC Code. Conzinc Riotinto of Australia Limited (CRA) released ‘A Guide to the Understanding of Ore Reserve Estimation’ (King, McMahon and Bujtor, 1982). Drawing on extensive experience gained within the CRA Group, the authors covered a wide range of issues related to ore reserve estimation and reporting. Among these were two which particularly helped to advance evolution of the JORC Code: i.
the definition of a reserve as a resource, which may be known or unknown, on which ‘investigational work has established a basis for decisions as to technological and economic feasibility’, thus developing further the principle proposed in USGS and USBM (1980); and
ii.
emphasis on the principle that ore reserve estimation involves much more than just a geologist’s calculations.
By the mid 1980s it was becoming apparent that, although companies with established ore reserves were able to report more or less in accordance with the then AusIMM-AMIC guidelines, companies with deposits at an exploration stage with no defined reserves were having difficulty conforming. Apart from making it difficult for such companies to properly inform the public, this was also having an adverse effect on their ability to raise exploration funds. JORC recognised that the creation of a separate category to cover reporting of tonnage and grade estimates prior to the establishment of technical and/or economic viability, as proposed by the USGS-USBM and CRA, would substantially resolve these difficulties. It also recognised that increasing stock exchange requirements for full and material disclosure in public reports, and the federal government’s preference for self-regulation rather than by decree, necessitated the establishment of a more comprehensive and enforceable set of standards. A major revision of the existing guidelines was commenced in 1987, and in 1989 the first edition of the JORC Code was released (JORC, 1989).
INTRODUCTION OF THE JORC CODE INITIATIVES The 1989 version of the JORC Code differed from previous JORC reports in that it: 1.
formally introduced the concept in Australasia of Mineral Resources as a precursor to Ore Reserves (Fig 1), Mineral Resources representing geologically-defined, in situ material and Ore Reserves being derived from the Resources by the consideration of economic and production-related factors;
2.
restricted the term Ore Reserves to what had previously been termed recoverable ore reserves;
FIG 1 - Reporting terminology
46
Geology of Australian and Papua New Guinean Mineral Deposits
THE JORC CODE, 1987–1997
3.
eliminated the category of Possible Ore;
4.
introduced the concept of Pre-Resource Mineralisation, to be changed, in 1996, to Exploration Results; and
5.
became a binding minimum standard for members of The AusIMM, representing individuals, and AMIC, representing companies.
In a very important development, the JORC Code also became binding on companies reporting to the ASX when it was, in July 1989, incorporated in full into the ASX Listing Rules, and later into the New Zealand Stock Exchange (NZSX) Listing Rules. The Code and Guidelines are the only inclusion of an externally sourced document in the ASX and NZSX Listing Rules. Furthermore, Australia and New Zealand are the first countries to have their Mineral Resource and Reserve reporting standards, as set by industry, as an integral inclusion in their Stock Exchange Listing Rules. Guidelines to the Code were published in 1990 (JORC, 1990) and the Code and Guidelines were revised and released in a combined form in 1992 (JORC, 1992). During 1990, the AIG became one of JORC’s parent organisations and in 1992 the definition of a Competent Person was expanded to include Corporate Members of the AIG. In 1993, an Appendix covering diamond reporting was issued (JORC, 1993) and in 1996, the JORC Code was again slightly revised (JORC, 1996), incorporating the Diamond Appendix and changing the term Pre-Resource Mineralisation to Exploration Results, with restrictions on its use to disallow public reporting of tonnage and grade estimates in that category. A major review of the Code and Guidelines is currently under way. The purpose of the review is to address constructive suggestions which have been made by the mining industry and investment community since the last major revision in 1992 and to enable Australia to maintain its leadership position on the international front (see comments under The Future). It is intended that a new edition will be released in July 1999.
The Guidelines provide recommended forms of clarifying statements. 5.
Lack of reference, when necessary, to assessment criteria (Table 1 of the Code). Any report of Exploration Results must include details of the sampling of the mineralisation such as drilling or sampling density, drilling or sampling techniques, sample and assay quality and so on; reports of Mineral Resources and/or Ore Reserves should state the nature of the data on which the estimates are based and mention any assessment criteria for which inadequate or only poor quality data are available.
6.
Inadequate and/or incorrect statements regarding the Competent Person. The ASX Listing Rules require inclusion of a statement in public reports which contain information on Mineral Resources and Ore Reserves to the effect that the requirements of the Code with respect to the Competent Person have been met.
7.
Statements to the effect that Resource and/or Reserve figures have been ‘estimated’(or even worse, ‘calculated’) in accordance with the Code. The JORC Code does not regulate estimation techniques; it establishes a system of Resource and Reserve classification and sets minimum standards for public reporting.
8.
Lack of rounding of Resource/Reserve figures. If tonnages and grades are not rounded so as to reflect the uncertainty surrounding their estimation, then an unrealistic degree of accuracy is implied.
CODE REGULATION FUNCTIONS It is important to understand what the JORC Code is, and is not, intended to regulate. The Code: 1.
establishes and prescribes the minimum standards for public reporting of Mineral Resources and Ore Reserves in Australasia;
2.
sets out a system for the classification of tonnage (or volume) and grade estimates as either Mineral Resources or Ore Reserves and for subdivision of each into categories which reflect different levels of certainty or confidence;
3.
describes the qualifications and experience required for a Competent Person as corporate membership of The AusIMM or AIG together with at least five years relevant experience, and sets out the responsibilities of the Competent Person with regard to classification and reporting of Mineral Resources and Ore Reserves; and
4.
provides a summary list of the main criteria which the Competent Person(s) should consider in the course of preparing estimates of Mineral Resources and Reserves.
COMMON EARLY REPORTING DIFFICULTIES Full compliance with the JORC Code did not happen overnight, and during the first two to three years after its introduction, nonconforming reports were occasionally, and usually inadvertently, released to the public. JORC has worked with the ASX in helping to educate companies releasing such reports and the incidence of non-compliance is now very low. Some of the more common errors have been: 1.
Lack of categorisation, with reports containing only total figures for Mineral Resources or Ore Reserves rather than a breakdown into the constituent categories.
2.
Use of incorrect terms, such as ‘probable resources’, ‘geological resources’, ‘in situ reserves’, ‘mining reserves’, ‘possible reserves’ which have no meaning under the JORC Code, but which were often used in preJORC Code days.
3.
4.
Reporting of contained metal only, without also reporting tonnages and grades. The Guidelines state that such reporting ‘deprives the public of vital information’ and ‘is not in accordance with the Code’. It is very relevant to the potential investor to know whether a quoted quantity of contained metal is the product of a large tonnage of low grade material or a small tonnage of high grade material. Lack of explanation of the relation between reported Mineral Resources and Ore Reserves. Reports often did not make it clear whether the stated Mineral Resources were inclusive of, or additional to stated Ore Reserves.
Geology of Australian and Papua New Guinean Mineral Deposits
The Code does not: 1.
seek to regulate the procedures used by Competent Persons to estimate and classify Mineral Resources and Ore Reserves; or
2.
seek to regulate companys’ internal classification and/or reporting systems.
It follows, therefore, that the phrase often included in public reports that ‘the Mineral Resources/Ore Reserves have been estimated in accordance with the JORC Code’ is, in fact, meaningless. Resources and Reserves may be classified and reported in accordance with the Code, but not estimated.
47
P R STEPHENSON and N MISKELLY
Although the JORC Code provides a minimum set of standards to be applied in public reporting, companies may (and, in fact, are encouraged by JORC and ASX to) provide information above and beyond that required by the Code. For technical mining persons compiling reports under the Code, it is important to reiterate that, although the JORC Code provides guidelines for the estimation and classification of Resources and Reserves, it is not a restrictive set of requirements in relation to these matters (as it has often, in the authors’ experience, been misinterpreted). It is primarily a means of setting minimum standards for how important technical information is conveyed by the mining industry to its prime sources of finance, the investment community.
CLASSIFICATION OF MINERAL RESOURCES APPLICATION OF THE CODE This is an area where, for each case, the sound judgement of a Competent Person is critical. Therefore it is emphasised that the views and opinions expressed in this section are those of the authors and should not be interpreted as ‘official’ guidance from JORC. Formal advice of a general nature is available from JORC, although it should be noted that the committee is not authorised to provide rulings for specific situations. The JORC Code is not concerned with the precise procedures used by an estimator in classifying Resources and Reserves. However, Resource and/or Reserve classification is nevertheless very important as it directly affects the proportion of Resources convertible to Reserves (since Resources classified as Inferred may not be converted to Reserves), and the company executives’, investors’ or financiers’ view of those Reserves. How does a Competent Person translate such definitions into criteria for application to Resource-Reserve estimates and how should potential investors and others interpret the results? It is important to appreciate that every mineral deposit is unique and that the tonnage-grade estimates are based on a tiny sample of the deposit and are dependent on an interpretation of the geology of the deposit. This is why the Code gives the responsibility for Resource estimation and classification to a Competent Person and why it is essential that the Competent Person retains control over all matters affecting these important activities. In terms of the basis upon which classification is carried out, the Competent Person should not abrogate his or her responsibility to make professional, experience-based judgements by allowing Resource-Reserve classification to be determined solely on the basis of statistical parameters, however those parameters may be derived. Over reliance on statistically-based classification criteria can result, and has, to the authors’ knowledge resulted, in an erroneous or even nonsensical Resource classification with all the serious consequences which can flow from misunderstanding the confidence which applies to the Resource-Reserve estimates. Translating classification guidelines into classification criteria should not just be a matter of picking a statistical property of each Resource block (although such data may be useful as a basis for classification), but should instead be an experience-based judgement, based on an assessment of all the data on which the estimate is based (including the quality of those data) and which keeps in mind the uses to which the estimates might be put.
48
It is also important to appreciate that the main reason why tonnage and grade estimates are classified is to provide company executives and others making mining investment decisions, particularly external investors, with a basis for assessing relative risk. It is useful, indeed essential, to bear this in mind when classifying Resources and Reserves.
MEASURED OR INDICATED RESOURCES? Key words which will usually give guidance in deciding whether estimates should be classified as Measured Resources or Indicated Resources are contained in the Guidelines to the Code under the description of Measured Resources: ‘. . . any variation from the estimate (ie the estimate of Measured Resources) would be such as not significantly to affect potential economic viability’. In other words, a Resource may be classified as Measured if confidence in the estimate is such that additional technical information would not significantly affect technical or economic decisions made on the basis of the estimate. It is important for both compilers and readers of Resource-Reserve reports to appreciate that this does not equate to perfect knowledge of the Resource or absolute confidence in the accuracy of the estimate. Indeed, in the authors’ experience, the accuracy which might attach to an appropriately classified Measured Resource estimate might range up to ±15%. A useful technique which might help in deciding between the Measured and Indicated Resource categories is to try to imagine the effect of infill drilling or sampling (Stephenson, 1995). If it is felt that closer drilling or sampling would not greatly affect the geological interpretation and/or confidence in grade distribution, or, even if it could affect the interpretation or grade distribution, would not result in a significantly different estimate of tonnage, grade, shape and location of the mineralised bodies, then (assuming that the quality of the data on which the estimate is based are acceptable) the particular section of the deposit under question may reasonably be classified as Measured.
INDICATED OR INFERRED RESOURCES? Again some key words in the description of Indicated Resources are contained in the Guidelines: ‘Confidence in the estimate (ie in the estimate of Indicated Resources) would be such as to allow the application of technical and financial parameters and to enable an evaluation of economic viability’. The description of Inferred Resources, on the other hand, contains the phrase ‘Caution should be exercised if this category is considered in preliminary economic studies’. Again it might be useful to consider the likely effect of further drilling or sampling (Stephenson, 1995). If it is thought that additional drilling or sampling could significantly affect the shape and/or distribution of the mineralised zones, but not substantially affect the tonnage-grade estimate, then the portion of the deposit in question can probably be classified as Indicated. If, however, it is thought that the tonnage and grade as well as the shape and distribution of mineralised zones could be substantially affected by further drilling, then the relevant portion of the deposit is probably not defined adequately to allow an evaluation of economic viability and should probably be classified as no better than an Inferred Mineral Resource.
Geology of Australian and Papua New Guinean Mineral Deposits
THE JORC CODE, 1987–1997
INFERRED RESOURCES OR EXPLORATION RESULTS? The term Pre-Resource Mineralisation, introduced in the 1989 edition of the Code, was changed in 1996 to Exploration Results, with restrictions on its use. The reason for this change was that some companies were publicly reporting tonnagegrade estimates for Pre-Resource Mineralisation when the intention of JORC in providing this category was to assist companies to report discoveries of mineralisation at a very early stage of exploration when drilling and sampling results would be insufficient to allow an estimate of tonnes and grade to be made with confidence. It might help if mineralisation at the Exploration Results stage is thought of as ‘partially identified mineralisation considered worthy of further exploration’. The most important criterion to be considered in separating mineralisation at the Exploration Results stage from Inferred Resources relates to assumptions regarding continuity. As used in the JORC Code, continuity has two components - continuity of geological features and continuity of metal values. At the Exploration Results stage, the authors suggest that there would usually be doubts as to assumptions that could be made with respect to both of these components due to the sparsity and/or quality of data. At the Inferred Resource stage, there would usually be some confidence in assumptions of geological continuity but possibly some doubts regarding assumptions of continuity of metal values, and possibly other concerns of a technical nature. In both situations there must be sufficient sampling data available on which to base the judgement of continuity. Inferred Mineral Resources may also reasonably be estimated on the basis of little or no sampling data where the mineralisation being considered covers extensions beyond identified Indicated and/or Measured Mineral Resources. Knowledge of the adjoining Resources would usually be sufficient to support such estimates of Inferred Resources. When in doubt, the Competent Person should remember that the Code only allows public statements of Mineral Resources to be made when there are ‘reasonable prospects for eventual economic exploitation’ of the Resources. If the situation being considered by the Competent Person is on the borderline between mineralisation at the Exploration Results stage and Inferred Resources, and there is concern that publicly releasing a tonnage and grade estimate might be misleading or might create unrealistic expectations in the minds of readers of the report, then the mineralisation should probably not be classified as an Inferred Resource.
CURRENT STATUS OF THE CODE The JORC Code has been operating successfully for eight years and has attracted worldwide attention and acclaim. This can be attributed to a number of factors (Stephenson and Miskelly, 1997; Stephenson, 1997), discussed briefly below.
REGULATORY BACKING The decision by the ASX to append the JORC Code in its entirety to its Listing Rules was probably the single most important development in making the Code an effective tool for setting and maintaining public reporting standards in Australia. Its adoption by the ASX, and through the ASX by the Australian Securities Commission (ASC), has given it regulatory backing on a national basis and application of the
Geology of Australian and Papua New Guinean Mineral Deposits
Code is not, therefore, bedevilled by the varying statutory and reporting regimes which may apply at a State or Territory level. No other country (except New Zealand) in which a classification and/or reporting standard has been developed has managed to establish this degree of intimate linkage with the main national regulatory authority. In addition, through the change to ASX Listing Rules introduced in 1995, the name(s) of the Competent Person(s) responsible for preparation of Resource-Reserve estimates reported to the public must be published. Awareness that their name will appear in print has made individuals in Australia careful to ensure that they have both the qualifications and experience necessary to act as Competent Persons in relation to the Resources or Reserves being reported.
AVOIDANCE OF PRESCRIPTIVE REQUIREMENTS There are probably two ‘end members’ of the ways in which the development of standards covering public reporting of Mineral Resources, Ore Reserves and exploration results can proceed: 1.
to make the definitions and operational requirements as tight and prescriptive as possible, thus leaving Competent Persons little choice in their actions; or
2.
to keep the definitions and operational requirements relatively non-specific and non-prescriptive, thus allowing Competent Persons considerable freedom to exercise their professional judgement, but ensuring that they can be held to account for their actions.
The JORC Code is very much of type 2 and this has enabled it to be successfully applied to a wide range of commodities, deposit types and economic situations.
DISCIPLINING COMPETENT PERSONS As stated above, one of the key elements of the JORC Code is that it does not attempt to prescribe the requirements for the estimation and classification of Mineral Resources and Reserves, a procedure which would be fraught with difficulty and controversy. Instead it defines the qualifications of the estimators (the Competent Persons), and allows them freedom to use their experience to decide appropriate estimation and classification approaches. This system is only likely to be effective if the Competent Persons can be made to account for their actions. Obligatory membership of either The AusIMM or AIG provides the mechanism by which Competent Persons can be brought to account, since both organisations are national professional bodies which have effective and enforced codes of ethics. Although the requirement for the Competent Person to be a Corporate Member of The AusIMM or AIG has not always been a popular constraint, there is no doubt that it has been instrumental in achieving high standards of reporting in Australasia. In several cases, the Ethics Committees of The AusIMM or AIG have investigated complaints made in respect of reporting by ‘Competent Persons’ and action has been taken when deemed justified. Such action would not have been possible if the persons concerned had not been members of The AusIMM or AIG. JORC, mining industry organisations and regulatory authorities regard the Competent Person provisions of the JORC Code and ASX Listing Rules as a critical cornerstone of the reporting regime and are adamant that any future changes to
49
P R STEPHENSON and N MISKELLY
the JORC Code must not include modifications which result in effective watering down of these provisions.
OTHER FEATURES Origin of the Code The Code was developed by industry on the basis of long, well documented experience, and then adopted by the ASX and NZSX, rather than being forced on the industry by regulatory authorities. The Code is therefore user-friendly to the mining industry and also meets the needs of readers of reports on Resources, Reserves and exploration results.
Nature of the Joint Ore Reserves Committee JORC, which at the time of writing comprises 16 regular members and two alternate members, is a permanent rather than an ad hoc committee, with members representing a diverse range of organisations and professions. Organisations represented on the committee are The AusIMM, AIG, MCA, Mineral Industry Consultants Association (MICA), ASX and Securities Institute of Australia (SIA). Ten committee members are practising geologists or have a geoscientific background, three are mining engineers, one is a metallurgist, one is an institutional investment analyst and one is a stockbroker (and a member of ASX). There is also a mixture of company employees or executives and specialist consultants which encourages a balanced consideration of issues.
Communication and revision of the Code JORC maintains constant communication with industry (via various industry journals) and the ASX and conducts regular reviews and updates of the Code and Guidelines. Proposed amendments are widely circulated for comment before adoption. Familiarity with application of the Code has taken time, and education of compilers, publishers and readers of reports has been a constant objective of JORC. In 1997, JORC, in conjunction with the prestigious Annual Report Awards Committee, presented the inaugural JORC Award to Emperor Gold Mines Limited for excellence in annual reporting of Mineral Resources and Ore Reserves. It is anticipated that this award will become much sought after by companies in the years to come thereby further encouraging improvements in public reporting standards.
ACCEPTANCE OF THE CODE The JORC Code works not because its words have some magic power, but because the manner and degree of its adoption by industry and regulatory authorities have created an environment in Australia in which companies and individuals recognise the benefits to be gained from a high standard of public reporting, while also recognising the professional, legal and financial risks involved in any attempts to flout those standards.
THE FUTURE The JORC Code is not a static creation. It is regularly being reexamined to determine its relevance to the mining and investment industries and revised editions are released when appropriate. JORC meets regularly to consider matters raised by its members and other interested parties and to maintain its commitment to communication with industry and regulatory authorities.
50
A major revision of the Code and Guidelines is currently under way with a new edition planned for release in July 1999. It should be emphasised, however, that although there may be changes to the wording and layout of the documents in response to international developments (see below) and suggestions for improvements made by interested parties, there will be no change to the fundamental structure of the Code or to its operating environment. It works well and JORC has no intention of tampering with its basic functions. At the time of writing, major developments are occurring on the international scene (IMM, 1991; Miskelly, 1994; SME, 1995, CIM, 1996; Miskelly, 1997; Stephenson and Miskelly, 1997). Stimulated by the success of the JORC Code, Australia is taking a leading role in an initiative by the Council of Mining and Metallurgical Institutions, an international umbrella body representing many professional mining institutes in the western world, to develop a universal set of reporting standards for resource and reserve which would be adopted in most countries. By the time this paper appears, the first steps in this process, development of an agreed classification and reporting framework and agreement on the definitions for Resources, Reserves and categories of these, may have been successfully brought to completion. The challenge will then be for the participating countries (currently Australia, USA, UK, Canada and South Africa) to cooperate and coordinate in order to bring about as much uniformity in the guidelines supporting each country’s Code or standards as possible. This may be a medium to long term process. Independent of the international moves, the success of the JORC Code in Australasia and concerns over public reporting standards arising from the major Busang scandal in Indonesia in 1997, have stimulated considerable international interest in the Code. Several countries and major international mining companies, including the Anglo American, Rio Tinto and Ashanti groups, have publicly stated their intention to adopt or adapt the JORC Code for their reporting purposes. JORC, and the authors in particular, have taken every opportunity at national and international forums to explain the operating regime of the Code to ensure that any other countries or organisations adopting it have the opportunity to derive the maximum benefit from Australia’s long learning experience. Australia has come a long way since the mid 1980s, when inconsistent classification and public reporting standards were commonplace, and even further since the late 1960s and early 1970s, when the industry was reeling from the lack of credibility created in the minds of the general (and investing) public by activities stemming from the Poseidon boom and bust. The JORC Code has brought about improvements in the uniformity and quality of classification and public reporting standards in Australia which are now the envy of the world. Australia can be justifiably proud of its achievements with the Code. It is a great credit to all participants in the reporting regime, including mining industry organisations (The AusIMM, MCA and AIG), which have provided the essential industry commitment, and regulatory authorities (especially the ASX), which have been prepared to break with established practice in order to provide the Code with the regulatory backing so vital to its success. In particular, it stands as a monument to the many far-sighted and dedicated members of JORC who worked hard over the years to develop the Code and, by so doing, to enhance the industry’s credibility in the eyes and minds of the investing public.
Geology of Australian and Papua New Guinean Mineral Deposits
THE JORC CODE, 1987–1997
ACKNOWLEDGEMENTS The authors wish to express their appreciation to D H Mackenzie, a fellow member of JORC, who kindly reviewed drafts of this paper.
REFERENCES CIM, 1996. Mineral Resource/Reserve Classification - Categories, Definitions and Guidelines for Public Reporting, Ad Hoc Committee on Mineral Resource Classification of the Canadian Institute of Mining Metallurgy and Petroleum. Danvers Power, F, 1905. Prospectuses, in Transactions of the Australasian Institute of Mining Engineers, 1905. Reprinted in The AusIMM Proceedings, 299(1): pp 5–12. Hughes, F E (Ed), 1990. Geology of the Mineral Deposits of Australia and Papua New Guinea (The Australasian Institute of Mining and Metallurgy: Melbourne). IMM, 1991. Definitions of Resources and Reserves, (The Institution of Mining and Metallurgy: London). JORC, 1972. Report by Joint Committee on Ore Reserves, Joint Committee of The Australasian Institute of Mining and Metallurgy and Australian Mining Industry Council. JORC, 1975. Report by Joint Committee on Ore Reserves, Joint Committee of The Australasian Institute of Mining and Metallurgy and Australian Mining Industry Council. JORC, 1981. Report of the Joint Committee of The Australasian Institute of Mining and Metallurgy and Australian Mining Industry Council, Joint Committee of The Australasian Institute of Mining and Metallurgy and Australian Mining Industry Council. JORC, 1989. Australasian Code for Reporting of Identified Mineral Resources and Ore Reserves, Joint Committee of The Australasian Institute of Mining and Metallurgy and Australian Mining Industry Council. JORC, 1990. Guidelines to the Australasian Code for Reporting of Identified Mineral Resources and Ore Reserves, Joint Committee of The Australasian Institute of Mining and Metallurgy and Australian Mining Industry Council. JORC, 1992. Australasian Code for Reporting of Identified Mineral Resources and Ore Reserves, Joint Committee of The Australasian Institute of Mining and Metallurgy, Australian Institute of Geoscientists and Australian Mining Industry Council. JORC, 1993. Australasian Reporting of Diamond Exploration Results, Identified Mineral Resources and Ore Reserves, Joint Committee of The Australasian Institute of Mining and Metallurgy, Australian Institute of Geoscientists and Australian Mining Industry Council.
Geology of Australian and Papua New Guinean Mineral Deposits
JORC, 1996. Australasian Code for Reporting of Identified Mineral Resources and Ore Reserves, Joint Committee of The Australasian Institute of Mining and Metallurgy, Australian Institute of Geoscientists and Minerals Council of Australia. King, H F, McMahon, D W and Bujtor, G J, 1982. A guide to the understanding of ore reserve estimation, Supplement to The AusIMM Proceedings, 281, March 1982. Miskelly, N, 1981. Ore reserve reporting practices of major Australian mining companies, in Proceedings 1981 AusIMM Annual Conference, pp 133-140 (The Australasian Institute of Mining and Metallurgy: Melbourne). Miskelly, N, 1994. International standard definitions for reporting of Mineral Resources and Reserves - some suggested definitions for consideration, in Proceedings of The Fifteenth Congress of the Council of Mining and Metallurgical Institutions, September 1994. Republished in The AusIMM Bulletin, 6, December 1994, pp 28-30 (The Australasian Institute of Mining and Metallurgy: Melbourne). Miskelly, N, 1997. International standard definitions for reporting of Mineral Resources and Reserves, in Proceedings of The Australasian Gold Conference, Kalgoorlie, 4-6 March 1997 (The Australasian Institute of Mining and Metallurgy: Melbourne). SME, 1995. Guide for reporting exploration information, Resources and Reserves, Society of Mining Engineers. Stephenson, P R. 1995. Reporting using the “Australasian Code for Reporting of Identified Mineral Resources and Ore Reserves”, in Proceedings of Understanding Resources, Short Course on Resource Estimation Practices, November 1994, pp 9.1-9.8 (ECS Mining Consultants: Bowral, NSW). Stephenson, P R, 1997, in press. Australia - bench-marking effective and respected technical reporting standards, in Proceedings of Assaying & Reporting Standards Conference, Singapore, November 1997. Stephenson, P R and Glasson, K R. 1992. The history of ore reserve classification and reporting in Australia, in Proceedings 1992 AusIMM Annual Conference, pp 121-125 (The Australasian Institute of Mining and Metallurgy: Melbourne). Stephenson, P R and Miskelly, N, 1997, International standards for reporting of Mineral Resources and Reserves - status, outlook and important issues, in Proceedings World Gold ‘97 Conference, pp 265-273 (The Australasian Institute of Mining and Metallurgy: Melbourne). USBM and USGS, 1980. Principles of a resource/reserve classification for minerals, US Geological Survey Circular 831.
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Cucuzza, J and Goode, A D T, 1998. Australian mineral exploration research, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 53–60 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Australian mineral exploration research 1
by J Cucuzza and A D T Goode
1
INTRODUCTION In 1996–97 mineral exploration expenditure in Australia was of the order of $640M. Associated research and development (R&D) expenditure, most of it eligible for the 125% tax concession, is categorised by two types (Fig 1). 1.
2.
Competitive R&D, the outcomes of which are critical to the company’s future business. This is carried out internally and not shared with other companies (eg Goode et al, 1983; Goode, 1984). The available resources determine to what extent the work is carried out internally or out-sourced through one-on-one arrangements with research institutions. Companies like BHP and Rio Tinto have corporate laboratories and are thus able to undertake a significant proportion of their R&D internally. Others, like Normandy and MIM, undertake significant internal R&D despite not having formal corporate laboratories. While it is difficult to obtain accurate statistics on the amount of corporate internal R&D being conducted, Mackenzie and May (1992) suggested that it was in the range of 4 to 10% of total exploration expenditure or $22–55M. Pre-competitive or multi-client collaborative R&D. This occurs when a group of companies is prepared to share the costs, risks and outcomes of the research. The work carried out in this area is predominantly generic and tends to underpin the internal R&D carried out by the companies. The outcomes from this lower risk R&D provide companies with the basic know-how from which they develop their own competitive solutions. The Australian Mineral Industries Research Association (AMIRA) estimates that the total level of exploration oriented collaborative R&D in 1996–97 was of the order of $4M, which is dominantly managed by AMIRA. In the same period, exploration R&D through AMIRA was $3.36M. This represents approximately 0.5% of total exploration expenditure. However, if the level of in-kind contributions, such as access to data, field sites and time of company experts (estimated as an additional 15–50% of in-kind sponsorship towards each project) is taken into account, this increases to some 0.7% of exploration expenditure. A very small component of the collaborative field is occupied by the untied donations made by companies to Key Centres, ASEG Research Foundation, and scholarships to individual research students and such like.
The remainder of this paper will deal only with collaborative research efforts associated with mineral exploration in Australia. Collaborative research has contributed to the
1.
Research Coordinator, AMIRA, Level 9, 128 Exhibition Street, Melbourne Vic 3000.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Exploration and R & D expenditure in Australia in 1996–97.
prosperity of the mineral exploration industry in Australia in five critical ways: 1.
through the development of new technologies;
2.
through the transfer of technology developed in one sector to another;
3.
through training of company personnel;
4.
through training of students as future employees; and
5.
by sustaining a healthy R&D infrastructure in Australia. The benefits of collaborative research to industry include:
1.
sharing of risk and minimising individual exposure to failure;
2.
sharing of costs - more done for less;
3.
access to a wider range of expertise, allowing broader inputs by access to many different experts, thereby building larger, more effective projects;
4.
more efficient utilisation of the R&D infrastructure by focussing and directing work to industry needs; and
5.
the significant leverage that can therefore be achieved in research expenditure.
AMIRA was formed in 1959 by the mining industry to facilitate the development of collaborative R&D in Australian research institutions (Mackenzie and May, 1992).
CURRENT POSITION A significant growth in exploration R&D spending in geoscience in Australia relative to total exploration expenditure began in the early 1980s (Fig 2; see also Goode, 1987). This
53
J CUCUZZA and A D T GOODE
inhibits the application of the traditional exploration technology of geological mapping and geochemical and electrical geophysical surveys. 4.
Recognition of the economic potential of the regolith itself, particularly for gold, which needs better understanding.
These problems have in turn been translated by industry into rankings of exploration research priorities. Industry surveys conducted by AMIRA in 1992 and 1997 indicate that the early recognition of the presence of an ore bearing system, and the understanding of and dealing with the regolith, have remained the two most important priorities (Table 1). TABLE 1 Industry surveys of exploration priorities. Fig 2 - AMIRA exploration business unit expenditure, 1964–1997. Exploration problem
was driven by recognition of the need to develop exploration technologies suitable to Australian conditions. Prior to this time most technologies were imported from North America where technique development reached its zenith in the 1960s and 1970s. This change is well exemplified by the number of Australian exploration technology papers presented at conferences in North America over the last 20 years: 2 at Exploration ‘77, 11 at Exploration ‘87, and 28 at Exploration ‘97. It has been this support from industry that has led to major advances in exploration technology in Australia, to the stage where it is now the international leader in geochemistry, geophysics, remote sensing and image processing, as well as taking a leading role in ore deposit modelling studies. Many of these developments have either been the direct result of or have been underpinned by collaborative research. AMIRA has played a major role in shepherding many of these developments to fruition. Over recent years AMIRA has been supported by important government funding through agencies such as the Australian Research Council (ARC), now Strategic Partnership with Industry-Research & Training (SPIRT) and the Minerals & Energy Research Institute of Western Australia (MERIWA). In-kind contributions from research contractors have significantly leveraged industry’s contributions. International recognition of the success of this collaborative program is indicated by the setting up of comparable organisations overseas (eg CAMIRO in Canada, MIRO in Great Britain). Australia now exports exploration technology and expertise overseas as part of the globalisation of the industry. It also retains a significant and coherent R&D infrastructure, and an important culture for collaborative research.
AUSTRALIAN EXPLORATION PROBLEMS The direction of exploration research in Australia over the last decade has been largely controlled by the increasing recognition of the key issues facing the industry in this country:
1992 ranking
1997 ranking
Recognition of an ore bearing system
2
1
Understanding and dealing with the regolith
1
2
Area selection
5
3
Value from drilling
3
4
Availability of basic geoscientific data
4
5
Data management, integration and interpretation
6
6
AMIRA’S CURRENT EXPLORATION PORTFOLIO AMIRA projects can be broadly grouped into four main types: development of technologies, ore deposit studies, regional studies and regional data compilations. Given that the bulk of collaborative industry research is conducted through AMIRA, it is therefore useful to analyse AMIRA’s current geoscience portfolio (Table 2) in terms of commodity or deposit type, discipline, region, problem type and sponsor type relative to expenditure. Cucuzza (1997a) presented the results of this analysis based on projects current at the end of 1996. In general these results reflect the above priorities, although expenditure on Archaean gold in WA is surprisingly low, probably reflecting the past decade of relatively easy discovery of regolith-hosted deposits and therefore low research needs. The current relatively low spending on the regolith ironically probably reflects the past successes of CSIRO in gold exploration in WA, and the ongoing focus of research through the Cooperative Research Centre for Landscape Evolution and Mineral Exploration (CRC LEME). Specifically the results indicate: 1.
Commodity - dominated by gold and base metals.
2.
Discipline - mainly geochemical, geophysical and ore deposit models.
1.
The size of the country, compared to the exploration and research resources available, requires increases in efficiency and cost effectiveness of exploration.
3.
Region - generally reflecting the areas active for exploration, predominantly in Australia but with some increasing focus overseas.
2.
Increasing maturity of exploration in areas of outcrop, which leads to increased activity in areas of thin to thick cover and at greater depths.
4.
Problem - dominated by area selection and recognition of ore-bearing systems.
5.
3.
Widespread complex regolith and transported cover of various ages and properties (eg conductivity), which
Sponsor - the bulk of funding being provided by the major mining houses, with increasing participation rate from ‘non-Australian’ companies.
54
Geology of Australian and Papua New Guinean Mineral Deposits
AUSTRALIAN MINERAL EXPLORATION RESEARCH
TABLE 2 AMIRA mineral exploration projects, at end of 1996. Project
Title
Institution
P223C
Full domain 3-D TDEM modelling and inversion
CRC AMET
P291A
Faulting and mineralisation in western Tasmania
Uni Tasmania
P340
Computer assisted learning and skills development for the resources industries
Learning Curve Pty Ltd
P383
Improved resource evaluation using GIS: A pilot study
Uni WA
P384A
Sediment-hosted base metal deposits
CODES
P388
Development of reference material for mineral exploration and mining
Ore Research and Exploration P/L
P390A
Geological, tectonic and metallogenic relations of mineral deposits in mainland SE Asia
CODES
P392
Regolith landform and geological mapping using Air SAR/TOPSAR
CSIRO
P407
CRC AMET geophysical methods program
CRC AMET
P408
Resistate indicator minerals in porphyry Au-Cu exploration
CSIRO CRC LEME
P409
Geochemical exploration in areas of transported overburden, Yilgarn Craton and environs, WA
P413A
Optimisation of the open file data resource: Eastern Queensland mineral provinces
Terra Search
P415
Interpretation and modelling aids for three component downhole TEM data
Monash Uni
P417
Exploration in regolith-dominated terrain, North Queensland
CRC LEME
P418
Integrated structural and geophysical modelling
Monash Uni
P425
Magmatic and hydrothermal evolution of major intrusive-related gold deposits
ANU, CSIRO and Klondike
P426
Magnetic petrology applied to geological interpretation of magnetic surveys
CSIRO
P431
Geoscience data model
AusDEC
P435
Mineral mapping with field spectroscopy for exploration
CSIRO
P436
The application of geophysics to mine planning and operations: Stage II
CMTE
P437
Mineralised volcanic and sedimentary environments in the Eastern Goldfields
Uni. WA
P438
Cloncurry base metals and gold
James Cook Uni
P439
Studies of VHMS-related alteration: geochemical and mineralogical vectors to mineralisation
CODES
P441
A comprehensive test of EM modelling and interpretation software
Rudgeofisika NPO
P444
A novel gravity gradiometer for the resource industry
CSIRO and Melbourne Uni
P446
Field trialing next generation magnetics
CSIRO
P454
Computer applications in structural geology and orebody modelling
Uni WA
P460
Capacitive electrodes and IP arrays
Macquari e Uni
P462
Geophysical autonomous model aircraft acquisition (da GAMA)
CRC AMET
P 465
ARIES-1 feasibility study
CSIRO, AUSPACE, COSSA, ACRES
P467
Data translator for interoperability among mining packages and other application software
CSIRO
P476
Image processing and interpretation of airborne EM data for regolith/geological mapping
CRC AMET
P477
Application of MWD and LWD technology in minerals exploration and mining
CMTE
P478
Victorian gold - timing relationships and emplacement
Uni Ballarat
P480
User-friendly isotope technologies in mineral exploration: Proterozoic of northern Australia
CSIRO
P481
Three dimensional multi-component electromagnetic interpretation
CRC AMET
P482
Characterisation and metallogenic significance of Archaean granitoids of the Yilgarn Craton, WA
AGSO
P504
Supergene mobilisation of gold in the Yilgarn Craton
CRC LEME
AMIRA’s current geoscience portfolio can also be assessed in numerical and financial terms: • Number of current projects - 34 • Number of sponsor companies - 69 • Number of sponsor government agencies - 8 • Industry contributions in cash - $12.4M • Industry contributions in kind - >$5.2M • Government contributions in cash - $4.4M
Geology of Australian and Papua New Guinean Mineral Deposits
Government contributions in kind - >$0.2M • Research contractor contribution in kind - $16.4M • Average project duration - 2.5 years • Average number of sponsors per project - 8.7 The total value of projects is therefore more than $38.6M, giving industry overall a ‘research leverage’ of 1.2, while individual companies obtain an average collaborative ‘leverage’ of 17.4 for their research dollar. McKinsey & Company recognise three types of innovation
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J CUCUZZA and A D T GOODE
based on the degree of newness and the value created (Grady et al, 1993): • Innovations which offer some newness, perhaps improvement over an existing technology and which create some value, are termed Incremental Innovations, and deliver small changes to existing technologies. Examples include improvements in hardware for airborne magnetic and radiometric surveys. • Innovations creating breakthroughs with substantial value are Transformational Innovations. These deliver major, discontinuous improvements to technologies, eg development of atomic absorption spectroscopy at the CSIRO during the 1950s. • Innovations with significant elements of both are called Substantial Innovations. The availability of carbon in leach and carbon in pulp technology allowed the mining of hitherto uneconomic gold deposits. The availability of a cost-effective airborne gravity gradiometer for mineral exploration may also be considered an innovation of this type. The current AMIRA projects can be plotted on an innovation matrix, assuming they are successful in achieving their objectives (Fig 3). The majority of projects tend to be incremental in nature, as would be expected from the role of collaborative research as pre-competitive and relatively low risk in nature. Only a handful of AMIRA projects can be classified as Substantial Innovations and possibly only one as Transformational.
the productivity of virtually every sector of industry including exploration. Complementary technologies such as GPS and faster processors have significantly enhanced field acquisition, modelling capability and instrumentation. Recently, independent technological advances in microbeam analytical techniques and field portable spectrometry are underpinning research into recognition of vectors to ore that may be provided by subtle changes in major and trace element composition of specific alteration minerals. Current AMIRA projects are being carried out at a variety of organisations reflecting the broad and vigorous research base in Australia: • Three Cooperative Research Centres • Two divisions of CSIRO • Five universities • Australian Geological Survey Organisation • Three consulting organisations • One overseas institution
RESEARCH INSTITUTIONS Australia has been very well served by its various research institutions over the past few decades, although most of the mineral exploration research has been centred on a few key establishments. During the last financial year, AMIRA geoscience research spending was distributed as follows: • 42.6% at universities • 40.9% at CSIRO • 9.8% at Cooperative Research Centres • 6.6% others
KEY CENTRES AND SPECIAL RESEARCH CENTRES
Fig 3 - Mapping of AMIRA geoscience projects on the innovation matrix (note that many projects overlap).
The 1990s are clearly the technological age, and there has been continued development of a number of important technologies used throughout the exploration industry eg GPS, PCs and workstations. Many in-house and collaborative initiatives are underway to develop new airborne EM systems, airborne gravity gradiometers and so on. Companies are now scouring the world not only for the best technologies but also for the best researchers. There is an increasing awareness of the importance of the spillover effect and some companies are endeavouring to monitor technology development in other fields (the spillover effect results from technological breakthroughs from various fields of endeavour finding application in other quite different fields). The availability of micro-miniaturisation and personal computers has increased
56
At the beginning of the last decade, the Federal Government sought to stimulate excellence in exploration geoscience research by providing special funds to set up several Key Centres. The initial centres were the Key Centre for Ore Deposit Studies (CODES) at the University of Tasmania, the Key Centre in Strategic Mineral Deposit Research at the University of Western Australia, and the Key Centre in Economic Geology at James Cook University of North Queensland. These key centres have now lapsed, although CODES has been recently designated a Special Research Centre (SRC). In addition, the Tectonics SRC has been established at UWA, while another new key centre has been formed at Macquarie University as the Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC). Universities, through the key centres, along with the CSIRO, have provided the main thrust in exploration geoscience research over the last decade. In addition, significant research is also carried out by individuals at most other Australian universities, but particularly at Monash, Australian National University and Curtin. An increasing amount of work is being carried out at the Cooperative Research Centres. Although there are some 26 earth science schools throughout Australia only a few have been involved with AMIRA on a consistent basis. Collaborative research at universities received a boost at the end of the 1980s when the ARC introduced the Collaborative Research Grants and the Post
Geology of Australian and Papua New Guinean Mineral Deposits
AUSTRALIAN MINERAL EXPLORATION RESEARCH
Graduate Research Awards, both directed to research undertaken collaboratively with industry. These incentives are now available under the new SPIRT Scheme. Over recent years some earth science schools have attempted to form alliances. Perhaps the most successful is the Victorian Institute of Earth and Planetary Sciences (VIEPS), a combination of the geoscience departments at the universities of Melbourne, Monash and La Trobe. There is no doubt that there will be increasing pressure for the formation of these alliances as the federal government continues to seek efficiencies and areas to reduce spending.
Recently the CRCs have also come under pressure, with the federal government indicating its desire for the centres to become self funding. It is difficult to see how this can be achieved.
SUCCESS STORIES During the 1980s and 1990s the Australian mineral industry has seen advances which have radically increased the efficiency and cost effectiveness of exploration, and assisted in the emergence of Australia as a world leader in exploration technology. Several examples are described below.
CSIRO
ELECTROMAGNETIC (EM) GEOPHYSICS
CSIRO had traditionally attracted the largest share of AMIRA support. In order to reposition itself into a more responsive and effective institution it has recently been restructured into five alliances composed of 22 sectors. Alliances are groupings focussing on common industries. Thus the Minerals and Energy alliance encompasses the Coal and Energy, Mineral Exploration and Mining, Mineral Processing and Metal Production, and Petroleum sectors. The Mineral Exploration and Mining sector calls on expertise from nine different divisions, including Exploration and Mining, and Mathematical and Information Sciences. CSIRO will continue to be an important research source for industry.
In the early 1980s EM contributed to some major mineral discoveries such as Hellyer, Benambra and Water Tank Hill (Eadie, 1997). EM modelling software can play a key economic role in the planning, evaluation, and interpretation phases of an exploration or delineation program. EM modelling is invaluable in deciding what system should be used for a given exploration problem. Once a survey is completed, a combination of accurate modelling and practical interpretation schemes can give valuable information on the existence of an orebody and its likely location and size.
AUSTRALIAN GEOLOGICAL SURVEY ORGANISATION (AGSO) AGSO has also undergone significant changes recently as a result of increasing funding pressure from government. It provides independent geoscientific information to government, industry and the community and key areas of responsibility include minerals, land and water, national databases, geohazards, petroleum and marine, and international. AGSO is a major participant in all three exploration oriented CRCs, and thus contributes significantly to collaborative research. Like many other government-funded institutions, the organisation is at the crossroads. However, there is a real need now, as there was some 50 years ago when it was formed, for an effective federal geoscience survey to undertake the basic information gathering and archiving needed to underpin exploration.
COOPERATIVE RESEARCH CENTRES Perhaps the greatest change in the research infrastructure in Australia in recent years has been the development of the cooperative research centre concept. The aims of the CRC program were threefold: to foster greater cooperation between research institutions, to foster closer links with industry and to provide a basis for further postgraduate education. Each CRC was designed to become a specialised centre catering to major perceived future needs of the industry. The mineral exploration industry is currently serviced by three key CRCs (Table 3): Landscape Evolution and Mineral Exploration (CRC LEME) specialising in regolith studies, Australian Mineral Exploration Technologies (CRC AMET) specialising in electrical geophysics, and Australian Geodynamics (AGCRC) specialising in tectonic studies. All involve cooperative ventures by CSIRO, AGSO and universities, and all have been very influential in the development of recent exploration thinking.
Geology of Australian and Papua New Guinean Mineral Deposits
Successful AMIRA projects over last 15 years have delivered a suite of software that allow geophysicists to model EM responses of complex 3D and 2D geology, incorporating topography, complex regolith plus deformed target hosts. Ten years ago modelling was restricted to simple geometries and our knowledge of the physics of propagating EM fields in the earth was imperfect. However, as a result of the work carried by A Raiche, J Buselli and others at the CSIRO and now the CRC AMET, coupled with the major advances in desktop computing, geophysicists can now expect to model near-real geology. The software developed in these AMIRA projects has been commercialised through ENCOM technology in a package called EMVision.
REMOTE SENSING, MINERAL MAPPING AND IMAGE PROCESSING Australia has led the world in the application of remote sensing to mineral exploration. A series of AMIRA projects resulted in the development of Australia’s digital image processing facility for remote sensing (Bailey, 1989). This work, led by A Green and J Huntington at CSIRO, also had major spinoffs in exploration technologies. The development of image processing technology has revolutionised the way all forms of digital data are visualised and interpreted. This technology has placed what was once in the domain of specialists into the hands of the geologist. Another important spinoff was the development of noninvasive spectroscopy of geological materials, particularly the PIMA field instrument and the CO2 laser spectrometer. As a result, field-based mineral mapping can quickly and cheaply recognise important alteration minerals (Huntington, 1997). Several AMIRA projects have demonstrated that certain mineral species can be identified from airborne platforms through analysis by innovative software. Currently CSIRO is developing highly sensitive, high-resolution profiling spectrometers for low altitude surveys. Ultimately this will form part of the GIMMS (geophysically integrated mineral mapping spectrometer) commercial instrument. The next step
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TABLE 3 The Cooperative Research Centres. CRC
Date of Inception
Partners
Mission
Australian Mineral Exploration Technologies (CRC AMET)
1992
CSIRO, Macquarie, Curtin, AGSO, WA Geological Survey, AMIRA, and World Geoscience
Develop and deliver dramatically improved electromagnetic (especially airborne electromagnetic) methods for exploration in environments characterised by complex, conductive regolith cover
The Australian Geodynamics Cooperative Research Centre (AGCRC)
1992
CSIRO, Monash, La Trobe, AGSO, Digital Equipment Corporation
Develop concepts, sciences, and technologies that assist the Australian minerals and petroleum industry to discover new deposits
Landscape Evolution and Mineral Exploration (CRC LEME)
1995
CSIRO, ANU, AGSO
To generate new knowledge through its research activities in regolith-landscape evolution, to identify the resulting implications in minerals exploration and mining, to develop new exploration models, and to devise new or improved geochemical exploration methods
is spaceborne remote sensing designed specifically for exploration. In a recent AMIRA project, a feasibility study was undertaken for ARIES-1, a satellite-borne system (Cucuzza, 1997b). The aim of the study was not only to define the user requirements, system specifications and the design to meet these requirements, but also to carry out a series of simulations showing potential ARIES products. Such technology will be invaluable in cheap regional mineral mapping, particularly in recognising and analysing major alteration systems.
REGOLITH GEOCHEMISTRY A succession of regolith-related geochemical projects carried out under the leadership of R E Smith and C R M Butt at CSIRO and more recently CRC LEME have had a major impact on gold exploration procedures. Of particular recent impact has been the development of calcrete sampling techniques in the Yilgarn and Gawler gold provinces, where surface geochemical sampling can now be effective even in areas of transported cover (Smith, 1996). Over 60 companies have contributed approximately $5M to these projects since 1987. The outcomes from this work have had a significant role in the discovery of several new gold deposits containing many millions of ounces of new resources.
LEAD ISOTOPE GEOCHEMISTRY The pioneering work of B Gulson and later G Carr of CSIRO through a series of AMIRA projects in the late 1970s and 1980s led to the lead isotope characterisation of weathered surface mineralisation and gossans (Gulson, 1986). This technique is now used by many companies, particularly base metal explorers, as a key method in discriminating priority targets, resulting in an increased chance of economic success as well as reducing the chance of fruitless exploration on minor deposits.
ORE DEPOSIT MODELS Significant advances in the understanding of ore deposit systems have also taken place over the last decade. Notable examples include volcanic- and sediment-hosted massive sulphide base metal deposits at the CODES Special Research Centre (University of Tasmania), Archaean lode gold deposits at the University of Western Australia, ironstone-hosted
58
copper-gold systems at James Cook University, and komatiitehosted massive nickel sulphide deposits at CSIRO.
FUTURE DEVELOPMENTS THE CHANGING ENVIRONMENT In recent years the exploration industry has had to adjust to significant changes in the business environment.
Increased cost efficiencies Some of these changes have been driven by the need for companies to focus on core competencies, to maximise return on assets and thus shareholder value. Accompanying these changes has been an alignment of corporate R&D with business goals. This has resulted in significant downsizing of corporate laboratories and the devolving of responsibility for R&D matters to operating sites or business units. This is consistent with the new paradigm suggested by Rouseel, Saad and Erickson (1994) in the Third Generation R&D model, in which R&D is 'strategic and purposeful', and is developed in 'partnership' with the business strategy. The downside of this is that the operations are now more than ever focussing on productivity, industrial relations and other essentially shortterm profitability-motivated issues. Consequently, the changing environment is creating cultures which value and reward short-term results at the expense of longer term goals typical of the research scene. The contraction of corporate laboratories seen over the last several years has a parallel in the public sector institutions. AGSO, the State geological surveys, CSIRO and the universities have come under severe funding pressure as governments attempt to reduce their deficits and impose ‘user pays’ principles and accountability. This phenomenon is by no means restricted to Australia; the USGS, Geological Survey of Canada and others are also facing the same issues. The result is that many government programs will be significantly curtailed if not terminated. However, it should also be noted that significant state government ‘initiatives’ in recent years have had a major impact on the provision of new regional geological and geophysical information to the benefit of the industry, as has the National Geological Mapping Accord by AGSO and the State geological surveys.
Geology of Australian and Papua New Guinean Mineral Deposits
AUSTRALIAN MINERAL EXPLORATION RESEARCH
Increased competition A general move to a more competitive climate over the last number of years has meant an increasing shift to internal R&D. In a customer survey carried out by AMIRA in 1996, responses from over 100 companies across all sectors of the minerals industry indicated that twice as many companies expected to carry out more internal and one-on-one research in the next five years than expected to do more collaborative research. The drivers for this appear to be the need to enhance competitive advantage and in part to shorten the technology development cycle.
last several years that have particularly impacted on exploration. These include: 1.
the emergence of specialist consultants offering services either on an exclusive or collaborative basis;
2.
the emergence of the Internet as a means of facilitating communications and thus developing networks and alliances that foster collaboration around the world; and
3.
increasing importance of intellectual property in defining the relationship between research providers and clients.
THE FUTURE FOR MINERAL EXPLORATION RESEARCH
Reduced tax incentives The introduction by the Federal government of the 150% tax incentive for R&D in the late 1980s was a significant stimulus to research in Australia. However, the government has recently decreased the incentive to 125% and scrapped the eligibility of syndication as a means of funding R&D and commercialising the outcomes. The tax concession has provided an incentive to broaden the scope of many projects, has encouraged some companies to increase their R&D expenditure, and encouraged others take on higher risk projects. The full impact of these changes has yet to be determined.
Globalisation One of the most fundamental changes in recent years has been the end of the Cold War and the fall of Communism, which has seen the opening up of the countries that hitherto were closed to Western investment. Many of these underdeveloped countries, which contain highly prospective terrains, are competing in the global market place by introducing competitive taxation regimes and mining policies to attract foreign companies. The liberation of economies has also eased restrictions on majority ownership, the transfer of investment capital and repatriation of profits. As a result of these factors, over the last decade there has been an increasing amount of Australian exploration funds going into overseas exploration. This trend has been exacerbated by the increasing exploration maturity of easily explored regions in Australia, as well as by land access and title security issues. Recent Minerals Council of Australia figures indicate that some 40% of the 1995–96 total Australian exploration expenditure was spent overseas. Over the last five years, overseas expenditure has increased by 50% while overall exploration expenditure in Australia has increased by 30%. Companies like BHP, Rio Tinto, WMC, Normandy, North and others have moved away from an Australian-centric focus to a global one. Australian companies are now exploring in every continent of the world except Antarctica. It is clear that globalisation has increased unabated and is likely to continue to do so in the foreseeable future. This has led to reduced exploration expenditure in Australia by the major mining houses, historically the main supporters of collaborative research in Australia. This, of course, has serious implications for AMIRA and Australian research institutions. Increasingly research will need to be focussed on overseas problems. In the immediate future, companies are particularly interested in fast assessment capabilities for new greenfield belts in these newly ‘opened’ countries as a means of recognising high priority ground or alternatively sterilising it for the short term.
Research directions over the next decade will be guided largely by the current needs of industry, although there will always be a potential unexpected impact from innovations through spillover effects and complementary technologies developed in such diverse fields as medical imaging and defence R&D. The results of the survey of the exploration and research managers carried out by AMIRA in 1997 can therefore be used as a guide to the types of technologies that will be required, and no doubt developed, given sufficient funding. The responses to the survey were diverse yet there were common threads that allowed them to be grouped and ranked under six general headings (Table 1). Based on these priorities, we see the need for: 1.
the development of geochemical and geophysical technologies that will provide vectors to ore and allow the explorers to distinguish weakly mineralised from strongly mineralised environments;
2.
continued research on the regolith to allow improved exploration for deposits hidden below the regolith; and
3.
the development of better screening techniques to allow explorers to more quickly and efficiently identify poorly known mineral provinces (particularly overseas) that are more likely to host major high grade deposits.
The increasing globalisation of the industry, while impacting on the Australian research scene, has opened up new challenges for organisations like AMIRA and the research institutions. Australian explorers have successfully utilised Australiandeveloped technologies in many similar environments overseas (eg use of regolith geochemistry in West Africa). However, there are likely to be fresh problems in relatively underexplored and unfamiliar climatic or topographic terrains in underdeveloped countries, eg the wet tropics of SE Asia, South America and Africa and the arid alpine regions of South America and central Asia. Similarly, relatively unfamiliar ore deposit types (eg epithermal gold, porphyry copper-gold) in these lesser explored areas also provide new research opportunities. Another key problem in many underexplored overseas countries is the lack of readily available quality regional geological and mineral deposit data sets. Within Australia continued efforts will be required to assist exploration for gold and base metal ore deposits at depth or under cover. Within Australia’s leading gold province, the Yilgarn, there is an emerging need for increased understanding of its geological development in order to better predict the location of new gold (and base metal) deposits, as the era of ‘easy’ discovery of surface deposits has essentially passed. Tools will be required that will not only allow explorers to handle multiple data sets efficiently, but will also permit
Gunthorpe (1997) identified other developments over the
Geology of Australian and Papua New Guinean Mineral Deposits
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manipulation and visualisation in more than two dimensions. There will be a need to exploit the synergies between the different data sets, taking into account the fact that each data set responds to a different physical property, has different vertical and horizontal resolutions, and different depth penetrations. Automation will be an important issue as larger multivariate data sets are increasingly acquired. With the existing and ever increasing size of data sets, there will be a need to provide tools that will allow automatic ‘first pass’ interpretation either individually or synergystically. These tools will ensure that data are efficiently processed and interpreted in shorter time frames so that decisions on prospective areas can be made more quickly. There will be a continual need to develop non-invasive airborne technologies that will be able to operate cost effectively in a variety of inhospitable regions, particularly rugged terrains. In mineral exploration and mining, maximum use of drill holes is not being made, particularly compared to the petroleum industry. The next five or so years may see the development of logging-while-drilling technologies that will assist explorers and miners make significantly better use of drill holes.
THE FUTURE FOR COLLABORATIVE RESEARCH In the current business climate, companies now expect that research projects must focus on real needs, ie the outcomes must add value to their business, research objectives must be well defined, projects must be well managed, and results delivered in a timely fashion and in a way that can readily be appropriated for use. How is collaborative R&D going to fare in this new environment? In a recent survey of exploration companies, over 75% of respondents agreed on the key criteria required from collaborative projects to attract financial support from companies: • The research project must be directly relevant to the company’s business • The science must be of the highest quality • The project leader must have a good scientific reputation or track record of success • The research project must have clear and measurable objectives • The components of the project must be relevant to answering the problem being addressed (as opposed to being assembled to provide funding for less relevant areas of science) • There must be a clear and acceptable strategy to transfer the outcomes to users • The research team assembled for the project must be of high calibre • The project objectives must be achievable within the budget and time proposed • The benefits and deliverables must be clearly articulated. The challenge is clearly before all those involved in the Australian mineral exploration and research business to meet these criteria, and provide the research support so beneficial to industry in the past.
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CONCLUSIONS Collaborative research has had a major impact on the exploration methodology utilised by mining companies in Australia, and on subsequent exploration successes. While this may be expected to continue, the major new exploration research challenges and opportunities will be increasingly found in the international arena, particularly in the newly accessible but poorly explored countries of Asia, South America and Africa. Within Australia the emphasis for research will increasingly focus on exploration for ore deposits under cover or at depth. AMIRA will continue to act as the prime focus for collaborative R&D, although it too must adapt to the changing business landscape.
ACKNOWLEDGEMENTS We would like to thank AMIRA for permission to publish this paper. In its preparation we have drawn from many sources; in particular, we would like to acknowledge the contributions of present and past colleagues at AMIRA and in industry and academia.
REFERENCES Bailey, J D, 1989. The Australian Landsat TM story: A commercial success, in Proceedings of the Seventh Thematic Conference on Remote Sensing for Exploration Geology, October 2–6, 1989, Calgary, Alberta, Canada, pp 1359–1366. Cucuzza J, 1997a. AMIRA mineral exploration research and development, in Minfo, New South Wales Mining and Exploration Quarterly, 55:4–8. Cucuzza J, 1997b. Seeing red (and green and blue): the Australian resource information and environmental satellite (ARIES-1) Project, Groundwork, September 1997, 1(1):18–19. Eadie, E T, 1997. Benefits from AMIRA programs in geophysics, in Proceedings of the 38th AMIRA Annual Technical Meeting, Adelaide, pp E6.1–E6.6. Goode, A D T, 1984. New directions in mineral exploration technology, in Technical Meeting on Exploration - Minerals and Petroleum (Abstracts), pp 1–10 (AMIRA: Melbourne). Goode, A D T, 1987. New techniques in mineral exploration - hi-tech overkill, value for money or necessary for survival? in Risk and Survival in the Mining and Petroleum Industries, pp 41–43 (The Australasian Institute of Mining and Metallurgy: Melbourne). Goode, A D T, Bumstead, E D, Bye, S M, Edwards, A C and Harman, P G, 1983. New technology in mineral exploration, BHP Technical Bulletin, 27:5–12. Grady, D, Lautenschlager, H, Murray, J and Thompson, R, 1993. Unlocking innovation - challenging conventional wisdom about what leaders do, McKinsey & Company Report (unpublished). Gulson, B L, 1986. Lead Isotopes in Mineral Exploration, Developments in Economic Geology, 23 (Elsevier: Amsterdam). Gunthorpe, R J, 1997. Exploration research - the industry interface, in Proceedings of the 38th AMIRA Annual Technical Meeting, Adelaide, pp E4.1–E4.8. Huntington, J, 1997. Field, air and spaceborne mineral mapping for exploration, in Proceedings of the 38th AMIRA Annual Technical Meeting, Adelaide, pp E3.1–E3.12. Mackenzie, D H and May, J R., 1992. Pre-competitive exploration research in Australia - A 30 year assessment, in Proceedings 1992 AusIMM Annual Conference, Broken Hill, pp 163–173 (The Australasian Institute of Mining and Metallurgy : Melbourne). Rouseel, P A, Saad, K N and Erickson, T J, 1994. Third generation R&D: managing the link to corporate strategy (Arthur D Little, Inc). Smith, R E, 1996. Regolith research in support of mineral exploration in Australia, in Journal of Geochemical Exploration, 57: 159–173.
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Pirajno, F and Preston, W A, 1998. Mineral deposits of the Padbury, Bryah and Yerrida basins, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 63–70 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Mineral deposits of the Padbury, Bryah and Yerrida basins 1
by F Pirajno and W A Preston
2
INTRODUCTION The Palaeoproterozoic Padbury, Bryah and Yerrida basins are situated along the northern margin of the Yilgarn craton, about 750 km NNE of Perth, WA, within the southern part of the 2200–1800 Myr Capricorn Orogen (Myers, 1993). These basins were formerly included in a single tectonic unit, the Glengarry Basin, which was defined by Gee and Grey (1993). The mineral deposit styles of the Bryah, Padbury and Yerrida basins and their immediate basement include mesothermal gold-only lodes, volcanogenic massive sulphide (VMS) copper-gold, shear zone–hosted copper, sedimentary-hosted lead, supergene-enriched manganese, banded iron formation (BIF) iron ore, and talc in metasomatised dolomitic rocks. A regional model for the genesis of the epigenetic mineral deposits of the region is proposed in this paper.
STRATIGRAPHY AND TECTONIC SETTING Rocks of the Glengarry Basin were formerly bracketed under the Glengarry and Padbury groups (Gee and Grey, 1993). On the basis of recent detailed geological mapping, integrated with aeromagnetic, Landsat, petrological, geochemical and geochronological studies, three lithostratigraphic domains are now recognised to constitute the former Glengarry Basin (Pirajno et al, 1996; Occhipinti et al, 1997). They are the Bryah and Padbury groups in the west and the Yerrida Group in the east (Fig 1). These groups have distinctive stratigraphic, structural and metamorphic characteristics and are now recognised to be three separate basins. The Yerrida Group comprises a sag basin succession of siliciclastics and evaporites and a volcano-sedimentary rift basin succession. The Bryah Group comprises a rift basin succession dominated by mafic–ultramafic volcanic rocks of oceanic affinity, and the Padbury Group includes a turbiditic sequence deposited in a peripheral foreland basin (Martin, 1994). Granitic inliers, the Marymia and Goodin granitic inliers (Fig 1) supplied some of the detritus to the succession in these basins. An important tectonic unit is the Peak Hill Schist, which hosts a number of economically significant gold deposits. This unit was formerly called the Peak Hill Metamorphic Suite and was assigned to the Glengarry Group (Gee, 1987; 1990; Gee and Grey, 1993). It may represent the highly deformed and 1.
Project Manager, Geological Survey of Western Australia, 100 Plain Street, East Perth WA 6000.
2.
Formerly Manager Resources Assessment and Advice, Geological Survey of Western Australia, now Manager Projects - North and Inland Division, Department of Resources and Development, 168-170 St George’s Terrace, Perth WA 6000.
Geology of Australian and Papua New Guinean Mineral Deposits
mylonitised southwestern tip of the Marymia Inlier, and is either in fault contact or tectonically interleaved with rocks of the Bryah Group (Pirajno and Occhipinti, 1995). Thus, although stratigraphically not belonging to either the Bryah or Padbury basins, the Peak Hill Schist (and its contained mesothermal gold-only deposits) and the Bryah and Padbury rocks, belong to a complex but coherent package of deformed and metamorphosed rocks. As such, the rocks of the Bryah and Padbury groups and the Peak Hill Schist can be considered in terms of a single tectono-metamorphic domain. Small slivers of granitic rocks of the Narryer Gneiss terrane could also be included in this domain as they are tectonically interleaved and metamorphosed with Padbury and Bryah rocks (Fig 1). The tectonic history of the Bryah, Padbury and Yerrida basins and their interaction with basement inliers are linked with the oblique continental collision between the Pilbara and Yilgarn cratons, between 2000 and 1800 Myr (Tyler and Thorne, 1990; Myers, 1993; Tyler et al, in press). Models that attempt to explain the geodynamic evolution of the Bryah, Padbury and Yerrida basins have been proposed by Pirajno et al (1995) and Pirajno (1996). These models envisage the development of the Bryah and Yerrida basins either in a back arc setting, prior to collision of the Pilbara and Yilgarn cratons, or as pull apart structures formed during sinistral strike-slip movements, relating to the oblique collision between the Yilgarn and Pilbara blocks. In both models thinning and rupture of Archaean continental crust would have occurred, with the localised development of small oceanic basins (Pirajno, 1996). The Padbury Basin is a tectonic unit which is considered to have originated in a foreland tectonic setting (Martin, 1994), in which the Bryah Group acted as a basement.
MINERAL DEPOSITS The mineral resources of the Peak Hill Schist, Bryah, Padbury and Yerrida basins are considerable, given the relatively small total area of these units of about 20 000 km2 and include gold, copper, lead, manganese, iron and talc. The distribution of the deposits is shown in Fig 1. Mineral production and defined resources within these tectonic units at December 1995 are in Tables 1 and 2. Defined resources are likely to increase with continuing exploration.
GOLD DEPOSITS The most significant production has come from the Peak Hill–Jubilee, Harmony, Fortnum and Labouchere– Nathans–Deep South gold deposits, and from the volcanogenic Horseshoe Lights gold-copper deposit. The total gold produced is 47 t, with a total identified endowment (production plus remaining resources) of approximately 73 t of contained gold (Table 1).
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F PIRAJNO and W A PRESTON
FIG 1 - Location and geological map of the Glengarry region, showing distribution of mineral deposits. Deposits are indicated thus: 1. Peak Hill; 2. Jubilee; 3. Mount Pleasant; 4. Harmony (New Baxter’s Find); 5. Labouchere; 6. Nathans-Deep South; 7. Fortnum group; 8. Horseshoe Lights; 9. Horseshoe gold; 10. Wilthorpe; 11. Wembley; 12. Mikhaburra; 13. Cashman; 14. Ruby Well; 15. Mount Padbury-Mount Fraser group (including Elsa); 16. Horseshoe manganese; 17. Ravelstone; 18. Robinson Range BIF (no specific locality); 19. Thaduna; 20. PGE-bearing gossan; 21. Magellan.
64
Geology of Australian and Papua New Guinean Mineral Deposits
MINERAL DEPOSITS OF THE PADBURY, BRYAH AND YERRIDA BASINS
TABLE 1 Total estimated gold endowment of the Padbury, Bryah, Yerrida groups and Peak Hill Schist. Deposit Name
Production (P) to 31.12.1995 Ore (kt)
Mount Fraser Baxter centre
Contained metal (kg)
Alluvial (kg)
Dollied (kg)
2.7
1.263
Remaining Resources (R) Total contained metal (kg)
Ore (kt)
Contained metal (kg)
Contained metal (kg)
263
1232.7
261 637
1197.5
Mount Seabrook centre
2
38.4
0.6
0.157
39.1
39.1
Ravelstone centre
5
105.8
0.6
3.161
109.5
109.5
1.5
1.5
879.1
879.1
Harmony Contact
Wilthorpe centre Peak Hill District sundry
1236.7
Total (P+ R)
1236.7 2362
< 500 t
1.5
3
770.6
91.4
17.185
7940
9137.5
Ruby Well centre
16
147.1
39.4
18.603
205
205
Horseshoe centre
26
102.4
53
92.4
247.7
247.7
Horseshoe Lights
1777
8949.2
8949.2
8949.2
Labouchere centre
2995
6934.1
6934.1
463
1338
8272.6
Fortnum
2600
6973.1
6973.1
5098
12 000
18 973.1
(595)
(1301)
(1301)
5095
21 541.6
507
764
22 492.6
391
1617
1617
Fortnum Peak Hill centre
124
62.9
21 728.6
Fiveways Jubilee Total
12 745
46 796.5
312
195.7
47 304.2
250
595
595
9666
25 555
72 859.2
Notes: Data from MINEDEX database, Western Australia Department of Minerals and Energy. Brackets enclose Measured plus Indicated Resources only.
The most important mineral deposits exploited to date have been the mesothermal-style gold-only lodes, which occur in the Peak Hill Schist, Bryah and Padbury groups. If the area occupied by these rocks alone (approximately 6000 km2) is taken into account, then the identified contained gold per unit area is 11.8 kg gold/km2. The past and present producers include Peak Hill, Jubilee and Mount Pleasant in the Peak Hill Schist; Harmony, Mikhaburra, Wembley, Cashman and Ruby Well in the Bryah Group; and Horseshoe, Labouchere–Nathans–Deep South and Fortnum in the Padbury Group. The Wilthorpe deposit is hosted by Late Archaean granitic rocks, which are tectonically interleaved with rocks of the Bryah and Padbury groups. The Labouchere–Nathans–Deep South and Fortnum area contains the region’s largest endowment of 28.5 t of contained gold, with an ore grade of about 2.4 g/t. Approximately half has been exploited, largely between 1989 and 1995, with the majority of the remaining resource in the Fortnum area. Details of the Labouchere–Deep South and Fortnum deposits can be found in Hanna and Ivey (1990) and Hill and Cranney (1990) respectively. The area around Peak Hill, including Ravelstone, has produced nearly 22 t of gold from ore at an average grade of 4 g/t, over half of which has been extracted in the last ten years. Remaining resources contain about 3 t of gold. Apart from a small production at the turn of the century, exploitation at the Harmony deposit (New Baxter’s Find) has only recently commenced. The Harmony deposit has a total endowment of about 9.2 t, at an ore grade of 3.5 g/t gold. Details of the Peak Hill and Harmony gold deposits are presented elsewhere in this publication by Hills et al.
Geology of Australian and Papua New Guinean Mineral Deposits
The lode deposits of the Peak Hill area are hosted by mylonitic schist, metasedimentary and/or metavolcanic rocks, or along their contact zones. They are spatially associated with high strain zones and hydrothermal alteration dominated by pyrite, quartz-muscovite, biotite and alkali feldspars. The mineralisation occurs in both ductile and brittle-ductile shears as at Peak Hill, and in discrete brittle fractures, as at Cashman, indicating a relationship of structural style with the rheology of the host rocks. The development of ductile, brittle-ductile and brittle structures (zones of high permeability) was accompanied by infiltration of hydrothermal fluids which produced alteration and mineralisation. The precise timing of the mineralisation is difficult to ascertain. Windh (1992) suggested syn-D2, but from field and petrological observations it is more likely that circulation of mineralising fluids took place during a continuum related to D1-D2 tectonism and metamorphism, under conditions of ductile or brittle-ductile regimes. Perhaps some remobilisation into brittle structures occurred during D3. Lead isotope palaeo-isochrons suggest that mineralisation in the Bryah and Padbury basins took place between 1920 and 1700 Myr (Windh, 1992; Thornett, 1995). The results of lead isotope studies also indicate that the lead was derived from Yilgarn Craton rocks (Dyer, 1991; Windh, 1992; Thornett, 1995), although there is a suggestion that the lead from the Peak Hill deposit, on the southwestern tip of the Marymia Inlier, is similar to that from galena in the Marymia deposit (McMillan, 1993). The nature of the mineralising fluids is poorly constrained. Alteration assemblages at Peak Hill and Mount Pleasant indicate that the fluids were enriched in iron, potassium, sodium, sulphur, boron, carbon dioxide, silica and water
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F PIRAJNO and W A PRESTON
TABLE 2 Total estimated endowment of copper, lead, silver, manganese, iron and talc in the Padbury, Bryah and Yerrida groups. Deposit Name
Production (P) to 31.12.1995 Ore or conc (t)
COPPER AND CUPREOUS ORE Thaduna Wonyulgunna Holdens Find Peak Hill sundry Horseshoe Lights Total (Total)
Contained metal or mineral (t)
33 811 8 7 63 223 807
2823.1 3.5 1.1 22.3 46 398.4
257 696
49 248.4
LEAD Magellan within which SILVER Horseshoe Lights Peak Hill general (including Horseshoe pre-1970)
(kg)
Total MANGANESE Horseshoe Mount Fraser Mount Padbury Ravelstone (Peak Hill) Total (Total)
(kg) 51 975.11 20 201.69
Remaining Resources (R) Ore or conc (t)
Total endowment (P + R)
Contained metal or mineral (t)
Contained metal or mineral (t)
500 000
17 000
3 050 000 + (3 570 000)
30 000 + (26 000)
19 823 4 1 22 76 398 + (26 000)
3 550 000 + (3 570 000)
47 000 + (26 000)
96 248 + (26 000)
(210 000 000)* 5 500 000 (7 090 000)
(3 780 000)* 394 000 284 000
(3 780 000)* 394 000 284 000
(kg)
(kg)
(kg) 51 975.11 20 201.69
72 176.8 489 895
203 899
228 7319 76 237
108 3498 36 938
573 679
244 443
IRON ORE Robinson Range TALC Mount SeabrookLivingstone
441 326
72 176.8 80 000 (205 000) 32 000 5000
21 000 (100 000) 9000 2000
224 899 (100 000) 9108 5498 36 938
117 000 (205 000)
32 000 (100 000)
276 443 (100 000)
(10 000 000)*
(6 000 000)*
(6 000 000)*
1 750 000
2 191 326
Notes: Data from MINEDEX database, Western Australia Department of Minerals and Energy Deposit name in bold type is discussed in text Brackets enclose Inferred Resources additional to Measured and Indicated Resources Asterisk indicates global resource estimate. Global resource estimate for the Magellan lead deposit is from McQuitty and Pascoe (this volume).
(Thornett, 1995). Fluid inclusion studies of mineralised materials from the Fortnum and Labouchere gold deposits (Dyer, 1991; Windh, 1992) indicate that the ore fluids were water- and carbon dioxide–rich with salinities of 7–12 wt% and 5–17 wt% NaCl equivalent respectively. Microthermometric measurements (Dyer, 1991; Windh, 1992) gave temperatures ranging from approximately 170 to 320oC.
SHEAR ZONE–HOSTED COPPER DEPOSITS Shear zone–hosted copper mineralisation is found in a group of five deposits, of which Thaduna is the largest and, in relative terms, economically the most important (Blockley, 1968; Marston, 1979; Gee, 1987). The deposits are situated along a steeply dipping NW-trending shear zone, which cuts through turbidite facies rocks of the Thaduna Formation of the Yerrida Group. The Thaduna mineralisation is within a quartz-filled shear zone, and is associated with hydrothermal graphite, chlorite and calcite. Primary mineralisation consists mainly of
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chalcopyrite and pyrite, with chalcocite and covellite occurring as supergene sulphides. The ore contains anomalous gold values to 260 ppb. The Thaduna mine was a producer of oxide ore (malachite, cuprite, chrysocolla), which was mined to a depth of about 50 m. Between 1942 and 1971 the mine produced about 340 000 t of oxide ore, averaging approximately 8.3% copper (28 000 t of contained copper), mainly for use as a fertiliser. Defined resources remaining at Thaduna are estimated at 17 000 t of contained copper at an average grade of 3.4% (Table 2).
EPIGENETIC LEAD MINERALISATION The unusual Magellan deposit, containing lead carbonate (cerussite), sulphate (anglesite), oxide (plattnerite, PbO2) and phosphate (pyromorphite), is in the SE of the Yerrida Basin. Its discovery was announced by Renison Goldfields Consolidated in 1993. The total resource is estimated to be approximately 210 Mt at 1.8% lead (McQuitty and Pascoe, this publication), with defined ore zones at a 2% cutoff containing an Indicated Resource of 5.5 Mt at 7.1% lead and an Inferred Resource of
Geology of Australian and Papua New Guinean Mineral Deposits
MINERAL DEPOSITS OF THE PADBURY, BRYAH AND YERRIDA BASINS
7.09 Mt at 4% lead (Renison Goldfields Consolidated, 1995). The mineralisation is hosted by silicified immature wacke and stromatolitic carbonate units of the Yelma Formation of the Earaheedy Group. In the Yerrida Basin the Yelma Formation is present as scattered outliers, unconformably overlying black shales of the Maraloou Formation of the Yerrida Group (Fig 1). A Pb-Pb model age of the carbonate ore material gave an age of 1650 Myr (Pirajno et al, 1995). The immature wacke host is characterised by selective sericitic and kaolinitic alteration. The lead minerals are paragenetically late and occur as replacements in the matrix of the host rock. Trace element analyses of ore materials indicate anomalous abundances of barium in the range 1000–1828 ppm, manganese 1900–3672 ppm and copper 257–400 ppm. No sulphides or other metals are present. The origin of the Magellan deposit is not known. It is possible, however, that this mineralisation is associated with the migration of low temperature late basinal fluids along permeable rocks such as the wacke. The lack of sulphides and the unique presence of oxide minerals would suggest that the deposit is the result of palaeoweathering processes, under physico-chemical conditions which were conducive to the oxidation and subsequent mobilisation of lead.
VOLCANOGENIC COPPER-GOLD DEPOSIT Horseshoe Lights has produced nearly 9 t of gold, of which almost 2.4 t were recovered from the copper concentrate produced between 1988 and 1993. The deposit is hosted by felsic schist of the Narracoota Formation of the Bryah Group. The mineralisation consists of massive sulphides overlying and/or flanked by disseminated and stringer sulphides, and the ore minerals are mainly chalcocite, pyrite, and chalcopyrite. Native copper is also present (Parker and Brown 1990). The host rocks are mylonitised chlorite schist, kaolinite-sericite schist and quartzsericite schist. The geometry of the ore zones, the alteration patterns, the predominantly felsic composition of the host rocks, and the metal association (copper, gold, silver, lead, zinc) suggest that the deposit was originally of the VMS type, but was subsequently enriched by supergene processes. Average ore grade was approximately 8 g/t gold, 10% copper and 300 g/t silver. The stringer mineralisation has a gold grade averaging between 0.2 and 0.3 g/t. Production ceased in 1994, and the remaining resources (Table 2) are of low grade mineralisation which is currently being investigated for exploitation by an acid leach and electrowinning process.
SUPERGENE MANGANESE DEPOSITS The region hosts a historically important manganese field, first recognised in 1905, with deposits in the Mount Fraser, Mount Padbury, Ravelstone (Peak Hill) and Horseshoe areas. The mineralisation is of supergene origin and is related to manganiferous and hematitic shale and BIF of the Horseshoe Formation of the Bryah Group, and to units of the Padbury Group. The chief ore mineral is pyrolusite. The ore is lateritic, locally pisolitic, and in places forms caps overlying the primary manganese-rich sedimentary material. In some localities, notably at Horseshoe, there is evidence that some enrichment may have taken place in a palaeodrainage channel, lake or swamp environment (McLeod, 1970). In the Ravelstone area,
Geology of Australian and Papua New Guinean Mineral Deposits
just north of the Peak Hill gold deposit, the supergene manganese enrichment appears to have a structural control. The Horseshoe area has been the main producer from two deposits, 2 and 3 km to the north and NW of Horseshoe townsite. The main production period was from 1948 to 1971, when 490 000 t were mined, all but 5000 t of which being classified as metallurgical grade ore. The enriched ore was 3 to 4.5 m thick and typically extended over lengths of 400 to 500 m. The North deposit averaged 30 m in width, whilst the South deposit was fan-shaped, opening from 20 m to 300 m wide at its maximum extent. Ore consisted of mixed manganese and iron oxides, with highly variable manganese and iron contents. Grades progressively decreased from 42% in 1948 to 35–38% after 1966. There are several small deposits in the Mount Fraser–Mount Padbury area, about 30 km west of Peak Hill. They contain patches of high grade ore within large deposits of ferruginous and manganiferous material. Production of high grade ore has been sporadic since 1949, and amounts to over 80 000 t at grades in excess of 46% manganese. A mining operation was commenced at Millidie (Elsa deposit) in the early 1990s, but this has not progressed to a full scale commercial operation. Defined resources of high grade ore in the area are estimated at approximately 200 000 t. In the Ravelstone area, immediately NNW of Peak Hill, mining took place between 1956 and 1964, producing nearly 2300 t of battery grade ore at 70–90% manganese oxide. Manganese production at Ravelstone was from three leases, where manganese orebodies trended approximately east over lengths to 100 m and widths to 30 m, but the deposits were generally small and narrow. The manganese ores of the region are characterised by high barium content, from 3000 ppm to 3.0% at Mount Fraser, 3000 ppm to 1% at Horseshoe and 4000 to 9000 ppm at Ravelstone.
IRON ORE DEPOSITS The Robinson Range Formation of the Padbury Group contains units of BIF, in which there are areas of supergene enrichment of hematite and goethite. These constitute a potential iron ore resource, estimated at approximately 10 Mt, with grades greater than 60% iron (Table 2). Enrichment occurs above two BIF units, approximately 100 m thick, separated by a hematitic shale. Iron grades of the primary BIF vary between 20 and 50%. Zones of hematite and hematite-goethite surficial enrichment contain grades in excess of 50%, as determined from the sampling of one of some 200 small pods of potentially ore grade material (Sofoulis, 1970).
TALC DEPOSITS Talc occurs in the Mount Seabrook–Livingstone–Trillbar region metasedimentary rocks, possibly of the Padbury Group. Talc is hosted by metasomatised dolomite to the west of Fig 1. The talc occurs as a series of steeply plunging lenses (Lipple, 1990). The Mount Seabrook deposit was discovered in 1965 and produced over 440 000 t of talc, mostly of cosmetic grade, between 1973 and 1995. Resources are defined at Mount Seabrook and Livingstone and amount to 1.75 Mt of ore (Table 2), with a significantly greater potential as the orebodies are open along strike.
67
F PIRAJNO and W A PRESTON
DISCUSSION AND CONCLUSIONS The Horseshoe Lights VMS-type copper-gold deposit is synvolcanic and pre-orogenic. All other deposits are of epigenetic origin, and syn- to post-orogenic. Figure 2 schematically depicts a simple regional model of ore genesis for the epigenetic mineral deposits in the Bryah–Padbury–Peak Hill Schist (BPPS) tectonometamorphic domain and the Yerrida Basin. The BPPS was subjected to dynamothermal metamorphism to mid greenschist facies. At least two phases of metamorphism are recognised with the prograde phase overprinted by a retrograde phase. Geothermometry and geobarometry studies in the area around Peak Hill by Thornett (1995) indicate temperatures of around 500–620oC for peak prograde metamorphism and 6.5 to 7 kb for minimum pressure of the prograde assemblages.
Hydrothermal solutions responsible for the emplacement of mesothermal lodes are generated in tectonically active regions and are associated with compressional and extensional tectonics (Kerrich and Cassidy, 1994). The mesothermal-style gold-only lodes of the Bryah and the Padbury basins were formed in a compressional setting characterised by thin skinned thrusting associated with prograde and retrograde mineral assemblages (Pirajno, 1996). Dyer (1991) concluded that the hydrothermal mineralisation in the Labouchere–Fortnum area was generated by the mixing of two fluids of different density and salinity. Deeply sourced, hot and saline carbon dioxide–bearing fluids mixed with cooler and less saline, nearsurface aqueous fluids. The available evidence points to the conclusion that the mineralising fluids were at first generated during compression and dehydration, and moved along ductile to brittle structures. During phases of extension, meteoric fluids would have infiltrated along the same structures and mixed with the hotter metamorphic solutions. The whole mechanism could have been repeated in the next phase of compression and extension, leading to multiphase ore genesis processes, in which the latest phase leaves the most detectable imprint. There is no obvious link with magmatic activity. In the relatively undeformed Yerrida terrane, expulsion of late basinal (connate?) fluids possibly occurred from the zone of compression associated with the Goodin Fault towards the SE, and along permeable horizons. These fluids may have been linked to the emplacement of the Magellan lead mineralisation.
FIG 2 - Schematic diagram depicting a model of ore genesis for epigenetic mineral deposits in the Peak Hill Schist, Bryah, Padbury and Yerrida basins.
The Yerrida Basin and BPPS were tectonically juxtaposed along the NE-trending Goodin Fault. Deformation which affected the BPPS was transmitted across the Goodin Fault for a few kilometres into the Yerrida terrane. This deformation becomes weaker eastward from the Goodin Fault. A possible genitic model would be the generation of fluids during phases of dynamothermal metamorphism in the BPPS with the metamorphic fluids largely responsible for the emplacement of the mesothermal gold-only and shear zone–hosted deposits. The paragenesis of the alteration assemblages associated with the mesothermal deposits is dominated by biotite, pyrite, quartz, white mica, chlorite, albite, tourmaline and carbonate. Textural relationships suggest that metamorphism and mineralisation were broadly contemporaneous, although alteration was, in most cases, during retrograde to peak metamorphism. Exceptions to this are localised zones of sodium metasomatism (with albite and arfvedsonite), which overprint the retrograde assemblages. The sodium metasomatism may be related to central zones of higher temperatures within the mineralised structures. There is no evidence in either the Bryah or the Padbury basins of granitic plutons intruding the volcano-sedimentary successions, and therefore a possible role of granitic magmatism as one of the heat and metal sources for the hydrothermal solutions is, based on our present knowledge, excluded.
68
The BPPS tectonometamorphic domain and the Yerrida Basin have areas of potential mineralisation, other than those described in this paper. The base metal mineral potential of these terranes has been discussed by Pirajno and Occhipinti (1995). Based on the recent geological mapping, geochemical and petrological data, they envisage that potential exists for black shale–hosted platinum group elements (PGE), and a gossan containing anomalous abundances of PGE was identified (Fig 1), for carbonate-hosted lead-zinc, and possibly nickel-copper magmatic sulphides. Mineral exploration is actively progressing and it is likely that discoveries will be made in the area formerly known as Glengarry Basin.
ACKNOWLEDGEMENTS This paper is published with the permission of the Director of the Geological Survey of Western Australia, Dr P Guj, whose useful comments on this paper are gratefully acknowledged. The authors are grateful to P Carroll and his staff for preparing the illustrations.
REFERENCES Blockley, J G, 1968. Diamond drilling at the Thaduna copper mine, Peak Hill goldfield, Western Australia Geological Survey Annual Report, 1967:53–57. Dyer, F L, 1991. The nature and origin of gold mineralisation at the Fortnum, Nathans and Labouchere deposits, Glengarry Basin, Western Australia, BSc Honours thesis (unpublished), The University of Western Australia, Perth. Gee, R D, 1987. Peak Hill, Western Australia - 1:250 000 geological series, 2nd edn, Geological Survey of Western Australia, Explanatory Notes SG 50–8. Gee, R D, 1990. Naberru Basin, Geological Survey of Western Australia Memoir 3:202–210.
Geology of Australian and Papua New Guinean Mineral Deposits
MINERAL DEPOSITS OF THE PADBURY, BRYAH AND YERRIDA BASINS
Gee, R D and Grey, K, 1993. Proterozoic rocks on the Glengarry 1:250 000 sheet: stratigraphy, structure and stromatolite biostratigraphy, Western Australia Geological Survey Report 41. Hanna, J P and Ivey, M E, 1990. Labouchere and Deep South gold deposits, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 667–670 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Occhipinti, S, Grey, G, Pirajno, F, Adamides, N, Bagas, L, Dawes, P and Le Blanc Smith, G, 1997. Stratigraphic revision of the Yerrida, Bryah and Badbury basins (formerly Glengarry basin), Geological Survey of Western Australia Record 1997/3. Parker, T W H and Brown, T, 1990. Horseshoe gold-copper-silver deposit, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 671–675 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Hill, A D and Cranney, P J, 1990. Fortnum gold deposit, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 665–667 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Pirajno, F, 1996. Models for the geodynamic evolution of the Palaeoproterozoic Glengarry Basin, Western Australia, Western Australia Geological Survey Annual Review 1995–96, pp 96–103.
Kerrich, R and Cassidy, K F, 1994. Temporal relationships of lode gold mineralization to accretion, magmatism, metamorphism and deformation - Archaean to present: a review, Ore Geology Reviews, 9:263–310.
Pirajno F, Adamides, N G, Occhipinti, S A, Swager, C P and Bagas, L, 1995. Geology and tectonic evolution of the early Proterozoic Glengarry Basin, Western Australia, Western Australia Geological Survey Annual Review 1994–95, pp 71–80.
Lipple, S L, 1990. Talc, in Geology and Mineral Resources of Western Australia, Western Australia Geological Survey Memoir 3:678–679.
Pirajno, F, Bagas, L, Swager, C P, Occhipinti, S A and Adamides, N G, 1996. A reappraisal of the stratigraphy of the Glengarry Basin, Western Australia Geological Survey Annual Review 1995–96, pp 81–87.
Marston, R J, 1979. Copper mineralization in Western Australia, Geological Survey of Western Australia, Bulletin 13. Martin, D McB, 1994. Stratigraphy and sedimentology of the Early Proterozoic Labouchere Formation, Padbury Group - constraints on the tectonic setting of the northern Yilgarn Craton, PhD thesis (unpublished), The University of Western Australia, Perth.
Pirajno, F and Occhipinti, S, 1995. Base metal potential of the Palaeoproterozoic Glengarry and Bryah basins, Western Australia, AIG Bulletin, 16:51–56. Renison Goldfields Consolidated, 1995. Annual Report (Renison Goldfields Consolidated Ltd: Sydney).
McLeod, W N, 1970. Peak Hill, Western Australia - 1:250 000 geological series, Geological Survey of Western Australia Explanatory Notes SG 50–8.
Soufoulis, J, 1970. Iron deposits of the Robinson Range, Peak Hill Goldfield, WA Geological Survey of Western Australia Record 1970/6.
McMillan, N H, 1993. Structure, metamorphism, alteration and timing of gold mineralisation at Marymia Gold Project in the Marymia Dome, in An International Conference on Crustal Evolution, Metallogeny and Exploration of the Eastern Goldfields, Extended Abstracts (Comps: P R Williams and J A Haldane), pp 243–244 (Geological Society of Australia: Perth).
Thornett, S, 1995. The nature and timing of gold mineralisation in the Proterozoic rocks of the Peak Hill District, MSc thesis (unpublished), The University of Western Australia, Perth.
Myers, J S, 1990. Gascoyne Complex, in Geology and Mineral Resources of Western Australia, Western Australia Geological Survey Memoir 3:198–202 Myers, J S, 1993. Precambrian history of the west Australian craton and adjacent orogens, Annual Review Earth and Planetary Science, 21:453–485.
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Tyler, I M, Pirajno, F, Bagas, L, Myers, J S and Preston, WA, in press. The geology and mineral deposits of the Proterozoic in Western Australia, AGSO Journal of Australian Geology and Geophysics, 17(3). Tyler, I M and Thorne, A, 1990. The northern margin of the Capricorn Orogen, Western Australia - an example of an early Proterozoic collision zone, Journal of Structural Geology, 12:685–701. Windh, J, 1992. Tectonic evolution and metallogenesis of the early Proterozoic Glengarry basin, Western Australia, PhD thesis (unpublished), The University of Western Australia, Perth.
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Vickery, N M, Buckley, P M and Kellett, R J, 1998. Plutonic gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 71–80 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Plutonic gold deposit 1
2
2
by N M Vickery , P M Buckley and R J Kellett INTRODUCTION The Plutonic deposit is 180 km NE of Meekatharra, WA, at lat 26ο15′S, long 119o36′E on the Peak Hill (SG 50–8) 1:250 000 scale map sheet (Fig 1). It was discovered in 1988 by Great Central Mines NL (GCM) and was the first gold discovery within the Plutonic Well greenstone belt. The property is now owned and operated by Plutonic Resources Ltd (PRL). Open cut and underground mining operations are currently in progress at Plutonic, and mining is planned at four deposits within 10 km of the initial discovery at Perch, Area 4, Zone 550 and Trout (Figs 1 and 2). Production commenced in 1990 and one million ounces of gold were produced to January 1996. Production during 1996
was 183 700 oz of gold from 2.02 Mt at a grade of 3.6g/t Au. Total mineral resources at 31 December 1996 were 35 Mt containing 7.3 Moz of gold (Table 1).
EXPLORATION HISTORY The Archaean age of the Plutonic Well greenstone belt was first recognised by International Nickel Australia (Inco), who explored the area for nickel from 1969 to 1977. The first licence for gold exploration was granted in July 1986 to Redross Consultants Pty Ltd and Oscar Aamodt, with GCM through a two year option first acquiring a 95% interest, then the remaining 5% of this licence (Bucknell, 1995).
1.
Project Geologist, Plutonic Operations Limited, PO Box 670, West Perth WA 6872.
In 1987, an arsenic and gold anomaly was detected by geochemical sampling involving 52 soil, 79 rock chip and 9 stream sediment samples. Grid based mapping, soil and lag geochemical surveys followed which led to the discovery of the deposit (Bucknell, 1995).
2.
Senior Geologist, Plutonic Operations Limited, PO Box 670, West Perth WA 6872.
In 1988, an initial program of 39 reverse circulation (RC) holes was drilled for 1948 m with three holes returning
FIG 1 - Location and regional geological map indicating the location of the Archaean Marymia Inlier within the sedimentary basins of the Proterozoic Capricorn Orogen.
Geology of Australian and Papua New Guinean Mineral Deposits
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N M VICKERY, P M BUCKLEY and R J KELLETT
TABLE 1 Ore Reserves and Mineral Resources, Plutonic gold deposits, at December 1996. Deposit
Category
Ore (000 t)
Grade Contained (g/t Au) gold (oz)
Ore Reserves Plutonic main pit
Proved and probable
2099
3.9
266 400
NW extensions
Probable
970
7.9
246 400
Zone 061
Probable
180
6.1
35 300
Zone 124
Probable
185
13.9
82 700
Plutonic underground
Zone 550
Probable
336
12.4
134 000
Salmon
Proved and probable
966
2.3
71 600
Area 4
Proved and probable
676
2.0
44 500
Trout
Proved and probable
210
3.2
21 300
Perch
Proved and probable
1452
2.0
92 100
Stockpiles
2192
1.9
133 300
Total Ore Reserves
9266
3.8
1 127 600
Mineral Resources (Inclusive of Ore Reserves) Measured, indicated and inferrred
2128
3.9
269 800
NW extensions
Indicated and inferred
2350
7.8
587 800
Zone 061
Indicated and inferrred
2535
5.8
467 100
Zone 019
Inferred
Zone 124 (including zone 550)
Measured, indicated and inferred
Plutonic main pit Plutonic underground
6600
10.2
2 160 000
12 588
7.7
3 117 200
Stockpiles
—
1704
1.9
105 300
Salmon
Measured indicated and inferred
1995
2.2
143 300
Area 4
Measured and indicated
985
2.1
67 800
Trout
Measured, indicated and inferred
668
2.5
53 100
Perch
Measured and indicated
2559
1.7
143 900
Plutonic East underground
Indicated and inferred
1036
6.4
212 000
35 148
6.5
7 327 300
Total Mineral Resources
REGIONAL GEOLOGY The deposit is in the southwestern corner of the Archaean Plutonic Well greenstone belt, a northeasterly trending elongate belt approximately 50 km long within the Marymia Inlier (Fig 1). The greenstone belt comprises metamorphosed Archaean mafic, ultramafic, sedimentary and felsic rocks with prominent Proterozoic dolerite dykes. Banded iron formation units occur only in the northeastern portion of the belt. Metamorphism is predominantly upper greenschist but locally lower amphibolite facies. The Marymia Inlier is an Archaean basement remnant within the Proterozoic Capricorn Orogen and contains the Plutonic Well and Baumgarten greenstone belts which both host gold mineralisation (Gee, 1987). The Capricorn Orogen is the result of the oblique collision of the Yilgarn and Pilbara cratons between 2000 and 1600 Myr, which resulted in the opening up of the Ashburton (representing a possible foreland basin along the margin of the Pilbara Craton), Glengarry and Earaheedy basins (Myers, 1990). The tectonic significance of these basins is not clear, though Tyler and Thorne (1991) have postulated that the Glengarry Basin may represent a back-arc basin, formed in an extensional tectonic setting and Windh (1992) interpreted the Glengarry Basin as an intracratonic rift system. The westernmost outcropping portion of the Capricorn Orogen is the Gascoyne Complex and to the east are the Glengarry and Earaheedy Basins, which consist of sedimentary sequences of 2000 to 1600 Myr age, locally including deformed and metamorphosed sediments and basement domes (Gee, 1990). The Glengarry and Earaheedy Basin sediments unconformably overly the Yilgarn Craton rocks to the south and are overlain to the north and east by the Bangemall, Officer and Savory Basins. The Earaheedy Basin overlies the Glengarry Basin to the east.
PREVIOUS DESCRIPTIONS
Source: Plutonic Resources Limited, Annual Report, 1996.
significant mineralisation. By the end of March 1989 approximately 700 RC holes for 20 000 m had been drilled and an initial resource was estimated at 4.1 Mt at 3.6 g/t gold for 475 000 contained oz.
72
In May 1989, Pioneer Minerals Exploration Ltd (later PRL) entered into an agreement with GCM to purchase 51% of three mining leases covering the deposit with the right to 51% of the surrounding GCM tenements. After assuming management of the project, PRL subsequently acquired 100% ownership in December 1989 and bought out a residual royalty interest in 1991. By September 1989 further drilling had defined an aggregate reserve and resource of 4.6 Mt at 3.5 g/t for 515 600 oz of contained gold (Bucknell, 1995). Construction of the plant began in February 1990 and the first gold was poured in August of that year.
Little has been written to date on the Plutonic deposit. Bucknell et al (1996) in an abstract volume provided a brief description of the deposit. Bucknell (1995) gave a detailed account of the discovery, definition and continuing exploration activities at the deposit and environs. The regional geology of the Plutonic mine region is mentioned by Subramanya et al (1995). B Willott (unpublished data, 1992) discussed the local stratigraphy and geology in detail. M Rowley (unpublished data, 1991) described the lithological, structural, mineralogical and alteration features of the deposit. Mining methods were described by B Davis (unpublished data, 1991). Bruce, Rowley and Bradford (1991) discussed the inception of mining and early operations at the deposit.
Geology of Australian and Papua New Guinean Mineral Deposits
PLUTONIC GOLD DEPOSIT
FIG 2 - Simplified geological plan of the Plutonic gold mine and environs.
ORE DEPOSIT FEATURES LOCAL STRATIGRAPHY Gold mineralisation is hosted by metabasalt in a moderately north dipping Archaean greenstone sequence consisting predominantly of low potassium tholeiitic metabasalt, ultramafic rocks (metaperidotite and metakomatiitic basalt) and minor siliceous, graphitic and sulphidic metasediments. To the east the greenstone package also contains sedimentary (possibly metapelite) and felsic volcanic rocks. Felsic porphyries and Proterozoic dolerite dykes locally crosscut this sequence. The geology of the Plutonic mine and surrounding deposits is shown in Fig 2, informal mine stratigraphic nomenclature is shown in Fig 3, and a cross section of the mine is shown in Fig 4. The contact of the greenstone rocks with the surrounding Archaean granite is complex. The west and NW contact is a thrust, in which the granite overlies the greenstone with the contact dipping at approximately 45°. To the south and SE, the contact is less clearly understood and is believed to be in part a fault contact with intrusion and assimilation of the greenstone by the granite. The Basal sediment is the lowest recognised unit of the package and is a fine grained pelitic metasedimentary sequence of unknown thickness, consisting of siltstone and minor graphitic shale. The rocks are finely laminated, biotite rich, pyrrhotitic and commonly garnetiferous. Minor intercalations of amphibolite are common and felsic porphyry and Proterozoic dolerite dykes intrude the sequence. The Basal mafic unit is the lowest mafic member of the sequence, overlies the Basal sediment and is characterised by an intense, pervasive layer-parallel foliation with concordant
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 3 - Schematic stratigraphic column of the Plutonic gold mine and environs.
73
N M VICKERY, P M BUCKLEY and R J KELLETT
FIG 4 - Cross section A-B through Plutonic main pit, looking NNW, location on Fig 2.
carbonate and quartz-carbonate veining. Locally intense silica, epidote and biotite alteration occurs in bands parallel to the foliation, and the mineral assemblage includes hornblende, tremolite, actinolite, plagioclase and garnet. Sulphides include pyrrhotite, pyrite, chalcopyrite, galena and sphalerite, typically occurring as fine disseminations and in sulphide-rich veins and breccia zones. The Hanging Wall ultramafic and Footwall ultramafic units are stratigraphically above and below the Mine mafic unit and are thick sequences of metakomatiitic basalt, metaperidotite and metapyroxenite flows. The mineral assemblages include chlorite, talc, carbonate and tremolite with accessory actinolite and magnetite. Relict olivine has been observed in thin section and is partially to completely serpentinised. Serpentinisation is common in the metakomatiite flows and is restricted to zones less than 5 m thick. Carbonate veining is abundant in the talcrich metakomatiites and the extensive spinifex texture is commonly folded and contorted. The ultramafic units show evidence of intense, predominantly ductile deformation. Folding, crenulation, boudinaging of carbonate veining and locally developed ultramafic melanges are common. Mineralisation is exclusively hosted by the Mine mafic unit, a sequence of upper greenschist to lower amphibolite grade basaltic flows of variable thickness sandwiched between the Hanging Wall and Footwall ultramafic units. The mineral assemblage includes hornblende, actinolite, plagioclase and garnet with alteration assemblages of chlorite, biotite, carbonate, epidote and albite. Sulphides include pyrrhotite, pyrite and arsenopyrite and rare sphalerite and galena. The Mine mafic unit is characterised by several laterally continuous interflow sedimentary marker horizons. These are: 1.
the Banded zone, a banded cherty horizon 0.5 to 1 m thick which may represent an altered interflow tuffaceous felsic unit; and
2.
the Double shale, Middle shale and Lower shale which are fine grained, strongly siliceous, graphitic and sulphidic shales containing minor garnet porphyroblasts. The shales are well laminated and strongly deformed, with intrafolial folds developed. Small scale faulting and
74
offsets are common and unit contacts are typically sheared. Sulphides include pyrrhotite, pyrite and sphalerite. The uppermost 5 to 30 m of the Mine mafic unit comprises a distinctive coarse grained hornblende-rich amphibolite which typically shows a high-magnesium basalt composition. Underlying this zone are basalts of low potassium tholeiitic composition which are massive to moderately foliated and comprise distinct (though unmappable) volcanic flows. The mafic rock below the Double shale marker horizon is termed the Lower Mine mafic unit. It is characterised by large porphyroblastic garnets associated with carbonate and pyrrhotite alteration, and by coarse grained recrystallisation of amphiboles (typically hornblende). The Overthrust mafic unit overlies the Hanging Wall ultramafic unit and is of variable thickness, comprising upper greenschist to lower amphibolite facies metabasalt interbedded with thin zones of siliceous and graphitic sediment and rare ultramafic rocks. Proximal to the Plutonic mine, the Overthrust mafic unit is thrust over the Hanging Wall ultramafic unit oblique to the granite thrust contact which dips at 45o and trends NNE on the northwestern margin of the greenstone belt. This thrust is called the Quartz Hill thrust. Medium grained and massive to moderately foliated, the Overthrust mafic unit comprises hornblende and interstitial plagioclase and quartz with alteration characterised by carbonate, biotite, chlorite, epidote and silica. Moderate to strong chlorite and carbonate alteration is pervasive throughout the unit which also contains zones of locally intense biotite and rare siliceous alteration. Carbonate veins are commonly folded and contorted, generally parallel to the foliation. The dominant sulphides are pyrrhotite, chalcopyrite and pyrite. Archaean granite overlies the Overthrust mafic unit to the NW of the mine and consists predominantly of coarse grained, weakly foliated to massive biotite monzogranite (Gee, 1987). The contact with the underlying greenstone package is strongly sheared and a narrow mylonitic zone is developed. The nature of the granite-greenstone contact to the south and east is not clearly understood.
Geology of Australian and Papua New Guinean Mineral Deposits
PLUTONIC GOLD DEPOSIT
MINERALISATION Plutonic Gold mineralisation within the Plutonic deposit occurs in several forms, the most common being a series of discrete, subparallel, NW-trending multiple lodes that generally dip at approximately 40 to 50o to the NE (Fig 2). Lode thickness varies between 1 and 10 m and single lodes of strike continuity of several hundred metres are not uncommon. This geometry of mineralisation has proved to remain consistent in depth for 1.5 km to the north and 1.5 km to the NW of the Main pit area. An auriferous laterite deposit is also developed on the southern flank of the deposit. The laterite deposit covered approximately 25 ha and consisted of a higher grade zone surrounded by a lower grade halo. Phyllic-propylitic style hydrothermal alteration and possible later metamorphism resulted in the development of an albitechlorite-epidote-carbonate assemblage with accompanying biotite (typically phlogopite), sulphides, trace tourmaline and scheelite. Lodes typically have strongly albitised central zones containing between 5 and 10%, locally to 20%, sulphides dominated by arsenopyrite and pyrrhotite with lesser amounts of pyrite, chalcopyrite and sphalerite. Lodes generally grade outwards to a phlogopite selvage containing subordinate calcite, chlorite and amphibole. Gold is present dominantly as native gold disseminated throughout the groundmass or within the silicate gangue, or to a lesser extent within the arsenopyrite crystal lattice or rarely within pyrite. Little, if any gold is intimately associated with pyrrhotite. Other styles of mineralisation at Plutonic, whose relationship to the dominant lodes is undetermined, include chloriteamphibole (typically tremolite with lesser actinolite and hornblende)-carbonate±phlogopite alteration with a sulphide assemblage dominated by pyrrhotite, chalcopyrite and pyrite. Quartz-carbonate±chlorite veins are ubiquitous throughout the mine area but are rarely associated with gold mineralisation.
Area 4 Area 4 is interpreted as the southern portion of the Plutonic deposit which has been displaced approximately 3 km to the east. Mineralisation is similar to that at the Plutonic mine, consisting of a series of moderately dipping stacked lodes which dip and strike obliquely to the stratigraphic succession. Ore zones occur exclusively within the Mine mafic unit and are intensely albite-biotite-carbonate altered with associated pyrrhotite, arsenopyrite and pyrite. Gold may be visible (usually fine grained) or intimately associated with arsenopyrite. Immediately south of Area 4, the Footwall ultramafic unit outcrops and may be considered to form a domal structure. The Mine mafic unit overlies this unit and thickens dramatically to the north, where mineralisation is associated. A change in the strike of this unit, from east to SE and of dip from north to NE may in part cause the thickening of the unit. Subvertical contacts and drag features are associated with the MMR fault in the northern portion of Area 4.
Perch The Perch deposit is immediately SE of Area 4 and is hosted within the Mine mafic unit. The pit geology is dominated by the Mine mafic unit which forms a dome in the west and a series of small folds in the central portion of the deposit, with fold axes
Geology of Australian and Papua New Guinean Mineral Deposits
oriented east. The south wall of the pit comprises the Footwall ultramafic unit and granite. Mineralisation occurs predominantly on the flanks of these structures and may be structurally controlled by small subparallel faults oriented approximately perpendicular to the fold axes. Lodes appear to strike east throughout the pit. The synformal fold axes are typically barren and lower grades occur also at the top of the dome. Mineralisation in the oxide zone is associated with goethitic alteration and occurs in kaolinitic, limonitic and hematitic mafic rocks. Primary lodes are similar to those at the Plutonic deposit and are associated with biotite-carbonatealbite alteration with associated pyrite, pyrrhotite and lesser arsenopyrite.
Salmon The geology of the Salmon deposit (Fig 1) comprises a sequence of mafic and ultramafic flows with interbedded graphitic shales, siltstones and mafic tuffs. This sequence is folded into a series of antiforms and synforms, crosscut by felsic dykes and truncated to the south by the MMR fault. Mineralisation is associated with mafic units in an albitecarbonate-chlorite-biotite-stilpnomelane assemblage with ilmenite/rutile rich zones and is also partly associated with quartz veining. Lodes dip concordantly with the stratigraphic succession at approximately 60o.
Zone 124 Mineralisation within Zone 124 is similar to that at the Plutonic deposit. The typical Plutonic-style lodes with intense albitecarbonate-phlogopite alteration and associated arsenopyritepyrrhotite and pyrite are the most common. Native gold also occurs as fine grains disseminated within quartz-carbonatechlorite-epidote altered mafic rocks and within quartzcarbonate veins in mafic rocks. As with Plutonic-style mineralisation, gold is also present in solid solution within the arsenopyrite and occurs in multiply stacked lodes within the Mine mafic unit, which dip gently to the NNW. In the northern portions of Zone 124, the lodes are more complicated with most of the abovementioned styles occurring together. Late stage gold remobilisation within quartz veins in contact with Proterozoic dolerite dykes occurs, and visible gold is associated with pyrrhotite. Lodes are also structurally more complex, possibly related to thrusting events and associated rotation in the northern portions of Zone 124 is observed.
Zone 550 The geology at Zone 550 is dominated by a series of low angle thrusts, striking approximately NE and dipping NW, resulting in the disruption of the greenstone package which is observed to the south in Zone 124 and at the Plutonic deposit. These thrusts are believed responsible for the repetition and attenuation of the mine sequence, including up to three or four fragments of mineralised Mine mafic unit that are thrusted up into the Hanging Wall ultramafic and Overthrust mafic units. The thrusting is interpreted to form a complex antiformal stacking which is bound by roof and floor thrusts. The Quartz Hill thrust, easily traceable further south and causing the thrusting of the Overthrust mafic onto the Hanging Wall ultramafic unit, does not appear to follow this contact
75
N M VICKERY, P M BUCKLEY and R J KELLETT
further north. In Zone 550 the dramatic thickening of the Hanging Wall ultramafic unit may be due in part to the Quartz Hill thrust, truncating the Mine mafic unit and juxtaposing the Hanging Wall and Footwall ultramafic units. Marker horizons within the Mine mafic unit also become disrupted and are discontinuous in Zone 550. Evidence for rotation of the mafic thrust slices also occurs. Lode styles at Zone 550 vary as at the Plutonic Deposit and Zone 124. Banding of the mafic units with accompanying intense albite-carbonate-phlogopite alteration, accessory scheelite and sulphides including arsenopyrite, pyrrhotite and pyrite is the most abundant mineralisation style. Epidotechlorite-carbonate altered mafic rocks also host mineralisation with arsenopyrite and lesser pyrrhotite, pyrite and chalcopyrite. Remobilised, fine to medium grained visible gold within massive to locally brecciated quartz veins is relatively common and pyrite and pyrrhotite are associated with the mineralisation.
Zone 019 Geological interpretation of Zone 019 has revealed a major NE-trending fault downthrown to the NW on the NW margin of the zone (possibly related to the Eastern Haul Roads fault system). This results in the repetition of the Mine mafic unit and substantial deepening of this unit. Thickening of the Hanging Wall ultramafic unit to the NE in Zone 019 may result in a thinning of the Mine mafic unit, possibly due to the Hanging Wall ultramafic–Mine mafic contact being a thrust. Areas within Zone 019 and Zone 124 with diminished thicknesses of Mine mafic unit may be explained by this theory, where contacts are steeper and higher grade, narrow lodes occur. The absence of marker horizons such as the Banded zone and Double shale within Zone 019 has hampered the interpretation of this occurrence. A number of styles of mineralisation have been identified within Zone 019. The major type of mineralisation is the typical Plutonic-style lodes, dominated by albite-biotite-carbonate alteration and by a sulphide assemblage consisting of arsenopyrite-pyrrhotite-chalcopyrite-pyrite with little or no visible gold. Minor mineralisation is represented by a second style as recognised within the Northwestern Extensions area, immediately NW of the Plutonic Main pit. This style consists of a chlorite-carbonate±biotite alteration with a sulphide assemblage dominated by pyrrhotite±chalcopyrite± pyrite. This second style more commonly contains visible gold. Rarely, visible gold is contained within quartz-carbonatechlorite or quartz-carbonate veins not associated with alteration styles as described above. This style of mineralisation is considered to be atypical of Plutonic lode types.
Trout The greenstone sequence at Trout (Fig 1) is moderately westerly dipping and consists of a hanging wall ultramafic unit underlain by a mafic unit (termed the Trout mafic unit), undifferentiated sediments and a lower ultramafic unit. Mineralisation is hosted by the Trout mafic unit which unconformably overlies strongly sheared metasediments, thickens to the SE and is intruded by a quartz-feldspar porphyry/felsic volcanic unit. The sequence is truncated to the north by an east trending fault in which movement is possibly sinistral with displacement of the sequence to the west.
76
Mineralisation consists of discrete, weakly sulphidic lodes and quartz veining with a sulphide assemblage comprising pyrite and arsenopyrite, hosted predominantly in quartz which is commonly goethitic. Lodes are narrow, conspicuous, typically anastomosing and extend laterally for up to 300 m. They strike NW and dip to the SE at approximately 20 to 30o. Visible gold is associated with goethitic quartz veins.
STRUCTURE The Plutonic mineralisation bears the imprint of Archaean and Proterozoic structural events and the relatively shallow dip of the host sequence sets it apart from most greenstone belts within the Yilgarn Craton. Early outcrop mapping indicated a strong Proterozoic structural influence with the recognition of post-mineralisation faulting and thrusting. During an Archaean (D1) event an initial phase of folding (F1) produced large scale open folds. A subsequent incremental Archaean event, deemed D2, initially refolded the F1 axial planes and resulted in a dome and basin fold system with F2 fold axes plunging gently NW. Further deformation during D2 initiated thrusting and the Overthrust mafic unit was emplaced over the Hanging Wall ultramafic unit. During this time the Mine mafic unit behaved as a relatively brittle layer between the more ductile Hanging Wall and Footwall ultramafic units, with stress preferentially partitioned into the antiformal dome areas which acted as frontal ramps. The thicker and more homogeneous Overthrust mafic unit remained relatively undeformed while the Mine mafic unit beneath was subjected to further fold tightening, faulting and mineralising fluid ingress. The D2 thrusting event is thought to be responsible for strong layer-parallel shear zones developed at formation contacts that had large ductility contrasts. These include the Hanging Wall ultramafic–Upper Mine mafic contact, the Banded zone, Double and Middle shale contacts and the Lower shale–Footwall ultramafic contact. Within this thrust-driven compresional regime, localised linking shears developed within the more brittle Mine mafic unit, allowing mineralising fluids travelling through the layer parallel shear zones to deposit gold within the linking shears. Structural mapping of the lodes and the enclosing Mine mafic unit reveals that the lodes have the same orientation as the axial planes of the F2 folds, ie plunging gently NW, striking NW and dipping NE. Examination of oriented thin sections reveals quartz pressure shadows around arsenopyrite aggregates within fine grained, strongly schistose ore zones indicating a dextral sense of movement. Pressure shadows are also crenulated by a later deformation, indicating that arsenopyrite crystallisation and related gold mineralisation predate at least two further stages of deformation. Samples have also been observed that contain a strong foliation defined by tabular arsenopyrite crystals indicating syndeformational mineralisation. The D3 event is responsible for the strong overprint observed within the Archaean structures. The dominant effect of D3 is the thrust emplacement of granite over the greenstone sequence west and NW of the mine (Fig 3). The overthrust granite was first recognised within the NW Extensions and Zone 019 areas in diamond hole PEDD0019 which intersected 320 m of granite, a further 170 m of Overthrust mafic unit before reaching high grade lodes within the Mine mafic unit, at a depth greater than 800 m. The granite contact with the Overthrust mafic unit dips WNW at between 40 and 45o.
Geology of Australian and Papua New Guinean Mineral Deposits
PLUTONIC GOLD DEPOSIT
At Zone 550, 2.5 km north of the Main pit, a D3 thrust surface has intersected stacked lodes of the Zone 124 deposit. Adjacent to and parallel with the overthrust granite, a thrust duplex has developed with the strongly mineralised Mine mafic unit emplaced over the northern extensions of the main body of the Mine mafic unit. The thrust has the effect of remobilising gold into quartz-rich veins and alteration zones and brings mineralisation, which occurs between 300 and 400 m depth at Zone 124, to within 70 m of the surface. The D3 event is thought to be related to the Capricorn Orogeny at 1720 Myr and is thus of Proterozoic age. D4 faulting offsets the overthrusted granite south of the Laterite pit. The east-striking D4 fault, known as the MMR fault, is a steeply north-dipping sinistral strike-slip fault with a small reverse, north block up component. The southern extensions of the Plutonic system have been identified some 2.5 km east of the mine and crop out south of the MMR fault as the Area 4 and Perch deposits. Numerous smaller scale faults also occur with orientations similar to the MMR fault. Proterozoic dolerite dykes crosscut all earlier features and occur as 10 to 20 m thick vertically or steeply north dipping, east striking dykes or as thinner sill-like bodies along rock contacts.
METAMORPHISM AND ALTERATION An early hydrothermal event characterised by phyllicpropylitic-carbonate alteration occurred prior to, or simultaneous with major regional scale deformation. Peak metamorphism occurred subsequent to this with minor retrogression and later metasomatic alteration (M Rowley, unpublished data, 1991). Hydrothermal alteration resulted in the mineral assemblage sericite, chlorite and quartz with accessory carbonate, sulphides, gold, tourmaline and scheelite. Arsenopyrite, pyrrhotite and lesser pyrite, chalcopyrite and sphalerite are present predominantly as fine grained disseminations in the groundmass and rarely in veins. Deformation has resulted in the formation of at least two penetrative foliations and textural reconstitution (P Ashley, unpublished data, 1990). Foliations are defined by aligned layers of silicate minerals, amphibole and recrystallised quartz, and hydrothermal veins are commonly concordant with the foliation. The subsequent regional metamorphism has resulted in the partial to complete destruction of the hydrothermal assemblage, for example, replacement of sericite by phlogopite, chlorite and clinozoisite (P Ashley, unpublished data, 1990). Metamorphism of the metabasaltic (amphibolitic) rocks hosting the deposit is typically upper greenschist to lower amphibolite facies. Mineral assemblages include hornblende, actinolite, tremolite, plagioclase, garnet and tourmaline with accessory quartz, apatite, ilmenite (commonly altered to titanite), carbonate and epidote. Sulphides include pyrrhotite, pyrite and chalcopyrite. Possible vesicles are present and are filled with quartz and carbonate with titanite rims. Porphyroblastic phases include hornblende and garnet and these are poikiloblastic and/or partially replaced, with prominent reaction rims. Retrogression is marked by the partial replacement of garnet by hornblende and chlorite, and the replacement of hornblende by chlorite and biotite. Metasomatic reactions are widespread throughout the mafic and ultramafic rocks, evidenced by assemblages such as carbonate, epidote, tourmaline, axinite, quartz and garnet. The retrogressive replacement of axinite
Geology of Australian and Papua New Guinean Mineral Deposits
(amphibolite facies) by tourmaline and epidote (greenschist facies) is common in alteration zones throughout the mafic assemblage. Vein assemblages of carbonate, chlorite, adularia and prehnite also indicate retrogression. Mass balance calculations carried out on altered amphibolite rocks across an ore zone reveal addition of potassium, sulphur and barium, and depletion of magnesium, titanium, phosphorus and possibly manganese. Variable enrichment and depletion of silica, calcium, aluminium, iron and sodium were observed without a clear trend. The ultramafic units comprise interflows of metaperidotite, metapyroxenite and metakomatiite. Komatiitic flows are partially to completely serpentinised and spinifex texture is well developed. Chlorite or talc after olivine, and tremolite after feather quenched pyroxene are abundant with prominent antigorite or magnetite after secondary olivine. In the ultramafic rocks, euhedral, well zoned tourmaline (uvitedravite) crystals are present in distinct bands at the base and top of the units and a metasomatic origin is favoured. Metamorphism of hydrothermal assemblages indicates a pre- to syn-metamorphism and deformation age for alteration and possibly for the mineralisation.
GEOCHEMICAL DATA A geochemical study of the mafic rocks at the Plutonic deposit revealed a low potassium tholeiitic composition typical of modern-day island arc or ocean floor basalt. Basalts range in composition from high iron to high magnesium and many major element and trace element plots were used (cautiously), including those of Jensen and Langford (1985), Hallberg (1984), Pearce and Cann (1973), Meschede (1986), Pearce, Gorman and Birkett (1975), Mullen (1983) and Floyd and Winchester (1975, 1978). In some diagrams a large scatter was observed and this may be due to the alteration, metamorphism, weathering and antiquity of the rocks. Samples from the Overthrust mafic, Mine mafic and Basal mafic units were all plotted and in most cases a strong overlap was observed. A series of Harker variation diagrams using major element geochemistry also failed to reveal significant differences between these mafic units. Rare earth element spider diagrams were also used and failed to show patterns similar to those of modern-day oceanic or continental basalts (Fig 5).
AGE DETERMINATIONS Pb DATING OF GALENA There have been several attempts to date the mineralisation and the host greenstone assemblage at the Plutonic deposit. Galena is not directly associated with gold mineralisation at Plutonic, however lead data using galena from brecciated quartz veins and wall rock samples were obtained and returned ages of 1700 to 1600 Myr. These ages are much younger than those obtained from the nearby Marymia gold deposit and various Yilgarn Craton deposits, where galena associated with gold mineralisation was dated between 2660 and 2630 Myr (McMillan, 1993). Interpretation of the ages derived from the Plutonic galena samples favours a hydrothermal event associated with the Capricorn Orogeny. Scatter within this group suggests that the lead was sourced from a local environment and that hydrothermal activity was localised.
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FIG 5 - Mantle normalised trace element variation diagram for the mafic units within the Plutonic sequence, indicating the similar patterns for all units. Data from Sun and McDonough (1989).
Pb DATING OF PYRITE Pyrite associated with gold mineralisation were dated measuring the relative increases in the radioactive isotopes 206 Pb and 207Pb relative to the stable 204Pb isotope, and gave an age of 2080 Myr. The favoured interpretation, taking into account the style of mineralisation and the dating of nearby deposits in the Plutonic Well greenstone belt, is that the age of pyrite is approximately 2630 Myr, with a younger pyrite of 1720 Myr (incorporating lead from surrounding rocks). A regional resetting of Pb dates at 1720 Myr may be a result of the Capricorn Orogeny, associated with the remobilisation of gold and the introduction of new pyrite into the system.
DATING OF ZIRCON Zircon dating (using U-Th-Pb isotope methods) of a quartzfeldspar-chlorite schist (possibly a felsic porphyry intrusive) from the Barramundi prospect approximately 9 km to the east of the Plutonic deposit (Fig 1), was carried out by McMillan and McNaughton (1995). Forty zircon grains were analysed from which two main age populations were obtained, at about 2740 Myr and about 2660 Myr. Both groups displayed zircon morphologies typical of an igneous origin, with the older population showing partial resorption of rims. Taking into account the zircon morphologies and the geological context of the Plutonic Well greenstone belt, a magmatic event at about 2660 Myr is favoured, with a previous magmatic event at about 2740 Myr (McMillan and McNaughton, 1995). These data suggest that the rocks at Barramundi are Archaean in age. Because the precise geological setting of the sample is not known (it could be high level intrusive or extrusive), the age of 2660 Myr is either the minimum age of the greenstone belt or the exact age.
78
MINE GEOLOGICAL METHODS OPEN PIT METHODS A plan of the Plutonic Main pit and surrounding prospects is shown in Fig 2. Grade control and ore definition are undertaken by two different methods. Within the oxide and transitional parts of the orebody, small diameter RC drill holes are used, with samples collected every metre. A 6 by 3 m pattern of vertical holes is drilled over the areas where mineralisation is expected. Ore boundaries are interpreted on sections prior to marking flitches for bench layout of ore blocks. As the rock hardness increases with depth, blast hole rigs for grade control sampling are used. A 3 by 3 m blast hole pattern is drilled and composite samples are taken, representing 0–3 m and 3–6 m depths, from which ore boundaries are then interpreted for each flitch. Ore block boundaries are marked out by survey using different coloured tapes for each ore grade class.
UNDERGROUND METHODS Underground mining at the Plutonic mine is currently targeting the NW Extension lodes, immediately NW and down dip of the Main pit (Fig 3). In this area the lodes dip at approximately 70o and levels are therefore developed at 15 m intervals. At each production level, crosscuts are driven from the decline across strike to intersect the mineralised zones. Ore drives are driven from each crosscut following the mineralised zone along strike. Sludge sample holes are drilled at regular intervals to ensure that no ore is left in the immediate hanging wall and footwall of the advancing strike drive. Diamond drilling is also undertaken from various ore drives, stockpile and drilling cuddies to test for the existence of any parallel or subparallel lodes that have
Geology of Australian and Papua New Guinean Mineral Deposits
PLUTONIC GOLD DEPOSIT
not been identified by the initial surface diamond drilling, and to delineate and evaluate the lode system on the next levels. To the north of the NW Extensions the lode dip changes from steep to subhorizontal, requiring different mining methods. Steeply dipping sections will ideally be mined using long hole open stope methods if there is sufficient regularity of shape and continuity between levels. Subhorizontal lodes will be mined by conventional room and pillar methods. Lodes that dip at approximately 33o (lower than the normal rill angle) are common and are too flat for long hole stoping. The mining method used in these areas will be a form of step mining. The mining methods used at Plutonic are based on the application of mechanised bulk mining methods to limit the manpower numbers and to maximise the productivity, thereby improving the overall efficiency of the underground mining operation.
ACKNOWLEDGEMENTS Plutonic Resources Limited are acknowledged for permission to publish.The authors would like to acknowledge the encouragement, support and help of W Bucknell, M Rowley and F Jockel. N McMillan and N McNaughton are likewise acknowledged for their help with the study and age dating carried out at the Key Centre for Strategic Mineral Deposit Studies at the University of Western Australia. The paper was reviewed by D Murphy and M Rowley. The exploration staff involved with the discovery of the deposit and continued success of the mine are gratefully acknowledged.
REFERENCES Bruce, P, Rowley, M and Bradford, P, 1991. Plutonic gold project inception to operation, in Proceedings World Gold ‘91, Cairns, pp 347–353 (The Australasian Institute of Mining and Metallurgy: Melbourne). Bucknell, W R, 1995. Discovery and definition of the Plutonic gold deposit, in New Generation Gold Mines: Case Histories of Discovery, pp 16.1–16.8 (Australian Mineral Foundation: Adelaide). Bucknell, W R, Jockel, F C M, Kellett, R J, Vickery, N M and Buckley, P M, 1996. The geology of the Plutonic deposit, Western Australia, Geological Society of Australia Abstracts, 41:67. Floyd, P A and Winchester, J A, 1975. Magma type and tectonic setting discrimination using immobile elements, Earth and Planetary Science Letters, 27:211–218. Floyd, P A and Winchester, J A, 1978. Identification and discrimination of altered and metamorphosed volcanic rocks using immobile elements, Chemical Geology, 21:291–306. Gee, R D, 1987. Peak Hill, Western Australia - 1:250 000 geological series, Geological Survey of Western Australia, Explanatory Notes SG 50-8.
Geology of Australian and Papua New Guinean Mineral Deposits
Gee, R D, 1990. Nabberu Basin, in Geology and Mineral Resources of Western Australia, Geological Survey of Western Australia Memoir 3, pp 202–210. Hallberg, J A, 1984. A geochemical aid to igneous rock type identification in deeply weathered terrain, Journal of Geochemical Exploration, 20:1–8. Jensen, K S and Langford, F F, 1985. Geology and petrogenesis of the Archaean Abitibi Belt in the Kirkland Lake area, Ontario, Ontario Geological Survey Miscellaneous Paper 123. McMillan, N M, 1993. Structure, metamorphism, alteration and timing of gold mineralisation at Marymia Gold Project in the Marymia Dome, in An International Conference on Crustal Evolution, Metallogeny and Exploration in the Eastern Golfields, Extended Abstracts (Eds: P R Williams and J A Haldane), pp 243–244 (Geoconferences WA Inc: Perth). McMillan, N M and McNaughton, N J, 1995. The post magmatic history of felsic rocks from the Archaean Marymia Dome: A SHRIMP study of the relationship between zircon morphology, Th/U and the geological history, in Australian Conference on Geochronology and Isotope Geoscience, Workshop Programs and Abstracts (Curtin University of Technology: Perth). Meschede, M, 1986. A method of discriminating between different types of mid ocean ridge basalts and continental tholeiites with the Nb-Zr-Y diagram, Chemical Geology, 56:207–218. Mullen, E D, 1983. MnO/TiO2/P2O5: a minor element discriminant for basaltic rocks of oceanic environments and its implications for petrogenesis, Earth and Planetary Science Letters, 62: 53–62. Myers, J S 1990. Capricorn Orogen, in Geology and Mineral Resources of Western Australia, Geological Survey of Western Australia, Memoir 3, pp 197–198. Pearce, J A and Cann, J R, 1973. Tectonic setting of basic volcanic rocks determined using trace element analyses, Earth and Planetary Science Letters, 19:290–300. Pearce, T H, Gorman, B E and Birkett, T C, 1975. The TiO2-K2O-P2O5 diagram: A method of discriminating between oceanic and nonoceanic basalts, Earth and Planetary Science Letters, 24:419–426. Subramanya, A G, Faulkner, J A, Sanders, A J and Gozzard, J R, 1995. Geochemical mapping of the Peak Hill 1:250 000 sheet, Geological Survey of Western Australia Explanatory Notes SG 50–8. Sun, S S and McDonough, W F, 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes, in Magmatism in the Ocean Basins (Eds: A D Saunders and M J Norry), pp 313–345, Geological Society of Australia Special Publication 42. Tyler, I M and Thorne, A M, 1991. The northern margin of the Capricorn Orogen, Western Australia - an example of an Early Proterozoic collision zone, Journal of Structural Geology, 12:685–701. Windh, J, 1992. Tectonic evolution and metallogenesis of the Early Proterozoic Glengarry Basin, Western Australia, PhD thesis (unpublished), The University of Western Australia, Perth.
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Harper, M A, Hills, M G, Renton, J I and Thornett, S E, 1998. Gold deposits of the Peak Hill area, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 81–88 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Gold deposits of the Peak Hill area 1
2
3
by M A Harper , M G Hills , J I Renton and S E Thornett INTRODUCTION The Peak Hill mining centre is 130 km NNE of Meekatharra, WA, at lat 25o38′S, long 118o43′E on the Peak Hill (SG 50–8) 1:250 000 scale and Bryah (2646) 1:100 000 scale map sheets (Fig 1). In recent years gold has been mined by Peak Hill Resources from the Peak Hill-Fiveways and Jubilee (J1) pits and ore is currently being produced from the Harmony pit, 9 km WSW of Peak Hill town site. Gold was also produced by Barrack Mines Limited (Barrack) from Mount Pleasant, 2 km SE of the town site.
4
Peak Hill-Fiveways and J1 orebodies was 4 117 700 t of ore for a recovery of 12.9 t (414 620 oz) of gold. Production from the Harmony pit up to June 1997 has totalled 4.9 t (156 600 oz) from 1 304 100 t of ore. Total production for the Peak Hill area, including historical mining, exceeds 850 000 oz of gold. Using a 1 g/t gold lower cutoff grade, the remaining resources and reserves for the Peak Hill area as at 31 December 1996 are given in Table 1.
MINING AND EXPLORATION HISTORY
Records of the WA Department of Minerals and Energy indicate that mining at Mount Pleasant between 1974 and 1988 produced a total of 441 kg (14 180 oz) of gold from 146 000 t of ore. During the period 1988 to 1995, total production from the
Gold was discovered at Peak Hill in 1892 and in the following 20 years the area became a major mining centre, with underground workings extending to depths of greater than 150 m. According to Gee (1987) the historical gold production from the Peak Hill field was approximately 8.2 t (263 600 oz).
1.
Mine Geologist, Peak Hill Resources, PO Box H560, Perth WA 6001.
2.
District Geologist, Plutonic Operations Limited, 24 Outram Street, West Perth WA 6005.
3.
Senior Mine Geologist, Peak Hill Resources, PO Box H560, Perth WA 6001.
4.
Senior Geologist, Piquero Geological Consultants, 53 Philip Road, Dalkeith WA 6009.
Modern exploration and mining in the area commenced in 1974 at Mount Pleasant, with Barrack Mines Limited producing gold ore from the Mount Pleasant pit until 1988. In 1978 Peko Wallsend Operations Limited (now North Limited) signed an option agreement with Peak Hill prospector R Otway and commenced exploration, focusing on the area of the Peak Hill workings and regional evaluation. This led to the discovery of a mineable gold orebody at Peak Hill. At the time
FIG 1 - Location map and regional geological map, Peak Hill area.
Geology of Australian and Papua New Guinean Mineral Deposits
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M A HARPER et al
TABLE 1 Resources and reserves, Peak Hill area. Ore (000 t)
Gold grade (g/t)
Contained gold (oz)
Inferred and Indicated Resources Enigma North
797
2.0
51 900
Peak Hill-Fiveways (Mini Pit and Deeps)
278
2.6
23 000
Jubilee (J2 zone)
40
2.8
3 600
Harmony
889
4.2
120 000
Harmony Southwest Laterites1
259
0.9
7 500
Fiveways Deeps
307
4.2
41 500
Jubilee (J3 zone)
247
2.1
16 700
Measured Resources
Probable and Proven Reserves Harmony
606
2.6
50 700
Harmony Southwest Laterites
38
1.4
1 700
Total Stockpiles1
983
1.4
45 400
1.
Calculated using a 0.5 g/t gold lower cutoff grade. Source: Plutonic Operation Limited, Annual Report 1996.
of the discovery, Grants Patch Mining Limited was extracting gold from tailings dumps at Peak Hill, for a total production of 10 413 oz. A 50:50 joint venture agreement was reached between Peko Wallsend and Grants Patch Mining Limited, and in 1988 the partners commenced mining under the unincorporated joint venture name of Peak Hill Resources. Grants Patch Mining Limited is now owned by Plutonic Operations Limited. A second joint venture between Afmeco Pty Ltd (50%) and Peak Hill Resources (50%), known as the Baxter joint venture, covered tenements to the west of Peak Hill. This included the Harmony deposit, which was discovered in 1991, and was in effect until 1994 when Afmeco’s interests in the area were taken over by Plutonic (Baxter) Pty Ltd. All tenements from the two original joint ventures are now covered by a single agreement known as the Peak Hill (1995) joint venture, owned 66.67% by Plutonic Operations Limited and 33.33% by North Limited. Since 1988, exploration by Peak Hill Resources and Afmeco Pty Ltd has used geological mapping, a variety of surface sampling techniques and shallow drilling to define geochemical anomalies. Rotary air blast (RAB) and aircore drilling were the main tools used to locate anomalous gold values in areas of shallow bedrock or within the regolith profile, with deeper follow up drilling, mainly by face sampling reverse circulation (RC) hammer, to define gold resources. Drilling has led to the discovery of significant additional gold resources, resulting in production from cut backs to the Peak Hill pit, an open pit at adjacent Fiveways, a small pit at Jubilee (J1 deposit) and currently from the Harmony pit, known as the Contact deposit by Afmeco Pty Ltd (Fig 1). It is notable that the Contact (Harmony) deposit was discovered by Afmeco Pty Ltd in an area with neither bedrock exposure nor previously known mineralisation.
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Plutonic Operations Limited assumed exploration responsibility in 1995 and has systematically explored the depositional plain of the Baxter area by angled RAB drilling to point of refusal, with holes of average length 70 m and commonly exceeding 100 m. This and previous work has outlined a 7 km long zone of gold mineralisation known locally as the Enigma structural zone, which contains ten discrete anomalous areas.
PREVIOUS DESCRIPTIONS The Peak Hill Goldfield was first investigated by government geologists with Maitland (1904) reporting on the geology of the gold bearing lodes and the progress of mining and exploration. Montgomery (1909) described mining at Peak Hill during a time of waning gold production. Two editions of the Peak Hill 1:250 000 scale geological map and explanatory notes have been published (MacLeod, 1970; Gee, 1987). The Peak Hill area has recently been remapped by the Geological Survey of Western Australia (GSWA) as part of the Bryah 1:100 000 geological sheet (Pirajno and Occhipinti, 1996, in press). All regional stratigraphic and structural nomenclature used in this paper follows Pirajno and Occhipinti (in press). Windh (1992) carried out mapping of weathered exposure in the Peak Hill pit and regional structural mapping of the Glengarry Fold Belt. Research on the geological setting and origin of gold deposits of the Peak Hill area was undertaken by Thornett (1995). Much detailed unpublished work has also been carried out by the staff of Afmeco Pty Ltd, North Limited and Plutonic Operations Limited and by consultants to those companies. The early exploration at Peak Hill was summarised by R W Baxter, P M McInerney and P M Kenny (unpublished data, 1989), and work by P L Kitto and R Forster on the geology and reserves of the Fiveways deposit was described by R Forster (unpublished data, 1991). A synoptic report by Afmeco Pty Ltd staff (unpublished data, 1992) and a report by R Grivas, C Nouel and D Virlogeux (unpublished data, 1993) outlined the exploration history and characteristics of the Contact (Harmony) deposit.
REGIONAL GEOLOGY The Peak Hill area is in the western part of the Nabberu Basin (Gee, 1990) in an area which has previously been known as the Glengarry Fold Belt or Glengarry Basin. Archaean rocks of the Marymia Inlier lie approximately 30 km ENE of Peak Hill. Recent mapping by the GSWA of the Bryah 1:100 000 scale sheet has led to the subdivision of the Glengarry Basin into three separate Proterozoic basins; the Yerrida, Bryah and Padbury basins. The name Glengarry Basin has been abandoned. Possible relationships between the Archaean and Proterozoic rocks in the area and detailed descriptions of the rock types are presented by Pirajno and Occhipinti (in press). Pirajno and Preston (this publication) discuss the regional setting of mineral deposits of these basins. Three of the four gold deposits described in this paper (Peak Hill-Fiveways, Jubilee and Mount Pleasant) are within deformed metamorphic rocks of unknown Precambrian age which Pirajno and Occhipinti (in press) have named the Peak Hill Schist (Fig 1). The fourth gold deposit, Harmony, is hosted by Palaeoproterozoic rocks in the Bryah Basin.
Geology of Australian and Papua New Guinean Mineral Deposits
GOLD DEPOSITS OF THE PEAK HILL AREA
The rocks of the Peak Hill Schist were previously known as the Peak Hill Metamorphic Suite and were assumed to be Proterozoic in age. Based on local mapping and aeromagnetic interpretation, Thornett (1995) suggested that the rocks of the Peak Hill Schist represent the deformed southwestern end of the Archaean Marymia Inlier. Field observations and the interpretation of Landsat images and aeromagnetic survey data led Pirajno and Occhipinti (in press) to support this structural interpretation, but because the absolute age of the Peak Hill Schist is unknown they have preferred not to assign these rocks to the Archaean. At Peak Hill-Fiveways and Jubilee the rocks of the Peak Hill Schist are dominated by muscovite schist, commonly with biotite, chlorite, magnetite, plagioclase and/or garnet. Graphite schist, amphibole-rich mafic schist and metadolerite are also present. The Mount Pleasant pit exposes a more carbonate-rich sequence, with lenses of marble within biotite-chlorite-carbonate-magnetite schist, with some plagioclase, muscovite, garnet and hornblende. Narrow quartzite units are present at Peak Hill and Mount Pleasant. Four phases of deformation are evident in the Peak Hill Schist (Thornett, 1995). The earliest of these produced a layerparallel fabric (S1) and is associated with early thrusting. Thornett (1995) has recognised three later phases of deformation (D2 to D4), with D2 manifested as asymmetric recumbent folds and the two younger deformations producing interference folds which define a broad regional domal structure in the Peak Hill Schist. The Peak Hill area has a regionally anomalous metamorphic grade, with coexisting hornblende and plagioclase at the Peak Hill-Fiveways deposit indicating that prograde metamorphism has reached amphibolite facies. Geothermometry on prograde assemblages provides temperatures of 535 to 620oC. Phengite geobarometry, on corresponding assemblages, indicates high minimum pressures of 6.5 to 7 kb. The Harmony deposit is west of Peak Hill in Palaeoproterozoic rocks of the Bryah Group (Pirajno et al, 1996) within the Bryah Basin. In the vicinity of the Harmony deposit the Bryah Group comprises turbiditic sedimentary rocks of the Ravelstone Formation, previously considered part of the Thaduna Greywacke, and mafic-ultramafic volcanic rocks and schist of the Narracoota Formation. Deformation is less intense than at Peak Hill-Fiveways but the S1 foliation is clearly developed and folded into a series of open, NWplunging folds. The rocks are of greenschist facies metamorphic grade, representing a lower grade than the rocks at Peak Hill-Fiveways.
ORE DEPOSIT FEATURES The gold orebodies within the Peak Hill Schist in the vicinity of Peak Hill occupy different stratigraphic positions but have a number of shared characteristics. All are within strongly altered zones which contain albite porphyroblasts, iron-rich chlorite, biotite and carbonate, and lesser garnet, tourmaline, graphite and pyrite. Although mafic rocks are generally rare in the Peak Hill Schist, they are commonly associated with gold mineralisation. The main examples are hornblende-zoisite rich metadolerite bodies at Jubilee and Peak Hill-Fiveways, and hornblende-rich mafic schist at Peak Hill-Fiveways and Mount Pleasant. Mineralisation shows a strong relationship to D4 structures, and is developed on the hinges of D4 folds, especially antiforms, and associated with D4 faults and shear zones. All three mined orebodies also show a strong spatial
Geology of Australian and Papua New Guinean Mineral Deposits
relationship to graphitic schist, with graphitic horizons commonly adjacent to or along strike from mineralised lodes (J I Renton, unpublished data, 1993). The orebodies in the Peak Hill Schist are described below in order of apparent stratigraphic position, starting with the lowest (Mount Pleasant) and progressing to the highest (Jubilee), with summaries derived from Thornett (1995). This is followed by a description of the Harmony deposit, which is close to the upper contact of the overlying Narracoota Formation.
MOUNT PLEASANT The rocks in the Mount Pleasant pit comprise the inferred lowest stratigraphic succession exposed in the Peak Hill Schist and are known locally as the Core sequence (Fig 1). The pit straddles a series of open, upright, macroscopic F3 folds, with subhorizontal hinges trending NNW. The schists are multiply deformed and cut by abundant quartz-carbonate veins which commonly contain coarse muscovite. The rocks exhibit a strong early S1 schistosity which is refolded by a series of tight mesoscopic F2 folds of 1 to 3 m amplitude. These folds are asymmetrical and show a dominant east vergence, but are irregular in profile suggesting their development as sheath folds. Mine records indicate that mineralisation comprised two flat-lying to shallowly-dipping ore zones. The weathered southern zone, the higher of the two, occupied the southern end of the pit, was 1 to 13 m thick and dipped northwards at 5 to 25o. The northern zone was characterised by a series of vertically stacked mineralised horizons, elongated NW and dipping at about 10o to the NW. The average gold grade reported from historical underground workings was 9 g/t, but average grades mined in the Barrack open cut were closer to 2 g/t. Most of the mineralisation in the northern zone was in fresh but extremely altered rocks, with typical alteration minerals including albite porphyroblasts, iron-rich chlorite, biotite, muscovite and hornblende. Graphitic rocks occur in two parts of the pit; one in an area of old underground workings within near-surface, bleached kaolinitic rocks and the other near the pit floor, in association with a major late shear zone, which dips at 6 to 16o to the NW. In both cases zones of gold mineralisation lie adjacent to the graphite-bearing rocks, but not within them. Gold was reportedly associated with pyrite and rare galena, in flat lying to vertical quartz-carbonate veins, pyritic veinlets in shear zones, and within vein quartz boudins.
PEAK HILL-FIVEWAYS In the adjoining Peak Hill and Fiveways pits the main host rock to mineralisation is locally named the Mine sequence. It is west dipping at 20 to 50ο, intensely altered, and composed of muscovite, biotite, chlorite and graphite. These west-dipping strata form the western limb of an antiform, with the hinge passing through the SE part of the Peak Hill pit (Fig 2). The top of the Mine sequence is commonly marked by a deformed, weakly graphitic quartzite, with an overlying Hanging Wall sequence of magnetite-rich muscovite schist, hornblende-rich mafic schist and metadolerite. The footwall to the Mine sequence is mainly quartz-rich muscovite schist which is referred to as the Intermediate sequence (Figs 2 and 3). Much of the gold mineralisation is located in mesoscopic to submesoscopic quartz veins within NNE (015o)-striking, westdipping (28 to 45 o) mineralised shear zones. Within the Mine
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M A HARPER et al
boundaries. Gold-bearing veins composed mainly of fine, acicular crystals of tourmaline (dravite) are abundant in the south of the Peak Hill pit and tourmaline is found in mineralised rocks throughout the area. XRD and SEM analyses of ore specimens (Thornett, 1995) show that visible gold is commonly associated with gold-free telluride minerals. Numerous lead and bismuth tellurides have been identified, including altaite (PbTe), rucklidgeite [(Bi, Pb)3Te4], tellurobismuthite (Bi2Te3) and tetradymite (Bi2Te1.65S1.35). Other gangue minerals include galena, molybdenite, pyrrhotite, chalcopyrite, fluorite and the secondary minerals chrysocolla, malachite, psilomelane and crystalline pyrolusite. Arsenopyrite has not been identified in the Peak Hill-Fiveways area.
FIG 2 - Interpreted geological plan, Peak Hill-Fiveways pit.
Analyses of gold and silver in bullion derived from the Peak Hill pit (Thornett, 1995) show that it contains <5% silver within 50 m of the surface, with the silver content reaching a maximum of 29.5% at a depth of 77 m and then gradually diminishing to 8% at a depth of 120 m. These analyses suggest gold enrichment in weathered rocks above the water table (ie within 50 m of the surface), with a marked increase in the content of silver and other metals in weathered rocks just below the water table. Gold fineness has been determined from samples containing visible gold, with the highest fineness of 975 to 985 from fresh drill core at a depth of 225 m below surface. Samples collected from 114 and 124 m below surface have fineness in the range of 944 to 964. Corresponding bullion analyses indicate a much lower fineness of around 890, suggesting that some of the silver in the bullion may be derived from native silver or gold-free silver compounds.
JUBILEE
FIG 3 - Schematic section on 10275 N, Peak Hill-Fiveways pit, looking north.
sequence, mineralisation tends to concentrate into upper (hanging wall) and lower (footwall) lodes through most of the deposit, but gold distribution is also affected by late stage, NEtrending faults. Structural studies by N J Archibald (unpublished data, 1990, 1992) showed these crosscutting faults to be sites of high grade ore shoots in the Peak Hill pit. Similar faults in the Fiveways pit are not closely associated with high gold grades, but the northernmost of these faults appears to define the local limit of the Fiveways lodes. Gold mineralisation at Peak Hill-Fiveways commonly occurs as ‘leaf’ or ‘paint’ gold in dark grey quartz veins, but is also found within masses of chlorite or chlorite-kaolin and as disseminations within chlorite- or biotite-bearing schist. Calcite veins are commonly associated with ore, and in some areas vughs in these veins are filled with coarse tabular crystals of low albite (Thornett, 1995). Pyrite and marcasite are present in quartz veins but are not always closely associated with gold. Where gold does occur in pyrite it forms tiny inclusions, discontinuous microveinlets and thin rims on pyrite grain
84
At Jubilee, altered muscovite schist is intruded by a body of essentially undeformed metadolerite which has a stratigraphic thickness up to 250 m. The mineralised schist is highly altered, with alteration assemblages including epidote or zoisite, ironmagnesium rich chlorite, muscovite, biotite, poikiloblastic albite and some garnet. Mineralisation is adjacent to both the hanging wall and footwall contacts of the metadolerite and the highest gold values appear to be associated with flexures in the metadolerite contact. The Jubilee pit area straddles an open, moderately to steeply NW-plunging antiform, the hinge of which continues southward to the eastern side of the Peak HillFiveways pit. Historical underground workings at Jubilee focussed on glassy to saccharoidal quartz veins within muscovite schist. Gold was found in the quartz veins and the adjacent narrow (<10 cm) sheared, wall rock selvages. All of the old shafts were located close to the metadolerite contact and the gold-bearing quartz veins had highly variable orientations. Mining at the Jubilee pit has exposed a complex vein stockwork which persists along the whole southern margin of the metadolerite, but has the greatest concentration of veins in antiformal hinges. Most veins are dilational, with little evidence of displacement of S1 across the veins. Some appear to be tensional fissures and a small number of veins lie in a saddle reef configuration, subparallel to S1. Most of the visible gold at Jubilee is present as fine specks or leafy plates and is dark yellow, suggesting a high degree of purity. Pyrite is the dominant gangue mineral and gold-bearing veins commonly contain abundant limonite and manganese minerals, with minor amounts of graphite and magnetite.
Geology of Australian and Papua New Guinean Mineral Deposits
GOLD DEPOSITS OF THE PEAK HILL AREA
HARMONY The mineralisation at the Harmony pit is hosted by a SWdipping sequence of intensely altered metabasalt and metadolerite at the top of the Narracoota Formation, with very minor mineralisation extending into the overlying pelitic metasedimentary rocks of the Ravelstone Formation (Fig 4). Talc-bearing ultramafic rocks form the footwall to mineralisation.
stockwork which is associated with higher ore grades. Most veins show evidence of post-emplacement fracturing and resilicification. The mineralisation at Harmony is accompanied by pervasive silicification and carbonate alteration. Below 515 m RL (40 m depth) the ore zone comprises a core of highly carbonated metabasalt surrounded by a chloritic halo. This grades into a zone of extreme limonite-kaolinite-hematite alteration adjacent to the sheared hanging wall contact. In their petrographic study of early diamond drill core, A Pacquet and J Reyx (unpublished data, 1992) recognised an alteration sequence in the mafic rocks involving early albitisation of plagioclase and silicification, followed by crystallisation of muscovite and iron-magnesium chlorite, with later weathering to smectite group clays. In the ultramafic footwall rocks, albitisation and silicification were accompanied by early recrystallisation of magnetite, followed by talc formation and later chloritisation, with local development of epidote. These rocks have generally not been altered to clay but do contain late dolomite and calcite. The pelitic metasedimentary rocks of the hanging wall have undergone strong alteration to sericite and chlorite, with later weathering to smectite group minerals. Detailed examination of the gold mineralisation (A Pacquet and J Reyx, unpublished data, 1992) revealed probable primary gold inclusions in pyrite, which contain up to 24% silver. Other elements which occur in trace quantities in the mineralisation are arsenic, tellurium, zinc and lead. Ore-forming minerals which accompany pyrite include pyrrhotite, pentlandite, chalcopyrite, supergene copper sulphides and scheelite.
FIG 4 - Interpreted geological plan, Harmony pit.
The deposit is on the western limb of a NW-plunging antiform within a more regional NW-trending monoclinal flexure named the Enigma structural zone (Afmeco Pty Ltd, unpublished data, 1992). Mineralisation is largely controlled by brittle structures, including quartz veins, which are commonly hematitic, and carbonate-filled ‘crackle’ breccia. The most intense veining and accompanying alteration and mineralisation occur in the mafic rocks at the sheared interface with the hanging wall metasedimentary sequence, where the competent mafic rocks are inferred to record the highest strain. Mineralisation is predominantly associated with subvertical NE-trending quartz veins, north-trending semi-stratabound veining at the hanging wall contact and NW-trending lodes parallel to the Enigma structural zone, which together in plan view form an ‘A frame’ shape (N J Archibald, unpublished data, 1996) as shown on Fig 4. A high proportion of the NEtrending veins have the hanging wall contact zone as their point of origin and they are interpreted to be en echelon tensional veins. These veins mostly dip steeply SE although a few NWdipping veins are noted as are some veins which extend into the metasedimentary rocks of the hanging wall. The above vein arrays locally become closely spaced, producing a complex
Geology of Australian and Papua New Guinean Mineral Deposits
The Harmony deposit was totally concealed by thick, transported and residual regolith and upon discovery presented no surface indication of the existence of mineralisation. The regolith cover comprises varying thicknesses of Quaternary hardpan and colluvium-alluvium above lateritised and weathered bedrock that extends to depths in excess of 60 m. The orebody includes a near surface, subhorizontal zone of supergene enrichment and a separate zone of low grade gold mineralisation within lateritic duricrust to the SW of the current pit design (Fig 5).
FIG 5 - Schematic section, Harmony pit, looking NW.
The thick overburden and lack of surface evidence for mineralisation rendered the Harmony deposit an ideal site for investigating geochemical metal dispersion in both transported and residual regolith. The Baxter area, which surrounds the Harmony deposit, was studied as part of a joint research project between AMIRA and the Cooperative Research Centre for Landscape Evolution and Mineral Exploration (Robertson, Phang and Munday, 1996). A complex palaeotopography was revealed by logging of the main regolith units in existing drill
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holes. The Harmony deposit is on a WNW-trending palaeohigh flanked by palaeovalleys containing up to 20 m of colluvium. A major palaeovalley, filled with mottled clay sediments, lies beneath the colluvial blanket to the SW of Harmony and is parallel to the trend of the palaeohigh. A less pronounced palaeovalley drains NNE from Harmony and clays filling it are visible in the NE pit wall. Geochemical sampling of predominantly ferruginous materials, including buried lateritic duricrust, indicated that both gold and tungsten are anomalous in close vicinity of the Harmony deposit, whereas arsenic, antimony and selenium are anomalous along the Enigma structural zone. Hydrogeochemical studies found the ground water to be pH neutral with a low salinity (Gray, 1995). Ground waters have extremely low gold concentrations, but in common with other sites in the northern Yilgarn, contain elevated selenium, molybdenum, tungsten and to a lesser extent rubidium in areas of gold mineralisation.
CONDITIONS AND AGE OF MINERALISATION Lead isotope studies and petrographic descriptions of fluid inclusions in samples from the Mount Pleasant and the Peak Hill-Fiveways pits have been carried out by Thornett (1995). The age determinations of galena and lead-bearing tellurides from gold mineralised zones at Peak Hill and Mount Pleasant suggest a mineralisation age of 1900 to 1800 Myr. The data also suggest that deposits in the Marymia to Peak Hill area may have a distinctly different lead source from other deposits in the Glengarry-Nabberu region. Fluid inclusions from mineralised vein quartz at Mount Pleasant and Peak Hill were found to be too small for heating-freezing studies but petrographic characterisation revealed liquid-rich, two-phase inclusions which were probably aqueous in most mineralised veins. Fluid inclusions commonly contain a cube-shaped daughter mineral which is assumed to be sodium chloride. Vapour rich inclusions are common and suggest that the mineralising fluids were trapped in a relatively low pressure environment. The presence of some three-phase inclusions, often with vibrating vapour bubbles, implies that CO2 is present in liquid and vapour states. Geothermometry of retrograde chlorite in alteration zones with high grade gold mineralisation (Thornett, 1995) indicates formation temperatures of 250 to 375oC. The presence of pure low albite in mineralised veins implies a maximum temperature of 400oC. Development of euhedral, medium to coarse grained, comb-textured quartz in cavities and open spaces, and the presence of vapour-rich fluid inclusions, suggest low to moderate confining pressures.
ORE GENESIS Intense alteration which accompanies gold mineralisation at Peak Hill is associated with early (D1) thrust surfaces and shear zones, and late (D4) brittle and ductile structures represented by shear zones, folds and faults. High temperature, but apparently retrograde, alteration assemblages (garnet-biotite) associated with early (D1) shear zones, suggest that hydrothermal fluids have infiltrated along these conduits since early in the deformational history. It is also possible that early mineralisation of these structures took place. Shear zones which formed late in the deformational history, such as the flat-
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lying shear zone in the floor of the Mount Pleasant pit, have also acted as fluid channelways. Late stage brittle deformation, associated with D4 at Peak Hill and manifested as crosscutting faults and dilational openings in shear zones and fold hinges, has provided sites for gold mineralisation at the Peak Hill and Harmony deposits. Work by Thornett (1995) suggests that during D4 deformation hot and saline hydrothermal fluids infiltrated newly opened fluid pathways, causing intense alteration of wall rocks. The mineralogy of the alteration zones suggests that these fluids were rich in iron, potassium, sulphur, water, carbon dioxide, boron, fluorine and probably sodium. The alteration assemblages, which contain up to 80% chlorite and biotite and relatively scarce carbonates, imply a high H2O:CO2 ratio. This is supported by the relative abundance of aqueous fluid inclusions compared to CO2 phases. The most likely ligands for gold transport in fluids of low temperature or pressure in equilibrium with pyrite or pyrrhotite, such as those at Peak Hill, are reduced sulphur complexes (Seward, 1973). Deposition has probably involved crystallisation of pyrite from the gold-bearing fluids causing destabilisation of the gold-bearing sulphide complexes and precipitation of gold. In the deposits within the Peak Hill Schist, gold may also have been precipitated as a result of the interaction of gold-bearing fluids with graphite. As primary gold-rich C-O-H fluids pass through graphitic wall rocks, the fluid-rock interactions may generate fluids which are rich in methane (Cox et al, 1991). As these methane-rich fluids pass out of the graphitic zone they may re-mix with, and destabilise, the primary fluid, causing gold precipitation. Lead isotope data (Thornett, 1995) indicate that galena and lead tellurides, which are intimately associated with gold mineralisation at Peak Hill and Mount Pleasant, have a source which is consistent with that of lead from the Marymia deposit. As the Marymia deposit is hosted by Archaean rocks, an Archaean basement source for at least part of the Peak Hill mineralising fluid is also implied. The age of mineralisation for Peak Hill is broadly synchronous with other Glengarry and Nabberu deposits at 1900 to 1800 Myr, although a precise age cannot be defined from the available data.
MINE GEOLOGICAL METHODS Grade control at Harmony uses RC drill holes on a 5 by 4 m staggered grid pattern on 5 m benches. Holes were initially drilled towards grid east, but as the controls on grade distribution became apparent the direction of the holes was changed to mine grid north (which is 30o west of AMG north). All holes are angled at 60ο and sampled with six 1 m samples per hole. All data are interpreted on stacked cross sections, the resultant ore zones wire framed and intersected horizontally to produce flitch plans from which the final interpretation of ore blocks is made. Statistical analysis of grade control data (P Kitto, unpublished data, 1996) demonstrated the need for a 17.5 g/t gold top cut to grade control samples as opposed to the top cut of 85 g/t which had been applied to exploration and development samples. Mining is carried out by contractors on 2.5 m flitches. Higher grade ore (>2 g/t gold) is taken to the Peak Hill mill for processing and lower grade ore ( <2 g/t gold) is stockpiled at Harmony.
Geology of Australian and Papua New Guinean Mineral Deposits
GOLD DEPOSITS OF THE PEAK HILL AREA
ACKNOWLEDGEMENTS
Maitland, A G, 1904. State aid to boring, Peak Hill, in Geological Survey of Western Australia, Annual Report 1904, pp 149–151.
This paper is published with the permission of Plutonic Resources Limited and North Limited. The authors gratefully acknowledge the contributions of past and present staff of North Limited (Geopeko), Afmeco Pty Ltd, Grants Patch Mining Limited, Plutonic Operations Limited, and of N J Archibald of Fractal Graphics, whose work has contributed to the geological knowledge of the deposits.
Montgomery, A, 1909. Report on the state of mining progress in certain centres in the Murchison and Peak Hill Goldfields, in Western Australia Department of Mines, Report of the State Mining Engineer, 1909, pp 63–81.
The efficiency of mining by Peak Hill Resources was recognised by two industry awards in 1996; ‘Gold Mine of the Year’ by the publishers of the Gold Mining Journal and ‘Outstanding Producer under 150 000 oz’ by the convenors of the Diggers and Dealers Forum.
Pirajno, F and Occhipinti, S, 1996. Bryah, WA - 1:100 000 geological series, Western Australia Geological Survey.
REFERENCES Cox, S F, Wall, V J, Etheridge, M A and Potter, T F, 1991. Deformation and metamorphic processes in the formation of mesothermal veinhosted gold deposits - examples from the Lachlan Fold Belt in central Victoria, Australia, Ore Geology Reviews, 6:391–423. Gee, R D, 1987. Peak Hill, Western Australia - 1:250 000 geological series, 2nd edn, Geological Survey of Western Australia Explanatory Notes, SG 50–8. Gee, R D, 1990. Nabberu Basin, in Geology and Mineral Resources of Western Australia, Geological Survey of Western Australia, Memoir 3, pp 202–210. Gray, D J, 1995. Hydrogeochemical dispersion of gold and other elements at Baxter, Western Australia, CSIRO Exploration and Mining Report 169R, CSIRO-AMIRA Project 409 (unpublished).
Pirajno, F, Bagas, L, Swager, C P, Occhipinti, S and Adamides, N G, 1996. A reappraisal of the stratigraphy of the Glengarry Basin, Western Australia, Western Australia Geological Survey, Annual Review 1995–96, pp 81–87.
Pirajno, F and Occhipinti, S, in press. Bryah, Western Australia - 1:100 000 geological series, Western Australia Geological Survey Explanatory Notes 2646. Robertson, I D M, Phang, C and Munday, T J, 1996. The regolith, geology and geochemistry of the area around the Harmony gold deposit (Baxter Mining Centre), Peak Hill, Western Australia, Cooperative Research Centre for Landscape Evolution and Mineral Exploration, restricted report 5R (unpublished). Seward, T M, 1973. Thio complexes of gold and the transport of gold in hydrothermal ore solutions, Geochimica et Cosmochimica Acta, 37:379–399. Thornett, S E, 1995. The nature and timing of gold mineralisation in the Proterozoic rocks of the Peak Hill District, MSc thesis (unpublished), The University of Western Australia, Perth. Windh, J, 1992. Tectonic evolution and metallogenesis of the Early Proterozoic Glengarry Basin, Western Australia, PhD thesis (unpublished), The University of Western Australia, Perth.
MacLeod, W N, 1970. Peak Hill, Western Australia - 1:250 000 geological series, Geological Survey of Western Australia Explanatory Notes, SG 50–80.
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Byass, A P and Maclean, D R, 1998. Nimary gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 89–96 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Nimary gold deposits 1
by A P Byass and D R Maclean
1
INTRODUCTION
EXPLORATION AND MINING HISTORY
The deposits are 45 km NE of the historic Wiluna gold mines and adjoin the Jundee gold deposit operated by Great Central Mines NL (Phillips, Vearncombe and Murphy, this publication). Nimary is at AMG coordinates 258 000 m E, 7 078 000 m N and lat 26o22′S, long 120o22′E on the Wiluna (SG 51–9) 1:250 000 scale map sheet (Fig 1).
There are no historic workings in the Nimary area, however the St Ives, Empire and Gourdis deposits about 35 km south of Nimary–Jundee have yielded 300 000 oz of gold (Davis and Farrelly, 1993) through sporadic mining by several companies, including Asarco Australia Ltd and Wiluna Gold Ltd in the early 1990s. The area has been explored for base metals and gold by a number of companies and individuals since the late 1970s. Regional exploration using laterite and lag sampling was conducted by Hunter Resources NL in the Nimary area in 1990 (Lewington, 1995). Anomalous arsenic and gold values were found over much of the Nimary area. Anomalies were followed up with 343 rotary air blast (RAB), 21 reverse circulation (RC) and 4 diamond drill holes. Positive results were obtained from drilling and an aeromagnetic survey was flown over the Nimary–Jundee area, although it should be noted that interpretation of aeromagnetic survey data was not used as a targeting tool for the discovery of the Nimary deposit. Continued drilling delineated four separate targets (now individual open cuts) by 1991, and a resource of 290 000 t at 3.9 g/t was estimated. Through 1992 and into early 1993 RAB and RC drilling continued. Eagle became involved with the tenement from 1993 in a joint venture agreement with Matlock Mining NL and Hunter Resources NL. With investment by Eagle, increased drilling extended the resource, and the Nimary deposits entered the production feasibility stage in late 1994. Production commenced in late 1995 with the first gold pour at Christmas 1995. Eagle obtained full ownership of the Nimary deposits in 1995 by buying out the other joint venture partners.
REGIONAL GEOLOGY FIG 1 - Geological sketch map of the Yilgarn Craton, showing the Norseman–Wiluna greenstone belt and location map for the Nimary deposits, adapted from Hagemann, Gebre-Mariam and Groves, 1994.
Nimary is owned and operated by Eagle Mining Corporation NL (Eagle). Production is by open cut mining in several pits, with open ended deeper mineralisation holding potential for increased resources and future underground operations. Ore is treated by gravity and CIL methods. At the end of March 1997, Nimary had produced 150 454 oz of gold from 967 200 t of ore at a head grade 5.33 g/t, at an average recovery rate to bullion of 91%. The remaining Indicated and Measured Resource is 6 593 000 t at 4.71 g/t for 998 380 oz of contained gold. All orientations and directions in this paper are in reference to local mine grid, in which grid north is 38° to the west of AMG north. 1.
Development Geologist, Eagle Mining Corporation NL, 1 Sleat Road, Applecross WA 6153.
Geology of Australian and Papua New Guinean Mineral Deposits
The Nimary deposits are within the Lake Violet greenstone belt, in the Archaean Yilgarn Craton. The Yandal belt divides into the Lake Violet and Millrose greenstone belts before being overlain to the north by Proterzoic sediments of the Nabberu Basin. The Yandal greenstone belt can be subdivided into a westerly assemblage of basalt and ultramafic rocks and an easterly assemblage containing mafic and intermediate igneous rocks (Barley et al, 1989). The westerly association contains tholeiitic and high-magnesium basalt, komatiite and minor rhyolite and sulphidic shale. The basalt is commonly pillowed, interlayered with sulphidic shale and intruded by synmagmatic sills. Overlying intermediate to silicic pyroclastic and associated sedimentary rocks are characteristic of a submarine, extensional tectonic setting (Barley and Groves, 1990). The mafic and intermediate volcanic rocks of the easterly association are typical of a volcanic back arc basin or terrestrial setting (Hallberg, 1985). The Lake Violet greenstone belt, which is part of the westerly submarine association of the Yandal belt, contains very low grade metamorphic rocks that have undergone low-strain ‘static’ metamorphism in the interior of the greenstone
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domains. It is bounded to the east by the Number Five shear and the west by the Moilers shear. Laterally extensive thrust faults trend NNW through the greenstone belt. There is evidence of the several deformation events assigned to the Yilgarn Craton by Barley and Groves (1990) and Hammond and Nisbett (1992).
ORE DEPOSIT FEATURES STRATIGRAPHY There is little outcrop in the Nimary area due to deep weathering and cover by transported material. Intense weathering has produced a regolith that averages 60 m and can extend up to 100 m depth. Drilling and limited subcrop mapping allow the Nimary area to be divided into westerly felsic- and easterly mafic-dominated sequences (Fig 2), with the eastern, mafic-dominated package the host to mineralisation. The eastern package comprises Archaean basalt, dolerite and gabbro with interflow sulphide-rich chert and shale, intrusive felsic porphyry and extrusive felsic volcanic rocks. Rare polymicitic breccias containing felsic and mafic igneous rocks are conformable with the strata. Postmineralisation mafic and intermediate porphyry and postcratonisation Proterzoic dolerite dykes crosscut all rock boundaries and the gold mineralisation. The principal rock types at Nimary are pillowed and tholeiitic basalt and dolerite. The sequence strikes broadly NE youngs and dips westward at 50–70o (Fig 3). Tholeiitic basalt has massive to pillowed, variolitic and ocellar textures that grade conformably into massive tholeiitic dolerite, indicating a layered sill or basalt flow relationship. Gradational contacts between basalt and dolerite and rare gabbroic units are shown in deep drill core from the Nim 3 deposit (Figs 2 and 3). Rock types also change in a gradational manner from basalt- or dolerite-dominated in the west to predominantly dolerite or gabbro in the east. The mafic rock composition also changes from tholeiitic to higher magnesium towards the Jundee deposit. Intrusive felsic porphyritic rocks are abundant in the Nim 3 and Nim 4 areas, but they are less volumetrically important at Nim1–2. Felsic volcanic rocks and interbedded black shale units are present in the western margins of the mafic-rich host package at Nim 4. There is a local pervasive cleavage in basalt and dolerite around the Nim 1–2 orebody, otherwise all rock types are unstrained and retain primary igneous textures outside fault zones. The present mineralogy includes relict primary minerals and those attributable to very low grade, prehnite-pumpellyite to lower greenschist facies regional metamorphism (Binns, Gunthorpe and Groves, 1976) and subsequent hydrothermal alteration. All rock types except post-mineralisation mafic porphyry and post-cratonisation dolerite dykes have been metamorphosed and the prefix ‘meta’ is dispensed with for the purposes of this paper.
STRUCTURE The regional and mine scale felsic-mafic package contact at Nimary (Fig 2) cuts through the western part of the Nim 4 deposit and may be the local expression of the Pope Well fault. Limited exposures of foliation within the heavily weathered sections do not indicate if this contact is the local representation of this laterally extensive regional feature. Aeromagnetic imagery does not highlight the contact of rock packages as defined by drill chip and suboutcrop mapping.
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FIG 2 - Geological map of environs of Nimary orebodies.
The host rock package at Nimary has undergone weak deformation resulting in limited folding, and the development of foliation outside fault zones in the Nim 1–2 area. Detailed
Geology of Australian and Papua New Guinean Mineral Deposits
NIMARY GOLD DEPOSITS
FIG 3 - Schematic cross section on A-B, through Nim 3 and Nim 4 orebodies. Location on Fig 2.
understanding of the relationships between controlling structures within separate orebodies is limited, and hampered by the relatively shallow depth of open cut operations and the distance between orebodies. The local structural features of the Nimary orebodies are discussed separately below, although discussion is limited to those deposits which are being mined. Structural features of note within diamond drill core and open pit mining operations are also listed. The strike of mineralisation in all orebodies appears to swing from the broad easterly strike and subvertical dip of mineralising structures in Nim 4 to a NE strike in Nim 3 and further to a north to NNW trend in Nim 1–2. Understanding of the relationships between the dominant structures that host individual lodes is incomplete.
INDIVIDUAL DEPOSIT FEATURES Nim 1–2 deposit The Proved and Probable Ore Reserve in March 1997 was 372 000 t at 7.66 g/t for 91 610 oz of contained gold. Mineralisation at Nim 1 and 2 is hosted by tholeiitic, pillowed basalt and massive tholeiitic dolerite. The Nim 1–2 deposit contains premineralisation felsic porphyry and post-mineralisation mafic porphyry dykes. The orebodies are divided by two parallel WNW-striking Proterozoic dolerite dykes (Fig 2). The main lode in Nim 1 dips at 65–80° to the SW and strikes at 310° (Fig 4A, B). Dilational veins within the ore zone dip steeply towards 210° with subsidiary breccia veins dipping at moderate angles towards 310°. Secondary mineralised structures strike WNW and dip vertically parallel to the crosscutting Proterozoic dolerite dykes. Mineralised fault zones crosscut the steeply NW to NNW dipping and NE-striking, pervasive foliation in Nim 1. Mineralisation within Nim 2 is hosted by a northstriking, steeply westerly dipping shear zone with secondary structures dipping vertically and striking WNW, parallel to the trend of the Proterozoic dolerite dykes.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 4 - Equal area, southern hemisphere stereonet projections (left) and rose diagrams (right) of structural data from Nim 1–2 and Nim 3 deposits. A. Orientations of orebodies and NW striking Proterozoic dolerite dykes. B. Brecciated, dilational, mineralised veins within Nim 1 orebodies. C. Colloform and crustiform veins in Nim 3. D. Colloform and crustiform veins in Nim 3. E. Planar caarbonate veins associate with regional fracture trends in Nim 3. F. Brecciated, mineralised veins in Nim 3.
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Nim 3 deposit The Proved and Probable Ore Reserve in March 1997 was 2 787 000 t at 6.36 g/t for 569 880 oz of contained gold. Mineralisation is hosted by basalt and dolerite and often spatially associated with weakly mineralised, intrusive felsic porphyry. The mineralised porphyry is invariably in contact with mineralised mafic rocks, with the bulk of mineralisation within the mafic rocks. Common quartz-carbonate colloform banding structures up to 2 m thick are present within all rock types, and consistently dip between 50 and 70o to the west or WSW (Fig 4C, D). These colloforn structures often host low grade (<1.0 g/t gold) mineralisation. The main trend of the mineralisation is within laterally discontinuous NE-striking and moderately NW dipping fault zones (Fig 4E, F), and subparallel to westerly dipping, NNE- to north-striking felsic porphyries. Foliation within Nim 3 is not well developed.
Nim 4 deposit The Proved and Probable Ore Reserve in March 1997 was 1 424 000 t at 3.69 g/t for 168 940 oz of contained gold. Mineralisation is hosted by basalt or dolerite and felsic porphyry. A deep weathering profile has developed an intense supergene dispersion that mimics a deeper, primary, steep to subvertical, southerly dipping, east striking structural feature. In the western part of Nim 4, felsic-rich sedimentary rocks of the westerly sequence host supergene mineralisation.
WEATHERING AND METAMORPHIC ALTERATION Weathering is strongly developed and there is also a thin (<5 m) layer of transported overburden above the Nim 3 and 4 deposits. Small ferruginised chert ridges outcrop to the north and west of Nim 1–2 within 500 m of the pit. The deep oxide weathering extends to a maximum 80 m depth. Metamorphism within the Nimary deposits is prehnitepumpellyite to lower greenschist facies. Metamorphic assemblages in unaltered wall rock consist of chloritised amphibole and pyroxene, with minor carbonatisation and calcite alteration. The saussuritisation of plagioclase cores and the pervasive hematite alteration of basalt in ore and unmineralised rock at Nim 1–2 provide evidence for minor sea-floor style alteration by meteroic water. Similar changes have been documented at Wiluna and a sea-floor alteration origin has been postulated (Hagemann, 1992; Hagemann, Gebre-Mariam and Groves, 1994). Further research is being conducted, based on fluid inclusion and isotope geochemistry, to more fully understand the crustal level of mineralisation. Hydrothermal alteration and mineralisation can be mineralogically and spatially divided into four styles. The division is based upon the presence or absence of index minerals or mineral phases, bleaching of host rock, level of igneous texture preservation or replacement, and vein morphology or composition. The four styles are: 1.
Regional background, regional metamorphism with prehnite-pumpellyite facies mineral assemblages.
2.
Distal: calcite-chlorite±quartz and/or pyrite (nonmineralised).
3.
Intermediate: limited hydrothermal bleaching and minor mineralisation.
4.
Proximal: intense hydrothermal alteration, associated with structural deformation and gold mineralisation.
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MINERALISATION AND ASSOCIATED ALTERATION Gold mineralisation at Nimary is hosted by fault breccias, hydrobreccias in dilation zones, fracture-fill veins and shear zones that are enveloped by zoned alteration haloes. Alteration haloes overprint the regional metamorphic mineral assemblages. There is a direct control of alteration zoning by structure. Gold mineralisation is associated with a specific phase of gold-silica-carbonate-sulphide rich veins that is found within distinctive and highly constricted proximal alteration haloes. Proximal alteration is characterised by a lack of dominant vein orientation in the intensely brecciated implosion style hydrobreccias (Fig 5A) that host the main lodes at Nim 1–2, 3 and 4. Proximal alteration in mafic rocks is typically found in fault breccias. The transition from low grade (distal or intermediate) alteration to proximal alteration mineralogy is locally sharp, often over <10 cm, with a dramatic increase in the frequency of fractures, onset of intense brecciation, microfracturing and extensive silica-carbonate±albite replacement of primary minerals, producing an intense bleaching of the host rock to a pale yellow-green-grey. Alteration minerals within ore zones in decreasing order of abundance are carbonate (dolomite, ankerite and minor calcite), quartz, sericite±fuchsite, albite, chlorite, arsenopyrite, pyrite±rutile and hematite. Vein matrices mainly comprise quartz-carbonate (dolomite and ankerite)-arsenopyrite-pyrite-sericite±albite±chlorite. Wall rock fragments in breccias are extensively sulphidised (arsenopyrite-pyrite and carbonate altered). The presence of arsenopyrite and albite, and increased silica, sericite (±fuchsite) levels are indicative of proximal alteration and gold mineralisation (Fig 6). The intermediate style is a transition from distal to proximal style alteration. Intermediate alteration is characterised by an increase in fracturing, vein formation and bleaching, and the replacement of primary igneous textures. Typical alteration minerals in decreasing order of abundance are ankerite (replacing calcite), dolomite, muscovite (sericite), chlorite, quartz, albite, pyrite and arsenopyrite. Pyrite is the dominant sulphide and arsenopyrite is rare. Distal alteration of basalt or dolerite is defined by pervasive chlorite-carbonate alteration that produces a dark to mid green colour in mafic rock. Alteration minerals present, in decreasing order of abundance, are calcite, ankerite, chlorite, quartz and muscovite (sericite), ±pyrite, rutile and chalcopyrite. Calcite and ankerite are present in both wall rock and veins. Quartz, carbonate and sericite form after plagioclase, chlorite replaces amphibole, and rutile (±leucoxene) partially replaces ilmenite and magnetite. Pyrite is common as euhedral, framboidal clusters, with rare chalcopyrite in wall rock, or within carbonate±quartz veins (Fig 6).
ORE GENESIS Gold mineralisation at Nimary is a late stage tectonic and alteration event. Structures hosting gold mineralisation are undeformed and crosscut the earlier foliation at Nim 1–2. There are also only minor post-mineralisation veins and fractures in the mineralised zones (late thin chlorite±carbonate±hematite fracture veins) that are strongly associated with Nim 1–2 mineralisation. The mechanism of gold mineralisation is likely to be a combination of pressure
Geology of Australian and Papua New Guinean Mineral Deposits
NIMARY GOLD DEPOSITS
FIG 5a - Intensely brecciated, silica-carbonate flooded, mineralised bleached basalt. Angular clasts of carbonate and sulphidised wall rock are in a matrix supported quartz-ankerite-dolomite-albite dominated vein matrix. Nim 1 main lode.
FIG 6 - Idealised sequence of zoning, from least altered rock wall through increasing metasomatism in basalt, from Byass (1996). Line thickness signifies relative abundance of mineral. Dashed lines indicate that a mineral may or may not be present. FIG 5b - Quartz dominated, quartz-carbonate crustiform veins, a common low pressure feature at Nim 3.
FIG 5c - Brecciated, carbonate- and silica-rich, quartz-carbonate colloform veins, mineralised breccia from Nim 3.
spatially associated with silica-rich felsic rocks that have a more plastic deformation style are examples of this. The textures of mineralised breccias suggest a high fluid pressure that has shattered wall rock and precipitated gold rapidly into vein matrices and within altered wall rock clasts. There is no dominant orientation for mineralised veins within breccia zones indicating that σ1 was similar to σ3. This is reflected in the lack of localised deformation in the host rock package. The low pressure, quartz-carbonate, colloform, crustiform vein structures (Vearncombe, 1993) at Nim 3 (Fig 5B) are premineralisation and have a dominant ~60o dip towards 260 o. The main mineralising event at Nim 3 formed at ~80o dip towards 320o and brecciated the quartz-carbonate colloform structures where the two structures coincided (Fig 5C). Mineralisation often follows earlier open, colloform structures over tens of metres as crack-seal vein structures. The paragenetic sequence for vein types at Nimary is:
FIG 5 - Photographs of half diamond drill core illustrating ore and vein textures within Nim 1–2 and Nim 3. Drill core is NQ2 and the vertical height of the core is 47.6 mm from base to top.
reduction, a linked drop in the temperature of the mineralising fluid and favourable iron:magnesium ratios in basalt and dolerite. Pressure reduction is a result of small lensoid dilation zones within complex conjugate fractures as in the Nim 1–2 basalt and dolerite and preferential fracturing within brittle rocks at rock unit contacts. Rheology differences, such as the brittle fracturing and increased permeability within mafic rocks
Geology of Australian and Papua New Guinean Mineral Deposits
1.
Chlorite-carbonate veins, linked to regional metamorphism and deformation. Examples are the foliation-parallel chlorite and buck quartz veins within Nim 1–2.
2.
Carbonate±chlorite±quartz±pyrite distal type alteration veins.
3.
Carbonate-quartz colloform or crustiform veins. These possible arise from an earlier pulse of fluid with lower P–T conditions than the mineralising fluid. They have different vein orientations from the main gold-bearing veins, though they can host low grade mineralisation grade (<1.0 g/t).
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4.
Auriferous quartz-carbonate-pyrite-arsenopyrite± sericite ±fuchsite±hematite±albite veins and breccia matrix. These overprint and brecciate the earlier quartzcarbonate colloform structures at Nim 3, and have an identical mineralogy and vein morphology at Nim 1–2, Nim 3 and Nim 4.
5.
Chlorite±hematite±carbonate veins with aligned orientation, mostly noted at Nim 1–2. They are possibly related to the post-mineralisation mafic porphyry or Proterozoic dolerite and D4 deformation.
The Jundee gold deposit is directly adjacent to the Nimary deposits but does not have the same paragenesis and structural control of ore deposition seen at Nimary. This probably reflects differing lithostratigraphy as the sequence deepens towards the east, changing from the Nimary host sequence to more massive dolerite and gabbro. The well documented Wiluna gold deposits (Hagemann, 1992; Rawlinson, 1994; Chanter et al, this publication) 40 km south of Nimary share many features of ore genesis, alteration and host rocks with the Nimary deposits (Byass, 1996). There is a strong association between intrusive felsic porphyry contacts at Nim 3 and to a lesser extent Nim 4 and the mafic rocks that host mineralisation. Both the felsic and mafic rocks may be mineralised, with the majority of the mineralisation being hosted by basalt and dolerite (Figs 2 and 3). The spatial association between felsic porphyry and basalt or dolerite is more pronounced at Nim 3 and 4 due to the proportionately larger volumes of felsic igneous rock there. The spatial association of felsic porphyry with mineralisation may be the result of mineralising fluids exploiting pre-existing weaknesses in the strata that were the channels of intrusion for the felsic porphyry. A second theory incorporates the rheology contrast and the high iron:magnesium ratios of the mafic rocks, compared with felsic porphyry. Deformation due to uneven strain rates is known to allow higher permeability in more easily fractured and brittle basalt when compared with the more plastic deformation in more silicic felsic rock types (Eisenlohr, Groves and Partington, 1989; Sibson, 1991). The higher permeability and fluid flow through the mafic rock will also expose more of the auriferous fluid to high iron levels and thus aid in the deposition of gold. There is a common association of arsenopyrite, pyrite and to a lesser extent hematite with gold in the ore zones (Byass, 1996). This is seen in phase separation type deposits, that are the result of a very high fluid pressure and low confining pressure system that can fracture giving a rapid drop in P–T conditions and precipitation of gold from solution (Vearncombe, 1993). Strongly oxidised, post-mineralisation chlorite±carbonate ±hematite veins are concentrated in the Nim 1–2 lodes and may be derived from intrusion of Proterozoic dolerite dykes and late Archaean mafic porphyry dykes. The Archaean mafic, porphyry dykes that post-date mineralisation have been dated at 2660±6 Myr.
GEOLOGICAL AND MINING METHODS Resource development RC and diamond drilling is on a 25 by 25 m pattern. Geostatistically derived 3D block models of the four deposit areas form the basis for the present pit designs and mine planning. Mineralisation is constrained according to geological features and gold grade trends.
94
Mining and blasting are carried out by a contractor. Ore and waste are mined by bottom loading excavators on 2 m flitches for oxide material and 2.5 m flitches for fresh rock. Grade control is by campaigns of 15 or 18 m deep, inclined RC holes to cover benches 12 or 15 m high, on a 5 by 4 m pattern. Samples are assayed by aqua regia-AAS for gold only. Threedimensional block models are constructed using the MED system and grades are interpolated using ordinary kriging. Grade control plans are digitised using block model grades, plan overlays of composite grade control holes and bench projections of cross sectional outlines. Composite detailed pit floor geology mapping is carried out during every campaign. Ore is mined as high grade (>3 g/t) and medium grade (>0.8 g/t). Mining is supervised by a grade control geologist or pit technician. Medium grade ore (comprising 0.83 Mt at 1.4 g/t) is stockpiled, as is mineralised waste of grade 0.3–0.8 g/t. Prior to the mining of transition and fresh ore, which are blended for milling, campaign milling by grade control benches was practisd for reconciliation purposes. Early reconciliations tended to underestimate gold content, which was ascribed to the presence of very high grade, constricted, discontinuous structures and high variance nugget values for supergene ore. The gravity component of gold production has averaged 21% since the start of production.
ACKNOWLEDGEMENTS The authors gratefully acknowledge Eagle Mining Corporation NL for permission to publish this information. Special thanks should also be given to C Schauss, G Pigott, A Timmins, A Hawkins and S Hyland and the technical staff and management of the Development and Regional Exploration departments for the input and advice. A special note of thanks to H Dixon for her photographic work and Perth-based drafting staff for their patience and effort in the preparation of this paper. Thanks also to C Yeats from the University of Western Australia for the zircon age dating.
REFERENCES Barley, M E, Eisenlohr, B N, Groves, D I, Perring, C S and Vearncombe, J R, 1989. Late Archaean convergent margin tectonics and gold mineralisation: a new look at the NorsemanWiluna Belt, Western Australia, Geology, 17:826–829. Barley, M E and Groves, D I, 1990. Deciphering the tectonic evolution of Archaean greenstone belts: the importance of contrasting histories to the distribution of mineralisation in the Yilgarn Craton, Western Australia, Precambrian Research, 46:3–20 Binns, R A, Gunthorpe, R J and Groves, D I, 1976. Metamorphic patterns and development of greenstone belts in the Eastern Yilgarn Block, Western Australia, in The Early History of the Earth (Ed: B F Windley), pp 303–313 (John Wiley and Sons: New York). Byass, A P, 1996. The nature of, and controls on, mineralisation at the Nimary lode-gold deposit, Northern Yandal Belt, Western Australia, BSc Honours thesis (unpublished), The University of Western Australia, Perth. Davis, R J and Farelly, C T, 1993. The geology and geological practices at the Wiluna mining operations, Western Australia, in Proceedings International Mining Geology Conference, pp 115–123 (The Australasian Institute of Mining and Metallurgy: Melbourne). Eisenlohr, B N, Groves, D I and Partington, G A, 1989. Crustal-scale shear zones and their significance to Archaean gold mineralisation in Western Australia, Mineralium Deposita, 24:1–8.
Geology of Australian and Papua New Guinean Mineral Deposits
NIMARY GOLD DEPOSITS
Hagemann, S G, 1992. The Wiluan lode-gold deposits, Western Australia: A case study of a high crustal level Archaean lode-gold system, PhD thesis (unpublished), The University of Western Australia, Perth. Hagemann, S G, Gebre-Mariam, M and Groves, D I, 1994. Surface water influx in shallow-level Archaean lode gold deposits in Western Australia, Geology, 22:1067–1070. Hallberg, J A, 1985. The Geology and Mineral Deposits of the Leonora–Laverton area, Northeastern Yilgarn Block, Western Australia (Hesperian Press: Perth). Hammond, R L and Nisbett, B W, 1992. Towards a structural and tectonic framework for the Central Norseman-Wiluna Greenstone Belt, Western Australia, in The Archaean: Terrains, Processes and Metallogeny, Publication 22, pp 39–49 (Geology Department, Key Centre and University Extension, The University of Western Australia: Perth).
Geology of Australian and Papua New Guinean Mineral Deposits
Lewington, G L D, 1995. The discovery of the Nimary gold camp, in New Generation Gold Mines: Case Histories of Discovery, pp 4.1–4.10 (Australian Mineral Foundation: Adelaide) Rawlinson, G, 1994. The nature and controls of gold mineralisation and related hydrothermal alteration within the Happy Jack South ore body Wiluna, Western Australia, BSc Honours thesis (unpublished), The University of Western Australia, Perth. Sibson, R H, 1991. Fault structure and mechanics in relation to greenstone gold deposits, in Greenstone Gold and Crustal Evolution, NUNA Conference Volume (Eds: F Robert, P A Sheahan and S B Green), pp 46–53 (Geological Association of Canada, Mineral Deposits Division: Ontario). Vearncombe, J R, 1993. Quartz vein morphology and implications for formation depth and classification of Archaean gold-vein deposits, Ore Geology Reviews, 8:407–424.
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Phillips, G N, Vearncombe, J R and Murphy, R, 1998. Jundee gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 97–104 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Jundee gold deposit 1
2
by G N Phillips , J R Vearncombe and R Murphy
3
INTRODUCTION The deposit is owned by Great Central Mines Limited (GCM), and is 600 km north of Kalgoorlie and 50 km NE of Wiluna, WA (Fig 1), at lat 26o23′S, long 120o36′E on the Wiluna (SG 51–9) 1:250 000 scale and the Millrose (3045) 1:100 000 scale map sheets. The Jundee and Nimary deposits are part of a single goldfield and have several similarities, but Nimary is described separately (Byass and Maclean, this publication). Total Measured, Indicated and Inferred Resources remaining at Jundee are 36 Mt at 2.4 g/t gold at a cutoff of 0.5 g/t at December 1996 for 86 t of contained gold (2.8 Moz). Production to December 1996 totals 6.8 t. These resources include Proved Reserves at December 1996 of 10 Mt of ore at 2.0 g/t and Probable Reserves of 2.5 Mt at 2.4 g/t, for a total of 26 t of contained gold. The total endowment at Jundee (production plus resources) is 93 t of gold (3.0 Moz).
EXPLORATION AND MINING HISTORY Jundee is a discovery in a greenstone belt with few historic workings, sparse outcrop, a truncated laterite profile, and several metres of transported alluvial cover (Eshuys and Lewis, 1995). It was discovered by surface geochemical sampling, rotary air blast (RAB) drilling, and extensive follow up drilling. The gold potential of the Yandal belt was recognised in the 1980s and this led to ground acquisition, surface work and drilling by GCM (Eshuys et al, 1995). The Jundee area was previously explored for base metals by Chevron Exploration Corporation in the 1980s, prior to the ground being granted to Mark Creasy in 1986 (Lewington, 1995; Wright and Herbison, 1995). The initial exploration program consisted of 1169 lag samples on lines 100 to 400 m apart. Follow up RAB drilling yielded four holes out of 83 with intervals containing more than 1 g/t gold, and one hole with 2 g/t. The Jundee tenement was joint ventured in December 1991 to GCM, who exercised an option to purchase in 1995. In October 1997, GCM took over Eagle Mining and control of Nimary. A program of 152 RAB holes commenced in February 1992, in which 17 holes contained 4 m composite samples of grade 1 g/t, and a best result of 4 m at 14.8 g/t. Reverse circulation (RC) and diamond drilling were used to follow up anomalous areas and a major exploration program continued under the 1.
General Manager, Great Central Mines Ltd, PO Box 3, Central Park Vic 3145.
2.
Consultant, Vearncombe & Associates Pty Ltd, 14a Barnett Street, Fremantle WA 6160.
3.
Chief Geologist, Jundee Gold Operations, Great Central Mines Limited, PO Box 1652, Subiaco WA 6904.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Location map and geological map of the Yandal greenstone belt based on an interpretation of gravity and aeromagnetic data, Landsat imagery, drill hole logging and surface mapping.
leadership of Ian Herbison until mining began in 1995. By the end of June 1996, there had been over 140 km of RAB, 600 km of RC and 23 km of diamond drilling at Jundee. Mining commenced at Jundee in August 1995, and the first gold was poured in December 1995. The 2 t of gold from Jundee is probably the highest first quarter production recorded by any Australian gold mine. Currently five open pits are being mined at Jundee with further pits planned, and some of these pits join with production areas of the adjacent Nimary mine.
97
G N PHILLIPS, J R VEARNCOMBE and R MURPHY
Current production rate at Jundee is 2.5 Mtpa at a grade of approximately 3 g/t. Jundee is presently one of Australia’s top ten gold producers based on production for the December 1996 quarter.
REGIONAL AND LOCAL GEOLOGY The Yandal greenstone belt in the NE part of the Yilgarn Block is up to 30 km wide, elongate NNW, with Archaean granitoids on the east and west flanks, and in some internal sections (Fig 1). In the northern part of the Yandal belt around Jundee, the oldest components are inferred to be banded iron formation (BIF) and ultramafic to mafic rocks (Fig 2). These form a recognisable, linear magnetic unit of 80 km strike length on the western margin of the belt, and are juxtaposed against mylonitic granitic rocks to the west. East of this is a mafic sequence dominated by tholeiitic and high-magnesium basalt and dolerite, with minor sediment. Overlying the mafic-rich package is an upper greenstone sequence dominated by felsic to intermediate volcanic rocks with minor chert, basalt and shale. The upper greenstone sequence contains a few discrete horizons of felsic volcanic composition, but the majority is more andesitic. Jundee is close to the eastern contact of the upper greenstone sequence with the ultramafic to mafic sequence (Figs 1 and 2).
facies with stable actinolite, chlorite, albite and quartz in mafic rocks compatible with the pattern suggested for this part of the Yilgarn Block by Binns, Gunthorpe and Groves (1976). Outcrop in the Jundee area is sparse, weathering is extensive, and much of the area is covered by up to 10 m (typically about 5 m) of transported material.
LITHOLOGY Three identifiable domains are important in the Jundee–Nimary goldfield (Fig 3), with two in the maficdominated greenstone package and the third being the felsicrich package. To the east of Jundee–Nimary, ultramafic rock and high-magnesium basalt contribute to a distinctive domain with strata-parallel highly magnetic layers. The 2 km wide, central domain includes the mineralised interval of predominantly tholeiitic basalt and dolerite, with lesser black shale and porphyritic dykes. The domain of variably foliated rocks of the upper greenstone sequence is west of Nimary. There are several ultramafic layers east of the Jundee–Nimary mineralisation, which are traceable by their aeromagnetic signature for at least 10 km in a NNW direction.
FIG 3 - Geological map of the Jundee–Nimary goldfield based on interpretation of detailed aeromagnetic data, extensive logging of RAB and RC chips, and mine mapping. Pit identifiers are: CR, Cook-Read; NW, Northwest; Mz, Menzies; Cn, Curtin; D, Deakin; B, Barton; F, Fisher; M, Main; L, Lyons; 1-2-3-4, NIM 1 to 4 pits.
FIG 2 - Geological map of the north Yandal greenstone belt based on relogging of several thousand RAB holes, interpretation of gravity and aeromagnetic data, and limited surface mapping. Moilers is a prospect of GCM; Two Hills and Gourdis are a prospect and a former mine of Wiluna Mines Ltd.
With the exception of some dykes, all components of the greenstone belt are regionally metamorphosed. The metamorphic grade in the Jundee area is lower greenschist
98
High magnesium basalt occurs between the ultramafic horizons and is dominated by felted masses of actinolite with some chlorite, opaque minerals and virtually no plagioclase. Pseudomorphs after pyroxene and olivine are recognisable in some samples. Tholeiitic basalt dominates the Jundee–Nimary goldfield. It is fine grained, commonly carbonate veined, and has well-preserved pillow structures. Plagioclase laths, interstitial actinolite and chlorite, minor quartz and opaque phases dominate the mineralogy. The analysed samples (Table 1, samples 1 and 2) have a strong tholeiitic affinity, and variable chromium content. Secondary minerals include epidote, titanite, carbonates, pumpelleyite and zeolites.
Geology of Australian and Papua New Guinean Mineral Deposits
JUNDEE GOLD DEPOSIT
TABLE 1 Chemical compositions of representative rock types from Jundee. 1 Lithology
2
Basalt Basalt
3
4
5
6
7
8
9
10
11
Dolerite
Dolerite
Dolerite
Dolerite
Dolerite
Dolerite
Dolerite
Feldsparquartz porphyry
Lamprophyre
50.3
SiO2
48.9
51.2
42.2
40.2
47.3
47.6
46.8
49.9
51.9
66.6
TiO2
0.7
1.2
0.4
0.4
1.0
0.7
2.0
1.3
1.6
0.3
0.8
Al2O3
13.7
13.6
7.1
8.0
14.3
13.5
13.2
13.5
12.8
14.4
12.1
FeO
9.1
12.6
12.3
10.9
11.4
9.3
13.6
16.3
14.1
2.8
6.3
MnO
0.2
0.2
0.2
0.2
0.2
0.1
0.2
0.3
0.2
0.1
0.1
MgO
4.4
4.9
22.8
18.0
9.1
6.8
5.9
5.5
3.1
1.3
7.0
CaO
10.9
8.6
4.2
7.7
8.8
9.3
6.7
0.9
5.7
4.0
8.9
Na2O
2.7
2.6
0.0
0.0
2.2
1.9
2.4
1.7
2.7
4.1
3.6
K2O
0.0
0.6
0.0
0.0
0.6
0.1
0.9
0.2
0.5
1.5
1.3
P2O5
0.1
0.1
0.0
0.0
0.0
0.0
0.1
0.2
0.2
0.1
0.9
S
0.1
0.0
0.0
0.
0.4
0.1
0.8
0.0
0.1
0.1
0.1
LOI
9.4
4.6
10.6
13.5
4.7
10.9
6.8
9.8
6.7
4.4
8.3
Total
100.1
100.4
99.8
99.1
100.0
100.4
99.5
99.6
99.5
99.8
99.6
Rb
1
19
1
3
18
4
18
3
8
40
24
Cs
0
1
0
1
2
1
1
0
1
2
2
Sr
136
253
57
70
27
139
164
79
172
260
838
Ba
68
143
33
18
218
58
471
61
197
289
1375
Cr
342
53
2881
1972
413
51
10
39
0
12
330
Ni
100
40
901
668
166
77
56
44
10
10
229
V
232
268
150
171
207
252
703
357
215
27
108
Cu
97
116
145
63
74
163
153
168
90
12
39
Zn
65
110
116
95
143
83
110
133
154
33
92 14
Pb
1
3
1
3
3
2
2
2
3
3
As
59
6
13
74
9
22
73
63
6
8
58
Zr
50
96
24
29
70
44
87
114
153
127
268
Nb
3
0
0
3
4
0
3
5
7
0
9
Th
0
1
0
1
1
0
1
1
2
2
30
La
2.9
7.6
2.6
2.8
2.6
3.1
5.5
8.5
11.5
14.1
162.0
Ce
6.8
17.7
5.8
6.4
5.4
7.4
13.3
19.5
27.8
25.6
330.0
Nd
5.3
11.7
3.6
4.4
4.7
5.6
9.7
12.8
18.4
9.8
158.0
Sm
1.6
3.2
0.9
1.3
1.7
1.5
2.9
3.5
4.8
1.6
25.7
Eu
0.6
1.1
0.3
0.5
0.8
0.7
1.1
1.3
1.7
0.5
6.0 16.5
Gd
2.2
4.0
1.0
1.5
2.6
2.1
4.0
4.1
5.9
1.2
Er
1.6
3.0
0.8
1.0
2.3
1.5
2.8
3.0
4.3
0.4
3.0
Tb
0.4
0.7
0.2
0.3
0.5
0.4
0.7
0.7
1.1
0.2
1.9
Yb
1.5
2.8
0.8
1.0
2.2
1.5
2.7
3.0
4.1
0.3
2.2
Lu
0.2
0.4
0.1
0.2
.4
0.2
0.4
0.5
0.6
0.0
0.3
Y
14
25
7
9
2
13
25
24
63
4
34
MgO/(MgO+FeO) atomic
46
41
77
75
59
56
43
38
28
45
66
Sample locations: Main pit - 2, 3, 4, 6, 8, 9, Barton pit - 5, 7, 11, NW pit - 1, Deakin pit - 10. Analytical methods: XRF - Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, Ba, Cr, V, Zr, Nb; Th; LECO - S; ICPMS - Rb, Cs, Sr, Ni, Cu, Zn, Pb, As, rare earth elements.
Sedimentary rocks in the Jundee–Nimary goldfield include black shale, sandstone and pyritic coarse grained sandstone. They contain original sedimentary features including graded bedding, as well as secondary replacement features such as pyrite after discrete granules. A thick shale unit has been the preferential site of dolerite emplacement into the sequence (Figs 3 and 4).
Geology of Australian and Papua New Guinean Mineral Deposits
Dolerite, as sill-like bodies several hundred metres thick and a few kilometres long, was intruded near the stratigraphic level of a major shale such that there is now sedimentary rock to the west, east, and internal to dolerite. The dolerite sills are differentiated and include cumulate, ophitic, intergranular and granophyric sections with highly variable proportions of original olivine, pyroxene, plagioclase, titanomagnetite and
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G N PHILLIPS, J R VEARNCOMBE and R MURPHY
FIG 4 - Geological sketch maps of some current open pits in the Jundee–Nimary goldfield showing main rock types, structural features and location of mineralisation. The pit outlines are approximate and change with time.
100
Geology of Australian and Papua New Guinean Mineral Deposits
JUNDEE GOLD DEPOSIT
quartz, representing a spectrum from peridotite and pyroxenite to plagioclase-quartz granophyre. Parts of some sills are moderately magnetic. Alteration products of the dolerite include chlorite, carbonate, quartz, leucoxene, titanite, white mica and pyrite. Coarse grained rocks of ultramafic composition are interpreted to be part of the dolerite sill sequence. These rocks have well-preserved igneous textures throughout much of the area between the local high strain zones, and contain olivine orthocumulates with pyroxene. Olivine is replaced by serpentine and talc, and pyroxene is replaced by chlorite, tremolite and opaque minerals. Porphyry dykes with variable proportions of feldspar phenocrysts and quartz are widespread throughout the goldfield, and include bodies hundreds of metres thick. Feldspar-rich dykes are important hosts to gold mineralisation in the Deakin pit, and have ‘magnesium numbers’ [ie MgO/(MgO+FeO) calculated on an atomic basis] similar to the tholeiitic mafic rocks, although with lower total iron and magnesium contents (Table 1, sample 10). Dykes with abundant quartz appear to be unmineralised. Other dykes of intermediate composition have feldspar phenocrysts, a chlorite rich matrix and common hematite alteration; these are typically crosscutting, unfoliated and unmineralised. Dykes with inferred lamprophyric affinity appear to be relatively widespread and of varied composition (Table 1, sample 11). Unfoliated mafic dykes to tens of metres thick crosscut the sequence, and are inferred to be Proterozoic in age based on their lack of foliation, preservation of primary pyroxene, and similarity to known Proterozoic dykes further south in the Yilgarn Block. These are broadly NE- and east-striking with steep dips, and are continuous for many hundreds of metres.
PETROLOGY The textural and mineralogical variation within the dolerites is reflected in their bulk rock composition. Preliminary whole rock chemical analyses of basalt and dolerite from Jundee confirm their tholeiitic nature, and illustrate the strong differentiation trend within the dolerite (Table 1, samples 3 to 9, Fig 5), potentially representing several sills. Increasing differentiation in this suite is reflected by an increasing concentration of silica, titanium, iron, zirconium, niobium, yttrium and rare earth elements (REE), and decreasing values for magnesium, calcium, chromium, nickel and magnesium number. Vanadium is strongly enriched in sample 7, and dolerite and basalt have considerable variation in chromium and nickel content. On a chondrite normalised REE plot, there is a tenfold variation of REE contents in the dolerite, reflecting differentiation (Fig 5). With progressive differentiation there is an increase in REE concentration, and a change from a flat REE pattern to one of enrichment in light REE. The REE values for basalt are within the range of values for the dolerite. There is considerable similarity between the compositional trend in the Jundee dolerites and that in the Triassic Palisades Sill of New Jersey, described as a classic differentiated tholeiitic sill (Walker, 1940). The Palisades Sill also shows a tenfold range of REE concentrations and light REE enrichment with differentiation. The Palisades dataset does not extend to the low REE concentrations and flat patterns seen in the data for the least differentiated Jundee dolerites.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 5 - Preliminary geochemical diagrams of tholeiitic basalt and dolerite from Jundee. Top - chondrite normalised rare earth element diagram (Boynton, 1984) showing the compositional range of basalt and dolerite from Jundee. For comparative purposes, the compositional range of the Golden Mile Dolerite at Kalgoorlie and Triassic Palisades Sill of New Jersey are also shown as these are well documented examples of differentiated tholeiitic sills. Bottom - MgO vs Zr Harker-type diagram showing the systematic element variations with increasing degree of differentiation (ie decreasing MgO).
The Golden Mile Dolerite at Kalgoorlie is also chemically similar to Jundee dolerites, particularly regarding the flat REE patterns of the least differentiated dolerite and the overall range of REE concentrations (Phillips and Gibb, 1993). The Golden Mile Dolerite is more iron rich than the Jundee dolerites, has a higher overall REE content, and does not show any light REE enrichment trend with differentiation.
STRUCTURE The Moilers shear zone on the western margin of the greenstone belt, a syncline in the upper mostly-felsic sequence and a heterogeneously-developed schistosity at Jundee were all formed during the D1 ductile deformation, and trend subparallel to the strike of the belt at about 340o. Symmetrical mylonite fabrics in the Moilers shear zone and chocolate-tablet boudinage structures in the Moilers BIF suggest flattening strains associated with D1. Lineations are generally subhorizontal, and rare kinematic indicators suggest sinistral
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G N PHILLIPS, J R VEARNCOMBE and R MURPHY
motion. The boundary between the mafic- and felsicdominated sequences of the greenstone belt, just west of Nimary, is parallel to the D1 schistosity. The nature of this boundary remains unresolved, in part due to minimal exposure. Apart from the 340o-trending shear zones, the rocks of the Jundee–Nimary goldfield are typically of low strain and preserve widespread original magmatic and sedimentary features. Crosscutting brittle faults, with trends between 040 and 060 o (NE) and between 080 and 100o (east) are recognisable on aeromagnetic imagery. The faults traverse regional granitoids, the greenstone belt, and crosscut mylonitic shear zones, clearly indicating that they are due to a later (D2) deformation, and are not splays off, or internal features to, ductile shear zones. Some of the fault planes are occupied by Proterozoic dolerite dykes. In the open pits at Jundee these faults are accompanied by multiple vein sets with similar orientations. Where kinematic evidence is available, the 040–060o fault set appears to be dextral, and the 080–100o fault set sinistral, and they are here interpreted to be a conjugate set. These fault and vein structures are critical to the distribution of the gold mineralisation.
ORE DEPOSIT FEATURES NATURE OF MINERALISATION Mineralisation at Jundee comprises quartz-calcite-pyrite veinlets with associated chlorite-carbonate-pyrite-muscovite alteration in a brittle deformation environment. However, in detail, there is some variation in the mineralisation related to host rocks and hosting structures (Fig 4). Significant mineralisation occurs in basalt in the Northwest-NIM 1-2 and NIM 3 pits, in dolerite in the Main and Barton pits, and in porphyry dykes in the Deakin pit, whereas the sedimentary rocks, extrusive ultramafic rocks, high-magnesium basalts and felsic dykes are poorly represented as host rocks to date. Four directions of auriferous veins dominate the mineralisation, and these are NNW-, east-, NE- and northtrending (Table 2). The relative importance of the vein strike directions varies between pits, and is not well understood in the unmined areas. Some NE- and east-trending mafic dykes show a close spatial relationship to gold mineralisation in the Northwest–NIM 1-2 pit, and are probably Proterozoic in age,
TABLE 2 Summary of mineralisation in the Jundee–Nimary goldfield. Name of mineralised zone Main
Host environment
Gold endowment (oz)1
Dolerite sill series
423 000 1 036 000
Style2 5 % arsenopyrite, brecciated.
Mineralisation trends3 (importance) NNW: Primary E: Secondary NE: Secondary
Barton Deeps
Dolerite sill series
Barton
Dolerite sill series Feldspar-quartz porphyry
201 000
Northwest
Tholeiitic basalt Feldspar-quartz porphyry
314 000
Muscovite-rich
NNW: Primary E: Secondary NE: Secondary
Deakin
Feldspar-quartz porphyry
258 000
5% arsenopyrite, strong chlorite alteration
NNW: None recorded E: Primary NE: Secondary N: Secondary
Cook–Reid
Unmined
170 000
NNW: Primary E: Primary
Fisher
Unmined
85 000
E: Primary NE: Secondary
Lyons
Unmined
61 000
NNW: Primary E: Secondary
Curtin
Unmined
55 000
NE: Primary
Menzies
Unmined
59 000
NNW: Primary
NIM 1,2
Tholeiitic basalt
110 000
E: Primary NNW: Secondary
NIM 3
Tholeiitic basalt
450 000
N: Primary E: Secondary
NIM 4
Tholeiitic basalt
160 000
NIM 6, NIM 7 1. 2. 3.
102
NNW: Primary E: None recorded NE: None recorded
120 000
‘Ounces’have been calculated from the December 1996 restricted uncut to 0.5 g/t resource statement of Great Central Mines Limited; and from the June 1996 quarterly report on resources by Eagle Mining Corporation. Dominant style of mineralisation: quartz-calcite veinlets with fine grained pyrite and associated chlorite-carbonate alteration in a brittle deformation environment are common features of virtually all mineralisation. Mineralised trends are defined thus: NNW: 310 to 330 strike with steep west dip E: 080–100/steep N and S dips NE: 040–060/steep SE and NW dips N: 010/60W
Geology of Australian and Papua New Guinean Mineral Deposits
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reactivating (or using) existing mineralised Archaean faults. Recent drilling well below the oxidised zone will provide the necessary constraints for detailed cross sections in the future. The strong structural control of veinlets and mineralisation in the weathered zone exposed in the open pits indicates that Jundee is essentially a weathered primary gold deposit rather than a supergene occurrence, and as such, has significant depth potential dictated by the main lithological and structural features.
CONTROLS Geological factors on different scales have contributed to the location and size of the Jundee–Nimary gold deposits. Several of these factors are common to other major goldfields, and it would appear that the Jundee–Nimary field owes its importance to a combination of geological factors, rather than a single unusual or unique factor. On a regional scale, the Jundee–Nimary goldfield is part of a line of anomalies and prospects on an inferred shear zone trending SE for 80 km (Figs 1 and 2). The nature of this trend is not well understood. The goldfield is also in a ‘wedge’ of mafic rocks at the convergence of felsic and ultramafic rocks formed immediately adjacent to the boundary between the two major lithological subdivisions within the greenstone succession (Figs 2 and 3). On a district scale, the basalt and dolerite hosts are important because their mechanical properties during greenschist facies deformation favoured brittle failure, and their high iron content led to enhanced gold precipitation during fluid–wall rock interaction. Porphyry dykes are also inferred to be favourable structural sites for mineralisation because of their rheology. Individual orebodies, outlined on Fig 4, contain a multitude of mineralised fractures, with a strong concentration in four orientations (NNW, north, NE and east), but only the NNW orientation has a significant ductile component. The high number of fractures, and especially intersections of several fracture orientations, appear to enhance mineralisation. Several of the factors critical in the localisation of the Jundee–Nimary goldfield have already been identified as critical in the evolution of the giant Kalgoorlie and Timmins (Canada) goldfields (Phillips, Groves and Kerrich, 1996). In a study of these largest greenstone goldfields in Australia and Canada, it was concluded that no individual factor could account for their enormous size, but rather a combination of numerous factors was critical to their origin and size. At the site of deposition, iron-rich mafic rocks play a conspicuous role at Kalgoorlie, Timmins and Jundee–Nimary, with both tholeiitic basalt and dolerite being host rocks. Differentiated dolerites play an important role at Kalgoorlie and Jundee (Fig 5). All these iron-rich mafic rocks are favourable because of their reaction with gold-bearing fluids to facilitate gold precipitation, and their rheological characteristic of low tensile strength which enables them to fracture under high fluid pressure. Carbonaceous metasedimentary rocks are also locally important host rocks for mineralisation at Kalgoorlie and Timmins. Two further striking similarities are the district-scale setting of all three goldfields in wide parts of greenstone belts away from granite margins, and the structural isolation of a thick pod of competent rock (tholeiitic basalt and dolerite) that is now
Geology of Australian and Papua New Guinean Mineral Deposits
surrounded by incompetent rocks (sedimentary, felsic and ultramafic rocks). In the case of Jundee–Nimary, this isolation is brought about by the convergence of the felsic rocks west of Nimary and the ultramafic rocks east of Jundee (Figs 1 and 2). The orientation of all three goldfields with respect to the farfield stress direction is similar, and conducive to fracturing of the isolated, low tensile strength mafic package. In all three goldfields there is a multitude of individual ore structures at several orientations.
CONCLUSIONS Jundee and the adjacent Nimary deposit are major discoveries in a poorly exposed greenstone belt away from significant old workings. The combined resource exceeds 120 t of gold localised in numerous quartz-carbonate-pyrite brittle veinlets of four main orientations. The predominant host rocks are mafic in composition, and include tholeiitic basalt, dolerite and spatially associated feldspar-rich porphyritic dykes. The Jundee–Nimary goldfield embodies many of the regional setting, host rock, structural and alteration features found at other major Archaean gold deposits in the Yilgarn Block, including Kalgoorlie.
ACKNOWLEDGEMENTS The directors of Great Central Mines Limited are thanked for permission to publish this paper, and J Gutnick, E Eshuys, and M Reed are thanked for their continued enthusiastic support and co-operation during geological studies at Jundee. This geological summary draws extensively on the work by the team of geologists led by I Herbison during the exploration stage, and by all the geologists currently involved in the mine and mine-based exploration at Jundee. P Ash, T Butler-Blaxell, D Clark, D Compston, M Clough, S Hassen, I Herbison, D Hope, M Knaak, P Kroupa, L Starkey and J Wright have made significant contributions that have been utilised here. Cooperation from Nimary mine staff, especially N Lithgow, has allowed Jundee to be placed in a district geological perspective. The manuscript has been improved by critical reviews from P Ash, E Eshuys, C Lewis, M Reed and J Wright.
REFERENCES Binns, R A, Gunthorpe, R J and Groves, D I, 1976. Metamorphic patterns and development of greenstone belts in the eastern Yilgarn Block, Western Australia, in The Early History of the Earth (Ed: B F Windley), pp 303–316 (Wiley: London). Boynton, W V, 1984. Geochemistry of the rare earth elements: meteorite studies, in Rare Earth Element Geochemistry (Ed: P Henderson), pp 63–114 (Elsevier: New York). Eshuys, E, Herbison, I, Phillips, N and Wright, J, 1995. Discovery of Bronzewing gold mine, in New Generation Gold Mines: Case Histories of Discovery, pp 2.1–2.15 (Australian Mineral Foundation: Adelaide). Eshuys, E and Lewis, C R, 1995. New approaches to gold exploration, in Proceedings of the Outlook 95 Conference, pp 336–340 (Australian Bureau of Agriculture and Resource Economics: Canberra). Lewington, G, 1995. The discovery of the Nimary gold camp, in New Generation Gold Mines: Case Histories of Discovery, pp 4.1–4.10 (Australian Mineral Foundation: Adelaide). Phillips, G N and Gibb, H C F, 1993. A century of gold mining at Kalgoorlie, Economic Geology Research Unit, James Cook University of North Queensland, Contribution 45:1–68.
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Phillips, G N, Groves, D I and Kerrich, R, 1996. Factors in the formation of the giant Kalgoorlie gold deposit, Ore Geology Reviews, 10:295–317. Walker, F R, 1940. Differentiation of the Palisade diabase, New Jersey, Bulletin Geological Society of America, 51:1059–1106.
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Wright, J H and Herbison, I, 1995. The Yandal Belt: Preliminary exploration of the Jundee deposit, in New Generation Gold Mines: Case Histories of Discovery, pp 5.1–5.14 (Australian Mineral Foundation: Adelaide).
Geology of Australian and Papua New Guinean Mineral Deposits
Chanter, S C, Eilu, P, Erickson, M E, Jones, G F P and Mikucki, E, 1998. Bulletin gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 105–110 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Bulletin gold deposit 1
2
3
4
by S C Chanter , P Eilu , M E Erickson , G F P Jones and E Mikucki
5
INTRODUCTION The deposit is within the Wiluna Goldfield, 5 km SE of Wiluna, WA, at lat 26o38′S, long 120o15′E on the Wiluna (SG 51–9) 1:250 000 scale map sheet (Fig 1). Since mining commenced in 1896 the goldfield has produced in excess of 2.5 Moz and the Bulletin deposit is one of the three largest ore systems in the field. The deposit is hosted by tholeiitic to high magnesium basalt and dolerite that have undergone low grade (prehnitepumpellyite) facies metamorphism. Mineralisation is structurally controlled by the Happy Jack–Bulletin fault zone which forms part of the brittle-ductile Wiluna fault system (Fig 1), and is associated with strong to intense sericitecarbonate±chlorite alteration. In the high grade ore zones the dominant carbonate species is dolomite, and gold occurs as submicroscopic particles or chemically bound within fine grained pyrite and arsenopyrite. At December, 1997 the Bulletin deposit contained Proved and Probable Reserves of 1.9 Mt at an average grade of 8.5 g/t gold and an additional Inferred Resource of 0.6 Mt at 7.3 g/t gold. The deposit is currently being mined at a rate of 400 000 tpa from a decline which, when completed, will access the orebody to approximately 600 m below surface. Due to the extremely refractory nature of the primary mineralisation, Wiluna Mines operates an innovative bacterial oxidation plant, based on the Gencor BIOX® process.
EXPLORATION AND MINING HISTORY Wiluna’s mining history commenced with the discovery of gold in 1896 by prospectors G Woodley, J N Wotton and J L Lennon, after which three distinct periods of mining followed. In the first phase prospectors worked the near surface quartz reefs. In 1904, when larger lodes were discovered in the oxidised zone, mining was carried out from shafts and winzes. Underground driving and stoping in this period was generally restricted to above the 100 foot (30 m) level.
1.
Senior Mine Geologist, Wiluna Gold Pty Ltd, PO Box 9, Wiluna WA 6646.
2.
Research Fellow, Key Centre for Strategic Mineral Deposits, Department of Geology and Geophysics, University of Western Australia, Nedlands WA 6009.
3.
Geology Manager, Wiluna Gold Pty Ltd, PO Box 9, Wiluna WA 6646.
4.
Exploration Manager, Wiluna Mines Ltd, 10 Ord St, West Perth WA 6005.
5.
Research Fellow, Key Centre for Strategic Mineral Deposits, Department of Geology and Geophysics, University of Western Australia, Nedlands WA 6009.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Location and geological plan of the Wiluna area showing the Wiluna fault system and pits mined to date.
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Refractory sulphide ore was first encountered in 1907, and by 1908 Gwalia Consolidated was the main producing company in the area, operating a 30 stamp head battery and processing plant. However, the company was unable to treat the refractory primary sulphide ore and gold production progressively wound down from 1913 and ceased in 1924. During the first phase (1896 to 1924) approximately 186 000 oz of gold was produced from 380 000 t of high grade oxide ores (Edwards, 1953) of which approximately 15 000 t at a head grade of 9.3 g/t gold was mined from zones immediately south of the main Bulletin orebody. The second phase began with the advent of the sulphide flotation process which renewed interest in the treatment of the ore. In this phase Wiluna Gold Corporation, headed by Claude de Bernales, dewatered the underground workings in 1926 and commenced construction of a pilot plant utilising the new flotation technique. The treatment process, which involved roasting of the sulphide flotation concentrate in an Edwards roasting furnace and cyanidation of the resulting calcines, was successful and construction of a full scale treatment plant began in 1929. Full scale production commenced in 1931 and production peaked in 1938 when over 141 000 oz of gold were recovered from 660 000 t of ore. During the period 1936 to 1943 a total of 530 000 t of primary sulphide ore at a head grade of 7.5 g/t gold was mined from the southern Bulletin orebody by underground methods. At that time underground mining utilised open stoping by sublevel underhand benching with gloryholing in the upper part of the mines. Sublevels were silled out to the full width of the orebody and benched down to the level below. Pillars were extracted immediately following stoping wherever possible and at Bulletin, as was the case in most of the mines, the stopes were not backfilled. By 1940 head grades were dropping and wartime shortages were severely impacting on the profitability of the mines. A Commonwealth Government subsidy to produce arsenic oxide enabled production to continue through the war but underground mining ceased in 1947, with the final gold production in 1950. Total gold production from the field during this second period (1926 to 1950) exceeded 1.5 Moz. The third and current mining phase commenced in 1984 with retreatment of the tailings from the previous operations. Mining of oxide and sulphide ores recommenced in 1987 and 1993 respectively. By 1994 an open pit at Bulletin had produced 660 000 t of oxide ore at 2.8 g/t gold and 83 000 t of sulphide ore at 3.7 g/t gold. The main high grade section of the deposit was discovered in 1992–93 by deep diamond drilling below the open pit. Drilling delineated a pre-mining, Indicated Resource of 2 Mt at 8.7 g/t gold, with an additional Inferred Resource of 0.9 Mt at 9.5 g/t gold. The new resource lies to the north of the zone mined in the late 1930s and old development drives approach to within 50 m of high grade mineralisation. Diamond drilling was carried out on sections 25 m apart with a hole spacing of approximately 40 m in the upper part of the orebody. Drill spacing in the deeper parts of the lode is broader, on approximately 50 m section spacing with an average hole spacing of 80 m. A total of 52 diamond and 160 reverse circulation holes was used to define the resource. A small cut back of the Bulletin open pit was completed in 1994, and in 1995 decline development commenced to access the deeper zones of the deposit. By early 1996 all production at
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Wiluna was from underground sources and in April of that year the millionth ounce of gold was produced from the current operation. Treatment of the refractory primary ore recommenced in 1993 utilising a biological oxidation process, BIOX®, developed by Gencor. The process involves the oxidation, by bacteria, of a pyrite-arsenopyrite flotation concentrate which enables gold to be recovered by conventional cyanidation techniques. To the end of January 1997, a total of 486 500 t of development and stope ore from the Bulletin underground mine had been treated at a reconciled grade of 6.4 g/t gold.
PREVIOUS REPORTS The earliest reports on the Wiluna mine area are by Blatchford (1899), Gibson (1908) and Talbot (1913), all of which provide brief descriptions of the geology and progress of early mining activities. The first detailed studies on the geology of the mine area were carried out by R F Playter (unpublished data, 1936) and H J C Connolly (unpublished data, 1937). These studies described the various host units in detail and established the current mine sequence which has remained unchanged to this day. H J C Connolly (unpublished data, 1937) provided many key insights into understanding the mineralised structures at depth. Early mining methods are described by Norrie (1941). Further descriptions of the mine are presented in McGoldrick (1990) and Davis and Farrelly (1993). Hagemann et al (1992) and Hagemann (1992) expanded on the earlier structural studies and incorporated new geochemical, isotopic and fluid inclusion data to provide a framework for the genesis of the Wiluna deposits. P Eilu (unpublished data, 1996) recently completed a detailed alteration study of the Bulletin lode.
REGIONAL GEOLOGY The Bulletin deposit is within the Wiluna Goldfield at the northern end of the Norseman–Wiluna greenstone belt (Fig 1). The belt can be divided into a western segment of thick, extrusive ultramafic and mafic volcanic rocks, interlayered with deep water synvolcanic sediment, and an eastern segment typified by tholeiitic and calc-alkaline subaerial or shallow marine volcanic rocks (Bavington, 1981; Barley et al, 1989; Barley and Groves, 1990). Metamorphic grades increase from prehnite-pumpellyite facies in the north to amphibolite facies in the south. For a more detailed description of the belt see Williams (1974), Groves and Batt (1984) and Hallberg (1986) and references therein. Within the Wiluna Goldfield mineralisation is hosted by a supracrustal greenstone succession that has undergone prehnite-pumpellyite to lower greenschist facies metamorphism. Igneous structures and textures are well preserved and hence the prefix meta is omitted from rock names, but is implied. Mineralisation is structurally controlled by the Wiluna strike-slip fault system (Fig 1). Two major north-trending structures, the Graphite fault to the west and the Eastern lineament to the east, enclose a corridor 2 km wide and 5 km long which contains most of the mineralisation. The sense of movement on these structures is dextral with apparent displacement up to 6 km on the Eastern lineament and 1.3 km on the Graphite fault. Twenty-one gold occurrences have been identified and mined to date. They are predominantly associated with second to third order north- and NE-trending,
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BULLETIN GOLD DEPOSIT
brittle to brittle-ductile, dextral strike slip faults, with mineralisation localised at dilational bends or jogs along the faults, at fault intersections, horsetail splays and in subsidiary overstepping faults. Major host rock types are iron- and magnesium-tholeiitic basalt and komatiitic basalt, with lesser mineralisation associated with thin sediment bands and sequence-parallel felsic dykes.
ORE DEPOSIT FEATURES STRATIGRAPHY A generalised stratigraphic column for the Wiluna Goldfield is presented in Fig 2. The section mainly consists of a lower, 3500 m thick sequence of iron-, magnesium- and komatiitic-basalt and interlayered dolerite overlain by 4000 m of felsic to intermediate volcanic rocks. The upper sequence also contains thick komatiitic units and ultramafic bodies (not shown on Figs 1 and 2) which host nickel sulphide deposits to the south, as at Mount Keith and Honeymoon Well. Interflow sediment constitutes less than 1% of the volcanic pile. The mine sequence can be traced for at least 14 km to the north and 10 km to the south. The units typically strike between 300 and 350o and dip at 60–80o to the SW. The Bulletin deposit is hosted within the Unit 3 basalt of Fig 2, which was called Flow 3 in historical and mine nomenclature. This unit consists of a number of individual
flows of tholeiitic basalt, some 100 m thick. The rocks are generally fine grained and contain both massive and pillowed horizons. Interlayered with the basalt flows are medium to coarse grained dolerites between 5 and 50 m thick. Dolerites grade upward into finer grained basalt, although sharp contacts between these textural types are often present. P Eilu (unpublished data, 1996) has subdivided the rocks into an iron tholeiite and two magnesium tholeiite suites based on Jensen’s (1976) discrimination diagram and contrasting Al:Ti and Al:Zr ratios. All rock suites contain both basaltic and doleritic variants, supporting textural evidence of a direct genetic relationship between these rock types. Subgreenschist facies metamorphism has done little to modify pre-existing igneous textures in unstrained rocks. Doleritic rocks have subophitic textures and consist of clinopyroxene, prismatic subhedral olivine and plagioclase. Original ferromagnesian minerals are partially replaced by actinolite and chlorite, and plagioclase is often completely altered to albite and epidote. Titanite, magnetite and rutile are common accessory minerals (P Eilu, unpublished data, 1996). Pillowed units are typically fine grained with dark green microporphyritic and amygdaloidal pillow interiors, yellowgreen, epidote-rich pillow margins, and pale green to pale grey inter-pillow matrices of epidote, calcite, quartz, chlorite and traces of pyrite. Textures are dominantly intergranular and porphyritic, but in places are seriate and/or variolitic. The most common phenocryst is plagioclase, but locally there are completely chloritised olivine and clinopyroxene phenocrysts (P Eilu, unpublished data, 1996).
STRUCTURE AND OREBODY GEOMETRY The deposit is on the Happy Jack–Bulletin fault zone (BFZ), one of the major structures within the Wiluna Fault system. The BFZ is a 50 m wide zone of brittle to brittle-ductile deformation which trends at 045o and dips steeply to the east at 80o, with local westerly dipping sections caused by flexures in the fault plane. Stratigraphic offsets indicate that movement on the fault was dextral, with an apparent displacement of 700 m . The deposit consists of the main ore zone (lens 10) and five subsidiary ore zones. Over 90% of the delineated resource is contained within the main zone. This zone generally occupies the central portion of the BFZ, with its margins parallel to the main fault boundaries along strike and down dip. It has a strike length of 200 to 300 m and is continuous for at least 600 m down dip (Fig 3). Widths vary from 3 to 6 m in the upper parts, increasing to 30 m in the high grade core approximately 300 m below surface (Figs 3 and 4). Subsidiary zones are located closer to the fault margins and are typically 2 to 6 m wide, but tend to lack continuity along strike and down dip. Of the subsidiary zones only lens 22 is currently being mined. Deformation zones are characterised by moderate to intense hydraulic fracturing of the rock which gives it a brecciated appearance, or by the development of a strong mineral foliation. Foliation planes are generally subvertical, and strike between 345 and 360o. Mineralisation is open at depth and plunges steeply to the south at 70 to 80o (Fig 4), subparallel to the intersection between the BFZ and the sequence.
FIG 2 - Stratigraphic section for the Wiluna Goldfield.
Geology of Australian and Papua New Guinean Mineral Deposits
Within the hanging wall numerous smaller faults parallel to the main fault displace rock units by up to tens of metres. These faults are continuous along strike and down dip, and although the timing and sense of movement along the faults are uncertain, the similarity of their alteration assemblages
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suggests a close relationship between the hanging wall faults and the main fault zone. Quartz veins are common throughout the fault zone and occur as either rootless stringer veins parallel to foliation or as hydraulic breccia fill. Late stage quartz-carbonate veins 0.2 to 2.0 m thick parallel the main fault zone and often mark the fault boundaries. These veins are often brecciated and contain clasts of quartz and wall rock. They appear to be post-mineralisation in that they cut the 45o fabric and are devoid of mineralisation. Individual veins have strike lengths of 5 to 15 m, though link structures often connect the veins to form an almost continuous vein more than 50 m long. Very late, 5 to 10 mm thick quartzcarbonate veinlets cut all of the above. Post-mineralisation faulting is restricted to movement along the late stage quartz-carbonate veins and minor displacement of the lode adjacent to a 2 to 6 m wide Proterozoic dyke which cuts across and stopes out part of the orebody.
ALTERATION AND PRIMARY GEOCHEMICAL DISPERSION Wall rock alteration
FIG 3 - Cross section of the Bulletin deposit on local grid line 51 235 m N, looking north.
Wall rock alteration is more or less pervasive throughout the entire BFZ and surrounding wall rock, extending 25 to 150 m from the borders of the BFZ on the hanging wall side, and 10 to 20 m from the footwall contact. In detail, alteration assemblages are complexly zoned and in part host rock dependant. P Eilu (unpublished data, 1996) subdivided alteration assemblages into four main zones: distal chloritecalcite, intermediate calcite-dolomite, and proximal sericite and dolomite-sericite. Distal alteration assemblages are ubiquitous in low strain domains adjacent to the BFZ and minor hanging wall faults, whereas intermediate and proximal alteration zones are generally restricted to the BFZ. With increasing proximity to ore there is a change from carbonate-free through calcite and calcite-dolomite to dolomite bearing, and from sericite absent to sericite-bearing assemblages. Relict olivine, clinopyroxene and metamorphic actinolite are replaced by chlorite-calcite assemblages within distal chlorite-calcite zones. Epidote and titanite also become unstable in these rocks, and are replaced by chlorite-calcite and calcite-leucoxene, respectively. Albite persists throughout the zoned sequence but may be partially replaced by hydrothermal minerals. Transitional calcitedolomite alteration zones to 15 m wide are generally found marginal to the BFZ in strained and unstrained rock. They are characterised by a gradual increase in ankerite and ferroan dolomite at the expense of calcite in the host rocks.
Fig 4 - Longitudinal projection of the Bulletin deposit, looking west, showing lode distribution and gram.metre contours.
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Proximal alteration assemblages characterise the ore and high strain portions of the BFZ and hanging wall faults, occasionally forming zones of pervasive alteration to 30 m wide. Sericite zone assemblages dominate in the more weakly mineralised portions of the system. These zones are distinguished by abundant sericite (5–20 vol %) as disseminated grains or along anastomosing lamellae and foliation planes within a sheared matrix dominated by dolomite±chlorite±calcite (P Eilu, unpublished data, 1996). Ore grade rocks are characterised by intense dolomite-sericitesulphide alteration. These zones of extreme wall rock bleaching and sulphidation are marked by distinct increases in sulphide abundance and complete destruction of the remaining chlorite and calcite.
Geology of Australian and Papua New Guinean Mineral Deposits
BULLETIN GOLD DEPOSIT
Geochemical dispersion Low levels of arsenic, antimony and tellurium exhibit the most extensive lateral geochemical dispersion and may extend more than 200 m from the BFZ. Significant anomalies defined by tellurium (>10 ppb), arsenic (>28 ppm) and antimony (>2 ppm) extend 100, 60 and 40 m from the fault margins, respectively. Other elements such as gold, silver and tungsten can also provide local vectors to ore within the BFZ, but show little outward dispersion (P Eilu, unpublished data, 1996). Anomalies defined by molar indices related to carbonation (CO2:Ca) and potassic alteration (3K:Al; Kishida and Kerrich, 1987) provide good guides to ore but do not define anomalies that extend beyond visible alteration. Carbonation anomalies extend to 150 m from the BFZ, whereas potassic alteration anomalies are generally within 10 to 15 m of the fault.
MINERALOGY AND ORE TEXTURES Gold-related hydrothermal activity occurred in two stages, each with a distinct mineralogical association: Stage I goldpyrite-arsenopyrite, and Stage II gold-stibnite mineralisation (Hagemann, 1992). Stage I mineralisation is present in all of the Wiluna deposits, but Stage II appears to be spatially restricted to the Graphite and Moonlight fault systems and is in turn limited to the upper levels (<200 m depth) of their associated deposits (Hagemann, 1992). The Wiluna sulphide ores are extremely refractory, with most gold occurring either in solid solution or as submicroscopic particles within fine grained sulphides. Only Stage I is present at Bulletin, with pyrite and arsenopyrite the main sulphide minerals with trace amounts of stibnite, chalcopyrite, tetrahedrite, tennantite, cobaltite, covellite and digenite. Sulphides generally occur as fine disseminations and occasionally as fracture fill throughout the host rock, and constitute between 2 and 8 vol % of the mineralised rock mass. Arsenopyrite constitutes between 25 and 42 wt % of the total sulphide content and pyrite most of the remainder. Higher gold values are associated with increases in the arsenopyrite:pyrite ratio. Gold to sulphur ratios (ppm Au:wt % Stotal) are typically in the order of 3.2–3.4 in the upper levels of the mine and increase to 3.5–3.7 with depth as gold grade increases. Minor free gold is present, but is rare in rocks containing <10g/t gold. Within the foliated zones arsenopyrite needles lie parallel to and crosscut the fabric, indicating that deposition of sulphide and presumably gold was late in the tectonic cycle.
MINERALISATION CONTROLS AND ORE GENESIS Mineralisation within the Bulletin deposit is principally controlled by the BFZ. Fault planes within the Wiluna field typically flex along strike and down dip, and these flexures or rolls typically produce sites of dilatancy (H J C Connolly, unpublished data, 1937; Hagemann et al, 1992, in press). These flexures in conjunction with favourable host rock composition act to form the best ore zones. There is a strong correlation between mineralisation and the iron tholeiite suite of rocks and to a lesser degree the magnesium tholeiite-1 (Fig 5). Whether this association is due to the chemical or rheological properties of these units is uncertain. Mineralisation outside the BFZ is restricted to small hanging wall faults which are of very limited economic potential and rarely contain gold values above background levels.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 5 - Al2O3 vs TiO2 diagram for mineralised and unmineralised samples from Bulletin. Iron tholeiite, magnesium tholeiite 1 and magnesium tholeiite 2 fields as defined by P Eilu (unpublished data, 1996). Fields are based on conserved element ratios of least altered samples.
Chlorite, carbonate and arsenopyrite geothermometry indicate an average depositional temperature of 325±25oC for gold-pyrite-arsenopyrite within the Wiluna field (Hagemann et al, in press). Ore fluids associated with gold-pyritearsenopyrite mineralisation are neutral to slightly alkaline (pH 5.1–5.4), with salinities of 4 eq wt % NaCl, and XCO2 of 0.17. Calculated oxygen fugacities (ƒO2) vary from10-31 to 10-32.7 bar at 325oC (Hagemann et al, in press). Hagemann et al (1992), Hagemann, Gebre-Mariam and Groves (1994) and Hagemann et al (in press) interpreted the Wiluna deposits to be an epizonal, relatively shallow-level end member of the crustal continuum of Archaean lode gold deposits. Fluids were emplaced into brittle to brittle-ductile dilatant zones during alternating fluid flow and seismic cycles related to movement along the Wiluna strike slip fault system. A suction pump mechanism, originally proposed by Sibson (1990) for epithermal mineralisation, was proposed as the likely fluid infiltration mechanism.
MINE GEOLOGICAL METHODS Grade control methods at Bulletin are based on underground diamond drilling, face sampling and mapping. Diamond drilling is carried out at 15 m intervals along strike on each level before driving on ore commences. A flitch and sectional interpretation of the ore zone is completed and the drives are positioned to optimise the stope shape. Development drives are mined on survey lines. Diamond holes are also drilled between levels for modelling and grade estimation, with the final drill hole spacing approximately 15 by 15 m. Ore reserve estimation is carried out by constructing a threedimensional block model of the orebody, and wireframing. Gold and sulphur grades are estimated using an anisotropic, inverse distance cubed grade interpolation, with all modelling carried out using Surpac mining software. The bullion to ore reserve grade reconciliation to date is 97.8% and the bullion to grade control reconciliation is 98.5%. Statistically the Bulletin assay data display a strong correlation between gold and sulphur grades consistent with the occurrence of gold within fine grained sulphide minerals. Free gold particles are extremely rare and assays above 60 g/t gold account for less than 0.1% of the total data set. The coefficient
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of variation for gold is 1.33 for the exploration data set and as low as 0.64 for the grade control data from within the high grade core of the deposit where gold grades are remarkably consistent.
Groves, D I and Batt, W D, 1984. Spatial and temporal variations of Archaean metallogenic associations in terms of evolution of granitoid-greenstone terrains with particular emphasis to Western Australia, in Archaean Geochemistry (Eds: A A Kroner, G M Hanson, and A M Goodwin), pp 73–98 (Springer-Verlag: Berlin).
Each face of ore is mapped by a geologist and sampled according to geological boundaries. Back mapping and detailed structural traverse mapping are carried out to provide geotechnical information for stope design and long term mine stability.
Hagemann, S G, 1992. The Wiluna lode gold deposits, Western Australia: a case study of a high level Archaean lode-gold system, PhD thesis (unpublished), The University of Western Australia, Perth.
ACKNOWLEDGEMENTS The authors gratefully acknowledge the permission of Wiluna Mines Ltd to publish this information and particularly thank the management and staff of Wiluna Mines Ltd for their support and valuable comment. Special thanks are due to M GebreMariam and S G Hagemann for their review of the manuscript.
REFERENCES Barley, M E, Eisenlohr, B, Groves, D I, Perring, C S and Vearncombe, J R, 1989. Late Archaean convergent margin tectonics and gold mineralisation: a new look at the Norseman-Wiluna Belt, Geology, 17:826–829. Barley, M E and Groves, D I, 1990. Deciphering the tectonic evolution of Archaean greenstone belts: the importance of contrasting histories to the distribution of mineralisation in the Yilgarn Craton, Western Australia, Precambrian Research, 46:3–20. Bavington, O A, 1981. The nature of sulphidic sediments at Kambalda and their broad relationships with ultramafic rocks and nickel ores, Economic Geology, 76:1606–1628. Blatchford, T, 1899. A geological reconnaissance of the country at the heads of the Murchison, East Murchison and Peak Hill Goldfields, Geological Survey of Western Australia Annual Report 1899. Davis, R J and Farrelly C T, 1993. The geology and mining practices at the Wiluna mining operations, Western Australia, in Proceedings of the International Mining Geology Conference, pp 115–123 (The Australasian Institute of Mining and Metallurgy: Melbourne). Edwards, A B, 1953. Gold deposits of Wiluna, in Geology of Australian Ore Deposits, (Ed: A B Edwards), pp 215–223 (5th Empire Mining and Metallurgy Congress and The Australasian Institute of Mining and Metallurgy: Melbourne). Gibson, C G, 1908. Report on the auriferous deposits of Barrambie and Errolls (Cue District) and Gum Creek (Nannine District) in the East Murchison Goldfield, also Wiluna (Lawlers District), in the East Murchison Goldfield, Geological Survey of Western Australia Bulletin 34:17–40.
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Hagemann, S G, Gebre-Mariam, M and Groves, D I, 1994. Surface water influx in shallow-level Archaean lode gold deposits in Western Australia, Geology, 22:1067–1070. Hagemann, S G, Groves, D I, Ridley, J R and Vearncombe, J R, 1992. The Archaean lode-gold deposits at Wiluna, Western Australia: high-level brittle-style mineralisation in a strike-slip regime, Economic Geology, 87:1022–1053. Hagemann, S G, Ridley, J R, Mikucki, E, Groves, D I and Brown, P E, in press. The Wiluna lode-gold deposits, Western Australia: Hydrothermal alteration and mineralisation in an Archaean epizonal Au-As-Sb deposit, Economic Geology. Hallberg, J A, 1986. Archaean basin development and crustal extension in the north-eastern Yilgarn Block, Precambrian Research, 31:133–156. Jensen, L S, 1976. A new method of classifying subalkalic rocks, Ontario Division of Mines Miscellaneous Paper No 66. Kishida, A and Kerrich, R, 1987. Hydrothermal alteration zoning and gold concentration at the Kerr-Addison Archaean lode gold deposit, Kirkland lake, Ontario, Economic Geology, 82:649–690. McGoldrick, P, 1990. Wiluna gold deposits, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 309–312 (The Australasian Institute of Mining and Metallurgy: Melbourne). Norrie, A W, 1941. The operations of Wiluna Gold Mines Ltd at Wiluna, Western Australia, Associate Otago School of Mines (Mining Division) thesis, (unpublished), University of Otago, Dunedin. Sibson, R H, 1990. Faulting and fluid flow, in Short Course on Fluids in Tectonically Active Regimes of the Continental Crust (Ed: B E Nesbitt), pp 93–132 (Mineralogical Association of Canada: Vancouver). Talbot, H W B, 1913. The country north of Lake Way, Geological Survey of Western Australia Annual Report 1912, pp 12–13. Williams, I R, 1974. Structural subdivisions of the Eastern Goldfields Province, Yilgarn Block, Geological Survey of Western Australia Annual Report 1973, pp 53–59.
Geology of Australian and Papua New Guinean Mineral Deposits
Winnall, N J, Hibberd, T J, Thynne, D S and Wahdan E, 1998. Some gold deposits of the Bluebird, Nannine and Cuddingwarra goldfields, Murchison district, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 111–118 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Some gold deposits of the Bluebird, Nannine and Cuddingwarra goldfields, Murchison district 1
2
3
by N J Winnall , T J Hibberd , D S Thynne and E Wahdan
3
INTRODUCTION The gold deposits described are owned by St Barbara Mines Ltd (SBML). The Bluebird goldfield contains the Bluebird deposit (Walsh, 1990) and the new discoveries of South Junction and Bluebird East which are a few hundred metres west and east respectively of the Great Northern Highway 16 km south of Meekatharra, WA (Fig 1). The South Junction deposit is at lat 26o44′S, long 118o25′ and AMG coordinates 641 500 E, 7 043 000 N; the Bluebird East deposit is at lat 26o43′S, long 118o26′E and AMG coordinates 642 840 E , 7 044 060 N on the Belele (SG 50–11) 1:250 000 scale and the Meekatharra (2544) 1:100 000 scale map sheets. The Nannine Goldfield deposits are 37 km south of Meekatharra at lat 26o53′S, long 118o21′E and AMG coordinates 634 000 E, 7 025 000 N (Fig 2) on the Belele (SG 50–11), Glengarry (SG 50–12) and Cue (SG 50–15) 1:250 000 scale map sheets. The centre of the Cuddingwarra Goldfield is 12 km NE of Cue. The largest deposit is Black Swan South, at lat 27o23′S, long 117o48′E and AMG coordinates 579 500 E, 6 971 575 N on the Cue (SG 50–15) 1:250 000 scale map sheet. Resource and reserve data are presented in Table 1.
EXPLORATION AND MINING HISTORY In the Bluebird goldfield, the South Junction deposit was discovered in mid 1989. The initial indication was in a water bore drilled about 1 km south of the Bluebird open pit, which intersected gold mineralisation below a thick cover of colluvium and laterite. The Bluebird East deposit was found in October 1992 as a result of sampling of quartz float and systematic rotary air blast (RAB) scout drilling along a known mineralised trend. The deposits of the Nannine field were found in October 1890, and by 1893 alluvial workings had given way to underground mining. Modern systematic exploration began in 1981 when diamond drilling from the Aladdin underground 1.
Exploration Manager, St Barbara Mines Ltd, PO Box 105, Meekatharra WA 6642.
2.
Formerly Project Geologist, St Barbara Mines Ltd, now Great Central Mines Ltd, 46 Kings Park Road, West Perth WA 6008.
3.
Project Geologist, St Barbara Mines Ltd, PO Box 105, Meekatharra WA 6642.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Location map and geological plan of the Bluebird goldfield.
workings identified further mineralisation. Exploration on Baileys Island occurred between 1987 and 1989 after trench sampling located rich supergene mineralisation. Subsequent reverse circulation drilling delineated the Baileys Island South, Baileys Island East, Baileys Island Central and Baileys Island North deposits. Gold was first discovered at the Cuddingwarra field in 1891. The Black Swan South deposit was found by deeper drilling at gold anomalies defined by shallow RAB drilling.
PREVIOUS DESCRIPTIONS For the Bluebird goldfield, the South Junction deposit was described by Fiala (1992), and in annual reports of SBML from 1991 to 1995, and Bluebird East is described in the 1993 to 1995 annual reports.
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TABLE 1 Selected resource and reserve data. Measured Resources (including proved reserves) Mt
Grade (g/t gold
Contained gold (’ooo oz)
Mt
Bluebird East
0.755
3.0
73
2.449
South Junction
1.189
1.4
53
Nannine Reef
Inferred Resources
Indicated Resources (including probable reserves) Grade Contained (g/t gold gold) (’ooo oz)
Total Resources
Mt
Grade (g/t gold)
Contained gold (’ooo oz)
Mt
Grade (g/t gold)
Contained gold (’ooo oz)
0.077
1.0
3
3.281
1.5
194
1.5
118
2.283
1.5
108
3.472
1.4
161
0.396
1.9
24
0.396
1.9
24
0.315
2.6
26
Bailey Island East
0.315
2.6
26
Bailey Island South
0.073
3.4
8
0.073
3.4
8
Aladdin
0.140
3.0
14
0.140
3.0
14
5.341
1.58
272
Black Swan South
8.109
1.7
441
TOTALS
10.53
1.75
567
Proved Reserves
Probable Reserves
Contained gold (’ooo oz)
Mt
3.61
66
0.516
2.74
172
4.551
2.30
337
7.07
2.53
575
Mt
Grade (g/t gold)
Bluebird East
0.569
South Junction
1.950
Black Swan South TOTALS
Nannine Reef
0.058 0.574
1.6 3.10 1.73
The Cuddingwarra gold deposits were described by Woodward (1907) and Campbell (1908). The history of systematic exploration in the area is recorded by P M Leeming (unpublished data, 1985, 1986), B Anderson (unpublished data, 1987, 1988) and in annual reports of SBML for 1992 to 1995.
REGIONAL GEOLOGY The three goldfields are in the Wydgee–Meekatharra belt in the NW corner of the Archaean Yilgarn Block. The greenstone belt contains Murchison Supergroup rocks which are divided into the Luke Creek Group and the overlying Mount Farmer Group. The deposits are hosted by rocks of the Luke Creek Group, which is subdivided in Table 2. Five major periods of deformation have affected the Murchison province (Watkins et al, 1990), and all of the rocks have been metamorphosed to lower greenschist facies. Initial deformation (D1) comprising recumbent folding and probable thrusting was followed by easterlytrending upright tight folding (D2) and intense NNE- to NNW-trending, upright, tight and isoclinal folds (D3) which produced a basin and dome pattern. Development
26 6 32
2.30
29
8.109
1.7
441
15.786
1.71
868
Total Reserves
Grade Contained (g/t gold gold) (’ooo oz)
Various historic authors in Gibson (1904) and Adams (1945) mention the geology of Nannine while focusing on mining activity. Watkins et al (1990) describe a detailed investigation of the Murchison region and the Nannine deposit. Walsh (1990) reported on the geology and mining activities at the Caledonian deposit, Ellice (1990) recorded a detailed structural analysis of the Nannine area and Burrows (1992) reported on a detailed geochemical study at the Caledonian deposit.
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0.392 Mt
Grade (g/t gold)
Contained gold (’ooo oz)
1.085
2.63
92
1.950
2.74
172
0.058
3.10
6
4.551
2.30
337
7.644
2.47
607
TABLE 2 Subdivision of the Luke Creek Group (after Watkins et al, 1990). Windaning Formation
Banded iron formation(BIF) interlayered with felsic volcanic and volcaniclastic rocks, sediment and basalt
Gabanintha Felsic volcanic rocks and Formation volcanogenic metasediment, interlayered with mafic rocks Ultramafic rocks and high-magnesium basalt, intruded by felsic porphyry Golconda Formation Murrouli Basalt
Host to mineralisation in the Bluebird and Cuddingwarra goldfields
Mafic rocks interlayered Host to Nannine goldfield with BIF BIF-related mineralisation High-magnesium and tholeiitic basalt
of NNE- trending, mainly dextral, large- scale fault systems and shear zones (D4) followed which partially overlapped the D3 folding and resulted from the same east-west oriented stress field. Subsequent east- to SE-trending faults and shear zones (D5) developed in the northwestern part of the Murchison province.
BLUEBIRD GOLDFIELD The deposits are in the lower part of the Gabanintha Formation, which is a 5000 m thick sequence of largely ultramafic rock of
Geology of Australian and Papua New Guinean Mineral Deposits
GOLD DEPOSITS OF THE BLUEBIRD, NANNINE AND CUDDINGWARRA GOLDFIELDS
age 3052 to 2952 Myr (Watkins et al, 1990). The deposits are along the regional Gabanintha shear, which hosts about a dozen deposits in the 18 km between Bluebird and Meekatharra. The larger deposits are at the intersection of the shear and the Norie pluton, an ‘internal’ granitoid (Fig 1). At this intersection, the shear shows evidence of flexure and is 3 km wide.
NANNINE GOLDFIELD The field is adjacent to the Gabanintha shear on the southwestern side of the Norie pluton. It is flanked to the west and east by post-folding intrusive granitoids in which regional east-west compression during emplacement caused intense deformation. The compressive strain produced tight upright folds (D3) and major NNE- to NNW-trending faults. A major thrust fault developed between the Nannine and Baileys Island areas, resulting in syntectonic sinistral displacement along the Caledonian fault (Fig 2). Further strain was accommodated by dextral displacement along the Aladdin fault to the north of Nannine. Gold deposits occur along both the Caledonian and Aladdin faults.
CUDDINGWARRA GOLDFIELD The deposits are on the eastern limb of the Big Bell antiform near the western margin of the Wydgee–Meekatharra greenstone belt, and in the northern part of the north-trending Cuddingwarra shear. The goldfield provides a distinctive wedge shaped area of strongly-responsive aeromagnetic data, which is the typical signature of the host sequence. This sequence, assigned to the Gabanintha Formation, is bounded to the west by low hills underlain by metabasalt and to the east by a succession dominated by felsic tuff (Fig 3).
ORE DEPOSIT FEATURES BLUEBIRD GOLDFIELD ‘Contact metasomatic’ gold deposits are most common in this field, with mineralisation occurring at the contact between granitoid or felsic porphyry and altered ultramafic rocks. The altered ultramafic rocks were probably komatiite or komatiite protoliths, with subordinate metadunite or metaperidotite, metagabbro and metasediment. The alteration assemblage is quartz-carbonate (ankerite)-fuchsite±sericite±pyrite±limonite ±galena. Gold mineralisation is closely associated with shear structures, which have commonly been intruded by granitoid and felsic porphyry. Bedrock hosted and supergene gold mineralisation appear to be mainly confined to quartz-carbonate±fuchsite schist and felsic porphyry. Laterite-hosted supergene gold also occurs in this field. Most of the primary deposits are along local shear zones that appear to be closely related to the Gabanintha shear. The depth to the base of weathering is between 50 and 160 m.
South Junction The numerous mineralised zones were emplaced along three parallel and north trending structures (Fig 1). The mineralisation is largely hosted by quartz veins in shear zones, by quartz vein sets or stockworks and by laminated quartz sheets and veins. The styles of mineralisation are described below.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 2 - Geological plan of the Nannine goldfield.
West trend The Edin Hope West lode is a shear-hosted deposit comprising lenticular pods of quartz-carbonate-chlorite±pyrite mineralisation containing abundant hematite and limonite
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quartz-goethite±sulphide assemblage. Coarse gold to the diameter of a five cent coin is also present. Gangue minerals include quartz, ankerite, sericite, fuchsite, goethite or hematite, potassium feldspar and plagioclase, with minor monazite, barite, rutile, zircon and leucoxene. Supergene gold is virtually silver free. Galena is the dominant sulphide but chalcopyrite, sphalerite and tetrahedrite also occur. Cerussite, mottramite [(Pb, Cu, Zn) (V04) OH], limonite and anglesite are common in the oxidised zone. Silver occurs in solid solution in primary native gold, to approximately 15%, and as supergene native silver. Minor mineralisation is located in the North Porphyry lode and the quartz-carbonate altered North shoot.
East trend The trend contains the Porphyry lode, East Porphyry lode, East lode and Camp lode deposits. Porphyry lode and East Porphyry lode typically have grades between 0.5 and 1.5 g/t gold, and high tonnages. Gold appears to be associated with fine and erratic quartz veining and not necessarily with the sericite and pyrite±carbonate alteration products. The Porphyry lode contains broad ore zones which are 10 to 30 m wide. The East lode mineralised quartz veins are hosted by a quartzcarbonate±chlorite rock and ore grade tends to be proportional to vein density. The quartz stockworks of the South East lode are hosted by a quartz-carbonate±fuchsite rock and grades vary between 0.5 and 2 g/t gold.
Mottled laterite There are low to medium gold grades (0.5 to 3 g/t) in mottled laterite in an area about 150 by 150 m in the SE of the South Junction pit. This supergene gold is mostly within goethite or hematite pisolites and occasionally in the accompanying argillaceous matrix, and is also present in a goethite-quartzclay rock.
Bluebird East FIG 3 - Geological plan of the Cuddingwarra goldfield.
replacing carbonate and pyrite. The Edin Hope lode contains the North Pole vein, originally worked between 1910 and 1930. It is a 0.5 m wide, laminated quartz sheet within quartzstockworked quartz-carbonate rock. Here coarse grained gold is associated with rare fine-grained galena and pyrite.
Central trend The trend comprises the South shoot, Central shoot, North shoot, Polar Star North lode and North Porphyry lode deposits. The South, Central, and North Polar Star shoots contain the highest grade ore, to 6 g/t gold. They are ladder-like stockworks and crosscutting quartz vein sets, in two subparallel NE-trending shoots of sheared quartz-carbonate-fushsite rock. The ore shoots are about 140 m long and up to 40 m wide. Enclosing the quartz stockwork rock is an envelope of brown goethite or hematite which occurs as stains and dustings in chlorite, mica and carbonate and as pseudomorphs after pyrite and other sulphides. Primary and supergene gold occur as discrete anhedral grains, skeletal aggregates and blebs and clusters. The grains are commonly between 0.002 and 0.1 mm in diameter in a gold-
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The Bluebird East deposit is at the nose of the Norie pluton (Fig 1), and the deposit is cut in the north by a NE-trending dolerite dyke. There are four types of mineralisation.
Main lode The lode is closely associated with the contact between felsic porphyry and quartz-carbonate-fuchsite rock, with mineralisation in both rock types. The felsic porphyry consists of quartz, ankerite and albite, and shear-oriented sericite pseudomorphs after tabular shaped feldspar of granitoid origin. Quartz-carbonate-fuchsite haloes around the felsic porphyry replace ultramafic rock which consisted of serpentinite, magnetite and olivine. The carbonate minerals are dolomite and ankerite, formed by carbon dioxide metasomatism. The lodes are up to 20 m thick, tabular and often stacked. They have a 30 to 40o east dip in the north, and steepen to near vertical in the south end of the pit. The distribution of the lodes appears to have been controlled by NW-trending and steeply SW-dipping shear zones which are often displaced by NEtrending faults. In longitudinal projection (Fig 4) the lodes pitch at 40o towards the south and are displaced by the NEtrending reverse faults or thrusts that dip at 65o to the SE.
Geology of Australian and Papua New Guinean Mineral Deposits
GOLD DEPOSITS OF THE BLUEBIRD, NANNINE AND CUDDINGWARRA GOLDFIELDS
lineaments, the north-trending Gabanintha shear, NW- and NEtrending faults, and the intrusion of two post-folding granitoids to the east and west of Nannine.
Nannine Mineralisation at Nannine occurs along the contact of the Gabanintha shear in which NNW-trending BIF units dipping at 60° to the NNE are cut by NNE-trending faults and quartz-filled shear zones that dip 70o to the NNW. Mineralisation occurs in podiform shoots that plunge between 30 and 50o north, along the intersection of the Nannine quartz reef and BIF units. Most of the gold occurs native with minor gold tellurides, silver-gold tellurides and a trace of electrum. Gold also occurs as discrete grains to 20 µm in diameter as inclusions in sulphides. The ore contains pyrite, galena, molybedenite, sphalerite and chalcopyrite and minor amounts of rutile, tellurides and gersdorffite (NiAsS). Gangue minerals are quartz, ankerite, calcite, siderite, dolomite, limonite and chlorite. FIG 4 - Longitudinal section on 21 600 E looking east, showing geology of the main ore zone, Bluebird East.
NW lode The lode is subvertical, tabular, up to 30 m wide and capped by a 2 m layer of duricrust. The host is a quartz-carbonatehematite alteration zone enclosed in a quartz-carbonatechlorite±limonite rock.
Great Northern Highway reef Gold mineralisation occurs in a 1 to 2 m wide blue-grey quartz reef in the SW of the pit. The reef is enclosed by a zone of talcchlorite schist country rock, which is 1 m wide in the hanging wall. A NE-trending dolerite dyke separates the NW and Main lodes.
Laterite deposits There are up to three gold-mineralised layers in the laterite, which reaches a thickness of 30 m at the south of the pit (Fig 4). The upper layer is a ferruginous pisolitic laterite which contains most of the gold and this layer comprises quartz with exotic goethite and hematite, kaolinite and montmorillonite.
NANNINE GOLDFIELD The deposits of this field are hosted by Golconda Formation, in BIF units in the upper part of the formation and in interlayered mafic volcanic rock and BIF near the contact with the overlying Gabanintha Formation (Fig 2). The Nannine field has a complex structural history, and mafic and BIF units have been folded and faulted on a microscopic and macroscopic scale. The major Nannine thrust fault with 3 km of displacement has sheared across the Golconda Formation between Nannine and Baileys Island. This has resulted in structural thickening of the Gabanintha and Golconda formations south of Nannine (Fig 2) and caused 600 m of sinistral displacement along the Caledonian fault. Interpretation of detailed aeromagnetic data has identified several NNE-trending shears or faults which are coincident with known deposits and indicate positions of potential deposits. Structural controls of the distribution of mineralisation include NE- to NNE-trending aeromagnetic
Geology of Australian and Papua New Guinean Mineral Deposits
Aladdin A major NW-trending curvilinear fault (D4), the Aladdin fault, transgresses the Golconda Formation and intersects the Gabanintha shear at the Aladdin deposit. The fault changes orientation from a strike of 022o and an 80o west dip at the southern end, to 033o strike and 60o NNW dip in the centre, to 056o strike and 40o north dip at the northern end. In the northern and central parts of the pit the footwall is well foliated talc-chlorite schist. The hanging wall consists of silicified pyritic basalt and silicified BIF, both containing abundant pyrite and pyrrhotite. Extensive faults and shear zones disrupt the host rocks, and the BIF units are discontinuous along strike. There are quartz-filled faults and shear zones oblique to the main Aladdin shear. Mineralisation occurs at the intersection of the fault and BIF units, as a steeply north-plunging sulphiderich lode. The lode is predominantly limonite and goethite, with minor leucoxene, pyrite and chalcopyrite, in a zone of quartz-carbonate alteration.
Baileys Island Three deposits have been identified on Baileys Island, each with different structural and mineralisation features. The Baileys Island North deposit is at the intersection of north-trending and east-dipping BIF units with a NE-trending shear zone. The shear zone contains anastamosing quartz veins, 0.02–0.2 m wide, which intersect highly weathered silicified limonitic BIF and hematitic mafic rock. The mineralisation plunges steeply to the north at this intersection and is hosted by magnetite-chert horizons in the BIF. Gold occurs as free crystalline grains of supergene origin in discontinuous shoots. Small intrusions of felsic porphyry occur throughout the deposit and are related to the adjacent western granitoid pluton. The mineralisation in the Baileys Island Central pit occurs at the contact of BIF and mafic flows, adjacent to the intrusive granitoid contact. Boxworks of pyrite often contain ‘dendritic’ free gold. Baileys South and East deposits, which are essentially one deposit, occur at the intersection of a fault zone, BIF units and the Gabanintha shear. The mineralisation at the contact occurs in tabular, steeply SW-dipping lodes associated with BIF, which contains pyrite, magnetite and quartz veining (Fig 5).
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Away from the contact the mineralisation is associated with intense wall rock alteration comprising carbonisation, silicification and hematisation. Gold occurs as inclusions in pyrite and arsenopyrite, and boxworks after these sulphides often contain dendritic free gold. The mineralisation tends to be stratabound in magnetite-chert bands in BIF. The altered mafic volcanic rocks consist of quartz, siderite and chlorite, and the BIF contains pyrite and hematite near the contact with the shear zone. Away from the contact zone the mafic volcanic rocks consist of actinolite after clinopyroyxene and saussurite after plagioclase, titanite and chlorite. The BIF consists of siderite, magnetite, quartz, grunerite and minor chlorite (ripidolite) and sulphides.
FIG 6 - Cross section 28 600 N looking north, showing geology of the Black Swan South deposit.
Visible gold was noted in drill core, in two sets of narrow quartz veins which cut the porphyry. One set dips steeply to the east, and the other dips at 70o to the west. A later quartz vein set containing tourmaline is barren of gold mineralisation and dips steeply east. The dominant sulphide is pyrite, with subordinate pyrrhotite and arsenopyrite, all proximal to quartz veins. Minor galena and sphalerite occur in gold-bearing quartz veins. FIG 5 - Cross section 20 400 N looking north, showing geology of the Baileys Island East deposit.
CUDDINGWARRA GOLDFIELD The Black Swan laterite root zone, Black Swan South and Rapier deposits here are parts of a system of en echelon zones of mineralisation (Fig 3). The host is quartz and feldspar porphyry, intrusive into a sequence which is predominantly tholeiitic basalt, variably altered to chlorite±carbonate schist, and ultramafic rocks, of which some are altered to talc-chlorite±carbonate schist. At Black Swan South the porphyry is emplaced in a tholeiitic basalt of marine origin, as shown by pillows, variolitic and hyaloclastic textures and peperite. The porphyry is variably silicified and carbonated, and contains disseminated pyrite as the main sulphide. The porphyry bodies are up to 40 m wide (Fig 6) and trend north to NE. They have been variably silicified, and have a ‘bleached’ appearance near quartz vein sets. Alteration products are mainly quartz and carbonate with chlorite-carbonate proximal to quartz vein margins. A lopolithic structure that plunges SSW is interpreted in the north portion of the deposit. The Rapier root zone porphyry dyke, to 15 m wide, trends north and dips east.
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The auriferous quartz veins strike north, NE and NNW, with a predominant easterly dip, although some narrow mineralised quartz vein sets dip west. The most substantial gold concentrations occur at intersections of north- and NE-trending shear structures and in dilational zones at the intersections of north- and NE-trending porphyry dyke swarms. Lateritic gold deposits also occur at Black Swan and Rapier. The gold is in a pisolitic layer extending to 8 m below surface. A palaeochannel containing mottled clays to a depth of 30 m extends NE across the northern parts of the Black Swan South deposit in which the clays are barren of gold. The base of oxidation varies between 50 and 80 m depth.
ACKNOWLEDGEMENTS The authors gratefully acknowledge the permission of St Barbara Mines Ltd to publish this information. G Knox, J Wheaton, C Garrett, A Islam, T Swe and B Wolfe are thanked for their comments, suggestions and contributions.
REFERENCES Adams, C F, 1945. The future of gold mining in Western Australia, WA Mines Department Annual Report for 1944, pp 30–40. Burrows, M, 1992. The effects of wallrock chemistry on gold depositional mechanisms in the Nannine mining centre, Western Australia, BSc thesis (unpublished), The University of Western Australia, Perth.
Geology of Australian and Papua New Guinean Mineral Deposits
GOLD DEPOSITS OF THE BLUEBIRD, NANNINE AND CUDDINGWARRA GOLDFIELDS
Campbell, W D, 1908. Notes on the auriferous reefs of Cue and Day Dawn, Geological Survey of Western Australia Bulletin 7. Ellice, T J, 1990. The structural controls on gold mineralisation at Nannine, Mt Magnet Meekatharra greenstone belt, Western Australia, BSc thesis (unpublished), The University of Western Australia, Perth. Fiala, J D, 1992. South Junction gold deposit, Meekatharra, The AusIMM Bulletin, 3:85–96. Gibson, C G, 1904. The geology and mineral resources of part of the Murchison Goldfield, Geological Survey of Western Australia Bulletin,14:53–60. St Barbara Mines Limited, 1991, 1992, 1993, 1994, 1995. Annual reports (St Barbara Mines Limited: Perth).
Geology of Australian and Papua New Guinean Mineral Deposits
Walsh, J F, 1990. Caledonian gold deposit, Nannine, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 187–189 (The Australasian Institute of Mining and Metallurgy): Melbourne. Watkins, K P, Hickman, A H, Davy R, Ahmat, A L and Fletcher, I R, 1990. Geological evolution and mineralization of the Murchison Province Western Australia, Geological Survey of Western Australia, Bulletin 137:1–220. Woodward, H P, 1907. A report upon the geology together with a description of the productive mines of the Cue and Day Dawn districts, Murchison Goldfield, Cue and Cuddingwarra Centres, part one, Geological Survey of Western Australia, Bulletin 29: 80–93.
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Ross, D I and Smith, D W, 1998. Omega gold deposit, Gidgee, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 119–122 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Omega gold deposit, Gidgee 1
by D I Ross and D W Smith
2
INTRODUCTION The deposit is owned by Arimco Mining Pty Ltd, and is 92 km ESE of Meekatharra, WA, in the Wiluna district of the East Murchison mineral field. It is within the Glengarry (SG 50–12) 1:250 000 scale and the Yanganoo (2744) 1:100 000 scale map sheets at lat 26o50′S, long 119o24′E and AMG coordinates 736 300 E, 7 027 000 N (Fig 1). The gold mineralisation is associated with quartz-pyrrhotite veins hosted by banded iron formation (BIF). Open cut mining of a Proved Reserve of 190 000 t at 6.56 g/t gold commenced in April 1991 and ceased in December 1992. During this period 217 974 t were mined at a head grade of 6.10 g/t from which 1170 kg (37 600 oz) of gold were recovered. Trial underground mining was undertaken in the first half of 1996, to recover a further 101 kg (3200 oz) of gold from 25 832 t at a head grade of 4.64 g/t. Recent drilling has shown that the mineralisation continues at depth where an Indicated Resource of 287 500 t at 7.57 g/t has been outlined. The mineralisation remains open down plunge.
EXPLORATION AND MINING HISTORY The Omega area was first identified as having potential for BIFhosted gold deposits by George de San Miguel of Amoco Minerals Australia Company in 1984. Significant gold values in samples from reverse circulation percussion (RC) drill holes under old prospecting shafts in BIF at the Camp prospect 800 m north of Omega (Fig 2) during 1985, encouraged exploration north and south along the strike of the BIF. An exploration program involving mapping and soil and rock chip sampling detected the Omega deposit under a prominent hill of lateritised BIF. A soil sampling program using the bulk leach extractable gold (BLEG) technique with a 100 by 20 m sample spacing outlined a strong anomaly over the deposit, with a peak value of 60 ppb cyanide-soluble gold. A short ( <5 m) costean cutting a quartz vein on the eastern contact of the BIF was the only sign of previous exploration activity at the site. Further RC drilling in 1987 provided very encouraging results. Various RC drilling campaigns continued through to 1990, enabling estimation of an ore reserve and pit design. Metallurgical testwork revealed that both oxide and fresh ore were free milling, with an average gold recovery of 88% through a CIP plant. Open pit ore was trucked 45 km south to the Gidgee gold mine where it was blended with other ores prior to treatment. 1.
Senior Exploration Geologist, Gidgee Gold Mine, Arimco Mining Pty Limited, 5 Mill Street, Perth WA 6000.
2.
Project Geologist, Gidgee Gold Mine, Arimco Mining Pty Limited, PO Box 1095, Midland WA 6936.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Location and geological map, Gum Creek greenstone belt, after Beeson, Groves and Ridley (1993).
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the centre of the belt are considered to be part of the lower sequence, having been brought to the surface by major folding and faulting. Metamorphic grades range from lower greenschist in the middle of the belt to lower amphibolite on the margins and around the granitoid intrusives.
ORE DEPOSIT FEATURES LITHOLOGY The Omega area contains amphibolite after basalt and dolerite, BIF and minor ultramafic rocks. The strike of the rocks is generally north and dips are very steep to vertical. Metamorphic grade is lower amphibolite facies. The BIF hosting the Omega deposit has a strike length of 12 km and attains a maximum true thickness of 35 m. Thickness varies considerably along strike due to deformation and continuity is often interrupted by faulting. The surface expression of the Omega BIF is heavily lateritised, often with a pisolitic ironstone cap. More cherty outcrops are generally broken and vuggy. The Omega BIF contains three main types (G Lebas, unpublished data, 1993): magnetite-chert (oxide facies), amphibole-chert (silicate facies) and chlorite-amphibolegarnet metasediment. The oxide facies BIF is a distinctly banded, black and white coloured unit with frequent interruptions to the continuity of banding by folding and faulting. It contains up to 40% magnetite, 30 to 40% chert and <30% amphibole. At Omega the steeply dipping oxide facies BIF trends through the centre of the open pit and has amphibolite on both sides (Fig 2). The silicate facies is weakly banded with much of the original banding lost due to metamorphism, and is dominated by cummingtonite-grunerite with minor hornblende and chlorite. The chlorite-amphibole-garnet metasediment has relict banding, varies in thickness from centimetres to 5 m and is found predominantly on the contact between BIF and amphibolite. It contains hornblende, grunerite, biotite, chlorite, quartz and coarse porphyroblastic almandine garnet. The amphibolite in the open pit is a fine to medium grained, well foliated rock, the major constituents being actinolite and plagioclase. A thin ultramafic unit of talc-tremolite-chlorite schist is present in the western wall of the pit. FIG 2 - Geological plan of the Omega area.
REGIONAL GEOLOGY The deposit is at the northern end of the Gum Creek greenstone belt (Fig 1). The greenstone belt, 110 km long and up to 25 km wide, is at the northern end of the Southern Cross Province of the Archaean Yilgarn Craton. It is elongate NNW and contains a southerly plunging syncline in which volcanic and sedimentary rocks are bounded on the east and west by granitoids. The Gum Creek greenstone belt comprises a lower sequence of mafic and ultramafic extrusive and intrusive rocks interbedded with BIF, overlain by a sequence of felsic volcanic and mafic volcanic rocks and sediment, mainly black shale (Beeson, Groves and Ridley, 1993). Granitoid stocks and eaststriking Proterozoic dolerite dykes intrude both sequences. Although the structure is synclinal, the mafic volcanic rocks in
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FIG 3 - Cross section on 29 275 N, Omega deposit, looking north.
Geology of Australian and Papua New Guinean Mineral Deposits
OMEGA GOLD DEPOSIT, GIDGEE
FIG 4 - Longitudinal projection of the Omega deposit on 9900 E, looking west.
MINERALISATION TABLE 1 Mineralogy of the Southern ore shoot, Omega deposit 1.
There are two gold bearing shoots at the Omega open pit. The Southern shoot is at the point where the strike of the BIF turns from north to ESE. Drilling has shown that this change in strike direction is caused by an open fold which plunges southerly at 70o, with the gold-bearing shoot confined to the hinge area of the fold (Fig 3). Thus the Southern shoot has a strike length of only 30 m, a maximum width up to 20 m, but a down-plunge extent of more than 300 m (Fig 4). Weathering extends to approximately 60 m below surface. The Southern ore shoot consists of discontinuous quartzpyrrhotite veins to 6 m wide surrounded by a zone of pyrrhotite banding in BIF. The veins contain the highest gold grades, to 400 g/t, with visible gold being common. Pyrrhotite replacement of magnetite in the BIF extends several metres on either side of the veins and varies from a few per cent to almost total replacement of the BIF. Gold grade generally increases, from trace to 10 g/t gold, with the increase in pyrrhotite content. The mineralogy of the Southern ore shoot is listed in Table 1. Apart from gold, no other definitive or pathfinder elements are known for the Omega deposit.
Geology of Australian and Papua New Guinean Mineral Deposits
1.
MINERAL
ABUNDANCE
quartz
major
pyrrhotite
major
cummingtonite
minor
magnetite
minor
almandine
accessory
chlorite
accessory
native gold
trace
chalcopyrite
trace
breithauptite (NiSb)
trace
From D Mason, unpublished data, 1995.
The Northern shoot is also on an open fold in the BIF which has a steep southerly plunge. In the open pit it had a strike length of 40 m and a width of 10 m. Weathering extended to 80 m below surface. In fresh rock the mineralisation consists of
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vuggy quartz-pyrrhotite-pyrite veins. Currently the Northern shoot has not been found at depth.
ORE GENESIS The origin of the Omega gold deposit can be explained by the model accepted for many Archaean epigenetic lode gold deposits, deposition of gold in structurally prepared sites within an iron-rich host rock. Thus the sequence of events at Omega would be: 1.
Folding of the BIF produced dilatancy in the hinge area of folds.
2.
Mineralising fluids were introduced into these dilatant zones, gold being carried in solution as sulphur complexes.
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3.
The reaction of sulphur with magnetite in the BIF led to the formation of pyrrhotite.
4.
With the removal of sulphur from solution, gold was precipitated.
ACKNOWLEDGEMENTS The authors gratefully acknowledge the permission of Arimco Mining Pty Limited to publish this information. Special thanks are due to D W Otterman and G de San Miguel for their reviews of the manuscript.
REFERENCES Beeson, J, Groves, D I and Ridley, J R, 1993. Controls on mineralisation and tectonic development of the central part of the northern Yilgarn Craton, Minerals and Energy Research Institute of Western Australia, Report 109 (unpublished).
Geology of Australian and Papua New Guinean Mineral Deposits
Hazard, N J, 1998. Kingfisher gold deposit, Gidgee, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 123–126 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Kingfisher gold deposit, Gidgee by N J Hazard
1
INTRODUCTION The deposit is 125 km SE of Meekatharra and 100 km north of Sandstone, WA within the Black Range district of the East Murchison mineral field (Fig 1). It is at lat 27o15′S, long 119ο25′E on the Sandstone (SG 50–16) 1:250 000 scale and the Youno Downs (2743) 1:100 000 scale map sheets. It is the largest of the 11 ore deposits discovered within a 3 km radius of the Gidgee gold mine mill. Prior to the commencement of open pit mining in August 1990, the Measured Resource for the deposit was 1.46 Mt at 4.5 g/t gold (211 000 oz of contained gold), using a low cut of 1 g/t and a high cut of 30 g/t. At the completion of mining, in June 1996, 264 950 oz of gold had been produced from 2.01 Mt of ore from the Kingfisher open pit at a recovered grade of 4.1 g/t gold and 27 200 oz of gold had been produced from 60 000 t of underground ore at a recovered grade of 14.1 g/t.
EXPLORATION AND MINING HISTORY The history of mining and exploration of the early Gidgee deposits is detailed by Otterman (1990). The Kingfisher area is covered by a thin layer of sandy clay loam overlying up to 6 m of partly iron oxide- and/or carbonate-cemented sheetwash (the Wiluna Hardpan), making surface geochemical sampling ineffective. There was no outcrop or evidence of old workings. In 1984 lines of rotary air blast (RAB) drill holes, on 400 to 800 m spacing, with vertical holes to 21 m depth spaced 20 m apart on the lines, were drilled over a structural trend defined by detailed low level aeromagnetic data. The drilling tested a zone SE from the old Jonesville workings, on which the Gidgee gold mining operations are largely based. Weakly anomalous gold and arsenic values were obtained in holes on a number of lines with a best result of 0.3 ppm gold over a 3 m interval. Other exploration priorities delayed follow up drilling until September 1989 when more detailed RAB drilling was completed, providing a highest assay of 3.1 g/t gold over a 3 m interval. Follow up RAB drilling in February and March of 1990 outlined a continuous mineralised zone trending at 310o true over a strike length of 1500 m. The first reverse circulation percussion (RC) drilling program at Kingfisher commenced in April 1990, with the seventh and twelfth holes providing spectacular results. Hole JRC1072 intersected 29 m at 18.44 g/t gold, and JRC1077 intersected 22 m at 13.93 g/t gold within a 54 m mineralised interval. Early in the program, it was realised the holes were drilled down the dip of the mineralisation, and subsequent holes, from JRC1084, were drilled towards grid east or 055ο true. By the end of July 1990, a total of 380 RC holes and 15 1.
Exploration Manager WA, Golden Cross Resources NL, 15 Frinton Avenue, City Beach WA 6015.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Location and geological map of the Gum Creek greenstone belt, after Beeson, Groves and Ridley (1993).
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diamond core holes had been drilled and an interim pit commenced. After a series of extensions and one major cutback, the open pit eventually measured 960 m long by 300 m wide by 120 m deep. A number of deep RC and diamond drill holes identified a second south-plunging ore shoot beneath the central and southern portion of the open pit. An exploratory decline beneath the pit to determine the viability of underground mining of these shoots commenced in February 1994 from a portal within the pit. The underground development comprised five levels, at 15 m intervals, accessed by a decline. Average mining width was 2.5 m, typically extracting a narrow, high grade zone on the hanging wall contact of a mineralised quartz vein. At the time of writing, underground mining had halted and the feasibility of mining a third shoot, beneath and north of the central shoot, was being assessed.
REGIONAL GEOLOGY The Gidgee deposits are in the middle of the Gum Creek greenstone belt, in the northern part of the Southern Cross Province of the Archaean Yilgarn Block. The greenstone belt is a sigmoidal, south-plunging synclinorium elongate NW. It is enclosed to the east and west by granitoid and gneiss and intruded by a number of granitoid stocks (Fig 1). The succession, as described by Beeson, Groves and Ridley (1993), comprises five major volcanic-sedimentary stratigraphic units and related intrusives. The lowest sequence (unit 1), which crops out as ranges of low hills around the margins of the belt, includes basalt, ultramafic sills and banded iron formation. Overlying these is a monotonous sequence of basalts (unit 2) which are locally pillowed. The third unit recognised is a felsic volcanicsedimentary sequence dominated by tuff and tuffite, with coarse clastic rocks grading to siltstone and rare rhyolitic flows. A monotonous sequence of tholeiitic and high magnesium basalt, which hosts the Gidgee gold deposits, forms unit 4. Narrow zones of spinifex-textured ultramafic rocks (komatiites) are found locally within this unit. The fifth unit comprises mixed felsic and basaltic rocks overlain by clastic sediment including black shale. Massive layered pyroxenite-gabbro sills have intruded along rock contacts, particularly within unit 2 along the eastern margin of the belt. Later faulting and interference folding and the intrusion of monzogranite stocks have created areas of structural complexity in the central part of the belt, exposing what are interpreted as unit 4 mafic volcanic rocks, including the Gidgee gold mine sequence. A large proportion of the Gum Creek greenstone belt, especially away from the margins, is covered by unconsolidated and semiconsolidated alluvium and colluvium. Southeast-draining deep palaeochannels occur in the central and southern parts of the belt and are related to the Lake Mason drainage system.
ORE DEPOSIT FEATURES
FIG 2 - Geological plan of the Gidgee gold deposits interpreted from aeromagnetic data.
LITHOLOGY The Kingfisher mineralisation is hosted by a highly fissile, banded sericite-carbonate schist, which is immediately adjacent to the regional scale, NW-trending Kingfisher fault,
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which is on the western limb of an interpreted domed maficultramafic sequence (Fig 2). The sericitic schist was previously referred to as a mylonite by Hazard and Crook (1993).
Geology of Australian and Papua New Guinean Mineral Deposits
KINGFISHER GOLD DEPOSIT, GIDGEE
Additional diamond core and further petrography (D Mason, unpublished data, 1995) suggest that the level of deformation is not as extreme as previously thought, and that narrow banding within the schist is due to a layered tuffaceous unit of varying composition and not to tectonically induced structures. The west dipping schist has a massive, weakly foliated, amygdaloidal high magnesium basalt on the hanging wall and interbedded basaltic flows and tuffs on the footwall side (Figs 3 and 4). These flows grade into more massive basalt at depth, as evidenced in intersections in the deepest diamond drill holes.
STRUCTURE The Kingfisher fault zone strikes at 325o true through the Kingfisher pit and dips, on average, at about 60 o to the SW. Open folding and subsequent movement on the fault are considered to have created favourable sites for gold mineralisation, particularly where there is a 45 to 50o westerly dip. The central part of the Kingfisher orebody coincides with a brecciated, faulted and quartz veined zone of sericitic schist to about 30 m wide, with at least a further 30 m of deformed, sericitised rock in the footwall. The zone of deformation and alteration is narrower and more sharply defined in the northern part of the open pit. Primary gold mineralisation is aligned parallel to the Kingfisher fault, which was interpreted (APC Pty Ltd, unpublished data, 1991) from satellite imagery and mapping within the North End pit, 3 km to the north (J Baxter and J Reid, unpublished data, 1988), to have an apparent sinistral strike slip movement. Evidence from the Kingfisher pit and the nearby Hawk and Eagle pits, respectively south and north along strike, suggests there is also a strong reverse dip-slip component. Evidence includes drag features and quartz vein offsets on the southern wall of the Kingfisher pit and sigmoidal deformation and rotation of early phase, non-gold bearing quartz veins in the Eagle pit.
FIG 3 - Cross section of the Kingfisher deposit on line 10 260 N, looking NW.
Crosscutting dextral strike-slip faulting occurs oblique to the Kingfisher fault. It is these faults, which strike at 355o to 360o true and dip steeply west that are considered fundamental to the formation of the Kingfisher deposit. The most significant dextral fault, with respect to gold mineralisation, occurs in the southern portion of the pit at 10100 N. The intensity of veining and mineralisation decreases to the north and south away from this fault, which suggests that it was probably the conduit for the mineralising fluids. It cuts across veining and disrupts the mineralisation, which indicates that the fault was active both during and post-mineralisation.
MINERALISATION
FIG 4 - Geological plan and ore blocks, on 483 RL, Kingfisher open pit, with location of section line for Fig 3.
Due to the extensive alteration within the Kingfisher fault zone, it is difficult to recognise rock precursors. Multi-element geochemical analyses of fresh diamond core from holes on line 10 200 N indicate three rock types, identified by separate populations of chromium values and titanium:zirconium ratios. The titanium:zirconium ratio method used by Hallberg (1984) to categorise igneous rocks was modified thus: acid rocks <4< intermediate rocks <60< basic-ultrabasic rocks. Basic and ultrabasic rocks were differentiated on the basis of relatively high chromium values in the ultrabasics. The hanging wall high-magnesium basalt tends to have elevated chromium values in the range 300 to 700 ppm, and a titanium:zirconium ratio that plots on the andesite–basalt boundary. The sericite schist has low chromium values and titanium:zirconium ratios in the felsic range, suggesting that the precursor is a felsic tuffaceous unit. Basaltic flows and tuffs in the footwall typically have 70 ppm chromium and titanium:zirconium ratios within the basalt field.
Geology of Australian and Papua New Guinean Mineral Deposits
Gold mineralisation is directly related to quartz veining. The most significant veins tend to occur at the hanging wall contact and are 0.3 to 4 m wide. A stockwork zone, to 15 m wide, of quartz-ankerite veins extends into the footwall. The highest gold grades tend to be associated with quartz veining at the hanging wall contact, with lower grades associated with the footwall stockwork. At least two phases of veining have been identified. An early phase of milky quartz veining cuts across the foliation (D3) in the sericite schist (Beeson, Groves and Ridley, 1993). These early veins are also displaced along D3 planes. Carbonate (mainly ankerite) occurs within the veins and is most common along the vein margins, suggesting an early carbonate-rich phase. The second phase of veining comprises quartz-filled breccias and these are considered to be part of the metallogenic event. Breccia clasts comprise milled and angular fragments of country rock. These quartz-filled breccia lodes located near or on the hanging wall contact are considered to have been the final veining event as no other crosscutting veins have been recognised. Although these veins are dominated by quartz there is some carbonate gangue. The footwall vein stockwork predominantly comprises quartz and ankerite. Weak quartz veining occurs within the hanging wall metabasalt, however
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N J HAZARD
gold grades associated with these veins are generally low. The quartz veining can exhibit complex brittle fracture arrays, pinch and swell structures along strike and down dip and variations in dip between 45o and 70o to grid west (235o true). The sulphide content of the quartz veins is generally less than 1%, although locally fine grained disseminated and coarser grained euhedral pyrite comprise up to 20% of the vein fill. Higher sulphide content is more prevalent in the southern part of the pit. Scattered occurrences of arsenopyrite, galena, sphalerite and chalcopyrite have been observed in diamond drill core or thin section (D Mason, unpublished data, 1995). Trace amounts of azurite and malachite have been observed within microfractures in quartz veins. Dark green to black tourmaline in a massive, layered microcrystalline to finely acicular form is often associated with ore grade gold mineralisation in quartz veins, but can be found in barren quartz veins. Epidote and chlorite commonly occur on the margins and within the immediate wall rock of high grade quartz veins. Visible gold is usually associated with segregations of euhedral pyrite within massive quartz or in close proximity to the hanging wall contact. It also occurs within massive cryptocrystalline quartz, free of sulphides, and in massive tourmaline (S Dexter, personal communication, 1993).
ALTERATION AND WEATHERING The Kingfisher deposit is hosted by rocks which have been regionally metamorphosed to greenschist facies. Greenschist mineral assemblages have been overprinted by carbonatepotassium-sulphur metasomatism, which has different effects in different rock types. The hanging wall high-magnesium basalts have an alteration assemblage of albite-actinolite(epidote)-chlorite-calcite, whereas the footwall basaltic flows and tuffs have an assemblage of quartz-chlorite-sericitechloritoid-carbonate. Quartz- and/or carbonate-filled amygdales, to 2 cm maximum dimension, are common in the hanging wall high-magnesium basalts. These basalts are only weakly carbonated 20 m from the Kingfisher fault but become increasingly carbonated towards it. There is very little silicification in the hanging wall but strong epidotisation occurs locally, particularly in close association with high grade quartz veins. Alteration in the footwall is dominated by sericite, quartz and carbonate. Silicification is pervasive, but is intense adjacent to major quartz veins. A strong quartz-ankerite vein stockwork occurs in the footwall with associated carbonate alteration. Sericitisation is extremely strong within the schist and is pervasive throughout the footwall. Pyrite often occurs along the vein margins as fine disseminations and as euhedra of 1–2 mm diameter. Disseminated pyrite occurs in silicified zones and is often aligned along cleavage planes. In the Kingfisher pit, between 10 300 N and 10 500 N, limonite staining is prominent, reflecting a greater abundance of pyrite in this zone. Massive ironstones occur along flat lying joints and extensive limonitic staining of the footwall rocks has occurred. A major north-striking dextral fault occurs in this area and is considered to have been the conduit for sulphidising fluids. The area within 1 km of the Kingfisher deposit is flat and covered by reddish brown sandy soil over a 3 to 6 m layer of ferricrete and ferruginous gravels. The depth of cover increases away from the mineralised zone to the east, west and south. Weathering beneath the transported overburden is extreme, with saprolitic clays occurring to a minimum depth of 60 m from the surface. Adjacent to the main mineralised structures,
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weathering extends to more than 120 m depth. The pre-mining water table was at about 35 m below ground level. Mineralised zones at Gidgee, including the Kingfisher deposit, contain manganese oxides in the weathered zone. They occur as dendrites and films on fracture surfaces and in places are quite abundant. The primary source of the manganese is probably complex carbonates associated with the metasomatic alteration accompanying the gold mineralising event. Calcite, siderite and ankerite have been recognised in drill core. Limited microprobe work on drill core from the Gidgee environment (J Borner, unpublished data, 1985) indicated that other complex manganese-bearing carbonates are also present. Gold from the primary zone has been remobilised to form an enriched supergene blanket, commencing approximately 30 m below ground level and extending to the base of extreme weathering 70 m below surface. A large proportion of the open pit gold production from Kingfisher came from this enriched zone.
ORE GENESIS Epigenetic emplacement of gold in the primary zone at the Kingfisher deposit is considered to have occurred after regional dip-slip and strike-slip (D3) faulting (Beeson, Groves and Ridley, 1993) and during, or immediately after, crosscutting dextral faulting. The dextral faulting , which is likely to have been caused by the intrusion of a granite pluton west of Kingfisher, created dilatant zones within the sericitic schist. Mineralising fluids of probable metamorphic origin, with an early carbonate-rich phase, intruded these dilatant zones during the deformation process. Quartz lodes were formed parallel to the schist, with their strike length determined by the distance between dextral faults.
ACKNOWLEDGEMENTS The author gratefully acknowledges the permission of Australian Resources Limited to publish this paper. The author also wishes to acknowledge the work done by all of the contractors and staff who have built up the database of knowledge at Gidgee gold mine and in particular, the major contribution made by D Crook who was the Senior Exploration Geologist at Gidgee for much of the time. Special thanks are also due to J Harris and D Otterman, who reviewed the first draft of the manuscript.
REFERENCES Beeson, J, Groves, D I and Ridley, J R, 1993. Controls on mineralisation and tectonic development of the central part of the northern Yilgarn Craton, Minerals and Energy Research Institute of Western Australia, Report 109. Hallberg, J A, 1984. A geochemical aid to igneous rock type identification in deeply weathered terrain, Journal of Geochemical Exploration, 20:1–8. Hazard, N J and Crook, D J, 1993. Geology and mineralisation of the Kingfisher orebody, Gidgee Gold Mine, Gum Creek greenstone belt, Western Australia, in Proceedings Mineral Exploration and Geology in the Eastern Goldfields (Ed: I Roberts) pp 37–44 (Western Australian School of Mines: Kalgoorlie). Otterman, D W, 1990. Gidgee gold deposits, Jonesville, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 267–271 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Geology of Australian and Papua New Guinean Mineral Deposits
Phillips, G N, Vearncombe, J R, Blucher, I and Rak, D, 1998. Bronzewing gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 127–136 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Bronzewing gold deposit 1
2
3
by G N Phillips , J R Vearncombe , I Blucher and D Rak
4
INTRODUCTION The deposit is 500 km north of Kalgoorlie and 80 km NE of Leinster WA, at lat 27o23′S, long 120o59′E on the Sir Samuel (SG 51–13) 1:250 000 scale and the Mount Keith (3043) and Wonganoo (3143) 1:100 000 scale map sheets. Mount McClure gold mine is 8 km to the west and the Mount Joel deposit is 20 km to the NE (Fig 1). Total Measured, Indicated and Inferred Resources at December 1996 were 38.3 Mt at 2.9 g/t gold with a cutoff grade of 1 g/t, for 112 t (3.6 Moz) of contained gold, and production to December 1996 totalled 12.6 t. Proved Ore Reserves at December 1996 were 6.6 Mt at 4.8 g/t with Probable Ore Reserves of 5.4 Mt at 4.3 g/t, for a total of 55 t of contained gold. The total endowment at Bronzewing (production plus resources) is 124 t (4 Moz) of gold. Bronzewing is a recent discovery in an area of thick transported overburden and deep weathering, away from significant old workings. It is owned by Great Central Mines Limited, who discovered it by geological targeting, understanding the regolith character, regional reconnaissance drilling to bed rock, and extensive follow up drilling.
EXPLORATION AND MINING HISTORY There are few old gold workings in the Yandal belt compared to many other Archaean greenstone belts of WA. This lack of success by early prospectors, despite the proximity to major gold mining centres like Wiluna and Leonora, can be attributed to the thick blanket of alluvial cover over much of the belt, obscuring the prospective greenstone rocks. There was some base metal exploration in the belt in the 1970s both south of Bronzewing towards the area where Teutonic Bore copper-zinc mine was discovered (Hallberg and Thompson, 1985), and to the north around Jundee where Chevron Exploration Corporation was active. The potential for gold in the Yandal belt was significantly upgraded in 1987 once its similarities to the highly prospective Norseman–Wiluna belt were recognised. Prior to this, the Yandal belt had been compared with other greenstone belts of the Northeast Yilgarn Province, with an implication of
1.
General Manager, Great Central Mines Ltd, c/- PO Box 3, Central Park Vic 3145.
2.
Consultant, Vearncombe & Associates Pty Ltd, 14a Barnett Street, Fremantle WA 6160.
3.
Former Chief Geologist, Bronzewing Gold Mine, now Field Operations Manager, Astro Mining NL, PO Box 1594, West Perth WA 6872.
4.
Senior Exploration Geologist, Bronzewing Gold Mine, Box 1594, West Perth WA 6872.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Geological map of the Yandal greenstone belt showing the major rock units and larger gold deposits. Sundowner consists of three distinct areas of mineralisation (Old Sundowner, New Sundowner, Cyclonic) and Mount Joel consists of at least ten separate areas of mineralisation.
relatively low prospectivity. Key indicators of the Yandal belt’s prospectivity include the abundance of host rocks such as iron-rich tholeiitic basalt and differentiated dolerite that could promote gold deposition and host higher gold grades, throughgoing shear zones and faults, and limited banded iron formation in contrast to greenstone belts further east. In some areas not covered by transported alluvium, there was widespread quartz
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veining with sulphide-mica-carbonate alteration assemblages, and anomalous gold values. Most of these features were evident in the vicinity of Bronzewing, particularly around Mount Joel, and were supported at that time by new ideas linking granite-greenstone boundary patterns to gold deposit distribution through heterogeneous stress in greenstone belts (Houstoun, 1987; see Fig 1 of Phillips, Eshuys and Hellsten, 1996). Confidence in Yandal belt prospectivity has been justified with the discovery of 10 Moz of gold in the belt in the 1990s (Vearncombe, 1997). The history of exploration at Bronzewing is recorded by Eshuys et al (1995). Great Central Mines Limited targeted areas for exploration based on favourable geological parameters, and used drilling methods most appropriate for the regolith cover. Exploration started near Jundee where surface sampling had shown extensive anomalous gold values (Wright and Herbison, 1995), and at Mount Joel where Minsaco Resources had revealed significant mineralisation in a favourable geological setting. The earliest drilling by Great Central Mines at Bronzewing was a regional rotary air blast (RAB) drilling program in 1992 using fence lines for access. The sixty-fifth hole in this program intersected 12 m at 1.1 g/t gold from 72 m and 4 m at 1.7 g/t from 44 m, and was followed by further RAB, reverse circulation percussion and extensive diamond core drilling. The three main bodies of mineralisation at Bronzewing are Central zone, Discovery zone and Western zone. Production commenced on 4 November 1994 at the Laterite, Central and Discovery open pits; Central pit became the access to underground mining from mid 1995, with Discovery pit the main source of ore for the first two years of production, prior to major underground mining. Shoot 39 is a component of Central zone distinguished by its high grade and size. Bronzewing is currently among Australia’s top ten gold producers. Other producing gold mines in the Yandal belt (Fig 1) include Jundee (Phillips, Vearncombe and Murphy, this publication), Nimary, Mount McClure (Otterman and de San Miguel, 1995; Harris, this publication) and Darlot. A feature of the Yandal belt is the relatively high grade of its gold deposits compared to some other active mining provinces in Australia today. Future plans for Bronzewing focus on underground resources plus ore from surrounding deposits including Mount Joel (several areas), Sundowner (Cyclonic) and Mandilla Well.
PREVIOUS WORK Some aspects of exploration methodology and practice in the Yandal belt, especially around Bronzewing, have been documented by Eshuys and Lewis (1995), Eshuys et al (1995) and Phillips, Eshuys and Hellsten (1996). The nature of the regolith in the Bronzewing–Mount McClure district has been the focus of research at CSIRO (Varga, Anand and Wildman, 1996). Research at the Centre for Strategic Mineral Deposits at the University of Western Australia on the primary mineralisation has included studies of petrology, alteration and veining (Dugdale, 1996). Research at the Earth Sciences Department of The University of Melbourne has furthered the mineralogical and alteration studies including magnetic susceptibility and links to aeromagnetic responses (Kotz, 1996; Hollonds, 1997). Unpublished studies include the petrography of Bronzewing rocks by M Clough, the regional and local structural geology by J Vearncombe (partly described in Vearncombe, 1996), alteration by P Eilu, the geophysical setting of Bronzewing by L Starkey, and structural geology and reserve evaluation by the staff of Bronzewing gold operations.
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REGIONAL GEOLOGY The Yandal greenstone belt in the northeastern part of the Archaean Yilgarn Block of WA is approximately 250 km long and up to 30 km wide, and is surrounded by Archaean granites (Fig 1), most with deep weathering and/or considerable cover of alluvium.
LITHOLOGY The main rock types of the Yandal belt are komatiite flows and intrusions, high-magnesium and tholeiitic basalt and dolerite, intermediate to felsic volcanic rocks, internal granitoid plutons, and medium to fine grained clastic and chemical sedimentary rocks. Most rock types are metamorphosed; deformation is both heterogeneous and locally intense, although there are large tracts of virtually undeformed rock. The dominant mafic rocks are described later as they have a special role in guiding the focus of exploration, and understanding alteration and gold distribution. Ultramafic rocks are a minor part of the greenstone succession in the Bronzewing district, and include linear bodies immediately north and east of Bronzewing that are known from drilling and can be traced by their strong aeromagnetic signature. To the NE is a 100 m thick komatiite unit, the Bronzewing komatiite. Further ultramafic rocks in the west of Central and Discovery pits are less differentiated sections within a dolerite sill (see below). The komatiite is the most magnetic unit in the Bronzewing mine area, and significantly more magnetic than the ultramafic section of the Bapinmarra dolerite sill. High magnesium basalt is a lesser part of the succession and is most abundant from Bronzewing mine to the Sundowner area NE of Bronzewing. Its texture is dominated by felted masses of tremolitic amphibole, chlorite and minor plagioclase, and it is spatially related to the komatiite unit. Metasedimentary rocks are uncommon, and include minor black shale and mafic wacke in a mafic package that includes feldspar-porphyritic basalt just west of Central and Discovery zones, and in the Sundowner area. Several types of dyke rocks are recorded, including feldspar porphyry, quartz-feldspar porphyry and lamprophyre. Some of the dykes are deformed, strongly altered and veined in the vicinity of gold mineralisation. Granitoids form the cusp-shaped boundary to the greenstone belt 10 km north of Bronzewing (Fig 1). These external granitoids are generally more magnetic than the greenstone belt sequence except where cut, and presumably retrogressed, by later fault zones. North of Bronzewing and west of Mount Joel the external granitoid is inferred to dip beneath the greenstone sequence. Within the greenstone belt there are several small granitoid intrusions, including a sill-like biotite-hornblende granodiorite stock to the immediate east of Discovery zone. This stock is dominantly a feldspar-amphibole rock with minor quartz, in which chlorite alteration of amphibole is extensive. Its chemical composition is transitional between granodiorite and diorite, and trondjhemitic in places. It is weakly foliated throughout, and brecciated and strongly foliated near its lower margin. Felsic to intermediate volcanic rocks are widespread in the southern Yandal belt and near Mount McClure gold mine, but they appear to be absent in the Bronzewing mine area.
Geology of Australian and Papua New Guinean Mineral Deposits
BRONZEWING GOLD DEPOSIT
In the vicinity of the Bronzewing mine workings, the sequence (Fig 2) from east to west is: 1.
tholeiitic dolerite-basalt sequence with minor ultramafic rocks;
2.
ultramafic package of 100 m thickness that includes the Bronzewing komatiite;
3.
tholeiitic basalts, in places pillowed, and including Central zone and Discovery zone mineralisation;
4.
a differentiated tholeiitic dolerite sill, informally named the Bapinmarra sill, that varies from ultramafic to mafic to granophyric; and
5.
tholeiitic basalt with minor black shale, feldsparporphyritic basalt and Western zone mineralisation.
The tholeiitic basalt near Bronzewing is dominated by an ophitic texture of intergrown clinopyroxene and plagioclase that is now altered to actinolite, chlorite, epidote, plagioclase and quartz. Grain size is variable, from very fine grained to several millimetres diameter and much of this variation can be related to flow margins compared to flow centres. Pillow structures are common. The Bapinmarra dolerite sill is many tens of metres thick and is differentiated from cumulate to ophitic in texture. The ultramafic section varies from a cumulate peridotitic rock with mesh-like serpentine, magnetite and chromite after an olivinerich assemblage, to pyroxenite now represented by chlorite and amphibole, to carbonate-talc-phlogopite rock with no igneous textures preserved. The less highly magnesian rocks have tremolite-actinolite. The ophitic section of the sill is plagioclase-rich with amphibole, chlorite and minor quartz. Minor granophyre in the west has common actinolite and plagioclase, with chlorite, ilmenite and quartz; and there is distinctive granophyric intergrowth of plagioclase and quartz in places. Epidote, leucoxene, carbonate and pyrite are alteration products. The distribution of the cumulate, ophitic and granophyric sections of the sill indicate the top of the sill is to the west. Considerable mineralogical variation exists within mafic rocks in the Bronzewing district with evidence of higher metamorphic grade assemblages at Sundowner, Mount Joel and Woorana. At these sites the mafic rocks have a metamorphic texture, with coarser grained amphibole and plagioclase than found in the mafic rocks at Bronzewing. The amphibole includes both hornblende and cummingtonite (Hollonds, 1997). This higher grade metamorphic assemblage also occurs in veins and their selvages at Mount Joel where there is a close association of amphibole, magnetite and sulphides.
FIG 2 - Geological map of the Bronzewing mine area with major rock types, structural features and position of Western, Central and Discovery zones of mineralisation. To date, mining at Laterite pit has only been from the regolith zone.
Differentiation trends in the Bapinmarra dolerite sill suggest that the sequence is younging to the west in the vicinity of the open pits, and hence is locally overturned.
Tholeiitic mafic rocks Mafic rocks of tholeiitic composition are the most important host rocks for gold in the Yandal belt, and are dominant in the Bronzewing district, including Discovery zone, Central zone, Western zone, the major parts of Mount McClure, Sundowner and Mount Joel. Included in this group is tholeiitic basalt that can be subdivided on the basis of bulk rock composition (see below), and dolerite sills that have an overall mafic, tholeiitic composition but have differentiated portions ranging from ultramafic to intermediate in composition. Because of their importance as gold host rocks the mafic types are discussed in detail.
Geology of Australian and Papua New Guinean Mineral Deposits
The overall bulk rock composition of the basalt around Bronzewing is tholeiitic, but it can be divided into iron tholeiitic and magnesium tholeiitic groups based upon subtle differences in magnesium, titanium, chromium and especially Cr:Ti ratio (Table 1, analyses 1729 and 1741). On an east–west traverse 5 km north of Bronzewing, the iron tholeiite is over 1 km wide, and the magnesium tholeiite is approximately 500 m wide to the east. A north-trending boundary between the two rock groups is also inferred through Central and Discovery zones with magnesium tholeiite on the west of each mineralised zone. The iron tholeiites are characterised by higher titanium, aluminium, iron, zirconium, niobium, rare earth elements and yttrium. The importance of these chemical trends includes their use in stratigraphic reconstruction and their potential influence on gold precipitation. The iron-rich nature of the tholeiitic rocks is important because of the role of iron in wall rock sulphidation processes, and the link between sulphidation and gold precipitation (Phillips, 1996). Higher iron content generally favours the formation of iron sulphides by combination of iron from the wall rock, with sulphur from solution. The loss of sulphur from solution destabilises the gold-sulphur complex and leads to gold precipitation. An independent but equally important factor making the tholeiites important as gold host rocks is their tendency to fracture under elevated fluid pressure, leading to extensive fluid ingress. The two factors make tholeiitic rocks important gold targets, and the iron-rich nature almost certainly contributes to the generally high gold grades at these deposits.
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TABLE 1 Chemical composition of mafic rocks, Bronzewing and Mount Joel. Hole
BWRB1729
BWRB1741
GJRC 46
GJRC 41
A.
B.
Location
Bronzewing
Bronzewing
Mount Joel
Mount Joel
Scaling up
Implied gains
Northing
15400
15400
6100
6100
GJRC461.1/0.9
A.-GJRC41
Easting
16520
15320
9880
9740
Assuming constant Ti
(g /100g of rock)
Rock type
basalt
basalt
Chloritoid (from mafic schist)
amphibolite
SiO2
51.7
53.5
55.6
46.6
65.1
18.5
TiO2
0.7
1.0
0.9
1.1
1.1
0.0
AI2O3
13.9
14.6
14.3
14.7
16.7
2.0
FeO
9.6
9.9
8.7
10.4
10.2
-0.2
MnO
0.2
0.2
0.2
0.2
0.2
0.0
MgO
7.2
5.5
4.5
4.8
5.3
0.5
CaO
11.0
9.8
6.4
9.2
7.5
-1.7
Na2O
2.0
2.4
0.9
1.4
1.1
-0.3
K2O
0.1
0.2
0.8
0.7
0.9
0.3 0.0
P2O5
0.0
0.0
0.1
0.1
0.1
SO3
0.0
0.0
0.5
0.2
0.6
0.3
C
0.1
0.2
0.8
1.8
1.0
-0.9
LOI
4.5
3.1
6.7
10.1
7.8
-2.3
Total
100.9
100.3
100.7
101.4
117.8
16.4
Rb
8
6
28
23
33
10
Sr
87
126
124
251
145
-106
(g/t of rock)
Ba
25
20
197
217
231
14
Cr
346
240
239
247
280
33
Ni
108
131
140
117
164
47
Cu
66
55
124
111
145
34
Zn
71
89
107
77
125
48
Pb
1
2
5
3
6
3
W
6
1
4
1
4
3
Zr
34
61
60
66
70
4
Nb
1.0
4.0
4.0
4.0
4.7
0.7
La
2.8
7.5
3.6
4.3
4.2
-0.1
Ce
5.8
13.4
9.3
11.0
10.8
-0.2
Nd
5.0
13.5
7.0
8.6
8.2
-0.4
Sm
1.9
5.2
2.5
2.8
2.9
0.1
Eu
0.7
1.7
0.8
1.0
0.9
-0.1
Gd
2.4
7.0
2.6
2.7
3.0
0.3
Tb
0.4
1.2
0.4
0.4
0.5
0.1
Yb
1.5
4.4
1.8
1.4
2.1
0.7
Y
12.0
50.0
18.0
20.0
21.1
1.1
Explanation: Column A is the chloritoid sample scaled upwards to have matching Ti with the amphibolite. Column B is column A minus the amphibolite analysis and a measure of the material that needs to be added to the amphibolite to generate the chloritoid rock. Alteration involved substantial gain of Si and S and loss of Ca, Na and Sr. Fe, Zr, Nb, Y and lighter rare earth elements (La, Ce, Nd, Sm, Eu, Gd) appear to have been immobile.
Alteration of mafic rocks Mafic rocks are the main gold host rock in the Yandal belt, and their alteration has a direct influence on gold precipitation. Alteration also provides a larger exploration target than gold alone, allowing alteration distribution and intensity to play a significant role in area selection and exploration priorities.
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The mafic sequences in the Bronzewing district are cut by a series of north-trending shear zones comprising chlorite schist with quartz and carbonate, but without epidote or amphibole. These anastomosing shear zones coincide with slightly higher total magnetic intensity, destruction of original magmatic textures, and with most of the gold mineralisation and gold anomalies.
Geology of Australian and Papua New Guinean Mineral Deposits
BRONZEWING GOLD DEPOSIT
Chloritoid-bearing rocks are widespread at Mount Joel and have been recorded at Sundowner and Woorana, garnet and cordierite are recorded at Mount Joel and Woorana, and andalusite at Woorana. All these occurrences of aluminous minerals are in highly strained rocks in which primary textures have been overprinted by either a schistosity or a gneissic layering. The Mount Joel examples have a mesoscopic segregation defined by quartz, albite, biotite, amphibole and carbonate distribution giving a gneissic layering; original bedding was either absent or has been completely overprinted. The gneissic layers do not correspond to likely precursors in a sedimentary sequence. Determination of the precursor(s) of these rocks containing aluminous minerals has relied upon their distribution as highly lensoidal patches, their spatial association with shear zones and higher strain areas, and their bulk rock composition (Table 1). Samples rich in chloritoid were compared chemically to amphibolites which from field evidence, texture and whole rock composition were mafic igneous rocks (Table 1, analyses 46 and 41). To make valid comparisons between potentially altered rocks, an element balance was carried out on the assumption that titanium and other high field strength elements such as zirconium, niobium, lanthanum, cerium, neodymium, samarium, europium, gadolinium, terbium, ytterbium and yttrium had been immobile. This group of elements was chosen because they are likely to have remained immobile during alteration based upon their chemical properties and by analogy with their behaviour in other Archaean gold systems. The element balance shows there is great chemical similarity between the amphibolite and the chloritoid rock with respect to the high field strength elements (Table 1, 41 and column A: note there is some variability in aluminium content). In contrast, there are significant differences in those elements likely to have been mobile, such as silicon and some large ion lithophile elements. The chloritoid rock can be modelled by alteration of a mafic rock involving migration of silicon and a small group of large ion lithophile elements including potassium, sodium, calcium and strontium. The volumetrically dominant aspect of this alteration is the addition of silicon, but it is predominantly the removal of calcium that led to the stabilisation of chloritoid at the expense of amphibole. This interpretation of the chloritoid assemblages adds significantly to the known extent of mafic rocks, especially those close to gold mineralisation. It also suggests that there are some large alteration systems with dimensions of kilometres (as at Mount Joel).
METAMORPHISM Metamorphic grade in the Yandal belt varies from lower greenschist facies in the north around Jundee to amphibolite facies near granite margins further south, in a pattern first recognised by Binns, Gunthorpe and Groves (1976). In the Bronzewing mine area, mafic rocks have mineral assemblages indicating conditions below the greenschist–amphibolite facies boundary (actinolite), whereas higher metamorphic grades of upper greenschist to lower amphibolite facies are indicated around Sundowner and Mount Joel, where garnet, chloritoid, biotite and/or hornblende are recorded. At Mount Joel, amphibole, chloritoid and biotite overprint the main tectonic fabric indicating peak metamorphic conditions were late in the deformational history.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 3 - Map of Central and Discovery zones at 240 m and 460 m depth showing distribution of blocks of gold mineralisation (boundary of blocks is 0.3 g/t gold).
STRUCTURAL GEOLOGY Two distinct styles of regional-scale structures are recognised in the Yandal belt; ductile shear zones, and crosscutting faults and fractures. The Moilers, Moongarnoo and Celia shear zones (Fig 1) are greenstone belt–scale mylonite zones along the Yandal belt margins that show extensive flattening as well as strike-slip sinistral simple shear. Shear zones within the greenstone belt, including the shear zone at Bronzewing, show strike-slip dextral displacement. External and internal (to the greenstone belt) shear zones generally trend 000o or 160o and are ductile. Other large scale structures include folds, a synclinal structure which hosts felsic volcanic rocks and sediments west and south of Jundee–Nimary, and a south-closing fold interpreted from aeromagnetic data, which has Bronzewing on its western limb. The second set of greenstone belt-scale structures is defined by regional scale quartz veins, faults and displacements of rock unit boundaries. These are recognised on the aeromagnetic images as discrete crosscutting linears with offsets usually less than 100 m. Two directions are recognised: NE (040–070o) and ESE (090–120o). In general, the NE structures are dextral and
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the ESE structures are sinistral, and they probably represent a conjugate pair. A late Archaean age is ascribed to these structures, as regionally they are associated with quartz veins, and locally at Bills Find and Gourdis, with Archaean gold mineralisation. Some of these cross linears are positive magnetic anomalies reflecting Proterozoic dolerite dykes which appear to have intruded along pre-existing cross faults (Isles and Cooke, 1990). Cross faults and fractures which are not intruded by Proterozoic dolerites are magnetic lows. The Bronzewing corridor is an informal term for the northtrending shear zones through the three pits at Bronzewing. It is a composite ductile structure, comprising two subparallel shear zones on the eastern side of the Bapinmarra dolerite sill, at least one shear zone on the western side, and shear links between the principal shear zones (Fig 2). The results of the anastomosing pattern of shear zones are intra-shear pods of less deformed rock. At the mine scale, shear zones wrap around these unfoliated pods of basalt in oblique strike-slip relay ramps. The overall geometry is of low strain shear pods with pressure shadows on their northern, southern, top and bottom tips. Gold mineralisation in the Bronzewing corridor occurs where shear zones converge and diverge, and where northtrending shear zones are intersected by NE (040 to 070o)trending and/or ESE (090 to 120o)-trending cross faults, which are readily identified on detailed aeromagnetic imagery.
REGOLITH There is extensive younger cover on the Yandal belt including ephemeral lake systems, aeolian sand, transported alluvium and deep regolith profiles. Much of the Bronzewing district is covered by transported alluvium which includes palaeochannels, sheetwash, clays tens of metres thick, lake systems with calcreted channels, and siliceous hardpan (Eshuys and Lewis, 1995; Eshuys et al, 1995). The surficial geology has a significant impact on exploration methods. Areas of outcrop such as near Mount Joel are minor but include very fresh mafic rocks at the surface that proved critical in early interpretations of the prospectivity of the belt. Most of the laterite profiles are partially stripped or covered. The regolith profile in the Bronzewing area was influenced by the Lake Maitland drainage system to the NE and its tributaries including Bates Creek, which flows north on the western edge of Discovery zone. Beneath the modern alluvial plains of these tributaries is evidence of steep sided channels greater than 50 m deep, that in places are oblique to the modern watercourse flow directions. In the south, Discovery mineralisation is covered by more than 50 m of transported material, whereas Central zone has 10–20 m of alluvial cover, and Laterite pit has 1–2 m of cover (Eshuys et al, 1995; Varga, Anand and Wildman, 1996). In the Discovery pit area, weathering is up to 100 m deep with saprolite overlying basaltic bedrock. This residual component of the regolith is overlain by buried laterite (<5 m thick), clay and pisolith-bearing alluvium with megamottles (>30 m), indurated iron-rich transported material referred to as hardpan (2–5 m), and a thin soil layer (<1 m). The weathering process has a significant influence on the aeromagnetic response over the greenstone belt, and primary magnetic minerals, especially magnetite, are readily destroyed in the regolith to 100 m depth or more (Hollonds, 1997). Magnetic lateritic material distributed adjacent to the deep palaeochannels, central to these channels, and on residual hills, is the dominant shallow aeromagnetic feature in several areas.
132
ORE DEPOSIT FEATURES The mineralisation at Bronzewing is subdivided into Western zone, Central zone and Discovery zone, each with different geological and geometric characteristics. All three styles of mineralisation comprise quartz veins in dominantly mafic schist with the bulk of the gold within the quartz veins.
STRUCTURE Shear zones The principal structural feature at Bronzewing is ductile shear zones with intra-shear zone boudins of pillow lava and ultramafic rock. The shear zones are typically 30 m wide, but can be as much as 100 m. The general strike direction of the shear zones is north to NE but significant variations exist. The shear zone which hosts the Western zone ore trends between 010 and 020o. The shear zones through Central pit trend about 350 to 010o and through Discovery pit they trend between 340 and 010o. An important feature of the shear zones in all areas is their trend oriented at 130 to 150o. These SE trends usually link otherwise parallel shear zones and are mineralised in Discovery pit. Between the shear zones are boudins of less deformed rock, including well developed pillow basalts in Discovery pit, and dolerite and ultramafic units of the Bapinmarra sill in Central and Discovery pits, respectively. In the SW corner of Discovery pit, one shear zone trends about 040o on the northern side of an intra-shear zone pod, but schistosity in that shear zone is oriented between 350 and 010o oblique to the local trend of the shear zone, suggesting that the position is a pressure shadow on the northern side of the pod. In Discovery pit these pressure shadow positions are mineralised (Fig 2).
Schistosity and mineral elongation lineations Schistosity in the mafic schist is generally defined by chlorite and amphibole, and locally by biotite or white mica. Talc and chlorite define the schistosity in the ultramafic schist. The gross geometry suggests that the schistosity is mostly subparallel to the margin of the shear zones, although in detail the schistosity is commonly at an oblique angle to the margin. Mineral elongation lineations in the plane of the schistosity are defined by the orientation of grains of amphibole, chlorite and plagioclase. Mineral elongation lineations are subhorizontal indicating that ductile motion was strike-slip. In Western zone there are especially well developed composite schistosities, with the ore-bearing laminated vein along the C surface of the composite schistosity in the chlorite and biotite schists of the wall rocks. The S–C surfaces consistently and clearly demonstrate dextral motion. Similar kinematics are implied by S–C schistosities and asymmetric boudinage in shear zones in Central zone.
Folds Mesoscopic folds are rare at Bronzewing. However, prominent folds of the laminated quartz vein in Western zone are close to the central and southern limits of the ore zone. The laminated quartz vein is folded, and although locally the schistosity wraps around the outer fold nose, it remains unfolded in the core of the fold. C surfaces appear to be axial planar to the fold. The folds have locally variable plunge and are non-cylindrical. The prominent fold on the south end of Western zone plunges northwards at 45o. The folds are asymmetric, consistent with dextral shearing.
Geology of Australian and Papua New Guinean Mineral Deposits
BRONZEWING GOLD DEPOSIT
The Hook antiform (an informal term) is a regional scale fold that dictates rock distribution near the mine area. It is northtrending, open and shallow south-plunging, and its most obvious expression is folding in the Bronzewing komatiite (Fig 2).
NE-trending faults and fractures At Bronzewing most quartz veins, including those with gold, are parallel to schistosity and trend between 340 and 020o. The strike of many individual quartz veins and high grade ore shoots is parallel to this direction, yet the economic ore outline trends 050 to 090o for Shoot 39 (Fig 3) and in Discovery pit, subparallel to the cross faults and fractures. Most cross faults and fractures cut directly across the ore zones. However, as much of the mine development is in ore zones, the nature of the cross faults and fractures away from the ore zones is poorly understood. The overall geometry of the mineralisation suggests a link between mineralisation and cross faults and fractures, and this is developed as a model for mineralisation.
MINERALISATION Below 400 m depth, Central and Discovery mineralisation form linear, parallel zones up to 600 m long and dipping 70o to the SE (Fig 3). In the upper 300 m both zones take on a much more complex geometry controlled by multiple fault and shear zone orientations; some of the complexity may be due to more intense drilling at shallow levels. At the higher levels, weathering has led to gold redistribution on the scale of metres or tens of metres. To-date, Western zone is only known above 350 m depth.
Western zone In Western zone the envelope to mineralisation is generally NNE-trending, steeply east-dipping and its long axis has a subhorizontal to 10o south plunge. The mineralised laminated quartz vein transgresses rock unit boundaries, but is generally in mafic schists between ultramafic rocks to the east and mafic rocks to the west. The vein is mostly parallel to the C planes of the composite schistosity, and hence oblique to the S planes. It cuts several phases of earlier quartz veining including a quartz matrix-supported breccia. Chalcopyrite, pyrite, tellurides and gold are in brittle fractures in the laminated vein. Fractures are either sheeted and approximately NE-oriented, or in an erratic and random brittle fracture pattern.
Central zone (including shoot 39) Mineralised quartz veins are totally within a mafic sequence dominated by tholeiitic basalt with sporadic pillow structures. Open pit ore outlines trend northerly and some veins are 340otrending and parallel to the local schistosity. The general pattern is one constrained by the anastomosing shear zones. For the Central zone underground area, including Shoot 39, the mineralisation envelopes trend at approximately 070o, probably bounded and controlled by similarly oriented cross faults, but high grade shoots trend northerly and dip steeply east parallel to the schistosity. The Shoot 39 ore outline plunges at approximately 70o to the SE.
Discovery zone In the Discovery open pit, quartz vein mineralisation is predominantly within mafic rocks similar to Central zone, but a
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 4 - Map of the alteration assemblages in mafic rocks around the Bronzewing gold deposit, based on petrological studies of P Eilu and A Hebb. The pit outlines provide a guide to the main locus of mineralisation. Sample positions marked by +.
granitoid to the east is mineralised by gold and brecciated, especially around its lower margin. The mineralisation envelope has a complex shape showing components of more detailed controls (Fig 3). Grade control ore blocks have 340 to 000o orientations along the general trend of the shear zones, 045o orientations related to fabrics wrapping around shear pods and the NE cross faults and fractures, and 090o orientations which have no single geological control and are an artefact of the mix of 160o -trends and 045o -trends. Exploration drilling showed a strong east–west continuity. The ore outlines in upper parts of Discovery pit are complex and related to several structures, and to weathering.
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Other mineralisation Significant gold mineralisation has been identified at Sundowner, Mandilla Well and Mount Joel (Fig 1). At Sundowner, which includes the mineralisation at Old Sundowner, New Sundowner and Cyclonic, mineralisation comprises auriferous quartz veins in foliated tholeiitic basalt and dolerite surrounded by alteration haloes of biotite, pyrite and carbonate. The chloritic rocks near mineralisation are altered equivalents of the surrounding actinolitic rocks, similar to the alteration pattern around Bronzewing. At Mandilla Well, gold mineralisation is related to quartz veining in a porphyry dyke in a sequence of mafic rock and felsic schist. Mount Joel has at least ten zones of gold mineralisation over a strike length of 8 km. Altered dolerite, amphibolite and metabasalt near the major granite west of Mount Joel (Fig 1) host some of this mineralisation, whereas chlorite-bearing mafic schist further from that granite hosts the bulk of the known mineralisation. All Mount Joel mineralisation relates to quartz veining in mafic rocks, and in general, these quartz veins are relatively flat lying or parallel to the moderate to steeply dipping schistosity. Mineral elongation lineations are gently north and south plunging. Alteration in the mafic rocks is widespread and comprises muscovite, biotite, chlorite, amphiboles, carbonate, magnetite, pyrrhotite, pyrite and tourmaline. The scale of mica alteration at Mount Joel is substantially greater than that at most other Yandal deposits, and comparable to Bronzewing and Jundee. The scale of development of chloritoid-, garnet- and cordierite-bearing assemblages is also exceptional for the Yandal belt.
MINE- AND VEIN-SCALE ALTERATION A broad alteration halo surrounds the Bronzewing gold deposit and is well defined in mafic rocks. The progression of alteration assemblages towards mineralisation can be summarised as follows: 1.
actinolite-plagioclase (unaltered);
2.
chlorite-calcite magnetite);
(also
epidote,
VEINING albite,
leucoxene,
3.
ankerite-biotite (also ilmenite, pyrrhotite); and
4.
muscovite-pyrite (most altered and proximal to mineralisation, includes rutile).
There is minor overlap of assemblages near alteration zone boundaries. A slightly modified, but very similar, alteration scheme is developed from drill hole logging around the mine (Fig 4). Throughout the area of Central and Discovery pits, actinolite-bearing assemblages are uncommon, defining a zone of actinolite destruction several hundred metres in diameter. Magnetite appears to be stabilised in the outer alteration zones mostly outside the pit areas. More proximal, there is a general correspondence of ankerite, biotite, muscovite and pyrite with stronger gold mineralisation. Alteration around individual veins shows some variation laterally and with depth. Biotite-calcite assemblages dominate Central zone veins where bleaching is extensive, whereas Discovery zone shows a trend from muscovite-ankerite around upper level veins to more biotite-calcite assemblages around veins below 200 m depth. Veins within Western zone have much less bleaching and are surrounded by assemblages of chlorite and minor pyrrhotite.
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FIG 5 - Mohr diagrams showing four states of stress relative to failure for the Yandal Belt. 5a shows schematically the state of stress before conjugate failure. 5b is a representation of the stress to achieve conjugate failure. 5c is the scenario for conjugate failure and dilation on these structures as at Bills Find and Gourdis. 5d shows the Mohr circle for failure with tensional veins opening parallel to a foliation but perpendicular to the maximum compression as at Western and Central zones, Bronzewing.
Several styles of quartz veining have been identified (Dugdale, 1996), and these can be summarised as pre-mineralisation veins with epidote and calcite in basalt, post-mineralisation veins linked to silicification, and a more complex set of veins that are synchronous with, or close to, the period of gold mineralisation (despite showing complex crosscutting relationships). Individual veins range in thickness from centimetres to metres, and in richer ore zones such as Shoot 39, occur as dense vein arrays tens of metres wide. Veins synchronous with mineralisation have a diverse mineralogy dominated by quartz, carbonates (calcite and ankerite), albite, micas (muscovite and biotite), sulphides, scheelite, tellurides (especially of silver and bismuth), tourmaline and gold, and show boudinage and pressure solution features. The sulphide mineralogy is dominated by pyrite with widely distributed minor pyrrhotite and chalcopyrite. The lamination in the Western zone vein is due to the wallparallel inclusion of wall rock slivers, which are commonly richly mineralised with sulphides and gold. The preponderance of veins parallel to schistosity suggests very high fluid pressures in a regime where dilation was controlled by fluid overpressuring.
Geology of Australian and Papua New Guinean Mineral Deposits
BRONZEWING GOLD DEPOSIT
CONTROLS ON MINERALISATION
Host rock and fluid controls
Structural controls
Research into the nature of the ore fluid composition utilising fluid inclusions and thermodynamic modelling is proceeding at the universities of Western Australia and Melbourne, but alteration mineralogy and associated geochemistry alone provide important constraints on fluid composition. The inferred ore fluid was water–carbon dioxide dominated with hydrogen sulphide and minor gold complexed with sulphur. Low salinity is indicated by the gold-only nature of the deposit and the lack of daughter products in fluid inclusions.
Gold mineralisation in the Yandal belt occurs in three different geometrical forms: 1.
as laminated and other schistosity-parallel quartz veins along the line of the ductile NNW-trending shear zones (eg at Western zone and parts of Central and Discovery zones at Bronzewing, and at Dragon, Challenger and Lotus at Mount McClure);
2.
as quartz veins along NE-trending brittle cross faults (eg parts of Discovery at Bronzewing, Bills Find and parts of Jundee); and
3.
as quartz veins along SE-trending brittle cross faults (eg Gourdis, parts of Jundee).
Some old workings and prospects show a combination of these features. Deposits at Western and Central zones, Dragon, Success and Lotus are of type 1 but are truncated or terminate at cross faults with either NE or SE orientations. Bronzewing and Orelia-Calista (at Mount McClure) are on the same regional scale NE-trending magnetic linear. The broader structural setting of Bronzewing near a cusp in the granite-greenstone boundary (Figs 1 and 2) is unlike some other large gold deposits (eg Kalgoorlie, Jundee) which are in wide sections of greenstone belts, but is similar to many other gold deposits which show a concentration within 3 km of greenstone belt margins and owe their position to heterogeneous stress distribution due to the interaction of granite, greenstone, and deformation zones during regional compression (Ridley, 1993). The inferred structural history at Bronzewing involves a progression of ductile to brittle to ductile deformation, confirming a complex interplay of thermal input, strain rate and especially fluid pressure. It appears that very high, and slightly variable, fluid pressures account for many of the variations in vein geometry and distribution at Bronzewing. High angle cross faults and fractures are ubiquitously associated with mineralisation which itself occurs as veins reactivating the schistosity in generally north-trending shear zones. This relationship may be viewed in the Mohr system in normal stress σn –shear stress τ space (Fig 5). Shear failure without dilation on faults occurs when the Mohr circle intersects the failure curve with minimum normal stress σ3 positive. Dilatational shear failure along these faults occurs as the Mohr circle intersects the failure curve with the minimum normal stress σ3 negative. Reactivation of the pre-existing weakness along the north trending ductile shear zones is related to the stress state giving rise to tensional veins parallel to foliation and near perpendicular to the maximum compression σ1. This is achieved by supralithostatic fluid pressures shifting the Mohr circle to the left (σ3 and σ1 are negative) and by a reduction in the differential stress σ1 - σ3 by stress relief. Thus a sequential model can be developed of increasing fluid pressure and decreasing mean stress (σ1 + σ2 + σ3)/3 as conjugate faults develop, dilate and then reactivate pre-existing weaknesses (Fig 5). This last phase appears to be responsible for the higher gold grades and larger tonnages of gold. This whole approach explains many of the variations between mineralisation styles at Bronzewing and has been valuable in predicting further potential styles.
Geology of Australian and Papua New Guinean Mineral Deposits
Gold is both in quartz veins and in the surrounding alteration halo, with the economic bulk of gold being in the quartz veins. In detail, there is a close spatial relationship between the gold within the quartz veins and either strongly altered wall rock fragments and/or sulphides, particularly chalcopyrite. These relationships suggest that lowering of sulphur activity in the fluid is the overriding control on gold deposition, and this resulted from interaction with iron-rich host rocks and the deposition of sulphides in the quartz veins. The alteration scheme at Bronzewing is typical of many greenstone-hosted gold deposits. The progression of amphibole-epidote to chlorite-calcite to ankerite to mica-pyrite at Bronzewing resembles the alteration progression at Kalgoorlie (Phillips, 1986). There are subtle temperature and mineralogical variations between these two deposits including scale, but the overall scheme is one of broad carbon dioxide addition, and more local potassium and sulphur addition with gold.
GENESIS OF BRONZEWING GOLD MINERALISATION Despite a complex array of quartz veins bearing gold and a variety of alteration assemblages at Bronzewing and surrounding areas, the gold mineralisation process can be explained in terms of a single major event involving gold introduction. There is no diagnostic evidence that the variability of vein types, vein geometry or alteration assemblages represent significantly different periods of gold introduction or significantly different fluid types. Gold mineralisation post-dated the emplacement of the major rock types in the greenstone belt including the external granites, as the major structures influencing the gold continue through surrounding granites. Gold also post-dated the granitoid on the east of Discovery zone which is itself mineralised (Fig 2). The major mineralisation at Bronzewing is superimposed upon ductile north trending shear zones that are part of the regional structural framework; this is compatible with reorientation of the fabric in clasts of the breccia associated with Western zone mineralisation. Furthermore, the ore outlines and boundaries are influenced by the distribution of the brittle NE-trending and near ESE-trending cross faults and fractures, suggesting mineralisation occurred after the regional ductile deformation (ie involving reactivation) and probably synchronous with the brittle deformation. Isoclinal folding of the mineralised quartz vein of Western zone implies ductile deformation after mineralisation. The relationship of mineralisation to the thermal peak is less clear in that peak metamorphism is of approximately the same metamorphic grade as the alteration assemblage at
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Bronzewing, Sundowner and Mount Joel. The ductile folding of the Western zone quartz vein is compatible with (and possibly requires) the thermal peak outlasting mineralisation, and provides an explanation for alteration assemblages of variable grade between Bronzewing, Sundowner and Mount Joel. The balance of evidence favours a gold mineralisation event in the Bronzewing–Sundowner–Mount Joel area at greenschist facies conditions being overprinted by a thermal peak of slightly (Bronzewing) to considerably (Mount Joel) higher temperature. The genesis of the Bronzewing gold deposit is best explained by a metamorphic fluid infiltrating a sequence of mafic rocks. That fluid was predominantly H2O–CO2 bearing typical of the fluid found at many other greenstone gold deposits, and it was structurally focussed by an anastomosing set of shear zones and faults in the Bronzewing area. Fluid interaction with the ironrich basalt sequence led to precipitation of iron sulphides and gold. The formation of other sulphides such as chalcopyrite within quartz veins led to lower sulphide activity in the fluid and related gold precipitation. In summary, structure and host rock composition are the principal control on gold mineralisation at Bronzewing, which has a complex geometry of anastomosing but generally north oriented ductile shear zones, and oblique, often subtle, cross faults and fractures. The detailed controls on mineralisation vary between orebodies, but the preponderance of veins parallel to schistosity suggests very high fluid pressures. Dilation was controlled by fluid overpressuring and can not be modelled with simple geometry. Fluid channelling and access appear to be controlled by the high-angle cross faults and fractures. These structures and associated alteration are overprinted by the thermal peak of metamorphism.
ACKNOWLEDGEMENTS The authors wish to thank the Directors of Great Central Mines Limited for permission to publish this paper, and especially J Gutnick and E Eshuys for commitment to the concept that exploration success can be greatly enhanced by improved geological knowledge. P Hepburn Brown, M Reed and J Landmark greatly facilitated the geological work through their cooperation, and were supported by the staff at Bronzewing. This summary has drawn upon the work of many GCM geologists including B Davis, C Grant, D Ryan, E Sharpe, L Starkey and J Wright. The petrography of major rock types is based upon petrological reports by M Clough and A Dugdale, and the stratigraphy from work led by J Wright. The Bronzewing district geological map presents data from the relogging of 3000 RAB holes by a team of geologists in late 1995. Substantial geological input was provided by I Herbison, D Compston, T Smith, D Hope and several other regional exploration geologists working on tenements near Bronzewing. Regolith studies in the Bronzewing district by CSIRO, alteration studies by P Eilu (University of WA), a Ph D project focussing on fluid inclusions and isotopes by A Dugdale (University of WA), and two petrological Honours projects led by R Powell (University of Melbourne) have all contributed to understanding of the Bronzewing geology. Discussions with D Groves, P Eilu, E Mikucki and J Ridley have added significantly to the geological understanding of Bronzewing.
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REFERENCES Binns, R A, Gunthorpe, R J and Groves, D I, 1976. Metamorphic patterns and development of greenstone belts in the Eastern Yilgarn Block, Western Australia, in The Early History of the Earth (Ed: B F Windley), pp 303–316 (John Wiley: London). Dugdale, A L, 1996. Multiple vein arrays and zoned alteration at Bronzewing gold deposit, WA, Geological Society of Australia, Abstracts, 41:120. Eshuys, E, Herbison, I, Phillips, N and Wright, J, 1995. Discovery of Bronzewing gold mine, in New Generation Gold Mines: Case Histories of Discovery, pp 2.1–2.15 (Australian Mineral Foundation: Adelaide). Eshuys, E and Lewis, C R, 1995. New approaches to gold exploration, in Proceedings of the Outlook 95 Conference, pp 336–340 (Australian Bureau of Agriculture and Resource Economics: Canberra). Hallberg, J A and Thompson, J F H, 1985. Geological setting of the Teutonic Bore massive sulfide deposit, Archean Yilgarn Block, Western Australia, Economic Geology, 80:1953–1964. Hollonds, A N, 1997. Metamorphic studies and magnetic petrology in the Bronzewing district, WA, BSc Honours thesis (unpublished), The University of Melbourne, Melbourne. Houstoun, S M, 1987. Competency contrasts and chemical controls as guides to gold mineralization: an example from the Barberton Mountainland, South Africa, in Recent Advances in Understanding Precambrian Gold Deposits, Publication 11 (Eds: S E Ho and D I Groves), pp 147–160, (Geology Department and University Extension, The University of Western Australia: Perth). Isles, D J and Cooke, A C, 1990. Spatial associations between postcratonisation dykes and gold deposits in the Yilgarn Block, Western Australia, in Mafic Dykes and Emplacement Mechanisms (Eds: A J Parker, P C Rickwood and D H Tucker), pp 157–162 (Balkema: Rotterdam). Kotz, (now Hebb) A J, 1996. Hydrothermal alteration at Bronzewing gold mine and equilibrium thermodynamic modelling of alteration, BSc Honours thesis (unpublished), The University of Melbourne, Melbourne. Otterman, D W and de San Miguel, G F, 1995. The discovery and development of the Mt McClure gold deposits, in New Generation Gold Mines: Case Histories of Discovery, pp 3.1–3.15 (Australian Mineral Foundation: Adelaide). Phillips, G N, 1986. Geology and alteration of the Golden Mile, Kalgoorlie, Economic Geology, 81:779–808. Phillips, G N, 1996. Fluids and large gold deposits, in Mesothermal Gold Deposits: A Global Overview, Publication 27, pp 104–109, (Geology Department and University Extension, The University of Western Australia: Perth). Phillips, G N, Eshuys, E and Hellsten, K, 1996. Integrated exploration techniques for Archaean gold, Western Australia, in Proceedings 1996 AusIMM Annual Conference, pp 289–293 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Ridley, J R, 1993. The relations between mean rock stress and fluid flow in the crust: With reference to vein- and lode-style gold deposits, Ore Geology Reviews, 8:23–37. Varga, Z S, Anand, R R and Wildman, J E, 1996. Bronzewing gold deposit, CRC LEME Restricted Report 4R and CSIRO Exploration and Mining Report 253R: pp 1–14 (unpublished). Vearncombe, J R, 1996. Fluid pressure controls on faulting and shear zone reactivation in greenstone belt gold deposits, Western Australia, in The Role of Basement Reactivation in Continental Deformation, Abstract Volume, Tectonic Studies group, Geological Society of London (unpaginated). Vearncombe, J R, 1997. The Yandal Belt: from nothing to 10 million ounces in the 1990s, in International Liaison Group on Gold Mineralisation Newsletter, April edition. P 62. Wright, J H and Herbison, I, 1995. The Yandal Belt: Preliminary exploration of the Jundee deposit, in New Generation Gold Mines: Case Histories of Discovery, pp 5.1–5.14 (Australian Mineral Foundation: Adelaide).
Geology of Australian and Papua New Guinean Mineral Deposits
Harris, J L, 1998. Mount McClure gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 137–148 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Mount McClure gold deposits 1
by J L Harris
INTRODUCTION The deposits occur over a strike length of 25 km within the Yandal greenstone belt, close to its western margin (Fig 1). The plant and village are at lat 27ο25′S, long 120o56′E and at AMG coordinates 295 000 m E, 6 966 000 m N on the Sir Samuel (SG 51–13) 1:250 000 scale map sheet, about 60 km NE of Leinster and 400 km north of Kalgoorlie, in the Eastern Goldfields Province of WA. The mineral tenements cover an area of about 500 km2 and extend 13 km north and 32 km south of the treatment plant. They are within the Mount Keith (3043), Sir Samuel (3042), Wanggannoo (3143) and Darlot (3142) 1:100 000 scale map sheet areas. The Mount McClure mine is owned by Australian Resources Limited (63.35%) and Forrestania Gold NL (36.65%), with Australian Resources the manager of the project. At 30 June 1996, total Proved and Probable Ore Reserves for the Mount McClure project were 12.566 Mt at an average grade of 2.0 g/t gold, containing 808 000 oz of gold. Additional Indicated Resources were 1.975 Mt at an average grade of 2.0 g/t containing 127 000 oz of gold. For the year ending 30 June 1996, 1 071 098 t of ore were milled at an average grade of 3.2 g/t gold to produce 102 619 oz of gold at a recovery rate of 93% (Australian Resources Limited, 1996). From the start of gold production in 1992 to 30 June 1996, 3 082 611 t of ore at an average grade of 3.4 g/t gold have been milled and 320 484 oz of gold have been produced, at an average recovery of 95%. Current mine production is from the Cockburn and Lotus North open pits, which supply oxide ore, and from the Lotus Deeps Stage 1 underground mine which supplies primary ore. The carbon-in-pulp cyanide treatment plant has a nominal capacity of 1.1 Mtpa.
EXPLORATION HISTORY A detailed account of the exploration and development history of the Mount McClure gold deposits is provided by Otterman and de San Miguel (1995) and a much briefer outline is given here. Prior to the 1970s, the project area had no record of mineral exploration or mining. Some of the previously known gold mines within the Yandal greenstone belt include Darlot, ‘Old’ Bronzewing, Mount Joel and Corboys. Of these, Darlot was the most significant. After the initial gold discoveries at Mount McClure between 1987 and 1990 (Otterman and de San Miguel, 1995) major gold discoveries were made in the Jundee area in the northern part of the belt (Lewington, 1995; Wright and Herbison, 1995) and at Bronzewing, 8 km NE of Lotus (Eshuys et al, 1995).
1.
Consulting geologist, J L and B A Harris Geological Services, 14 Collier Street, Ardross WA 6153.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Location and regional geological map, Mount McClure area.
The Mount McClure area was explored for base metals in the period 1976–1983 by the Mount Isa Mines–Seltrust joint venture following their discovery of the Teutonic Bore volcanic-hosted massive sulphide deposit 80 km to the south. Western Mining Corporation (WMC) carried out an extensive electromagnetic (EM) geophysical survey for copper-zinc massive sulphide deposits over the northern half of the project
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J L HARRIS
area during this period. Selected EM anomalies were drilled and some outcrop and lag samples were assayed for gold and arsenic. The southern half of the area was explored by a Getty Oil–Spargos Exploration joint venture. In 1984, geologist Ian Hassell of Oresearch NL (later taken over by Forrestania Gold NL) located a gossanous chert horizon with anomalous gold values on the western side of the Yandal greenstone belt and applied for two Exploration Licences. In 1985 the prospect was joint ventured to Amoco Minerals Australia Company (afterwards Cyprus Gold Australia Company) on the recommendation of geologist George de San Miguel. The Cyprus equity in the project and management passed to Arimco NL in 1989 and most of the personnel involved in the project were transferred to Arimco (later to become Australian Resources Limited), thus maintaining continuity. Early reverse circulation (RC) drilling of the anomalous chert horizon gave disappointing results. A bulk cyanide leach extractable gold (BLEG) soil geochemical survey, which confirmed the WMC lag results, was expanded and three soil gold-arsenic anomalies (later shown to overlie the Success, Parmelia and Challenger gold deposits) were outlined. These anomalies were all in zones of sparse outcrop about 200 m to the east of the well-exposed gossanous chert horizon. This work was followed up by a systematic program of rotary air blast (RAB) and RC drilling. By 1988 the three abovementioned deposits and the Dragon gold deposit (Fig 1) had all been outlined using these methods. A 1989 feasibility study, based on a Proved and Probable Reserve in the four deposits of 1.198 Mt at 3.09 g/t gold, indicated that further ore reserves were needed to justify setting up a mining operation. Subsequent work concentrated on RAB gold anomalies in an area where transported overburden, to 40 m thick, made soil geochemistry ineffective. This resulted in the outlining of the Calista and Lotus gold deposits, respectively 10 km and 12 km north of Success. A second feasibility study, based on a reserve in six deposits of 2.1 Mt at a grade of 3.4 g/t was completed in mid 1991. Open pit mining began at Lotus in December 1991 and ore treatment commenced in April 1992 at a throughput of 575 000 tpa. In 1995, following the delineation of a Probable Ore Reserve of 11.264 Mt at 2.0 g/t at the Cockburn deposit, mill capacity was upgraded to 1.1 Mtpa. Cockburn includes the Calista deposit and its northern and northeastern extensions, known respectively as the Orelia and Cumberland gold deposits, outlined since 1991.
REGIONAL GEOLOGY
A second major shear zone, the Ockerburry fault, trends NNE through the central part of the Yandal greenstone belt, linking, on a regional scale, the Mount McClure fault with the major NW-trending Ninnis fault on the eastern side of the belt. Numerous east-, NE- and SE-trending linears, probably oblique faults, have been interpreted from air photos and aeromagnetic and Landsat TM imagery (H Davies, unpublished data, 1992). Of these, the most significant with respect to possible structural controls of gold mineralisation appear to be some of the NE-trending faults. An internal tonalite or granodiorite stock with an area of at least 40 km2 is centred 10 km east of Success. From the aeromagnetic pattern, this stock has disrupted the northerly trend of the surrounding greenstone. A smaller stock, about 4 km2 in area, is centred 4.5 km south of Anxiety Bore. Both stocks, which may be part of a larger granitoid pluton at depth and include feldspar porphyry and aplite as minor, generally marginal, phases, are associated with significant (+1 g/t) gold values.
PRECAMBRIAN STRATIGRAPHY Structures seen in diamond drill core from the Parmelia, Success and Lotus-Cockburn areas indicate that the volcanicsedimentary succession in the western part of the Yandal greenstone belt is east facing. The structures include thin (to about 10 cm) turbidity current graded beds, inclusions of fragments from an underlying unit in the basal zone of the immediately overlying clastic sedimentary unit, repeated cycles of upward coarsening pyroclastic fragments in felsic tuffs and some well preserved spinifex textured zones in komatiite flows. These features imply that west-dipping strata are overturned, most notably in the Lotus–Cockburn area. Close to the western margin of the belt, the strata strike generally NNW and dip at about 50o easterly. In the Lotus–Cockburn area the strata strike NW with moderate to steep westerly dips. From west to east, the broad stratigraphic succession is: 1.
felsic (andesitic-dacitic) pyroclastic rocks, including lithic lapilli tuff, with intercalated tuffaceous sediment, including carbonaceous (graphitic) units, thin banded cherts and some basalt and dolerite;
2.
komatiitic volcanic rocks of peridotite to basalt composition, often with spinifex (quench) textures and flow-top breccia structures. Intercalated with the komatiite lava flows are lenses of tuffaceous graphitic sediment;
3.
tholeiitic basalt and basaltic lithic lapilli tuff with some sulphidic cherty tuff intercalations;
4.
basaltic to andesitic volcanic rocks, mainly tuffs, which become more felsic upwards, with intercalated sulphidic cherty tuff bands;
5.
felsic (mainly dacitic) volcanic rocks, including lithic lapilli tuff and some probable lava flows, interbedded with felsic tuffaceous sediment; and
6.
basaltic to felsic volcanic rocks, including amygdaloidal basalt, andesitic to dacitic lava and lithic and feldspar crystal tuff and polymictic agglomerate, with some tuffaceous sediment intercalations.
STRUCTURE The Mount McClure joint venture area covers a strike length of 45 km on the western side of the Yandal greenstone belt of the Archaean Yilgarn Block (Fig 1). The Archaean volcanicsedimentary succession trends generally NNW with dips from near vertical to moderate easterly or westerly. The Mount McClure fault, a major NNW-trending shear zone, separates the greenstone belt from the Cocks–Satisfaction zone, a thick sequence of NNW-trending felsic and mafic gneiss on its western side (Wyche and Westaway, 1995). A large granitoid pluton lies between this zone and the Mount Keith greenstone belt, 25 km to the west.
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Apart from the internal granitoid stocks referred to above, the sequence is intruded by generally conformable dolerites up to at least 200 m thick. In some places, most notably in the Lotus–Cockburn area, the sequence has been intruded by
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT McCLURE GOLD DEPOSITS
swarms of intermediate dykes of varied texture and mineralogy. The classification of these dykes as intermediate is supported by petrological work and application of zirconium:titanium dioxide ratios (Hallberg, 1983). Based on multi-element assay data, the more mafic of these are almost certainly calc-alkaline lamprophyres (J L Harris, unpublished data,1994).
METAMORPHISM The greenstones appear to belong to the lower to mid greenschist facies of regional metamorphism. Prehnite has been reported and pale red garnet occurs in rare instances at Cockburn as scattered porphyroblasts in altered basaltic rock. These zoned garnets, with high manganese and calcium cores, probably reflect unusual rock chemistry rather than regional metamorphic grade (Pontifex and Associates, unpublished data, 1996). Andalusite has been observed in metapelite close to the western contact of the larger internal granitoid and is probably the result of contact metamorphism. The metamorphic grade is appreciably higher in the southern part of the tenement area. Compositionally-banded coarse grained hornblende-clinopyroxene-calcic plagioclase-quartz± biotite rocks from the footwall of the Dragon deposit, described as annealed mylonite after basalt (Pontifex and Associates, unpublished data, 1992), indicate a lower to middle amphibolite facies for this area, although garnet is notably absent. The metamorphic grade and degree of deformation also increase as the Mount McClure fault is approached. Some of the highly deformed chert horizons in this zone may be annealed mylonites rather than chemical sediments.
WEATHERING HISTORY AND LANDFORMS A detailed account of the weathering history, regolith and landforms within the Sir Samuel 1:250 000 sheet area has been given by Kojan, Faulkner and Saunders (1996). Williamson (1992) made use of the regolith classification system developed by the CSIRO to focus on the regolith over a 110 km2 area centred on the Calista deposit. The weathering history of the Yilgarn Block commenced about 250 Myr ago following the Lower Permian glaciation, as recorded by scattered outliers of glacigene sediments of the Patterson Formation, such as that at Ockerburry Hill, 23.5 km SSE of Mount McClure trig (Bunting and Williams, 1979). A lag of scattered, well rounded boulders, mainly of granitoids but also including other resistant rock types such as chert, vein quartz and banded iron formation (BIF), is widespread over the internal granitoid to the east of Success. In the northern half of the Yilgarn Block, alluvium and colluvium are frequently cemented by a mixture of hydrated iron oxides, calcrete and opaline silica to form an indurated layer, the Wiluna Hardpan (Bettenay and Churchward, 1974), which may be as much as 20 m thick and is generally covered by a thin (to 1m) layer of soil. Colluvial and alluvial deposits have formed on complete and truncated Tertiary laterite weathering profiles.
FIG 2 - Geological plan at 465 RL (50 m below ground level), Lotus–Cockburn deposits, showing position of cross sections on Figs 4, 5 and 6.
LOTUS AND COCKBURN DEPOSITS PHANEROZOIC STRATIGRAPHY The Lotus and Cockburn deposits are part of one mineralised system with an overall strike length of 3 km (Fig 2). They are
Geology of Australian and Papua New Guinean Mineral Deposits
separated by an interval of 600 m, referred to as ‘The Gap’, in which relatively sparse drilling has intersected only sporadic ore grade gold mineralisation. The Cockburn–Lotus area, which slopes very gently to the NE, is covered by the Wiluna
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Hardpan, a 4 to 15 m thick, crudely bedded, sandy to gravelly sheetwash alluvium, which is overlain by up to 1 m of sandy clay soil. The Wiluna Hardpan is mainly cemented by hydrated iron oxides (ferricrete) but zones of calcrete and silcrete also occur within it. The degree of cementation generally decreases with depth and the unit is thickest over the deeper lake sediments, described below.
These are interbedded with sulphidic cherty tuff and some clastic sedimentary units. The sequence has been intruded by penecontemporaneous dolerite sills and a suite of younger Archaean intermediate dykes. A generalised cross section of the Cockburn geology is shown in Fig 3. From west to east the sequence is as listed below.
In the Lotus pit area, the Wiluna Hardpan overlies mottled zone saprolite. To the south of this and in the southern and western parts of the Cockburn deposit, it is underlain by up to 10 m of partly transported, Tertiary pisolitic to nodular ferruginous laterite which carries significant gold values. Todate, 105 283 t of laterite ore has been mined from the Lotus West pit at an average grade of 1.94 g/t gold. In the northern part of the Cockburn deposit and in The Gap, the ferruginous laterite and underlying saprolite have been removed, by headward erosion of a post-Miocene drainage system, to a depth of up to 30 m. Prior to deposition of the Wiluna Hardpan this palaeochannel was filled by a variety of locally derived fluviatile and lacustrine sediment, including kaolinitic and plastic clay, quartzose sand and grit and coarser material comprising mainly ferruginous pisolites and fragments of nodular laterite and lateritised rock. These sediments are generally very poorly sorted and clasts tend to be matrix supported. Scour and fill structures are common and, except for a basal lag of coarser quartz and laterite fragments, an upward coarsening trend is evident in both clasts and matrix. Bands of secondary opaline silica occur within the succession, especially near the base. Pale green, soapy smectite clay occurs sporadically as a replacement mineral in the lower part of the lake sequence and in the immediately underlying pallid zone saprolite. The lake clays are distinguishable from the underlying saprolite by the presence of scattered rounded quartz grains and occasionally abundant transported pisolites, which have cutans largely removed. The kaolinitic lake clays have a distinctive silky lustre and subconchoidal fracture. The more plastic clays are generally recovered from RC drill holes as balls to about 5 cm diameter which may include dark carbonaceous or manganese oxide stained zones. There are usually subtle changes in the colour and texture of drill cuttings once the saprolite is intersected. The base of oxidation varies from about 55 m vertical depth in the Lotus pit to 90 m adjacent to the massive sulphide-chert horizon in the Cockburn deposit. It is defined as the point below which leaching of carbonate and oxidation of pyrrhotite to pyrite or marcasite has not occurred. Limonite staining on joints and fractures is also absent. The somewhat subjectively determined base of extreme weathering within the saprolite zone varies between about 30 and 60 m depth. Above this, textures in the clayey (kaolinitic) rock are largely destroyed during RC drilling, making recognition of parent rock types extremely difficult. The upper 20 m of the saprolite zone has undergone marked gold depletion in some cases, whereas there is often supergene enrichment of gold in the saprock zone.
ARCHAEAN STRATIGRAPHY The overturned Archaean volcanic-sedimentary sequence was deposited in a proximal volcanic environment in which lava flows and pyroclastic units predominate. There is an upward (easterly) trend from ultrabasic volcanic rock through tholeiitic basalt and basaltic tuff to andesitic and dacitic tuff and lava.
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FIG 3 - Generalised geological cross section, Cockburn, looking grid north (348 o magnetic).
Basaltic-peridotitic komatiite The peridotitic flows are generally altered to talc-carbonate rocks with olivine cumulate textures in less foliated zones. The more basaltic flows often have well developed spinifex textures and flow top breccia structures, which in some cases can be used to demonstrate an east facing. In the SW part of Cockburn there is an enclave of strongly foliated graphitic tuffaceous sediment and strongly tectonised, quartz-veined high magnesium basalt about 90 m thick.
Massive sulphide and chert Where relatively undeformed, this unit is about 2 to 10 m thick. It is a variably laminated chemical sediment of volcanic exhalative origin comprising mainly massive pyrrhotite with minor pyrite, chalcopyrite and sphalerite, intercalated with chert and cherty tuff bands. In many cases, especially where transposed into steeply dipping fault zones, the unit has been strongly sheared and comprises subrounded fragments of chert, komatiite, tholeiitic basalt and quartz in an annealed matrix of massive sulphides. Above the base of oxidation, the massive pyrrhotite is oxidised first to secondary pyrite or marcasite with distinctive atoll structures and a porous texture due to shrinkage, and then to a massive limonite gossan. On several sections there are a number of narrow zones of massive sulphide (mainly pyrrhotite) within the komatiite, for several tens of metres to the west of the main contact. Many of these appear to be tectonically emplaced offshoots of the massive sulphide–chert horizon, and some may be sulphidic interflow sediment.
Tholeiitic basalt The contact between the massive sulphide-chert horizon and the tholeiitic basalt is generally gradational, with the basal portion of the basalt a rubbly interflow breccia with a highly sulphidic and tuffaceous matrix. This grades into more massive
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT McCLURE GOLD DEPOSITS
flows with, in places, well preserved pillow structures. Disseminated sulphides, from 2 to 10%, are most abundant in interflow and interpillow breccias and comprise mainly pyrrhotite with lesser pyrite and minor to 1% chalcopyrite. This unit is up to 50 m thick.
Basaltic lithic lapilli tuff This unit is up to 50 m thick and is composed of unsorted lapillisized angular fragments of tholeiitic basalt in a basaltic matrix, with some erratic inclusions of larger lithic fragments of basalt and chert. It is moderately foliated and has 5 to 10% disseminated sulphides throughout, comprising mainly pyrrhotite with up to 1% chalcopyrite and minor pyrite. Some of the sulphide appears to be present as fragments of previously deposited massive sulphide bands. There are some intercalated cherty tuff bands with up to 20% sulphides, including minor sphalerite and rare arsenopyrite, and some thin, rubbly basalt lava flows.
Tholeiitic basalt with cherty tuff This unit is about 20 m thick and includes bands of sulphidic cherty tuff and basaltic tuff. It has well developed pillow structures in some instances and is generally less sulphidic than the two basaltic units described above.
Orelia dolerite This informally named unit has the overall form of a sill, although its contacts may be locally discordant. Where the Orelia dolerite is in contact with basalt, its chilled margins and the fact that it has undergone the same history of metamorphism and metasomatism as the basalt, make its contacts difficult to locate precisely. The dolerite generally has a coarser texture and only minor to 2% disseminated pyrite, and veining is also less well developed. Some zones within the Orelia dolerite have a coarse relict cumulate pyroxene texture not seen in any of the other dolerite sills in the area, and it may be more magnesian. The Orelia dolerite is up to 50 m thick but varies considerably in thickness along strike and down dip, and on some sections it is split by a raft of basalt and sulphidic cherty tuff.
some cherty bands. There are some zones of dacitic lithic lapilli tuff and on some sections, a fine grained dolerite to about 5 m thick. The sulphide content of Felsic zone 1 is generally less than 2% but may be up to 10% over narrow intervals, and it comprises pyrrhotite and pyrite with minor chalcopyrite and sphalerite. Soft sediment slump structures are common in some zones and stylolites are common. Occasional graded beds to about 5 cm thick indicate an east facing.
Cumberland dolerite The informally named Cumberland dolerite varies considerably in thickness along strike and down dip, from about 5 to 60 m. It is generally conformable with the enclosing strata and has sharp contacts with chilled margins. On some sections it is split by large rafts of the felsic volcanicsedimentary sequence. The Cumberland dolerite has scattered blue quartz eyes and granophyric textures in some zones, and is more like the Lotus dolerite than the Orelia dolerite. It generally contains only minor disseminated pyrrhotite and pyrite and a trace of chalcopyrite.
Felsic zone 2 This unit is about 80 m thick. It is similar to Felsic zone 1 except that it has a higher proportion of dacitic lithic lapilli tuff and relatively fewer bands of fine grained felsic tuffaceous sediment. The sulphide content is generally less than 2%, although it reaches 5% in some zones.
Lotus dolerite The informally named Lotus dolerite is about 175 m wide (at the 300 m RL on 52 875 N). It has chilled margins and appears to be largely conformable with the enclosing volcanicsedimentary succession. It is a quartz dolerite with scattered blue quartz eyes and zones of granophyric texture. Although there are marked variations in texture and grain size, there is no obvious indication of gravity differentiation within it. The Lotus dolerite is thought to be comagmatic with the tholeiitic basalts in the enclosing strata. It has only minor disseminated pyrrhotite and pyrite and a trace of chalcopyrite.
Amygdaloidal basalt Mixed zone This comprises interbedded basalt (often pillowed), basaltic tuff, sulphidic cherty tuffaceous sediment (rarely graphitic) and andesitic to dacitic tuff. There is a tendency for the unit to become increasingly felsic eastwards (stratigraphically upwards) and the contact with the overlying felsic unit is gradational. The sulphide content varies from minor disseminations in the basaltic tuff to semi-massive in some bands within the laminated cherty tuff. The main sulphide is pyrrhotite, although pyrite is dominant in some zones. Chalcopyrite and sphalerite are also generally present in varying amounts, to about 3% each. The basaltic tuff is typically highly foliated, moderately laminated and biotitechlorite altered. It may be derived from a high magnesium basalt and is found only in the Mixed zone. Soft sediment slump structures are common in the cherty tuff units.
Felsic zone 1 This unit is up to about 50 m thick and comprises interbedded dacitic tuff, and laminated felsic tuffaceous sediment with
Geology of Australian and Papua New Guinean Mineral Deposits
This tholeiitic basalt is characterised by an abundance of quartz-carbonate-sulphide filled amygdales, to about 2 cm but generally less than 1 cm in diameter. The rock is generally unfoliated and sparsely veined, with only minor disseminated pyrrhotite and pyrite. Its thickness is uncertain but is probably less than 20 m.
Andesitic volcanics The majority of the sequence intersected by core drilling on the east side of the Lotus dolerite is felsic pyroclastic rock and lava, mainly of andesitic composition, but including some dacite. These are interbedded with tuffaceous sediment to about 10 m thick and some dolerite units, which may be offshoots of the main Lotus dolerite. One notable member, to 20 m thick, is polymictic agglomerate with an average composition of andesite. Fragments include basalt, andesite and dacitic porphyry in an andesitic tuff matrix. These volcanic rocks generally have only minor disseminated pyrite and pyrrhotite with a trace of chalcopyrite and sphalerite but total sulphide reaches 5% in some zones.
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Acid dykes These rocks, to 20 m wide but generally much less, occur sporadically at Cockburn–Lotus and are intruded in a number of stratigraphic positions, including the komatiite sequence, the Mixed zone and Felsic zone 1. The acid dykes,which are usually highly sericitised and silicified, are distinguishable from dacitic volcanic rocks by an absence of laminations and an even peppering of disseminated subhedral pyrite to about 5%. A stockwork of thin, irregular quartz veins is usually present. The acid dykes appear to be penecontemporaneous with the volcanic-sedimentary succession and may have been feeders to some of the more acid volcanic units. They often carry significant gold values, to 12 g/t.
Intermediate dykes Swarms of intermediate dykes cut the volcanic-sedimentary succession, including the dolerites and acid dykes, and are most abundant at Lotus. These dykes all appear to be younger than the gold mineralisation, and vary in width from a few centimetres to more than 10 m, with the majority being 1 to 2 m wide. They generally have chilled margins and are very variable in texture and mafic index. Several pulses of intrusion are indicated by contact relations within composite dykes. The larger, coarser grained intermediate dykes have a granitoid texture and are called diorites. Other dyke rocks include feldspar- and amphibole- (altered to biotite-chlorite) phyric and non-porphyritic biotite-rich types and pale coloured, fine to medium grained intermediate dykes. Some of the biotite-rich dykes have scattered augen-like inclusions of quartz-carbonate to 5 mm diameter. Nearly all the dyke rocks have minor disseminated pyrite and are slightly to moderately foliated and carbonated. Based on limited petrographic and multi-element assay data, the more mafic dyke rocks are thought to be calc-alkaline lamprophyres. The diorite dykes are most obviously transgressive to the stratigraphy and foliation in the Lotus area, with the majority dipping at 40o to 60o to the SE (Fig 4). Many dykes strike subparallel to foliation but have a shallower easterly dip. The dykes have a mainly dilational effect on the enclosing rocks but significant displacement of ore zones is associated with some of the dykes.
FIG 4 - Cross section showing geology and ore blocks, Calista, looking grid north (348o magnetic).
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Xenoliths, mainly of dolerite and basalt but also of other rocks in the volcanic-sedimentary succession and granodiorite, are common, especially in the diorite dykes. Some of the variations in texture and mafic index in the diorite dykes are due to variable assimilation of xenolithic material into the melt. In some instances more than half of the volume of diorite dykes in drill core intersections comprises a variety of rounded to angular xenoliths, some of which must have been transported in the melt for a considerable distance. One of the most persistent rock units at Cockburn is a feldsparphyric intermediate intrusive which extends along strike and down dip over virtually the entire extent of the Cockburn deposit. It varies in thickness from about 30 cm to 5 m (average 1.5 m) and has a sill-like relationship to the enclosing strata, being emplaced in the eastern (upper) part of the Mixed zone, about 10 m from the contact with Felsic zone 1. This feldspar porphyry is slightly foliated, weakly carbonated and has minor to 1% disseminated pyrite and a trace of chalcopyrite. It is locally transgressive to structures in the enclosing strata and in some instances is split by a wedge of tuffaceous sediment. It is cut by other intermediate dykes, including biotite-rich types, all of which appear to be younger than the porphyry. Similar, apparently less persistent feldspar porphyry intrusives occur elsewhere at Cockburn–Lotus.
STRUCTURE The volcanic-sedimentary succession and associated dolerite sills in the Cockburn–Lotus area strike northwesterly and dip between 20o and 80o southwesterly. The shallower dips occur in the southern part of the Cockburn deposit, where the western limb and hinge zone of a broad antiform, plunging about 40o SE, are indicated. A major NNW-trending, steeply dipping shear zone cuts the sequence at the Lotus and Cockburn deposits. Because it is oblique to the regional trend, it progresses eastwards through the sequence as it is traced from the southern part of Cockburn northwards to Lotus. There is a tendency for the shear to follow particular units or contacts for some distance before stepping across to the next. Thus, in the SW part of Cockburn, higher grade gold values are associated with the massive sulphidechert horizon and are even found within the metakomatiite (Fig 4). In the northern half of Cockburn the gold mineralisation has moved away from this contact and is found increasingly in the Mixed zone and Felsic zone 1. In the northeastern part of Cockburn, significant gold values are found within the Cumberland dolerite and Felsic zone 2 (Fig 5), while at Lotus, where the shear zone is narrower and more clearly defined, gold mineralisation is hosted by the Lotus dolerite and by metavolcanic rock on its eastern side. Gold mineralisation at Cockburn, as indicated by anomalous (+0.1 ppm) gold values, is spread over a width of at least 300 m but is unevenly distributed within this structural corridor. Both the Orelia and Cumberland dolerites contain relatively low gold values at Cockburn, although some significant intersections have been made within them. The zones adjacent to the upper and lower contacts of these dolerites have significant gold values, regardless of rock type, provided they lie within the structural corridor. This is probably due to the competency contrast between the dolerite and the enclosing strata.
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT McCLURE GOLD DEPOSITS
FIG 5 - Cross section showing geology and ore blocks, OreliaCumberland, looking grid north (348o magnetic).
Variations in the apparent thickness of different rock units within the Orelia–Cumberland sequence may be due to facies changes in what was undoubtedly an unstable proximal volcanic environment of deposition, soft sediment deformation, ductile deformation of less competent units, and faulting. Drill core shows very few fractures below the limit of weathering. There are, however, some zones to about 15 m but generally less than 5 m true width, of strongly fractured and slickensided rock, especially in the metakomatiite and Felsic zone 1 units. These fracture zones may represent significant late stage faults which could be expected to displace rock units and mineralised zones. The anastomosing NNW-trending shear zone hosting the gold mineralisation at Lotus occurs largely within the Lotus dolerite and is 25 to 40 m wide. Ore occurs within the shear zone in the Footwall (western) and Hanging Wall (eastern) lodes (Fig 6). The Footwall lode dips at 70 to 80o east and the Hanging wall lode dips at 50 to 70o east, and becomes flatter with depth, resulting in a divergence of the lodes. Subsidiary lodes occur between the main lodes and to the west of the Footwall lode (the Footwall 1 lode). These lodes are generally narrower and less continuous than the main lodes. At depth the Hanging Wall lode occurs partly within metabasalt, while the Footwall lode remains just within the eastern margin of the Lotus dolerite. In the Lotus South zone, the lodes are less divergent and dip from 70o east to 80o west with increasing depth, and in the Lotus North zone, in many cases, only one lode, presumed to be the Footwall lode, is present. At both Lotus and Cockburn, ore shoots within the mineralised shear zone, defined by gold values of +5 g/t, plunge at about 40o southwards, parallel to the axis of the Cockburn antiform. There also appears to be a steeply north plunging control of high grade shoots in some zones at both deposits.
MINERALISATION The gold ore zones within the Lotus dolerite have 2 to 10% (average about 5%) pyrite as blebs and veinlets in quartz veins and altered dolerite. Minor to 1% chalcopyrite, and arsenopyrite as fine grained euhedra, are commonly associated with the pyrite, and trace sphalerite and galena are occasionally visible in the quartz veins. As only minor pyrrhotite, pyrite and chalcopyrite occur in the unaltered Lotus dolerite it is evident that higher sulphide concentrations are generally related to the gold mineralising event.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 6 - Cross section showing geology and ore blocks, Lotus, looking grid north (348o magnetic).
The Footwall lode at Lotus typically has one or more massive white quartz–minor carbonate-chlorite veins. There are generally some subsidiary subparallel quartz veins separated by strongly foliated and metasomatised metadolerite. The Footwall lode varies from about 2 to 5 m in width, with a fairly rapid transition to relatively unfoliated and unaltered metadolerite at its margins. The Hanging Wall lode is characterised by an abundance of veins of bluish white quartz–minor carbonate–chlorite, to about 5 cm wide, emplaced in a zone, to 10 m wide, of strongly foliated and metasomatised metadolerite or metabasalt. Although ptygmatically folded in part, these quartz veins are subparallel to foliation, giving the rock a distinctly banded appearance. Visible gold, as scattered blebs to 2 mm diameter, is most common in the larger quartz veins of the Footwall lode. Gold mineralisation at Cockburn is generally associated with a stockwork of irregular quartz-carbonate±chlorite veins, to about 2 cm but generally less than 5 mm wide, in pervasively altered but often only slightly foliated host rock. Bluish-white quartz veins with minor chlorite, carbonate and disseminated sulphides are also closely associated with significant gold values. These veins, which are up to 10 cm but generally less than 1 cm wide, are usually subparallel to foliation and are often boudinaged and/or ptygmatically folded,and occasionally contain blebs of gold to about 1 mm diameter. Sparsely distributed blebs, to 2 mm diameter, of a suite of soft, silvery minerals are associated with some exceptionally high grade (+10 g/t) gold zones at Cockburn, almost invariably with visible gold. Minerals identified by optical, X-ray microdiffraction and electron microprobe techniques include bismuthinite, hedleyite (Bi7Te3), tetradymite (Bi2Te2S) and native bismuth (E Nickel, personal communication, 1996;
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Pontifex and Associates, unpublished data, 1996). It was also noted that grains of native gold associated with the bismuth minerals had up to 11% silver in solid solution. Other sulphide minerals, occurring as accessory minerals closely associated with gold mineralisation in both quartz veins and acid dykes and reported from mineralogical work, include galena and molybdenite. Scheelite has also been reported in a quartz vein from the Mixed zone. Several of the units in the Cockburn succession are highly sulphidic, notably the massive sulphide-chert horizon, the basaltic lapilli tuff and associated cherty tuff bands and the cherty tuffaceous sediment in the Mixed zone. The most abundant sulphide is pyrrhotite, with lesser and often only minor pyrite, and generally trace to minor chalcopyrite and sphalerite. Galena and arsenopyrite are rare. Most of the sulphidic chert units have appreciable zinc and copper values, to about 3% zinc and 2% copper. Sphalerite is only moderately iron-rich, as indicated by its reddish brown colour. Most of the sulphides, especially pyrrhotite, chalcopyrite and sphalerite, are thought to be volcanogenic. There is no correlation between total sulphide content and gold values, as both sulphide-rich and sulphide-poor rocks may contain high or very low gold values. There is evidence however that pyrite has been introduced during the gold mineralising event, or alternatively that some pyrrhotite has been altered to pyrite.
METASOMATIC WALL ROCK ALTERATION At Lotus, because the shear zone and associated gold lodes are well confined and largely within the quartz dolerite, the metasomatic wall rock alteration which accompanied the gold metallogenic event is easy to recognise. There is a gradational change from unfoliated and unaltered metadolerite to strongly foliated and metasomatised quartz veined rock in the ore zones. The main alteration minerals are biotite, chlorite and carbonate (mainly calcite but also siderite in some zones) and there is an increase in sulphide content to about 5%. Primary interstitial ilmenite is progressively altered to leucoxene. The biotite is an indication of potassium metasomatism. In contrast to Lotus, a well developed foliation is less common in the mineralised zones at Cockburn, which are mainly associated with a stockwork of quartz and quartzcarbonate veins. Wall rock alteration in mafic rocks at Cockburn is similar to that at Lotus, with the development of chlorite, biotite and carbonate as a pervasive alteration assemblage in strongly mineralised zones, or as alteration selvages around veins in less altered rock. Phlogopite is developed in preference to biotite in quartz-veined komatiite, which may carry significant gold values within about 10 m of the massive sulphide–chert contact. Gold ore zones within felsic units at Cockburn, such as Felsic zone 1, are characterised by a prominent foliation, a predominance of blue quartz veins subparallel to foliation, and moderate to strong sericite alteration (reflecting the low iron content of the host rock), and silicification. Disseminated sulphides, mainly pyrite, are generally less than 3% and quartzcarbonate veining and carbonate metasomatism are virtually absent. Mineralised zones within the Cumberland dolerite are similar to some of the weaker mineralised zones within the Lotus deposit. They are moderately foliated, quartz veined and chlorite-biotite-carbonate altered and have up to 3% disseminated pyrite. However, gold values in these zones are generally fairly low.
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Secondary magnetite, as fine to medium grained disseminations to about 1%, is commonly developed in strongly metasomatised (biotite-chlorite-carbonate altered) metabasalt. Minor vein minerals that have been noted in drill core include muscovite, rutile and purple fluorite. At both Cockburn and Lotus there are zones of epidote veining accompanied by moderate to strong, pervasive hematite alteration. This style of alteration, which affects all rock types, does not appear to be related to significant gold values and is clearly a late stage metasomatic event. Strong carbonate-chlorite metasomatism is associated with late stage coarse grained quartz-calcite–greyish green chlorite–sulphide (pyrrhotite-pyrite-chalcopyrite) veins. These steeply dipping veins, which are up to 30 cm wide, trend grid east and cut all other rocks, including the intermediate dyke suite. They are undeformed and appear to represent the last significant Archaean event in the Cockburn–Lotus area.
SOUTHERN DEPOSITS The four southern gold deposits have many geological similarities. From north to south they are Success, Parmelia, Challenger and Dragon (Fig 1). All are close to the western margin of the Yandal greenstone belt and are essentially tabular bodies striking NNW, except for Dragon which strikes north. They dip at about 50o easterly, conformable with the foliation and bedding in the enclosing strata. Success, Parmelia and Challenger appear to be at the same stratigraphic horizon, about 200 m east of a prominent gossanous chert unit. The Dragon deposit appears to be slightly higher in the east-facing stratigraphic succession. Open pit mining has ceased at all four deposits, but it is likely that mining will recommence at Success and Challenger at least.
SUCCESS Here a total of 247 951 t of ore at an average grade of 3.49 g/t gold has been mined in an open pit to 45 m depth. The orebody is hosted by a strongly foliated, weakly to moderately graphitic felsic cherty tuff, about 15 m thick (Fig 7). This unit is overlain by a foliated, fine-grained chloritic tuffaceous sediment with some graphitic bands. Basalt units and thin dolerite intrusives are also present. The footwall is a strongly-foliated dacitic lithic lapilli tuff in which the fragments have undergone extreme attenuation in the plane of foliation.
FIG 7 - Cross section showing geology and ore blocks, Success, looking grid north (348o magnetic).
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT McCLURE GOLD DEPOSITS
The gold mineralisation is associated with numerous quartz (with minor carbonate-chlorite) veins to 10 cm thick, subparallel to the foliation. The veins are often boudinaged and/or ptygmatically folded. Disseminated pyrite-arsenopyrite to 10% and trace to minor chalcopyrite-sphalerite are associated with the mineralised zone, which shows moderate to strong biotite-chlorite-carbonate-sericite metasomatic alteration. Deep weathering is a feature of the Success and Parmelia deposits, and is described at the end of the section on Parmelia.
PARMELIA
holes at Success both the hanging wall and footwall units are fresh below about 80 m vertical depth. At Parmelia the base of oxidation follows the footwall contact down dip from about 80 m below ground level.
CHALLENGER A total of 138 946 t of ore at 2.37 g/t gold was mined from two small pits at Challenger, to depths of about 40 m, well above the base of oxidation (Fig 9). The rock types, structures and styles of mineralisation are similar to those at Success and Parmelia. Due to the lack of core drilling, knowledge of the stratigraphy at Challenger is limited.
The deposit has been mined in an open pit to a depth of about 90 m to produce 536 157 t of ore at 3.03 g/t gold. The geology of the Parmelia gold deposit is very similar to that of Success, except that there are two main ore zones, about 40 m apart, subparallel to bedding and foliation. The upper zone is in chloritic schist, while the lower zone is partly within ‘graphitic chert’, about 10 m thick, which separates the hanging wall sequence from the strongly foliated dacitic lithic lapilli tuff footwall unit, which is identical to the footwall unit at Success. Bands of graphitic sediment, to about 2 m thick, are intercalated with the chloritic schist, which is apparently a basaltic to andesitic tuffaceous metasediment of the hanging wall sequence. There is also a lenticular dolerite intrusive, several metres thick, above the upper ore zone. The graphitic chert at Parmelia comprises interbedded graphitic sediment and felsic tuff with some cherty bands. Graded bedding in this unit consistently indicates an east facing. As the upper ore zone is followed down dip to the east, it steepens to near vertical and merges with the lower zone, forming a U-shaped orebody in cross section (Fig 8). Enigmatically, no significant gold values have been intersected below this closure, although the enclosing strata continue uninterrupted.
FIG 9 - Cross section showing geology and ore blocks, Challenger, looking grid north (348o magnetic).
DRAGON This is the southernmost of the Mount McClure gold deposits, 24 km SSE of the mill. A total of 363 231 t of ore at an average grade of 3.60 g/t was mined by open pit to a depth of about 70 m. A prominent, north-trending chert ridge is about 300 m to the west of the deposit. This ridge is close to the faulted contact with the gneiss terrane on the western side of the Yandal greenstone belt. The mine sequence strikes north and dips at 45–60o to the east and is probably east facing. However the upper greenschist to amphibolite facies metamorphism and strongly developed foliation subparallel to the rock layering have largely destroyed the primary rock structures and textures. The structural and presumed stratigaphic footwall unit at Dragon is a tholeiitic basalt. In the southern part of the deposit it has been partly brecciated and carbonate altered. In the northern part of the deposit the basalt has been mylonitised, then annealed to form a crudely laminated, coarse grained rock in which diopside is common but garnet is notably absent.
FIG 8 - Cross section showing geology and ore blocks, Parmelia, looking grid north (348o magnetic).
Even in the deepest intersections (+100 m below ground level) of the Success and Parmelia mineralised zones, there is evidence of leaching of carbonate from the rock fabric and veins. Pyrrhotite, which oxidises readily to pyrite or marcasite, is absent from the ore zone although it is present as minor disseminations in the underlying fresh, weakly carbonated footwall felsic tuff at Success and Parmelia. In deeper core
Geology of Australian and Papua New Guinean Mineral Deposits
Overlying the footwall unit is a coarse grained ultrabasic schist comprising talc, tremolite, chlorite and carbonate, to 25 m thick, which is the main host for the gold mineralisation (Fig 10). Other minerals in this rock include anthophyllite and phlogopite, the latter indicative of potassium metasomatism. Intercalated with the ultrabasic schist are one or more bands of graphitic schist to about 1 m thick. These probably represent interflow sediments in a komatiitic lava sequence, although any direct evidence of volcanic origin, such as spinifex (quench) textures and flow-top breccia structures have been destroyed during metamorphism. The upper part of the ultrabasic unit
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J L HARRIS
As discussed by Perring, Groves and Ho (1987) Archaean lode gold deposits are most likely to be metamorphogenic. The observed relationships for the Mount McClure gold deposits are consistent with this origin during prograde regional metamorphism. The metamorphic fluids were channelled along brittle–ductile shear zones subsidiary to major ductile shear zones, such as the Mount McClure fault and the Ockerburry Fault, and deposited vein minerals and gold in dilational (low pressure) zones within or close to them, as well as causing extensive wall rock alteration. Folding and faulting at deposit scale and competency contrasts between different rock types are most likely to have generated the dilational zones.
FIG 10 - Cross section showing geology and ore blocks, Dragon, looking grid north (348o magnetic).
includes what appear to be basaltic tuff and tuffaceous sediment intercalations. The hanging wall sequence comprises basalt and a lenticular pyroxenite-dolerite intrusive to 10 m thick, as well as basic to intermediate tuffaceous sediment, and several graphitic sedimentary units to about 2 m thick. Gold mineralisation is mainly confined to the lower half of the ultrabasic unit, including its graphitic zones (Fig 10). The ore zone has a strike length of 600 m and dips subparallel to the enclosing strata and foliation. The northern half of the orebody is a single lode from 2 to 10 m thick. The southern half has three subparallel lodes, of average width 3 m. The gold mineralisation is associated with thin quartz veins, generally subparallel to foliation, with minor to 5% disseminated pyrite and arsenopyrite. Prominent north-striking white quartz veins, which do not contain significant gold values, cut the mineralised zone. Dragon is the least weathered of all the Mount McClure gold deposits and there is little supergene gold enrichment. There is however, abundant paint gold on fracture surfaces in some graphitic zones within the mineralised ultrabasic unit. Further evidence for redeposition of gold is the merging of the three primary lodes into one broader unit in the upper part of the weathered zone.
ORE GENESIS The bulk of the pyrrhotite-dominated sulphide mineralisation at Cockburn is thought to be volcanogenic and unrelated to the gold metallogenic event which occurred during deformation and regional metamorphism of the Yandal greenstone belt. A similar relationship is reported for volcanogenic base metal sulphide mineralisaion and primary gold ore at Mount Gibson (Yeats, 1995). Rock et al (1987) suggested that calc-alkaline lamprophyres, which often have a close spatial relationship to gold mineralisation, may be genetically related to it. At Cockburn–Lotus, gold deposition clearly predates the intrusion of the intermediate dykes, including the probable lamprophyres. However both the gold-bearing fluids and the intermediate dykes have invaded the same zone of structural weakness. On the other hand, the significant gold mineralisation within the tonalite or granodiorite stocks at Anomaly 45 and east of Success indicate that mineralisation was later than the emplacement of at least the internal granitoids.
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Iron-rich rocks such as tholeiitic basalt and quartz dolerite are considered most favourable for the deposition of gold by a process of sulphidation, producing pyrite from reduced sulphur-gold complexes (Phillips and Groves, 1983). Certainly tholeiitic basalt and the Lotus dolerite are two of the most important host rocks at Cockburn–Lotus. Other iron-rich rocks such as the massive sulphide-chert and sulphidic cherty tuff are also important host rocks at Cockburn. However, the importance of favourable structure is emphasised by the tendency for ore to occur in the iron- and sulphide-poor, but strongly foliated, felsic rocks on either side of the more massive Cumberland dolerite, rather than within it. The Orelia dolerite may be a relatively unfavourable host rock because of a higher magnesium:iron ratio. In the southern deposits, the graphite within the ore zone may have been a factor in localising gold mineralisation.
ACKNOWLEDGEMENTS The writer gratefully acknowledges the permission of Australian Resources Limited and Forrestania Gold NL to publish this paper. The work presented here is the result of a team effort. In particular D Otterman, G de San Miguel, P Brookes, H Little, R Gordine, T Tuffin, D Brinsden and M Kemsley, as well as the mine staff and support staff at the Perth office, have all made major contributions to the success of the Mount McClure Project. The contributions of petrological consultants I R Pontifex and A C Purvis of Pontifex and Associates Pty Ltd to the understanding of the Mount McClure geology are also acknowledged. Special thanks are due to B H Smith for reviewing the first draft of this paper.
REFERENCES Australian Resources Limited, 1996. Resources Limited: Sydney).
Annual Report (Australian
Bettenay, E and Churchward, H M, 1974. Morphology and stratigraphic relationships of the Wiluna Hardpan in arid Western Australia, Journal of the Geological Society of Australia, 21:73–80. Bunting, J A and Williams, S J, 1979. Sir Samuel, Western Australia 1:250 000 geological series, Geological Survey of Western Australia Explanatory Notes SG 51–13. Eshuys, E, Herbison, I D, Phillips, G N and Wright, J H, 1995. Discovery of the Bronzewing gold mine, in Proceedings, New Generation Gold Mines: Case Histories of Discovery, Perth 27–28 November 1995, pp 2.1–2.15 (Australian Mineral Foundation: Adelaide). Hallberg, J A, 1983. An aid to rock-type discrimination in deeplyweathered terrain, in Geochemical Exploration in Deeply Weathered Terrain (Ed: R E Smith), pp 63–66 (CSIRO Division of Mineralogy: Perth).
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Kojan, C J, Faulkner, J A and Saunders, A J, 1996. Geochemical mapping of the Sir Samuel 1:250 000 sheet, Geological Survey of Western Australia Explanatory Notes SG 51–13. Lewington, G L D,1995. The discovery of the Nimary gold camp, in Proceedings, New Generation Gold Mines: Case Histories of Discovery, Perth 27–28 November 1995, pp 4.1–4.10 (Australian Mineral Foundation: Adelaide). Otterman, D W and de San Miguel, G F, 1995. The discovery and development of the Mt McClure gold deposits, in Proceedings, New Generation Gold Mines: Case Histories of Discovery, Perth 27–28 November 1995, pp 3.1–3.15 (Australian Mineral Foundation: Adelaide). Perring, C S, Groves, D I and Ho, S E, 1987. Constraints on the source of auriferous fluids for Archaean gold deposits, in Recent Advances in Understanding Precambrian Gold Deposits, Publication No 11 (Eds: S E Ho and D I Groves), pp 287–306 (The Geology Department and University Extension, The University of Western Australia: Perth). Phillips, G N and Groves, D I, 1983. The nature of Archaean goldbearing fluids as deduced from gold deposits of Western Australia, Journal of the Geological Society of Australia, 30:25–29.
Geology of Australian and Papua New Guinean Mineral Deposits
Rock, N M S , Duller, P, Haszeldine, R S and Groves, D I, 1987. Lamprophyres as potential gold exploration targets: some preliminary observations and speculations, in Recent Advances in Understanding Precambrian Gold Deposits, Publication No 11 (Eds: S E Ho and D I Groves), pp 271–286 (The Geology Department and University Extension, The University of Western Australia: Perth). Williamson, A, 1992. Regolith-landform evolution and geochemical dispersion from the Calista gold deposit, Mount McClure district, Western Australia, BSc Honours thesis (unpublished), The University of Western Australia, Perth. Wright, J H and Herbison, I D, 1995.The Yandal belt - preliminary exploration of the Jundee deposit, in Proceedings, New Generation Gold Mines: Case Histories of Discovery, Perth 27–28 November 1995, pp 5.1–5.9 (Australian Mineral Foundation: Adelaide). Wyche, S and Westaway, J M, 1995. Geology of the southern part of the Yandal greenstone belt, Eastern Goldfields, Geological Survey of Western Australia 1994–1995 Annual Review, pp 94–97. Yeats, C J, 1995. 2.63 Ga lode-gold style mineralization overprinting a 2.93 Ga VHMS event at the Mount Gibson gold deposits, Yilgarn Craton, Western Australia, Geological Society of America Abstracts, 27(6):A–378.
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Smith, M E, 1998. Tuckabianna gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 149–154 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Tuckabianna gold deposits by M E Smith
1
INTRODUCTION The centre of the Tuckabianna group of gold deposits of Westgold Resources NL (Westgold) is 24 km east of Cue, WA, in the Murchison mineral field. The deposits are dispersed in a 30 km long zone between AMG coordinates 602 000 m E and 619 000 m E, 6 975 000 m N and 6 950 000 m N, or between lat 27o20′S and 27o34′S, long 118o02′E and 118o12′E on the Cue (SG 50–15) 1:250 000 scale and the Reedy (2543) and Wynyangoo (2542) 1:100 000 scale map sheets (Fig 1).
recovery of 90.1%. Ore has been mined from 22 separate open pits. Most of the gold production (90%) was from the large Tuckabianna mill but 658 000 t at 3.45 g/t gold came from the Comet and Pinnacles open pit mines and was treated in a smaller treatment plant, between 1989 and 1992. In the 12 months to 30 June 1996, 769 140 t of ore were milled at an average head grade of 2.78 g/t gold to produce 1985 kg (63 830 oz) of gold at a recovery of 92.8%. Mining terminated in April 1996 as open pit ore reserves were exhausted, but milling is expected to continue until March 1997. At 30 June 1996 total open pit, underground and stockpile Identified Mineral Resources were 5.65 Mt at 2.1 g/t gold. This comprised 4.26 Mt at 1.5 g/t gold of open pit resources, 762 000 t at 6.3 g/t gold of underground resources and 630 000 t at 1.14 g/t gold in stockpiles. The resources include Proved and Probable Ore Reserves of 1.14 Mt at 3.1 g/t gold comprising underground Probable Reserves of 513 000 t at 5.5 g/t gold and the stockpiled ore.
EXPLORATION AND MINING HISTORY Gold was discovered by prospectors at Tuckabianna in 1915 and was mined intermittently until the commencement of modern mining operations in 1988. Production to 1988 totalled about 53 000 oz of gold, from a series of shallow workings, generally on small high grade pods of mineralisation in outcropping banded iron formation (BIF). The average grade of this early production was approximately 18 g/t gold.
FIG 1 - Regional geological map of the Tuckabianna area (modified after J Hallberg, unpublished data, 1991).
Modern mineral exploration in the Tuckabianna region commenced in the early 1960s and focused on nickel and volcanogenic massive sulphide deposits. Exploration for gold commenced in the early 1980s by CSR Limited (CSR) and culminated in the discovery of an open pittable gold resource at Caustons in 1987, in a joint venture with Australmin Pacific NL (Australmin). CSR sold its interest in the project to Australmin in 1988, and Australmin later discovered the concealed deposits at Julies Reward, Caustons South and Tuckabianna West. Mining by open pit commenced in 1988. Australmin was acquired by Newmont Australia Pty Ltd (Newmont) in May 1990, which subsequently merged with BHP Gold Mines Limited to form Newcrest Mining Limited (Newcrest). Westgold purchased the operation from Newcrest in March 1994.
REGIONAL GEOLOGY From commencement of milling operations in the Tuckabianna region in November 1988 to the end of June 1996, 6 420 000 t of ore have been treated at an average grade of 2.65 g/t gold for the production of 15 336 kg (493 074 oz) of gold at a
1.
Exploration Manager, Westgold Resources NL, PO Box 7754, Cloisters Square, Perth WA 6850.
Geology of Australian and Papua New Guinean Mineral Deposits
The Tuckabianna deposits are in the Archaean Murchison Province within a NE-trending supracrustal greenstone sequence of volcanic, intrusive and sedimentary rocks. The greenstones are part of the Luke Creek Group, and the mineralisation is concentrated within the lower formations of the group (Golconda Formation and Gabanintha Formation) which dominate the greenstone belt here (Watkins, Tyler and Hickman, 1987; Watkins and Hickman, 1990). Detailed mapping at 1:25 000 scale has been completed over the area (J
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Hallberg, unpublished data, 1990, 1991). Hallberg disagrees with the previous formal stratigraphy and prefers to divide the rocks into two rock ‘associations’ as outlined below. The supracrustal rocks are divided broadly into associations separated by the major Tuckabianna Shear Zone and intruded by post-tectonic granitoids (Fig 1). Association 1 rocks to the east of the shear zone comprise BIF beds interlayered with mafic and ultramafic volcanic and intrusive rocks deformed into the asymmetric Kurrajong syncline. Association 2 rocks are west of the Tuckabianna Shear Zone and comprise felsic rocks of the Eelya complex, and mafic and ultramafic volcanic rocks. The association 2 rocks are characterised by an almost complete absence of BIF. Quartz porphyry and quartz-feldspar porphyry intrusives are commonly associated with the rocks which host gold mineralisation.
ORE DEPOSITS DEPOSIT TYPES Most of the gold produced to-date occurred in or near structurally deformed BIF along the western limb of the Kurrajong syncline where it is cut by the Tuckabianna Shear Zone. In addition to BIF hosted mineralisation, gold has been mined from deposits in other iron-rich sediments, mafic rocks, porphyry and tonalite. There has been significant gold production from laterite, and from alluvial wash within a Tertiary palaeochannel. A listing of deposits by type is in Table 1.
The Tuckabianna Shear Zone (also referred to as the Comet–White Well Shear Zone) is a 1 to 2 km wide NNEtrending zone of intense deformation and alteration along the 30 km length of the mine corridor. The Zone is a portion of the much larger Mount Magnet–Meekatharra Shear Zone which extends for at least 180 km between the two main mining centres and beyond. The Tuckabianna Shear Zone transects the western limb of the Kurrajong syncline with its SE margin corresponding to the prominent main zone of BIF. It is very poorly exposed and marked by deep weathering. There are also north- to NNW-trending faults and shear zones, with displacements up to several hundred metres. Granitoids to the east of the greenstones are pre- to syntectonic granodiorite, while those to the west are largely posttectonic. The Archaean rocks are cut by east-trending mafic dykes of presumed Proterozoic age. Association 2 rocks to the west of the Tuckabianna Shear Zone have been metamorphosed to lower–middle amphibolite facies. To the east of the Zone, the grade is generally greenschist facies but increases to amphibolite facies close to the granitoid contacts. All of the basement rocks have been extensively weathered and deeply oxidised during the Tertiary, and much of the ground is now covered by a complex regolith comprising residual and transported lateritic materials. TABLE 1 Tuckabianna gold deposits by deposit types. Deposit host rock
Gold deposit
Banded iron formation
Julies Reward, Caustons, Caustons North, Caustons South, Sherwood, Marion, Jaffas, Tucka Mining Centre, Tuckabianna West, Big John, Little John, Friars, Hamlet
Iron-rich sediment
Comet, Comet North, Pinnacles, Eclipse, Venus
Mafic rock
Minor ore in most of the larger BIF hosted deposits, Comet North, Jasper Queen
Porphyry
White Well, Jasper Queen
Granitoid
Rapier
Palaeochannel gravel
Jasper Queen
Laterite
Caustons, Caustons North, Caustons South, Julies Reward, Tuckabianna West, Sherwood, Friars, Hamlet, Katies, Jasper Queen, Big John, Little John
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FIG 2 - Simplified geological plan of the Tuckabianna West open pit showing several north trending BIF and sediment units.
Geology of Australian and Papua New Guinean Mineral Deposits
TUCKABIANNA GOLD DEPOSITS
BIF hosted mineralisation Between 70 and 80% of the gold produced from Tuckabianna has come from deposits hosted by BIF and associated laterite, in part of the sheared western limb of the Kurrajong syncline. The most significant producers have been the Julies Reward, Caustons and Tuckabianna West deposits. There are numerous BIF units in the mine sequence. In the zone between the Julies Reward and Tuckabianna West deposits, more than a dozen BIF units have been mapped over a 400 m wide zone. Several of these units are illustrated in the plan of the Tuckabianna West open pit (Fig 2). Individual units are less than 1 m to 25 m thick, and are usually separated by dolerite or gabbro, mafic schist (probably sheared dolerite or gabbro) and intrusive quartz-feldspar porphyry. Commonly only one or two (usually the westernmost) of these BIF units carries significant gold mineralisation. However, at Tuckabianna West economic gold mineralisation was present in three sedimentary units (Fig 2). The mineralised BIF units on the western limb of the Kurrajong syncline strike NE and dip SE at between 55 and 70o.
chlorite-amphibole±pyrite-pyrrhotite rock with minor stilpnomelane, retrograde almandine garnet, magnetite and chalcopyrite. The Hanging Wall lode is similar but contains more quartz and no garnet. A statistical analysis of mineralised intercepts of the mineralisation below the Comet pit has indicated an average thickness of 3.6 m for the combined lodes and intervening basalt. Gold mineralisation is intimately associated with pyrrhotite in both lodes, and coarse grained gold can occasionally be observed in drill core. The highest grades occur in well defined steeply dipping shoots. Diamond drilling below the Comet pit has outlined a total Resource (Measured + Indicated + Inferred) of 762 000 t at 6.3 g/t gold to a depth of 190 m below present ground level, which is 120 m below the base of pit. A Probable Ore Reserve of 513 000 t at 5.5 g/t gold has been estimated.
Mafic hosted mineralisation
In fresh mineralised BIF, the gold is associated with quartzcarbonate-pyrite-pyrrhotite stringers which disrupt, fracture and replace well laminated BIF. Higher grade zones are associated with an increase in quartz and sulphide content. In the upper 70 m, within the oxidised zone, mobilisation of gold by lateritic processes has made zones of economic mineralisation wider than those in the underlying primary zone.
Gold is associated with sheared mafic rocks in many of the predominantly BIF-hosted deposits, such as Caustons and Julies Reward. In most situations the mineralised mafic rocks are sheared dolerite adjacent to mineralised BIF and/or porphyry units (Fig 3). In the Jasper Queen deposit, high grade gold mineralisation occurs in sheared amphibolite in contact with sheared porphyry, and at Comet North gold is associated with sheared high-magnesium basalt adjacent to mineralised iron-rich sediments.
Iron-rich sediment hosted mineralisation
Porphyry hosted mineralisation
The deposits in the SW of the area (Comet, Comet North, Pinnacles, Eclipse and Venus) are principally hosted by ironrich sediment within a dominantly mafic sequence. The Comet is the largest of this group. Here mineralisation occurs in two horizons, the Footwall and Hanging Wall lodes which dip SE at 45o and are separated by a fine-grained massive and barren basalt 0.5–1 m thick. The Footwall lode is a banded quartz-
Narrow quartz porphyry and quartz-feldspar porphyry intrusives are invariably associated with the mineralised BIF in the larger gold deposits at Tuckabianna, and are themselves commonly mineralised. However, larger porphyry bodies occur throughout the greenstone sequence, and the mineralised porphyry at White Well and Jasper Queen has been mined in recent years.
FIG 3 - Caustons deposit, cross section on 12 100 m N, looking NE, showing gold mineralisation in BIF and adjacent mafic rock and porphyry.
Geology of Australian and Papua New Guinean Mineral Deposits
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White Well The deposit is 8 km NE of the Tuckabianna mill, within porphyry intruded into a sequence of andesites. The andesites are coarse grained and immature fragmental rocks to the west of the main Tuckabianna BIF horizons. The quartz-feldspar porphyry body which hosts the gold mineralisation is a SE dipping semiconformable body to 60 m thick, characterised by flattened ovoid quartz phenocrysts or amygdales. The gold occurs in a quartz vein stockwork in the porphyry body, over a strike length of approximately 1 km. The andesite and porphyry have been intensely deformed and deeply weathered to white kaolin clay, quartz and sericite. A portion of the deposit was mined in 1992 by Newcrest from a small trial pit, which produced 165 000 t at 1.69 g/t gold. The White Well Indicated and Inferred Resource, including the trial pit production, is 2.3 Mt at 1 g/t gold. The gold mineralisation has a strong supergene component and a marked depletion zone from surface to about 20 m depth over the entire length of the deposit.
Jasper Queen The deposit was outlined by Westgold in 1994 and mined in 1995–96. It was a concealed orebody 600 m SW of the historic Jasper Queen workings and 6 km NNW of the Tuckabianna mill. Three broad styles of mineralisation are present, namely in primary and oxidised lode, palaeochannel gravel and laterite. The lode mineralisation is predominantly within porphyry and to a lesser extent amphibolite along a highly sheared NEtrending contact zone (Figs 4 and 5). The higher grade primary gold mineralisation is associated with quartz and pyrite, and is confined to a 50 m long zone averaging 5 m thick at an average grade of 7 g/t gold. In the oxidised zone above 45 m vertical depth, the gold mineralisation (as defined by the 1 g/t gold contour) is dispersed over a 35 m wide zone.
Granitoid hosted mineralisation The northernmost deposit in the Tuckabianna group is at Rapier, which is 12 km north of the Tuckabianna mill. The mine operated during 1993 and produced 129 000 t of ore at 2.80 g/t gold. Rapier occurs in the northern portion of the historic Eelya mining centre with historic production of 682 oz from small pits and shafts over a strike length of 1.2 km. The Rapier mineralisation is entirely within a granitoid body, of tonalite to granodiorite composition. This intrusive is considered to be syntectonic (J Hallberg, unpublished data, 1991). The gold is in multiple, gently to steeply dipping, silicified and anastomosing shear zones, and is associated with quartz, pyrite and minor chalcopyrite, pyrrhotite and molybdenite. The deposit is adjacent to the intersection of north- and NW-trending structures. The depth of oxidation is only 20 to 30 m, and the sulphides are altered to limonite and the feldspars to kaolinite in the oxidised zone.
Palaeochannel hosted mineralisation At Jasper Queen, portion of the ore mined in 1995–96 by Westgold was a small but rich palaeochannel deposit very close to the Jasper Queen lode. The deposit was discovered by rotary air blast (RAB) drilling on a nominal 100 by 40 m spacing over outcropping mineralised laterite. The highest RAB intersection, in hole ATK5458, was 6 m at 59.2 g/t gold. A
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Fig 4 - Geological plan of Jasper Queen pit. FIG 5 - Jasper Queen deposit, cross section on 16 065 m N, looking NE. Fig 6 - Jasper Queen deposit, cross section on 5 545 m E, looking NW, showing profile of palaeochannel.
Geology of Australian and Papua New Guinean Mineral Deposits
TUCKABIANNA GOLD DEPOSITS
reverse circulation hole (ARC2904) twin of ATK5458 contained 7 m at 21.1 g/t gold. Final production figures for the palaeochannel are not available because portion of the ore is still stockpiled for processing. However, during January 1996, 55 000 t of palaeochannel ore was treated at a recovered grade of 9.0 g/t gold. The mineralised portion of the palaeochannel, now completely mined out, was 250 m long with an average width of 20 m. The channel cuts across the trend of the primary lode at a high angle and at this intersection the width increased to approximately 60 m (Figs 4 and 6). The average thickness of the channel was 5 m, and its base was approximately 20 m below the present land surface. The channel fill material comprises poorly sorted and slightly indurated sediment, from coarse gravel at the base grading upwards to coarse siltstone at the top. The basal gravel consists of subrounded quartz pebbles to 25 cm diameter, in a white to slightly greenish clay matrix with minor iron oxides. Gold is present throughout the entire channel profile but is concentrated in the basal portion, where it is commonly coarse grained and readily visible to the naked eye. Scanning electron microscope studies (L M Lawrance, unpublished data, 1996) indicate that gold is both alluvial and supergene. The alluvial gold occurs as subrounded grains containing minor silver and copper, and the supergene gold is commonly crystalline and low in silver and copper.
Laterite hosted mineralisation Most of the BIF hosted gold deposits at Tuckabianna are associated with economic lateritic gold mineralisation. The largest of these laterite sheets was at Tuckabianna West where a pre-mining Proved and Probable Ore Reserve of 582 000 t at 2.20 g/t gold was outlined in a flat-lying sheet over 600 m long, 100 m wide and up to 20 m thick. The next largest reserve was at Julies Reward which had a pre-mining Proved and Probable Ore Reserve of 124 000 t at 2.02 g/t gold.
The mineralised laterite has a close spatial relationship with the underlying mineralised rocks. At most deposits the underlying mineralised rocks are BIFs, but at Jasper Queen mineralised laterite is developed over the palaeochannel and lode gold deposits. At Tuckabianna West the laterite-hosted gold occurs downslope and to the west of the subcrop position of the largest and highest grade BIF horizon. At Julies Reward the laterite is more evenly distributed on either side of the highest grade BIF horizon. Economic gold mineralisation in the laterite rarely extends beyond 100 m from the mineralised BIF source. The lateritic profile varies considerably throughout the area but in general the mineralised portion comprises loosely compacted pisolites with variable amounts of BIF fragments and quartz vein material. The laterites are usually capped by 2 to 4 m of hardpan which is sub economic or barren. A section through the Sherwood deposit, from which 105 000 t at 2.3 g/t gold were milled, illustrates the thickness and grade variation within mineralised laterite over two BIF units (Fig 7). The laterites to the west and east of this section were uneconomic. All of the mineralised laterite mined to date at Tuckabianna is interpreted to be transported, but only over very short distances. No unequivocal residual ferruginous laterite has been recognised. The gold in these laterites is interpreted to be supergene and to have concentrated in the pisolitic horizons after transportation.
ORE GENESIS All primary gold mineralisation at Tuckabianna, with the exception of Rapier and Jasper Queen, is located within the Tuckabianna Shear Zone. Within this zone, the gold in the major BIF hosted deposits appears to be concentrated where the north- to NNW-trending faults intersect the main BIF units. The fracturing of the BIF units has provided channelways for the focussing of hydrothermal fluids which then preferentially precipitated gold in this iron-rich environment.
Fig 7 - Sherwood deposit, cross section on 15 104 m N, looking NE.
Geology of Australian and Papua New Guinean Mineral Deposits
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M E SMITH
At the deposits in the Comet and Comet North area the gold was concentrated in shears within, or adjacent and parallel to, iron-rich sediment. There is no obvious control by crosscutting structural features. The palaeochannel gold at Jasper Queen comprised alluvial and supergene types. The deposit is overlain by transported pisolitic gravel and is itself lateritised. The alluvial gold was closely related spatially to the primary gold lode at Jasper Queen and was probably derived from it by pre-Tertiary erosion. All of the lateritic gold occurrences are within transported pisolitic material and probably formed during a period of high ground water levels in the late Tertiary, after the main period of laterite development and subsequent erosion.
wishes to acknowledge the substantial work carried out by previous mine and exploration geologists from Australmin, Newmont and Newcrest, and the work of present Westgold geologists at Tuckabianna. P Dawson, R McLeod and D Kruger reviewed the manuscript and their comments are much appreciated.
REFERENCES Watkins, K P and Hickman, A H, 1990. Geological evolution and mineralisation of the Murchison Province, Western Australia, Geological Survey of Western Australia Bulletin 137. Watkins, K P, Tyler, I M and Hickman, A H, 1987. Cue, Western Australia (2nd edition) - 1:250 000 geological series, Geological Survey of Western Australia Explanatory Notes, SG 50–15.
ACKNOWLEDGEMENTS Special thanks are due to the management of Westgold for their support and permission to publish this paper. The author
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Geology of Australian and Papua New Guinean Mineral Deposits
Inwood, N A, 1998. New Holland, New Holland South and Genesis gold deposits, Lawlers, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 155–160 (The Australasian Institute of Mining and Metallurgy: Melbourne).
New Holland, New Holland South and Genesis gold deposits, Lawlers by N A Inwood
1
INTRODUCTION The deposits are in the Agnew-Lawlers district of the East Murchison mineral field, approximately 130 km NW of Leonora, WA, at lat 27o59′S, long 120o29′E or AMG coordinates 253 700 m E, 6 902000 m N (Fig 1) on the southern edge of the Sir Samuel (SG 51–13) 1:250 000 scale map sheet. All of the deposits are within mining lease M36/314, which is owned by Forsayth NL, a wholly owned subsidiary of Plutonic Resources Limited. From 1898 to 1996, 157 823 oz of gold were recovered from the Genesis, New Holland and nearby deposits from a reported ore production of 1 665 680 t at 3.09 g/t gold. Mineral Resources at 31 December 1996 were 668 000 t at 2.6 g/t gold for Genesis, 832 000 at 4.5 g/t for New Holland and 512 000 t at 9.2 g/t for the New Holland South underground deposit (Table 1).
open cut mining of the Emu and Great Eastern (Lawlers) orebodies by Western Mining Corporation Ltd and Forsayth NL respectively (G R Powell, unpublished data, 1991). Forsayth NL continued exploration throughout the Lawlers area and in 1987 began a stream sediment sampling program on the tenements near the Genesis deposit. Reconnaissance rotary air blast (RAB) drilling of subsurface laterite profiles led to the discovery in 1989 of a buried lateritic gold deposit, named the Waroonga Laterites (G R Powell, unpublished data, 1991). In 1990, while mining the Waroonga Laterites, Forsayth continued follow up reverse circulation (RC) drilling of previous RAB gold anomalies which led to the discovery of the Genesis deposit just to the west of the Waroonga Laterites (G R Powell, unpublished data, 1991).
EXPLORATION HISTORY
In 1991, exploration RAB and follow up RC drilling near the old New Holland workings delineated the New Holland orebody, which was subsequently mined by open cut methods from 1993 to 1994.
In 1894 gold was discovered in the Lawlers area, 11 km SW from the Genesis-New Holland deposits. Two significant deposits, Great Eastern (Lawlers) and Emu (formerly known as Waroonga), were mined until 1915. Mining of these and smaller deposits continued intermittently until the 1980s (G R Powell, unpublished data, 1992). From 1980 to 1985 gold exploration increased in the Lawlers area, culminating in the
The New Holland South orebody was discovered in 1995 by deep RC drilling designed to follow up a southerly dipping, highly mineralised trend within the New Holland mineralised zone. Further drilling during 1996 established that the New Holland South mineralisation extended over 300 m to the south of the planned base of the New Holland pit, and was a direct continuation of the New Holland orebody.
1.
Project Geologist, Lawlers Gold Mine, PMB 47, Leinster WA 6437.
Mineral resource estimates were based on RC holes drilled at 10 to 20 m spacing on east-west section lines, which were spaced at 20 m for New Holland, 40 m for New Holland South
TABLE 1 Historical production data and current resource estimates for the New Holland, Genesis and New Holland South deposits. HISTORICAL PRODUCTION Deposit
Ore treated
Genesis New Holland New Holland
491 903 4888 1 168 889
Total
1 665 680
(t)
Recovered gold grade (g/t)
Gold produced (oz)
3.03 15.55 2.86
47 919 2444 107 460
2.95
157 823
Production period 1992–1993 1898–1946 1993–1996
CURRENT RESOURCES Deposit Genesis New Holland New Holland South
Upper cut (g/t Au) 20.0 30.0 35.0
Resource type Measured and Indicated Measured and Indicated Indicated
Total
Resource (t)
Grade (g/t Au)
Contained gold (oz)
668 000 832 000 512 000
2.60 4.50 9.20
55 800 120 400 151 400
2 012 000
5.06
327 600
Source: Extract from Plutonic Resources Limited, Annual Report, 1996.
Geology of Australian and Papua New Guinean Mineral Deposits
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FIG 1 - Location and regional geological map of the Lawlers region (Plutonic Exploration, unpublished data, 1997).
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Geology of Australian and Papua New Guinean Mineral Deposits
NEW HOLLAND, NEW HOLLAND SOUTH AND GENESIS GOLD DEPOSITS, LAWLERS
and 15–20 m for Genesis. All RC holes were sampled at 1 m intervals and assayed for gold by fire assay. To December 1996 more than 28 exploration, resource definition, infill and sterilisation RAB and RC drilling programs totalling 51 000 m had been drilled in the New Holland-Genesis area.
REGIONAL GEOLOGY The Lawlers gold project area is in the northern sector of the Norseman–Wiluna greenstone belt of the Yilgarn Craton, WA. This sector is known as the Agnew–Wiluna greenstone belt. The belt comprises a sequence of mafic to ultramafic volcanic rocks and associated interflow sediments which have been folded to form the Lawlers Anticline (Fig 1) which plunges in a northerly direction at 30–40o. The core of the anticline was intruded by a tonalite-granodiorite complex, the Lawlers Tonalite, which was in turn intruded by late stage leucogranites. The mafic and ultramafic units of the Lawlers Anticline are overlain to the north and west by a 1500 m thick sequence of fine to coarse grained clastic sediment termed the Scotty Creek Formation. The dominant structure in the Lawlers area is the Waroonga shear zone, a zone of intense deformation 1–2 km wide, which separates supracrustal rocks in the east from the granitoid intrusives to the west. The Waroonga shear zone is part of the regional Mount Ida lineament, which is over 200 km long. A smaller shear, known as the Emu shear zone, separates the Scotty Creek Formation from the sediment and greenstone of the Lawlers Anticline to the east. The Genesis, New Holland, Glasgow Lass, Dobra Serica, Hidden Secret and Golden Swan deposits are all within medium to coarse grained units of the Scotty Creek Formation. The linear trend of gold mineralisation from the Genesis deposit in the north to the Golden Swan deposit in the south is informally known as the Glasgow Lass trend.
LOCAL GEOLOGY LITHOLOGY AND STRATIGRAPHY The Scotty Creek Formation trends north to NNW and consists of a predominantly ultramafic-derived basal conglomerate followed by a coarsening upward sequence of arkose and tonalite-clast conglomerate. The clastic material is thought to have been deposited in a terrestrial environment and derived from mafic and ultramafic rocks and tonalite, which suggests that the sequence is unconformable on the Lawlers greenstone sequence and tonalite (Platt, Allchurch and Rutland, 1978). Sedimentary structures within the Genesis and New Holland pits are generally well preserved with bedding dipping at 80 to 90o to the west. Bedding faces to the west. These sediments are in part overlain by Permian fill and fluvioglacial sediments which are partially overlain by Tertiary to Quaternary alluvial and colluvial sediments (P V Reynolds, unpublished data, 1994). Mineralisation in the Genesis, New Holland and New Holland South deposits is hosted by a series of elongate north to NW trending, steeply dipping, medium to coarse grained and moderately sorted metagreywacke units, referred to locally as ‘sandstones’ (Figs 2 and 3). They contain detrital quartz, feldspar and minor pebbles of granite, with varying levels of biotite and chlorite alteration. The coarser grained units are subparallel, from 10 to 50 m wide and are observed over strike
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 2 - Geological plan, New Holland and Genesis pits.
lengths of up to 1500 m. They are interbedded with finer grained sandstones and siltstones at their margins. In the New Holland–Genesis area the coarser grained sandstone units are separated by siltstone and fine grained sandstone from tens to hundreds of metres thick. The finer grained units range from well to poorly sorted, have a variable matrix content and range in composition from arenite to greywacke. Biotite alteration is characteristic, but in zones of veining or strong shearing chlorite alteration is more common. The fine grained sandstone and siltstone units are usually well sorted, strongly biotite altered and have a well developed foliation.
STRUCTURE The Genesis–Hidden Secret mineralised trend was formed in a low strain zone between the Waroonga shear to the west and the Emu shear to the east (M D Jones, unpublished data, 1992).
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Gold mineralisation also occurs along the near vertical contacts between the finer grained sediment and the medium to coarse grained sandstone units (D Riddington, unpublished data, 1993). Individual veins are up to 1 m thick and contain visible free gold grains to 3 mm in diameter. In longitudinal projection, deeper mineralised zones to the south of the Genesis pit plunge at ~15o to the south. Individual quartz veins within this zone, however, seem to have a flat to gentle ENE dip similar to the veins in the Genesis and New Holland pits.
MINERALISATION
FIG 3 - Cross section on 11 560 m N, New Holland pit, looking north.
Four phases of deformation are recognised near the New Holland and Genesis deposits (Table 2). Gold mineralisation is associated with tensional S3 cleavage structures created during reactivation of NW- to NNE-oriented D1 faults in areas of high competency contrast between the fine grained sandstone and siltstone and coarse to medium grained sandstone. The presence of bedding-parallel shearing in the finer grained units indicates they have taken up strain and behaved in a ductile manner. The coarser grained units, however, were relatively less deformed during D2 and more susceptible to brittle fracturing during D3 and have deformed by opening up tension fractures, producing ladder zones of auriferous quartz veins (Fig 3) which dip at around 30o to the ENE (M D Jones, unpublished data, 1992).
QUARTZ VEINING Gold mineralisation in the Genesis and New Holland orebodies is closely associated with quartz veining within the medium to coarse grained sandstone units (Fig 2). In the Genesis pit veining occurs primarily as ladder sets dipping at 32o towards 072o true for the main higher grade gold bearing veins, and dipping at 70 o towards 168o true for a lower grade vein set.
The high grade, shallow NE-dipping quartz veins at Genesis have been characterised by D’Ercole (1992) into quartz-calcite and feldspar-quartz vein sets. The quartz-calcite veins consist of quartz, calcite, and biotite±feldspar, arsenopyrite, apatite, prehnite and chlorite. Quartz forms over 60% by volume of the veins and the feldspars are microcline and albite. The feldsparquartz veins consist of microcline, quartz and albite, ±biotite, arsenopyrite, chlorite, muscovite and calcite. Feldspar forms over 50% by volume with less than 20% by volume quartz present. Accessory minerals in both vein types include rutile, sphene, apatite and scheelite. The steeply south dipping veins are characterised by a lack of arsenopyrite and lower gold grades. These veins are composed of potassium feldspar and chlorite, ±biotite, calcite, pyrite, pyrrhotite and chalcopyrite, with trace cubanite and pentlandite (D’Ercole, 1992). High grade quartz veins in the New Holland pit often contain up to 2–3% galena, which primarily occurs as vug and fracture fill with minor disseminated euhedral crystals. The quartz veins are typically weakly ferruginised along fractures and contact boundaries and have a weak foliation present as subparallel fractures 1 to 10 cm apart. In the fresh rock portion of the New Holland pit, individual shallow, NE-dipping quartz veins to 10 m wide by 40 m long can be recognised in the pit floor. These can form an intense zone of veining of dimensions to 40 by 160 m in the western sandstone unit, and to 20 by 80 m in the eastern sandstone unit (Fig 2). The zones of veining have a lensoid shape parallel to the eastern contact of the coarse- to medium-grained sediments. The steeper, southerly dipping veins sets tend to contain only low grade ore blocks up to 5 by 10 m, which parallel the strike of the veining.
TABLE 2 Geological and structural history of the Genesis-New Holland area, after D’Ercole (1992) and M D Jones, unpublished data (1992). GEOLOGICAL EVENT
STRUCTURES
Deposition of the greenstone sequence D1. East-west brittle ductile compression (greenschist facies)
AGE (Myr) ca 2700
North trending folds which plunge at 30–40o north with S 1 axial planar cleavage. 320o and 255o sinistral? faulting. Bedding (S0) trends at 320o and dips at 60 to 70 o NE
Intrusion of the Lawlers Tonalite
2652±20
Uplift and erosion of greenstones. Interpreted deposition of the Scotty Creek Formation >2567±14
Intrusion of leucogranite o
o
D2. Regional deformation, Waroonga shear and peak regional metamorphism (amphibolite facies)
S2 regional foliation, 70 –80 north plunging isoclinal folds, north trending faults and shears
D3. Fault reactivation and retrograde metamorphism with quartz veining and gold mineralisation
Dip slip reactivation of D1, NW and NNE? faults, 20o–30o S3 cleavage hosts veining and mineralisation
D4. Jointing or veining
Late stage jointing and quartz veining
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ca 2630
Geology of Australian and Papua New Guinean Mineral Deposits
NEW HOLLAND, NEW HOLLAND SOUTH AND GENESIS GOLD DEPOSITS, LAWLERS
An economic mineralised envelope to 60 by 200 m in plan view is centred around the zones of quartz veining in the western sandstone unit. This has a southerly dip and forms the major part of the mineralisation within the New Holland pit and the New Holland South orebody. In the New Holland pit, high grade (5 to 10 g/t gold) ore blocks within the mineralised envelope can have dimensions to 35 by 60 m, with low grade (1 to 5 g/t gold) ore blocks generally less than 15 by 30 m. Individual ore blocks within this area tend to strike NW, due to a combination of the SSE strike of the western sandstone unit in this area and the NW strike of the constituent, high grade quartz veins. The gold from the New Holland and Genesis orebodies is mostly free milling with particles to 3 mm in diameter, and is accompanied by arsenopyrite, galena and scheelite in the quartz veins. Up to 2% subhedral to euhedral disseminated arsenopyrite is present in the host rock near higher grade zones. Pyrite and sphalerite are present in small quantities (D Riddington, unpublished data, 1993).
ALTERATION Two alteration types characterised by the presence of chlorite and biotite were recognised by D’Ercole (1992). Chloritealtered units are characterised by their quartz, chlorite, muscovite, arsenopyrite, albite and minor microcline content. Chlorite and muscovite define a generally weak foliation. Arsenopyrite is the most abundant sulphide mineral (1–2% by volume) and two generations are recognised. The first consists of grains within the metamorphic fabric which often show pressure shadows, and the second consists of grains which overprint the fabric and contain abundant inclusions of calcite, quartz, albite, muscovite and opaque minerals. This style of alteration is commonly found in the coarse to medium grained sandstones, or where shearing is present. Biotite-altered units are characterised by quartz, albite, biotite, arsenopyrite and minor calcite. Quartz is present as detrital and metamorphic types and the biotite forms subidioblastic laths that define a weak to moderate foliation. The arsenopyrite content is variable, from trace to 5% by volume. Both the coarser sandstones and finer sediments exhibit this type of alteration. Alteration haloes associated with the two types of mineralisation in the New Holland and Genesis deposits are poorly developed. D’Ercole (1992) recognised two types of alteration haloes. The first consists of biotite and arsenopyrite±calcite and quartz. The second consists of localised muscovite and chlorite alteration, and is associated with strong shear textures and lower grades in the Genesis orebody. Muscovite and chlorite alteration in the New Holland orebody often occurs in high grade, sheared quartz vein sets.
PARAGENESIS D’Ercole (1992) proposed that quartz veining was an early stage event, and gold mineralisation was a later stage, infilling event. Two stages of arsenopyrite formation are recognised. The early stage comprises coarse subhedral to euhedral grains, and deformation of the grains was followed by fill of quartz and gold. The later stage formed late in the paragenetic sequence as subhedral grains crosscutting the wall rock fabric.
Geology of Australian and Papua New Guinean Mineral Deposits
Calcite formed in two stages, the first consisting of fine to coarse grained subhedral to anhedral grains, with the later stage fine grained, interstitial crystals associated with grain boundaries. Chlorite occurs as a late stage vein mineral and is associated with minor amounts of galena, rutile, sphene, sphalerite, bismuth, gold and pyrite (D’Ercole, 1992).
ORE GENESIS This section is based on the genetic model proposed by D'Ercole (1992) who proposed that mineralisation at the Genesis deposit took place at temperatures from 206 to 380oC at 1–2 kb and approximately 10 km depth under the influence of geochemical and structural controls. Mineralisation occurred after peak metamorphism and was synchronous with much of the gold mineralisation elsewhere in the Yilgarn Block (Groves et al, 1995). Mineralisation is related to reactivation of NW-trending D1 faults and the subsequent creation of S3 structures in the coarse grained units. High fluid pressures dilated the S3 cleavage structures, resulting in a drop in pressure, phase separation and subsequent precipitation of the quartz. The gold was present as bisulphide complexes and precipitated during phase separation as a result of increased pH and O2 fugacity. Three phases of fluids are envisaged during phase separation: 1.
aqueous low to moderate salinity with K+, Na+, Cl- and Mg2+;
2.
low salinity with CO2 and H2O; and
3.
non-saline, CO2 rich with minor CH4.
Isotopic studies indicates that the Genesis and New Holland ore fluids were derived from a mixture of surface waters and more saline, deeply derived fluids.
MINE GEOLOGICAL METHODS Recent mining of the New Holland and Genesis deposits has been by open cut methods. Grade control drilling in the Genesis pit was carried out using angled RC holes, sampled every metre, for the top 60 m (500–440 m RL) of mainly oxide material, then sampling 2.5 m composites from blast hole rigs for the next 10 m (440–430 m RL) in transitional to fresh rock. The method of grade control was changed from RC to blast hole sampling due to increasing hardness of the fresh host rock and mining constraints on the time allowed for grade control drilling. Due to economic and mining factors, grade control in most of the New Holland pit was by blast hole sampling. Interpretation of ore blocks was based upon flitch and sectional interpretation of RC and blast hole assays and was adjusted on the ground to suit the geology where necessary. The grade control blocks in both pits were marked out on the ground using tape and white lime and a spotter was present during mining.
ACKNOWLEDGEMENTS The author wishes to acknowledge the geological staff of Plutonic Resources Limited for their help in preparation of this paper. Special thanks to M Rowley, J Stott and J Webster for comments and editing, and J Cannon for help with the research.
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REFERENCES D’Ercole, C, 1992. Nature of Archaean lode-gold mineralisation in metasedimentary rocks of the Genesis Mine, Lawlers, Western Australia, BSc Honours thesis (unpublished), The University of Western Australia, Perth. Groves, D I, Ridley, J R, Bloem, E M, Gebre-Mariam, M, Hagemann, S G, Hronsky, J M A, Knight, J T, McNaughton, N J, Ojala, J, Vierlreicher, R M, McCuaig, T C and Nolyland, P W, 1995. Lodegold deposits of the Yilgarn block: products of Late Archaean crustal-scale overpressured hydrothermal systems, Early Precambrian Processes, 95:155–172. Platt, J P, Allchurch, P D and Rutland, R W R, 1978. Archaean tectonics in the Agnew Supracrustal belt, Precambrian Research, 7:3–30.
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Broome, J, Journeaux, T , Simpson, C, Dodunski, N, Hosken, J, De Vitry, C and Pilapil, L, 1998. Agnew gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 161–166 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Agnew gold deposits 1
2
3
4
4
6
by J Broome , T Journeaux , C Simpson , N Dodunski , J Hosken , C De Vitry 5 and L Pilapil . INTRODUCTION The deposits are 128 km NW of Leonora, WA, 23 km west of Leinster and up to 9 km from the old Agnew township, at lat 28o01′S, long 120o30′E, on the Leonora (SH 51–1) 1:250 000 scale map sheet. Redeemer is the largest deposit and is at AMG coordinates 252 500 m E, 6 893 000 m N, 6 km SW of Agnew township (Fig 1).
In June 1996 the Agnew Gold Operation of WMC Resources Ltd (WMC) reported total Proved and Probable Ore Reserves of 7.1 Mt at 4.5 g/t gold for 32 000 kg of contained gold, and total additional Indicated and Inferred Resources of 7.6 Mt at 4.04 g/t gold for 31 000 kg of contained gold (WMC Limited, 1996). These reserves and resources are within the Redeemer, Emu, Cox-Crusader, Claudius Creek, Deliverer, Pilgrim, Zone 2 and Zone 3 deposits. As of June 1996, a total of 9.7 Mt had been mined from the Redeemer, EMU, Cox-Crusader and Deliverer deposits, at an average recovered grade of 3.46 g/t gold to yield 33 600 kg of gold after ore treatment. The Agnew deposits share similar structural settings within several host rock types. To facilitate description, these are divided into northern or ‘Redeemer’ and southern or ‘Cox’ area lodes.
EXPLORATION AND PRODUCTION HISTORY EMU The Emu deposit is the most northerly of those described below. It was discovered in 1897, and was mined until 1911 by Waroonga Gold Mines (Aoukar and Whelan, 1990). In 1935 it was acquired by East Murchison United Gold Mines (EMU) and a treatment plant was built. Over the following decade nine underground levels were developed until persistent flooding in 1948 led to mine closure. All associated mining leases were subsequently acquired by Messrs Trundle and Cock. In 1976, WMC entered into an option-purchase agreement for the Emu leases. Between 1986 and 1992 WMC mined a total of 2.7 Mt averaging 2.36 g/t gold from near surface, open pittable lodes and exploratory underground development began in 1989. Production of underground ore totalled 72 086 t at 2.78 g/t gold. The Emu mine was closed in 1992 and allowed to flood. This deposit was described by Aoukar and Whelan (1990) and will not be discussed further. FIG 1 - Location map and geological setting of the Agnew gold deposits. 1.
Mine Geologist in Charge, Agnew Gold Operation, WMC Resources Ltd, PMB 10, Leinster WA 6437.
2.
Exploration Geologist, Agnew Gold Operation, WMC Resources Ltd, PMB 10, Leinster WA 6437.
3.
Project Geologist, Agnew Gold Operation, WMC Resources Ltd, PMB 10, Leinster WA 6437.
4.
Mine Geologist, Agnew Gold Operation, WMC Resources Ltd, PMB 10, Leinster WA 6437.
5.
Chief Geologist, Agnew Gold Operation, WMC Resources Ltd, PMB 10, Leinster WA 6437.
6.
Mine Geologist, Rocky's Reward, Leinster Nickel Operation, WMC Resources Ltd, PO Box 22 Leinster WA 6437.
Geology of Australian and Papua New Guinean Mineral Deposits
REDEEMER, ZONE 2 AND ZONE 3 WMC Ltd entered into a joint venture with Nord Australex Pty Ltd in 1985, to explore for Emu style mineralisation south of the Emu mine. A 4.1 ppm gold in soil anomaly found in April 1985 led to the discovery of the Redeemer orebody. Between 1987 and 1989 the Redeemer open pit yielded a total of 1.6 Mt of ore, averaging 3.51 g/t gold. A northern pit extension was completed in 1990 for an additional 0.5 Mt at 3.59 g/t gold. Underground development began at Redeemer in late 1989, to access the West and North Pipe lodes. Underground mining continues today with the decline currently at 500 m below surface, at the 9960 m RL (40 m below sea level). To June 1996, sublevel caving techniques have been used to extract a total of 4.3 Mt at 3.90 g/t.
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J BROOME et al
During 1985–1987 subsidiary deposits (Zones 2 and 3) were identified during exploratory drilling at Redeemer. Underground mining development of the Zone 2 and Zone 3 orebodies is limited to a few headings and where they are intersected by the Redeemer decline.
COX–CRUSADER The Cox orebody, due south of the Deliverer deposit, was discovered in 1987 by Asarco Australia Ltd during exploratory rotary air blast drilling. In the same year Asarco and Forsayth Mining Services commenced the Cox open pit, producing 177 047 t at an average grade of 5.57 g/t gold. Subsequent exploration by WMC led to an intersection of the Cox shoot within WMC leases, 700 m north of the Asarco holdings. WMC purchased the Cox leases from Asarco in 1992, and mining at the extended Cox open pit was completed in 1994. A decline was then commenced to access deeper underground reserves which now form the Crusader orebody. The Crusader decline is currently 250 m below surface with production to June 1996 totalling 640 000 t at 5.9 g/t.
DELIVERER AND PILGRIM
FIG 2 - The stratigraphy and lateral correlation of the Agnew gold deposits.
Anomalous values in a soil geochemical survey led to the discovery of the Deliverer orebody in April 1985. An open pit was developed to 50 m in 1990, which produced 126 000 t at 4.15 g/t gold. Exploratory drilling at Deliverer led to the discovery of the nearby Pilgrim orebody which is being upgraded to an open pittable reserve (1996) using data from further drill holes. Current mine designs incorporate both orebodies into a single, large open pit.
CLAUDIUS CREEK The deposit was identified in 1990 as a result of drilling in the Cox area by WMC. Further exploratory drilling has resulted in the definition of an open pit Probable Reserve with an underground Indicated Resource.
REGIONAL GEOLOGY STRATIGRAPHY The Agnew gold deposits are within the NW sector of the Norseman-Wiluna greenstone belt in the Eastern Goldfields Province (Fig 1). The succession in the Agnew mining area comprises basal metamorphosed basalt, gabbro, dolerite and ultramafic flows of the informally named Lawlers Greenstone formation (Fig 2). These are unconformably overlain and/or fault bounded by conglomerate, arenaceous metasediment and greywacke of the Scotty Creek Formation (Aoukar and Whelan, 1990; R P A Perriam, unpublished data, 1996).
STRUCTURE AND METAMORPHISM The Agnew mining leases are on the western limb of a large open fold, the Lawlers Anticline, which plunges 30o to the north (R P A Perriam, unpublished data, 1996). It is crosscut by the prominent NNE-trending Emu shear zone, which separates metavolcanic from metasedimentary formations (Aoukar and Whelan, 1990). Non-coaxial deformation in the Pilgrim shear is also observed in ultramafic rocks within the southern lease areas. To the west, Scotty Creek metasediment is in fault
162
FIG 3 - Cross section of the Redeemer area looking north showing the geology and relative position of the Redeemer, Zone 2 and Zone 3 orebodies.
contact (Waroonga shear) with a flanking granite-gneiss terrain (Aoukar and Whelan, 1990). The greenstones of the Agnew area have upper greenschist–lower amphibolite metamorphic grades.
Geology of Australian and Papua New Guinean Mineral Deposits
AGNEW GOLD DEPOSITS
REDEEMER AREA DEPOSITS REDEEMER MAIN LODE Geology The stratigraphic sequence in the mine area is dominated by sediment of the Scotty Creek Formation. Basal ultramafic conglomerate is overlain by intermediate mafic conglomerate and upper sandstone sequences (Figs 2 and 3). The ultramafic conglomerate has a variable thickness, to a maximum of 30 m. The matrix consists of well foliated chlorite-talc-actinolite assemblages, and the clasts are highly deformed ultramafic, mafic and rare felsic rocks. The mafic paraconglomerate is 50 to 60 m wide and has three distinct facies: 1.
a basal conglomerate with a mafic matrix and ultramafic, mafic and minor felsic clasts;
2.
a clast-poor central conglomerate; and
3.
an upper, granitoid clast-rich conglomerate.
Clasts within the mafic conglomerate range in size from pebbles to boulders and occur as intensely deformed ultramafic clasts to 2 m long, moderately flattened mafic clasts to 50 cm in diameter, and elongate well rounded, pebble to boulder size granite clasts. Coarse clast dimensions reflect a local talus provenance, linked to uplift and erosion of underlying ultramafic and mafic rocks. R P A Perriam (unpublished data, 1996) suggests that the felsic clasts were derived from the Lawlers Tonalite, which forms the core of the Agnew anticline. The overlying sandstone is composed of arenite and greywacke which form a series of upward fining sequences (J L Dugdale, unpublished data, 1992).
Structure The Main lode is hosted by a structurally complex, open flexure within overturned mafic conglomerate. It has a 150 m strike length, an average width of 50 m and plunges steeply northward at 60 to 70o. To date the deposit has been traced for 1000 m down dip. Mineralisation is concentrated in lenses proximal to the hanging wall (ultramafic conglomerate) and footwall (sandstone) contacts. These may be joined by cross lodes, developed along anastomosing shear zones of strike 150 to 160o which transect the mafic conglomerate and dip steeply eastward. The Main lode is marked by a pervasive, steep northerly trending foliation within the mafic conglomerate, which lies parallel to the fold axis of the Lawlers Anticline and subparallel to bedding. Subsequent compression has led to the development of a subordinate, NW-trending foliation. The intersection of these secondary and tertiary structural elements defines a lineation which plunges conformably with the Main lode.
Mineralisation Gold mineralisation within the Redeemer lodes is marked by variably intense biotite-amphibole enrichment and minor secondary sulphide, silica and/or carbonate minerals. Secondary magnetite is observed within the core of the mineralised lodes, proximal to the hanging wall. Native gold often occurs as <100 µm diameter grains interstitial to coarse
Geology of Australian and Papua New Guinean Mineral Deposits
actinolite and biotite or as fine grains within them. It may also crystallise in spatial association with tellurobismuthite (Bi2Te3), joseite [Bi3Te(Se,S)], arsenopyrite, chalcopyrite, molybdenite and scheelite (J L Dugdale, unpublished data, 1992). High grade zones adjacent to the deposit hanging wall often have visible gold developed along shear surfaces and biotite cleavage planes. Gray (1995) has proposed a threefold hydrothermal zoning model for Redeemer, namely: 1.
a mineralised, biotite-amphibole core;
2.
an enveloping partially mineralised, amphibole-biotitechlorite zone; and
3.
an outer, weakly mineralised, amphibole-chlorite zone.
Geochemistry Characteristic major and trace element geochemical signatures are preserved in a variety of rock types at Redeemer, eg ultramafic rock - chromium, nickel, magnesium; mafic rock iron, titanium; felsic rock - zirconium, aluminium. Gold correlates positively (Fig 4) with potassium among major elements and with tellurium, bismuth, selenium±tungsten, molybdenum and copper among the trace elements (De VitrySmith, 1994). Although auriferous arsenopyrite lodes are observed locally at Redeemer, arsenic and gold are not uniformly enriched across the entire mine. Similarly, surface arsenic anomalies may not correspond uniquely with elevated gold values.
Ore genesis Whereas the structural and metamorphic setting of the Redeemer mine is consistent with classic mesozonal gold deposits (Gebre-Mariam, Hagemann and Groves, 1995), the ore mineralogy suggests an atypical depositional history. De Vitry-Smith (1994) suggests that unique fluid–wall rock interaction led to increases in fluid ƒO2 and pH. Destabilisation of aqueous gold-bisulphide complexes followed with the precipitation of native gold and protracted sulphur solubilisation. Similarly, wall rock composition most likely suppressed formation of quartz-carbonate precipitation in favour of amphibole (eg actinolite)±biotite and carbon dioxide generation. The conditions of ore deposition at Redeemer have been constrained in part using amphibole-plagioclase geothermometry (Spear, 1980). Results indicate that in shears and dilational zones, where fluid flow peaked, gold precipitated at or near 520oC. This is consistent with the presence of complex sulphide minerals such as joseite A (Bi4TeS2) which is only stable above 475oC (Verryn, Merkle and Gruenewaldt, 1991).
ZONE 2 AND ZONE 3 Geology The Zone 2 and Zone 3 orebodies are hosted by two felsic conglomerate end members of the Scotty Creek Formation (Figs 2 and 3) within 100 m of the Redeemer Main lode. Both conglomerate units are stratabound by adjacent arenite and greywacke which comprise a series of upward fining sequences (J L Dugdale, unpublished data, 1992). The Zone 2 conglomerate averages 5 m in width and consists of granule to cobble sized tonalite clasts within a quartz-feldspar matrix,
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with lesser interstitial biotite-chlorite-actinolite alteration (B Pidgeon, unpublished data, 1996). It is laterally discontinuous and grades into medium to coarse grained sandstone. The Zone 3 conglomerate averages 12 m in width and has both granite and tonalite clasts in a similar matrix (B Pidgeon, unpublished data, 1996).
Structure The Zone 2 and Zone 3 ore shoots are related to flexures analogous to those within the Redeemer Main lode. These structures also plunge northward at 60 to 70o, but may flatten progressively with depth (B Pidgeon, unpublished data, 1996). The mineralisation is hosted within semi-brittle deformation zones, formed by dilational shearing (J L Dugdale, unpublished data, 1992; B Pidgeon, unpublished data, 1996).
Mineralisation In contrast to the adjacent Redeemer Main lode, gold is spatially associated with arsenopyrite-bearing quartz veins (J L Dugdale, unpublished data, 1992). Mineralised horizons often display a marked increase in hydrothermal alteration. Analysis of quartz vein orientations in Zone 2 identifies a preferred mineralisation trend, bearing 150 to 170o with a steep easterly dip. The Zone 3 mineralisation is comparatively more irregular than Zone 2, limiting its overall prospectivity.
COX AREA DEPOSITS GEOLOGY The rock sequence in the Cox area (Figs 1 and 2) strikes north, dips at 50 to 60o and youngs westward (Fig 5). It comprises a basal tholeiitic pillow basalt, commonly metamorphosed to massive amphibolite, and marine sediment. Accessory phases in the basalt include interstitial quartz, chlorite along sheared surfaces and rare euhedral almandine. Secondary alteration assemblages contain amphibole (actinolite-hornblende)chlorite-epidote-magnetite with minor pyrrhotite-pyritechalcopyrite. Unmineralised carbonate vein and lesser quartz
FIG 4 - Selected element and oxide analyses through the Redeemer Main lode from sandstone to mafic conglomerate to ultramafic conglomerate (De Vitry-Smith, 1994).
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FIG 5 - Cross section of the Cox area looking north, illustrating the geology and relative positions of the Cox–Crusader, Deliverer and Pilgrim orebodies.
Geology of Australian and Papua New Guinean Mineral Deposits
AGNEW GOLD DEPOSITS
vein sets are also present. The basalts contain interflow marine sediment, sulphidic (pyrrhotitic) carbonaceous shale and stratiform (ponded) dolerite (R P A Perriam, unpublished data, 1996). The marine sediments exhibit strong foliation, pervasive biotite veining and amphibole-biotitechlorite±garnet alteration.
extend up and down dip along the ultramafic rock–basalt contact. Gold may also occur in tension fractures perpendicular to local shears, on shallow northerly plunging mullion surfaces and occasionally along the ultramafic rock–basalt contact.
The basalts are overlain by a 40 to 60 m thick ultramafic sequence, comprising serpentinised and/or carbonatised dunites and orthocumulate peridotites (R P A Perriam, unpublished data, 1996). Typical alteration assemblages include magnesite±antigorite-talc-dolomite-ankeritemagnetite±chlorite as matrix or vein constituents. The overlying ultramafic komatiite flows are 30 to 110 m thick. These lavas display cumulate bases with spinifex-textured flow tops and thin, brecciated, glassy chill margins. Alteration assemblages include amphibole-talc-chlorite-magnetitecarbonate±antigorite.
The orebody contains native gold as fine to very fine grained disseminations. Associated alteration comprises epidotemagnetite-chlorite±actinolite-biotite, indicative of a high temperature (>400oC) and a high ƒO2 depositional environment. The presence of actinolite-quartz-potassium feldspar-biotite-garnet assemblages within the mafic volcanic host rock is consistent with amphibolite facies grade metamorphism (J L Dugdale, unpublished data, 1993). Multielement geochemical studies have outlined a gold-bismuthtellurium-selenium-molybdenum-tungsten association.
The 10 to 14 m thick Pilgrim shear zone marks the contact between the cumulate ultramafic and overlying komatiitic flows. Within the lower and middle sections the shear is marked by the development of a wedge of intense talcmagnesite-carbonate-biotite alteration. Upper sections are more intensely deformed and may contain mylonitisation with coincident chlorite-amphibole-talc-biotite alteration. The penultimate layer in the Cox area comprises an 80 m thick volcano-sedimentary pile dominated by high magnesium basalt, interflow marine sediment and shale. It is variably crosscut by felsic intrusions and gabbro, and also exhibits an altered basal ultramafic assemblage (Figs 2 and 5). The Cox area sequence is capped by a 110 m thick sheared mafic conglomerate, representing the southern extension of the Redeemer Main lode host rock.
STRUCTURE AND MINERALISATION Gold mineralisation in the Cox area occurs predominantly within the basal tholeiitic basalt at or close to its contact with the cumulate ultramafic unit. It is commonly associated with interflow sediment and dolerite. Compressional tectonics led to the formation of prominent flexures, with rheological contrasts between rock types inducing tension and fracturing providing dilational sites for fluid ingress. These flexures subsequently formed the locus of mineralisation, with gold grade and lode dimensions peaking at the point of maximum curvature (Fig 5). Some of the Cox area deposits (ie Crusader, Deliverer, Claudius) may contain up to five discrete lodes.
Cox–Crusader The Cox–Crusader orebody is 3 km south of the Redeemer mine on the western limb of the Lawlers Anticline.
Structure The orebody is a 20 to 30o plunging, north-trending shoot, associated with a flexure in the ultramafic rock–basalt contact (Fig 2). Its long axis parallels the axial plane of the Lawlers Anticline and may have been generated during the same compressional event. Cross sectional width varies between 2 and 30 m, with down dip extensions to 50 m. Mineralisation is contained largely within three parallel lodes associated with interflow sediment and structures generated by the controlling flexures. The lodes contain high grade pods within the flexure core which are linked to thin and lower grade limbs which
Geology of Australian and Papua New Guinean Mineral Deposits
Mineralisation
Deliverer and Pilgrim The Deliverer and Pilgrim orebodies are 300 m north of the Cox open pit (Fig 1). The Deliverer orebody is hosted by the basal tholeiitic basalt (Fig 2). It contains three high grade shoots at or near the ultramafic rock–basalt contact. The shoots plunge 45o to the NW and are related to a prominent flexure in the westerly dipping contact. Individual lodes vary from lenticular to elliptical in cross section with 5 to 12 m true widths and have been traced for over 70 m down dip. To date the Deliverer orebody has been intersected for 300 m down plunge. Mineralisation is associated with chlorite-epidote-magnetite alteration and veining. Interpreted ore genesis is consistent with the larger Cox–Crusader deposit. The Pilgrim orebody is in the ultramafic rock hanging wall of the Deliverer orebody. It is hosted by the heavily chloritised Pilgrim shear, at the boundary between basal cumulate and upper ultramafic flow zones. The mineralisation varies from 5 to 7 m true width and dips at 60o to the west. The highest grades are coincident with flexures in the contact between mafic and ultramafic rocks and mineralisation is associated with chlorite±biotite.
Claudius Creek The orebody is 700 m north of the Cox open pit (Fig 1). The lode is hosted within the basal tholeiitic basalt (Fig 2) and parallels a flexure in the basalt–ultramafic rock contact, which plunges at 30o to the NNW. The flexure is prominent up plunge and marked by the development of an 18 m thick lode, which formed where rheological contrast peaked between the basalt and ultramafic units. Down plunge, the flexure becomes less pronounced with the intersection of five parallel lodes, 3 to 8 m thick, which have a uniform lenticular cross section. Mineralisation consists of very finely disseminated native gold and is associated with amphibole-chlorite-biotite alteration.
SUMMARY 1.
The Agnew gold deposits are restricted to volcanic and sedimentary rocks within the informally named Lawlers Greenstone and the Scotty Creek formations, along the western limb of the Lawlers Anticline.
2.
Gold mineralisation is controlled by and contained within prominent structurally prepared zones, contact flexures and allied shears. Rheological contrast played a frequent and critical role in their formation.
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3.
Mineralisation post-dates formation of the Lawlers Anticline and peak metamorphism.
4.
Mineralisation assemblages are either quartz-sulphide depleted as at the Redeemer Main, Cox–Crusader, Claudius Creek, Deliverer and Pilgrim lodes or quartzsulphide enriched as at the Zone 2, Zone 3 and Emu lodes (Aoukar and Whelan, 1990).
ACKNOWLEDGEMENTS This paper is published with the permission and encouragement of WMC Limited. The authors gratefully acknowledge the cartographic support provided by J Bailey and M Dwyer.
REFERENCES Aoukar, N and Whelan, P, 1990. Emu gold deposit, Agnew, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 323–329 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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De Vitry-Smith, C, 1994. Genesis of the high temperature, sulphur depleted Redeemer-Main gold deposit, Agnew-Lawlers Region, Western Australia, BSc Honours thesis (unpublished), The University of Western Australia, Perth. Gebre-Mariam, M, Hagemann, S G and Groves D I, 1995. A classification scheme for epigenetic Archaean lode-gold deposits, Mineralium Deposita, 30:408–410. Gray, A, 1995. Characterisation of alteration at the Redeemer gold mine, Agnew-Wiluna Greenstone Belt, BSc Honours thesis (unpublished), The University of Western Australia, Perth. Spear, F S, 1980. NaSi → CaAl exchange equilibria between plagioclase and amphibole, an empirical model, Contributions to Mineralogy and Petrology, 72:33–41. Verryn, M C, Merkle, K W and Gruenewaldt, G V, 1991. Gold and associated minerals of the Waaikraal deposit, northeast of Brits, Bushveld Complex, European Journal of Mineralogy, 3:451–466. WMC Limited, 1996. Annual Report to Shareholders (WMC Limited: Melbourne).
Geology of Australian and Papua New Guinean Mineral Deposits
Hayden, P and Steemson, G, 1998. Deflector gold-copper deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 167–172 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Deflector gold-copper deposit 1
by P Hayden and G Steemson
2
INTRODUCTION The deposit is in the southern Murchison district of WA, 70 km NNE of Morawa and 50 km SW of Yalgoo (Fig 1). It is at lat 28o40′S, long 116o22′E or AMG coordinates 439 000 m E, 6 828 300 m N, on the Yalgoo (SH 50–2) 1:250 000 scale and the Mellenbye (2140) 1:100 000 scale map sheets. The property is held by Gullewa Gold NL.
1.
literature search of previous exploration (1990);
2.
compilation and interpretation of data from aeromagnetic surveys by previous explorers in 1991, from which Deflector was targeted as a dilational jog in a NEtrending fault (G Hewlett, personal communication, 1997);
3.
reconnaissance aircore drilling across the target area during 1991 which encountered anomalous gold and copper values in bedrock at Deflector;
4.
infill rotary air blast drilling which defined a bedrock gold-copper geochemical anomaly at Deflector within a 2 by 1 km anomalous area encompassing Deflector and the nearby Titian, Tintoretto and Bellini prospects; and
5.
extensive reverse circulation (RC) and minor diamond drilling which delineated the Deflector lodes, leading to the first resource estimation in 1993.
Evaluation of the deposit to December 1996 has involved 402 RC holes and 15 diamond drill holes, totalling 23 000 m. Minor ground geophysical and geochemical surveys have been carried out, but have not contributed significantly to evaluation of the deposit.
REGIONAL GEOLOGY
FIG 1 - Location map and interpreted bedrock geology of the Gullewa greenstone belt.
The Indicated plus Inferred Mineral Resource at Deflector is 665 000 t at 4.6 g/t gold and 1.8 % copper. This includes 460 000 t at 4.6 g/t gold and 2.3% copper in West lode. The deposit is open along strike and at depth, and further drilling is proposed prior to a production decision.
EXPLORATION HISTORY The Deflector lodes are buried beneath shallow alluvium on a flat, mulga-covered plain. Conventional prospecting was thus ineffective, and there are no old workings at the site. The deposit was found by Sons of Gwalia Ltd during extensive exploration of the Gullewa greenstone belt between 1990 and 1993. The stages leading to the discovery were:
1.
Formerly Senior Development Geologist, Gullewa Gold NL, now Consulting Geologist, 33 Torwood Drive, Gooseberry Hill WA 6076.
2.
Chief Geologist, Gullewa Gold NL, PO Box 398, Cloverdale WA 6985.
Geology of Australian and Papua New Guinean Mineral Deposits
The Deflector lodes are in the Gullewa greenstone belt, within the Murchison Province of the Archaean Yilgarn Craton. The main elements of the regional geology are shown on Fig 1 which was compiled from a variety of company and public sources. The stratigraphic sequence at Gullewa comprises a lower group of mafic and ultramafic greenstone with minor local banded iron formation, overlain by intermediate and felsic volcanic rocks and then an upper association of clastic sediment including shale, sandstone and conglomerate. The upper sediment sequence may have been deposited in a younger, second order unconformable basin. In the main lobe of the belt, the strata are folded into a major, east-trending synform with the clastic sediment in its core. The mafic sequence in the northern limb of the synform has a complex deformation history involving the development of subsidiary folds, shear zones and the emplacement of various intrusive bodies. A major north-trending fault, the Salt River lineament, passes through the eastern part of the belt. Secondary, NE-trending shears splay off the west side of this structure and are associated with much of the gold mineralisation. The greenstone belt is surrounded by granitic rocks, and there are minor granite and felsic porphyry intrusions within the belt. Several gold prospects are closely related to these porphyries, and aeromagnetic and drilling data suggest that blind intrusions occur at shallow depth below or close to other
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prospects. A stock of mildly alkaline trachyandesite called the Gearless Well intrusion by Johnson, Cooper and Blight (1989) occurs immediately SE of Deflector, and intermediate biotite porphyry dykes coeval with this body are widespread. The greenstone and granite are cut by Proterozoic dolerite dykes with a variety of trends. The east trending dykes are part of the normal pattern of the Yilgarn Craton. Dykes trending NE are characteristic of the Murchison Province and some of them follow northeasterly trending shears which control the distribution of gold deposits. The north dyke trend is characteristic of the western margin of the Yilgarn Craton, and Deflector is only 70 km east of the Darling Fault Zone which marks this margin. Most of the greenstone belt is covered by remnants of the Tertiary laterite blanket, or by broad swathes of Quaternary alluvium and colluvium. The bedrock geology of these areas is interpreted from aeromagnetic survey and drilling data.
ORE DEPOSIT FEATURES LOCAL GEOLOGY Cover The Deflector area is within a wide drainage system of sheetwash plus braided channel deposits which transports alluvium from the NW, through Deflector, towards the Salt River. The system is 12 km long, and is 5 km wide at Deflector. Near Deflector the alluvium is a medium to coarse grained, immature, loamy sand consisting largely of angular granitic quartz and feldspar grains, minor ironstone granules and silt. It is reddish brown in colour, and below about 50 cm depth is cemented into a horizontally stratified hardpan. Ironstone tends to be more abundant towards the base of the alluvium, and fragments of fresh basalt occur within a metre of the bedrock contact. On the western side of Deflector, over West lode, the alluvium is generally 3 to 5 m thick. It progressively thickens to the east, averaging about 6 m over Central lode, and reaches about 10 m over Contact lode on the eastern side of the deposit. In general, Deflector sits on a palaeotopographic rise, with the alluvial cover becoming thicker away from the deposit in all directions and typically 20–30 m thick a few hundred metres from the deposit. The alluvium directly overlies bedrock, and lateritic ferricrete which may have originally covered the bedrock has been stripped by erosion. The underlying saprolite has also been stripped to varying degrees.
Rock types Bedrock geology is shown on Fig 2. The NW side of Deflector is underlain by a large area of metabasalt which has a stratigraphic thickness of at least 300 m. This sequence strikes 045o magnetic and dips steeply to the SE. On the SE side of the deposit the metabasalt is in contact with a sedimentary unit dominated by siltstone. The metabasalt is generally fine grained, black, extremely hard and lacks cleavage. Limited petrography and major element analyses suggest it is high magnesium basalt. About half the basalt units have a needle-like texture reminiscent of spinifex texture in komatiites, and the others have poorly developed pillows. One unit of ‘spinifex’ basalt has been delineated immediately west of the basalt-siltstone contact;
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FIG 2 - Bedrock geological plan at the base of alluvium, compiled from drilling data, Deflector deposit.
with more work the entire Deflector basalt pile could be subdivided into spinifex and pillowed units. The sedimentary sequence is heterogeneous and rock identification is difficult. The major rock type is siltstone, mostly black and graphitic, but locally pale brown or grey. It has no cleavage or fissile bedding and has probably been hornfelsed. The sequence is cut by pale felsic (quartz-albite) porphyries and dark grey to black, intermediate biotite porphyries. The latter are lamprophyric (minette), and are probably offshoots of the Gearless Well trachyandesite which lies about 300 m SE of the basalt-sediment contact. Biotite (phlogopite?)-rich rocks occur locally within the sedimentary pile and are thought to be the product of potassic metasomatism associated with the biotite porphyries. Porphyry dykes are most abundant east of the contact, but thin porphyries occur throughout the entire Deflector area. Drilling indicates a dome or pipe-like intrusion of black biotite
Geology of Australian and Papua New Guinean Mineral Deposits
DEFLECTOR GOLD-COPPER DEPOSIT
porphyry centred at 19 380 N, 9830 E. Its roof is about 10 m below the surface. West lode appears to cut straight through this intrusion.
Weathering On the western side of the Deflector deposit, near West lode, the alluvium directly overlies fresh or weakly weathered bedrock which consists of hard, black metabasalt. There is minor development of saprolite, 10 to 15 m deep, near Central lode and the thickness of weathered bedrock steadily increases to the SE, reaching 25 m at Contact lode. Superimposed on this general trend is a very marked tendency for deeper weathering down the mineralised lodes (Fig 3). The lack of saprolite above West lode greatly diminishes the chance of lateral secondary dispersion of gold from the lode into the adjacent bedrock. This explains why West lode is only weakly expressed in the Deflector bedrock geochemical anomaly defined by the early shallow reconnaissance drilling.
In the immediate vicinity of Deflector the greenstones are sandwiched between two plutons - a granitic intrusion 500 m NW of the deposit and the Gearless Well intrusion 300 m to the SE (Fig 1). The host rocks are very massive. Cleavage and lineation are rare, and even within the lodes are weakly developed or absent. This may be partly due to hornfelsing by one or both of the nearby intrusions, but the preservation of delicate igneous spinifex textures in basalt suggests that these rocks never suffered pervasive deformation. An important structural element is the contact between the metabasalt and the metasediment, which dips eastwards at 70o (Fig 3). It shows a sharp sinistral deflection between 19 340 N and 19 400 N, apparently due to a fault which strikes north and dips eastward. The true displacement may be east block north or east block down. The horizontal offset of Contact lode is 70 m, and the fault may also affect West lode at 19 480 N. The fault plane is mineralised in places.
Metamorphism
Structure Deflector is within greenstone strata which strike at 045o and dip steeply to the SE. This sequence probably also youngs to the SE, with the metasediment overlying the metabasalt. The dominant fault in the Gullewa belt is marked by the Salt River lineament. Disruption of stratigraphic trends (Fig 1) clearly demonstrates that this is a major north-trending structure, at least 80 km long, which bisects the belt. Aeromagnetic images clearly show a secondary lineament which branches off the Salt River structure and trends to the SW passing through the Deflector corridor. A perceived bend in this lineament, interpreted as a dilational jog, led to the initial targeting of the Deflector area. Other prospects in the Gullewa belt are also associated with NE-trending shears.
Metamorphism appears to have been of low grade and static style. The mafic mineral assemblage is tremolite-actinolite, chlorite and epidote which suggests greenschist facies. The lack of cleavage, preservation of delicate igneous textures and predominance of brittle fracture rather than shearing are consistent with low metamorphic grade. The siltstones east of the Deflector contact appear to have been hornfelsed, and the hardening of the metabasalt is probably also due to contact metamorphism.
MINERALISATION Lode nomenclature The gold-copper mineralisation at Deflector falls naturally into three zones: West, Central and Contact (Figs 2 and 3). West
FIG 3 - Cross-section at grid line 19 300 N through the Deflector lodes, looking NE, with typical intersections.
Geology of Australian and Papua New Guinean Mineral Deposits
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lode is the major lode and the only one with potentially economic copper grades. It is essentially a single tabular lode with one major branch (West branch). Central lode contains two en echelon lodes (Central and South Central). Contact lode lies on the basalt-siltstone contact, with smaller parallel lodes in the footwall and hanging wall of the contact. There are also two small bodies of flat lying supergene gold mineralisation associated with Contact lode.
Lode composition Typical unweathered intersections comprise altered, quartzveined metabasalt with minor to moderate amounts of sulphides. Intersections of massive quartz reef and massive sulphide have also been drilled, but these are minor, local features.
Overall geometry The Deflector lodes are all tabular sheets of mineralisation. They are slightly sinuous, with a moderate variation in thickness and they all strike NE, parallel to the enclosing strata. The host rocks are thought to dip steeply to the SE. West lode dips vertically to steeply NW, and is thus transgressive. Central lode is near vertical, and the Contact lodes dip east at about 70o, parallel to the basalt-siltstone contact. The lodes are regularly spaced with 60 to 80 m between West and Central, and Central and Contact. Mineralisation has been traced to date over a strike length of 500 m in all three structures, although drilling of Central and Contact is very sparse over some of this length. Fractures and veinlets occur in the country rock around the lodes. Oriented drill core shows one steep set of fractures and veinlets parallel to the lodes and a second set with flat to moderate westerly dips. There is no systematic difference in orientation between fractures and veinlets, or different types of veinlet.
Internal structure In contrast to most gold deposits of the Yilgarn Craton, the Deflector lodes do not appear to occupy shear zones and the host rocks lack cleavage. The lodes are not generally bounded by shear planes, and shearing within the lodes varies from weak to absent. Lineation is rare. The lode material is fractured rock with quartz-sulphide vein material filling the opened spaces,
and brecciation is common. Quartz and orthoclase form subhedral crystals to 10 mm and sometimes show crustiform layering suggesting open space filling.
Weathering and supergene effects There is a strong tendency for deep weathering down the lode structures. These narrow zones of strong weathering follow all the lodes, typically to about 50 m depth. The weathering coincides strongly with the mineralised rock, rarely extending more than a metre or two beyond the lode margins except at very shallow depths. The weathering pattern alone clearly defines the lode on many cross sections. It is anticipated that during mining the lode will be clearly visible to 50 m depth as a band of brown, clayey, weathered rock surrounded by harder black basalt. Mining dilution may be chiefly controlled by changes in rock weathering adjacent to the lode. Supergene effects are manifest by vertical redistribution of gold, copper and silver within the lodes, and lateral transport of gold out of the lodes. The latter effect is significantly developed only at Contact lode (Figs 2 and 3). Here there are two bodies of flat-lying supergene gold mineralisation, up to 8 m thick, developed between 13 and 26 m below the surface. The two bodies are attached to two segments of the main Contact lode which are separated by a fault. They extend eastward from the lode for about 20 m, presumably indicating an easterly movement of ground water at the time they formed. The modern water table lies 4.4 m below the surface. Surprisingly, the supergene bodies cut across weathering zones, with gold occurring in rock ranging from weakly to strongly weathered. Supergene enrichment within the lodes has been studied in detail only at West lode. It is clearly seen on a longitudinal projection (Fig 4) and plots of grade against depth (Fig 5). The latter shows a concentration of high gold, copper and silver values at 30 m depth. The enrichment does not form a continuous strip throughout the lode, but is developed within the three best shoots at 19 120 N, 19 300 N and 19 460 N. Smaller clusters of high assays occur at 45 m and 60 m depth but their origin is uncertain. There is no evidence to suggest that all high gold values at Deflector are due to supergene enrichment; some of the highest values occur in fresh rock.
FIG 4 - Longitudinal projection of West lode, contoured with the gold content of the lode in gram.metres. Contours of gold grade, copper grade and lode width all show broadly the same pattern.
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DEFLECTOR GOLD-COPPER DEPOSIT
FIG 5 - Variation in gold, silver and copper grade with depth in West lode intersections. Note supergene enrichment at 30 m.
Variation in grade and thickness The spatial distribution of gold grades (Fig 4) shows the following features: 1.
the pattern is complex and rather irregular;
2.
the broadest areas of gold mineralisation, as defined by the lower contours, seem to plunge shallowly to the south;
3.
smaller areas, within higher grade contours, have near vertical plunges. This includes the rich 19 120 N, 19 300 N and 19 460 N shoots;
4.
the very highest grades mostly occur in small supergene pods at 30 m depth; and
5.
changes from very high to low grades often occur over very short distances.
The spatial distribution of copper grades is very similar to that of gold, and the same comments apply. This suggests that copper and gold are highly correlated in the lodes. One difference is that there appears to be some depletion of copper values in the top 10 to 15 m of West lode, whereas no such depletion zone is apparent for gold. The true thickness of West lode ranges to 10 m. The pattern of the thickness variation is strikingly similar to those of gold and copper grades, indicating that lode width and grade are correlated. There are very abrupt changes in lode thickness in some areas.
Copper mineral zoning In the primary (unweathered) mineralisation at Deflector the only significant copper mineral is chalcopyrite. In the weathered zones above, a variety of other copper minerals have been developed by supergene processes. The frequency of occurrence of each mineral in West lode has been plotted within 5 m vertical depth intervals (Fig 6). There is an oxidised zone from the base of alluvium to a depth of 35 to 40 m. Below this a transitional zone, dominated by chalcocite, extends to 70 m, followed by the primary chalcopyrite mineralisation. The pattern appears to be simple, and consistently developed throughout the length of the lode.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 6 - West lode copper mineralogy against depth, as recorded in drill intersections.
In the oxide zone the main copper minerals are malachite and chrysocolla of roughly equal abundance. Azurite is rare. Native copper is common in the lower part of the oxide zone, at about 30 m depth, particularly in and close to the three richest shoots, and is closely associated with cuprite. Limonitic ironstone after pyrite is developed throughout the oxide zone. The transition zone is characterised by chalcocite and pyrite, with most of the chalcocite a sooty powder. Hard chalcocite lumps commonly enclose kernels of chalcopyrite which they have partly replaced. A transition from limonite to pyrite occurs at the top of the transition zone. The only common sulphides in the primary zone are chalcopyrite and pyrite.
Hydrothermal alteration Alteration may occur as pervasive metasomatic alteration, where fluid has soaked throughout the rock, or as veinlets where the fluid has deposited minerals along fractures in the rock only. There is little obvious sign of pervasive alteration adjacent to the lodes. The rock within the lodes is always chloritised, biotite alteration occurs locally and carbonate is rare. The wall rock immediately outside the lode is hard, black, unfoliated metabasalt apparently similar to that distant from the lode. Petrography and major element analyses show the rock to be high magnesium basalt with 9 to 13% MgO. However, the silica and potassium contents are anomalously high, up to 55% and 3% respectively. The basalts may be impregnated with very fine grained silica and a potassic phase (orthoclase?) which are not obvious in hand specimen or even thin section. Silicification may explain the great hardness of the Deflector metabasalt which is difficult and expensive to drill. White and yellow veinlets occur throughout the host rocks. They are mostly only a few millimetres thick, and fill sharpedged, brittle fractures. The white veinlets are very widespread and they contain quartz, orthoclase and calcite, singly or in any combination. Some contain pyrite and/or chalcopyrite. The yellow veinlets contain mostly epidote, usually with pyrite.
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In many drill intersections it appears that the yellow veinlets form a halo around the lodes, extending metres or tens of metres into the wall rocks, with the white veinlets extending further out. However, other lode intersections have no yellow veinlets. Where white veinlets surround the mineralisation their pyrite and chalcopyrite content increases close to the lode.
The Gearless Well trachyandesite has been dated by the rubidium-strontium method (Johnson, Cooper and Blight, 1989). Two biotite-whole rock measurements gave an age of 2188 Myr. If this date is correct, and the Deflector biotite porphyries are the same age, and the lode does cut across them, then the mineralisation is Proterozoic.
The mineral assemblage introduced by the hydrothermal fluid (quartz, orthoclase, chlorite, epidote, biotite, calcite and pyrite) suggests low temperature, greenschist conditions. The brittle fracture style of the lode and veinlets suggests relatively shallow depth.
No significant gold mineralisation of definite Proterozoic age is currently known in the Yilgarn Craton. Zircon dating of the Gearless Well intrusion and the Deflector biotite porphyries is desirable. However, until the mine is developed it may be difficult to prove beyond doubt whether the lode cuts the porphyries, or is cut by them.
Trace elements Deflector has a fairly simple geochemical signature. Copper, silver and sulphur are strongly anomalous, and correlate strongly with gold. Arsenic, bismuth, tellurium, selenium, molybdenum, tungsten and cobalt are mildly anomalous and also correlate with gold. Antimony, mercury, zinc, lead and tin are not present in anomalous amounts. It is rare for any sulphides to be seen in the lodes except pyrite and the copper minerals. Possible economic by-products include silver and cobalt. Silver is strongly depleted in the upper part of the oxide zone to a depth of 15 m (Fig 5) and concentrated in the lower part, at around 30 m depth where values from 50 to 300 g/t are common. Silver assays correlate very strongly with copper and less strongly with gold. Silver-rich electrum has been seen in the chalcocite-pyrite zone but the silver content of gold from the primary ore is not known. The distribution of cobalt is known from relatively few samples but values to 0.2% have been returned from West lode.
Age of mineralisation The Deflector basalt and sediment are cut by minor intrusions of dark grey biotite porphyry (minette). General appearance, petrography and major element chemistry strongly suggest that these porphyries are offshoots of the Gearless Well trachyandesite. The gold-copper mineralisation appears to be younger than the biotite porphyries. The Deflector mineralisation is associated with a halo of disseminated pyrite and also quartzpyrite-chalcopyrite veinlets. Such pyrite and veinlets are as abundant in the dykes as they are in the host basalts. West lode apparently cuts through a pipe-like body of biotite porphyry, centred on 19 380 N, 9825 E with gold mineralisation being intersected in drill hole GWC324 within the pipe.
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CONCLUSIONS The discovery of the Deflector deposit is a classic case of the use of geological concepts to find orebodies in concealed terrain. The deposit nicely conforms to the criteria many geologists currently use to target gold deposits. Specifically, Deflector is within 5 km of a major primary fault, immediately adjacent to a secondary splay fault, and possibly related to a dilational jog. It is close to a dolerite dyke, an internal granite, and intimately associated with lamprophyric intrusions. Deflector has some unusual features compared with most other gold deposits in the Yilgarn Craton. The most obvious is its high copper content - West lode has an average grade of 2.3% copper, with local grades over 30%. The silver content is also unusually high. The deposit appears to have formed in a low temperature, relatively shallow environment where brittle fracture was more important than shearing. Circumstantial evidence suggests Deflector may be younger than most Yilgarn gold deposits, but further work is required to confirm this.
ACKNOWLEDGEMENTS This paper is published with the permission of Gullewa Gold NL. It draws on the data and insights of many geologists who worked at Deflector for Sons of Gwalia Ltd and Gullewa Gold NL. The authors thank P Mazzoni and G Hewlett for constructive reviews of the text. The figures were prepared by R Roberts.
REFERENCES Johnson, G I, Cooper, J A and Blight, D F, 1989. The geology and geochronology of a Proterozoic trachyandesite plug, Murchison Province, Yilgarn Block, Western Australia, Australian Journal of Earth Sciences, 36(3):319–336.
Geology of Australian and Papua New Guinean Mineral Deposits
Fairclough, M C and Brown, J C, 1998. Tarmoola gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 173–178 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Tarmoola gold deposit 1
by M C Fairclough and J C Brown
2
INTRODUCTION The deposit is 29 km NW of Leonora, at lat 28o40′S, long 121o10′N, or AMG coordinates 320 500 E, 6 827 500 N, on the Leonora (SH 51–1) 1:250 000 scale and the Leonora (3140) 1:100 000 scale map sheets (Fig 1). The Tarmoola mine is wholly owned and operated by Mount Edon Gold Mines (Aust) Ltd (MEGM), and is the fifteenth largest gold mine in Australia based on reserves, and one of only nineteen gold mines in Australia with more than 1 Moz of reserves in a single deposit. The Tarmoola orebody is beneath the site of the historic King Of The Hills workings and immediately adjacent to a previously mined laterite-hosted gold deposit, to which much of the presently exploited style of mineralisation is probably unrelated.
EXPLORATION AND DEVELOPMENT The King Of The Hills mineralisation was discovered in 1897 and mined intermittently by prospectors to 1918. During this period approximately 12 000 oz of gold were won from ore of average grade 20 g/t. One of the earliest significant exploration efforts for gold was by Esso Exploration and Production Australia Inc in 1980–81 who concluded that the known mineralisation had limited depth potential. During 1983, Anaconda Australia Inc conducted minor exploration in the area but considered that there was little possibility of a near surface open-pittable resource. Subsequently however, as a result of extremely encouraging assays from samples of the chert outcrops, major work was carried out in 1984 by the Agnew Joint Venture Partners (BP Minerals, AIH and Seltrust Mining Ltd), who delineated an estimated resource of 184 000 t of ore grading 4.55 g/t of gold. This resource was subsequently purchased by Kulim Ltd who mined 85 623 t to produce more than 18 000 oz during 1985–86 in a joint venture with Sons of Gwalia NL. In mid 1987, MEGM farmed into the tenements surrounding the King Of The Hills site in a joint venture with Townson Holdings Pty Ltd and, over the next two years, delineated combined Reserves and Resources of 2.95 Mt of laterite and oxide ore at King Of The Hills West. The discovery was a gradual process, through systematic soil sampling, rotary air blast (RAB) and reverse circulation (RC) drilling, during late 1987 and early 1988. The mineralisation envelope was recognised as a combination of both flat-lying and steeply west-dipping vein sets with a strong supergene enrichment component. Host rocks to this shallow mineralisation were identified as strongly carbonated and sheared ultramafic schist and basalt, in contrast to the chert-related historic production. 1.
Structural Geologist, Mount Edon Gold Mines (Aust) Ltd, PO Box 133, Greenwood WA 6024.
2.
Project Geologist, Mount Edon Gold Mines (Aust) Ltd, PO Box 133, Greenwood WA 6024.
Geology of Australian and Papua New Guinean Mineral Deposits
Kulim Ltd transferred its interest in the King Of The Hills project in early 1988 to Arboyne NL who, during 1989, were able to delineate a new reserve of about 266 000 t at an average grade of 4.1 g/t in addition to the remaining 180 000 t at 5.1 g/t. MEGM commenced prestripping on the adjacent leases in May 1989, with full scale mining and stockpiling starting in August of that year and commissioning of the processing plant in May 1990. Mining by Arboyne NL brought the cumulative total production from King Of The Hills to more than 400 000 t at 4.4 g/t. However, falling head grades, problems with pit dewatering and the consequent withdrawal of Sons Of Gwalia NL from the joint venture ultimately led to the project being sold to MEGM in January 1990. The integrated operations at King Of The Hills, King Of The Hills Extended and King Of The Hills West were renamed Tarmoola after Tarmoola Station to the SW of the deposit. Reserves and resources have increased rapidly as mining progressed into the primary zone and the character of the orebody changed with depth. In October 1996 Proved and Probable Reserves for the Tarmoola pit were 12.973 Mt at 2.6 g/t for 1.1 Moz of contained gold, with additional total resources of 17 Mt at 1.97 g/t for 1.1 Moz. With the recent upgrade of the Tarmoola processing plant to a throughput of 1.7 Mtpa, this represents at least a further nine years of mine life at current mining schedules. The MEGM operation is now a 1.4 km long open cut which has yielded approximately 400 000 oz of gold to January 1997. Production for the 1994–95 financial year was 57 378 oz, with a projected output for 1996–97 of 107 500 oz.
REGIONAL GEOLOGY The deposit lies within the Leonora 1:250 000 scale and 1:100 000 scale map sheet areas (Fig 1), for which the regional geology has been investigated by Thom and Barnes (1977) and Williams (in press) respectively. The deposit is hosted within the Norseman–Wiluna greenstone belt of the Eastern Goldfields Province (EGP). As is the case with much of the Yilgarn Block, the belt consists of several relatively narrow linear greenstone belts which anastomose around large elongate gneissic granitoid domes. Regional metamorphic grade does not generally exceed greenschist facies, but reaches upper amphibolite facies along major granite-greenstone contacts (Williams and Currie, 1993).
STRUCTURE In recent years several authors have proposed increasingly complex structural and tectonic histories for some areas of the EGP. A broad consensus has begun to emerge which proposes local development of early low angle, east trending extensional (De) and contractional (D1) structures (Hammond and Nisbet, 1992; Williams and Whittaker, 1993), the second of which
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FIG 1- Location map and regional geological setting of the Tarmoola gold deposit, after Hallberg and Thompson (1985) and Pratt and Jankowski (1993).
resulted in structural repetition of greenstone stratigraphy by thrusting (Martyn, 1987; Swager and Griffin, 1990). The gross NNW structural and stratigraphic grain of the EGP is a result of major east-west transpression during D2 resulting in upright folding, reverse and sinistral shear zone development and a penetrative foliation. Subsequent brittle reactivation of earlier ductile shears and development of discordant, high level, subvertical conjugate faults occurred during D3, under an inferred stress field coaxial to that of D2.
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The Tarmoola deposit is west of a concentrated belt of D2 and reactivated earlier structures, the Keith–Kilkenny tectonic zone (KKTZ) of Hallberg (1985) which is equivalent to the Yerilla Terrane of VanderHor and Witt (1992). To the south, the supracrustal sequence is sheared against the Raeside Batholith by the Sons of Gwalia shear zone (SOGSZ). The SOGSZ is an early extensional De structure and hosts several significant gold deposits including Sons of Gwalia (Williams, Nisbet and Etheridge, 1989) and Harbour Lights (Skwarnecki, 1987, 1988; Vearncombe, 1992).
Geology of Australian and Papua New Guinean Mineral Deposits
TARMOOLA GOLD DEPOSIT
Relevant detailed structural work for localities in the Leonora area has been described by Passchier (1990, 1992, 1994) who outlined a structural history involving early nappestyle tectonics followed by discordant regional scale upright folding. The Tarmoola deposit is situated within the closure of the gently north-plunging F 2 Tarmoola antiform, the eastern limb of which is delineated by the KKTZ. The core of the Tarmoola antiform is a D2 strain shadow, with the intensity of D2 related deformation increasing to the west and east. Preserved, folded segments of the D1 faults and the early extensional (De) SOGSZ occur within the Tarmoola antiform (Fig 1).
LOCAL GEOLOGY ROCK TYPES Gold mineralisation at Tarmoola is largely shear-hosted, occurring along a shallowly dipping to subvertical granitegreenstone boundary to the NE of the deposit and within granite-hosted, subvertical, brittle shear zones to the SW. Neither of these ore zones has a significant local surface expression. The stratigraphic sequence comprises strongly silica-altered tholeiitic basalt, moderately to intensely foliated ultramafic rocks and minor folded chert, overlying the Tarmoola granodiorite. The granodiorite is an elongate NE striking pluton with steeply dipping margins to the NE and east (Figs 2 and 3), and a domed geometry to the SW. These areas and associated mineralised zones are locally referred to as the ‘North East extension’, ‘Eastern flank’ and ‘Southwest granite’ respectively (Fig 4). Depths to the granodiorite surface as indicated by diamond and percussion drilling delineate the granodiorite geometry, although the shape of the western margin of the granite is poorly understood (Fig 4). Depth to the
FIG 2 - Geological plan of the Tarmoola gold deposit.
base of oxidation is a complex interaction between degree of silicification in the protolith, granite geometry and extent of shearing, and ranges from near surface down to approximately 80 m.
FIG 3 - Representative cross section A-A′ of the Tarmoola deposit. The section is orientated north-south (local grid) and the section line is shown on Fig 2.
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The Southwest granite is flanked by a SE-dipping brecciated fault zone within the overlying greenstone sequence. The granite in this area is crosscut by a series of brittle, 10 m wide, subvertical fracture zones, characterised by steeply dipping, millimetre to decimetre scale NE and NW striking conjugate hydrofractures and consequent stockwork style mineralisation. Layer parallel chert and metasediment occur as metre scale units within the greenstone. They occur along the NE margin of the pit and appear as fine to coarsely laminated, fine-grained banded units, often containing strong interlaminar sulphide mineralisation.
STRUCTURE
FIG 4 - Contour map of depth to granite surface as shown by the first granite intercepts in RC and diamond drill holes. Contour interval is 10 m and the shaded portion is the current pit area.
Whole rock geochemical analyses of the Tarmoola granodiorite have returned silica, and sodium and potassium oxide compositional ranges of 66–78 wt % and 5–7 wt % respectively. These values give a granodioritic to leucogranitic composition, and A-F-M ternary plots show a calc-alkaline compositional trend. Compositionally bimodal, metre scale dykes including felsic and dioritic members crosscut the granodiorite. In the case of the dioritic dykes, their lack of any foliation and crosscutting nature indicate a later emplacement than the felsic dykes and the host granodiorite, postdating at least the local De event. Further, their whole rock trace element composition, specifically their relative enrichment in yttrium, indicates a separate parentage. The whole rock composition of the felsic dyke suite is compatible with that of the Tarmoola granodiorite, and although they crosscut the granodiorite, they also occur within the greenstone, where they have a prominent foliation. It is likely that they are contemporaneous with, or closely postdate intrusion of the granodiorite, and are derived from a related magma. The northern portion of the contact between the granodiorite and overlying mafic-ultramafic sequence is a zone of intense deformation (the mine site expression of the SOGSZ) which is largely localised within the greenstone and is the host for the North East extension mineralisation. The style of deformation adjacent to the pluton is protolith-dependent but ranges laterally from ductile shearing to brecciation, largely related to ultramafic and mafic rock types respectively. The thickness of the zone also varies, from less than a metre along the subhorizontal to gently NE-dipping pluton roof, to 80 m or more adjacent to the steeply dipping eastern margin. The shape of this zone is geometrically consistent along strike, but it is unclear whether it is the result of metasomatic alteration due to comagmatic fluids associated with the emplacement of the granodiorite, or whether it is a reflection of the geometry of the dilatant zone on the flanks of the rigid pluton under extension. The outer margin of this zone is transitional into relatively undeformed, albeit foliated, country rock.
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Structural features at Tarmoola are controlled by a combination of extension and compression directions and by the presence of the relatively rigid Tarmoola granodiorite. Overprinting relationships and U-Pb isotope geochronology (D Nelson, unpublished data, 1996) indicate that the intrusion of the granodiorite was syn- to pre-kinematic with respect to the first recognisable event in the structural evolution of the deposit. This early deformation is manifest in the Tarmoola pit as an intense, shallowly north-dipping foliation, particularly adjacent to the granodiorite, the composite nature of which indicates top to the NW to NE movement (P Williams, unpublished data, 1994). In association with irregular northdipping en echelon vein arrays, it is clear that the earliest recognisable structural event at Tarmoola was, on average, top to the north extension (referred to as De). Occasional contradictory senses of movement, as indicated by foliation asymmetry and rare crenulations, can be related to a second set of south-dipping sigmoidal en echelon tension gashes which overprint the extensional features. The presence of listric north-dipping thrust planes and asymmetric quartz porphyroclasts in the pit confirms that early extension was succeeded by broadly coaxial top to the south compression (D1) resulting in local ‘basin inversion’. Along the eastern margin of the granodiorite in the North East extension, highly discordant east-dipping D2 reverse faults and associated subparallel veins strike approximately north and are consistent with the King Of The Hills trend. The intensity and frequency of these interpreted D2 features increases towards the east, ie towards the KKTZ. These reverse faults clearly crosscut both the De and D1 structures. In places, D1 thrust faults have been partly reactivated during this event, with partial rotation into the D2 trend and the development of an east-plunging slickenside lineation. Features of this type which crosscut the granodiorite have resulted in the tectonic intercalation of greenstone and granodiorite with significant associated sulphidic quartz veining and silicic alteration of the wall rock along the eastern contact. This superposition of multiple deformations around the rigid pluton has resulted in a complex geometry along the eastern granodiorite contact. Subvertical SE-trending brittle faults with minor sinistral offsets are encountered toward the southern limits of the pit and overprint all other major structural features and are denoted D3. Minor block faulting, which does not appear to be significant with respect to mineralisation, also occurs. The orientation of each generation of structures varies significantly around the granodiorite margin, such that overprinting relationships and average orientations are more indicative of the true structural setting.
Geology of Australian and Papua New Guinean Mineral Deposits
TARMOOLA GOLD DEPOSIT
CONTROLS OF MINERALISATION The North East extension mineralisation at Tarmoola is largely concentrated within the sheared greenstone, although discrete high grade mineralised structures occur within the underlying granodiorite. Irregular and diffuse zones of low grade gold mineralisation occur in broad sections of the granodiorite and are manifest as pervasive microscopic fracturing and mineralisation by gold-bearing microveinlets. Both the early north-dipping (D1) and late east-dipping (D2) reverse faults are mineralised by high grade quartz(±carbonate) veins with pyrite, chalcopyrite, galena, sphalerite and trace stibnite. The second generation of contractional faults also occur within the granodiorite and host discrete, high grade zones of mineralisation. The two generations of structures combine to form a broad arcuate zone of mineralisation which abuts the granodiorite contact for approximately 400 m of strike length and 150 m down dip. Smaller scale subparallel en echelon mineralised veins occur within the granodiorite up to approximately 150 m from the contact. In general, south-dipping D1 en echelon tension gashes host the majority of the gold within north-dipping mineralised envelopes, which correspond to D1 thrust zones with associated thrust-parallel and oblique en echelon vein sets. Later mineralised structures are manifest as subvertical hydrofractures axial planar to the Tarmoola antiform and surface traces of fold axes in chert, which are largely highly discordant to early thrusts. Both the D1 and D2 generations of mineralised structures display a close correlation with the associated sulphides. The degree of remobilisation of gold between separate generations of structures is presently unknown, but is unlikely to account for all mineralisation given the extreme differences in stress field orientations inferred for different mineralised structures. Mineralisation in the Southwest granite is similarly quartz vein hosted. The sulphides in this area include pyrite and sphalerite, but are notably deficient in chalcopyrite, a major ore indicator in the North East extension. Trace amounts of arsenopyrite have also been recorded in veins within the Southwest granite. The geometry of the domed granite surface has resulted in major fracturing of the rock, probably under compression, resulting in the development of stockwork-style vein systems, striking NW and partitioned within two larger scale, subvertical, NW-striking structural corridors. Preliminary evidence suggests that these fault zones display minor (<20 m) sinistral offsets and are up to 50 m wide with currently defined strike lengths of 200 m. Adjacent to the granodiorite contact, and occurring along its SE dipping southern flank, is a lenticular, high grade ore zone comprising brecciated and rehealed ultramafic rocks. This zone is similar to the highly deformed, mineralised greenstone suite in the North East extension. This zone of brecciation extends to the NE along the Eastern flank for a known strike length of approximately 400 m.
ALTERATION Intense silicification and carbonation of the ultramafic and mafic sequence is present within the contact shear zone. The intensity of alteration increases towards the granodiorite contact, with significant associated replacement and recrystallisation. An alteration assemblage of sericite-quartzpyrite occurs in the granitoid, forming a metasomatised, siliceous skin 20 to 50 m wide adjacent to the granodiorite
Geology of Australian and Papua New Guinean Mineral Deposits
surface. Although pervasive over much of this zone, in several cases the alteration occurs as a discrete halo around quartz veins, indicating a spatial and probably temporal connection between the two features. The intensity of alteration decreases with distance from the contact, beneath which the granodiorite retains its primary mineralogy of quartz, hornblende, plagioclase, potassium feldspar and biotite. There is no direct relationship between secondary alteration and gold grade, and work to date indicates that all granodiorite-hosted gold at Tarmoola is confined to quartz veins, with negligible wall rock mineralisation.
ORE GENESIS Preliminary thermometric fluid inclusion studies have been conducted at Tarmoola to provide indications of the source of the ore forming fluids. The results are equivocal, but have succeeded in defining two distinct types of fluid inclusions in mineralised veins, both in the granodiorite and the overlying greenstone. Type I forms a mixed population of low temperature (homogenisation temperatures <190oC), low salinity (6 equivalent wt % NaCl) fluid inclusions divisble into those carrying sodium chloride salts and those with sodium and potassium chloride solutes. Type II fluid inclusions are of low to moderate salinity (1–14 equivalent wt % NaCl), moderate to high temperature ( 290oC), and carbon dioxide rich. No true trapping temperatures for either class of fluid inclusion were obtained from this study. Proportionally the Type II fluid inclusions dominate, and are more likely to be directly related to gold mineralisation. Preliminary conclusions from this study are that, given the high carbon dioxide content of the fluid inclusions, it is unlikely that the mineralising fluids were derived entirely from magmatic fluids from the Tarmoola granodiorite.
MINE GEOLOGICAL METHODS Modelling predictions of grade and tonnes for the supergene component of the Tarmoola orebody in recent years have been adequately constrained using a geostatistically calculated block model with minimal geological control. Recent mine development has led to a transition from broad, dispersed, strongly supergene-influenced oxide ore to more discrete high grade mineralised structures in the sulphide zone at depth. This provided the impetus for a major re-evaluation of geological and geostatistical modelling techniques. The implementation of tighter geological constraints on the model has been coupled with a significant change in grade control and mining methodology. In the past, 5 m benches were sampled and mined in 2.5 m lifts. The current methods involve sampling of vertical blastholes at 1 m intervals, with 5 m benches extracted in three equal lifts. This method was adopted to increase the sampling density and to allow for more selective sampling of the ore, thereby reducing dilution.
DISCUSSION AND CONCLUSIONS Much of the insight into the geology and structural setting of the Tarmoola deposit was generated via regional studies and the relationship of other orebodies to structure in the area. Strong similarities exist with Sons Of Gwalia (Williams, Nisbet and Etheridge, 1989) and Harbour Lights (Vearncombe, 1992), particularly with respect to the early structural history. Similarly, the Granny Smith deposit (Ojala et al, 1993) is a
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strong geometric analogy for the later structural events that tend to control the geometry of the Tarmoola deposit, ie westverging D2 and D3 reverse faults and later discordant conjugate strike slip faults. Integration of local structural information from Tarmoola with the regional structural framework has enabled the generation of a working model for mineralisation, which is not easily reconcilable with traditional viewpoints concerning a single, late stage mineralising event (Witt, 1991; Groves, 1993). The current geological model for Tarmoola is for emplacement of the Tarmoola granodiorite immediately prior to, or early in the development of the mine site portion of the SOGSZ. The granodiorite is considered to have been intruded near the top of the subsurface extension of the shear zone system during top to the north extension early in the history of the EGP. Tarmoola occurs on the northern margin of this ‘tongue’ of granitoid. Shear strain was locally concentrated within an ultramafic sequence and subsequently along the granitoid-greenstone contact. Mineralisation is related to strain perturbations along the irregularly-shaped pluton roof of the Tarmoola granodiorite and around its northern edge during subsequent inversion related to southwards directed D1 thrusting and thus predates regional D2 folding. Subsequent mineralisation is more typical of EGP gold deposits and occurs in the form of relatively late, steeply dipping, NNW to north striking D2 faults and veins adjacent to the Tarmoola granodiorite. Tarmoola therefore represents a unique combination of the early structural characteristics of Sons of Gwalia and Harbour Lights deposits, and the later features of the Granny Smith deposit.
ACKNOWLEDGEMENTS The management of Mount Edon Gold Mines (Aust) Ltd is thanked for allowing publication, for the first time, of some of the details on the Tarmoola deposit. The cumulative knowledge of past and present mine geologists, particularly S Coyle and R Schellekens, is also acknowledged. P Williams of Etheridge Henley Williams Geoscience Consultants provided much insight to the geology of Tarmoola which contributed to in house geological models. We are indebted to B Hill of Geologists Australia for supplying pit maps and sections, and to G Plowright for efficient drafting of figures. Critical reviews by D Greenaway and other MEGM geologists are gratefully acknowledged. The opinions presented are not necessarily those of the individuals mentioned above.
REFERENCES Groves, D I, 1993. The crustal continuum model for late-Archaean lode-gold deposits of the Yilgarn Block, Western Australia, Mineralium Deposita, 28:366–374. Hallberg, J A, 1985. Geology and Mineral Deposits of the LeonoraLaverton Area, Northeastern Yilgarn Block (Hesperian Press: Perth). Hallberg, J A and Thompson, J F H, 1985. Geologic setting of the Teutonic Bore massive sulfide deposit, Archaean Yilgarn Block, Western Australia, Economic Geology, 80:1953–1964. Hammond, R L and Nisbet, B W, 1992. Towards a structural and tectonic framework for the central Norseman-Wiluna greenstone belt, Western Australia, in The Archaean: Terrains, Processes and Metallogeny, Publication 22 (Eds: J E Glover and S E Ho), pp 39–49, (The Geology Department and University Extension, The University of Western Australia: Perth).
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Martyn, J A, 1987. Evidence for structural repetition in the greenstone sequence of the Kalgoorlie District, Western Australia, Precambrian Research, 37:1–18. Ojala, V J, Ridley, J R, Groves, D I and Hall, G C, 1993. The Granny Smith gold deposit: the role of heterogeneous stress distribution at an irregular granitoid contact in a greenschist facies terrane, Mineralium Deposita, 28:409–419. Passchier, C W, 1990. Report on the geology of the Leonora area, Western Australia, Bureau of Mineral Resources Geology and Geophysics Record 1990/59 (unpublished). Passchier, C W, 1992. The nature of high-strain zones in the LeonoraLaverton area, Western Australia, Bureau of Mineral Resources Geology and Geophysics Record 1992/53 (unpublished). Passchier, C W, 1994. Structural geology across a proposed Archaean terrane boundary in the eastern Yilgarn craton, Western Australia, Precambrian Research, 68:43–64. Pratt, J D R and Jankowski, P, 1993. The geology and grade control at Bannockburn gold mine, Leonora District, Western Australia, in International Mining Geology Conference, Kalgoorlie-Boulder, WA, 5–8 July, 1993 (Eds: I Robertson, W Shaw, C Arnold and K Lines), pp 125–132 (The Australasian Institute of Mining and Metallurgy: Melbourne). Skwarnecki, M S, 1987. Controls on Archaean gold mineralisation in the Leonora district, Western Australia, in Recent Advances in Understanding Precambrian Gold Deposits, Publication 11 (Eds: S E Ho and D I Groves), pp 109–136 (The Geology Department and University Extension, The University of Western Australia: Perth). Skwarnecki, M S, 1988. Alteration and deformation in a shear zone hosting mineralisation at Harbour Lights, in Advances in Understanding Precambrian Gold Deposits, Volume 2, Publication 12 (Eds: S E Ho and D I Groves), pp 111–129 (The Geology Department and University Extension, The University of Western Australia: Perth). Swager, C and Griffin, T J, 1990. An early thrust duplex in the Kalgoorlie-Kambalda greenstone belt, Eastern Goldfields Province, Western Australia, Precambrian Research, 48:63–73. Thom, R and Barnes, R G, 1972. Leonora, Western Australia - 1:250 000 geological series, Geological Survey of Western Australia Explanatory Notes SH 51–5. Van der Hor, F and Witt, W K, 1992. Strain partitioning near the KeithKilkenny Fault Zone in the central Norseman-Wiluna Belt, Western Australia, Bureau of Mineral Resources Geology and Geophysics Record 1992/68 (unpublished). Vearncombe, J R, 1992. Archaean gold mineralisation in a normalmotion shear zone at Harbour Lights, Leonora, Western Australia, Mineralium Deposita, 27:182–191. Williams, P R, in press. Leonora, Western Australia - 1:100 000 geological series, Australian Geological Survey Organisation Map Commentary. Williams, P R and Currie, K L, 1993. Character and regional implications of the sheared Archaean granite-greenstone contact near Leonora, Western Australia, Precambrian Research, 62:343–365. Williams, P R, Nisbet, B W and Etheridge, M A, 1989. Shear zones, gold mineralisation and structural history in the Leonora district, Eastern Goldfields Province, Western Australia, Australian Journal of Earth Sciences, 36:383–403. Williams, P R and Whittaker, A J, 1993. Gneiss domes and extensional deformation in the highly mineralised Archaean Eastern Goldfields Province, Western Australia, Ore Geology Reviews, 8:141–162. Witt, W K, 1991. Regional metamorphic controls on alteration associated with gold mineralisation in the Eastern Goldfields Province, Western Australia: Implications for the timing and origin of Archaean lode-gold deposits, Geology, 19:982–985.
Geology of Australian and Papua New Guinean Mineral Deposits
Newton, P G, Gibbs, D, Grove, A, Jones, C M and Ryall, A W, 1998. Sunrise-Cleo gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 179–186 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Sunrise-Cleo gold deposit 1
2
3
4
5
by P G N Newton , D Gibbs , A Grove , C M Jones and A W Ryall INTRODUCTION The deposit is approximately 55 km south of Laverton in the Eastern Goldfields Province of WA at lat 29o05′S, long 122o25′E and AMG coordinates 443 250 E, 6 782 800 N, on the Edjudina (SH 51–6) 1:250 000 scale and the Lake Carey (3339) 1:100 000 scale map sheets (Fig 1). It straddles an ownership boundary and is being mined simultaneously by the Granny Smith Joint Venture (GSJV: Placer Pacific Ltd 60% and Delta Gold NL 40%), who refer to their part of the deposit as ‘Sunrise’, and Acacia Resources Limited, who refer to their portion as ‘Cleo’. At Sunrise, the Identified Mineral Resource (including Reserves) at July 1996 was 6.0 Mt at 2.6 g/t gold at a 1.0 g/t cutoff (Table 1). Production from Sunrise from July 1995 to July 1996 was 1.2 Mt at 2.85 g/t, yielding 110 000 oz of gold, leaving a Probable Reserve of 3.6 Mt at 3.1 g/t. The Identified Mineral Resource for Cleo at December 1996, prepared using a geostatistical indicator-kriging approach, is 11.5 Mt at 3.6 g/t gold at a 1.0 g/t cutoff. The total contained gold for the SunriseCleo deposit exceeds 1.8 Moz making it one of the most significant recently discovered gold deposits in the Yilgarn Craton, and additional drilling by both parties is expected to significantly increase this figure. The Sunrise-Cleo deposit is atypical of most gold deposits in the Yilgarn Block, because mineralisation is developed in the Archaean sequence and in the overlying transported cover. In addition, gold mineralisation is controlled by a shallowlydipping thrust duplex system, locally termed the Sunrise shear, unlike the majority of Archaean lode gold deposits in the Yilgarn Block, which have moderately- to steeply-dipping mineralised structures. Acacia and the GSJV have exchanged geological and mining data for the Sunrise-Cleo deposit, in order to maximise the recovery and value of the resource to both parties. Mining commenced for the GSJV in May 1995 and Acacia began prestrip mining and project construction in October 1996.
1.
Centre for Strategic Mineral Deposits, Department of Geology and Geophysics, University of WA, Crawley WA 6907.
2.
Chief Mine Geologist, Acacia Resources Limited, 3 Richardson Street, West Perth WA 6005.
3.
Geologist, Acacia Resources Limited, 3 Richardson Street, West Perth WA 6005.
4.
Mine Geologist, Placer (Granny Smith) Pty Ltd, Granny Smith Mine, PO Box 33, Laverton WA 6440.
5.
Principal Exploration Geologist, Delta Gold NL, 46-50 Kings Park Road, West Perth WA 6005.
Geology of Australian and Papua New Guinean Mineral Deposits
EXPLORATION AND MINING HISTORY DELTA GOLD NL–PLACER PACIFIC LIMITED Title acquisition in 1983 by Canyon Resources Pty Ltd, the parent company of the then unlisted Delta Gold NL, targeted areas of outcropping banded iron formation (BIF) and areas with favourable magnetic signatures. Early prospecting by composite float sampling provided some promising results, but it was not until August 1988 that anomalous gold zones were delineated by a 500 by 500 m offset spaced bulk leach extractable gold (BLEG) soil sampling program. Follow up sampling on 250 by 100 m spacing produced values to 50 ppb cyanide-soluble gold, and subsequent rotary air blast (RAB) drilling in April 1989 produced a best result of only 4 m at 0.28 g/t gold, from a site several hundred metres NW of the current deposit and hosted by transported sediment. In July 1990, the first economic supergene gold mineralisation was encountered within transported sediment, by aircore and reverse circulation (RC) drilling. Following this discovery, the northern (Sunrise) part of the deposit was progressively delineated by RC drilling (Table 1). In late 1992 a six hole RAB profile was completed over an untested soil anomaly. This gave the first intersection of supergene mineralisation in the main part of the deposit as 4 m at 16.1 g/t in hole SDR 144. This extension of the Sunrise supergene mineralisation led to a major increase in drilling activity, and by November 1993 the mineral resource had been increased to 2.3 Mt at 4.2 g/t (Table 1). By July 1994, further drilling under the new GSJV had increased this to 4.8 Mt at 3.0 g/t. The Sunrise mine was officially opened in May 1995. Ore is trucked 32 km to the Granny Smith mill, and annual gold production exceeds 100 000 oz.
ACACIA RESOURCES LIMITED Billiton Australia Ltd, the predecessor to Acacia, began ground acquisition in the area in 1987. Scout RAB drilling quickly defined the Golden Delicious deposit approximately 8 km NE of Cleo, and exploration efforts were initially focussed on this area. Tenement-wide RAB and aircore drilling returned anomalous results in the Cleo area in 1991, including 4 m at 0.78 g/t. The significance of these results was not recognised until 1993 when the fourth hole of an RC drilling program intersected 54 m at 10 g/t. Intensive exploration in the Cleo area followed with results shown in Table 1. Acacia commenced production in February 1997, with a mill throughput of approximately 1 Mtpa, generating annual gold production of approximately 100 000 oz.
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FIG 1 - Geological map of the Laverton–Linden region showing mineral deposits and resource estimates, modified from D Gibbs (unpublished data, 1995) and Ojala (1995).
180
Geology of Australian and Papua New Guinean Mineral Deposits
SUNRISE–CLEO GOLD DEPOSIT
TABLE 1 Resource and reserve estimates for the Sunrise-Cleo gold deposit from 1992 to 1997. Resources
Reserves
Manager
Date
Measured Indicated ('000 t at g/t) ('000 t at g/t)
Inferred ('000 t at g/t)
Delta Gold NL
Aug '92
171 at 2.8
260 at 2.3
Delta Gold NL
Nov '93
478 at 3.2
1365 at 4.2
GSJV
June ‘94
1800 at 2.4
3000 at 3.3
GSJV
June '95
180 at 3.2
2670 at 3.1
2160 at 4.2
GSJV
June '96
4210 at 2.7
1760 at 2.4
GSJV
Oct ‘97
2 805 at 2.7#
6768 at 2.9
Proven ('000 t at g/t)
Probable ('000 t at g/t)
Total Total Reserve ('000 t at g/t)
Resource + Contained Reserve ('000 t gold ('000 at g/t) 1. ounces)
SUNRISE
413 at 5.5
150 at 3.2 2805 at 2.7#
431 at 2.3
32
2256 at 4.2
305
4800 at 3.0
456
2180 at 3.1
2330 at 3.1
5020 at 3.6
575
3580 at 3.1
3580 at 3.1
5970 at 2.6
500
3943 at 3.4
6748 at 3.1
9573 at 2.8
872
3200 at 4.2
455
CLEO Acacia Resources
Dec '94
800 at 5.0
2400 at 4.3
Acacia Resources
Dec '95
1 172 at 4.8
1967 at 3.9
2555 at 3.9
1286 at 4.5
1890 at 4.0
3176 at 4.2
5694 at 4.1
750
Acacia Resources
Dec ‘96
2 120 at 4.4
5494 at 3.4
8264 at 2.5
1931 at 4.7
3953 at 3.9
5884 at 4.1
15 878 at 3.1
1354
Acacia Resources
Sept ‘97
2 160 at 4.3
6860 at 4.4
9330 at 3.9
*
*
*
18 350 at 4.2
2460
1. Resources include reserves
#
Stockpiles
* To be calculated Dec '97
PREVIOUS DESCRIPTIONS Gellatly et al (1995) discussed the exploration history of the Sunrise deposit in the only paper published to date. Within the GSJV area, early studies by J Standing (unpublished data, 1993) and V J Ojala (unpublished data, 1994) were valuable in interpreting the complex structural history of the deposit, while similar work for Acacia was completed by D Gibbs (unpublished data, 1995) and A Grove, R Belcher and J Jessop (unpublished data, 1995). Regolith interpretations were completed for GSJV by J King (unpublished data, 1994) and L M Lawrance (unpublished data, 1994), and for Acacia by Keserue-Ponte (1995).
REGIONAL GEOLOGY The area which includes the Sunrise-Cleo deposit extends northwards to Laverton and south to include the Red October, Butcher Well and Linden deposits and is known as the Laverton–Linden region of the Archaean Eastern Goldfields Province (Fig 1). Ojala (1995) recognised three distinct structural-stratigraphic domains referred to as the Margaret, Laverton and Burtville domains which broadly correspond to the domain outline of Gower (1976) and Hallberg (1985) (Fig 1). Sunrise-Cleo is within part of the structurally complex Laverton Domain whose boundaries were interpreted by Ojala (1995) based on structural style. It is equivalent to the ‘structural corridor’ described by Hall and Holyland (1990) and the ‘Laverton Deformation Zone’ of Hronsky (1993). The Laverton Domain forms a major, broad structural break between the Margaret and Burtville Domains, and is dominated by acid to intermediate volcanic rocks and volcaniclastic sedimentary rocks compared with mafic rocks and BIF in the Margaret Domain and ultramafic and mafic rocks in the
Geology of Australian and Papua New Guinean Mineral Deposits
Burtville Domain. In addition, the Laverton domain is structurally complex and characterised by tight folding and thrusting. Four tectonic events are recognised in the area and are broadly consistent with events interpreted throughout the Yilgarn Craton: (i) isoclinal folding and layer parallel contractional faults (D1); (ii) two progressive phases of regional shortening (D2, D3) which produced upright folds and widely-spaced transcurrent faults; and (iii) late extension which produced a series of east-striking normal faults which are commonly filled by Proterozoic dolerite, lamprophyre and carbonatite dykes. A number of gold deposits lie within, or near, the margins of the Laverton Domain, including Lancefield, Childe Harold, Granny Smith, Keringal, Red October and the Laverton and Barnicoat groups (Fig 1). Most of these deposits are hosted by sedimentary rocks, which is a distinctive feature of the Laverton–Linden region relative to other deposits in the Yilgarn Block. The Keringal and Red October deposits are the only significant gold deposits (>1 t gold) that are not closely associated with BIF, chert or clastic sedimentary rocks.
ORE DEPOSIT FEATURES LITHOLOGY AND STRATIGRAPHY Apart from a small outcrop of BIF at Sunrise Hill, there is virtually no outcrop in the immediate mine area. The surface of the area is characterised by sand and gypsiferous dunes, salt pans and sheet wash alluvium, which overlie a 20 to 60 m thick sequence of clay and lateritic gravel. Consequently, the understanding of the underlying Archaean sequence is primarily based on drilling information and aeromagnetic interpretation, and exposures in the stage 1 GSJV pit.
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The local stratigraphy is dominated by a group of sedimentary, volcanic and volcaniclastic rocks, generally of acid to intermediate composition, which is separated from ultramafic dominated packages by two north trending lineaments (Fig 2). The aeromagnetic pattern of the area is characterised by discontinuous NE-trending magnetic highs that correspond to shallowly-dipping, thrust-faulted banded iron formation (BIF). Several NNE-trending thrust faults, which are subparallel to bedding, are interpreted from drilling information and aeromagnetic data, and are closely associated with mineralisation, not only at Sunrise-Cleo, but also at Golden Delicious and Pink Lady. In addition, a series of northand NW-trending faults disrupt the BIF package and show some characteristics similar to the Boogardie breaks which disrupt the folded BIF sequence at Hill 50, Mount Magnet (Thompson et al, 1990).
goethitic mottles in clays, with a base that is commonly ferricreted (J King, unpublished data, 1994). The unconformable contact at the base of the transported sequence is iron-rich and irregular (Fig 4a). The nature of the contact indicates that topographic relief in excess of 30 m existed prior to deposition of the laterite and clay-rich sediments. A southerly draining palaeochannel trends south to SW through the deposit area.
Archaean rocks The upper part of the Archaean sequence is saprolitic and extends to the base of weathering approximately 60–80 m below the surface. Archaean rocks consist of a shallowlydipping sequence of interbedded sedimentary, volcaniclastic and volcanic rocks of acid to intermediate composition, which includes agglomerate, coarse ash tuff, sandstone, crystal tuff, and intrusive rocks. The volcaniclastic rocks are generally thick bedded to massive, with finer grained units being laminated and gradational along strike to interbedded BIF facies. In general the sequence fines upwards, with an increasing dominance of finer grained thinly-bedded volcaniclastic rocks and BIF units within the upper part of the sequence, and a relative reduction in coarser grained sedimentary rocks. BIF units that are not repeated by folding are typically 2 to 10 m thick and consist of interbedded cherty and magnetitehematite mesobands, although rare jasperoidal and more massive magnetite beds are developed. The BIF units commonly grade into magnetite-rich tuffs. The intermediate volcaniclastic rocks are generally grouped together as a package, despite local variations in texture, and are dominated by lithic and crystal tuffs. Distinctive marker units, which can be correlated from section to section, include a polymictic agglomerate consisting of chloritic bombs in a matrix of felsic lapilli and a locally derived sedimentary rock consisting of angular clasts of jasper and magnetite.
FIG 2 - Aeromagnetic interpretation of the Sunrise Dam area, modified from D Gibbs (unpublished data, 1995).
The rocks within the deposit include interbedded BIF, volcaniclastic rocks and acid to intermediate volcanic rocks, which have been intruded by a mafic unit and quartz-feldspar porphyries (Fig 3a).
Transported overburden The transported sedimentary profile can be broadly divided into: 1.
dune sands: the uppermost stratigraphic unit, which are generally less than 3 m thick, and are associated with the eastern margin of Lake Carey; and
2.
lateritic sediments: range from 20 to 60 m thick, and consist of green-grey to white clay with interbedded lateritic pisolitic gravel lenses, transported saprolite blocks and other residual materials. Several ferruginous horizons are developed within the profile and are characterised by the development of hematitic and
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A 20 to 40 m thick intrusive of mafic composition post-dates the volcano-sedimentary package on the western side of the Cleo deposit. The unit is altered to a distinctive soft chloritecarbonate rock that strikes approximately north and dips steeply west. Quartz-feldspar porphyries intrude the sequence at both deposits and post-date the mafic intrusive at Cleo. The porphyries are typically pink to pale grey and coarse grained, with rounded, strongly resorbed quartz and plagioclase phenocrysts to 10 mm in diameter. The porphyries vary in thickness from 10 cm to several metres and occur as flat to steeply-dipping, north striking sets. Intrusive contacts of the porphyries are commonly strongly deformed and altered.
STRUCTURE Two main phases of structures are recognised at Sunrise-Cleo. D1 structures include the Sunrise shear, which dips shallowly towards the west to NW and has a thickness of 10 to 20 m, although the associated carbonate-sericite alteration halo is commonly 30 to 40 m thick (Fig 3a). Smaller-scale shear zones above the Sunrise shear are both parallel and discordant to the shallowly-dipping succession. Early recumbent (F1) folds have axial planar surfaces that parallel the Sunrise shear. A penetrative L-S fabric (S1) is generally parallel to this structure and has a stretching lineation that plunges shallowly towards
Geology of Australian and Papua New Guinean Mineral Deposits
SUNRISE–CLEO GOLD DEPOSIT
FIG 3 - Composite east-west cross section on 6 782 825 N through the Sunrise-Cleo deposit showing: (a) the major rock types and structures; and (b) the distribution of mineralisation above a 0.3 g/t cutoff.
NNW. In cross section, the geometry of D1 structures is similar to a thrust duplex or imbricate stack, which may reflect the D1 regional-scale tectonic event. Southeast-directed reverse movement on the Sunrise shear is inferred from kinematic indicators, and is consistent with the geometry of the recumbent folds. However the penetrative stretching lineation plunges shallowly to the north parallel to the fold axes, which implies a significant component of extension oblique to the direction of tectonic transport. This can be interpreted as a result of: (i) deformation involving limited thrust movement on the shear, together with north–south extension and limited vertical extension; or (ii) reactivation on the shear planes with a north–south component of displacement. Second phase (D2) structures are upright to inclined folds with north-striking axial planar surfaces and gently northplunging axes. A spaced cleavage (S2) is associated with these upright folds and locally crenulates the shallowly-dipping S1 penetrative fabric. Quartz veins are abundant through the deposit. Most, regardless of host rock, are 0.5–2 cm thick and less than 1 m in length. The nature of the veins is strongly controlled by the rheological properties of the host rock. In BIF and thick unaltered volcaniclastic sequences, many shear veins form parallel to bedding, although there are a significant number of oblique, extension veins sub-perpendicular to bedding. The majority of veins in BIF are not folded and there are several examples of veins which cut across a fold profile. In comparison, strongly foliated and sericite-carbonate altered volcanic units have many sigmoidal, rootless and folded veins
Geology of Australian and Papua New Guinean Mineral Deposits
characteristic of ductile deformation. Most veins are parallel to the penetrative foliation and vein fragments are elongated parallel to the stretching lineation.
GOLD MINERALISATION Gold mineralisation in the area occurs intermittently along a 4.5 km long NE-trending corridor, coincident with the strongly magnetic BIF sequence that extends from Cleo in the SW through the Sunrise and Black Magic prospects to Pink Lady in the NE (Fig 2). At Sunrise, a significant proportion of the resource consists of secondary mineralisation developed within the transported cover (Figs 3b and 5). Supergene gold mineralisation occurs within horizontal blankets related to iron redox fronts and associated water tables (L M Lawrance, unpublished data, 1994). At least two distinct horizons are noted, each 2 to 12 m thick. The upper horizon occurs 5 to 15 m from the surface and the lower at 20 to 40 m depth. The plan dimensions of this supergene mineralisation are approximately 600 m NNE by 200 m width (Fig 5). It occurs in distinct northern and southern zones in plan, narrowing towards the Cleo boundary. This style of mineralisation is also recognised at Cleo, but it is subeconomic there. Below the unconformity, mineralisation is developed in both oxidised and fresh bedrock, which has a shallow westerly dip (Fig 3b). In plan view, the dimensions of bedrock mineralisation are approximately 1.6 km NE by 0.7 km width (Fig 5).
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There are three styles of primary gold mineralisation at Sunrise-Cleo, although all three are contemporaneous and the differences are largely the result of host rock type:
quartz textures and commonly has ptygmatic fold forms (Figs 4g, h). The alteration zones around these veins are notably depleted in silica and rich in pyrite.
1.
At Sunrise, the steeply-dipping high grade gold-bearing veins are located in a 20 to 40 m wide NE-trending corridor in volcaniclastic rocks within the hanging wall of the Sunrise shear (Fig 5). This corridor can been traced for at least 300 m along strike.
Pyrite replacement of BIF accounts for the majority of gold mineralisation and is well developed where bedding-parallel shear zones follow the contact of BIF with less competent units. At Sunrise high gold grades correlate with areas of structural thickening due to thrustrelated folding. A correlation with folding is recognised at Cleo, although the bulk of the BIF-hosted mineralisation is located adjacent to bedding parallel and subparallel shear zones located along the footwalls of subhorizontal BIF units. The internal texture of mineralised BIF varies between two end members: (i) with dominantly bedding-parallel veins and minor discordant fracture-fill veins cutting preserved sedimentary fabrics (Fig 4b); and (ii) chaotic discordant quartz-siderite-pyrite vein networks/arrays, small scale faulting and brecciation (Fig 4c). High gold grades and zones of ‘pyrite flooding’ are associated with both end members. Pyrite is the dominant sulphide and occurs as aggregates of granular, subhedral to euhedral grains which commonly replace primary magnetite. Rare anhedral blebs of pyrrhotite and chalcopyrite occur as inclusions within pyrite. Associated with pyrite replacement textures are interlocking mosaics of siderite and quartz, which dominate the gangue mineralogy. In fresh rock, free gold is common in BIF and located at pyrite grain margins, while in supergene ore abundant remobilised free gold occurs along discordant fractures.
2.
Sunrise shear mineralisation occurs within the basal shear zone and subsidiary shears, and overall dips shallowly west to NW, broadly parallel to bedding. Although there is reasonable grade continuity parallel to the north-plunging stretching lineation, grade distribution is erratic, and is associated with quartzankerite±pyrite veining and pervasive ankerite, silica, sericite and pyrite alteration of dominantly intermediate volcaniclastic host rocks (Figs 4d, e). Most veins within the shear zones are dismembered and rotated by discrete shear zones, indicating that the shear zones were active during and after mineralisation. In addition, sulphides are deformed and show well developed pressure shadows. Gold is located within the veins and associated with the discrete shear zones, and at microscopic scale occurs as inclusions within pyrite grains and adjacent to grain margins.
3.
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Thin quartz-carbonate vein mineralisation occurs at Sunrise and Cleo, although it is more common at Sunrise. The veins are generally less than 2 cm thick and are characterised by their steep, variable dip and high (in places extremely high) gold grade. The composition and internal vein texture of the veins are variable, although they are grouped together due to unresolved timing relationships. Breccia veins of ankerite, quartz and pyrite have randomly oriented wall rock inclusions, and commonly have sigmoidal forms, indicating, in different cases, reverse or normal movement on bedding planes. Laminated veins consist of quartz and ankerite, with chlorite dominant in the laminations, and show evidence of multiple veining episodes (Fig 4f). A third vein type, characteristically rich in free gold, has chalcedonic
ORE GENESIS ORE IN TRANSPORTED OVERBURDEN Work by L M Lawrance (unpublished data, 1994) suggests that the supergene gold distribution within the transported overburden is related to local physical transport of mineralised material during channel infill. Local high grades are influenced by topography and occur at the base of a palaeochannel. Some of the gold has been chemically reworked above an iron redox front, with changing profile hydrology during the development of the nearby saline lake system west of the deposit. The preferential development of the supergene mineralisation in the transported material at Sunrise compared with Cleo is probably related to the intersection of the Sunrise shear with the unconformity east of Sunrise (Fig 3a) and the presence of the palaeochannel which is discontinuous into Cleo.
ORE IN ARCHAEAN ROCKS The Sunrise shear is the most important structure in the deposit, as it controls the geometry of the mineralisation and was the main conduit for the gold-bearing hydrothermal fluids. Areas of high gold grade, both within the Sunrise shear and in the hanging wall rocks, are related to a flexure in dip and change in strike direction of the shear. In this model, reactivation of the shear during a later deformation (D3?) created a local extensional environment, and gold-bearing fluids were focussed into this dilational area (V J Ojala, unpublished data, 1994). Both the structural geometry and the model for mineralisation at Sunrise-Cleo show several important similarities with the thrust fault controlled gold deposits within the Hyde-Macraes Shear Zone, New Zealand (Teagle, Norris and Craw, 1990). A dilational area is created above the area of shallow dip, and discordant veins can form as both fault veins and extensional veins to accommodate movement on the basal thrust. Mineralised veins postdate the formation of D1 and D2 folds, consistent with a late structural timing of mineralisation. However, extensive vein deformation and pressure shadow development around sulphides suggests that mineralisation was synchronous with a deformation that had a component of north–south extension. The location of thin, relatively competent units (eg BIF and porphyry) adjacent to less competent units such as volcaniclastic and sedimentary rocks controls the location of the shear zones, as shear strain is taken up along the contact of the competent units. Fluid movement along these layer-parallel shear zones is reflected by bedding parallel veins in BIF and zones of pyrite flooding. The correlation of high gold grade with folding reflects pre- to syn-mineralisation structural thickening. The folded BIF units were obliquely oriented in relation to a horizontal stress field, allowing the build up of high fluid pressures, followed by hydraulic fracturing and fluid flow.
Geology of Australian and Papua New Guinean Mineral Deposits
SUNRISE–CLEO GOLD DEPOSIT
FIG 4 - Photographs of typical textures and geological relationships at Sunrise-Cleo. All holes drilled at 60ο dip towards 090ο, showing the northern half of the core with the down hole direction towards the right. a. Southwest-facing photograph of the Sunrise pit in April 1996. Note the irregular unconformity at the base of the cover sequence and the shallow-dipping Archaean rocks. b. Complete pyrite replacement of BIF with preserved sedimentary textures. c. Complexly brecciated and pyritised BIF. d. Typical ore textures in the Sunrise shear and subsidiary shears. e. Textures developed in lower strain areas of the Sunrise shear. f. Laminated quartz-carbonate-chlorite vein parallel to core axis. g. Ptygmatically folded quartz-ankerite-chlorite vein in lapilli tuff. h. Discordant, chalcedonic quartz-carbonate vein in BIF with well-developed pyrite-carbonate alteration halo.
Geology of Australian and Papua New Guinean Mineral Deposits
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ACKNOWLEDGEMENTS Permission to publish by Acacia Resources Limited, Delta Gold NL and Placer Pacific Limited is gratefully acknowledged. The authors wish specially to note the contribution of the following people who have helped to improve the quality of this manuscript: A Ross, W Allardyce, H Dorsett-Bain, G Fahey, D Groves, J Holmes, S Jackson, L Jepsen, K Kenny, D Keough, J King, A McKay, V J Ojala, J Ridley and J Standing. M Barker is thanked for his efforts in producing the Figures. P Newton's research is supported by the Key Centre for Strategic Mineral Deposits and an Australian Postgraduate Research Award, with further logistical and financial support from the GSJV. FIG 5 - Surface plan showing the distribution of gold mineralisation in the overburden and bedrock at the Sunrise-Cleo deposit.
MINE GEOLOGICAL METHODS Geological control at Sunrise is based on logging of blast hole cuttings combined with pit mapping. The resultant interpretation is used in conjunction with blast hole assay grade distribution to arrive at the final ore block design. In the transported material, pisolite content exhibits a strong correlation with grade and allows efficient visual grade control during mining. Within the underlying Archaean sequence, mapping of BIF units is used to designate ore boundaries. As not all of the BIF units are mineralised, methods are being sought to delineate the mineralised units so that selective mining procedures can be applied. Ore blocks are exposed and mined from the hanging wall (west) in an effort to minimise dilution. The high grade zones are mined on day shift only, and a geotechnician is present where visual control is possible. The highest grade ore to date has come from the area immediately above the unconformity in pisolitic gravels. Early grade control drilling involved variable depth (12 to 20 m) angled aircore holes on a 12.5 by 8 m pattern, with 1 m sample intervals. In areas of consolidated pisolite and saprock, 2 m composite samples are taken from 41/2 inch (112 mm) vertical aircore blast holes drilled on a 6 by 5 m pattern. This technique has been used for grade control from 12 m below the surface. The difficulty in collecting an uncontaminated sample of the transported clays led to various experiments with sample collection methods. A two tier splitter producing a 75:25 split from a 2 m composite sample was shown to give the best result.
REFERENCES Gellatly, D C, Ion, J C, Ryall, A W, Thomson, R M and Holmes, J S, 1995. The Sunrise gold deposit: A greenfields discovery, in New Generation Gold Mines: Case Histories of Discovery, pp 10.1–10.9 (Australian Mineral Foundation: Adelaide). Gower, C F, 1976. Laverton, Western Australia - 1:250 000 geological series, Geological Survey Western Australia Explanatory Notes SH 51-2. Hall, G C and Holyland, P W, 1990. Granny Smith gold deposit, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 519–524 (The Australasian Institute of Mining and Metallurgy: Melbourne). Hallberg, J A, 1985. Geology and Mineral Deposits of The LeonoraLaverton Area, Northeastern Yilgarn Block, Western Australia (Hesperian Press: Perth). Hronsky, J M A, 1993. The role of physical and chemical processes in the formation of gold ore-shoots at the Lancefield gold deposit, Western Australia, PhD thesis (unpublished), The University of Western Australia, Perth. Keserue-Ponte, F, 1995. Lithogeochemical characteristics of the transported regolith at the Cleo prospect, Sunrise Dam, Laverton, Western Australia, BAppSc Honours thesis (unpublished), Curtin University, Perth. Ojala, V J, 1995. Structural and depositional controls on gold mineralisation at the Granny Smith Mine, Laverton, Western Australia, PhD thesis (unpublished), The University of Western Australia, Perth. Teagle, D A H, Norris, R J and Craw, D, 1990. Structural controls on gold bearing quartz mineralisation in a duplex thrust system, Hyde-Macraes Shear Zone, Otago Schist, New Zealand, Economic Geology, 85:1711–1719. Thompson, M J, Watchorn, R B, Bonwick, C M, Frewin, M O, Goodgame, Y R, Pyle, M J and MacGeehan, P J, 1990. Gold deposits of Hill 50 gold mine NL at Mount Magnet, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 221–241 (The Australasian Institute of Mining and Metallurgy: Melbourne).
At Cleo, Acacia began prestrip mining in October 1996, and is yet to finalise grade control and general mining methods. It is likely to involve RC grade control drilling with indicatorkriging interpolation of grade.
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Joyce, R M, Woodhouse, W K and Young, C H, 1998. Lights of Israel gold deposit, Davyhurst, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 187–190 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Lights of Israel gold deposit, Davyhurst by R M Joyce1, W K Woodhouse2 and C H Young3 INTRODUCTION The deposit is at Davyhurst, 110 km NW of Kalgoorlie and 55 km SW of Menzies, WA. It is on the Kalgoorlie (SH 51–9) 1:250 000 scale and the Davyhurst (3037) 1:100 000 scale map sheets (Fig 1) at lat 30o02′S, long 120ο38′Ε οr AMG coordinates 274 240 E, 6 675 865 N.
Two phases of open pit mining (1988–1989 and 1993–1994) by the Bardoc-Davyhurst joint venture (Aberfoyle Limited 57%, Aurora Gold Ltd 43%) produced 1.65 Mt of ore at 2.79 g/t gold, which was treated at the Bardoc mill 80 km to the SE. Following the recognition of a significant shallowly plunging extension to the deposit, decline access for underground mining commenced in late 1994 to provide access to an Indicated and Inferred Resource of 2.81 Mt at 4–5 g/t gold, with additional Inferred Resources of approximately 1 Mt at 4–5 g/t. In October 1995, Aberfoyle increased its interest to 100%, and production from underground mining at a rate of approximately 400 000 tpa commenced in 1996. Consolidated Gold NL (Aberfoyle approximately 19.8%) purchased the mine in late 1996.
EXPLORATION AND MINING HISTORY Recorded historical production from Lights of Israel was 34 kg gold from 3846 t of ore between 1906 and 1913 (Wyche and Witt, 1994). Hellsten et al (1990) detailed the recent exploration history of the Davyhurst area including the recommencement of mining activity by Western Mining Corporation in 1986 at the nearby Waihi and Golden Eagle deposits. They also describe the exploration and development of resources for open pit mining at the Lights of Israel deposit and associated Makai deposit by Aberfoyle and Hill Minerals (later Aurora Gold), and resource drilling at Great Ophir by Billiton Australia and Jones Mining (Figs 1 and 2) which had a
FIG 1 - Regional geological map of the Davyhurst area (after Wyche and Witt, 1994).
1.
Exploration Manager Gold, Aberfoyle Limited, 1 Altona Street, West Perth WA 6005.
2.
Geologist, Aberfoyle Limited, 1 Altona Street, West Perth WA 6005.
3.
Exploration Manager Project Development, Aberfoyle Limited, Level 31, 525 Collins Street, Melbourne Vic 3000.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 2 - Geological plan of the Davyhurst gold deposits.
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pre-mining ore reserve of 552 000 t at 2.67 g/t gold. The Great Ophir deposit was mined between late 1988 and early 1991 and produced 685 065 t at 2.83 g/t gold. In late 1993, treatment of Great Ophir low grade stockpiles commenced at Bardoc. A total of 237 121 t at 1.42 g/t gold was milled. The original Lights of Israel pit was developed on the near surface parts of the Lights of Israel and the Makai shoots (Fig 3). Detailed observation of ore geometry, foliation and structure resulted in the recognition of potential for shallowly north plunging shoots beyond the limits of the pit. Between 1993 and mid 1994 drilling of 182 reverse circulation and/or diamond holes for 43 000 m traced the Lights of Israel shoot for a distance of over 1500 m down plunge from the limit of open pit mining (Fig 3). At the limit of drilling information, mineralisation is at approximately 650 m below surface. The Makai shoot was defined for about 300 m down plunge but grade continuity was low.
defined by Wyche and Witt (1994) and Swager et al (1990). At surface, this terrane boundary is correlated with the Ida Fault, a crustal scale, east dipping, reverse fault system recognised in deep seismic imaging (Goleby et al, 1993). The Davyhurst deposits occur within a subdivision of the Kalgoorlie Terrane, the Coolgardie Domain, which is bounded by the Zuleika Shear to the east and the Ida Fault to the west. In the environs of Davyhurst the major Ida Fault, Zuleika Shear, Kunanalling Shear and Bullabulling Shear converge where the greenstone belt is about 10 km wide. The Goldfields seismic section shows the Ida Fault as the major, east dipping structure which truncates the other largely west dipping faults. A zone of intensely deformed amphibolite facies metamorphic rocks occurs along the eastern margin of the greenstone sequence. These rocks are the ‘Eastern Sequence’ of Hellsten et al (1990), within which the Lights of Israel, Makai, Great Ophir and Golden Eagle gold deposits occur.
ORE DEPOSIT FEATURES LITHOLOGY AND ALTERATION The Lights of Israel deposit occurs in a shallowly dipping biotite-rich schist zone within a sequence of sheared mafic amphibolites. Primary volcanic textures such as pillow lava and breccia units suggest that the amphibolites, now composed of hornblende and plagioclase, are metamorphosed tholeiitic basalts. All mineralised rock types are interpreted as alteration products of precursor amphibolite or feldspar porphyry intrusives. The mineralised shear zone at Lights of Israel is up to 30 m thick and contains a higher grade zone of alteration, termed quartz-feldspar lode (QFL), usually towards the base of the biotite-rich schist, which comprises the bulk of the alteration zone. The QFL consists of a plagioclase-quartz-calcite-biotiteactinolite schist with up to 10% pyrite and pyrrhotite. QFL is generally associated with high gold grades, and has gradational contacts with the surrounding biotite schist (Fig 4).
FIG 3 - Plan of contoured gram.metres, Lights of Israel deposit.
PREVIOUS DESCRIPTIONS Gibson (1904) described the early mining history of the Davyhurst district. Hellsten et al (1990) described the results of more recent exploration and mining activity at several deposits at Davyhurst, including Lights of Israel. More recently Wyche and Witt (1994) documented results of regional geological mapping of the Davyhurst area.
REGIONAL GEOLOGY
FIG 4 - Cross section A-A′ on 10 375N, Lights of Israel deposit, looking north.
Davyhurst is located in a narrow, approximately 10 km wide, belt of Archaean greenstones between two extensive granite or granite gneiss domains (Fig 1). The greenstone belt includes the major terrane boundary between the Eastern Goldfields (Kalgoorlie Terrane) and Southern Cross (Barlee Terrane) as
The biotite-rich schist is an intensely foliated and lineated rock composed of fine laminae of biotite-plagioclaseactinolite-calcite-quartz-pyrite or pyrrhotite with zones of tourmaline.
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shear zone. The contorted banding and folding are most intense immediately adjacent to the high grade ‘keel’ zone (Fig 4).
MINERALISATION
FIG 5 - Longtidudinal projection B-B′ through the Lights of Israel ore shoot.
The biotite schist alteration zone at Lights of Israel forms an irregular tabular body which dips at approximately 30o towards 280ο. The alteration assemblage (biotite, plagioclase and calcite) is typical of amphibolite metamorphic grade Archaean gold deposits and major element values indicate introduction of carbon dioxide, silica and potassium, as documented in many other Archaean deposits (Mikucki, 1996). A series of feldspar porphyry bodies occurs within the mineralised schist package and the surrounding amphibolite. Three major porphyry horizons are known, two in the hanging wall of the mineralised shear zone and one within and along the footwall (Fig 5). A hole drilled several hundred metres into the footwall of the mineralised shear zone intersected two additional thin porphyries. Porphyry within the mineralisation is foliated, biotite-altered, and commonly tightly folded, and has a close spatial relationship with zones of higher grade gold mineralisation. The contact between porphyry and the QFL type alteration is gradational in some places and sharp in others. The mineralised shear zone was possibly initiated along a porphyry-amphibolite contact, with progressive deformation and introduction of fluid leading to pervasive alteration of the shear zone, the intrusion of further porphyry dykes into dilatant zones, and gold mineralisation. The porphyry may have contributed potassium through breakdown of potassium feldspar phenocrysts (P R Williams, unpublished data, 1994).
STRUCTURE The mineralised zone is locally discordant with regional foliation trends in the surrounding amphibolites, which tend to dip more steeply, at 50ο towards 255o (J R McIntyre, unpublished data, 1993). Intense deformation has produced intrafolial folds and rootless folded quartz-feldspar veins. Late stage faults offset the biotite schist envelope. These structures generally have limited strike slip movement with a dip slip component of up to 20 m, generally north block down. The biotite-rich mineralised rocks and the surrounding amphibolites and porphyries are characterised by strong to intense foliation, and by a mineral elongation lineation which plunges at approximately 20ο towards 340o. Although essentially tabular, the biotite schist zone is characterised by flexures and zones of intensely contorted banding, and the porphyries, particularly the footwall porphyry, are tightly folded with axes parallel to the main foliation in the mineralised
Geology of Australian and Papua New Guinean Mineral Deposits
A zone of higher grade gold values within the Lights of Israel deposit (the Lights of Israel shoot) has an elongate form, traceable by drilling for over 1500 m down plunge (Fig 5). It is up to 25 m thick in the keel (average 8 to 10 m) and over 100 m wide, and plunges parallel to the stretching lineation. Gold grades correlate well with intensity and thickness of alteration. The shoot is asymmetric, being thickest adjacent to the keel, and thinning gradually to the east, up dip, and abruptly westwards towards Great Ophir (Fig 4). The highest grades occur in very sulphidic QFL, generally close to the base of the biotite schist package. The northernmost holes show a slight change in the alteration pattern, with a progressively greater proportion of unaltered porphyry in the keel. Petrographic examination reveals two dominant habits of pyrite and pyrrhotite, as thin irregular aggregates on schistose laminae and as coarser euhedra. Chalcopyrite and sphalerite are found in trace amounts. Fine, discrete gold grains occur most commonly on silicate or carbonate grain boundaries, with minor amounts on sulphide grain boundaries. There is virtually no petrographically visible gold within the sulphide grains. Gold grains vary from <1 to 480 µm in diameter and are mostly <20 µm (K G McQueen, unpublished data, 1993). Textural relationships between gold and associated minerals suggest that most of the gold should be free milling and this is evidenced by recoveries above 90%. The lack of retrograde minerals, the plunge of the Lights of Israel shoot parallel to the stretching lineation in the host rocks, and the association of alteration minerals with this lineation invite comparisons with the Sons of Gwalia and Harbour Lights deposits at Leonora which are also in amphibolite grade rocks. At these deposits mineralisation is interpreted as being synchronous with ductile shearing and peak metamorphism (Vearncombe, 1992; Williams and Whitaker, 1993).
MINE GEOLOGICAL METHODS Underground mining of the shallowly dipping, gently plunging shoot is carried out via decline access with level development, predominantly footwall drives, every 15 m vertically. Visual geological control is used for the placement of development headings. Stoping is carried out using up hole retreat methods, with stope design based on a Datamine block model, grade contoured sections and detailed lithological mapping. Grade control utilises rock chip sampling and blast hole sludge sampling to infill the relatively coarse resource drilling pattern, on an approximate 50 by 25 m grid.
ACKNOWLEDGEMENTS The authors wish to thank Aberfoyle Limited for permission to publish this paper. Significant contributions to the understanding of the geology and economic potential of the Lights of Israel deposit were made by previous mine geologists C Robinson and G Howard, and by Aurora Gold personnel, particularly R Williams. Contributions from others in the exploration team responsible for resource and exploration drilling, and from previous mine geologist C Allison, are also acknowledged.
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REFERENCES Gibson, C G, 1904. The geological features and mineral resources of Mulline, Ularring, Mulwarrie and Davyhurst, North Coolgardie Goldfield, Geological Survey of Western Australia Bulletin 12. Goleby, B R, Drummond, B J, Swager, C P, Williams, P R and Rattenbury, M S, 1993. Constraints from seismic data on the regional and district scale structure of the Eastern Goldfields Province, in An International Conference on Crustal Evolution, Metallogeny and Exploration of the Eastern Goldfields, Extended Abstracts 1993, pp 85–90, AGSO Record 1993/54. Hellsten, K J, Colville, R G, Crase, N J and Bottomer, L R, 1990. Davyhurst gold deposits, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 367–371 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Swager, C P, Griffin, T J, Witt, W K, Wyche, S, Ahmat, A L, Hunter, W M and McGoldrick, P J, 1990. Geology of the Archaean Kalgoorlie Terrane - an explanatory note, Geological Survey of Western Australia Record 1990/12. Vearncombe, J R, 1992. Archaean gold mineralisation in a normalmotion shear zone at Harbour Lights, Leonora, Western Australia, Mineralium Deposita, 27:182–191. Williams, P R and Whitaker, A J, 1993. Gneiss domes and extensional deformation in the highly mineralised Archaean Eastern Goldfields Province, Western Australia, Ore Geology Reviews, 8:141–162. Wyche, S and Witt, W K, 1994. Davyhurst, Western Australia - 1 : 100 000 geological series, Geological Survey of Western Australia Explanatory Notes 3037.
Mikucki, E J, 1996. Hydrothermal alteration in Archaean lode gold deposits, in Mesothermal Gold Deposits: A Global Overview, Publication 27, pp 42–46 and 72–74 (The Geology Department and University Extension, The University of Western Australia: Perth).
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McIntyre, J R and Czerw, A, 1998. Mount Dimer gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 189–194 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Mount Dimer gold deposits 1
by J R McIntyre and A Czerw
2
INTRODUCTION The deposits are 100 km NE of Southern Cross, WA, in the Yilgarn mineral field, at lat 30o23′S, long 119o50′E, on the Jackson (SH 50–12) 1:250 000 scale map sheet (Fig 1). The mine is owned by Tectonic Resources NL and is significant because it is the largest in current and historical terms of only three modern gold mining operations in the extensive Marda greenstone belt. During the year to 30 June 1996, 33 136 oz of gold and 50 946 oz of silver were produced from 175 702 t of ore. This brings the total production since inception to 82 362 oz of gold and 115 300 oz of silver. At June 30, 1996 remaining Proved and Probable Ore Reserves totalled 165 280 t at 7.1 g/t gold, for a total production plus remaining resources of 120 000 oz of contained gold.
EXPLORATION HISTORY There are no records of exploration or production prior to exploration for gold by Western Mining Corporation Ltd (WMC) in the late 1980s. Extensive soil sampling and follow up drilling led to the discovery of lateritic and underlying quartz-vein hosted gold mineralisation. After mining and heap leaching the lateritic ore, WMC sold the property to Glengold Holdings Pty Ltd, who in 1992 delineated open pit and underground resources for the vein systems. Glengold sold the property to Tectonic Resources NL in 1994, and mining commenced in March of that year. The mine is managed on behalf of Tectonic by National Mine Management Pty Ltd. Subsequent exploration by National Mine Management and Continental Resource Management on behalf of Tectonic Resources led to the development and mining of several satellite lode and laterite deposits, the most significant being the Golden Slipper.
REGIONAL GEOLOGY The Mount Dimer deposits lie at the eastern end of the Archaean Marda greenstone belt (Fig 1). This belt comprises a succession of ultramafic and mafic volcanic and chemical sedimentary rocks (‘the lower greenstone cycle’). These rocks are unconformably overlain by a sequence of clastic sediments, felsic volcanics and possibly comagmatic granitoid, including the Marda Complex of Hallberg, Johnston and Bye (1976), and are grouped here in the informal ‘upper greenstone cycle’. The greenstone sequence is bounded to the south by granite, with the boundary marked by a structurally complex zone
1.
2.
Formerly Exploration Manager, Tectonic Resources NL, now Exploration Manager Cove Mining NL, 24 Kings Park Road, West Perth WA 6872. Formerly Mine Geologist, National Mine Management, now Emperor Mines Ltd, 50 Margaret Street, Sydney NSW 2000.
Geology of Australian and Papua New Guinean Mineral Deposits
Fig 1 - Regional geological map of the Marda greenstone belt.
informally named the ‘Dimer lag’. To the east and west the sequence is bound by the Yendilberin and the Koolyanobbing shear zones respectively. The Dimer lag is steeply north dipping and is folded into broad antiformal and synformal structures, with steep stretching lineations and north block down sense of movement, ie normal movement. The structure truncates layering in the overlying greenstone pile. Four generations of structure are recognised by the authors, listed in order of decreasing age: 1.
early layer parallel structures along the base of the greenstone, eg the Dimer lag, which truncate layering;
2.
layer parallel structures within the lower greenstone cycle which folded and stacked the sequence, and are visible in the multiple banded iron formation (BIF) horizons;
3.
NW- to NNW-trending structures which folded and truncated the earlier faults, such as the Koolyanobbing and Yendilberin shear zones, and probably the Aurora fault; and
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4.
minor discrete late faults which offset the earlier structures with small displacement.
Two suites of granite are recognised on the basis of their character as interpreted from regional aeromagnetic images. The earlier suite comprises granite and granitic gneiss with a prominent tectonic grain, which is intruded by a later granite suite, comprising massive, generally lobate granite bodies.
been mined as a ‘laterite’ resource, although it forms the eastern edge of a colluvial blanket. Radiometrically anomalous potassic clays are developed on the surface of some of these palaeochannels. 4.
Recent lag gravels are developed in residual areas and loamy soils are developed in depositional areas, the latter with strongly developed pedogenic carbonate horizons at or near surface.
ORE DEPOSIT FEATURES LITHOLOGY AND STRUCTURE REGOLITH Although the landscape is overall one of low relief and currently a residual to weakly depositional environment, a complex regolith has been developed over the Archaean rocks. Several discrete regolith units are recognised, listed here in order of probable decreasing age: 1.
‘Billy’ is a colluvial unit, comprising poorly sorted rounded clasts of vein quartz in a quartz sand matrix which is locally siliceous and ferruginous. This unit is generally 1 to 2 m thick and lateritised in places.
2.
A well developed lateritic profile, comprising pisolitic to nodular ferruginous duricrust over mottled leached rock and saprolite, is locally well developed, especially over the Lφ1, Lφ2 and Lφ3 lodes. The duricrust over these and other lodes, eg Anomaly Two, is well mineralised.
3.
Colluvial to alluvial fill in palaeochannels, ranging to 50 m thick over the ultramafic units, mantles much of the surface of the mine leases away from the main lodes. The palaeochannel fill comprises coarse grained polymictic conglomerate interbedded with clay rich and minor clean quartz sand layers, which appear to cut into the laterite profile, by a process involving some erosion and redeposition of lateritic material. For example, a colluvial blanket of gold bearing material is developed on the west flank of the Golden Slipper open pit, and has
A geological interpretation of the bedrock geology of the mine leases is presented in Fig 2, compiled from drill hole information, mapping in open pits and underground workings and interpretation of geophysical data. A layered greenstone sequence of ultramafic and mafic rocks, including pillowed basalt, dolerite and minor high magnesium rock, is intruded by abundant granite sills and dykes. The granite has intruded mafic rocks above and below the ultramafic unit and forms approximately 60% of the areal rock mass. The mafic rocks form abundant xenoliths in the granite, with a broadly east trending orientation, but the geometry of the mafic xenoliths is not well constrained. The ultramafic body has remained essentially intact and forms a coherent tabular body (or laterally extensive screen) in the granite. The southern edge of the ultramafic unit is now taken as the main granite-greenstone contact, and marks the trace of the Dimer lag mapped in Fig 1. To the SW of the mine leases mapping and drill hole traverses indicate that the ultramafic body is moderately to steeply NW to NNW dipping, although underground exposures of the lodes indicate that the ultramafic contact is steeply south dipping. Several generations of granite are recognised, including irregular bodies with a moderate pervasive foliation intruded by sills and dykes of massive granite. Pegmatitic phases are developed locally.
Fig 2 - Geological plan of the Mount Dimer lease area.
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The granite and mafic rocks are typically massive, with narrow shear zones developed in the granites or along the margins of the xenoliths. These shear zones dip steeply to the east or west (the lode structures) or moderately to the north (layer-parallel shear zones). Where mapped in the Golden Slipper pit, the layer-parallel structures dip 45o north and contain a strong stretching lineation plunging 45o north. No consistent sense of overprinting has been recognised and the layer-parallel and lode structures truncate one another. A pervasive layer-parallel foliation is developed in the ultramafic body, dipping steeply to the south in the vicinity of the main lodes, as opposed to north to NW dipping on a regional scale.
Lφ1, Lφ2 AND Lφ3 LODES Structure Four principal lode gold deposits and several minor laterite deposits have been mined within the lease area, and several lode structures are currently being evaluated (Fig 2). The bulk of production to date has been from the Lφ1, Lφ2 and Lφ3 lode structures. These three lode structures comprise steeply east dipping, 340o striking narrow (0.5–1.0 m wide where unmineralised) but intense shear zones in granite and mafic rock, spaced approximately 110 m apart. A fourth lode structure, Lφ4, has recently been located approximately 110 m to the east of the Lφ1 lode. The Lφ1, Lφ2 and Lφ3 lodes outcrop, with Proved Ore Reserves established to the 10 level (180 m below surface) and Probable Ore Reserves established to the 13 level in the Lφ1 and Lφ2 lodes (220 m below surface). The geology of the Lφ1 and Lφ2 lodes on the 7 level is shown on Fig 3. The lode structures have a strike length of approximately 250 m and are terminated to the north by the steeply south-dipping ultramafic unit. The South bounding fault (SBF) terminates the lodes to the south (Fig 4). The lode structures swing into the SBF with a sinistral sense of movement and both weakly displace the ultramafic contact (with a dextral sense of movement) and are offset in a sinistral sense along it. Multiple lodes, although of low grade, are developed on Lφ1 at its intersection with the SBF, and subparallel to it. The lode structures contain a strong stretching lineation plunging 46 to 343o, with S–C fabrics in lower strain zones indicating a reverse to right lateral sense of movement. The lineations are consistent in plunge along strike between the bounding structures and up and down dip between the 7 and 13 levels. In detail the lode structures contain splays and bifurcations, and change in dip and strike along the lode. Locally the lodes ‘roll over’ in section, producing steeply west dipping structures. A series of moderately NE dipping, 310o trending structures, the Boobook lodes, are developed at irregular intervals on the Lφ1 and Lφ2 lodes. The intersections of the Boobook structure and the principal lodes plunge steeply north, generally steeper than the stretching lineation. Ore grade mineralisation is developed in the lode close to this intersection line. In the upper levels on the Lφ2 structure two principal lodes are developed, the Lφ2 structure North (above the 7 level), and the Lφ2 North A (below the 7 level). The intersection of the two lodes is more steeply north plunging than the stretching
Geology of Australian and Papua New Guinean Mineral Deposits
Fig 3 - Detailed geological plan of the Lφ1 and Lφ2 lodes, 7 level.
lineation, and higher grades and widths of mineralisation are developed in both lodes adjacent to the intersection. A series of 45o west-dipping shear zones and laminated quartz-sulphide veins were recorded during mine development, and although mineralised (to 90 g/t gold in rock chip samples) no development has yet been undertaken.
Mineralisation Mineable mineralisation is contained in elongate shoots of strike length from 5 to 50 m and width from 0.5 to 6.5 m, which generally plunge at 45o to the north (Fig 4). The shoots steepen in plunge in the lower levels and locally reverse in plunge against the ultramafic contact. Although the overall plunge direction of the mineralised shoots is parallel to the stretching lineation, on the Lφ2 lode the plunge is steeply north, parallel to the intersection of the North and North A lode structures. The gold mineralised shoots are associated with the development of foliation-parallel laminated quartz-sulphide veins and locally veins of massive sulphide, and screens of altered wall rock within the otherwise narrow lode structure. Within the lode structure one or more laterally extensive veins may occur and are called hanging wall or footwall lodes. These are often separated by up to 3 m of altered wall rock, typically a biotite-bearing amphibolite, as in the Lφ1 lode south of the crosscut on the 7 level (Fig 3). In some stopes both lodes may be mined, in others only one will be above the cutoff grade, with mining often alternating between hanging wall and footwall along strike. Locally the laminated veins are isoclinally folded, with the fold hinges varying in plunge but generally clustering around the plunge of the stretching lineation.
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Fig 4 - Longitudinal projection of the Lφ1 lode.
The Lφ1, Lφ2 and Lφ3 lode mineralisation consists of laminated veins containing a quartz-pyrite-sphalerite-galenasilver-gold assemblage. Gold mineralisation is almost entirely within the veins. The narrow wall rock alteration zones in granite are not mineralised and in mafic rocks they are only weakly mineralised, to 0.1 g/t gold. The primary sulphide assemblages are, in order of decreasing abundance, pyrite, sphalerite, galena, pyrrhotite and chalcopyrite, with trace bornite, tetrahedrite and covellite. A silver to gold ratio of 1.5:1 is reported in the mill returns and electrum is observed in the ore. Gold grades show a strong correlation with zinc.
GOLDEN SLIPPER LODE Structure o
The lode is hosted by a steeply west dipping, 340 striking shear zone in granite and along granite-mafic xenolith contacts (Fig 2). The structure can be traced northwards to the ultramafic contact, but is truncated to the south against a north dipping layer-parallel shear zone in a large mafic xenolith. The structure truncates (with right lateral sense of drag in the foliation) two similar layer-parallel shear zones, each located on 45° north dipping granite-mafic xenolith contacts (shown schematically in Fig 2). These two east-striking mafic xenoliths are separated by approximately 50 m of granite, foliated along the contacts.
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Mineralisation The Golden Slipper structure contains poddy quartz veins and narrow, low grade gold mineralisation. One thick high grade shoot is developed in altered wall rock where the structure juxtaposes granite (hanging wall) against the granite located between the mafic xenoliths (footwall of the shear). The high grade shoot is associated with up to 20% disseminated pyrite in a biotite-altered and strongly sheared granite. The shoot is 15 m long, up to 10 m wide and has a plunge parallel to the intersection of the Golden Slipper structure with the layer parallel shear zones. The Golden Slippper lode is characterised by abundant disseminated pyrite with only minor galena and sphalerite. The bulk geochemical composition reflects this, with almost negligible arsenic and low overall silver, lead and zinc values.
ALTERATION AND METAMORPHISM The Lφ1, Lφ2 and Lφ3 lode structures are surrounded by narrow intense alteration zones, 20 to 50 cm wide, comprising sericite-silica-pyrite alteration in granite and biotite-calcitepyrite alteration in the basaltic precursors. A broader halo of much weaker hydrothermal alteration is developed around the intense proximal alteration, comprising minor quartz-calcite veining and the development of trace disseminated biotite in
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT DIMER GOLD DEPOSITS
vein margins. The alteration is pre-peak metamorphism and deformation and recrystallisation of vein quartz and of sericite to coarse grained muscovite is evident. Peak metamorphism reached the greenschist-amphibolite transition, with characteristic assemblages including biotite-epidote-green hornblende in metabasalt and muscovite-calcite-quartzplagioclase-potassium feldspar in metagranite. A weak to strong schistosity is developed as a crenulation cleavage in samples exhibiting an earlier alteration assemblage and cleavage. Minor post-foliation albite veins were observed locally. The Golden Slipper lode is contained in a zone of intense silica-biotite-pyrite alteration in mafic rock or silica-sericitebiotite-pyrite in granitic host rock. The silica occurs as both fine foliation-parallel veinlets and pervasive silicification of the wall rock.
GEOLOGICAL EVOLUTION AND GENETIC MODEL The deposits occur in a massive granite in the footwall of the Dimer lag, a regional scale structural zone forming the southern margin of the Marda greenstone belt. From regional geological observations, the Dimer lag is interpreted to be an extensional detachment, ie, a layer-parallel to low angle extensional shear zone. This is analogous to the ‘DE’ structures identified in other areas of the Yilgarn (Hammond and Nisbet, 1992; Williams and Currie, 1993; Williams and Whitaker, 1993). The antiformal geometry of the Dimer lag in the vicinity of the mine is due to syn-extensional folding of the extensional detachment, as described by Mancktelow and Pavis (1994). There may have been additional tightening of the fold during later movement along the Yendilberin shear zone. Granite emplacement along the Dimer lag was concurrent with the deformation event and concluded with the emplacement of the massive granites hosting the lodes late in the DE deformation event. The massive granite was emplaced in the core of the antiformal structure, the most dilational part of the antiform. Within the massive granite, strain along the Dimer lag was compartmentalised into narrow discrete structures such as the lodes and layer parallel shears. The lodes comprise an array of narrow and intense high strain zones, with a net 45 o to the NNW movement direction and east block south sense of movement, which overprint the layer parallel structures. The lodes are best developed where the dip of the Dimer lag (taken to be the ultramafic contact in underground exposure) is steep, towards the south, at variance with the generally north to NW dip of the structure. This reversal in dip along the ultramafic contact has created a compressional jog in the structure in the vicinity of the lodes. Within the lode structures mineralised shoots have several controls:
Geology of Australian and Papua New Guinean Mineral Deposits
1.
the primary control on the ore shoots is the extension direction;
2.
secondary control is the intersection of splay structures, generally steeper than the extension direction; and
3.
in the Golden Slipper lode the ore shoot is parallel to the intersection of layering and structure, but located where the Golden Slipper structure juxtaposes granite against granite.
Mineralisation is pre-peak metamorphism, which reached upper greenschist to lower amphibolite facies, probably associated with deformation along the Yendilberin shear zone.
MINE GEOLOGICAL METHODS Underground access to the lodes is by decline and crosscuts, located between the Lφ1 and Lφ2 lodes, with lodes accessed by strike drives, typically established at 13 to 15 m vertical intervals. A retreat stoping method is employed once the strike drive has reached the extremities of the orebody. As there are hanging and footwall lodes a significant geological presence is maintained and all faces and backs are mapped in detail. Bazooka holes are drilled into the foot and hanging walls to test for splay structures (eg Boobook lodes) and for lode-parallel footwall and hanging wall lodes.
ACKNOWLEDGEMENTS The authors acknowledge Tectonic Resources NL for permission to publish this information and thank the geological staff, particularly D Tholen and J Sharpe, for their contributions to the geological understanding of the mine.
REFERENCES Hallberg, J A, Johnston, C and Bye, S M, 1976. The Archaean Marda Igneous Complex, Western Australia, Precambrian Research, 3:577–595 Hammond, R L and Nisbet, B W, 1992. Towards a structural and tectonic framework for the central Norseman-Wiluna greenstone belt, Western Australia, in The Archaean: Terrains, Processes and Metallogeny, Publication 22 (Eds: J E Glover and S E Ho), pp 39–49 (The Geology Department and University Extension, The University of Western Australia: Perth). Mancktelow, N S and Pavis, T L, 1994. Fold fault relationships in low angle detachment systems, Tectonics, 13(2):668–695. Williams, P R and Currie, K L, 1993. Character and regional implications of the sheared Archaean granite greenstone contact near Leonora, Western Australia, Precambrian Research, 62:343–365. Williams, P R and Whitaker, A J, 1993. Gneiss domes and extensional deformation in the highly mineralised Archaean Eastern Goldfields Province, Western Australia, Ore Geology Reviews, 8:141–162.
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Geology of Australian and Papua New Guinean Mineral Deposits
Glasson, M J, Henderson, R G and Tin, M, 1998. Broads Dam gold deposts, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 197–200 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Broads Dam gold deposits 1
2
by M J Glasson , R G Henderson and M Tin
3
INTRODUCTION The deposits are near Broads Dam in the Coolgardie mineral field, 40 km WNW of Kalgoorlie, WA. They are centred on lat 30ο25′S, long 121o06′E on AMG coordinates 318 150 E, 6 616 900 N on the Kalgoorlie 1:250 000 (SH 51–9) and 1:100 000 (3136) scale map sheets. To the end of 1993 a total of 697 000 t of open pit ore of head grade 4.13 g/t gold had been treated for a recovery of 2717 kg (87 365 oz) of gold, with 60% derived from the Broads Dam 2 deposit. A further 150 000 t of low grade ore of estimated grade 1.5–1.7 g/t gold and 200 000 t of subgrade ore around 1 g/t gold were stockpiled on site. The low grade ore is currently being treated by heap leaching.
DISCOVERY AND MINING HISTORY Broads Dam 1, a blind ‘new generation’ gold deposit, was discovered by Noranda in 1986 by rotary air blast (RAB) drilling through transported cover along strike from gold and arsenic soil geochemical anomalies which were delineated south of Broads Dam. After reverse circulation (RC) drilling on a 20 by 25 m spacing, mostly south of Broads Dam, Noranda estimated a total Indicated Resource of 480 000 t at 3.9 g/t gold. Metall Mining Australia Pty Ltd and Thyssen Schachtbau Gmbh purchased the property from Plutonic Resources Ltd and Money Mining NL early in 1991. Prestripping of the Broads Dam 1 deposit south of Broads Dam (Fig 1) commenced in March 1991 and the pit was completed within 12 months. The larger Broads Dam 2 or Dragline deposit, about 120 m along strike to the north of Broads Dam, was discovered in mid 1991 by infill and deeper drilling around mineralised intercepts in the shallow Noranda drill holes. Prestripping of Broads Dam 2 began in November 1991 and a cutback was completed in March 1993 taking the western portion of the pit to 98 m depth. The Broads Dam 1 pit was backfilled with waste from Broads Dam 2. Early in 1993 a small ore reserve was defined at Broads Dam West, 400 m across strike to the SW of the Broads Dam 1 pit (Fig 1), after following up a number of significant drill hole intersections obtained by Noranda. Mining at Broads Dam West commenced in August 1993 and was completed in October. The Broads Dam tenements were sold to Australasian Gold Mines NL in 1994. 1.
Formerly Chief Geologist, Metall Mining Aust Pty Ltd (MMA), now Exploration Director, Kilkenny Gold NL, PO Box 1135, West Perth WA 6872.
2.
Formerly Senior Mine Geologist MMA, now Senior Geologist, Exploration and Mining Consultants, PO Box 4583, Kalgoorlie WA 6430.
3.
Formerly Senior Geologist MMA, now Senior Geologist, Lone Star Exploration NL, Level 5, 15 William Street, Perth WA 6000.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Regional and local location plans, broads Dam gold deposits.
GEOLOGICAL SETTING The deposits occur in a sequence of mafic, ultramafic and sedimentary rocks within the Kalgoorlie–Ora Banda sector of the Norseman–Wiluna greenstone belt, within the Archaean Yilgarn Block.
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M J GLASSON, R G HENDERSON and M TIN
The Broads Dam area is near the western margin of the Kurrawang Syncline. Within the core of the syncline the Kurrawang formation, which was deposited in a late Archaean sedimentary basin, unconformably overlies Archaean greenstone. Metasediment of the Kurrawang formation, including magnetic pebble conglomerate, crops out in the east of the area. Further westwards the underlying greenstone comprises mostly fine grained metasediment with intercalated mafic volcanic rocks, komatiites and dolerites. The greenstone sequence is steeply dipping and is cut by a number of discordant felsic dykes. Gold mineralisation at Broads Dam is associated with the Zuleika Shear Zone, a concordant NW-trending strike-parallel regional shear that is related to gold mineralisation 6 to 11 km to the NW at the Zuleika and Hawkins Find deposits and 3 km to the SE at the Blue Funnel deposit.
ORE DEPOSIT FEATURES The mine sequence at Broads Dam comprises NW-striking, steeply dipping fine-grained foliated metasediment, including black shale and talc schist, and quartz dolerite. All Archaean rocks are strongly weathered to a depth of at least 25 m and the upper portion of the weathering profile has been truncated. The upper part of the regolith comprises transported, indurated Tertiary lateritic residuum with an average thickness of 5 m overlain by 2 m of unconsolidated Quaternary sand. Gold mineralisation in the Broads Dam I and 2 deposits mainly occurs within sheared and altered silicified zones, and in quartz stockworks developed in quartz dolerite sills(?), and occasionally within silicified ultramafic rock and metasediment. Due to the intense weathering the primary mineralisation has undergone strong near-surface depletion. As a result, the upper 20 to 25 m consisting of strongly leached pallid clay and saprolite is virtually devoid of mineralisation. Supergene ‘blankets’ were well developed in places, from 25 to 35 m depth, especially in the NW end of the Broads Dam 2 deposit (Fig 3). The quartz dolerite host rocks are generally funnel shaped and consequently ore width diminishes with depth. Gold mineralisation is fine grained in the supergene zone. In the primary zone gold is associated with pyrite, and with finely disseminated arsenopyrite and minor chalcopyrite in the altered stockwork zones. The Broads Dam 1 and 2 deposits are separated by a 100 m zone of barren country rock (Fig 2), which is located beneath the original Broads Dam, and is more deeply weathered and devoid of dolerite host rock. It is suspected that a crosscutting structure separates Broads Dam 1 and 2 deposits, and although the host dolerites are lithologically almost identical, their structural and stratigraphic relationships are uncertain. During mining all of the Broads Dam open pits were subjected to extremely high saline ground water inflows indicating that the Zuleika Shear Zone in this region is a major aquifer. Standing water level in the abandoned open pits is at 35 m below surface.
HOST ROCK AND ORE PETROLOGY Quartz dolerite ore The less-altered quartz dolerite host rock contains feathery to spheroidal granophyric quartz associated with incipiently sericitised plagioclase. Alteration usually comprises apple
198
FIG 2 - Simplified geological plan at 450 m RL (50 m) depth), Broads Dam 1 and 2 open pits.
green, aluminium-rich iron chlorite ± minor biotite that has penetrated along microfractures in the matrix and locally replaces the original ferromagnesian minerals. Relict skeletal magnetite has been preserved and is often enveloped by chlorite and minor biotite. The quartz content varies from 10 to 20% confirming a differentiation trend within the dolerite host. Original sulphides have been pervasively replaced by limonitegoethite. A penetrative schistosity followed by iron chlorite
Geology of Australian and Papua New Guinean Mineral Deposits
BROADS DAM GOLD DEPOSITS
FIG 3 - Cross section along 19 360 m N, looking north, showing gold grade distribution, Broads Dam 2.
and sericite is observed in some samples confirming the presence of brittle-ductile shears. Carbonate veins locally cut across the foliation which accompanies attenuation and fracturing of the various quartz and plagioclase components.
Altered quartz dolerite ore The altered dolerite host is penetrated by fine granoblastic quartz veins but has relict spheroidal granophyric textures enveloping equant to lath like, incipiently sericitised plagioclase, characteristic of the unmineralised dolerite. The ferromagnesian component has been replaced by biotite, chlorite or carbonate. The more strongly-altered quartz dolerite contains an interstitial fine granular mosaic of secondary plagioclase associated with granular carbonate, which appears to have penetrated along microfractures in the matrix. Sulphides are associated with the granular carbonate alteration phase. Fine grained euhedral arsenopyrite is locally enveloped by pyrrhotite which, in turn, has been replaced by colloform, birds eye–textured secondary pyrite. Gold grains to 30 µm diameter have been observed as inclusions in arsenopyrite that has replaced leucoxene and/or relict skeletal magnetite in the matrix. Gold grains are occasionally visible in thin quartz stringers within the strongly altered dolerite. A petrological study was not undertaken for the supergene ore due to its strongly weathered nature. The supergene gold ore was free milling, with metallurgical recoveries in excess of 95%, and was very pure. Metallurgical recoveries were lower in the primary ore due to its semi-refractory nature.
Geology of Australian and Papua New Guinean Mineral Deposits
BROADS DAM 1 DEPOSIT Quartz dolerite is the dominant rock in the Broads Dam 1 open pit, as two steeply west dipping units separated by a central zone to 18 m thick comprising talc schist and metasediment (Fig 2). The margins of the two dolerite units at their contact with this central zone are strongly sheared, silicified and altered, and host two parallel lodes to 15 m thick. Both lodes are generally conformable with the dolerite margins, dip steeply to the west and plunge steeply southwards. The eastern lode is continuous for the length of the open pit whereas the western lode is developed in the southern portion of the deposit only. Both lodes thin with depth and appear to pinch out to the south, due to thinning of the host dolerite. Deeper drilling at approximately 80 m depth beneath Broads Dam 1 intersected only narrow dolerite with low grade mineralisation.
BROADS DAM 2 DEPOSIT Here the host dolerite has been folded into a south-plunging antiformal(?) structure which has been truncated by erosion. As a result the single lode at the southern end of the deposit splits into two lodes as the limbs diverge northwards (Figs 2 and 3). Mineralisation in the eastern lode occurs mostly along the western margin of the dolerite, at the contact with ultramafic rock and black shale. In contrast, the western lode covers almost the entire width of the dolerite and extends into the black shale on the NE side. Other narrower, subvertical lodes are also developed within the dolerite in the footwall to the east of the
199
M J GLASSON, R G HENDERSON and M TIN
main east lode (Fig 3). The western limb of the dolerite is funnel shaped (Fig 3) and thins markedly with depth. It is near vertical in the southern end of the deposit but rolls westwards at depth in the north. Two deep holes were drilled to intersect the west limb ore zone at 150 m depth, however neither dolerite nor gold mineralisation were intersected, implying that this limb of the dolerite pinches out or is faulted off at depth. The western limb of the dolerite was the most important source of ore in the Broads Dam 2 open pit. Here the bulk of the mineralisation was confined to several vertically stacked subhorizontal lodes cutting almost the entire width of the dolerite and exhibiting a shallow southerly plunge. Vertical lodes appear more important at depth where the dolerite is thinner.
BROADS DAM WEST DEPOSIT This small orebody comprised an upper and lower blanket of supergene mineralisation developed over a narrow and high grade steeply dipping crosscutting quartz vein system trending 15o magnetic. The veining cuts across a deeply weathered sequence of ultramafic rocks. Primary mineralisation within the quartz vein plunges steeply to the SW. The highest grades occur at the southern end of the vein system although further south the vein system thins markedly and diminishes in grade. The Broads Dam West structure transects the main Broads Dam mineralised trend between the Broads Dam 1 and 2 deposits, and may be responsible for the apparent structural dislocation in this area.
200
BROADS DAM SOUTH AREA Patchy and generally low grade mineralisation along strike to the south of Broads Dam 1 is confined to steeply westerlydipping zones of silicified ultramafic rock and talc schist. Several zones of higher grade and width are of too limited strike length and were too deep, due to the strong surface depletion, to have been economically viable at the time the other deposits were mined. The absence of the favourable quartz dolerite host rock has apparently precluded the formation of any significant gold deposits in this portion of the shear zone.
ORE GENESIS The controls on gold mineralisation at Broads Dam appear to be the Zuleika Shear Zone, the presence of structurally brittle and chemically receptive quartz dolerite host rocks and their competency contrast with the enveloping metasediment and ultramafic rocks. A crosscutting structure associated with the Broads Dam West deposit may have also been instrumental in localising gold mineralising solutions.
ACKNOWLEDGEMENTS This paper is published with the permission of Australasian Gold Mines NL. The authors would like to thank Dr C Rugless for petrographic descriptions of the dolerite and for critically reviewing the manuscript.
Geology of Australian and Papua New Guinean Mineral Deposits
Beckett, T S, Fahey, G J, Sage, P W and Wilson, G M, 1998. Kanowna Belle gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 201–206 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Kanowna Belle gold deposit 1
2
3
by T S Beckett , G J Fahey , P W Sage and G M Wilson INTRODUCTION The deposit is 18 km NE of Kalgoorlie and 2 km west of the historic gold mining centre of Kanowna, WA, at lat 30o37′S, long 121o35′E and AMG coordinates 363 410 E, 6 612 540 N, on the Kurnalpi (SH 51–10 ) 1:250 000 and Kanowna (3236) 1:100 000 scale map sheets (Fig 1). The project is owned by the Golden Valley Joint Venture (50% North Gold WA Ltd and 50 % Delta Gold NL).
4
At the end of June 1996 the total Measured, Indicated and Inferred Resources remaining were 19.8 Mt at 5.9 g/t gold for 3.8 Moz of contained gold, remaining open pit Proved and Probable Reserves were 4.5 Mt at 4.9 g/t gold, with underground Proved and Probable Reserves for the first mining block directly beneath the open pit of 2.9 Mt at 7.0 g/t gold (Table 1). The deeper resource blocks were being defined by underground diamond drilling in mid 1997. The deposit is concealed by 40 m of gold-depleted cover, and with its proximity to the historically worked and explored Kanowna mining field, proves it to be appropriately termed the ‘one the old-timers missed’.
EXPLORATION AND MINING HISTORY Gold was discovered at Kanowna in 1893, with output peaking in about 1900 and rapidly declining after 1914. Production was initially from quartz reefs along the White Feather structure and later from deep lead and cemented alluvials adjacent to the Kanowna townsite (Fig 1). Historic gold production from the Kanowna district totalled in excess of 500 000 oz (Maitland, 1919).
FIG 1 - Regional geology and location of the Kanowna Belle deposit after Swager and Griffin (1990) and N J Archibald and J R Thornett (unpublished data, 1995).
In 1993 the deposit had an estimated pre-production resource of 22 Mt at 5.7 g/t gold for 4 Moz of contained gold to a vertical depth of 1000 m. Prestrip mining commenced in November 1992, leading to the first gold pour in September 1993. Ore mined to the end of June 1996 totalled 4.3 Mt at 3.86 g/t for 420 000 oz of gold recovered. Ore mined from July 1995 to July 1996 was 1.3 Mt at 4.03 g/t for 153 000 oz of gold recovered. 1.
Senior Mine Geologist, Kanowna Belle Gold Mines, PO Box 1622, Kalgoorlie WA 6430.
2.
Chief Geologist Mining, Delta Gold NL, PO Box 98, West Perth WA 6872.
3.
Manager Mining, Kanowna Belle Gold Mines, PO Box 1622, Kalgoorlie WA 6430.
4.
Senior Geologist, Delta Gold NL, PO Box 152, Kalgoorlie WA 6430.
Geology of Australian and Papua New Guinean Mineral Deposits
Modern day gold exploration was sporadic through the late1970s and early 1980s. Canyon Resources Pty Ltd - the precursor of Delta Gold NL (through the driving force of David Gellatly) recognised the potential for open pit deep lead and bed rock gold mining at Kanowna and aquired tenements between 1982 and 1983. Peko Wallsend Ltd (now North Mining Ltd) also became involved in 1983 in the search for deep lead gold. After minor successes the companies agreed to join forces as the Golden Valley Joint Venture in order to exploit the QED supergene deposit overlying deep lead channels. North Mining Limited became mine manager and Delta Gold NL the exploration manager (Gellatly et al, 1995). Between 1989 and 1993 the small QED heap leach operation processed 870 000 t of ore and recovered 75 000 oz of gold. Early systematic exploration of the Kanowna Belle area comprised vertical rotary air blast (RAB) drilling on 1000 by 200 m spacing directed towards defining deep lead targets and associated primary mineralisation. In mid 1987 drilling intersected gold mineralisation, including 2 m at 11g/t from 52 m depth and 4 m at 3 g/t from 28 m depth. In late 1987, follow up RAB drilling in this area intersected anomalous gold values towards the base of some holes, however other holes failed to penetrate the 40 m thick zone of leaching and gold depletion characteristic of the Kanowna Belle deposit (Thomson and Peachey, 1993). Surface geochemical programs confirmed anomalous values in the vicinity of the earlier RAB intersections, with analysis of minus 80 mesh soil samples for total gold giving the strongest response. This work defined a bullseye anomaly of greater than 60 ppb gold with a diameter of 350 m and a peak value of 150
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T S BECKETT et al
TABLE 1 Kanowna Belle resources and reserves at 30 June 1996. RESOURCES
Measured
Indicated
g/t
Mt
g/t
Moz
Mt
g/t
Moz
Mt
g/t
Moz
Mt
g/t
Kanowna Belle
1.0–2.0
8.09
5.9
1.535
0.48
3.2
0.05
11.27
6.0
2.181
19.84
5.9
3.766
Kanowna Belle stockpiles
various
1.58
1.7
0.086
—
—
—
—
—
—
1.58
1.7
0.086
9.67
5.2
1.621
0.48
3.2
0.05
11.27
6.0
2.181
21.42
5.6
3.852
Total
Grade Contained gold
Ore
Total
Ore
Deposit
Grade Contained Ore gold
Inferred
Cutoff g/t
Grade Contained Ore gold
Grade Contained gold Moz
RESERVES
Proved Cutoff
Ore
Probable
Grade Contained Ore gold
Deposit
Total
Grade Contained gold
Ore
Grade Contained gold
g/t
Mt
g/t
Moz
Mt
g/t
Moz
Mt
g/t
Gold
Kanowna Belle open pit
1.35
4.44
4.9
0.700
0.1
2.9
0.010
4.54
4.90
0.709
Kanowna Belle underground
2.00
2.96
7
0.666
-
0
—
2.96
7.00
0.666
Kanowna Belle stockpiles
various
1.58
1.7
0.086
-
0
—
1.58
1.70
0.086
8.98
5.0
1.451
0.1
2.9
0.010
9.08
5.00
1.461
Toyal
ppb gold, compared to a local background of 10 to 20 ppb (Thomson and Peachey, 1993). Further RAB drilling confirmed significant gold mineralisation. Reverse circulation (RC) drilling commenced in late 1989, and a major drill-out program comprising RC and diamond core drilling commenced in January 1990. By September 1991, 418 RC holes for 41 978 m and 80 cored holes for 10 400 m had been used to delineate a Measured, Indicated and Inferred Resource of 11.2 Mt at 5.2 g/t gold. While mining feasibility studies were in progress, deeper core drilling continued to outline resources for future underground mining. By the time the deep drilling program was completed in 1993, the Measured, Indicated and Inferred Resource had been drilled on 80 by 80 m spacing to a depth of 1000 m, with the resource still open at depth and along strike to the east. The total premining resource at Kanowna Belle had then increased to 22 Mt at 5.7 g/t for a total of 4 Moz gold.
REGIONAL GEOLOGICAL SETTING The deposit lies within the Boorara Domain of the Kalgoorlie Terrane, one of the recognised structural divisions of the Eastern Goldfields Province (Swager and Griffin, 1990). The stratigraphy within the domain comprises a greenstone succession ranging from a basal basalt through chert and ultramafic rocks into upper felsic volcanic rocks and sediment of the Gindalbie Formation. The Gindalbie Formation is correlated with the Black Flag Group found in the surrounding Ora Banda, Kambalda and Coolgardie domains. The dominant structural feature in the Boorara Domain is the Scotia–Kanowna anticline, a large upright fold with a granitic core, that plunges shallowly to the SE. The regional structure has been described previously by Archibald (1990) and Swager and Griffin (1990). Recent work by N J Archibald and R J Thornett (unpublished data, 1995) has identified features within the area which have been attributed to a deformation event that is earlier than any previously documented (Table 2).
202
TABLE 2 Summary of the deformation sequence for the Kanowna area. Tectonic events
Deformation
Intrusion and metamorphism
D4
Brittle faults; dextral reactivation of D2 structures
Retrograde metamorphism
D3
Upright open folds, NW high angle reverse faults and high strain zones
Structurally late porphyry intrusions; metamorphic peak; low-mid greenschist facies
D2
NE arcuate thrust ramp complexes with shallow plunging folds
Kanowna Belle type porphyry intrusions
D1
Not seen on a macroscale, but recognised on outcrop scale as layer parallel foliations and pressure solution structures in highly incompetent units
Source: Summarised from N J Archibald and J R Thornett (unpublished data, 1995); note that the D2 deformation is analogous to the D1 deformation of Swager and Griffin (1990).
Metamorphism within the Boorara Domain is recognised as lower-mid greenschist facies, indicated by the assemblage albite-chlorite-actinolite-clinozoisite in metabasalt (Taylor, 1984). However there is an increase to lower amphibolite facies adjacent to the granitic core.
MINE GEOLOGY STRATIGRAPHY The mine sequence comprises a package of rocks within the greenstone succession comprising a moderately (55o) SW to south dipping sequence of sedimentary conglomerate and volcaniclastic rocks of the Gindalbie Formation, intruded by
Geology of Australian and Papua New Guinean Mineral Deposits
KANOWNA BELLE GOLD DEPOSIT
numerous felsic porphyries (Fig 2). Two distinct lithological domains are recognised within the mine area, a footwall sequence and a hanging wall sequence, which are juxtaposed along a major NE-trending structural feature, the Fitzroy shear zone (FSZ), a reactivated D2 structure.
a felsic-dominated clast-supported conglomerate similar to the Cemetery conglomerate. The Lowes sandstone is a finer grained unit, characterised by rare ragged quartz eyes in a strongly sericitised quartzo-feldspathic matrix. It has a strike extent of approximately 300 m and a true width up to 60 m, and tapers with depth. The Grave Dam grit occupies the remainder of the hanging wall sequence. It is a felsic lithic tuff displaying rapid lateral and vertical facies variations into clast-rich and clast-poor variants, with changes in the degree of rounding and size of the clasts from small gravel or pebbles to boulders. Bedding features are absent or poorly developed. The Kanowna Belle porphyry, the primary host for mineralisation, contains aproximately 10–20% of subhedral to euhedral albitic plagioclase phenocrysts to 5 mm diameter, and is locally quartzphyric. The matrix is a very fine grained intergrowth of plagioclase and quartz and the main accessory minerals are magnetite, rutile, apatite, pyrite, sphene and zircon. Chemically the porphyry has a granodiorite composition with around 66 wt % silica (Ren and Heithersay, in press). Fine grained fuchsite-altered clasts derived from digested ultramafic xenoliths are common. The porphyry in the upper portion of the orebody is primarily on the hanging wall of the FSZ. At about 560 m below surface, the porphyry has been offset by the FSZ and is mainly in the footwall of the fault. It has a strike extent greater than the orebody, a true thickness to 60 m and a down dip extent in excess of 1000 m (Fig 3).
FIG 2 - Kanowna Belle geological sequence and legend for Fig 4.
The footwall sequence is dominated by the interbedded mafic-dominated Golden Valley conglomerate and the felsicdominated Cemetery conglomerate. These units are often fault bounded but also exhibit conformable contacts which dip 50o to the SW. Coarse bedding can be seen in numerous exposures with beds to 1 m thick comprising coarser and finer cycles of deposition in which individual clasts to 1.5 m diameter are common. The hanging wall sequence contains cyclic sequences of felsic sediment and volcaniclastic rocks with minor mafic rocks, intruded by felsic porphyries. The sequence dips moderately SSE, subparallel to the FSZ. Four main units are mapped, namely the Kanowna Belle porphyry, QED rudite, Lowes sandstone and Grave Dam grit, which is the dominant unit. All units host mineralisation, however approximately 70% of the total resource is contained within the Kanowna Belle porphyry. The QED rudite occurs in the western half of the deposit, having a strike extent of 200 m and a true thickness to 30 m. It is
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 3 - Kanowna Belle cross sections looking grid west showing mineralisation (after Ren and Heithersay, 1996).
Numerous aphyric felsic dykes intrude the hanging wall sequence above the Kanowna Belle porphyry, and are similar in chemical composition (A Ross, personal communication, 1995). The dykes trend approximately east and dip steeply to the south. They attain widths of 20 m and strike lengths in excess of 300 m. A major unmineralised felsic intrusive,
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dipping moderately to the SW, is exposed in the western wall of the pit. Minor dolerite dykes also intrude the hanging wall sequence.
STRUCTURE Gold distribution and wall rock alteration at Kanowna Belle are directly related to the FSZ and oblique structures developed in the hanging wall of this shear. Shears and faults have largely controlled the emplacement of felsic porphyries, the predominant host rock. The FSZ is the most readily identifiable structure within the deposit. It is a prominent mylonitic shear and fracture zone developed over widths to 10 m, characterised by ductile c/s shear fabrics, and it dips at 60o to the SSE. N J Archibald (unpublished data, 1993) has interpreted this structure to be one of a series of regional arcuate NE-trending thrust ramps formed during D2 deformation. The coincidence of the Kanowna Belle deposit and the intersection of the FSZ with the SE-plunging hinge of the Scotia–Kanowna anticline has been recognised by N J Archibald and J R Thornett (unpublished data, 1995) as a likely site for reactivation and opening of the fault. A similarly oriented but more discrete structure, the Hanging Wall shear (HWS), defines the upper contact of the Kanowna Belle porphyry and the Lowes sandstone (Fig 4). It is a ductile shear zone, 1 to 2 m wide, which can be traced to considerable depth subparallel to the FSZ. Deformation within the open pit comprises a series of NWtrending (D3) features including a pervasive foliation, dextral fault offsets and narrow moderately dipping shear and breccia zones within both the hanging wall and footwall units. Felsic
and rare mafic dykes have intruded along these structures within the hanging wall sequence. These features both laterally confine and to some degree control the gold mineralisation. Brittle late-stage reverse movement within the FSZ during D4, marked by a fault ‘pug’ gouge horizon to 25 mm thick known as the Fitzroy fault, shows that this fault postdates mineralisation and overprints earlier structural episodes. The Fitzroy fault is remarkably planar over the strike and depth extent of the mineralisation and its position can be accurately predicted to 1000 m depth. In the deeper, eastern portions of the deposit, the FSZ and the Fitzroy fault are increasingly separated from one another (G J Fahey, A W Ryall, L Widenbar and N J Archibald, unpublished data, 1993).
LODE CHARACTERISTICS A variably undulating supergene ore zone called the supergene blanket occurred above the primary shoots at 35 to 45 m depth. Above this, the saprolite was strongly depleted with gold, averaging <0.01 g/t. A silcrete hardpan above the saprolite covered the hanging wall sequence and was weakly mineralised in three locations, directly overlying the uppermost position of the main shoots which were 5 to 12 m below surface. The supergene mineralisation extended in places to 100 m north of the FSZ and had plan dimensions of 600 by 250 m and a thickness of 1 to 10 m. Anomalous mineralisation associated with the supergene blanket follows the FSZ for 200 m east of the Kanowna Belle pit. Four principal lodes or shoots are recognised within the open pit (Fig 4), namely Lowes, Troy, Hanging Wall and Hilder. Over 80% of the known resource is contained within the Lowes
FIG 4 - Kanowna Belle open pit geology projected to 10 280 m RL, showing major rock units, structures and orebodies.
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KANOWNA BELLE GOLD DEPOSIT
shoot, which, for the top 400 m, is in the hanging wall of the Fitzroy fault. Below this depth the mineralisation narrows, transgresses the fault, and below 560 m depth is predominantly in the footwall of the fault. The shoot dips at 60o to the SSE, is tabular in shape, has an average strike length of 300 m, a true thickness of 5 to 50 m and a down dip extent in excess of 1000 m (Figs 3 and 5).
The Hilder shoot occurs solely in the footwall of the Fitzroy fault in coarse conglomerates, and is the least understood. It is oblique to the Lowes Shoot, strikes SSE and dips steeply to the WSW. It has a strike length of 30 m, a true thickness of 3 to 15 m and a down dip extent of 150 m. Due to the lower grade of the Hilder shoot (2–3 g/t), the mineralisation is uneconomic below a relatively shallow depth.
ORE FABRIC There are two distinct styles of ore fabric at Kanowna Belle. The most significant is within the Lowes shoot hosted by the Kanowna Belle porphyry and Lowes sandstone. The fabric is characterised by enhanced microfracturing, veining and alteration of the host rocks. Ore is associated with silicified zones containing abundant quartz-carbonate-pyrite veins and minor secondary breccias. Intense, texturally destructive alteration was developed immediately above the FSZ, as closely spaced crackle veining in zones to several metres wide. The highest gold grades occur in more restricted brecciated zones subparallel to the dominant structural trends. These are characterised by rotation and partial rounding of wall rock fragments in a quartz-sericite-carbonate±pyrite matrix. In general terms the degree of veining and brecciation decreases away from the FSZ. Pyrite is the principal, and often the only sulphide present, occurring as fine grained subhedral disseminations and local fracture fillings. In the Lowes Shoot gold has a strong refractory component due to <50 µm inclusions of gold in pyrite.
FIG 5 - Three-dimensional model showing pit mapping, mineralised outlines at 1 g/t cutoff and proposed underground development.
In the upper part of the orebody the Lowes shoot is predominantly hosted by the Kanowna Belle porphyry and Lowes sandstone, with lesser mineralisation within the QED rudite. At depth, where the mineralisation is in the footwall of the Fitzroy fault, the host rocks are the Kanowna Belle porphyry and intensely sheared Golden Valley conglomerate. Mineralisation occurs as two or more subparallel lenses separated by barren zones. The Lowes mineralisation is characterised by consistency of ore grade across its width, averaging 4 g/t gold, however the higher grades (+10 g/t gold) tend to be associated with two orientations. One is subparallel to the FSZ, the other is parallel to D3 structures and dips at moderate angles to the SSW. The Troy shoot is the highest grade shoot, averaging 15 g/t gold, and with the Hanging Wall shoots, strikes obliquely to the Lowes shoot, and dips steeply to the SW. It has a strike length of 100 m, a true thickness from 2 to 25 m, a down dip extent of at least 160 m and plunges to the SSE. The shoot is hosted mainly within the Grave Dam grit, with its geometry controlled by D3 shearing. The Hanging Wall shoots lie SW of, and roughly parallel to, the Troy shoot, in two anomalous corridors (Fig 4). They vary in strike length from 10 to 100 m, in thickness from 2 to 20 m, and in down dip extent from 20 to several hundred metres. The principal host rock for mineralisation is the Grave Dam grit, with minor amounts hosted within felsic and mafic intrusives.
Geology of Australian and Papua New Guinean Mineral Deposits
In the Troy and Hanging Wall shoots the ore fabric is quite different, comprising a stacked series of moderately to steeply SW dipping veins. The mineralised zones are characterised by discrete quartz-carbonate-pyrite±graphite dilational veins, commonly within narrow silicified haloes, and occur in moderately carbonate-sericite altered felsic volcaniclastic rocks. These veins comprise an inner chalcedonic core rimmed by pyrite. Gold grade is related to vein density and alteration intensity, with zones 1–2 m thick of intense silicification corresponding with gold grades >100 g/t. Native gold occurs on the margins of these veins as <1 mm grains, and much of this gold is free milling. The major accessory sulphide species is arsenopyrite, and the sulphur content of the ore is approximately 1%.
ALTERATION The main alteration minerals at Kanowna Belle are sericite, carbonate (ankerite) and pyrite, with localised narrow and discontinuous zones of moderate to strong silicification and albitisation. Three alteration assemblages are recognised: outer sericite-carbonate to carbonate-chlorite, intermediate sericite-carbonate-pyrite and inner pyrite-albite-silica. Chlorite-carbonate alteration may be regional alteration rather than a peripheral alteration to gold mineralisation (Ross, 1993), similar to the Golden Mile regional alteration halo. Zones of pervasive carbonate-sericite alteration extend up to 300 m from the defined mineralisation, but the intensity of alteration does not always directly correspond to gold grade as some rock types are preferentially altered (Ross et al, in press). Silicification is most commonly developed within the porphyries, close to their margins or adjacent to the FSZ. It often appears to overprint earlier carbonate-sericite alteration,
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and is associated with elevated pyrite content and albitisation of feldspar phenocrysts. Although the intensity of alteration is not always related to gold grade, the presence of intense pyrite and silica-albite alteration with hydrothermal brecciation (crackle veining texture) is directly associated with high grades. Hematitisation is a minor alteration style which forms a broad rim to the mineralisation. It is generally associated with chloritic zones in both the hanging wall and footwall positions. This alteration style is manifested by a fine dusting in remnant feldspars, giving them a distinctive pink colour, and is best developed in the Hilder shoot.
MINE GEOLOGICAL METHODS Grade control sampling in the open pit is by angled RC drill holes on variable hole spacing, depending on the complexity of the ore shoot boundaries. This sampling method is independent of ore production blast holes, allows wider hole spacing in the more continuous parts of the orebody, and averages an 8 by 5 m pattern. Samples are assayed for gold and sulphur. The ore above the run of mine cutoff grade of 1.3 g/t gold is also categorised by sulphur content and sericite content. Ore excavation is supervised by geological pit technicians and geologists, with pit mapping carried out on a regular basis to gain a better understanding of the controls of mineralisation, and as an aid to ore categorisation.
DISCUSSION
ACKNOWLEDGEMENTS The authors gratefully acknowledge the permission of North Ltd and Delta Gold NL to publish this paper. The paper consolidates the work of a number of dedicated geoscientists, in particular T Peachey and A Cowden who contributed greatly to the geological understanding of Kanowna Belle prior to, and during, the early feasibility stage. Contributions are acknowledged from N J Archibald for continuing contributions to the geological framework of Kanowna Belle and critical comments for this paper, and from T Ryall and J Harris. N Douglas is thanked for her patient and persistent typing of the text. D Banks greatly enhanced the paper by drafting the figures, with also a word of thanks to S Nicholls and P Ketelaar of Fractal Graphics for their help in generating the three dimensional image, and to S Ren of North Ltd for the preparation of Fig 3.
REFERENCES Archibald, N J, 1990. Tectonic-metamorphic history of the Lake Lefroy area: implications for greenstone belt evolution in the Kambalda–Norseman region, Western Australia, in Third International Archaean Symposium, Perth, 1990 (Eds J Glover and S Ho), pp 457–458 [Geoconferences (WA) Inc: Perth]. Gellatly, D C, Peachey, T R, Ryall, A W and Beckett, S, 1995. Discovery of Kanowna Belle gold deposit - one the old-timers missed, in New Generation Gold Mines: Case Histories of Discovery, pp 14.1–14.10 (Australian Mineral Foundation: Adelaide).
Investigation of the age and genesis of mineralisation at Kanowna Belle is continuing. As the open pit reaches maturity and the underground development continues, a greater understanding of the deposit and implications for exploration will also develop.
Heithersay, P S, Ren, S K and Baxter, R W, 1994. Towards an understanding of Kanowna Belle, the Kanowna Belle gold deposit and implication to Archaean gold metallogeny of the Yilgarn Craton, Western Australia, in 12th Australian Geological Convention Perth, September 1994, Geological Society of Australia, Abstracts, 37: 496–497.
Heithersay, Ren and Baxter (1994) and Ren and Heithersay (1996) provided some geological, geochemical and paragenetic constraints for the Kanowna Belle gold deposit. They concluded that two generations of gold mineralisation are evident. In an early event, low grade refractory gold was deposited in a halo around an oxidised granodiorite porphyry that had intruded a rapidly developing volcano-sedimentary pile. This was followed by a regional structure that remobilised and refined gold into narrow shear veins and faults, locally upgrading the grade of the deposit so that in essence it is a reworked porphyry gold deposit.
Maitland, A G, 1919. The gold deposits of Western Australia, extract from the Mining Handbook, Geological Survey Memoir 1, Chapter II, Economic Geology Part III, section I (Hesperian Press: Perth).
The authors, however, suggest that Kanowna Belle is a typical Archaean structure-controlled gold deposit. The Kanowna Belle porphyry is spatially related to D2 thrust ramps, indicating it was intruded during late D2 deformation. Mineralisation is related to the reactivation of the D2 thrust ramps during D3. The arcuate nature of the thrust ramps helped create a major dilational zone at Kanowna Belle allowing excessive fluid access. High fluid pressures led to hydraulic microfracturing and brecciation of the porphyry. The FSZ was reactivated during D4 creating the Fitzroy fault pug which overprints well developed D2 shear fabrics and offsets both the Kanowna Belle porphyry and mineralisation. The known resource at Kanowna Belle will be mined well into the next century and is the subject of ongoing research programs. That such a significant and exciting orebody has been discovered in the much worked and explored Kalgoorlie area has led to a reappraisal of the potential of the region.
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Ren, S K and Heithersay, P S, in press. The Kanowna Belle gold deposit and its implications to Archaean gold metallogeny, Yilgarn Craton, Western Australia, in Proceedings of the 9th IAGOD Symposium, 1994, Beijing (E Schweizerbart’sche Verlagsbuchhandlung: Stuttgart). Ross, A, 1993. Archaean lode gold deposits of the Kanowna region, in Gold Mineralisation at Various Regional Metamorphic Grades: An International Conference on Crustal Evolution, Metallogeny and Exploration of the Eastern Goldfields, Excursion 4 Guidebook, Kalgoorlie ‘93 (Eds: J R Ridley and D I Groves), pp 101–135 (Australian Geological Survey Organisation: Canberra). Ross, A, Fahey, G J, Beckett, T S and Vanderhor, F, in press. Kanowna Belle, in Systematic Documentation of Archaean Gold Deposits of the Yilgarn Block (Eds: F Vanderhor and D I Groves), MERIWA Project 195 report. Swager, C P and Griffin, T J, 1990. An early thrust duplex in the Kalgoorlie–Kambalda greenstone belt, Eastern Goldfields Province, Western Australia, Precambrian Research, 48: 63–73. Taylor, T, 1984. The palaeoenvironment and tectonic setting of Archaean volcanogenic rocks in Kanowna District, near Kalgoorlie, Western Australia, MSc thesis (unpublished), The University of Western Australia, Perth. Thomson, R M and Peachey, T R, 1993. The Kanowna Belle case study; the discovery of a concealed ore body, in An International Conference on Crustal Evolution, Metallogeny and Exploration of the Eastern Goldfields, Extended Abstracts (Eds: P R Williams, and J A Haldane), pp 229–231 Australian Geological Survey Organisation Record 1993/54.
Geology of Australian and Papua New Guinean Mineral Deposits
Lea, J R, 1998. Kundana gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 207–210 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Kundana gold deposits by J R Lea
1
INTRODUCTION The deposits are 25 km WNW of Kalgoorlie, WA, in the Coolgardie Goldfield, at lat 30o41′S, long 121o15′E, on the Kalgoorlie (SH 51–9) 1:250 000 scale map sheet (Fig 1). They are owned by Goldfields Pty Ltd, a member of the RGC Group. Mining of the first of a number of open pits commenced in 1988 and since 1993 two of the orebodies have been mined underground. Production to July 1996 totalled 2 569 000 t at 4.8 g/t gold for 394 000 oz gold. In 1995–1996 the mill treated 524 000 t at 6.6 g/t gold for 104 000 oz gold. Remaining Proved and Probable Reserves at 30 June 1996 were 2 805 000 t at 7.6 g/t gold (G B Hart, unpublished data, 1996).
Further exploration by the WFJV and Pancontinental Mining Ltd, which acquired the tenements in 1990, again using soil sampling and RAB drilling, led to the discovery of the Strzelecki, 21 Mile and Barkers deposits which had not been located by the early miners. The significance of the Barkers deposit was not recognised until the initial 50 g fire assays were repeated using 1 kg screen fire assaying which gave a truer indication of the high but irregular gold grades. Continuing exploration using mainly surface and underground diamond drilling has tested the mineralisation to 500 m below surface and many of the areas covered previously by shallow RAB drilling are being reappraised in the light of recent geological understanding.
PREVIOUS DESCRIPTIONS Most of the descriptions of the work at Kundana are in unpublished company reports. The most significant work is by H R Hadlow (1990) which contains the most comprehensive account of the geology and structural history. A J Warden of the Western Australian School of Mines is currently researching the alteration patterns and mineralisation characteristics at Kundana.
REGIONAL GEOLOGY
FIG 1 - Location map, Kundana deposits.
EXPLORATION HISTORY Gold mining commenced in the Kundana area in 1894, when the area was known by a number of names, including 21 Mile (from Coolgardie), Barker’s Find and White Flag. Over 30 small scale mining operations commenced and the area was covered by hundreds of shafts and shallow workings. By 1897, when the township of Kundana was gazetted, the field was virtually abandoned, the miners being beaten by harsh conditions, low gold grades and high ground water inflow (Freeman and Thom, 1988). Records for this period are sparse though total gold produced is thought to be less than 10 000 oz. From the 1960s regional exploration for gold and nickel was undertaken by a number of companies but systematic exploration did not commence until 1987. Using soil sampling, costeaning and rotary air blast (RAB) drilling the White Flag joint venture (WFJV: Tern Minerals NL and Kalbara Mining NL) while exploring near historic workings, located the North Pit and South Pit deposits. Later drilling programs defined a total reserve of 1 Mt at 3.8 g/t gold leading to a feasibility study. 1.
Chief Geologist, Paddington Gold Pty Ltd, PO Box 1161, Kalgoorlie WA 6430.
Geology of Australian and Papua New Guinean Mineral Deposits
Mineralisation at Kundana straddles the craton-scale Zuleika Shear Zone which can be traced for at least 250 km and separates the Ora Banda and Coolgardie domains in the northern Kalgoorlie Terrane of Swager et al (1990). The Kundana deposits are hosted by a structurally prepared sequence of sediment, volcaniclastic, mafic and ultramafic volcanic and intrusive rocks typical of the greenstone sequences in the Archaean Yilgarn Block. Three main phases of deformation resulted in: 1.
broad upright folding, after ENE–WSW transpression, such as the Powder Sill and Kurrawang synclines;
2.
oblique movement along the NNW-trending Zuleika Shear Zone during which gold-rich quartz veins were emplaced in dilatant zones along sheared contacts between rock units; and
3.
NNE-trending brittle faults which offset the sequence and the mineralisation.
ORE DEPOSIT FEATURES STRATIGRAPHY Exposure around Kundana is sparse with the majority of the area covered by colluvium or laterite over deeply weathered bedrock. Hence the local geology has been defined using mining exposure and drill hole data. Rock contacts generally strike NW and dip steeply west with the bulk of the sequence overturned and younging to the east.
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FIG 2 - Surface geological plan, Kundana area.
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The sequence from west to east is as follows (Fig 2). Komatiite forms a 750 m thick prominent ridge at the SE end of the Powder Sill syncline, with a serpentine-tremolite-chloritetalc assemblage within which strong lateritisation has destroyed most textures. Felsic volcanic rock and derived sediment such as felsic tuff, with sandstone and mudstone, conformably overlie the komatiite with a thin black shale at the contact. The assemblage is generally deeply weathered and up to 1000 m thick. Dolerite intrudes the felsic volcanic rock and is a medium to coarse grained granophyric quartz dolerite, commonly with chlorite-biotite alteration, which varies in thickness up to 800 m. High magnesium basalt follows. It is a fine to medium grained, massive, high-magnesium tholeiitic basalt to 100 m wide with strong chlorite alteration and an intrusive or extrusive origin. Cat Rock basalt is a distinctive unit to 100 m thick with coarse (to 3 cm) plagioclase phenocrysts set in fine grained matrix. It has a probable extrusive origin. Black shale, to 20 m thick, is a graphitic, pyritic, variably silicified and strongly sheared unit. It contains dismembered and folded chert bands, and is host to the South and North Pit deposits. Intermediate volcanic rocks complete the sequence. They are up to 700 m thick and sparsely outcropping, consisting of a sequence of andesitic to dacitic volcaniclastic rocks with minor derived sediment including agglomerate, lithic tuff and siltstone.
STRUCTURE The Kundana sequence is interpreted to form part of an upright isoclinal anticline between the Powder Sill syncline to the west and the Kurrawang syncline to the east (Hadlow, 1990). Rock unit contacts trend at 300 to 330 o with steep westerly dips. The dominant subvertical foliation, associated with the Zuleika Shear Zone, trends from 320 to 350o. A steep north-plunging lineation and other kinematic indicators suggest sinistral movement with west side up and to the south. The difference in orientation between rock contacts and structural trends is crucial for the formation of the Kundana orebodies. This relationship coupled with rock competency contrasts affected the degree of dilation and hence the emplacement of gold-rich quartz veins wide enough to be economically mined. The bedding-foliation intersection results in the mineralisation having a steep southerly plunge. Late brittle dextral faulting trending at 10 to 20o offsets rock unit contacts and mineralisation.
MINERALISATION The bulk of the mineralisation at Kundana is in the form of thin and planar laminated quartz veins <2 m thick which dip moderately to steeply west (Fig 3) with strike lengths to 600 m. All the mineralisation is currently open at depth. The veins are along or near at least three distinct rock unit contacts, and tend to pinch out along strike as the dilational zones change orientation and contract. The wall rocks exhibit strong localised shearing and alteration (carbonate and sericite) but typically have negligible gold content. Maximum vein width is 4 m although they are generally much thinner.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 3 - Schematic cross section, Kundana deposits, looking NW.
There is no quantifiable relationship between vein width and grade. The veins contain free gold with trace sulphides such as pyrite, chalcopyrite, galena, sphalerite and pyrrhotite, and minor localised scheelite and tourmaline. Secondary supergene mineralisation has developed variably near surface with depletion in places to 30 m. A second style of mineralisation is present at Gabbro Hill where deformation has produced a quartz stockwork in the dolerite, but this is currently poorly defined and understood. There are five main orebodies at Kundana. The South lode is a black shale hosted laminated quartz vein averaging 1.5 m wide with a 600 m strike length and a 75–85o west dip. Gold grades average 10–15 g/t, with a 1–2 m wide zone of quartz stockworking surrounding the vein with grades to 2 g/t gold. It was mined by open cut and subsequently underground. The North lode lies along strike from South lode and has similar grades. It is a shale hosted vein 400 m long and dipping 75o west. It is more structurally complex than South lode, with northerly-striking crosscutting shears causing localised isoclinal folding of the shale and wall rock and disruption to the mineralisation. It was mined in North pit. At Strzelecki the main and footwall veins are near the contact between the quartz dolerite and felsic volcanic rock, and dip at 55o west. The main vein is 500 m long, averages 0.7 m wide and consistently contains 60–90 g/t gold. The footwall vein, 30 m to the east, has a strike length of 200 m, an average width of 0.4 m and an irregular gold distribution with grades of 1–60 g/t gold. All the gold is contained in the veins with minimal surrounding alteration or wall rock mineralisation. The veins were mined by open cut and subsequently underground. Barkers orebody is at the western contact of the dolerite. Here the vein dips 70o west and has a strike length of 600 m although it is poorly mineralised in places. The vein averages 0.4 m wide and commonly has grades of over 60 g/t gold. It is currently being mined by open cut with plans for future underground extraction. Lying on the same contact as Barkers but not in a dilational orientation, the 21 Mile orebody consists of shallow supergene mineralisation with gold probably remobilised from Barkers and hence with no underlying source. It is to be open cut mined in conjunction with Barkers orebody. Several other mineralised zones exist along strike from the known orebodies but have not been thoroughly tested.
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MINE GEOLOGICAL METHODS
ACKNOWLEDGEMENTS
The thin but high grade veins at Kundana pose geological and mining challenges both in open pits and underground. The challenge is to maximise recovery while minimising dilution. In the open pits a method of selective blasting was used in which the hanging wall material was blasted initially and removed, hence exposing the ore which was then blasted and mined separately. This was effective in reducing dilution. The planar and non-disrupted nature of the veins enables their extraction from underground using an uphole bench retreat method. With underground mechanised mining the process for extracting thin veins is still being improved, with blast hole drilling accuracy being essential in thin veins and ground support imperative particularly in the shale hosted South lode. It is likely that other mining methods involving backfill will be needed in future to assist in the support of the hanging wall.
The author gratefully acknowledges the permission of Goldfields Pty Ltd to publish this information and recognises the ideas contributed by a number of past and present Kundana geologists, in particular H R Hadlow.
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REFERENCES Freeman, A T and Thom, J H, 1988. Kundana, a Brief History of an Early Gold Mining Area in the Eastern Goldfields of Eastern Australia (Scott Four Colour Print: Perth). Hadlow, H R, 1990. Structural controls on mineralisation in the Kundana South pit, Coolgardie Goldfield, WA, MSc (Prelim) thesis (unpublished), The University of Western Australia, Perth. Swager, C P, Ahmat, A L, Griffin, T J, McGoldrick, P J, Witt, W K and Wyches, S, 1990. Geology of the Archaean Kalgoorlie Terrane, Western Australia - An explanatory note, Geological Survey of Western Australia Record 1990/12.
Geology of Australian and Papua New Guinean Mineral Deposits
Hemming, G R, 1998. Geko gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 211–214 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Geko gold deposit by G R Hemming1 INTRODUCTION The deposit is 60 km west of Kalgoorlie, in the Coolgardie mineral field, WA, at lat 30o52′S, long 120o54′E on the Kalgoorlie (SH 51–9) 1:250 000 scale and Dunnsville (3036) 1:100 000 scale map sheets (Fig 1). It is 15 km north of the Resolute Samantha Bullabulling gold mine and 40 km SSE of the recent Reptile Dam gold discovery. The Geko deposit is owned by Nexus Minerals NL (Nexus, 70%) and Conquest Mining NL (Conquest, 30%).
blast drilling located a 1000 m long zone trending 080o which contained grades to 2.5 g/t gold over 38 m widths directly over the centre of Geko. Further drilling included aircore holes on a 50 by 50 m grid and 60o angled reverse circulation (RC) percussion and diamond holes to 180 m vertical depth. Newcrest tested a 350 m strike length at 20 to 40 m hole spacing along 50 m spaced lines. In 1992, Newcrest sold the property to Royal Harry Mines Pty Ltd (Royal Harry). In 1995 Nexus entered an agreement to earn 70% from Royal Harry, and Conquest acquired 30% prior to a drilling program, which concentrated on drilling the supergene zone of the deeply lateritised deposit. The Nexus-Conquest joint venture completed 21 665 m of RC and diamond drilling prior to October 1996 on a 25 by 10 m spacing to move to reserve status. This work has tested 1 km of the 4 km of prospective structure. Preliminary feasibility studies show a waste to ore ratio of 3.5 to 1. Testwork on the mottled and saprolite zones of the deposit gave heap leach recoveries ranging from 75 to 98% by agglomerating the ore, whereas recoveries to 98% were suggested for the transitional and primary zones by conventional CIP processing (P Seen and J Angove, unpublished data, 1996).
REGIONAL GEOLOGY
FIG 1 - Locality map and regional geological sketch map, Geko deposit.
Geko is in greenstone near the Ida fault on the western edge of the Bullabulling Domain in the Archaean Kalgoorlie Terrane and on the southern margin of an embayment in the Silt Dam monzogranite, a post-tectonic intrusive. The deposit may lie on the north-trending Bullabulling shear which localises gold mineralisation at Bullabulling. This shear may continue due north to the Jaurdi deposit or NNW to the Reptile Dam deposit.
Measured, Indicated and Inferred Resources estimated in October 1996 are 2.47 Mt averaging 1.45 g/t gold for 115 500 oz of contained gold (Table 1). There is no previous mining activity and the search for additional resources continues.
At Geko the host ultramafic and mafic sequence has been folded into an east-trending elongate structure, currently interpreted as synclinal, in a predominantly north striking region (Fig 2). The mineralised folded and sheared mafic host is completely contained within the ultramafic body and has a strike length of over 4 km.
EXPLORATION AND MINING HISTORY
ORE DEPOSIT FEATURES
The camp sites of early prospectors are the only evidence of historic exploration activity. Their loaming probably located low grade surficial pisolite lateritic gold 200 m north of Geko. Newcrest Mining Limited (Newcrest) joint ventured the ground in 1988 after a soil anomaly with a peak value of 52 ppb gold had been located over Geko. Termed the ‘408 Prospect’ Newcrest continued with auger drilling over an area concealed by alluvium coincident with a regional aeromagnetic feature related to an ultramafic body. Follow-up vertical rotary air 1.
Senior Geologist, Nexus Minerals NL and Conquest Mining Limited, 47 Ord Street, West Perth WA 6005.
Geology of Australian and Papua New Guinean Mineral Deposits
ROCK TYPES Gold mineralisation at Geko occurs in three adjacent geological settings: 1.
in lateritised moderately to steeply dipping quartzsericite mafic schist overlying a large ultramafic body (Figs 2 and 3);
2.
in the ultramafic rock 10 to 30 m below the mafic rock contact; and
3.
in a gold-enriched horizon overlying both deposits at 15 to 20 m depth, where gold is dispersed in grit at the base of a palaeochannel cut into the weathered Archaean rocks.
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G R HEMMING
FIG 2 - Geological plan of Geko deposit at approximately 10–20 m below surface, with location of section line.
212
Geology of Australian and Papua New Guinean Mineral Deposits
GEKO GOLD DEPOSIT
FIG 3 - Cross section on 1480 E local grid, Geko deposit, looking NE.
Geology of Australian and Papua New Guinean Mineral Deposits
213
G R HEMMING
TABLE 1 Geko and Humpback resource details.
Zone
Palaeochannel
Measured Resource
Indicated Resource
Inferred Resource
Ore (t)
Grade (g/t Au)
Ore (t)
Ore (t)
–
–
Grade (g/t Au)
Total Resource
Grade (g/t Au)
Ore (t)
Grade (g/t Au)
58 500
1.9
30 000
1.4
88 500
1.73
Mottled
23 300
1.44
191 100
1.3
118 000
1.4
332 400
1.34
Saprolite
38 700
1.39
264 300
1.3
136 000
1.6
439 000
1.40
Transitional
1 021 300
1.40
286 600
1.7
308 000
1.5
1 616 000
1.47
Total
1 083 300
1.40
800 500
1.5
592 000
1.5
2 475 900
1.45
Notes. 1.Source J L Baxter (unpublished data, 1996). 2.Upper cut 19 g/t gold. 3.Lower cut; 0.4 g/t gold, palaeochannel and mottled zone; 0.5 g/t gold, saprolite zone; 0.7 g/t gold, transitional and primary zones.
4.Bulk density - palaeochannel - 2.0; mottled zone - 1.5; saprolite - 2.2; transition and primary - 2.8 5.Resource estimate by computer block modelling, grades approximate. The mafic-hosted deposit contains little quartz vein material and gold is associated with sulphides. In the primary zone Geko gold mineralisation is in a sericite-quartz schist, but immediately NW of Geko the Humpback mineralisation is hosted by ultramafic rocks. Amphibolite facies metamorphism of sediment with a mafic volcanic association has produced a fine to medium grained sericite-quartz-hornblende schist with sulphide aggregates comprising 1 to 5% by volume.
STRUCTURE Mineralisation is thought to be controlled by an 080o trending shear, coincident in part with the ultramafic-mafic contact, over a length of about 4 km. The 45 to 90o south dipping contact is directly overlain by intensely altered quartz-sericite-biotitehornblende-pyrite schist. The Humpback ultramafic-hosted gold mineralisation occurs 10 to 40 m beneath the contact, associated with parallel shearing (Fig 2). The deposit comprises many overlapping zones of mineralisation dipping at 20 to 30o south within the 45 to 90o south dipping schist. Higher grade shoots plunging 30 o west have been noted.
WEATHERING ZONES The primary gold deposits are overprinted by a deep weathering profile forming extensive horizontal gold-enriched laterite layers over a length of 800 m and a maximum width of 100 m, concealed by some 20 m of transported alluvium. The lateritic profile is developed to about 80 m depth (Fig 3). There is a remnant of gold-mineralised pisolite regolith extending over an area of 300 000 m2 to a depth of 5 m at Geko North, about 300 m north of Geko. Drilling here showed up to 2.4 g/t gold over a 4 m hole length, from surface. In the mottled zone this supergene gold enrichment zone is flat topped, consistently begins at approximately 25 m vertical depth and extends to a maximum depth of 55 m. It is up to 100 m wide horizontally by 20 m thick vertically. Drilling results show a maximum of 19 m grading 2.82 g/t gold from a vertical depth of 35 m.
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Gold enrichment in the saprolite zone lies directly below the mottled zone. It is approximately 60 m wide horizontally by 20–30 m thick, at grades comparable to the mottled zone. The transitional oxide zone extends below the saprolite zone to a maximum vertical depth of 120 m and has a maximum horizontal width of 60 m. The best intersection was 37 m grading 2.42 g/t gold from a depth of 85 m.
PALAEOCHANNEL The intense weathering over the Geko mineralisation has allowed deeper erosion in a shallow broad drainage valley developed directly over the primary deposits. Gold is concentrated at the base of the palaeochannel in gritty sediment at 15 to 20 m depth, generally at grades above 1 g/t gold and to a maximum of 6 m at 18.41 g/t gold from a depth of 19 m, as reflected in the slightly elevated resource grade for this ore type (Table 1).
PRIMARY MINERALISATION The primary zone was been tested to 180 m vertical depth and showed wide zones of mineralisation with poorly defined high grade shoots. One such interval was 6 m at 15.80 g/t gold, from a depth of 80 m. A westerly plunge is interpreted for these shoots, which may be tested by exploration below the Silt Dam monzogranite where reverse faulting has rafted the granite over greenstone.
ACKNOWLEDGEMENTS The author gratefully acknowledges the permission of Newcrest Mining Limited, Nexus Minerals NL and Conquest Mining Limited to publish the information. Thanks are also extended to the management and staff for their support and to service groups D Rafty and Associates, Continental Resource Management Pty Ltd, Oretest Pty Ltd and Micromine Proprietary Ltd, for their contribution and permission to cite their work.
Geology of Australian and Papua New Guinean Mineral Deposits
Ivey, M E, Fowler, M J, Gent, P G and Barker, A J, 1998. Centurion gold deposit, Binduli, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 215–218 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Centurion gold deposit, Binduli 1
2
3
by M E Ivey , M J Fowler , P G Gent and A J Barker INTRODUCTION The deposit is 12 km SW of Kalgoorlie in the Eastern Goldfields of WA (Fig 1), at lat 30o50′S and long 121ο25′E on the Kalgoorlie (SH 51–9) 1:250 000 scale map sheet. Centurion is one of several recently discovered gold deposits which collectively form the Binduli project, and is wholly owned by Croesus Mining NL.
4
Modern open pit production from Binduli commenced in March 1993, and to July 1997, 1 815 087 t of ore grading 2.3 g/t gold had been milled for 3860 kg of recovered gold (92% recovery). The total Measured, Indicated and Inferred Resource for Binduli at July 1997 was 19.3 Mt at 1.4 g/t gold for 27 000 kg of contained gold. The Centurion resource comprises 2.6 Mt at 1.9 g/t gold for 4900 kg of contained gold.
EXPLORATION HISTORY The Binduli mining centre has been intermittently explored since its discovery in 1897, and production to 1942 is reported to be 7 kg of gold from 358 tons of ore (Department of Mines, 1954). The Binduli area was explored for base metals by BHP Ltd and Dampier Mining Pty Ltd in the late 1960s. Systematic exploration for gold began in 1987 when Defiance Mining NL acquired title to the area and undertook an intensive gold exploration program concentrating on four known gold prospects in the centre of the mining centre. Australian Consolidated Minerals Limited (ACM) entered into a joint venture with Defiance in 1990 and commenced more regional style exploration, initiating extensive minus 80 mesh soil sampling for gold on 200 by 50 m centres. Soil anomalies were identified over a 10 km strike length on the western side of the project area. Follow-up rotary air blast (RAB) drilling to a set depth of 40 m was carried out at all +30 ppb anomalies and numerous narrow, low grade intercepts were reported. ACM withdrew from the joint venture in 1991.
FIG 1 - Regional geology and location map, Binduli mining centre.
1.
Exploration Manager, Croesus Mining NL, 39 Porter Street, Kalgoorlie WA 6430.
2.
Senior Exploration Geologist, Croesus Mining NL, 39 Porter Street, Kalgoorlie WA 6430.
3.
Senior Geologist, Croesus Mining NL, 39 Porter Street, Kalgoorlie WA 6430.
4.
Formerly Project Geologist, Croesus Mining NL, now Exploration Geologist, Kalmet Resources NL, 10 Spring Street, Sydney NSW 2000.
Geology of Australian and Papua New Guinean Mineral Deposits
Croesus Mining NL purchased the Binduli titles from Defiance in 1993 and commenced open pit mining of the previously defined 80 000 t Pitman deposit. Exploration by Croesus commenced at the same time, with work concentrating on following up the western area of anomalism identified by ACM. Drill testing to unweathered rock by RAB and later aircore drilling identified several new gold-bearing zones in saprolite. Resource definition drilling of these zones in mid 1993 outlined supergene gold mineralisation at the Centurion and Ben Hur deposits (Fig 2). The Centurion deposit is more important as it has significant primary mineralisation below the supergene zone. Primary mineralisation at Centurion is divided into Western contact mineralisation (WCM) and Eastern contact mineralisation (ECM). The WCM was discovered as deeper drilling tested the primary zone directly beneath the main supergene blanket. The ECM was discovered in 1995, two years after the commencement of mining at Centurion, when a series of stacked, gold-bearing quartz veins exposed by mining were drill tested from the pit floor. The ECM contains approximately 50 000 oz of gold in 12 g/t ore, and is distinctive for its high pyrite and base metal content. ECM and WCM are both free milling ores.
215
M E IVEY et al
FIG 2 - Schematic geological map of the Binduli deposits.
REGIONAL GEOLOGY The Binduli deposits are in the Kalgoorlie Terrain (Swager et al, 1995) of the Norseman–Wiluna greenstone belt within the Eastern Goldfields Province of the Archaean Yilgarn Block. The Centurion deposit (Fig 1) is within the Black Flag Group of the Ora Banda Domain (Swager et al, 1995), one of the four major domains which comprise the Kalgoorlie Terrain. The regional stratigraphic succession (Witt, 1994; Swager et al, 1995) shows the lowest exposed unit to be a thick basaltic sequence overlain by extensive komatiite flows which are overlain in places by an upper basaltic unit. The upper part of the succession comprises felsic volcaniclastic and pyroclastic rocks of the Black Flag Group, which are unconformably overlain by the conglomeratic Kurrawang Formation immediately to the west of Binduli. The Binduli area is within a broad antiform adjacent to the Kurrawang Syncline (Hunter, 1993), formed during ENEWSW compression which produced upright folds with northto NW-trending fold axes. Continued ENE-WSW shortening has resulted in NW- to NNW-trending strike-slip faults and shear zones (Swager et al, 1995). Major north- to NW-trending structures are interpreted to the west and east of Binduli, with the Zuleika Shear Zone about 8 km to the west and the Abattoir shear about 3 km to the east.
ORE DEPOSIT FEATURES PRIMARY HOST ROCKS The Centurion mine sequence consists of felsic volcaniclastic sedimentary and pyroclastic rocks dipping at 60o SW which are intruded by feldspar porphyry. A thin continuous unit of graphitic black shale occurs on the eastern side of the deposit (Fig 3).
216
FIG 3 - Simplified geological map, Centurion deposit, with location of cross sections Figs 4 and 5.
Western contact mineralisation The WCM zone (Figs 3 and 4) is hosted by felsic volcaniclastic sedimentary and tuffaceous rocks and feldspar porphyry. Mineralisation is related to shallowly SW-dipping quartz veins with associated bleaching, silicification and pyrite alteration. The mineralisation has a northerly trend which is oblique to the NNW strike of the host rocks. The felsic pyroclastic rocks are massive to weakly foliated and well bedded, and comprise fine grained ash tuff to coarse grained crystal tuff, lapilli tuff and felsic agglomerate, with individual units to 30 m thick. These units are mostly plagioclase- and quartz-rich with a quartzofeldspathic groundmass making up 20 to 40% of the rock, plus variable amounts of sericite and carbonate. Felsic agglomeratic rocks contain large, subangular to rounded clasts to 2 m wide, commonly within a fine grained quartz-feldspar matrix. The clasts are commonly felsic porphyry. The porphyry bodies are usually 40% plagioclase phenocrysts from about 0.5 to 4 mm diameter in a quartzfeldspar matrix. Phenocrysts of quartz and potassium feldspar are also present and sericite is commonly developed. The porphyry is variably deformed with strong shearing common on the margins, but is internally massive and conformable with the sequence on its western contact (Fig 4). The porphyry body is interpreted to be a NNW-trending elongated boudin.
Eastern contact mineralisation The ECM occurs within a small juxtaposed flat-lying lens of fine grained sediment within felsic rocks along a well defined,
Geology of Australian and Papua New Guinean Mineral Deposits
CENTURION GOLD DEPOSIT, BINDULI
FIG 4 - Cross section through the Centurion deposit, location on Fig 3.
steeply dipping, felsic volcanic–black shale contact immediately above a porphyry intrusive (Fig 4). This unit has a maximum strike length of 240 m, is up to 40 m wide and between 5 and 20 m thick. This dark grey to black, silicified sediment has a distinct sulphide banding which is thought to represent bedding. Footwall to the ECM sediment host is a grey-green, sericitic and variably silicified porphyry which becomes strongly hematitic away from the mineralised zone. Hanging wall rocks comprise a wide agglomerate unit and felsic volcaniclastic rocks which contain WCM style veining. The agglomerate contains clasts of felsic tuff, sediment and porphyry within a fine grained matrix.
STRUCTURAL CHARACTERISTICS Gold mineralisation in the WCM zone is associated with a brittle quartz-vein array dominated by shallowly dipping quartz veins. The ore envelope has a strike length of about 400 m and a maximum width of 100 m, and plunges at about 20o to the SSE. Bedding-parallel faults with a reverse sense of movement are common, often offsetting the flat-lying quartz veins up to 4 m (Fig 5). Thin mineralised quartz veins are developed along the reverse fault structures. The graphitic black shale on the eastern side of the Centurion deposit dips steeply east, parallel to the contact with the felsic volcanic unit. A pervasive, vertical to steeply NE-dipping S2 or S3 cleavage, developed in the black shale, represents a zone of strong deformation with a mineral elongation lineation plunging at 15 to 20o SE. Clasts within the agglomerate unit plunge at 15 to 20o to the SSE. Sampling of quartz veins shows a strong relationship between gold and quartz veins. Common quartz vein
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 5 - Cross section through Western contact mineralisation at the Centurion deposit, location on Fig 3.
orientations for WCM are 320o strike and 25ο SW dip and 330o strike and 65ο SW dip. Quartz vein widths range from 0.1 to 10 cm with an average of 3 cm, with rare veins to 100 cm wide. Veins have a frequency of one to two per metre and are commonly spaced greater than 40 cm apart. They can be traced laterally over 30 m, with vein margins ranging from straight to slightly irregular. The ECM has a strike length of 240 m. It trends parallel to the black shale on its eastern side and plunges moderately to the NNW. Various quartz vein sets are present, however their relationship to the gold mineralisation is unclear. Vein orientations are mostly flat, plunging west to NW with vein widths from <1 to 50 cm, averaging 10 cm.
217
M E IVEY et al
ALTERATION AND MINERALOGY Wall rock alteration in the WCM is characterised by a pale grey bleaching due to silica-sericite-pyrite alteration, with accessory carbonate and biotite proximal to quartz veins. This alteration style occurs in all rock types. There is a direct correlation between vein width and the width of the alteration halo. The major opaque mineral is pyrite which ranges locally to 10% proximal to veining, with magnetite, chalcopyrite and galena occurring as accessory minerals. The ECM is intensely silicified and highly sulphidic. The sulphide assemblage consists of pyrite with accessory chalcopyrite, galena and sphalerite, and minor fluorite.
WEATHERING AND SUPERGENE MINERALISATION All rock types have been extensively weathered, with the base of oxidation ranging from about 40 m in porphyry to 60 m in volcaniclastic rocks. The upper part of the weathering profile has been truncated to the upper saprolite zone, which has a thin covering of aeolian sand and playa lake clay. The upper part of the preserved profile has been intensely leached and weathered to kaolinite, quartz and clay. Supergene mineralisation related to the primary WCM in the upper part of the profile occurs as subhorizontal zones from 3 to 18 m thick which are interpreted to have formed at palaeoredox fronts in the profile. This mineralisation trends north to NW and is up to 200 m wide. In the lower part of the weathering profile at the transition zone or lower saprolite, mineralised zones display relict primary textures and orientations. Supergene mineralisation is not well developed above the primary ECM.
MINERALISATION CONTROLS
limb of the antiform. Late subvertical movement along this shear may have resulted in the development of the shallowly SW-dipping quartz vein set. These veins cut through all rock types and are generally less than 10 cm wide. Primary mineralisation is associated with this quartz vein array with variable bleaching of host rocks adjacent to veining. Silica-sericite-pyrite alteration is dominant, with weak carbonate alteration present. Widespread hematite and magnetite alteration is also present, although its association with gold mineralisation is unclear. Sulphide and base metal content are very low except in the high grade parts of the ECM. The mineralised zone has an overall north alignment, crosscutting the NNW-striking sequence. Mineralisation at all the Binduli deposits is confined to a strike-parallel corridor over 8 km long, and all of the deposits are immediately adjacent to porphyry bodies.
MINE GEOLOGICAL METHODS Mining at Binduli has been by open cut methods. Ore is hauled to the Croesus Hannan South treatment plant 19 km to the SE by road where the ore is treated by conventional CIP methods. Bench height is 2 m to allow for accurate excavation of the subhorizontal supergene mineralisation. A 5 by 5 m pattern of aircore drilling is used for grade control purposes, with holes drilled at -60ο east and sampled on 1 m intervals.
ACKNOWLEDGEMENTS This paper is published with the permission of Croesus Mining NL. W Lally and H Wiechecki are thanked for their valuable contribution in the early stages of exploration at Binduli.
REFERENCES Department Of Mines, 1954. List of Cancelled Gold Mining Leases.
Primary gold mineralisation at Binduli is hosted by a wide range of rock types of the Black Flag Group. Mineralisation is best developed on or adjacent to major rock unit contacts. In plan, mineralisation is oblique to the strike of host rocks and has a northerly trend. Mineralised shallowly SW-dipping quartz veins occur throughout the deposit.
Hunter, W M, 1993. Geology of the granite-greenstone terrane of the Kalgoorlie and Yilmia 1:100 000 sheets, Western Australia, Geological Survey of Western Australia, Report 35.
Feldspar porphyry is interpreted to have conformably intruded the felsic sequence in the hinge of an antiform. East of the porphyry, the east-dipping black shale is strongly deformed and thus may represent a shear zone which truncates the eastern
Witt, W K, 1994. The geology of the Bardoc 1:100 000 sheet, Western Australia, Geological Survey of Western Australia, Explanatory Notes, 3137.
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Swager, C P, Griffin, T J, Witt, W K, Wyche, S, Ahmat, A L, Hunter, W M and McGoldrick, P J, 1995. Geology of the Archaean Kalgoorlie Terrane - an explanatory note., Geological Survey of Western Australia, Report 48.
Geology of Australian and Papua New Guinean Mineral Deposits
Copeland, I K and the Geological Staff of New Hampton Goldfields NL, 1998. The Jubilee gold deposit, Kambalda, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 219–224 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Jubilee gold deposit, Kambalda 1
by I K Copeland and the Geological Staff of New Hampton Goldfields NL
2
INTRODUCTION The deposit is 35 km south of Kalgoorlie, WA, and 15 km north of Kambalda within the Archaean Norseman–Wiluna granitegreenstone terrain, at lat 31o00′S, long 121o36′E on the Widgiemooltha (SH 51–14) 1:250 000 and Lake Lefroy (3235) 1:100 000 scale map sheets. The deposit is one of many, including the Kalgoorlie ‘Golden Mile’, within the Boulder–Lefroy fault zone (Fig 1). Jubilee forms part of a major mineralised system which extends for over 4 km and includes recent and current open pit mining operations at New Celebration, Mutooroo, Hampton Boulder and Golden Hope. Jubilee was explored, evaluated and mined by Hampton Areas Australia Ltd (HAAL) between 1984 and 1996 with the final phases of open pit mining being carried out by New Hampton Goldfields NL from June 1996 to May 1997. The mineralised system is subvertical and extends for at least 200 m below the initial pit floor. As underground mining of the adjacent Hampden Boulder deposit had reached 600 m depth by early 1997, exploration and evaluation of the deeper section of the orebody is ongoing.
EXPLORATION AND PRODUCTION HISTORY Jubilee is on freehold Location 48, one of many such Locations pegged and purchased by the Hampton Land and Railway Syndicate in 1882 as pastoral leases. Following discovery of gold in the region in the 1890s (Gresham, 1991) the owners commenced some involvement in prospecting but it was not until 1919, after a period of decline in gold mining, that discoveries such as Celebration, Dawns Hope and White Hope were made. The Celebration mine, discovered in September 1919 by Hansen and Ireland, was named to mark the victory in the Great War. A town called Celebration City was established adjacent to the Celebration mine, and served as centre for several mines. In 1920 gold was discovered at Golden Hope and Mount Martin. By the mid 1920s a falling gold price and a decline in water availability saw most of the mines in the area close and Celebration City became a ghost town. Interest was revived in early 1980 when a joint venture was formed between HAAL and Newmont Australia Ltd covering Locations 48 and 50. A joint venture covering the entire Location 50 was initially established but HAAL subsequently elected to excise small areas from Location 48 after recognition of their substantial exploration potential. The Jubilee lease was one such excised area. By 1984 soil sampling, rotary air blast and percussion drilling outlined major gold resources for the
1.
Formerly Chief Geologist, Jubilee Operations. Present address, Kundana Gold Pty Ltd, PO Box 622, Kalgoorlie WA 6430.
2.
9 Havelock Street, West Perth WA 6005.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Location map.
joint venture partners at Celebration and Hampton Boulder (Norris, 1990) and for HAAL at Jubilee. Open pit mining commenced at Jubilee in November 1987. The mill, commissioned in early 1988, was originally designed to treat 600 000 tpa but was upgraded in 1989 to 1.1 Mtpa in response to a major increase in the mineable ore reserves in Jubilee open pit. The Jubilee pit was mined to 210 m depth and its final surface dimensions were approximately 650 by 300 m. During the ten year pit life a total of 10 323 518 t of ore were mined and 693 689 oz of gold were recovered, for a recovered grade of 2.09 g/t (M D Burrows, unpublished data, 1997; I K Copeland and G Pickens, unpublished data, 1992).
219
I K COPELAND and THE GEOLOGICAL STAFF OF NEW HAMPTON GOLDFIELDS NL
REGIONAL GEOLOGY The Boulder–Lefroy fault zone (Swager, 1989) extends SSE from the Kalgoorlie area through the Jubilee–New Celebration and Kambalda mining districts to the Victory–St Ives mining district, a distance of over 70 km (Fig 1). The fault zone is one of many tectonic zones which broadly parallel the dominant strike of the volcano-sedimentary sequences in the Norseman–Wiluna greenstone belt (Ho, Groves and Bennett, 1990). A consolidated interpretation of the nature and movement history on the Boulder–Lefroy fault zone has not been produced but most workers have concluded that it is a long-lived structure with a complex history of movement (Archibald, 1990; Keele, 1991). The greenstone sequence in the Kalgoorlie–Kambalda region comprises a lower basalt unit dominated by tholeiitic and high magnesium basalts overlain by a relatively thick sequence, often exceeding 1 km, of ultramafic rocks with a high proportion of komatiite flows and thin interflow sediments. This komatiite unit is overlain by another basalt sequence, the upper basalt unit, again comprising both high magnesium and tholeiitic basalt, characterised by many extensive coarser grained mafic units inferred to be intrusive sills. The uppermost unit is a felsic volcanic and sedimentary unit which includes a wide range of volcaniclastic and sedimentary rocks and is widely referred to in the literature as the Black Flag beds. Mafic sills also frequently occur in this stratigraphic interval. Within the broad corridor centred on the Boulder–Lefroy fault zone, the lower stratigraphic units are extensively exposed and often intruded by relatively thin, semiconcordant felsic units commonly referred to as porphyries. Thick sequences of the Black Flag beds flank this Boulder–Lefroy corridor to the east and west.
ORE DEPOSIT FEATURES The area between the Celebration and Golden Hope pits (Fig 2) is characterised by anastomosing segments of the Boulder–Lefroy fault zone which predominantly transect the lower parts of the stratigraphic sequence and are locally known as the Hampton Boulder fault zone. The Jubilee–Hampton Boulder mineralised system is flanked to the east and west by extensive ultramafic units. A substantial, coarse grained, differentiated mafic unit abuts the main mineralised zone to the west.
GEOLOGY OF THE JUBILEE PIT The Jubilee deposit is within the Hampton Boulder fault zone. There are differentiated dolerite and felsic sedimentary rocks in the western side of the pit whereas highly altered ultramafic rocks predominate on the eastern side (Fig 3). Mineralisation is centred upon a series of broadly concordant felsic porphyry dykes dipping at around 85o west, which were intruded along the Hampton Boulder fault zone (Fig 4) into deformed and highly schistose rocks. Gold is typically associated with silicification and pyrite. The geology of the pit is discussed below with emphasis on structural characteristics which are regarded as the principal controls of gold mineralisation.
Lithological sequence (east to west) Ultramafic rocks The east wall of the Jubilee pit is characterised by fine grained, homogeneous ultramafic rocks (Uk, Figs 3 and 4). Fresh
220
FIG 2 - Regional geology of the Celebration–Jubilee–Dawn’s Hope district.
samples comprise pale green, equigranular talc-chloritecarbonate schist after komatiitic ultramafic extrusives. The unit has been strongly deformed, resulting in the destruction of many of the primary textures. However, komatiitic flow characteristics with preserved flow morphologies (A and B zones) are identifiable. There is also a change from higher to lower magnesium komatiites with olivine-dominant flow top spinifex zones in the centre of the pit changing to pyroxenedominant spinifex zones in the SE of the pit (N J Archibald, unpublished data, 1992). The Footwall shear zone separates this unit from the central domain and main ore zones (Fig 3).
Schist zone This is a zone of schistose rocks (Ubs and Uts, Figs 3 and 4) intruded by several semiconcordant felsic porphyry bodies, and bounded to the east by the Footwall shear and to the west by the Mylonite fault. The zone is inferred to represent the trace of the Hampton Boulder fault zone and comprises highly deformed and often strongly altered rock types. The foliated schists consist of carbonate, chlorite, biotite and talc, in varying proportions and include fine to coarse disseminated pyrite. Carbonate alteration is very strong and carbonate and quartzcarbonate veining is common.
Porphyries Several felsic and intermediate porphyries, aligned parallel to the Hampton Boulder fault zone, have intruded the sequence (Gpm, Figs 3 and 4). Boudinage is common within the porphyries whose thickness varies from <1 m to 50 m. The porphyries have strong strike and dip continuity (>400 m), and pinch and swell along strike and down dip.
Geology of Australian and Papua New Guinean Mineral Deposits
JUBILEE GOLD DEPOSIT, KAMBALDA
The felsic porphyry is medium to coarse grained and contains subhedral relict phenocrysts of orthoclase, now replaced by carbonate. The groundmass is fine grained and consists of carbonate, sericite, quartz and disseminated euhedral sulphides. Intermediate porphyries are similar in texture to the felsic porphyries but are characterised by an abundance of biotite, either fresh or pseudomorphing amphibole. Subhedral phenocrysts of feldspar give the rock a porphyritic appearance and the groundmass consists of biotite-carbonatechlorite±tourmaline and acicular actinolite. Pyrite is fine to medium grained and comprises up to 5% of the rock. Both felsic and intermediate porphyries have quartz, carbonate and quartz-carbonate veining. Gold grades in the felsic porphyry are generally lower than in intermediate porphyry.
Intermediate sediments Separating the porphyry-intruded schist zone and the intermediate sediments is the highly strained Mylonite fault zone, now represented by biotite-feldspar-quartz gneiss.
FIG 3 - Generalised geological plan, Jubilee pit, with location of section line for Fig 4.
Sedimentary structures are present in weathered rock but fresh hand specimens are almost featureless. The sediments (Fss, Figs 3 and 4) are thin to thick bedded with horizons of flattened angular pebble-sized clasts of adjacent sediments (N J Archibald, unpublished data, 1992). The sediments are thickest in the central and southern portions of the pit and are bounded by shears on the hanging wall and footwall. This unit underlies the Jubilee Dolerite.
Jubilee Dolerite The Jubilee Dolerite (Dg, Figs 3 and 4) is strongly differentiated. A basal (eastern) pyroxenite, now tremolitechlorite±carbonate schist, passes into a melanocratic dolerite (micro-norite) which contains equant blastophenocrysts after pyroxene. The dolerite becomes more equigranular with increasing interstitial magnetite and granophyre before becoming a magnetite granophyre and passing upwards into a fine grained magnetite-rich dolerite characterised by bladed pyroxene pseudomorphs, and finally into a dolerite which becomes progressively finer grained towards the top contact. This zoning is a composite reconstruction from pit mapping and can not be observed within a single unbroken section. The zoning is similar to that observed in the Defiance Dolerite in the vicinity of the Victory-Defiance gold deposits at St Ives, south of Kambalda (N J Archibald, unpublished data, 1992).
Felsic and intermediate porphyries The contact between the Jubilee Dolerite and the overlying ultramafic rocks is marked by the Pisces shear. This contact is strongly disrupted by highly deformed and metasomatised felsic (quartz-feldspar) and intermediate (biotite-feldspar) intrusive porphyries which are not shown individually on Fig 3.
Ultramafic rocks
FIG 4 - Cross section through the central part of the Jubilee pit, looking NNW.
Geology of Australian and Papua New Guinean Mineral Deposits
Blocky talc-carbonate-chlorite rocks (Uk, Fig 3) overlie the Jubilee Dolerite to the west. Original komatiitic flow textures have not been recognised but the rocks are compositionally low magnesium komatiites.
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I K COPELAND and THE GEOLOGICAL STAFF OF NEW HAMPTON GOLDFIELDS NL
Structure The Jubilee pit can be broadly subdivided into eastern, central and western structural domains (Fig 3). The eastern domain rocks are east younging, based on komatiite flow morphologies, whereas the western domain rocks are west younging, based on the differentiation sequence within the Jubilee dolerite. Thus the Hampton Boulder fault zone separates blocks with opposing younging directions, and can be inferred to represent the faulted-out hinge zone of an antiform. In general, the structure within the pit is strongly controlled by the distribution of the high strain shear zones and associated splay faults. The nature of the rock type being transected is reflected in the nature of structures and tectonite fabric produced. There is a strong contrast between the deformation style developed in the eastern domain when compared with that developed in the central and western domains.
Eastern domain The eastern domain is bounded on its western boundary by the Footwall shear, a major shear zone to 20 m wide consisting of talc-carbonate-chlorite schist. The shear zone is steeply dipping and kinematic fault direction indicators suggest a complex movement pattern. The dominant later movements are west block down. Invariably this movement folds an earlier, now flatter foliation. Extreme examples demonstrate strong vertical extension with flattened orange pip–shaped boudins of talc-chlorite-carbonate rock being refolded into steeply-plunging sheath folds. Some of the anastomosing shears within the Footwall shear show minor strike-slip movement. Both dextral and sinistral displacements are observed with sinistral movement predominant. Many shears to the east form plumose structures radiating from the Footwall shear (Fig 3). Flat NW-dipping shears become progressively steeper, curving in and more asymptotic as they get close to the main shear. Grossly these shears have the morphology of upward palm or geopetal structures generally attributed to wrench structures, although at Jubilee it is demonstrably clear that such structures have been generated by steeply dipping, not strike-slip, movements. Whether such structures have been generated solely by a single fault episode is unclear, but they probably formed by rotation of earlier shears by later steeply-dipping faults. The complex generation of slickensides on fault surfaces may attest to the latter. The plumose faults are commonly cut by steep structures subparallel to the Footwall shear. The intersection of such steep shears with the flatter shears dipping towards the pit results in significant geotechnical problems. The talcose, chloritic nature of the shear planes, when lubricated by water, provides little or no shear resistance along the fault planes. Thus failure along ‘greasy heads’ has been a continual problem over the life of the pit. Fortunately, planned pit excavation was completed without substantial sterilisation of ore by pit wall failure.
Central domain The central domain represents the Hampton Boulder fault zone and is bounded by the Footwall shear to the east and the Mylonite fault adjacent to the biotite-feldspar sedimentary gneiss or metasediment to the west. The two fault zones are slightly divergent at the southern end of the pit, leaving a wedge of lower strain tremolite-talc-chlorite rocks which thin out
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towards the middle of the pit (Fig 3). The central domain makes an acute angle with the overall enveloping surface of layered rocks in the western domain. Mesoscopic fabric relationships within the central domain are complex. Fault kinematic indicators indicate multiple movement rather than a single generation of faulting. These movements apparently spanned a range of metamorphic and/or hydrothermal conditions: 1.
Fabrics developed in the Mylonite fault are biotitefeldspar±andalusite stable indicating this zone of high strain developed synchronously with peak (upper greenschist–lower amphibolite facies) metamorphism. Biotite and/or amphibole mineral lineations associated with this fabric are always steep.
2.
Many prograde metamorphic fabrics developed in both intermediate lamprophyre porphyries and boudins of sediment are retrogressed to biotitechlorite±carbonate±pyrite assemblages.
All fabrics indicate that strong near-vertical extension dominated within the central domain. Early-formed biotite lineations are steep and boudin necks within deformed porphyries are subhorizontal, with flat-lying extensional quartz veins commonly developed within the necks. The relationship of these structures to mineralisation is discussed later.
Western domain The western domain is characterised by heterogeneous deformational fabrics in strongly foliated, high strain zones transecting less deformed rocks. The main deformation zone, the Pisces shear, separates the Jubilee Dolerite from the overlying talc-carbonated low magnesium komatiites. The shear is oriented slightly oblique to the central domain, resulting in a fault-bounded wedge-shaped segment of Jubilee Dolerite and sediment that is thickest in the southern portion of the pit and wedges out to the north. Zones of very high strain, with dominant planar foliations, as along the Pisces shear, are separated from zones of lower strain where foliation development is irregular and commonly anastomosing. The Pisces shear, especially in the northern portion of the pit, is transected by a series of steep conjugate strike-slip faults with <1 m displacement and very strong extensional jointing. These structures, combined with the intense foliation, make these fabrics particularly conducive to toppling failure, as seen in the northwestern wall of the Jubilee pit.
MINERALISATION Gold mineralisation is strongly focussed by the structures of the Hampton Boulder fault zone and is distributed through highly schistose rocks intruded by semiconcordant felsic and intermediate porphyries. The main ore zone in the Jubilee pit has a strike length of 800 m and an average width of around 40 m. The northerly continuation of the ore zone into the adjoining Hampton Boulder pit extended for a further 1100 m and was generally narrower. The northern end of the Jubilee pit is defined by the tenement boundary. At the southern end the ore zone narrows to 3 m and is associated with the contact between porphyries. Jubilee mineralisation is concentrated in the central domain and is associated with the felsic and intermediate porphyries, probably owing to their competent nature and chemical composition compared to the enveloping schists.
Geology of Australian and Papua New Guinean Mineral Deposits
JUBILEE GOLD DEPOSIT, KAMBALDA
Coarse grained free gold is very rare. Gold mainly occurs as particles of 2–10 µm diameter in pyrite and quartz-carbonate veins. Pyrite is the predominant sulphide and occurs as fine subhedral grains oriented along foliation planes or within quartz-carbonate veins. Chalcopyrite, sphalerite, galena and pyrrhotite are present in trace amounts. Magnetite is common in the dolerite but relatively rare in the ore hosts. The high grade ore shoots step in an en echelon manner, which accounts for the apparent lateral displacement of the New Celebration from the Jubilee mineralisation. At Jubilee, large grade variations are evident across strike and small variations occur along strike and down dip. Broadly the patterns of gold mineralisation are as follow: 1.
2.
3.
Mineralisation is grossly controlled by the Mylonite fault and the Footwall shear and appears best developed where these structures coalesce, eg at the northern end of the Jubilee pit. In detail, gold distribution is highly influenced by rock type, eg the highest grades are developed in altered intermediate porphyries, whereas in the silicified felsic porphyries grades are lower. Spatially there are a number of empirical observations which singly or severally may indicate influences on the deposition of gold. These are: a)
The best gold grades are developed where the Jubilee Dolerite abuts the Mylonite fault.
b)
The grades are generally higher within the zone where the Mylonite fault and the Pisces shear are juxtaposed. This is also the area with many smallscale second order cross faults and orthoclase and/or biotite-chlorite-pyrite alteration.
c)
Gold grades are weakest in the central portion of the pit where the hanging wall sediments and silicified quartz porphyries attain their maximum thickness.
d)
e)
4.
5.
Silicification, either pervasively or as quartz veins along tensional openings, is not by itself an indication of mineralisation. The presence of sulphides associated with this silicification is a better guide. The gold mineralisation appears to be late in the metamorphic-structural sequence and is associated with retrogression assemblages and largely potassium-sulphur metasomatism, reflected in biotite-chlorite-orthoclase-sulphide assemblages.
Mineralisation adjacent to the Pisces shear is localised along flat to moderately dipping quartz extension veins which are generally only mineralised where they transect the magnetite-rich granophyric zones of the Jubilee Dolerite. Within the main ore zone of the central domain, three different patterns of gold distribution have been identified from detailed variography in blast hole gold assays. Gold distribution and controls within the central silicified quartz porphyry are significantly different from those within the flanking western and eastern porphyry ore zones. Recognition of these ore types greatly aided grade control and resource and reserve estimation.
The inability to correlate the observed distribution of gold mineralisation with observed mesoscopic structures prevents a thorough understanding of the precise controls of gold
Geology of Australian and Papua New Guinean Mineral Deposits
desposition. Although specific controls of gold distribution have not been identified, the majority of available data points to mineralisation being strongly focussed by the Hampton Boulder fault zone. Early formed fabrics indicate its instigation was before or synchronous with peak metamorphism, although later fabrics, including those related to gold deposition and metasomatic alteration, are largely retrograde. Although the precise structural controls of localisation of gold mineralisation are unclear, the strong coincidence of conjugate cross faults of limited strike-slip displacement may indicate that depositional controls are related to late-stage lock-up structures (akin to kink bands) within a reactivated fault zone. The grade of gold mineralisation is closely linked to rock type, with higher grades associated with biotite-chlorite-carbonate±magnetite-sulphide rocks (altered intermediate porphyry and dolerite).
NATURE OF THE BOULDER–LEFROY FAULT ZONE AT JUBILEE The mineralisation at Celebration which is the northern continuation of the Jubilee deposit is controlled by the Pisces shear zone whereas the Mylonite and Footwall shear zones are the major influencing structures at Jubilee. These zones form part of a system of anastomosing shears within a high strain zone representing the Boulder–Lefroy fault zone. This structure has been long recognised as a significant feature which has had a profound influence on the localisation of gold mineralisation in the Paddington, Golden Mile, New Celebration–Jubilee and Victory–Defiance districts. However, very little detailed work has been undertaken to define the nature of the fault and the mechanisms of its formation. Recent consensus in the literature (Swager, 1989; Keele, 1991) is that it is a crustal wrench fault, and most workers have inferred multiple episodes of movement. Mapping in the Jubilee pit has provided some detail of the nature of the Boulder–Lefroy fault zone in the Jubilee–New Celebration area. Pertinent observations are: 1.
The fault is a major structure which results in juxtaposition of rocks of opposing facing ie west of the Mylonite fault everything is west younging, east of the Footwall shear the sequence is east younging. The enveloping surface to the succession is subparallel to the fault zone.
2.
The fault appears to be a relatively long-lived structure with early fabrics related to the Mylonite fault developed synchronous with the metamorphic peak. These are overprinted by later fabrics associated with retrogression.
3.
Both prograde and retrograde fabrics are dominated by structures indicating that dominant movement and extension was steep to vertical, as defined by steep mineral lineations, flat extension veins and orientation of boudin necks. Strike-slip movement appears late in the fault kinematic history and related to minor movement subparallel to the fault along pre-existing shears or on conjugate cross faults cutting the main shear fabrics at high angles.
The fabrics are not consistent with a crustal wrench which should be dominated by subhorizontal to horizontal fabrics and kinematic movement indicators. They indicate instead that relative to the present steep attitude of the faults, movement was largely dip slip. A more consistent explanation for the
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I K COPELAND and THE GEOLOGICAL STAFF OF NEW HAMPTON GOLDFIELDS NL
origin of the Hampton Boulder fault is that it is either a reoriented thrust which has been steepened during subsequent deformation, or it is a major lateral ramp (transfer fault) interconnecting low level thrusts (Archibald, 1990).
CONCLUSION The Jubilee–Hampton Boulder mineralised system is one of the many major deposits within the Boulder–Lefroy corridor. Whereas the morphology of these deposits is quite variable in detail, the influence of the Boulder–Lefroy fault zone and the predominant control on distribution of gold by the component segments of the fault zone is well illustrated in the Jubilee pit. With production from the corridor (Ho, Groves and Bennett, 1990) comprising Paddington (1.2 M oz), Kalgoorlie Golden Mile (45 M oz), Hampton Boulder-Jubilee (1.5 M oz), VictorySt Ives (1.5 M oz) and Junction (1 M oz), continuing exploration along this corridor seems assured for many years to come.
MINE GEOLOGICAL METHODS Conventional drill and blast methods were employed on 5 m flitches. Ore block definition was based on both lithological and grade control assay data. A top grade cut of 10 g/t and average bulk density of 2.77 t/m3 were used, and high (>1.2 g/t), medium (0.8–1.2 g/t) and low (0.4–0.8 g/t) grade ore categories were adopted. Over the life of the pit reconciliation between predictions from block models and actual production was excellent with increases in both tonnes (15%) and grade (3%).
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ACKNOWLEDGEMENTS The authors would like to thank New Hampton Goldfields NL for facilitating the compilation and publication of this information. The early contribution by N J Archibald to the understanding of the geology and structure at Jubilee is acknowledged.
REFERENCES Archibald, N J, 1990. Tectonic-metamorphic history of the Lake Lefroy area: Implications for greenstone belt evolution in the Kambalda-Norseman Region, in Third International Archaean Symposium, Perth 1990: Extended Abstracts Volume, pp 457–458 (Geo Conferences (WA) Inc: Perth). Gresham, J J, 1991. Kambalda, History of a Mining Town (Western Mining Corporation: Melbourne). Ho, S E, Groves, D I and Bennett, J M, (Eds), 1990. Gold Deposits of the Archaean Yilgarn Block, WA: Nature, Genesis and Exploration Guides, Publication 20 (Geology Department and University Extension, The University of Western Australia: Perth). Keele, R, 1991. Regional tectonic setting and kinematics of gold deposits in the Yilgarn Block, in Structural Geology in Mining and Exploration, Kalgoorlie 1991, Extended Abstracts Volume, Publication 25, pp 18–19 (Geology Department and University Extension, The University of Western Australia: Perth). Norris, N D, 1990. New Celebration gold deposits, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 449–454 (The Australasian Institute of Mining and Metallurgy: Melbourne). Swager, C, 1989. Structure of Kalgoorlie Greenstones - Regional deformation history and implications for structural setting of the Golden Mile deposits, Geological Survey of Western Australia, Report 25.
Geology of Australian and Papua New Guinean Mineral Deposits
Newton, P G N, Smith, B, Bolger, C and Holmes, R, 1998. Randalls gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 225–232 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Randalls gold deposits 1
2
3
by P G N Newton , B Smith , C Bolger and R Holmes INTRODUCTION The deposits are approximately 80 km SE of Kalgoorlie in the Eastern Goldfields of WA at lat 31o05′S, long 122o12′E and AMG coordinates 425 000 E, 6 565 000 N, on the Widgiemooltha (SH 51–14) 1:250 000 scale and the Mount Belches (3335) 1:100 000 scale map sheets (Fig 1). The Randalls group consists of five recently active mines, Cockeyed Bob, Maxwells, Rumbles, Santa-Craze and Santa North. For the purposes of this contribution, only Santa-Craze will be described as it has similar geological characteristics to the other deposits. The term Santa-Craze is used for the Santa Claus and Craze orebodies which are within a single open pit. Open cut mining of the Randalls deposits began in December 1993 by the Mount Monger Gold Project (MMGP; General Gold NL 50% and Ramsgate Resources NL 50%) and until December 1996 had produced 4655 kg of gold after the treatment of 1.64 Mt of ore. Gold production for the 1996 calendar year was 1637 kg from 585 000 t of ore. The total Mineral Resource for the Randalls group of deposits at March 1997 was 3 222 700 t at 3.2 g/t for 10 300 kg of contained gold, which included a total Proved and Probable Ore Reserve of 887 700 t at 3.3 g/t for 2900 kg of contained gold. Open cut mining at Santa-Craze ceased in 1996, and during the three year life of the deposit it produced approximately 1.3 t of gold. A small, high grade Indicated and Inferred Resource of 73 000 t at 9.3 g/t for 680 kg of contained gold remains below the pit floor and remains a possible underground target. Gold mineralisation at all of the Randalls deposits is hosted by iron-rich pelites and banded iron formation (BIF) and is spatially related to sulphidation haloes developed around shallowly-dipping extensional quartz veins. Given that the mineralisation styles are similar in all deposits, the description of Santa-Craze represents a generalised view.
EXPLORATION AND MINING HISTORY Production in the Randalls area began in the early 1900s and is documented as 12 400 oz gold from 38 000 t of ore (Griffin, 1989). The majority of production was from the Santa Claus deposit, but included ore from Dunlevy, Karnilbinia (Rumbles), Floradora and Browns. 1.
Research Geologist, Centre for Strategic Mineral Deposits, Department of Geology and Geophysics, The University of Western Australia, Crawley WA 6907.
2.
Formerly Senior Project Geologist, Mount Monger Gold Project, now c/o Resource Services Group Pty Ltd, 168 Egan Street, Kalgoorlie WA 6430.
3.
Chief Geologist, Mount Monger Gold Project, 145 Boulder Road, Kalgoorlie WA 6430.
4.
Mine Geologist, Mount Monger Gold Project, 145 Boulder Road, Kalgoorlie WA 6430.
Geology of Australian and Papua New Guinean Mineral Deposits
4
Modern exploration began when the tenements were purchased by Newmont Pty Ltd in 1978 and has subsequently involved several companies (Table 1). The Newmont bedrock geochemical sampling at Santa Claus and Rumbles located a maximum gold value of 600 ppb, with high values largely correlated with the old workings. The EM survey covered the strike extensions of the Santa Claus BIF and detected two weak conductors which were not targeted for drilling. Extensive reverse circulation (RC) drilling by Nord Resources during 1982–1986 mainly targeted the old workings around Santa-Craze, and the data were used in resource estimations using longitudinal projection. Mawson Pacific Ltd continued RC drilling during 1986–1990 and in late 1990 entered into a joint venture agreement with Newcrest Mining Ltd, with Newcrest as project manager. Newcrest conducted extensive mapping, geochemical sampling and drilling at Randalls Dam, SantaCraze and at other BIF targets. RC and diamond drilling at Santa-Craze defined an Indicated and Inferred Resource of 245 000 t at 4.35 g/t gold for 34 300 contained oz. Significant intercepts including 4 m at 35.8 g/t and 9 m at 18.4 g/t were drilled at Cock-eyed Bob. Newcrest relinquished the tenements in mid 1993 and they were then acquired by the MMGP, who subsequently carried out resource estimates, further drilling and geochemical surveys at various prospects. The Santa-Craze and Cock-eyed Bob pits were mined in 1993 followed by Santa North (1994), Rumbles (1995) and Maxwells (1996). Geochemical sampling was particularly successful in discovering the Maxwells deposit, which has a reduced magnetic signature. Ore was trucked 40 km to the Mount Monger plant and treated at 500 000 tpa. Mining was suspended at Randalls in June 1997 while the MMGP focussed on an exploration program aimed at providing sufficient reserves to justify building a treatment plant at site.
PREVIOUS DESCRIPTIONS Newton et al (in press) describe the geological and geostatistical parameters of the Santa-Craze deposit. Data are also available from annual reports compiled mainly by Newcrest and the MMGP.
REGIONAL GEOLOGY The deposits are hosted by quartz-magnetite-amphibole BIF units that form a minor component of the Mount Belches beds, a 3.5 km thick sequence of graded feldspathic sandstone and interbedded laminated siltstone. Dunbar and McCall (1971) interpret the metasediments as a turbidite sequence deposited in a deep water basin. Swager (1997) places the sequence between the Gindalbie and Kurnalpi terranes and notes that the eastern boundary of the Gindalbie terrane is probably buried underneath this sedimentary succession.
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FIG 1 - a. Aeromagnetic interpretation of the Randalls belt, b. Geology of the Randalls mining centre showing the major deposits and prospects. TABLE 1 Summary of previous exploration and mining activities in the Randalls area. PERIOD
MANAGER
1978–1979
Newmont Pty Ltd
WORK COMPLETED Mapping and soil sampling.
1980–1981
Succo Gold NL
Soil sampling, 18 RC holes on various prospects.
1982–1986
Nord Resources (Pacific) Pty Ltd.
Soil sampling, 176 RC and 13 diamond drill holes. Resource evaluations and feasibility studies.
1983
Newmont Holdings Pty Ltd
Ground magnetic surveys, bedrock geochemical RAB drilling (542 holes).
1984
Maitland Mining NL
Ground magnetic surveys, 10 RC holes.
1984
Coopers Resources NL
Regional assessments of the area south of Randalls.
1986–1990
Mawson Pacific Ltd
68 RC holes and one diamond drill hole, mostly at Randalls and Karnilbinia mining centres. Ore reserve estimations and feasibility studies.
1990–1993
Newcrest
Extensive RC and diamond drilling at Randalls, Cock-eyed Bob and Wheeler. RAB drilling along the Bare Hill Shear (Randall fault).
1993–present
Mount Monger Gold Project
Extensive RC and diamond drilling at Santa-Craze, Maxwells, Rumbles, Cock-eyed Bob. Regional geochemical sampling, ground magnetic surveys. Open-cut mining at Santa-Craze, Maxwells, Cock-eyed Bob, Rumbles and Santa North.
Source: G Davis, unpublished data, 1991
The regional deformation sequence for the Kalgoorlie–Norseman area (Swager, 1989, 1997) is applicable to Randalls and consists of early dominantly layer-parallel contractional faults (D1), followed by two progressive phases of regional shortening (D2, D3) which produced a series of NWtrending folds and widely-spaced transcurrent faults. A final deformation event (D4) produced north-striking faults oblique to the earlier fold trends, generally with dextral movement senses. Peak regional metamorphism accompanied D2 deformation and in the Randalls area is generally lower amphibolite grade.
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The most obvious structural feature in the Mount Belches area is spectacular large-scale chevron folding of BIF (Fig 1). The fold shapes approach isoclinal near the Randall fault, and the fold axial planes change from NW-striking at Santa-Craze to a strike slightly east of north adjacent to the fault. The folds plunge moderately to steeply towards the north (Griffin, 1989), and as both limbs are east dipping, are therefore overturned. The Randall and Wilson faults are prominent north- to NWstriking structures which define the boundaries of the Mount Belches beds and are subparallel to bedding and the fold axes.
Geology of Australian and Papua New Guinean Mineral Deposits
RANDALLS GOLD DEPOSITS
Several smaller scale faults of similar orientation are interpreted from ground magnetic images in the Randalls area. A NE-striking fault set strikes at high angles to the BIF sequence and shows similarities to the Boogardie breaks that disrupt the BIF sequence at Hill 50, Mount Magnet (Thompson et al, 1990). The continuity of the NE-striking faults on magnetic images is generally less than 2 km. The chevron folds are likely to reflect regional D2 shortening, and D3 deformation at Randalls is represented by north- to NW-trending structures. Oblique faults with similar orientations to those at Randalls are noted throughout the Kalgoorlie area and are interpreted to be due to a slight rotation in the main shortening direction during D3 (Swager et al, 1992). Gold mineralisation in the Mount Belches beds is restricted to the BIF units at Randalls, although there has been only minor systematic exploration in other psammite-dominated parts of the domain.
ORE DEPOSIT FEATURES The Santa-Craze deposit contains lodes on opposing limbs of a tight, moderately to steeply north-plunging chevron fold that closes 200 m north of the orebody (Fig 2). The two mappable rock units are a mixed silicate- and oxide-facies BIF, and a sequence of clastic metasedimentary rocks, herein termed psammite. At Craze the mean orientation of the stratigraphic succession is 83ο/068o (dip/dip direction) compared with 75o/057ο at Santa.
LITHOLOGY AND STRATIGRAPHY Two BIF packages are recognised, a lower package which outcrops sparsely and hosts mineralisation at Maxwells, and an outcropping upper package which hosts gold mineralisation at Santa-Craze, Rumbles, Cock-eyed Bob and Santa North. At Santa-Craze the upper package of BIF consists of three to four
mappable units (Fig 2) which vary in thickness from 1 to 8 m and grade from quartz-magnetite rocks to iron-rich pelite with up to 40% chlorite and amphibole. BIF units are interbedded with, and surrounded by psammite units to 7 m thick that are interbedded with thinner metasiltstone and pelite units.
STRUCTURE Despite the tight shape of the fold structure north of the SantaCraze deposit, mesoscopic folds in the BIF units and an axial planar cleavage in the psammites are rarely noted. Therefore, the plunge of the fold can only be inferred from a stereonet analysis of bedding orientations, which gives a plunge of 70 ο towards 010ο, and limited regional data which suggest a moderate to steep plunge to the north (Griffin, 1989). The younging direction from graded bedding is consistently towards NE at Santa and towards WNW at Craze, conclusive evidence that the fold is an anticline. A localised foliation, defined by chlorite, is generally subparallel to bedding and developed in psammite units near the contact with BIF. No penetrative linear fabrics are developed. Brittle structures are common at Santa-Craze and include shallowly dipping extensional quartz veins, NE-striking oblique faults and layer-parallel shear zones. A summary of the important brittle structures is given below and their three dimensional relationships are summarised in Fig 3. Quartz veins are preferentially developed in the BIF units and generally form planar arrays, often as ladder sets, that have a shallow dip towards the SW at Santa and Craze (Fig 4a). The veins are characterised by regular planar walls and range in thickness from a few centimetres to one metre, with lateral dimensions to 10 m. Vein margins are undeformed, or are rarely offset by layer-parallel movement on bedding surfaces. Within the veins, the quartz grains are generally recrystallised, but where preserved, the fibres are generally oriented at a high angle to the vein margin. The majority of veins are restricted to
FIG 2 - Simplified geological map of the Santa-Craze deposit showing a. the position of the Craze and Santa orebodies relative to the fold closure and the pit shell in plan view, b. geology of Craze, c. geology of Santa Claus.
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layer-parallel movement, based on rare offsets of shallowlydipping veins, is generally <0.5 m. The local structural synthesis and its relationship to the regional sequence of Swager (1997) is summarised as: 1.
early chevron folding which equates to D2;
2.
D3 strike-slip movement on north- to NE-striking faults;
3.
oblique movement on the north to NE faults (D4);
4.
extensional vein formation which post-dates movement on faults and is very late in the structural history; and
5.
minor movement on north- to NE-striking faults which is probably a continuation of D4.
The NNW-striking faults are probably compensation structures during D2 folding and have been reactivated during subsequent deformations. FIG 3 - Schematic block diagram of the Santa-Craze deposit. The longitudinal projection shows that the intersection of veins with the BIF plunges shallowly towards the south, but in cross section the veins are oblique to the stratigraphic layering.
BIF horizons (Fig 4b), although thicker veins extend into the psammite for distances typically less than 10 m. Thin, beddingparallel veins show mutually crosscutting relationships with shallowly-dipping veins, but are less common and generally discontinuous along strike. Bedding-parallel veins are best developed near the hanging wall contact of the BIF. Several oblique faults which strike between 010 and 060o and dip subvertically are exposed in the pit or interpreted from ground magnetic images. The faults rarely have significant quartz fill, and alteration around them is minor. Displacements on the structures are generally less than 1 m. However, the Santa North fault shows 30 m of dextral strike-slip movement of the BIF marker units (Fig 2) which is close to a true displacement given that the slickensides on the fault plane pitch subhorizontally. The Santa South fault(s) also shows significant offsets of the succession, with a displacement in plan view of 30 m in a sinistral sense (Fig 2). Prominent slickensides pitch steeply to the south on the fault plane, and therefore true displacement is approximately 60 m in an oblique-normal sense. A second, apparently earlier subhorizontal striations are preserved on the Santa South fault plane and are similar to those on the Santa North structure. Several north- to NE-striking faults outcrop at Craze, although none significantly displace the stratigraphic succession. The shallowly-dipping quartz veins both cut and are cut by the oblique faults. An important example of this relationship is noted at Santa where several veins cut the Santa South fault and are not offset, despite the significant amount of movement along the structure (Fig 4c). There are, however, examples of veins which are displaced by oblique faults, generally less than 50 cm, in all cases with a south block up movement sense. A set of NNW-striking quartz-vein filled faults can be traced in the pit walls at the south end of Santa (Fig 4d). Slickensides on the fault surfaces pitch shallowly to the north and calcitefilled step outs indicate a dextral sense of movement. However, where the faults project to the BIF, there is no significant offset of these units. Decimetre wide layer-parallel shear zones are common at the contacts of BIF with psammite. The plunge of striations on these shear zones is variable, and therefore suggests a complex history of movement on bedding planes. The amount of late
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MINERALISATION Economic gold mineralisation is only developed in BIF, is controlled by late, shallowly-dipping quartz veins that formed after the folding, and is associated with pyrrhotite-arsenopyrite alteration haloes developed adjacent to the veins. The individual veins rarely carry economic gold grades, but the sulphide haloes in the BIF extend for 10–40 cm away from the veins and are rich in gold (Fig 5a, b). The vein control of gold grade is also apparent on a longitudinal projection (Fig 6), which shows the plunge of ore shoots parallel to the intersection of the veins with the BIF. Interestingly, the two zones of higher gold content on Fig 6 are adjacent to the Santa South and Santa North oblique faults. The increased gold content is related to greater vein density (Fig 3), and neither veins nor alteration follows the faults. Below 410 m RL, the zone of mineralisation plunges at 50o towards the NW (Fig 6), the plunge corresponding to the intersection of the NNW-trending faults with the BIF (Fig 3). In this area there is also an increase in the density of subhorizontal quartz veins near the faults, though the faults are also filled with quartz veins. Shallowly-dipping quartz veins extend from the faults for up to 10 m (Fig 4d). Due to limited exposures, the crosscutting relationships of these structures with other faults are unclear. Pyrrhotite and arsenopyrite are the most common sulphide minerals and are best developed in polymineralic mesobands in BIF, where they replace iron silicates and oxides. In contrast, monomineralic bands of tightly interlocked grains (eg magnetite) have limited sulphide development. Pyrrhotite typically forms decimetre-scale haloes around veins and is intimately associated with euhedral arsenopyrite and occasionally with secondary magnetite (Figs 4e, f). In thin section, pyrrhotite occurs as inclusion-rich, anhedral aggregates that replace primary magnetite and iron silicates and also as euhedral pseudomorphs of arsenopyrite grains. Arsenopyrite is typically coarse grained (>2 mm), euhedral and contains blebs of pyrrhotite. Gold mineralisation has a positive correlation with sulphide content, especially arsenopyrite. Minor free gold is located near vein selvages associated with hydrothermal chlorite, and in thin section occurs as 5–20 µm inclusions in pyrrhotite and arsenopyrite. Gold has a fineness of 989 and is free milling, with approximately 60% recovered by gravity (W J Shaw, S Khosrowshahi and P G N Newton, unpublished data, 1994). Gold recovery from the treatment plant averages between 92 and 96%.
Geology of Australian and Papua New Guinean Mineral Deposits
RANDALLS GOLD DEPOSITS
a. Slightly sigmoidal subhorizontal ladder veins in footwall psammite. Taken looking SW.
b. Typical ore face at Santa-Craze showing the sharp contact between the BIF and the footwall psammite (Ps) and the selective development of veins in the BIF. Larger veins continue into the psammite, but only for limited distances. Taken looking SE.
c. Timing relationships of NE-trending oblique faults and shallowlydipping vein sets. The fault next to the geological hammer offsets the succession by 30 m, yet the veins are not offset by this fault. Taken looking south.
d. NNW-striking quartz-filled faults with subhorizontal extension veins developed in the footwall to faults. Taken looking south.
e. Decimetre-scale pyrrhotite (Po) replacement of magnetite (Mt) adjacent to a shallowly-dipping quartz-ankerite vein (Drill hole
f. Development of pyrrhotite alteration halo adjacent to a shallowlydipping quartz vein with arsenopyrite (Apy) more dominant with increasing distance from the vein (drill hole RDD14, 207.5 m).
FIG 4 - Photographs illustrating the macroscopic vein features at Santa-Craze.
ALTERATION The BIF mineral assemblage in areas of gold mineralisation consists of varying proportions of magnetite, cummingtonite, hornblende, actinolite, biotite, chlorite, carbonate and iron sulphides. This is compared with the unmineralised BIF assemblage of magnetite-quartz-chlorite-iron amphibole which is characteristic of upper greenschist metamorphic
Geology of Australian and Papua New Guinean Mineral Deposits
grade. Cummingtonite is often rimmed by hornblende and together they form distinctive stellate clusters which replace primary magnetite. Tremolite or actinolite is associated with granoblastic quartz and carbonate which cut across the weakly foliated chloritic matrix. Two types of chlorite are recognised; a fine grained weakly foliated type, and late stage platy grains which are associated with biotite idioblasts and carbonatetremolite or actinolite intergrowths.
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Fig 5a - Results of face sampling from 20 142 N and 20 110 N, 465 m RL at Santa showing decrease in grade away from vein.
Fig 6 - Longitudinal projection of gold grade x thickness in the plane of the western BIF at Santa. Drill intercepts are plotted as cumulative metal content derived by multiplying gold grade (g/t) by thickness, using a minimum value of ≥1 m at ≥ 1 ppm gold. Since the BIF thickness is relatively uniform throughout, no correction for thickness is applied.
At the scale of the Randalls group of deposits, there is a broad association of gold deposits with anticlinal fold closures, notably the Santa-Craze, Santa North, Rumbles and South Rumbles deposits. However, at the deposit scale, there is no evidence of syn-folding mineralisation. The association of deposits with fold closures is due to influences of the fold forms on stress distribution. This is discussed in further detail below. The subhorizontal veins are the main control of gold mineralisation at the deposit scale and post-date the main phase of folding and oblique faulting. Gold precipitation adjacent to these veins was most likely the result of infiltration of a sulphur-bearing hydrothermal fluid along microcracks in the host rock adjacent to veins, and of precipitation via fluid–wall rock desulphidation processes (Groves, Barley and Ho, 1989).
Fig 5b - Diagrammatic representation of the different scales of sulphidation haloes proximal to subhorizontal veining.
Empirically, the oblique faults control areas of increased veining, and are therefore an indirect control of higher gold content. Pre-veining strike-slip movement on the oblique faults juxtaposed blocks of thin BIF units along strike against thick, less competent psammite. Under conditions in which the minimum principal stress (σ3) is parallel to layering, large variations in the magnitude and orientation of the principal stresses are predicted in thin competent layers (Ridley, 1993). In this orientation a competent BIF unit would be a zone of low mean stress and high permeability, and as such would influence hydrothermal fluid pathways, with fluid flow focussed to these low mean stress regions. Some areas of the psammite immediately adjacent to the BIF were also under conditions favourable for periodic tensile fracturing.
Hydrothermal alteration of the psammites is characterised by development of biotite porphyroblasts and of chlorite, sericite and carbonate. In many cases the primary sedimentary textures are preserved; reverse grading is commonly observed due to preferential biotite development in the finer grained sections of the graded bed.
The north plunge of the ore zone at the south end of Santa between 300 m RL and 410 m RL (Fig 6) suggests that the NNW-trending faults were hydraulically open during mineralisation. Where the structures leave the BIF at 410 m RL, the upward movement of fluid formed flat-lying hydraulic fractures in which fluid–rock interaction was more efficient, hence the higher gold content.
ORE GENESIS
MINE GEOLOGICAL METHODS
The model developed here for the emplacement of gold mineralisation at Santa-Craze is based largely on structural features and is discussed in more detail in Newton et al (in press).
Grade control drilling was on a 7 by 2.5 m pattern using open percussion holes of 89 mm diameter. Drilling was completed for a 5 m bench using holes 6 m long and inclined at 70o towards 230o, allowing for 1 m subdrill. Samples to 3 kg were collected
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RANDALLS GOLD DEPOSITS
every metre using a cyclone, riffle split 75:25 and pulverised using a LM2 bowl pulveriser to -75 µm. A 50 g fire assay for gold was carried out and all grade control samples were logged for rock type, weathering and quartz content. Ore block volumes and grade estimates were produced using a sectional approach which integrated geological data and assay values. A 5 m flitch was produced from the sections which was interpreted and estimated using a polygonal method. Mining was highly selective and involved pre-split drilling of the hanging wall contact, followed by removal of waste on 2.5 m flitches. The ore was then pre-split drilled, blasted and trucked using 75 t dump trucks. Ore blocks were only mined during day shift under supervision of a geologist. High grade ore with >1.8 g/t gold was trucked to the mill, whereas medium (1.2–1.8 g/t) and low grade (0.8–1.2 g/t) ores were stockpiled.
ACKNOWLEDGEMENTS Permission to publish by the Mount Monger Gold Project is gratefully acknowledged. The authors wish specially to note the contribution of D Groves and J Ridley who have helped to improve the quality of this manuscript. The staff at the Randalls mine, notably S Coxhell, S Millner, W Cooper, P Fox and L Votiva are thanked for their assistance. P Newton's research is supported by the Centre for Strategic Mineral Deposits and an Australian Postgraduate Research Award, with further logistical and financial support from the MMGP.
REFERENCES Dunbar, G J and McCall, G J H, 1971. Archaean turbidites and banded ironstones of the Mount Belches Area, WA, Sedimentary Geology, 5:93–133.
Geology of Australian and Papua New Guinean Mineral Deposits
Griffin, T J, 1989. Widgiemooltha, Western Australia - 1:250 000 geological series, Geological Survey of Western Australia Explanatory Notes SH 51–14. Groves, D I, Barley, M E and Ho, S E, 1989. Nature, genesis and tectonic setting of mesothermal gold mineralization in the Yilgarn Block, Western Australia, in The Geology of Gold Deposits: The Perspective in 1988 (Eds: R R Keays, W R H Ramsay and D I Groves), Economic Geology Monograph, 6:71–85. Newton, P G N, Ridley, J R, Groves, D I, Khosrowshahi, S and Smith, B, in press. Integration of directional variography and structural geology: an example from the Santa-Craze BIF-hosed Au deposit, near Kalgoorlie, Western Australia, Chronique de la Recherche Miniere. Ridley, J R, 1993. The relationship between mean rock stress and fluid flow in the crust: with reference to vein- and lode-style gold deposits, Ore Geology Reviews, 8:23–37. Swager, C P, 1989. Structure of Kalgoorlie greenstones - Regional deformation history and implications for the structural setting of the Golden Mile gold deposits, Geological Survey of Western Australia, Report 25. Swager, C P, 1997. Tectono-stratigraphy of late Archaean greenstone terranes in the southern Eastern Goldfields, Western Australia, Precambrian Research, 83:11–42. Swager, C P, Witt, W K, Griffin, T J, Ahmat, A L, Hunter, W M, McGoldrick, P J and Wyche, S, 1992. Late Archean granitegreenstones of the Kalgoorlie Terrane, Yilgarn Craton, Western Australia, in The Archean:Terrains, Processes and Metallogeny (Eds: J E Glover and D I Groves), pp 107–122 (Geology Department and University Extension, The University of Western Australia: Perth). Thompson, M J, Watchorn, R B, Bonwick, C M, Frewin, M O, Goodgame, Y R, Pyle, M J and MacGeehan, P J, 1990. Gold deposits of Hill 50 Gold Mine NL at Mount Magnet, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 221–241 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Nguyen, P T, Donaldson, J S and Ellery, S G, 1998. Revenge gold deposit, Kambalda, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 233–238 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Revenge gold deposit, Kambalda 1
2
3
by P T Nguyen , J S Donaldson and S G Ellery INTRODUCTION
EXPLORATION HISTORY
The deposit is 13 km SE of Kambalda, WA (Fig 1), at lat 31o17′S and long 121o45′E on the Widgiemootha (SH 51–14) 1:250 000 scale and Lake Lefroy (3235) 1:100 000 scale map sheets. Revenge gold mineralisation is associated with a complex shear zone and quartz vein system in mainly mafic host rocks. Mining by WMC Resources began in 1991 in an open pit, and underground development followed in 1992. Total production to December 1996 was 2.37 Mt at 4.64 g/t gold, with a further 833 000 t at 4.50 g/t of total Proved and Probable Reserves and 1.18 Mt at 4.58 g/t of total Indicated and Inferred Resources remaining. A description of other Kambada gold deposits is given by Roberts and Elias (1990) and by Watchorn (this publication).
The Revenge deposit was discovered in 1984 during exploratory diamond drilling in the Lake Lefroy area, in a program designed to test several magnetic anomalies in favourable rock types adjacent to the Playa shear zone. The area is overlain by 1 to 20 m of conductive lake sediment with >50 m Tertiary channel sediment to the south. The discovery was made by drill hole LD7010, with 6.95 m at 6.30 g/t gold, which intersected the N01 lode of the Revenge deposit at about 155 m below the lake surface. Revenge was the first 'blind' deposit in the Kambalda region to be discovered by conceptual geophysical exploration modelling (J C Donaghy and A Cowden, unpublished data, 1986). Subsequently, similar geophysical models were used in the discovery of several gold mineralised structures in the Lake Lefroy area. Additional diamond and percussion drilling in the Revenge area has identified several gold mineralised structures, which are interpreted to be conjugate sets of reverse shear zones (Nguyen et al, 1995), occuring in an area about 2 by 2 km.
REGIONAL GEOLOGY The Kambalda region is in the southern part of the Norseman–Wiluna greenstone belt within the Archaean Yilgarn Craton. The region has a corridor of mafic-ultramafic rock bounded by two major NNW-trending regional structures, the Boulder–Lefroy Fault to the east and the Merougil Fault to the west. The geology and stratigraphy of the Kambalda area have been extensively studied and documented by Gresham and Loftus-Hills (1981), M J Donaldson, L S Mitchell and E M O'Connor (unpublished data, 1983), N J Archibald (unpublished data, 1985), Clark (1987), A Cowden and N J Archibald (unpublished data, 1989) and Roberts and Elias (1990).
FIG 1 - Locality map of Kambalda–St Ives area.
1.
Tectonics Special Research Centre, Department of Geology and Geophysics, The University of Western Australia, Nedlands WA 6907. Now, Senior Geologist, WMC Exploration, WMC Resources Limited, Belmont WA 6984.
2.
Senior Mine Geologist, WMC St Ives Gold, WMC Resources Limited, Kambalda WA 6442.
3.
Senior Evaluation Geologist, WMC St Ives Gold, WMC Resources Limited, Kambalda WA 6442.
Geology of Australian and Papua New Guinean Mineral Deposits
The Kambalda–Tramways region has been polydeformed and metamorphosed, with metamorphic grade ranging from upper greenschist to lower amphibolite facies (Binns, Gunthorpe and Groves, 1976; Wong, 1986; Clark, 1987). Gresham and Loftus-Hills (1981) first established the structural history of the Kambalda area, as four generations of folds: early isoclinal recumbent, upright NNW-trending, NNEtrending, and late, poorly developed south-plunging folds. A regional structural study by N J Archibald (unpublished data, 1985) and mine structural studies by Clark (1987, Victory gold mine) and Fogarty (1993, Long nickel mine), also defined four different periods of deformation. However, early northtrending recumbent folds and late second-order splay faults were included in the studies of N J Archibald (unpublished data, 1985) and Clark (1987), and ENE- and WSW-dipping shear zones and a late north–south shortening event were included in the study of Fogarty (1993).
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In contrast, three episodes of deformation were recognised by Clout (1990), and only two by J R Vearncombe (unpublished data, 1987) and Woodfull (1993). Despite these differences, three similar events have been recognised by most authors: early thrusting and overturned folding, NNW-trending upright folding, and late faulting associated with gold mineralisation. Recent work in the Kambalda area has established four main episodes of deformation (Nguyen et al, 1995). The first, D1, produced regional south-over-north thrusts (eg Foster thrust) and the second, D2, produced upright, NNW-trending, gently SE-plunging folds. The third event, D3, involved brittle-ductile
oblique-sinistral wrench faulting associated with the development of gold-bearing reverse shear zones, and the fourth event, D4, involved dextral± reverse reactivation of early structures.
LOCAL GEOLOGY HOST ROCKS The geology of the Revenge area is shown in Fig 2. The data were obtained from open pit and underground exposures, extensive percussion and diamond drilling, and geophysical surveys. Gold mineralisation occurs in the Kambalda
FIG 2 - Geological plan of the Revenge area (updated from Western Mining Corporation surface mapping, drilling data and geophysical data) and cross section 6 537 300 N, looking north, showing location of the mineralised shear zones.
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REVENGE GOLD DEPOSIT, KAMBALDA
Komatiite, Devon Consols Basalt, Kapai Slate, Defiance Dolerite, Paringa Basalt and felsic-intermediate porphyries. This is very similar to the setting of the Victory–Defiance deposits to the SE (Clark, 1987). Detailed petrography of these rocks has been described by Gresham and Loftus-Hills (1981), Clark (1987) and Cotnoir (1989). The Revenge deposit consists of mineralised structures mainly developed on the eastern limb of the NNW-trending, shallowly SSE-plunging Delta Island anticline. The Kambalda Komatiite has been intersected in several drill holes to the NW and is overlain by the Devon Consols Basalt, Kapai Slate, Defiance Dolerite and Paringa Basalt. The Defiance Dolerite is the most important host to the main mineralised structures (N01, S01, N22). Smaller structures occur within the Devon Consols Basalt (Delta, W44, W45, W46, W48, W66), Kapai Slate (Delta, West Revenge) and Paringa Basalt (S01, N22).
1990). The slate is commonly silicified at the surface, but in underground exposures and in diamond drill holes it comprises numerous 1 to 8 m intervals of laminated carbonaceous sulphidic phyllite interbedded with sulphide-magnetite bearing albite-quartz chert. These sedimentary intervals are commonly separated by intermediate to felsic sills to 70 m thick.
Defiance Dolerite
The area is bounded to the east by the steeply east-dipping (70 to 80o) Playa shear zone, which separates the Kambalda Komatiite in the hanging wall from younger units in the footwall. The Playa shear zone has been recognised as a second-order regional NW-trending shear zone (T P Nguyen, D Goodwin, C Wilkinson and J Withers, unpublished data, 1992). It is beneath Lake Lefroy on the eastern side of the Revenge deposit, and extends for about 10 to 15 km NW, to just south of the Kambalda Dome.
The Defiance Dolerite is exposed at the Revenge open pit and in underground development of the N01, S01 and N22 areas. It comprises medium- to coarse-grained differentiated mafic units. The Dolerite is up to 300 m thick on the eastern side of the anticline and directly overlies the Kapai Slate. This apparent thickening of the Defiance Dolerite is interpreted to reflect the repetition of the succession and the complexity introduced by the mineralised reverse shear zones. On the western side of the Delta Island anticline the Dolerite is thinner and less differentiated, with an unknown lateral extent. It is the main host to gold mineralisation at Revenge and has been recognised as a favourable rock type for brittle structures associated with gold mineralisation in the Victory–Defiance–Orchin area (Clark, 1987; Roberts and Elias, 1990). The petrography of the Defiance Dolerite has been studied in detail by J C Donaghy and A Cowden (unpublished data, 1986), Clark (1987) and Carey (1994).
Kambalda Komatiite
Paringa Basalt
The Tripod Hill Komatiite, the upper member of the Kambalda Komatiite, is a sequence of strongly foliated ultramafic rocks on the eastern side of the Playa shear zone. It consists of 1–10 m thick, low MgO (15–30 wt %) flows with a total thickness of about 400 m. Felsic and intermediate intrusions, to 150 m thick, are common in the Tripod Hill Komatiite, which has been metamorphosed to a talc-chlorite-actinolite-tremolitedolomite assemblage.
The Paringa Basalt is exposed in the southern corner of the Revenge open pit and in underground development of the S01 and N22 areas. Drilling indicates significant lateral extension of the Paringa Basalt to the south and west. The true thickness of the Paringa Basalt is not known in the Revenge area, but elsewhere in the Kambalda region, it is a 1 to 1.5 km thick sequence of lava flows (A Cowden and N J Archibald, unpublished data, 1989). The rock is characterised by radiating needles of chlorite and amphiboles (after acicular pyroxene) in a fine grained matrix. Pillow basalt flows with chlorite-albite rich varioles are common, although they are less prominent than those in the Devon Consols Basalt. Laminated cherty to carbonaceous sedimentary units 1–5 m thick are common at the base of the Basalt.
The lower member, the Silver Lake Peridotite, is not exposed in the Revenge area. However, in numerous deep diamond drill holes (eg CD2001, 1000 m depth), the lower member is present as high MgO (30–40 wt %), 20–30 m thick, talc- and magnesite-rich flows, conformably underlying the Tripod Hill Komatiite. The thickness of the Silver Lake Peridotite in the Revenge area varies from 50 to 100 m, with several intervals 0.5 to 4.0 m thick of interflow cherty-chloritic sediment.
Devon Consols Basalt The Devon Consols Basalt is exposed at the northeastern side of Delta Island, across the main decline, and in underground development of the W45 and W66 declines. The Basalt is about 150 m thick in the Revenge area and hosts the Delta, W44, W45, W46, W48 and W66 mineralised structures. The Basalt conformably overlies the Tripod Hill Komatiite with a gradational contact and consists of massive and pillowed basalt flows. The pillows vary in diameter from 10 cm to 2 m and have dark green mafic margins, which contain pale yellow-green felsic zones with significant epidote varioles to 1.5 cm in diameter.
Kapai Slate The Kapai Slate is exposed on the southeastern part of Delta Island, in the Delta open pit and at the W66 decline. It conformably overlies the Devon Consols Basalt and is an important regional stratigraphic marker in the Kambalda–Kalgoorlie area (Clark, 1987; Cotnoir, 1989; Clout,
Geology of Australian and Papua New Guinean Mineral Deposits
Intermediate and felsic intrusive suites In the Revenge area, intermediate intrusions, given the local name xenolithic diorite, commonly intrude the Kapai Slate and other units from the Kambalda Komatiite to the Paringa Basalt. They are lamprophyric rocks (Perring, 1988), which occur as sills to 70 m thick within the Kapai Slate and the Paringa Basalt, or as narrower plugs in the Devon Consols Basalt and the Defiance Dolerite. Felsic intrusions, locally named quartzalbite dykes, include upright, WNW-trending dykes, which are exposed throughout the mine area. They vary in thickness from 1 to 10 m with strike lengths from 10 m to 1 km. The quartzalbite dykes are younger and crosscut the xenolithic diorites.
DEFORMATION HISTORY As for the rest of the Kambalda region, the Revenge mine area has a complex deformation history. The terms D1 to D4 are used here to correspond to similar events at regional scale. Shearing associated with gold mineralisation is the dominant structural feature in the mine area. However, other episodes of deformation are recognised.
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D1 phase
GOLD MINERALISATION
Evidence for the D1 phase of thrusting and isoclinal folding is not well developed in the Revenge mine area except for the bedding-parallel foliation within the Kapai Slate. This earliest foliation is common in the laminated cherty sediment units, where chlorite-biotite, quartz-albite and magnetite-pyrrhotite show a preferred orientation parallel to the compositional layering of iron oxides and silicates. Poorly defined isoclinal microscopic folds are recognised in the iron oxide layers. This early fabric is folded by the NNW-trending upright D2 folds and is crosscut by the subvertical S2 fabric and the D3 mineralised shear zones.
Gold mineralisation in the Revenge area is associated with foliated and altered host rocks and with accompanying quartz vein systems.
D2 phase This deformation is typified by the macroscopic, open, upright Delta Island anticline (Fig 2). The axial surface of the anticline trends NNW and its axis plunges at 15–20o toward the SSE. Subvertical spaced cleavage is well defined, especially in the Kambalda Komatiite and Kapai Slate, where it crosscuts the early D1 fabric and is cut and displaced by a later D3 shear fabric. NNW-trending, subvertical 20–50 cm wide foliated zones and west-trending subvertical 20–50 cm wide carbonate veins, which are also associated with the D2 event, are common.
D3 phase A complex system of interlinked shear zones associated with the D3 deformation is recognised. Two main types of shear zone can be distinguished: regional NW-trending, subvertical shear zones, and smaller, more localised east- and west-dipping gold-mineralised shear zones. The Playa shear zone is exposed in the 196N01-North airleg drive of the Revenge mine, about 90 m below Lake Lefroy, with Defiance Dolerite in the footwall and strongly mylonitic Kambalda Komatiite in the hanging wall. S–C fabrics in this locally steep east-dipping structure indicate sinistral reverse movement in the shear zone (T P Nguyen and J S Donaldson, unpublished data, 1995). The Playa shear zone is barren to weakly mineralised (<0.5 g/t) in the Revenge area.
East-dipping shear zones These mineralised shear zones are NNW- to NNE-trending, 40–45 o east-dipping and are characterised by moderate to high grade gold mineralisation of 4–5 g/t associated with quartz veins and albite-pyrite rich altered wall rocks. These structures comprise the N01-S01 and N22 lodes, which are hosted mainly by the Defiance Dolerite in the eastern part of the mine, and the W44, W45, W46 and W48 lodes, which are hosted by the Devon Consols Basalt to the west (Fig 2). The N01-S01 shear is the most important mineralised structure in the Revenge mine. It is 1 to 12 m wide and extends for about 900 m along strike. The N01-S01 shear is continuous, with the northern part termed N01 and the southern part S01. The N22 shear is a hanging wall structure, varying in width from 1 to 8 m and extending along strike for about 400 m. Paringa Basalt and felsic porphyry, which occur in the southern end of the N22, host minor proportions of the mineralised structures. A typical cross section of the east-dipping shear zones is shown schematically in Fig 3, in which the shear zone is associated with strongly foliated altered wall rocks. Complex quartz vein systems, consisting of shear veins and extensional veins, are associated with the east-dipping shear zones. The shear veins, which are 5 to 70 cm thick and 10 to 100 m in strike length, commonly occupy the centre of the shear zones. The extensional veins are north- to NE-trending and dip east at 5–25o. They vary in thickness from 1 to 30 cm, and extend for up to 20 m from the centre of the shear zones. They commonly cut the shear veins and have a visible albite-carbonate-pyrite alteration halo.
West-dipping shear zone
D4 phase
The west-dipping shear zone is north- to ENE-trending and dips WNW at 10–15o. This shear zone comprises two sections: W66 to the west (Fig 2) and C02 to the east. It is about 200 to 300 m below the surface, and extends up to 300 m in strike length with thickness from 1 to 20 m. The shear zone is principally hosted by the Devon Consols Basalt but extends eastward into the Kapai Slate and the intermediate porphyry which has intruded the base of the Kapai Slate. Towards the NW the structure intersects the Kambalda Komatiite, where it thickens and has a lower gold grade (average 1 g/t) compared with the average grade of the structure of 3.5 g/t. Mining and drilling to date have not yet defined the total extent of the W66 lode.
This event is characterised by prominent NNW-trending dextral faults, which crosscut and offset the mineralised shear zones and folds. These faults are well defined on the regional magnetic images and have a 1 to 30 km strike length. Weak mineralised west-trending faults of 10–200 m strike length with dominant sinistral reverse displacement are evident at the mine scale (eg Alimak fault). Local NE-trending, subvertical mineralised quartz veins which crosscut the main mineralised shear zone and quartz vein system are also interpreted to be D4 veins.
The shear zone is characterised by subhorizontal undulating surfaces with a large brecciated zone filled with quartz veins 0.5 to 5 m wide at the centre of the shear (Fig 4). Geometrical relationships and kinematic indicators indicate that W66 is a D3 thrust zone with about 70 m displacement, with approximately ESE–WNW subhorizontal shortening and subvertical extension at the time of formation. The vein system in the westdipping shear zone is dominated by quartz with subordinate carbonate and albite. Shear veins are well developed in the W66 shear zone and typically occupy its core. Individual shear veins extend for about 50 to 100 m along both dip and strike.
The gold mineralised shear zones in the Revenge area have two principal orientations: NNW- to NNE-trending and 45o east-dipping, and north- to ENE-trending and 15o west- to NWdipping. Geometries and structural data indicate that they are dominantly reverse shear zones, which formed as a conjugate set during ESE–WNW directed subhorizontal shortening. A complex quartz vein network is associated with these shear zones including shear veins or fault-fill veins and extension veins.
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Geology of Australian and Papua New Guinean Mineral Deposits
REVENGE GOLD DEPOSIT, KAMBALDA
FIG 3 - Schematic diagram showing a typical cross section of an east-dipping shear zone, looking north (modified from Nguyen et al, 1995).
FIG 4 - Schematic diagram based on mosaic photographs of a cross section of the W66 shear zone, looking north (modified from Nguyen et al, 1995).
Thick breccia veins commonly form as large dilational jogs on the releasing side of the curved shear surfaces. Extensional veins in the W66 shear zone are less well developed than in the east-dipping shear zones. They range from 1 to 20 cm wide, vary in strike from 005 to 040°, and dip at 10 to 30° to the east. Most extensional veins cut the shear veins and have prominent albite alteration haloes. Disseminated gold is associated mainly with altered wall rock, especially in the albite-biotite-dolomite zones, whereas visible gold to 10 µm in diameter is common in pyrite-rich quartz veins. In both cases pyrite is the best mineralogical
Geology of Australian and Papua New Guinean Mineral Deposits
indicator of high gold content, as gold is commonly present as inclusions in pyrite or coats and infiltrates pyrite grains along fractures.
ALTERATION The alteration assemblage associated with the Revenge mineralised structures is similar to that described for the Victory mine area (Clark, 1987; Roberts and Elias, 1990), which is zoned and overprints the regional metamorphic asemblages. The outer chlorite zone extends from 10 to 30 m
237
P T NGUYEN, J S DONALDSON and S G ELLERY
from the centre of the shear zones, and consists mainly of chlorite with lesser amounts of biotite and dolomite. Closer to the shear zone the chlorite is replaced by biotite, carbonate and minor albite, pyrite and magnetite, which are strongly aligned with the shear fabric. The inner albite zones are commonly adjacent to the quartz veins. These are bleached zones of albiteankerite-dolomite-quartz-pyrite with minor biotite, muscovite and calcite. There is local overprinting of late biotite on albite in places.
ORE GENESIS Gold mineralisation at Revenge is mainly confined to the westand east-dipping reverse shear zones, which formed as a conjugate set during a relatively late ESE–WNW subhorizontal shortening. These are brittle-ductile shear zones with localised dilatant zones resulting in ore localisation. Host rock composition and fluid–wall rock interaction are also important factors of ore development. Pyrite and gold are interpreted to have formed by desulphidation reactions between the sulphurrich hydrothermal fluid and the iron-rich wall rock at the margin of the veins (Groves et al, 1984; Phillips and Groves, 1984).
MINING METHOD The Revenge open pit is in the southern part of Lake Lefroy, where water levels vary seasonally from 0 to 1 m deep. A bund wall 2 m high by 6 m wide was built around a designed open pit of 800 m by 500 m surface dimensions. Plastic liner, mud and clay were placed on the inner side of the wall to a depth of 3 to 4 m to prevent the inflow of surface water before excavation commenced. The underground workings were designed to mine the bulk of the 45o-dipping N01-S01 ore body utilising Alimak horizontal longhole stoping. Other areas of the mine utilise jumbo room and pillar stoping or conventional longhole stoping according to the local dips of the orebodies.
ACKNOWLEDGEMENTS We thank Western Mining Corporation Limited for permission to publish these data. The assistance of many WMC geologists, especially R Watchorn, K Hein, C Quinney and R Millar is greatly appreciated. Thanks are extended to C McA Powell and D I Groves for criticism of the manuscript.
REFERENCES Binns, R A, Gunthorpe, R J and Groves D I, 1976. Metamorphic patterns and development of greenstone belts in the Eastern Yilgarn Block, Western Australia, in The Early History of The Earth (Ed: B F Windley), pp 303–313 (John Wiley and Sons: New York).
238
Carey, M L, 1994. Petrography and geochemistry of selected sills from the Kambalda-Kalgoorlie region, WA, BSc Honours thesis (unpublished), Australian National University, Canberra. Clark, M E, 1987. The geology of the Victory gold mine, Kambalda, WA, PhD thesis (unpublished), Queens University, Ontario. Clout, J M F, 1990. Regional geological setting of the Kambalda nickel and gold deposits, in Geology of Kambalda Nickel and Gold Deposits, Third International Archaean Symposium Excursion 3 Guide Book, pp 3–13 (Geological Society of Australia, Western Australian Division: Perth). Cotnoir, A, 1989. The nature of Kapai Slate Formation and its role in the genesis of gold mineralisation at the Victory mine, Kambalda, MSc thesis (unpublished), University of Melbourne, Melbourne. Fogarty, S M, 1993. Structural controls of Long Shoot, Kambalda, Western Australia, BSc Honours thesis (unpublished), The University of Western Australia, Perth. Gresham, J J and Loftus-Hills, G D, 1981. The geology of the Kambalda nickel field, WA, Economic Geology, 76:1373–1416. Groves, D I, Phillips, G N, Ho, S E, Henderson, C A, Clark, M E and Woad, G M, 1984. Controls on distribution of Archaean hydrothermal gold deposits in Western Australia, in Gold ’82: The Geology, Geochemistry and Genesis of Gold Deposits (Ed: R P Foster), pp 669–712 (A A Balkema: Rotterdam). Nguyen, T P, Powell, C McA, Harris, L B and Hein, K A A, 1995. Development of vein systems in shear zones at Revenge mine, Kambalda, Western Australia: evidence of palaeoseismic events, in Clare Valley International Conference, Specialist Group in Tectonics and Structural Geology, Abstracts, pp 117–118, Geological Society of Australia. Perring, C S, 1988. Petrogenesis of the lamprophyre ‘porphyry’ suite from Kambalda, Western Australia, in Recent Advances in Understanding Precambrian Gold Deposits, Publication No 12 (Eds: S E Ho and D I Groves), pp 277–294 (Geology Department and University Extension, The University of Western Australia: Perth). Phillips, G N and Groves, D I, 1984. Fluid access and fluid-wall rock interaction in the genesis of the Archaean gold-quartz vein deposit at Hunt mine Kambalda, Western Australia, in Gold ‘82: The Geology, Geochemistry and Genesis of Gold Deposits (Ed: R P Foster), pp 389–416 (A A Balkema: Rotterdam). Roberts, D E and Elias, M, 1990. Gold deposits of the Kambalda-St Ives Region, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 479–491 (The Australasian Institute of Mining and Metallurgy: Melbourne). Wong, T, 1986. Metamorphic patterns in the Kambalda area and their significance to Archaean greenstone belts of the KambaldaWidgiemootha area, BSc Honours thesis (unpublished), The University of Western Australia, Perth. Woodfull, C, 1993. Structural controls on gold mineralisation at the Victory gold mine, Kambalda, Western Australia, MSc thesis (unpublished), Melbourne University, Melbourne.
Geology of Australian and Papua New Guinean Mineral Deposits
Kriewaldt, M, 1998. Nelson’s Fleet gold deposit, St Ives, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 239–242 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Nelson’s Fleet gold deposit, St Ives by M Kriewaldt
1
INTRODUCTION The deposit is about 100 km south of Kalgoorlie and 20 km SE of Kambalda, WA, at lat 31o18′S, long 121 o48′E on the Widgiemooltha (SH 51–14) 1:250 000 scale and the Lake Lefroy (3235) 1:100 000 scale map sheets (Fig 1). The Nelson’s Fleet mining lease M15–570 is held by Viking Resources Ltd, a subsidiary of Centamin NL. At December 1994, Nelson’s Fleet had a Measured Resource to 150 m depth of 2.5 Mt grading 2.7 g/t gold at a cutoff of 1 g/t. The deposit is completely concealed beneath younger sediment, whereas the original finds at nearby St Ives were associated with scattered low outcrops (Clarke, 1925).
1966). The area now known as Nelson’s Fleet was explored by Aztec Mining Company Ltd in a joint venture with Centamin from the mid 1980s, being known as the Nobodies Fault prospect until October 1990. The first discovery of gold was in reverse circulation (RC) drill hole LS5 drilled in May 1987. The 2 m section from 52 to 54 m assayed 0.43 g/t gold in a felsic rock immediately below the weathering front, followed by 0.33 g/t gold from 62 to 64 m and 0.43 g/t from 84 to 86 m associated with as much as 5% pyrite in white and pink altered rocks. Rotary air blast (RAB) hole 97 drilled in March 1987 had provided a sample with 0.92 g/t gold but check assaying failed to substantiate this value. However, a value of 0.1 g/t gold for the interval 44 to 46 m for this hole is listed in an Aztec report. RC hole LS2 drilled in June 1987 intersected 1.55 g/t gold over the 2 m from 48 to 50 m in deeply weathered Archaean bed rock. Holes 97 and LS2 were sited to test a ‘subtle magnetic anomaly’. Auger sampling of soils in April 1988 indicated a zone about 600 by 100 m in which all samples assayed 9 ppb gold or more. One spot high of 16 ppb gold was explored in October 1988 by three RC drill holes. The highest gold value from these three holes was 0.21 g/t for the 2 m from 28 to 30 m. Subsequent aircore drilling over much of Nelson’s Fleet in early 1990 provided many samples of weathered bed rock with over 0.1 g/t gold, the best of which were 13 g/t gold from 39 to 40 m and 11.6 g/t gold from 28 to 30 m in two holes drilled 100 m apart through superficial cover 24 to 27 m thick. Free gold was reported from the second hole. Diamond drill hole D1 tested the area between these two holes in May 1990. It intersected 26 m, from 59 to 85 m, with an average of 2.1 g/t gold. This hole showed a bed rock intercept with the potential for ore grade, and is considered to be the discovery hole. Because of non-correlation of gold values in weathered rock with gold values in bed rock, the follow up drilling at Nelson’s Fleet was not restricted to zones with ≥0.1 g/t gold. A similar situation holds at the Revenge gold mine 5 km WNW of Nelson’s Fleet (Woodall, 1993).
FIG 1 - Location map and geological plan of the Nelson’s Fleet deposit.
DISCOVERY AND EVALUATION HISTORY Up to the 1960s the St Ives district had produced around 550 kg of gold from the treatment of about 44 200 t of ore (Sofoulis,
1.
Consulting geologist, 57 Kishorn Road, Mount Pleasant WA 6153.
Geology of Australian and Papua New Guinean Mineral Deposits
The prospect was tested by 285 RAB and aircore holes for 13 187 m, 91 RC holes for 7257 m and 32 diamond drill holes, mostly NQ cored, for 4540 m. Sites of the cored holes are shown on Fig 1. The resource was estimated in 1994 using data from the diamond drill holes and 60 RC holes on a spacing of 25 to 50 m. Gold assays were by inductively coupled plasma optical emission spectroscopy. Prefeasibility studies based on open pit mining of 30 000 oz of contained gold were conducted in 1995.
REGIONAL GEOLOGY The St Ives area is in the Kambalda Domain of the Kalgoorlie Terrane (Swager and Griffin, 1990; Swager et al, 1992).
239
M KRIEWALDT
Nelson’s Fleet is hosted by the Archaean ‘Black Flag unit’ stratigraphically above the units that host the Kambalda nickel deposits and the St Ives gold deposits. The Black Flag unit is towards the top of the Archaean Kalgoorlie succession, and is a sedimentary sequence of epiclastic rocks with interlayered felsic volcanogenic rocks, dominantly volcaniclastic. Metamorphic grade in the Lake Lefroy area is generally upper greenschist and lower amphibolite facies. Following intrusion of granite there was intense deformation with the development of tight, upright folds and steep cleavage. Major wrench faults of the St Ives area include the Boulder–Lefroy Fault (Griffin, 1990). The Paddington, Mount Charlotte, Golden Mile, Victory, Defiance, Revenge, Orion and Junction gold deposits are all associated with the 150 km long Boulder–Lefroy Fault which passes just west of Kalgoorlie and east of Kambalda (Woodall, 1990). The Boulder–Lefroy Fault may be associated with the Nelson’s Fleet deposit, but the evidence is equivocal. It is variously plotted as close to the east of the Victory deposit, 3 km SW of Nelson’s Fleet (Griffin, 1990; Roberts and Elias, 1990), as cutting across the Nelson’s Fleet property (N J Archibald, unpublished data, 1990; Eisenlohr, Groves and Partington, 1989), and as close to the west of Nelson’s Fleet (Swager et al, 1992; Griffin, 1990). The age of the ore host rocks at Nelson’s Fleet is about 2700 Myr; of the intrusive granites about 2600 Myr; and of the postmineralisation dyke about 2400 Myr (Swager et al, 1992; Griffin, 1990). Gold mineralisation probably occurred 30 Myr after peak metamorphism at 2.25 kb and 435oC during uplift (Woodall, 1990). Witt (1993) considered that ‘gold mineralisation occurred toward the end of, and is an integral part of, a regional carbonation event, for which carbon isotope data indicate a mantle origin’.
LOCAL GEOLOGY SUPERFICIAL COVER The present land surface is generally about 290 m above sea level. The landform comprises sand sheets, sand rises, sandy clay flats (ephemeral swamps) and the Nelson’s Fleet saline clay pan. There are no bed rock outcrops of any kind. The landform has developed on a sequence of clays and sands that varies in thickness from around 20 m to over 50 m. A widespread layer of angular quartz fragments, possibly an old surface lag deposit, rests unconformably on deeply weathered Archaean bed rock and is overlain by clean quartz sand with interbedded white clay. The basement rocks beneath these cover beds are commonly deeply weathered and clayey for about 20 to 40 m depth below the unconformity. The position of the unconformable contact between the cover sediment and the weathered basement is not readily recognised. The white clays with sand beds and basal conglomerate are taken to be part of the Eocene Hampton Sandstone of the Eucla Basin which has been recognised in an ancient drainage west of Kambalda (Clarke, 1993; Jones, 1990). In some holes at Nelson’s Fleet the white beds assigned to the Hampton Sandstone are overlain by beds of red and mottled red and white clay with some red sand. These red beds are taken to be part of the fluviolacustrine Oligocene-Miocene Revenge Formation (Clarke, 1993, 1994). The Hampton and Revenge formations at Nelson’s Fleet are overlain by red clayey sand, and this is overlain by younger surficial materials.
240
BED ROCK GEOLOGY Bed rock configuration Drilling has shown a ‘buried hill’ trending about NNW on the western side of the lease with a spur to the ENE above a dolerite dyke (Fig 1). The top of the buried hill is about 20 to 30 m below surface and coincides with the presently known gold mineralisation. The hill trends parallel to the regional schistosity. Depth to bed rock in the NE of the lease away from the buried hill is about 50 m.
Lithology Host rocks for gold mineralisation at Nelson’s Fleet are quartzo-feldspathic schists intruded by massive, unfoliated, felsic and granitic rocks.
Veining Cores from some holes (for example D21) have a microcrenulated foliation, and the foliation is kink folded in some zones. Details of foliation and quartz veining, as logged in diamond drill core from holes D1 to D9 and D12 are presented below from a study by R J Wilson and M Caswell (unpublished data, 1990). Precisely oriented core data were limited. Bearings are from true north, and the direction of dip is only a general indication. From a plot of 681 determinations, the dominant foliation has a strike of around 330o and a dip of 60o NE. Zones where the foliation has a dip of 50o NE, and in which the strike of the foliation is around 290o, were recognised in holes D1 and D6. Three main sets of quartz veins were recognised: Set
Strike
Dip
Thickness
1
340ο
70oNE
5 to 30 mm
2
300o
15oSW
10 to 50 mm
3
o
70oSE
5 to 100 mm
060
Veins of set 1, which parallel the regional foliation and are in general only weakly auriferous, are cut by veins of set 3, which have 5 to 10% pyrite and are highly auriferous in places. Set 2 veins have ‘bucky’ quartz with pyrite as cubes to 3 mm diameter, and are auriferous.
Alteration In drill chips and core from Nelson’s Fleet there is a clear distinction between unaltered dark grey and green country rock and pale-coloured, altered, mineralised rock. The zones of altered rocks are tens of metres thick, and commonly coloured red, pink, and pale grey. Boundaries between the unaltered and altered rock are commonly sharp, or gradational over a narrow interval, with altered and unaltered sections alternating in some places at intervals of less than one metre. The altered zones commonly contain bleached zones a few metres thick. These are usually auriferous and contain schist with small flakes of white micas. Many contain up to 5% pyrite as small cubes less than 1 mm in diameter. Some of the bleached zones have distinctive shiny white micas which have been logged as sericite. The pink and grey zones also contain sections which are quartz veined and silicified, and many of these are auriferous. Pale, honey-coloured, siliceous lodes, such as in diamond drill hole D21, are notably auriferous.
Geology of Australian and Papua New Guinean Mineral Deposits
NELSON’S FLEET GOLD DEPOSIT, ST IVES
Classes of alteration styles used on Aztec diamond drill sections include:1.
intense silicification, with quartz veins to 10 cm thick, 3 to 5% pyrite as cubes and disseminated, and pervasive hematisation;
2.
less intense silicification, with thin and minor (0.5–3%) quartz veins, 0.5–5% pyrite, and variable and moderately pervasive hematisation;
3.
moderate silicification and hematisation, with thin and minor quartz veins and 0.5–2% pyrite;
4.
moderate silicification and sericitisation, with thin and minor quartz veins and 0.5–2% pyrite; and
5.
strong silicification and clay alteration haloes around (0.5–5%) quartz veins, with weak to moderate sericitisation.
Pyrite The auriferous rocks at Nelson’s Fleet are not all notably pyritic. Even though pyrite is conspicuously visible as cubes to 10 mm diameter in some auriferous sections, pyrite rarely exceeds 5% of the mineralised section. Bleached zones commonly have disseminated pyrite as cubes of less than 1 mm diameter. Pyrite cubes to 1 cm diameter associated with veins of quartz, and of quartz and dolomite, are less common. Drusy coatings of pyrite and marcasite were logged in a few sections. The sulphur content of 24 auriferous samples analysed was commonly less than 0.1% (9 samples) and is at most 1.0%. These samples were altered but not apparently weathered rock. Veins with clear selenite were also logged, eg at 232 m hole depth in D30. These may have been formed by the action of sulphuric acid from the oxidation of pyrite on veins with dolomite or calcite (Simpson and Gibson, 1912).
Mineralisation and resource zones Gold mineralisation at Nelson’s Fleet is in lodes of quartzveined and silicified quartzo-feldspathic schist with generally less than 5% pyrite. Supergene alteration of lode minerals appears to extend for at least 100 m vertically below the top of the weathered bed rock. The resource zones are interpreted to be irregular tabular bodies dipping at about 60o towards about 035o true (Fig 2). The zones are within an envelope about 500 m long and up to 100 m wide.
Post-mineralisation dyke A steeply dipping post-mineralisation dyke of dolerite and gabbro about 40 m wide which strikes at 050 o to 060o and dips 70o SE cuts the Nelson’s Fleet mineralised zone in the northwestern sector of the property. At the Defiance and Victory pits, about 3 km SW from Nelson’s Fleet, there is a steeply dipping Proterozoic dyke of quartz dolerite about 40 m wide cutting northeasterly through the orebodies (Glacken, 1987). Aeromagnetic data indicate that the dolerite dykes at Nelson’s Fleet and at the Defiance and Victory pits are parts of the same dyke.
ACKNOWLEDGEMENTS This report is published with the permission of Centamin NL, and has been reviewed by W J Lorimer. The figures were
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 2 - Cross section on 47 550 N looking NNW, Nelson’s Fleet deposit.
drafted by S E Clark, and word processing was by W M Graham. This paper largely draws on unpublished information gathered since 1986, first by Aztec Mining Company Ltd and later by Centamin Limited. Unpublished contributions to the knowledge of the Nelson’s Fleet area by N C Archibald, J Ashley, S M Belford, J E Borner, M Caswell, J Coggon, G S Compton, B Davies, B Jones, A C Purvis, C R Ringrose, C Stoakes, G Sylvester and R J Wilson are acknowledged.
REFERENCES Clarke, E de C, 1925. The geology of a portion of the East Coolgardie and North-East Coolgardie Goldfields including the mining centres of Monger and St Ives, Geological Survey of Western Australia Bulletin 90. Clarke, J D A, 1993. Stratigraphy of the Lefroy and Cowan palaeodrainage channels, Western Australia, Journal of the Royal Society of Western Australia, 76:13–23. Clarke, J D A, 1994. Evolution of the Lefroy and Cowan palaeodrainage channels, Western Australia, Australian Journal of Earth Sciences, 41:55–68. Eisenlohr, B N, Groves, D I and Partington, G A, 1989. Crustal-scale shear zones and their significance to Archaean gold mineralisation in Western Australia, Mineralium Deposita, 24:1–8. Glacken, I M, 1987. Victory-Defiance mine description, in Symposium Extended Abstracts and Excursion Guide - Mine Descriptions for Second Eastern Goldfields Geological Field Conference Kalgoorlie, 1 - 4 April 1987 (Eds: W K Witt and C P Swager), pp 130–134 (Eastern Goldfields Geological Discussion Group and Geological Society of Australia, Western Australian Division: Perth). Griffin, T J, 1990. Geology of the granite–greenstone terrane of the Lake Lefroy and Cowan 1:100 000 sheets, Western Australia, Geological Survey of Western Australia Report 32.
241
M KRIEWALDT
Jones, B G, 1990. Cretaceous and Tertiary sedimentation of the western margin of the Eucla Basin, Australian Journal of Earth Sciences, 37:317–329.
Witt, W K, 1993. Gold mineralisation in the Menzies–Kambalda region, Eastern Goldfields, Western Australia, Geological Survey of Western Australia Report 39.
Roberts, D E and Elias, M, 1990. Gold deposits of the Kambalda–St Ives region, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 479–491 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Woodall, R, 1990. Gold in Australia, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 45–67 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Simpson, E S and Gibson C G, 1912. Contributions to the study of the geology and ore deposits of Kalgoorlie, East Coolgardie Goldfield, Part 1, Geological Survey of Western Australia Bulletin 42.
Woodall, R, 1993. The multidisciplinary team approach to successful mineral exploration, SEG Newsletter No 14, July 1993, pp 1, 6–11 (Presented as keynote address at the Society of Economic Geologists International Conference ‘Integrated Methods in Exploration and Discovery’, Denver, Colorado, USA, 17–20 April 1993).
Sofoulis, J, 1966. Widgiemooltha, Western Australia - 1:250 000 geological series, Bureau of Mineral Resources Geology and Geophysics, Explanatory Notes SH 55–14. Swager, C P and Griffin, T J, 1990. Geology of the Archaean Kalgoorlie Terrane (northern and southern sheets), 1:250 000 geological map, Geological Survey of Western Australia. Swager, C P, Witt, W K, Griffin, T J, Ahmat, A L, Hunter, W M, McGoldrick, P J and Wyche, S, 1992. Late Archaean granitegreenstones of the Kalgoorlie Terrane, Yilgarn Craton, Western Australia, in The Archaean: Terrains, Processes and Metallogeny, Publication 22 (Eds: J E Glover and S E Ho), pp 107–122 (The Geology Department and University Extension, The University of Western Australia: Perth).
242
Geology of Australian and Papua New Guinean Mineral Deposits
Watchorn, R B, 1998. Kambalda–St Ives gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 243–254 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Kambalda–St Ives gold deposits by R B Watchorn1 INTRODUCTION
390 000 E
370 000 E
The production from gold deposits of the Kambalda–St Ives area is managed by the St Ives Gold (SIG) operations of WMC Resources Limited. The deposits occur in an area 60 to 100 km south of Kalgoorlie, on the Widgiemooltha (SH 51–14) 1:250 000 scale and the Lake Lefroy (3235) 1:100 000 scale map sheets (Fig 1), with the centre of the area at approximately lat 31o30′S, long 121o30′E.
6 550 000 N
KAMBALDA NICKEL PLANT
KAMBALDA
10
0 KILOMETRES
RED HILL
REDOUTABLE
BAHAMA
y fro Le
INTREPIDE
SANTA ANA
REVENGE THUNDERER SIRIUS ORCHIN BRITANNIA LIFEBOAT VICTORY DEFIANCE
DELTA SOUTH
ke La
St Ives Gold Administration ARGO
ST IVES GOLD PLANT
APOLLO 6 520 000 N
Perth
Kalgoorlie Kambalda
JUNCTION
WIDGIEMOOLTHA
FIG 1 - Location map, Kambalda–St Ives gold deposits.
The region has a history of gold exploration and production dating from 1897, and has been Australia’s largest nickel producing area since the discovery of nickel at Kambalda in 1966. This paper is the companion of the paper on the nickel deposits of the region by Stone and Masterman (this publication) and of the detailed description of the Revenge gold deposit (Nguyen, Donaldson and Ellery, this publication). This paper focusses on the gold deposits discovered since 1988 which are much more varied, both structurally and
1.
Chief Geologist, St Ives Gold, WMC Resources Ltd, PO Kambalda, Kambalda WA 6444.
Geology of Australian and Papua New Guinean Mineral Deposits
EXPLORATION AND MINING HISTORY The history of exploration was fully described by Elias (1990) and is only briefly summarised here. Prospectors fanning out from Coolgardie and Kalgoorlie discovered gold in the Kambalda–St Ives region in 1897, initially at Red Hill, Victory, Orchin and Delta Island and then at Ives Reward in 1919. The combined production from these mines, until production ceased in the 1930s, was 40 000 oz. The first nickel ore in Australia was discovered at Kambalda in 1966, only a few hundred metres from the Red Hill gold mine. Mining of the Lunnon shoot began in 1967 ( Elias, 1990).
Kambalda Nickel Operations Administration
CAVE ROCKS
stratigraphically, than those found previously. However, the overall stratigraphy and structure as described by Roberts and Elias (1990), are still valid and will not be repeated in detail. The deposits containing 100 000 oz of gold found after 1986 are described.
Over the next 13 years, patches of rich gold ore were sporadically intersected in the Lunnon, Hunt and Fisher nickel mines. The St Ives area was briefly investigated for its gold potential in 1974. However, except for the 1973–1976 period, the general outlook for gold prior to 1979 was poor, and even the mines on the Kalgoorlie Golden Mile were closed. Only two gold mines were operating in 1977; at Mount Charlotte in Kalgoorlie and at Norseman. An increase in the price of gold in 1979–80 led to renewed interest in the gold potential of the Kambalda–St Ives area. In 1979, the Hunt gold deposit was found beneath the Hunt nickel orebody and in 1980 the Victory deposit was discovered. These were followed in rapid succession by the discovery of the Defiance, Orchin, Orion, Britannia and Sirius deposits in the Victory area, the Junction orebody to the south and the Revenge orebody to the north. These deposits, found before 1987, were briefly discussed by Roberts and Elias (1990). Since 1988 there have been further major discoveries at Intrepide, Cave Rocks, Delta South, Argo, Apollo, Britannia, Sirius, Thunderer, Lifeboat, Redoutable and Santa Ana-Bahama, and major extensions to most of the known orebodies have been delineated (Figs 1 and 2). The discoveries and extensions to known deposits have allowed ore production to be expanded from 1.5 Mtpa in 1988 to 3.1 Mtpa in 1998. This has taken production from the 1988 level of 140 000 oz/yr to 435 000 oz in 1997, which at the time of writing will make SIG Australia’s second largest gold mining operation. The SIG reserves and resources at June 1997 were: • Proved and Probable Ore Reserves: 26.9 Mt at 4.16 g/t gold, • Indicated and Inferred Resources: 40.3 Mt at 2.78 g/t gold, and • Total Reserves and Resources: 67.2 Mt at 3.33 g/t gold (7.2 Moz).
243
6 550 000 N
375 000 E
R B WATCHORN
LEGEND Proterozoic dykes
TABLE 1 Kambalda-St Ives cumulative gold production to June 1997.
Granitoid
KAMBALDA
Merougil beds Black Flag Group
Mine
Ore mined (’000 t)
Dolerite includes Condenser and Junction Dolerites
Redoutable
Kalgoorlie Group Paringa Basalt Kapai Slate Defiance Dolerite Devon Consols Basalt
Formidable Bahama
Intrepide
Kambalda Komatiite
Santa Ana Lunnon Basalt
0
South Delta
Revenge
Orchin
5 kilometres AMG grid
Britannia, Sirius Victory
Lunnon Basalt
St Ives mill Argo
6 525 000 N
Ives Reward
Gold produced (’000 oz)
Open pit Redoutable
1264
3.0
122
Argo
567
3.0
55
Clifton Blue Lode
162 175
3.3 3.1
17 17
Orchin
809
2.8
73
North Orchin
231
3.4
25
Defiance
3046
3.1
311
Victory
624
3.2
65
Orion
386
3.1
39
Delta Is
792
2.3
59
Sirius
898
2.7
80 13
Britannia
Junction
Recovered grade (g/t Au)
94
4.1
Cave Rocks
149
3.7
18
Revenge
843
4.8
131
Junction
1077
4.1
141
Lifeboat
235
2.1
16
Low grade
2971
1.1
120
14 323
2.8
1302
Victory
8309
3.6
979
Junction
3773
4.7
570
Revenge
2256
4.2
307
Britannia
639
9.6
197
Hunt
652
2.5
52
Sub total underground
15 629
4.2
2105
Total
29 952
3.5
3407
Sub total open pits Underground FIG 2 - Regional geological map, Kambalda–St Ives gold deposits.
Cumulative Kambalda–St Ives production (Table 1) plus reserves and resources is 97.2 Mt at 3.47 g/t gold for 10.8 Moz of contained gold.
PREVIOUS INVESTIGATIONS There has been extensive documentation of the geology of the gold and nickel deposits and the stratigraphy of the Kambalda–St Ives region by WMC geologists since 1966. This documentation and descriptions of numerous external studies have been summarised by A Cowden and N J Archibald (unpublished data, 1989), Elias (1990), Roberts and Elias (1990) and Hudson (1990).
REGIONAL GEOLOGY STRATIGRAPHY The Kambalda–St Ives region forms part of the Norseman–Wiluna greenstone belt which comprises regionally extensive volcano-sedimentary packages. These were extruded and deposited in an extensional environment at about 2700–2660 Myr (Barley and Groves, 1990). The stratigraphic succession (Table 2, Figs 2 and 3) is dominated by the Kalgoorlie sequence, which has been described in detail by A Cowden and N J Archibald (unpublished data, 1989) and Roberts and Elias (1990), and will only be briefly discussed in this paper. The succession consists of the Kalgoorlie Group volcanic rocks and the Black Flag Group felsic volcanic rocks and sediments, overlain by the post-tectonic Merougil beds.
244
Kalgoorlie Group The Kalgoorlie Group consists of mafic rocks, to ultramafic basalt, to komatiite lava flows with minor interflow sediment and a prominent sedimentary marker bed. The following is a brief summary of the units, starting with the lowermost members. The Lunnon Basalt is a >2000 m thick sequence of thin lava flows with thin interflow sediments and pillowed tholeiite. This unit forms the footwall to the stratigraphic succession and has been dated at 2720 Myr (Roddick, 1984). It is conformably overlain by the Kambalda Komatiite. The Kambalda Komatiite, a series of 800–1200 m of high magnesium ultramafic flows, has been dated at 2709±4 Myr (Claoue-Long, Compston and Cowden, 1988). The nickel orebodies formed on or just above the contact with the Lunnon Basalt. There is a gradational contact with the overlying 60–100 m Devon Consols Basalt, a high magnesium basalt characterised by numerous ocelli and minor interflow sediments. These have been dated at 2693±30 Myr (Compston et al, 1986a). The Kapai Slate is a remarkably persistent 5–25 m marker horizon mapped from north of Kalgoorlie (Travis, Woodall and
Geology of Australian and Papua New Guinean Mineral Deposits
6 550 000 N
IN TR R ED E I D O E U TA B E
E EN R E
IC TO R
O
AR
N O TI C N U
REPUBLICAN THRUST
6 525 000 N
KAMBALDA––ST IVES GOLD DEPOSITS
AMBA DA DOME
FOSTER THRUST
AMBA DA RANODIORITE
TRAMWAYS THRUST
MA OR THRUSTS
ERTICA SCA E
RANITOID
DE ONS CONSO S BASA T
B AC
AMBA DA OMATIITE
HORIZONTA SCA E
25
0
A ROU
ARIN A BASA T CONDENSER DE IANCE DO ERITE
UNNON BASA T
FIG 3 - Schematic longitudinal projection of the Kambalda–St Ives gold deposits, looking west.
TABLE 2 Summary of Kambalda-St Ives Archaean stratigraphy (modified after Roberts and Elias, 1990). Group
Formation
Member
Thickness (m)
Merougil beds Black Flag Group
2000–3000 Morgan Island Epiclastic
500
Argillite, wacke, minor rudite
Newtown Felsic Volcanics
1000
Felsic and intermediate intrusives, volcanic conglomerate, breccia and tuff
4–500
Intrusive tholeiitic differentiated gabbro
Condenser and Junction Dolerites Paringa Basalt
500–1000 Defiance Dolerite
Kapai Slate Kalgoorlie Group
Description Conglomerate, terrigenous arenite, arenite
Devon Consols Basalt Kambalda Komatiite
300
Differentiated dolerite or flow
1–10
Carbonaceous and sulphidic argillite, minor chert
Victory Dolerite
60–100
High-Mg pillowed, variolitic basalt, thin differentiated dolerite
Tripod Hill
20–1000
Thin komatiite flows (1–10 m)
Silver Lake
50–200
Thick komatiite flows, high-Mg cumulate zones (50–200 m)
>2000
Pillowed to massive tholeiitic basalt
Lunnon Basalt
Bartram, 1971) to south of St Ives. Its lithology varies from pyritic graphitic slate to magnetite-bearing laminated chert. It separates the dominantly low silica lavas of the underlying formations from the high silica, high magnesium lavas of the overlying formations, and has been dated at 2692±4 Myr (Claoue-Long, Compston and Cowden, 1988). Conformably overlying the Kapai Slate is the Paringa Basalt, a l000–1500 m thick, siliceous high-magnesium basalt. It consists of variolitic pillowed flows and minor dolerite sills
Geology of Australian and Papua New Guinean Mineral Deposits
Siliceous high-Mg basalt, minor interflow sediment
with numerous bands of laminated and 2 to 5 m thick cherty interflow sediment, and has been dated at 2690±5 Myr (J M F Clout, unpublished data, 1991).
Black Flag Group The Black Flag Group consists of a felsic volcanic and sedimentary succession more than 1 km thick, conformably overlying the Kalgoorlie Group. These have been dated at 2676±4 Myr (J C Claoue-Long, I H Campbell and R Hill,
245
R B WATCHORN
unpublished data, 1991). Two formations were defined in the Group in the Kambalda area by Gemuts and Theron (1975); a lower dominantly felsic volcanic unit (Newtown Felsic Member) and an upper dominantly sedimentary unit (Morgan’s Island Epiclastic).
Merougil beds The Merougil beds comprise conglomerate, terrigenous arenite, pebbly arenite and arenite and correspond to the Kurrawang beds west of Kalgoorlie (Griffin, Hunter and Keats, 1983). The unit is estimated to be more than 2000 m thick in the region. The Merougil beds are the youngest unit in the Kambalda area and unconformably overly the Black Flag Group. The beds lack a distinct penetrative fabric, and this, in conjunction with discordant shallow dips, indicates syn- to post-tectonic deposition.
Kambalda intrusive rocks Doleritic to gabbroic dykes and sills, including the Defiance, Condenser and Junction dolerites, intrude the Kalgoorlie and Black Flag groups. The stratiform but discontinuous Defiance Dolerite is believed to have formed by in situ fractionation of the base of the Paringa Basalt. It is up to 300 m thick and is dated at 2693±50 Myr (Compston et al, 1986b). The 500 m thick Condenser Dolerite is stratigraphically and chemically equivalent to the Golden Mile Dolerite at Kalgoorlie. It was intruded transgressively, but is essentially conformable with the contact between the Paringa Basalt and the Black Flag Group. The Junction Dolerite is stratigraphically equivalent to the Condenser Dolerite. It is highly differentiated and has been subdivided into four zones of which Zone 4, a coarse-grained, magnetite-rich, quartz granophyric zone is the favoured host for high grade ore. The regional succession is intruded by at least four distinct episodes of igneous intrusions. The first comprises thin mafic to intermediate, aphyric, fine grained, shear- and layer-bounded sills and dykes which have been affected by all major deformations. The second comprises large (to 300 m thick) subconcordant lamprophyric xenolith-bearing sills with felsic differentiates. A kersantite of this suite has been dated at 2684±6 Myr (Perring, 1988). They intrude the Kambalda Komatiite and the Kapai Slate, and are present as sills or dykes at the Paringa Basalt–Black Flag Group contact in the Lake Lefroy area. The Kambalda Granodiorite and a set of essentially upright, felsic dykes of at least two generations comprise the third episode. These intrusions are dated at 2662±6 Myr (Compston et al, 1986a) These rocks have only weakly developed fabrics which are discordant to earlier (D1 and D2) structures, and intrude along and are deformed by later (D3) structures (Clark, Archibald and Hodgson, 1986). The fourth intrusive event resulted in emplacement of numerous Proterozoic dolerite dykes trending east and ENE. Some have a strong positive magnetic signature and others have a reversed magnetic response suggesting several emplacement regimes. These are dated at 2420±30 Myr (Turek, 1966) and 2042±45 Myr (Compston, 1980).
246
METAMORPHIC AND TECTONIC FRAMEWORK The Kambalda–St Ives region is structurally complex with polyphase deformation accompanying and post-dating regional metamorphism. The regional metamorphic grade reached lower amphibolite facies at 520–550oC and 2–3 kb (Donaldson, 1983; Wong, 1986). The earliest recognised deformation in the Kambalda area (D1) comprises major thrusts with a mylonitic fabric, forming a SE to NW thrust-repeated succession. Major thrusts have been identified, spaced 5–10 km apart, at Foster, St Ives, Tramways and Republican Hill (Fig 3). Recumbent open to tight folds formed during D1. These are commonly dislocated by thrusts which are subparallel to their axial surfaces. Subsequent NNW-trending deformation formed upright open folds (D2). The regional structure is dominated by a broad south-plunging antiform. D2 was synchronous with, but slightly later than, peak metamorphism (Gresham and LoftusHills, 1981). The D3 deformation is characterised by major NNW- and north-trending anastomosing shear zones such as the Boulder–Lefroy and Zulieka shear zones. There are several generations of shear zones and their fabrics have chloritic retrograde metamorphic assemblages. Structures associated with ore deposits are generally late stage D3 shears and fractures, and usually represent third and fourth order splays off the major NNW shear zones. These late stage shears often reactivate earlier D1 or D2 shears, which may also be mineralised. The last generation of major faults (D4) trend NNE, generally with fairly minor (50–200 m) dextral movement.
ORE DEPOSIT FEATURES The locations of the deposits are shown in Figs 1 and 2 and their characteristics are listed in Table 3.
DEPOSIT HOST ROCKS Ore deposits formed in nearly all stratigraphic units. The hosts, in order from the earliest units, and the contained deposits are: • Lunnon Basalt: Hunt; • Kambalda Komatiite: Redoutable, Victory–Repulse and Red Hill; • Devon Consols Basalt: Revenge (W45) and Britannia; • Kapai Slate: Victory, Clifton, Blue Lode, Delta South and Repulse footwall lode; • Defiance Dolerite: Revenge N01 and N22, Defiance, Thunderer, Orchin and North Orchin; • Paringa Basalt: Defiance, Sirius, Apollo and Santa Ana; • Condenser Dolerite: Argo and Junction, and Cave Rocks in an analogous dolerite; and • Intermediate and felsic intrusive rocks: Intrepide and Victory Flames.
BASALT HOSTED DEPOSITS Sirius The Sirius orebody was discovered in 1988 and the top 90 m was worked in an open pit in 1989–90. The depth of weathering reaches 40 m with considerable supergene enrichment of gold.
Geology of Australian and Papua New Guinean Mineral Deposits
KAMBALDA––ST IVES GOLD DEPOSITS
TABLE 3 Characteristics of the Kambalda–St Ives gold deposits, modified after Roberts and Elias (1990). Deposit
Host rocks
Main structure
Lode type
Regional metamorphic assemblage
Lunnon Basalt
Shear, NNW, 7oW
Q vns, bx
hb, ac, cl, pl, Q cl, bt, ank, ca ank, bt, ab, py
Devon Consols Basalt/Kambalda Komatite
Shear, NNW, 70o E
Q vns, mylon, bx
tc, cl, ank, tm
cl, dl, tc, ab
Kambalda Komatite
Shear
Q-cb vns
sp, tc,cl,tm
Orion
Devon Consols Basalt/inflow sediment
Shear, NNW, 60oW
Q vns, mylon
Orchin
Defiance Dolerite/Paringa Basalt
Shear, NE, 45oSE
North Orchin
Defiance Dolerite/Kapal Slate
Victory Defiance
Alteration zoning
Premining reserve ( t gold)
Mining method
3-15
U/G
dl, cl, ab, tc, py, po
<3
Minor open pit historical
cl,dl,tc
cl,dl,py
<3
Historical
ac,cl,pl
cl,mt,dl
bt,dl,ab,py, ms
3–15
Open pit
Q vns, mylon
hb,ac,cl,pl
cl,dl,bt
bt,dl,ab,py
3–15
Open pit
Shear, N,60oE
Q vns, mylon
hb,ac,cl,pl,mt,Q
cl,mt,bt
bt,dl,ab,py, ms
3–15
Planned U/G
Kapai Slate
Complex shear, NE
Q vn array
Q,ab,mt,py
ac,cl,mt
ms,ab,py
Defiance Dolerite/Paringa Basalt Devon Consols Basalt
Complex N,E
Stacked Q bx,vns
hb,ac,cl,pl
cl,bt,dl
Dl,bt,ab,py
Compex N,E
Mylon
hb,ac,pl,cl
cl,bt,dl
dl,bt,ab,py
Revenge
Defiance Dolerite
Shear, ENE, 45oSE mylon
Stacked Q bx, vns
hb,ac,pl,cl
cl,bt,dl
Junction
Junction Dolerite
Shear, NNW, 40oNE
Q vns, bx, mylon
hb,ac,pl,cl
Sirius
Paringa Basalt
Shear, NNW, 60oE
Q vn array
Britannia
Paringa Basalt
Shear NNW, 80oE
Repulse footwall lode
Kambalda Komatite/Devon Consols Basalt
Santa Ana
Outer Hunt Ives Reward
Red Hill
Inner
Open pit, U/G >31
Open pit, U/G
dl,bt,ab,py
15-31
Open pit, U/G
cl,di
Cl,dl,ab,po, ac
>31
Open pit, U/G
Q,cl,py
cl
bt,ab,py
25
Open pit, U/G
Q vn, py
q, cl
cl,bt,mt
ab,bt,py,po
10
Open pit, U/G
Shear NNW 10oE
Q vn, py shear
Q,ab,cl
cl,bt,mt
ab,bt,py
15
U/G
Paringa Basalt/ trondhjemite
Q vn, NNW shear, 70oE
Q vn, shear
cl
cl,dl
bt,ab,py
9
Open pit
Argo/Apollo
Condenser Dolerite/Paringa Basalt
N-S shear, 40oW
Q vn,py,shear
cl,hb
cl,dl
ab,bt,py,as
10
Open pit, U/G
Cave Rocks
Dolerite
Shear, NNW, 85oE
Q vns
cl,hb,ac,pl
cl,dl
Bt,ab,Q,po, as
9
Open pit, U/G
Felsic/ intermediate intrusions
NNW Q vn stockwork
Q
pl,hb
cl
py,ab,bt
12
Open pit
Kambalda Komatite intermediate/ felsic intrusions
NNW shear, 60oE
Q vns
tc,dl
cl
bt,py,ab
4
Open pit
Intrepide
Redoutable
Q - quartz, ac - actinolite, cl - chlorite, hb - hornblende, pl - plagioclase, tc - talc, ank, ankerite, tm - tremolite, ca, - calcite, sp - serpentine, mt magnetite, ab - albite, py - pyrite, bt - biotite, dl - dolomite, po - pyrrhotite, ms - muscovite, ep - epiodite, sn - sphene, bx - breccia, mylon mylonite, vns - veins, as - arsenopyrite.
In the last six years the orebody has been accessed and intensively drilled from underground. The Sirius orebody is hosted by the Paringa Basalt on the strongly sheared east limb of the major D2 fold in the Victory area. In this area the Repulse shear zone acted as a sole thrust, with the Victory, Britannia and Sirius shears being associated hanging wall splays (Fig 4).
Geology of Australian and Papua New Guinean Mineral Deposits
The 100–200 m wide Sirius shear dips 30–60o ENE, and has a strong, late, flat, west-dipping crenulation cleavage overprinting a strong chlorite-biotite foliation (D Barrett, unpublished data, 1993). The 400 m long, 100 m thick by 300 m deep orebody is localised where the Sirius shear changes up dip from a 35o east dip to subvertical (Fig 5). Multiple generations of quartz veins have been emplaced, thrust folded
247
383 500 E
382 000 E
R B WATCHORN
shear zones. The shear zones contain 0.3–2 g/t gold and are associated with sparse quartz veining. Most of the gold is associated with biotitisation of the wall rock. Except for a high grade, laminated, crack-seal quartz vein at the north of the orebody, where gold is associated with the laminations in the vein, most of the quartz veins have low gold grades.
THUNDERER
534 500 N
The orebody is open to the north, south and down dip. The current plan is to mine the top 250 m as an open pit, and the deeper parts and the thinner ore zones to the north and south from underground.
NORTH ORCHIN BRITANNIA SIRIUS Cross section on Fig 5
LIFEBOAT
Britannia REPULSE
The Britannia lodes are 200 m west of Sirius in the footwall of a major listric shear zone (Figs 4 and 5) and were discovered in 1987. Initially (1987–1990) worked in a pit, the orebody was drilled and accessed by underground development in 1993.
VICTORY DEFIANCE 533 000 N
FO ST
FLAMES E R
N31 ST RU TH
CONQUEROR 0
750 metres
LEGEND Proterozoic dyke Defiance Dolerite, Zone 1 Defiance Dolerite, Zones 2-5
Lamprophyre
Kapai Slate
Felsic intrusive
Devon Consols Basalt Tripod Hil Komatiite
Black Flag Group Paringa Basalt
Dolerite
Victory-Defiance premined ore reserve boundary Kambalda local grid
Footwall basalt
FIG 4 - Geological plan of the Victory area, showing gold deposits.
and brecciated. The final set of flat, SW dipping veins was emplaced after the major movement. Higher grade (2–5 g/t) gold mineralisation occurs as lenses of intense biotite-quartz veining, with 2–10% pyrite, within
DE IANCE
The orebody is hosted by the 100 m wide, major early chloritic Britannia shear, which is subvertical at the surface and flattens to 45o at depth. The shear marks the contact between the Devon Consols Basalt and the Paringa Basalt. It is one of the earlier (D3) shears in the area and is intruded by sheared, late stage, felsic dykes. Elsewhere the intrusive rocks crosscut and thus post-date the major NNW folding (D2). The orebodies are focussed on the felsic intrusive contacts (Fig 6) and the ore zones have a gentle north plunge, in sympathy with late flat NNW-dipping veins. Steeply plunging, en echelon, high grade shoots within this zone are associated with magnetite and massive euhedral sulphides, including 0.5–1 cm pyrite grains with <10% interstitial pyrrhotite grains. Relict sheared magnetite-rich sediments in the footwall of the shear zone provide an iron-rich setting that enhanced gold deposition
ICTOR BRITANNIA SHEAR
SIRIUS SHEAR
RE U SE AU T
00 RL
OTENTIA ORE SHOOTS AT DE TH
Flames porphyry
E END
Proterozoic dyke
Proterozoic dyke
Basalt
Gold mineralisation
Kapai Slate
Defiance Dolerite
Devon Consols Basalt
Lamprophyre
-400 RL
200 metres Victory local grid
Footwall basalt
5 900E
Paringa Basalt
Tripod Hill komatiite 0
5 500 E
Flames intrusive
FIG 5 - Cross section along local grid line 11 400 N, looking north, showing the Defiance, Victory, Britannia and Sirius gold deposits.
248
Geology of Australian and Papua New Guinean Mineral Deposits
3 5 500 E
BAHAMA SHEAR ZONE
AL PH FA A IS ULT LA Z O ND NE
60
3 500 E
6 000 E
SIRIU S S HEA R
5 500 E
12 000 N
5 750 E
KAMBALDA––ST IVES GOLD DEPOSITS
82
9 5.5 44
12 4.9 65
85
11 750 N
BAHAMA TRONDHJEMITE
SIRIUS IT
70
BRITANNIA IT
50 2.3 33 2.3
60
85
15 4.4
45 8.1
75 5 000 N
11 500 N
ORION IT
SIRIU S SH EAR
75
13 1.8
8 8.2
C 5
LEGEND Shear zone
SANTA ANA PIT OUTLINE
Cross section Fig 8
LEGEND Trondh emite
24 2.5 35 7.7
Lamprophyre Paringa Basalt Kapai Slate
Proterozoic dyke
11 250 N
0
100 metres
Porphyritic felsic intrusive Paringa Basalt Devon Consols Basalt
Devon Consols Basalt Kambalda Komatiite 3 5500 E
Gold mineralisation
SANTA ANA SHEAR ZONE
0 2 SANTA ANA TRONDHJEMITE
0
Down hole drill metres gold grams per tonne 200
metres
Kambalda local grid
Victory local grid
FIG 7 - Simplified geological plan of the Santa Ana gold deposit. FIG 6 - Geological plan of the Britannia and Sirius gold deposits.
carbonate shear, especially along the trondhjemite contact (Fig 8). High gold grades coincide with sulphide concentrations and biotite wall rock alteration but rarely occur in quartz veins. The high grade (12 g/t) of the Britannia ore is related to the introduction of late potassic (biotite forming) and sulphidic ore fluids which overprinted even the late flat tension-quartz veins. The orebody, like Sirius, is weathered to about 40 m depth, with considerable supergene enrichment, and is open to the north and south.
Santa Ana and Bahama The Santa Ana deposit, discovered in 1995, 5 km NW of the Revenge mine, is on the contact between the Paringa Basalt and trondhjemite stocks which intrude the Paringa Basalt. The Santa Ana lode shears occur over a 1 km length at Santa Ana, to the south, and a 1.5 km length at Bahama, to the north. Bahama is interpreted as an extension of the Santa Ana mineralisation, on the north side of the Alpha Island fault (Fig 7). The Condenser Dolerite is absent in this area, and a trondhjemite intrusion occurs to the west of a sheared zone of Paringa Basalt. Dykes and sills of intermediate lamprophyre have been intruded into the sheared zone. The Santa Ana lodes are hosted by two or three NNWtrending 60–85o east-dipping shears similar to the Junction shear. The shears are displaced by a NE–NNE trending shear, known as the Alpha Island fault. This shear zone has an implied early dextral movement of about 200 m, with the initial movement possibly contemporaneous with the initiation of the lode shears (Fig 7). The Santa Ana mineralisation is associated with chloritecarbonate-quartz±biotite wall rock alteration accompanying quartz-carbonate-pyrite-pyrrhotite veins (R S Morrison, unpublished data, 1997). The highest grades are associated with the thicker quartz veins within the broad chlorite-
Geology of Australian and Papua New Guinean Mineral Deposits
Weathering to 70 m has resulted in the development of significant supergene enrichment.
KAPAI SLATE HOSTED DEPOSITS Repulse footwall lode This lode was discovered in 1994 within the listric Repulse shear zone. This shear zone flattens from near vertical in the east wall of Defiance pit to subhorizontal in the Repulse area (Figs 4 and 5). In the East Repulse area the Repulse shear zone occurs along the contact of the Devon Consols Basalt–Kapai Slate sequence with the Kambalda Komatiite. It represents the thrust surface of the Kambalda Komatiite over the Devon Consols Basalt. The Devon Consols Basalt–Kapai Slate package is totally disrupted and forms a cataclastic zone with intense folding, alteration and quartz veining for 100 m beneath this contact. Within this disrupted zone the many NNW-trending, subvertical felsic intrusions have been brecciated and rotated to horizontal. Where the Kapai Slate occurs in the disrupted zone, high grade gold mineralisation (5–10 g/t) coincides with abundant euhedral pyrite. This zone of intense disruption and contrasting rock competencies caused a refraction of the Repulse shear zone and a focussing of mineralising fluids. The blocks of ironrich Kapai Slate formed a chemical trap for desulphidation and gold precipitation. This resulted in a lozenge-shaped orebody following the Kapai Slate beneath the komatiite contact.
DOLERITE HOSTED OREBODIES A focussed search for dolerite-hosted orebodies commenced in 1981. The Junction deposit was discovered in 1986 and was
249
375 200 E
SUPERGENE MINERALISATION IN O IDE ARCHAEAN
375 000 E
374 800 E
R B WATCHORN
300 m RL
SURFACE BASE OF SEDIMENT
MINERALISATION IN TRONDHJEMITE 200 m RL
LAMPROPHYRE INTRUSIONS SANTA ANA TRONDHJEMITE
PARINGA BASALT SHEAR ZONE HANGING WALL LODE IN PARINGA BASALT SHEAR ZONE
LEGEND Shear Trondh emite
MAIN LODE IN PARINGA BASALT SHEAR ZONE
100 m RL
Lamprophyre Paringa Basalt 200 Kambalda local grid
0 metres
0
APPRO IMATE APOLLO PIT OUTLINE
ARGO OPEN PIT
526 000N
The Apollo mineralisation is similar to Argo, but the alteration and shear intensity are not as strong as at Argo. Both the Argo and Apollo orebodies are open down dip.
250
FAR EAST ARGO
Proterozoic dyke
525 500N
r miner al d shea
LEGEND Mineralised shear Inferred mineralised shear
ised
Cross section Fig 8
Inferre
The Argo shear mineralisation is about 30 m wide with an inner high grade (4–6 g/t) albitic core, a moderate grade biotitic zone and an outer low grade chlorite zone. The gold is associated with sulphides, particularly pyrite, but also with occasional arsenopyrite euhedra. Highest grades and widths are associated with the granophyric zone of the Condenser Dolerite where disruptions of the more brittle, quartzmagnetite rich zone has resulted in stacked quartz breccia shear lodes. The ore zone is generally 2–6 m wide. Mineralisation is subeconomic where the Argo shear intersects the overlying Black Flag Group and lower grade in the underlying Paringa Basalt. There has been considerable leaching of the Argo deposit in the oxidised area with supergene enrichment above the oxide–fresh rock boundary.
250 metres
Argo and Apollo The Argo deposit (Fig 9), discovered in 1993, is 3 km south of the St Ives gold plant (Fig 2) and is within the 300 m thick SW dipping Condenser Dolerite. It is on the west limb of the Kambalda–St Ives antiform (Fig 2). The main Argo shear, which hosts the mineralisation, strikes north and dips moderately to the west. A subparallel mineralised structure, the Apollo shear, is 150 m to the east (Figs 9 and 10).
384 250 E
383 750 E
described by Roberts and Elias (1990). It is hosted by the Condenser Dolerite, a transgressive sill intruded along the contact of the Paringa Basalt and Black Flag Group (Carey, 1994). At Junction, this sill has been thrust or folded and is located 2 km into the Black Flag beds. The iron-rich and rheologically competent natures of differentiation zones within the Condenser Dolerite provide an attractive host for gold mineralisation.
383 250 E
FIG 8 - Representative cross section of the Santa Ana gold deposit, on local grid line 540 700 N, looking north.
Flames Intrusive Condensor Dolerite Black Flag Group Paringa Basalt Kambalda local grid
FIG 9 - Geological sketch plan of the Argo–Apollo area.
Cave Rocks The deposit (D Barrett, unpublished data, 1992) is 5 km west of Kambalda, adjacent to the Zuleika Shear Zone. The stratigraphic succession is similar to the Kambalda–St Ives area, but the sequence is disrupted, with widely disparate units juxtaposed by faulting.
Geology of Australian and Papua New Guinean Mineral Deposits
KAMBALDA––ST IVES GOLD DEPOSITS
A
300 R
S
AR O SHEAR 50 0
0 0 202 3
0 33
22 0
0 2
200 metres
303
AO O SHEAR
050
H
LEGEND Drill hole trace
0R
Quaternary sediment
2
Gold mineralisation
3
0
Shear Condenser Dolerite
3 3 00 E
Black Flag Group 0 2
Down hole drill metres gold grams per tonne Kambalda local grid
362 000 E
The westernmost unit is an ultramafic body (Kambalda Komatiite?) faulted against a dolerite which is compositionally similar to the Condenser Dolerite. To the east are packages of alternating 10–100 m thick, subvertical units of sediment similar to the Black Flag beds, and dolerite (Fig 11). The middle amphibolite facies metamorphic grade is higher than at St Ives.
547 000 N
e t lod Wes 546 500 N
0
250 metres
LEGEND
KOMATIITE–FELSIC INTRUSIVE HOSTED OREBODIES
Inferred fault Strike, vertical dip Pit outline
The first gold mines in the Kambalda area (Red Hill and Westralia, from 1896) had rich but patchy grades and produced more than 30 000 oz of gold (Fig 1). These deposits were on the contact between east-dipping felsic intrusions and the Kambalda Komatiite, where transpression and the rheology contrasts produced dilatant zones for gold mineralisation. In the early 1970s, further gold mineralisation on this felsic intrusive–komatiite contact was found on 7 level in the adjacent Silver Lake nickel mine. Subsequent drilling of the southern extensions of the Lunnon and Hunt nickel deposits showed that gold mineralisation in felsic intrusive rocks extended southwards for over 4 km. The 1992–95 follow up of these intersections resulted in the discovery of the Intrepide (D B Goodwin, P Nguyen, J Withers and C Wilkinson, unpublished data, 1993), Formidable and Redoutable orebodies.
Geology of Australian and Papua New Guinean Mineral Deposits
e t lod Eas
The mineralisation consists of two, probably en echelon orebodies known as the East and West lodes (Fig 11). Mineralisation comprises two subvertical, 2–10 m wide lenses with a gentle southerly plunge (Fig 12) in a 5–50 m wide chlorite-rich shear. The central core of the shear has quartz veins with biotite wall rock alteration with albite alteration in the high grade zones. The ore mineralogy is similar to that at Argo, with pyrrhotite and some euhedral arsenopyrite.
362 500 E
FIG 10 - Cross section of the Argo and Apollo gold deposits on local grid line 543 460 N, looking north.
546 000 N
Mullock dump outline Metadolerite gabbro Sediment black shale, chert, wacke Ultramafic rock Kambalda local grid
FIG 11 - Simplified geological plan of the Cave Rocks gold deposit.
Intrepide The deposit, discovered in 1992 (D B Goodwin and M Reston, unpublished data, 1994), is in a large intermediate intrusive plug in the Kambalda Komatiite on the crest of the D2 Kambalda antiform (Figs 2 and 13).
251
546 600 N
546 400 N
350 m RL
546 800 N
R B WATCHORN
NATURAL SURFACE
18.0 10.03 ORE SHOOT
PIT OUTLINE
250 m RL
1.5 35.44 3.2 2.4
ORE SHOOT 4.71 4.79
2.0 5.19 1.5 5.13
4.0 12.04
150 m RL
5.3 2.6 FW 2.4 68.1 FW 21.0 6.7
CURRENT RESER E 5 0 000 2 3 1.80
5 3.77
LEGEND
5 1.84 55.5 4.96 FW
4.0 1.8
5.9 2.02
Interpreted gold mineralisation outline
3.48 9.12 ORE SHOOT 3.63 3.0 7.7 13.7
18.8 2.85
Probable ore reserve outline
50 m RL
Drill hole position
ORE SHOOT
0 2
100
0
Metres true width Au grams per tonne
metres
Kambalda local grid
LEGEND
377 000 E
376 000 E
TO
375 000 E
FIG 12 - Longitudinal projection of the West lode at Cave Rocks gold deposit, looking west.
KA MB
Gold mineralisation Black Flag group
ALD
REDOUTAB E IT
A
Dolerite C
Felsic intrusive Devon Consols Basalt Paringa Basalt Kambalda Komatiite
The quartz stockwork has three predominant vein orientations, at 173o/36oE, 340ο/19oW and 234ο/65oS. Abundant visible gold is associated with the quartz veins, but like other St Ives deposits most of the gold occurs in pyrite and the albitic wall rock alteration. There is a significant supergene gold layer at 30 m below surface, below an intensely leached and silicified zone, and within the oxidised intrusion.
AMG grid
Redoutable ORMIDAB E RO ECT 100
0
A HA IS AND
T IV
LT
INTRE IDE IT
HA
NT
BETA IS AND
PLA
ISL
LD GO
ALP
6 541 500 N
ES
DF AU
6 542 500 N
S TO
AN
metres
FIG 13 - Simplified geological plan of the Intrepide and Redoutable gold deposits.
The orebody is a gently southerly plunging and NNW striking, 400 m long, 100 m wide and 200 m deep quartz stockwork body, with accompanying strong albite alteration and 2–5 % pyrite. High grade (4–6 g/t) biotite-rich shear zones in the south of the orebody may originate from the initial deformation that fractured the intrusion and enabled formation of the stockwork lode.
252
The deposit is 2 km NNW of Intrepide, and in the same shear zone. Here the shear is very broad and individual shear zones are difficult to define. Mineralisation occurs within the entire 100–250 m wide, east-dipping zone, within which occurs a 400 m long, 50 m wide and 100 m deep, gently southerly-plunging ore envelope (Figs 13 and 14). Subvertical ore lenses have formed within the ore envelope due to the interaction of the NNW- and NNEtrending shear systems and large NW-trending intermediate and felsic dykes and small plugs. Higher grade (3–10 g/t) ore formed on the margin of these intrusives, with the highest grade lenses associated with strong albite alteration, quartz veining and 2–10% pyrite. About 70–80% of the gold is associated with pyrite. Visible gold is contained within quartz veins and minor interflow sediment in the ultramafic rocks. Most of the shear zone, including the igneous intrusions, has a strongly foliated biotite alteration associated with lower grade mineralisation. Minor barren chlorite alteration zones are also present. The original target for exploration in the Redoutable area was the intersection of the NNW-trending Lunnon shear zone and the NNE-trending Alpha Island fault. Open pit mining confirmed a broad, dextral swing in the ore zone but did not show a sharply defined offset on the Alpha Island fault. Several thin, NNE-trending faults were mapped, which do not displace
Geology of Australian and Papua New Guinean Mineral Deposits
8 1.5
Ba
4 1.0
se
1 2.7
2 4.61 2 9.20
KD
34
04
25 33 KD
24 KD
7 3.1 1 5.65
LUNNON SHEAR
33
23 33 n
oxidatio
of 1 4.59
4 1.5
KD
22 KD
33
21 33 KD
20 33
33
18 33
KD
SURFACE
KD
KD
KD
33
17
300 m RL
19
375 500 E
375 600 E
KAMBALDA––ST IVES GOLD DEPOSITS
9 1.9
ALPHA ISLAND FAULT ZONE
9 6.0
LEGEND
200 m RL
Gold mineralisation
D3 S3 Fabric
0
Felsic intrusive
100
Ultramafic rock
METRES
E-W REVERSE SHEAR
9 6.0
Downhole metres gold g t Kambalda local grid
FIG 14 - Cross section of the Redoutable gold deposit on AMG grid line 544 000 N, looking north.
the felsic and intermediate dykes. The Alpha Island fault diffuses into the ductile ultramafic rocks and is probably synchronous with the NNW Lunnon shear zone (D3).
mineralisation, which was associated with non-foliated biotite overprinting all D3 fabrics at Victory, is dated at 2601±3 Myr (Compston, 1980; Clark, 1987).
There is significant supergene mineralisation within the saprolite, beneath 5–10 m of lake sediment.
Gold mineralisation over a 30 Myr period is consistent with Kalgoorlie where the Golden Mile telluride mineralisation is dated at 2627±3 Myr and with the mineralisation associated with the quartz stockwork at Mount Charlotte, which is dated at about 2602±8 Myr (Kent and McDougall, 1995).
HOST STRUCTURES North- to NNW-trending and east- or west-dipping (45o) thrust zones, 2–10 m wide, host most of the gold deposits at Revenge, Defiance, Repulse, Orchin, Junction, Santa Ana, Apollo and Argo. Steep, wide, NNW-trending fault zones also host orebodies at Cave Rocks, Ives Reward, Hunt and Britannia. Gold deposits associated with stockworks and vein arrays were formed in brittle host rocks such as the Kapai Slate at Victory and in felsic and intermediate intrusions at Intrepide.
General studies by WMC geologists, particularly on the Victory, Revenge, Junction and Britannia deposits, have established the following gold mineralisation sequence: 1.
Emplacement of steep, 280–300o trending, pre-gold crosscutting carbonate-quartz veins at the Victory and Revenge deposits.
2.
Development of the main shear systems containing the mylonitic quartz-molybdenite lode at Junction, and the cataclastic Repulse shear system at the Victory and Britannia deposits. The systems were formed during, or immediately after, the intrusion of felsic and intermediate dykes and sills at about 2651 Myr (Clark, 1987). At Junction, a mylonitic quartz-molybdenite lode is cut by a diorite intrusion, indicating contemporaneity of mineralisation and igneous intrusive activity. At Britannia the felsic intrusions, associated with a regional crosscutting dyke swarm, were intruded along the Britannia shear, indicating contemporaneity with the earlier shear system. In the Victory area, during the early, normal movement of the Repulse shear, felsic dykes with <1 g/t gold were intruded. The main shearing was associated with retrogressive chlorite alteration.
3.
Carbonate stockwork and breccia lodes with <0.5 g/t gold were emplaced in the shear systems late in the chloritic alteration event.
Gold deposits with composite features occur where host rocks have contrasting competencies, such as where felsic or intermediate dykes intrude Kambalda Komatiite at Redoutable and Red Hill, or where felsic dykes intrude the Kambalda Komatiite–Devon Consols Basalt contact, as at VictoryRepulse.
ORE GENESIS AND TIMING The main gold mineralisation in the Kambalda–St Ives area was emplaced over a period of >30 Myr following peak metamorphism at 2660 Myr (Compston et al, 1986b) and after D2 upright folding. The mineralisation also followed the intrusion of the intermediate and felsic dykes at 2671–2651 Myr (Clark, 1987). Carbonate alteration surrounding the gold lodes is dated at 2627 Myr and the foliated biotite alteration associated with the major quartz-biotite lode shears at Defiance is dated at 2629 Myr (Clark, 1987). The last episode of gold
Geology of Australian and Papua New Guinean Mineral Deposits
253
R B WATCHORN
4.
5.
Breccias and widespread quartz veining then formed in the major shears. These were associated with, or postdated, the foliated biotite alteration, which was emplaced under ductile conditions during which the earlier quartz veins were also folded. The breccias were brittly deformed later, marking the rheological deformation transition. In the Revenge W45 area the gold grade is about 4–6 g/t and very erratic where the biotite lodes have not been overprinted by late biotite-albite alteration. In areas of albite-dominant alteration the gold grades are significantly higher (6–8 g/t) and more consistent. In restricted areas a late stage of biotitic alteration followed earlier veining, and in the Britannia deposit is accompanied by massive pyrite euhedra and gold grades >15 g/t.
CONCLUSIONS The last five years at the SIG operation have been characterised by an increased rate of discovery of gold deposits. Orebodies now have been found in every rock unit and in diverse structural settings. A common feature of many of the deposits is a NNWtrending structure associated with rocks of contrasting competency. Such an environment allows passage of ore fluids in either dilatant, compressive or transpressive regimes. The chemical composition of the host rock, although important, does not have an overriding influence in gold deposition.
ACKNOWLEDGEMENTS WMC Resources Ltd is acknowledged for permission to publish this compilation of the continuing careful documentation of the geology of the Kambalda region by WMC and external geologists. The excellent work of the SIG geological staff and the SIG-KNO exploration team over the five years since the formation of SIG operations is acknowledged. Without their work the discoveries on which this paper is based would not have been made. The encouragement and contributions of K Hein, P Ellis, J Chapman, P McGeehan, E Baltis, P Nguyen, E Ainscough, J Osborne, J Donaldson, B Dumpleton, S Ellery, E Poole, R Morrison and all the current staff at SIG operations have greatly improved this manuscript. Thanks to A Brown who drafted the figures and D Barclay who typed the manuscript.
REFERENCES Barley, M E and Groves, D I, 1990. Archean metal deposits related to tectonics: evidence from Western Australia, in Third International Archean Symposium (Eds: J E Glover and S E Ho), pp 343–345 (Geoconferences (WA) Incorporated: Perth). Carey, M, 1994. Petrography and geochemistry of selected sills from the Kambalda-Kalgoorlie region, WA, BSc Honours thesis (unpublished), Australian National University, Canberra. Claoue-Long, J C, Compston, W and Cowden, A, 1988. The age of the Kambalda greenstones resolved by ion microprobe: implications for Archean dating methods, Earth and Planetary Science Letters, 89:239––59. Clark, M E, 1987. The geology of the Victory gold mine, Kambalda, Western Australia, PhD thesis (unpublished), Queen’s University, Kingston, Ontario. Clark, M E, Archibald, N J and Hodgson, C J, 1986. The structural and metamorphic setting of the Victory gold mine, Kambalda, Western Australia, in Gold ’86, an International Symposium on the Geology of Gold Deposits (Ed: A J MacDonald), pp 243–254 (Gold ‘86: Toronto).
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Compston, W, 1980. History of the crust and mantle of Kambalda using isotopic age determinations, in Western Australian Institute of Technology 1980 Seminar series (abstract). Compston, W, Pidgeon, R T, Williams, I S and Kinney, P D, 1986a. A further occurrence of detrital zircons older than 4100 Ma in Western Australia, in The Australian National University, Research School of Earth Sciences, Annual Report 1985, pp 80–81. Compston, W, Williams, I S, Campbell, I H and Gresham, J J, 1986b. Zircon xenocrysts from the Kambalda volcanics: age constraints and direct evidence for older continental crust below the Kambalda-Norseman greenstones, Earth and Planetary Science Letters, 76, pp 299–311. Donaldson, M J, 1983. Progressive alteration of barren and weakly mineralised Archean dunites from WA, PhD thesis (unpublished), The University of Western Australia, Perth. Elias, M, 1990. Kambalda gold deposits - history of exploration, in Geological Aspects of the Discovery of Some Important Mineral Deposits in Australia (Eds: K R Glasson and J H Rattigan), pp 43–47 (The Australasian Institute of Mining and Metallurgy: Melbourne). Gemuts, I and Theron, A, 1975. The Archean between Coolgardie and Norseman - stratigraphy and mineralisation, in Economic Geology of Australia and Papua New Guinea, Vol 1 Metals (Ed: C L Knight), pp 66–74 (The Australasian Institute of Mining and Metallurgy: Melbourne). Gresham, J J and Loftus-Hills, G D, 1981. The geology of the Kambalda nickel field, Economic Geology, 76:1373–1416. Griffin, T J, Hunter, W M and Keats, W, 1983. Geology of the Kalgoorlie-Widgiemooltha district, in Eastern Goldfields Geological Field Conference, 1983, Abstract and Excursion Guide (Ed: P C Muhling), pp 7–8 (Geological Society of Australia: Perth). Hudson, D R, 1990. Nickel sulphide deposits of Western Australia-an introduction, in Geological Aspects of the Discovery of Some Important Mineral Deposits in Australia (Eds: K R Glasson and J H Rattigan), pp 393–394 (The Australasian Institute of Mining and Metallurgy: Melbourne). Kent, A J R and McDougall, I, 1995. Ar-Ar and U-Pb age constraints on the timing of gold mineralisation in the Kalgoorlie goldfield, Western Australia, Economic Geology, 90:845–859. Perring, C S, 1988, Petrogenesis of the lamprophyre ‘porphyry’ suite from Kambalda, Western Australia, in Recent Advances in Understanding Precambrian Gold Deposits, Publication 12 (Eds: S E Ho and D I Groves), pp 277–294 (Geology Department and University Extension, The University of Western Australia: Perth). Roberts, D E and Elias, M, 1990. Gold deposits of the Kambalda-St Ives region, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 479–491 (The Australasian Institute of Mining and Metallurgy: Melbourne). Roddick, J C M, 1984, Emplacement and metamorphism of Archean mafic volcanics at Kambalda, Western Australia - geochemical and isotopic constraints, Geochimica Cosmochimica Acta, 48:1305–1318. Travis, G A, Woodall, R and Bartram, G D, 1971. The geology of the Kalgoorlie Goldfield, in Symposium on Archean Rocks, Perth 1970 (Ed: J E Glover), Geological Society of Australia Special Publication, 3:175–190. Turek, A, 1966. Rubidium-strontium isotopic studies in the Kalgoorlie-Norseman area, Western Australia, PhD thesis (unpublished), Australian National University, Canberra. Wong, T, 1986. Metamorphic patterns in the Kambalda area and their significance to Archean greenstone belts of the KambaldaWidgiemooltha area, BSc Honours thesis (unpublished), The University of Western Australia, Perth.
Geology of Australian and Papua New Guinean Mineral Deposits
Crookes, R A and Dunnet, D, 1998. Yilgarn Star gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 255–260 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Yilgarn Star gold deposit 1
by R A Crookes and D Dunnet
2
INTRODUCTION The deposit is 20 km SE of Marvel Loch, WA, within the Yilgarn mineral field, at lat 31o32′S, long 119o41′E in the area of the Southern Cross (SH 50–16) 1:250 000 scale and the Cheritons Find (2834) 1:100 000 scale map sheets (Fig 1).
2.71 Mt at 5.93 g/t gold, Indicated Resources are 1.22 Mt at 7.54 g/t gold and Inferred Resources are 0.86 Mt at 8.66 g/t gold. Mining commenced in November 1991 and to December 1996, 466 911 fine oz of gold had been produced from 3.36 Mt of ore.
EXPLORATION AND MINING HISTORY The earliest significant exploration recorded in the Yilgarn Star tenement area was by Union Miniere Development and Mining Corporation Ltd in joint venture with Laporte Titanium Ltd, who unsuccessfully explored for nickel sulphide mineralisation in ultramafic rocks from 1969 to 1972. CRA Exploration Pty Limited (CRAE) explored for gold along the ultramafic rock–sediment contact zone within the eastern arm of the Parker Dome in 1981–1982. The southern part of the area now covered by the Yilgarn Star mine lease formed a small part of the tenement investigated by CRAE. Extensive soil and laterite cover and low assay results from a small number of rock chip samples discouraged CRAE from undertaking detailed exploration. The Yilgarn Star deposit was a virgin find made about 1.5 km WNW of the old workings at Harris Find. The prospectors who mined gold at Harris Find up to the early 1980s were commissioned by Salokin Nominees Pty Ltd in September 1986 to prospect the surrounding Harris Find exploration licence E77/135. This prospecting work included reconnaissance geological mapping at 1:25 000 scale and systematic rock chip sampling, which yielded a 2.47 ppm gold assay of a quartz sample collected from the surface above the Yilgarn Star deposit.
FIG 1 - Location and regional geological map of the Banker saddle region, Marvel Loch, after Keats (1991).
The mine is operated by Yilgarn Star Pty Ltd, a joint venture between Gasgoyne Gold Mines NL (50%), Orion Resources NL (45%) and Gemini Mining Pty Ltd (5%), with Orion managing the production JV. At December 1996 Proved Reserves are 1.59 Mt at 3.68 g/t gold, Probable Reserves are
1.
Chief Mine Geologist, Yilgarn Star Pty Ltd, PMB 6, Southern Cross WA 6426.
2.
Managing Director, Orion Resources NL, PO Box 169, Applecross WA 6153.
Geology of Australian and Papua New Guinean Mineral Deposits
In April 1987 Salokin and Harris Gold NL (Salokin) set out a grid on 200 by 40 m spacing for 41 line km over part of the Yilgarn Star tenement, and collected 549 soil samples which were analysed for cyanide-soluble gold and for total arsenic, zinc, copper and lead. In September 1987 an additional 48 km of gridding was completed and a further 706 samples were collected and analysed. These programs defined a strong geochemical anomaly in which the +400 ppb cyanide-soluble gold contour, trending about 305o magnetic, covered an area 1200 by 120 m. A maximum gold value of 0.60 ppm was recorded, and the 0.20 ppm gold contour extended over a length of 600 m. This was the surface expression of the Yilgarn Star orebody, but due to the stock market crash in late 1987 the anomaly was not drill tested for almost two years. A joint venture was established between Salokin and Gasgoyne Gold Mines NL in February 1989, whereby Gasgoyne completed gridding on 40 m line spacing and a program of open hole percussion drilling. In total, 143 holes for 5427 m were drilled along the 1240 m length of the anomaly, to 40 m depth. Gold grades of >1 g/t over economic widths were identified in 24 of the 32 sections drilled (D J Porter, unpublished data, 1989).
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In late December 1989 Orion Resources NL entered into a subscription agreement with Gasgoyne to fund reverse circulation percussion (RC) drilling at 40 m and subsequently 20 m centres. The results of the drilling were used to delineate a resource of 1.163 Mt at 5.1 g/t gold (C C Schaus, unpublished data, 1992). Orion acquired a direct 45% share of the Yilgarn Star Project by September 1990 and assumed management control of the production joint venture in January 1991. Additional RC drilling in 1991 outlined Proved and Probable Reserves of 1.536 Mt at 4.17 g/t gold. Mining by open cut methods to 130 m vertical depth was completed in October 1995. Ore production from underground mining commenced in March 1995, with the full production rate of 0.55 Mtpa achieved in mid 1996. The resource potential is estimated to be high as the entire deposit remains open at depth and along strike in both directions. Drilling data are available to 740 m vertical depth, highlighting the strong downdip continuity of gold mineralisation, the mine sequence and the associated alteration system.
REGIONAL GEOLOGY The Marvel Loch–Yilgarn Star area is in the central-southern part of the Archaean Yilgarn Craton and within the Southern Cross Province. The mineralisation and regional geology of the Southern Cross greenstone belt have been well documented (Keats, 1991). The Marvel Loch–Yilgarn Star area falls into three structural domains shown in Fig 1 (R Marston, unpublished data, 1993): 1.
2.
3.
The Northern domain is an area between Marvel Loch and the Yilgarn Star haul road and comprises an arcuate belt of rocks striking around the southern perimeter of the Ghooli Dome granitoid batholith. The Central domain is a latitudinal zone about 5 km wide, north to south, extending from Great Victoria in the west to the Yilgarn Star mine in the east, which contains a prominent east-trending Proterozoic mafic dyke near its northern boundary, close to the Yilgarn Star haul road. The Southern domain is a narrow belt of rocks striking around the northern perimeter of the Parker granitoid dome.
It is likely that the present form of the area is due to progressive ENE to WSW compressional deformation involving polyphase folding and major shear coupling, directed by forces associated with the granite emplacement. The regional stratigraphic sequence is dominated by a range of metasedimentary and metavolcanic rocks, with a complex history of structural deformation, metamorphism and metasomatism. The metasedimentary sequence comprises a package of alternating schistose rocks, originally shale, siltstone, sandstone, wacke and conglomerate, and many have the graded and thinly bedded appearance of turbidites. Semipelitic quartz-mica schist is the most common outcrop. Iron-rich cherty and carbonaceous chemical sediments are typically developed at the base of this sequence, with layered calc-silicate amphibolites locally referred to as ‘banded amphibolites’. These rocks are interlayered with more siliceous metasediment and may represent tuffaceous mafic rocks, metamorphosed marly sediment or hydrothermal alteration zones.
256
A local structural unconformity with pronounced faulting and folding is often apparent between the metasediment and a mafic-ultramafic rock sequence. These metavolcanic rocks comprise a pile of tholeiitic to magnesian mafic and ultramafic rocks of extrusive and intrusive origins containing interflow pelitic metasediments. Several horizons of oxide-type banded iron formation and chert occur in the lower half of the sequence. Dark green homogenous amphibolites are the most common outcrops (Barnes and Schaus, 1993). The major contrast in ductility between the two sequences, coupled with the prevalence of pelitic, carbonaceous and sulphidic rocks at the base of the metasedimentary sequence, has resulted in the focus of folding and faulting at this major rock contact. This in turn has influenced the movement of metamorphic hydrothermal fluids and the formation of mineral deposits (R Marston, unpublished data, 1993).
DEPOSIT GEOLOGY LITHOLOGY The deposit lies along the NW-trending Yilgarn Star shear zone (YSSZ), which is bound to the stratigraphic contact of a thick unit of altered metagreywacke with ultramafic amphibolechlorite rocks. A wedge of skarn-banded gneiss occurs along this contact in the central and southern sectors of the orebody. The distribution of these rock types is shown in Fig 2, which highlights the summary geology as mapped in the open pit at 2320 m RL (mine datum) and an interpretation from drill holes projected in cross section to 500 m depth. The mine sequence is shown in Fig 3. The uppermost part of the exposed sequence, units A to F, is a series of interbedded altered amphibolites and graphitic schists, which are generally unmineralised. Units G to L comprise 120 m of altered metagreywacke overlying and often incorporating unit M. Units M to S are of economic importance and are detailed below. Unit M is a knotted mica schist, which forms the hanging wall to much of the gold mineralisation. It is a fine grained quartz-andalusite-biotite-muscovite-tourmaline schist, interlayered with quartz-muscovite schist and minor quartzplagioclase gneiss. Minor quartz-actinolite lenses are common in the lower parts of this unit. Pyrrhotite stringers 2–3 mm thick are found throughout and constitute between 1 and 3% of the rock volume. Unit N, the contact skarn, comprises massive pyrrhotite in bands averaging 1 m thick, but varying from 0.1 to 8.0 m. It contains bands of garnet-cummingtonite and olivine-calcite skarn and garnet-rich quartz-actinolite schist. Minor constituents are calcite, magnetite and rare arsenopyrite. Unit O is a skarn-banded gneiss comprising massive bluegrey, indurated quartz-actinolite schist, with individual beds to 3 cm thick. Minor fuchsite bands and quartz veins occur throughout. This unit displays crosscutting brittle fractures which have been subsequently transgressed by the anastomosing YSSZ. Interlayered with quartz-actinolite schist are bands of brown to black quartz-biotite schist with minor groundmass diopside. Quartz and diopside veins to 1 m thick are common. Crosscutting faults host intense biotite alteration and quartz veining. Pale green, layer-parallel diopside bands, with abundant pyrrhotite and minor groundmass carbonate are common throughout, and contain gold, nickel, zinc, lead, bismuth and silver mineralisation.
Geology of Australian and Papua New Guinean Mineral Deposits
YILGARN STAR GOLD DEPOSIT
FIG 2 - Yilgarn star geological plan at 2320 m RL, showing simplified geology and ore blocks, and cross section on line 10 900 m N, looking north.
At the base of the skarn-banded gneiss sequence is a 3 to 5 m thick zone of fuchsite alteration and quartz veining in a foliated quartz-diopside and quartz-biotite groundmass. Minor grey to white quartz veins are also present, generally as 5 to 10 cm stringers with minor pyrrhotite. Grey-green quartz-diopside and brown quartz-biotite constitute the groundmass in this subunit and occur as 10 to 30 cm thick laminae. The fuchsite subunits are thought to represent the southward continuation of
Geology of Australian and Papua New Guinean Mineral Deposits
the sericite-rich shears that dominate the shear zones in the northern half of the pit. Unit P is the Yilgarn Star shear zone (YSSZ). It transgresses several units and is characterised by an alteration assemblage of potassium-iron-magnesium-chromium phyllosilicates, calcsilicates, iron oxides and iron-arsenic sulphides. This assemblage overprints the metamorphic rocks at the main contact, and subsidiary alteration zones occur on parallel and
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regional folding event disclosed in premetamorphic structures within the ultramafic rocks, followed by sinistral brittle-ductile transcurrent shearing (D1 as mapped in the pit, but D2 regionally), then dextral ductile shearing occurring during preto post-peak metamorphism (D3), and a later period of brittle fracturing. The dominant structural style in the metasediment is tight, shear related NW-trending isoclinal folding, with a strong, penetrative WSW-dipping axial planar foliation. The foliation contains a steeply north-pitching penetrative mineral stretching lineation. The mean bedding orientation swings between 240o/76o and 275o/76o (all bearings refer to mine grid, where GN = 315o true) and the dominant north-trending schistose foliation has an approximate mean orientation of 267 o/74o. Fig 4 highlights the significant structural form surfaces. The footwall ultramafic rocks are massive to weakly foliated, and where present, the millimetre scale foliation is defined by penetrative layering of tabular chlorite and elongate tremolite. A strong mineral lineation also pitches steeply NW. The YSSZ is an oblique-slip WNW-trending ductile-brittle shear system that is the focal point of extensive metasomatism, veining and mineralisation. The alteration assemblages indicate a long history of progressive deformation and fluid flow, influenced by both D2 and D3 shearing events. Shallowly plunging boudins and the growth of fibrous minerals on fractures that dip gently SE in the YSSZ suggest an extensional regime. The shear zone margins appear to dip west less steeply than the internal foliation, indicating west-block-up movement (with an accompanying sinistral component). Most of the long axes of the boudins are concordant with the steep NW lineation, as are the axes of deformed quartz veins.
FIG 3 - Stratigraphic section for the mine sequence at Yilgarn Star.
horse-tailing structures in the footwall and hanging wall. Deformed hydrothermal veining is an integral component of the YSSZ and its gold lodes (Barnes and Schaus, 1993). Unit Q, the quartz-diopside-carbonate alteration zone, is a narrow band of fine grained, pale green diopside-quartzcarbonate alteration associated with the YSSZ at the top of the ultramafic unit. Unit R typically comprises a tension vein array of calciteolivine-magnetite±quartz veins, with a granoblastic texture and irregular wall rock contacts. Unit S is the predominant host to these veins. Unit S comprises a tremolite-chlorite schist, a weakly foliated amphibole-chlorite metakomatiite with widely spaced magnesian skarn bands. This basal footwall ultramafic sequence is known to extend for 2 km NW of the Yilgarn Star pit and for over 0.5 km to the SE. It has a total apparent thickness of 600 to 800 m and is progressively thinner towards the north. The unit is believed to be a metamorphosed magnesian basalt and contains chlorite and tremolite as the main constituents with minor amounts of calcite, diopside, feldspar, ankerite, phlogopite, magnetite and quartz.
STRUCTURE The complex history of structural deformation is well preserved, with four possible events having occurred: an early
258
Late structural elements (ie post-dating the YSSZ) are all brittle in character and occur predominantly in the ultramafic rocks. Subordinate transverse trending foliation and fractures are present, spatially limited to within about 100 m of the contact zone. Laterally persistent fractures typically oriented at 222o/78o occur immediately below the YSSZ. A significant proportion have subhorizontal slickensides which may be mineral lineations. Faults rarely pass completely through the YSSZ, implying that the YSSZ is the site of mechanical decoupling between the strong footwall rocks and the weaker hanging wall rocks. Within near–hanging wall mineralised and altered metasediments, transverse structures are evident as a schistose foliation and quartz veins, typically oriented at 237o/76o. Sinistral shearing is invoked as being crucial for their development. Fractures and veins are predominantly oriented normal to one of the foliation planes and tend to occur as steep and flat dipping groups within an overall dispersion predominantly oriented normal to the principal north trending foliation. A high proportion of the flat dipping structures include quartz and/or carbonate fill and locally have a 30o SE or NW dip. These fractures and veins are interpreted as extensional features, preferentially developed normal to the locally dominant horizontal stress field. Their density and style similarly reflect the relative brittleness of their host rock.
MINERALISATION Gold is associated with structurally controlled dilatant sites in the veined, ductile-brittle YSSZ. The level of gold mineralisation is closely linked to the intensity of shearing and concentration of veining. The deposit is broadly 1.2 km long
Geology of Australian and Papua New Guinean Mineral Deposits
YILGARN STAR GOLD DEPOSIT
FIG 4 - Structural element form surfaces at Yilgarn Star.
and dips 76o towards 268o parallel to S0 to a vertical depth of at least 740 m. It varies in width from less than 1 m to a series of individual mineralised structures that are combined as mining blocks up to 30 m wide. The orebody is divided into three primary domains along strike: 1.
Northern or Premier lode mineralisation transgresses the YSSZ in quartz-diopside-carbonate altered ultramafic rock and may extend to 10 m below the main shear zone. Intense diopside alteration and carbonate-olivinemagnetite veining correlate with the highest gold grades. The YSSZ and immediate hanging wall sediment comprise variably sheared sericite-andalusite-biotitefuchsite schist containing elongate andalusite porphyroblasts and biotite-rich laminae. Pyrrhotite to 5% occurs as 1 to 3 mm disseminations. The presence of quartz-pyrrhotite veins, generally to 10 cm wide, often with hydrothermal biotite selvages, signifies gold mineralisation, with grades reaching 50 g/t.
2.
The Central lodes are a series of mineralised (axial planar?) shears in muscovite-andalusite-biotite schist, up to 30 m above the main YSSZ, which form the bulk of the lode system. Pelitic beds have preferentially deformed, with intense shearing anastomosing around more competent psammitic beds, creating a focus for potassic alteration and gold mineralising fluids. Quartz-pyrrhotite veins parallel the shear surfaces and can occur in brittle fractures within rafts of indurated quartz-actinolite schist. Quartz-andalusite-biotite schist beds also host gold, confined to small brittle faults from 2 to 40 m long. These faults are characterised by zones of sinuous grey and milky white quartz veins and stringers which are between 5 and 50 cm wide.
Geology of Australian and Papua New Guinean Mineral Deposits
3.
The complex Southern lode system comprises two main lodes along the upper and lower contacts of units N and O (Fig 2). In addition to the mineralisation styles of the Premier lode, gold occurs in carbonate-olivine-magnetite veins, as vein packages paralleling steep 290o to 320o trending faults. This ore type is concentrated within 15 m of the metasediment–ultramafic rock contact and occurs as distinctive black and white spotted, lenticular granular veins, with traces of pyrrhotite. The carbonate content varies from 10 to 70% and is commonly about 30%. Gold content also varies greatly, and south of 10 300 N this rock becomes the dominant ore type with individual veins containing coarse particulate gold of 1–3 mm diameter and grades in excess of 50 g/t. North of 10 300 N gold grades range between 0.5 and 15 g/t, corresponding to increased carbonate content and reduced magnetite. In unit O, the skarn-banded gneiss, gold also occurs in sheared sericite-andalusite-biotite-fuchsite schist and in lenses of randomly distributed biotite-quartz schist.
The prime control of gold mineralisation is the sheared metasediment–ultramafic rock contact zone. All economic resources occur within 50 m of this contact, and all substantial lenses and pods of gold mineralisation appear to be physically connected to this contact.
ACKNOWLEDGEMENTS Permission to publish by Orion Resources NL, Gasgoyne Gold Mines NL, and Gemini Mining Pty Ltd is gratefully acknowledged. The authors wish specially to thank past and present mine geologists for their contributions to the geological development of the project.
259
R A CROOKES and D DUNNET
REFERENCES Barnes, J F H and Schaus, C C, 1993. Exploration and resource implications of going underground, in Open Pit to Underground: Making the Transition (Eds: W J Shaw and S E Ho), pp 1–15, AIG Bulletin 14. Keats, W, 1991. Geology and gold mines of the Bullfinch-Parker Range region, Southern Cross province, Western Australia, Geological Survey of Western Australia Report 28.
260
Geology of Australian and Papua New Guinean Mineral Deposits
Shedden, S H, 1998. Two Boys gold deposit, Higginsville, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 261–264 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Two Boys gold deposit, Higginsville by S H Shedden 1 INTRODUCTION The deposit is within a 19.2 ha mining lease at the Higginsville mining centre, 112 km south of Kalgoorlie and 45 km north of Norseman, WA, at AMG coordinates 379 200 m E, 6 487 300 m N and lat 31o44′S, long 121o43′E on the Widgiemooltha (SH 51–14) 1:250 000 scale and the Cowan (3234) 1:100 000 scale map sheets (Fig 1). Mining lease M15/231 is owned by Gindalbie Gold NL, is operated under a 50:50 production joint venture between Gindalbie Gold NL and Barminco Pty Ltd (the Two Boys Joint Venture) and is 1.5 km SE of the Resolute Ltd gold treatment plant at Higginsville (Fig 2).
FIG 2 - Local geological map and mine site plan, Higginsville (after Resolute Ltd, 1996).
EXPLORATION AND MINING HISTORY Mining at Higginsville commenced about 1900 and has continued sporadically to the present. Major open pit mining commenced in 1988, following the discovery of the Poseidon and Poseidon South gold deposits by Samantha Exploration NL (now Resolute Ltd).
FIG 1 - Location and regional geological map of the Higginsville area (after Resolute Ltd, 1996).
Mine development by the Two Boys Joint Venture commenced on 18 January 1997. Concurrently, underground operations were being developed at the neighbouring Poseidon South and Chalice gold mines, both operated by Resolute Ltd. These events marked a new era of undergound mining in the Higginsville district. 1.
Managing Director, Gindalbie Gold NL, PO Box 10400, Kalgoorlie WA 6430.
Geology of Australian and Papua New Guinean Mineral Deposits
Two Boys was discovered by prospectors in 1933, and produced a reported 4714 oz of gold between 1933 and 1964 from the oxidised portion of a shallow dipping reef structure. The average grade of mined ore during that period has been calculated at 19.77 g/t gold (Gindalbie Gold NL, 1994). In 1966 the Two Boys mining lease was purchased by prospector W T Trythall who worked the deposit via a vertical shaft to about 50 m depth and explored it by several diamond drill holes during the following years. In 1983 Samantha Exploration NL consolidated ownership of most of the Higginsville mining centre, including the Two Boys mining lease, and commenced systematic exploration. Two Boys was purchased from Samantha and Trythall by Gindalbie Gold NL (GBG) in 1994. To that time, three successive drilling programs had resulted in an Inferred and Indicated Resource estimate of 109 000 t averaging 4.99 g/t
261
S H SHEDDEN
gold (uncut), based on 100 reverse circulation (RC), diamond and open-hole percussion drill holes totalling 5047 m. The occasional high grade intercept did not encourage an early resumption of mining. Drilling around the old workings was difficult because of mine openings and the presence of clay.
by dolerite and gabbro. The greenstones are bounded by contemporaneous basaltic volcanic rocks and younger Archaean clastic sediment of the Black Flag beds, and the sequence has been subjected to upper greenschist to lower amphibolite facies regional metamorphism (Griffin, 1989).
During 1994 GBG drilled a further 57 RC drill holes, testing the mineralised zone to a maximum vertical depth of about 100 m. The Inferred and Identified Resource was then estimated to be 330 000 t averaging 4.38 g/t gold (uncut) or 3.14 g/t gold after a top cut to 16 g/t gold and a 1 g/t lower cutoff. This was generally based on a nominal drill intercept spacing of 20 by 25 m. Drilling to the end of 1994 totalled over 10 000 m in 157 holes (S H Shedden, unpublished data, 1995).
The Higginsville belt is bounded to the east by the NNWtrending Zuleika Shear Zone, which may be traced for more than 100 km. The greenstones have been subjected to at least three phases of deformation. The earliest phase (D1) is defined by north–south regional shortening which generated south over north low angle thrusting. The second phase (D2) consists of a ENE-trending regional shortening event which resulted in open upright folding about a NNW-trending fold axis and low to high angle reverse thrusts along fold limbs. Second order splay faults were also generated as low angle thrusts. The third phase (D3) consists of ESE–WNW regional shortening which generated sinistral strike-slip shear zones along NNW-oriented thrusts and along the fold limbs formed during D2. Second and third order splay faults off major NNW-oriented structures are common and generally occur as low angle thrusts during D3 (D Goodwin, personal communication, 1997).
As the bulk of the resource was only accessible by underground mining, attention then focussed on a high grade zone in the eastern sector of M 15/231. Drilling had initially suggested that this zone was limited to around 15 000 t at an average grade of about 20 g/t gold. To facilitate planned mining of the high grade zone, GBG entered into a production joint venture with underground mining contractor Barminco Pty Ltd. Barminco had the right to conduct check drilling, which commenced in November 1995. The first nine holes formed a pattern around the high grade zone at a spacing of 12.5 by 10 m. These holes confirmed the presence of the high grade zone and drilling continued, testing the mineralised body to about 187 m vertical depth, beyond which it remains untested. By June 1996, 64 RC drill holes totalling 7572 m had been completed in this program. An Inferred and Indicated Resource for the high grade zone was then estimated to be 220 000 t at an average uncut grade of 19.9 g/t gold (for 141 000 contained oz) or 15.6 g/t gold after a top cut to 80 g/t gold, within a total Inferred and Indicated Resource of 570 000 t at an average uncut grade of 9.2 g/t gold, equal to 169 000 contained oz (R G Colville, unpublished data, 1997). Mine development commenced in January 1997 based on a Proved and Probable Ore Reserve estimated to be 230 000 t at 12.29 g/t gold after a top cut of 80.0 g/t gold and a 4 g/t lower cutoff (R G Colville, unpublished data, 1997). The expected recovery of 95% should enable 86 000 oz to be produced. As the main mineralised body and subsidiary mineralised zones have yet to be fully explored and diamond drilling has intersected a favourable structure at depth below the mineralised shear, significant extensions to known mineral resources are anticipated. Drilling and development by Barminco have resulted in the third and perhaps ultimate Two Boys mining operation. Mine access will be by a 5.5 by 5.5 m decline with a gradient of 1 in 7. The deposit is planned to be mined by a combination of mechanical long hole overhand retreat and conventional airleg stoping commencing in about June 1997. The ore will be treated under a custom milling arrangement at Coolgardie, 110 km from the mine site.
REGIONAL GEOLOGY Two Boys is within the Archaean Yilgarn Block and the Kalgoorlie Terrane of the Norseman–Wiluna greenstone belt (Fig 1). The Higginsville area is underlain by a fault bounded, thrust repeated, NNW-trending 5 km wide sector of the greenstone belt. The Archaean greenstones dominantly comprise metamorphosed high magnesium basalt, minor komatiite flows and minor interflow clastic sediment, intruded
262
ORE DEPOSIT FEATURES REGOLITH Two Boys is covered by a lateritic weathering profile to about 50 m thick. The upper 2 to 3 m consists of intensely ferruginised and calcareous clays which overlie a mottled zone to 40 m thick. A narrow, poorly developed pallid zone 2 to 3 m thick is commonly observed in most drill holes, and overlies a weathered bed rock zone of variable thickness (S H Shedden, unpublished data, 1995).
LITHOLOGY The Two Boys lease (Fig 3) is underlain by a sequence of high magnesium basalt, gabbro and minor sediment. The sequence includes a quartz gabbro unit, the Fairplay gabbro, which cuts the SW corner of the tenement (Figs 2 and 3). A NW strike and subvertical dip have been interpreted from drill intercepts of a narrow metasedimentary unit. High magnesium basalt is the dominant rock type. It varies texturally from a fine grained, variolitic rock to a doleritic granophyre, with the strongest gold mineralisation developed within the granophyric phases.
STRUCTURE The sequence is cut by a low angle shear zone formed during D2 as a low angle thrust, known as the Two Boys shear zone (TBSZ). The TBSZ strikes east and has an overall dip of about 27 to 30o to the NNE. The TBSZ hosts the Two Boys gold deposit, which occurs as lenses of quartz-vein reef of variable width and extent with sheared, altered and mineralised selvages (Fig 4). The TBSZ crops out within the southern boundary of the mining lease and is marked by a prominent quartz reef to 4 m thick (Fig 3). A second significant reef structure 100 m north of the TBSZ outcrop strikes NNW with a shallow easterly dip. Drilling of this structure to-date has intersected alteration and subeconomic gold mineralisation.
Geology of Australian and Papua New Guinean Mineral Deposits
TWO BOYS GOLD DEPOSIT, HIGGINSVILLE
is incomplete. The TBSZ extends beyond the limits of the Two Boys mining lease and is being tested elsewhere by drilling (Resolute Ltd, 1996, 1997). Adjacent to the TBSZ foliation of the host high-magnesium basalt is intense over several metres into the wall rock, and is associated with intense biotite-chlorite-carbonate-sericitepyrite alteration. The alteration is the key identifier of mineralisation in drill holes. Mineralisation occurs in quartz veins and sheared selvages and is often visible as coarse free gold grains to several millimetres in diameter. Carbonate, arsenopyrite and pyrite are essential accessories to gold. Carbonate is widely and irregularly distributed as coarse grained intergrowths with quartz and as disseminated fine grained masses throughout the altered zone. Arsenopyrite occurs as euhedral crystals to several millimetres wide. Fine grained euhedral pyrite has a much wider distribution than arsenopyrite, occurring throughout the alteration zone. Minor scheelite, as discrete crystals within quartz veins, is also frequently associated with the mineralisation.
FIG 3 - Surface plan and projection of mineralisation, Two Boys gold mine.
Subsidiary zones of gold mineralised quartz-carbonate veining have been intersected above and below the Two Boys reef. Little is known of the lower zones due to the lack of drill penetration past the main shear. The upper subsidiary mineralised zones do not display the same continuity, thickness or grade as that within the TBSZ ore zone, but economic mineralisation is evident and remains to be fully tested.
ORE CONTROLS The primary gold mineralisation at Two Boys was emplaced in dilatant lenses within the TBSZ which has been demonstrated to persist for several hundred metres down dip to the north. The TBSZ is interpreted as a D2 low angle thrust. During D3, the structure was reactivated, forming dilational sites and allowing subsequent gold deposition post-D3. The development of gold mineralisation within the TBSZ is enhanced in upward flexures and appears also to favour doleritic phases of the host high-magnesium basalt. Gold mineralisation is notably weak where the TBSZ extends through the Fairplay gabbro to the west.
FIG 4 - Cross section between holes BTB004 and BTB030, looking NW.
MINERALISATION Modelling of the surface of the gold-bearing quartz vein within the TBSZ has indicated that vein thickness and gold grade increase consistently with upward flexures in the quartz vein. Whereas the TBSZ and the associated quartz veining, wall rock alteration and gold mineralisation are laterally persistent throughout the lease area, higher grade gold mineralisation is confined to two main areas in the east and west of the lease. The full extent of higher grade mineralisation is unknown as drilling
Geology of Australian and Papua New Guinean Mineral Deposits
Primary gold mineralisation at Higginsville, although persistently associated with intense wall rock alteration and ubiquitous arsenopyrite-pyrite-scheelite, occurs in a range of rock types and structures. Hosts include quartz gabbro at Poseidon South, Poseidon and Fairplay and coarse grained tremolitic ultramafic rock at Erin (Fig 2). The epigenetic deposition of gold mineralisation at Higginsville was therefore largely structurally controlled, with the mineralising fluids derived from a common source. Mineralising fluids were channelled along second or third order structures related to the regional scale Zuleika Shear Zone, with higher grade ore formed in low pressure dilatant zones within granophyric host rocks where compression resulted in brittle failure. The spatial and timing relationships between the brittle failure zones and the enclosing ductile-deformed shear zone have not yet been studied. Although earlier mining exploited oxidised ore zones at Two Boys, there is little evidence in drill holes of any significant secondary enrichment of gold relative to the grade of the primary zone.
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ACKNOWLEDGEMENTS This paper is published with the permission of Gindalbie Gold NL. The Two Boys story is one of persistent, if sporadic, exploration and mining over 64 years. The effort and commitment of the management of Barminco Pty Ltd, in particular R G Colville, W T Trythall’s long term faith in the ability of Two Boys to yield a mineable gold deposit, and the ready assistance of Resolute Ltd during the exploration and development phase are acknowledged.
264
REFERENCES Gindalbie Gold NL, 1994. Prospectus (Gindalbie Gold NL: Perth) Griffin, T J, 1989. Widgiemooltha, Western Australia - 1:250 000 geological series (2nd edition), Geological Survey of Western Australia, Record 1989/4. Resolute Ltd, 1996, 1997. Quarterly reports to the Australian Stock Exchange.
Geology of Australian and Papua New Guinean Mineral Deposits
Archer, N R and Turner, B J, 1998. Norseman gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 265–272 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Norseman gold deposits 1
by N R Archer and B J Turner
2
INTRODUCTION
Harlequin, Bullen and OK underground mines (Fig 1), the Scotia operation having been temporarily suspended in 1995.
The deposits are on the Norseman (SI 51–20) 1:250 000 scale and Norseman (3233) 1:100 000 scale map sheets at lat 31o12′S, long 121o47′E, AMG coordinates 385 000 E and 6 436 000 N, at the southern end of the Eastern Goldfields Province of Western Australia (Fig 1). Total production from the field to June 1996 is nearly 5 Moz of gold, and Central Norseman Gold Corporation (CNGC) has produced more than 80% of this since it commenced production in 1935. CNGC production in 1995–96 was 119 603 oz from 306 440 t of ore at an average recovered grade of 12.1 g/t gold, predominantly from the
Published resources for CNGC at June 1996 are shown in Table 1.
EXPLORATION AND MINING HISTORY GENERAL The Norseman Goldfield was discovered in 1894 and most mining, and 85% of gold production, has taken place on the Mararoa and Crown reefs in the main field, and at the North Royal and Princess Royal reefs, about 10 km to the north (Fig 1). Although some regional exploration was carried out before 1983, no significant mineralisation was discovered outside the main field or Royal areas. Early exploration at the major reefs had been successful by using the concepts of a ‘favourable’ stratigraphic sequence and down plunge repetitions (Thomas, Johnson and MacGeehan, 1990). This ‘favourable bed’ hypothesis continued to heavily influence exploration thinking by CNGC and, with the inaccessibility of the surrounding salt lake environment, tended to restrict most exploration to the lower portion of the Woolyeenyer Formation. Exploration was heavily biased towards looking for north striking and easterly dipping, high grade quartz veins.
BULLEN MINE
FIG 1 - Location and general geological map, Norseman area.
1.
2.
Formerly Manager, Geology and Exploration, Central Norseman Gold Corporation, PO Box 56, Norseman WA 6443. Now Consulting Geologist, Longbow Geological Services, 33 Highbridge Way, Karringup WA 6018. Mine Geologist, Central Norseman Gold Corporation, PO Box 56, Norseman WA 6443.
Geology of Australian and Papua New Guinean Mineral Deposits
The long history of the field and the high grade, nuggety nature of the orebodies resulted in much exploration being carried out by underground driving along the known reefs, with only a few drill holes. This means that even now in the centre of the main field the drilling coverage is sparse. Easterly striking veins in the main field were known from the earliest days but their importance as ore sources has only recently been recognised. These ‘cross links’ quite commonly pinch out before they intersect the main north striking reefs and would not have been found by driving on the main reefs. The Bullen deposit was found in 1990 and the Viking ‘re-discovered’ in 1986 by correlating ‘spurious’ intersections in diamond drill holes drilled to test targets on the main reefs. Mining has now taken place on these cross links at the St Patrick’s mine, the Alimak stope in the Regent mine, the Viking mine (Royal Standard reef) and at the Bullen mine (Bluebird link). High gold grades make these very profitable operations and attractive exploration targets. The lack of effective drilling in the main field means that many such opportunities remain.
OK MINE About 2 km to the south of the main field, in the OK and Cumberland areas, the reefs predominantly strike east and are subvertical. The OK mine was originally worked in the 1930s, but lay idle until 1980 when the shaft was re-opened by CNGC
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N R ARCHER and B J TURNER
TABLE 1 CNGC June 1995 and June 1996 Ore Reserves and Mineral Resources. June 1995 Ore (‘000 t)
Gold grade (g/t)
June 1996 Contained gold (‘000 oz)
Ore (‘000 t)
Gold grade Contained gold (g/t) (‘000 oz)
RESERVES UNDERGROUND
Proved Probable
SUBTOTAL OPENCUT
Proved Probable
SUBTOTAL TOTAL PROVED AND PROBABLE
220 610
16.9 14.7
119 288
303 547
15.4 12.4
150 220
830
15.3
409
852
13.5
370
10 70
5.0 7.6
2 17
0 106
0.0 14.4
0 49
80
7.3
19
106
14.4
49
910
14.6
428
958
13.6
419
830 1430 1600
7.1 6.0 6.9
190 277 355
1040 1410 1540
6.4 5.9 6.6
213 266 326
3860
6.6
822
3990
6.4
805
3910 1160 670
0.7 3.5 1.5
93 132 32
3910 1110 440
0.7 2.8 3.2
93 99 45
5740
1.4
257
5460
1.3
237
9600
3.5
1079
9450
3.5
1042
10 510
4.5
1507
10 408
4.4
1461
RESOURCES UNDERGROUND
Measured Indicated Inferred
SUBTOTAL OPEN CUT
Measured Indicated Inferred
SUBTOTAL TOTAL RESOURCES GRAND TOTAL
to mine remnant ore from the OK Main reef. Underground drilling of the east striking tensional Main reef led to the discovery of the 300o striking O2 reef.
SCOTIA MINE Rotary air blast drilling beneath Tertiary cover, approximately 30 km south of the main field, on the southern margin of a WNW-striking magnetic zone identified by E S T O’Driscoll, resulted in a strong arsenic anomaly (M W Nevill, unpublished data, 1983). Further work, including drilling under old workings, led to the discovery of the Scotia deposits. This complex vein system occurs within the lower Woolyeenyer Formation to the south of the NE-striking, Proterozoic Dambo fault.
Follow up aircore and diamond drilling defined a reserve by the end of September 1993 and underground production commenced in June 1995. Sailfish, Harlequin, and another low grade resource at Cobbler were originally defined as high priority targets by CNGC geologists.
PRODUCTION AND MINING Recent production data from CNGC operations is shown in Table 2 and the contribution of each of the major orebodies to CNGC’s production history is shown in Fig 2.
HARLEQUIN MINE The projection of known mineralised trends has targeted potential for gold orebodies in salt lake covered greenstone to the west and NW of Norseman. In addition gold-bearing quartz veins, similar in style to those at Norseman, occur at Higginsville, 30 km NNW of the North Royal mine across Lake Cowan. In 1990, a dedicated Hagglund mounted drill rig and air compressor commenced aircore testing on the salt lake beneath Tertiary sediment and Recent mud. This was the first successful drill rig of its type used on salt lakes in Australia. Holes were drilled on regional traverses at a spacing of 2 km by 400 m, and at closer spacings, to test targets defined by interpretation of magnetic data to confirm CNGC’s regional geological understanding. Within six months of drilling the first aircore hole a resource had been identified at Sailfish. In December 1992, the first aircore hole on a ground magnetic target at Harlequin intersected 10 m grading 8.1 g/t gold.
266
FIG 2 - CNGC production record, 1937–1996.
Prior to the opening of the Harlequin mine in 1995 the Bullen mine (Bluebird link) was the most important ore source for CNGC (Fig 2) during some of the more difficult years in the
Geology of Australian and Papua New Guinean Mineral Deposits
NORSEMAN GOLD DEPOSITS
TABLE 2 OK, Scotia, Bullen and Harlequin production from 1984 to 1996. OK MINE
SCOTIA MINE
Gold Contained gold grade (oz) (g/t)
BULLEN MINE
Tonnes (‘000)
Gold grade (g/t)
Contained gold (oz)
HARLEQUIN MINE
Tonnes (‘000)
Gold grade (g/t)
Contained gold (oz)
Tonnes (‘000)
Gold Contained gold grade (oz) (g/t)
Year
Tonnes (‘000)
1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996
7203 8060 4973 13 139 20 478 51 758 47 801 54 665 43 231 60 673 43 305 44 189 31 443
11.9 11.5 14.4 12.9 11.4 8.7 8.4 8.2 8.5 8.8 8.3 12.8 8.1
2758 2974 2301 5467 7524 14 512 12 958 14 413 11 713 17 128 11 571 17 788 8179
1477 76 912 164 397 238 298 89 294 56 090 49 785 48 669 67 211 18 656
5.1 9.4 5.6 4.6 5.7 8.2 6.1 6.1 6.8 3.4
244 23 185 29 571 35 511 16 344 14 861 9717 9514 14 041 2052
14 146 52 693 72 157 54 414
21.2 20.6 19.4 17.6
9 638 34 869 44 316 30 706
7346 178 082
19.5 12.6
4538 72 034
Totals
430 918
9.3
129 286
810 789
5.9
155 040
193 410
19.2
119 529
185 428
12.8
76 572
history of the operation. The deposit has a 40o dip and is mined by airleg, room and pillar methods. At the OK mine the very narrow quartz widths (average 0.3 m), highly variable grades and slow mining rate mean that mining is only marginally profitable. Longhole mining methods are used. A small underground resource remains at Scotia. Two open cut mines operated in the late 1980s (Fig 2) and an underground mine was accessed via a decline from the base of Pit 3 from 1989 and exploited a complex vein system (Fig 3).
The Harlequin orebody is wider than the average Norseman vein and most stopes are mined by long hole methods. The main ore shoot on the HV1 vein is commonly between 150 and 200 m long. Structural complexity, wide ore zones (commonly more than 4.5 m) and multiple reefs in some areas increase the amount of gold which can be recovered per vertical metre. Other recent gold producers include Australis NL, which produced from low grade (1 to 3 g/t) deposits in the Noganyer Formation during the 1980s, and more recently Australasian Gold Mines at the Red, White and Blue deposit, also in the Noganyer Formation. A number of small prospector-scale mines have also been worked.
PREVIOUS DESCRIPTIONS The Norseman mines have been the subject of many geological studies over their 100 year history. Thomas, Johnson and MacGeehan (1990) highlighted the important prior work. Since then significant advances in our understanding have been made. Perring and McNaughton (1990) used lead isotope studies to show that significant remobilisation of oreassociated metals (and possibly gold) occurred during the Proterozoic. Age dating constraints were imposed as a result of work by Hill, Campbell and Compston (1992), Kent (1994) and McCuaig (1996). The concept of a crustal continuum of deposit types from Scotia in the south to the North Royal in the north was proposed by McCuaig et al (1993).
REGIONAL GEOLOGY Thomas, Johnson and MacGeehan (1990) gave a good description of the regional geology. Doepel (1973) completed mapping of the Norseman 1:250 000 scale sheet (SI 51–2) while more recently McGoldrick (1993) completed mapping of the Norseman 1:100 000 scale sheet (3233). Significant new data have become available through aircore drilling by CNGC in areas covered by lakes and Tertiary and Recent sediment.
FIG 3 - Schematic cross section on 6 406 650 m N, Scotia orebodies (‘A’ to ‘F’ lodes), looking north.
Geology of Australian and Papua New Guinean Mineral Deposits
Studies of regional scale, publicly available, aeromagnetic data together with closer spaced company surveys, have advanced the regional understanding. S G Peters (unpublished data, 1991) and others, including D W Haynes (unpublished data, 1991), and L A Offe, S G Peters, J S Chapman and N W
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Brand (unpublished data, 1991) have recognised that the folding of the Noganyer Formation described by Keele (1984) is actually the core of a regional antiform, and that rocks previously assigned to the Mount Kirk, Buldania and Killaloe formations (Fig 4) are probably able to be correlated. A number of CNGC geologists and P McGoldrick (personal communication, 1993) have proposed thrust repetitions of stratigraphy, but these ideas are yet to be tested. Another significant advance in the regional geological understanding has been the recognition that the isolated outcrop of ultramafic rock on a small island near Jimberlana Station is part of a regionally extensive, probably extrusive unit, which has been informally named the Talbot Island
ultramafic (L A Offe, S G Peters, J S Chapman and N W Brand, unpublished data, 1991). The unit can be traced on images of magnetic data as a magnetic high, and it forms a marker horizon of the antiformal structure. The Talbot Island ultramafic is known to contain disseminated nickel sulphide mineralisation (predominantly millerite), as first noted during regional mapping by P McGoldrick (personal communication, 1990).
ORE DEPOSIT FEATURES The most common gold mineralisation style consists of quartz veins hosted within metamorphosed Archaean mafic rocks. The Mararoa and Crown reefs are near north striking and easterly dipping quartz veins which occur over 3.5 km strike length. Higher grade ore shoots occur where the veins intersect coarser grained, mafic (E Cameron, unpublished data, 1968) or ultramafic dykes (J S Chapman and L A Offe, unpublished data, 1993) which have intruded relatively fine grained, metamorphosed (upper greenschist to lower amphibolite facies) basalt. The Royal orebodies are also predominantly north striking, but are structurally more complex quartz veins and shears which occur over a similar strike extent to the veins in the main field. Higher grade sections are hosted within coarser grained mafic intrusive rocks (‘gabbros’) as in the main field.
BULLEN MINE The mine accesses the Bluebird link orebody which is a near 070o striking and 40o SE dipping, predominantly tensional, laminated quartz vein within the favourable sequence. The vein is hosted within a similarly oriented medium to coarse grained gabbro, and is bounded to the east by the Mararoa reef and is either bounded, or offset, at its western end by the Bluebird shear (Fig 5). The vein does not outcrop and feathers out near 100 m vertical depth. In the upper two levels the orebody has a strike length of nearly 200 m but by the 19 level (405 m below surface) this has increased to nearly 400 m, reflecting the divergence of the bounding structures (Fig 6). Despite the greater strike length the ounces per vertical metre have remained constant with depth. Although the geometry of the ore body is relatively simple, significant variations in dip occur.
FIG 4 - Norseman ‘stratigraphy’ (thrust repetitions are likely to occur), with positions of mineralisation. Modified after Doepel (1973).
268
FIG 5 - Schematic relationship between Bluebird link and bounding shears, Bullen mine (not to scale).
Geology of Australian and Papua New Guinean Mineral Deposits
NORSEMAN GOLD DEPOSITS
The quartz lenses at Scotia are small tabular bodies with strike lengths of less than 100 m and down dip extents of less than 40 m, plunging to the north at approximately 20o. The quartz averages 1.1 m wide and is massive and dark coloured, a feature which has been attributed to strain (J Skeet, unpublished data, 1988; R S Waugh, unpublished data, 1991). The grade distribution is atypical of Norseman orebodies. The quartz has a lower grade than other Norseman deposits but a much lower coefficient of variation. The gold is free milling, but is very fine grained and has a much lower silver content than in the main field (J Skeet, unpublished data, 1988; S B Luitjens, unpublished data, 1991). Free gold has only very rarely been observed. The predominantly biotite-hornblende and plagioclase (McCuaig et al, 1993) alteration halo, which is generally 0.7 m wide, carries grades of 1 to 3 g/t gold. Other alteration minerals include epidote, ilmenite, actinolite, clinopyroxene (diopside), calcite, microcline, zoisite and garnet. Common accessory minerals in the ore veins are carbonate, scheelite, pyrite, pyrrhotite and chalcopyrite, with trace amounts of galena and arsenopyrite. FIG 6 - Plan view of Bullen mine showing high, medium and low grade zones.
The vein averages 0.7 m in width and has very little associated shearing, reflecting its extensional character. Like other Norseman orebodies the gold is free milling and commonly visible and usually concentrated within laminations in the vein. Minor biotite alteration occurs intermittently around the vein, extending for no more than a metre into the country rock. Common accessory vein minerals include carbonate, scheelite, pyrite, pyrrhotite, galena and sphalerite.
OK MINE The OK mine exploits the O2 reef, a shear hosted vein near the top of the favourable sequence. The reef pinches out above 2 level at about 55 m below surface, but continues to the 21 level, 460 m below surface. The Main reef is barren below the 5 level, 150 m below the surface. The gold in the O2 reef is free milling and hosted by a very narrow (0.3 m average width) laminated quartz vein which is commonly surrounded by a selvage to 2 m wide of predominantly biotite alteration. The veins are most commonly hosted by fine grained metamorphosed basalt or relatively fine grained intrusive rocks. Accessory minerals include carbonate, scheelite, pyrite, chalcopyrite and arsenopyrite. The O2 and Main reefs are among the most nuggety at Norseman and definition of ore blocks is extremely difficult, so that the assigning of grades from drill holes to sections of reef is almost impossible. Dilution during mining tends to lessen the effects of this variation due to the incorporation of low grade material from the alteration halo.
Two major brittle fault sets occur in this area. These are the FN series which strike 010 o, dipping 50o to the west, with reverse movements varying from 1 to 40 m. These commonly host pegmatite intrusions, which have been dated at 2621±98 Myr (Kent, 1994). The EW series are, as the name implies, an east-striking subvertical fault set with dextral movement to 200 m, with the largest of these intruded by a Proterozoic dyke.
HARLEQUIN MINE The most important ore bearing quartz vein so far known at Harlequin is HV1, a 070o striking and 50o SE dipping and relatively wide (average 4 m width, maximum 11 m) quartz vein. A generalised cross section through this orebody is shown in Fig 7. The vein orientation is similar to that of the cross links (including the Bluebird link) in the main field but
SCOTIA MINE Gold mineralisation at Scotia is significantly different to that in the main Norseman field. The ore has a similar stratigraphic setting in coarse grained mafic intrusive rocks of the lower Woolyeenyer Formation, but has been subject to midamphibolite facies metamorphism (McCuaig et al, 1993). The Scotia orebodies are hosted by a weak to moderate, 1 to 20 m wide north striking, easterly dipping (average 50 o) shear zone, which in the mine area is hosted by a subparallel coarse grained mafic intrusive unit (Fig 3).
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 7 - Cross section on 385 125 m E at Harlequin HV1 prospect, looking west.
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N R ARCHER and B J TURNER
HV1 is more structurally complex and has more associated shearing. Intense biotite and arsenopyrite alteration persists for several metres around the orebody, and commonly contains high gold values. As is common elsewhere at Norseman the ore grade portions of the HV1 vein are hosted by coarser grained mafic intrusions. Structural investigations suggest that the geometry of the Harlequin vein sets may not be the same as in the main field. However, a north striking and east dipping vein, HV6, has been located to the west of HV1, in the Harlequin West area, and an Indicated Resource and Probable Reserve have been defined at depth. Quartz veins at HV6 are hosted by zones of strong shearing which are commonly 10 m wide and can be as much as 30 m wide. The host rock for HV6, even in the ore grade zones, is basalt. The ore shoot lies close and subparallel to the intersection of HV6 and a northerly striking body of microgranodiorite. The first ore-hosting vein located at Harlequin has a similar orientation to the veins which have been mined last in the main field. The Harlequin orebody occurs in the Desirable pillow lavas, a part of the stratigraphic succession long thought to be unfavourable for gold mineralisation. This highlights the fact that although the ‘favourable bed’ hypothesis has been useful as a guide to ore, the position in the stratigraphy should be viewed as less important than structurally prepared zones.
ORE GENESIS, MODELS AND CONTROLS ON MINERALISATION BULLEN MINE D N Kelly (unpublished data, 1992) suggested that this vein is emplaced in a tensional opening formed by movement on the Mararoa reef and Bluebird shear. The location of this opening has been influenced by the presence of a favourably oriented, relatively coarse-grained mafic intrusive. The relationship of the Bluebird link to the Mararoa reef and Bluebird shear is shown in Fig 5.
OK MINE The steep, thin, near east striking veins at the OK are hosted within fine grained basalt. The block of country around the OK and Cumberland mines, and south of a WNW-striking fault known as E-fault, predominantly hosts east oriented reefs. Efault terminates the north striking reefs in the main field to the north.
COMMON FEATURES All the Norseman reefs share common features which give clues to their genesis: 1.
2.
270
Most of the high grade ore zones occur where veins intersect ‘gabbro’ intrusions, and specific oriented contacts are particularly favourable. This is most likely a result of competency contrasts which allow preferential propagation of cracks and other openings within the coarser grained rocks, and the amount of veining is controlled by the orientation of the contact relative to stress directions. Zones where NNE- and west-dipping felsic, dacitic porphyries are intersected by the reefs tend to be zones of intense structural complexity and gold grades are even
more variable than usual. In some reefs these can be zones of high grades, and in others, low grades. This reflects the geometry relative to the local direction of maximum compression, and therefore whether the structures are tight or open. 3.
Most reefs have only very narrow (a few metres at most) alteration selvages. In some cases these selvages host high gold grades but in all cases the grade drops off very quickly away from the quartz vein. The northern deposits usually have wider alteration haloes caused by more reaction of ore fluids with host rocks (McCuaig et al, 1993).
The orebodies are almost completely structurally controlled. Ore bearing fluids with fluid inclusion compositions which suggest a mixed source (R T Bills, unpublished data, 1990) deposited gold and other minerals when sudden pressure release occurred, either due to seismic events (Sibson, 1990) or when fluids gained access to pre-existing openings. Minor wall rock reaction also took place and the alteration assemblage (including the abundance of gold and other sulphides) was determined by the wall rock composition (McCuaig, 1996). The gold is not uniformly distributed and the gold in the orebodies is normally nuggety. The erratic distribution of gold on the Bluebird link, for example, is shown in Fig 6. The orebodies are now believed to have been emplaced close to the time of peak metamorphism as described by McCuaig (1996). The V0 and V2 veins described by Thomas, Johnson and MacGeehan (1990) are now believed to be of the same age, resulting from the same stress field and are tensional and shear veins respectively. The strike of the main regional compressive stress direction is thought to have been between NNE and ENE (N R Archer, unpublished data, 1991) but locally blocks of country have vein sets which give evidence of different orientations. East–west compression is suggested at the OK mine (P Bird, unpublished data, 1990). Complex reef shapes which include buckling, faulting, ramping and multiple crosscutting vein sets are perhaps due to rotation of blocks of country during the vein-forming deformation event. Major reorientations of stress directions may have resulted from failure of regional scale faults which may be existing, or new, blockbounding structures. Proterozoic orogenic activity associated with the Albany–Fraser Orogen has caused later faulting and redistribution of ore-associated minerals and possibly gold (Perring and McNaughton, 1990; Kent, 1994; McCuaig, 1996). Archaean shear zones have provided preferential pathways for movement of Proterozoic fluids associated with the Jimberlana Dyke suite. Diopside- and microcline-bearing assemblages have been noted in shears near the Jimberlana Dyke (N R Archer, unpublished data, 1990) and at Scotia near a Proterozoic intrusive (B J Turner, personal communication, 1996). Local gold enrichment, not due to weathering, is also known near the edge of the Jimberlana Dyke and is further evidence of remobilisation of gold during the Proterozoic.
MINE GEOLOGICAL METHODS Traditional mine geological methods, such as back mapping, plan and longitudinal projections, classical and other varieties of polygonal ore reserve estimation methods are used. Inherent problems of grade prediction persist and driving along the reefs remains the only way to assign reliable grades to ore blocks.
Geology of Australian and Papua New Guinean Mineral Deposits
NORSEMAN GOLD DEPOSITS
Plans are in place to apply computer-based three dimensional modelling techniques, with the greatest benefit expected to be in communicating the complex geometry of reefs, such as in the Harlequin West area.
and mine geologists have added to the understanding of Norseman geology. More recently K Johnson, D N Kelly, S G Peters and L A Offe have made significant contributions. C Stephens is thanked for his technical review of the paper.
CONCLUSIONS
REFERENCES
Each of the more recently exploited Norseman orebodies is different to some extent from the veins which have been the big producers in the past. At the Bullen mine cross links are 070o striking and SE dipping tensional veins which are part of the same shear-vein system as the north striking producers, but their importance was not recognised in the past.
Doepel, J J G, 1973. Norseman, Western Australia - 1:250 000 geological series, Geological Survey of Western Australia Explanatory Notes SI 51–2.
At the OK mine the major producing vein is the 300o striking, subvertical, shear hosted O2 reef, in contrast to the east striking subvertical tensional reefs (Main reef) which have been historical producers. At Scotia, south of the Proterozoic Dambo fault, the structurally complex orebodies contain a number of small lensoidal ore shoots affected by localised Proterozoic alteration. Mining at Harlequin has so far been on the 070° striking and SE dipping HV1 vein, but continuing exploration has defined mineralised veins with other orientations. Although the Harlequin area has many similarities in geometry to the main field, the differences include more shearing associated with veins in the HV1 orientation. The Scotia, Bullen, OK and Harlequin mines have so far produced only about 500 000 oz, or 12%, of the gold mined from the Norseman Goldfield. However each is contributing to a new phase in the history of the field. Not only are different styles of mineralisation proving to be important but a much larger area of fertile country is now known to exist and many other promising prospects are being tested. Technological advances have made it possible to explore even the most hostile of environments.
ACKNOWLEDGEMENTS Central Norseman Gold Corporation and WMC Resources are thanked for permission to publish the paper. Many exploration
Geology of Australian and Papua New Guinean Mineral Deposits
Hill, R I, Campbell, I H and Compston, W, 1989. Age and origin of granitic rocks in the Kalgoorlie-Norseman region of Western Australia: Implications for the origin of Archaean crust, Geochimica et Cosmochimica Acta, 53:1259–1275. Keele, R A, 1984. Emplacement and deformation of Archaean goldbearing quartz veins, Norseman, Western Australia, PhD thesis (unpublished), University of Leeds. Kent, A J R, 1994. Geochronological constraints on the timing of Archean gold mineralization in the Yilgarn Craton, Western Australia, PhD thesis (unpublished), Australian National University, Canberra. McCuaig, T C, 1996. The genesis and evolution of lode gold mineralization and mafic host lithologies in the late-Archaean Norseman Terrane, Yilgarn Block, Western Australia, PhD thesis (unpublished), University of Saskatchewan, Saskatoon. McCuaig, T C, Kerrich, R, Groves, D I and Archer, N, 1993. The nature and dimension of regional and local gold-related hydrothermal alteration in tholeiitic metabasalts in the Norseman Goldfields: the missing link in crustal continuum of gold deposits, Mineralium Deposita, 28: 420–435. McGoldrick, P, 1993. Geology of the Norseman 1:100 000 sheet (Geological Survey of Western Australia: Perth). Perring, C S and McNaughton, N J, 1990. Proterozoic remobilization of ore metals within Archaean gold deposits: lead-isotope evidence from Norseman, Western Australia, Australian Journal of Earth Sciences, 37: 369–372. Sibson, R H, 1990. Faulting and fluid flow, in Tectonically Active Regimes of the Continental Crust (Ed: B E Nesbit), Mineralogical Association of Canada Short Course 18, pp 93–132. Thomas, A, Johnson, K and MacGeehan, P J, 1990. Norseman gold deposits, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 493–504 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Mulholland, I R, Cowden, A, Hay, I P, Ion, J C and Greenaway, A L, 1998. Ninbus silver-zinc deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 273–278 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Nimbus silver-zinc deposit 1
2
3
4
by I R Mulholland , A Cowden , I P Hay , J C Ion and A L Greenaway INTRODUCTION The deposit is 17 km ESE of Kalgoorlie, WA, at about lat 30o47′S, long 121o39′E or AMG coordinates 370 500 E, 6 592 500 N (Fig 1), on the Kurnalpi (SH 51–10) 1:250 000 scale and the Kanowna (3236) 1:100 000 scale map sheets. It is about 2 km NE of the historic Boorara mining centre and straddles the old Boorara–Bulong water pipeline track. The deposit is owned by Archaean Gold NL.
5
An Identified Mineral Resource totalling 929 000 t at 270 g/t silver and 0.3 g/t gold has been estimated for the oxide and transition zone portions of the deposit at the Discovery, Western and Eastern zones (Table 1, Fig 2). This represents about 8.14 Moz of contained silver and 9900 oz of contained gold, or 118 400 oz of gold equivalent where gold equivalent = gold + silver/75. No resource estimate has yet been made for the sulphide mineralisation, exploration of which is continuing. TABLE 1 Nimbus resource summary. Resource category
Ore type
Ore (’000 t)
Ag Au Ag (g/t) (g/t) (’000 oz) 370
0.3
5078
Au (oz)
Oxide Transition
70.0
340
0.4
754
820
Indicared
Oxide
89.5
110
0.2
318
590
305.5
190
0.4
1892
4010
Inferred
Oxide
15.0
40
0.6
21
280
Transition
23.0
100
0.4
76
290
929.0
270
0.3
8139
9900
Total oxide and transition resources
FIG 1 - Location map and regional geology (from Ahmat, 1995).
Contained metal
Measured
Transition
426.0
Grade
3910
Notes: 1. Based on an undiluted block model 2. Ag grade is cut to 3000 g/t in the Eastern zone and 2200 g/t in the Discovery zone 3. Lower cutoff is 0.5g/t gold equivalent, where gold equivalent = gold + silver/75 4. There may be discrepancies in totals due to rounding.
Nimbus is the first example of high-grade silver-zinc mineralisation found in the Kalgoorlie area, and it may epitomise a new province of volcanogenic massive sulphide (VMS) deposits. Within the oxide zone, to 90 m depth, outcropping silver and gold mineralisation is underlain by a blanket of transition zone silver-gold mineralisation at the base of complete oxidation. In fresh rock, high grade silver-zinclead sulphide mineralisation has been intersected. 1.
Formerly Development Manager, Archaean Gold NL, 18 Richardson Street, West Perth WA 6005.
2.
Formerly Managing Director, Archaean Gold NL, 18 Richardson Street, West Perth WA 6005.
3.
Formerly Project Geologist, Archaean Gold NL, 18 Richardson Street, West Perth WA 6005.
4.
Formerly Chief Geologist, Archaean Gold NL, 18 Richardson Street, West Perth WA 6005.
5.
Formerly Exploration Geologist, Archaean Gold NL, 18 Richardson Street, West Perth WA 6005.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 2 - Schematic drilling plan, Nimbus deposit, showing mineralised zones projected to surface.
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EXPLORATION HISTORY The Nimbus area was previously explored for precious and base metals, and minor anomalous silver and arsenic values in soil had been identified before the involvement of Archaean Gold NL. The current leases were pegged by a prospector, C J Hake, in 1991. Mr Hake and partners recovered over 160 oz of gold from a quartz outcrop some 300 m south of the outcrop of the Discovery zone. In 1993 an Archaean Gold geologist, Patrick Cheetham, while reviewing the prospect with Mr Hake, recognised a heavy grey-green mineral present in panned cuttings of a drill hole as possibly being silver chloride. Analysis of cuttings and nearby outcrops provided silver values to 390 ppm and an option to purchase was negotiated. Archaean Gold commenced exploration at Nimbus in July 1994 using 200 by 40 m spaced soil sampling followed by infill soil sampling. By December 1994 widespread gold and/or silver anomalies in soil with peak values of 119 ppb gold (BCL) and 13 000 ppb silver (AAS) had been outlined in the vicinity of the discovery outcrop. In March 1995, rotary air blast (RAB) drilling at the soil anomalies in the Discovery zone intersected silver values of >1000 g/t over wide downhole intervals (>40 m), accompanied by highly anomalous zinc, gold and lead values in completely oxidised rock. In June 1995 additional drilling of silver anomalies resulted in the discovery of the Eastern zone. Gossan outcrop at the Eastern zone was mildly anomalous, with silver values to about 7 ppm. Since discovery, 32 538 m of RAB, 18 449 m of reverse circulation (RC) and 3214 m of diamond core drilling have been used to delineate the three areas of oxide and transition zone mineralisation at the Discovery, Western and Eastern zones and to test deeper sulphide mineralisation at Nimbus Deeps (Fig 2). A prefeasibility study into mining of the oxide and transition zone mineralisation is in progress.
REGIONAL GEOLOGY Nimbus is within the uppermost felsic units of the Boorara Domain of the Archaean Kalgoorlie Terrane (Swager et al, 1990) (Fig 1), adjacent to the Boorara Shear (Ahmat, 1995). Stratigraphy is poorly understood and it is difficult to distinguish between intrusive and extrusive felsic igneous rocks and between flows, tuffs and volcaniclastic sediment because of the effects of metamorphism, weathering and the lack of critical exposures (Ahmat, 1995). No formal stratigraphic names have yet been allocated to the Nimbus host rocks.
Nimbus is hosted by feldsparphyric felsic rocks of rhyolitic to dacitic composition, including flow-banded lava, breccia (hyaloclastite and autobreccia) and fine grained felsic volcaniclastic sandstone. Siliceous layered chert, black shale and sulphide-rich variants of these sediments are also present. Primary flow brecciation and cooling fractures and/or intrusive flow brecciation occur along the glassy contacts of some volcanic units. Glassy chilled margins to fragments are common. This assemblage of host rocks, although unclear in its geometry, is typical of that hosting VMS deposits. Regional greenschist facies metamorphism is represented by the development of sericite, chlorite and quartz. All volcaniclastic rocks and most sulphide-rich rocks are often schistose with an upright penetrative fabric, defined by sericite and/or chlorite, which dips at 65o towards 225o magnetic. In more competent chert and felsic rock this fabric is weakly developed or absent. A strong stretching lineation dipping at 30o towards 135o magnetic lies in the plane of the fabric. The orientation of unit boundaries is generally unclear but most readily definable units at surface, such as chert outcrops and gossan float trails, are parallel to this regional fabric.
MINERALISATION Secondary silver mineralisation occurs in two zones - within the completely oxidised weathering zone (termed oxide mineralisation), and at or near the base of oxidation (termed transition zone mineralisation). Sulphide zone mineralisation occurs within fresh rock below the transition zone.
Oxide zone Oxide mineralisation at the Discovery, Western and Eastern zones (Figs 2, 3 and 5) is a maximum of 80 m wide, 50 m deep and 120 m long. There is a higher grade core of silver-gold mineralisation where values of silver in the range 500–3000 g/t are common, possibly representing relict primary mineralisation. For example hole BOC011 returned 23 m at 960 g/t silver, BOD028 returned 39 m at 890 g/t silver, BOD059 returned 42 m at 1170 g/t and BOM002 returned 34 m at 1464 g/t silver. Hydromorphic dispersion is probably responsible for the gradual decrease of silver and gold values in weathered rock around this higher grade core.
ORE DEPOSIT FEATURES LOCAL GEOLOGY The base of complete oxidation is at about 80 to 100 m below surface with all rocks above this completely weathered to kaolinite, iron oxide and quartz. Primary rock type in the weathered zone is difficult to determine with certainty. A 10 to 20 m thick zone of partially weathered material lies below the base of oxidation and above fresh rock. In this zone rock textures are better preserved and rock types are more recognisable.
274
FIG 3 - Schematic cross section A – A′ looking north. Location on Fig 2.
Geology of Australian and Papua New Guinean Mineral Deposits
NIMBUS SILVER-ZINC DEPOSIT
TABLE 2 Nimbus Deeps representative drilling results. Hole No
Interval (m)
Zn (%)
Pb (%)
Ag (g/t)
BOD022
18
1.6
0.7
276
BOD036
41
4.3
0.7
353
BOD044
25
3.6
0.2
148
BOC0682
11
6.8
0.6
254
BOC076
8
2.3
0.4
99
BOD100
50
6.8
1.1
219
BOD103
31
1.8
0.2
275
BOD167
1
6.6
1.0
350
BOD168
5
17.9
2.0
943
BOD168
11
5.7
0.9
106
BOD169
36
7.2
1.6
399
BOD174
5
1.0
-
24
BOD174
9
2.3
0.5
102
BOD175
9.6
10.1
7.4
907
BOD175
8
2.3
0.2
128
BOD175
11.3
1.2
0.1
80
BOD177
5.7
12.4
4.1
1200
1
1. BOD prefix indicates diamond drill hole 2. BOC prefix indicates RC drill hole.
FIG 4 - Schematic cross section B – B′ looking NW. Location on Fig 2.
5). The soft clays and earthy gossans developed at this level contain native silver and secondary copper minerals, principally chalcocite.
Sulphide zone
Transition zone
Sulphide mineralisation occurs beneath and down plunge from the oxide and transition zone mineralisation at Discovery, Eastern and Western zones. Individual sulphide bodies vary in true thickness from 2 to 25 m (Fig 4), have a horizontal extent of >100 m (Fig 2) and a vertical extent of at least 50 to 80 m (Figs 4 and 5). Representative assay values from drilling are given in Table 2.
Transition zone mineralisation occurs at or near the base of oxidation and there is some local continuity with oxide mineralisation above and sulphide mineralisation below (Fig
Weak alteration forms a halo up to 700 m wide centred on the mineralisation. Strong sericitisation of feldspar plus disseminated euhedral pyrite and yellow tourmaline, albite and carbonate are typical.
The principal silver-bearing mineral within the oxide zone has been identified as cerargyrite [Ag(Cl,Br,I)]. A complex lead-iron-antimony-copper sulpho-arsenate, electrum, and iron and manganese oxides also occur.
FIG 5 - Schematic longitudinal projection C – C′ looking NE. Location on Fig 2.
Geology of Australian and Papua New Guinean Mineral Deposits
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Intense alteration occurs within 50 to 100 m of sulphide-rich mineralisation. Carbonate and sulphides are abundant and they replace and destroy pre-existing textures. Green and black chlorite and either fuchsite or biotite develop as vein microselvages and may define the schistose fabric. Two styles of sulphide mineralisation have been observed; massive sulphide and sulphide breccia.
Massive sulphide Massive sulphide occurs as either massive banded sphaleritegalena or as massive pyrite, in layers of 2 to 10 m true thickness. The massive banded sphalerite-galena may contain high silver values in addition to high base metal values, for example hole BOD175 intersected 9.6 m at 10.1% zinc, 7.4% lead and 907 g/t silver, and hole BOD177 intersected 5.7 m at 12.4% zinc, 4.1% lead and 1200 g/t silver (Table 2). Sphalerite is the dominant mineral, forming an even grained mosaic of 0.2–0.3 mm diameter equant crystals and is the host to other sulphides, principally pyrite, galena and boulangerite, as intergrowths in the sphalerite. Silver-rich tetrahedrite and pyrargyrite are sometimes present. Arsenopyrite is consistently present as fine idiomorphic rhombs in the other sulphides, and up to 10% quartz is common as gangue. In most mineralised zones there are cataclastic microstructures and pressure shadows associated with more competent primary relic features such as crystals and clasts. Massive pyrite is usually barren of zinc, lead and silver.
Sulphide breccia Sulphide breccia represents widespread sulphide replacement of a pre-existing breccia zone, probably a hyaloclastite or a stringer zone. Mineralisation is usually thick (25 to 30 m true thickness) but of lower grade than the massive sulphide type, for example hole BOD 100 intersected 50 m at 6.8% zinc, 1.1% lead and 219 g/t silver (Table 2). Sulphide breccia originally consisted of fragments of felsic porphyritic host rocks in a matrix of quartz and chlorite. The breccia matrix and then the rock fragments have been altered and progressively replaced by pyrite and at a later stage by sphalerite. At the lower margins of the breccia, alteration intensity is low and angular rock fragments are easily identifiable. Alteration intensity increases upwards and rock fragments and original textures are progressively destroyed. Towards the top of each sulphide breccia zone the intensity of sphalerite replacement increases such that within 2 to 10 m of the top of the zone (Fig 6) there is near total replacement of matrix and rock fragments by sphalerite and pyrite. There are at least two generations of sphalerite, a brownish (higher iron) variety being later than honey coloured (lower iron) sphalerite. Galena, boulangerite, arsenopyrite, pyrite and pyrargyrite occur in minor to trace quantities. Minerals identified are pyrite, sphalerite, galena, silverbearing tetrahedrite [(Cu,Fe,Zn,Ag)12Sb 4S13], boulangerite (Pb5Sb4S11), pyrargyrite (Ag3SbS3), arsenopyrite (FeAsS), jalpaite [(Ag,Cu)2S], native silver, amalgam (Ag,Hg), bournonite (PbCuSbS3), chalcopyrite, electrum and cinnabar.
FIG 6 - Schematic section through sulphide mineralisation.
276
Geology of Australian and Papua New Guinean Mineral Deposits
NIMBUS SILVER-ZINC DEPOSIT
ORE GENESIS Nimbus displays many features of a VMS type deposit. The two mineralised components of the deposit, massive sulphide and sulphide breccia, have a different dip, but similar strike and plunge. At Discovery zone the association of flat-lying banded massive sphalerite and galena with overlying chert and argillite may indicate a classic VMS exhalative position. The underlying (subvertical) sulphide breccia either represents sulphide replacement of a hyaloclastite (quench breccia) which acted as a fluid conduit, or it may simply be a sphalerite-rich stringer feeder zone beneath the exhalative position now represented by massive sphalerite-galena and pyrite. Oxide mineralisation represents hydromorphic redistribution and possible enrichment of relatively immobile elements such as silver, lead and gold from weathered massive sulphide and sulphide breccia. Copper and zinc are almost totally removed from the oxide zone by weathering of the original primary sulphide mineralisation. Transition zone mineralisation has been formed by supergene weathering at or about the water table.
PLANNED MINING OPERATIONS Metallurgical studies for prefeasibility purposes indicate that the oxide mineralisation is amenable to a standard cyanide leach, similar to most gold ores, with recoveries of 95% silver and 80% gold. Recovery of silver and gold from the leach
Geology of Australian and Papua New Guinean Mineral Deposits
solution will probably be by zinc precipitation using a Merill Crowe circuit. Other process options such as flotation and direct reduction are also being investigated.
ACKNOWLEDGEMENTS The authors acknowledge the permission of Archaean Gold NL to publish this paper. Nimbus was not discovered and developed by one individual, but rather a team of people, working together with a common discovery goal. In addition to the authors, this team included P L Cheetham and J Stockley. Mineragraphic examinations by R Townend and petrographic work by M Maxwell aided geological interpretations, as did discussions with visiting geologists. Thanks are also extended to Genalysis Laboratory Services who developed a new silver fire assay technique, P Benjamin who drafted the figures and K Spiers who typed the manuscript.
REFERENCES Ahmat, A L, 1995. Kanowna, Western Australia - 1:100 000 geological series, Western Australia Geological Survey, Explanatory Notes 3236. Swager, C P, Griffin, T J, Witt, W K, Wyche, S, Ahmat, A L, Hunter, W M and McGoldrick, P J, 1990. Geology of the Archaean Kalgoorlie Terrane - an explanatory note, Western Australia Geological Survey Record 1990/12.
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Parks, J, 1998. Weld Range platinum group element deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 279–286 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Weld Range platinum group element deposit 1
by J Parks
INTRODUCTION The Archaean Weld Range ultramafic-mafic complex is about 700 km NNE of Perth, WA, in the Murchison Province, some 70 km NNW of the township of Cue and about 1 km north of the banded iron formation (BIF) ridges of the Weld Range. The complex is at lat 26o50′S, long 117o45′E on the Belele (SG 50–11) 1:250 000 scale and the Madoonga (2444) 1:100 000 scale map sheets (Fig 1). Platinum group element (PGE) mineralisation has been identified within a laterally persistent olivine-clinopyroxenite unit with a strike length of 15 km and in the overlying regolith. An Inferred Resource has been estimated for the supergene 1.
Senior Geologist, Battle Mountain (Australia) Inc, 106 Dalrymple Road, Currajong Qld 4814.
deposit which is the first potentially economic discovery of supergene PGE in Australia (Table 1). Studies including metallurgical testing are continuing. Sparse drill testing of mineralisation in fresh rock shows apparent continuity which is typical of this style of mineralisation, but the testing is presently insufficient to allow resource estimation. The hard rock mineralisation remains open at depth and for a further 4 km of strike length. The Weld Range complex is enclosed by tenements subject to a joint venture between Sons of Gwalia Ltd (SOG) with 65% equity who manage the joint venture, and Dragon Mining NL (Dragon) with 35%. The original joint venture from 1991 was between Dragon and Austmin Gold NL (Austmin), which became a subsidiary of Burmine Ltd in 1993. SOG assumed control following a merger with Burmine Ltd in May 1996.
FIG 1 - Location and simplified geological map, showing location of drill hole traverses and cross section lines on Figs 3 and 5.
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TABLE 1 Inferred Resources at different cutoff grades, Weld Range supergene PGE deposit. Cutoff Pt+Pd+Au Pt (g/t)
Grade Pd (g/t)
Au
Mt
Combined Contained oz Pt+Pd+Au Pt+Pd+Au (g/t)
0.50
0.6
0.5
-
14.76
1.1
522 000
0.75
0.8
0.6
-
9.53
1.4
429 000
1.00
0.9
0.7
-
6.30
1.6
324 000
2.00
1.5
1.1
0.1 1.38
2.7
120 000
EXPLORATION HISTORY The ultramafic rocks of the Weld Range complex have been explored for nickel sulphides, initially by International Nickel Australia Ltd between 1969 and 1971 and then by Australian Consolidated Minerals NL (ACM) in 1971, subsequently in joint venture with Broken Hill Proprietary Ltd (BHP) from 1972 to 1973. Only rare grains of sulphide (in part supergene) were identified, although significant nickel mineralisation was discovered in laterite in some localities, but was considered too discontinuous to be economically viable. Petrography by BHP on fresh drill hole samples showed the rocks to be olivine and olivine-chromite cumulates with clinopyroxene (diopside)olivine cumulates along the southern margin near the interpreted top of an ultramafic lopolithic intrusion (C J Palethorpe, unpublished data, 1972). Traces of chalcopyrite were noted in two samples from the olivine-clinopyroxene unit, with up to 1000 ppm copper obtained from drill hole samples of ultramafic clays developed above the same unit. No significance was attached to these weak copper enrichments and no samples were analysed for PGE. Potential for chromite mineralisation was noted but not tested due to the thickness of laterite cover. Additional exploration in the area by the ACM–BHP joint venture was directed at volcanogenic copper-zinc deposits in rocks interpreted as felsic volcanic breccias intruded by mafic to intermediate rocks flanking the ultramafic intrusion to the SW. No mineralisation was located, however petrography of selected samples showed that the rocks including the polymict breccias are mafic. Rocks described include basalt with crescumulate textures (bladed pyroxenes indicative of supercooling) and uralitised feldspathic pyroxenite. Regional mapping showed that these rocks are part of the greenstone sequence (Elias, 1982; Watkins and Hickman, 1990). Potential for PGE mineralisation in the area was not recognised until the late 1980s when Dragon and Austmin acquired adjacent tenements enclosing the ultramafic complex. Dragon initially focussed on the chromium potential of the laterites in an area investigated by CRA Limited in 1976–1977. CRA had concluded that most of the chromium was associated with iron oxides and not recoverable as chromite. Dragon has now identified an Inferred Resource of 31 Mt at 3.6% chromium at a 2% chromium cutoff to 12 m depth within an area of 5.4 km2 excised from the joint venture (Fig 1). Laterite samples collected from the area of the chromium resource were assayed for gold and the full suite of PGE. No anomalous PGE values were obtained, but it was recognised that other PGE targets within the intrusion remained untested (Morgan, 1987).
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However, conventional models for PGE mineralisation based on stratiform layered mafic-ultramafic complexes such as the Bushveld Complex were not thought applicable, due to the apparent absence of gabbroic rocks. Importantly the gabbroic top to the ultramafic cumulates was not recognised until field reconnaissance in 1990 by Geological Consultants International on behalf of Austmin identified rare outcrops of gabbro near the SW margin of the intrusion. At this time the polymict breccias and associated rocks in the SW corner of the intrusion were re-interpreted as a marginal facies to the ultramafic-mafic complex rather than part of the greenstone sequence. This meant that conventional PGE models could be applied and gave significance to the weak copper sulphide mineralisation previously identified at the top of the ultramafic sequence. The exploration concept was subsequently validated when drilling by Austmin, as managers of the joint venture with Dragon, intersected PGE mineralisation in the regolith and in fresh rock immediately below the gabbro contact. To date, 24 diamond drill holes for 3740 m, 41 reverse circulation holes for 3690 m and 367 rotary air blast/aircore holes for 15 428 m have been drilled by the joint venture partners, mostly directed at the known mineralisation.
REGIONAL GEOLOGY The age and contact relationships between the Weld Range complex and the country rocks are not precisely known, due to the paucity of outcrop. The complex is bounded to the south by apparently concordant dolerites and banded iron formation units which constitute the Weld Range. Granite flanks the complex to the north and west (Fig 1). Regional geological mapping and interpretation places the Weld Range complex in the Gabanintha Formation of the Luke Creek Group, the lowermost of the two greenstone sequences in the Murchison Supergroup (Watkins and Hickman, 1990). The Gabanintha Formation is overlain by the Windaning Formation, which in this locality constitutes the jaspilitic BIF and dolerite forming the core of the Weld Range greenstone belt. A U-Pb age of 3000 Myr has been obtained from rocks in the upper part of the Luke Creek Group (Watkins and Hickman, 1990). The Gabanintha Formation in the greenstone belts up to 150 km south of Meekatharra is characterised at or near its base by a laterally extensive volcanic-intrusive ultramafic complex up to 1400 m thick. Weld Range is a discrete ultramafic-mafic intrusive complex to 8 km thick. Complexes of such thickness have not previously been recognised in the Luke Creek Group.
DEPOSIT GEOLOGY The geology of the complex is poorly understood due to sparse exposure and lateritic weathering to more than 80 m in the centre of the intrusion. BHP aeromagnetic data show that the complex is triangular in plan with the widest point at the stratigraphic top with a strike length of about 15 km. Available data suggest it is a recumbent lopolith such that the layering now dips at 60 to 85o south. Greenschist facies metamorphism has mostly destroyed primary silicate minerals although olivine and clinopyroxene grains commonly have fresh cores, and primary textures are typically well preserved. In some places the rocks have been intensely carbonate-talc altered with almost total destruction of minerals and texture. Shearing has been noted locally.
Geology of Australian and Papua New Guinean Mineral Deposits
WELD RANGE PLATINUM GROUP ELEMENT DEPOSIT
Outcrop within the complex, with the exception of the marginal rocks, is restricted to a hill of lateritic duricrust. Silicified ultramafic caprock showing relict igneous textures is exposed in places on the southern flanks of this hill. The laterite profile developed over the complex has been variably stripped, but generally to the level of the upper saprolite (Harrison, 1993). Lateritic gravel, assorted locally derived alluvium and colluvium, calcrete and aeolian sand overlie this truncated surface to depths of over 20 m. Depths of both weathering and transported overburden thin to less than 2 m over the SW of the intrusion.
LITHOLOGY AND STRATIGRAPHY Layering is evident in aeromagnetic data, but the drill hole data do not allow a reliable characterisation of these layers, which appear to reflect in part varying degrees of serpentinisation. The complex can be subdivided into a lower Ultramafic series to 5 km thick and an overlying Mafic series to 3.5 km thick (Fig 1). A Marginal series, consisting of breccia, fine grained mafic rocks and feldspathic pyroxenite, has only been observed on the SW margin of the intrusion, but may be more extensive. The Ultramafic series is rhythmically layered, whereas the Mafic series appears to be mostly massive. The upper part of the Ultramafic series and the entire gabbro sequence have been extensively intruded by a variety of mostly aplite dykes or sills. These sills are broadly parallel to the sequence but in detail pinch, swell, anastomose and bifurcate, but do not appear to be associated with any stoping or loss of sequence. Aplite units to 34 m thick disrupt the PGE mineralised zone. The relationship between the aplite and the granitic country rocks is not known. The contact between the Ultramafic and Mafic series is locally intruded by aplite and typically displays greater alteration than the adjacent sequence, obscuring primary contact relationships. However, the contact appears to be sharp and faulted at least in part. Difficulties in correlation between drill holes further indicate that it may be unconformable.
Ultramafic series The Ultramafic series can be subdivided into a lower olivinechromite zone and an overlying wehrlite zone. The olivinechromite zone forms the greater part of the sequence. The first appearance of cumulus clinopyroxene marks the base of the wehrlite zone, which is a maximum of 500 m thick.
Olivine-chromite zone BHP petrographic data show the olivine-chromite zone consists mostly of olivine-chromite cumulates and subordinate chromite-olivine cumulates and olivine adcumulates. No discrete chromitite layers or chromite adcumulates have been observed. Textures are typically mesocumulate to adcumulate with original interstitial material consisting of orthopyroxene, clinopyroxene and plagioclase, with rare biotite.
Wehrlite zone The wehrlite zone, which hosts the PGE mineralisation, consists of fine to medium grained (1–2 mm) olivine and clinopyroxene cumulates. Textures are mesocumulate to adcumulate. Clinopyroxene adcumulates and olivine adcumulates (dunite) are a minor component and where present appear to lack lateral continuity. Orthopyroxene is locally a
Geology of Australian and Papua New Guinean Mineral Deposits
minor (to 15%) cumulus phase. In some places it forms anhedral poikilitic grains 1 cm across. Rare chromite has been observed in only two samples: intergrown with sulphide in a mineralised sample from near the western end of the complex, and as very fine grained inclusions within clinopyroxene in a mineralised sample from the central part of the intrusion. Interstitial minerals are orthopyroxene, hornblende and possibly rare plagioclase. Green-brown hornblende and subordinate red-brown hornblende (kaersutite?), are a common although minor (<5%) component of the mineralised zone. They occur as rims around pyroxene grains, as grains between pyroxene crystals and as small optically-continuous inclusions within pyroxene crystals. The rocks intersected in cored drill holes display irregular rhythmic layering developed on a scale of millimetres to metres, mostly defined by modal variations in the relative proportions of olivine and clinopyroxene. There are no mineral composition data available to determine if the layering is also cyclic. Correlation of individual layers is not possible with the current wide drill hole spacing.
Alteration Clinopyroxene has been partially altered to tremolite and chlorite with minor magnetite and also locally carbonate. Olivine is mostly pseudomorphed by antigorite and fine grained ragged magnetite. The former presence of orthopyroxene is inferred from pseudomorphs of fine grained talc, tremolite, chlorite and local cummingtonite. Plagioclase has been tentatively identified in a few samples and is totally altered to chlorite.
Mafic series Information on the mafic rocks is only available from sparse drill holes into the lower 2 km of the Mafic series. They appear to be a monotonous sequence of massive, medium to coarse grained (to 4 mm diameter) plagioclase-clinopyroxene and plagioclase-clinopyroxene-orthopyroxene cumulates. Hornblende is a minor interstitial phase in the lowermost part of the sequence, and no primary oxide phases have been observed. There are gradations in the mode of the constituent phases with local development of gabbro tending to anorthosite over a scale of a few centimetres and rare weak igneous lamination.
Alteration Plagioclase is completely saussuritised to assemblages of epidote, carbonate, muscovite, clinozoisite, chlorite, talc and quartz, and identification of the former mafic phases is difficult. Pyroxene is mostly pseudomorphed by amphibole, generally tremolite, with minor chlorite and carbonate. Orthopyroxene or possibly olivine is inferred from circular areas of carbonate, minor tremolite and chlorite.
Aplite The aplites mostly consist of fine grained (0.5–1.0 mm) igneous textured quartz-feldspar (mostly microcline)-biotite rocks. The feldspar is generally altered to sericite and chlorite with biotite partly altered to chlorite. Metamorphic-textured assemblages of prehnite, epidote, quartz, chlorite, titanite, zircon and minor garnet are interpreted to be the metasomatised equivalent. Rare occurrences of quartz-feldspar graphic
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pegmatite are also considered to be part of the aplite suite. Locally the later intrusive rocks are more mafic, and contain needles of metamorphic hornblende, or less commonly tremolite pseudomorphing inferred igneous hornblende, and are probably metamorphosed dacites. Alteration of the ultramafic rocks is commonly more intense within several centimetres of the aplites.
MINERAL CHEMISTRY The compositions of olivine and chromite as determined from electron microprobe analyses of fresh drill hole samples from near the basal margin of the intrusion do not conclusively determine the parental magma affinities of the complex, but show that it is most closely related to komatiites and stratiform intrusions. Plots of chromite compositional data show overlap with the fields for the Great Dyke (Fig 2a), WA komatiites (Figs 2b, c, d) and stratiform intrusions (Figs 2c, d). Olivines from olivine adcumulate rocks have high forsterite contents ranging between 88.2 and 89.8%. If it is assumed that these were the first rocks to crystallise, then the magnesium:iron ratio in the parent magma can be calculated. Roeder and Emslie (1970) have determined a partition coefficient (Kd) of 0.30 for iron and magnesium between olivine and liquid in basaltic melts, but it may be as high as 0.33 for komatiitic magmas (Arndt and Nisbet, 1982). At Weld Range the mole ratio of MgO:FeO in the parent magma is 0.73:0.27. This figure is at the magnesian end of the range of parental magma compositions (0.62–0.73) proposed for the Stillwater Complex (Helz, 1985). The preponderance of olivine-chromite cumulates, rare pyroxene cumulates and the relatively thin mafic sequence at Weld Range are typical of sequences developed from differentiation of komatiitic magmas as at Agnew (Hill, Gole and Barnes, 1987), and further suggest a highly magnesian parent magma.
FIG 2b - Cr-Fe-Al diagram for Weld Range chromite. Komatiite data from Donaldson (1983), Windimurra data from Ahmat (1986).
FIG 2c - Cr/Cr+Al versus Mg/Mg+Fe2++Mn+Ni+Zn diagram for Weld Range chromite. Boninite data from Cameron, Nisbet and Dietrich (1979), alpine and stratiform data from Greenbaum (1977), komatiite data from Donaldson (1983), Windimurra data from Ahmat (1986).
FIG 2a - Cr2O3 versus Al2O3/Al2O3+Cr2O3+Fe2O3 diagram for Weld Range chromite. Variation diagram from Thayer (1970), Windimurra data from Ahmat (1986), Manitoba data from Bliss and MacLean (1975).
282
FIG 2d - Fe3+/Cr+Al+Fe3+ versus Mg/Mg+Fe2++Mn+Ni+Zn diagram for Weld Range chromite. Alpine and stratiform data after Greenbaum (1977), komatiite data from Donaldson (1983), Windimurra data from Ahmat (1986).
Geology of Australian and Papua New Guinean Mineral Deposits
WELD RANGE PLATINUM GROUP ELEMENT DEPOSIT
The clinopyroxene at Weld Range has been described as diopside, however microprobe analyses are required to determine if it is endiopside (ie magnesium rich), which is the clinopyroxene characteristic of komatiitic rocks.
PGE MINERALISATION Primary mineralisation The primary PGE mineralisation at Weld Range is hosted by a clinopyroxene-olivine cumulate and less commonly by a pegmatoidal clinopyroxene-(olivine) orthocumulate, in the upper part of the wehrlite zone. The mineralisation does not crop out. The mineralised zone, as defined by grades in excess of 1 g/t combined precious metals, has a true thickness of 13 to 18 m with its top 8–17 m below the gabbro contact. Widely spaced drilling has determined the persistence of this layer over 11 km of strike length to a depth of 250 m (Fig 3).
Mineralogy The host wehrlite is fine grained and contains olivine and clinopyroxene in variable proportions with clinopyroxene generally dominant. Locally the mineralised zone tends to clinopyroxenite and less commonly to thin intervals of dunite. Hornblende is a common but minor component. There does not appear to be any systematic variation in rock type that can be related to the PGE mineralisation. The upper part of the
mineralised zone contains minor sulphides, but sulphides are absent in the lower part of the zone and the footwall to the mineralisation can not be identified visually. The presence of hornblende may characterise the mineralised zone, but there are no petrographic data available from the footwall wehrlite to support this. There is no obvious textural relationship between hornblende and sulphides, but such a relationship, if it existed, may have been obscured by metamorphic recrystallisation. The sulphide content is typically trace to 1 modal %, rarely up to 5%. The sulphides are dominantly chalcopyrite (90%) with subordinate pentlandite (mostly metamorphic), bornite, pyrrhotite, violarite, pyrite, gersdorffite (NiAsS), sphalerite, and possible valleriite [2(Fe,Cu)2S2.3(Mg,Al)(OH)2] and millerite. Sulphides are typically ragged, fine to very fine grained and disseminated throughout the host rock along cleavage traces, pyroxene grain boundaries and less commonly as fine stringers or coarser interstitial grains. In samples with higher sulphide content, lobate sulphide-magnetite intergrowths are interpreted as metamorphically recrystallised and compositionally modified igneous sulphide aggregates (M Gole, unpublished data, 1992). A small number of qualitative analyses of the platinum group mineral (PGM) species using a scanning electron microscope (SEM) identified grains to 10 µm diameter of moncheite [Pt(Te,Bi)2] associated with chalcopyrite-magnetite intergrowths, and in another sample a 1 µm grain of possible taimyrite [(Pt,Pd,Cu) 3(Sn,Sb)].
FIG 3 - Schematic drill hole cross sections, looking ENE.
Geology of Australian and Papua New Guinean Mineral Deposits
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High grade mineralisation has also been intersected within a pegmatoid unit, to 0.7 m true thickness, located near the base of the main mineralised zone in the central part of the intrusion. Values to 10.2 g/t palladium, 8.2 g/t platinum, 1.2 g/t rhodium, 0.62 g/t iridium, 0.22 g/t ruthenium, 0.05 g/t osmium and 230 ppm copper have been obtained. The pegmatoid consists of 60% clinopyroxene and tremolite pseudomorphs after clinopyroxene, which form interlocking tabular grains up to 2 cm long, containing numerous inclusions of pale brown hornblende. Antigorite and magnetite pseudomorph small (2 mm) rounded olivine grains. Accessory ilmenite forms large (6 mm) altered grains. Ore minerals in the pegmatoid consist mainly of spottily distributed 1 cm diameter intergrowths of bornite with minor magnetite, pyrite, pentlandite, ilmenite, chalcopyrite, tremolite needles and trace amounts of galena, sphalerite, chromite and PGM. The PGM are located within chalcopyrite and less commonly pentlandite. Taimyrite is the most common PGM and occurs as stubby tabular grains to 110 µm diameter containing inclusions of bornite and rarely tremolite. Another variety of moncheite [(Pt,Pd)Te2], osmium metal and possible acanthite [(Ag,Cu,Fe)S] were also identified, in addition to a number of alloys comprising various combinations of iridium, copper, nickel, osmium, cobalt, platinum, antimony, tin and bismuth.
Grade distribution The typical distribution of platinum, palladium, gold and sulphides as indicated by copper abundance through the mineralised zone is shown on Fig 4. Platinum and palladium are present in about equal amounts and have a strong
correlation, but exhibit slight variations in their relative ratios over intervals of several metres. There is a tapering of values at the top and bottom of the mineralised zone, with palladium anomalism commencing lower in the sequence with a concomitant persistence of platinum to a higher stratigraphic interval. Gold and copper values show a reasonable correlation and have an offset relationship to platinum and palladium, attaining maximum values higher in the sequence than peak PGE values. Copper reaches its highest concentration immediately below the gabbro contact and shows a sharp drop in concentration to about 600 ppm immediately above this contact, with a gradual diminution in values above this. Within this broad distribution there are localised high grade ‘spikes’ where all four elements may exhibit a sympathetic relationship. These spikes, except in the pegmatoid, can not be consistently correlated with either rock type or between drill holes.
Supergene mineralisation Platinum, palladium and gold are dispersed into a subhorizontal enrichment zone in the saprolite above the primary wehrlite zone (Fig 5). The depth of weathering is 45–50 m in the vicinity of the supergene mineralisation but aplite units are typically more deeply weathered. Weathering is also slightly deeper directly over the mineralised wehrlite due to the minor sulphide concentrations. The residual laterite profile has been eroded and covered by 2 m to over 20 m of lateritic gravel over the mineralised zone. Here the overburden increases in depth from south to north, corresponding to the present and inferred past drainage patterns which are also from south to north (Harrison, 1993). The mineralisation is dispersed to the north up to 120 m from the source wehrlite, but not to the south. The mineralisation is also dispersed into weathered aplite and to a lesser degree into the transported overburden. Gold is concentrated above an iron palaeoredox front in the upper saprolite, consistent with the weathering behaviour of gold elsewhere. Platinum and palladium have nearly identical dispersions and are concentrated about 10 m below this front in the lower saprolite above a manganese redox front, which contains up to 2 wt % manganese oxides. Harrison (1993) attributes the platinum and palladium distribution to adsorption on to manganese oxides. No discrete manganese oxides have been identified by SEM studies, but there are a number of manganese-rich iron oxides, which also contain barium. Most of the manganese oxides are interpreted to be finely dispersed throughout the clay surfaces, with the remainder substituted into the iron oxide lattice of the manganiferous oxides or intimately intergrown with the iron oxides (Harrison, 1993).
Mineralogy SEM investigations have identified numerous 1–5 µm diameter gold grains within crevices in clays and less commonly within oxide minerals. A single grain 10 µm in diameter containing gold and osmium was observed in a fracture in smectite clays from immediately below the unconformity. A grain of platinum 8 µm in diameter within a crevice in clays has also been identified.
ORE GENESIS
FIG 4 - Geochemical profile for drill hole WRD-03, on line 7080 m E. Aplite units removed for clarity; assay values on log scale.
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The origin of stratabound PGE mineralisation in layered intrusions is conjectural, but is variably attributed to crystal fractionation, magma mixing and hydrothermal processes, or combinations thereof. The Weld Range deposit is similar to
Geology of Australian and Papua New Guinean Mineral Deposits
WELD RANGE PLATINUM GROUP ELEMENT DEPOSIT
FIG 5 - Interpreted platinum and gold supergene distribution on cross section 9800 m E, adapted from Harrison (1993).
deposits in the Great Dyke of Zimbabwe and in the Munni Munni Complex in that the PGE are associated with minor sulphides at a comparable stratigraphic position. The similarity of stratabound PGE deposits worldwide indicates that similar processes apply on a global scale. There are insufficient data from Weld Range to enable substantial contribution to debates on their origin, but hydrothermal processes possibly had a role at Weld Range, even if only to remobilise and redistribute primary igneous sulphides and PGE. The widespread occurrence of hornblende both as rims and intergranular grains is interpreted as evidence for considerable late igneous hydrothermal activity (M Gole, unpublished data, 1992). However, crystal fractionation appears to have been the most important mechanism. The PGE, gold and copper are incompatible in silicate melts and are progressively enriched during fractional crystallisation of the magma. Sulphur saturation and subsequent scavenging of the chalcophile metals by the immiscible sulphide phase occurred near the top of the wehrlite zone. Sulphide crystallisation may have been triggered by changes in magma composition due to either pyroxene crystallisation or supersaturation with plagioclase immediately before crystallisation of the Mafic series. The appearance of platinum and palladium lower in the sequence than gold is attributed to the greater chalcophile nature of the PGE. The rarity of nickel sulphides is further evidence of a protracted history of silicate crystallisation, as nickel preferentially partitions into ferromagnesian silicates in the absence of a sulphide phase, resulting in depletion in nickel in the residual melt. Olivine from near the base of the olivinechromite zone contains high concentrations of NiO (0.39 to 0.47 wt %) confirming that sulphur saturation had not occurred at this level. This also explains the absence of sulphides at the base and along the margins of the intrusion, as discovered by early nickel explorers.
Geology of Australian and Papua New Guinean Mineral Deposits
ACKNOWLEDGEMENTS The permission of Sons Of Gwalia Ltd and Dragon Mining NL to publish this information is gratefully acknowledged. SOG provided for drafting of figures. BHP loaned archived thin sections and polished thin sections for petrography and microprobe analyses. Special thanks to R Hill for assisting with interpretation of mineral chemistry data, and to R England for clarifying rarer PGM formulae. M Gole carried out all petrography and SEM studies on the primary mineralisation. Special acknowledgment to R England, J Brigden, M Hoyle and B Agar for reviewing an earlier version of this manuscript.
REFERENCES Ahmat A L, 1986. The age, structure and petrology of the Windimurra Complex, Western Australia, PhD thesis (unpublished), The University of Western Australia, Perth. Arndt, N T and Nisbet, E G, 1982. What is a komatiite?, in Komatiites (Eds: N T Arndt and E G Nisbet), pp 19–28 (George Allen and Unwin: London). Bliss, N W and Maclean, W H, 1975. The paragenesis of zoned chromite from central Manitoba, Geochimica et Cosmochimica Acta, 39:973–990. Cameron, W E, Nisbet, E G and Dietrich, V J, 1979. Boninites, komatiites and ophiolitic basalts, Nature, 280:550–553. Donaldson, M J, 1983. Progressive alteration of barren and weakly mineralized Archaean dunites from Western Australia: a petrological, mineralogical and geochemical study of some komatiitic dunites from the Eastern Goldfields Province, PhD thesis (unpublished), The University of Western Australia, Perth. Elias, M, 1982. Belele, Western Australia - 1:250 000 geological series, Geological Survey Western Australia Explanatory Notes SG 50–11. Greenbaum, D J, 1977. The chromitiferous rocks of the Troodos ophiolite Complex, Cyprus, Economic Geology, 72:1175–1194.
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Harrison, B C, 1993. Precious metals in the supergene environment, Weld Range, Western Australia, BSc Honours thesis (unpublished), The University of Western Australia, Perth. Helz, R T, 1985. Compositions of fine-grained mafic rocks from sills and dikes associated with the Stillwater Complex, in The Stillwater Complex, Montana: Geology and Guide (Eds: G K Czmanske and M L Zientek), pp 97–117, USGS Special Publication 92. Hill, R E T, Gole, M J and Barnes, S J, 1987. Physical Volcanology of Komatiites: a Field Guide to the Komatiites between Kalgoorlie and Wiluna, Eastern Goldfields Province, Yilgarn Block, Western Australia, Excursion Guide Book No 1 (Geological Society of Australia, Western Australian Division: Perth).
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Morgan, K H, 1987. Dragon Resources NL Prospectus (Dragon Resources NL: Perth). Roeder, P L and Emslie, R F, 1970. Olivine-liquid equilibrium, Contributions to Mineralogy and Petrology, 29:275–289. Thayer, T P, 1970. Chromite segregations as petrogenetic indicators, in Symposium on the Bushveld Igneous Complex and Other Layered Intrusions, Special Publication No 1 (Eds: D J L Visser and G von Gruenewaldt), pp 380–390 (Geological Society of South Africa: Johannesburg). Watkins, K P and Hickman, A H, 1990. Geological evolution and mineralization of the Murchison Province, Western Australia, Geological Survey of Western Australia, Bulletin 137.
Geology of Australian and Papua New Guinean Mineral Deposits
Morant, P, 1998. Panorama zinc-copper deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 287–292 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Panorama zinc-copper deposits by P Morant
1
INTRODUCTION The deposits are about 120 km SE of Port Hedland in the Pilbara mineral field of WA, centred at lat 21o12′S, long 119o14′E on the Marble Bar (SF 50–8) 1:250 000 scale and North Shaw (2755) 1:100 000 scale map sheets (Fig 1). They were formed at about 3240 Myr, and are thus amongst the oldest volcanogenic massive sulphide (VMS) deposits in the world. 1.
Principal Geologist, Sipa Resources Limited, PO Box 1183, West Perth WA 6872.
The Panorama project, owned by Sipa Resources Limited and subject to a farm in agreement with Outokumpu Zinc Australia Pty Limited, comprises approximately 2000 km2 of mineral tenements. Volcanogenic zinc-copper mineralisation has been intersected by drilling over a strike length of about 30 km, at the Sulphur Springs, Kangaroo Caves, Bernts, Breakers, Jamesons, Man O’War, Anomaly 45 and Roadmaster deposits. Indicated and Inferred Resource estimates for three deposits were announced to the Australian Stock Exchange in March 1997:
FIG 1 - Location and regional geological map of the Panorama zinc-copper deposits.
Geology of Australian and Papua New Guinean Mineral Deposits
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P MORANT
• Sulphur Springs – 2.8 Mt at 10.7% zinc and 0.6% copper and 2.5 Mt at 4.0% copper and 1.1% zinc • Kangaroo Caves – 1.7 Mt at 9.8% zinc and 0.6% copper • Bernts – 0.6 Mt at 7.8 % zinc and 0.3% copper.
EXPLORATION HISTORY The Panorama project area was explored intermittently before the 1970s for base and precious metal mineralisation, but the only previous production was from small gold workings. Stream sediment sampling and regional magnetic and electromagnetic surveys were the main exploration methods used. The first discovery of VMS mineralisation in the area was the Cardinal gossan, 35 km SSW of Sulphur Springs (Fig 1), in the early 1970s, but drilling did not identify a base metal resource. The only systematic regional geological mapping before 1992 was at 1:250 000 scale by the Geological Survey of Western Australia (Hickman and Lipple, 1978). Sulphur Springs, the most advanced deposit of the Panorama project, was discovered in 1984 by Harry Wilhelmij, who found magnesium sulphate deposits in a creek bed. Anomalous base metal values had been obtained from this creek in an earlier stream sediment sampling program. Denis O’Meara subsequently found the Sulphur Springs gossan, which crops out over an area of about 300 by 100 m. The area was explored in 1986–1988 for base metals and gold by Miralga Mining NL and Homestake Australia Limited. After unsuccessful drilling attempts at Sulphur Springs, the Miralga tenements were farmed out to a joint venture between Sipa Resources Limited and Ashling Resources NL, a wholly owned subsidiary of Burmin Exploration and Development plc. The Sipa–Ashling JV bought Miralga’s Pilbara tenements in 1989, and continued to explore until the end of 1992. The Outokumpu Group became involved in the project in early 1993 after Sipa had made a takeover of Burmin, the largest shareholder of which was the Outokumpu subsidiary, Tara Mines plc. Exploration by Sipa–Ashling between 1989 and April 1993 and Sipa–Outokumpu since 1993 has included about 50 000 m of diamond and reverse circulation drilling in about 200 drill holes. Geological mapping has been the main exploration approach since 1989. Geochemical exploration has included rock, soil and stream sediment sampling surveys. Soil sampling in this mainly erosional regolith regime has been particularly effective in locating sulphide-defining alteration, whereas interpretation of stream sediment data is limited by short dispersion trails for copper and lead, and lithological variation in background abundances. Airborne, ground and down hole geophysical surveys have been used extensively at Panorama but their effectiveness has been constrained by the mineralogy and texture of the ore for magnetic, electromagnetic and gravity methods, graphite in sedimentary rocks and in fault zones for electromagnetic methods, and rugged topography for gravity surveys.
REGIONAL GEOLOGY STRATIGRAPHY The deposits are in Archaean greenstone of the Pilbara Craton (Hickman, 1983). Metamorphic grade and strain are typically very low to low, such that primary textures and structures are well preserved in the VMS mineralisation (Vearncombe et al,
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1995), and in volcanic and sedimentary rocks. The volcanic succession that hosts the mineralisation and the Strelley Granite have been dated at 3235–3238 Myr by SHRIMP U-Pb methodology (University of Western Australia, unpublished data, 1996). These ages are inconsistent with the interpretation of Hickman (1983) that the felsic volcanic rocks that host the mineralisation should be assigned to the Wyman Formation, which has been dated by U-Pb methodology at 3325 Myr (Thorpe et al, 1990). The host volcanics are therefore informally called the Strelley succession (Morant, 1995). The VMS mineralisation is associated with felsic volcanic and volcaniclastic rocks within a north to NE trending tectonostratigraphic domain, the Strelley domain, that is bounded by regional-scale faults (Fig 1). At six of the deposits mineralisation occurs near the top of the Strelley succession, a volcanic-dominated sequence that includes the laccolithic Strelley Granite, beneath a succession of dominantly epiclastic sedimentary rock, the Gorge Creek Group. At Roadmaster the mineralisation occurs in chert about 1 km beneath the top of the Strelley succession. Mineralisation at Bernts is hosted by pumiceous rhyolitic breccias in the core of a regional syncline, adjacent to the faulted eastern margin of the Strelley domain. The Mid to Late Archaean De Grey Group (Lalla Rookh Sandstone) and Fortescue Group unconformably overlie the Strelley succession and Gorge Creek Group. A submarine setting below wave-base during deposition of the Strelley succession and the Gorge Creek Group is inferred from pillow lava, suspension-settled mudstone, turbiditic sandstone and coarse debris flow deposits. The host volcanics to VMS mineralisation are in the upper part of the Strelley succession, above the level of the Strelley Granite. The <1.5 km thick succession of tholeiitic volcanic rock above the Strelley Granite shows marked facies variations from north to south, particularly with respect to the felsic rocks (Fig 1). Andesitic volcanics are ubiquitous but are much more abundant in the north. Pillow lava, pillow breccia and hyaloclastite mainly occur towards the top of the andesitic unit, which is up to 1 km thick. Rhyodacite sills characterise the northern sector between Roadmaster and Sulphur Springs, whereas dacite sills have been emplaced near the top of the volcanic succession between Roadmaster and Breakers (Fig 1). A shallow intrusive origin for these amygdaloidal felsic volcanic rocks is indicated by contact relations with overlying strata and the scarcity of cogenetic in situ or reworked volcaniclastic deposits. The dacite sills are up to 250 m thick and are continuous along strike for up to 15 km. They host much of the VMS mineralisation and footwall alteration at Sulphur Springs (Fig 2) and Kangaroo Caves (Fig 3). Rhyolite domes and non-welded, pumice-rich rhyolitic volcaniclastic deposits increase in abundance south of Kangaroo Caves and are predominant south of Breakers. The volcanic facies at Man O’War represents a subaqueous rhyolite dome complex, which partly intrudes thick (cogenetic?) pumice-rich mass flow deposits (Y Goto and J McPhie, unpublished data, 1995). The lower part of the volcanic succession has been intruded by a comagmatic sill complex that has components that range from peridotite to rhyodacite in composition. Mafic and intermediate components of the sills can be rich in magnetite. The sill complex has been intruded by the magnetite-bearing Strelley Granite. Subvolcanic characteristics of the Strelley Granite include a 300 m thick granophyric upper marginal
Geology of Australian and Papua New Guinean Mineral Deposits
PANORAMA ZINC-COPPER DEPOSITS
FIG 2 - Geological plan and section of the Sulphur Springs deposit, section looking NW.
Geology of Australian and Papua New Guinean Mineral Deposits
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FIG 3 - Geological plan and section of the Kangaroo Caves deposit, section looking NW.
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Geology of Australian and Papua New Guinean Mineral Deposits
PANORAMA ZINC-COPPER DEPOSITS
zone, a core characterised by irregular porphyritic texture, and miarolitic cavities. Mineralisation in the Strelley Granite includes vein hosted copper-zinc-lead-silver-tin (eg Wheal of Fortune deposit) and gold, and gold associated with disseminated sulphides. A regionally extensive marker chert at the top of the Strelley succession comprises silicified sediment with locally abundant black kerogenous chert veining. The marker chert defines a transition from volcaniclastic to epiclastic sedimentation. The marked regional angular discordance in bedding of up to 15o between the Strelley succession and the onlapping Gorge Creek Group (Fig 3) implies an unconformable relationship. The Gorge Creek Group mainly consists of turbiditic sandstone, wacke and ferruginous mudstone with minor conglomerate lenses. These sediments are intruded by laterally extensive differentiated sills that include dolerite, pyroxenite and peridotite.
REGIONAL CONTROLS ON VMS MINERALISATION The close spatial relationship at all the main deposits between mineralisation and structures that may represent growth faults indicates a primary structural control of the distribution of mineralisation. Five of the main deposits (Roadmaster, Sulphur Springs, Kangaroo Caves, Breakers and Man O’War) are associated with faults that radiate from the Strelley Granite. These deposits are spaced at 5 to 7 km intervals (Fig 1), which may reflect the size of regional hydrothermal cells. At Sulphur Springs and Kangaroo Caves mineralisation is associated with laterally extensive dacite sills that intrude pillowed and hyaloclastic andesitic volcanic rocks, under a carapace of silicified volcaniclastic and epiclastic sediments, the marker chert (Figs 2 and 3). In contrast, the host rocks to most of the mineralisation at Man O’War, Breakers, Anomaly 45, Jamesons and Bernts are rhyolitic volcanic rocks including locally abundant pumice-rich volcaniclastic rocks. Mineralisation at the main deposits is associated with feldspar destructive chlorite-quartz alteration. Distal from mineralisation, sericite-quartz±carbonate and alkali feldsparcarbonate±pyrite (spilitic) alteration facies prevail. The regional alteration facies overprint the volcanic minerals and texture (C Brauhart, unpublished data, 1996).
ORE DEPOSIT FEATURES The Panorama VMS deposits are of the zinc-copper type (Large, 1992) and sulphide mineralisation is typically zoned downwards from zinc (±lead)-rich to copper-rich. They are low in gold (generally containing ≤0.2 g/t), silver (generally <40 ppm) and lead (generally <1 %), compared with most other Australian VMS deposits (Large, 1992). The gossans are variably siliceous and ferruginous, and contain secondary copper, zinc and lead minerals. They are typically highly anomalous in lead and silver, but strongly depleted in zinc. Values for gold, tin, bismuth, antimony, arsenic, mercury and barium may also be strongly anomalous. At Sulphur Springs (Fig 2) and Kangaroo Caves (Fig 3) a laterally extensive dacite sill has been intruded near the top of a sequence of pillowed and hyaloclastic andesite. The 200 m thick sill is variably amygdaloidal, generally aphyric and commonly perlitic textured. The overlying marker chert, a composite unit to 100 m thick at Sulphur Springs, has silicified
Geology of Australian and Papua New Guinean Mineral Deposits
volcaniclastic (including andesitic shard-rich sandstone) and epiclastic (including black mudstone, sandstone and breccia) components that are disrupted by chert veining. There are local stratigraphic variations above the marker chert. At Sulphur Springs (Fig 2) a sequence of interbedded polymict megabreccia, sandstone and black mudstone has been intruded by quartzphyric, spherulitic and perlitic textured rhyodacite. At Kangaroo Caves (Fig 3) feldsparphyric calcalkaline rhyodacite breccia, that is interpreted to be an extrusive dome (Y Goto and J McPhie, unpublished data, 1995) locally overlies the marker chert. The hanging wall sediment of the Gorge Creek Group has been intruded by differentiated sills that range from dolerite to peridotite. Zinc-copper mineralisation at Sulphur Springs occurs immediately beneath the marker chert, within the marker chert, and rarely in the hanging wall rhyodacite. Mineralisation within the marker chert is generally zinc-rich, whereas the much larger accumulation beneath the marker chert has a massive zinc-rich cap, a massive copper-rich middle and a stringer style copper-bearing zone at the base. The zinc and copper orebodies may be separated by up to 10 m of subeconomic mineralisation. Much of the high grade zinc mineralisation at Kangaroo Caves is hosted by the marker chert in a shallowly NE-plunging shoot that has been intersected by drilling up to 1.5 km down plunge from the gossan outcrop (Fig 3). Ore grade copper-zinc mineralisation is not as well developed in strongly altered volcanic rock beneath the marker chert, compared with Sulphur Springs. The sulphide assemblage at Sulphur Springs and Kangaroo Caves typically comprises pyrite, low-iron sphalerite, chalcopyrite and galena, with minor arsenopyrite and tennantite-tetrahedrite. Quartz, chlorite, sericite, dolomite and barite are the main gangue minerals, although barite is only locally abundant. The very low strain and metamorphic grade have allowed excellent preservation of the sulphide textures, from which textural zoning has been recognised (Vearncombe et al, 1995). Dendritic, colloform and botryoidal textures that characterise the upper parts of the mineralisation may have formed by open space precipitation of sulphides. Some delicate sulphide textures, such as spherical pellets and globular and stromatolitic types, have been interpreted by Vearncombe et al (1995) as analogues of sulphide chimneys at present-day submarine hydrothermal vents. Barite is also preserved in the upper parts, as tabular blades, rosettes and colloform masses. The lower parts of the massive to semi-massive sulphides are characterised by massive, granular, honeycomb and filigree sulphide textures, as well as veining. Stringer zones comprise volcanic rocks that are strongly altered to chlorite, sericite, carbonate, pyrite and quartz, with veins that contain pyrite, chalcopyrite and some sphalerite. Textural evidence for a mainly replacement origin for the mineralisation at Sulphur Springs and Kangaroo Caves includes the preservation of volcanic textures such as perlite and shards in massive to semi-massive sulphides. Lithogeochemical evidence includes the preservation of immobile element ratios in texturally similar volcanic rocks. For example, the titanium:zirconium ratio is in the range 14 to 18 and the scandium:zirconium ratio is in the range 0.05 to 0.06 for dacite. These ratios are virtually independent of the intensity of alteration.
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Footwall alteration zoning extending away from mineralisation typically comprises chlorite-pyrite, sericitepyrite, sericite-carbonate and albite-carbonate, all with quartz. Hanging wall alteration related to VMS mineralisation does not occur in the Gorge Creek Group sedimentary rocks.
5.
Hydrothermal fluids may have penetrated the marker chert and debouched onto the sea floor, forming analogues of sulphide chimneys at present-day submarine hydrothermal vents.
ACKNOWLEDGEMENTS ORE GENESIS The 3240 Myr old Panorama zinc-copper deposits are of VMS style, as shown by: 1.
The close association with felsic volcanic rock, generally at the top of the volcanic-dominated Strelley succession.
2.
A submarine environment, although the depth of sea water is not well constrained.
3.
The association with hydrothermal alteration.
4.
Vertical metal zoning from a zinc (± lead)-rich top to a copper-rich base.
5.
Sulphide textures that indicate replacement of volcanic rock, open space precipitation, and perhaps exhalation onto the sea floor.
regionally
zoned
intense
The Sulphur Springs and Kangaroo Caves deposits are interpreted to have formed as follows: 1.
Intrusion of dacite near the top of a pile of pillowed and hyaloclastic andesite overlain by volcaniclastic and epiclastic sediment.
2.
Silicification of the mainly fine grained sediment immediately overlying the volcanic sequence, with concomitant chert veining, forming an impermeable carapace, the marker chert.
3.
Replacement of volcanic and volcaniclastic rocks beneath, and possibly within the marker chert, by sulphides, silicates and carbonates.
4.
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Open space precipitation of sulphides, mainly within the marker chert.
Sipa Resources Limited and Outokumpu Mining Australia Pty Limited are acknowledged for permission to publish. Many employees of Sipa Resources, Ashling Resources NL and Outokumpu have contributed to the project since 1989. J McPhie and Y Goto of CODES at the University of Tasmania have considerably advanced the understanding of the volcanic facies architecture of the Strelley succession. A Black, C Brauhart, R Buick, M Doepel and D Mueller provided helpful comments on the manuscript.
REFERENCES Hickman, A H, 1983. Geology of the Pilbara Block and its environs, Geological Survey of Western Australia Bulletin 127. Hickman, A H and Lipple, S L, 1978. Marble Bar, Western Australia 1:250 000 geological series, Geological Survey of Western Australia, Explanatory Notes, SF 50–8. Large, R R, 1992. Australian volcanic-hosted massive sulphide deposits: features, styles and genetic models, Economic Geology, 87:471–510. Morant, P, 1995. The Panorama Zn-Cu VMS deposits, Western Australia, Australian Institute of Geoscientists Bulletin, 16:75–84. Thorpe, R I, Hickman, A H, Davis, D W, Mortensen, J K and Trendall, A F, 1990. Application of recent zircon U-Pb geochronology in the Marble Bar region, Pilbara Craton, to modelling Archean lead evolution, in Third International Archaean Symposium Extended Abstracts Volume, pp 11–13 (Geoconferences: Perth). Vearncombe, S, Barley, M E, Groves, D I, McNaughton, N J, Mikucki, E J and Vearncombe, J R, 1995. 3.26 Ga black smoker-type mineralisation in the Strelley Belt, Pilbara Craton, Western Australia, Journal of the Geological Society of London, 152:587–590.
Geology of Australian and Papua New Guinean Mineral Deposits
McQuitty, B M and Pascoe, D J, 1998. Magellan lead deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 293–296 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Magellan lead deposit 1
by B M McQuitty and D J Pascoe
2
INTRODUCTION
EXPLORATION HISTORY
The deposit is 30 km NW of Wiluna, WA, at lat 27o14′S, long 119o57′E (Fig 1) on the Glengarry (SG 50–12) 1:250 000 and the Merewether (2844) 1:100 000 scale map sheet areas, and in the SE corner of the Proterozoic Yerrida Basin (Pirajno and Preston, this publication). It is a subhorizontal sheet of lead oxide mineralisation within 45 m of the surface, with favourable metallurgical qualities.
The deposit was discovered by RGC geologists in 1991 by following up regionally anomalous lead values in rock chip samples by rotary air blast drilling. Holes drilled at the northern end of lines along the southwestern edge of a low hill obtained values between 0.1 and 1.2% lead. Cerussite in the matrix of outcripping sandstone was then identified. A reverse circulation drilling program in late 1991 located a large, near surface, low grade lead resource with indications of high grade pods. Subsequent drilling defined an area measuring 1 km north by 0.5 km east which hosts the bulk of the high grade resource. The present drill hole spacing does not preclude the possibility of other substantial high grade pods of mineralisation. Following this discovery, drilling of the equivalent formation to the host rocks at Magellan in the Earaheedy Basin, 120 km to the NE, located the Iroquois prospect (Fig 1). This is a shallow, tabular, lead oxide mineral occurrence similar to Magellan, but lower in grade. Drilling down dip of Iroquois beneath cover sequences intersected primary galena, sphalerite and pyrite as veins and disseminations in a dolomite host rock (R Feldtmann, unpublished data, 1994). This style of mineralisation has now been detected over a 30 km strike length of the southern margin of the Earaheedy Basin.
REGIONAL GEOLOGY
FIG 1 - Location and regional geology, Yerrida and Earaheedy basins.
At a 0.1% lead cutoff, the deposit has an Inferred Resource of 210 Mt at 1.8% lead (D J Pascoe and W B Edgar, unpublished data, 1995). Data from closer spaced drilling within part of this body has allowed the estimation at a 3% lead cutoff of an Indicated Resource of 4.53 Mt at 7.8% lead and an Inferred Resource of 3.36 Mt at 4.5% lead (Fig 2). 1.
Supervising Geologist, RGC Exploration Pty Ltd, PO Box 322, Victoria Park WA 6979.
2.
Principal Geologist, RGC Exploration Pty Ltd, PO Box 322, Victoria Park WA 6979.
Geology of Australian and Papua New Guinean Mineral Deposits
The Magellan deposit is in the SE corner of the Yerrida Basin, one of several Proterozoic basins between the Pilbara and Yilgarn cratons (Fig 1), which is discussed in more detail by Pirajno and Preston (this publication). The generalised stratigraphic column for the Magellan region is shown in Fig 3. Archaean granite and greenstone form the basement. They are unconformably overlain by the Finlayson Member (about 600 m thick) of the Juderina Formation which comprises a lower sequence of mature quartz sandstone and an upper sequence of siltstone and extensively silicified stromatolitic dolomite. The upper members of the Juderina Formation are missing here and the Finlayson Member is unconformably overlain by black carbonaceous shale of the Maraloou Formation (250 m thick). Dolerite sills of the Killara Formation (>700 m thick) intrude the Maraloou Formation to the north of Magellan. The Magellan deposit is hosted by carbonate and sandstone horizons which unconformably overlie Yerrida Group sediments and have been recognised as correlatives of the Yelma Formation of the Earaheedy Group (Le Blanc Smith et al, 1995). The Yelma Formation at Magellan is preserved as an outlier, forming a WNW-trending mesa of approximately 5 by 2.5 km, unconformably overlying Maraloou Formation (Fig 2). Other outliers of Yelma Formation occur in the region, unconformably overlying either Finlayson Member or Maraloou Formation, and drill testing of these over an area of 200 km2 has indicated that they have a consistently elevated lead content.
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FIG 2 - Lead accumulation, Magellan deposit.
ORE DEPOSIT FEATURES LITHOLOGY At Magellan the Yelma Formation consists of a basal sandstone unit, which fines upward into a siltstone and claystone, which are overlain by a quartz-clay breccia. Unaltered stromatolitic dolomite, locally with vugs filled with crystalline quartz, occurs at the same stratigraphic level as the quartz-clay breccia unit. When fresh, the basal sandstone is a grey-green, medium- to coarse-grained carbonaceous quartz wacke, from 10 to 15 m thick and containing minor interbeds of conglomerate and siltstone. Where oxidised, the sandstone has a tan colour and contains elevated amounts of lead. The sandstone is transitional to a tan siltstone in the upper 2 to 4 m. The quartz-clay breccia overlying the basal sandstonesiltstone unit is up to 35 m thick. The breccia comprises fragments of silicified stromatolitic carbonate, chert, siltstone, vuggy euhedral crustiform quartz and colloform banded quartz. Breccia fragments range from 10 mm to 10 cm in diameter and are contained within a clay matrix. Intense Tertiary weathering has silicified the upper 20 m of the quartz-clay breccia unit to form a silcrete layer (Fig 4).
STRUCTURE Major NNW- and east-trending fault systems transect the Yerrida Basin and the underlying Archaean basement beneath the Magellan deposit. Bedding dips in all formations are very low and rarely exceed 5o. The Yelma Formation outlier at Magellan occurs as a gentle syncline with a NW-trending axis.
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FIG 3 - Generalised stratigraphy, SE Yerrida Basin.
Geology of Australian and Papua New Guinean Mineral Deposits
MAGELLAN LEAD DEPOSIT
FIG 4 - Cross section on 3350 N of the Magellan deposit, looking north.
MINERALISATION Lead mineralisation at Magellan occurs as a subhorizontal sheet, contained entirely within the Yelma Formation. The lead minerals, in order of decreasing abundance, are cerussite, anglesite, pyromorphite and coronadite (PbMn8O16). Plattnerite (PbO2) has also been noted (Pirajno and Preston, this publication). Cerussite is by far the dominant mineral, however the deposit displays some vertical zoning, with an anglesitedominant zone above its thickest parts (Fig 4). The majority of the lead oxide mineralisation is hosted by the quartz-clay breccia unit. The sandstones and claystones beneath the quartzclay breccia also contain significant lead oxide mineralisation but only where weathering has occurred, indicating the strong involvement of supergene processes in redistribution of the lead mineralisation. The upper, silcrete portion of the quartzclay breccia unit contains only low levels of lead. Zinc grades are very low. The deposit has a Zn:Zn+Pb ratio of the order of 1:1000. Zinc grades are strongly depleted in the upper portion of the deposit and tend to increase gradually towards the base of oxidation, eg from 13 ppm Zn to 309 ppm zinc in DDH22 (Fig 4). The Maraloou Formation carbonaceous shales contain up to 0.5% zinc immediately beneath the highest grade parts of the Magellan deposit.
DEPOSIT MODEL The host breccia is interpreted to be the residue of a primary carbonate-hosted lead sulphide deposit after solution collapse, which occurred during and after formation. However no unequivocal boxwork textures after sulphide minerals have been recognised. The lead oxide mineralisation at Magellan and Iroquois, and associated lead-zinc sulphide mineralisation on the southern margin of the Earaheedy Basin, constitute evidence for a basinwide mineralising system hosted by Yelma Formation carbonates. Observations from over 50 drill holes completed in
Geology of Australian and Papua New Guinean Mineral Deposits
the region down dip from Iroquois suggest that weathering of the carbonates results in a progressive volume reduction and concentration of sulphide and insoluble silica residue. As weathering progresses, the sulphides are oxidised, leaving silica fragments in a clay matrix enriched in base metal oxides. This process results in an approximate tenfold increase in base metal grades. The Magellan deposit is interpreted to relate to a primary lead-zinc sulphide deposit of moderate base metal content that was enriched in lead as a result of volume reduction by weathering processes as described above. During intense Tertiary weathering the downward mobilisation of lead and zinc occurred, with zinc being almost totally leached from the host rocks. Elevated levels of zinc in the Maraloou Formation immediately below the deposit may be due to the reduction of zinc in solution by carbon in the black shales. The restriction of elevated levels of lead to the oxidised portions of the basal sandstone unit of the Yelma Formation is further evidence of the role of weathering in the redistribution of base metals. The widespread lead mineralisation within the Yelma Formation suggests that sheet-like hydrothermal fluid flow occurred along a permeable horizon. The driving mechanism for such flow may have been a distal orogenic event, eg the Mid Proterozoic Capricorn Orogen. Primary fluid inclusion homogenisation temperatures from samples of Magellan quartz fall mainly in the range 180 to 220oC, with some as high as 260oC. Salinities are in the range 9 to 15 eq wt % NaCl (S W Halley, unpublished data, 1992; P K Seccombe and J Lu, unpublished data, 1993). The salinities are within the range of those expected for a typical basinal brine. Two samples collected by the Geological Survey of Western Australia yielded Pb-Pb dates of 1650 Myr (Le Blanc Smith et al, 1995). The lead isotopic ratios indicate that the lead was sourced from Archaean basement, or from Proterozoic sediment derived from Archaean basement (G A Carr and J A Dean, unpublished data, 1992).
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The Magellan lead deposit formed from a primary carbonatehosted lead-zinc sulphide deposit of Mississippi Valley (MVT) affiliation that was oxidised and upgraded by intense Tertiary weathering. The widespread occurrence of lead-zinc mineralisation within carbonates of the Yelma Formation represents a significant, new Mid Proterozoic MVT province.
who contributed to the advancement of geological understanding of the Magellan lead deposit, and thank F Pirajno of the Geological Survey of Western Australia for reviewing the manuscript.
ACKNOWLEDGEMENTS
Le Blanc Smith, G, Pirajno, F, Nelson, D and Grey, K, 1995. Base metal deposits in the Early Proterozoic Glengarry terrane, Western Australia, Western Australia Geological Survey Annual Review 1993–94, pp 59–62.
The authors acknowledge the permission of RGC Exploration Pty Ltd to publish this paper. They also acknowledge all those
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REFERENCES
Geology of Australian and Papua New Guinean Mineral Deposits
Gole, M J, Andrews, D L, Drew, G J and Woodhouse, M, 1998. Honeymoon Well nickel deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 297–306 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Honeymoon Well nickel deposits 1
2
3
by M J Gole , D L Andrews , G J Drew and M Woodhouse
4
INTRODUCTION The deposits are approximately 45 km south of Wiluna, WA, at lat 26o40′S, long 120o25′E on the Wiluna (SG 51–9) 1:250 000 scale and Wiluna (2944) 1:100 000 scale map sheets (Fig 1). They are the most northerly of numerous known nickel deposits within the 2700 Myr old Agnew–Wiluna greenstone belt (Marston, 1984). Both disseminated and massive nickel sulphide deposits are present within the deformed and metamorphosed Honeymoon Well ultramafic complex and are hosted by distinctly different komatiitic rocks reflecting formation in markedly different volcanic settings (Gole et al, 1996). The deposits are held by Outkumpu Mining Australia Pty Limited. At December 1995 an Indicated Resource of 155.5 Mt at 0.71% nickel in four main deposits at 0.4% nickel cutoff to a depth of 300 m was reported. This includes a massive sulphide resource of 2.5 Mt at 3.4% nickel. The project is the subject of a mining feasibility study.
EXPLORATION HISTORY The first nickel exploration in the area was by Delhi Oil Pty Ltd and Vam Ltd in 1970. CRA Exploration negotiated a joint venture with these parties and in the period 1972–75 employed widely-spaced vertical percussion drilling to provide geochemical and geological data for areas of ultramafic rock that had been outlined by ground magnetic data. Diamond drilling, targeting lateritic nickel-copper geochemical anomalies along parts of the eastern and western contacts, intersected disseminated sulphide bodies with up to 1% nickel. These bodies are parts of the now-defined Hannibals, Harakka and Corella deposits (Fig 1). Sporadic percussion and diamond drilling continued to 1985. A re-interpretation of Honeymoon Well geology (Gole and Hill, 1988), some encouraging drill results including 130 m at 1% nickel, and a joint venture agreement with Outokumpu Exploration Australia Pty Limited provided impetus for further exploration. Since 1989 intensive exploration has discovered two additional deposits and increased resources in the previously known deposits. As a result the overall resource has increased from 10 Mt at 1% Ni in 1989 to the current 155.5 Mt at 0.71% nickel. 1.
Consultant, Martin Gole and Associates, 8 Landor Road, Gooseberry Hill WA 6076.
2.
Principal Geologist, Rio Tinto Exploration Pty Limited, 37 Belmont Avenue, Belmont WA 6014.
3.
Chief Geologist, Rio Tinto Exploration Pty Limited, 2 Kilroe Street, Milton Qld 4064.
4.
Supervising Geologist, Outokumpu Mining Australia Pty Ltd, 141 Burswood Road, Victoria Park WA 6100.
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FIG 1 - Location map and geological plan of Honeymoon Well at 100 m below surface.
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This paper is based on information from over 830 diamond drill holes and a similar number of percussion holes as well as detailed aeromagnetic data. Such data were unavailable for previous studies of the Honeymoon Well deposits (Donaldson and Bromley, 1981; Gole and Hill, 1990).
REGIONAL SETTING At Honeymoon Well the Agnew–Wiluna greenstone belt (Fig 1) is 6 to 7 km wide and composed of a west-younging sequence of a lower basalt and gabbro unit, including a laterally persistent basaltic komatiite flow, felsic to intermediate volcanic and volcanoclastic rocks, a heterogeneous ultramafic (komatiite) sequence and a western felsic and basalt sequence (Liu, Hickman and Langford, 1995). Local east-younging is due to minor folds associated with fault zones. The ultramafic sequence, from 1.5 to 3.0 km wide, consists of a diverse suite of metamorphosed komatiites including spinifex-textured rocks, olivine orthocumulates (oOC), mesocumulates (oMC) and adcumulates (oAC). Minor augite and augite-plagioclase cumulates define fractionated cyclic units within the sequence. The regional distribution of rock types, particularly within the Honeymoon Well ultramafic complex, shows that the sucession, especially the komatiites, has been repeated by faulting. Repetition is interpreted to have occurred within a D1 thrust duplex. Folding and deformation during D2 resulted in the formation of NNW-trending shear zones and strike-slip faults that further duplicated the sequence. Much of the ultramafic sequence is thus interpreted to be allochthonous. The stratigraphic repetition accounts, in part, for the anomalous width of the ultramafic sequence at Honeymoon Well compared to the remainder of the belt. The main metamorphic alteration appears to be associated with D2 shear and strike-slip fault development (Eisenlohr, 1992). Metabasalt assemblages indicate greenschist facies metamorphic grade (Donaldson and Bromley, 1981). Ultramafic rocks have been altered to serpentine-dominant and local talc-carbonate assemblages, with complete alteration of igneous silicate minerals and partial to complete reconstitution of igneous chromite and sulphides.
HONEYMOON WELL GEOLOGY COVER The ultramafic bedrock is covered by cemented sand and grit from 1 to 25 m thick, transported clay and minor basal gravel to 40 m thick, and a weathered residual regolith 30 to 60 m thick (Mitchell, 1994). These hamper attempts to understand bedrock geology, which is interpreted from drill hole and detailed aeromagmetic data.
ULTRAMAFIC ROCK TEXTURES Drill hole core and cuttings have been logged for igneous and metamorphic textures, rock and vein mineralogy, and assayed for a range of sulphide- and lithology-related elements. Igneous rock names are used where appropriate in this paper despite complete metamorphic reconstitution of the igneous silicate minerals and partial to complete reconstitution of oxides and sulphides.
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Igneous textures are widely preserved within bedrock, with foliated and shear fabrics restricted to relatively discrete zones. Olivine adcumulates display polygonal-textured olivine pseudomorphs and have negligible igneous porosity. Olivine grain sizes range up to 2.5 cm but are mostly in the range 4 to 10 mm. In serpentine-rich rocks stichtite after chromite forms lobate, fibrous aggregates at former olivine triple-point junctions. Olivine mesocumulates consist of olivine pseudomorphs that are mostly 3 to 8 mm across with highly variable stichtite aggregate size and content (0.5 to 5%). Olivine orthocumulates have a wide variation in igneous porosity and texture. Low igneous porosity orthocumulates generally have coarser, more even-grained olivine pseudomorphs whereas high igneous porosity rocks show a wide range in olivine grain size, sometimes within the same rock, and with grain shapes ranging from euhedral to hopper to harrisitic. The high porosity rocks are mostly associated with spinifex-textured rocks. In a few localities 20 to 40 m thick sequences of thin spinifextextured flows are well preserved. Mostly, however, deformation has destroyed spinifex textures except in scattered and relatively small low strain pockets. Some very distinctive, recrystallised spinifex-textured rocks and associated oOC show evidence of partial melting and high temperature metamorphism (Gole, Barnes and Hill, 1990). These rocks form part of a marker horizon that is critical in the stratigraphic reconstruction of the complex. Many of the talc-carbonate rocks are foliated, reflecting penecontemporaneous alteration and deformation within fault or shear zones permeable to carbon dioxide-bearing fluids. Lizardite [Mg3Si2O5(OH)4] +graphite schists reflect alteration and deformation within water-rich fluid regimes.
DISTRIBUTION OF ULTRAMAFIC ROCKS Coarse-grained oAC forms the core and makes up the bulk of the komatiite complex (Fig 1), with oMC, oOC, spinifextextured rocks and minor, medium grained oAC occurring around the margins and as narrow horizons within the complex. Dips are highly variable, from 30o to vertical. No traceable stratigraphic layering has been recognised within the main mass of oAC, but it has been recognised in various states of preservation within the marginal ultramafic sequences. The Harrier and Corella sulphide deposits are interpreted to be hosted by stratigraphically equivalent sequences and are linked by a discontinuous horizon containing recrystallised spinifex-textured flows, intermediate to mafic volcanic rocks and minor sedimentary rock units. Recrystallised spinifex rocks also occur within the Hannibals deposit located along the western contact of the ultramafic complex. The two main ultramafic horizons in the south of the area are thought to be stratigraphically equivalent units that have been duplicated by a strike-slip fault along the east side of the Harrier deposit which extends northwards along the eastern contact of the komatiite complex (Fig 1). The host sequences of the Hannibals–Harakka and Wedgetail sulphide deposits occur as thin, fault-bounded units along parts of the western ultramafic contact that are interpreted to be thin D1 thrust slices.
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Olivine and minor pyroxene spinifex-textured flows on the westernmost ultramafic contact at Harrier define the top of the main oAC-bearing unit. Spinifex flows, augite and augiteplagioclase cumulates and felsic rocks that form a structurally disrupted block within the northern part of the complex are also probably remnants of this upper contact. This top unit is, however, not preserved elsewhere within the ultramafic complex. The thickness of the oAC in the central and northern part of the area is thought to be a result of repetition by thrust faults of an originally extensive and thick oAC body. The abrupt northern termination of the ultramafic complex, where the width thins from 3 km to nil over 1.5 km of strike, occurs at the intersection of these thrust faults with an interpreted D2 strikeslip fault that defines much of the eastern and northeastern ultramafic contact.
OTHER ROCKS Rocks in contact with, and in places included within the ultramafic complex consist of a wide range of volcanic and volcaniclastic rocks that include tholeiitic basalt and related gabbro, andesite, dacite and minor rhyolite (Harrison, 1995; Russell, 1995). Minor pyritic black shale, chert and possible conglomerate are also present, as are rare, discontinuous, thin layers of basaltic and peridotitic komatiite.
relatively large (to 5 mm diameter) due to the greater space between olivine grains. Metamorphic reconstitution of the rocks has, in many cases, greatly modified the shape and the mineralogy of the aggregates. In the disseminated sulphide deposits pentlandite and heazlewoodite [NiS2] are the dominant primary sulphides. In parts of the deposits minor to trace amounts of pyrrhotite and chalcopyrite are also present but these minerals are absent within large blocks of the mineralisation. A very high proportion of chalcopyrite occurs in veins along with pyroaurite, magnetite, brucite, carbonate and in places iron sulphides. Massive sulphide is considerably more iron-rich than disseminated sulphide and comprises pyrrhotite, pentlandite, pyrite and minor chalcopyrite. The depth to the oxide–supergene boundary usually ranges from 60 to 100 m. The width of the supergene zone is variable, being mostly 3 to 25 m. The transition zone is generally 20 to 30 m thick. Supergene sulphide assemblages of violarite, pyrite and millerite occur in the upper supergene zone as well as in patches deep within the deposits.
DEPOSIT GEOLOGY Nickel sulphides at Honeymoon Well occur in two deposit types: 1.
disseminated sulphides with trace to 5 modal per cent in olivine-rich cumulates (osMC, osAC, osOC) in the Hannibals, Harrier, Corella, and Harakka deposits, and
2.
sulphide-rich rocks comprising massive sulphide, sulphide breccia and olivine-sulphide cumulates (osC, osOC) hosted by spinifex-textured flows in the Wedgetail deposit.
MINERALOGY OF ULTRAMAFIC ROCKS Olivine adcumulates and oMC are dominated by lizardite assemblages and do not retain igneous minerals except for minor to trace amounts of partly altered chromite (Donaldson and Bromley, 1981). Antigorite-carbonate and particularly talc-carbonate assemblages, although they may dominate in large volumes of rock, are mostly associated with veins, faults and shear zones. Olivine orthocumulate and most spinifex-textured rocks consist of variable proportions of serpentine (mostly antigorite), tremolite, chlorite, carbonate, magnetite and relict igneous chromite. In places oOC contains minor igneous kaersutite (titaniferous hornblende) and rarely clinopyroxene. Recrystallised spinifex-textured rocks generally retain coarsegrained metamorphic pyroxenes and only rarely other minerals from their high temperature metamorphic assemblage (Gole, Barnes and Hill, 1990). In some areas, including much of the Wedgetail deposit and parts of the Harrier deposit, late arsenic-gold-carbonate bearing fluids have further altered the rocks. The lizardite-dominant rocks in particular have a high vein density with the principal vein minerals being pyroaurite [Mg6Fe2(CO3)(OH)16.4H2O], iowaite [Mg4FeOC1(OH) 8.24H2O], brucite, carbonate, magnetite and serpentine. In antigorite and particularly talc-carbonate rocks the vein mineralogy is less complex being dominated by carbonate and magnetite.
NICKEL SULPHIDES In olivine sulphide mesocumulate and adcumulate (osMC, osAC), the sulphide aggregates are moulded around pseudomorphs of closely packed olivine and hence have a general lobate shape. In olivine sulphide orthocumulates (osOC) the aggregates tend to be more blebby and may be
Geology of Australian and Papua New Guinean Mineral Deposits
Harakka is relatively small and is not described in detail.
DISSEMINATED SULPHIDE DEPOSITS Hannibals deposit The deposit is along the western contact of the ultramafic complex within a fault-bounded, west-younging sequence of oMC and minor oAC, oOC and spinifex-textured rocks that can be traced for about 2.4 km along strike (Fig 1). The deposit, which contains an Indicated and Inferred Resource of 36.1 Mt at 0.70% nickel at a 0.4% nickel cutoff, to 300 m depth, is divided into western and eastern overlapping fault blocks which are stratigraphically equivalent and have been repeated along a central fault zone which is probably a reactivated thrust (Figs 2 and 3 ). The contact against the central oAC is along a major, discrete, relatively planar fault with mylonitic fabric preserved in places. This fault transgresses the mineralised sequence at a low angle to the stratigraphy and is interpreted to be a well preserved D1 thrust fault. To the west the mineralised sequence is in fault contact with tholeiitic basalt flows. The ultramafic rocks along this contact are mostly schists derived from spinifex-textured flows. In the eastern block the stratigraphy of the sulphide-bearing sequence is relatively intact and the igneous layering is subvertical. A high grade core with 1% nickel within the mineralisation plunges in a general SE direction. Thin oOC and spinifex-textured rock units occur lateral to the mineralised rocks suggesting that the sulphides occur in a 30 to 80 m deep, 150 m wide channel.
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FIG 3 - Cross section 13 800 N, Hannibals deposit, looking NW.
oMC. Many of the sharp mineralisation contacts are on minor faults. Down dip and along strike from the central portions of the mineralisation the sulphide layers thin and generally become lower grade and interdigitate with barren ultramafic rocks. The mineralisation can be divided into two types:
FIG 2 - Geological plan of Hannibals deposit at 100 m below surface.
In the western fault block the lithological distribution appears to be more complicated due to possible layer-parallel faulting. The block extends southward for several hundred metres as a 20 to 60 m wide sequence between the central fault and metabasalt to the west. A high grade nickel core appears to be subhorizontal within the mineralised sequence. The mineralised sequence bottoms at depth against the folded, western metabasalt-ultramafic fault contact and is truncated to the east by a fault contact with the central oAC.
Mineralisation Mineralisation consists of disseminated sulphides containing trace to about 3 modal per cent sulphide with nickel grades to 2.4%. The sulphide-bearing units are igneous horizons with gradational to sharp contacts with barren or weakly mineralised
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1.
High copper mineralisation, containing 300–2000 ppm copper, having relatively low nickel:sulphur ratios (range 0.5–1.3) and with the sulphide mineralogy dominated by pentlandite with trace chalcopyrite and in places pyrrhotite.
2.
Low copper mineralisation containing <40 ppm copper (most samples below the detection limit of 5 ppm), having high nickel:sulphur ratios (range 1.2–2.5), in which the sulphides are mostly nickel-rich pentlandite and heazlewoodite. This type forms 60% of the deposit.
The high copper ore is commonly hosted in black lizardite with a significant proportion of antigorite-carbonate assemblages with veins containing a relatively high proportion of carbonate. The low copper ore mostly occurs in dark to light green lizardite, has a lower proportion of antigorite and the vein mineralogy is dominated by brucite and pyroaurite or iowaite. Based on copper content, the contact between ore types is relatively sharp (2 to 6 m) whereas other geochemical and mineralogical differences are gradational over 5 to 50 m.
Harrier deposit The deposit, in the south of the area along the eastern contact of the central oAC-bearing ultramafic unit (Fig 1), contains an
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Indicated and Inferred Resource of 43.0 Mt at 0.64% nickel at a 0.4% nickel cutoff, to 300 m depth. The nickel sulphides are hosted by osMC with minor osC and osOC. The deposit extends continuously along strike for at least 1.7 km and varies in width from 30 to 140 m (Fig 4).
FIG 5 - Cross section 10 600 N, Harrier deposit, looking NW.
into barren oMC and then, in places, into oAC. North of 11 000 N there is a horizon within the dominantly olivine cumulaterich ultramafic sequence, composed of spinifex-textured flows, andesitic volcanic rocks and minor pyritic and cherty sedimentary rocks (Fig 4). This unit is up to 100 m thick and dips at 60–75o to the east although younging directions derived from the flows are westerly. North of 11 500 N the oAC on the western side of this horizon thickens into the oAC body that forms the core of the Honeymoon Well complex. This internal horizon extends, in various states of preservation, to the Corella deposit 6 km to the north (Fig 1). The nickel sulphide mineralisation north of 10 700 N is constrained to one osMC unit with minor layers of osOC in the north. To the south other sulphide-bearing units are also present (Fig 5) that appear to be igneous layers rather than structural duplications of the extensive osMC horizon.
FIG 4 - Geological plan of Harrier deposit at 100 m below surface.
To the east, the ultramafic sequence is in fault contact with metamorphosed andesitic to dacitic lava and tuff, minor sedimentary rocks and a thin, discontinuous spinifex-textured komatiite (Russell, 1995). In the south this contact dips at 40o east and is subparallel to both the ultramafic and intermediate volcanic sequences (Fig 5) whereas to the north it is subvertical and truncates the 60 to 80o east-dipping ultramafic sequence. Although part of the ultramafic sequence has been removed by faulting, the succession to the west of the fault appears to be relatively intact. The nickel sulphide-bearing sequence grades
Geology of Australian and Papua New Guinean Mineral Deposits
The mineralogy of the host sequence is dominated by lizardite-rich assemblages similar to those at Hannibals. However a significant proportion of the host sequence in the central and narrowest part of the deposit has been altered by carbonate-bearing fluids. This has produced a zoned pattern with lizardite assemblages giving way to antigorite-carbonate assemblages which, in places, envelop talc-carbonate rocks.
Mineralisation The sulphide content typically varies between 1 to 5 modal per cent with grades to 2.9% nickel being highest in osOC. The mineralisation can be divided into high copper and low copper zones generally similar to those within Hannibals deposit. Low copper mineralisation occurs as discontinuous, patchy areas generally on the margins of the high copper zones. Within the
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primary zone pentlandite+trace chalcopyrite and pentlanditeheazlewoodite are the dominant sulphide assemblages in lizardite and most antigorite-rich rocks. In talc and some antigorite-rich rocks the sulphides are pentlandite, millerite, trace chalcopyrite, pyrrhotite and patchy gersdorffite, niccolite and maucherite (Ni11As8).
0.62% nickel at a 0.4% nickel cutoff, to 300 m depth, and extends for 1400 m with widths of 20 to 100 m (Fig 6). The deposit is strongly deformed and sheared with many units, including mineralised rocks, occurring as fault-bounded boudins on various scales. Units interdigitate as a result of both original stratigraphic relationships and structural dislocation. Because of the shearing the vein density and thickness are significantly higher at Corella than at the other deposits. The Corella sequence contains a wide variety of rock types including oAC, osAC, osC, oMC, osMC, high porosity osOC, oOC and minor, partially remelted and recrystallised spinifextextured rocks, eg in drill hole 74HWD003 (Gole, Barnes and Hill, 1990). Minor andesite and basalt are also present and are generally spatially associated with the spinifex textured rocks. The spinifex textured rocks show both westerly and easterly younging directions. The latter are thought to be due to local folds associated with shear zones. The Corella ultramafic sequence is subvertical over the extent of the deposit although in the south the deeper parts of the sequence dip to the west. To the east the ultramafic rocks are in fault contact with basalt and minor gabbro in the northern part and with andesite to the south (Fig 6). In plan this fault is subparallel to the mineralised sequence except in the south where the fault cuts across to the west and truncates the sequence. In cross section however, the fault has variable attitudes. In the north it dips to the west and truncates the mineralised sequence at depth and in the south it dips to the east away from the mineralisation (Fig 7).
FIG 6 - Geological plan of Corella deposit at 100 m below surface. FIG 7 - Cross section 16 300 N, Corella deposit, looking NW.
Corella deposit The deposit is along the NE contact of the ultramafic complex. It contains an Indicated and Inferred Resource of 53.5 Mt at
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Due to this fault the lowermost part of the ultramafic sequence is missing. Within the preserved part of the sequence the interpreted succession is, from east to west, barren oAC, the
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mineralised sequence of osC-osAC, then osMC, with overlying osOC and oOC. This sequence was originally capped by spinifex textured flows and mafic to intermediate volcanic rocks. Up sequence and to the west of these rocks are minor oMC, then medium grained oAC that grades into the very coarse grained oAC that forms the central core of the ultramafic complex. The spinifex textured and non-ultramafic volcanic rocks within the Corella ultramafic sequence are relics of an internal horizon that can be traced just to the west of the Harrier deposit (Fig 1). As at Harrier this horizon represents a hiatus in the development of the komatiite volcanic pile. Its patchy distribution at Corella is a function of probable extensive thermal erosion by magma from which the overlying oAC unit formed, as well as later structural dislocation. Between the shears and faults within the ultramafic rocks, igneous textures are generally well preserved. The metamorphic mineralogy of the osOC and oOC consists dominantly of antigorite and carbonate with minor chromite, chlorite and rarely kaersutitic amphibole. This assembalge pseudomorphs the igneous texture. The originally more olivine-rich rocks (osAC, oAC, osC, osMC, oMC) are mostly altered to green, lizardite-brucite-stichtite assemblages. In places these assemblages are altered to non-pseudomorphic antigorite-carbonate and talc-carbonate assemblages. Talccarbonate alteration is restricted in its distribution, occurring mostly along the eastern ultramafic fault contact. This alteration mostly affects unmineralised rocks and thus is generally outside the limits of the nickel sulphide resource.
The deposit comprises disseminated and massive sulphides hosted by a north-striking, west-younging, moderate to steep easterly dipping sequence of oOC and spinifex-textured rocks. In the south the mineralised sequence is in fault contact with the central oAC (Fig 8). At depth and to the north the sequence is separated from the oAC by a fault wedge of felsic volcanic rocks. To the west the mineralised sequence is in fault contact with variably deformed felsic to mafic metavolcanic and minor metasedimentary rocks that typically have a strong subvertical cleavage (Harrison, 1995). North of the inflection in the oAC contact around section 18 800 N (Fig 1) the mineralised sequence pinches out at depth where the faults that mark the eastern and western contacts join. On some sections a thin layer of massive sulphide breccia extends down dip along the combined fault plane. South of 18 800 N the sequence is open below the depth of drilling (300 to 500 m vertical depth).
Mineralisation The mineralised sequence varies along strike being dominated by osOC and lesser osMC in the north to osAC and lesser osOC in the south. To the north the proportion of sulphide in the sequence drops dramatically although the barren and weakly mineralised parts of the sequence continue as a horizon 10 to 30m thick. To the south the mineralised sequence is partially truncated by the eastern boundary fault but is open at depth. The thicker mineralised parts of the deposit, in the area of 16 100–16 200 N and 16 700–17 000 N appear to result from stratigraphic repetition by faulting, presumably related to early thrusting. Although the thicker parts of the mineralisation appear to be coherent blocks, a combination of original stratigraphy and particularly deformation and disruption has resulted in numerous discontinuous lenses of mineralisation occurring in otherwise barren rocks. This is particularly the case on the sections south of about 16 400 N. As at the other disseminated deposits the mineralisation can be divided into high copper and low copper types which are generally similar to those within the other deposits.
MASSIVE SULPHIDE DEPOSITS Wedgetail The deposit is along the NW margin of the oAC complex (Fig 1). It has a strike length of 1.7 km, varies in width from 10 to 80 m, and contains an Indicated and Inferred Resource of 22.9 Mt at 1.08% nickel at a 0.5% nickel cutoff, to 300 m depth, including 2.5 Mt at 3.36% nickel.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 8 - Cross section 18 300 N, Wedgetail deposit, looking NW.
Olivine orthocumulates and osOC mostly contain nonpseudomorphic antigorite assemblages with only minor preservation of lizardite assemblages. Talc-carbonate assemblages are generally confined to contacts, although much of the southernmost part of the mineralised sequence is altered to assemblages containing talc, carbonate and chlorite.
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Mineralisation Nickel grades within the mineralised sequence generally increase from east to west, with massive sulphide present in places along or close to the western faulted contact. Sulphide breccia, which comprises 30 to 90% sulphide and contains 1 to 20 cm sized, angular clasts of ultramafic and country rock, also occurs along this contact. The massive sulphide–sulphide breccia varies in thickness from a few centimetres to a maximum of 12 m. No massive sulphide is recognised in its original stratigraphic position and all is thought to have been remobilised along faults and shears. Primary zone sulphides are pyrrhotite, pentlandite, pyrite, minor chalcopyrite and trace gersdorffite. Massive sulphide consists of interlocking polygonal shaped, relatively coarse grains (to 1.5 mm diameter) of pyrrhotite, pentlandite and minor pyrite. Chalcopyrite forms small intergranular, irregularly shaped grains. A preferred alignment of grains or compositional layering is occasionally present. In osOC the sulphides are strongly intergrown with metamorphic gangue minerals and form irregular aggregates that mostly do not show typical magmatic shapes, although such textures are preserved in places. Very minor low copper ore, consisting of pentlandite and trace heazlewoodite, is restricted to low grade zones along the eastern contact of the mineralised sequence. Wedgetail ores are significantly more iron- and sulphur-rich than those of the other Honeymoon Well deposits. On a 100% sulphide basis massive sulphide contains about 9% nickel and has a nickel:sulphur ratio of 0.17 to 0.4, whereas sulphides in osC and osOC contain 11% nickel and the nickel:sulphur ratio mostly ranges from 0.2 to 0.9 with only very minor areas with higher values.
GEOLOGICAL MODEL The current model for the evolution of the Honeymoon Well ultramafic complex involves the initial formation of two ultramafic volcanic horizons. 1.
The lowermost unit contains all the known Honeymoon Well nickel sulphide deposits. Lateral facies variations in volcanic environment appear to have been marked, ranging from continuous turbulent flow to episodic emplacement of lava flows (Hill et al, 1990, 1995), and account for the marked differences in host rock types and mineralisation styles of the sulphide deposits. The unit is capped by spinifex-textured rocks which are in turn overlain by andesitic volcanic and minor metasedimentary rocks. The base of the unit is not preserved but originally the unit was probably, in places, more than 250 m thick.
2.
The upper unit formed within a few tens of years after the lower unit (Gole, Barnes and Hill, 1990). Its character varies along strike and in the Honeymoon Well area contains a coarse grained oAC body, originally 1000 m thick by 6 km long in apparent cross section, formed in a massive lava channel (Hill et al, 1990). To the south the oAC body thins into an oOC-dominated sequence whereas to the north proximal lateral equivalents are not preserved. The unit is capped by spinifex-textured flow rocks and, in places, particularly to the north, by fractionated sequences of pyroxenite and gabbro. Thermal erosion occurred along the basal contact of the unit during emplacement and the immediately
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underlying rocks were partially melted and metamorphosed at high temperatures during subsequent cooling of the volcanic pile. The stratigraphic succession at Honeymoon Well is generally similar to that proposed by Barnes, Gole and Hill (1988) for the Perseverance nickel mine area in the southern part of the Agnew–Wiluna greenstone belt. Early deformation of the Honeymoon Well sequence occurred within a D1 thrust duplex and the geometry of this duplex was probably greatly influenced by the size and competency of the unmetamorphosed oAC body. The oAC unit itself was stacked and thin thrust slices of the lower mineralised ultramafic horizon were emplaced along the top (now western) margin of the duplex. Strike-slip faulting and minor folding during D2 were accompanied by lower greenschist facies metamorphism. Strike-slip faulting resulted in further duplication of the ultramafic sequence as well as displacement of the northern proximal equivalents of the Honeymoon Well ultramafic horizons. At Wedgetail the bulk composition of the massive sulphide reflects pyrrhotite accumulation from a sulphide melt whereas the composition of net-textured sulphides more closely approaches that of the sulphide liquid (Zientek et al, 1994). Apart from relatively minor redistribution of copper and the addition of arsenic to parts of the deposit there appears to have been relatively little metamorphic adjustment of the sulphide bulk composition. The nickel-rich bulk sulphide compositions in the disseminated sulphide deposits are partly a function of exchange, during post-magmatic cooling, between sulphides and a relatively large volume of olivine, resulting in the sulphides gaining nickel and losing iron (Binns and Groves, 1976). Some of these sulphides have been markedly affected by metamorphic processes. Major faults near the sulphide deposits acted as pathways for migration of metamorphic fluids. These fluids have caused extensive metasomatism and resulted in intense veining, plus complete leaching of copper and partial loss of iron, sulphur and zinc from parts of the sulphide deposits and of iron from some of the barren ultramafic rocks. In the disseminated sulphide deposits this has formed a zoned pattern in copper content, whole rock nickel:sulphur ratios and primary sulphide assemblages. The sulphide zones are defined by pentlandite-minor pyrrhotitechalcopyrite, pentlandite-only, pentlandite-heazlewoodite and heazlewoodite-only assemblages. Zones containing pyrrhotite-chalcopyrite represent the least altered sulphide assemblages whereas heazlewoodite-only assemblages occur in the most strongly leached rocks.
ACKNOWLEDGEMENTS Outokumpu Mining Australia Pty Limited is thanked for permission to publish. Many CRAE and OMA geologists have worked on the project over the past 25 years and we would like to acknowledge their contributions to the understanding of the geology of Honeymoon Well.
REFERENCES Barnes, S J, Gole, M J and Hill, R E T, 1988. The Agnew nickel deposit, Western Australia. I. Stratigraphy and structure, Economic Geology, 83:524–536.
Geology of Australian and Papua New Guinean Mineral Deposits
HONEYMOON WELL NICKEL DEPOSITS
Binns, R A and Groves, D I, 1976. Iron-nickel partition in metamorphosed olivine-sulphide assemblages from Perseverance, Western Australia, American Mineralogist, 61:782–787. Donaldson, M J and Bromley, G L, 1981. The Honeymoon Well nickel sulphide deposits, Western Australia, Economic Geology, 76:1550–1565. Eisenlohr, B N, 1992. Contrasting deformation styles in superimposed greenstone belts in the northern sector of the Norseman-Wiluna Belt, Yilgarn Block, in The Archaean: Terrains, Processes and Metallogeny, Publication No 22 (Eds: J E Glover and S E Ho), pp 191–202 (The Geology Department and University Extension, The University of Western Australia: Perth) Gole, M J, Andrews, D L, Drew, G J and Woodhouse, M, 1996. Geology and nickel sulphide mineralisation, Honeymoon Well, Western Australia, in Proceedings 1995 AusIMM Annual Conference, pp 367–370 (The Australasian Institute of Mining and Metallurgy: Melbourne). Gole, M J, Barnes, S J and Hill, R E T, 1990. Partial melting and recrystallisation of Archaean komatiites by residual heat from rapidly accumulated flows, Contributions to Mineralogy and Petrology, 105:704–714. Gole, M J and Hill, R E T, 1988. Geology of the Honeymoon Well area, Agnew-Wiluna belt, Western Australia, with implications for nickel exploration, CSIRO Division of Exploration Geoscience, Report MG68R. Gole, M J and Hill, R E T, 1990. The refinement of extrusive models for the genesis of nickel deposits: implications from case studies at Honeymoon Well and the Walter Williams Formation, Western Australia Minerals and Energy Research Institute, Report No 68.
Hill, R E T, Barnes, S J, Gole, M J and Dowling, S E, 1990. The physical volcanology of komatiites in the Norseman-Wiluna belt, Western Australia, Geological Society of Australia, Western Australian Division, Excursion Guide Book 1. Hill, R E T, Barnes, S J, Gole, M J and Dowling, S E, 1995. The volcanology of komatiites as deduced from field relationships in the Norseman-Wiluna greenstone belt, Western Australia, Lithos, 34:159–188 Liu, S F, Hickman, A H and Langford, R L, 1995. Stratigraphic correlations in the Wiluna greenstone belt, Geological Survey of Western Australia Annual Review 1994-95, pp 81–88. Marston, R J, 1984. Nickel mineralization in Western Australia, Geological Survey of Western Australia Mineral Resources Bulletin 14. Mitchell, A C, 1994. Mineralogical hosts for nickel in the regolith over the Harrier nickel-sulfide deposit, Honeymoon Well, Western Australia, Preliminary PhD thesis (unpublished), The University of Western Australia, Perth. Russell, S C, 1995. A petrological and geochemical investigation of felsic volcanic country rocks of the Harrier nickel deposit, Honeymoon Well, Western Australia, BSc Honours thesis (unpublished), Curtin University, Perth Zientek, M L, Likhachev, A P, Kunilov, V I, Barnes, S-J, Meier, A L, Carlson, R R, Briggs, P H, Fries, T L and Adrian, B M, 1994. Cumulus processes and the composition of magmatic ore deposits: examples from the Talnakh district, Russia, in Proceedings of the Sudbury Noril’sk Symposium Special Publication No 5 (Eds: P C Lightfoot and A J Naldrett), pp 373–392 (Ontario Geological Survey: Sudbury).
Harrison, S, 1995. Gold mineralisation of the Wedgetail deposit, Honeymoon Well, Wiluna, Western Australia, BSc Honours thesis (unpublished), Curtin University, Perth.
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Hopf, S and Head, D L, 1998. Mount Keith nickel deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 307–314 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Mount Keith nickel deposit 1
by S Hopf and D L Head
2
INTRODUCTION The deposit is owned by WMC Resources Ltd and is 94 km NNE of Leinster in the Northeastern Goldfields of WA (Fig 1). It is on the Sir Samuel (SG 51–13) 1:250 000 and Mount Keith (3043) 1:100 000 scale map sheets at lat 27o14′S, long 120o32′E. The deposit is an example of the large tonnage, low grade (0.5–1.5% nickel) disseminated nickel sulphide deposits hosted by very large, layered bodies of olivine cumulate, the Class 2 deposits of Hill and Gole (1990). These were formerly known as ‘intrusive-dunite associated’ (Binns, Groves and Gunthorpe, 1977; Marston et al, 1981) or ‘dunite-hosted’ deposits (Donaldson et al, 1986). Recent studies suggest that the Mount Keith deposit is hosted within an extrusive komatiite (Dowling and Hill, 1990).
Nickel mineralisation at Mount Keith extends for over 2 km along strike and continues to a depth of at least 500 m, the base of drilling. As of June 1996, Measured, Indicated and Inferred Resources totalled 459 Mt at 0.60% nickel using a 0.26% recovered nickel cutoff grade. The Resources include Proved and Probable Ore Reserves of 173 Mt at 0.61% nickel, at a 0.26% recovered nickel cutoff grade. Production for the 1995–96 year was 29 677 t of nickel in concentrate from 7.95 Mt of ore at a head grade of 0.60% nickel. From the start of production in October 1994 to the end of February 1997 the mine has produced 73 682 t of nickel in concentrate.
EXPLORATION AND MINING HISTORY The Mount Keith nickel mineralisation was first identified in November 1968 by J T Jones of Mount Keith Station, who found sulphides while drilling an outcrop of ultramafic rock. Metals Exploration NL and Freeport of Australia jointly acquired the tenements late in November 1968 and began extensive exploration. The deposit was defined by magnetic and induced polarisation surveys, shallow rotary drilling, detailed surface mapping and follow up diamond drilling. In mid 1969, disseminated sulphides were intersected by diamond drill hole MKD5. By June 1972, after 32 diamond drill holes totalling 14 133 m had been drilled, a total resource of 263 Mt averaging 0.6% nickel using a 0.4% nickel cutoff was defined (Burt and Sheppy, 1975; Marston, 1984). A vertical shaft was then sunk to a depth of 153 m to provide bulk samples for metallurgical testwork (Burt and Sheppy, 1975). Metals Exploration was acquired by Australian Consolidated Minerals Ltd (ACM) in 1976. Exploration had been put on hold in 1973 when the price of nickel dropped, but resumed in 1981. In 1988 ACM became sole owner of Mount Keith, and in 1989 it entered into a joint venture with Outokumpu Oy that covered the mining, processing and marketing of nickel from the deposit. WMC Resources Ltd acquired the ACM interest in Mount Keith in September 1991. The Outokumpu interest was acquired in 1993, in exchange for supplying 14 000 tpa of nickel in concentrate for ten years. The development of Mount Keith began in 1993, the first ore was mined in July 1994, and milling operations commenced in October 1994.
FIG 1 - Location and simplified geological map of part of the Agnew–Wiluna greenstone belt showing the location of the Mount Keith nickel deposit (after Dowling and Hill, 1990). 1.
Formerly Project Mineralogist, Mount Keith Nickel Operation, now Project Exploration Geoscientist, WMC Resources Limited, Hedges Gold Mine, Private Bag 601, Post Office, Pinjarra WA 6208
2.
Chief Geologist, WMC Resources Limited, Mount Keith Nickel Operation, PO Box 238, Welshpool Delivery Centre WA 6986.
Geology of Australian and Papua New Guinean Mineral Deposits
The ore is mined by conventional open cut methods. Following an upgrade of the concentrator completed in February 1997, the operation is now treating ore at 10.5 Mtpa for an annual production of 42 000 t of nickel, compared to the original planned capacity of 6.6 Mtpa ore for 28 000 tpa of nickel. Concentration of the ore is carried out using conventional bulk sulphide flotation techniques (George, 1996). Concentrate from Mount Keith is pentlandite-rich and contains 20% nickel. The majority of the concentrate is trucked to the Kalgoorlie nickel smelter, and 14 000 tpa is railed under contract to the port of Esperance for shipment to the Outokumpu smelter in Finland.
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S HOPF and D L HEAD
Since the start of exploration in 1968 to the end of February 1997, 234 diamond drill holes totalling 76 000 m and 291 reverse circulation percussion drill holes totalling 33 700 m have been completed. Additional geotechnical drilling to the end of 1996 totalled 5900 m of HQ triple tube core, with hole depths from 150 to 300 m.
PREVIOUS INVESTIGATIONS The results of early studies on the mineralisation at Mount Keith and descriptions of the regional geology have been reported by Burt and Sheppy (1975), Elias and Bunting (1978), Bunting and Williams (1979) and Groves and Keays (1979). A summary of the exploration history was presented by Marston (1984). A detailed account of the geology, mineralisation, geochemistry, komatiite volcanology and genesis of the deposit has been provided by Dowling and Hill (1990). Since mining commenced, more data have become available on the deposit and more detailed work has been carried out (Bongers, 1994; Fardon, 1995; Seymon, 1996).
REGIONAL GEOLOGY The Mount Keith deposit is in the narrowest part of the Agnew–Wiluna greenstone belt, in the northern part of the Eastern Goldfields Province of the Archaean Yilgarn Craton (Fig 1). The Agnew–Wiluna belt extends over 200 km and ranges in width from 5 to 25 km. It contains deformed and metamorphosed felsic to intermediate volcanic and volcaniclastic rocks, basalt, gabbro, high-magnesium basalt, komatiite and minor units of clastic and chemical sedimentary rocks (Fig 1). The metamorphic grade increases from prehnitepumpellyite facies in the north, near Wiluna, to lower amphibolite facies towards Leinster in the south. In the Mount Keith area the grade is mid greenschist facies.
1979; Groves and Hudson, 1981; Marston et al, 1981), as subvolcanic sill-like feeder chambers for overlying spinifextextured komatiites (Lesher and Groves, 1986; Naldrett and Turner, 1977), or as extrusive cumulate bodies formed in lava lakes (Donaldson et al, 1986). Research by Hill, Gole and Barnes (1987) has shown that these bodies have gradational contacts with the overlying and underlying strata, and it is presently considered that the adcumulate bodies occupy major flow channels formed by the thermal erosion of floor rocks during ultramafic eruptions (Dowling and Hill, 1990).
ORE DEPOSIT FEATURES STRATIGRAPHY The regolith profile extends for 80 to 120 m below the surface. Within the regolith there are both transported (20 to 40 m thick) and residual components. The lowermost ultramafic unit (Fig 2), the early flows of Dowling and Hill (1990), is not exposed but is recognised in drill core. It is overlain by mafic volcanic rocks and felsic volcaniclastic sedimentary rocks which are exposed in the eastern wall of the open pit (Fig 3).
The lower part of the greenstone sequence in the Mount Keith area is characterised by interbedded volcanic and pyroclastic rocks, shale and chert. The upper part contains a thick sequence of pillowed basalt, a thick succession of volcaniclastic rocks, a zone dominated by komatiite, and an upper series of thin komatiite flows, layered gabbro, and highmagnesium and tholeiitic basalt. The komatiites can be correlated for over 100 km of strike, from south of Perseverance (Libby et al, this publication) to Honeymoon Well (Gole et al, this publication), and face and dip steeply to the west (Hill, Gole and Barnes, 1987, 1989). The interval dominated by komatiitic rocks is up to 2.5 km thick and contains up to five individual komatiite flow units. Typically the komatiitic rocks consist of spinifex-textured flows and olivine orthocumulate. Three west-facing komatiite units (Eastern, Central and Western ultramafic units, Fig 1) and a thin komatiite unit (early flows) of local extent are present in the Mount Keith area (Dowling and Hill, 1990). The units are juxtaposed in places or separated by volcanic and sedimentary rocks. The sequence is variably deformed with the most intense deformation along narrow shear zones. Nickel mineralisation occurs within the thicker zones of the komatiitic units at Mount Keith, and at Honeymoon Well and Six Mile Well to the north of the area shown in Fig 1. These zones are elliptical in outline and are occupied by concordant bodies of layered adcumulate–mesocumulate (Dowling and Hill, 1990). They have previously been interpreted as dykelike intrusions (Burt and Sheppy, 1975; Groves and Keays,
308
FIG 2 - Schematic stratigraphic column for the greenstone sequence in the Mount Keith area.
The MKD5 ultramafic unit (Fig 2), the Eastern ultramafic unit of Dowling and Hill (1990), is the host to the Mount Keith deposit. It is completely serpentinised and has a maximum thickness of about 650 m. The unit can be divided into three lithologically distinct zones (Fig 2):
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT KEITH NICKEL DEPOSIT
1.
a basal olivine orthocumulate;
2.
a central zone containing an unmineralised, coarse grained olivine adcumulate which grades upwards into a layered olivine-sulphide adcumulate–mesocumulate (Mount Keith orebody); and
3.
an upper orthocumulate containing zones of gabbroic ultramafic differentiate.
Unmineralised porphyritic olivine rock occurs in three distinct stratiform layers from 0.05 to 1.50 m thick in the upper part of the olivine adcumulate and the basal part of the olivinesulphide mesocumulate (Seymon, 1996). The MKD5 ultramafic unit is also cut by several mesocratic hornblendeplagioclase dykes to 5 m thick, and a rodingite dyke <1 m wide, which consists of diopside, grossular, plagioclase and chlorite (Dowling and Hill, 1990). Both the hanging wall and footwall contacts of the MKD5 ultramafic unit are extensively sheared and carbonated. The hanging wall is exposed in the western pit face. A pyrite-rich sediment and chert layer with a maximum thickness of 10 m occurs at the contact in the pit area, but is absent to the south of the pit. It is generally overlain by a thin carbonaceous shale layer, up to 1 m thick. The shale is overlain by a sequence of intermediate volcaniclastic rocks up to 50 m thick, and a sequence of basalt and dolerite up to 50 m thick. The volcaniclastic rocks contain interbedded thin layers of black shale and, in the upper part, are intercalated with the overlying mafic rocks. The sequence of mafic rocks is followed by the third and uppermost komatiitic unit, the Central ultramafic unit.
Geology of Australian and Papua New Guinean Mineral Deposits
HOST ROCK ALTERATION The sequence at Mount Keith has been metamorphosed to mid greenschist facies and igneous minerals have been completely replaced. Ultramafic rocks of the MKD5 unit have been serpentinised, and the overlying mafic and volcaniclastic rocks have been replaced by albite-actinolite-epidote-chlorite assemblages. No relict olivine has been observed in the ultramafic rocks. The majority of the serpentinised ultramafic cumulates in the upper part of the MKD5 unit are lizardite-rich (59 to 73%) and contain hourglass- and mesh-textured lizardite pseudomorphs after olivine, with finely dispersed magnetite (1 to 6%). The intercumulus areas contain hydroxycarbonates (8 to 23%), brucite (4 to 11%) and pentlandite (1 to 6%). The hydroxycarbonates include pyroaurite [Mg6Fe2(CO3) (OH)16·•4H2O] and distinctive violet-coloured stichtite [Mg6Cr2(CO3)(OH)16•4H2O], which replaces chromite. Carbonate alteration of the ultramafic rocks occurs in or adjacent to strongly fractured or sheared zones. Three main types of alteration have been recognised, though a gradation exists from one type to the other. Intense carbonate±talc alteration occurs along the margins of the MKD5 ultramafic unit and along faults and fractures. Adjacent to these zones, particularly in the lower part of the MKD5 unit, the carbonate alteration is weaker and the rocks contain lizardite and antigorite with smaller amounts of carbonate (proximal carbonate alteration). Farther away from the fracture zones the serpentinite contains abundant hydroxycarbonates and trace amounts of carbonate (distal carbonate alteration).
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MINERALISATION Distribution and ore types Nickel mineralisation at Mount Keith is almost entirely hosted by the central olivine adcumulate–mesocumulate (Figs 2 and 3) in the middle and upper part of the MKD5 unit. Primary nickel-bearing sulphides occur in lobate aggregates in the interstices between former olivine grains (Fig 5a). The aggregates are generally 0.1 to 0.2 cm in diameter, but may be up to 0.6 cm. Typical whole rock nickel grades range from 0.1 to 1.0% and average 0.6%, and are highest in non-carbonated, lizardite-rich adcumulate–mesocumulate. Compared to most Class 1 nickel deposits (Hill and Gole, 1990), the Mount Keith disseminated sulphides are richer in nickel and are characterised by higher nickel:sulphur and nickel:copper ratios, of 1.02 and 54.2 respectively (Keays and Davison, 1976). The higher nickel content is also reflected by a higher pentlandite:pyrrhotite ratio, at 2:1 (Fardon, 1995). Nickel grades show an irregular distribution within the mine, but most of the higher grades (0.65% nickel) are concentrated in the central part of the MKD5 unit (Figs 3 and 4). Pentlandite is by far the most common nickel sulphide mineral and is associated with pyrrhotite and magnetite. Millerite is much less abundant but is an important component of up to 20% of the orebody, particularly in the lower part of the MKD5 unit. Heazlewoodite and godlevskite are minor components closely associated with millerite. Arsenic-bearing minerals gersdorffite, maucherite, niccolite and cobaltite are also present, mainly in carbonated zones. Chalcopyrite, chalcocite, and valeriite are generally present in small quantities. Violarite after pentlandite, and marcasite and/or pyrite after pyrrhotite are present in the weathering zone and throughout the deposit in zones through which fluids have circulated. FIG 4 - Plan view of bench 364 RL (about 180 m below the surface) showing the hanging wall and footwall sequences of the MKD5 unit, and the distribution of modelled kriged nickel grades. Most of the high nickel zones are in the central part of the MKD5 unit.
The intense carbonate alteration contains magnesite+talc± chlorite±antigorite±magnetite, and is interpreted to have formed as a result of interaction with oxidising CO2-rich fluids (Eckstrand, 1975). There is a wide range in the degree of carbonation, and magnesite and talc are generally more abundant in the most strongly carbonated rocks. The presence of elevated arsenic levels in carbonated zones (average 100 ppm) suggests that the CO2-bearing fluids also introduced arsenic into the orebody. Antigorite-magnesite-hydroxycarbonate-magnetite assemblages in proximal carbonate alteration formed as a result of alteration by oxidising fluids with lower CO2 contents. This resulted in the partial recrystallisation of lizardite to interlocking aggregates of bladed antigorite, with associated fine grained magnesite (1 to 29%). Generally the magnetite in these rocks (2 to 11%) is not in former olivine grains but occurs along the grain boundaries. Serpentinites with distal carbonate alteration are dominated by lizardite (52 to 61%) and hydroxycarbonates (30 to 37%). Pyroaurite is the main hydroxycarbonate and it generally occurs along former olivine grain boundaries. Brucite, pentlandite, and magnesite are present in minor amounts up to 6%, 6% and 5%, respectively.
310
Below the depth of oxidation two distinct nickel-bearing assemblages are present: 1.
pentlandite-pyrrhotite±magnetite; and
2.
pentlandite-millerite±heazlewoodite±magnetite.
The pentlandite-pyrrhotite assemblage is dominant and represents the primary magmatic assemblage. There is a distinct zoning in the sulphide mineralogy of the orebody. In the stratigraphic upper part of the orebody, the sulphide assemblage is pentlandite-pyrrhotite. The proportion of pyrrhotite gradually decreases with depth and it is absent from the lowermost part of the orebody. The pentlandite-millerite assemblage occurs in the eastern, stratigraphically lower part of the orebody, where it alternates with pentlandite-rich ore in strongly carbonated zones. Magnetite is closely associated with the nickel-bearing sulphides in both assemblages, but magnetite content is lower in the pentlandite-pyrrhotite assemblage. Heazlewoodite is present as submicroscopic (to 1 µm wide) oriented intergrowths with millerite (Fig 5c). Nickel is not only present in discrete sulphide minerals, but is also contained in the structure of serpentine group minerals (nickel substitution for magnesium), and to a lesser extent in magnetite (nickel substitution for Fe2+). The nickel content of lizardite is up to 0.5%, and appears to be lower for antigorite.
Vertical zoning in the weathering profile Three mineralogically defined zones are recognised in the weathering profile above the primary ore zone: oxide, upper transition and lower transition zones (Figs 3 and 6). The oxide
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT KEITH NICKEL DEPOSIT
FIG 5(a) - Photomicrograph of an intercumulus sulphide-oxide bleb in lizardite-rich dunite showing a typical geometric intergrowth of magnetite (mg) and pentlandite (pn). A rim of magnetite is partially developed around the aggregate. Plane polarised reflected light. Scale bar = 0.3 mm. FIG 5(b) - Photomicrograph of an intercumulus sulphide aggregate in serpentinised dunite with proximal carbonate alteration. The aggregate consists of millerite pseudomorphs after pentlandite. The pentlandite octahedral cleavage is partly preserved (arrow). Note the variation in brightness of the millerite which is due to different orientations of individual millerite grains. Crossed polarised reflected light. Scale bar = 0.3 mm. FIG 5(c) - Backscattered electron image of a millerite (ml) aggregate which contains oriented intergrowths of heazlewoodite (hz) and is rimmed by magnetite (mg). Scale bar = 20 µm. FIG 5(d) - Intensely carbonated dunite with small scale pentlandite-gangue intergrowths. Note the deformation of the pentlandite (arrow). Plane polarised reflected light. Scale bar = 0.1 mm.
zone is the thickest part of the weathering profile. It extends to a depth of approximately 70 m below the surface, but is deeper in fracture zones and in the hanging wall and footwall contacts of the MKD5 ultramafic unit. The oxide zone–upper transition zone boundary is marked by the disappearance of visible oxidation effects. The upper transition zone is about 40 m thick on average but reaches 90 m in faults. The lower transition zone is generally narrow, has an average thickness of about 20 m and is characterised by the presence of partially altered primary sulphides. In the oxide zone the sulphides in both ore types have been completely weathered to oxides. Below the oxide zone the two nickel assemblages show characteristic differences (Fig 6) which reflect the absence of pyrrhotite in the pentlanditemillerite assemblage. Violarite is the main nickel-bearing mineral in the upper transition zone for both assemblages. Weathered pentlandite-pyrrhotite ore also contains marcasite, pyrite and soluble nickel sulphates, whereas weathered
Geology of Australian and Papua New Guinean Mineral Deposits
pentlandite-millerite ore contains only magnetite and violarite. The lower transition zone assemblages are similar to those in the upper transition zone, but primary nickel sulphides are also present. The proportion of primary nickel sulphides increases gradually towards the lower transition–primary ore boundary.
Ore mineral textures Variations in sulphide occurrence, texture and chemical composition are interpreted to have resulted from varying degrees of carbonation of the host. In non-carbonated, lizardite-rich ultramafic cumulate the sulphides occur at triple point junctions. With increasing degree of carbonation there are progressively more sulphides within former olivine grains and along grain boundaries. Coupled with the change in occurrence there is a decrease in sulphide grain size with increasing intensity of carbonation. In non-carbonated and weakly carbonated lizardite-rich ultramafic cumulate the sulphides occur as lobate to spherical
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S HOPF and D L HEAD
FIG 6 - Diagrammatic mineral zoning in the weathering profile at Mount Keith. The opaque mineral assemblages within the different zones of the weathering profile are given for the pentlandite-pyrrhotite and pentlandite-millerite ore assemblages. Note the deeper weathering in faults and fractures (F).
shaped aggregates at triple point junctions, suggesting a primary igneous origin. The sulphide aggregates are up to 0.6 cm in diameter and consist of a magnetite rim and a core containing geometric intergrowths of pentlandite (grain size to 250 µm) and magnetite±pyrrhotite (Fig 5a). Exsolution lamellae of pentlandite in pyrrhotite are rare, and pentlandite and pyrrhotite generally occupy distinct areas within aggregates. In zones of proximal carbonate alteration, the bleb-like interstitial sulphide aggregates are partly replaced by carbonates and, in the stratigraphic lower part of the orebody, pentlandite is partially or completely replaced by millerite (Fig 5b), heazlewoodite and godlevskite. The sulphides have a bimodal grain size distribution with an average grain size of 40 to 60 µm for the coarser fraction, and 1 to 10 µm for the finer fraction. The bimodal distribution is caused by the presence of fine grained millerite and pentlandite within former olivine grains and along former olivine grain boundaries. The fine grained millerite and pentlandite are interpreted to have formed from the nickel released during the replacement of lizardite by antigorite (Dowling and Hill, 1990). In intensely carbonated serpentinised ultramafic cumulate, interstitial coarse grained sulphide aggregates are rarely preserved. More typically, the sulphides occur as discrete grains to 40 µm in diameter and in 1 to 5 µm intergrowths with carbonate, talc or serpentine group minerals (Fig 5d). Pentlandite is the sole nickel sulphide and is not associated with magnetite, however a large proportion of the nickel is contained in the arsenic-bearing minerals. The small scale, lamellar sulphide-gangue intergrowths are deformed (Fig 5d),
312
suggesting remobilisation of the nickel prior to or during deformation.
Ore mineral composition Pentlandite in primary lobate aggregates in non-carbonated lizardite-rich ultramafic cumulate contains 21 to 26% nickel (Table 1). By contrast, pentlandite associated with millerite in ultramafic cumulate with proximal carbonate alteration has a nickel content to 43% (Table 1). The higher nickel content and the replacement of pentlandite by millerite (Table 2) indicate that nickel and sulphur were remobilised under oxidising conditions during formation of the proximal carbonate alteration. The conversion of lizardite to antigorite under
TABLE 1 Representative electron microprobe analyses (EDS) of pentlandite from Mount Keith. Number1
Ni %
Fe %
S%
Co %
Total %
1
21.4
42.9
32.8
1.02
98.1
2
2
26.4
38.3
32.8
1.0
98.5
3
42.5
23.2
32.3
1.1
99.1
4
42.9
23.9
32.0
0.3
99.1
1.
Analyses 1 and 2 are of pentlandite not associated with millerite, and analyses 3 and 4 are from pentlandite associated with millerite.
2.
Cobalt content determined qualitively.
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT KEITH NICKEL DEPOSIT
TABLE 2 Representative electron microprobe analyses of millerite from Mount Keith. Number
Ni %
Fe %
S%
Co %
1
63.1
0.7
34.6
1.1
Total % 99.5
2
63.7
0.8
34.4
0.4
99.3
3
63.9
0.9
34.8
0.1
99.7
4
64.2
0.3
34.5
0.6
99.6
oxidising conditions would have released some of the nickel (up to 0.5%) formerly contained in the lizardite structure, and this may have provided some of the nickel required for the formation of millerite.
ORE GENESIS The nickel mineralisation at Mount Keith is widely interpreted to have developed within a large active komatiite lava channel (Dowling and Hill, 1990; Hill, Barnes and Perring, 1996). Lava flow may originally have been focussed along a topographic low which was further enlarged by thermal erosion of the footwall rocks, forming a broad, deep lava conduit. Initial supercooling on the margins of the channel would have resulted in the development of orthocumulate (the outer layer of the MKD5 ultramafic unit), thus insulating the hot lava in the centre of the channel. By contrast, adcumulate–mesocumulate would have formed within the channel where cooling rates were lower and turbulent flow was maintained. Slow cooling in the centre of the lava channel would have resulted in the crystallisation of olivine, and a progressive enrichment of sulphur in the residual melt until sulphur saturation occurred. Further cooling and crystallisation of olivine beyond this point would have resulted in the exsolution of a sulphide melt. The sulphide content of the Mount Keith ore is low and the mineralisation is concentrated in the centre of the adcumulate–mesocumulate, which suggests that sulphur saturation was not reached until a large proportion of the lava had crystallised.
In intensely carbonated serpentinite, the grain size of the recrystallised sulphides is extremely small, and the nickel grades are low, which suggests that nickel has undergone small scale remobilisation, possibly due the breakdown of nickel sulphides under CO2–rich conditions.
ACKNOWLEDGEMENTS This paper is published with the permission of WMC Resources Ltd and is a summary of the current understanding of the deposit. Assistance in drafting by P Bebbington is gratefully acknowledged. Acknowledgement is also made to the staff of Mount Keith Nickel Operation for providing information on the mine. Thanks are due to J Reeve, J Hronsky, J Libby and B Stone for comments on the manuscript.
REFERENCES Binns, R A, Groves, D I and Gunthorpe, R J, 1977. Nickel sulphides in Archaean ultramafic rocks of Western Australia, in Correlation of the Precambrian (Ed: A V Sidorenko), pp 349–380 (Nauka, 2: Moscow). Bongers, E A, 1994. A structural interpretation of the Mount Keith region, BSc Honours thesis (unpublished), Flinders University, Adelaide. Bunting, J A and Williams, S J, 1979. Sir Samuel, WA - 1:250 000 geological series, Geological Survey of Western Australia Explanatory Notes, SG 51–13. Burt, D R L and Sheppy, N R, 1975. Mount Keith nickel sulphide deposit, in Economic Geology of Australia and Papua New Guinea, Vol 1 Metals (Ed: C L Knight), pp 159–168 (The Australasian lnstitute of Mining and Metallurgy: Melbourne). Donaldson, M J, Lesher, C M, Groves, D I and Gresham, J J, 1986. Comparison of Archean dunites and komatiites associated with nickel mineralisation in Western Australia, Mineralium Deposita, 21:266–305. Dowling, S E and Hill, R E T, 1990. Rivers of fire: The physical volcanology of komatiites of the Mt Keith region, NorsemanWiluna belt, Western Australia, CSIRO Division of Exploration Geoscience, Restricted Report 103R (unpublished). Eckstrand, O R, 1975. The Dumont serpentinite: A model for control of nickeliferous opaque mineral assemblages by alteration reactions in ultramafic rocks, Economic Geology, 70:183–201.
Subsequent reactions involving olivine have been an important factor in upgrading the nickel content of the ore, particularly as the proportion of sulphides in the rock is small (< 5%), and a large proportion of the total nickel in the rock may have originally been contained in igneous olivine. The enrichment of the sulphides in nickel may be related to reequilibration of the olivine with a residual sulphide-rich melt prior to complete crystallisation, and/or the release of nickel contained in olivine and serpentine minerals during serpentinisation and carbonation.
Elias, M and Bunting, J A, 1978. Wiluna, WA - 1:250 000 geological series, Geological Survey of Western Australia Explanatory Notes, SG 51–9.
The replacement of olivine by lizardite, and of lizardite by antigorite, probably resulted in the release of nickel, which may have then been incorporated in pentlandite. Thus the nickel content of pentlandite could have increased progressively during serpentinisation and carbonation. More oxidising conditions in zones surrounding fractures would have favoured the formation of antigorite, as well as the replacement of pentlandite by millerite. The upgrade in nickel tenor was coupled with a decrease in grain size of nickel sulphides in ore assemblages formed under more oxidising conditions. In particular, fine grained pentlandite and millerite appear to have formed within former olivine grains, possibly from the nickel released in reactions involving olivine or serpentine minerals.
Groves, D I and Hudson, D R, 1981. The nature and origin of Archaean strata-bound and volcanic-associated nickel-iron-copper sulphide deposits, in Handbook of Strata-bound and Stratiform Ore Deposits, Vol 9 (Ed: K H Wolf), pp 305–410 (Elsevier: Amsterdam).
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Fardon, M C, 1995. The Mount Keith nickel sulphide deposit: Effects of primary magmatic and post-depositional processes on sulphide composition and mineralogy, BSc Honours thesis (unpublished), Monash University, Melbourne. George, C, 1996. The Mt Keith operation, in Proceedings Nickel ‘96, Publication Series No 6/96 (Eds: E J Grimsey and I Neuss), pp 19–23 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Groves, D I and Keays, R R, 1979. Mobilization of ore-forming elements during alteration of dunites, Mt Keith-Betheno, Western Australia, Canadian Mineralogist, 17:373–389. Hill, R E T, Barnes, S J and Perring, C S, 1996. Komatiite volcanology and the volcanogenic setting of associated magmatic nickel deposits, in Proceedings Nickel ‘96 (Eds: E J Grimsey and I Neuss), pp 91–95 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Hill, R E T and Gole, M J, 1990. Nickel sulphide deposits of the Yilgarn Craton, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 557–559 (The Australasian Institute of Mining and Metallurgy: Melbourne). Hill, R E T, Gole, M J and Barnes, S J, 1987. Physical volcanology of komatiites - a field guide to the komatiites between Kalgoorlie and Wiluna, Eastern Goldfields Province, Yilgarn Block, Western Australia, Excursion Guide Book No 1 (Geological Society of Australia, Western Australian Division: Perth). Hill, R E T, Gole, M J and Barnes, S J, 1989. Olivine adcumulates in the Norseman-Wiluna greenstone belt, Western Australia: Implications for the volcanology of komatiites, in Magmatic Sulphides – The Zimbabwe Volume (Eds: M D Prendergast and M J Jones), pp l89–214 (The Institution of Mining and Metallurgy: London). Keays, R R and Davison, R M, 1976. Palladium, iridium and gold in the ores and host rocks of nickel sulfide deposits in Western Australia, Economic Geology, 71:1214–1228.
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Lesher, C M and Groves, D I, 1986. Controls on the formation of komatiite associated nickel-copper sulfide deposits, in Geology and Metallogeny of Copper Deposits (Eds: G H Friedrich, A J Naldrett, J D Ridge, F M Vokes, A D Genkin and R H Sillitoe), pp 43–62 (Springer-Verlag: Berlin). Marston, R J, 1984. Nickel mineralization in Western Australia, Geological Survey of Western Australia Mineral Resources Bulletin, 14:66–71. Marston, R J, Groves, D I, Hudson, D R and Ross, J R, 1981. Nickel sulphide deposits in Western Australia: A review, Economic Geology, 76:1330–1363. Naldrett, A J and Turner, A R, 1977. The geology and petrogenesis of a greenstone belt and related nickel sulfide mineralization at Yakabindie, Western Australia, Precambrian Research, 5:43–103. Seymon, A R, 1996. Petrology and geochemistry of porphyritic olivine-rich rocks, Mount Keith, WA, BSc Honours thesis (unpublished), Monash University, Melbourne.
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De-Vitry, C, Libby, J W and Langworthy, P J, 1998. Rocky’s Reward nickel deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 315–320 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Rocky’s Reward nickel deposit 1
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by C De-Vitry , J W Libby and P J Langworthy INTRODUCTION The deposit is 17 km north of Leinster and 380 km north of Kalgoorlie, WA (Fig 1) in the Archaean Agnew-Wiluna greenstone belt. It is on the Sir Samuel (SG 51–13) 1:250 000 scale map sheet at lat 27o48′S, long 120o42′E or AMG coordinates 272 900 E, 6 923 640 N. The Rocky’s Reward and Perseverance (Libby et al, this publication) deposits are mined by the Leinster Nickel Operation of WMC Resources Ltd.
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Production at Rocky’s Reward is currently at a rate of 800 000 tpa, and to June 1997 a total of approximately 3.2 Mt of ore had been mined at a grade of 2.85% nickel. At June 1996 remaining Proved and Probable Reserves were 6.4 Mt at 2.22% nickel. Rocky’s Reward is part of a single, komatiite-hosted nickel sulphide mineralised system that includes the Perseverance deposit and is of a magnitude unprecedented in the rest of the world. The deposit occupies a zone of massive and disseminated sulphide mineralisation that is continuous over a strike extent in excess of 3.5 km.
EXPLORATION AND MINING HISTORY Despite being only 2 km north of the Perseverance deposit, Rocky’s Reward was not discovered until 1984, 13 years after significant mineralisation was first identified at Perseverance. The Rocky’s Reward deposit is the most recently discovered nickel sulphide orebody in the Agnew–Wiluna greenstone belt. The Perseverance deposit was discovered in 1971 by Australian Selection Pty Ltd. In 1973 shallow drilling in the area of Rocky’s Reward identified anomalous nickel and copper in regolith. However, the area was considered unprospective as ultramafic rocks were not intersected during this phase of drilling. After the formation of the Agnew Mining Company (AMC) in 1974 to manage the Perseverance (Agnew) deposit and surrounding leases, exploration focussed on numerous other occurrences of nickeliferous sulphide in the belt (Marston, 1984), and exploration in the Rocky’s Reward area was discontinued. However, in 1984 mapping and sampling of gossanous outcrops north of Perseverance by geologist Grant (Rocky) Osborne revealed a large nickeliferous gossan. Drilling under the gossan in that year intersected massive nickel sulphide mineralisation, and follow up drilling intersected the significant nickel sulphide mineralisation now known as the Rocky’s Reward deposit.
FIG 1 - Location map and regional geological map, Eastern Goldfields Province.
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Mine Geologist, WMC Resources Ltd, Leinster Nickel Operation, PO Box 22, Leinster WA 6437.
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Senior Research Geologist, WMC Resources Ltd, WA Operations North -Exploration, PO Box 22, Leinster WA 6437.
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Exploration Manager, WMC Resources Ltd, WA Operations North - Exploration, PO Box 22, Leinster WA 6437.
Geology of Australian and Papua New Guinean Mineral Deposits
A major feasibility study was initiated almost immediately and confirmed the economic potential of the deposit via conventional open pit mining. However, the ensuing depressed nickel price meant that mining was postponed until WMC purchased the Perseverance deposit and surrounding leases from AMC in December 1988. WMC started open pit mining of the Rocky’s Reward deposit almost immediately, and in 1989 the first ore was processed through the nickel concentrator at Perseverance. In 1991 the open pit was completed, having produced 1.8 Mt of ore at an average grade of 2.1% nickel, and mining changed to a wholly underground operation using longhole (open and backfill) and room-and-pillar stoping. Access to the underground operation is via two declines with in-pit portals.
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PREVIOUS DESCRIPTIONS Little information has been published on the Rocky’s Reward deposit despite its significance as a major example of massive nickel sulphide mineralisation in the Yilgarn Craton. The first published description focussed solely on the deposit was provided by Woodhouse (1988). Reid (1995) presented a structural analysis of the Rocky’s Reward deposit and described many of the characteristics of the mineralisation. The geophysical properties and response of the Rocky’s Reward mineralisation and associated rocks were presented by Mutton and Williams (1994). Sulphide geochemistry and comparisons with the Perseverance deposit were provided by Barnes, Gole and Hill (1988b). Other descriptions of Rocky’s Reward were provided by CSIRO workers in descriptions of the Perseverance deposit and the surrounding ultramafic horizons (Barnes, Gole and Hill, 1988a). Results of recent work on the regional and local geology of the Perseverance-Rocky’s Reward region are presented by Libby et al (this publication) to which the reader is referred for detail.
ORE DEPOSIT FEATURES LOCAL GEOLOGY Nickel sulphide mineralisation at Rocky’s Reward is hosted by two thin komatiitic ultramafic horizons that are wholly within a thick succession of interbedded felsic volcaniclastic sediments. The sediments contain subordinate horizons of sulphidic black shale, exhalative massive iron sulphide and chert and are intruded by subvolcanic felsic porphyry. Multiple phases of hydrothermal alteration are evident throughout the felsic succession which has generally been metamorphosed to low amphibolite facies, producing variable assemblages containing biotite and amphibole (actinolite, hornblende or gedrite) with localised garnet and minor andalusite.
The mineralised ultramafic body within this succession occupies a narrow, high strain corridor in which polyphase deformation and strain partitioning have resulted in the complex folding of all rocks and the development of discordant zones of localised high strain. The high strain corridor can be traced to the south where it appears to be continuous with the well mineralised 1A structure at Perseverance (Fig 2). Extending north from Rocky’s Reward, highly strained rocks can be traced to the well mineralised Sir Samuel area (Marston, 1984). Despite the apparent continuity of the high strain corridor between Perseverance and Rocky’s Reward, the mineralised ultramafic body at Rocky’s Reward is attenuated and pinches out immediately south of the deposit. Although ultramafic rock has not been intersected in drilling between the northern extent of mineralisation in the 1A structure and the southern extent of Rocky’s Reward, the link between the two is indicated by sporadic gossanous outcrops after nickel sulphide and an anomalous surface geochemical response along the length of the zone. Ultramafic rocks in the Rocky’s Reward mine area have been hydrothermally altered, metamorphically recrystallised and serpentinised. Unlike Perseverance, primary igneous textures are rarely preserved within the ultramafic horizons. Instead, the ultramafic rocks are dominated by coarse-grained randomly oriented metamorphic olivines, now pseudomorphically replaced by serpentine minerals. Hydrothermal alteration is common throughout the ultramafic rocks and has resulted in the development of carbonate-rich assemblages which predominantly occur as narrow zones along the margins of the ultramafic horizons. Textural and mineralogical relationships indicate that the main phase of hydrothermal alteration occurred pre- to syn-peak metamorphism (Gole, Barnes and Hill, 1987) and that the current assemblages are the result of pervasive retrograde serpentinisation. This serpentinisation
FIG 2 - Geological map of Perseverance-Rocky’s Reward area.
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has resulted in the development of lizardite, magnetite, brucite and pyroaurite after olivine, and talc after magnesium silicate minerals like anthophyllite and enstatite.
MINERALISATION Mineralisation at Rocky’s Reward occurs as massive and disseminated sulphides typical of the Class 1 style of mineralisation as defined by Hill and Gole (1990). Massive sulphide mineralisation typically contains 4.0 to 6.0% nickel, and disseminated sulphide mineralisation contains 0.7 to 4.0%. Massive sulphide mineralisation has an ore tenor (defined as wt % nickel in 100% sulphide) in the order of 5.5–6.0%, and the tenor in disseminated mineralisation is in the order of 6–7%.
DEPOSIT GEOMETRY Mineralisation at Rocky’s Reward is situated within three ultramafic layers, referred to locally as ‘surfaces’ (Figs 3 and 4). The two eastern surfaces, termed the upper and lower, are folded about an upright to inclined F2 axis (Figs 3 and 4) such that they form two vertically stacked mineralised surfaces that plunge at 10–15o to the north. These surfaces are truncated against a steeply dipping zone of high strain, termed West fault, that contains abundant remobilised massive sulphide and tectonic slices or lithons of the adjacent ultramafic and felsic rocks and forms the third surface.
In both massive and disseminated sulphide, primary sulphides comprise pyrrhotite and pentlandite with trace quantities of chalcopyrite. In massive sulphide ore these minerals are recrystallised into a medium to coarse grained, interlocking to granoblastic texture. However, sheets of massive sulphide often preserve a relict ductile tectonite fabric expressed by localised mineralogical banding of alternating coarse pyrrhotite- and pentlandite-rich layers, the presence of layer-parallel trails of spinel and inclusions of deformed and attenuated felsic and ultramafic slices. In contrast, disseminated sulphide mineralisation generally occurs as coarse, triangular sulphide aggregates interstitial to, and as poikilitic inclusions within, serpentine pseudomorphs after coarse, blade-textured metamorphic olivine. Chalcopyrite is common, though in low abundance within the deposit, and tends to concentrate in highly structured zones containing physically remobilised sulphides and quartz. Alteration has had a marked impact on sulphide mineralogy within the ore zone. Hydrothermal alteration introduced arsenic, and resulted in the formation of gersdorffite, cobaltite and niccolite in minor quantities associated with carbonate-rich assemblages. Supergene alteration has resulted in mixed assemblages containing violarite, bravoite, millerite, pyrite, marcasite and magnetite. Nickel et al (1977) described similar assemblages at Perseverance.
FIG 3 - Cross section on grid line 222 400 N, Rocky’s Reward open pit, looking north.
FIG 4 - Geological plan on 10 360 RL (160 m below surface), Rocky’s Reward deposit.
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Upper surface The upper surface at Rocky’s Reward (Figs 3 and 4) is the thickest and most continuous ultramafic horizon in the mine environment. It contains two distinct, stacked horizons of disseminated sulphide mineralisation with lesser quantities of massive sulphides occurring as discrete, structurally remobilised stringer zones within small-scale fold closures and along steeply to moderately dipping faults that transect the ultramafic surface. The main fold that defines the geometry of this surface plunges at 10–15o to the north, and can be traced continuously down plunge for more than 3.5 km. The western and eastern limbs of this fold are also laterally continuous, but are restricted in their vertical extent (Fig 3). The eastern limb of the fold pinches out at depth, and the western limb is truncated at ground level by erosion. To the north of Rocky’s Reward disseminated sulphide mineralisation in the eastern limb diminishes, and is truncated by a laterally extensive discordant shear zone that juxtaposes sulphidic black shale, unmineralised ultramafic rock and basaltic komatiite against the mineralised upper surface.
Lower surface Although the geometry of the lower and upper surfaces is similar, the lower surface (Figs 3 and 4) is significantly thinner, and is dominated by massive sulphides. Within the lower surface the highest concentrations of massive sulphide occur in the western fold limb and hinge zone, whereas the eastern limb contains tectonically intermixed ultramafic rock, massive sulphide and felsic volcaniclastic sediment. Like the eastern limb of the upper surface, the eastern limb of the lower surface pinches out at depth. This surface essentially forms a large scale, matrix-supported tectonic breccia, in which massive sulphide forms the matrix to numerous clasts or boudins of ultramafic and felsic country rock. Local concentrations of massive sulphide in fold closures and along faults transecting the surface produce a series of localised, shallow angle (10–15o), north-plunging shoots. Unlike the upper surface, the lower surface does not persist for a significant distance north of the immediate mine environment. Instead, it appears to pinch out against, or merge with, the upper surface. To the south of Rocky’s Reward the lower surface exits at ground level, where it was expressed prior to mining as a significant nickeliferous gossan.
West fault surface West fault (Figs 3 and 4) is the dominant source of high-grade massive sulphide mineralisation in the Rocky’s Reward mine. In the immediate mine area mineralisation within the shoot is characterised by numerous clasts of felsic and ultramafic rocks set in a matrix of massive sulphide. The presence of highly deformed rocks adjacent to the shoot, and the occurrence of numerous tight to isoclinal folds with amplitudes in excess of 10 m within it, attest to the high strains attained along this structurally complex ore zone. The clasts of country rock in massive sulphide are rounded to flattened in cross section and are generally highly elongate parallel to the plunge of the upper and lower surface folds. Individual clasts range in size from centimetre diameter to in excess of 4 m wide and 50 m long, and appear to represent a combination of rootless folds enclosed in remobilised massive
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sulphide and lithons of country rock scaled from the walls of the shoot and included in the massive sulphide by hanging wall and footwall faults, splays and piercements. To the north of the mine area nickel mineralisation continues along the West fault but diminishes into a series of discrete, discontinuous pods of massive sulphide and ultramafic rock that are elongate parallel to the upper and lower surfaces. In the southern part of the mine, the high strain zone associated with the West fault appears to continue south of Rocky’s Reward, but does not contain appreciable mineralisation south of its intersection with the lower surface.
Relationships between the mineralised surfaces at Rocky’s Reward Mapping of mine production faces indicates that massive sulphide and ultramafic rock of the lower surface is continuous into West fault such that the two surfaces represent stratigraphic equivalents joined across a tight synformal fold closure at the base of West fault. This, combined with the geometry in the mine environs suggests that West fault and the lower surface occupy a west-verging asymmetric fold that has been locally attenuated by high strains along West fault. The relationship between the upper and lower surfaces at Rocky’s Reward remains equivocal. Various models have been proposed suggesting that they represent separate lava flows separated by a phase of sedimentation, or that they represent the same horizon that has been either locally thrust stacked or folded early in the structural evolution of the area. However, due to the high degree of tectonic disturbance, metamorphic recrystallisation and alteration at Rocky’s Reward, many of the key criteria for determining the relationship between these surfaces have been largely obliterated. Recent work comparing the nature of the two surfaces suggests that they may have an opposed facing, and occupy the limbs of an early recumbent fold. This interpretation is based on the spatial relationship between unmineralised ultramafic rock and disseminated sulphide mineralisation within the horizons which, if it reflects primary igneous layering, may indicate facing. In the upper surface, disseminated sulphide mineralisation generally occurs above unmineralised ultramafic rock, whereas in the lower surface, the reverse is the case. The position of this fold with respect to the surfaces at Rocky’s Reward is indicated in Figs 3 and 4 by the trace of F1.
ORE GENESIS Mineralisation at Rocky’s Reward occupies a high strain corridor that can be traced for more than 15 km, from south of the Perseverance orebody to the Sir Samuel area. Almost all significant high grade nickel sulphide occurrences within the southern part of the Agnew–Wiluna greenstone belt are confined to this corridor, which is characterised by discrete shear zones with intervening lower strain zones of folded rocks. Deformation within this corridor has resulted in a complex geometry at Rocky’s Reward which is locally controlled by rheology contrasts between the ultramafic, massive sulphide and felsic rocks. This deformation has resulted in attenuation of the mineralised ultramafic body, large scale remobilisation of massive sulphide mineralisation and at least two stages of folding.
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ROCKY’S REWARD NICKEL DEPOSIT
The earliest phase of deformation at Rocky’s Reward is expressed by a layer-parallel tectonite fabric within massive sulphide mineralisation and along the margins of the ultramafic horizons of the upper and lower surfaces, and is associated with intense carbonate alteration. This fabric has been metamorphically recrystallised, and is folded about the shallowly-plunging, upright F2 folds. Continued deformation resulted in the partitioning of strain and reactivation of earlier structures along the long limbs of F2 folds.
mentioned in the text. N J Archibald is thanked for many stimulating and informative discussions. L Nicoletti is gratefully thanked for the generation of figures for this manuscript.
The mineralised surfaces at Rocky’s Reward are interpreted to represent a single, mineralised ultramafic horizon that has been complexly deformed, altered and locally repeated by F1 folding to form two vertically stacked surfaces. The presence of highly tectonised boundaries on all ultramafic rock contacts, their narrower width and restricted lateral and vertical extent are consistent with the mineralised ultramafic body at Rocky’s Reward occupying a discrete shear zone within the regional high strain corridor. This shear zone can be traced south to the 1A structure at Perseverance, where its contained mineralisation is similar in all respects to that at Rocky’s Reward. This shear zone resulted in the development of the early tectonite fabrics, was folded at Rocky’s Reward about a shallowly-plunging F2 axis, and is truncated against West fault.
Barnes, S J, Gole, M J and Hill, R E T, 1988b. The Agnew nickel deposit: Part II. Sulphide geochemistry with an emphasis on the Platinum-group elements, Economic Geology, 83:537–550.
The ultramafic body and mineralisation at Rocky’s Reward are interpreted to represent a tectonic slice derived from the base of a large lava channel complex in which sulphur saturation was attained and Class 1 mineralisation accumulated (Barnes, Gole and Hill, 1988a). Although the existence of one of these channels is demonstrated at Perseverance, the magnitude of displacement on the shear zones, and therefore the link between the Perseverance channel complex and Rocky’s Reward remains equivocal. However, the proximity of Rocky’s Reward to the Perseverance channel complex and dextral displacement along the high strain zones linking them (Eisenlohr, 1987; N J Archibald, personal communication, 1997), is consistent with the derivation of Rocky’s Reward from the mineralised zone at Perseverance.
ACKNOWLEDGEMENTS The authors would like to thank WMC Resources Ltd for permission to publish this manuscript, and especially J S Reeve and J M A Hronsky for their valuable comments and support. Special mention must also be made of the numerous WMC geologists and external researchers who have contributed to our understanding of the deposits and region but who are not
Geology of Australian and Papua New Guinean Mineral Deposits
REFERENCES Barnes, S J, Gole, M J and Hill, R E T, 1988a. The Agnew nickel deposit: Part I. Structure and stratigraphy, Economic Geology, 83:524–536.
Eisenlohr, B, 1987. Structural geology of the Kathleen Valley-Lawlers region, Western Australia, and some implications for Archaean gold mineralisation, in Recent Advances in Understanding Precambrian Gold Deposits, Publication 11 (Eds: S E Ho and D I Groves), pp 85–95 (The Geology Department and University Extension, University of Western Australia: Perth). Gole, M J, Barnes, S J and Hill, R E T, 1987. The role of fluids in the metamorphism of komatiites, Agnew nickel deposit, Western Australia, Contributions to Mineralogy and Petrology, 96:151–162. Hill, R E T and Gole, M J, 1990. Nickel sulphide deposits of the Yilgarn Craton, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 557–559 (The Australasian Institute of Mining and Metallurgy: Melbourne). Marston, R J, 1984. Nickel Mineralisation in Western Australia, Geological Survey of Western Australia, Mineral Resources Bulletin, 14:56–63. Mutton, A J and Williams, P K, 1994. Geophysical response of the Rocky’s Reward nickel sulphide deposit, Leinster, Western Australia, in Geophysical Signatures of Western Australian Mineral Deposits, Publication 26 (Eds: M C Dentith, K F Frankcombe, S E Ho, J M Shepherd, D I Groves and A Trench), pp 85–95 (The Geology Department and University Extension, University of Western Australia: Perth; and Australian Association of Exploration Geophysicists, Special Publication 7). Nickel, E H, Allchurch, P D, Mason, M G and Wilmshurst, J R, 1977. Supergene alteration at the Perseverance nickel deposit, Agnew, Western Australia, Economic Geology, 72:184–203. Reid, R J, 1995. Structural evolution of the Rocky’s Reward komatiiteassociated nickel sulphide ore body, Yilgarn Block, Western Australia, BSc Honours thesis (unpublished), James Cook University of North Queensland, Townsville. Woodhouse, M, 1988. The discovery of Rocky’s Reward nickel deposit, Western Australia, in Research and Development for the Minerals Industry Conference ‘88, pp 94–98 (Western Australian School of Mines: Kalgoorlie).
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Libby, J W, Stockman, P R, Cervoj, K M, Muir, M R K, Whittle, M and Langworthy, P J, 1998. Perseverance nickel deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 321–328 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Perseverance nickel deposit 1
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by J W Libby , P R Stockman , K M Cervoj , M R K Muir , M Whittle and 4 P J Langworthy INTRODUCTION
(WMC). The other is the Rocky’s Reward mine 2 km to the north (De-Vitry, Libby and Langworthy, this publication).
Formerly known as the Agnew nickel deposit, Perseverance is 15 km north of Leinster and 380 km north of Kalgoorlie, WA (Fig 1). It is in the Agnew–Wiluna greenstone belt, on the Sir Samuel (SG 51–13) 1:250 000 scale map sheet at lat 27o49′S, long 120o42′E or AMG coordinates 272 900E, 6 923 640 N. Perseverance is one of two deposits currently in production at the Leinster Nickel Operations of WMC Resources Ltd
The deposit is currently being mined underground with Proved and Probable Reserves to June 1996 totalling 31.3 Mt at 1.65% nickel. Nickel mineralisation occurs in massive and disseminated sulphides hosted by serpentinised ultramafic rock (olivine mesocumulate to adcumulate). Production from Perseverance has totalled approximately 10.6 Mt at 2.1% nickel to June 1997. Treatment of the ore is by conventional sulphide flotation with a four-stage cleaning process, and concentrate is thickened and dried on site. Dried concentrate is transported by road and rail for smelting at the WMC Kalgoorlie nickel smelter.
EXPLORATION AND MINING HISTORY Exploration for base metal mineralisation was initiated in the Agnew–Wiluna greenstone belt by Australian Selection Pty Ltd (Selcast) in the 1960s. However, it was not until the discovery of nickel mineralisation at Kambalda in the mid 1960s that the nickel potential of the Norseman–Wiluna greenstone belt was realised. After mapping an abundance of ultramafic rocks in the greenstone belt, reconnaissance geochemical sampling identified weak nickel anomalies in the Perseverance area. Follow-up rock chip sampling led to the discovery of nickeliferous gossan in April 1971 and drilling under the gossan in May of that year intersected nickeliferous sulphides in the first drill hole. By December 1973 a total of 216 diamond drill holes for 64 844 m had been completed at the deposit (Marston, 1984), confirming it as one of the largest economic accumulations of nickel sulphide mineralisation of its type in the world.
FIG 1 - Location map and regional geological map, Eastern Goldfields Province.
1.
Senior Research Geologist, WMC Resources Ltd, WA Operations North - Exploration, PO Box 22, Leinster WA 6437.
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Senior Mine Geologist, WMC Resources Ltd, Leinster Nickel Operation, PO Box 22, Leinster WA 6437.
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Mine Geologist, WMC Resources Ltd, Leinster Nickel Operation, PO Box 22, Leinster WA 6437.
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Exploration Manager, WMC Resources Ltd, WA Operations North - Exploration, PO Box 22, Leinster WA 6437.
Geology of Australian and Papua New Guinean Mineral Deposits
After the sinking of an exploration shaft and bulk metallurgical testing confirmed the economic viability of the Agnew nickel deposit, Mount Isa Mines Ltd acquired a 40% interest in the project from Selcast, and the Agnew Mining Company (AMC) was formed to manage the operation. In February 1976 mining of massive sulphide mineralisation was given the green light, and the first ore was extracted in early 1978. In order to access the ore and undertake further exploration AMC developed a decline to 400 m depth and constructed a shaft to 1162 m depth. Mining of the Agnew deposit used a range of methods including vertical crater retreat, long hole cut and fill stoping and post-pillar cut and fill stoping. The mine closed in August 1986 due to depressed nickel prices after 3.72 Mt of ore at 2.56% nickel were mined for 67 112 t of nickel in concentrate. The operation was placed on care and maintenance until December 1988 when WMC acquired the project. The Agnew deposit was renamed Perseverance, and in late 1989 a conventional open cut mine was begun over the AMC underground development.
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During open cut mining WMC continued underground development and rehabilitation in preparation for full scale underground mining, using long hole open stoping on the main massive sulphide shoot, known as the 1A shoot, and sublevel caving within disseminated sulphide mineralisation. In 1993 the concentrator was expanded from 1.0 Mtpa capacity to 2.0 Mtpa, and mining of the open cut was completed in February 1995.
PREVIOUS DESCRIPTIONS The Perseverance (Agnew) nickel deposit and surrounding region have been well described by numerous authors. General descriptions of the deposit are provided by Martin and Allchurch (1975), Billington (1984), Marston (1984) and Barnes, Gole and Hill (1988a). Nickel et al (1977) describe the sulphide assemblages with an emphasis on alteration. Researchers from the CSIRO and other institutions (Hill, Gole and Thompson, 1985; Barnes, Gole and Hill, 1988b, c; Barnes, Lesher and Keays, 1995; Rutter, 1995; Coffey, 1995) describe various aspects of the volcanological significance, controls of mineralisation, ore stratigraphy, lithogeochemistry and structural geology of the deposit. Alteration and metamorphism at the deposit are discussed by Binns and Groves (1976), Donaldson (1982) and Gole, Barnes and Hill (1987). Although a number of models have been proposed for the genesis of the mineralisation (eg Marston et al, 1981; Donaldson et al, 1986), the most recent and robust model is that of Barnes, Gole and Hill (1988a). Descriptions of the regional geology of the area surrounding the Perseverance and Rocky’s Reward deposits are provided by Naldrett and Turner (1977), Bunting and Williams (1979), Platt, Allchurch and Rutland (1978), Eisenlohr (1987) and McCluskey (1996).
REGIONAL GEOLOGY The Perseverance and Rocky’s Reward deposits are within a regionally extensive ultramafic horizon close to the eastern margin of the Agnew–Wiluna greenstone belt (Fig 2). Eisenlohr (1987) divided the southern part of the greenstone belt into western and eastern successions based on structural and stratigraphic constraints. All known nickel mineralisation, from Honeymoon Well in the north (Gole et al, 1996; Gole et al, this publication) to Weebo Bore in the south (Marston, 1984), occurs within the eastern succession.
The eastern succession is dominated in the west by tholeiitic basalt, pillowed in places, with abundant sulphidic interflow sediment, spinifex-textured komatiite and minor, intercalated felsic sediment. To the east the succession is dominated by felsic, predominantly rhyodacitic to dacitic, volcaniclastic sediment and lavas with intermittent horizons of ultramafic (komatiitic) rocks and black sulphidic, graphitic shale. Basaltic komatiite, mafic intrusive rocks and felsic intrusive porphyries are minor components of the package in the southern part of the belt. All known nickel sulphide mineralisation in the eastern succession is in the ultramafic units within the felsic sediment-dominated package. Reliable dating of the greenstone succession is restricted to the felsic volcaniclastic rocks in the immediate vicinity of the Perseverance and Rocky’s Reward deposits. U-Pb in zircon dating has returned ages of 2720±14 Myr (D Nelson, personal communication, 1996) and 2702±7, 2707±8, 2707±8 and
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FIG 2 - Geological map of the Agnew–Wiluna greenstone belt between Weebo Bore and Mount Keith.
2706±6 Myr (R I Hill and I H Campbell, unpublished data, 1991). The eastern succession is structurally complex, and appears to have been deformed by the series of events documented for the greenstone belts of the Eastern Goldfields Province of the Yilgarn Craton (Swager et al, 1995). Locally, Platt, Allchurch and Rutland (1978), Eisenlohr (1987), Bongers (1994), Coffey (1995), Reid (1995) and WMC geologists have documented a similar sequence of deformation events for the area between Agnew and Wiluna. Results of these studies suggest a deformation sequence as follows: 1.
D1: Essentially north–south compression resulted in low angle thrust faults and associated inclined to recumbent folds.
2.
D2: WSW- to WNW-oriented bulk inhomogeneous shortening resulted in dominantly vertical movements in the greenstone belt. Structures developed during this event include upright, north- to NNW-trending regional scale folds, and high strain corridors dominated by flattening along transposed fold limbs and imbrication of the greenstone succession along steep to moderately west dipping reverse shear zones.
3.
D3: A change in deformation style to dominantly horizontal movement within the greenstone belt resulted in the development of NE-trending dextral and NWtrending sinistral strike-slip shear zones and associated mesoscopic en echelon fold arrays.
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4.
D4: Extensional collapse within the greenstone belt along steeply dipping north- to NNW-trending faults.
5.
Post-D4: A range of late structures formed, including the intrusion of east-trending Proterozoic dykes, many of local significance only.
ORE DEPOSIT FEATURES PREAMBLE The Perseverance nickel deposit represents the largest single accumulation of Class 1 (Hill and Gole, 1990) komatiite-hosted nickel sulphide mineralisation in the world, and is the largest known occurrence in the Yilgarn Craton where significant volumes of both Class 1 (massive and heavily disseminated sulphide) and Class 2 (weakly disseminated sulphide) mineralisation coexist. The work of researchers from the CSIRO in the mid to late 1980s (eg Barnes, Gole and Hill, 1988a) led to the development of a genetic model for the Perseverance deposit and Perseverance ultramafic complex as a whole, based largely on the geometry and relationships of units within the Perseverance ultramafic complex. To date this model has remained largely unchallenged and has formed the basis of genetic models for numerous other occurrences of dunite-associated, komatiitehosted nickel sulphide mineralisation. Recent work by WMC geologists and associated researchers has led to a more detailed understanding of the relationships between mineralised and unmineralised ultramafic rocks associated with the Perseverance nickel deposit, and the superimposed effect of deformation on the distribution of these rocks. Based on these relationships, it has been possible to reconstruct the general nature of the Perseverance ultramafic complex prior to deformation and refine the genetic model for mineralisation at the Perseverance deposit. This paper presents the results of recent studies at Perseverance, and discusses the results in the context of previous descriptions and models for the evolution of the
Perseverance deposit, and specifically that of Barnes, Gole and Hill (1988a). Detailed descriptions of the character of mineralisation, ore mineralogy, alteration and local stratigraphy are not provided in this paper as they have been adequately described elsewhere.
LITHOLOGY The deposit is hosted by a regionally extensive ultramafic horizon (Fig 3) that occupies the overturned eastern limb of a regional D2 anticline. On a regional scale the succession is characterised by a thick layer of olivine ortho- to mesocumulate, capped by a succession of thin, spinifex-textured flows. At Perseverance the lower cumulate horizon thickens considerably into a zone containing a thick core of olivine adcumulate. The ultramafic rocks that occupy this thickening are termed the Perseverance ultramafic complex. The Perseverance orebody is a zone of high-grade massive and disseminated nickel sulphide mineralisation (Class 1 of Hill and Gole, 1990) situated within an extensive sheet of weak nickel sulphide mineralisation (Class 2). The sheet of weak mineralisation is similar in character to the Mount Keith orebody (Hopf and Head, this publication) and is confined to the stratigraphic base (the western contact) of the ultramafic complex (Fig 3). The Perseverance deposit occupies a structurally complex position within the weakly mineralised sheet, in which deformation has resulted in folding of the orebody, the physical remobilisation of massive sulphide mineralisation into fault bounded lodes and dilatant fold hinges, and the division of the main disseminated mineralisation into a series of vertically stacked imbricate lenses (Billington, 1984). Within the mine environment the disseminated mineralisation forms a distinct shoot that plunges at 70o to the south, extending from the surface to at least 1100 m depth which is the base of exploration drilling. In contrast, the majority of massive sulphide mineralisation occurs in a series of individual, fault-bounded sheets that generally dip steeply to the west and strike north.
FIG 3 - Geological map of the Perseverance–Rocky’s Reward area.
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Ultramafic rocks in the mine area have all been variably affected by metamorphic recrystallisation, including the growth of ‘blade-textured’ metamorphic olivine (Barnes, Gole and Hill, 1988a), serpentinisation (Donaldson, 1982), carbonate alteration (Gole, Barnes and Hill, 1987) and oxidation and/or supergene alteration (Nickel et al, 1977). However, despite these effects, primary igneous cumulate textures are usually well preserved, particularly away from the effects of strain partitioning and alteration along the margins of the ultramafic complex.
Perseverance ultramafic complex The complex (Fig 3) is a zone of thickening in the regionally extensive ultramafic horizon which hosts the Perseverance orebody. The complex comprises a thick, largely intact accumulation of olivine-rich ultramafic rocks (adcumulate to mesocumulate) in which primary igneous cumulate textures are well preserved. The stratigraphic top of the complex on the eastern margin is truncated by a major north-trending fault, termed the ‘East Perseverance fault’, that juxtaposes orthocumulate-dominant, spinifex-textured komatiite against the adcumulate to mesocumulate units that comprise the main part of the complex. To the west of the East Perseverance fault the ultramafic complex is divided into a central, essentially unmineralised domain, and a western domain of variably mineralised ultramafic rocks along the stratigraphic base on the western margin of the complex (Fig 4). The central domain is characterised by a cryptically layered succession of coarse grained (>0.5 cm) adcumulate to mesocumulate which has been variably serpentinised. Igneous olivine is extensively preserved in the core of the domain. Although predominantly unmineralised, two zones of sulphide mineralisation to 10 m wide occur within this domain. These zones locally contain coarse grained and blebby pentlandite interstitial to coarse cumulus olivine, and despite only limited exploration drilling in this part of the complex, appear to form laterally continuous horizons that define an internal stratigraphic succession. In the north of the complex, these zones are oblique to, and truncated against the basal western margin of the ultramafic complex,
whereas to the south the zones can not be traced due to a lack of drilling. The western mineralised domain (Fig 4) is a 2.5 km long sheet of variably mineralised adcumulate to mesocumulate along the base of the ultramafic complex. The domain is dominated by trace to weakly disseminated sulphide, with heavily disseminated (net textured) and massive sulphide mainly restricted to the Perseverance ore environment. Within this domain, systematic variations in the abundance of intercumulus sulphide and the grain size of ex olivine grains, now pseudomorphed by serpentine minerals, define a distinct, laterally continuous mineralised sequence. The domain is thickest, to 250 m wide, south of Perseverance and is truncated by the East Perseverance fault. North of Perseverance the domain forms a zone to 25 m wide within deformed and carbonate-altered ultramafic rocks extending from the F1–F2 shoot described below. At least five stratigraphic horizons are distinguishable in the southern part of the western domain. These horizons comprise three zones containing trace to weakly disseminated intercumulus sulphide which are separated by a zone of weakly to moderately disseminated sulphide and a zone containing only traces of sulphide. The three westernmost horizons can be traced northwards to Perseverance where they form an integral part of the local ore succession and envelop the orebody. The Perseverance orebody is wholly contained within the stratigraphic horizon characterised by weakly to moderately disseminated sulphide mineralisation, termed the main mineralised horizon (MMH), and occurs as a lensoid zone of heavily disseminated sulphide mineralisation within a locally thickened portion of the horizon. In the southern part of the western domain, the sulphide stratigraphic succession is apparently transgressed by unmineralised ultramafic rock of the central domain. This transgression does not correspond with any significant structural features, and is interpreted to have formed as a result of primary igneous processes during the development of the ultramafic complex. The rapid change from mineralised western domain ultramafic rock to essentially unmineralised central domain ultramafic rock, across the domain boundary,
FIG 4 - Geological plan, at 320 m below surface, of the Perseverance ultramafic complex, showing distribution of mineralisation types.
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and the apparent truncation of the western domain sequence by the central domain, suggests that the two domains represent distinct facies within the ultramafic complex which reflect a substantial change in the processes and conditions during development of the complex.
DEPOSIT GEOMETRY The Perseverance orebody is at a perturbation in the western contact of the ultramafic complex (Fig 4) in an area which has undergone a high degree of structural modification.
The mine environment is divided into three domains (Fig 5); the F1–F2 shoot, the 1A shoot and the main disseminated domain. Each domain displays a range of relationships that provide information critical to understanding the genetic and structural evolution of the deposit. The main disseminated domain highlights primary stratigraphic relationships between rock units within the Perseverance ultramafic complex; the F1–F2 shoot highlights relationships between the ultramafic and adjacent felsic rocks; and the 1A shoot highlights a range of structural relationships and controls of massive sulphide mineralisation.
Main disseminated domain The Main disseminated domain (Fig 5) hosts the main ore source at Perseverance. It comprises heavily disseminated sulphide mineralisation enclosed within a laterally extensive zone of moderately to weakly disseminated sulphide (MMH). Despite the high degree of structural modification within the mine environment, this domain has generally undergone a low degree of strain, and primary igneous textures and relationships are widely preserved. Heavily disseminated mineralisation at Perseverance forms a high grade core to the MMH that plunges at 70o to the south and dips steeply to the west. Faults associated with the 1A structure and remobilised sulphide in the F1–F2 shoot (see below) divide the domain into a series of vertically imbricate slices. These faults are predominantly manifest as narrow zones of either foliated and carbonate-altered ultramafic rock to 3 m wide, or narrow zones of blade-textured ultramafic rock.
The main disseminated domain is divided into the central, marginal and low grade zones, that differ in their sulphide abundance and character (Fig 5). The central and marginal zones are the source of ore grade mineralisation (>1.0% nickel), whereas the low grade zone contains subgrade mineralisation and is continuous with the weakly disseminated mineralisation of the MMH. The central zone forms a high grade core to the main disseminated domain. Nickel grades within this domain are consistently greater than about 1.5% nickel (>20 vol % sulphide), and sulphide aggregates form a distinctive and consistent net or semi-net texture (ie sulphides wrapping igneous ex olivine grains) of pyrrhotite and pentlandite in a ratio of ~4:1. Although primary textures are ubiquitous within this domain, metamorphic and deformation fabrics are locally developed. Examples include zones of foliation crenulated with axes parallel to the main fold in the F1–F2 shoot and defined by anastomosing sulphide aggregates between ex olivine grains; intensely carbonated, tremolite-chlorite rich mylonitic ultramafic along narrow zones to 3 m wide; and linear zones of blade-textured metamorphic ex olivine.
Geology of Australian and Papua New Guinean Mineral Deposits
The marginal zone fully encloses the central zone and is characterised by systematic decimetre- to metre-scale layering defined by variations in sulphide content. On the eastern margin of the orebody, where there has been limited structural disturbance, this layering appears to be systematic and cyclic. With few exceptions, each cycle is characterised by an eastward gradation from heavily to weakly disseminated sulphide, with the contacts between cycles represented by an abrupt change in sulphide content, occasionally occupied by a stringer of massive sulphide to 3 cm wide. The outer limit of this zone is marked by the last occurrence of heavily disseminated sulphide and corresponds with a sharp decrease in sulphide abundance, from 10–15 vol % to <5%, a natural break in nickel grade between 0.75 and 1.0% nickel, and a change in the sulphide assemblage from domination by pyrrhotite to domination by pentlandite (Rutter, 1995). The low grade zone consists of weakly disseminated sulphide mineralisation and forms an uneconomic mineralised envelope around the main disseminated mineralisation. The zone typically contains 0.5–0.75 wt % nickel and less than 5 vol % sulphides and is characterised by lobate, pentlandite-rich (pyrrhotite:pentlandite <2:1) aggregates in the interstices between ex olivine grains. This mineralised zone can be traced to the north and south of the Perseverance orebody where it corresponds to the MMH.
F1–F2 shoot The F1–F2 shoot (Fig 5) is a steeply-plunging fold closure in the contact between the Perseverance ultramafic complex and adjacent felsic footwall rocks. Ultramafic rocks within this domain are characterised by strong tectonite fabrics and variable degrees of carbonate alteration. They range from serpentine-rich, high magnesium ultramafic (adcumulate to mesocumulate) to tremolite-chlorite rich ultramafic rock, probably representing komatiite flow tops. Primary igneous textures are rarely preserved within this domain. Mineralisation within this domain is dominated by massive sulphide, which is either remobilised along steep, west-dipping faults that cut across the felsic-ultramafic contact, or as folded sheets parallel to the contact. Contact-parallel mineralisation is generally within a few metres of the felsic-ultramafic contact and is often spatially associated with parallel zones of intensely foliated and carbonated ultramafic rock. Felsic rocks within this shoot form a succession dominated by fine grained, biotite-rich quartzofeldspathic sediment, and includes a sheet of dolerite which has intruded adjacent to the felsic-ultramafic contact. All rock types are truncated against the felsic-ultramafic contact and have been intruded by a large, pipe like body of felsic porphyry in the main fold closure of the felsic-ultramafic contact. The relationships between high-magnesium ultramafic rocks, tremolite-chlorite rich ultramafic rocks and massive sulphide in this shoot were used by Barnes, Gole and Hill (1988a) as criteria for determining the facing directions. The interpretation of facing directions underpinned their model for the development of the Perseverance ultramafic complex and its contained mineralisation. Although the local preservation of delicate skeletal chromite grains within contact-parallel massive sulphide implies that at least some of the massive sulphide in the F1–F 2 shoot is not significantly deformed, observations by WMC geologists and other researchers (Rutter, 1995; N J Archibald, personal
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communication, 1997) cast some doubt over the interpretation that the tremolite-chlorite rich rocks in the F1–F2 shoot ubiquitously represent in situ lava flow tops. Inconsistent geochemical data (Barnes, Lesher and Keays, 1995), the development of similar rock types along altered shear zones within the main mineralised domain and the common occurrence of symmetrically zoned alteration haloes about them, suggest that many of the tremolite-chlorite rich ultramafic rocks represent shear zones in which there has been mobility of elements such as aluminium, magnesium and calcium. As a result the interpretation of gross facing directions within this zone remains equivocal.
1A shoot This shoot (Fig 5) is the largest source of massive sulphide mineralisation in the Perseverance deposit. The domain is represented by a major fault zone called the 1A structure, that contains remobilised massive nickel sulphide mineralisation and numerous tectonic slices or lithons of ultramafic and felsic rocks that are all enclosed within foliated and mylonitic felsic sediment. The 1A structure is discordant to all major rock contacts in the F1–F 2 and main disseminated domains. Massive sulphide and ultramafic lithons extend along the 1A structure and can be traced for over 500 m north of the F1–F 2 domain where they are entirely enclosed within felsic volcaniclastic sediment (Fig 4). The presence of isoclinally folded ultramafic lithons and felsic volcaniclastic horizons within the zone attest to the high strain nature of the shoot. The 1A structure differs in its style and contained rocks north and south of its intersection with the felsic-ultramafic contact in the F1–F2 domain. To the north the structure is characterised by abundant massive sulphide and ultramafic lithons, whereas to the south it is dominated by thin, highly attenuated ultramafic lithons and contains only small volumes of massive sulphide. A lower proportion of mylonitic felsic rocks along the northern part of the 1A structure suggests that the majority of the strain in this area was accommodated by deformation of the massive sulphide. Locally, a set of moderately to steeply plunging, NNEoriented D3 en echelon folds cross the 1A structure and its contained rock units and isoclinal fold limbs. These folds are extensively developed within the felsic rocks of the F1–F2 shoot where they are represented by a pervasive, decimetrescale crenulation and large (>10 m amplitude) chevron folds. Although these folds locally cross the 1A structure, the majority detach and die out along the massive sulphide within the structure. To the north and east of the 1A structure these folds appear to decrease in abundance, whereas to the west of the 1A structure similar folds are much less extensive and generally plunge at a shallow to moderate angle. Accompanying these folds is a series of shear zone splays that are preferentially developed within the felsic rocks of the F1–F2 shoot. The splays offset the rock units with a sinistral sense, and generally bend into and die out along the 1A massive sulphide shoot.
FIG 5 - Geological plan of the Perseverance deposit, at 420 m below surface, showing distribution of domains, shoots and mineralisation types (N J Archibald, unpublished data, 1996).
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As in the F1–F2 shoot, Barnes, Gole and Hill (1988a) interpret facing directions in this shoot based on the relationships between massive sulphide and interpreted komatiitic flow tops. Given the high degree of strain within the 1A domain and extensive remobilisation of massive sulphide, interpretations of facing direction within this domain remain equivocal at best.
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ORE GENESIS MODEL Barnes, Gole and Hill (1988a) interpreted the ultramafic rocks in the Perseverance mine environment to have been deposited during two separate komatiite eruptive episodes separated by a period in which a thick succession of felsic volcaniclastic sediment was deposited in the F1–F 2 shoot domain.They suggested that the mineralisation constituting the Perseverance orebody was deposited during the first volcanic event, and weakly disseminated mineralisation immediately overlying the deposit and extending to the north and south was deposited during the second volcanic event. The juxtaposition of the two volcanic successions and mineralisation styles was interpreted to have resulted from thermal erosion through the felsic volcaniclastic succession by channelised lavas of the second volcanic event. Relationships described in this paper necessitate minor modifications to the model of Barnes, Gole and Hill (1988a), as follows: 1.
2.
Ore grade mineralisation at Perseverance formed during the same volcanic event as the weakly disseminated mineralisation. This is evidenced by the definition of a distinct mineralised stratigraphic succession within the western domain of the Perseverance ultramafic complex which fully encloses the main disseminated domain (Fig 4). The favoured interpretation is that the orebody represents a subchannel within the MMH, and that lava flowing within this channel carried a higher proportion of immiscible sulphide droplets compared to the rest of the MMH. The current spatial relationships between the felsic volcanic succession in the F1–F 2 shoot, the Perseverance ultramafic complex and rocks within the 1A shoot are entirely the result of tectonic emplacement, and do not represent primary stratigraphic layering. This is suggested by the highly tectonised nature of the 1A structure and felsic-ultramafic contact in the F1–F2 shoot, combined with the truncation of the felsic succession in the F1–F2 shoot against these tectonised zones (Fig 5). Kinematic indicators in the 1A mylonite (N J Archibald, personal communication, 1997) and the restriction of massive sulphide and ultramafic lithons to the north of the F1–F2 domain, suggest that the 1A mylonite had a dextral sense of displacement and that its contained rock units were derived from the truncated portion of the F1–F 2 shoot.
3.
Massive sulphide in the F1–F 2 shoot occurs stratigraphically below the main disseminated mineralisation, rather than above it as suggested by Barnes, Gole and Hill (1988a). Although the preservation of skeletal chromite in the contact-parallel massive sulphide of the F1–F 2 shoot indicates that it has not been significantly remobilised, the equivocal nature of the facing criteria presented by Barnes, Gole and Hill (1988a) precludes reliable interpretation. The location of the massive sulphide body below the main disseminated mineralisation is consistent with the overall eastward facing of the Perseverance ultramafic complex and observations in other komatiite-hosted nickel deposits (eg Kambalda; Fig 5 in Stone and Masterman, this publication).
Based on the features described above it has been possible to reconstruct the primary relationships between the rock units of the Perseverance ultramafic complex (Fig 6). This reconstruction shows that the ultramafic units, including the Class 1 mineralisation of the Perseverance deposit, are intimately related and form part a single ultramafic complex. This complex is interpreted to represent a single, large lava channel complex developed during a major eruptive phase. The variations in the primary rock types of the ultramafic complex are considered to reflect changes in the processes and conditions within the channel during lava flow. A model is proposed whereby early development of the complex involved the crystallisation of olivine and precipitation of nickel sulphides in a wide zone of channelised lava flow to form an extensive sheet of weak to trace disseminated sulphide mineralisation. Episodic variation in the conditions and processes (eg discharge, temperature, turbulence) occurring in the channel complex during the eruptive event resulted in changes in the abundance of precipitated sulphides to form a distinct mineralised sequence. The deposit formed during one of these events in which an extensive sheet of weakly to moderately disseminated sulphide was precipitated. The localisation of the mineralisation, its heavily disseminated nature and association with massive sulphides suggests that the mineralisation formed in a subchannel during this event, and that significant sulphur oversaturation resulted in the development of large volumes of immiscible sulphide that accumulated at the base of this channel.
FIG 6 - Diagrammatic reconstruction of the Perseverance ultramafic complex, not to scale.
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Continued olivine crystallisation associated with reduced discharge within the channel resulted in a restriction of the channel width and a shift of the channel axis to the north of its current position. The subsequent lavas rarely attained sulphur saturation, and a largely unmineralised adcumulate succession was deposited. As channel topography decreased, due to filling by crystallisation, flow became less channelled and sheets of komatiitic lava with spinifex-textured flow tops were deposited over the top of the channel. Subsequent deformation resulted in the interleaving of felsic and ultramafic rocks, folding and imbrication of the orebody and remobilisation of massive sulphide mineralisation into fault-bounded shoots and dilational fold closures. Mineralisation along the Perseverance to Sir Samuel high strain corridor, including the Rocky’s Reward deposit, may in part represent tectonic fragments of a single mineralised system dispersed along a zone of large displacement.
ACKNOWLEDGEMENTS The authors would like to thank WMC Resources Ltd for permission to publish this manuscript, and especially J S Reeve and J M A Hronsky for their valuable comments and support. Special mention must also be made of the numerous WMC geologists and external researchers who have contributed to our understanding of the deposits and region but are mentioned in the text. L Nicoletti is thanked for the generation of figures for this manuscript. N J Archibald of Port Management Services Pty Ltd is acknowledged for his observations and thoughts during relogging of diamond core, mapping and geological modelling of parts of Perseverance.
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Swager, C P, Griffin T J, Witt W K, Wyche S, Ahmat A L, Hunter W M and McGoldrick P J, 1995. Geology of the Archaean Kalgoorlie Terrane - An Explanatory Note, Geological Survey of Western Australia, Report, 48:3–15.
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Fazakerley, V W and Monti, R, 1998. Murrin Murrin nickel-cobalt deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 329–334 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Murrin Murrin nickel-cobalt deposits 1
by V W Fazakerley and R Monti
2
INTRODUCTION
EXPLORATION AND MINING HISTORY
The deposits are about 210 km NNE of Kalgoorlie and 50 km east of Leonora in the North Eastern Goldfields region of WA at lat 28o50′S, long 121o54′E, and approximate AMG coordinates 6 810 000 m N, 392 000 m E on the Laverton (SH 51–2) 1:250 000 scale and the Minerie (3240) 1:100 000 scale map sheets (Fig 1).
The Murrin Murrin area was first explored in the late 1890s. A number of small gold mines were worked by prospectors and small syndicates, with reported historical production between 1897 and 1946 totalling 104 016 oz of gold from 136 374 t of ore at an average recovered grade of 23.7 g/t (Department of Mines, 1954). Prospecting in the 1890s also led to the discovery of small lodes of copper–zinc sulphide mineralisation within shales in felsic volcanic rocks at the Anaconda, Rio Tinto and Nangeroo deposits. A total of 47 000 t of ore was mined for recovery of 4300 t of copper at an average grade of 9% between 1899 and 1908 (Gower, 1976).
The tenements are held by Anaconda Nickel NL. Geological investigations between 1994 and 1996 resulted in resource estimates for nine lateritic nickel-cobalt deposits. At a 0.8% nickel cutoff the resources at May 1996 were 125.2 Mt at 1.02% nickel and 0.065% cobalt; 66.4 Mt in the Measured category and 58.8 Mt in the Indicated category. A bankable feasibility study was completed in 1996 and the project is scheduled to be in production by the beginning of 1998.
Modern exploration commenced in 1964, when Selection Trust initiated regional exploration to search for copper–zinc massive sulphide deposits hosted by felsic volcanic and volcaniclastic rocks. In 1969 exploration was redirected towards nickel sulphide deposits but only minor low grade nickel sulphide mineralisation was found (Uren, 1975). However, at this time the nickeliferous clays that host the Murrin Murrin lateritic deposits were discovered, in an area near the current Murrin Murrin 2 deposit. Since ‘no economic process to extract the Ni from the clays’ was thought to exist at the time, no further work was carried out on these discoveries (Uren, 1975). Recognition of the pressure acid leach process and favourable economic forecasts led Anaconda to target the Murrin Murrin area for lateritic nickel-cobalt deposits in late 1993. Limited geological mapping and reverse circulation drilling was carried out prior to a public issue of shares and listing of the company on the Australian Stock Exchange in March 1994. Subsequent exploration work included acquisition and interpretation of geophysical data, geological mapping, petrological, mineralogical and multi-element geochemical investigations, excavation of costeans and trial pits and major drilling programs. Drilling included 4487 m of rotary air blast-percussion drilling in 278 holes, 118 065 m of reverse circulation drilling in 4082 holes and 262.1 m of diamond core drilling in 12 holes.
REGIONAL GEOLOGY
1.
Senior Geologist, Anaconda Nickel NL, PO Box 1456, West Perth WA 6872.
Murrin Murrin lies near the middle of the Archaean Norseman–Wiluna greenstone belt. The deposits are hosted by a laterite profile developed on Archaean serpentinised peridotite rocks, that are within a sequence dominated by immature feldspathic, clastic and volcaniclastic sediment derived from felsic volcanic centres. The sequence also contains mafic volcanic and intrusive rocks (Fig 1) and has been intruded by porphyritic granodiorite and felsic to mafic dykes.
2.
Chief Geologist, Anaconda Nickel NL, PO Box 1456, West Perth WA 6872.
Metamorphism was generally synchronous with regional deformation. It is characterised by relatively low pressure and
FIG 1 - Geological map (after Hallberg, 1985) and location of deposits, Murrin Murrin.
Geology of Australian and Papua New Guinean Mineral Deposits
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V W FAZAKERLEY and R MONTI
high geothermal gradients that produced assemblages from prehnite-pumpellyite through greenschist to upper amphibolite facies (Barley et al, 1990). Structurally the NNE-striking Murrin Murrin ultramaficrich zone is sandwiched between a steeply north-plunging regional scale anticlinorium to the east, and a similar large synclinorium to the west. The sequence forms a corridor between major NNE-trending, westerly dipping strike faults. These faults are splays off the major NW-trending Keith–Kilkenny tectonic zone to the SW. The supracrustal sequence is folded into NNE-trending synclinal structures exhibiting plunge reversals, indicating the presence of cross folding, possibly related to emplacement of the granodiorite.
ORE DEPOSIT GEOLOGY BASEMENT
for field identification, combined with its mineralogy, competency, hardness and colour. This zone is dominated by serpentine with lesser amounts of chlorite and smectite. At Murrin Murrin North and particularly at the Murrin Murrin 2 deposit chlorite is more abundant and commonly a ferruginous saprolite has been formed.
Smectite zone The smectite zone is the most prominent feature of the regolith profile. The smectites are coloured dark green to bright apple green, chocolate brown to brown, and black where manganese oxides are abundant. The zone consists of about 90% smectite with minor chlorite, serpentine and accessory iron oxides and oxyhydroxides (eg goethite, hematite) and manganese and chromium oxides. The lower boundary of the smectite zone on the saprolite is sharp or gradational over a few metres. In the
The peridotite hosting the Murrin Murrin deposits is a serpentinised, medium to coarse grained olivine cumulate rock. Cumulate textures consistent with the ‘B’ zone of a flow and bladed ‘A’ zone spinifex textures, indicate a komatiitic (extrusive) origin. The ultramafic units have significant textural layering concordant with the surrounding stratigraphic sequence at scales from centimetres to several hundred metres. The units also exhibit lateral facies variation and textural changes from olivine orthocumulate through mesocumulate to adcumulate. In the Murrin Murrin area successive prograde and retrograde serpentinitic textures have been documented. Initial serpentinisation has altered olivine to antigorite, pseudomorphing the polyhedral olivine grains. Intercumulus pyroxene (if present) was altered to fibrous tremolite±chrysotile or lizardite. With retrograde alteration antigorite was replaced by chrysotile, lizardite and fibrous chrysotile±brucite, and tremolite after clinopyroxene was retrogressed to fibrous talc and chrysotile. Successive phases of retrograde alteration formed brucite, lizardite and chrysotile assemblages. Metasomatism was not widespread, and allowed formation of talc±carbonate±biotite-phlogopite phases (Pathfinder Exploration Pty Ltd, personal communication, 1995). The complex structural history has resulted in two areas of peridotite outcrop designated Murrin Murrin North and South, separated by about 9 km.
DESCRIPTION OF MAIN REGOLITH UNITS The generalised regolith profile throughout the project area contains four main units. The profile is, in places, complicated by the irregular interlayering of these and other minor zones.
Ultramafic rocks The basement consists of a fresh to slightly weathered, locally silicified ultramafic rock. The boundary between saprolite and ultramafic rock is gradational and tends to be subjectively assigned.
Saprolite zone The saprolite zone is logged from drill hole cuttings as a zone retaining primary rock textures. This is the principal criterion
330
FIG 2 - Generalised laterite profile, Murrin Murrin North.
Geology of Australian and Papua New Guinean Mineral Deposits
MURRIN MURRIN NICKEL-COBALT DEPOSITS
FIG 3 - Typical cross section, Murrin Murrin 4 viewed from the SW.
latter case decreasing amounts of serpentine plus more smectite and chlorite accompany destruction of relict textures of the host rock. The upper contact of the smectite zone is usually sharp. The boundary may, in places, be characterised by a thin mixed zone of ferruginous smectite clay that has similar chemical composition and structure to the underlying smectite clay, but has slightly different physical properties, being generally less plastic, containing less moisture and having a higher iron oxide content.
Ferruginous zone The ferruginous zone consists of a zone of goethite and hematite containing minor clay that may be intermixed with ferruginous and kaolinitic clay at deeper levels. A mixed chlorite-kaolinite clay rich zone termed plastic clay is locally interlayered within the ferruginous zone. Overlying this less consolidated zone is a duricrust (hardcap) of iron oxides. Deposition of carbonates, silica and sulphates has locally produced calcrete, magnesite, silcrete and gypsum. Colluvial material overlies the duricrust.
GEOLOGY OF MURRIN MURRIN NORTH Murrin Murrin North comprises a Y-shaped area of ultramafic rock. The area contains six deposits covering the nose (Murrin Murrin 4) and parts of the western (Murrin Murrin 2, 8 and 9) and eastern (Murrin Murrin 1 and 7) limbs of a synclinal peridotite body (Fig 1). The deposits are continuous with one another, although a north trending dextral fault (not shown on Fig 1) displaces Murrin Murrin 7 and 8. The boundaries between deposits shown on Fig 1 are placed at tenement rather than geological boundaries. The western limb consists predominantly of olivine adcumulate and mesocumulate bodies at Murrin Murrin 8, passing laterally into orthocumulates in Murrin Murrin 9. Murrin Murrin 2 is almost entirely underlain by orthocumulates and mesocumulates. The eastern limb is dominated by olivine
Geology of Australian and Papua New Guinean Mineral Deposits
adcumulates and mesocumulates. Murrin Murrin 4 represents the southern extension of the fold closure, where the predominant basement rock types are olivine orthocumulates and mesocumulates. Murrin Murrin North contains irregular outcrops of felsic volcanic rocks, gabbro and pyroxenite, entirely surrounded by olivine cumulates, reflecting the structural complexity of the area. The deposits occur in areas characterised by lateritic and sandy soils interspersed with low lateritic rises. Trial pits were excavated at the Murrin Murrin 2 and 7 orebodies and contributed considerably to the detailed knowledge of the laterite profile which is generalised in Fig 2. The mineralisation at Murrin Murrin 4 is predominantly contained within the smectite zone whereas in other deposits the mineralisation is continuous through the smectite, saprolite and ferruginous saprolite zones. The ferruginous saprolite zone is predominant at Murrin Murrin 2. The ferruginous zone is characterised at Murrin Murrin 4 and to a lesser extent at the other Murrin Murrin North deposits by a discontinuous plastic clay zone. Figure 3 shows a typical cross section of Murrin Murrin 4.
GEOLOGY OF MURRIN MURRIN SOUTH The Murrin Murrin South area contains three deposits covering the eastern (Murrin Murrin 5) and western (Murrin Murrin 3 and 6) limbs of a tightly folded ultramafic body (Fig 1). The ultramafic unit is a closely folded syncline, trending NNE and plunging to the south. In the core of the fold at the stratigraphic top of the sequence, there is a gabbro-dolerite unit. The western limb consists of three conformable lithostratigraphic units: 1.
a lower olivine orthocumulate layer;
2.
a middle adcumulate unit; and
3.
an upper layered olivine orthocumulate, exposed as outcrops of saprolite and serpentinite.
Pyroxenites and olivine-pyroxene spinifex rocks occur along the basal western contact with felsic volcanics, suggesting multiple flows.
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The area is characterised by a series of NNE-trending, parallel lateritic ridges, dissected by numerous creeks. The ridges rise to a maximum of 35 m above the surrounding plain. A generalised profile for Murrin Murrin South is shown on Fig 4. The ferruginous smectite zone is poorly developed; silica and magnesite are common in the saprolite zone. The Murrin Murrin 6 deposit occurs as a small hill to the SW of Murrin Murrin 3. The laterite between Murrin Murrin 3 and 6 has been stripped by erosion. Figure 5 shows a typical cross section for Murrin Murrin 3.
MINERALISATION The nickel-cobalt mineralisation as indicated by the 0.5% nickel contour (Figs 3 and 5) is hosted primarily in the smectite, saprolite and ferruginous saprolite zones, with lesser amounts within the ferruginous and ferruginous smectite zones. Within these zones significant concentrations of nickel are contained primarily in smectite clays with lesser amounts in manganese oxides, serpentine and chlorite. The bulk of the cobalt in the laterite profile is associated with manganese oxide minerals. The deposits are essentially subhorizontal. Samples submitted for X-ray diffraction, scanning electron microprobe and energy dispersive spectroscopy show that the highest nickel grades are contained in the iron end member of the smectite clay group, nontronite, and also locally in iron rich montmorillonite. A mixed magnesium-iron chlorite is the other main secondary silicate present. Nickel was also detected in iron and manganese oxides, chromite, palygorskite (a fibrous or lath-like magnesium-rich silicate) and silica. In the saprolite zone serpentine and chlorite are the silicate major phases with minor magnesium-rich smectite (K C Camuti, personal communication, 1996; R Townend, personal communication, 1995).
ACKNOWLEDGEMENTS The authors wish to thank Anaconda Nickel NL for permission to publish. Thanks are also extended to all those who worked so hard on the project. FIG 4 - Generalised laterite profile, Murrin Murrin South.
FIG 5 - Typical cross section, Murrin Murrin 3 viewed from the SSW.
332
Geology of Australian and Papua New Guinean Mineral Deposits
MURRIN MURRIN NICKEL-COBALT DEPOSITS
REFERENCES Barley, M E, Groves, D I, Hallberg, J A, Libby, J W and McNaughton, N J, 1990. Geology and Late Archaean tectonic evolution of the Yilgarn Craton, in Gold Deposits of the Archaean Yilgarn Block, Western Australia: Nature, Genesis and Exploration Guides, Publication 20, pp 19–29, (The Geology Department (Key Centre) and The University of Western Australia: Perth). Department of Mines, 1954. List of Cancelled Gold Mining Leases Which Have Produced Gold (Government Printer Western Australia: Perth).
Geology of Australian and Papua New Guinean Mineral Deposits
Gower, C F, 1976. Laverton, Western Australia — 1 : 250 000 geological series, Bureau of Mineral Resources Explanatory Notes SH 51–2. Hallberg, J A, 1985. Geology and Mineral Deposits of the LeonoraLaverton Area, Northeastern Yilgarn Block, Western Australia (Hesperian Press: Perth). Uren, B J, 1975. Murrin Project; progress report March 1975, Northern Selcast (Pty) Ltd, Department of Minerals and Energy Western Australia reference A5747 (unpublished).
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Geology of Australian and Papua New Guinean Mineral Deposits
Hellsten, K, Lewis, C R and Denn, S, 1998. Cawse nickel-cobalt deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 335–338 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Cawse nickel-cobalt deposit 1
2
by K Hellsten , C R Lewis and S Denn
3
INTRODUCTION The deposit is 50 km NW of Kalgoorlie, WA, in the Broad Arrow mineral field, at lat 30o3′S, long 121o08′E or AMG coordinates 320 000 m E, 6 638 000 m N, on the Kalgoorlie (SH 51–9) 1:250 000 scale and the Bardoc (3137) 1:100 000 scale map sheets (Fig 1). The project is owned by Centaur Mining and Exploration Limited (Centaur) who also own and operate the adjacent Ora Banda and Mount Pleasant gold operations which are 7 km to the west and 15 km to the SE respectively.
portion of the Cawse Central area known as Bunyip Dam and the Central pits. Proved and Probable Reserves based on Measured and Indicated Resources in these areas are 24.6 Mt at 1.0% nickel and 0.08% cobalt for 246 000 t of contained nickel and 19 680 t of contained cobalt. This provides for at least 20 years of production at the initial treatment rate of 500 000 tpa of leach feed.
EXPLORATION HISTORY In the late 1960s WMC Limited outlined a resource of approximately 30 Mt grading 1.3% nickel and 0.08% cobalt at Siberia, which is NW and directly along strike from Cawse and within the same prospective stratigraphic group. Small zones of siliceous nickel-cobalt-manganese ore at the Linger and Die, SM7, Gulch and Patch prospects were mined from 1978 to 1980 by shallow (<20 m) open pits (Loftus-Hills, 1975; Elias, Donaldson and Giorgetta, 1981). During the late 1980s Newcrest Mining Limited produced gold from a small open pit adjacent to old workings in granite immediately to the east of the Bunyip Dam resource, and completed a large soil sampling program over the Cawse Central area. Several gold anomalies in granite immediately to the south of the open pit were tested by drilling, without success. Nickel results in soils were relatively low over the ultramafic sequence due to the presence of transported overburden and siliceous caprock.
FIG 1 - Location and geological map, Cawse deposits.
A feasibility study was completed in November 1996 and construction of the processing plant and associated infrastructure is scheduled to commence in May 1997. Production is forecast for the third quarter of 1998. The total Identified Mineral Resource is 217 Mt at 0.7% nickel and 0.04% cobalt (Fig 1). Mining studies have focussed on a 1.
General Manager Operations, Centaur Mining and Exploration Ltd, Level 8, 580 St Kilda Road, Melbourne Vic 3004.
2.
Geologist, Centaur Mining and Exploration Ltd, Level 8, 580 St Kilda Road, Melbourne Vic 3004.
3.
Senior Exploration Geologist, Centaur Mining and Exploration Ltd, 67–71 Dugan Street, Kalgoorlie WA 6430.
Geology of Australian and Papua New Guinean Mineral Deposits
Centaur applied for Prospecting Licences 24/2996–3005 in November 1992 in order to explore for gold-bearing palaeochannel gold mineralisation. Drilling commenced in March 1993 with a single traverse of seven reverse circulation (RC) holes at 200 m or 100 m spacing. Drilling failed to intersect palaeochannel sediments (S Lynn, unpublished data, 1993) but four holes intersected elevated nickel values in limonitic clay, with the best intercept being 8 m at 2.2% nickel from 19 m in KSC3105. The discovery area, some 2.5 km north of the Bunyip Dam pit, became known as the Nickel grid. Follow up drilling comprising 13 RC holes intersected a best result of 27 m at 1.9% nickel from 28 m depth. From April to August 1994 exploration focussed on the delineation of the high grade nickel laterite mineralisation at Nickel grid. Diamond core and RC drilling defined a NEtrending high grade zone associated with a major bedrock shear defined by talc-rich clays. More broadly spaced step out drilling completed at this time outlined a NNW-trending, subhorizontal blanket of 0.7–1.1% nickel mineralisation overlying the dunite sequence of the Walter Williams Formation, adjacent to its contact with the granite to the east. Between September 1994 and mid 1995 broadly spaced RC drilling continued over the Cawse Central and Cawse Extended areas, and by July 1995 a Measured and Indicated Resource of
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K HELLSTEN, C R LEWIS and S DENN
48 Mt at 1.0% nickel and 0.08% cobalt had been outlined. The project was placed on a fast track assessment program in September 1995 culminating in completion of the feasibility study in November 1996.
and includes shear surfaces on granite-greenstone contacts and most strike-parallel shear zones. Local faulting trending north and NE has been correlated with D3 and D4. The regional drainage pattern is also subparallel to these structures (Brand, Butt and Hellsten, 1996).
ORE DEPOSIT FEATURES REGOLITH STRATIGRAPHY The Cawse project area is on the SW side of the Goongarrie–Mount Pleasant Anticline (Fig 1). This regional fold has a width of about 20 km and has a granitoid core overlain by a greenstone sequence. The greenstone sequence progresses from mafic and ultramafic rocks at the base to felsic rock and sediment further up the profile. This sequence is typical of other Norseman–Wiluna greenstone belt successions and can be loosely related to the Kalgoorlie–Kambalda succession (Swager et al, 1990). The mafic–ultramafic sequence on the SW side of the anticline is 10 km thick. This contrasts with the eastern side where the greenstone belt is much more attenuated (Witt, 1990). The oldest unit in the stratigraphic sequence is the Pole Group. This basaltic unit does not outcrop in the project area but has been intersected in drill holes in an area 5 km north of the Nickel grid where it is juxtaposed with granite at the western margin of a major north-trending fault (Fig 1). The ultramafic rocks of economic interest in the area are contained within the Linger and Die Group, which is divided into the Walter Williams Formation and the overlying Siberia Komatiite. The Walter Williams Formation structurally abuts the granitoid in the Goongarrie–Mount Pleasant Anticline, except where the Pole Group is preserved.
The project area lies within an erosional-depositional regime adjacent to a low ultramafic and greenstone strike ridge which closely follows the NW trend of the granite-greenstone contact. This ridge is capped by a lateritic duricrust composed of iron oxide nodules and ferruginous saprolite. The hill flanks and valleys are draped by calcareous red clay soil. A complete lateritic profile is preserved, with partial truncation only associated with the drainage system (Hellsten, Lewis and Brand, 1995; Hellsten and Lewis, 1996). The depth of weathering at Cawse varies according to rock type and the intensity of shearing. It is typically 80 to 85 m, occasionally more than 100 m above the dunite, and shallows to 30 m over the adjacent basalt and granite. The full profile at Cawse (Fig 2) as developed over the Walter Williams Formation consists from the surface of: 1.
transported soil and alluvial silt and clay, 0 to 3 m thick;
2.
lateritic duricrust, 0 to 10 m thick, with a pisolitic ironstone cemented in part near the surface to form a ferricrete layer;
3.
a limonitic zone, 10 to 30 m thick, containing residual clay with finely-divided iron oxide particles imparting an orange or yellow colour. This forms the bulk of the limonitic ore zone and varies considerably in colour and induration according to the relative abundance of iron, silica, manganese oxides and magnesite.
The Walter Williams Formation is divided into a discontinuous basal orthocumulate which grades into an adcumulate dunite, and an overlying orthocumulate. The adcumulate and orthocumulate are separated by a zone of harrisitic textured olivine (Witt, 1990). The contact between the Walter Williams Formation and the overlying Siberia Komatiite is a ‘complex zone of peridotite, pyroxenite, gabbro and high magnesium basalt’ (Witt, 1990). The Siberia Komatiite is a 2600 m thick series of ultramafic flows with olivine spinifex textures, and includes minor high magnesium basalt and gabbro (Witt, 1990). The ultramafic sequence is overlain by basalt and topped by andesite and sediment. Various bodies intrude the basalt and andesite pile including the Ora Banda, Mount Pleasant and Mount Ellis sills and later east-trending dolerite dykes of Proterozoic age. Regional metamorphic grade is greenschist to lower amphibolite.
STRUCTURE Mapping by the Geological Survey of Western Australia and interpretation of aeromagnetic data and drill core indicate that the Cawse area has a complex structural history. The earliest D1 event is characterised by NE to SW crustal shortening producing thrust sheets. The D2 event produced large upright folds including the Goongarrie–Mount Pleasant Anticline. The D3 deformation produced sinistral wrench faulting along NW- to NNWtrending shears. The D4 movement was dominantly vertical FIG 2 - Typical weathering profile, Cawse deposits.
336
Geology of Australian and Papua New Guinean Mineral Deposits
CAWSE NICKEL-COBALT DEPOSIT
A 1 to 4 m thick unit of indurated silica and manganese oxide with pisolitic, crustiform or botryoidal form, known as the siliceous cobalt (or SiCo) zone, is consistently developed at the top of the limonite zone. It is dark brown to blue-black in colour and contains variable but generally high grades of nickel and cobalt; and 4.
a clay zone, 10 to 30 m thick, composed of lime green, mottled green and brown nontronitic clays with variable proportions of silica overprinting. Talcose clays occur in subvertical shear zones within this horizon.
MINERALISATION Nickel-cobalt mineralisation is present as a series of subhorizontal layers over a strike length of at least 30 km. It is stratabound and almost entirely overlies the dunite of the Walter Williams Formation. Higher grade zones are commonly associated with shear and crosscutting structures or dykes which have enhanced the weathering process. The mineralisation predominantly occurs as nickel- and cobaltbearing limonite clays which extend from 8 to 10 m below surface, to a maximum depth of 50 m. There are four zones of nickel-cobalt enrichment: 1.
a subhorizontal blanket of limonitic clay which extends over the strike length of the project areas and contains nickel and cobalt;
2.
a siliceous manganese-nickel-cobalt horizon, 1 to 10 m thick, at the top of the limonite zone;
3.
shear controlled high grade nickel mineralisation in talcose zones; and
4.
nontronite ore, comprising green to chocolate-brown smectitic clays.
The intense weathering of the Walter Williams Formation has led to a reduction in magnesium content from 35% to 2.3% and a three- to five-fold increase in nickel and cobalt grades. The limonitic nature of the Cawse deposit is not typical of Western Australian laterite deposits which are generally hosted by nontronitic clays. The Cawse laterites are derived from a low aluminium dunite rather than a peridotitic protore as at Bulong and Murrin Murrin (Fazakerley and Monti, this publication). Typical data for the four ore types are provided in Figs 2 and 3 and Tables 1 and 2.
CONCLUSIONS The deposit was found by Centaur Mining and Exploration Limited by exploration drilling in areas of alluvial cover, and as such represents a ‘blind’ discovery. It is a major nickel and cobalt resource containing over 1.3 Mt of nickel and 75 000 t of cobalt. The mineralisation is largely hosted by limonitic clay above an adcumulate dunite of the Walter Williams Formation. It is a unique deposit in that ore can easily be upgraded by screening to remove the low grade siliceous fraction. Higher grade nickel and
FIG 3 - Longitudinal projection on line 10 680 m E, Bunyip Dam deposit, looking west. TABLE 1 Ore type data. Ore type and % of resource Limonitic, 78% Siliceous cobalt, 4% Talc zone (shear), 10% Nontronite, 8%
Regolith zone
Depth
Ore minerals
Comments
Clay zone and limonite zone
15–55 m
Fe oxides and minor Mn oxides
Overlies dunite
Mottled upper limonite boundary
<20 m
Mn oxides and minor Fe oxides
Similar to the Siberia Ni-Co laterites
Shear in limonite zone
15–55 m
Nickel chlorite
Controlling structure for high grade talc zones
Clay zone
10–55 m
Nontronite clays
Geology of Australian and Papua New Guinean Mineral Deposits
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K HELLSTEN, C R LEWIS and S DENN
TABLE 2 Chemical composition of ore types.
Ore type and % of resource
Ore grade (%)
Average composition (%) MgO
MnO2
Fe2O3
SiO2
Al2O3
0.8–1.5 Ni
1.5
0.2
24
62
2.0
>1 Ni 0.1–1.5 Co
0.8
4.4
28
54
1.5
Talc zone (shear), 10%
1–2 Ni
5–20
0.3
15
46
2.8
Nontronite, 8%
1–2 Ni
3–15
0.3
23
40
5.9
Limonitic, 78% Siliceous cobalt, 4%
cobalt zones are present in siliceous horizons in the upper part of the lateritic profile, and some are associated with deeper bedrock structures. The low magnesium and aluminium levels of the ore result in low acid consumption and only minor scale formation during processing.
ACKNOWLEDGEMENTS The authors gratefully acknowledge the permission of Centaur Mining and Exploration Limited to publish this paper. J Gutnick and E Eshuys are thanked for their continued support. Cartographic services were provided by L Seager and staff and secretarial expertise was supplied by L Hogeboom.
REFERENCES Brand, N, Butt, C R M and Hellsten, K J, 1996. Structural and lithological controls in the formation of the Cawse nickel laterite deposits, Western Australia - implications for supergene ore formation and exploration in deeply weathered terrains, in Proceedings Nickel ’96 - Mineral to Market (Eds: E J Grimsey and I Neuss), pp 185–190 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Elias, M, Donaldson, M J and Giorgetta, N, 1981. Geology, mineralogy and chemistry of lateritic nickel-cobalt deposits near Kalgoorlie, Western Australia, Economic Geology, 76:1775–1783. Hellsten, K J and Lewis, C R, 1996. Cawse nickel laterite deposit, presented at ‘Australian Nickel Conference 1996’, organised by Alta Metallurgical Services, Perth. Hellsten, K J, Lewis, C R and Brand, N W, 1995. Cawse nickel laterite deposit, presented at ‘Australian Nickel Conference 1995’, organised by Alta Metallurgical Services, Perth. Loftus-Hills, G D, 1975. Ora Banda nickel laterite deposits, in Economic Geology of Australia and Papua New Guinea, Vol 1 Metals (Ed: C L Knight), pp 1010–1011 (The Australasian Institute of Mining and Metallurgy: Melbourne). Swager, C P, Witt, W K, Griffin, T J, Ahmat, A L, Hunter, M, McGoldrick, P J, Morris, P A and Wyche, S, 1990. Geology of the late Archaean Kalgoorlie granite-greenstone terrane, in Third International Archaean Symposium, Perth, 1990, Excursion Guidebook (Eds: S E Ho, J E Glover, J S Myers and J R Muhling), pp 203–304 (The Geology Department and University Extension, The University of Western Australia: Perth). Witt, W K, 1990. Geology of the Bardoc 1:100 000 sheet, Western Australia Geological Survey Record, 1990/14.
Geology of Australian and Papua New Guinean Mineral Deposits
Hicks, J D and Balfe, G D, 1998. Silver Swan, Cygnet and Black Swan nickel deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 339–346 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Silver Swan, Cygnet and Black Swan nickel deposits 1
by J D Hicks and G D Balfe
2
INTRODUCTION The deposits are in the Eastern Goldfields of WA, 43 km NNE of Kalgoorlie, at lat 30o24′S and long 121o39′E on the Kurnalpi (SH 51–10) 1:250 000 scale and Gindalbie (3237) 1:100 000 scale map sheets (Fig 1). The exploration and development project, referred to as Black Swan, is a 50:50 joint venture between Mining Project Investors Pty Ltd (MPI) and Outokumpu Exploration Ventures Pty Ltd (OEV). The project area encompasses three discrete sulphide nickel resources, namely the Silver Swan massive sulphide deposit, the Cygnet high grade disseminated deposit and the Black Swan disseminated deposit. MPI through its wholly owned subsidiary Black Swan Nickel Pty Ltd is manager of the Joint Venture. At 30 June 1996, Silver Swan contained an Indicated Resource of 450 000 t grading 14.0% nickel, for that part of the deposit between 190 m and 550 m depth (1175 RL to 815 RL). The resource equates to a Probable Ore Reserve of 655 000 t at 9.5% nickel. The deposit is open at depth. At a cutoff grade of 0.75% nickel, Cygnet contains a total Indicated and Inferred Resource of 3.4 Mt grading 1.42% nickel, including a discrete high grade zone containing 963 000 t grading 2.47% nickel. At a cutoff grade of 0.4% nickel, Black Swan contains an Inferred Resource of 7 Mt grading 0.8% nickel. Development of the Silver Swan mine began in March 1996 with the commencement of a footwall decline. Commercial production is anticipated to commence in May 1997 and Cygnet may be developed later, using access from the Silver Swan decline. At present there are no plans to develop Black Swan.
EXPLORATION HISTORY The area was first explored for nickel by Australian Anglo American Limited (AAA) in a joint venture with Whim Creek Consolidated NL and Freeport of Australia Incorporated, from 1967. A nickel-copper geochemical soil anomaly was outlined towards the end of 1969 which led to the discovery of the Black Swan disseminated nickel sulphide deposit in 1970. The geology of the Black Swan deposit was described by Groves, Hudson and Hack (1974). AAA drilled three diamond drill holes to the north of Black Swan in 1974, and one of these intersected 17.9 m of disseminated sulphide mineralisation grading 2.17% nickel.
1.
Senior Geologist, Mining Project Investors Pty Ltd, PO Box 749, West Perth WA 6872.
2.
Exploration Manager, Mining Project Investors Pty Ltd, PO Box 749, West Perth WA 6872.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Location and regional geology, Boorara Domain, after Swager and Griffin (1990).
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J D HICKS and G D BALFE
In 1994 the MPI–OEV joint venture recognised the significance of the historical AAA drill results and targeted the Black Swan area for further exploration. Diamond drilling began in April 1995, to follow up the 1974 high grade intersection. The second hole (BSD 15) intersected the Silver Swan massive sulphide shoot 400 m to the north of Black Swan on 9 May 1995 as a 2.45 m core length of massive sulphide grading 16.7% nickel. An additional 131 diamond drill holes (comprising 68 prime holes and 63 wedged daughter holes) of total length 39 000 m were drilled during the following 14 months, permitting resource estimates for Silver Swan and the adjacent disseminated sulphide zone to the south, which MPI named Cygnet. At Black Swan, several additional diamond drill holes have been drilled but no new detailed geological studies have been completed by the MPI–OEV joint venture.
REGIONAL GEOLOGY The Black Swan project nickel deposits are hosted by Archaean greenstones of the Eastern Goldfields of WA, in a region largely covered by salt lakes and transported soil. The area was mapped by the Geological Survey of Western Australia (GSWA) in 1969–1970 as part of the Kurnalpi 1:250 000 scale map sheet (Williams, 1970). Williams recognised two main lithological associations, the Morelands and Gindalbie formations. The latter, of felsic metasediment association, contains the Black Swan komatiite (BSk) host to the Black Swan, Silver Swan and Cygnet deposits. It also hosts the Kanowna Belle gold deposit 25 km to the SSW of Black Swan (Fig 1). Recent 1:100 000 scale mapping of the Kalgoorlie region by the GSWA has led to the subdivision of the region into six tectono-stratigraphic domains (Swager and Griffin, 1990) which comprise the tectono-stratigraphic Kalgoorlie Terrane. The Black Swan area is in the upper greenschist, lower amphibolite facies Boorara Domain. The major structural feature of this domain is the Kanowna–Scotia anticline which has the BSk on its east facing, east dipping limb. To the east of Black Swan the Boorara Domain is separated from the Kurnalpi Terrane by the Mount Monger–Moriarty shear and to the west it is separated from the Kambalda Domain by the Boorara shear.
LOCAL GEOLOGY BLACK SWAN KOMATIITE The Silver Swan, Cygnet and Black Swan nickel deposits are hosted by the BSk. Several small exposures of serpentinised olivine cumulate and talc-carbonate altered rock are the only surface expressions of the komatiite and most of the area, including the Silver Swan and Cygnet deposits, is covered by several metres of transported or residual lateritic soils. Thus geological knowledge of the komatiite has been largely derived from exploration drill holes. Within the project tenements the BSk strikes NW and dips steeply NE. It faces NE, is 150 to 600 m thick and has a strike length of 3 km. Large areas of the komatiite have been subjected to intense carbonation, altering the komatiite to a carbonate-talc±quartz-sericite assemblage. Carbonate is the dominant alteration mineral. It is typically an intermediate member of the magnesite-siderite series and forms a mosaic of 0.5 to 1.0 mm diameter grains completely replacing the
340
komatiite. Fine matted talc flakes are intergrown with carbonate in places to form a significant component of the rock. Quartz is a minor constituent, as optically continuous patches which replace and envelop the carbonate. Minor sericite (hydromuscovite) is a widespread alteration product, particularly about the Cygnet deposit. Two small, irregular bodies of serpentinite, near the northern and southern ends of the project tenements, survived the carbonation event. The mineralogy of the two serpentinite bodies is dominated by antigorite-carbonate-talc±chrysotile. The majority of the Black Swan disseminated deposit, 400 m to the SE of Silver Swan, is within the southernmost serpentinite body. Relict coarse grained (10 to 20 mm diameter former olivine crystals) ortho-mesocumulate textures are common in the two serpentinite areas and thin flow, spinifex textured rocks are recognised in drill holes 900 m to the south and 1400 m to the north of Silver Swan. Elsewhere in the BSk very few komatiite textures survived the carbonation event. Where carbonate alteration was less intense along the basal contact of the BSk, relict fine grained orthocumulate textures (1 to 2 mm) with occasional coarser grained hopper olivines and rare, very fine spinifex textures are preserved in a zone to 2 m thick. Thin zones containing relict orthocumulate textures (2 to 5 mm) have been recognised approximately 100 m stratigraphically above the Silver Swan and Cygnet deposits. The BSk is underlain by a thick sequence of acidintermediate felsic volcanic rocks. They are dominantly felsite, trachyte and other sodic lavas, with minor welded tuff, tuff and agglomerate lavas. All have been extensively sericitised and carbonated, which has destroyed many textural and compositional features of the sequence. There are considerable colour contrasts, due to the pervasive introduction of dark green chlorite and a late stage overprint of the chloritoid ottrelite [(Fe2+,Mg, Mn)2 (Al, Fe3+)(OH)4 Al2O3 (SiO4)2] in favourable locations. These changes are augmented in places by thin shear planes coated with graphite films and ultrafine grains of opaque rutile. Coarse subrounded pseudoclastic features in the lavas are interpreted to be due to flow brecciation. The upper levels of the BSk and the overlying rocks are poorly known, but limited outcrop and drill hole data indicate that the komatiite is overlain by a felsic volcanic sequence similar to the footwall sequence.
INTRUSIVES Three compositionally similar, but texturally variable felsic rock types are recognised within the BSk and the footwall volcanic sequence adjacent to the Silver Swan and Cygnet deposits. All are compositionally similar to the footwall volcanic sequence, ranging from porphyritic sodic trachyte to quartz trachyte. Field relationships and textural differences indicate a probable mixed intrusive-extrusive origin for some of these felsic rocks. Two of the three felsic rock types form a thick, complex zone within the BSk, approximately 100 m stratigraphically above the Silver Swan and Cygnet deposits. Individual contacts within the zone are often strongly transgressive, but overall the zone dips more or less conformably with the komatiite (Figs 2, 3 and 4). Contacts with the komatiite are generally sharp with distinct alteration haloes. Occasionally random chlorite laths, after amphibole, are present in these contact altered zones and may represent relict spinifex textures.
Geology of Australian and Papua New Guinean Mineral Deposits
SILVER SWAN, CYGNET AND BLACK SWAN NICKEL DEPOSITS
FIG 2 - Simplified Silver Swan geological cross section on 11 770 N, looking NW.
FIG 4 - Simplified Cygnet geological cross section on 11 650 N, looking NW.
FIG 3 - Simplified geological plan of the Silver Swan, Cygnet and Black Swan areas on 1100 RL.
Geology of Australian and Papua New Guinean Mineral Deposits
Large areas of the zone are considered to be intrusive and consist of coarse (2 to 3 mm diameter), randomly oriented albite phenocrysts, with occasional quartz phenocrysts set in a groundmass of fine matted albite laths. Patches of fine grained replacive chlorite and dolomite occur throughout, with leucoxenised primary opaque minerals. Ultrafine grained pyrite and chalcopyrite are associated with the chlorite. The remaining areas consist of felsic material that exhibits a pronounced coarse, clast supported clastic texture which is considered to be the result of extrusive processes. Contact relationships between these two texturally different rock types are unclear. The third felsic rock type consists of thin intrusive bodies of fine grained quartz trachyte. They have been observed in the BSk and the footwall volcanic sequence and consist of fine
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J D HICKS and G D BALFE
grained albite phenocrysts in a random mass of subhedral albite grains and occasional interstitial quartz. Fine grained aggregates or veinlets of pale replacive chlorite occur throughout, and carbonate and sericitic alteration is extensive. Fine euhedral pyrite crystals are conspicuous.
of 75 m are attained in these zones, but thicknesses are generally less than 10 m. Within the neck zone the strike length of the deposit is reduced to 30 m, but a horizontal thickness of 20 m is attained. The shape of the deposit below 815 RL (550 m below surface) is unclear due to insufficient drill hole data.
Based on their compositional similarity a close genetic link is indicated between the footwall volcanic sequence and the three felsic rock types.
Silver Swan is a discrete, single body of massive sulphide mineralisation with sharp, relatively undeformed contacts. Minor and thin (1 to 20 cm wide) brittle fractures and ductile shears are recognised within the sulphide body. The immediate hanging wall komatiite is devoid of matrix and disseminated mineralisation. Coarse grained skeletal chromite is occasionally present in the massive sulphides immediately adjacent to the hanging wall contact. The felsic volcanic footwall contact is typically less well defined with small, semirounded felsic substrate inclusions frequently persistent for 1 to 2 m into the massive sulphides. Magnetite often forms thin skins on the contact surfaces between massive sulphide and felsic substrate. Below the deposit, sporadic nickeliferous sulphide disseminations and sulphide filled veinlets and fractures often persist for several metres into the felsic substrate. Nickel grades in this zone rarely exceed 1–2%, but arsenic (in gersdorffite) levels are often significantly elevated.
STRUCTURE A detailed structural understanding of the Silver Swan and Cygnet deposits is still evolving due to the lack of outcrop and the limited distribution of drill holes. No structural features have been identified that are likely to significantly impact on mine development. As a general rule the rock mass characteristics at the two deposits and adjacent rock types are very good, with competent, moderate to high strength rocks throughout.
WEATHERING Beneath the thin surface soil veneer, all basement rock types are weathered to varying depths. The present day ground water table is approximately 30 m below the surface. Weathering is generally limited to depths of 40 to 60 m over the footwall felsic volcanic sequence and large felsic areas within the BSk. Localised pockets of much deeper and more intense weathering occur within the BSk, especially along the footwall contact and above the Cygnet deposit (Fig 4). Narrow tongues of weathered rock extend from these pockets to depths approaching 200 m. The weathered profile is characterised by complete clay alteration near the surface, through a broad saprolitic zone to slightly weathered iron-stained rock near the base of oxidation. Within the saprolite zone the partial dissolution of carbonate grains often gives the komatiite a pitted or vuggy appearance. The distribution of nickel mineralisation in the weathered zone is poorly known due to the scarcity of drill hole data.
DEPOSIT GEOLOGY SILVER SWAN Geology The Silver Swan deposit is a discrete, steeply plunging 550 m long shoot of massive pyrrhotite- pentlandite- pyrite± chalcopyrite- magnetite-ferrochromite mineralisation on the footwall contact of the BSk, below the base of oxidation (Fig 2). It has been intersected by drill holes between 190 and 740 m (1175 to 625 RL) depth, although most intersections are between 1175 and 815 RL. Between the surface and 1175 RL the deposit is believed to be truncated by felsic intrusives, and at depth it appears to continue below 625 RL. The deposit attains a maximum horizontal thickness of 20 m and a strike length of 75 m. It dips NE between 45 and 75o. Three broad zones can be recognised between 1175 and 815 RL. A central (1040 to 940 RL) narrow neck zone separates an upper (1175 to 1040 RL) from a lower (940 to 815 RL) zone. In plan view the deposit has a consistent lens shape in the upper and lower zones, becoming poddy and more irregular in the neck zone. The upper zone dips at 70o towards 064o, and the lower zone dips at 68o towards 094o. Maximum strike lengths
342
Up to three thin massive sulphide zones have been encountered in carbonate-altered BSk within 20 m of the deposit. Similarly, in several locations significant, albeit thin, massive sulphide zones have also been encountered in the footwall felsic volcanic sequence several metres below the deposit. Overall, these occurrences are minor. They may represent primary mineralised trap sites for sulphide-rich liquid or structurally remobilised apophyses from the deposit. Approximately 50 m to the north of the Silver Swan deposit, the basal contact of the BSk is abruptly truncated (folded?) eastward in a complex series of felsic rocks that thin and eventually terminate stratigraphically above the deposit (Fig 3). The felsic rocks are considered to be intrusive, but they also exhibit extrusive features in places. Several thin zones of nickeliferous sulphide have been encountered in drill holes through these transgressive felsic rocks, particularly those nearest the Silver Swan deposit. The felsic rocks have an easterly dip and the overall structure appears to plunge steeply to the north. Differences in strike and dip between the felsic rocks and the deposit result in felsic rock types forming the immediate hanging wall to the deposit in several places. Above 1175 RL the felsic rocks dip more shallowly than the deposit, and are responsible for truncating the deposit near the surface. No evidence of these felsic rocks is evident below about 800 RL.
Mineralisation The massive sulphides are typically coarse grained, without any consistent compositional banding or layering. The sulphides usually have a ‘lattice’ texture of alternating stringers and subparallel lenses of pentlandite and pyrrhotite. Violarite, chalcopyrite, pyrite and gersdorffite are present in minor to trace amounts. Individual pentlandite lenses are typically 2 to 5 mm thick and several centimetres long and consist of 0.1 to 2 mm diameter granular pentlandite crystals. The pyrrhotite lenses comprise coarse granular aggregates of pyrrhotite in addition to minor stringers or flames of pentlandite and pyrite. Replacement of pyrrhotite by pyrite is locally significant, particularly towards the top of the massive sulphides, where 0.5 m thick pyrite-rich zones are occasionally present.
Geology of Australian and Papua New Guinean Mineral Deposits
SILVER SWAN, CYGNET AND BLACK SWAN NICKEL DEPOSITS
Chalcopyrite occurs as thin lenses intergrown within pyrrhotite and pentlandite, and also as 2 to 20 µm intergrowths in pyrite. A later, remobilised pyrite-chalcopyrite association is recognised within the massive sulphides. It occurs primarily as coarse grained aggregates within and around the felsic substrate inclusions concentrated towards the base of the deposit, and as thin veinlets within the 1 to 20 cm thick brittle fracture and ductile shear zones that occur infrequently throughout the deposit. A strong spatial correlation exists between these two areas within the massive sulphide mineralisation and the presence of gersdorffite, the principal arsenic-bearing mineral at Silver Swan.
increase in sulphide content. As a general rule nickel grades increase from the hanging wall and strike extremities of the lens towards the central zone. The average grade of the high grade zone is 2.5% nickel compared with 1.0% nickel for the remainder. Sulphur levels mirror this trend, averaging 4.5% in the high grade zone and 1.95% outside. The high grade zone also contains an occasional thin (< 0.5 m) vein of semimassive to massive sulphide. As a general rule the veins are restricted to the more sulphide-rich areas of the high grade zone. They are both randomly distributed and oriented in drill core and appear to be the product of the remobilisation and reconcentration of sulphides.
Violarite is typically rare within the deposit. It has been observed replacing pentlandite in core from two drill holes, but without any appreciable supergene alteration of other sulphides.
The gangue comprises coarsely crystalline, interlocking plates of the iron-bearing magnesite breunnerite [(Mg, Fe, Mn) CO3] with trace to minor amounts of interstitial replacive quartz. Minor sericite (hydromuscovite) is widespread and locally patches of fine, matted talc flakes are intergrown with the breunnerite and are a significant component of the rock. The rock exhibits a uniformly random fabric, and no primary komatiite textures have been recognised.
Table 1 summarises the analytical data for 53 diamond drill hole intersections of the Silver Swan deposit.
CYGNET
The gangue also contains fine grained magnetite and occasional chromite crystals, to 0.7 mm diameter, mantled with magnetite. Fine grained, bladed to prismatic hematite, sometimes intergrown with sulphides or hosting fine sulphide cores, is also conspicuous.
Geology This disseminated sulphide nickel deposit is immediately to the SE of Silver Swan, and 5 to 10 m stratigraphically above the basal contact of the BSk (Fig 3). It is a coherent, uniform lens of disseminated pyrite-millerite±vaesite (NiS2) mineralisation. The lens, which partly overlaps the Silver Swan deposit (Fig 5) has a strike length of 190 m (11 560 to 11 750 N) and a maximum thickness of 40 m. It dips at about 70o to the NE (Fig 4) and has been delineated by diamond drill holes between 1240 and 940 RL (Fig 5). Mineralisation persists below 940 RL but widths and grades diminish (Fig 4). There is a sharp boundary between oxidised and unoxidised rock at the base of oxidation.
Mineralisation Pyrite is by far the most abundant sulphide and the host to nearly all other sulphides. Minor to trace amounts of millerite, vaesite, siegenite [(Co,Ni)3S4], pentlandite, violarite, pyrrhotite, chalcopyrite and gersdorffite are also present. The sulphides generally have a granular texture and occur as either small, randomly scattered composite grains or as individual grains. There is considerable variation in texture and grain size. The sulphides are clearly enclosed within the breunnerite grains of the host rock. Pyrite grains vary from 200 µm to 1 mm diameter in low nickel grade areas and are up to 15 mm diameter in the central, high grade zone.
The lens is defined by a grade boundary of 0.75% nickel. Outside the lens nickel grades are typically between 0.2 and 0.3%. Within the lens a central zone of higher grade disseminated mineralisation is present along the footwall (west) side of the lens (Fig 4). The high grade zone reflects an
TABLE 1 Drill hole analytical data summary, Silver Swan. Element
Units
No of assays
Length weighted average grade
No of 1 m assay composites
Average composite grade
Coefficient of variation
Ni
%
453
13.66
399
13.72
0.23
Cu
ppm
453
4696
399
4647
0.56
Co
ppm
453
2157
399
2155
0.54
As
ppm
453
1867
399
1850
3.13
Zn
ppm
453
78
399
77
1.81
Cr
ppm
453
671
389
690
2.07
Mn
ppm
30
507
28
509
0.46 0.19
S
%
453
32.98
399
33.08
Au
ppb
271
112
-
-
-
Pt
ppb
271
340
244
339
0.66
Pd
ppb
270
706
244
711
1.65
Fe
%
162
39.65
140
39.47
0.18
Mg
%
160
0.61
141
0.62
3.21
Geology of Australian and Papua New Guinean Mineral Deposits
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J D HICKS and G D BALFE
Siegenite generally occurs as lamellar intergrowths within millerite. There are traces of relict pentlandite and pyrrhotite in the central high grade zone. Chalcopyrite occurs in trace to minor amounts as either fine individual grains in gangue or as intergrowths within pyrite. Gersdorffite occurs as fine intergrowths within chalcopyrite or less commonly as free grains. Table 2 summarises the analytical data available from 43 diamond drill hole intersections of the Cygnet deposit.
BLACK SWAN Geology Black Swan is a ‘low grade’ disseminated sulphide nickel deposit 400 m to the SE of Silver Swan, and 50 m stratigraphically above the basal contact of the BSk. It consists of a central elongate zone of disseminated pyrite-milleritemagnetite-violerite mineralisation, surrounded by several discrete, much smaller mineralised zones (Fig 3). The majority of the deposit is within the southern serpentinite area of the BSk. The central elongate zone has a strike length of approximately 350 m (11 050 to 11 400 N) and a maximum thickness of 130 m. It dips NE and has been delineated by a series of rotary, RC and diamond drill holes between surface and about 250 m depth. The mineralisation is generally patchy, with sulphide-rich zones alternating with sulphide-poor zones over intervals of several metres. On virtually all drill sections there is a consistent, recognisable trend of diminishing mineralised widths with increasing depth.
Mineralisation Groves, Hudson and Hack (1974) recognised two sulphide types at Black Swan; a predominant interstitial type and a relatively rare droplet type. Interstitial sulphides occur as aggregates (< 2 mm diameter) which are generally interstitial to the olivine pseudomorphs and droplet types consist of spheroidal aggregates (to 10 mm diameter) that are typically the same size as, and occur in similar textural positions to the pseudomorphs after olivine. Both sulphide types consist predominantly of pyrite, with subordinate millerite and magnetite, and minor violarite and chalcopyrite. In the carbonated rocks surrounding the serpentinite body, magnetite is virtually absent and chalcopyrite and violarite are more abundant than in the serpentinite. Vaesite was also recognised in these carbonated rocks intergrown with violarite and pyrite.
DISCUSSION
FIG 5 - Longitudinal projection showing Silver Swan and Cygnet resource outlines and exploration drill hole intersections, looking SW.
Pyrite-millerite±magnetite is the most frequent composite intergrowth combination followed by pyrite-vaesite±hematite. As a general rule pyrite-millerite is dominant in the central high grade zone and pyrite-vaesite is restricted to areas outside this zone. Vaesite is typically present to the exclusion of millerite.
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The Silver Swan and Cygnet deposits have several unusual features compared with other nickel sulphide deposits in WA. Principal among these are the mineralogy of the Cygnet deposit and the absence of matrix and or disseminated mineralisation immediately above the Silver Swan deposit, which suggest that decoupling occurred between the sulphides and the komatiite liquid. A more detailed geological understanding of the deposits will result from the development of the Silver Swan mine and ongoing geological investigations. At the completion of the initial exploration and evaluation phases, the Silver Swan deposit can be described as a discrete body of massive sulphide mineralisation at the base of the BSk.
Geology of Australian and Papua New Guinean Mineral Deposits
SILVER SWAN, CYGNET AND BLACK SWAN NICKEL DEPOSITS
TABLE 2 Drill hole analytical data summary, Cygnet. Element
Units
No of assays
Length weighted average grade
No of 1 m assay composites
Average composite grade
Coefficient of variation
Ni
%
1477
1.47
1398
1.46
0.71
Cu
ppm
1477
805
1398
804
1.12
Co
ppm
1477
267
1398
265
0.48 3.47
As
ppm
1458
177
1379
176
Zn
ppm
1458
44
1379
44
0.53
Cr
ppm
1458
674
1379
674
1.50
Mn
ppm
132
588
130
587
0.58
S
%
1458
2.65
1379
2.63
0.74
Au
ppb
149
14
-
-
-
Pt
ppb
135
57
132
58
0.59
Pd
ppb
135
116
132
118
0.65
Fe
%
831
6.59
792
6.57
0.31
Mg
%
766
18.96
729
19.00
0.10
A precursor trough structure to the mineralisation has not been recognised at this stage. The massive sulphides are without recognisable compositional layering or banding and are generally devoid of inclusions except near the basal contact, where small semirounded inclusions of felsic substrate are not uncommon. These inclusions are indicative of sulphide liquid pooling and partial melting of the felsic substrate during formation. The presence of thin magnetite skins on many initial felsic substrate–massive sulphide surfaces is further evidence of a primary undisturbed sulphide liquid–felsic volcanic contact (R Hill, personal communication, 1996). Coarse skeletal chromite in contact with overlying komatiite in several areas of the deposit is indicative of a primary undisturbed sulphide liquid–komatiite contact (Barnes, Gole and Hill, 1988). All these features are consistent with the deposition, in a suitable trap site, of a bed load of sulphide liquid from a flowing lava, possibly within a lava tube (R Hill, personal communication, 1996). Continued komatiite volcanism led to the subsequent formation of the Cygnet and Black Swan disseminated deposits. Large areas of the BSk were subsequently subjected to an intense carbonation event. The Silver Swan massive sulphides were largely unaffected by the carbonation process due possibly to their self-buffering potential. The carbonation process, which is an oxidising process and was ongoing after emplacement of all felsic rock types within the BSk, may be related to a regional gold-arsenic mineralising event. It is believed to be responsible for the introduction and redistribution of arsenic-bearing nickeliferous sulphides in and about the deposits. Downward percolation of liquid sulphide is also a probable mechanism for the redistribution of nickeliferous sulphides into the felsic substrate beneath the Silver Swan deposit. At Cygnet the self-buffering potential of the disseminated sulphides was significantly less, consequently the sulphides were more susceptible to alteration during the carbonation event. The oxidising carbon dioxide–rich fluids are believed to
Geology of Australian and Papua New Guinean Mineral Deposits
be responsible for the alteration and redistribution of a body of disseminated pentlandite-pyrrhotite mineralisation and the formation and eventual encapsulation of the present pyritemillerite (±magnetite) sulphide minerals in a magnesite dominant host. Groves, Hudson and Hack (1974) describe in detail a similar process during serpentinisation and talccarbonate alteration of the Black Swan disseminated deposit. Pyrite-vaesite±hematite was formed at the highest oxidising levels of the carbonation process, and where the original Cygnet sulphide content was low. During the alteration process sufficient remobilisation of sulphides occurred in the more sulphur-rich portions of the central, high grade zone to form the thin semi-massive to massive sulphide veins.
ACKNOWLEDGEMENTS The authors gratefully acknowledge the permission of MPI and OEV to publish this paper and the contributions made by geoscientists from both companies. In particular the authors acknowledge the efforts of S Dowling (MPI) and J Vesanto (OEV) for their contributions to project generation and during initial project work.
REFERENCES Barnes, S J, Gole, M J and Hill, R E T, 1988. The Agnew nickel deposit, Western Australia: Part II. Sulfide geochemistry, with emphasis on the platnium group elements, Economic Geology, 83:537–550. Groves, D I, Hudson, D R and Hack, T B C, 1974. Modification of ironnickel sulfides during serpentinization and talc-carbonate alteration at Black Swan, Western Australia, Economic Geology, 69:1265–1281. Swager, C P and Griffin, T J, 1990. Geology of the Archaean Kalgoorlie Terrane (northern and southern sheets), 1:250 000 geological map, Geological Survey of Western Australia. Williams, I R, 1970. Kurnalpi, Western Australia – 1:250 000 geological series, Bureau of Mineral Resources Geology and Geophysics, Explanatory Notes, SH 51–10.
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Stone, W E and Masterman, E E, 1998. Kambalda nickel deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 347–356 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Kambalda nickel deposits 1
by W E Stone and E E Masterman
2
INTRODUCTION The komatiite-associated nickel sulphide deposits of the Kambalda district are mined by Kambalda Nickel Operations (KNO), a wholly owned subsidiary of WMC Holdings Limited (WMC). The deposits are to the north and south of Kambalda township, 60 km south of Kalgoorlie, WA, at lat 31o13′S, long 121o40′E, and on the Widgiemooltha (SH 57–14) 1:250 000 scale and the Lake Lefroy (3235) and Cowan (3234) 1:100 000 scale map sheets (Fig 1).
Thirty years after discovery, the Kambalda nickel district remains the largest producer of nickel in Australia. In the year ending July 1996, KNO produced 1.27 Mt of ore at 3.27% nickel for 40 000 t of nickel metal in concentrate, from 11 operating mines. As of July 1996, Proved and Probable Reserves were 10.5 Mt at 3.1% nickel. Total historical production from 1967 to 1996 is approximately 34 Mt at 3.1% nickel, for more than 1 Mt of nickel metal in concentrate. The pre-mining resource (production plus present Measured and Indicated Resources) is approximately 67 Mt at 2.9% nickel. This paper documents the outstanding stratigraphic, volcanological, geochemical, and mineralisation characteristics of the recent discoveries and/or developments at Helmut, Mariners, Blair and Coronet, and at the existing Carnilya Hill mine. It also presents the pertinent results of studies of the Kambalda massive sulphide fabrics (Cowden and Archibald, 1991), ore tenor variations (Lesher and Campbell, 1993), and Re-Os isotope data (Foster et al, 1996; Lambert et al, in press). The discussion focusses on the impact of this information on the previous ore environment models and the ore genesis models derived for the district.
EXPLORATION HISTORY Since the description by Cowden and Roberts (1990), exploration in the Kambalda nickel district has resulted in the discovery of significant iron-nickel sulphide deposits in the Tramways belt (Helmut mine), at the Widgiemooltha dome (Mariners mine and Miitel prospect), and the Golden Ridge–Carnilya Hill belt (Blair mine), as well as at the Kambalda Dome (Coronet mine). These successes have largely resulted from the use of ore environment models and derivative ore genesis models developed for earlier known deposits in the Kambalda Dome area (Ross and Hopkins, 1975; Marston and Kay, 1980; Gresham and Loftus-Hills, 1981). The Helmut deposit is in the Tramways belt, 45 km south of Kambalda (Fig 1). Helmut was initially intersected in WMC diamond drill holes in 1980. Follow up diamond and percussion drilling and downhole electromagnetic (EM) surveys from 1993 to 1995 delineated a pre-mining resource of 1.1 Mt at 2.8% nickel. Production commenced in 1996, with total production to 1997 of 25 000 t at 1.86% nickel. In view of the large resource, Helmut will be a major contributor to future nickel production. FIG 1 - Geological setting and location of nickel deposits in the Kambalda Dome, St Ives, Tramways, Widgiemooltha dome, and Golden Ridge–Carnilya Hill areas, Kambalda nickel district.
1.
Senior Research Geologist, Kambalda Nickel Operations, WMC Resources Ltd, Kambalda WA 6442.
2.
Geologist-in-Charge - Blair–Carnilya Hill, Kambalda Nickel Operations, WMC Resources Limited, Kambalda WA 6442.
Geology of Australian and Papua New Guinean Mineral Deposits
Mariners is on the eastern flank of the Widgiemooltha dome, 70 km SW of Kambalda. It was initially intersected in 1974 in diamond holes drilled by an exploration partnership between Anaconda, CRA, and Union Minière Development and Mining, but the deposit was not delineated until 1989, during a program of diamond drilling by WMC. Production commenced in 1991, with total production to 1997 of 650 000 t at 2.8% nickel. The present pre-mining resource equals 1.3 Mt at 3.5% nickel and the orebody is open down plunge. Mariners will be a major contributor to future nickel production, but about 22% of the ore will contain 500 ppm arsenic.
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The Blair deposit is in the Golden Ridge–Carnilya Hill belt, 40 km north of Kambalda (Fig 1). The deposit was discovered by percussion and diamond drilling in 1982 following the identification of nickel gossans in the area, and production commenced in 1989. Total production to 1997 is 660 000 t at 2.60% nickel. The present pre-mining resource equals 1.0 Mt at 3.2% nickel and the orebody is open down plunge. The Coronet deposit is on the NW flank of the Kambalda Dome (Fig 2). It was discovered by diamond drilling in 1993, 16 years after the last previous discovery on the Dome (Beta prospect). Production commenced at Coronet in 1996, with total production to 1997 of 11 000 t at 2.27% nickel. The present pre-mining resource is 600 000 t at 3.5% nickel and the orebody is open down plunge.
the Kambalda Dome, St Ives, Tramways, Widgiemooltha dome, and Golden Ridge–Carnilya Hill areas (Fig 1). In the Kambalda Dome area (Fig 2), the deposits are most closely associated with the Lunnon Basalt (footwall) and the Kambalda Komatiite (host rock and hanging wall). The volcanic and sedimentary sequence accumulated in the 40 Myr interval from 2710 to 2670 Myr (Fig 2). The sequence was subsequently intruded by granitoid, metamorphosed up to amphibolite facies, and complexly faulted and folded during four episodes of deformation (Gresham and Loftus-Hills, 1981; McQueen, 1981; N J Archibald, unpublished data, 1985). During metamorphism and deformation the komatiitic rocks hydrated to antigorite+chlorite+magnetite assemblages or carbonated to talc+magnesite or dolomite±chlorite assemblages. The geological evolution of the other areas in the Kambalda nickel district is broadly similar to that in the Kambalda Dome area. The Kambalda Komatiite (Fig 3) comprises the Silver Lake Member (Cowden and Roberts, 1990) and the overlying Tripod Hill Member (Thomson, 1989). The iron-nickel sulphide mineralisation is generally restricted to the lowermost Silver Lake Member flows. In each flow, lateral and vertical variations in composition, degree of differentiation, and distribution of interflow sedimentary units define channel flow facies and sheet flow facies (Cowden, 1988; Lesher, 1989).
FIG 2 - Geological map of the Kambalda Dome area showing the distribution of rock units, and the iron-nickel sulphide deposits in plan projection. The stratigraphic scheme is from Cowden and Roberts (1990) and the geochronological data are from Compston et al (1986), Claoué-Long, Compston, and Cowden (1988), and J M F Clout (unpublished data, 1991). The age of the Lunnon Basalt is uncertain. Ages derived for xenocrystic zircons from the basalt are between 3432±6 Myr and 3370±4 Myr and between 2730±39 Myr and 2667±18 Myr (Compston et al, 1986).
The Carnilya Hill deposit is in the Golden Ridge–Carnilya Hill belt, 20 km ENE of Kambalda (Fig 1). The deposit was discovered in 1970 by BHP, as a result of drilling nickel gossans, and production commenced in 1980. Total production to 1997 is 1.29 Mt at 3.47% nickel. The present pre-mining resource is 1.30 Mt at 4.7% nickel.
REGIONAL GEOLOGY AND MINERALISATION STYLES The Kambalda nickel district is in the south-central part of the Norseman–Wiluna greenstone belt, Yilgarn Craton (Gee et al, 1981; Cowden and Roberts, 1990) and contains ore deposits in
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FIG 3 - Schematic stratigraphic and volcanic model of the typical Kambalda-style nickel sulphide ore deposit, adapted from Gresham and Loftus-Hills (1981).
The channel flow facies is dominated by up to 100 m of magnesian (to 48% volatile-free MgO) komatiitic flows with thick olivine cumulate zones and relatively thin spinifextextured zones. The facies is traceable for up to 15 km in length and 500 m in width, occupies trough structures in the underlying Lunnon Basalt (Fig 3), and contains felsic ocelli at its lateral margins (Frost and Groves, 1989). These features suggest that the channel flow facies represents the main locus of turbulent lava flow (Cowden and Roberts, 1990). The sheet flow facies consists of thin, less magnesian (≤36% volatile-free MgO) komatiite flows with thin cumulate zones and relatively thick spinifex-textured zones. They are separated by thin sulphidic sedimentary units (Bavinton, 1981), and lack associated mineralisation (Fig 3). These features suggest that the sheet flow facies represents a regime of laminar lava flow in areas flanking channels. The iron-nickel sulphide deposits are restricted to the base of the lowermost channel facies flow as ‘contact ore’ and more
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sedimentary substrate by sulphur-undersaturated lava, and subsequent iron-nickel sulphide saturation and accumulation on the channel floor.
rarely, to the base of overlying flows as ‘hanging wall ore’ (Fig 3). Contact ore provides 80% of the resources, as tabular to ribbon-like bodies up to 3 km long, 300 m wide, generally <5m thick and <0.5 to 10 Mt in size (Marston and Kay, 1980). It also occurs as pod-like bodies hundreds of metres long distributed along a linear trend (Evans, Cowden and Barratt, 1989). Individual sulphide (pyrrhotite+pentlandite±pyrite± chalcopyrite) bodies are zoned, with a zone to 2 m of massive sulphide at the base overlain in sequence by matrix sulphide to 2 m thick and then by disseminated sulphides and/or in places blebby sulphides. The massive zone contains from 30 to 70% of the total nickel in the ore profile. The fabrics shown by the massive sulphide zone, such as monomineralic layering and foliated pyrite lenses, preserve the entire deformational sequence (Cowden and Archibald, 1987).
ORE DEPOSIT GEOLOGY The recently discovered and/or developed deposits and prospects and the known Carnilya Hill deposit have important geological characteristics that differ significantly from the typical Kambalda-style deposit (Table 1), which render aspects of the descriptions of previous ore environment and genetic models inadequate and possibly inaccurate. Documentation of the atypical characteristics will therefore advance an understanding of ore environments in all the areas of the district. Understanding of the ore environments is also advanced by consideration of more general structural and geochemical studies (Cowden and Archibald, 1991; Lesher and Campbell, 1993; Foster et al, 1996; Lesher and Stone, 1996; Lambert et al, in press) undertaken since the deposits were described by Cowden and Roberts (1990).
Significant systematic differences exist in tenor (nickel content in 100% sulphides) between individual deposits (Cowden, 1986) and, locally, between volcanic channels (Marston and Kay, 1980). The tenor variations have been attributed by Cowden and Woolrich (1987) to variations in f(O2):f(S2) within the sulphide-silicate system during lava emplacement and ore genesis.
HELMUT
The features of the iron-nickel sulphide deposits suggest formation by igneous processes (Ross and Hopkins, 1975; Gresham, 1986; Cowden, 1988; Lesher, 1989). The genesis of the deposits has been explained by two competing models, illustrated by Stone (1996). Briefly, traditional models (Lesher et al, 1981; Cowden 1988) are based on a proximal komatiitic eruptive centre, an intrinsic source of sulphur (komatiite lava or magma), lava channelling in palaeotopographic lows, and accumulation of the sulphides at the flow base at the transition between the turbulent flow regime in the channel and the laminar flow regime in the flanks. In contrast, more recent models (Lesher, 1989 and references therein) are based on a distal komatiitic eruptive centre, an extrinsic source of sulphur, channelling by thermal erosion and assimilation of sulphidic
The deposit occurs within talc-magnesite-magnetite altered olivine orthocumulate rock, 3 to 7 m up section from the footwall basalt contact (Fig 4). The host flow unit appears to be up to 110 m thick and 400 m wide, and may be the largest channellised olivine cumulate sequence in the Kambalda nickel district. It is overlain by interflow sulphidic to carbonaceous to cherty sedimentary rocks (S Olsen, unpublished data, 1995), and passes laterally to highly brecciated flow units in the flanking facies (Hollamby, 1995). The deposit is dominated by dense disseminated to matrix mineralisation rather than massive mineralisation and ore tenor is low, but increases from <5% to 8–12% with increase in total sulphide content. The ore
TABLE 1 Characteristics of the recently discovered and/or developed iron-nickel sulphide deposits that differ from the typical Kambalda-style deposit. Deposit name
Helmut
Ore position
Mariners
Blair
Coronet
Carnilya Hill
Kambalda Dome mainly contact
internal
contact
contact
mainly contact
contact
Ore trend
linear
arcuate
linear
linear
linear
linear
Ore morphology
poddy
poddy
ribbon-like
poddy
ribbon-like
ribbon to tabular
ds and mx
mx and ds
ms and mx
ms and mx
ms and mx
Ms and mx
ds to mx to ms to mx to ds
ms to mx to ds
ms to mx to ds
ms to mx to ds
ms to mx to ds
ms to mx to ds
ms>mx>ds
ms<mx
ms<mx
ms<mx
ms<mx
ms<mx
25
280
315
300
Dominant mineralisation style Zoning Tenor variation Ni:As ratio Pd (ppb) Host unit Host unit thickness (m)
up to>10 000
up to 600
up to 700
up to 3000
lowermost high Mg flow
lowermost high Mg flow
lowermost high Mg flow
lowermost high Mg flow
low-Mg flow on contact
lowermost high Mg flow
110
~100?
60
40–60
2–20
30–60
Flanking fancies
thin, brecciated flows
thin, spinifex textured flows
thin, spinifex textured flows
Sediment in ore environment
yes
yes
yes
no
no
generally no
Major hanging wall structure
none recognised
Albatros fault
none recognised
Loreto thrust
none recognised
none recognised
Note: ms = massive, mx = matrix, ds = disseminated
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FIG 4 - Cross section of the Helmut–Schmitz area, looking north, showing the internal position of the Helmut deposit and the large apparent size of the associated trough structure, modified from S Olsen (unpublished data, 1995).
profile consists of a small core of massive mineralisation which passes sequentially outwards to matrix mineralisation and then to disseminated mineralisation (Fig 5). The apparent internal occurrence of the deposit, abnormal thickness and width of the host unit, presence of sedimentary rocks up section in the channel flow facies, brecciated flow units in the sheet flow facies, predominance and distribution of disseminated and matrix mineralisation in conjunction with the low nickel tenor are features that differ from the typical Kambalda-style deposit (Table 1).
MARINERS The deposit appears to be arcuate in longitudinal section (Fig 6) and locally is markedly enriched in arsenic (Table 1) with a nickel:arsenic ratio of 25:1 and to 30 000 ppm arsenic (N Poll, unpublished data, 1992). It is also enriched in nickel sulphidearsenide phases such as gersdorffite and niccolite, and in platinum group elements (PGE), particularly palladium (to >10 000 ppb). However, the tenor is only medium at 8 to 12% nickel. The high arsenic values occur mainly in foliated ore, particularly within NNE-trending, steep easterly-dipping latestage faults (Fig 7). The deposit is also characterised by erratic variations in thickness and continuity of ore pods and the presence of a 25 m thick pyrrhotitic sedimentary rock unit in the hanging wall. The arcuate shape, enrichments in arsenic and palladium in medium tenor ore, complex variations in distribution, and presence of a sedimentary rock unit in the hanging wall are features of Mariners that differ significantly from the typical Kambalda-style deposit.
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FIG 5 - Detailed cross section of the Helmut deposit, looking north, showing its internal position, dominance of disseminated and matrix ore, and apparent upwards zoning of disseminated to matrix to massive to matrix to disseminated ore, adapted from S Olsen (unpublished data, 1995).
BLAIR The deposit is immediately underlain by sedimentary rocks rather than tholeiitic basalt. This footwall sedimentary sequence (Fig 8) is up to 15 m thick and is composed of carbonaceous pelite and smaller amounts of pelletoidal pelite, epidosite and cherty rock which stratigraphically overlie
Geology of Australian and Papua New Guinean Mineral Deposits
KAMBALDA NICKEL DEPOSITS
FIG 6 - Longitudinal projection looking west, showing the overall arcuate distribution and the irregular pod-like shape of the Mariners orebodies. The white lines represent the locations of the 10 sill drive and the 980 sill drive backs mapping illustrated in Fig 7.
FIG 7 - Geological maps of the 10 and 980 level sill drives at Mariners deposit showing the strong spatial association of high (>500 ppm) arsenic values and NNE-trending, east-dipping faults. The fault control on the distribution of the 02 and 03 orebodies, which could originally have been a single orebody, is also noteworthy.
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tholeiitic basalt. In lateral flanking environments the sedimentary sequence thickens and the pelites are interbedded with volcaniclastic greywackes (Marchiori, 1995). The presence of a pelite footwall, the carbonaceous composition of the pelite, and lateral increase in the thickness of the footwall sedimentary sequence are characteristics of Blair that differ from the typical Kambalda-style mineralisation.
CORONET The deposit occurs beneath overthrust Lunnon Basalt (Fig 9). The overthrust, with at least 700 m of subvertical offset, is called the Loreto thrust (R I Williams, unpublished data, 1994) and most probably formed during the thrust-stacking D1 episode (Cowden and Roberts, 1990). Earlier, the Coronet–Loreto area (Fig 2) had been interpreted as the closure of a north-plunging faulted syncline called the Otter trough (J J Gresham, unpublished data, 1978; Cowden and Roberts, 1990). The thrust-hidden position of Coronet is a characteristic of its general structural setting that is anomalous compared to the typical Kambalda-style deposit.
CARNILYA HILL
FIG 8 - Geological plan showing the stratigraphic and structural setting of the Blair deposit. Note that the immediate footwall is sedimentary rock (modified from Marchiori, 1995).
The deposit has a medium to high tenor at about 10 to 16% nickel (S Macklin, unpublished data, 1989), with massive mineralisation in a low magnesium (<20% volatile-free MgO) amphibole-chlorite altered picrite and pyroxenite unit which is up to 20 m thick. This unit (Fig 10) underlies and is in sharp contact with unmineralised high-magnesium talc-chlorite
FIG 9 - Cross section of the Coronet deposit, looking north, concealed from surface by overthrust Lunnon Basalt.
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Geology of Australian and Papua New Guinean Mineral Deposits
KAMBALDA NICKEL DEPOSITS
flowing komatiite lava as immiscible sulphide droplets. In consequence, variations in ore tenor within and between the nickel sulphide deposits are controlled by variations in silicate lava: sulphide liquid ratio (R factor; 100 for the low tenor ores and 500 for the high tenor ores), as well as by variations in f(O2):f(S 2) ratio (Cowden and Woolrich, 1987). This model (illustrated in Stone, 1996), emphasises the importance of timing and mechanism of sulphide saturation in concentration of the sulphide to form ore and the availability of an external sulphur source, and predicts the concentration of nickel sulphide mineralisation processes in distal volcanic settings.
FIG 10 - Diagrammatic cross section of the Carnilya Hill deposit at the contact of low magnesium rocks and footwall basalt, down section of unmineralised high magnesium rocks (komatiitic olivine cumulate).
altered komatiitic olivine cumulate with 32 to 52% volatile-free MgO (S Macklin, unpublished data, 1989). The presence of the medium to high tenor mineralisation in the low-magnesium host rocks immediately down section of unmineralised highmagnesium rocks is a characteristic of Carnilya Hill that is different from the typical Kambalda-style mineralisation.
GENERAL FEATURES The significance of fabric relationships in the massive sulphide zones (Cowden and Archibald, 1987) to genetic models for the troughs at Kambalda Dome has been discussed by Cowden and Archibald (1991). They interpret the sulphide fabrics to indicate that the pinch-out structures that bound the troughs (Fig 3) are complex D2 fold-thrust couples, which form curvilinear traces that transgress orebody trends. These foldthrust couples are considered to have preferentially nucleated at massive sulphide contacts during peak metamorphic conditions synchronous with D2. On the basis of these relationships and interpretations, Cowden and Archibald (1991) concluded that the trough structures at Kambalda may not be volcanic channels, but instead a product of deformation. Alternatively, the troughs could be tectonically modified volcanic channels (Cowden and Roberts, 1990).
NICKEL DEPOSIT GEOCHEMISTRY The ore tenor variations question has recently been addressed by Lesher and Campbell (1993). According to their model, sedimentary sulphide will not be assimilated by komatiite lava, but instead melts and remains as a dense sulphide liquid at the channelled flow base, or is incorporated into the turbulently
Geology of Australian and Papua New Guinean Mineral Deposits
The Re-Os isotopic system is a robust tracer of iron-nickel sulphide ore genesis and a sensitive monitor of crustal assimilation processes. Re-Os isotopic studies of iron-nickel sulphide mineralisation at Kambalda Dome have been conducted by Foster et al (1996). Ore samples from the Coronet, Victor, Hunt and Otter deposits (Fig 2) have low Re:Os ratio values and a near-chondritic (nonradiogenic) initial Os isotopic composition (γOs values) of -3 to +21. However, a sample from a sulphidic sedimentary rock unit flanking the Hunt deposit has very high Re:Os ratio values and an extremely non-chondritic (radiogenic) initial γOs value of +900. These data, according to Foster et al (1996), constrain bulk assimilation of sulphidic sediments and selected sulphide sedimentary components, and limited sedimentary-derived fluids or volatiles to <0.5% and the content of average crustal material to <2% at R factors of 100–500.
ORE ENVIRONMENTS STRATIGRAPHIC SETTING The presence of the brecciated lateral flow units appears to be unique to Helmut. These units could represent disrupted solid levees lateral to constructional channels during lava flow (Lesher and Arndt, 1995). However, this interpretation, as pointed out by Hollamby (1995), is inconsistent with the presence of interflow sedimentary rocks in the ore environment, which indicates a low energy depositional environment and possibly quiescent intervals in an eruptive cycle when lava flow ceased and lavas solidified. Alternatively, Hollamby (1995) interpreted the lateral relationship to the ore environment to indicate that the brecciated units represent lava tube margins disrupted during lava resurgence. If correct, this model suggests that the brecciated units could indicate major resurgent lava tubes, which, by analogy to Helmut, could host major deposits. The features of the hanging wall sedimentary rocks at Helmut and Mariners and the footwall sedimentary rocks at Blair suggest a stratigraphic rather than a tectonic relationship with the komatiitic rocks and mineralisation. If this interpretation is correct, the presence of these sedimentary rocks is inconsistent with the previous ore environment models (Fig 3), where the lack of sedimentary rocks is a fundamental characteristic of the channel facies component. Furthermore, sedimentary rocks are footwall to other deposits in the district (Fig 1), as at Mount Edwards, Wannaway, Foster, Edwin, Ken and McMahon (M St J Turner, unpublished data, 1983; Evans, Cowden and Barratt, 1989). Consequently, the lack of sedimentary rocks as an indicator of ore environments should be reassessed, at least in exploration of the Tramways belt, Widgiemooltha dome, and Golden Ridge–Carnilya Hill areas. At Carnilya Hill, the spatial association of medium to high tenor mineralisation with the low magnesium rocks is markedly
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inconsistent with the previous ore environment models, in which the mineralisation is associated with the highest magnesium rocks at the base of the lowermost komatiite unit (Fig 3). The Carnilya Hill low magnesium rocks are considered to represent a discrete flow unit emplaced before the onset of komatiite volcanism. Consequently, highest MgO enrichment in the lowermost komatiitic unit as an indicator of ore environments should be reassessed, at least in exploration of the Golden Ridge–Carnilya Hill area.
STRUCTURAL SETTING The previous ore environment models, in which basalt is restricted to the stratigraphic footwall, except in pinch-outs, and the sulphide deposits consist of linear arrays of either ribbon-like orebodies or regular pod-like orebodies, are contradicted by the overthrust of Lunnon Basalt at Coronet and the arcuate shape and complex distribution of the ore pods at Mariners. The presence of the overthrust at Coronet has important implications for descriptive ore environment models, because similar structures could conceal major additional deposits in other areas of the district. In addition, if some of the troughs or parts thereof are severely deformed, this could explain the arcuate shape and complex ore distribution of Mariners. If correct, this interpretation implies that the shape and distribution of the more deformed ore environments differ markedly from the less deformed ore environments. Obviously the lack of superimposed structural complexity in the previous ore environment models should be reassessed.
MINERALISATION POSITION AND STYLE The presence of well preserved pseudomorphs of tabular, possibly crescumulate olivine zones in the base together with the lack of spatially associated faults suggests that the internal position of the Helmut deposit results from volcanic rather than later tectonic processes. If correct, this interpretation requires emplacement of the barren footwall before the mineralised hanging wall, possibly as a separate flow. A similar model has been proposed by Groves et al (1986) to explain the origin of the hanging wall ore at the Lunnon deposit (Fig 2), where spinifex textures and hanging wall ore were present in and on top of the lowermost komatiite unit, contact ore was present at the base of that unit, and those orebodies passed laterally to sedimentary units. Obviously, Helmut is not an example of the typical Kambalda-style hanging wall ore. In the previous genetic models (Lesher, 1989; Cowden and Roberts, 1990), formation of the massive sulphide zone was followed by sequential formation of the matrix sulphide zone and the disseminated sulphide zone. Lesher and Campbell (1993) attribute this zoning to formation of the massive zone as a dense layer of sedimentary-derived sulphide melt at the flow base, followed by emplacement of the overlying matrix mineralisation zone and disseminated mineralisation zone as immiscible sulphide droplets settled from the turbulently flowing lava. However at Helmut, disseminated and matrix mineralisation appear in part to predate massive mineralisation. If igneous in origin, the formation of this atypical ore profile requires very wide variations in the relative proportions of olivine and iron-nickel sulphide accumulated. As previously noted by S Olsen (unpublished data, 1995), the Helmut mineralisation style is in many ways more comparable to the iron-nickel sulphide mineralisation at the Perseverence deposit
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in the north of the Norseman–Wiluna greenstone belt (Hill et al, 1995; Libby et al, this publication) than deposits at Kambalda Dome.
ORE GENESIS In the most recent ore genesis model (Lesher and Campbell, 1993), the nickel tenor of matrix and particularly disseminated mineralisation can be significantly higher than massive mineralisation, because R factors should be higher during formation and accumulation of these iron-nickel sulphides. However, the observed tenor variations and therefore related R factors at Helmut are opposite to this prediction. Consequently, the typical relationship between mineralisation style and nickel tenor variation is not entirely universal and should be reassessed in any further development and refinement of ore genesis models, at least for use in exploration of the Tramways belt. In the strictest context of the most recent ore genesis model (Lesher and Campbell, 1993), the dominance of matrix and disseminated sulphides, in conjunction with the high arsenic and PGE contents, if igneous in origin, suggests high R factors during mineralisation at Mariners. However, this possibility is inconsistent with the medium nickel tenor of the deposit. Alternatively, the strong spatial association with the NNEtrending faults strongly implies that at least the high arsenic content is secondary in origin (N Poll, unpublished data, 1992; Goodgame, 1997) and related to hydrothermal fluid flux along those structures and alteration of pentlandite to gersdorffite and niccolite. If correct, the possibility that arsenic and PGE enrichment might be hydrothermal rather than igneous in origin should be seriously considered, at least in the case of Widgiemooltha dome deposits, if not in general. The significance of the Os isotopic results to the most recent genetic models and derived exploration techniques has been hotly debated. The disparate Os isotopic compositions of the ore deposits and associated sulphidic sedimentary rock led Foster et al (1996) to conclude that the iron-nickel sulphide deposits formed from uncontaminated komatiite melts, and hence a crustal source of sulphur may not be required for sulphur saturation nor concentration of sulphide in a limited zone to form ore. Alternatively, they propose that sulphur saturation may have resulted from cooling, decompression, or changes in fO2 and fS2 in the komatiite melt, and that concentration of the sulphide resulted from fluid dynamic processes during emplacement of the komatiite lava. In response, Lesher and Stone (1996) argued that the radiogenic initial γOs isotopic signature of the original sedimentaryderived sulphide would be diluted to the observed nonradiogenic initial γOs signature at R factors of 100–500. However, according to Lambert et al (in press), R factors of 8000 are required to explain the observed signature. If correct, the models of Foster et al (1996) and Lambert et al (in press) favour intrinsic sulphur source-based genetic models (Cowden, 1988) and emphasise the importance of fluid dynamic processes in sulphide concentration, and, importantly, permit mineralisation in proximal volcanic and even plutonic settings, with or without associated sulphidic sedimentary rocks.
ACKNOWLEDGEMENTS This paper is published with permission of WMC Resources Limited and with support, encouragement, and critical
Geology of Australian and Papua New Guinean Mineral Deposits
KAMBALDA NICKEL DEPOSITS
comments from J Reeve, R Behets, G Chapman, A Hill, M Larson, D Mapleson, B Palich and A Restell. Preparation of the figures by KNO Geological Spatial Information Section is acknowledged.
Gresham, J J, 1986. Depositional environments of volcanic peridotiteassociated nickel deposits with special reference to the Kambalda dome, in Geology and Metallogeny of Copper Deposits (Eds: G Friedrich, A Genkin, A J Naldrett, J D Ridge, R H Sillitoe and F M Vokes), pp 63–90 (Springer–Verlag: Berlin).
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Compston, W, Williams, I S, Campbell, I H and Gresham, J J, 1986. Zircon xenocrysts from the Kambalda volcanics: age constraints and direct evidence for older continental crust below the Kambalda–Norseman greenstone belt, Earth and Planetary Science Letters, 76:299–311.
Hollamby, J A, 1995. The volcanology of the Tramways area, Kambalda, Western Australia: A petrographic and textural study of the Silver Lake Member of the Kambalda Komatiite Formation, BSc thesis (unpublished), Monash University, Melbourne.
Cowden, A, 1986. The geochemistry, mineralogy and petrology of the Kambalda iron-nickel sulphide deposits, Western Australia, PhD thesis (unpublished), The University of Western Australia, Perth. Cowden, A, 1988. Emplacement of komatiite lava flows and associated nickel sulfides at Kambalda, Western Australia, Economic Geology, 83:436–442. Cowden, A and Archibald, N J, 1987. Massive-sulfide fabrics at Kambalda and their relevance to the inferred stability of monosulfide solid-solution, Canadian Mineralogist, 25:37–50. Cowden, A and Archibald, N J, 1991. Massive sulphide fabrics at Kambalda: Sensitive records of deformation history, in Extended Abstracts, Structural Geology in Mining and Exploration, Publication No 25, pp 99–102 (The University of Western Australia: Perth). Cowden, A C and Roberts, D E, 1990. Komatiite hosted nickel sulphide deposits, Kambalda, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 567–581 (The Australasian Institute of Mining and Metallurgy: Melbourne). Cowden, A and Woolrich, P, 1987. Geochemistry of the Kambalda iron-nickel sulfides: implications for models of sulfide-silicate partitioning, Canadian Mineralogist, 25:21–36. Evans, D M, Cowden, A and Barratt, R M, 1989. Deformation and thermal erosion at the Foster nickel deposit, Kambalda, Western Australia, in Magmatic Sulphides - The Zimbabwe Volume (Eds: M D Prendergast and M J Jones), pp 215–219 (The Institution of Mining and Metallurgy: London). Foster, J G, Lambert, D D, Frick, L R and Maas, R, 1996. Re-Os isotopic evidence for genesis of Archaean ores from uncontaminated komatiites, Nature, 382:703–706. Frost, K M and Groves, D I, 1989. Ocellar units at Kambalda: evidence for sediment assimilation by komatiite lavas, in Magmatic Sulphides - The Zimbabwe Volume (Eds: M D Prendergast and M J Jones), pp 207–214 (The Institution of Mining and Metallurgy: London). Gee, R D, Baxter, J L, Wilde, S A and Williams, I R, 1981. Crustal development in the Yilgarn Block, Western Australia, in Archaean Geology, Special Publication 7 (Eds: J E Glover and D I Groves), pp 43–56 (Geological Society of Australia: Perth). Goodgame, R, 1997. The distribution and origin of arsenic and platinum group element mineralization in the Mariners nickel sulphide deposit, Widgiemooltha, Western Australia, PhD thesis (unpublished), University of Oregon, Eugene, Oregon.
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Lambert, D D, Foster, J G, Frick, L R, Hoatson, D M and Purvis, A C, in press. Application of the Re-Os isotopic system to the study of Precambrian magmatic sulphide deposits of Western Australia, Australian Journal of Earth Sciences. Lesher, C M, 1989. Komatiite-associated nickel sulfide deposits, Reviews in Economic Geology, 4:45–101. Lesher, C M and Arndt, N T, 1995. REE and Nd isotope geochemistry, petrogenesis and volcanic evolution of contaminated komatiites at Kambalda, Western Australia, Lithos, 34:127–157. Lesher, C M and Campbell, I H, 1993. Geochemical and fluid dynamic modeling of compositional variations in Archean komatiite-hosted nickel sulfide ores in Western Australia, Economic Geology, 88:804–816. Lesher, C M, Lee, R F, Groves, D I, Bickle, M J and Donaldson, M J, 1981. Geochemistry of komatiites at Kambalda: I. Chalcophile element depletion - a consequence of sulfide liquid separation from komatiitic magmas, Economic Geology, 76:1714–1728. Lesher, C M and Stone, W E, 1996. Exploration geochemistry of komatiites, in Trace Element Geochemistry of Volcanic Rocks: Applications for Massive Sulphide Exploration Short Course Notes 12 (Ed: D A Wyman), pp. 153–204 (Geological Association of Canada: Winnipeg). Marchiori, K, 1995. The stratigraphy of the Blair nickel mine, Kalgoorlie terrane, Western Australia, with special reference to the sedimentary rocks, BSc thesis (unpublished), Monash University, Melbourne. Marston, R J and Kay, B D, 1980. The distribution, petrology and genesis of nickel ores at the Juan complex, Kambalda, Western Australia, Economic Geology, 75:546–565. McQueen, K G, 1981. Volcanic-associated nickel deposits from around the Widgiemooltha Dome, Western Australia, Economic Geology, 76:1417–1443. Ross, J R and Hopkins, G M F, 1975. Kambalda nickel sulphide deposits, in Economic Geology of Australia and Papua New Guinea, Vol 1 Metals (Ed C L Knight), pp 100–121 (The Australasian Institute of Mining and Metallurgy: Melbourne). Stone, W E, 1996. Sulphur sources for komatiite-associated Ni sulphide ores in the Kambalda mining district, Western Australia, in Proceedings Nickel’96 (Eds E J Grimsey and I Neuss), pp 103–109 (The Australasian Institute of Mining and Metallurgy: Melbourne). Thomson, B, 1989. Petrology and stratigraphy of some texturally well preserved thin komatiites from Kambalda, Western Australia, Geological Magazine, 126:249–261.
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Buck, P S, Vallance, S A, Perring, C S, Hill, R E and Barnes, S J, 1998. Maggie Hays nickel deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 357–364 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Maggie Hays nickel deposit 1
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by P S Buck , S A Vallance , C S Perring , R E Hill and S J Barnes
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INTRODUCTION
EXPLORATION HISTORY
The deposit is 500 km east of Perth, WA, at lat 32o15′S, long 120o30′E, on the Lake Johnston (SI 51–1) 1:250 000 scale map sheet (Fig 1). It is owned by Forrestania Gold NL (Forrestania) with 50% equity and QNI Limited (QNI)) with 50%, and managed by Forrestania. The deposit had an Inferred and Indicated Resource at 26 July 1996 of 12 Mt at 1.55% nickel, and is open at depth and along strike to the south.
Serious mineral exploration in the Lake Johnston greenstone belt began in 1966, when Laporte Mining (Laporte) commenced exploration for nickel sulphide deposits. In 1969, Union Miniere and Laporte formed a joint venture which in 1971 located the first evidence of nickel sulphides in the belt at Maggie Hays as 6.1 m at 0.98% nickel in drill hole LJ3 and 2.4 m at 1.0% nickel in LJ6. This mineralisation is shallow and located 600 m south of the Maggie Hays deposit. In 1974, Amoco Minerals Australia Incorporated (Amoco) acquired portions of the Lake Johnston belt, to test beneath the mineralisation discovered earlier at Maggie Hays. Initial follow up diamond drilling in 1974 intersected subgrade mineralisation. Between 1978 and 1980, shallow rotary air blast (RAB) drilling by Amoco encountered anomalous nickel-copper values in the weathered zone, particularly along the basal (eastern) contact of the ultramafic unit which hosts the Maggie Hays deposit. Diamond drilling at the northern end of the prospect in 1981 intersected nickel sulphide mineralisation in nine holes, with a best intersection of 4.15 m at 5.8% nickel. Amoco subsequently curtailed exploration because of the complexity of the mineralisation and depressed nickel prices. At least one of the Amoco drill holes intersected the up dip portion of the Maggie Hays deposit. Throughout the 1980s exploration in the Lake Johnston belt was carried out by a number of companies with a focus on gold rather than nickel. Drilling by Cyprus Minerals (Cyprus) encountered anomalous nickel to 1% over down hole widths to 30 m in the weathered zone, flanking the upper margin of the deposit.
FIG 1 - Location and regional geological map, Lake Johnston greenstone belt, after Gower and Bunting (1976).
This description is based on the results of exploration by Forrestania since 1991 and collaborative research by the CSIRO Division of Exploration and Mining (Buck et al, 1996).
1.
Exploration Manager, Forrestania Gold NL, 15 Ord Street, West Perth WA 6005.
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Senior Project Geologist, Maggie Hays Nickel NL, 15 Ord Street, West Perth WA 6005.
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Research Scientist, Magmatic Ore Deposits, CSIRO Division of Exploration and Mining, Underwood Avenue, Floreat Park WA 6014.
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Group Leader, Magmatic Ore Deposits Group, CSIRO Division of Exploration and Mining, Underwood Avenue, Floreat Park WA 6014.
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Principal Research Scientist, Magmatic Ore Deposits Group, CSIRO Division of Exploration and Mining, Underwood Avenue, Floreat Park WA 6014.
Geology of Australian and Papua New Guinean Mineral Deposits
In 1991 Forrestania purchased the Maggie Hays prospect and carried out further diamond drilling which was not successful in locating significant additional mineralisation. However, it was subsequently concluded that numerous holes failed to adequately test the stratigraphic base of the host ultramafic unit. Deeper drilling in 1993 finally succeeded in locating the main zone of disseminated mineralisation in part of the deposit. The discovery of the Maggie Hays deposit was the culmination of 22 years of exploration, during which the deposit eluded discovery because it commences at 100 to 180 m beneath the surface and the geometry of the mineralised contact is complex. In 1994, Forrestania, through its fully owned subsidiary Maggie Hays Nickel NL, entered into a joint venture agreement with Gencor Ltd, to explore the Lake Johnston belt and continue the evaluation of the deposit. Gencor earned its 50% interest by solely contributing to exploration and demonstrating that its BioNIC technology can successfully treat Maggie Hays concentrate. In 1995, the joint venture discovered significant strike extensions of massive nickel sulphide mineralisations hosted by felsic volcanic rocks, north of the disseminated zone. In late
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1997 diamond drilling of the Maggie Hays deposit was still progressing, thus the boundaries of the deposit have yet to be firmly delineated and the geology fully understood. In 1997 Gencor’s interest in the deposit was transferred to QNI.
REGIONAL GEOLOGY The deposit is in the Lake Johnston greenstone belt, which forms a part of the Southern Cross Province in the Archaean Yilgarn Block, between the southern portions of the Southern Cross–Forrestania greenstone belt to the west and the Norseman–Wiluna greenstone belt to the east. The geology of the belt was first documented by Honman (1914). In 1971 the belt was mapped by the Geological Survey of Western Australia (Gower and Bunting, 1976), and found to comprise a folded, west-facing, homoclinal sequence of felsic to mafic volcanic and sedimentary rocks, including banded iron formation (BIF), and ultramafic units interpreted to be of intrusive origin. The greenstone belt is a series of irregular, cuspate to linear shaped blocks separated and intruded by granitoid plutons (Fig 1). The most continuous portion of the belt, which hosts the Maggie Hays deposit, occurs between Round Top Hill and Mount Glasse, a distance of about 60 km. The bedrock geology is widely masked by lateritic duricrust, deep oxidation and a variety of transported materials. Weathering of the ultramafic rocks is particularly intense with widespread development of silica-rich bands (‘caprock’) through the saprolite profile, beneath the surface duricrust layer. Gower and Bunting (1976) subdivided the stratigraphic succession into the Maggie Hays, Honman and Glasse formations, from east to west (Fig 1). Each contains an ultramafic unit, termed the Eastern (EUU), Central (CUU) and Western (WUU) ultramafic units by Forrestania and CSIRO (Perring, Barnes and Hill, 1994). The Maggie Hays Formation is the oldest, and predominantly consists of basalt, along the eastern margin of the belt. The EUU, which occurs as a discontinuous series of lenses <80 m thick, is located towards the top of the formation. Surface exposures of this ultramafic unit are sparse. Chips from shallow RAB drill holes suggest that it is a series of thin differentiated komatiitic flows with variable magnesium contents. The overlying Honman Formation comprises a lower sequence of undifferentiated felsic volcanic rocks which are overlain by the CUU, the most prominent of the three ultramafic units and host to the Maggie Hays deposit. This ultramafic unit, which occurs throughout the belt over thicknesses to 600 m, is also the most magnesium rich and consists predominantly of olivine cumulates. The base of the Glasse Formation is marked by the first appearance of a regionally persistent BIF, as a layered series of units intercalated with the lower flows of the WUU. The WUU, which is up to 400 m thick and occurs continuously throughout most of the belt, consists of a fractionated series of thin komatiitic flows having varying magnesium contents. The upper portion of the formation is mainly basalt. The greenstone belt has been affected by early regional thrusting with movement primarily concentrated along the the BIF horizon. The Maggie Hays deposit and the host CUU are
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truncated at depth by an east-dipping reverse fault system. In addition to early thrusting, there is evidence for several other later fault sets having east, west-northwesterly and northeasterly orientations. Numerous narrow, mafic Proterozoic dykes with northwesterly and easterly orientations transect the greenstone belt and flanking granitoids. The well known east trending, mafic to ultramafic Jimberlana Dyke intrudes across the belt approximately 1.4 km north of the Maggie Hays deposit (Fig 1).
ORE DEPOSIT FEATURES STRATIGRAPHY The Maggie Hays area spans a distance of about 12 km between the Jimberlana Dyke in the north and a granitoid pluton north of Maggie Hays Hill which locally disrupts the stratigraphic succession (Fig 1). The bedrock geology is masked by a laterite profile and pediment or colluvium cover. The laterite comprises a nearsurface ferruginous duricrust, underlain by a broad mixed zone of siliceous caprock and nontronite clays and finally, a narrow saprolite zone. The ferruginous duricrust is only partially preserved because of erosion, which has caused deposition of a ferruginous pediment or colluvium lag downslope. To overcome the masking effects of laterite and transported cover, an understanding of the geology has relied on detailed logging of widespread RAB, reverse circulation percussion and diamond drill holes and costeans. The sequence (Fig 2) strikes northwesterly, with dips varying from 60° east in the north to 80° west in the south. From east to west it comprises felsic volcanic rocks, the thick, lenticular CUU cumulate flow unit, an interlayered series of BIF and felsic volcaniclastic units, and a sequence of thin differentiated komatiitic flows of the WUU. Spinifex textures in the WUU indicate that the sequence faces west and is partly overturned, particularly in the northern part.
Felsic volcanic rocks A thick sequence of felsic volcanic rocks forms the stratigraphic footwall to the CUU (Fig 2). In the north, where the succession is overturned to the west, the felsic sequence structurally overlies the CUU. The most prominent rock type is porphyritic, with up to 15% plagioclase phenocrysts in a fine-grained quartz-feldsparmuscovite-biotite groundmass. Vague broad layering, which may represent bedding, occurs locally. Many of the primary textural and structural features have often been overprinted by metamorphic recrystallisation, making it difficult to determine if the units are of lava flow or volcaniclastic origin. An exception are several volcaniclastic units which consist of subangular, poorly sorted lithic lapilli (<64 mm) supported by a fine grained (<1 mm) matrix suggesting a laharic, mass flow origin. Fine grained, thin to laminar bedded volcaniclastic rocks of probable subaqueous origin occur in association with the interpreted laharic units.
Central ultramafic unit (CUU) The unit attains a maximum thickness of 400 m in its northern part, at the Maggie Hays deposit (Fig 2), and thins progressively towards the south. Drill hole data are too sparse
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MAGGIE HAYS NICKEL DEPOSIT
FIG 2 - Geological plan of Maggie Hays deposit, modified from Perring, Barnes and Hill, (1994) and Perring (1995a).
to indicate the internal stratigraphy of the CUU in detail. The evidence suggests that it consists dominantly of olivine adcumulate and mesocumulate with up to 2% cumulus chromite (Perring, Barnes and Hill, 1994). In the northern part of the unit the basal stratigraphic zone consists of olivine orthocumulate, to 50 m thick, which may be underlain by, or intercalated with, pyroxene-bearing orthocumulate units 5–10 m thick. Geology of Australian and Papua New Guinean Mineral Deposits
There are limited data regarding the top of the CUU. Available information suggests that the olivine mesocumulates and adcumulates fractionate upwards into pyroxene±olivine cumulates and pyroxene-plagioclase cumulates (Perring, Barnes and Hill, 1994; Perring, 1995a). Locally the olivine mesocumulates to adcumulates are capped by a thin flow top.
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Geochemistry of the CUU Komatiitic samples from the CUU have been analysed by XRF by the CSIRO Division of Exploration and Mining (Perring, 1995b) with recalculation to 100% on a volatile- and sulphidefree basis. Samples containing more than 1.5% sulphur were filtered from the suite. Elements compatible with olivine, such as magnesium and nickel, are elevated in olivine adcumulates, becoming progressively less in olivine mesocumulates, orthocumulates, pyroxenites and komatiitic flow margins. In contrast, incompatible elements, such as calcium, aluminium and titanium, are most enriched in fractionated rocks which approach the composition of the komatiitic liquid (Perring, 1995b; Buck et al, 1996). The constant Al2O3:TiO2 ratio, close to 12.5, indicates that the Maggie Hays rocks are cogenetic and that the komatiitic source lavas were depleted in aluminium with respect to the expected composition of a primitive mantle melt. There are also ultramafic rocks depleted in aluminium at Forrestania, in the adjacent greenstone belt to the west, and at Ruth Well in the Pilbara (Nesbitt, Sun and Purvis, 1979). This contrasts with komatiites in the Norseman–Wiluna belt which are not depleted in aluminium.
BIF and felsic volcaniclastic rocks The CUU is capped by BIF and felsic volcanic units (Fig 2) which occur as a layered series intercalated with basal flows of the WUU. The BIF horizon, which is up to 150 m thick, has variable mineralogical characteristics. In less structurally affected zones it consists of alternating layers of quartzmagnetite-actinolite-quartz-pyrrhotite±pyrite and quartzactinolite±garnet±diopside±magnetite. Structurally affected zones have undergone intense hydrothermal alteration which consists dominantly of silica and lesser chlorite, garnet, pyrrhotite and pyrite. Brecciation may be a feature of the altered zones, as a result of hydraulic fracturing by hydrothermal fluids. The felsic volcanic rocks associated with the BIF horizon (Fig 2) commonly have an aphyric texture and consist of a fine grained quartz-plagioclase-muscovite assemblage. Although metamorphism has obliterated primary textures, the protolith is interpreted to be of tuffaceous origin.
Western ultramafic unit (WUU) The WUU is less well understood than the CUU. The unit is best exposed by costeans several kilometres south of the deposit, where it is about 250 m thick. Here it consists of a pile of thin, differentiated komatiitic flows with well preserved olivine spinifex-textured flow tops and olivine orthocumulate to mesocumulate flow bases. The proportion of olivine cumulate phases is highest towards the stratigraphic base (east), because of higher magnesium levels during the early phase of volcanism.
to amphibolite facies and the majority of rock types are metamorphic (Binns, Gunthorpe and Groves, 1976; Perring, Barnes and Hill, 1994). Tremolite-chlorite rich rocks are the metamorphic equivalent of the chilled base or spinifex textured top of a komatiite flow unit. Where spinifex textures are preserved, igneous pyroxene is pseudomorphed by tremolite-actinolite and igneous olivine by serpentine and/or chlorite plus magnetite. Rocks devoid of relict igneous textures are commonly pale grey in colour and consist of the assemblage tremolite-chlorite-magnetite±metamorphic olivine-talc. Assemblages developed after olivine orthocumulates comprise subequal proportions of metamorphic olivine and tremolite-chlorite. Other phases which may be present in minor amounts are anthophyllite, talc and chromite. The metamorphic olivine component imparts distinctive macroscopic textures which are typically porphyroblastic, although ‘ribbons’ of metamorphic olivine are also common. Metamorphic rocks derived from olivine mesocumulate protoliths are typified by the magnesian silicates metamorphic olivine, anthophyllite, cummingtonite and talc, with 5 to 25% tremolite+chlorite and minor chromite. Rocks with an adcumulate composition have been reconstituted to metamorphic olivine (variably retrogressed to serpentine) with minor amounts of anthophyllite (<10%, variably retrogressed to talc), talc (<5%), chlorite (<5%) and chromite (<3%). Relict olivine adcumulates are also present. The relict igneous olivine in these rocks is recognisable by its dark brown colour, which contrasts with the clear, colourless metamorphic equivalent.
STRUCTURE There are at least two generations of faults, a NW-trending reverse fault set, related to early thrusting, and a later, crosscutting, NNW-trending, steeply dipping set (Fig 2). Reverse faults substantially disrupt the stratigraphic continuity of the CUU along strike to the north and at depth (Figs 2 and 3). The faults dip at 50–60o to the east and although drilling information is limited, particularly from within the footwall fault block, they appear to be developed as a compound zone with a close spatial association with the BIF horizon (Fig 2). To the south the fault zone appears in places to demarcate a high angle discordance between stratigraphic dips in the hanging wall and footwall fault blocks. In the hanging wall block, stratigraphic dips of the CUU and footwall felsic volcanic rocks tend to be steeper and therefore strongly discordant to the fault zone orientation. In contrast, stratigraphic dips of the BIF and WUU in the footwall block appear to be at shallower angles to the fault zone.
Metamorphosed ultramafic rocks
The CUU at Maggie Hays appears to be restricted to the hanging wall fault block. The fault-controlled termination of the CUU down dip has a south-plunging pitch line which reaches the surface at about 83 000 N (Figs 2 and 4).
The ultramafic rocks at Maggie Hays have a wide range of compositions from olivine-poor, spinifex textured rocks and pyroxene cumulates to highly magnesian olivine adcumulates. Relict igneous olivine is preserved in olivine adcumulates at the core of the CUU. However, the area has been metamorphosed
Based on aeromagnetic data, the NNW-trending faults (Fig 2) appear to be late, regional tectonic features, possibly crosscutting both the greenstone belt and flanking granitoid plutons. Minor faults of this set, with offsets of less than 100 m, locally disrupt the stratigraphic continuity at Maggie Hays.
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but is a gradual boundary over which the sulphide content progressively diminishes. The northern and southern limits of the deposit have yet to be fully closed off by drilling, however a progressive narrowing in thickness of the mineralisation along strike suggests that the termination of the deposit is being approached. The deposit consists of distinctly different southern and northern zones, each having a unique host lithology and geometry of mineralisation.
Southern zone The southern zone (Figs 2 and 4) has a strike extent of approximately 850 m and a down dip extent of approximately 350 m. Dips vary along the length of the zone, from subvertical and mainly to the west in the southern part, becoming overturned to the west and dipping at about 60° east in the northern part. Mineralisation occurs at the base of the thickest portion of the CUU, down dip from a roll in the contact which may represent a palaeotopographic high (Fig 3). The mineralisation appears in part to occupy an internal, subtle trough-shaped depression.
FIG 3 - Cross section 82 700 N, Maggie Hays deposit, looking WNW. Location on Fig 2.
MINERALISATION Distribution The Maggie Hays deposit is a tabular body about 1400 m long (Fig 2). The up dip extremity of the deposit, between 100 and 180 m below the present ground surface, is not sharply defined
The zone is truncated at depth by an east-dipping reverse fault system (Fig 3). The south-plunging pitch line marking the termination of the CUU and the mineralisation against the fault plane, defines both the down dip and northern limits (83 000 N) of the southern zone (Fig 4). The mineralisation in the southern zone consists of a zone of dominantly disseminated (<40%) sulphides, to about 40 m thick, stratigraphically underlain over a wide area by a narrow basal massive sulphide zone to 7 m thick. The characteristics of the mineralised section can vary according to the distribution of the various types of ultramafic cumulates, as generalised in Table 1.
FIG 4 - Longitudinal projection of Maggie Hays deposit, looking SSW.
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TABLE 1 Generalised stratigraphic profile of the southern ultramafic-hosted mineralisation.
Top
Base
Stratigraphic section Barren adcumulate Olivine-sulphide adcumulate
Olivine-sulphide mesocumulate Olivine orthocumulate Massive sulphide Chill zone pyroxenite (tremoliteactinolite)
Sulphide content Decreases with increasing stratigraphic height from 15–35% towards the base to 5–10% towards the top 15–40% 15–40% 80–100% <10%
Thickness
To 40 m
<10 m <5 m <1 m
In places the massive sulphides contain inclusions of chill zone material and are locally remobilised into the footwall felsic volcanic rocks. The bulk of the disseminated mineralisation is continuous and forms a main, lower zone of nickel concentration. Locally the mineralisation also occurs in several subsidiary, lenticular hanging wall zones.
Northern zone The zone (Figs 2 and 4) is contiguous with the southern zone, the boundary between them being the south-plunging pitch line of the fault termination of the CUU and disseminated mineralisation. The northern zone remains partially open down plunge to the south, beneath the disseminated zone. The northern limit of the zone is characterised by a gradual thinning of the mineralisation but has yet to be closed off conclusively by drilling. The northern zone is entirely hosted by the felsic volcanic sequence (Fig 5), which forms the stratigraphic footwall to the CUU further south, and is spatially controlled by the reverse fault system which truncates the CUU at depth (Fig 3). The northern zone averages about 3 m in thickness with a maximum thickness of 9.5 m. It comprises massive sulphide layers to 3 m thick and broader zones of stringer sulphides, which appear to represent a matrix enclosing felsic clasts. In part the mineralisation appears to be several stacked zones of uncertain lateral continuity. Based on the apparent strong structural controls, it is suggested that the northern zone may have formed during early deformation, by the remobilisation of sulphides from the southern zone, or alternatively, may represent massive sulphides left behind following movement of the overlying mineralised CUU during thrusting.
Sulphide mineralogy and chemistry Mineralogical studies have been reported by Perring, Barnes and Hill (1994); Perring (1995b); Perring and Hill (1995), and Barnes, Perring and Reilly (1995). In the disseminated zone the sulphides occur as fine grained single crystals or polycrystalline aggregates of pyrrhotitepentlandite±pyrite and chalcopyrite. The aggregates may occur as fine grained blebs (<0.5 mm), interstitial to silicate phases or occluded within them, or as coarser grained (>1 mm) triangular grains in aggregates interstitial to metamorphic olivine blades.
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FIG 5 - Cross section 83 200 N, Maggie Hays deposit, looking WNW. Location on Fig 2.
The hypogene assemblage is dominated by pyrrhotite with lesser pyrite and chalcopyrite. Pyrrhotite, pentlandite and chalcopyrite occur as anhedral crystals, whereas pyrite is either coarse grained and subhedral or occurs in fine grained intergrowth with chalcopyrite. The grain size is typically finer in disseminated mineralisation than in the basal massive mineralisation. The pyrrhotite:pentlandite ratio in the southern zone is relatively constant, averaging about 4:1 in the disseminated mineralisation. In contrast, the basal massive sulphides tend to be more pyrrhotite rich with pyrrhotite:pentlandite ratios as high as 12:1.
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Veinlets of metamorphic magnetite crosscutting and partially replacing the sulphide blebs are relatively common. The evidence suggests that metamorphic oxidation of magnetite formed after sulphide and shows a positive correlation with retrograde serpentinisation of metamorphic olivine. Macroscopic evidence indicates that the northern zone has higher pyrite and chalcopyrite contents than the massive mineralisation in the southern zone. This increase in the pyrite content is accompanied by elevated cobalt levels. Supergene alteration involving the replacement of pentlandite and to a lesser extent pyrrhotite, by violarite, and pyrrhotite by pyrite, occurs locally to depths in excess of 400 m. There is no distinct near-surface supergene zone, as the bulk of the mineralisation occurs below the water table. Barnes, Perring and Reilly (1995) found that pentlandite commonly contains between 34 and 36% nickel but levels can be as low as 23% nickel. The bulk of the deposit which occurs below the base of weathering consists of primary sulphides, in which high nickel pentlandite is more characteristic. Lowernickel pentlandite species appear to be restricted in distribution, and to coincide with supergene alteration in which pentlandite is partially replaced by violarite. The nickel content of pentlandite does not vary significantly with variations in sulphide concentration. Nor are there any apparent significant systematic relationships between variations in host rock type and pentlandite composition. Cobalt levels in pentlandite vary from 0.5 to 2.69%. Variation in the nickel content is the only significant compositional variable in pyrrhotite, between 0 and 1.2%, with lower values occurring in supergene assemblages. The only significant compositional variable in pyrite is cobalt, which varies from 0.2 to 3%. The most significant and systematic variations in cobalt content are between the southern and northern zones of the deposit, with the northern zone characterised by higher cobalt concentrations. The nickel content of silicate and oxide phases is commonly low, with values less than 0.1%.
VOLCANOLOGY The thickness and broad lateral extent of the CUU indicates that it formed from a continuous and voluminous eruption of magnesium-rich komatiitic lava. Where the unit is thickest, in the vicinity of the Maggie Hays deposit, it is interpreted that this is indicative of a large lava channel and the thinner, southern zone may represent progression to a flanking sheet-flow facies (Hill et al, 1990; Perring, Barnes and Hill, 1994). The felsic substrate, upon which the komatiitic lava erupted, would have been susceptible to thermal erosion. Although the lack of marker horizons within the felsic substrate makes this difficult to prove, the thicker portion of the CUU may indirectly indicate stronger thermal erosion of the substrate with the development of a large lava channel which resulted in focussed flow. The fractionated cumulates at the top of the CUU may have formed from local ponding of the lava in a stagnant lava lake environment (Perring, Barnes and Hill, 1994). The vigorous CUU volcanic event was initially followed by alternating periods of quiescence and episodic, less vigorous volcanism, marking the commencement of the thinly layered, differentiated WUU komatiitic sequence. Chemical
Geology of Australian and Papua New Guinean Mineral Deposits
sedimentation, resulting in the formation of BIF, and concomitant felsic volcanism, took place during quiescent periods in the WUU ultramafic volcanic event. As time progressed, the tempo of komatiitic volcanism became more continuous, forming the upper series of thinly differentiated flows of the WUU. The bulk of the WUU flow units may have formed in a sheet flow environment with the thicker, more magnesium-rich cumulate units occurring at the base of the unit, confined locally in channels, similar to what has been described by Cowden and Roberts (1990) at the base of the Silver Lake Member at Kambalda.
ACKNOWLEDGEMENTS This paper is published with the permission of Forrestania Gold NL and the CSIRO. Input by T Hack, J Kilroe and G Kelly is gratefully acknowledged in the documentation and understanding of the deposit and its setting. Thanks also to G Plint, K Wild, Northpoint Cartographics and Geocad for assisting with the preparation of this paper.
REFERENCES Barnes, S J, Perring, C S and Reilly, N S, 1995. Nickel and cobalt distribution between sulphide and gangue minerals in the Maggie Hays nickel deposit, CSIRO Exploration and Mining Report 180R (unpublished). Binns, R A, Gunthorpe, R J and Groves, D I, 1976. Metamorphic patterns and development of greenstone belts in the Yilgarn Block, Western Australia, in The Early History of the Earth (Ed: B F Windley), pp 303–313 (Wiley: New York). Buck, P S, Vallance, S A, Perring, C S Hill, R E and Barnes, S J, 1996. Geology of the Maggie Hays komatiitic nickel sulphide deposit Western Australia, in Proceedings Nickel ‘96, Mineral to Market (Eds: E J Grimsey and I Neuss), pp 111–120 (The Australasian Institute of Mining and Metallurgy: Melbourne). Cowden, A and Roberts, D E, 1990. Komatiite hosted nickel sulphide deposits, Kambalda, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 567–582 (The Australasian Institute of Mining and Metallurgy: Melbourne). Gower, C F and Bunting, J A, 1976. Lake Johnston, Western Australia - 1:250 000 geological series, Geological Survey of Western Australia Explanatory Notes, S1 51–1. Hill, R E T, Barnes, S J, Gole, M J and Dowling, S E, 1990. Physical Volcanology of Komatiites, Excursion Guide Book No 1 (Geological Society of Australia, Western Australian Division: Perth). Horman, C S, 1914. The Breiner Range country, Dundas Goldfield, Western Australia Geological Survey Bulletin 59, Miscellaneous Report No 46. Nesbitt, R W, Sun, S-S and Purvis, A C, 1979. Komatiites: geochemistry and genesis, Canadian Mineralogist, 17:165–186. Perring, C S, 1995a. Geology of the Maggie Hays prospect, CSIRO Exploration and Mining Report 102R (unpublished). Perring, C S, 1995b. Update on the whole-rock silicate and sulphide chemistry of the Maggie Hays Nickel Deposit, Lake Johnston Greenstone Belt, WA, CSIRO Exploration and Mining Report 154R (unpublished). Perring, C S, Barnes, S J and Hill, R E T, 1994. The volcanology and genesis of komatiitic rocks and associated nickel sulphides at Maggie Hays, Southern Cross Province, Western Australia, CSIRO Exploration and Mining Report 22R (unpublished). Perring, C S and Hill, R E T, 1995. Preliminary report on the sulphide petrography at Maggie Hays, CSIRO Exploration and Mining Report 101R (unpublished).
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Frost, K M, Woodhouse, M and Pitkäjärvi, J T, 1998. Forrestania nickel deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 365–370 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Forrestania nickel deposits 1
2
by K M Frost , M Woodhouse and J T Pitkäjärvi
3
INTRODUCTION The deposits are centred at lat 32o35′S, long 119o44′E or AMG coordinates 757 000 E, 6 392 000 N, 90 km east of Hyden, WA, on the Hyden (SI 50–4) 1:250 000 scale map sheet (Fig 1). Mining by Outokumpu Mining Australia Pty Ltd (Outokumpu) from September 1992 at the Digger Rocks, Cosmic Boy and Flying Fox deposits (Fig 1), was the first production of nickel sulphide ore from the Southern Cross Province. Ore production in 1996 was 660 700 t at 1.78% nickel for 9513 t of nickel metal recovered, and the total production from September 1992 to December 1996 was 2.5 Mt at 1.72% nickel for 30 600 t nickel metal recovered. Ore is currently produced from the Cosmic Boy and Flying Fox underground mines and production is expected to recommence in 1998 from the underground Digger Deeps mine, accessing ore from below the base of the Digger Rocks pit. At January 1997 the Proved and Probable Ore Reserves were estimated to be 1.1 Mt at 1.9% nickel.
EXPLORATION AND DEVELOPMENT HISTORY In the heyday of Western Australia’s nickel boom the Forrestania greenstone belt was the target of aggressive exploration by Amax Exploration (Australia) Inc. In October 1969 the first nickel gossan was discovered on the lower contact of a prominent ultramafic trend at the New Morning prospect, although it took two diamond drilling programs to locate high grade (>5% nickel) massive sulphides, which indicated the potential of the belt for more significant deposits. In 1970, Amax joint ventured with Amoco Minerals Australia Company to explore a 90 km strike length of the greenstone belt, and over a seven year period the Cosmic Boy, Digger Rocks, South Digger Rocks and Flying Fox deposits and most of the other known nickel sulphide occurrences were located. The sparsely exposed ultramafic belts were largely explored by drilling, using about 33 500 rotary air blast (RAB) holes on traverses mostly 60–120 m apart to locate nickel pathfinder geochemical anomalies in weathered ultramafic bedrock. The geochemical targets were tested by 650 vertical percussion drill holes and over 800 diamond drill holes, to define a total nickel sulphide resource of about 11.5 Mt at 2% nickel in eight deposits (Table 1).
1.
Senior Project Geologist, Outokumpu Mining Australia Pty Ltd, 141 Burswood Road, Burswood WA 6100.
2.
Supervising Geologist, Outokumpu Mining Australia Pty Ltd, 141 Burswood Road, Burswood WA 6100.
3.
Senior Project Geologist, Outokumpu Mining Australia Pty Ltd, 141 Burswood Road, Burswood WA 6100.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Location and geological map of the Forrestania greenstone belt showing location of nickel sulphide mines and prospects.
Amax sold its interest in Forrestania to Metals Exploration Limited (MEL) in 1981, and with Cyprus (later Arimco) the joint venture partners focussed on ways to extract nickel from the three main deposits at Cosmic Boy, Digger Rocks and Flying Fox, although no further developments were forthcoming and the deposits remained uneconomic. The focus was shifted to gold exploration during the 1980s, and the 1986
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TABLE 1 Pre-mine total nickel sulphide resources, Forrestania base metals project. Deposit
Mineral Resources1 (Mt)
Nickel grade (%)
Nickel cutoff grade (%)
Cosmic Boy2
4.0
2.4
0.9
Digger Rocks2
1.8
1.5
1.0
2
0.3
6.1
1.2
South Digger Rocks3
2.9
1.6
1.2
Beautiful Sunday3
0.5
1.4
1.0
New Morning3
0.3
5.3
1.0
Flying Fox
Seagull4 Liquid Acrobat4 TOTAL 1. 2. 3. 4.
1.0
1.3
1.0
0.7
1.4
1.0
11.5
2.0
Combined Measured, Indicated and Inferred Resources. Feasibility study resource estimates. Outokumpu resource estimates. Amax resource estimates shown in Marston (1984).
Aztec Mining discovery of the Bounty gold deposit close to the former Amax Mount Hope prospect highlighted the gold potential of the belt. As significant gold deposits were not found, MEL sold its 50% interest in the Forrestania base metals joint venture to Outokumpu Australia Pty Ltd in 1989, retaining gold rights in selected tenements under a separate joint venture. Outokumpu attained 100% ownership from Hudspeth and Co (formerly held by Arimco) in 1991, and the Forrestania base metals project commenced mining in September 1992, with the first nickel sulphide ore won from the Digger Rocks open pit some 20 years after the first drill holes intersected nickel sulphides. Although the Amax-led approach to exploration was very successful at Forrestania, few companies would have sufficient exploration funds today to blanket drill greenstone belts of the size of the Forrestania belt. A more cost effective approach at the initial target selection stage is to develop deposit models based on a sound understanding of the controls of nickel sulphide mineralisation and the nature of the komatiite sequences in Archaean greenstone belts. In this way the more expensive exploration techniques such as geophysical and geochemical surveys and drilling can be focussed at a prospect scale to locate sulphide mineralisation. To achieve the first aim, the CSIRO Magmatic Ore Deposits group was involved early in the Outokumpu-managed phase of exploration to undertake mapping (mostly using RAB drill cuttings) and to develop deposit models based on known nickel sulphide deposits in the complexly deformed and high metamorphic grade Forrestania greenstone belt. This research has added considerable value to understanding the geology and metamorphic alteration of the belt, and has allowed the application of volcanological facies models to select prospective areas of komatiite sequences to focus exploration for nickel sulphide deposits. Much of the geological understanding presented below has benefited from a close working association between the CSIRO Magmatic Ore Deposits group and Outokumpu project and mine geologists.
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PREVIOUS DESCRIPTIONS Aspects of the Forrestania nickel deposits were outlined by Purvis (1978), Leggo and McKay (1980), Marston et al (1981) and Marston (1984). Porter and McKay (1981) presented a comprehensive description of the geology and mineralisation of the Forrestania greenstone belt. At this time the generally accepted origin of the dunite host rocks at Forrestania, and at other nickel mineralised districts in the Yilgarn Block, was that the dunites were emplaced as largely concordant sill-like bodies, close to the base of the ultramafic belts, and could have acted as feeder chambers to overlying komatiite volcanics (Marston et al, 1981). This hypothesis is now challenged by the results of: 1.
detailed mapping in many of the ultramafic belts which clearly shows that olivine adcumulate to mesocumulate bodies (so called dunite) are an integral component of komatiite flow units a few tens of metres to hundreds of metres in thickness and can be traced laterally into multiple flow units, demonstrating their time stratigraphic equivalence; and
2.
fluid dynamic and mathematical modelling of komatiite lavas that supports volcanological models for the crystallisation of olivine from continuous flow of hot, highly magnesian komatiite lavas along preferred lava pathways.
It follows that the diverse olivine-cumulate textural units in thick flow units and associated sequences of thin spinifextextured komatiite rocks were formed in dynamic flow environments in which many different volcanic facies were present (Hill et al, 1995). Recent studies of the Digger Rocks deposit by Woodhouse, Cox and Cotton (1992), and results of detailed mapping of the ultramafic belts at Forrestania presented by Perring, Barnes and Hill (1995), support a primary volcanic setting for all mineralised komatiite sequences previously thought to be hosts of intrusive deposits.
REGIONAL GEOLOGY The regional geology of the Forrestania greenstone belt has been previously outlined by Porter and McKay (1981), Chin, Hickman and Thom (1984), Woodhouse, Cox and Cotton (1992) and Perring, Barnes and Hill (1995). The following description is taken principally from Woodhouse, Cox and Cotton (1992) and Perring, Barnes and Hill (1995) to present the recent interpretation of the evolution of this complex greenstone belt. The 2900 Myr old Forrestania greenstone belt is the southern extension of the Southern Cross greenstone belt, a 300 km long and up to 40 km wide supracrustal belt bounded by Archaean granitoid and gneiss, intruded by less deformed granite and pegmatite and cut by east-trending Proterozoic dolerite dykes. Detailed mapping in the Forrestania area has shown that the belt comprises a lowermost sequence of tholeiitic basalt, with up to six ultramafic members, and numerous thin banded iron formation (BIF) and chert units (Fig 1). This largely volcanic pile is overlain by psammitic to pelitic schists which occupy the core of a regionally north-plunging syncline. Dips are moderate to steep and locally overturned, and only the Western ultramafic belt faces east, showing that the synclinal axis is between the Western ultramafic belt and the Takashi belt (Fig 1).
Geology of Australian and Papua New Guinean Mineral Deposits
FORRESTANIA NICKEL DEPOSITS
The belt has mostly been subjected to upper amphibolite facies metamorphism and multiple deformation events which have extensively modified primary rock textures. For example, spinifex textures are only rarely preserved in thin komatiite flow rocks and these rocks are commonly recrystallised to assemblages of tremolite, chlorite, serpentine, anthophyllite, enstatite and metamorphic olivine. Original olivine cumulate textures are only preserved in the cores of the very thick olivine adcumulate to mesocumulate bodies, and elsewhere these rocks comprise bladed to granular metamorphic olivine, serpentine, talc, anthophyllite and enstatite. Basalt is also recrystallised to amphibolite, although rare pillow structures point to a history of subaqueous basaltic volcanism. The coarsely banded BIFs are commonly well preserved, although psammitic to pelitic rocks are strongly recrystallised to quartzmuscovite-sillimanite schists. The structural evolution of the belt is less clearly understood, as much of the geology has been compiled from sparse outcrop and shallow drill hole data. However, a recent study of the structural geology of the Cosmic Boy deposit (T Koistinen, unpublished data, 1997) provides evidence of at least three deformation events. An early deformation is shown by a weak fabric, parallel to magnetite and chert sedimentary layers in BIF. Two subsequent folding events associated with strong metamorphic fabrics are recognised in BIF and hanging wall amphibolite at Cosmic Boy, with the earlier event possibly related to isoclinal folding along shallow plunging axes, and the later event manifest in the broad upright synclinal structure of the belt. Faults are either prograde annealed structures localised along rock contacts and mostly oriented parallel to strike, or late brittle retrograde faults, of which the NNW set is dominant, that have had the strongest influence on the present complexity of the nickel sulphide orebodies.
ORE DEPOSIT FEATURES All of the nickel sulphide deposits of the Forrestania greenstone belt occur within the Eastern and Western ultramafic belts, mostly in the lowermost one or two ultramafic units. The Cosmic Boy and Digger Rocks deposits are in the Eastern ultramafic belt, and the Flying Fox deposit is in the Western ultramafic belt (Fig 1). The geology of the Cosmic Boy and Flying Fox deposits is described below, and a detailed description of the Digger Rocks deposit is in Woodhouse, Cox and Cotton (1992).
COSMIC BOY DEPOSIT Exploration The deposit was discovered by diamond drill testing of a strong copper-nickel geochemical anomaly defined from 18 m spaced RAB drill hole traverses, in an area of small isolated gossan outcrops on the eastern contact of the lowermost ultramafic unit. The first diamond drill hole, CBD1, intersected +2% nickel in what is now known as the hanging wall ore zone, but the targeted ‘basal’ ultramafic unit proved to be barren. The basal ore zone was only discovered by drilling south of CBD1, despite the lack of encouragement from shallow RAB drilling of this contact. The explanation for the relatively isolated geochemical anomaly on the basal ultramafic contact is that the basal ore zone pitches SW, below the level of RAB drill testing. Gossans were also discovered at the southern limit of the hanging wall orebody showing a more irregular distribution of mineralisation within this ultramafic unit.
Geology of Australian and Papua New Guinean Mineral Deposits
Geological setting The deposit is within an area of the Eastern ultramafic belt dominated by relatively thin (<60 m thick) but laterally extensive ultramafic units, interlayered with BIF and tholeiite basalt (Figs 2 and 3). Dips are about 50o west and the sequence is generally west facing, as shown by the location of the basal orebody on the eastern contact of the lowermost ultramafic unit, and the overall evolution of the ultramafic pile to more evolved, less olivine-rich, thin spinifex-textured komatiite flow units towards the western margin of the belt. The lowermost ultramafic unit consists of several discrete lenses of olivine mesocumulate to 900 m strike length which are linked by differentiated flow units to 50 m thick. Other smaller olivine mesocumulate lenses occur in the lowermost ultramafic unit, and within most other ultramafic units, but these are a relatively minor component of the ultramafic rocks.
Mineralisation Cosmic Boy is the largest nickel sulphide resource at Forrestania, containing about 4.0 Mt at 2.4% nickel in two parallel ore zones, and production is currently from the larger and more easily mined basal orebody. The basal orebody is about 800 m in strike length and extends from the weathering base to about 500 m depth, at the base of a 40 to 60 m thick olivine mesocumulate unit, overlying a prominent BIF unit. The up and down dip limits of the mineralisation are defined by a gradual thinning of the sulphide zone, although the fault termination of the basal ore zone at the northern and southern margins and internal barren, faulted contacts indicate a strong degree of structural control of the present ore outline. The ore has a sharp faulted contact against the footwall BIF, and the hanging wall contact is defined by a gradual reduction in nickel grade to the background level of about 0.3% in barren ultramafic rock. Thin, irregular mafic and felsic dykes were preferentially intruded along the ultramafic-BIF contact, and have contributed to significant ore dilution. The orebody shows the modifying effects of late stage faulting which has resulted in the local pinching and swelling of ore and high ground stress. Primary sulphide mineralisation consists of a strong dissemination of pyrrhotite-pentlandite±pyrite and chalcopyrite between 20 and 40% by volume, with nickel grade between about 0.7 and 5%. Violarite is a significant sulphide phase in the supergene-altered ores. Sulphides form either triangular domains between bladed metamorphic olivine-talc grains, or partially connected networks moulded around granular silicate grains. No massive sulphides have been encountered in the basal ore zone, which is unusual in basal contact–hosted deposits. The hanging wall ore zone has only been investigated from surface drilling and minor underground development, but initial indications show a very complex arrangement of thin discontinuous lenses of disseminated sulphides hosted by a 10–20 m thick olivine mesocumulate unit. The location of the hanging wall ore zone, close to the ‘upper’ contact of the second ultramafic unit (Figs 2 and 3), and the similar strike length of the near surface mineralisation, can be explained, in part, by early isoclinal folding of the basal ore zone along a shallowly plunging axis, although a fold hinge between the ore zones has not been recognised at the current level of mining.
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FIG 3 - Cross section on 10 420 m N, Cosmic Boy mine, looking north.
change to thin flow units associated with dominantly tholeiitic basalt, in contrast to the lowermost sequence of thick ultramafic flow units with abundant BIF. A 5 m thick ultramafic unit contains abundant low grade disseminated sulphides and isolated intersections of massive sulphides to 1 m thick, although no additional economic mineralisation has been located.
FLYING FOX DEPOSIT Exploration The deposit was discovered late in the main phase of exploration at Forrestania, in 1977, despite indications from the New Morning deposit in the early 1970s that the belt was prospective for high grade massive sulphides. Closely spaced traverses of RAB drill holes defined a strong coincident copper-nickel geochemical anomaly, comparable in metal concentrations to the anomaly over the New Morning mineralisation, which also occurs in a similar position on the western (ie basal) contact of the lowermost ultramafic unit. Massive nickel sulphides were subsequently found by drilling beneath the RAB geochemical target and demonstrated the effectiveness of this exploration strategy.
Geological setting FIG 2 - Surface geological plan of the Cosmic Boy mine area, with location of Fig 3.
Exploration drilling has intersected nickel sulphides within successively higher ultramafic flow units in the Cosmic Boy mine area. The geology at this higher position shows an abrupt
368
The deposit is towards the middle of the Western ultramafic belt, about 30 km NW of Cosmic Boy (Fig 1). The sequence in this part of the Western ultramafic belt is east facing and dips at o about 40 east. It comprises a strongly foliated, well layered sequence of intercalated ultramafic flow units, basalt, sulphidic chert and psammitic to pelitic metasediment, overlying a thick succession of psammitic schist (Fig 4). The lowermost ultramafic unit is the thickest, to 50 m, and most olivine-rich of
Geology of Australian and Papua New Guinean Mineral Deposits
FORRESTANIA NICKEL DEPOSITS
sulphide deposits which are well represented in the Archaean Yilgarn Block of Western Australia and in many other greenstone belts worldwide. The Forrestania deposits are also interpreted to show a strong volcanic control of the setting of the ores and their host rocks, after allowing for the modifying effects of high grade metamorphism and multiple deformation events, in common with deposits from Kambalda and the Agnew–Wiluna greenstone belt. For example, mapping of the ultramafic belts at Forrestania by Perring, Barnes and Hill (1995) shows that the komatiite sequences can be divided into several distinct associations of rock types, and that these in turn, indicate a wide range of volcanological regimes under which olivine crystallised.
FIG 4 - Cross-section on 28 400 N, Flying Fox mine, looking north.
the entire sequence, and is capped by pyroxenite and thin komatiite flow rocks. The ultramafic rock types show a typical evolution up the stratigraphic section from relatively thin, olivine-rich flow units to multiple thin-flow sequences of less olivine-rich flows. The volcanosedimentary succession is cut by flat lying granitoid and pegmatite dykes at repeated regular depth intervals, and faults located on the margins of these dykes show a consistent displacement of the lower block to the east. The rocks are overlain by about 10 m of transported aeolian quartz sand derived from granitoid rocks which outcrop a few kilometres west of the deposit.
Mineralisation The deposit is nearly mined out and the total production is expected to be about 200 000 t at 3.1% nickel. The orebody is a high grade massive sulphide deposit containing 5 to 9% nickel, that strikes north for 400 m, dips between 25 and 80° east and extends from the weathering base to about 200 m depth (Fig 4). The up and down dip margins of the massive sulphide ore are bounded by two prominent granitoid sheets, and the orebody is split into two pods by an east-trending Proterozoic dyke. The massive sulphide ore has a wide range of thicknesses from 0.1 to 10 m (average 1.5 m), and parts of the deposit are faulted into the footwall metasedimentary rocks and disrupted by thin granitoid sills. Disseminated sulphide mineralisation to 1.5% nickel occurs in the basal ultramafic host unit above the massive sulphide orebody, and as a low grade halo on its north, south and up dip margins. The primary massive sulphides are totally replaced by a supergene assemblage of pyrite-violarite, and large pyrite porphyroblasts show a metamorphic overprint.
DISCUSSION Nickel sulphide deposits at Forrestania belong to the economically important class of komatiite-hosted nickel
Geology of Australian and Papua New Guinean Mineral Deposits
The olivine adcumulate to mesocumulate bodies that host the nickel sulphide deposits at Cosmic Boy, Flying Fox and other deposits at Forrestania, were likely to have formed in preferred lava pathways which facilitated the crystallisation of thick cumulate piles from continually replenished lavas. Strongly differentiated thin flow komatiites that surround olivine adcumulate to mesocumulate bodies, and occur as thick monotonous sequences higher in the volcanic pile, formed from the episodic emplacement of less olivine-rich lavas. However, other critical ingredients are required to form nickel sulphide deposits in preferred lava pathways, such as an available source of sulphur, sulphur saturation and equilibration of sulphide melts with replenished lavas, and an effective trap for the dense sulphide melts. The ultramafic belts at Forrestania are commonly interlayered with thin units of sulphidic substrate rocks which demonstrates the wide availability of sulphur in the komatiite flow environment. The numerous occurrences of nickel sulphide mineralisation are evidence of widespread sulphur saturation of komatiite lavas, although all of the economic deposits are restricted to the Eastern and Western ultramafic belts. The variation in the style of nickel sulphide mineralisation at Forrestania is interpreted to reflect the dynamic environment of accumulation of magmatic sulphides in komatiite lavas. Massive sulphide deposits are interpreted to form at an early stage of flow in preferred lava pathways, or at least before significant olivine crystallisation, probably through ground melting of the sulphidic sediment substrate over which the lavas flowed. Disseminated sulphide deposits are a mixture of mostly olivine and sulphide droplets in variable proportions, and formed during olivine crystallisation, either early in the flow event to collect as dense disseminated ores in the basal sections of komatiite flows, or later to form internal low grade mineralisation by cotectic olivine-sulphide crystallisation.
ACKNOWLEDGEMENTS This paper is published with the permission of Outokumpu Mining Australia Pty Ltd. The authors acknowledge the technical support of project and mine geologists from Forrestania Nickel Mines.
REFERENCES Chin, R J, Hickman, A H and Thom, R, 1984. Hyden, Western Australia - 1:250 000 geological series, Geological Survey of Western Australia Explanatory Notes SI 50–4. Hill, R E T, Barnes, S J, Gole M J and Dowling, S E, 1995. The volcanology of komatiites as deduced from field relationships in the Norseman-Wiluna Belt, Western Australia, Lithos, 34:159–188.
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Leggo, M D and McKay, K G, 1980. Forrestania nickel deposits, Yilgarn Block, WA, Journal of Geochemical Exploration, 12:178–183.
Porter, D J and McKay, K G, 1981. The nickel sulfide mineralisation and metamorphic setting of the Forrestania area, Western Australia, Economic Geology, 76:1524–1549.
Marston, R J, 1984. Nickel mineralization in Western Australia, Geological Survey of Western Australia Mineral Resources Bulletin 14.
Purvis, A C, 1978. The geochemistry and metamorphic petrology of the Southern Cross - Forrestania Greenstone Belt at Digger Rocks, Western Australia, PhD thesis (unpublished), University of Adelaide, Adelaide.
Marston, R J, Groves, D I, Hudson, D R and Ross, J R, 1981. Nickel sulfide deposits in Western Australia: a review, Economic Geology, 76:1330–1363. Perring, C S, Barnes, S J and Hill R E T, 1995. The physical volcanology of Archaean komatiite sequences from Forrestania, Southern Cross Province, Western Australia, Lithos, 34:189–207.
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Woodhouse, M, Cox, R and Cotton, R E, 1992. Economic geology of the Digger Rocks nickel deposit, Forrestania, Western Australia, The AusIMM Proceedings, 297(2):31–43.
Geology of Australian and Papua New Guinean Mineral Deposits
Waters, P J, 1998. The Y2-3 and Y10 iron ore deposits Yarrie, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 371–374 (The Australasian Institute of Mining and Metallurgy: Melbourne).
The Y2–3 and Y10 iron ore deposits, Yarrie by P J Waters
1
INTRODUCTION The deposits are in the NE corner of the Pilbara Craton, 200 km east of Port Hedland, WA at lat 20o36′S, long 120o18′E on the Yarrie (SF 51–1) 1:250 000 scale map sheet (Fig 1). The deposits are on the southern (Y2–3) and northern (Y10) margins of the Yarrie plateau (Fig 2).
FIG 2 - Map of fault structures in the Nimingarra–Yarrie area.
EXPLORATION HISTORY
FIG 1 - Location map and basic geology of the northeastern Pilbara Craton.
Open cut mining commenced at Yarrie in late 1993. Both mines are operated by BHP Iron Ore Pty Ltd on behalf of the Mount Goldsworthy Mining Associates Joint Venture which comprises BHP Minerals Pty Ltd 85%, C I Minerals Australia Pty Ltd 8% and Mitsui–Itochu Iron Pty Ltd 7%. Total Measured Resources at the Y2–3 bedrock deposit pre-mining were 65 Mt at 64.7% iron, 0.034% phosphorus, 4.95% silica and 1.38% alumina. Measured Resources at Y10 totalled 23.6 Mt of hematite conglomerate ore at 61.2% iron, 0.090% phosphorus, 6.5% silica and 3.0% alumina before mining commenced. At May 1996, 14.5 Mt of ore had been mined from the Y2–3 deposit and 1.3 Mt from the Y10 deposit.
1.
Superintendent Geological Services, BHP Iron Ore, PO Box 655, Newman WA 6753.
Geology of Australian and Papua New Guinean Mineral Deposits
In the late 1950s, D F D Rhodes Pty Ltd obtained a Temporary Reserve over the main Yarrie plateau, with Goldsworthy Mining Limited taking up the adjoining Kennedy Gap Temporary Reserve, covering the Y10 deposit, in 1960. In 1961 D F D Rhodes Pty Ltd sold the Temporary Reserve to National Bulk Carriers, who subsequently formed the Sentinel Mining Company. During the period from 1961 to the early 1970s, Sentinel Mining conducted shallow airtrack conventional drilling on the outcropping mineralisation at Y3, and Goldsworthy Mining Limited completed a similar campaign on the Y10 deposit. In the early 1970s, Sentinel Mining Company was bought out by Goldsworthy Mining Limited. Between 1970 and 1990, Goldsworthy Mining Limited conducted a number of small scale drilling and bulk sampling campaigns on the known Y3 and Y10 deposits. By 1986, a resource of 12.4 Mt at 61.9% iron, 0.042% phosphorus, 7.1% silica and 2.03% alumina was estimated for the Y3 area. Indicated resources at Y10 were 24.9 Mt at 60.8% iron, 0.089 % phosphorus, 6.6% silica and 3.8% alumina. Goldsworthy Mining Limited had previously mined the Shay Gap, Sunrise Hill and Nimingarra deposits in this area (Fig 2) and the Mount Goldsworthy deposit further to the west. BHP bought Goldsworthy Mining Limited in 1990, and in 1991 commenced regional mapping, an aeromagnetic survey (Kerr et al, 1994) and a systematic reverse circulation (RC) drilling program in the Nimingarra–Yarrie area. The Yarrie plateau was identified as a priority drill target after a review of the geology, which indicated that the Y3 deposit had the
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potential to continue to the west and to depth. Subsequent RC drilling intersected substantial mineralisation to the west of the Y3 deposit, which became known as the Y2 deposit. Further drilling established that the two areas were continuous and the entire deposit is now known as the Y2–3 deposit, the second largest bedrock resource known in the area, the largest being the Goldsworthy deposit (Fig 1). Drilling at Y3 was closed down to a 25 by 25 m grid and at Y2 to a 35 by 35 m grid prior to mining.
interbedded shale and sandstone, and intruded by dolerite sills. The formation is unconformably overlain by near horizontal Permian Paterson Formation sediment and Tertiary limestone, calcrete and pisolitic material.
The Y10 conglomerate deposit was re-examined in 1992, and 6000 m of drilling on a 50 by 50 m grid was completed over the orebody.
LOCAL GEOLOGY
REGIONAL GEOLOGY The Yarrie deposits are in the NE part of the Pilbara Block (Fig 1). Archaean granitoids are overlain by the Middle to Late Archaean greenstone sequence of the Warrawoona Group (or Megasequence), a dominantly volcanic succession. Unconformably overlying the Warrawoona Group is the Gorge Creek Group (or Megasequence), consisting of clastic sediment and banded iron formation (BIF), with interbedded mafic volcanic rocks. These units have moderate to steep dips. Iron ore mineralisation is hosted by the Cleaverville Formation of the Gorge Creek Group (Fig 3).
Dykes and veins of dolerite, quartz, pegmatite and granite of probable Proterozoic age intrude and dissect Archaean units. Structurally the region is dominated by a series of near vertical faults, with displacements exceeding 1 km (Fig 2).
STRATIGRAPHY Granite of the Warrawagine batholith forms the southern margin and basement to the Yarrie plateau (Fig 4). The granite is unconformably overlain by a 5 to 15 m thick quartzite unit, which is succeeded by 400 m of BIF. Outcropping BIF dominates the surface exposed on the Yarrie plateau. These sediments are part of the lower Cleaverville Formation, and this sequence can be correlated with the footwall sequence at Mount Goldsworthy, some 100 km west of Yarrie. Interbedded discontinuously throughout the lower portion of the BIF are thin mudstone units, which can be correlated with the mudstone-chert marker band prominent in the Cleaverville Formation at Cundaline, Shay Gap and Nimingarra (Podmore, 1990), 30 to 50 km west of Yarrie. To the north of the plateau BIF is succeeded by red shale of the upper Cleaverville Formation. The dips of the BIF, quartzite and shale vary from 20o to 50o towards north to NW.
FIG 4 - Cross section through Y3 (eastern) part of the Y2–3 orebody, looking west.
On the NE flank of the Yarrie plateau the basal conglomerate of the Eel Creek Formation unconformably overlies the BIF. The conglomerate dips at 5 to 15o to the NE, with thickness reaching 30 m. This basal unit is a BIF-rich conglomerate, with localised hematite concentrations. It has no cross bedding, upward fining or other sedimentary structures. Above this unit lie shales, siltstones and dolerite sills of the Eel Creek Formation.
MINERALISATION FIG 3 - Stratigraphic column of the Yarrie area (after Kerr et al, 1994).
Unconformably overlying this Archaean succession is the gently dipping Middle Proterozoic Eel Creek Formation of basal conglomerate, locally hematitic, grading up into
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Iron ore has been discovered in four settings in the Shay Gap–Yarrie area: 1.
Archaean or early Proterozoic stratabound lode deposits;
2.
Proterozoic conglomerates (Eel Creek Formation);
3.
Mesozoic–Tertiary crustal deposits; and
4.
Tertiary scree (detrital deposits).
Geology of Australian and Papua New Guinean Mineral Deposits
THE Y2–3 AND Y10 IRON ORE DEPOSITS, YARRIE
Stratabound lode deposits are the most attractive exploration targets, as they are by far the largest and highest grade. The crustal ores are thought to equate to the martite-goethite ores, and the lode ores to the Proterozoic burial metamorphic, microplaty hematite ores of the Hamersley Basin.
1.
an early, mostly isoclinal set;
2.
a prominent set of mesoscale folds plunging north to NW, overturned to the east;
3.
a less prominent mesoscale set that is overturned to the west;
Iron ore mineralisation at Y2–3 is a large, continuous, stratabound lode deposit containing hematite, with relict banding from original BIF. The ore is in the basal part of the BIF unit, over a strike length of 2200 m. A thin basal shale, directly overlying the lowermost quartzite unit, is the usual footwall to the mineralisation, but to the north mineralisation is terminated at a dolerite-intruded fault zone (Fig 4).
4.
a large, open, upright set which plunges at 32o north; and
5.
late, large scale open folds, which trend east and affect the entire Yarrie plateau sequence, resulting in an overall northerly regional dip.
The eastern and outcropping Y3 part of the Y2–3 deposit comprises the originally delineated 1000 m length of the deposit. The orebody outcrops on the southern margin of the plateau and dips conformably at 10 to 15 o north, before being terminated to the north by a dolerite intruded east-trending normal fault (Fig 4). Minor interbedded shale bands to 3 m thick occur erratically throughout the orebody. To the west, mineralisation in Y2 swings around to strike NE, dipping at 25 to 35o NW for a further 1200 m beyond the original limit of Y3. To the NW, mineralisation is terminated at depth by an 050o trending dyke-intruded high angle fault zone (Fig 5). This zone is characterised by sheared shale, ore and dolerite, which forms the deepest part of the orebody, some 250 m below the surface.
The deformation history is complex, with multiple fold and joint sets due to a series of deformational events. The relative timing of the fold sets is not well understood due to the lack of well developed axial plane cleavages.
Faulting The main faults terminating the Y2–3 mineralisation down-dip trend at 050 and 070o. Striations, shearing and splays in the fault zone suggest a dominant strike-slip component of displacement, and there is evidence of several periods of reactivation. Dolerite dykes intrude the faults, with little postintrusion movement along their planes (T M Johnson, unpublished data, 1995). Also evident throughout the pit are low angle thrusts, with displacement less than 20 m. R Mason (personal communication, 1995) suggested that the thrusts may be related to the main strike-slip fault system, and may have formed in association with a ‘restraining bend’ at the intersection of the 050 to 070o trending fault zones. The structural complexity, and the differences in rock competency between BIF and ore and the less competent shales, dolerites and granites makes geotechnical design critical for slope stability. The Y10 hematite conglomerate dips at 5 to 15o to the north, and has no post-depositional structures.
ORE GENESIS FIG 5 - Cross section through Y2 (western) part of the Y2–3 orebody, looking SW.
The Y10 deposit is in the basal conglomerate of the Eel Creek Formation, which onlaps the northern part of the Yarrie plateau (Fig 2). The deposit comprises moderately hard, rounded to ellipsoidal hematitic pebbles and cobbles, with average iron content 63%, cemented by a fine grained hematite-sand matrix. The thickness of the hematite conglomerate is approximately 6 m, but varies from 1.5 to 12 m. Several minor interbedded shale bands, generally less than 0.25 m thick, are contained within the orebody. The orebody onlaps Archaean shales to the west, and BIF of the Cleaverville Formation to the south and east.
STRUCTURE The Yarrie area is structurally more complex than the Shay Gap–Nimingarra area.
Folding Since mining commenced at Y2–3, pit face mapping has helped to identify the structural setting of the Yarrie plateau. T M Johnson (unpublished data, 1995) has identified five fold sets:
Geology of Australian and Papua New Guinean Mineral Deposits
The Y2–3 deposit is believed to have formed by a process of supergene enrichment similar to that proposed by Morris (1985) for the bedded iron ores of the Hamersley Province. Ore formation involved metasomatic replacement of cherty BIF to create an initial martite-goethite type ore. Subsequent burial metamorphism converted the goethite to a martite-microplaty hematite ore. Though there is no direct age determination of the ore forming process, it is believed to have taken place at the same time as formation of the high grade microplaty ores of the Hamersley Province. Microscopic analysis of ore from near the dolerite-intruded fault zone indicates that a third phase of ore genesis may have taken place. I J Tehnas (unpublished data, 1992) has proposed that the dolerite intrusion and associated hot fluids forced along pre-existing fractures and voids created a secondary hypogene alteration environment, which caused minor recrystallisation of colloform goethite and masking of other textures. The Y10 hematitic conglomerate was formed by the erosion of the Y2–3 deposit, with transport of the hematite pebbles 7 to 8 km north, where they were deposited in an alluvial fan environment (P J Waters, unpublished data, 1993). Hematite and siliceous matrix components are assumed to have been derived from the same area. As there is no evidence to suggest
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P J WATERS
that any hematite mineralisation has occurred after deposition of the conglomerate, the bedrock mineralisation at Y2–3 must have occurred before the deposition of the Eel Creek Formation.
ACKNOWLEDGEMENTS BHP Iron Ore is acknowledged for permission to publish. Thanks to M Kneeshaw, G Townshend and T M Johnson for proofreading the text and providing additional information and thoughts, and the staff at BHP Iron Ore, Newman, are also thanked for assistance in preparation of the paper.
REFERENCES Kerr, T L, O’Sullivan, A P, Podmore, D C, Turner, R and Waters, P, 1994. Geophysics and iron ore exploration: Examples from the Jimblebar and Shay Gap–Yarrie regions, Western Australia, in Geophysical Signatures of Western Australian Mineral Deposits, Publication 26, pp 355–367 (The Geology Department and University Extension, The University of Western Australia: Perth). Morris, R C, 1985. Genesis of iron ore in banded iron-formation by supergene and supergene-metamorphic processes - a conceptual model, in Handbook of Strata-Bound and Stratiform Ore Deposits, Vol 13 (Ed: K H Wolf), pp 73–235 (Elsevier: Amsterdam). Podmore, D C, 1990. Shay Gap–Sunrise Hill and Nimingarra iron ore deposits, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 137–140 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Geology of Australian and Papua New Guinean Mineral Deposits
McKenna, D M and Harmsworth, R A, 1998. Brockman No 2 detritals (B2D) iron ore deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 375–380 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Brockman No. 2 detritals (B2D) iron ore deposit 1
by D M McKenna and R A Harmsworth INTRODUCTION The deposit is 55 km NW of Tom Price township in the Pilbara region of WA, at lat 22o26′S, long 117o20′E on the Mount Bruce (SF 50–1) 1:250 000 scale and the Jeerinah (2353) 1:100 000 scale map sheets (Fig 1). The deposit was discovered by Hamersley Iron Pty. Limited (Hamersley) geologists in June 1986 and mining commenced in March 1992. Hamersley mining operations produce around 55 Mtpa of high grade sized lump ore (minus 31.5 mm plus 6.3 mm) and fine ore (minus 6.3 mm). The B2D mining operation is designed primarily to produce lump ore, and during 1996 B2D contributed 3.4 Mt of lump and 1.9 Mt of fine ore from a crusher feed of 6.9 Mt. The B2D products are blended at the port of Dampier with those from Hamersley's four larger mining operations at Mount Tom Price, Paraburdoo, Channar and Marandoo. It connects to the Hamersley rail system via a 44 km spur line, with a total rail distance from B2D to Dampier of 291 km. The iron content of the B2D lump product, generally 63.0 to 65.0%, and the amount produced are varied to achieve Hamersley’s target shipping grades. A proportion of the higher grade fines, generally grading 61.5 to 62.0% iron, is also recovered as a saleable product. However much of the fine ore 1.
Consultant, Hamersley Iron Pty. Limited, GPO Box A42, Perth WA 6001.
2.
Principal Geologist, Hamersley Iron Pty. Limited, GPO Box A42, Perth WA 6001.
2
produced has low iron (<61.5%) and high impurity levels and is discarded. Fines are mainly derived from the detrital matrix which is softer or more brittle than the hematite clasts, inherently of lower grade, and reports preferentially in the minus 6.3 mm product after crushing and screening. From a total B2D Measured Resource of 38 Mt at 62% iron it is anticipated that four pits will provide 25 Mt of saleable products of which around 18 Mt will be lump ore and 7 Mt fine ore.
EXPLORATION AND MINING HISTORY CONCEPTS Hamersley identified and tested five relatively small detrital deposits close to the Mount Tom Price and Paraburdoo mines during the 1970s and early 1980s. This experience demonstrated that saleable lump ore could be obtained from this type of feed by simple crushing and screening. The lump product was not of direct shipping quality but was suitable for blending to supplement overall lump ore production. All of the fines from these deposits were discarded because of low iron content and high impurity levels. During the 1980s Hamersley decided to search within 50 km of its main railway line for detrital ore deposits. A combination of the following broad geological features was used to select target areas: 1.
Remnant outcrops of hematite-goethite mineralisation on adjacent hill slopes of the Brockman Iron Formation, the source of the detrital deposits.
FIG 1 - Geological map of portion of the north limb Brockman Syncline showing B2D Pits 1 to 4 and remnant mineralisation on Brockman Iron Formation.
Geology of Australian and Papua New Guinean Mineral Deposits
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D M McKENNA and R A HARMSWORTH
2.
Hematite conglomerate outcrops near the base of hill slopes at plain level. This hard rock type is the compacted derivative of the hematite detritals.
3.
Potential for buried topographic traps or basin structures below plain level.
The area chosen for the first systematic testing was the well defined Brockman valley on the northern limb of the Brockman Syncline. Limited drill testing of a gravity anomaly a few years earlier had intersected low grade detritals between two hematite conglomerate outcrops near the middle of the southern margin of the valley, and this enhanced the area as an exploration target.
EXPLORATION AND EVALUATION The scout drill pattern chosen was holes on 200 m intervals on lines 400 m apart along the valley. A total of 20 km strike length was tested initially with five or six reverse circulation (RC) holes per line, a spacing chosen to increase the chance of locating concealed high grade (+60% iron) deposits larger than about 5 Mt. The discovery hole for the largest of the high grade lenses at B2D was drilled during June 1986, and isolated high grade intersections were also found over the entire 20 km strike length. Subsequent ground checking and drilling identified a high grade zone 3 km long with the most potential for an economic deposit, and this zone contains the B2D mining operation (Fig 1).
Formation (230 m thick) and the Brockman Iron Formation (620 m). These host the supergene ‘BIF-derived’ hematite and hematite-goethite iron deposits of the province (Morris, 1985). Lying conformably between the Marra Mamba and Brockman Iron Formations is a sequence of dolomite, dolomitic shale, shale, chert and thin BIF units forming the Wittenoom Formation (150–350 m), Mount Sylvia Formation (30 m) and Mount McRae Shale (50 m). When fresh, the BIF is a banded magnetite-hematite-chertcarbonate-silicate assemblage containing around 30% iron. Supergene enrichment processes have locally oxidised the magnetite to hematite (martite), and the gangue minerals have been largely replaced by goethite and in part leached out, resulting in enriched hematite-goethite deposits. The Marra Mamba and Brockman hematite-goethite mineralisation flanking the B2D deposit is the result of this process. A prominent feature of the province is a mature Tertiary land surface — the Hamersley surface of Campana et al (1964). This surface is mantled by a carapace of ferricrete and characterised by gently rounded and domed hills. Abrupt erosion scarps or breakaways also occur as a result of later landscape rejuvenation (Fig 2).
The scout drilling phase was followed by four years of staged evaluation activities. The scout drill hole spacing was progressively reduced to a final 100 by 50 m grid focussed on the 3 km long high grade resource. A total of 23 714 m of RC drilling was completed in 537 holes during the exploration and evaluation phases. A shaft sinking program followed, to obtain bulk samples, and comprised 15 vertical shafts totalling 556 m over a 17 km strike length. Five of the shafts were in the 3 km long high grade resource. The bulk sample testing was designed to verify the accuracy of chemical estimates of the resource and to determine the proportions and physical properties of both lump and fines products. Physical properties are as important as chemical composition in the marketing of iron ore. A preliminary feasibility study in late 1989 identified three detrital ore lenses at B2D. They were predicted to yield 15 Mt of lump ore product, based on shaft bulk sampling. Mining commenced in June 1992, and predictions of lump yield have subsequently been revised to 18 Mt due to development of an additional high grade deposit at Pit 4 and higher than expected lump ore recovery. Pits 1, 3 and 4 are currently being mined and Pit 2 and part of Pit 1 are being back filled with waste from adjacent mining areas as part of an environmental management plan.
REGIONAL GEOLOGY The stratigraphy of the Hamersley Iron Province, which covers an area of nearly 80 000 km2, has been described by MacLeod (1966), Trendall and Blockley (1970) and Trendall (1983). The 2500 Myr old Hamersley Group consists of a 2500 m thick sequence of banded iron formation (BIF), dolomite, shale and acid volcanic rocks intruded by dolerite sills and dykes. There are three major BIF formations in the Group, with the two of most economic interest being the Marra Mamba Iron
376
FIG 2 - Landforms of portion of the Brockman Syncline looking south. Note the rounded hills capped by the Hamersley surface, breakaways and stream patterns draining towards Pits 1 (left) and 2 (right).
The B2D deposit occurs in the SW of the province which is characterised by a series of large scale folds whose axes trend east, with axial lengths of 10 to 100 km. The Brockman Syncline, which hosts the B2D deposit, is typical of these major folds. Small scale folds trending between 115 and 120o are common in the Hamersley Group on the limbs of major structures (Fig 1). Strike slip faults with a NW trend are common, with movement (mostly dextral) up to several kilometres. A second set of strike slip faults has sinistral movement and trends between north and NE. All these structures influence stream direction and therefore deposition of younger detrital deposits. Three Tertiary detrital types, as defined by Morris (1994), occur in the B2D area and are described briefly below in decreasing order of age: 1.
Ochre-rich detritals of Eocene to Miocene age. Deposits of this type are derived from the Marra Mamba deposit on the northern edge of the Brockman valley (Fig 1) and are currently uneconomic.
Geology of Australian and Papua New Guinean Mineral Deposits
BROCKMAN NO. 2 DETRITALS (B2D) IRON ORE DEPOSIT
2.
Channel iron deposits (CID) of Middle to Upper Miocene age. Examples of this pisolitic goethite-hematite type are the Beasley River deposit 25 km to the SW of B2D and the larger Robe River and Yandicoogina deposits.
3.
Hematite-rich detritals and hematite conglomerate of Pliocene age, derived from nearby mineralised Brockman Iron Formation outcrops such as the B2D deposit.
Colluvium of Quaternary age covers most of the B2D deposit.
ORE DEPOSIT FEATURES ECONOMIC SETTING AND LITHOLOGY The ore beds consist of a shallow blanket of hematite-rich outwash scree draped against the Brockman Iron Formation escarpment in the strike valley between the escarpment of the Brockman Iron Formation immediately to the south and a low strike ridge of Marra Mamba Iron Formation to the north. The valley floor comprises colluvium unconformably overlying easily weathered dolomite and shale of the Wittenoom Formation, Mount Sylvia Formation and the Mount McRae Shale. The B2D deposit comprises four discrete lenses, each being that portion of the total detrital pile defined initially by a lower cutoff of 60% iron. Impurity levels of alumina, silica, loss on ignition and phosphorus must also be within acceptable limits.
The detrital ore is derived from the pre-existing ferruginous carapace and ferricrete of the adjacent Hamersley surface developed on Brockman Iron Formation. The detritals occur close to the base of the steep hill slope and have little outcrop, as they are overlain and almost totally concealed by an unconformable blanket of colluvium, named ‘siliceous detritals’ (SD) at B2D. Two typical cross sections of the ore lenses, lower grade detritals and SD are shown in Fig 3. The sections illustrate the highly variable thickness and shape of ore types over short strike intervals (200 m in this case). These features to a large degree dictate the selection of bench heights and mining equipment. There are two principal ore types distinguished by their mineralogy and tenacity of the matrix which binds the individual hematite clasts.
Hematite conglomerate (HC) This ore type, also known locally as canga, varies from indurated with minimal friability to very hard. The HC is developed from a compacted hematite detrital which has been cemented by goethitisation of matrix and clast margins. HC mainly underlies hematite detritals but small intercalations of HC may occur within HD as localised bands to 2 m thick. The contacts dip gently north and away from the southern flanking escarpment of the Brockman Iron Formation (Fig 3). The cementing medium is mainly a mixture of goethite, kaolinite, silica and fine hematite.
FIG 3 - Typical cross sections of B2D Pit 1 illustrating orebody shape and iron grade variability within lithological units. Location of section lines on Fig 1.
Geology of Australian and Papua New Guinean Mineral Deposits
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D M McKENNA and R A HARMSWORTH
FIG 4 - Rice bubble canga.
FIG 6 - Cemented red matrix canga.
FIG 5 - Welded canga.
Four main varieties of HC are identified in the field, based mainly on physical features. Properties such as hardness, brittleness and the strength of the clast–matrix bond dictate the degree of fragmentation during mining and the lump recovery and iron grade of products derived from the crushing and screening operations. Rice bubble canga consists of well defined hematite clasts cemented by hematite or goethite (Fig 4). Porosity is generally variable and dependent on clast packing characteristics and degree of leaching of matrix, and the rock is generally hard and brittle. Welded canga is generally more massive than the other varieties and consists of tightly packed hematite clasts with vague outlines due to goethitisation (Fig 5). The matrix is goethite, quartz and clay, and porosity is significantly less than in the rice bubble canga. Cemented red matrix canga has relatively unaltered hematite clasts cemented by a ferruginous kaolinitic matrix (Fig 6) and iron grade is normally higher than in the previous types. Goethitic canga consists of hematite clasts supported in a goethite matrix with some clay and quartz (Fig 7). The clasts are partially leached and altered along fracture planes. The matrix does not separate readily from the clasts, and consequently results in a high proportion of composite particles after crushing, leading to a lower grade lump product than that from other varieties.
Hematite detritals (HD) This ore type consists of a loose to tightly compacted assemblage of hematite clasts within a red hematitic and kaolinitic matrix. The contact with HC is quite sharp. A feature of this type is that the component clasts are well sorted, and in the highest grade areas hematite clasts comprise the most
378
FIG 7 - Goethitic canga.
abundant component. In the lower grade areas the hematite is accompanied by significant quantities of silica present as BIF or chert particles.
Siliceous detritals (SD) The SD unconformably overlies and to a large extent conceals the HC and HD. It consists of a chaotic assemblage of poorly sorted angular to subangular clasts of BIF and goethite with some hematite, chert and reworked HC rocks. The average iron content of this material is significantly less than the ore beds, reaching 55% in patches near the orebody but averaging around 40%. Metallurgical testing of this material has been aimed at recovering the contained lumpy hematite, but results show that processing is currently uneconomic due to the low recovery of hematite.
DIMENSIONS AND INTERNAL CHARACTERISTICS Controls on deposition The hematite-rich ferruginous carapace and ferricrete of the Hamersley surface was the source of the hematite in the ore lenses (Fig 2). This surface was dissected by rejuvenated stream systems in the late Eocene and Miocene, and the dominant basin-wide fault and joint system provided planes of weakness to direct actively down-cutting streams to the strike valley below. Preferential chemical erosion of dolomite and
Geology of Australian and Papua New Guinean Mineral Deposits
BROCKMAN NO. 2 DETRITALS (B2D) IRON ORE DEPOSIT
shale prepared the valley at the base of the resistant Brockman Iron Formation escarpment to become a trap for the stream debris. Deposition occurred near the foot of the range front to form hematite deposits of roughly tabular shape in plan view. The base of the deposits reflects the gentle basinal shape of the underlying valley floor. The HD material deposited on the basement mainly comprised well rounded and closely sized hematite clasts within a red ferruginous clay matrix with some BIF and chert clasts towards the top of the sequence. Morris (1994) offers evidence that the HC within the orebody was derived from HD by cementation during hiatuses in deposition. These events were followed by a further hiatus in the cycle of sedimentation, before Recent stream rejuvenation resulted in further active erosion through the parent carapace, ferricrete, and underlying BIF and goethite. This mixture of rocks, with minor amounts of reworked HC, was deposited unconformably as colluvial cover on the HC and HD sequence.
Hydration of hematite conglomerate The quantity of goethite in HC frequently increases as the iron content decreases with depth due to downward percolating water and the consequent hydration of hematite. A stage is reached where the HC becomes physically weak because of high goethite content and is unacceptable as lump ore, even though the iron content may be more than 60%. A second adverse feature of this material type is that lump ore recovery from hydrated HC is usually uneconomically low. As a consequence of these undesirable features, the lower grade cutoff for HC has a loss on ignition (LOI) component in addition to iron content, and the LOI maximum cutoff ranges between 6.0 and 7.0%. A typical drill hole section from surface through SD, HD and an HC-goethite complex is shown in Table 1 and Fig 3, and illustrates the relation between decreasing iron in HC with increasing levels of LOI towards the base of the orebody.
Formation of pisolites Dimensions The four lenses at B2D are tabular and dip at 10 to 15 o north and away from the range front. The deposit within Pit 1 (Fig 1) comprises roughly equal proportions of HD and HC ore as defined by the 60% iron lower cutoff and will yield around 25 Mt of crusher feed. It is around 1500 m long, 300 m wide and 30 m thick, and consists of a series of overlapping adjacent fans deposited by several streams draining northwards (Fig 2). The eastern end of the pit yields the highest grade at about 63.5% iron and the western end around 61.5% iron. Pits 2, 3 and 4 are sited on smaller lenses, each deposited by a single stream draining the southern carapace. Each pit will contribute around 2 to 4 Mt of high grade crusher feed. The deposits are 400 to 500 m long, 200 to 300 m wide and approximately 20 m thick, and consist mainly of HC with minor HD. The eastern edge of the HC in Pit 2 terminates against the wall of a buried gorge at an indentation in the range front and has a sharp, near vertical contact with the shale or dolomite country rock (Figs 1 and 2).
Diagenetic processes Chemical and physical changes have occurred within the detrital pile after deposition, and three which affect the economics of the iron resource in the B2D deposits are as follows.
Phosphorus depletion Phosphorus is an undesirable contaminant in iron deposits. The ferruginous parent rocks of the Hamersley surface from which the B2D deposit was developed are characterised by a high phosphorus content of between 0.1% and 0.15%, compared with 0.06% in the B2D deposit. Phosphorus tends to associate preferentially with goethite in the parent hematite-goethite ore (Morris, 1985). Phosphorus depletion in the lump material is due to prolonged dehydration of some of this goethite to hematite in the hardcap prior to erosion, effectively ‘unlocking’ the phosphorus, with further dehydration taking place during transport of clasts and following deposition of the detrital pile. Leaching of goethite from detrital clasts in situ by circulating groundwaters may also be a significant factor in reducing the level of phosphorus.
Geology of Australian and Papua New Guinean Mineral Deposits
Pisolites are found in varying concentrations throughout the detrital deposit. The pisolites are mostly loose with a clay matrix, and are present from trace quantities to 90% of the rock mass. Individual pisolites are normally less than 3 mm diameter, but some reach 5 mm, and they occasionally occur as tightly cemented masses which, when broken, resemble lumps of massive iron ore. Pisolites are always well rounded, with a high degree of sphericity, and their internal structure normally comprises concentric layers of hematite, goethite and kaolinite around a nucleus. The pisolites have been studied in considerable detail by Morris (1994) and others, and opinions on their origin vary. There is evidence of clastic origin and derivation from ferruginous soil profiles and deposition simultaneously with the detrital succession (Morris 1994). There is also field evidence that many were formed in situ by circulating fluids in the detrital pile. Regardless of origin, the presence of high and unexpected concentrations of pisolites in crusher feed may result in unacceptably low recoveries of lump product. Thus pisolite concentration in material scheduled as crusher feed must be carefully measured and recorded during bench face mapping and drill hole logging.
MINE GEOLOGICAL METHODS Geological activities are concentrated on ore block selection and reconciliation between predicted and actual plant output. Bench face mapping and ore reserve determinations are routine practice. Drilling and blasting are done on 10 m benches, and digging on 3.3 m sub-benches to assist selective separation of ore and waste. All blast holes are sampled in 3.3 m intervals, and assays are used in ore block delineation. Additional input to delineation of ore boundaries is provided by geological logging of blast drill hole cuttings. Accurate reconciliation of predicted ore production with plant output is possible because crusher feed is sourced from single ore blocks, and also because there are no stockpiles between crushers and screens. The plant in effect is a 'straight through' system from primary crusher to loadout. Samples of products are assayed at 1000 t intervals of lump production. Measurements of the high temperature properties of lump product are also done on a regular basis.
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TABLE 1 Summary of drill hole B2D DHA722 showing material type and chemical composition. Material
Depth (m)
Per cent - dry basis LOI
SiO2
Al2O3
46.7
5.1
23.1
4.0
0.09
60.5
2.4
5.9
4.1
0.07
From
To
Fe
SD <55% iron
0
10.5
HD >60% iron
10.5
15.0
P
HC > 60% iron
15.0
19.5
60.9
5.0
2.8
3.6
0.07
Goethite
19.5
24.0
57.2
9.5
2.9
3.9
0.06
HC >60% iron
24.0
31.5
63.2
4.1
1.8
2.4
0.06
HC hydrated >60% iron
31.5
37.5
61.7
6.3
1.2
2.6
0.06
Goethite
37.5
55.5
58.7
9.3
1.8
3.2
0.07
Chert/shale basement
55.5
63.0
38.1
8.3
22.5
11.7
0.03
ACKNOWLEDGEMENTS The authors thank Hamersley Iron Pty. Limited for permission to publish this paper. It has benefited from discussions with technical officers in several divisions within Hamersley, in particular those in the Brockman Mine Operations area. A Connor is thanked for reviewing the paper and contributing many useful comments. J Clout of the CSIRO Division of Minerals kindly supplied photographs used for Figs 4 to 7.
REFERENCES Campana, B, Hughes, F E, Burns, W G, Whitcher, I G and Muceniekas, E, 1964. Discovery of the Hamersley iron deposits (Duck Creek–Mt Turner area), The Australasian Institute of Mining and Metallurgy Proceedings, 210:1–30.
Morris, R C, 1985. Genesis of iron ore in banded iron-formation by supergene and supergene- metamorphic processes — a conceptual model, in Handbook of Strata-Bound and Stratiform Ore Deposits Vol 13 (Ed: K H Wolf), pp 73–235 (Elsevier: Amsterdam). Morris, R C, 1994. Detrital iron deposits of the Hamersley Province, CSIRO Division of Exploration and Mining, restricted report 76R (unpublished). Trendall, A F, 1983. The Hamersley Basin, in Iron-Formation : Facts and Problems (Eds: A F Trendall and R C Morris), pp 69–129 (Elsevier: Amsterdam). Trendall, A F and Blockley, J G, 1970. The iron formations of the Precambrian Hamersley Group, Western Australia; with special reference to crocidolite, Geological Survey of Western Australia Bulletin 119.
MacLeod, W N, 1966. The geology and iron deposits of the Hamersley Range area, Western Australia, Geological Survey of Western Australia Annual Report 1962:44–54
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Paquay, R D and Ness, P K, 1998. Hope Downs iron ore deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 381–386 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Hope Downs iron ore deposits by R D Paquay1 and P K Ness2 INTRODUCTION The Hope Downs deposits are located 75 km NW of Newman, WA (Fig 1), at lat 23o00′S, long 119o50′E on the Roy Hill (SF 50–12) and Newman (SF 50–16) 1:250 000 scale and the Weeli Wolli (2752) and Ophthalmia (2751) 1:100 000 scale map sheets (Figs 1 and 2). They are within exploration licences E47/243, E47/308 and E47/597, owned by Hancock Prospecting Pty Ltd.
Limited exploratory percussion drilling was carried out in 1987, but it was results from percussion drill hole HPD 1015 drilled in 1987 that gave exploration at Hope Downs its real impetus. This hole was drilled to 100 m and later deepened to 178 m. It intersected mineralisation with an average grade of 62.1% iron and 0.053% phosphorus from surface to 176 m (Paquay, 1995). Low level aeromagnetic surveys were flown over the Hope Downs area in 1988 and 1996. Exploration was at first concentrated at Hope North, and in 1991 a 100 m decline was excavated, from which a bulk sample for metallurgical testing was taken. Investigation of the Hope South area began in 1992, and by mid 1994 more than 80 000 m of reverse circulation percussion drilling and 3000 m of diamond drilling had been completed at the two deposits. Two discrete orebodies were recognised, both with an east-west strike length of more than 5 km (Fig 2). Detailed geological mapping of the area was carried out in 1995. During the period 1993–94 eight winzes were sunk at Hope North and four at Hope South, to a maximum depth of 81 m, for a total depth of 643 m. Bulk sample composites were metallurgically tested. Resource evaluation of Hope North and South orebodies began in 1994, and pre-mining feasibility work commenced in 1996.
FIG 1 - Location map of the Hamersley Iron Province.
The total mineral resource at Hope Downs is 551 Mt above a 58% iron cutoff, including Proved Reserves at Hope North of 337 Mt at 61.6% iron, 2.9% silica, 1.5% alumina, 0.06% phosphorus and 7.1% LOI, at a cutoff of 58% iron, and at Hope South of 185 Mt at 61.6% iron, 3.5% silica, 1.8% alumina, 0.06% phosphorus and 6.2% LOI, at a cutoff of 59% iron.
EXPLORATION HISTORY Hope Downs was held by Hancock interests as Temporary Reserve 5072H prior to 1971. The area was joint ventured with Pacminex Pty Ltd who undertook an assessment of the area (Pacminex, unpublished data, 1972). Resource estimates at the time were 90 to 135 Mt at an average grade of +57% iron and 0.046% phosphorus. Pacminex relinquished their interest in 1973. TR5072H was replaced by exploration licence E47/308 following the change in the Mining Act in 1982. No further exploration was carried out until 1985.
1.
Chief Geologist, Hancock Prospecting Pty Ltd, PO Box Locked Bag No 2, West Perth WA 6872.
2.
Senior Geologist, Hancock Prospecting Pty Ltd, PO Box Locked Bag No 2, West Perth WA 6872.
Geology of Australian and Papua New Guinean Mineral Deposits
REGIONAL GEOLOGY Rocks of the Hamersley Group occur within the Hope Downs area, dominated by extensive outcrops of Marra Mamba Iron Formation in the core of a major fold known as the Weeli Wolli Anticline (Fig 2). The deposits occur in the Marra Mamba Iron Formation and consist of a banded hematite-goethite assemblage which extends to depths of more than 270 m below surface. Further areas of Marra Mamba Iron Formation are known at the Hope 2 and Hope 3 lease areas, 20 km and 35 km south of Hope Downs. Marra Mamba–type ores have a high goethite (FeO2H) content and a variable proportion of limonite (Feα (OH)1ψ.H2O). The mineral limonite, typified by its yellow biscuity appearance, is composed of lepidocrocite (γ-FeO.OH) and goethite. The Marra Mamba Iron Formation is Late Archaean to Early Proterozoic, of age 2650 to 2450 Myr (Morris, 1985). Radiometric dating at Mount Tom Price gives an age of 2470 Myr for the Dales Gorge Member (Trendall et al, 1990). However it is arguable whether mineralisation of the Marra Mamba Iron Formation at Hope Downs is of Proterozoic, Palaeozoic or Mesozoic age. There are two main sets of structures in the Hamersley Iron Province: large-scale dome and basin folds (pre-Ashburton Basin), and a north-trending fold belt (Tyler and Thorne, 1990). The main deformations of the Hamersley Group (D2 and D3
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FIG 2 - Regional structural map of Hope Downs area in relation to iron orebodies.
folding) took place during the Capricorn Orogeny at about 2000 Myr. A further deformation (D4) period took place subsequent to the Capricorn Orogeny and resulted in cross folding and faulting trending about 040o, which formed dome and basin structures.
thick, and shale NS-3 is 400 mm thick. Other shale macrobands are normally less than 200 mm thick. Limonite within both deposits is associated with high alumina values contained in the shale macrobands. The Mount Newman Member is conformably overlain by the Wittenoom Dolomite Formation.
LOCAL STRATIGRAPHY WITTENOOM DOLOMITE FORMATION The geology at Hope Downs is described in terms of ‘bedded’ units and ‘detrital’units. The bedded units include the Marra Mamba hosted hematite-goethite mineralisation. The detrital units include loose and cemented scree, surficial canga, ooidal/pisolitic/peloidal channel iron deposits (CID), alluvial fans, and red-ochre conglomerates and calcrete which occur in the alluvial valley on the northern side of the bedded mineralised zones.
The West Angela Member, the lowest unit of the Wittenoom Dolomite Formation, is 45 m thick at Hope Downs. It is composed of chert-shale and BIF macrobands. The BIF interbeds are 2 to 10 m thick, and are generally mineralised. The remainder of the Wittenoom Dolomite Formation is dolomite, dolomitic chert-shale and minor BIF.
TERTIARY TO RECENT MARRA MAMBA IRON FORMATION The Marra Mamba Iron Formation is the lowermost formation of the Hamersley Group (Fig 3), and contains three members: 1.
2.
3.
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The Nammuldi Member is the lowermost and conformably overlies the Roy Hill Shale of the Jeerinah Formation. It is 109 m thick in the project area and contains banded iron formation (BIF) and chert with 18 internal shale macrobands. The BIF and shale are more carbonate rich toward the base. The MacLeod Member is about 52 m thick at Hope Downs. It contains 13 shale macrobands, MS-1 to MS-13 generally <1 m thick, within BIF and chert. The chert and BIF are generally podiform in appearance. The Mount Newman Member is locally 65 m thick and essentially comprises monotonous BIF, with eight shale macrobands (NS-1 to NS-8), and is the major host to iron ore mineralisation at Hope Downs. Shale macrobands are thin compared to other areas within the province. Shale macrobands NS-4 and NS-6 are each 650 mm
Carbonaceous sediment within CID at Hope North deposit has been dated as Late to Early Miocene (Morris, 1994). The palaeolandscape at the base of the detrital cover indicates a very active rejuvenated river with numerous oxbow lakes and coarse gravels. Upstream the sediments indicate a change to passive sedimentation and low water flows indicated by smaller and more rounded grains. The thin surface canga, loose pisolitic scree and BIF scree are much younger, of Pliocene to Pleistocene age. They formed during rejuvenation due to uplift and further erosion, which produced coarser, more angular material.
LOCAL STRUCTURE FOLDING The Weeli Wolli Anticline is the major D3 structure in the Hope Downs area. The central part of the anticline has been eroded, removing the Mount Newman Member and most of the
Geology of Australian and Papua New Guinean Mineral Deposits
HOPE DOWNS IRON ORE DEPOSITS
FIG 3 - Regional stratigraphy of the Hamersley Group, left, (modified after Harmsworth et al, 1990) and type section of the Marra Mamba Iron Formation at Hope Downs, right.
Geology of Australian and Papua New Guinean Mineral Deposits
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R D PAQUAY and P K NESS
MacLeod Member, and occasionally exposing inliers of the underlying Jeerinah Formation. The anticline is folded asymmetrically about its east trending axis with a south dipping o limb which dips at 15 to 30 , and a steeper northern limb. The o axial planes of the larger D3 folds generally dip at about 85 towards the south. Virtually all the significant Marra Mamba deposits in the eastern Hamersley Iron Province are located on the north facing limbs of similar structures (Harmsworth et al, 1990), including Hope Downs. The larger folds are doublyplunging both to the east and the west, and normally have steep dips on the northern and shallow dips on the southern limbs. At Hope North and Hope South a broad D3 anticline forms the main ridge to the south of each orebody. On the northern side of each anticline the bedding is folded to the north and is buckled in a series of open folds contained in an envelope that dips to the north (S Ballantine and A Murie, unpublished data, 1994). At about 150 m depth at Hope North, and at about 80 to 100 m depth at Hope South, the bedding is synclinally folded (D2 folds) to dip to the south, then dips gently to the north. Folding is complex and typified by refolded asymmetrical (D2) folds of 40 to 100 m wavelength that have a steep northern limb and a flat southern limb. The D3 folds are generally more symmetrical with subvertical axial planes and are the main control of mineralisation, and the D2 and D3 folds are subparallel. The main structural features at Hope Downs are shown superimposed on the local geology in Fig 4. The first deformation (D1) consists of rodding, boudinage and small-scale folds with amplitudes and wavelengths of <0.5 m. The D1 folds are bedding-parallel, tight to isoclinalrecumbent and are rare. The only D1 folds evident at Hope
Downs are either small scale in the decline at Hope North, or in the West Angela Member on the southern limb of the Weeli Wolli Anticline. Outcrop scale D2 folds vary from less than 1 to 100 m in amplitude. They plunge at between 2 and 10o toward 060 to 080° at Hope North and at about 9o toward 80–105o at Hope South (A C Duncan, unpublished data, 1994). The D2 event is a thrust-like deformation produced by a south over north movement. The D3 event was not as intense as D2, but it did produce significant folds and is thought to be the main control on the Weeli Wolli Anticline (A C Duncan, unpublished data, 1994). The outcrop-scale D3 folds have amplitudes of 40 to 200 m and wavelengths up to 50 m, and an axial plane strike of 080–100o at Hope North and 100–120o at Hope South. The D3 event produced significant east trending upright folds on all scales. The D4 deformation is characterised by a regional joint and fold set that trends at 350 –045o and cuts across other fold styles to create the dome and basin effect. Joints associated with D4 and D5 are open and subvertical. A NNW trending dolerite dyke 1 km south of the Hope South orebody may be emplaced in a D5 structure, and has the same trend as the dolerite dykes described at Paraburdoo, Channar, and the Southern Batter fault at Mount Tom Price. Aeromagnetic signatures from Hope Downs indicate that some of the D4 structures are displaced by D5 movement. The enriched portions of the Marra Mamba Iron Formation are in synclines within D2 and D3 deformation zones and extend to 270 m below the surface. The deformation zones are about 1
FIG 4 - Geological map of Hope Downs showing main structural trends (left) and typical geological cross sections A-A′, B-B′for each orebody, looking west (right).
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Geology of Australian and Papua New Guinean Mineral Deposits
HOPE DOWNS IRON ORE DEPOSITS
km wide and appear to occur at a regular spacing of approximately 4 km with an amplitude of approximately 200 m (Ness, 1995). The presence of tight synclines has resulted in higher levels of iron enrichment, particularly in the Mount Newman Member. Typical cross sections through the two orebodies are shown in Fig 4.
FAULTING Faulting is minor at Hope Downs (J R Vearncombe, unpublished data, 1993). East-trending steep dipping normal faults were mapped in the Hope North decline, and appear to be axial plane slippages. Displacements are generally small, between 1 and 2 m. At Hope South a set of small scale reverse faults is recognised, but these appear to have little effect on the orebody. These faults are probably a result of small-scale reverse faulting in D2 hinge zones due to refolding by D3.
ORE DEPOSIT FEATURES STRUCTURE AND STRATIGRAPHY Discontinuous high grade iron mineralisation occurs around the margins of two easterly plunging D3 antinclines, with the more extensive high grade zones along their northern flanks. Hematite-goethite enrichment at Hope North extends continuously along the 5.5 km strike length of the northern limb and occurs preferentially in the synclinal troughs and flexures on the limb. The orebody has a width of approximately 250 m except in the central area where it widens to nearly 1 km over a strike length of 1 km (Fig 4). The highest part of the orebody is the outcrop in the mid-central area at 680 m RL, and the lowest mineable mineralisation intersected by drilling is at 370 m RL in the eastern syncline. The plain level to the north of the North deposit is at about 600 m RL. The local water table fluctuates seasonally about 570 m RL. The Hope South deposit is 5 km south of Hope North and has a similar structural setting. Hematite-goethite mineralisation extends over 5 km along the northern flank of the southern anticline with a width of approximately 200 m. Mineralisation widens to 1.2 km where the Mount Newman Member is relatively flat lying as it wraps around the nose of the D3 anticline at the eastern end of the deposit. In contrast to the D3 anticline at Hope North, discontinuous hematite-goethite occurs along the southern limb of the Hope South anticline. This mineralisation appears to have a 20 to 30 m true thickness and to vary in grade and quality. The bulk of the mineralisation at Hope North and South is confined to the Mount Newman Member and the base of the West Angela Member. The Nammuldi Member is only enriched at Hope South. The MacLeod Member is partially mineralised at both deposits. The main difference between the two deposits is that stripping by erosion is much less at Hope North than at Hope South. Hope South has a low grade lateritised or hydrated hardcap to 30 m depth, overlying a high grade hematitegoethite orebody. There is some thin hardcap at Hope North but it is discontinuous, near-surface ( <15 m thick) and is generally siliceous compared with the bulk of the orebody.
study confirmed that the ores were of the Mesozoic, nonmetamorphosed type, derived by supergene enrichment of the Marra Mamba Iron Formation. Characteristically the Hope Downs ores contain less ochreous goethite and are less friable than other Marra Mamba deposits. Hope Downs ore has been subjected to greater levels of ground water leaching which has tended to dissolve out the metasomatic goethite and replaced the BIF matrix, thus increasing the residual hematite. Voids have been filled by later generations of goethite effectively recementing the ores to give an improved lump (-30 mm +6 mm) to fines (-6 mm) ratio (Morris, 1985). Where numerous phases of mineralisation have occurred the voids have been filled (flooded) with goethite resulting in a less porous and denser rock. In some cases goethite has reverted to hematite. The Hope Downs deposits have undergone at least five, and possibly up to seven, enrichment events. Most other Marra Mamba ores have only one or at most two documented mineralising phases. Structure is an important factor in the localisation of the enrichment zones at Hope Downs and in the subsequent ground water modification of the ore. At both Hope North and Hope South tight synclines tend to contain mineralisation with higher iron grades, and the enriched zones tend to parallel the axis of the major folds. The bulk of the iron mineralisation follows the form, trend and plunge of the broad D2 and D3 synclines (Ness, 1995). It is unlikely that the deposits would be sufficiently mineralised without the large scale folds to control and channel the water and allow enrichment of the BIF. Hematite-rich goethitic ores at Hope Downs contrast markedly with the extremely friable and limonitic Marra Mamba ores typified by Mount Tom Price Marra Mamba and Orebody 29 at Newman, or the Marandoo ore with significant amounts of residual partially eroded kenomagnetite. Such metastable components in ore can be related to the evolution of the deposit during the enrichment stage, and the later weathering of the deposit, and thus to its maturity. The Hope Downs ores have had a different evolutionary sequence than those of other Marra Mamba deposits. Multiple supergene enrichment phases have replaced much of the initial texture leading to more dense, less limonitic ores and loss of much of the classic textural evidence that has been used at other deposits to determine the genetic model.
ORE GENESIS The essential conditions for supergene iron ore formation of the type found at Hope Downs are: 1.
an iron-rich aquifer (BIF) both as a source of iron and a target for enrichment;
2.
impervious layers such as shale above and below as aquicludes;
3.
a favourable structure such as a plunging syncline, and deep water access, for example along a fault-engendered fracture zone (as at Mount Tom Price or Mount Whaleback), or fold axis fractures, to initiate the process;
4.
specific geochemical conditions including exposure of the BIF to the atmosphere;
5.
suitable electrochemical conditions; and
MINERALOGY AND PETROLOGY
6.
tectonic stability for long periods.
R C Morris (unpublished data, 1995) carried out a detailed petrographic investigation of the Hope Downs deposits. The
The broad supergene process has led to the production of hematite-goethite ore which has been modified by surface
Geology of Australian and Papua New Guinean Mineral Deposits
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exposure and continued ground water movement to produce a range of very dense to porous ores. For iron ore mineralisation to occur, the host BIF generally requires some confining and controlling structure to channel water so that supergene-type enrichment can take place. Axial plane fracturing and water movement through the Wittenoom Dolomite is perhaps one requirement for initiating the replacement process in BIF.
MINING GEOLOGY FACTORS In both deposits the friability of the mineralisation increases slightly, and the lump per cent decreases marginally with depth. In the resource estimation process ordinary kriging has been used for grade interpolation. Detailed variography was used to establish the continuity of grades and ore zones for each ore type and for both bedded and detrital units in both deposits. Eleven assay determinations (Fe, LOI, SiO2, Al2O3, P, Mn, TiO2, CaO, MgO, S, K2O) are carried out on 2 m sequential samples from each hole and related to the rock type, adjacent holes and cross sections. The Hope Downs ores are generally lower in contaminants (alumina, silica and manganese) and are of higher density and lower porosity than many other Marra Mamba-type ores. The high LOI factor is due to goethite. The Hope North deposit contains on average 25%, and Hope South deposit 35% free limonite as stringers in hematitegoethite. As a comparison, other Marra Mamba deposits have over 50% limonite. Hope Downs lacks any obvious lower limonite zone. The Hope Downs deposits contain both massive and friable iron ores. As a result the deposits have a high variability in lump (-30 mm +6.0 mm) distribution, from 25% to 55%. Hope North has on average 41% lump and Hope South 38% lump. The deposits are composed of hematite-martite-goethite rather than limonite-goethite (martite is a rhombohedral crystalline form of hematite). The higher iron grades are related to the most friable ore but fines mineralisation is generally hematitic rather than limonitic.
have contributed to the discovery and development of the Hope Downs deposits, and thank D Clarke for her assistance in the production of this paper.
REFERENCES Duncan, A C, 1994. Structural Geology of Hope Downs, Hancock Prospecting Pty Ltd, (unpublished report). Harmsworth, R A, Kneeshaw, M, Morris, R C, Robinson, C J and Shrivastava, P K, 1990. BIF-derived iron ores of the Hamersley Province, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: FE Hughes), pp 617–642 (The Australasian Institute of Mining and Metallurgy: Melbourne). Morris, R C, 1985. Genesis of iron ore in banded iron-formation by supergene and supergene-metamorphic processes - a conceptual model, in Handbook of Strata-Bound and Stratiform Ore Deposits, Volume 13 (Ed: KH Wolf), pp 73–235 (Elsevier: Amsterdam), Morris, R C, 1994. Detrital iron deposits of the Hamersley Province, CSIRO Division of Exploration and Mining, restricted report 76R (unpublished). Ness, P K, 1995. Ore resources in the iron ore industry: A case study of Hope Downs Iron Ore Project, MSc thesis (unpublished), The University of Western Australia, Perth. Pacminex Pty Ltd, 1971. Exploration for Iron Ore, TR 5072/74H, West Pilbara Goldfields, W4, Quarterly Report (unpublished report). Paquay, R D, 1995. The Hope Downs iron ore deposit Western Australia: geology and exploration history, MSc thesis (unpublished), The University of Western Australia, Perth. Trendall, A F, Compston, W, Williams, I S, Armstrong, R A, Arndt, N T, McNaughton, N J, Nelson, D R, Barley, M E, Beukes, N J, De Laeter, J R, Retief, E A and Thorne, A M, 1990. Precise zircon UPb chronological comparison of the volcano-sedimentary sequences of the Kaapvaal and Pilbara Cratons between 31 and 2.4 Ga, in Correlation of the Fortescue Group (Eds: D R Nelson, A F Trendall and J R De Laeter), pp 105–110, Western Australian Minerals and Energy Research Institute, Report No 61. Tyler, I M and Thorne, A M, 1990. The Northern margin of the Capricorn Orogen, Western Australia - an example of an Early Proterozoic collision zone, Journal of Structural Geology, 12 (5/6): 685–701. Vearncombe, J R, 1993. Structural Geology of the Hope Downs Deposits, Hancock Prospecting Pty Ltd, (unpublished report).
ACKNOWLEDGEMENTS This paper is published with the permission of the Chairman of Hancock Prospecting Pty Ltd, Gina Rinehart. The authors recognise the contribution made by numerous geologists who
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Rogers, K A, 1998. Speewah fluorite deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 387–392 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Speewah fluorite deposit by K A Rogers
1
INTRODUCTION The fluorite deposits owned by Elmina NL are in the Speewah Valley, 110 km SW of Kununurra in the Kimberley region of WA (Fig 1). The main deposits are at lat 16o26′S, long 127o58′E on the Lissadell (SE 52–2) 1:250 000 scale map sheet. Total Inferred, Indicated and Measured Resources are 3.87 Mt at 25% CaF2 and there is potential for much larger resources.
EXPLORATION HISTORY Exploration in the Speewah area has been carried out since the early 1900s for gold, base metals, uranium, heavy mineral sands, tin, rare earths, diamonds, fluorite and barite. Fluorite was first discovered in 1905 (Simpson, 1951). Exploration by Great Boulder Mines Ltd and North Kalgurli Mines Ltd during 1972–1973 identified nine fluorite veins in the Main zone, containing Proved and Probable Reserves of 1.641 Mt at 47% CaF2 in veins A, B and C (K Schultz, unpublished data, 1973). Elmina acquired the Speewah deposits in 1987. Data from drilling at A, B and C veins were used to increase the Resource to 1.89 Mt Measured, 0.41 Mt Indicated and 1.59 Mt Inferred at 25% CaF2, using a 13% CaF2 cutoff grade. Exploration between 1993 and 1996 showed that the fluorite mineralisation and associated quartz veining and alteration are indicative of a high-level epithermal system with potential for gold, silver and base metal mineralisation (Alvin, 1993). It has also identified additional fluorite vein systems which could significantly increase the resource at Speewah. Metallurgical testwork demonstrated that Speewah fluorite can be upgraded by conventional froth flotation to a 97–98% CaF2 acid grade concentrate. Several studies were carried out on the preferred mining and processing methods, including value-adding the fluorite into aluminium fluoride (Rogers, 1993). Elmina is currently considering a 65 000 tpa acid grade fluorite operation supplying the domestic demand of 20 000 tpa and export markets.
REGIONAL GEOLOGY The deposits are in the western edge of the Halls Creek Mobile Zone and on the SE side of the Speewah Dome (Fig 1). They are structurally controlled by NNE- and north-trending faults and brittle fractures which appear to be part of the regional scale Greenvale Fault. Folding of Early Proterozoic units of the Kimberley Block has produced a series of broad domes and basins, of which the Speewah Dome is the main example. It is 32 km long by 13 km wide, with its long axis striking slightly east of north.
1.
Exploration Manager, Elmina NL, PO Box 1193, West Perth WA 6872.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Location and regional geological map, Speewah area.
Fluorite mineralisation is mainly hosted by normal faults where they intersect Kimberley Block units, which are predominantly sediment and minor volcanic rocks, intruded by the differentiated Hart Dolerite sill. The mineralisation occurs mostly within the Speewah Dome, with minor occurrences elsewhere along the Greenvale Fault. It crops out as large veins to 10 m wide and 700 m long which are discordant with the host rocks, indicating an epigenetic origin for the deposits.
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The stratigraphy of the western part of the Halls Creek Mobile Zone in the Speewah area is summarised in Table 1.
ORE DEPOSIT FEATURES LITHOLOGY There is ample exposure in the Speewah area as the area has a rugged topography, in which the Speewah Valley occupies the central eroded and lower lying part of the Speewah Dome. Primary minerals and textures are well preserved in outcrop, although there are several secondary mineral assemblages, the most common being sericite±epidote after plagioclase, and chlorite±biotite after hornblende or pyroxene. The units surrounding the fluorite deposits include the Valentine Siltstone and Lansdowne Arkose of the Speewah Group, the intrusive Hart Dolerite and Yilingbun Granophyre, a locally developed unit called the Doon Doon breccia, and the Antrim Plateau Volcanics. All of these units host varying amounts of fluorite mineralisation. The Hart Dolerite is exposed in the core of the Speewah Dome, and is surrounded by prominent mesas of gently dipping sediment. The main dolerite intrusion was emplaced within the Valentine Siltstone, and other sills have been intruded at higher stratigraphic levels. The dolerite is up to 1800 m thick, with the upper part a granophyric unit to 240 m thick (Plumb, 1968). It contains numerous large blocks of Proterozoic sediment. G S
Eupene (unpublished data, 1970) described the dolerite as a differentiated laccolithic sill, for which intrusion produced the doming at Speewah and the granophyre. The sill intrusion process was thought to comprise filter-press action from the central portions of the dolerite sill after the outer carapace had solidified. Three units have been identified by Alvin (1993): olivine dolerite, a basal unit rich in magnetite and titaniferous magnetite; quartz dolerite, the main outcrop type; and microdolerite with chilled dolerite at sedimentary contacts. The Yilingbun Granophyre is the new name given to the granophyric phase of the Hart Dolerite (Alvin, 1993). It includes syenogranite and alkali feldspar granite exposed in the eastern half of the Speewah Dome, which stand out as a pinkred unit in contrast to the brown-black Hart Dolerite. The most abundant minerals in the granophyre are quartz and potassium feldspar, with ubiquitous granophyric texture. Potassium feldspar has a persistent dusted appearance, possibly due to submicroscopic hematite. The two gradational end members of the Yilingbun Granophyre are a lower mafic unit comprising mainly potassium feldspar, quartz, plagioclase, pyroxene, apatite and hornblende, and an upper felsic unit comprising potassium feldspar, quartz, albite, epidote and apatite. The granophyre contains three inclusion types - large quartz sandstone roof inclusions, small mafic (?dolerite) inclusions, and small inclusions of syenitic composition. Hornblende in the mafic unit is associated with the only fluorite noted in the
TABLE 1 Stratigraphy of the Halls Creek Mobile Zone in the Speewah area. Age
Stratigraphic name
Lithology
Tertiary ?
Doon Doon breccia
Tectonic, hydrothermal and fluidised breccias; breccia and pebble dykes, contains Yungal carbonatite
Lower Cambrian
Antrim Plateau Volcanics
Tholeiitic basalt lava, some vesicular, amygdaloidal, and agglomeratic; minor chert and sandstone
Early Proterozoic
Yilingbun Granophyre
Syenogranite and alkali feldspar granite, granophyric
Hart Dolerite
Quartz dolerite, basal olivine dolerite, microdolerite on contacts
Pentecost Sandstone
Quartz sandstone and feldspathic sandstone, minor siltstone, glauconitic sandstone
Elgee Siltstone
Red siltstone, minor fine grained sandstone, green shale, algal dolomite
Warton Sandstone
Quartz sandstone and feldspathic sandstone, minor purple shale
Carson Volcanics
Basalt, feldspathic sandstone, minor micaceous sandstone, chert, chloritic siltstone
King Leopold Sandstone
Quartz sandstone and granule sandstone, conglomerate; micaceous siltstone
Lumen Siltstone
Micaceous siltstone, shale, minor sandstone
Lansdowne Arkose
Feldspathic to quartz sandstone and arkose, minor micaceous siltstone
Valentine Siltstone
Green siltstone and mudstone, grey-purple tuffaceous unit
Tunganary Formation
Quartz sandstone, feldspathic sandstone, arkose, minor siltstone
O’Donnell Formation
Greywacke, silty sandstone, quartz sandstone, minor green shale, fine grained sandstone
Whitewater Volcanics
Quartz-feldspar porphyry, feldspar porphyry and acid volcanic pyroclastics
Bow River Granite
Granite and granodiorite, some porphyritic
Kimberley Group
Speewah Group
Source: Plumb (1968).
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Geology of Australian and Papua New Guinean Mineral Deposits
SPEEWAH FLUORITE DEPOSIT
Yilingbun Granophyre. The mafic unit contains <5 % magnetite and ilmenite and some specimens have a high magnetic susceptibility. Field relationships, petrographic studies and whole rock analyses suggest that the Hart Dolerite and Yilingbun Granophyre are genetically related, but possibly not through simple in situ fractionation as in the case of differentiated sills. A direct contact between the Hart Dolerite and Yilingbun Granophyre is not evident at Speewah. The Hart Dolerite crops out topographically higher and lower than the granophyre, with slightly discordant contacts, suggesting that the granophyre may have intruded the Hart Dolerite. The Lansdowne Arkose outcrops along linear ridges. It hosts fluorite mineralisation and has similar bedding orientations to the Valentine Siltstone. It comprises a variable sequence of pink arkose, subarkose, and feldspathic to quartz sandstone. The Valentine Siltstone is a soft and friable unit of mostly green siltstone and mudstone, with minor grey-purple tuffaceous sediment. It crops out below the Lansdowne Arkose and dips between 10 and 20o to the west and SW. The Doon Doon breccia is associated with fluorite mineralisation in the Speewah area (Alvin, 1993). It is found along fault zones, cropping out as linear ridges to 50 m high or as low subvertical ledges. It consists of tectonic and hydrothermal breccia, breccia dykes and pipes, and rare pebble dykes. Dolerite within the tectonic and hydrothermal breccias is highly altered, and veined by calcite, quartz and fluorite. These breccia zones are also veined by a brown, highly silicified rock that has a subvertical dyke-like form. The veins or dykes are up to 1 m wide, comprising about 80% angular, poorly sorted, poorly packed quartz clasts and lithic fragments, supported by a brown oxide or carbonate matrix. Some have rounded lithic fragments and resemble pebble dykes. The crosscutting, intrusive nature of this dyke-like rock suggest that it is a fluidised breccia or breccia dyke which has filled pre-existing fractures in the host rock. Breccia dykes are, like other breccias capping intrusives, considered important indicators of buried plutons and the presence of metallic mineralisation (Laznicka, 1988). Felsic pyroclastic fragmental rocks form small irregular intrusives cut by fluorite-quartz veins at West vein. They contain a heterolithic assemblage of sericite-altered angular fragments of Yilingbun Granophyre, Hart Dolerite, sandstone, chert and mylonite in a matrix of quartz sand, very fine grained silica, and carbonate and fluorite cement. Traces of chalcopyrite and pyrite are found in the siliceous varieties, which may represent high level hydrothermally altered vent breccias. A carbonatite dyke, to 15 m wide, named the Yungal carbonatite, is hosted by Doon Doon breccia in West Ridge. Marginal zones of the dyke appear to have a laminated structure, comprising mainly manganiferous calcite and rounded potassium feldspar–quartz-plagioclase clasts. Small veins, 10 to 20 cm wide, with potassium feldspar rims and calcite cores are common in the outer laminar zones. This assemblage is similar to the potassium metasomatism known as fenitisation which is commonly associated with carbonatites. The central portion of the carbonatite dyke has a massive structure, comprising mainly interlocking, coarse grained, euhedral manganiferous calcite with little or no clastic
Geology of Australian and Papua New Guinean Mineral Deposits
component. Apatite is an accessory mineral in the central portion of the dyke. Calcite veins with a bladed crystal texture crosscut this carbonatite dyke. Associated with the carbonatite dyke at West Ridge is an unusual clastic-fluorite dyke composed of rounded potassium feldspar fragments cemented by purple fluorite. Gold values of 0.2 g/t are found in this rock.
STRUCTURE The Greenvale Fault forms the eastern margin of the Kimberley Block and is a composite system of intersecting faults (Fig 1). An older set strike at 210o, and comprise shear zones to 2 km wide, with complex normal and reverse vertical thrusting. Younger splay faults trend north and dip steeply east or west as narrow zones of brecciation and quartz veining. They show horizontal displacements of up to 6 km but only small vertical movement, and in the Speewah Valley are known to contain breccia dykes, fluorite veins, base metal mineralisation and a carbonatite dyke. All the main rock units in the immediate deposit area have been cut by the two fault sets. The faults cut all the Early Proterozoic rocks and appear to control the westernmost extent of the Antrim Plateau Volcanics, but they also cut these volcanic rocks. The present shape of the Speewah Dome is also controlled by these faults. These observations suggest the possibility of several phases of reactivation of the faults over a long period. The Valentine Siltstone and Lansdowne Arkose, which host a large proportion of the fluorite mineralisation, crop out as linear, NNE-trending ridges associated with faults of the same orientation. From the juxtaposition of lithostratigraphic units the movement on these mineralised faults is interpreted to be normal. Primary bedding structures in the sediment dip to the SE between 10 and 20o, with the subtle igneous layering of the Hart Dolerite having a similar orientation. All rocks have excellent textural preservation indicative of low regional strain.
METAMORPHISM The regional, prograde, prehnite-pumpellyite facies metamorphic mineral assemblage comprises small spherulitic clusters of prehnite, with chlorite and epidote. Retrograde metamorphism includes sericitisation of feldspars, as in the Hart Dolerite. The Lansdowne Arkose and Valentine Siltstone have hornfelsic textures representing contact metamorphic zones 8 to 10 m wide at their contacts with the Hart Dolerite and Yilingbun Granophyre.
MINERALISATION AND RESOURCES Fluorite occurs as narrow, tabular, steeply dipping high grade veins from less than 1 m to 10 m thick, flanked by lower grade stockwork and stringer veins forming a mineralised envelope to 25 m wide. The fluorite is mostly grey or white in outcrop, with minor pale green and purple varieties, and contains minor base metal mineralisation. The gangue mainly consists of quartz in the high grade veins, with lesser barite and silicified fragments of country rock. There are several periods of fluorite veining associated with several phases of quartz and barite veining. Gold values to 0.27 g/t are found in some fluorite veins. Within the Speewah area, fluorite mineralisation is presently known in four areas, namely the Main, West, Northwest and
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K A ROGERS
Central zones (Fig 1). These occur along faults which also contain breccia dykes and epithermal quartz veins.
Main zone A major NNE-trending splay fault of the Greenvale Fault occurs along a granophyre–sediment contact on the SE edge of the Speewah Dome. Along this structure there are at least nine fluorite vein sets over a strike length of 7.5 km. The A, B and C veins form a prominent linear ridge some 2 km long at the northern end of this trend (Fig 2) and average 35 to 40 m elevation above the level of the Dunham River.
FIG 3 - Cross sections of A (top) and B (bottom) veins, Main zone deposits, looking NE.
Northwest zone
FIG 2 - Geological map of the Main zone deposits, with location of cross sections on Fig 3.
The A vein is about 500 m long, the B vein to the south is some 700 m long, and both are 20 to 25 m wide. Between them lies the smaller and narrow C vein. None of the veins have been closed off at depth. The dip is predominantly subvertical in A vein, and averages 65o east in B vein (Fig 3). The D, E, F, G and South veins have been recorded in subparallel fault zones, but their extent has not been defined by drilling.
West zone The West vein is about 5 km south west from B vein and just east of the prominent West Ridge (Yungal) structure (Fig 1). It outcrops discontinuously over a 200 m strike length with a northerly trend, and varies from 1 to 3 m in width. It occurs in a pervasively silicified shear zone within felsic pyroclastic breccia and pyritic dolerite. The fluorite is white and occasionally purple in colour, and is associated with minor disseminated galena, chalcopyrite and pyrite.
390
The zone is 5 km north of West zone, along the West Ridge fault (Fig 1). Three north-trending subparallel veins outcrop intermittently here in a 50 m wide zone, and have strike lengths from 30 to 330 m. Widths vary from 0.3 to 6.5 m and the mineralisation comprises both fluorite and barite lenses and pods.
Central zone The Central zone is a north trending ridge formed by the confluence of the West Ridge structure and a northerly trending splay off the ABC vein structure, about 14 km NNW of B vein (Fig 1). It contains a narrow fluorite-quartz vein about 150 m long, paralleling a breccia-pebble dyke. The northern end of the vein comprises brecciated quartz dolerite with traces of galena, and to the south it is a breccia with fluorite cement and fluorite veinlets.
Fluorite vein textures The most common structures and vein textures are symmetrical laminations, colloform and crustiform banding, spider vein brecciation, vuggy textures, and quartz pseudomorphs after bladed calcite. These textures are indicative of ore deposition in
Geology of Australian and Papua New Guinean Mineral Deposits
SPEEWAH FLUORITE DEPOSIT
open spaces under low confining pressure, and the deposit is therefore interpreted to have been deposited in an upper crustal environment, probably at 1 to 2 km depth and at a pressure of <1 kb.
Fluid inclusion studies Fluid inclusions in fluorite and barite are aqueous and mainly fluid rich. The inclusions in fluorite have moderate to high salinity, between 15 and 20 but up to about 30 eq wt% NaCl, and have trapping temperatures of about 150 to 170oC. Barite fluid inclusions have relatively low salinity (6–14 eq wt% NaCl) and trapping temperatures of about 280oC. These temperatures, particularly those of the main stage fluorite mineralisation, are within the temperature range of known epithermal deposits. Low temperature analysis of fluid inclusions indicates that they are characterised by a complex calcium-rich fluid, suggesting that both calcium and fluorine were transported in the fluid to the depositional site, rather than added to the fluid by interaction with calcium-rich wall rocks such as the Hart Dolerite. The coexistence of vapour-rich and fluid-rich inclusions is rare, and suggests that ore fluid boiling was not the dominant control of ore deposition. Instead, a model of simple cooling along the ore fluid flow path is proposed for fluorite deposition.
AGE OF FLUORITE MINERALISATION Galenas hosted by the fluorite have consistently given lead model ages between 15 and 131 Myr (Alvin, 1993). Recent direct dating of fluorite utilising the Sm-Nd method gave a 120 Myr age (M P Alvin, personal communication, 1997). These dates support a younger hydrothermal event than previously reported, with the as yet undated Yungal carbonatite representing the only exposed potential igneous source of the ore fluids. Importantly, there is alkaline igneous activity, including the emplacement of carbonatites, in the West Kimberley diamond province which overlaps in time with the galena model ages from the Speewah deposits. Samples of galena collected from a flat lying ‘bucky’ quartz sulphide vein at Martins prospect (Fig 1) gave a lead model age of 1500 Myr. No fluorite is associated with this vein, although it is cut by subvertical thin epithermal quartz veins found elsewhere in the Speewah area with fluorite. One fluorite vein cuts the Antrim Plateau Volcanics, which clearly dates the fluorite mineralisation as post-Cambrian.
ORE GENESIS The fluorite and associated copper-lead-silver mineralisation have been considered to be related to late stage hydrothermal activity associated with emplacement of the granophyric phase of the Hart Dolerite sill (Blockley, 1972). More recently, Alvin (1993) has shown that the Speewah fluorite is an epigenetic epithermal deposit, with field, geochemical, and ore fluid constraints indicating that the fluorite mineralisation is igneous related. The ore fluid composition at Speewah is similar to that of other hydrothermal deposits which are associated with igneous activity. Spatial associations between the fluorite mineralisation and the Yilingbun Granophyre (syenogranite) and the Yungal
Geology of Australian and Papua New Guinean Mineral Deposits
carbonatite, combined with high fluorine concentrations in both the Yilingbun Granophyre (110–540 ppm) and the carbonatite (190–520 ppm) suggest that a genetic association with the fluorite mineralisation is possible for both units. The similarity between chondrite-normalised REE patterns of the fluorite and carbonatite is more suggestive of an association between them, than between the fluorite and the Yilingbun Granophyre. Crosscutting structural relationships indicate that the fluorite mineralisation post-dates all the Early Proterozoic and Cambrian rocks. The close spatial relationship between Doon Doon breccia, itself associated with the carbonatite, and the fluorite mineralisation, provide additional support for a link between the mineralisation and the carbonatite. Lead model ages support a young, possibly Tertiary to Cretaceous, age for the fluorite mineralisation. The widespread breccia dykes support the presence of buried plutons. Recent finds of quartz syenite, diorite and quartzfeldspar porphyry rubble in soil covered areas suggest there may be other alkaline source rocks in the Speewah area. Further geochemical, isotopic and geochronological research is required to unequivocally determine the igneous source of ore fluids and age of the Speewah deposit. Present data favour the carbonatite as the source, particularly as carbonatites are associated with fluorite deposits elsewhere.
A NEW GEOLOGICAL MODEL Recent field work has identified 42 km of epithermal quartz (adularia-fluorite-barite-carbonate) vein and breccia dyke systems within the Speewah Dome, over an area of 30 km by 8 km. This represents a very large hydrothermal system. Vein textures and breccias are indicative of high level structures, and include evidence of volcanic pyroclastic vent-like bodies. Surface sampling has identified anomalous copper, gold, silver, lead and antimony values associated with fluorite. A new geological model has been developed which involves vertical zoning down the vein structures, with various levels prospective for gold and base metal mineralisation. The low gold values in outcrops may be indicative of bonanza style gold mineralisation at depth. Interpretation of airborne magnetic data and aerial photographs has identified circular and curvilinear structures, suggestive of ring structures within the Speewah Dome. Fractures related to caldera collapse, resurgence and subsequent rifting may have served as conduits for mineralising solutions in later hydrothermal episodes, including fluorite veins, gold, silver and base metals in epithermal veins and breccias, and possible porphyry coppergold systems below the breccia dykes over intrusives.
ACKNOWLEDGEMENTS The author gratefully acknowledges the permission of Elmina NL to publish this information. Special thanks are due to M Alvin for his ideas and research work.
REFERENCES Alvin, M P, 1993. The nature, depositional conditions, and source of ore fluids and solutes of the Speewah fluorite deposit, East Kimberley Region, Western Australia, BSc Honours thesis (unpublished), The University of Western Australia, Perth.
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Blockley, J G, 1972. The Speewah Fluorite Deposit, Western Australia Geological Survey, Mineral Resources Report No 27. Laznicka, P, 1988. Breccias and coarse fragmentites: petrology, environments, associations, ores, in Developments in Economic Geology, 25, pp 583–585 (Elsevier: New York). Plumb, K A, 1968. Lissadell, Western Australia - 1:250 000 geological series, Bureau of Mineral Resources Geology and Geophysics, Explanatory Notes SE 52–2.
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Rogers, K A, 1993. Proposed fluorite and aluminium fluoride operations at Speewah, Western Australia, in Mining and Metallurgical Practices in Australasia (Eds: J T Woodcock and J K Hamilton), pp 1361–1364 (The Australasian Institute of Mining and Metallurgy: Melbourne). Simpson, E S, 1951. Minerals of Western Australia, Vol 2, pp 289–290 (Hesperian Press: Perth).
Geology of Australian and Papua New Guinean Mineral Deposits
Hughes, F J, 1998. Perseverance gold deposit, Tarcoola, in Geology of the Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 395–400 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Perseverance gold deposit, Tarcoola by F J Hughes
1
INTRODUCTION The deposit is 3 km west of the township of Tarcoola, at lat 30o43′S, long 134o32′E, or AMG coordinates 454 700 E and 6 602 500 N, on the Tarcoola (SH 53–10) 1:250 000 and Tarcoola (5836) 1:100 000 scale map sheets. The historic Tarcoola Blocks mining area is a few hundred metres to the SE of Perseverance (Fig 1). Before the discovery of Olympic Dam in 1975, the Tarcoola district was South Australia’s only major goldfield. The area is currently being explored by Grenfell Resources NL (Grenfell). Historically, about 57 000 oz of gold was produced from the Tarcoola Blocks mines, from narrow quartz veins in sedimentary rocks, at an average recovered grade of 42.8 g/t. Gold mineralisation at Perseverance occurs within broader shear zones in sediment and underlying granitic rocks, and other styles of mineralisation have now been recognised. Combined Measured, Indicated and Inferred Resources of 1.545 Mt at 1.8 g/t gold for a global total of 89 000 oz of
1.
Consultant Geologist, Grenfell Resources NL, PO Box 1056 West Perth WA 6872.
contained gold have been defined (Table 1) at Perseverance and the nearby subsidiary deposits of Last Resource and Wondergraph (F J Hughes, unpublished data, 1995). Drilling at these deposits in 1996 and 1997 indicated potential for an increase in the resource, although further drilling is required to fully quantify it.
EXPLORATION AND MINING HISTORY Gold was discovered at Tarcoola in 1900. Recorded production in the district was over 75 000 oz at an average recovered grade of 37.5 g/t, mainly between 1901 and 1918. The largest of the many small producers was the Tarcoola Blocks mine which produced about 57 000 oz from 41 600 t of ore at a recovered grade of 42.8 g/t gold. A ‘lode’ at Perseverance was discovered in 1920, and recorded production was about 5400 oz from 5000 t of ore at a recovered grade of 33.6 g/t. The Tarcoola district was not tested by drilling until 1985, when Aberfoyle Exploration Pty Ltd, in joint venture with Afmeco Pty Ltd, completed 1127 m of percussion drilling in 21 holes and intersected minor gold mineralisation. Drilling was principally targeted at the northerly-trending structures on which many of the old workings were situated. It was
FIG 1 - Location map and geological plan, Perseverance area, Tarcoola district (after Daly, 1985).
Geology of Australian and Papua New Guinean Mineral Deposits
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F J HUGHES
recognised that the distribution of gold was not solely confined to quartz veins, but occurred also in metasediment, altered granite and mafic rocks (Aberfoyle, unpublished data, 1982–1985). From 1985 to 1991, BHP Gold Ltd, in joint venture with Aberfoyle and Afmeco, conducted reverse circulation (RC) drilling and diamond drilling programs at the Perseverance and Last Resource areas. They completed around 8750 m in 146 holes, which mainly targeted north-trending structures. This phase of exploration resulted in a Measured and Indicated Resource of 260 000 t at 3.7 g/t. Preliminary metallurgical testwork indicated that the ore was amenable to standard processes for the recovery of gold (BHP, unpublished data, 1986–1989). D B Clarke, trading as Queens Road Mines (QRM), purchased the tenements in 1991, and subsequently sold participating interests to Grenfell and Imdex NL. The Tarcoola Joint Venture (TJV) was then formed, with QRM as operator. Grenfell, as a partner of the TJV, was involved in exploration of the Tarcoola district from 1991 until early 1996. Activities included several phases of RC and rotary air blast (RAB) drilling, and soil geochemical surveys at Perseverance, Last Resource and other nearby areas of old workings. Approximately 28 000 m in 340 RC holes and 17 000 m in 820 RAB holes were drilled, and 8400 soil samples were collected for geochemical analysis.
FIG 2 - Drill hole plan and plan projection of shallow resources, Perseverance and Last Resource ore zones.
This period of exploration defined the sheet-like gold mineralisation near the granite-sediment contact, and led to estimation of resources at the Perseverance, Last Resource and Wondergraph deposits (Table 1). The program identified broad auriferous shear zones, and recognised that much of the gold so far encountered at Perseverance was at or near the contact between the sediment and the underlying granite (F J Hughes, unpublished data, 1995). TABLE 1 Perseverance area, Measured, Indicated and Inferred Resources at 0.5 g/t gold cut-off, 1995. Deposit name
Resource category
Perseverance
Measured including
1300 620
1.8 3.01
75 000 60 000
Last Resource
Indicated and Inferred
220
1.6
11 000
Wondergraph
Inferred
25
3.5
3000
1545
1.8
89 000
Total 1.
Resource (’000 t)
Gold Contained grade (g/t) gold (oz)
Cutoff 1.1 g/t gold.
In 1996 Grenfell purchased the other joint venture partners’ interests, and began a regional exploration program involving airborne and ground magnetic surveys and calcrete sampling. They also completed a six hole diamond drilling program at Perseverance, primarily aimed at investigating the broad scale geological controls of mineralisation. A total of 1800 m was drilled, and additional high grade gold and subsidiary base metal mineralisation was encountered within a broad NEtrending shear zone in granite, beneath and along strike from the established Perseverance resource (Figs 2 and 3).
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FIG 3 - Schematic cross section, Perseverance deposit, looking north.
REGIONAL GEOLOGY The Perseverance area is in the central Gawler Craton, which is inferred to comprise extensive Archaean basement rocks, mainly gneiss and granite, with variously deformed Lower and Middle Proterozoic sediment, volcanic and intrusive rocks (Daly, 1985; Daly, Horn and Fradd, 1990).
Geology of Australian and Papua New Guinean Mineral Deposits
PERSEVERANCE GOLD DEPOSIT, TARCOOLA
The Archaean assemblage is known locally as the Mulgathing Complex and is dated at around 2350 Myr. Proterozoic rocks in the region include the Hutchison Group, whose most prominent component is the Wilgena Hill Jaspilite, and the Tarcoola Formation, dated at 1655 Myr, which unconformably overlies the Hutchison Group. Part of the sediments of the Tarcoola Formation may have been deposited contemporaneously with acid and basic magmatic rocks of the Hiltaba Granite Suite and/or Gawler Range Volcanic ‘event’, at 1516–1592 Myr. The Tarcoola Formation is the main historic host for gold mineralisation in the district, and comprises the relatively thin Peela Conglomerate, overlain by up to 400 m of the Fabian Quartzite Member, and approximately 400 m of the Sullivan Shale Member. The Lower Proterozoic Hiltaba Granite Suite occurs throughout the Gawler Craton, and is inferred, mainly from aeromagnetic data, to consist of discrete equidimensional plutons with diameters of around 15 km. The genetically related Gawler Range Volcanics occur to the north of Perseverance, and are interpreted to be quite extensive (Daly, Horn and Fradd, 1990).
LOCAL GEOLOGY Historic production of gold from the Tarcoola Blocks area was from near-vertical, northerly trending quartz veins crosscutting the Tarcoola Formation, with minor amounts from nearby, altered and relatively undeformed granite (Ridgway and Johns, 1949). Daly, Horn and Fradd (1990) regarded the granite as part of the Hiltaba Granite Suite, intrusive into the Tarcoola Formation, and inferred that the emplacement of the granite caused tension fractures, which provided sites for gold mineralisation. Hein, Both and Bone (1994) on the basis of field relationships, suggested that the granite pre-dated the Tarcoola Formation, and that the contact was a low angle fault. R Thom (unpublished data, 1993) adopted the latter view and developed a structural interpretation involving thrusting subparallel to the granite-sediment contact and the layering in the Tarcoola Formation. Limited age dating information suggests a Hiltaba age for the underlying granite, and field relationships and the general characteristics of gold mineralisation support the view that the granite pre-dated the Tarcoola Formation, and therefore also pre-dated the mineralisation. Diamond drilling at Perseverance by Grenfell in 1996 and 1997 revealed that the contact between the granite and sediments is a zone characterised by a carbonate-chloritesericite schist. This observation supports the interpretation proposed by R Thom (unpublished data, 1993), whereby the granite-sediment contact and the well documented, layer parallel intra-sediment faults at Tarcoola Blocks (Ridgway and Johns, 1949) represent thrusts, probably with movement sense from SE to NW. These thrusts are dominantly parallel to the dip of the layering in the Tarcoola Formation (locally 40-45°). Folding perpendicular to the inferred thrust direction and the development of conjugate faults and tensional openings (the historic quartz reefs) are remarkably consistent with this broad structural interpretation.
Geology of Australian and Papua New Guinean Mineral Deposits
ORE DEPOSIT GEOLOGY LITHOLOGY The Perseverance deposit is hosted by Tarcoola Formation sediment and the underlying granite. The Tarcoola Formation has been divided into three members (Daly, 1984, 1985). The Peela Conglomerate, at the base, is a discontinuous thin layer of conglomerate and breccia containing angular to rounded clasts of quartzite and jaspilite or banded iron formation, and granitic detritus (J A Hallberg, unpublished data, 1996). The Fabian Quartzite Member consists of 12 units of laminated to thick-bedded quartzites interlayered with sandstone, micaceous siltstone and carbonaceous shale. Units 9 and 10 were historically called the ‘Back Slates’ and ‘Front Slates’ respectively (Ridgway and Johns, 1949), and are significant because gold production from the Tarcoola Blocks was largely derived from the sites of intersection of these units with the crosscutting NNE- to NNW-trending quartz veins. At depth in the Tarcoola Blocks and south of Perseverance, the shale and siltstone are carbonaceous and pyritic, but at Perseverance the shale has been intensely altered to kaolin and sericite. The overlying marine Sullivan Shale Member outcrops to the south of Perseverance and the Tarcoola Blocks, and consists of finely laminated carbonaceous shale and siltstone (Daly, 1985). The contact between the underlying relatively unaltered granite and the Tarcoola Formation is roughly parallel to the strike of the sediments. The granite is generally coarse-grained, potassium feldspar and quartz rich, and contains biotite- and/or chlorite-rich phases and finer grained derivatives. Drilling by Grenfell in 1996 and 1997 intersected a thick zone of carbonate-chlorite schist at the contact between the sediment and the underlying granite. Where present, the Peela Conglomerate is always at the base of this unit, at the granite contact (Fig 3). The nearby Last Resource deposit is entirely within a very coarse grained feldspar-rich granite. Several late stage dykes and sills, including basalt, andesite, dolerite, and a fine-grained ‘monzogranite’ intrude the Tarcoola Formation and the granites.
STRUCTURE In the vicinity of Perseverance, the Tarcoola Formation is folded along an east-trending axis, with the Perseverance and Tarcoola Blocks deposits situated on the moderately dipping southern limb of an anticline. There is a large NE-trending basement fault west of Perseverance (Fig 1), and Daly, Horn and Fradd (1990) suggest that sinistral movement along this fault produced the open spaces in which gold was deposited at the Tarcoola Blocks. Bogacz (1988) suggested that later compression produced reverse faults parallel to the strike of the sediments, such as the Main Slide fault and the Pug Seam fault in the Tarcoola Blocks (Ridgway and Johns, 1949). The carbonate-chlorite schist at Perseverance may be a result of reverse thrusting of the sediments over the granite. Apart from this trend and the predominant ENE trends reflected in the Tarcoola Formation at the Tarcoola Blocks, NE-trending faults, fractures and occasional large scale dykes are common, and at Perseverance an interpreted NNE fault set is evident. Northwesterly trends have been inferred from gold grade distributions (F J Hughes, unpublished data, 1995), and from close-spaced ground magnetic data. These trends and the
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NE-trending structures have been inferred to be a conjugate set (Isles, Hughes and Anderson, 1996). Mineralisation at Perseverance and Last Resource is largely concentrated in the broad NE-trending fracture zones, and along NW trends such as the Bonanza zone (Fig 2).
was found near surface in quartz veins. Grenfell’s drilling before 1996 identified other styles of mineralisation (Hughes, unpublished data, 1995). An interpretation was developed which suggested two important controls on gold distribution. These were:
ALTERATION
1.
the NNE trending Perseverance zone, causing a distinct elongation and enhancement of gold mineralisation.
Gold mineralisation at Perseverance is associated with strong kaolin, sericite and goethite alteration of the sediment at or near the granite-sediment contact. Gold is also found in the granite at Perseverance, Last Resource and Wondergraph, in zones characterised by intense sericite alteration, with local quartzepidote-pyrite alteration. These alteration zones are very consistent along strike; in particular, the alteration zone associated with the Perseverance fault can be traced to the NE, from south of Perseverance for a distance of approximately 600 m.
2.
NW trending structures evident locally in the grade distributions, and particularly in a very high grade subzone of Perseverance known as the Bonanza zone (Fig 2). The existence of these structures is supported by high-sensitivity ground magnetics.
MINERALISATION Prior to 1996, mineralisation was believed to be concentrated in the broadly strike-perpendicular quartz reefs within the Tarcoola Formation, and in ‘layer-parallel’ zones at or near the granite-sediment contact. Minor mineralisation had been worked in zones of shearing and alteration within the granite. Previous explorers at Perseverance had followed the NNEtrending line of workings of the Perseverance ‘lode’, and gold
A diamond drilling program was designed to investigate these controls on mineralisation, together with those coincident with, and subparallel to the surface of the granitesediment contact, at depth and along strike from Perseverance. Several holes encountered high grade (10–30 g/t) intervals apparently confined within the broad envelope of the Perseverance zone (Fig 4). Sulphides including pyrite, galena and sphalerite are relatively abundant within the zone, and lesser amounts of arsenopyrite, chalcopyrite and bornite have been noted. Massive sulphides in association with high grade gold mineralisation have been found to the south of the previously defined resource. Holes drilled beneath Perseverance and to the north have encountered more disseminated mineralisation, still within fractured and altered granite.
FIG 4 - Longitudinal projection A-B through main mineralised zones, Perseverance deposit, looking NW.
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PERSEVERANCE GOLD DEPOSIT, TARCOOLA
DISCUSSION
REFERENCES
The following view of gold mineralisation at Perseverance and more generally the Tarcoola District has been formed:
Bogacz, W, 1988. Structural studies of the Tarcoola gold deposit. Report prepared for Tarcoola Gold Ltd, Dep Mines Energy South Aust Open File Envelope 6858 (unpublished).
1.
Structure rather than granite intrusion is likely to be the main factor influencing gold mineralisation.
2.
The Perseverance Fault is a broad zone encompassing often high grade gold mineralisation, and present over a strike length of at least 500 m.
3.
Structural elements observed at Perseverance, Tarcoola Blocks and the district in general may be part of a single, dominantly compressive event.
Further field and laboratory observation together with an ongoing drilling program will continue the evolution and development of the geological model for gold mineralisation in the Tarcoola District.
ACKNOWLEDGEMENTS The author would like to thank Grenfell Resources NL for facilitating the compilation and publication of this information. The considerable contribution from the geological and technical staff of Euro Exploration Services (South Australia) is also acknowledged.
Geology of Australian and Papua New Guinean Mineral Deposits
Daly, S J, 1984. Wilgena 1 well completion report, Department Mines Energy South Australia, Rep 84/13 (unpublished). Daly, S J, 1985. Tarcoola South Australia 1:250 000 - geological series, Geological Survey of South Australia, SH 53–10. Daly, S. J, Horn, C M and Fradd, W P, 1990. Tarcoola goldfield, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1040–1053 (The Australasian Institute of Mining and Mwtallurgy: Melbourne). Hein, K A A, Both, R A and Bone, Y, 1994. The geology and genesis of the Tarcoola gold deposits, South Australia, Mineraluim Deposita, 29:224–236. Hughes, F J, 1995. A Preliminary Resource Estimate of the Perseverance Gold Deposit EL 1827 Tarcoola SA Report prepared for the Tarcoola Joint Venture, July 1995 (unpublished). Isles, D J, Hughes, F J and Anderson, C G, 1996. Gold mineralisation and exploration in the Tarcoola District, Resources ’96 Convention Abstracts (Comp: W P Priess), pp 70–74, Department Mines and Energy South Australia. Ridgway, J E and Johns, R K, 1949. Fabian (Tarcoola Blocks) gold mine, Mining Review South Aust, 88: 170–194.
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400
Geology of Australian and Papua New Guinean Mineral Deposits
Morris, B J, Davies, M B and Newton, A W, 1998. Iron ore deposits of the northern Gawler Craton, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 401–406 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Iron ore deposits of the northern Gawler Craton 1
2
by B J Morris , M B Davies and A W Newton
3
INTRODUCTION The search for iron ore deposits by Mines and Energy, South Australia (MESA) in the northern Gawler Craton is part of the South Australian Steel and Energy Project (SASE). It is a joint project with Meekatharra Minerals Ltd and Ausmelt Ltd, to produce pig iron near Coober Pedy using coal from the Phillipson Coalfield and the Ausmelt direct smelting technology. Four iron deposits, all close to the Stuart Highway and the Tarcoola–Alice Springs Railway (Fig 1), have been identified: 1.
Hawks Nest, 115 km SSE of Coober Pedy at lat 30o00′S, long 135 o08′E on the Billa Kalina (SH 53–7) 1:250 000 scale map sheet.
2.
Giffen Well, 142 km south of Coober Pedy at lat 30o18′S, long 134o35′E on the Tarcoola (SH 53–10) 1:250 000 scale map sheet.
3.
Peculiar Knob, 87 km SE of Coober Pedy at lat 29o35′S long 135o23′E on the Billa Kalina (SH 53–7) 1:250 000 scale map sheet.
4.
Sequoia, 106 km SSW of Coober Pedy at lat 29 o53′S, long 134o18′E on the Coober Pedy (SH 53–6) 1:250 000 scale map sheet.
The Hawks Nest and Giffen Well deposits are held by MESA and the Peculiar Knob and Sequoia deposits are on Exploration Licences held by Normandy Exploration Ltd–Metals Exploration Ltd and Minotaur Gold NL respectively.
EXPLORATION HISTORY Large iron ore deposits were first recognised on the Gawler Craton in the Middleback Range near Whyalla in the late 1890s. The Broken Hill Proprietary Co Ltd (BHP) pegged the first claim in 1897 and commenced mining at Iron Knob in 1899. Further deposits were discovered in 1920 and the host ironstone has now been traced for 60 km (Drexel, 1982). Since 1900 about 200 Mt of high grade hematite ore have been mined, primarily for the BHP steelworks at Whyalla. From aeromagnetic surveys in the late 1950s iron formations were recognised in the Tarcoola (Wilgena Hill) and Mulgathing (Mount Christie) areas in the north and NW of the Gawler Craton. Whitten (1965) investigated several of the 1.
Senior Geologist, Mines and Energy South Australia, PO Box 151, Eastwood SA 5063.
2.
Consultant Geologist, 7 Fern Road, Crafers SA 5152.
3.
Manager, Mineral Resources, Mines and Energy South Australia, PO Box 151, Eastwood SA 5063.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Regional geological map and location of iron ore prospects, northern Gawler Craton.
larger deposits, and although no systematic evaluation was undertaken a 20 Mt resource at 40% iron was inferred for Mount Christie, and 65 Mt at 40% iron, above plain, was inferred at Wilgena Hill. Over the past ten years exploration in the northern Gawler Craton has not specifically targeted iron ore but has focussed on magnetic and gravity anomalies as indications of Olympic Dam style base metal–gold and banded iron formation (BIF) hosted gold mineralisation. However numerous magnetite and hematite occurrences have been found as steeply dipping layers or in breccias beneath 8 to 100 m of mostly soft sediment cover. Iron ore resources were indicated by CRA Exploration Pty Limited (CRAE) at Hawks Nest (Sugden, 1990) and Peculiar Knob (Finch, 1986). In 1993 detailed aeromagnetic surveys were flown over the northern Gawler Craton, at 400 m line spacing and 80 m mean terrain clearance, as part of the MESA South Australian Exploration Initiative, resulting in high quality aeromagnetic images. MESA commenced exploration for iron ore in January 1995, targeting aeromagnetic anomalies and known iron ore
401
B J MORRIS, M B DAVIES and A W NEWTON
occurrences. To August 1996 regional and prospect exploration comprised 812 line km of ground magnetic surveys, 6309 gravity stations and 36 377 m of reverse circulation (RC) and diamond drilling.
REGIONAL GEOLOGY The Gawler Craton (Fig 1) is a stable region of crystalline basement of Archaean (2700 Myr) to Mesoproterozoic (1450 Myr) age (Drexel, Preiss and Parker, 1993). Regional aeromagnetic and geological surveys (Ambrose and Flint, 1981; Benbow, 1982; Daly, 1985; Cowley and Martin, 1991) have shown that the Craton is host to two deformed metasedimentary sequences that contain BIF. These rocks are sparsely exposed, being generally overlain by thin soil and flatlying Cretaceous sediment.
FIG 2 - Ground magnetic and drill hole locality plan, Hawks Nest deposit.
The older sequence occurs in the southern, central and western parts of the Craton. These Archaean rocks contain BIF generally less than 50 m thick with discontinuous strike lengths of up to 500 m. The Sequoia and Mount Christie occurrences are of this age. Following the Late Archaean Sleafordian Orogeny (2640–2300 Myr), which reached granulite facies metamorphism, Palaeoproterozoic sediment, including BIF, was deposited in extensional elongate shallow water basins. The sediments were deformed and metamorphosed to amphibolite facies, and extensively invaded by granites during the Kimban Orogeny (1850–1700 Myr). The Palaeoproterozoic (1900 Myr) BIF units are most extensive in the eastern and northern parts of the Craton. These are up to 700 m thick and persist along strike for up to 25 km, and are present at Peculiar Knob, Hawks Nest, Giffen Well and Wilgena Hill and in the Middleback Range.
LOCAL GEOLOGY FIG 3 - Cross section on 511 600 E, looking east, Hawks Nest deposit.
HAWKS NEST DEPOSITS Lithology Exposure of the Palaeoproterozoic BIF is sparse and limited to a few outcrops in a gently undulating area covered by a few metres of Quaternary red-brown sandy ferruginous clay and up to 20 m of flat-lying Cretaceous white, porcelaneous claystone and shale. The clay has a distinctive surface lag of ‘buckshot’ hematite gravel and eroded BIF fragments.. The subsurface extent of BIF is estimated from aeromagnetic, ground magnetic and gravity responses and from drilling results (Fig 2). Several BIF units are evident but it is uncertain whether these are separate units or structural repetitions. The aeromagnetic anomaly extends along strike to the NE for 20 km but the depth of cover increases towards the NE to over 100 m. Figure 3 is a typical cross section showing the BIF horizons interlayered with a steeply dipping sequence of metasediment that comprises metasiltstone, conglomerate, calc-silicate, basalt and carbonate layers. The sequence generally strikes northeasterly and is near vertical. It is folded and disrupted by northwesterly-trending brittle faults and basic dykes of the Gairdner Dyke Swarm. The BIF is generally brecciated, has well developed microfaulting and microfolding, and has been partly to completely oxidised to hematite in the vicinity of faults. The unoxidised magnetite-BIF zones stand out in Fig 2
402
as discrete magnetic anomalies. Surface oxidation extends to a depth of 30 to 35 m and changes magnetite to hematite, goethite and limonite.
Mineralisation Three types of iron ore are indicated from the 110 RC hammer holes totalling 11 912 m and four diamond drill holes totalling 350 m: 1.
Low grade magnetite-BIF bodies from 150 to 500 m wide occur as unoxidised sections of the BIF horizons (Fig 2). They are finely laminated (0.2–5 mm) and composed of quartz and microgranular magnetite (0.05 mm in diameter) which dominate in alternate bands. Some bands contain appreciable amphibole, predominantly cummingtonite and minor actinolite. The amphiboles are randomly oriented and also occur as narrow crosscutting veinlets. Calcite is present as late stage veinlets. The BIF typically comprises 25–50% magnetite, 30–55% quartz, 10–25% amphibole and 2–5% carbonate. Modelling of ground magnetic and gravity data indicates the bodies have an estimated depth extent of 500 to 1000 m. Inferred Resources to 100 m below the level of oxidation are shown in Table 1. The magnetite-BIF
Geology of Australian and Papua New Guinean Mineral Deposits
IRON ORE DEPOSITS OF THE NORTHERN GAWLER CRATON
TABLE 1 Inferred Resources, Northern Gawler Craton iron ore deposits. Body
Ore Type
Strike Length (m)
Width (m)
Depth (m)
Resource (Mt)
Hawks Nest Kestrel
magnetite-BIF
2000
300–600
130
260
Goshawk
magnetite-BIF
1500
250–350
130
148
Harrier
magnetite-BIF
650
250
130
54
Eagle
magnetite-BIF
1400
200
130
92
Kite
magnetite-BIF
600
150
130
30
Falcon
magnetite-BIF
500
150
130
25
hemitite
450
10–50
120
5
magnetite-BIF
3000
250
150
240
hemitite
1000
15–36
120
14
magnetite-BIF
1000
2–45
145
20
Buzzard Giffen Well Peculiar Knob Sequoia
bodies represent a low grade (35–40% iron) Inferred Resource of about 600 Mt that may be readily beneficiated to a high grade product. The average composition of the largest magnetite-BIF body, Kestrel, is shown in Table 2. 2.
The high grade magnetite Kite body (Fig 2) has drill intersections with 34% to 69.3% iron, and Table 2 shows the average composition. The body has a ground magnetic and gravity expression and contains a significant Inferred Resource (Table 1) that may require only limited beneficiation. A high grade, 24 m thick magnetite body of unknown lateral extent was intersected in the Kestrel body. This layer averages 67.2% iron, 3.22% silica, 0.40% alumina and 0.11% phosphorous pentoxide and represents direct feed magnetite ore.
3.
The high grade hematite Buzzard body (Fig 4) is a tectonically brecciated BIF adjacent to a northeasterlytrending fault zone that has been oxidised to hematite and leached of silica. The hematite comprises homogeneous, massive microplaty hematite as subangular to subrounded breccia clasts (1–10 mm in diameter) randomly disposed through a matrix of microcrystalline hematite. Remnant accessory magnetite is present as inclusions in hematite. Drill intersections range from 48.6% to 67.2% iron with an average composition as shown in Table 2. This body represents direct feed hematite ore, with an Inferred Resource of 5 Mt.
GIFFEN WELL DEPOSIT Lithology Surface exposure of Palaeoproterozoic BIF is limited to a few outcrops along a gentle topographic rise. Quaternary redbrown sandy ferruginous clay with a distinctive surface lag of buckshot hematite gravel and eroded BIF fragments covers the area to a few metres depth. The associated aeromagnetic anomaly strikes NNE and is 6 km long. However ground magnetic, gravity and drilling results (Fig 5) indicate that near surface BIF, with a depth extent of 500–1000 m and 150–240 m wide is confined to a strike length of about 3 km.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 4 - Drill hole locations and plan view of mineralisation, Buzzard deposit, Hawks Nest.
A cross section (Fig 6) shows that the BIF is part of a conformable package of NNE-striking and steeply east dipping metasediment intruded by Mesoproterozoic Hiltaba Suite granite. A lamprophyre dyke of minette to vogesite type has been identified in diamond drill core. The BIF is laminated (bands 1–8 mm thick), moderately to highly fractured, and microfolded. The stratigraphic facing is unknown. The BIF is equivalent to the Hawks Nest BIF and is of mid to upper amphibolite metamorphic grade. Surface oxidation extends to 30–50 m depth and has altered primary magnetite-BIF to hematite, goethite and limonite BIF.
Mineralisation The ore zone of low grade magnetite-BIF was investigated with 25 angled RC hammer drill holes totalling 1991 m and a 114 m
403
B J MORRIS, M B DAVIES and A W NEWTON
FIG 5 - Ground magnetic and drill hole locality plan, Giffen Well deposit.
deep angled diamond drill hole. It is a homogeneous regular micro- to meso-layered sequence of fine granular magnetite (0.01–0.05 mm in diameter), and chert that often incorporates fine amphibole. The thicker and more magnetite-rich layers consist of aggregated magnetite grains about 0.08 mm in diameter. Some of the layering is repeated as rhythmic bands. Two species of amphibole (actinolite and cummingtonite) are widespread through the chert layers and fill microfractures. Apatite occurs as an accessory and there are late stage crosscutting calcite veinlets. The BIF typically comprises 20–50% magnetite, 30–55% cherty quartz, 10–35% amphibole, 0–3% apatite and 0–3% carbonate, and the average composition is shown in Table 2. An Inferred Resource of 240 Mt (Table 1), to 100 m below the level of oxidation, has been estimated, and may be readily beneficiated to a high grade product.
FIG 6 - Cross section on A-A′, looking north, Giffen Well deposit.
PECULIAR KNOB DEPOSIT Lithology This area is completely covered by Quaternary red-brown clay, sand and silt, over the Cretaceous Bulldog Shale, a pale brown shale with minor gypsum. The flat-lying shale overlies basement to a depth of 12 m at the western end of the deposit, and thickens to 32 m at the eastern end. Basement comprises a northeasterly-trending Palaeoproterozoic metasedimentary sequence of BIF, quartzite and quartz-microcline-sillimanite gneiss with a metamorphic grade of upper amphibolite facies. Mesoproterozoic porphyritic granodiorite of the Balta Granite intrudes the metasediment and has intense chloritic alteration.
TABLE 2 Whole rock analyses, Northern Gawler Craton iron ore deposits. Deposit
Fe%
SiO2% AI2O3% P2O5% CaO%
K2O%
Na2O%
MgO%
MnO%
TiO2% SO3%
LOI%
Hawks Nest Kestrel
36.4
38.2
0.97
0.13
3.63
0.12
0.09
3.26
0.23
0.05
0.20
1.05
Kite
50.5
24.2
0.82
0.25
0.79
0.12
0.06
1.08
0.10
0.03
—
—
Buzzard
60.2
11.0
1.41
0.08
0.15
0.21
<0.05
0.17
0.01
0.04
0.03
1.01
Giffen Well
36.5
42.7
0.11
0.18
1.38
0.04
0.14
2.80
0.06
<0.01
0.09
0.23
DD1(56.6–104.7m)
61.0
11.2
0.12
0.03
0.13
0.18
0.01
0.09
0.03
0.14
—
0.21
DD2(41.9–54.6m)
67.2
2.4
0.18
0.04
0.13
0.23
0.01
0.19
0.04
0.10
—
0.94
DD3(49.5–75.7m)
65.8
4.7
0.09
0.04
0.12
0.17
<0.01
0.13
0.02
0.11
—
0.07
SE1(112–164m)
39.0
37.76
1.56
0.09
2.78
0.57
0.20
2.31
0.19
0.08
0.15
0.01
SE2(82–126m)
31.4
41.9
4.37
0.11
3.71
1.05
0.40
2.35
0.17
0.21
0.39
0.61
SE3(52–68m)
33.1
43.32
3.47
0.13
2.45
0.90
0.62
2.07
0.10
0.20
0.21
0.50
SE4(84–100m)
27.6
47.23
5.61
0.17
2.55
1.76
0.80
2.78
0.08
0.26
0.19
0.08
SE6(38–88m)
25.0
47.43
6.10
0.13
4.54
1.56
0.85
2.87
0.21
0.29
0.26
0.53
SE8(78–110m)
25.9
48.75
5.48
0.18
2.46
0.83
0.83
2.83
0.10
0.31
0.25
1.29
Peculiar Knob
Sequoia
404
Geology of Australian and Papua New Guinean Mineral Deposits
IRON ORE DEPOSITS OF THE NORTHERN GAWLER CRATON
Mineralisation The ore zone of massive specular hematite (Figs 7 and 8) was investigated by MESA with three angled diamond drill holes totalling 397 m and 14 RC hammer holes totalling 920 m. Five diamond drill holes, including two drilled by CRAE (Finch, 1986), and seven RC hammer holes have intersected the ore zone which was formed by the hydrothermal alteration of a BIF. Leaching of silica and concentration of iron produced a coarse mosaic of specular hematite (0.3 to 4 mm in diameter) with minor remnant banding and some residual magnetite. Metamorphic hematite has largely replaced the magnetite with the remainder subsequently replaced by martite. Minor interstitial subrounded quartz grains (0.1 to 0.4 mm diameter) are present.
FIG 8 - Log of drill hole PKDDH 3 on line 10 200 N, Peculiar Knob deposit.
meta-igneous sequence from a differentiated basic intrusion. The bedrock is generally oxidised to about 45 m depth.
Mineralisation
FIG 7 - Drill hole locations and projected geological plan, Peculiar Knob deposit.
Mineralisation is high grade, averaging 63.2% iron, with low alumina and phosphorous contents. Typical analyses are shown in Table 2. To a depth of 100 m below the cover there is an Inferred Resource of 14 Mt (Table 1) that may be beneficiated by simple two stage crushing and screening.
SEQUOIA DEPOSIT Lithology The prospect lies beneath a gentle topographic high about 1000 m long, 200 m wide and rising 15 m above the surrounding plain. There is about 10 m of Quaternary cover comprising redbrown sandy clay with quartz and ironstone pebbles, with a friable calcrete cement at surface and also immediately above bedrock. A few small exposures of BIF are recorded on the Coober Pedy 1:250 000 scale geological map (Benbow, 1982). The bedrock comprises Archaean BIF, banded quartzmagnetite-diopside-hypersthene-amphibole gneiss, with interlayers of quartz-microcline-plagioclase gneiss and plagioclase pegmatoid. The sequence may be a high grade
Geology of Australian and Papua New Guinean Mineral Deposits
The magnetite-BIF zone has a well defined magnetic and gravity expression and was tested with eight angled RC hammer holes totalling 1167 m and one angled diamond drill hole totalling 112 m (Fig 9). The BIF layers vary from 2 to 50 m wide, are laterally discontinuous, strike north, dip at 70–80o west, and are hosted by banded gneiss. BIF comprises 1 mm to 1 cm wide bands of quartz-actinolite-magnetite, actinolitemagnetite, and biotite-quartz-magnetite. Quartz is often lensoid and accessory minerals include apatite, garnet, and pyrite. Magnetite grains are 0.1 to 0.5 mm in diameter with some disseminated as rounded porphyroblasts to 1 mm diameter. There are typically two magnetite-rich zones, to 45 m wide, that vary laterally in width and grade. Grades vary from 21% to 39% iron (average 28% iron) and selected whole rock analyses are shown on Table 2. The deposit contains an Inferred Resource of 20 Mt (Table 1) to 100 m below the level of oxidation and may be readily beneficiated to a high grade product.
BENEFICIATION Preliminary beneficiation testing has been carried out by Amdel Ltd on diamond drill core of low grade magnetite-BIF (35–40% iron) from the Hawks Nest, Giffen Well and Sequoia deposits, to produce a high grade product (+60% iron). Results indicate that two stage grinding and magnetic separation produces a high grade magnetite concentrate with good iron recovery rates. The main impurity is silica, of which 40–50%
405
B J MORRIS, M B DAVIES and A W NEWTON
Minotaur Gold NL is acknowledged in publishing information on the Peculiar Knob and Sequoia prospects respectively. Thanks are extended to Analabs Pty Ltd and Amdel Laboratories Ltd for geochemical, mineralogical and beneficiation testing and to Pontifex and Associates Pty Ltd and Mason Geoscience Pty Ltd for petrological studies. Thanks are extended to J Hough, G W Ferris, R Shaw and P Polito for geological support, to J T Woodcock (CSIRO) for technical advice and to P P Crettenden, M W Flintoft and D M Russell for their technical support.
REFERENCES Ambrose, G J and Flint, R B, 1981. Billa Kalina, South Australia 1:250 000 geological series, Geological Survey South Australia Explanatory Notes SH 53–7. Benbow, M C, 1982. Coober Pedy, South Australia - 1:250 000 geological series, Geological Survey South Australia Explanatory Notes SH 53–6. Cowley, W M and Martin, A R, 1991. Kingoonya, South Australia 1:250 000 geological series, Geological Survey South Australia Explanatory Notes SH 53–11. Daly, S J, 1985. Tarcoola map sheet SH 53–10, Geological Atlas 1:250 000 geological series (Geological Survey South Australia: Adelaide). Drexel, J F, 1982. Mining In South Australia - A Pictorial History, Mines and Energy South Australia Special Publication No 3. Drexel, J F, Preiss, W V and Parker, A J, 1993. The Geology of South Australia, Volume 1, The Precambrian, South Australian Geological Survey Bulletin 54. FIG 9 - Stacked ground magnetic and Bouguer gravity profiles, and summary drill hole sections, Sequoia deposit.
Finch, I D, 1986. 10th quarterly report for Engenina EL1145 South Australia, by CRA Exploration Pty Ltd, Mines and Energy South Australia Open File Envelope 4248 (unpublished).
occurs as liberated grains and a simple reverse flotation process may improve the product grade further.
Sugden, S P, 1990. Twentieth and final quarterly report for Hawks Nest EL1277, South Australia for period ending 25 Feb, 1990, by CRA Exploration Pty Ltd, Mines and Energy South Australia Open File Envelope 5431 (unpublished).
ACKNOWLEDGEMENTS The authors gratefully acknowledge the permission of Mines and Energy South Australia, Meekatharra Minerals Ltd and Ausmelt Ltd to publish this paper. The permission of Normandy Exploration Ltd–Metals Exploration Ltd, and
406
Whitten, G F, 1965. Iron ore deposits in South Australia outside the Middleback Ranges, in Geology of Australian Ore Deposits (Ed: J McAndrew), pp 309–311 (Eighth Commonwealth Mining and Metallurgy Congress and The Australasian Institute of Mining and Metallurgy:Melbourne).
Geology of Australian and Papua New Guinean Mineral Deposits
Miller, G C, Kirk, C M, Hamilton, G and Horsburgh, J R, 1998, Brocks Creek gold deposits, Pine Creek, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 409–416 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Brocks Creek gold deposits, Pine Creek 1
2
3
by G C Miller , C M Kirk , G Hamilton and J R Horsburgh INTRODUCTION The deposits were discovered by Cyprus Gold Australia Corporation (Cyprus) and Solomon Pacific Resources NL (Solomon) in 1991–1993 but now are owned by Acacia Resources Limited. They are 125 km SSE of Darwin, NT, at about AMG coordinates 761 250 m E, 8 510 000 m N, and lat 13o28′S, long 131 o25′W on the Pine Creek (SD 52–8) 1:250 000 and the Batchelor (5171) 1:100 000 scale map sheets (Fig 1). The nearest settlement is Adelaide River, 50 km to the NW. The Brocks Creek mineralisation is quartz vein type, hosted by metasediment of the Palaeoproterozoic South Alligator Group in the central Pine Creek Geosyncline. Solomon commenced production from the Faded Lily pit in April 1996 based on a Proved and Probable Ore Reserve of 6.1 Mt at 1.73 g/t gold (after cutting grades of +20 g/t to 20 g/t) at the Faded Lily and Alligator deposits. These reserves were contained within a total Measured, Indicated and Inferred Resource of 17.8 Mt at 1.74 g/t gold (grades cut) which included satellite
4
gold deposits at Zapopan, Rising Tide and Burgan (Table 1). Acacia’s production to the end of 1996 was 745 460 t at an average recovered grade of 1.46 g/t gold. TABLE 1 Brocks Creek Measured, Indicated and Inferred Resources, sectional estimates at March 1996. Pit or prospect (date of estimate)
Cutoff (g/t gold)
Mt
Grade Contained (g/t gold) gold (oz)
Faded Lily (March ’96)
0.5
8.08
1.80 467 000
Alligator (June ’95)
0.5
6.32
1.57 319 000
Rising Tide (March ’96)
0.5
2.50
1.36 109 000
Zapopan (March ’96)
1.0
0.48
4.80
74 000
Burgan (June ’95)
0.5
0.30
2.38
23 000
Sub total Dumps (June ’95) Total
17.68 0.14 17.82
1.75 992 000 1.30
6000
1.74 998 000
In the recent past, open pit gold mines were operated by Dominion Mining Limited at Woolwonga, 15 km NE of Brocks Creek, and at Cosmo Howley, 10 km to the SW (Fig 1).
EXPLORATION AND MINING HISTORY In 1871 a gold bearing quartz reef was discovered near the western end of a low WNW-trending ridge system near the Overland Telegraph line, by a party that included Messrs Burgan and Herbert (B Pedersen-McLaren, unpublished data, 1996). This was to become the John Bull reef (Fig 2). The area was later named Brocks Creek after William Brock, a government officer associated with the Overland Telegraph.
FIG 1 - Location map, Brocks Creek deposits.
1.
Regional Manager (SE Asia), Cyprus Gold Australia Corporation, Level 4, 5 Mill Street, Perth WA 6000.
2.
Geological Consultant, 27 Wharf Road, Surfers Paradise Qld 4217.
3.
Formerly Managing Director, Solomon Pacific Resources NL, now Director, Cullen Resources NL, PO Box 42, Lindfield NSW 2070.
4.
Formerly Executive Chairman, Solomon Pacific Resources NL, now Director, Cullen Resources NL, PO Box 42, Lindfield NSW 2070.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 2 - Regional geological map of Brocks Creek district (after Cyprus mapping, 1992).
409
G C MILLER et al
Discovery of the Faded Lily line of reef, 4 km ESE of John Bull along the crest of the ridge system, soon followed. This was the prelude to a long period of eluvial working by Chinese miners and lode mining by English companies and Chinese tributers until about 1914. From incomplete historical records, gold production is estimated at 1028 kg from John Bull, Crocodile, Alligator, Faded Lily and Zapopan (Balfour, 1981; Ahmad et al, 1993). The most productive mine was Zapopan, 0.5 km ESE of Faded Lily, where stoping of high grade reefs produced approximately 823 kg of gold from 41 000 t of ore.
Alligator and Zapopan stood at 9.3 Mt at 2.68 g/t gold (2.37 g/t after top cutting) or 802 000 contained oz (708 000 oz after cutting), at which time a 39.9 km2 mineral lease was applied for.
The Brocks Creek area was subsequently explored for gold and base metals during the 1970s and 1980s by Geopeko Ltd, CRA Exploration Pty Limited, CSR Ltd, Goldfields Exploration Pty Ltd, Newmont Limited, Pacific Goldmines NL and Zapopan NL. Most of these companies carried out some drill testing with mixed results, notably Newmont in 1984 and Pacific Goldmines in 1986. Both of these drilling programs were at Faded Lily, under the terms of a joint venture with Top End Mineral Ventures Pty Ltd, the owner of nine small tenements totalling 2.2 km2 covering the John Bull–Crocodile–Alligator–Faded Lily line of workings. At this time Zapopan held almost all of the surrounding ground (30 km2) and subsequently (1988–1990) outlined an estimated resource with 66 000 oz of contained gold (J Goulevitch and D J Holden, unpublished data, 1988).
A positive feasibility study of mining the Faded Lily and Alligator deposits was completed in mid 1995 at which time a total of 30 400 m had been drilled on 25 m spaced traverses. Mining by open pit was scheduled at 1 Mtpa for six years, commencing at Faded Lily, with processing by gravity concentration and carbon in leach. Construction of the plant, access road and tailings dam was completed in March 1996. Gold production commenced in April 1996, and by the end of September 1997 a total of 2287 kg of gold had been produced from 1 533 000 t of ore at a recovered grade of 1.51 g/t gold.
In joint venture with Pacific Goldmines on the Top End ground, Cyprus carried out some pre-drilling exploration (grid rehabilitation, an aeromagnetic survey and a gradient array IP survey) in 1987 before buying out Pacific’s interest in early 1988. A dispute with Top End, whose interest had by then been reduced to a royalty, caused a cessation of work until 1990, when the Supreme Court ruled in Cyprus’ favour. In a 75–25 joint venture with Top End, Cyprus carried out further pre-drilling work in 1990, followed in 1991 by a 3200 m reverse circulation (RC) percussion drilling program, testing costean intercepts and IP chargeability anomalies, mainly at Faded Lily. The program resulted in a number of encouraging intercepts, including 14 m at 14.28 g/t gold (or 7.67 g/t after a 20 g/t top cut) at Faded Lily, and suggested the presence of at least a small gold resource. In early 1992 Solomon, with whom Cyprus was already in joint venture at the eastern end of the Brocks Creek–Zapopan mineralised trend, effectively replaced Top End. Drilling was then recommenced on Faded Lily, concurrent with costeaning the laterite- and scree-covered flanks of the WNW-trending Alligator ridge. The Alligator deposit was discovered beneath auriferous scree and laterite cover later that year by drill testing a bedrock costean intercept on the southern flank of the Alligator ridge, 100 m south of the crest. Late in 1992 the Zapopan NL tenement holdings in the district were acquired by the Cyprus–Solomon joint venture. The Cyprus–Solomon joint venture evaluated the Faded Lily and Alligator deposits in more detail during 1993 and early 1994. Exploration of other gold targets was also conducted using soil and bed rock geochemical sampling, IP surveys, costeaning, geological mapping and RC percussion drilling. This led to the discovery in 1993 of gold mineralisation at Burgan, between Alligator and Faded Lily and at Rising Tide, 2.5 km to the north, with costeaning and soil sampling respectively defining drill targets. By late 1993 the total Measured, Indicated and Inferred Resource for Faded Lily,
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In mid 1994 Cyprus sold its 75% interest in Brocks Creek to Solomon, who moved to 100% ownership and took over management. Closer spaced drilling commenced immediately and by the end of the year the resource base had increased to 17.8 Mt at 1.74 g/t gold (1 Moz contained gold) which also included the Rising Tide and Burgan deposits (Table 1).
In March 1996, Acacia Resources Limited announced a takeover for all the of the issued shares of Solomon Pacific Resources NL listed on the Australian Stock Exchange. The takeover was ultimately successful in June 1996 thereby transferring ownership of Brocks Creek by way of Solomon to Acacia.
PREVIOUS DESCRIPTIONS There are no major published descriptions of the Brocks Creek gold deposits. An earlier report of geology and mineralisation of the Brocks Creek district was written by Sullivan and Iten (1952). Historical information on Brocks Creek gold mining is mainly in records held by the Northern Territory Department of Mines and unpublished research by B Pedersen-McLaren (1996).
REGIONAL GEOLOGY The deposits are in the Pine Creek Geosyncline, one of the major mineral provinces of the N T, which extends over 66 000 km2 from Katherine in the south to Darwin in the north. The main components are a series of late Archaean basement domes overlain by a Palaeoproterozoic sedimentary and volcanic sequence deposited in a shallow intracontinental rift. Regional metamorphic grade is greenschist. The South Alligator Group near the base of the sequence, the generally accepted subdivision of which is shown in Table 2, dominates the
TABLE 2 South Alligator Group stratigraphy, from Nicholson, Ormsby and Farrar (1994). Unit Mount Bonnie Formation Gerowie Tuff Koolpin Formation
Rock type
Thickness (m)
Greywacke, carbonaceous argillite, argillite, chert, cherty tuff, BIF.
150–400
Argillite, chert, cherty tuff.
200–400
Carbonaceous argillite, chert, BIF, silicified dolomite.
300–1000
Geology of Australian and Papua New Guinean Mineral Deposits
BROCKS CREEK GOLD DEPOSITS, PINE CREEK
stratigraphy of the Brocks Creek area and includes highly carbonaceous argillite, evaporite, carbonate and banded iron formation (BIF), particularly in the Koolpin Formation at the base of the Group.
Koolpin Formation, low over the Gerowie Tuff or lower Mount Bonnie Formation and variable (mainly due to magnetite in greywacke) over the upper Mount Bonnie Formation and overlying Finniss River Group sediments.
In the Brocks Creek region metasediments of the South Alligator Group and overlying Finniss River Group (Burrell Creek Formation) are folded along ESE-trending axes. Deformation during the Top End Orogeny at 1870 to 1800 Myr resulted in tight folds, one of which is the Brocks Creek–Zapopan anticline, the axis of which can be traced over about 12 km, and lower greenschist facies metamorphism. Recent AGSO work (L Wyborn, A Budd and I Bastrakova, unpublished data, 1996) indicates two main episodes of granitic intrusion, around 1850 and 1820 Myr. The younger episode included emplacement of the Burnside Granite, an I-type, essentially reduced (non-magnetic), granodioritic intrusion approximately 90 km2 in area which crops out 3 km north of Brocks Creek (Fig 2). Contacts with the surrounding intruded metasediment are mostly concordant. Prominent regional faults in the area include the NNW-trending Pine Creek Shear to the east of the Burnside Granite and the north-trending Mount Shoobridge Fault to the west. Faulting in the immediate Brocks Creek area appears to be mainly parallel to the ESE fold axes.
Aeromagnetic data suggest that the Brocks Creek-Zapopan anticline is a subsidiary structure on the southern flank of a more open anticline, the axes possibly merging in the Zapopan area as illustrated on Fig 2.
The central Pine Creek Geosyncline is a major gold province, reviewed by Nicholson and Eupene (1990) and Ormsby, Nicholson and Butler (1994). Of the four styles of gold mineralisation generally recognised, two are considered to encompass the major deposit types, the most important being the quartz vein set or stockwork style at Mount Todd, Enterprise, Union Reefs and Woolwonga. Second in importance is the stratiform style represented by Cosmo Howley in part, Golden Dyke, Mount Bonnie and Beef Bucket (Rustlers Roost). Many of the deposits are on low ridges along anticlinal axes. At Brocks Creek both major styles are represented. The Alligator, Faded Lily and some of the Zapopan gold lodes are largely concordant quartz vein sets or stockworks localised along the WNW-trending Brocks Creek–Zapopan anticline (Fig 2), and hosted in argillite-greywacke units in a mappable greywacke-argillite-tuff-chert package which probably occupies an upper Gerowie Tuff or lower Mount Bonnie Formation stratigraphic position. There are also probable stratiform gold lodes, comprising thin pyrite-garnet-chloritechert (‘silicate-sulphide facies BIF’) units at Zapopan. The Rising Tide lodes appear stratabound in Koolpin Formation carbonaceous sediment, but are probably structurally controlled. With the notable exception of Rising Tide, most of the known gold orebodies and occurrences are either along the Brocks Creek–Zapopan anticlinal axis or within 150 m of it on its south side. Distinctive features of the Brocks Creek area on regional magnetic images include: 1.
the low magnetic signature of the Burnside Granite, contrasted with a high magnetic intensity thermal metamorphic aureole; and
2.
contrasting magnetic signatures outlining broad scale folding of the South Alligator Group metasediments. These are generally high (probably representing indigenous pyrrhotite in argillite, pyrrhotite and magnetite in BIF and Zamu Dolerite intrusions) over the
Geology of Australian and Papua New Guinean Mineral Deposits
ORE DEPOSIT FEATURES LOCAL STRATIGRAPHY The mineralised sequence in the Brocks Creek–Zapopan anticline has been informally subdivided into six units whose thickness ranges from 10 to 120 m as shown in Table 3. The lower and upper units (1 and 6) are largely undifferentiated. The sequence consists of argillite, often highly carbonaceous particularly near its base, with variable proportions of interbedded greywacke, chert and tuff. There are thin BIF beds near the top of the sequence. Rock types were described from thin sections by I R Pontifex (unpublished data, 1993, 1994). TABLE 3 Stratigraphic sequence Brocks Creek–Zapopan anticline. Unit
Thickness (m)
Rock type
Contained gold deposits
6 (younger)
>120
Argillite-chert- Zapopan (top of BIF unit)
5
2–10
Argillite
4
9–13
Argillitegreywacke
3
13–22
Argillite-cherttuff
2
11–32
Argillitegreywacke
Alligator, Burgan, W Faded Lily (Footwall)
1 (older)
>120
Argillite-chert
John Bull, Crocodile, Alligator (historical)
Alligator, Burgan, W Faded Lily (Hanging wall), E Faded Lily
The Faded Lily, Alligator and Burgan mineralisation is mainly hosted by argillite-greywacke units 2 and 4 (Fig 3). Argillite is typically micaceous and variably carbonaceous (graphitic where sheared), sericitic, dolomitic, silty and tourmalinitic. The greywackes are poorly sorted quartzofeldspathic lithic sediments with recrystallised quartz±feldspar+mica matrices. In contrast with greywacke higher in the South Alligator Group they are typically non magnetitic. The cherts are composed of micromosaic quartz, plagioclase, sericite and biotite, probably with an original tuff component. Mafic dykes are found in all units, but preferentially along the anticline axial zone, are typically composed of fine grained biotite or sericite, amphibole and quartz, and may be of lamprophyric origin (I R Pontifex, unpublished data, 1993, 1994).
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OREBODY GEOMETRY Quartz veining is broadly continuous for 6 km along the Brocks Creek–Zapopan anticline axis, from west of John Bull to east of Zapopan (Fig 2). The vein system is parallel or subparallel to strike, up to 120 m wide at Faded Lily, and much of it gold bearing, with orebodies defined to date at Faded Lily, Alligator and Zapopan.
FIG 3 - Geological plan of Faded Lily and Alligator open pits.
The Zapopan mineralisation (J Goulevitch and D J Holden, unpublished data, 1988) is hosted by an argillite-tuff-chert-BIF package near the top of unit 6. Mineralisation at Rising Tide, 3 km north of the Brocks Creek–Zapopan anticlinal axis is hosted by pyrrhotitic, carbonaceous rocks near the base of the Koolpin Formation, adjacent to intrusive Zamu Dolerite and close to the southern contact of the Burnside Granite (Fig 2).
Gold mineralisation at Faded Lily extends discontinuously over a strike length of approximately 1050 m and down dip for at least 200 m. The main (footwall) zone of mineralisation at Western Faded Lily largely consists of multiple 50-60o south dipping, 35o east plunging, broadly concordant, parallel ore lenses occurring over a strike length of 200 m and across a width of 30 to 40 m. Individual lenses comprise regular and sheeted vein sets and range from 5 to 20 m thick (Fig 4). There are also slightly discordant vein sets, having a slightly steeper dip and a more southerly strike than the host rocks, which are commonly boudinaged and up to 1 m thick and 25 m long. A large part of the resource delineated at Faded Lily is in this western part of the footwall zone (Fig 5). A hanging wall zone with ore lenses 5 to 10 m thick is less well defined (Fig 4). These have dips and plunges similar to the footwall zone lenses.
LOCAL STRUCTURE The mine sequence is folded into the tight ESE-trending, D2 Brocks Creek–Zapopan anticline which is overturned to the north and plunges 35o ESE. The southern limb dips at 40 to 60o south and the northern limb dips at 60 to 70o south. Based on geological mapping and more particularly on airborne magnetic data, this anticline appears to be on the southern limb of a more open regional fold. A NE-trending lineament referred to informally as the ‘Chinese–Burnside linear’ can be seen on 1:100 000 scale Landsat TM imagery. The structure coincides with the linear SE-trending boundary of the Burnside Granite and passes through or close to Alligator and Rising Tide as well as a satellite deposit (Chinese Howley) at Cosmo Howley (Fig 1). It may represent a deep-seated zone of fracturing. C Hy (unpublished data, 1993) identified three parallel regional WNW-oriented zones of shearing or faulting. The central zone was informally named the ‘Brocks Creek shear zone’, which was suggested to be a reverse fault and a major mineralising control, and to be largely coincident but slightly asymmetrical with the axis of Brocks Creek–Zapopan anticline. Hy suggested this asymmetry to account for Faded Lily and Zapopan being in the anticline axial zone and Alligator on its southern limb. One of the authors (GCM) disagrees with this in part, and suggests that the very strong IP chargeability anomaly along the axis of the anticline north of Alligator indicates a zone of strong shearing through carbonaceous argillite-chert rocks of unit 1, with the absence of significant mineralisation in this zone tied to the absence of greywacke. He also considers the location of Alligator in an east-west sense to be controlled by fracturing along the Chinese–Burnside linear. The main (footwall) zone at western Faded Lily is located in a recognisable anticlinal fold closure, beneath a cherty tuff bed. Zapopan also appears to be in a fold closure. Rising Tide, as well as being close to the Chinese–Burnside linear, appears to be (?thrust) fault controlled.
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FIG 4 - Cross sections on 10 150 E (Faded Lily) and 8550 E (Alligator) showing distribution of mineralisation. Key to rock types: arg, argillite; gw, greywacke; cht, chert; tf, tuff.
The Alligator lodes are largely stratabound in a zone to 60 m wide, extend for approximately 700 m along strike and have been traced down dip for at least 200 m. The main zone of mineralisation is 200 m long, has a true width of 40 to 60 m and a dip of about 55o SW (Figs 4 and 5). Compared with Faded Lily, Alligator has wider, more consistent zones of mineralisation (eg 60 m at 1.97 g/t gold) with lower, middle and
Geology of Australian and Papua New Guinean Mineral Deposits
BROCKS CREEK GOLD DEPOSITS, PINE CREEK
Tourmaline is often associated with graphitic alteration in sheared carbonaceous argillite and in the vicinity of quartz veining, and appears to be particularly strongly developed on the northern side of the Faded Lily deposits, closer to the Burnside Granite. This spatial association of granite, tourmaline and gold mineralisation is also seen at the Esmeralda prospect, south of Union Reefs (G Miller, unpublished data, 1992). At Rising Tide, petrographic studies of host rocks (I R Pontifex, unpublished data, 1993, 1994) show alternating metamorphic assemblages of quartz-feldspar-biotitehornblende±cummingtonite and clinopyroxene-scapolitefluorite. The scapolite is commonly altered to clay. Pontifex concluded that the host rocks represent skarn-related metasomatic alteration involving enrichment in calcium, iron, sodium, fluorine and sulphides. There is also tourmaline alteration of argillite units. The depth of oxidation at Brocks Creek is about 35 m.
MINERALISATION
FIG 5 - Longitudinal projections of drill hole pierce points and gold factor values, Faded Lily footwall zone and Alligator deposits.
upper ore zones identifiable on drill sections. Like Faded Lily, Alligator ore zones plunge at about 35–40o ESE (Fig 5). Mineralisation is open down dip and down plunge. The Zapopan lodes have been identified over a strike length of 300 m, with historical mining in about 100 m. The lodes comprise either largely concordant quartz veins 1.5 to 2.5 m thick or stratiform sulphidic chert bands to 0.4 m thick, and have been intersected in drilling to 120 m below surface. There appear to be two main dip directions: the lodes south of the anticlinal axis dip 60–65o south and strike close to east, while those to the north appear to have steeper southerly dips and a slightly more southerly strike. The Rising Tide deposit comprises two subparallel tabular zones at the contact of carbonaceous argillite and Zamu Dolerite, dipping 25o SE and striking 065o and varying in thickness to 15 m. Mineralisation has been defined over a strike length of 400 m and to a vertical depth of 80 m.
METAMORPHISM AND ALTERATION Chiastolite spotting in the more argillaceous units, cordierite in more siliceous rocks and hornfelsing of argillite in the Alligator-Faded Lily mineralised package are thermal (contact) metamorphic effects surrounding the Burnside Granite. Hydrothermal alteration assemblages at Faded Lily and Alligator include quartz, pyrite, arsenopyrite (more at Alligator than Faded Lily), sericite, muscovite, clay, chlorite, graphite and tourmaline. Cordierite is often altered to a sericite-biotitechlorite assemblage. Clay alteration, particularly of mica, is commonly associated with shearing and faulting. Locally developed garnet is a feature of the alteration assemblages at Zapopan which are otherwise broadly similar to Alligator and Faded Lily and are dominated by silica, pyrite and arsenopyrite. Garnet is also occasionally seen at Alligator.
Geology of Australian and Papua New Guinean Mineral Deposits
Gold mineralisation at Faded Lily and Alligator occurs within quartz veins, along vein margins and within graphitic shear zones, and has a close affinity with pyrite and arsenopyrite. Ore zones are characterised by low sulphide content (generally 1.0 to 1.1% sulphide sulphur was reported in the 1996 feasibility study) but with up to 10% pyrite and 5% arsenopyrite. Galena and sphalerite are present in trace to minor amounts. In the weathered zone, mineralisation consists of limonite, goethite and minor scorodite. There is strong scorodite mineralisation associated with sheeted quartz veining (and local high grade gold to 41.8 g/t over 1m) in the axis of the anticline at Crocodile, 500 m west of Alligator (Fig 3). Small grains of visible gold are a relatively common feature of the higher grade zones of Faded Lily, Alligator and Zapopan. Studies of polished thin sections of two oxide and two primary ore concentrate samples recorded numerous gold grains (I R Pontifex, unpublished data, 1993) mostly in the 40 to 60 µm size range, with 96% of grains observed >20 µm and 22% >80 µm in diameter. Gold also occurs in microclusters to 2 mm across, with individual grains ranging in diameter from 2 to 120 µm. Two-thirds of the grains seen were within host rock fragments such as limonite-goethite, oxidised or limonitised greywacke, unoxidised greywacke and pyrite or arsenopyrite. Fire assays of sieved, sized splits of samples of oxide and primary mineralisation showed that most of the gold at Faded Lily and Alligator is in the coarse fraction. At Faded Lily 62% of the gold was in the +1 mm fraction (47% of the sample mass) with only 4% of the gold in the -75 µm fraction (15% of mass). Recoveries of 54 to 63% from gravity concentration tests on primary ore reflect the free gold mode of occurrence. Gold encapsulated in sulphides was rarely seen. Based on analyses of primary ore samples, Alligator contains on average 4300 ppm arsenic, 100 ppm copper, 134 ppm lead, 468 ppm zinc and less than 5 ppm silver. In contrast, Faded Lily ore contains significantly lower arsenic (1000 ppm), slightly lower base metals (36 ppm copper, 104 ppm lead, 289 ppm zinc), but similar levels of silver. The Zapopan gold mineralisation has not been studied to the same extent but is, in part, associated with quartz veins as at Alligator and Faded Lily. It also occurs in apparent stratiform mode in pyritic chert units (silicate-sulphide facies BIF) to 40
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cm thick, which are high grade (15 g/t) and appear to be folded over the axis of the anticline. The Rising Tide gold mineralisation appears to be associated with quartz-pyritepyrrhotite-arsenopyrite vein sets, though the pyrrhotite may represent an original, indigenous component of the host Koolpin Formation carbonaceous shale as in the Moline area (G Miller, unpublished data, 1988).
1994). The stratiform BIF lodes at Zapopan are very similar to occurrences at Gandys Hill and International. Rising Tide is different from most Pine Creek Geosyncline gold deposits but has elements of similarity (host rock type, geometry) with Tom’s Gully (Tsuda, Mizuta and Sakurai, 1994).
ORE GENESIS
The presence of significant fine grained free gold in the Brocks Creek deposits provided a challenge in obtaining representative drill samples and gold assays. Exhaustive studies were carried out during the exploration phase into various drilling, sampling and sample preparation techniques, which included sending large (8 kg) RC percussion drill hole samples to the laboratory and assaying whole core. During the feasibility study, the protocol of drilling RC to water influx and then coring to the finish was strictly adhered to. Standard samples were 4 kg of RC cuttings split at the rig or of half core.
In recent publications by the Northern Territory Geological Survey, Bajwah (1994) and Ahmad et al (1993) reported results of stable isotope, fluid inclusion, geological and geochemical studies of Pine Creek type gold–quartz vein deposits. They suggested a common link between vein formation, granitic magmatism, contact metamorphism and the structural deformation event, with at least four cycles of fluid release. The genesis of the Cosmo Howley gold deposits, 10 km SW of Brocks Creek, was discussed by Henley, Matthai and Kavanagh (1994) and Matthai, Henley and Heinrich, (1995). Mixing of hot magmatic (I-type granite intrusion) and metamorphic fluids along bedding concordant fractures in the Howley Anticline is considered to have resulted in gold deposition in and close to carbonaceous metasediment, with the deposits being classified as very high temperature hypothermal vein systems. Other gold deposits of the Pine Creek Geosyncline are considered to be related to similar high temperature granite intrusion processes. The Alligator and Faded Lily gold deposits are associated with veins of quartz-pyrite-arsenopyrite±tourmaline within the Burnside Granite thermal metamorphic aureole, the auriferous vein sets being found in various rock types but best developed in sheared alternating greywacke and graphitic argillite. Though no detailed research has been carried out, petrographic studies indicate a hydrothermal system at Faded Lily and Alligator. C Hy (unpublished data, 1996) concluded from detailed observations that arsenopyrite and gold mineralisation at Faded Lily and Alligator occurred at all stages of the shear deformation. Multiple structural (dextral reverse shearing), rock type (greywacke) and/or chemical (carbon) controls on mineralisation were proposed. Fault components of the Brocks Creek shear zone rather than the Brocks Creek–Zapopan anticline axis per se may control the location of the main Alligator and Faded Lily deposits. Faded Lily is close to the fold axis whereas Alligator is situated in the southern fold limb. The shear zone provided access for mineralising fluids to the host greywacke and argillite in two different structural positions. The origin of the quartz lode-associated gold mineralisation at Zapopan appears to be essentially the same as at Faded Lily and Alligator. The apparent stratiform mineralisation in the silicate-sulphide facies BIF units may have a synsedimentary origin. At Rising Tide gold-sulphide mineralisation appears to be clearly associated with skarn related metasomatic alteration. The common occurrence of fluorite in the host rocks implies influx of magmatic fluids from the intruding granite into the contact zone sediment. On the basis of ore type, host rock type and structural setting, Faded Lily and Alligator are broadly similar to the Woolwonga (Kavanagh and Vooys, 1990) and Enterprise (Cannard and Pease, 1984) deposits and to the Gandys Hill-International deposit, immediately north of Enterprise (G Miller, unpublished data, 1984; H Davies, unpublished data, 1987) and to the Union Reefs deposit (Hellsten, Wegmann and Giles,
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GEOLOGICAL EVALUATION PRACTICE
The initial (1991) RC percussion holes were drilled using the then conventional crossover sub, whereas the early 1992 drilling program used a face sampling hammer and a riffle wet splitter, and from late 1992 onwards a face sampling hammer was combined with additional compressor capacity, to keep the holes dry to greater depths, and a rotary wet splitter. For the same mineralised zone, comparative twin and triplet drill holes showed that mineralised wet sample intervals from RC drill holes with the face sampling hammer-rotary splitter combination were comparable with core but lower grade (and probably more reliable) than face sampling hammer-riffle split results. The latter were lower grade and generally indicated a thinner mineralised zone than the early crossover sub RC drill intercepts. It is suggested that the wet riffle splitter upgraded the gold content of the samples by taking a split from the outside of the wet sample stream, in which, because of the centrifugal effect combined with a short distance between the base of the cyclone and the splitter (material still spinning), there was a disproportionate amount of the coarse, heavy fraction known to contain most of the gold. From early 1994 the combination of RC drilling to water influx and core finish together with drilling of twin holes to check earlier wet sampling provided a sound basis for the feasibility study. Resource Service Group completed the feasibility study and concluded that the results from wet RC, dry RC and core samples were representative of the same population. The effort put into drilling, sampling and assay methods at the exploration stage was continued into the mining stage with grade control drilling conducted by an RC drill rig under close supervision in dry conditions, noting any factors that could affect the integrity of the samples. The grade control drilling was supplemented by geological mapping and pit sampling, including face sampling of vein sets and host rocks, and blast hole sampling in the zones of interest outside the established ore zones.
ACKNOWLEDGEMENTS This paper is published with the permission of Acacia Resources Limited. The input of geologists A Buskas, R Bertleson, S Mottram and B Smith to the exploration success at Brocks Creek is acknowledged. Contributions from geologists D Wegmann and M Hancock during the evaluation stage, and from P Van Den Oever during the early mining stage, are also acknowledged.
Geology of Australian and Papua New Guinean Mineral Deposits
BROCKS CREEK GOLD DEPOSITS, PINE CREEK
REFERENCES Ahmad, M, Wygralak, A, Frencz, P F and Bajwah, Z U, 1993. Pine Creek, Northern Territory - 1:250 000 metallogenic map series, Northern Territory Geological Survey Explanatory, Notes SD 52 –8. Bajwah, Z U, 1994. A contribution of geology, petrology and geochemistry of the Cullen Batholith and related hydrothermal activity responsible for mineralisation, Pine Creek Geosyncline, Northern Territory, Northern Territory Geological Survey, Report 8. Balfour, I S, 1981. Recorded mineral occurrences in the Northern Territory, Northern Territory Geological Survey Technical Report GS 81/17 (unpublished). Cannard, C J and Pease, C F D, 1984. Enterprise gold mine, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 773–778 (The Australasian Institute of Mining and Metallurgy: Melbourne). Hellsten, K, Wegmann, D and Giles, D, 1994. Union Reefs: A project assessment and development case history, in Proceedings 1994 AusIMM Annual Conference (Ed: C P Hallenstein), pp 37–44 (The Australasian Institute of Mining and Metallurgy: Melbourne). Henley, R W, Matthai, S K and Kavanagh, M E, 1994. Hypothermal vein mineralisation at the Cosmopolitan Howley gold deposit, Northern Territory, The AusIMM Bulletin, 5:65–69.
Matthai, S K, Henley, R W and Heinrich, C A, 1995. Gold precipitation by fluid mixing in bedding-parallel fractures near carbonaceous slates at the Cosmopolitan Howley gold deposit, Northern Australia, Economic Geology, 90:2123–2142. Nicholson, P M and Eupene, G S, 1990. Gold deposits of the Pine Creek Inlier, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 739–742 (The Australasian Institute of Mining and Metallurgy: Melbourne). Nicholson, P M, Ormsby, W R and Farrar, L, 1994. A review of the structure and stratigraphy of the Pine Creek Geosyncline, in Proceedings 1994 AusIMM Annual Conference (Ed: C P Hallenstein), pp 1– 9 (The Australasian Institute of Mining and Metallurgy: Melbourne). Ormsby, W R, Nicholson, P M and Butler, I K, 1994. A review of the structure and stratigraphy of Central Pine Creek Geosyncline, in Proceedings 1994 AusIMM Annual Conference (Ed: C P Hallenstein), pp 11 –19 (The Australasian Institute of Mining and Metallurgy: Melbourne). Sullivan, C J and Iten, K W B, 1952. The geology and mineral resources of Brocks Creek District, Northern Territory , Bureau of Mineral Resources Geology and Geophysics, Bulletin 12. Tsuda, K, Mizuta, T and Sakurai, M, 1994. Mineralisation of the Toms Gully Deposit, Northern Territory, in Proceedings 1994 AusIMM Annual Conference (Ed: C P Hallenstein), pp 89–96 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Kavanagh, M E and Vooys, R A, 1990. Woolwonga gold deposit, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 747–750 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Newton, P G N, Switzer, C, Hill, J, Tangney, G and Belcher, R, 1998. Union Reefs gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 417–426 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Union Reefs gold deposit 1
2
3
4
by P G N Newton , C Switzer , J Hill , G Tangney and R Belcher INTRODUCTION The deposit is approximately 185 km SE of Darwin, NT, at lat 13o42′S, long 131o47′E and AMG coordinates 801 700 E, 8 482 000 N, on the Pine Creek (SD 52–8) 1:250 000 and Pine Creek (5270) 1:100 000 scale map sheets (Fig 1). It is 100% owned and operated by Acacia Resources Limited.
5
173 000 oz. Exploration since 1994 using diamond and reverse circulation (RC) drilling has enabled a significant upgrade in the Mineral Resource and Ore Reserve (Table 1). Mill throughput is currently 1.7 Mtpa and is scheduled to increase to 2.1 Mtpa in late 1997. Union Reefs is one of several recently developed gold mines in the Pine Creek Geosyncline, in addition to Mount Todd (Ormsby et al, this publication) and Brocks Creek (Miller et al, this publication). The deposit has gross features typical of gold deposits in the Pine Creek Geosyncline, ie the mineralisation has a spatial association with compressional structures such as anticlinal crests and reverse shear zones and is hosted within a mixed package of pelite and psammite that forms part of a palaeoturbidite sequence. However, the smaller scale features of the deposit are anomalous and their description will help understanding gold mineralisation processes at Union Reefs and in the Pine Creek Geosyncline.
EXPLORATION AND MINING HISTORY
FIG 1 - Location and regional geological map, modified from Nicholson and Eupene (1984) and Nicholson, Ormsby and Farrar (1994).
With production in 1996 of 2.71 t or 87 000 oz of gold from 1 943 430 t of ore, Union Reefs is currently the largest gold producer from the Pine Creek Geosyncline and the third largest in the Northern Territory behind The Granites (6.88 t) and Tanami (3.78 t). Total gold production from 1994 is 5.37 t or 1.
Research Geologist, Centre for Teaching and Research in Strategic Mineral Deposits, Department of Geology and Geophysics, University of Western Australia, Crawley WA 6907.
2.
Formerly Geology Superintendent, Union Reefs Gold Mine, Acacia Resources Ltd, now Principal Geologist, MIM Exploration, 55 Little Edward Street, Spring Hill Qld 4000.
3.
Formerly Senior Mine Geologist, Union Reefs Gold Mine, Acacia Resources Ltd, now c/o Billiton Bogoau Gold Limited, PO Box 16075, Airport Post Office, Accra Ghana, West Africa.
4.
Project Geologist, Union Reefs Gold Mine, Acacia Resources Ltd, PO Box 194, Pine Creek NT 0847.
5.
Mine Geologist, Union Reefs Gold Mine, Acacia Resources Ltd, PO Box 194, Pine Creek NT 0847.
Geology of Australian and Papua New Guinean Mineral Deposits
Gold was discovered at Union Reefs in December 1873 by prospectors Adam Johns and Phil Saunders (Jones, 1987), and since then approximately 1600 pits, shafts, adits and open cuts to 61 m deep have been worked (Shields, White and Ivanac, 1967). Most of the claims were held by European and Chinese miners until 1892, but most had been purchased by Chinese miners by 1894. Much of the gold obtained during this time was not recorded, but the data for the period 1884 to 1910 show production of 48 000 oz of gold from 58 000 t of ore for a recovered grade of 26 g/t (Hossfeld, 1936). Diamond drilling programs completed at Union Reefs between 1905 and 1964 (Brown, 1906; Jensen, 1915; Shields, White and Ivanac, 1967) include two government-funded holes drilled in 1905–1906, believed to be the first exploration holes in the Northern Territory (Hellsten, Wegmann and Giles, 1994). Drilling during the 1960s by the Bureau of Mineral Resources identified a resource at Crosscourse. This was followed by further drilling in 1969–1970 by Central Pacific Minerals NL, who drilled four diamond drill holes, three at Crosscourse and one at Lady Alice North (J W Shields, unpublished data, 1970). Between 1984 and 1988, 25 exploration holes were drilled by Enterprise Gold Mines NL at Ping Ques and Crosscourse which gave encouraging results and led to a resource estimate (Hellsten, Wegmann and Giles, 1994). Enterprise considered mining the deposit as a small heap leach operation, but could not reach an agreement with the neighbouring leaseholder (Mineral Horizons NL) to provide ground for infrastructure. In 1988, Mineral Horizons drilled 68 percussion holes along the northern half of the Union and Lady Alice lines of mineralisation (M G Mulroney, unpublished data, 1988).
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TABLE 1 Progressive Mineral Resources and Ore Reserves for Union Reefs from 1994 to 1997. Identified Mineral Resources Year
Inferred (Mt at g/t Au)
Indicated (Mt at g/t Au)
Measured (Mt at g/t Au)
Total (Mt at g/t Au)
Contained gold (Moz)
Dec 1994
2.90 at 2.10
5.10 at 2.40
3.40 at 2.50
11.40 at 2.35
0.86
Dec 1995
2.35 at 2.20
6.42 at 2.20
3.21 at 2.22
11.98 at 2.20
0.85
Dec 1996
9.15 at 1.98
6.86 at 1.84
4.66 at 2.20
20.67 at 1.97
1.31
June 1997
3.89 at 2.02
10.04 at 1.93
4.23 at 2.08
19.06 at 1.98
1.21
Ore Reserves Year
Probable (Mt at g/t Au)
Proved (Mt at g/t Au)
Total (Mt at g/t Au)
Contained gold (Moz)
Dec 1994
4.40 at 2.20
3.70 at 2.23
8.10 at 2.21
0.57
Dec 1995
4.19 at 2.10
3.34 at 2.06
7.56 at 2.10
0.51
Dec 1996
5.30 at 1.80
3.97 at 2.19
9.27 at 1.97
0.59
June 1997
8.21 at 1.92
3.79 at 2.07
12.00 at 1.97
0.76
No further work was carried out until 1991 when the Union Reefs leases were acquired by The Shell Company of Australia Limited who carried out detailed soil sampling, geophysical surveys and percussion and diamond drilling. By 1994 Shell Australia had defined a Measured and/or Indicated and/or Inferred Resource at Crosscourse and Union North of 11.5 Mt at 2.4 g/t and had completed a detailed feasibility study that led to completion of plant construction by early 1994 (Hellsten, Wegmann and Giles, 1994). Shell Australia transferred its mineral interests to Acacia Resources, which was then floated as a public company, in April 1994.
PREVIOUS DESCRIPTIONS Turner (1990) and Hellsten, Wegmann and Giles (1994) have published papers on the deposit. Prior to Shell-Acacia acquiring the project, J L Baxter (unpublished data, 1987) completed one of the few studies of the deposit. However, since acquisition, Shell and Acacia have commissioned unpublished studies by J S Donaldson (1992) on structure and geochemistry, by R N England (1992) on petrography, by D M Ransom (1992) on structure, by D Wegmann (1992) on structure and ore reserves, by R Gaze and S Khosrowshahi (1996) on ore reserves and by R Sliwa (1996) on structure.
REGIONAL GEOLOGY The deposit is within the Cullen mineral field, a well mineralised portion of the Pine Creek Geosyncline (Fig 1). The geosyncline is a Palaeoproterozoic inlier approximately 66 000 km2 in area (Needham, Stuart-Smith and Page, 1988; StuartSmith et al, 1993) which comprises a supracrustal sequence of metasedimentary and volcanic rocks which overlie Archaean granite migmatite complexes (Klominsky et al, 1996). Sedimentation and volcanism took place between 2000 and 1870 Myr in an intracratonic basin to provide a sequence approximately 10 km thick, and was followed by multiphase deformation, regional metamorphism and granitoid intrusion. The earliest deformation event recognised by Johnston (1984), Stuart-Smith et al (1993) and Partington and McNaughton (1997) is local recumbent folding and low angle thrusting. This was followed by a regional compression event which produced prominent upright, shallowly-plunging NW-
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trending folds and reverse faults such as the Howley Anticline and the Pine Creek shear zone. Local cross folding with easttrending axes post-dated this event and was followed by late faulting and reactivation of earlier structures. The Cullen Batholith was intruded late in the deformation sequence and has been dated at 1835 to 1800 Myr (Klominsky et al, 1996) and according to Stuart-Smith et al (1993) is composed of 23 interconnected granitic plutons. Contact metamorphic aureoles associated with the batholith generally overprint greenschist facies regional metamorphic minerals. Several types of mineralisation including gold, base metals, tin, tantalum and uranium are associated with the Cullen Batholith. Since gold mining began in the late 1800s, over 3 Moz have been produced from the region and present resources and reserves are greater than 4 Moz. Gold mineralisation occurs in linear belts which are spatially related to anticlinal crests, strike-slip shear zones and duplex thrusts (Partington and McNaughton, 1997). The Pine Creek shear zone is a major control of gold mineralisation at several deposits including Enterprise, Woolwonga and Union Reefs.
ORE DEPOSIT FEATURES The gold deposits at Union Reefs are within the 300 m wide Pine Creek shear zone, along the parallel Union and Lady Alice trends which strike at approximately 330o (Fig 2). Known gold mineralisation extends for over 3.5 km along strike, 400 m across strike and to a vertical depth of 300 m. The eastern line of workings follows the Lady Alice anticlinal hinge, but the major workings are developed almost entirely on the western limb of this plunging fold. The five areas of current mining at Union Reefs are Crosscourse, Western lenses, Ping Ques, Lady Alice and Prospecting Claim, and it is from these areas and Union North that most of the information in the following description is sourced.
LITHOLOGY AND STRATIGRAPHY The deposits are hosted by interbedded pelite and psammite units of the Burrell Creek Formation, a member of the Finniss River Group and the youngest of the Palaeoproterozoic geosynclinal sedimentary units of Stuart-Smith et al (1993). The fine to coarse grained psammites are dominantly
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workings which changes from 330o at Crosscourse to 345–350o at Union North and Lady Alice North. Bedding dips subvertically with a relatively consistent dip to the WSW (Fig 4a), although east of Lady Alice, bedding dips towards the ENE at 60–70o. A pronounced bedding-parallel slaty cleavage is developed in the pelites, and a pervasive foliation in psammites is defined by aligned chlorite and muscovite. The Lady Alice fold closure is characterised by several foliation-parallel shear zones which have transposed the anticlinal hinge. Stratigraphic younging in the western part of the deposit is consistently towards the west, but reverses several times in a 200 m section at Lady Alice. Rocks further east of the shear zones young consistently towards the east, hence the area is interpreted as a sheared out anticlinal closure. Mesoscopic fold closures are not preserved, and throughout most of the deposit bedding and cleavage are subparallel, which suggests that the folds are tight to isoclinal. Several exposures east of Lady Alice, near the fold closure, show a consistent obliquity of bedding and cleavage (Fig 3a), which intersect in an orientation of 40–60o towards the NNW (Fig 4b). This local plunge orientation of the Lady Alice anticline contrasts with a subhorizontal plunge of semi-regional scale folds interpreted from regional magnetic images (R Sliwa, unpublished data, 1996). A well-developed mineral elongation lineation plunges subvertically. It is shown by elongated quartz grains in coarsergrained psammite units and stretched arsenopyrite grains in mineralised areas (Fig 4c). Symmetrical pull-apart structures in the stretched arsenopyrite grains are commonly filled with quartz. Other linear features include subhorizontal striations on bedding planes which generally overprint the stretching lineation. Small-scale kink folds of bedding and the axial planar cleavage plunge steeply, and in some areas conjugate kink bands are developed which strike at 050 and 110o. FIG 2 - Location of resources, old workings and pit outline, Union Reefs gold mine.
greywacke beds from 2 cm to metres thick with well preserved sedimentary features including graded beds, flame structures and cross bedding. The pelites are laminated to thinly bedded, olive-green to brown, and consist of siltstone and sandy siltstone with rare phyllite in shear zones. Typically the pelites have sharp contacts with greywacke-dominated psammite units. Dolerite bodies are interpreted from magnetic images and have been intersected in recent drilling. The dolerites follow the main structural trend and are offset by minor cross faults.
Quartz vein morphology Quartz veining is widespread throughout the deposits and has a close association with gold mineralisation. Hellsten, Wegmann and Giles (1994) summarise the vein characteristics using three end-member styles (1–3 below) that form a continuum depending on the rock type and structural setting. Since mining began in 1994, some additional vein morphologies have been recognised (4–5 below), although these are relatively minor and have no clear association with gold mineralisation. 1.
Lode style veins are 1 mm to 2 m thick and are developed in layer-parallel shear zones in pelitic wall rocks. In high strain shear zones the veins form discontinuous, anastomosing arrays which are wrapped by the shear fabric. In less deformed areas, lode style veins are thicker, more continuous and form symmetrical boudins with subhorizontal axes (Fig 3b). Lode style veins are prominent at Lady Alice and Ping Ques, where they follow the Lady Alice anticline axial surface. Many old workings are located on this style of veins.
2.
Sheeted oblique veins are most common in interbedded sequences of psammite and pelite, and are commonly continuous into lode style veins. The veins occur as en echelon sets between 1 cm and 1 m thick, which crosscut the axial planar foliation and are generally parallel to an oblique fracture cleavage, along which bedding is offset
The sedimentary rocks of the Burrell Creek Formation are interpreted to be a turbidite sequence deposited in a deep water, high energy environment (Needham, Stuart-Smith and Page, 1988) and local features suggest a palaeocurrent direction towards the NW (Donaldson, 1992). Both psammite and pelite are mineralised, although the thicker psammite units are the preferred host to economic gold mineralisation.
STRUCTURE Folding and planar features Typical Union Reefs structures are shown in Fig 3. The stratigraphic succession strikes parallel to the trend of the
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FIG 3a
FIG 3c
FIG 3b
FIG 3d
FIG 3e
FIG 3f
FIG 3g
FIG 3 - Photographs of typical structural features at Union Reefs. All drill core is NQ oriented at -60o toward 241o, showing the northern half of the core with down hole direction towards the left of the page: a. Plan view of bedding and cleavage relationships in the fold hinge at Lady Alice North. Photo width - 60 cm; b. cross section view of lode style veins in layer parallel shear zone with subhorizontal boudin axes. Photo width - 1.8 m, taken looking SE; c. en echelon oblique veins in interbedded package of pelite and psammite. Photo width - 60 cm, taken looking SE; d. stockwork veins at E lens. Photo width - 3 m, taken looking NW; e. symmetrically folded quartz vein with NNW-plunging axis. Photo width - 60 cm, taken looking NW; f. subhorizontal extension veins splaying from bedding-parallel vein, URD 033, 156.5 m; g. stylolitised milky quartz vein, URD 033, 156.3 m.
420
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UNION REEFS GOLD DEPOSIT
with generally reverse movement. In pelites the oblique veins are typically <2 cm thick and are subparallel to bedding and the axial planar cleavage, but in psammites the veins are 5–10 cm thick, are at a high angle to bedding and typically dip to the NE and NW (Figs 3c and 4b). Sheeted veins occur throughout the deposits, but are best developed at Crosscourse South, Union North and Western Lenses. 3.
FIG 3h - stretched arsenopyrite crystals with quartz-filled pressure shadows, URD 086, 113.5 m.
Stockwork vein sets are largely restricted to the psammite-dominated sequences at E lens, the area of greatest vein density at Union Reefs. The veins form planar arrays (Fig 3d), but are also folded about subvertical axial surfaces with moderately NNWplunging axes (Figs 3e and 4d). The average axis of folded veins is broadly parallel to the plunge of the Lady Alice anticline. Folded veins are commonly transposed by reverse movement on bedding-parallel shear zones (see Fig 3 of Hellsten, Wegmann and Giles, 1994).
FIG 4 - Equal-area lower hemisphere stereoplots of the major structural elements at Union Reefs: a. poles to bedding, n = 74; b. poles to bedding (n = 19) and axial planar cleavage (n = 15) in the fold closure at Lady Alice North; c. linear features - mineral lineations (n = 12) and boudin axes (n = 34); d. plunge of folded quartz veins (n = 35) and poles to planar discordant veins (n = 36). Closed circle - bedding; open square - cleavage; open triangle - discordant vein; closed diamond - folded vein axis; cross - boudin axis; closed triangle - mineral stretching lineation.
Geology of Australian and Papua New Guinean Mineral Deposits
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4.
Subhorizontal veins are common throughout Union Reefs, but rarely have significant thickness or lateral continuity. They are generally less than 1 cm thick and commonly occur as fracture fill veins with quartz-pyrite filling. There are several examples of SE-dipping en echelon arrays of subhorizontal vein sets which splay from bedding parallel and oblique veins (Fig 3f). However, unlike oblique and lode style veins, subhorizontal veins are rarely deformed, and have preserved vein fibres oriented perpendicular to the vein margins.
5.
Pyritic veinlets less than 3 mm thick are common in drill core from E lens. R Sliwa (unpublished data, 1996) notes that the veins crosscut stretched arsenopyrite crystals but are also folded.
The quartz veins at Union Reefs have deformational textures including brecciated margins, foliated wall rock fragments (Fig 3g) and chlorite-sulphide stylolites. Only the subhorizontal sets show well preserved primary growth textures.
Shear zones and faults The Lady Alice shear is one of several bedding-parallel, steeply dipping shears in the Union Reefs area. Individual shear zones are generally 1–5 m wide and consist of anastomosing smallscale shears, an intense slaty cleavage and discontinuous quartz veins. The amount of movement on the shear zones is unclear, although it is possible that the Lady Alice shear, which transposes the anticlinal fold closure at Lady Alice, has juxtaposed significantly different levels of the succession (R Sliwa, unpublished data, 1996). Lineations on the shear planes are variable, although down dip is the most common orientation. Significant amounts of graphitic material are noted in the several shear zones at Crosscourse and Lady Alice. Faults have dominantly NNE and WNW trends and steep dips. At Crosscourse and Union North the movement on these structures is generally small, and slickensides on fault planes pitch at shallow angles. The NNE set has a dextral sense of movement and sinistral movement is recorded on the WNW set. Most faults are filled with clay gouge and minor quartz, although one larger NW-striking structure at E lens is 20–30 cm thick and filled with milky quartz and wall rock slivers. The faults generally crosscut all veins and show minor offsets. However, there are isolated veins which bend into the oblique faults, and areas where veins are preferentially distributed on one side of a structure.
Structural timing The slaty cleavage, Lady Alice anticline and layer-parallel shear zones, including the Pine Creek shear zone, represent the first deformation event recognised at Union Reefs, which is the equivalent of D2 in the synthesis of Partington and McNaughton (1997) and Stuart-Smith et al (1993). This is a significant compression event which produced the prominent folds and thrust faults in the Pine Creek Geosyncline including the Howley Anticline and Hayes Creek Fault. Partington and McNaughton (1997) note that many of the folds formed during this event are doubly plunging, asymmetric and noncylindrical, due to ramping of a sole thrust at depth through the succession. However at Union Reefs the style of folding is consistently isoclinal, as bedding and cleavage are parallel throughout the area. The fold plunge is constrained from bedding-cleavage obliquity in the hinge zone as 40–60 o towards the NNW.
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The majority of macroscopic veins formed during one event, as indicated by mutually inconsistent crosscutting relationships between all vein types. The contrasting vein styles (ie discordant versus bedding parallel) reflect the contrasting rock properties of the pelite and psammite and not different stages of veining. Quartz veining took place after the early stages of folding but ended before the final stages, as the discordant veins cut across the axial planar cleavage, and therefore post-date the earliest stages of folding. However, several deformation features of quartz veins including boudinage, folding and stylolitisation suggest that deformation continued after vein formation. The local stress configuration prior to, during and after veining is similar and involved coaxial flattening with ENEdirected principal stress direction (σ1) and subvertical σ3. Evidence for this style of deformation and stress orientations include: 1.
the lack of asymmetry in the deformation fabrics, eg the slaty cleavage in pelites;
2.
subhorizontal extension veins which splay from both discordant and lode style veins. The subhorizontal set formed during periods of supralithostatic fluid pressures, and subvertical quartz fibres suggest that the minimum principal stress (σ3) was subvertical;
3.
symmetically developed boudins with subhorizontal axes;
4.
subvertically stretched arsenopyrite and quartz grains with symmetrical pressure shadows; and
5.
near symmetrical folds of quartz veins.
To form inclined vein sets which parallel the regional fold plunge during subvertical extension, it is likely that the regional fold was in place prior to vein formation. The vein sets manipulated pre-existing weaknesses in the fold hinge such as bedding planes and radial fractures. Continued shortening of the fold or a separate folding event with identical stress axes to the first (type 0 fold interference pattern) will produce folded veins which also plunge parallel to the regional trend. The relative timing of the NNE- and WSW-striking oblique faults and their influence on vein formation is equivocal. The faults commonly offset oblique veins, which places them late in the structural history. However, the asymmetric distribution of veins around the faults and the continuity of some veins into the fault plane suggests that at least some of the structures were in place during vein formation. The opposing sense of strike slip movement on the structures is consistent with a conjugate set formed during ENE–WSW directed shortening.
WALL ROCK ALTERATION Wall rock alteration is pervasive at Union Reefs and is best developed in psammite units. The typical alteration mineral assemblage, in decreasing order of abundance, consists of quartz-muscovite-chlorite-arsenopyrite-pyrite±dolomite. In hand specimen altered psammite is pale green-yellow due to chlorite and muscovite, and extends throughout the intensely veined E lens. Outside E lens, alteration is more discrete and occurs as decimetre-scale bleached haloes around individual veins. Intense green alteration of pelites in shear zones is due to the development of fine grained chlorite. The wall rock sulphide assemblage is restricted to the immediate margins of the quartz veins and is dominated by arsenopyrite grains to 20 mm in diameter.
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UNION REEFS GOLD DEPOSIT
MINERALISATION Gold mineralisation is strongly associated with all types of quartz veining, and despite a broad association with increased muscovite-chlorite alteration, there are only minor amounts of gold outside the veins (Fig 5) which is a common feature of turbidite-hosted gold deposits throughout the world (R Goldfarb, personal communication, 1997). Statistically, gold mineralisation is best developed in the psammite units (Table 2), which not only reflects the greater amount of psammite in the mine sequence, but also the increased amounts of veining and arsenopyrite alteration. Pelitic units are also mineralised, but their average gold grade is lower, and the thickness of individual pelite units is typically less than the psammite units.
proximal to vein margins. Pyrite is closely associated with arsenopyrite and forms subhedral, fine grained aggregates. Pyrrhotite is rare, except in shear zones with significant veining and graphite, where it can account for 15% of the rock volume. Sphalerite and galena are in late stage veinlets which crosscut arsenopyrite crystals but also form aggregates with arsenopyrite, pyrite and minor chalcopyrite. Base metal sulphide precipitation is interpreted to have occurred over a protracted interval and to be broadly associated with the gold event. Hellsten, Wegmann and Giles (1994) noted that coarse grained gold is a characteristic feature of the mineralisation, and summarised the location and nature of gold at Union Reefs. It occurs within quartz-sulphide veinlets and at the brecciated margins of thicker, more continuous quartz-rich veins, which suggests that most gold was introduced during the later stages of veining. Gold particles are typically 25 µm to 1 mm in diameter and associated with fractures in arsenopyrite crystals (Donaldson, 1992). Approximately 25% of the gold recovered from the mill is from the gravity circuit and the remainder is free milling.
ORE GEOMETRY The geometry of gold mineralisation at Union Reefs reflects a range of features including the NNW-plunging regional fold structure, plunging folded quartz veins, bedding-vein intersection axes, subhorizontal boudin axes in lode style veins and oblique faults.
FIG 5 - The average gold grade from rock chip samples at Union Reefs showing: a. the strong vein control of gold grade; b. that economic gold grades are found in all of the three major vein types.
The vein minerals are quartz, dolomite, chlorite, pyrite, arsenopyrite, sphalerite, galena, marcasite and minor bismutite. Large pink albite aggregates occur within the larger veins. Arsenopyrite is the principal vein sulphide mineral, typically located along late quartz-chlorite fractures and as aggregates
Figures 6–8 show the distribution of gold mineralisation at Crosscourse in plan view, cross section and longitudinal projection. The lodes are generally parallel to the succession and typically have better down-plunge continuity compared with along-strike (Figs 6 and 8). At E lens the plunge of the main ore shoot is 65o to the NNW, compared with 40o at Ping Ques and a weakly constrained subvertical plunge at Western Lenses (Fig 8). The moderate to steep plunge towards the NNW mimics the plunge of the Lady Alice anticline and the folded quartz veins. In plan view, the ore shoots step in an en echelon manner along 270–300o trends (Fig 6) which are parallel to the strike of the sinistral fault set. This is an important empirical relationship, as at the Enterprise deposit, 13 km south of Union Reefs, gold mineralisation shows a strong relationship with NNE-trending oblique faults (C Brauhart, personal communication, 1997). Similar NNEtrending faults are present at Union Reefs but are interpreted as the conjugate of the faults with 270–300 o strike. Directional variograms calculated for the ore shoots generally confirm the subtle variations in ore-shoot plunge and are constantly re-evaluated as new batches of grade control and exploration data become available. Variograms calculated for different rock types reflect the markedly different geometry of mineralisation within the pelites and psammites. Highly-
TABLE 2 Statistics of gold grade in pelite and psammite samples, from grade control data. Rock
Number
Mean
Min
Max
Range
Variance
cv
Pelite
3588
0.11
0.01
5.70
5.70
0.09
2.62
Psammite
8194
0.48
0.01
11.8
11.8
0.80
1.86
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mineralisation in the Pine Creek Geosyncline is poorly described. Recent work has begun to resolve this problem (Matthäi et al, 1995; Klominsky et al, 1996; Partington and McNaughton, 1997), but further work is required to test these models. At Union Reefs, the structural and mineralogical features of the deposit are similar to many features at turbidite-hosted gold deposits throughout the world. At this stage only minor isotopic and thermodynamic data are available for the deposit, but the gross geological features suggest: 1.
There is a strong structural control of the location and geometry of the mineralisation. At the macroscopic scale, Union Reefs gold is broadly associated with an anticlinal closure, reverse shear zones and oblique faults, and at a smaller scale, with quartz veins. The hydrothermal fluids were focussed into the area during a horizontal compression event, and local fluid overpressuring resulted in synchronously-formed oblique, lode-style and flat-lying veins which fill fracture meshes.
2.
The proximity to the Cullen Batholith is critical, and it is probably the heat source for the hydrothermal fluids. A direct magmatic fluid source is possible (Matthäi et al, 1995) and is discussed in a global context by Sillitoe (1991), but there is a likely input from less saline metamorphic fluids (Partington and McNaughton, 1997). Groves et al (in press) also highlight the problem of whether to classify the many ‘intrusion-related gold deposits’ such as Union Reefs as typical ‘orogenic gold deposits’ formed during accretionary and collisional orogens.
3.
The wall rock alteration suggests the addition of significant amounts of potassium, carbon dioxide, iron, sulphur, gold, arsenic, lead, zinc and bismuth.
4.
Gold precipitation may have been controlled by fluid mixing between an oxidised magmatic-metamorphic fluid and a reduced fluid near the carbonaceous shales as at Cosmo Howley (see Matthäi et al, 1995).
FIG 6 - Plan view of geology, gold mineralisation and high grade shoots at Crosscourse, modified from P Collins (unpublished data, 1997).
MINE GEOLOGICAL METHODS
FIG 7 - Cross section through the Crosscourse pit looking NW, along local grid 6650 N.
strained pelitic sequences have a subhorizontal major axis of continuity, which reflects the subhorizontal boudin axes of many veins in layer-parallel shear zones in pelites. In contrast, the major axis of continuity in psammite units plunges at approximately 40o towards the NNW and reflects the plunge of folded veins and the intersection of the oblique veins with bedding.
ORE GENESIS Compared with other turbidite-hosted mineral fields throughout Australia and the world, as at Bendigo–Ballarat, Australia and Nova Scotia, Canada, the genesis of gold
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Geological control of ore grade at Union Reefs uses rigorous techniques and stringent procedures in order to reduce the risk of misclassification. These techniques have been adopted to counter the high nugget effect and erratic grade distribution which characterise the orebody. The economies of the mineralised zones are very susceptible to the edge effect and to dilution. Grade control drilling is conducted on a 10 by 4 m pattern using reverse circulation (RC) holes of 12.5 cm diameter. Drilling is completed for a 7.5 m bench using holes 10 m long and inclined at 60 o toward 241.5o, allowing for 1 m subdrill. Samples to 5 kg are collected every metre and pulverised using a Mixermill 1000 mill to 90% passing 75 µm. A 50 g fire assay for gold is carried out. All grade control samples are logged for rock type, weathering, quartz and sulphide content, all of which are incorporated into the geological model. Ore blocking and grade estimation are carried out using a conditional simulation package designed for a block size of 2 m (X) by 5 m (Y) by 2.5 m (Z) with sub-blocking to 0.5 m by 2.5 m by 2.5 m. Fifty individual simulations are run for each drilled area using grid nodes relevant to the sub-blocking pattern. The
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UNION REEFS GOLD DEPOSIT
FIG 8 - Longitudinal projections of gold grade by thickness in the planes of 4800 E, 4900 E and 5000 E, looking SW. Drill intercepts are plotted as cumulative metal content derived by multiplying grade (g/t) by thickness in metres and dividing by total true thickness. All data from exploration drilling.
results of the simulations are then averaged and optimised to the SMU block size taking into account the risk factors including current costs, average mill recovery and expected return. Three blocking levels of 2.5 m are created to represent the 7.5 m depth which has been drilled. Probability layers are produced which show the probability that each block will have at least a 50% chance that the grade will be above cutoff. As of February 1996, the following ore classification scheme was in place: Oxidised zone Run of mine (ROM) ore: >0.7 g/t
Mineralised waste: 0.3–0.7 g/t
Waste: <0.3 g/t
Mineralised waste: 0.5–0.7 g/t
ACKNOWLEDGEMENTS Permission to publish by Acacia Resources Limited is gratefully acknowledged. The authors wish specifically to note contribution to this manuscript by N Crase, D Groves and J Ridley. P Newton’s research is supported by the Centre for Teaching and Research in Strategic Mineral Deposits, AMIRA project P454 and an Australian Postgraduate Research Award.
REFERENCES
Transitional and fresh zone ROM ore: >0.7 g/t
using two PC185 excavators and 85 t dump trucks. Material being mined is continually spotted by technicians in order to maximise recovery and minimise dilution.
Waste: <0.5 g/t
Brown, H Y L, 1906. Northern Territory boring operations, South Australian Parliamentary Paper 55. Donaldson, J S, 1992. The Union Reefs Prospect, NT: structure and geochemistry of a turbidite hosted gold deposit, BSc Honours thesis (unpublished), University of Tasmania, Hobart.
The Vulcan blockout procedure is used by the geologist to slice through the mid bench of each 2.5 m flitch, and by creating a polygon around each classification type, the tonnes and grade of each ore type is available for blast design and mine planning. Conditional simulation using the grade control drill hole data has proved to be the most effective estimation procedure. Mining is completed over 3 by 2.5 m flitches for a 7.5 m bench
Geology of Australian and Papua New Guinean Mineral Deposits
Groves, D I, Goldfarb, R J, Gebre-Mariam, M, Hagemann, S G and Robert, F, in press. Orogenic gold deposits: A proposed classification in the context of their crustal distribution and relationship to other gold deposit types, Ore Geology Reviews. Hellsten, K, Wegmann, D and Giles, D, 1994. Union Reefs: a project assessment and development case history, in Proceedings 1994 AusIMM Annual Conference, pp 37–43 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Hossfeld, P S, 1936. The Union Reefs gold-field, Aerial, Geological and Geophysical Survey of Northern Australia, Northern Territory, Report No 2. Jensen, H I, 1915. Report on diamond drilling in the Northern Territory, Northern Territory Department of External Affairs, Bulletin 12. Johnston, J D, 1984. Structural evolution of the Pine Creek Inlier and mineralisation therein, NT, Australia, PhD thesis (unpublished), Monash University, Melbourne. Jones, T G, 1987. Pegging in the Northern Territory, in A History of Mining in the Northern Territory, 1870–1946 , p 234 (Northern Territory Government Printer: Darwin). Klominsky, J, Partington, G A, McNaughton, N J, Ho, S E and Groves, D I, 1996. Radiothermal granites of the Cullen Batholith and associated mineralisation (Australia), Czech Geological Survey, Special Papers 5, p 44 (Czech Geological Survey: Prague). Matthäi, S K, Henley, R W, Bacigalupo-Rose, S, Binns, R A, Andrew, A S, Carr, G R, French, D H, McAndrew, J and Kavanagh, M E, 1995. Intrusion-related, high temperature gold quartz veining in the Cosmopolitan Howley metasedimentary rock-hosted gold deposit, Northern Territory, Australia, Economic Geology, 90:1012–1045. Needham, R S, Stuart-Smith, P G and Page, R W, 1988. Tectonic evolution of the Pine Creek Inlier, Northern Territory, Precambrian Research, 40/41:543–564. Nicholson, P M and Eupene, G S, 1984. Controls on gold mineralisation in the Pine Creek Geosyncline, in Proceedings of Australasian Institute of Mining and Metallurgy Conference, 1984, pp 377–396 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Nicholson, P M, Ormsby, W R and Farrar, L, 1994. A review of the structure and stratigraphy of the Central Pine Creek Geosyncline, in Proceedings 1994 AusIMM Annual Conference, pp 1–9 (The Australasian Institute of Mining and Metallurgy: Melbourne). Partington, G A and McNaughton, N J, 1997. Controls on mineralisation in the Howley District, northern Australia: a link between granite intrusion and gold mineralisation, in Precambrian Mesothermal/Late-Orogenic Gold Deposits, pp 1–36 (Key Centre for Teaching and Research in Strategic Mineral Deposits, The University of Western Australia: Perth). Shields, J W, White, D A and Ivanac, J F, 1967. Geology of the gold prospects at Union Reefs, Northern Territory, Bureau of Mineral Resources, Geology and Geophysics, Report No 45. Sillitoe, R H, 1991. Intrusion-related gold deposits, in Gold Metallogeny and Exploration (Ed: R P Foster), pp 165–209 (Blackie and Son Ltd: Glasgow). Stuart-Smith, P G, Needham, R S, Page, R W and Wyborn, L A I, 1993. Geology and mineral deposits of the Cullen Mineral Field, Northern Territory, in Australian Geological Survey Organisation, Minerals Land Use Program, 229, p 127 (Australian Government Publishing Service: Canberra). Turner, I R, 1990. Union Reefs gold deposits, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 769–771(The Australasian Institute of Mining and Metallurgy: Melbourne).
Geology of Australian and Papua New Guinean Mineral Deposits
Ormsby, W R, Olzard, K L, Whitworth, D J, Fuller, T A and Orton, J E, 1998. Mount Todd gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 427–432 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Mount Todd gold deposits 1
2
3
3
by W R Ormsby , K L Olzard , D J Whitworth , T A Fuller and J E Orton
3
INTRODUCTION The deposits are 42 km NNW of Katherine, NT, at lat 14o08′S, long 132o06′E and AMG coordinates 187 000 E and 8 435 000 N on the Katherine (SD 53–9) 1:250 000 and the Katherine (5369) 1:100 000 scale map sheets (Fig 1). They comprise the Batman and the smaller Golf and Tollis satellite deposits, and are wholly owned and operated by Pegasus Gold Australia Pty Ltd. Tollis and Golf are approximately 1.5 km and 2.5 km respectively NE of Batman (Fig 2).
FIG 2 - Mount Todd geological map (modified after Poxon and Hein, 1994).
Open cut mining in Phase II will be at a rate of approximately 8 Mtpa. Proved and Probable Reserves for the Batman pit at 31 December 1995 were 97.6 Mt at 1.07 g/t gold. Ongoing exploration of the satellite prospects, particularly at Quigleys deposit (4 km NE of Batman) is aimed at defining further reserves to supplement ore from the Batman pit.
EXPLORATION AND MINING HISTORY
FIG 1 - Location map and regional geological map (after Poxon and Hein, 1994).
Mining commenced at the Batman pit in August 1993 and was carried out at Golf and Tollis in the 1996 dry season. To the end of August 1996, total mine production was over 13 Mt at approximately 1 g/t gold for recovery of 179 200 oz gold. Phase I of the project, which involved open pit mining, processing and heap leaching of oxide and transitional ore, was completed in August 1996. Phase II is scheduled to commence in November 1996 with the commissioning of a mill to process primary ore from the Batman pit, in which gold will be extracted via a four stage crushing, milling, and combined flotation and carbon-inleach process. 1.
Chief Geologist, Mount Todd Gold Mine, PMB 400, Katherine NT 0852.
2.
Senior Mine Geologist, Mount Todd Gold Mine, PMB 400, Katherine NT 0852.
3.
Mine Geologist, Mount Todd Gold Mine, PMB 400, Katherine NT 0852.
Geology of Australian and Papua New Guinean Mineral Deposits
The Mount Todd Goldfield was discovered in 1889 by Walden and Rennett (Kinhill Engineers Pty Ltd, unpublished data, 1989). Small-scale gold and tin mining took place mainly between 1902 and 1914 (K J Kenny, D Gibbs, D Wegmann, F Fuccenencco and N Hungerford, unpublished data, 1990). Gold was mined at Tollis, Quigleys, and Jones Brothers workings; the last is immediately east of Golf and was the largest mine. Incomplete records show production from Jones Brothers workings of about 27 kg of gold from 893 t of ore grading 30.2 g/t (Kruse et al, 1994). Intermittent prospecting, sampling and mining were carried out in later years and the Mines Branch completed five diamond drill holes at Jones Brothers in 1937 (Hossfeld and Nye, 1941). Modern gold exploration began in 1975. A number of companies conducted stream sediment sampling, rock sampling and geological mapping, culminating in diamond drilling at Quigleys. From late 1986 until 1988, Pacific Goldmines NL carried out small-scale open cut gold mining at Quigleys and Golf. Total production was 94 000 t at 3.6 g/t (Poxon and Hein, 1994). In February 1987, the Mount Todd Joint Venture (MTJV) was finalised between Zapopan NL and Billiton Australia Gold Pty Ltd with Billiton the operator. Bulk cyanide leach (BCL) analyses of bulk stream sediment samples in 1987 revealed a strong gold anomaly in the Batman area. Follow up BCL grid
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soil sampling, rock chip sampling and mapping led to percussion drilling at Batman in May 1988, when the discovery of significant gold mineralisation was confirmed. To the end of March 1990, the MTJV had completed 16 340 m of diamond drilling and 28 870 m of reverse circulation (RC) drilling, mainly at Batman but also at Tollis, Golf, Quigleys, and other nearby prospects (K J Kenny, D Gibbs, D Wegmann, F Fuccenencco and N Hungerford, unpublished data, 1990). The MTJV also completed an aerial magnetic survey, several ground electromagnetic (EM) surveys and regional geological mapping. The Batman prospect was found to coincide with strong magnetic and EM anomalies. Zapopan acquired Billiton’s equity in the project in 1992, and to the end of February 1996, had carried out a further 19 000 m of diamond drilling and 55 000 m of RC drilling in the Mount Todd area. In mid 1995, Pegasus Gold Australia Pty Ltd acquired Zapopan NL and became sole owner and operator of the project.
PREVIOUS DESCRIPTIONS Koerber (1989) initially reported on the structural setting of the gold mineralisation at Mount Todd. All work carried out by Billiton for the MTJV is summarised by K J Kenny, D Gibbs, D Wegmann, F Fuccenencco and N Hungerford (unpublished data, 1990). A detailed study of Batman, Golf and Quigleys was completed by Hein (1994) as part of a post doctoral thesis. Previous published papers on the Batman deposit are by Poxon and Hein (1994), Farrelly (1994) and Poxon (1994). The regional geology is summarised by Kruse et al (1994).
km west of Mount Todd (Fig 1). Most of the known gold mineralisation at Mount Todd occurs along a NE-trending zone coincident with a prominent aeromagnetic lineament which can be traced from Batman though the Tollis, Golf, Quigleys and Horseshoe deposits, to Driffield (Figs 1 and 2). Other mineralisation in the region includes tungstenmolybdenum-uranium in greisen in the Yenberrie Leucogranite, numerous tin occurrences, and gold at Driffield.
ORE DEPOSIT FEATURES BATMAN Stratigraphy and lithology The Batman sequence consists of interbedded greywacke, shale and minor tuff and has a true thickness of over 650 m (Fig 3). Four gross upward-fining cycles have been recognised, each commencing with a greywacke-dominant unit which grades through increasing shale interbeds to a shale-dominant unit (inset, Fig 4). Tuffs have been mapped in two of the shaledominant sequences.
REGIONAL GEOLOGY The Mount Todd deposits are within the southern portion of the Palaeoproterozoic Pine Creek Geosyncline in folded and metamorphosed greywacke, shale, siltstone and minor tuff of the Burrell Creek Formation. This formation, with the overlying Tollis Formation (Fig 2), comprises the Finniss River Group (Kruse et al, 1994), which represents the final phase of turbiditic deposition in the Pine Creek Geosyncline. The Palaeoproterozoic sequences are unconformably overlain by Cambrian to Ordovician carbonate and siliciclastic rocks of the Daly River Group SW of Mount Todd. The Finniss River Group has been regionally metamorphosed to lower greenschist facies (Kruse et al, 1994), with local contact metamorphism to hornblende hornfels facies (Hein, 1994). The Cullen Batholith locally comprises the small Yenberrie Leucogranite, approximately 3 km west of Batman, and the Tennysons Leucogranite further to the west. Emplacement of the Cullen Batholith has been dated at 1835 to 1820 Myr (Kruse et al, 1994). The dominant folds in the Mount Todd area have northtrending axes with moderate to steeply west-dipping axial planes (Fig 2). Axial plane cleavage is locally developed, particularly in fold hinges. This folding is referred to as D1 by Hein (1994), and corresponds regionally to D2 according to Nicholson, Ormsby and Farrar (1994) or D3 as described by Johnson (1984). This deformation has been attributed to the Nimbuwah event at about 1870 Myr by Needham, Stuart-Smith and Page (1988). A later D2 event resulted in open easttrending folds (Hein, 1994). The major 330 o trending, Pine Creek shear zone with an apparent sinistral movement of up to 2 km (Nicholson, Ormsby and Farrar, 1994) is approximately 8
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FIG 3 - Composite geological plan at 1146 RL, Batman deposit, showing location of cross section Fig 4.
Sixteen stratigraphic units have been mapped, mainly comprising interbedded greywacke and shale. They are named according to the main rock type, with an identifying number. Individual units range in thickness from 2 m for the T20 tuff unit to 271 m for the main interbedded shale and greywacke unit, SHGW23 (Fig 4).
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT TODD GOLD DEPOSITS
FIG 4 - Cross section on 10 100 m N, Batman deposit, looking north.
A pale to dark grey, medium to coarse and occasionally fine grained massive greywacke is the main rock type. It occurs as beds to 5 m thick and has a characteristic blocky fracture. The greywackes are moderately to poorly sorted and consist of detrital quartz grains and lithic fragments with variable quantities of detrital biotite, potassium feldspar and plagioclase. The lithic fragments are angular to subangular, and range from cherty, quartz-rich and carbonaceous sediment and metasediment to felsic and mafic volcanic rocks. The matrix is often schist, and consists of quartz and sericite with varying amounts of biotite, muscovite, potassium feldspar, chlorite and leucoxene with minor zircon and tourmaline. In X-ray diffraction analyses the greywackes average 78% quartz. The shales have distinctive bedding laminations which occur in groups from several centimetres to approximately 10 cm thick. They are pale to dark grey to green-grey, and normally a darker shade than the adjacent greywacke. Shales are often fissile, except where extensively recrystallised. Three pale grey to off-white tuff marker units have been identified (T18, T20 and T21 in Fig 4). In thin section, the tuffs mainly comprise cryptocrystalline quartz with lesser albite and leucoxene with minor chlorite, sericite, possible carbonaceous material and rare zircon.
Structure The Batman deposit is located on the eastern limb of a south plunging syncline (Fig 2). Bedding consistently strikes NW and dips moderately towards the SW (Fig 3). The mean bedding orientation is 325o strike and 51o dip with local variations between 310o and 340o in strike and 30o and 60o in dip (all structural measurements are with respect to grid north, which is
Geology of Australian and Papua New Guinean Mineral Deposits
magnetic north +3o). All of the beds are conformable, and graded bedding indicates younging towards the SW. Apart from local minor flexures where bedding orientations differ by up to 10o over 20 m, no folding has been recognised. Cleavage is not commonly observed, although Koerber (1989) and Hein (1994) have recorded a fracture cleavage attributed to D2. The cleavage has a mean strike of 319o and dip of 52o SW, with a refraction in the quartz-sulphide veins to 305o strike and 54o dip SW (Hein, 1994). Bedding-parallel faults preferentially occur within thin shale interbeds and are indicated by bedding-parallel shears, closely spaced fractures, fault gouge, wall rock and quartz-sulphide breccias, and quartz-sulphide veins. These faults vary in width from a few millimetres to 20 cm. The fault surfaces are commonly marked by quartz slickensides which have a consistent moderate plunge towards WSW to WNW. A sinistral and reverse sense of movement has been established by Hein (1994) and is consistent with the overall ore geometry. The lack of crosscutting marker features makes the magnitude of individual bedding-parallel movement difficult to establish, but observed offsets are all less than 1 m. Significant beddingparallel faults identified to date are shown in Figs 3 and 4, labelled FNW2 to FNW8. Many more bedding-parallel faults occur, especially within the SHGW23 unit, in which most of the many thin interbedded shales show evidence of faulting along contacts and laminations. Some significantly mineralised faults such as FNW1 are not always conformable with bedding, but vary slightly in strike and are sometimes steeply dipping. Two other minor fault orientations coincide with prominent joint directions and are also closely associated with the gold mineralisation. They
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strike NNE with a mean strike of 018o and dip of 64o to the east, and WNW with a mean strike of 295o and dip of 87o to the north. The NNE fault trend corresponds to the orientation of the main mineralised veins. Hein (1994) inferred normal fault displacement of approximately 3 m across mineralised veins in this set by measurements of vein quartz crystal rotation. Pit mapping and grade control sampling have indicated that the veins in this orientation occur within at least seven zones of increased gold grade and in part, increased vein density. The zones are between 5 and 50 m wide. The intervening areas are well jointed in the same orientation, with lower gold grades and partly reduced vein densities. Faults in this direction both displace and have been offset by bedding plane faults. The WNW-trending faults coincide with zones of intense jointing and have frequently caused minor sinistral displacement of the NNE vein set. These faults are often marked by a thin oxide coating or chlorite, sometimes by breccia and rarely by quartz veins and fault gouge. Three mapped faults of this type are shown as FE1 to FE3 in Fig 3. A set of subhorizontal faults with wavy surfaces and various orientations was identified by M C Bridges (unpublished data, 1989). These displace bedding-parallel and NNE veins, and generally have a northerly strike with a dip of approximately 15 o to the east. Hein (1994) noted other minor normal, reverse and strike slip faults with strike between NW and NE, and also ENE and easterly joint directions.
main body of mineralisation occurs in an envelope which trends at 10o and tapers to the north and south from a central horizontal width of approximately 150 m (Fig 3), and the current mine plan is to develop the open pit over the central 950 m of the deposit. It is open at depth, and overall dips at 75o to the east. Less continuous mineralisation extends for up to 250 m east of the main body.
Vein control Gold mainly occurs within quartz-sulphide veins at Batman. Although four contemporaneous gold-mineralised quartzsulphide vein sets have been recognised (Poxon and Hein, 1994), the dominant type has a mean strike of 018o and dip of 64o east and coincides with a major joint set. Veins in this orientation range from less than 1 to 50 mm thick, with most being between 1 and 10 mm. Individual veins are lenticular with large length to width ratios and mainly form parallel sheets, but can also be anastomosing. Vein spacing varies from a few centimetres to one m, but typically is 10 to 20 cm in the ore zones. The veins pinch out to narrow veinlets along strike and terminate either as joints or against the bedding-parallel or minor cross faults. Other gold mineralised quartz-sulphide vein orientations are: 1.
NNW–NNE strike with steep westerly, vertical, flat easterly and flat westerly dips;
2.
bedding parallel;
3.
ENE strike with southeasterly dip; and
Intrusives
4.
east strike with vertical dip.
Two <2.5 m thick and steeply dipping dykes have been mapped. A NNW-trending lamprophyre dyke in the SW corner of the pit is porphyritic, with olivine variably altered to tremolite and clinopyroxene phenocrysts in a microcrystalline groundmass of biotite, phlogopite, tremolite after clinopyroxene, quartz and feldspar. An approximately northtrending dolerite dyke also occurs in the NW corner of the pit.
The percentage of gangue and sulphide components within individual veins is highly variable. The main vein gangue mineral is quartz, with minor tourmaline, biotite, muscovite and chlorite. Other vein minerals in order of decreasing abundance (Hein, 1994) are pyrite, pyrrhotite, marcasite, chalcopyrite, arsenopyrite±bismuth, bismuthinite, bismuthrich galena, cubanite (CuFe2S3), talnakhite (Cu9Fe8S16), loellingite (FeAs2), pavonite (AgBi3S5) and hedleyite (Bi7Te3). Pyrrhotite is the main sulphide below 100 to 150 m. Towards the surface, pyrrhotite is replaced by pyrite until pyrite is dominant.
Metamorphism The sedimentary sequence in the Batman area has been metamorphosed, producing a quartz-cordierite-muscovitebiotite-chlorite assemblage identified by Hein (1994) as indicative of hornblende hornfels facies. Mapping by Hein (1994) showed a biotite facies isograd on the western side of Batman. Recent petrography and quantitative XRD analyses have confirmed the presence of biotite on the western side of the Batman pit, although biotite has also been found in other locations at Batman and no definitive isograd has been mapped. Other metamorphic minerals identified include sericite, chlorite, microcline, muscovite and adularia.This assemblage combined with the lack of cordierite and the fine-grained granoblastic texture of quartz grains has been interpreted as indicative of local retrogression in the vicinity of the mineralisation (J Taylor, unpublished data, 1990; R N England, unpublished data, 1990).
Mineralisation Distribution The Batman gold mineralisation has been delineated over a strike length of 1500 m and to a depth of 500 m by drilling. The
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In the oxide zone, which generally follows the surface topography to a depth of approximately 50 m, the sulphides are weathered to goethite, supergene marcasite and minor scorodite. Towards the base of the oxide zone, chalcopyrite is partially or wholly altered to chalcocite±covellite±bornite. Gold occurs as inclusions 2.5 to 30 µm in diameter mainly in quartz (R N England, unpublished data, 1990) but in close association with sulphides. An intimate association exists between gold and the bismuth minerals listed above. A broad association has been found between gold and copper-iron sulphide minerals. A significant proportion of gold occurs at the vein margins, and gold within the host rock is rare, consequently gold grade is partly a function of vein density. Alteration commonly extends into the wall rock for one to three times the vein width adjacent to the quartz-sulphide veins (Hein, 1994). Vein alteration envelopes can however be discontinuous or only partially present. The main alteration minerals are fine to extremely fine grained sericite±chlorite±quartz±disseminated sulphides (pyrite± pyrrhotite±arsenopyrite). Often a <1 mm selvage of chlorite occurs along the vein wall. Post gold mineralisation carbonate-
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT TODD GOLD DEPOSITS
sulphide veins mainly comprise calcite and sphalerite, with lesser pyrite, galena, arsenopyrite, chalcopyrite and quartz±pyrrhotite.
Control by rock type The composition and grade of the quartz-sulphide veins is partly influenced by rock type. Individual quartz-sulphide veins at hand specimen and core sample scale are often more sulphide-rich in shale compared with the adjoining greywacke (P Schwann, unpublished data, 1995). Selective sampling has shown a corresponding increase in gold grade in the sulphiderich, shale hosted portion of the vein compared with the same vein in the adjacent greywacke. Control by rock type is also shown, as significant ore occurs to the east of the main orebody preferentially within the shaledominant SH20, SH21 and SH22 units (Figs 3 and 4). Furthermore, many of the ore–waste contacts in that sequence are bedding conformable. There is a correlation between frequency of bedding-parallel faults and higher ore grades and widths. For example, the main part of the orebody occurs within the SHGW23 unit which comprises greywacke with numerous thin shale interbeds and coincident bedding-parallel fault contacts.
Ore boundaries Individual ore blocks, and the boundary of the main orebody, frequently terminate along the dominant vein and bedding plane–fault contacts. The intersection of these two orientations o plunges at approximately 34 towards the south and is consistent with geostatistically determined continuity. Significant bedding-parallel sinistral and reverse fault movement within the SHGW23 unit may account for the observed more northerly strike and steeper dip of the ore envelope in this region compared with the dominant vein orientation. Other influences of lesser importance to the shape of the mineralisation include the WNW-trending, steeply dipping fault and/or joint set.
GOLF The deposit has produced over 800 000 t of oxide ore grading 1.07 g/t from two open pits (Fig 5) and is open at depth. As at Batman, the deposit is within sediment of the Burrell Creek Formation. Rock types are similar to Batman, the main difference being a greater variation in grain size. The greywackes are coarse grained in part, the shales are very well laminated, and a massive fine to very fine grained siltstone has also been mapped. A definitive stratigraphic succession has not been established for Golf due to the unknown displacement by several major faults. The Golf deposit is situated in the core of a faulted, doubly plunging F1 anticline (Jones anticline, Fig 5). Except at the northern and southern ends, the bedding strikes at an average of 006o, and dips at 50o west on the western limb and 70o to 80o east on the eastern limb. The anticline plunges at approximately 30o to the north and south. Strong axial plane cleavage which dips at approximately 50o west is developed in the hinge zone. All structural elements in Golf have been refolded in a sigmoidal shape, with axial planes trending NNE at the extremities and north in the middle.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 5 - Composite geological plan at 1138 RL, Golf deposit, and cross section on 11 800 m N, looking north.
A west-dipping reverse fault zone averaging 20 m in width has been mapped in the centre of the anticline over a strike length of 850 m. The zone is defined by intense shearing, local brecciation, multiple individual fault planes, and an increase in vein density and grade of gold mineralisation. A NW (300o to 330o) trending and steeply SW dipping series of faults with relatively minor sinistral displacements crosscuts the reverse fault zone. A distinct and often intense WNW-striking, steeply dipping joint set occurs throughout the deposit. All of the structures are crosscut by a NNW-striking, steeply east dipping fault at the southern end of Golf, which has an apparent sinistral displacement of 30 m, and is commonly marked by brecciation. Gold mineralisation at Golf extends over a strike length of 850 m and maximum width of 50 m and is found within quartzsulphide veins. It occurs as sheeted to anastomosing veins in the central reverse fault zone, and as bedding-parallel veins on the eastern margin of the main fault zone and on both limbs of the anticline. The sheeted to anastomosing vein set contains most of the gold mineralisation. Individual veins mainly strike at 006o and dip 40o to 70o west, and vary in thickness from 1 to 50 mm. They are interconnected by a stockwork of variously oriented veinlets, 1 to 10 mm thick, and hosted by a massive greywacke in the core of the anticline. The bedding-parallel veins occur as two main types: 1.
Single, thicker quartz-sulphide veins such as Jones lode may be up to 1 m wide and may also be locally discordant to bedding. They occur to the east of and along the eastern margin of the main fault zone.
2.
Numerous 10 mm thick quartz-iron oxide (after sulphide) veins hosted by the laminated shales occur on both limbs of the anticline.
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There is a blue-grey alteration halo to 3 m wide around the larger mineralised quartz veins. Limited thin section work has identified mainly phyllosilicates, especially sericite and chlorite, as the alteration products.
TOLLIS Tollis has produced approximately 36 000 t of oxide ore grading 0.84 g/t to date. It is in a faulted anticlinal area adjacent to Golf (Fig 2). Gold mineralisation occurs within steeply dipping, near north-striking, thin sheeted quartz-sulphide veins. The intersection of normally west-dipping, NNEstriking shale-dominant units with the sheeted vein set locally enhances the gold grade, resulting in poddy areas of higher grade mineralisation.
ORE GENESIS It is postulated that alternating periods of east–west compression directed along a deep seated NE-trending basement fault resulted in a NE-trending wrench fault zone in the overlying sediment (Fig 2). The following structural elements mapped at Batman and Golf are consistent with formation during east–west directed compression and a resultant overall dextral wrench movement: 1.
NW-trending sinistral reverse faults;
2.
the NNE-trending doubly plunging anticline at Golf with axial planar reverse or thrust faulting; and
3.
WNW-trending, steeply dipping joints and faults.
Relaxation of the compression, possibly related to cooling of the Tennyson Leucogranite, facilitated the dilation of preexisting NNE-trending structures including the Golf reverse fault zone and the main joint set at Batman. The following is largely summarised from Hein (1994): 1.
2.
3.
The generation of dilatant structures above the basement structure coupled with a sudden reduction in pressure may have brought about channelling of fluid flow into the upper crust. The metal- and sulphur-enriched mineralising fluid was mainly derived from a mixed magmatic-metamorphic source. Rising fluids decompressed, causing separation of the vapour and fluid phases, concurrent with mineral precipitation. Phase separation and metal deposition accompanied a decrease in temperature from approximately 380 to 240oC.
4.
Gold precipitation occurred late in the development of the silicate-sulphide-carbonate assemblage from a hot (~250oC), acidic, hypersaline brine and was accompanied by precipitation of bismuth.
5.
Liquid bismuth may have scavenged gold and increased its concentration in the final precipitate (R N England, unpublished data, 1990).
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There are several possible explanations for the increase in sulphide content and gold grade within shale-hosted veins. P Schwann (unpublished data, 1995) suggested that the mineralising fluid reacted with iron-bearing minerals in the shales such as chlorite or amphibole or magnetite. Methane production in shales may have brought about sulphide, bismuth and gold precipitation, and Hein (1994) noted that gold and bismuth grains occurred with methane-rich fluid inclusions.
ACKNOWLEDGEMENTS The authors would like to thank Pegasus Gold Australia Pty Ltd for its support and permission to publish. K Hein and N Burn kindly reviewed the paper and provided assistance during its preparation. The paper has also benefited from discussions with J Goulevitch, P Schwann, A C Purvis and I R Pontifex. Cartographic and Survey Drafting Services drafted the illustrations.
REFERENCES Farrelly, C T, 1994. The mining geology and grade control practices of the Batman deposit, Mt Todd project, in Proceedings 1994 AusIMM Annual Conference, pp 103–108 (The Australasian Institute of Mining and Metallurgy: Melbourne). Hein, K A A, 1994. The structure and geochemistry of gold mineralisation in the Mt Todd goldfield, Pine Creek Inlier, Northern Territory, PhD thesis (unpublished), University of Tasmania, Hobart. Hossfeld, P S and Nye, P B, 1941. Second report on the Mount Todd auriferous area, Pine Creek district, Aerial Geological and Geophysical Survey of Northern Australia, Northern Territory Report 31. Johnson, J D, 1984. Structural evolution of the Pine Creek Inlier and mineralisation therein, Northern Territory, Australia, PhD thesis (unpublished), Monash University, Melbourne. Koerber, D, 1989. The structural setting of gold mineralisation at Mt Todd, Northern Territory, BSc (Honours) thesis (unpublished), University of New South Wales, Sydney. Kruse, P D, Sweet, I P, Stuart-Smith, P G, Wygralak, A S, Pieters, P E and Crick, I H, 1994. Katherine, Northern Territory - 1:250 000 geological series, Northern Territory Geological Survey Explanatory Notes SD 53–9. Needham, R S, Stuart-Smith, P G and Page, R W, 1988. Tectonic evolution of the Pine Creek Inlier, Northern Territory, Precambrian Research, 40/41:543–564. Nicholson, P M, Ormsby, W R and Farrar, L, 1994. A review of the structure and stratigraphy of the central Pine Creek Geosyncline, in Proceedings 1994 AusIMM Annual Conference, pp 1–10 (The Australasian Institute of Mining and Metallurgy: Melbourne). Poxon, R F, 1994. Exploration and development history of the Batman gold deposit, Mt Todd project, Northern Territory, in Proceedings Fourth Large Open Pit Mining Conference, pp 65–73 (The Australasian Institute of Mining and Metallurgy:Melbourne). Poxon, R F and Hein, K A A, 1994. The geology and gold mineralisation of the Batman Deposit, Mt Todd project, NT, in Proceedings 1994 AusIMM Annual Conference, pp 29–36 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Geology of Australian and Papua New Guinean Mineral Deposits
Morrison, I J and Treacy, J A, 1998. Gold Creek gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 433–438 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Gold Creek gold deposit 1
by I J Morrison and J A Treacy
2
INTRODUCTION The deposit is 20 km east of Katherine in the southernmost part of the exposed Pine Creek Inlier in the Northern Territory, at lat 14o26′S, long 132o26′E on the Katherine (SD 53–9) 1:250 000 and the Katherine (5369) 1:100 000 scale map sheets (Fig 1). It is wholly owned by Kalmet Resources NL (Kalmet). The total Measured, Indicated and Inferred Resource at Gold Creek is 4.3 Mt at 3 g/t gold (415 000 oz of contained gold) which includes a Measured Resource, within an open pit design, of 681 000 t at 7.03 g/t. The deposit remains open at depth, along strike, down dip and down plunge.
EXPLORATION HISTORY The historic Maud Creek Goldfield is approximately 1 km east of the deposit and was the scene of prospecting and minor gold production at the start of the 20th century. The tenements enclosing the Gold Creek deposit were explored by the Minerals Exploration and Development Group of CSR Ltd (MEDG) in 1985. Their bulk stream sediment sampling program produced a large gold anomaly coincident with the dolerite-hosted deposits of the Maud Creek Goldfield, and a sample from a creek 1 km to the west of the goldfield returned a result of 1.3 ppb cyanide soluble gold. Placer Exploration Ltd (Placer) purchased MEDG in 1987, and followed up the 1.3 ppb anomaly. They recognised that the creek drained a quartz-hematite breccia which subcrops over roughly 1 km of strike along the northerly-trending Gold Creek fault. Rock chip sampling of the breccia returned highly anomalous results, to 10 g/t gold. Subsequent drilling of the southern end of the breccia resulted in the discovery of the deposit. Placer decided to dispose of the Gold Creek deposit in 1992, which then contained a resource of 1 Mt at 4 g/t gold or 129 000 contained oz. In December 1992, Placer granted a five year option to Kalmet to purchase their Maud Creek tenements, including the Gold Creek deposit (Kalmet Resources, 1993), and the option was exercised in 1994 (Kalmet Resources, 1994). Exploration by Kalmet in 1994 increased the resource to 180 000 contained oz and also determined that the ore was refractory. In mid 1995, Kalmet commenced a major exploration program aimed at increasing the resource and overcoming the metallurgical problems. The current resource is 415 000 contained oz and metallurgical test work indicates that bacterial oxidation of a flotation concentrate, in association with gravity concentration, results in a carbon-in-leach 1.
Consultant Geologist, Lantana Exploration Pty Ltd, Box 5046 Mail Centre, Townsville Qld 4810.
2.
Executive Director, Kalmet Resources NL, 84 Dyson Street, Kensington WA 6151.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Regional geology and location plan for the Gold Creek area, after Needham et al, (1989).
recovery of over 90% of the gold. Drilling by Kalmet to the end of 1996 comprised 287 reverse circulation holes for 28 156 m and 33 diamond core holes for 2379 m.
REGIONAL GEOLOGY The Gold Creek gold deposit is within the Pine Creek Inlier, which has a Late Archaean granitic basement overlain by Early to Middle Proterozoic sedimentary and volcanic sequences. Volcanic activity is believed to be related to rifting, and volcanic rocks commonly occur beneath unconformities (Needham and De Ross, 1990).
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The deposit is in the Tollis Formation, the uppermost member of the Early Proterozoic El Sherana Group. The formation, which is up to 1700 m thick, consists predominantly of sediment, with interbedded intermediate and mafic volcanic rocks, including the 200 m thick Dorothy Volcanic Member which comprises basaltic lava, tuff and sills (Kruse et al, 1994). Metamorphic grade is low (Stuart-Smith and Needham, 1985). At Gold Creek the Tollis Formation is represented by lithic sandstone and siltstone to the west of the Gold Creek fault and mafic tuff to the east. The mafic tuff probably belongs to the Dorothy Volcanic Member (Figs 1 and 2).
the east, and less intensely altered and deformed footwall sediments to the west. The exposed part of the Gold Creek fault extends for about 2 km to the north of the Gold Creek orebody, but further to the north it is covered by Middle Proterozoic Kombolgie Formation sediment. The southerly extent is obscured by onlapping Cambrian basalt flows of the Antrim Plateau Volcanics which also form extensive cover on the footwall sediments to the west (Fig 2). In the immediate orebody environs, the highly deformed hanging wall mafic tuff is bound to the east by the relatively undeformed Maud Dolerite which lies about 250 m to the east of the Gold Creek fault (Fig 2). Mafic dykes are a relatively minor component of the host sequence. They trend subparallel to the Gold Creek fault and locally cut the orebody. The dykes are only very locally brecciated by late stage postmineralisation faults. Preliminary investigations have revealed sporadic mineralisation and associated quartz veining and brecciation along the exposed strike length of the Gold Creek fault, and to the south under basalt cover. Oriented drill core reveals that the intense foliation in the hanging wall mafic tuff generally dips moderately to steeply to the west and is overturned into a steep easterly dipping orientation within a few metres of the fault-bound lode (Fig 3). This rotation is consistent with a significant component of steep reverse movement along the Gold Creek fault. Given the foliated nature of pervasive alteration phyllosilicates in the hanging wall mafic tuff, there appears to be a temporal link between the ductile deformation, evidenced by the intense foliation, later brittle failure along the Gold Creek fault, and mineralisation
HOST ROCKS Hanging wall mafic tuff The mafic tuff is generally massive or indistinctly thickly bedded and clast rich, with lapilli-sized clasts in a fine ash(?)
FIG 2 - Geological plan of the Gold Creek deposit.
ORE DEPOSIT FEATURES GEOLOGICAL SETTING Ore grade gold mineralisation and the associated intense quartz veining are largely concentrated within a discrete lode, referred to as the contact mineralised zone (CMZ), at the southern end of the exposed extent of the Gold Creek fault (Fig 2). This fault forms a steep easterly-dipping, north-trending faulted contact between highly altered and deformed hanging wall mafic tuff to
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FIG 3 - Diagrammatic cross sectional relation between dominant foliation and the steep reverse Gold Creek Fault.
Geology of Australian and Papua New Guinean Mineral Deposits
GOLD CREEK GOLD DEPOSIT
matrix. Rare, laminated to thinly bedded siltstone intervals are generally less than 10 cm thick, and may represent primary ash beds or the localised reworking and deposition of a fine volcanic fraction. Undeformed clasts are typically subangular, sparsely to densely finely amygdaloidal and weakly to moderately porphyritic, with highly altered pyroxene(?) phenocrysts to 5 mm long. Clasts become progressively elongate parallel to the dominant foliation with increasing intensity of ductile deformation, and in highly deformed domains it is difficult to differentiate between primary and tectonic fragmental textures. The tuff has a high content of nickel, typically 500 to 700 ppm, and of chromium, typically 250 to 400 ppm. The tuff in the corridor between the CMZ and the Maud Dolerite is highly altered, with extensive pervasive chloritecarbonate dominant alteration and stockwork carbonate-quartz veins. This is overprinted by pervasive silica-sericite-fuchsitegraphite alteration and stockwork quartz veins, particularly approaching the upper CMZ contact and in tuff-hosted massive and stockwork quartz vein domains within the CMZ. Shear and fracture controlled intense graphite and chlorite alteration in the immediate CMZ environs post-dates the silica-sericite alteration.
ALTERATION Pervasive and vein-style alteration and associated mineralisation in the hanging wall mafic tuff are more intense and apparently more extensive than alteration of the footwall sediment and mafic dykes. The probable paragenetic sequence of alteration of the mafic tuff comprises: 1.
An extensive outer mid to dark green-grey zone of variably intense pervasive chlorite-carbonate-sericite alteration with commonly up to 10% veins of finely intergrown carbonate-quartz, and generally only traces of fine grained pyrite. The phyllosilicates are foliated, and veins are generally deformed, but locally crosscut the foliation, consistent with late syn- to post-ductile deformation alteration and veining. Localised pale greenish zones of silica-sericite alteration and associated stockwork quartz veins appear to post-date the more chloritic alteration. These zones contain elevated sulphides and gold grades and are probably temporally related to the silica-sericite phase of alteration discussed below.
2.
An inner pale green to yellow zone of variable silicasericite-carbonate-fuchsite-graphite alteration with associated quartz veins (commonly 10–20% veins), typically contains 1–3% combined pyrite, arsenopyrite and gersdorffite (R N England, personal communication, 1997), and highly anomalous gold grades. This zone commonly extends up to 20 m above the CMZ contact and is also evident in tuff-hosted domains in the CMZ. Quartz vein intensity, graphite alteration and silicification typically increase approaching the CMZ contact. Quartz veins typically crosscut the foliation and silicification appears to be texturally destructive. Graphite occurs as fine concentrations along foliation seams, as a fill phase in narrow breccia bands that are generally parallel to, but post-date the foliation, and as a minor component in some quartz veins. Sulphides occur as fine disseminations, patchy concentrations, and veinlets that clearly post-date the dominant foliation. Highly anomalous gold commonly forms continuous zones of hanging wall ore grade mineralisation that are locally gradational to CMZ mineralisation, particularly in the top 100 m of the deposit.
3.
Highly graphitic, chloritic and variably pyritic fractures or veinlets, and narrow shear zones to several centimetres thick, are relatively common for several metres above the CMZ contact. These zones crosscut quartz veins associated with the silica-sericite phase of alteration described above. Rarely, this graphite-chlorite phase of alteration is pervasive over several tens of centimetres. This alteration is commonly the dominant phase in the CMZ and immediate footwall environs.
4.
Localised zones of late-stage brown carbonate flooding and veining, with associated veinlets and disseminations of sphalerite, galena, chalcopyrite and specular hematite, occur sporadically within several tens of metres above the CMZ contact and very locally overprint CMZ mineralisation. These zones are generally 1–5 m wide and contain highly elevated silver values, commonly >50 ppm; limited analyses have revealed anomalous tin, locally to 1000 ppm. Patchy gold values in these zones may indicate localised dissolution and reprecipitation.
Footwall sediments The following observations are based on information from drilling that generally only extends 10–20 m into the footwall sequence. The footwall sequence dominantly comprises moderately and thickly bedded fine to coarse grained and gritty, poorly sorted lithic sandstone, and generally lesser laminated to thinly bedded mudstone and siltstone. Graded bedding is most common in the siltstone and fine grained sandstone units. Abundant quartz and feldspar grains and rare to abundant glass shards indicate a felsic volcanic provenance (R N England, personal communication, 1997). Sandstone is generally only weakly foliated, usually expressed as a fine fracture cleavage. In contrast, the finer sediments are generally highly foliated and fissile. This dominant foliation is generally steeply east-dipping and bedding-parallel, and forms an axial plane foliation to uncommon, tight to isoclinal, moderately south-plunging folds. Pervasive moderate to rarely intense silica-sericite alteration in the footwall, and within sediment-hosted domains within the CMZ, is overprinted by apparently shear and fracture controlled graphite and chlorite alteration within and immediately adjacent to the CMZ.
Mafic dykes Fine grained mafic or intermediate dykes are a relatively minor component of the host sequence. However, they comprise a significant part of the immediate footwall to the deposit in the top 100 m, and locally cut the CMZ. The dykes are several centimetres to several metres thick and trend subparallel to the Gold Creek fault, but dip more steeply to the east. Deep drilling does not extend far enough into the footwall to intersect the dykes at depth. Characteristically the dykes are relatively undeformed and clearly crosscut the intense alteration and deformation fabrics evident in the CMZ and host rocks. Very localised brecciation of the dykes is consistent with very late syntectonic intrusion. In the immediate CMZ environs the dykes are moderately to highly sericitised and weakly mineralised.
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Pervasive and fracture or shear controlled graphite and chlorite alteration of the footwall sediments, commonly with up to 5% disseminated and veinlet pyrite and arsenopyrite and generally highly anomalous gold grades, is largely restricted to the CMZ and immediate footwall environs. This alteration post-dates the pervasive sericite-silica alteration that extends to the limits of drill testing. Quartz vein intensity decreases abruptly, or rapidly over a few metres, beyond the CMZ contact and, through the bulk of the drill-tested footwall sequence, veining is typically weak and <2% by volume. Very localised zones of moderate to intense quartz veining in the footwall are generally brecciated, graphitic and chloritic, and contain elevated pyrite, arsenopyrite, and gold, locally to ore grade. In the immediate orebody environs, mafic dykes are highly sericitised and generally contain up to 2% finely disseminated pyrite and rare arsenopyrite, but are only very weakly mineralised in gold. Away from the orebody, dykes are generally only weakly or moderately chloritised and contain minor disseminated pyrite. Very minor white carbonate±sulphide veinlets represent the last identifiable alteration event throughout the host rocks and the CMZ. Rare gold in these late veinlets within the CMZ is probably the result of remobilisation.
MINERALISATION The bulk of the Gold Creek orebody is contained within the CMZ, which straddles the faulted contact between hanging wall mafic tuff and footwall sediment. Detailed drilling has shown that the orebody is moderately to steeply east-dipping, steeply south-plunging, generally 10 to 20 m thick and from 150 to 200 m long (Figs 4 and 5). In the top 50 m, over the central portion of the deposit, the orebody and host sequence are overturned and have a very steep westerly dip; this is tentatively ascribed to post-mineralisation drag folding associated with faulting at a low(?) angle to the Gold Creek fault. The orebody remains open at depth below RL -175 m or 300 m below surface and is open along strike below RL -100 m or 230 m below surface (Figs 4 and 5). South of approximately 9200 m N the host sequence and orebody appear to be offset to the west along a shallowly southdipping fault (Figs 4 and 5). This interpretation remains to be tested.
Primary ore The CMZ is characterised by multiple phases of stockwork (5–50% quartz) and massive (>50% quartz) veining, silica flooding, brecciation, commonly intense graphite and/or chlorite alteration, and roughly 5% total sulphides. Veining and mineralisation are hosted by highly altered hanging wall tuff, footwall sediments, brecciated mixed rock types and rock flour. Graphitic and sulphidic fractures and stylolites are common in the more massive quartz domains, and post-quartz deposition shears are characterised by brecciated to finely ground quartz in highly graphitic rock flour matrix. Figures 6 to 8 illustrate common ore textures. Pyrite and arsenopyrite, with gersdorffite in tuff-hosted domains, are the common sulphide phases and generally comprise roughly 5% by volume of the ore zone, although uncommon intervals of several tens of centimetres contain up to 50% pyrite. Sulphides occur as fine- to medium-grained disseminations, patchy concentrations, and thin veinlets that typically crosscut all tectonic fabrics. Significantly, the
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FIG 4 - Simplified cross sections, looking north, Gold Creek deposit.
sulphides are generally undeformed. In particular, coarse bladed arsenopyrite in intensely sheared graphitic zones is clearly post-deformation. Given the strong geochemical association between gold and arsenic, it is reasonably inferred that mineralisation substantially post-dates the most intense phases of deformation and quartz veining. Within the primary ore zone arsenic is highly anomalous, with values typically from 500 ppm to 2%, and it is the only element with an obvious close correlation with gold values. Copper is the only base metal that is consistently moderately anomalous in the orebody with values generally from 50 to 300 ppm and rare spikes to around 0.5%. The high values show no direct correlation with gold. With typical ranges shown in brackets, lead (10–100 ppm), zinc (10–50 ppm), antimony (10–50 ppm) and bismuth (<2 ppm) all display very local spikes, but show no correlation with gold values. Silver grades generally range from 1 to 15 ppm and show a crude correlation with elevated copper. In intervals with a significant mafic tuff component, nickel and chromium reflect the high values found throughout the hanging wall mafic unit. The host fault zone varies in character from a simple faulted juxtaposition of hanging wall and footwall rock types, to complex tectonically interleaved slices of hanging wall and footwall rocks. In the complex zones, crosscutting relationships reveal a protracted history of pre-, syn- and postmineralisation faulting. Highly graphitic and/or chloritic
Geology of Australian and Papua New Guinean Mineral Deposits
GOLD CREEK GOLD DEPOSIT
base of the zone is highly oxidised. Specular hematite is common along fine fractures. Significantly, visible flecks of secondary gold are fairly common within calcite veinlets and, although spectacular gold grades occur, detailed sampling suggests a degree of secondary remobilisation and possible gold depletion in the top 5 to 10 m of the deposit. This has not been quantified. The contact between the highly oxidised and the moderately oxidised transition zone ore generally occurs over several tens of centimetres. The transition zone is characterised by less intense and more patchy iron oxide staining and fracture coatings, the appearance of significant graphite, the disappearance of calcite over 1–2 m at the top of the zone and, in the tuff, the coincident emergence of disseminated and vein carbonate. Alteration phyllosilicates are generally only weakly weathered. Pyrite is typically only weakly oxidised and arsenical sulphides are rare. Localised zones of both highly oxidised and relatively fresh ore are common. There is no obvious enrichment or depletion of gold in the transition zone. The contact between transition ore and weakly oxidised to fresh ore is generally abrupt. Weak oxidation can occur sporadically for over 100 m below surface and is characterised by relatively minor iron oxide coatings along fractures, with very localised iron oxide staining restricted to fracture selvages.
MINERALISATION CONTROLS AND TIMING FIG 5 - Longitudinal projection, looking west, showing gram.metre contours, Gold Creek deposit.
Several key relationships and textures help to constrain the timing of, and controls on, mineralisation. 1.
Mineralisation is substantially concentrated within a steeply plunging dilation zone along the brittle Gold Creek fault, referred to as the CMZ. Localisation of the fault may be due to rheology contrasts between relatively undeformed footwall sediment to the west, and highly deformed hanging wall mafic tuff to the east. Rotation of the dominant foliation in the host rocks into the brittle fault indicates a strong component of steep reverse movement (Fig 3).
2.
The earliest phase of pervasive chlorite-carbonate alteration is foliated, and associated carbonate-quartz veining is mostly deformed, indicating that the hydrothermal process probably commenced during the latest stages of ductile deformation. This also suggests that brittle failure along the Gold Creek fault may represent the final stages of a protracted ductile-brittle deformation event.
3.
Significant gold mineralisation and associated pyrite and arsenical sulphides are associated with an early phase of quartz veining and silica-sericite-fuchsite-graphite alteration in the hanging wall tuff. This style of mineralisation is typical of the upper parts of the CMZ (Fig 6).
4.
The early phase of quartz veining crosscuts the dominant foliation but is commonly brecciated and silica flooded (Fig 7), particularly in the more massive quartz domains in the CMZ. The intensely silicified rock commonly contains finely disseminated sulphides, indicating a second phase of mineralisation.
5.
In more graphitic or chloritic domains, deformation produces highly fragmented quartz vein clasts in an intensely foliated graphitic or chloritic matrix. Undeformed fine to coarse grained pyrite and
domains are typically intensely sheared and contain rounded quartz clasts enclosed in a fissile graphitic-chloritic rock flour. Zones of massive quartz veining are commonly brittly brecciated and silica flooded. Both top and bottom contacts vary from abrupt, typified by post–veining and mineralisation faulting, to gradational, with ore grades extending for several tens of centimetres to several metres above and below the zone of intense quartz veining, graphitic alteration and brecciation that characterises the CMZ. Hanging wall ore, associated with stockwork quartz veins (generally 5–20% quartz), comprises roughly 20% of the measured open pit resource and, although generally forming discrete separate zones above the CMZ, is locally contiguous with the CMZ mineralisation. Ore grade mineralisation in the footwall sediment is typically sporadic and insignificant, and is generally associated with narrow zones of quartz veining, brecciation, and graphite and/or chlorite alteration.
Oxidised ore The orebody is typically highly oxidised to roughly 15 to 20 m below surface, then moderately oxidised (transition ore) to 25 to 30 m depth, passing abruptly to very weakly oxidised to fresh ore. Highly oxidised ore is characterised by intense pervasive iron oxide staining and coatings along fractures, partial to complete weathering of silicate minerals to clays, and the presence of abundant pervasive and veinlet iron oxide–stained calcite. Significant calcite is restricted to this highly oxidised domain and is possibly the product of leaching of magnesium and iron from primary carbonates and oxidation of graphite. Graphite is generally absent, and very rare relict pyrite near the
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FIG 6 - Core from drill hole MD29 from 311.55 m depth, grade 18.4 g/t gold, showing tuff-hosted massive quartz veining (>50% quartz) typical of the upper part of the CMZ.
FIG 8 - Core from drill hole MD28 from 181.1 m depth, grade 2.6 g/t gold, showing stockwork quartz veins in highly chlorite- and weakly graphite-altered sediment from the lower part of the CMZ. Crosscutting relationships reveal three main phases of quartz veining.
contributed to the hydrothermal fluids responsible for mineralisation. Minor mineralisation in the Maud Dolerite may be the product of remobilisation along fractures and shears that largely or wholly post-date the Gold Creek gold deposit.
ACKNOWLEDGEMENTS
FIG 7 - Core from drill hole MD24 from 265.9 m depth, grade 6.07 g/t gold, showing brecciated and silica-flooded massive quartz (>50% quartz) with late crosscutting graphite-sulphide stylolites and veinlets from within the CMZ.
arsenopyrite euhedra in the intensely foliated matrix clearly post-date this phase of deformation. These sulphides indicate another textural phase of mineralisation that may be temporally related to mineralisation associated with the intense silica flooding. 6.
7.
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The authors gratefully acknowledge the permission of Kalmet Resources NL to publish this information. Thanks are extended to M Forbes, previous Director of Exploration for Kalmet, for his early work on the assessment of the deposit, and B Smith of Rock Search Australia Pty Ltd for her contribution to the detailed evaluation of the deposit. R N England (consultant petrologist) and K Camuti of Lantana Exploration Pty Ltd gave their time freely for valuable discussions. Roger Merrison and Associates drafted the figures and Through The Looking Glass Photographics photographed the rocks.
REFERENCES Kalmet Resources NL, 1993. Annual Report (Kalmet Resources NL: Sydney).
Sulphides, graphite and chlorite are variously concentrated along fine late fractures or in veinlets, shears and stylolites (Fig 7) and appear to represent the last main phase of gold mineralisation, although these may be the product of remobilisation.
Kalmet Resources NL, 1994. Annual Report (Kalmet Resources NL: Sydney).
Mafic dykes locally intrude the CMZ and crosscut all major deformation fabrics and quartz vein phases. However, the dykes are moderately to highly altered and commonly contain several per cent of fine grained pyrite, rare arsenopyrite and very low gold grades. Also, very locally, they are tectonically brecciated. This is consistent with intrusion during the latest stages of deformation and fluid movement, although mineralisation in the dykes may be the product of remobilisation. It is speculated that the dykes are genetically related to the Maud Dolerite, a substantial intrusive body about 250 m east of the deposit. Prior to intrusion, the parent magma may have provided the necessary heat to generate fluid flow, and possibly
Needham, R S and De Ross, G J, 1990. Pine Creek Inlier - regional geology and mineralisation, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 727–737 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Kruse, P D, Sweet, I P, Stuart-Smith, P G, Wygralak, P E, Pieters, P E and Crick, I H, 1994. Katherine, Northern Territory - 1:250 000 geological series, NT Geological Survey, Explanatory Notes SD 53–9.
Needham, R S, Stuart-Smith, P G, Bagas, L and others, 1989. Geology of the Edith River region (1:100 000 scale map), Bureau of Mineral Resources, Canberra. Stuart-Smith, P G and Needham, R S, 1985. Pine Creek Geosyncline field guide for the tectonics and geochemistry of the Early to Middle Proterozoic fold belts, Darwin, Australia, August 7–14, 1985, Bureau of Mineral Resources, Geology and Geophysics, Record, 1985/26 (unpublished).
Geology of Australian and Papua New Guinean Mineral Deposits
Bosel, C A and Caia, G P, 1998. White Devil gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 439–442 (The Australasian Institute of Mining and Metallurgy: Melbourne).
White Devil gold deposit 1
by C A Bosel and G P Caia
2
INTRODUCTION The deposit is within the Tennant Creek, NT, gold-copperbismuth-(cobalt) mineral province at lat 19o29′S, long 133o54′E on the Tennant Creek (SE 53–14) 1:250 000 scale and Short Range (5659) 1:100 000 scale map sheets, 50 km NW of Tennant Creek township (Fig 1). The mine is owned by Normandy Gold Ltd and has been the main gold producer at their Tennant Creek operations since the mid 1980s.
purchase the leases, after encouraging drill intersections were made. Shaft sinking commenced in 1986 to access the narrow high grade upper levels of the Main zone orebodies. Subsequent discoveries led to the mining of the West lode open cut, which produced 28 660 t at 16.9 g/t gold, and from its base a decline was started in April 1988. Longhole stoping then gradually replaced benching and rill stope mining methods. The Pinter mineralisation (named after a miner, Les Pinter) was first intersected in late 1986 in drill hole WDRC-058, as 3 m at 14.5 g/t gold from 145 m. In March 1990, hole WDDD391 intersected 9 m at 43.0 g/t gold near 750 RL, leading to the decision to crosscut from the decline through the Eastern porphyry. To the east, Pinter B lenses merge into the Navsix breccia-style ore lenses, named after Peko’s old ‘Navigator Six’ designation of the aeromagnetic anomaly near this area. Subsequent ore delineation drilling showed that the Pinter–Navsix system contained some 12.44 t (400 000 oz) of gold. To date over 1180 reverse circulation or diamond drill holes have been drilled in the mine leases, a number justified by the high grade of the many ore lenses.
PREVIOUS DESCRIPTIONS
FIG 1 - Locality map, White Devil mine.
Ore production during the 12 months to 31 March 1996 was 162 600 t at 15.0 g/t gold, for 2.3 t (74 000 oz) of gold recovered. Total ore production to date is 980 000 t at 17.8 g/t gold for 17.14 t (551 200 oz) of gold recovered. The remaining Proved and Probable Reserve is 478 200 t at 11.6 g/t gold, equivalent to 5.53 t or 177 900 oz of contained gold. White Devil to date is the fourth largest gold producer in the Tennant Creek mineral province, after Warrego, Nobles Nob and Juno. Since 1992, most production has been from the eastern Pinter and Navsix ore lenses, the focus of this paper.
EXPLORATION AND MINING HISTORY The surface expression of the White Devil–Black Angel mineralisation was first pegged by prospectors in 1934 and worked intermittently until Peko-Wallsend Operations Ltd (Peko) acquired the leases in 1969 in order to search for copper. In 1986 Australian Development Ltd exercised an option to
1.
Project Geologist, Normandy NFM, The Granites Gold Mine, former Senior Mine Geologist at White Deveil Mine.
2.
Senior Mine Geologist, White Devil Mine, PO Box 294, Tennant Creek NT 0861.
Geology of Australian and Papua New Guinean Mineral Deposits
Honours theses by Nguyen (1987) and Edwards (1987) preceded a Masters thesis by Cozens (1992). Published studies include those by Ivanac (1954), Nguyen et al (1989), Edwards, Booth and Cozens (1990), Huston (1990), Huston, Bolger and Cozens (1993), Khin Zaw et al (1994), Huston and Cozens (1994) and Smith and Hall (1995).
REGIONAL GEOLOGY The deposit occurs within a large iron-magnesium-silica alteration complex (‘ironstones’), hosted by turbiditic sediments of the Palaeoproterozoic Warramunga Formation defined by Donnellan, Hussey and Morrison (1995). The Warramunga Formation is composed of alternating beds of siltstone and greywacke. These are metamorphosed to lower greenschist facies and exhibit a regional axial plane slatey cleavage (S1), striking east-west. A second semi-ductile to brittle deformation phase, of reverse dip-slip shearing, produces a S2 fabric, similar to S 1 in orientation (C Hy, unpublished data, 1988). In the mine area, this second phase generated a set of east-trending reverse shear zones, lifting the bedding (S0) and the cleavage (S1) subvertically. The shear zones, previously interpreted as an open anticline, are a splay of the Mary-Lane regional lineament and form a duplex structure. The alteration complex over 1 km in length and at least 0.6 km identified in depth, was developed in the duplex structure. Lensoidal gold orebodies are found within the ironstones, with the mineralising fluids access proceeding through the shearzones, into the pre-existing alteration complex.
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A set of NW-trending subvertical quartz-feldspar porphyry dykes cuts through the mine area, truncating and sinistrally offsetting several ore lenses (Fig 2). There has been some debate on the timing and significance of the porphyries (C Hy, unpublished data, 1988; Nguyen et al, 1989; Huston and Cozens, 1994). Current knowledge favours the view of C Hy that the porphyries postdate ironstone formation and goldcopper-bismuth mineralisation, and that they developed foliation and retrograde contacts during a later D3 strike-slip deformation. The longitudinal projection of the orebodies (Fig 3) bears this out in the ‘jigsaw’ appearance of the barren zones either side of the central porphyry dyke. Occasional thin lamprophyre dykes also crosscut the sediment and porphyries, and dip NW at shallow angles.
ORE DEPOSIT FEATURES
Gold and subeconomic copper and bismuth sulphide mineralisation is contained within the alteration complex. The gold is free milling, generally occurring as particles within magnetite grains or chlorite laths and often intergrown with chalcopyrite and bismuth sulphosalts. Cozens (1992) showed that there is no strong vertical metal zoning at White Devil, as is found at the Juno mine. Gangue minerals include magnetite, hematite, magnesian chlorite, iron chlorite, pyrite, siderite, talc, serpentine, quartz, sericite and dolomite. In the West and Main zones (Fig 4) the alteration complex dips steeply north and is characterised by magnetite-hematitequartz-chlorite ironstone, chlorite-magnetite stringer material, and chlorite schist (‘altered sediment’) units. Here, Edwards, Booth and Cozens (1990) described two main distinct styles of mineralisation, Main zone and Deeps. The ore lenses are generally within the chlorite-magnetite stringer unit and along the footwall margins of the ironstones.
The White Devil orebody comprises over 60 subvertical ore lenses, each defined by a 3.3 g/t gold cutoff over a minimum 2 m mining width. Typical dimensions are 60 m along strike by 50 m high by 3 m wide. In plan view (Fig 2), the composite body extends over a 650 m easterly strike length and is up to 80 m wide. Lenses are generally of a tabular sigmoidal shape and may merge into each other along strike or at depth. In longitudinal projection the mineralisation plunges east at 20o and extends to 350 m below surface (Fig 3). Significant gold intersections have been obtained from as deep as 430 m below surface, but they are patchy. The resource is divided by three major porphyry dykes into the Black Angel–West Porphyry, West, Main and Pinter–Navigator Six (shortened to Pinav and also referred to as the eastern lenses) ore zones.
FIG 2 - Simplified surface geology of White Devil mine showing the plan projection of the orebody and position of the cross sections.
FIG 4 - Cross section at 5155 E through the Main zone, looking WSW (A–A′ in Fig 2).
FIG 3 - Simplified longitudinal projection of the White Devil deposit, looking north.
440
In the Pinav zone (Fig 5) the alteration complex splays out at depth into three separate ‘root’ zones, with two subvertical zones largely comprising Navsix style breccias, and the major talc-dolomite rich Pinter C shear dipping moderately to the north. They coalesce at around 800 RL into the broad high grade Pinter C mineralisation. Ore from the Pinav zone may be of Deeps or Main zone style, or a unique ‘Navsix hydraulic breccia’ style. Early ore delineation work in 1991 suggested the presence of seven lenses, which were named Pinter A to F.
Geology of Australian and Papua New Guinean Mineral Deposits
WHITE DEVIL GOLD DEPOSIT
Today, the numerous eastern lenses are individually numbered and grouped into either the Pinter B or Pinter C group, based on their relative position and orientation, or Navsix group based on ore style. The main characteristics of these groups are as follows: 1.
Pinter B lenses are the eastern strike extension of the Main zone mineralisation style. Vertical to steep southerly dips predominate and the ore lenses are within the chlorite-pyrite-magnetite stringer halo of the southernmost extensive thin Pinter B ironstone, the ‘southern tabular ironstone’ of Cozens (1992), generally on the northern footwall side. Gold distribution can be very nuggetty, locally assaying to 5500 g/t over a 1 m length of drill core.
2.
Pinter C lenses, in contrast, appear to be the eastern strike extension of the Deeps mineralisation style (Edwards, Booth and Cozens, 1990), and contain similar small high grade bismuthinite concentrations and talc, but with considerably less magnetite-hematite and more pyrite. In hand specimen the ore typically has an alteration assemblage of pyrite and/or magnetite blebs and stringers within a chlorite, talc or talc-serpentine matrix. The lenses dip moderately to steeply north and drape the southern footwall underside of the largest White Devil ironstone mass, the ‘northern lensoidal ironstone’ of Cozens (1992). They directly underlie the anomalous copper-lead-zinc bearing, talc-carbonate hydrothermal vein complex of the Pinter C shear (Fig 5) which can be traced as a series of irregular carbonate masses plunging 30o west, parallel to and north of the gold mineralisation. The presence of numerous low angle, talc-filled shears has made mining of the Pinter C area difficult, however gold grades are very high, with typical stopes averaging 30 g/t gold.
3.
Navsix lenses occur along strike east of Pinter B and underneath Pinter C lenses. The ore in hand specimen is a clast supported, hydraulically fractured breccia, with numerous occasionally flattened angular to subrounded clasts of silicified Warramunga metasediment, hosted by a matrix of chlorite-sericite-magnetite-quartz stringers. The clasts often display centimetre-scale reaction rims of chlorite with disseminated magnetite. Fine grained native gold occurs in the chlorite-magnetite-quartz matrix, often intimately intergrown with bismuth sulphosalts and rare chalcopyrite. Current thinking is that the Navsix mineralisation represents the less sheared, lower temperature, lower fluid-to-rock ratio peripheral feeders to the richer Pinter mineralisation. Moderate silicification of, and lack of shear zones within, the Navsix ore makes for excellent ground conditions in mining, however grade distribution is more erratic than in the other lenses.
CONTROLS OF MINERALISATION The main controls of mineralisation at White Devil are structure, primary porosity and the close proximity of iron-rich rock types. The orebody was formed by a two stage hydrothermal process: an initial lower temperature, lower salinity fluid deposited a hematite lath ironstone precursor, later reduced to magnetite pseudomorphs. This was followed by a higher temperature, higher salinity sulphidised fluid which
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 5 - Cross section at 5305 E through the Pinav zone, looking WSW (B–B′ in Fig 2).
deposited the gold-copper-bismuth mineralisation (Huston, 1990; Huston, Bolger and Cozens, 1993; Huston and Cozens, 1994; Khin Zaw et al, 1994). Petrography by Cozens (1992) has shown that the ironstones originally consisted of interlocking masses of hematite laths which were later deformed, pseudomorphed and overgrown by magnetite prior to the introduction of the gold mineralising fluids. The flow rate and destination of these fluids would have been strongly influenced by structurally induced pathways coupled with the presence or absence of primary porosity in the proto-ironstone lath networks. Cozens (1992) noted that the gold and bismuth sulphosalts are often intergrown with undeformed chlorite within or in (pressure shadow?) gaps between the magnetite laths. Earlier workers C Hy (unpublished data, 1988) and Nguyen (1989) noted that the highest grade ore tended to be in areas of the greatest brittle or ductile deformation, eg in regular goldbismuth rich sigmoidal tension gashes in parts of the Deeps orebody. Most of the very high grade gold mineralisation occurs between 760 and 830 RL, which would appear to be the zone of maximum dip-slip sigmoidal rotational extension. T Hopwood (unpublished data, 1993) observed that there is only one prominent schistosity within the ore zones, no crenulate overprinting, and that vertical dip-slip movement along the shears has continued through semi-ductile into brittle strain conditions.
441
C A BOSEL and G P CAIA
MINE GEOLOGICAL PRACTICE The structural complexity, erratic grade distribution and multiple ore lenses at White Devil require tight geological control for successful mining. Ore delineation drilling at a 10 by 15 m spacing provides sufficient information for drives to be put in by hand held methods at 15 m spaced levels. Every drive face is chip sampled and each wall is drilled using extension steels on a 5 m grid spacing to 5.4 m depth. The boundaries of the thicker orebodies are defined by assays of sludge samples from these holes. Drive walls and backs are then stripped sufficiently to allow access for the ‘Simba’ production drilling rig. Areas of complex geology may then be drilled and sludge sampled using the Simba rig to better define ore boundaries between the levels. Lens outlines are wireframed using Datamine software, with ore reserves calculated from a block model constrained by the wireframe boundaries within which grades are interpolated by an inverse distance squared search ellipse. Stope reconciliations are routinely carried out, which also utilise data from a detailed stockpile management and batch milling system.
ACKNOWLEDGEMENTS The authors wish to thank Normandy Gold Ltd for permission to publish this paper, the many geologists who have previously worked at White Devil or on the deposit, and M Sarangay for drafting the figures. Unpublished work by consultants C Hy and T Hopwood is also acknowledged.
REFERENCES Cozens, G, 1992. Geology and mineralisation of the White Devil deposit, Tennant Creek, Northern Territory, Australia, MEcon Geol thesis (unpublished), University of Tasmania, Hobart.
Edwards, G C, 1987. Structural and geochemical controls on alteration and mineralisation, Tennant Creek goldfield, Northern Territory, B Sc Honours thesis (unpublished), Monash University, Melbourne. Edwards, G C, Booth, S A and Cozens, G J, 1990. White Devil gold deposit, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 849–855 (The Australasian Institute of Mining and Metallurgy: Melbourne). Huston, D, 1990. Paragenetic and fluid inclusion studies of the White Devil deposit and selected barren ironstones, University of Tasmania Proterozoic Gold-Copper Project, Workshop Manual 4 (unpublished). Huston, D, Bolger, C and Cozens, G, 1993. A comparison of mineralisation at the Gecko K-44 and White Devil deposits implications of ore genesis in the Tennant Creek district, NT, Australia, Economic Geology, 88:1198–1225. Huston, D and Cozens, G, 1994. The geochemistry and alteration of the White Devil porphyry: implications to intrusion timing, Mineralium Deposita, 29:275–287. Ivanac, J F, 1954. The geology and mineral deposits of the Tennant Creek Gold-field, Northern Territory, Bureau of Mineral Resources Geology and Geophysics Bulletin 22(1). Khin Zaw, Huston, D L, Large, R R, Mernagh, T and Hoffman, C F, 1994. Microthermometry and geochemistry of fluid inclusions from the Tennant Creek gold-copper deposits: implications for ore deposition and exploration, Mineralium Deposita, 29:288–300. Nguyen, P T, 1987. Structural geology and mineralisation of the White Devil mine, Tennant Creek, Northern Territory, Australia, BSc Honours thesis (unpublished), University of Adelaide, Adelaide. Nguyen, P T , Booth, S A, Both, R A and James, P R, 1989. The White Devil gold deposit, Tennant Creek, Northern Territory, Australia, in The Geology of Gold Deposits: The Perspective in 1988, Economic Geology Monograph 6 (Eds: R R Keays, W R H Ramsay and D I Groves), pp 168–179 (The Economic Geology Publishing Company: El Paso, Texas). Smith, P and Hall, D, 1995. Application of geophysics to the White Devil gold deposit, NT, Exploration Geophysics, 26:116–123.
Donnellan, N, Hussey, K J and Morrison, R S, 1995. Flynn 5759, Tennant Creek 5758: 1:100 000 geological map series, Department of Mines and Energy, Northern Territory Geological Survey, Explanatory Notes.
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Geology of Australian and Papua New Guinean Mineral Deposits
Tunks, A and Marsh, S, 1998. Gold deposits of the Tanami Corridor, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 443–448 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Gold deposits of the Tanami Corridor 1
by A Tunks and S Marsh
2
INTRODUCTION The deposits are 650 km NW of Alice Springs, NT, at lat 19o58′S, long 129o42′E or AMG coordinates 574 600 E and 7 791 950 N, on the Tanami (SE 52–15) 1:250 000 scale and the Tanami (4858) 1:100 000 scale map sheets (Fig 1).
Venture (TJV) and then by Zapopan NL from 1987 to 1994. Redback, Dogbolter and Jim’s Find, which are respectively 7, 10 and 20 km south from the Tanami ore treatment plant, are ‘greenfields’ discoveries by the current joint venturers, Otter Gold Mines Limited (60%) and Acacia Resources Limited (40%). This paper describes the geology of the deposits at the Tanami mine in ML 153 and south to Dogbolter in ML 167 (Fig 2). The joint venturers conduct exploration in the area as the Central Desert Joint Venture (CDJV), with mining carried out by the Tanami Mine Joint Venture (TMJV).
EXPLORATION AND MINING HISTORY Gold was discovered in the Tanami Desert in 1900 by an exploration party led by Alan Davidson. Following the discovery of a 9.6 kg rock containing 5.1 kg of gold in 1908, a gold rush ensued between 1910 and 1913 (Nicholson, 1990). Intermittent mining continued between 1913 and 1965 at the original leases, for production of 2500 oz of gold (Blake, Hodgson and Muhling, 1979). Between 1965 and 1985 a number of companies and the NT Department of Mines conducted gold exploration programs, which included diamond drilling, ground magnetic surveys, rock chip sampling and soil geochemical surveys (Nicholson, 1990). In 1985, Harlock Pty Ltd commenced systematic exploration within the Tanami mine area (Nicholson, 1990; Ahmad, 1995) which led to open pit mining in mid 1987 by the TJV. Zapopan NL acquired the TJV in 1990 and continued mining until March 1994, when gold reserves within ML 153 were judged to be uneconomic due to Zapopan’s limited tenure and level of royalty payments. From June 1987 until March 1994 the TJV operations produced 788 762 oz of gold.
FIG 1 - Locality map and regional aeromagnetic image and gravity contours of The Granites–Tanami region showing magnetic Mount Charles beds (stippled) against a backdrop of low magnetic stratigraphy. The Frankenia granite dominates the image, with the Tanami corridor forming an arcuate package on its western margin.
Four clusters of gold deposits, known as the Tanami Mine, Redback Rise, Dogbolter and Jim’s Find fields (Fig 1) have been intensively explored and mined over the past ten years. The Tanami mine was operated first by the Tanami Joint
1. 2.
Senior Geologist, North Limited, PO Box 231, Cloverdale WA 6105. Senior Geologist, Acacia Resources Limited, PO Box 36121, Winnellie NT 0821
Geology of Australian and Papua New Guinean Mineral Deposits
The CDJV partners have been exploring for gold in the areas around the Tanami mine since 1989. By the re-start of open pit mining in October 1995, the CDJV had discovered an Identified Mineral Resource (Measured, Indicated and Inferred) of 6.4 Mt grading 3.1 g/t gold for 633 000 contained oz, including Proved and Probable Reserves (Table 1) of 3.7 Mt at 3.4 g/t gold, for 411 000 contained oz.
TABLE 1 Proved and Probable Ore Reserves for deposits within MLs 167 and 168, at 30 June 1996. Deposit
Mt
Grade (g/t gold)
Estimated contained gold (’000 oz)
Dogbolter Area
1.033
3.8
126
Redback Area
1.614
3.4
177
Jim’s Find
1.050
3.2
108
Total
3.697
3.4
411
443
A TUNKS and S MARSH
FIG 2 - Geological map of the Tanami corridor in MLs 153 and 167. The sequence faces west and dips west at 50 to 70 o.
444
Geology of Australian and Papua New Guinean Mineral Deposits
GOLD DEPOSITS OF THE TANAMI CORRIDOR
REGIONAL GEOLOGY
3.
relict lateritic gravel and duricrust over the Redback Rise deposits; and
All of the gold deposits within the TMJV area are hosted by a laterally continuous, layered sequence of tholeiitic basalt and clastic sediment. The Tanami corridor is an arcuate package of these rock types adjacent to the western margin of the Frankenia granite, and hosts all of the deposits with the exception of Jim’s Find (Fig 1).
4.
a relict profile stripped to mottled clays, with up to 15 m of transported gravel and sand, from the Redback South to Dogbolter deposits. Islands of relict lateritic duricrust occur over the Dogbolter Main deposit.
The host rocks to all TMJV deposits within the corridor dip and face to the NW. Regionally two folding events can be recognised. First order F 1 folds are NNE-trending, open to tight, with a maximum wavelength to 8 km, although metrescale parasitic folds are also present and typically have a weakly developed axial planar cleavage. A shallowly SSWplunging F1 syncline is interpreted to the west of the Tanami mine corridor. The F2 event is either weak or non-existent in the Tanami area but is well developed at Jim’s Find and further south. The F2 folds are open to tight and NW-trending, and accompanied by an axial plane slaty cleavage. At the Tanami mine the cleavages and small-scale folds related to the two deformations are absent. The metamorphic grade is typically low and varies from subgreenschist to lower greenschist facies. Previous BMR mapping (Blake, Hodgson and Muhling, 1979) placed all of the known gold deposits within The Granites-Tanami Inlier in the Mount Charles beds, a subdivision of the Paleoproterozoic Tanami Complex. Nicholson (1990) suggested that the host rocks were not part of the Mount Charles beds, but were in fact the basal portion of the Birrindudu Group, and possibly part of the Gardiner Sandstone. This interpretation is not valid because a clear angular unconformity between the Gardiner Sandstone and the host rocks to mineralisation can be seen at several locations in the Tanami area, furthermore the mine sequence can be traced on aeromagnetic images along strike to Mount Charles, the type section for the Mount Charles beds as defined by Blake, Hodgson and Muhling (1979). However, disparities between the stratigraphy, structural style and metamorphic grade at Tanami and elsewhere in the south of the Inlier where exposed by mining (eg at The Granites and Dead Bullock Soak gold mines) suggest that not all mineralisation is in the Mount Charles beds. Our interpretation is that the host rocks at Tanami are younger than those at The Granites or Dead Bullock Soak mines and that they were deposited unconformably on a metamorphic basement during a post-orogenic period of intracontinental rifting. It is likely, therefore, that the host rocks to mineralisation at the Tanami mine represent the rift phase sediments of the Leichhardt rifting event, a continentwide basin-forming event which occurred after the Barramundi Orogeny (Etheridge and Wall, 1994).
LOCAL GEOLOGY REGOLITH The surface regolith material is variable. From north to south it consists of: 1.
relict lateritic gravel and duricrust over much of ML 153 (Fig 2);
2.
a relict profile stripped to the mottled zone, and covered by 3 to 6 m of transported sand and gravel between the Redback North and Redback Rise deposits;
Geology of Australian and Papua New Guinean Mineral Deposits
The complete regolith profile consists of a cap of transported and/or relict lateritic material 3 to 20 m thick, underlain by a mottled clay zone 6 to 30 m thick, then a lower saprolite clay zone down to approximately 70 m depth. Weathering down fault zones is commonly accompanied by a deeper saprolite profile to about 100 m depth.
BEDROCK LITHOLOGY Bedrock has been mapped from drill hole intersections, pit exposures and interpretation of detailed magnetic survey data. The layered sequence comprises interbedded extrusive basalt and fine to coarse grained turbiditic sediment which strike NNE to NE and dip consistently 50 to 70o NW (Fig 2). The basalts are variably pillowed, vesicular, massive and doleritic, with rare autoclastic breccias at the tops and bottom of some flows. The basalts consist of albitised laths of plagioclase with interstitial clinopyroxene and magnetite in a groundmass of chlorite, leucoxene and rare epidote and zeolites. The sediments comprise monotonously interbedded sandstone and siltstone with rare intervals of carbonaceous siltstone, purple mudstone and polymict intraclastic conglomerate. The combination of pillowed basalt with monotonously interbedded clastic sediment with abundant tractional sedimentary structures indicates deposition of the sequence in a subaqueous setting below wave base.
LOCAL STRATIGRAPHY The sequence in the Mount Charles beds can be divided as shown in Table 2. The stratigraphic names are informal but have been adopted by the CDJV for the Tanami corridor. The Bouncer basalt is a magnetic unit traceable for over 16 km from the Dinky pit in ML 153 (Fig 2) to Carlsberg. It is 85 to 130 m thick and contains up to eight basalt flows and autoclastic horizons with intercalated, laminated siltstone and sandstone beds to 6 m thick. The Hurricane sediment has been mapped from Tobruk to Carlsberg and varies between 120 and 200 m thick. It consists of a basal horizon, about 2 to 3 m thick, of laminated, bleached carbonaceous shale overlain by interbedded siltstone and sandstone. Below the Hurricane sediment is the Redback basalt complex, a 300 to 600 m thick magnetic unit of intercalated basalt and sediment. Underlying the Redback basalt complex is the Harleys sediment, which is a distinctive local marker horizon between the Dogbolter and Redback North deposits. It is 100 to 150 m thick and consists predominantly of interbedded siltstone and sandstone. The base of the Tanami corridor sequence is the Footwall basalt complex, a thick undifferentiated package of basalt, sediment and phyllitic rocks which is bounded to the east by the Frankenia granite dome. Intruding the sequence are numerous 1 to 3 m wide felsic dykes that form in three characteristic orientations; parallel to bedding, subparallel to the mineralised fault zones, and NWtrending and steeply NE dipping. Based on mutual crosscutting relationships with the mineralised fault zones, the dykes are
445
A TUNKS and S MARSH
TABLE 2 Stratigraphic distribution of deposits and their strike directions.
Stratigraphic name1
Gold deposit strike direction ML 153
ML 167
Hanging wall sediment Bouncer basalt 85–130 m thick
Hurricane sediment 120–200 m thick Redback basalt complex 300–600 m thick
Harleys sediment 100–150 m thick
Dinky (000 o)2, Gap (000 o) Repulse (000o), Dingo (000o), Miracle (060o), Bumper (020o), Bouncer (060o) Hurricane (000o) Dice (000o), Airstrip (000o), Lauries A, B, C (020o) Reward (020o), Battery (020o) Bastille (020o), Southern (020o) Second Southern (020o)
Lynx (060o), Wincy (060o), Redback SW (060o), Incy (060o) Katipo (060o), Legs (060o)
Temby North (020o), Central (020o), Assault (020o)
Dogbolter Main (020o), Funnel Web (020o), Trapdoor (020o), Dogbolter NE (060o)
Temby (020 o), Temby South (020o)
Harleys (060o), Huntsman/woman (020o, 060o)
Footwall basalt complex
Redback SE (060o), Money (060o)
Note: 1. Units top to bottom correspond to west to east on Fig 2. 2. Major vein orientation directions shown in brackets.
believed to have been intruded under the same regional stress conditions and approximately synchronous with the mineralisation (Tunks, 1996). These felsic dykes have similar trace element geochemistry to the nearby Frankenia granite, implying mineralisation was broadly contemporaneous with granite emplacement.
LOCAL STRUCTURE Based on overprinting relationships, orientation and associated alteration, four episodes of brittle faulting are recognised within the Tanami corridor: Type 1:
Pre-mineralisation, syndepositional, extensional growth faults.
Type 2:
Pre-mineralisation low-angle thrusting, characterised by low angle ramp and beddingparallel faulting. Thrust ramps are characteristically marked by up to 10 cm of foliated fault gouge.
Type 3:
Pre- to syn-mineralisation faulting, which formed a complex array of dominantly strike-slip faults. Mineralisation is preferentially associated with faults oriented at 350–000o, 020–040o and 060–070o which all dip to the east or SE. Trends of individual deposits are shown in Table 2. These fault zones are commonly accompanied by zones of pervasive wall rock alteration.
Type 4:
Post mineralisation faulting, which resulted in reactivation of pre-mineralisation thrusts, and developed an east-trending, subvertical fault set with minor dip-slip reverse displacement, which is inferred to be the last significant faulting episode at the Tanami mine.
446
ORE DEPOSIT FEATURES PRIMARY MINERALISATION AND ALTERATION Individual gold deposits within the corridor are characteristically small and poddy (<100 000 contained oz). The largest deposit found to-date is Hurricane-Repulse in ML 153, with about 250 000 oz and >1 km long, however, it is a combination of three shoots which were eventually mined from one pit. In ML 167, the deposits are 150 to 600 m long and 25 to 130 m deep, with Indicated and Inferred Resources ranging from 3000 to 67 000 oz of contained gold. Down-dip extensions of many of the deposits are still open and require further deep drilling to test their potential for underground mining. Gold mineralisation throughout the corridor is structurally controlled and hosted by basalt and medium to coarse grained sediment along Type 3 fault zones, veins and their associated alteration haloes. The mineralised faults are commonly associated with extensive development of quartz-carbonate extensional shear veins and breccia zones. Displacement across Type 3 structures is characteristically small and typically less than 10 m. Hydrothermal alteration haloes surrounding the mineralised Type 3 structures can be divided into an inner sericite-quartz-carbonate-pyrite zone, which ranges in width from <1cm around individual veins to 10 m around complex zones, and an outer chlorite-carbonate zone which forms a broad envelope more than 20 m wide around the mineralised fault zones. Gold occurs as coarse, free particles to 5 mm diameter within the quartz-carbonate veins and breccia zones, and also as micron-sized inclusions within pyrite and chalcopyrite, within the veins and in the altered wall rock. Alteration associated with extensive weathering and supergene alteration of the host
Geology of Australian and Papua New Guinean Mineral Deposits
GOLD DEPOSITS OF THE TANAMI CORRIDOR
sequence has resulted in the development of abundant clay minerals and minor remobilisation of gold from the primary fault structures. Maroon staining (hematite after pyrite) is a characteristic feature of this mineralisation within the oxidised zone, but is not ubiquitous. In plan, most of the deposits within ML 153, excluding Miracle and Bouncer (Fig 2), strike from 350o to 040o, whereas within ML 167 most of the deposits strike from 060o to 070o, with only Dogbolter Main, Funnel Web and Trapdoor deposits striking at 020o (Table 2). However in most deposits both fault sets can be recognised. In cross section, the orebodies contain one or more subparallel ore zones dipping to the east at a high angle to the stratigraphic layering (Fig 3). High grade SEplunging ore shoots of >5 g/t gold are commonly developed at the intersection of the 350o to 040o and 060o to 070o striking faults.
SUPERGENE MINERALISATION At the Redback Rise and Dogbolter deposits, blankets of supergene gold mineralisation were mined from the laterite and transported lateritic gravel. Shallow pits to a maximum depth of 5 m within ML 153 indicate that supergene ‘lateritic gold’ was also mined previously. The underlying mottled zone is commonly depleted in gold with respect to the laterite and subjacent saprolite zones. This is commonly seen in grade control data from the Redback and Dogbolter mines where up to 25 m wide, 60 m long, high grade (>5 g/t) pods were mined from the laterite; about 3 to 10 m wide, 40 m long, low grade (< 1 to 1.5 g/t) pods were mined in the mottled zone; and 15 to 25 m wide, 60 to 70 m long, 1.5 to 6 g/t pods were mined from the saprolite and fresh bedrock zones.
Geology of Australian and Papua New Guinean Mineral Deposits
Physical and chemical mobilisation of gold is inferred to have taken place in the uppermost part of the regolith profile, whereas in and below the saprolite zone there is little evidence of mobilisation. Microprobe analysis of gold grains from the saprolite zone at the Dogbolter deposit indicates that supergene gold does occur, although the gold remains largely confined to the primary structures.
GOLD DEPOSITION Gold deposition is interpreted to have occurred in two stages. The first involved deposition of quartz±sericite±pyrite ±chlorite±sphalerite±arsenopyrite±gold, with quartz, sericite and pyrite the dominant gangue minerals. The second stage included an assemblage of ankerite±quartz±chalcopyrite ±chlorite±gold±sericite±pyrite±calcite. Ankerite, quartz and chalcopyrite are the dominant gangue minerals in this stage (A Coote, unpublished data, 1995; Tunks, 1996). Based on detailed fluid inclusion microthermometry (Tunks, 1996), gold precipitation occurred at depths of 3 to 6 km and trapping temperatures of about 300oC. These conclusions are consistent with the observed alteration assemblage (sericite, ankerite, quartz and pyrite) at the Tanami mine and at other shear zone–hosted gold deposits with similar alteration assemblages as at the Golden Mile (Phillips, 1986). Low fluid salinities (about 5 eq wt % NaCl) and low carbon dioxide levels (∑C ≈ 0.5 molal) were a feature of the mineralising fluids, which are interpreted to be magmatichydrothermal and/or contact metamorphic fluids sourced from the thermal aureole of the Frankenia granite (Tunks, 1996).
447
A TUNKS and S MARSH
MINE GEOLOGICAL METHODS
REFERENCES
Conventional grade control and surface mining techniques are used. Ditchwitch trenching and sampling along 10 m spaced lines on the pit floor is the principal grade control technique, and is backed up by 10 by 10 m reverse circulation grade control drilling.
Ahmad, M, 1995. The Granites SF 52–3, Tanami SE 52–15, Northern Territory Geological Survey 1:250 000 Mineral Deposit Data Series.
ACKNOWLEDGEMENTS The authors gratefully acknowledge permission by Otter Gold Mines Limited and Acacia Resources Limited to publish this paper. Contributions from the exploration geologists in the Tanami over the last few years, including S Henderson, I Hart, K Stanton-Cook and A Coote, and D Cooke of the CODES Key Centre, University of Tasmania, have all been invaluable in improving our understanding of the area.
Blake, D H, Hodgson, I M and Muhling, P C, 1979. Geology of the Granites–Tanami Region, Northern Territory and Western Australia, Bureau of Mineral Resources Geology and Geophysics Bulletin 197. Etheridge, M A and Wall, V J, 1994. Tectonic and structural evolution of the Australian Proterozoic, Geological Society of Australia Abstracts, 37:102–103 Nicholson, P M, 1990. Tanami gold deposit, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 715–718 (The Australasian Institute of Mining and Metallurgy: Melbourne). Phillips, G N, 1986. Geology and alteration in the Golden Mile, Kalgoorlie. Economic Geology, 88:1084–1098. Tunks, A, 1996. Geology of the Tanami gold mine, PhD thesis (unpublished), University of Tasmania, Hobart.
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Geology of Australian and Papua New Guinean Mineral Deposits
Smith, M E H, Lovett, D R, Pring, P I and Sando, B G, 1998. Dead Bullock Soak gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 449–460 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Dead Bullock Soak gold deposits 2
1
3
by M E H Smith , D R Lovett , P I Pring and B G Sando INTRODUCTION The deposits are approximately 600 km NW of Alice Springs, NT, in the Schist Hills area of The Granites–Tanami Goldfield (Fig 1). They are at lat 20o31′S, long 129o57′E or AMG coordinates 598 000 m E, 7 730 500 m N, on The Granites (SF 52–3) 1:250 000 scale map sheet. Exploration and mining operations at Dead Bullock Soak (DBS) and The Granites are wholly owned by North Flinders Mines Ltd (NFM).
1
TABLE 1 Identified Mineral Resources, Dead Bullock Soak MLS 154, at 31 December 1996. Deposit and status
Ore ‘000 t
Gold grade (g/t)
Contained gold (oz)
Callie Open Cut < 180 m Measured Indicated Underground 180 - 300 m Indicated 300 - 540 m Inferred
665 3115
5.6 4.4
118 900 441 500
1231
7.7
304 300
4711
8.8
1 332 900
Gahn (open cut) Indicated
57
3.1
5 700
9779
7.0
2 203 300
225 2526
3.7 3.7
26 600 300 800
152 1137
3.8 3.3
18 300 120 800
Triumph Hill Measured Indicated
180 882
4.2 3.9
24 000 109 400
Colliwobble Ridge Measured Indicated
142 411
3.0 3.0
13 800 40 100
Fumarole Measured Indicated
89 295
2.9 2.9
8 400 27 500
198
2.9
18 500
Callie total
BIF-hosted deposits (open cut) Villa Measured Indicated Dead BullockRidge Measured Indicated
FIG 1 - Location map, Dead Bullock Soak area.
Avon Indicated
DBS ore is trucked 40 km east to The Granites where a CIP mill has been operating since 1986. NFM discovered gold in banded iron formation (BIF) at DBS in 1988 and at Callie in October 1991. Mining commenced at DBS in 1990 and at Callie early in 1992. Production to 31 December 1996 was 1.26 Mt at 3.9 g/t gold from the BIF deposits at DBS and 2.33 Mt at 5.8 g/t from Callie. Total Identified Mineral Resources for the BIF deposits and Callie at this date are shown in Table 1.
Sleepy Hollow Indicated BIF total DBS total
145
2.3
10 700
6382
3.5
718 900
16 161
5.6
2 922 200
EXPLORATION HISTORY BIF-HOSTED DEPOSITS
1.
Supervising Geologist, North Flinders Mines, 24 Greenhill Road, Wayville SA 5034.
2.
Senior Project Geologist, North Flinders Mines, 24 Greenhill Road, Wayville SA 5034.
3.
Project Geologist, North Flinders Mines, 24 Greenhill Road, Wayville SA 5034.
Geology of Australian and Papua New Guinean Mineral Deposits
Previous exploration in the DBS area comprises small bath tub sized prospecting pits dug when The Granites deposits were being worked, between 1910 and 1940 (Gee, 1911; Hossfeld, 1940). The only previous recorded company exploration was by Geopeko between 1969 and 1971 (A R Twiggs, unpublished data, 1971). NFM began systematic exploration in the Schist Hills area in 1988 in EL 2367 at the commencement of its regional exploration program in the Tanami region. Early
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attention was directed at this area because of the gold occurrences reported in the literature, the presence of geology similar to that of The Granites deposits, and the rare outcrops which provided an easy focus for the initial exploration. The first exploration consisted of locating and sampling the old prospecting pits which confirmed the presence of gold with grades to 10 g/t in oxidised amphibole-rich chert. Geological mapping and rock chip sampling on a 50 by 50 m grid followed. Vacuum drill sampling of bedrock complemented this work in areas without outcrop. The better mineralised parts of the BIF unit, referred to as the Schist Hills iron member (SHIM), were evaluated by a program of 15 reverse circulation (RC) drill holes at a spacing of 25 m on lines 50 m apart. The drill holes were inclined at 60ο and drilled to the north to 60 m depth. This work brought immediate success and was followed by a more extensive RC drilling program. By June 1989 exploration had sufficiently advanced to allow the estimation of an Indicated Resource of 600 000 t at 3.3 g/t gold at the Triumph Hill and Dead Bullock Ridge deposits. Additional deposits have since been discovered in the SHIM at Sleepy Hollow and Colliwobble Ridge. A mining lease (MLS 154) was applied for and granted in 1990 to allow the commencement of mining. By 1990 geological mapping of the DBS area allowed the recognition that the deposits were within or adjacent to easttrending anticlinorial fold closures. This is a common feature of some other Proterozoic and younger gold deposits in low grade metamorphic terrains, eg Pine Creek Geosyncline, NT, and Ballarat, Vic. In addition exploration at The Granites had demonstrated that there were two gold mineralised BIF horizons (Mayer, 1990). As a result, exploration at DBS became focussed on the poorly outcropping western extensions of the folds lower in the sequence. During 1990 mapping and sampling in this area identified a poorly mineralised lower BIF unit called the Orac formation. It was traced along strike by costeans 2 m deep and 1 m wide and shallow (10 to 20 m) rotary air blast drill holes. RC drilling of the better mineralised parts of the formation in 1991 led to the discovery of the Villa, Fumarole and Avon deposits. Exploration of the BIF deposits and adjacent sequence is continuing, directed at defining resources available for underground mining to 300 m depth.
CALLIE DEPOSIT The deposit was discovered in 1991, three years after exploration commenced at DBS. During those years exploration was directed at the discovery and delineation of outcropping gold mineralisation in the BIF at the eastern end of MLS 154 as discussed above. The concepts which drove this exploration were the testing of the major axial corridor within the DBS anticlinorium which hosted the Villa, Fumarole and Triumph Hill deposits, and the observation that the gold deposits of the Tanami region often had a clustered distribution, as at The Granites, Tanami and DBS deposits, and therefore that there may have been more deposits to find. As a consequence, a vacuum drill program designed to sample oxidised bedrock was initiated over the western half of the lease at a hole spacing of 200 by 25 m. This work defined a long sinuous bedrock anomaly of >50 ppb gold with the peak value of 7 ppm over the Callie orebody which is buried beneath several metres of sand and colluvium. A similar but less well defined arsenic anomaly was slightly offset from the gold anomaly. Given the tenor of the anomaly and its location within
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the prospective axial corridor, an RC drilling program of 30 holes inclined at 60 o to the horizontal, azimuth due north, 60 m deep, at a spacing of 15 m and on traverses 50 m apart was used to evaluate the vacuum drill hole anomaly. This program achieved some high grade intersections, including 42 m at 27 g/t gold and 23 m at 20 g/t gold, and resulted in an Indicated Resource of 550 000 t of 12 g/t gold being announced in October 1991. To enable this to be achieved the line spacing of the RC drilling was reduced to 25 m for resource estimation purposes. Within 12 months of the initial RC drilling program, the Callie deposit was shown to be in an easterly plunging anticline of fine grained metasiltstone stratigraphically below the BIFhosted deposits. The gold mineralisation was in a swarm of thin quartz veins within a particular sedimentary unit and around the crest of an easterly plunging anticline. Subsequent exploration has consisted of diamond drilling at 25 and 50 m intervals following the easterly plunge of the anticline. By late 1996 continuous mineralisation had been traced to about 700 m depth. Wide and high grade intersections of gold mineralisation similar to those discovered during the initial round of RC drilling continue to be encountered at these depths. Within the resource outline the orebody averages about 18 000 t/vertical m and 5000 oz/vertical m.
PREVIOUS DESCRIPTIONS The geology of The Granites–Tanami region has been described by Hodgson (1976), Blake, Hodgson and Muhling (1979) and Plumb (1990). Ding Puquan and Giles (1993) outlined the geological setting of gold mineralisation in the Tanami region. The gold deposits of Dead Bullock Soak have been previously described by Lovett et al (1993). Ireland (1995) presented a detailed account of the discovery of, and the exploration rationale for, the BIF-hosted deposits and the nearby Callie deposit. This paper updates the information of Lovett et al (1993) on the geology of the Villa, Triumph Hill and Dead Bullock Ridge deposits and of Ireland (1995) on Callie. Of the other BIFhosted deposits (Table 1) the geology of Fumarole remains unchanged from the 1993 description by Lovett et al. The Colliwobble Ridge deposit is geologically an extension of the Triumph Hill deposit and Avon and Sleepy Hollow are small, lower grade deposits which do not justify separate descriptions at this stage.
REGIONAL GEOLOGY STRATIGRAPHY The DBS area is dominated by a sequence of fine to medium grained clastic metasediment and minor chemical metasediment of the Mount Charles beds of the Palaeoproterozoic Tanami Complex (Fig 2). Three main units are recognised. The oldest is the Blake beds, a generally monotonous sequence of fine grained metapelite containing carbonaceous and rare chert horizons. The Blake beds host the Callie deposit which is about 1000 m to the west of the area of the BIF-hosted deposits. The Davidson beds conformably overlie the Blake beds and comprise the Orac formation and the overlying Schist Hills formation. The former hosts the Villa, Fumarole and Avon deposits and the latter the Triumph Hill,
Geology of Australian and Papua New Guinean Mineral Deposits
DEAD BULLOCK SOAK GOLD DEPOSITS
The earliest phase of deformation D1, was layer-parallel regional shearing which produced mostly weak layer-parallel foliation (S1) and was responsible for the boudinaging of original layer-parallel quartz veins within the Blake and Davidson beds. The second phase of deformation, D2, produced the major easterly plunging (originally NE-trending) F2 DBS anticlinorium. The plunge is between 40 and 75ο to the east (Fig 2) and the anticlinorium controls the gross configuration of the DBS geology. D3, the third phase of deformation, is responsible for the development of ESE-plunging F3 interference folds on the F2 anticlinorium. Subsequent deformations (D4–D6) are represented by several phases of folding and faulting which are described under the individual deposit headings.
METAMORPHISM FIG 2 - Geological map, Dead Bullock Soak mining lease with location of deposits.
Dead Bullock Ridge, Colliwobble Ridge and Sleepy Hollow gold deposits (Lovett et al, 1993). The youngest is the Madigan beds which conformably overlie the Davidson beds and consist of flysch greywacke and siltstone not known to be mineralised at DBS.
INTRUSIVES There are two significant intrusive events in the DBS area; the emplacement of a large body of semiconformable dolerite, the Coora dolerite, and the intrusion of erratically distributed lamprophyre dykes. The Coora dolerite is a large, complex sill-like body which for the most part occupies the contact between the Schist Hills and Orac formations. It is composed of coarse to medium grained equigranular amphibole, chlorite and feldspar with zones of feldspar phenocrysts. In parts it is intensely foliated and predates much of the folding and faulting at DBS. It is rarely gold mineralised. Where gold is present it is within quartz veining and confined to fault zones in the vicinity of the Villa deposit. Minor basic sills and dykes less than 5 m wide are relatively abundant, especially in the lower parts of the sequence. Some are comagmatic and others are compositionally and temporally different from the Coora dolerite and the End It All dolerite at Callie. The lamprophyres are minor, late-stage medium grained biotite-rich intrusives that have been intruded, generally as complex swarms, parallel to S5 and may constitute melts of enriched subcontinental lithosphere (A C Purvis, unpublished report, 1996). They generally post-date mineralisation, are rarely gold mineralised and are emplaced in any of the DBS rock types.
STRUCTURE The area has a complex structural history which has been integral to the formation of the DBS orebodies. Most of this history has been unravelled by Ding Puquan (unpublished data, 1992–1996).
Geology of Australian and Papua New Guinean Mineral Deposits
The regional metamorphic grade of the Tanami Complex surrounding DBS is lower greenschist facies characterised by sericite and chlorite. Studies by Ding Puquan (unpublished data, 1994) suggest that the metamorphic grade reached lower amphibolite facies in parts of the area as evidenced by the presence of amphibole in gold mineralised chert, the preservation of folded biotite within the Blake beds and the local development of garnet in higher parts of the stratigraphic succession. To some degree this apparently higher grade is a result of varying initial rock composition. It is also clouded by a combination of subsequent metasomatic and thermal metamorphic effects which are largely restricted to the Callie orebody and rapidly diminish in importance away from it. Garnets within the manganiferous chert unit of the Schist Hills formation indicate a lower amphibolite facies towards the top of the DBS succession. The Coora dolerite shows minor late stage low temperature chlorite-carbonate alteration as chloritisation with minor carbonate veining. Zones of metasomatic alteration strike east and are characterised by a strong late stage foliation, quartz veining and a thermally derived assemblage of biotite, muscovite, amphibole and magnetite. These alteration zones constitute ‘structural corridors’ within the axial planes of the anticlinorial folds. The Callie, Villa and Fumarole deposits and possibly the Dead Bullock Ridge and Triumph Hill deposits are within these corridors.
QUARTZ VEINING There are at least four types of quartz-dominant veins within the gold deposits at DBS. They are bedding-parallel, sheeted, random and buck. The bedding-parallel quartz veins are thought to be the oldest and it is possible that there are two generations. One generation was folded by the D2 deformation event that formed the tight to isoclinal folds, and the second are more like saddle reefs that formed late in this event. The concept of saddle reef veins is supported by a greater abundance and thickness of beddingparallel veins within fold hinge zones. The sheeted veins occupy a cleavage that was formed during a deformation that occurred after the saddle reef forming event, and are generally 5 to 30 cm thick. These veins were originally
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quartz-feldspar and host most of the gold at Callie and a lesser proportion at the Villa, Dead Bullock Ridge and Triumph Hill deposits. The random veins are similar to the sheeted variety in morphology and texture and may be of the same generation but occupy different structural positions. The buck quartz veins are typically large, milky white veins that are most likely to be the last generation of veins as they do not appear to be folded, boudinaged or recrystallised. This type includes the late stage gash veins of the Callie deposit.
ORE DEPOSIT FEATURES CALLIE Stratigraphy A cover of 1 to 3 m of aeolian sand and largely transported colluvium overlies the orebody. A patchy in situ ferricrete horizon underlies this cover and passes into a variably clay-rich bleached zone. Complete oxidation of bedrock extends to 50 to 60 m and partial oxidation to about 130 m depth. The Callie deposit is hosted by the Blake beds which are interbedded siltstone, carbonaceous siltstone and chert with minor calcareous horizons. Sedimentary structures are frequently preserved, particularly within the upper Blake beds, and include graded bedding, cross stratification and scour and fill structures. Graded beds frequently consist of a fine sandy base, extending into silt and overlain by fine carbonaceous silt. The graded sequence often appears similar to divisions c to e of the Bouma cycle. The depositional sequence of the Blake beds (Fig 3) consists of: 1.
siltstone, calcareous in part (lower Blake beds) overlain by
2.
thinly banded siltstone (Callie host unit) then
3.
interbedded to banded siltstone and carbonaceous siltstone (Magpie schist),
4.
a carbonaceous siltstone and boudinaged chert horizon (Callie boudin chert) and finally
5.
interbedded siltstone, carbonaceous siltstone and chert (upper Blake beds).
Within the Blake beds, the Callie boudin chert is the only significant marker horizon, being identified by its distinctive egg-shaped chert or quartz boudins within a carbonaceous, usually sulphidic (pyrite, pyrrhotite and arsenopyrite) siltstone. The Callie host unit is the preferred host to gold mineralisation at Callie and is identifiable by its characteristic 1 to 5 mm thick, pale siliceous bands which contain a slightly coarser fraction than the enclosing chlorite-sericite dominant interbeds. Fine grained titaniferous minerals, principally ilmenite and leucoxene, are found within these pale bands. The Callie host unit was originally carbonaceous in part.
Intrusives The semiconformable End It All dolerite lies between the Callie boudin chert and Magpie schist and can be locally transgressive. As the dolerite grossly conforms to the dominant folding episodes at DBS and possesses a weak but pervasive foliation, intrusion is believed to have taken place early in the deformational history.
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FIG 3 - Stratigraphic column (not to scale), Callie deposit.
Structure Several phases of deformation subsequent to D3 are seen within several hundred metres of Callie. The D4 deformation is represented by minor local WSW plunging F4 folds. D5 is represented by minor folds plunging steeply to the east and west. A prominent domainal ENE-trending S5 foliation is evident within ‘structural corridors’ corresponding to an ENEoriented, steeply southerly dipping vein set which hosts the Callie orebody. Finally D6 represents a period of semi-brittle deformation and is responsible for two sets of post-mineralisation faults and carbonate veins (with or without base metal mineralisation). An east- to ENE-trending fault set with southerly dips predates a north- to NW-trending set which dips moderately to the east (Figs 4 and 6). All the major north- to NW-striking faults have reverse movement which has significantly uplifted successively eastern blocks of the east-plunging folded sequence. This has resulted in the effective plunge of the Callie fold structure being less than it would have been without faulting. The cross sectional geometry of the Callie F2 fold varies from west to east. The Callie anticline is relatively higher than the Lantin anticline to the north (Fig 5) and the poorly understood Jupiter II anticline to the south. However down plunge to the east the F2 structure containing the Callie, Lantin and Jupiter II anticlines becomes more complex and the Lantin anticline develops at the expense of the Callie anticline. This is due to the interference between F2 and F3 folds.
Geology of Australian and Papua New Guinean Mineral Deposits
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corridor passes gradually from the Callie anticline, into the Lantin anticline. This increases the potential of the Lantin anticline for gold mineralisation (Fig 5). As a result it hosts the bulk of the Callie gold mineralisation in the Wilson shoot east of about 9900 E.
Metamorphism Following the regional prograde metamorphism to greenschist facies, retrograde metamorphism produced the commonly observed quartz-sericite-chlorite assemblage. A second pulse of thermal metamorphism was also followed by retrogression within this facies range.
FIG 4 - Geological plan, Callie deposit, 1380 RL (near surface).
G W Morrison (unpublished data, 1994) proposed a thermal metamorphic effect associated with the introduction of mineralisation, after the regional and retrogressive metamorphic episodes. This event may have been driven by the emplacement of a large igneous intrusion which focussed fluids within the D5 structural corridor. To date, no direct evidence for such an intrusion has been observed at DBS.
As a necessary precursor to vein development, they also exhibit the effects of thermal metamorphism. The corridors have an easterly orientation and thus transect the ESE plunge of the F3 fold axes at a low angle.
Thermal metamorphism appears to have produced a distinct domed zoning within the Callie orebody. Associated minerals are a function of the composition of the host rock and the metasomatic fluids introduced into the structural corridor. Beginning with the deepest parts of the orebody tested to date, the thermal metamorphic-metasomatic assemblages consist of amphibole-epidote-magnetite, amphibole-biotite, biotitemuscovite-ilmenite, chlorite-illite-leucoxene and clayleucoxene.
At Callie, two discrete corridors have been recognised. The Callie corridor is the best developed and is host to economically significant mineralisation. As a result of the difference between the easterly orientation of the structural corridor and the ESE strike of the F3 Callie and Lantin anticlines, the Callie
The zoning is suggestive of increased temperature with depth in the orebody. Magnetite exists only in the deeper parts of the lower Blake beds where it preferentially replaces original sedimentary carbonate in places. Amphibole is also largely restricted to the lower Blake beds, but also extends into the
Structural corridors Structural corridors at DBS are defined as discrete areas having concentrations of quartz-dominant veins, a well developed D5 foliation and metasomatic alteration.
FIG 5 - Serial geological cross sections on 9300 N, 9750 N and 10 000 N, Callie deposit, looking west. For location of 9300 N see Fig 4.
Geology of Australian and Papua New Guinean Mineral Deposits
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been extensively recrystallised and altered. Most of the veins have poorly developed alteration selvages. A depth-related zoning pattern of accessory minerals, similar to that within the host rocks, exists for the mineralised vein set. G W Morrison (unpublished data, 1994) proposed a threefold subdivision of mineral zones within the larger hydrothermal system. These include, starting at the lower levels of the deposit:
FIG 6 - Longitudinal projection in the plane of the Callie structural corridor, Callie deposit, looking north.
Callie boudin chert in the eastern parts of the orebody, and the biotite and chlorite zones can occur throughout the sequence. Magnetite, amphibole and biotite associated with thermal metamorphism and related metasomatism are invariably porphyroblastic, but amphibole and biotite are sometimes aligned with the D1 or D5 foliation.
Alteration Alteration is most strongly associated with the D5 thermal metamorphic and related metasomatic event. It is difficult to determine whether the mineral phases developed during this event are of thermal metamorphic or hydrothermal origin. Minerals related to metasomatism, however, may have formed in the later stages of the thermal event and include prismatic aluminium-rich amphibole, chlorite and biotite. Microprobe analyses of chlorite and biotite suggest that no significant compositional differences exist between the minerals associated with regional and thermal metamorphism and those formed by metasomatism (R Duckworth, unpublished data, 1994). Alteration selvages around mineralised veins are not widespread. In comparison, late stage, D6 carbonate veins and fracture fill generally show strong alteration selvages of yellow sericite and carbonate.
Veining The Callie orebody is essentially a sheeted quartz-vein hosted gold deposit within a complexly folded metapelite sequence. Four major pre- to post-mineralisation vein types have been recognised, including: 1.
Early quartz-only veins without alteration selvages. These are folded, frequently boudinaged and interpreted to predate the major deformation (D2). They equate to the bedding-parallel veins described earlier.
2.
Subhedral quartz-feldspar veins. These were emplaced early in D5 and occur as a parallel set within the Callie structural corridor. They are composed of quartz and feldspar with accessory epidote, tourmaline, biotite and amphibole and have no selvage.
3.
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The gold mineralised vein set, only found within the structural corridor. Originally the veins had a composition similar to vein type 2, with subhedral to euhedral grains, however the original components have
(i)
a biotite zone which has biotite-ilmenite alteration and biotite and epidote fill with pyrrhotite, chalcopyrite and gold;
(ii)
a chlorite zone which has chlorite-sphene-rutile alteration and quartz-chlorite-carbonate fill with pyrite-arsenopyrite-gold and minor chalcopyrite and sphalerite; and
(iii) a calcite-chlorite zone with chlorite-clayleucoxene alteration and quartz-calcite-chloritepyrite fill, which is occasionally mineralised. 4.
Late stage gash veins, associated with D6 structures. These post-mineralisation veins are generally not auriferous and represent a brittle deformational period after the thermal metamorphic event. They dominantly comprise calcite or ankerite-quartz-pyritechalcopyrite±sphalerite-galena and are frequently associated with sericitic alteration. These equate to the buck quartz veins described earlier.
The gold mineralised vein set is best developed within the structural corridor where thermal metamorphism has moderately hornfelsed the country rock and in so doing has increased its potential for fracturing and hence veining. It occurs as a parallel set oriented ENE and dipping steeply to the south, and a further package of more erratic veins is particularly developed within anticlinal crests. Within the corridor, veining exists within all parts of the sequence but is more prominent and better mineralised below the Callie boudin chert where less carbonaceous material exists. The carbon-rich units are more ductile and are therefore less susceptible to brittle deformation.
Fluid inclusions Studies of fluid inclusions within the four major vein types at Callie have identified six major fluid types. Pre-mineralisation stage D5 quartz veins tend to have aqueous inclusions and indicate low to moderate salinity but veins associated with mineralisation generally have inclusions with low to moderate salinity which also contain concentrations of CO2 and CH4. It has been suggested that the fluid characteristics of the mineralised veins indicate mixing of a deep seated saline fluid with a connate fluid containing CO2-CH4 derived from the metasedimentary sequence (G W Morrison, unpublished data, 1994), or indicate reaction of the saline fluid with carbonaceous country rock.
Mineralisation The process of mineralisation at Callie was a complex interplay of structural, stratigraphic and geochemical factors. Mineralisation associated with the D5 deformation is considered responsible for the Callie orebody, in contrast with the multiple phases recognised in the BIF-hosted deposits. Economic mineralisation at Callie is predominantly hosted by the Callie host unit and to a lesser extent by the lower parts of the Magpie schist and laminated horizons within the lower
Geology of Australian and Papua New Guinean Mineral Deposits
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Blake beds where they intersect the structural corridor. However, except for the intrusive dolerites and lamprophyres, significant mineralisation has been intersected in all parts of the sequence but mostly within the structural corridor. Mineralisation is thus broadly stratabound but in detail locally crosscuts the sequence. Semi-brittle, post-mineralisation D6 structures disrupt the orebody and are largely unmineralised. However some of the gold introduced during D5 has been remobilised into later structures. The D6 faulting has had a beneficial effect on the economics of the Callie orebody because the reverse movement has successively uplifted the easterly plunging structure and brought it closer to the surface (Fig 6) Major controlling factors on mineralisation at Callie are: 1.
the Callie structural corridor and contained D5 vein package;
2.
coarser grained parts of the sequence where centimetrescale graded beds have increased porosity and permeability, as in the Callie host unit, the lower part of the Magpie schist and the upper parts of the lower Blake beds, and thus are preferred hosts;
3.
intersections between quartz veins and bedding planes which are important loci for gold precipitation, consequently mineralisation favours structural positions which created the maximum number of intersections; and
4.
the chlorite and chlorite-biotite zones of the hydrothermal system containing the bulk of the mineralisation which diminishes in the biotite-amphibole zone.
When examined in detail, gold at Callie can be seen to be hosted by fine shears or fractures within recrystallised and deformed veins. Mineralisation may also extend into the wall rock within these fractures. It post-dates the veins but is part of the same complex D5 deformation. Associated with the gold within the fine fractures are iron silicates such as chlorite and amphibole and minor amounts of pyrite and chalcopyrite. Gold within veins is typically 0.2 to 2 mm in diameter but nuggets exceeding 40 mm in diameter have been found. Much of the gold in the Callie deposit is nuggetty and contrasts strongly with the finer grain size of the gold in all of the other deposits at DBS, and also at The Granites. Arsenopyrite is locally abundant but is not intimately associated with the gold mineralisation. The orebody is known to have a total strike length of at least 1500 m, including Gahn, which lies west of the Western shoot (Fig 4). It has been dismembered by D6 faults and shears into four zones; Gahn, Western shoot, Central shoot and Wilson shoot (Fig 6). The ore shoots arise due to the moderately ESEplunging nature of the F3 folds and their relationship with the east-striking structural corridor. From west to east the mineralisation gradually moves from the southern limb of the Callie anticline at Gahn, into the crest and northern limb of the Callie anticline in the Central shoot and finally into the Lantin anticline in the eastern parts of the Wilson shoot. There is no apparent secondary dispersion of gold in the oxidised zone. The exploration potential at Callie remains open to the east within the Wilson shoot and within the structural corridor to the west, and below the deposit where the corridor intersects favourable rock types.
Geology of Australian and Papua New Guinean Mineral Deposits
VILLA Stratigraphy The host sequence Villa is the Orac formation which is 40 to 50 m thick and consists of fine grained metapelite (quartz-chloritealbite-biotite+graphite schist) with several iron-rich units characterised by numerous thin chert layers. There are up to four distinct units of this type that are typically 1 to 5 m in true thickness, although in places they can be up to 10 m thick. The iron-rich units consist of almost pure chlorite with numerous 1–20 cm laminated chert bands and interbedded layers of amphibole and sometimes chlorite. The amphibole-rich layers commonly contain abundant pyrite and sometimes arsenopyrite in layers parallel to bedding. Trace amounts of fine grained magnetite occur in some chert and amphibole-rich layers. Rare examples of massive carbonate layers which are progressively replaced by amphiboles suggest that the iron-rich units may be metamorphosed or metasomatised carbonatefacies BIF. The schist horizons, which are interbedded with the iron-rich units, show little compositional variation. They consist mostly of quartz-chlorite-albite-biotite with the chlorite content increasing with increasing proximity to the chert. The biotite is the product of potassium metasomatism and occurs as a porphyroblastic overprint within the schist (N Tate, unpublished data, 1994). Small amounts of graphite commonly occur as thin laminae throughout these metasediments.
Structure The structure is the result of several phases of deformation. The first recognisable phase was a bedding-parallel shearing event (D1) that produced a widespread schistosity and boudinaged the chert horizons. The next major phase of deformation (D2) produced a series of tight to isoclinal folds (F2) that form the DBS anticlinorium. The D3, D4 and D5 events produced parasitic folds within the anticlinorium, but did not affect the gross morphology of the Villa structure to a great extent. The D5 event simply flexed existing structural features. Gold mineralisation was possibly introduced during several phases of deformation and remobilised during others. Finally, late stage thrust faulting (D6) disrupted the Villa deposit, but has little to no associated gold mineralisation. The resulting Villa structure is an east-striking, overturned, isoclinal F2 anticline which dips steeply to the south and plunges at 55–60o to the east within the DBS anticlinorium (Figs 7 and 8). The anticline is more open at the western end of the deposit where the southern limb swings to the south. Parasitic folding is common throughout the deposit.
Mineralisation The mineralisation is essentially stratabound and generally correlated with the units rich in amphibole and sulphide. The most continuously mineralised iron-rich unit is known as the lower Orac chert indicated by LOCU on Figs 7 and 8. This unit tends to be the thickest, 3 to 5 m in true thickness, of the ironrich units within the Orac formation. The others tend to be lower in grade, thinner, and more sporadic, with poor continuity along strike and down dip. Higher grade ore shoots occur within zones of greatest deformation such as anticlinal fold closures and parasitic flexures on the limbs.
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2.
larger grains, typically 0.1 to 1 mm in diameter, disseminated in amphibole-rich bands; and
3.
erratically distributed grains in deformed quartz and quartz-chlorite veins.
Settings 1 and 2 are within the D2 structural fabric and setting 3 is related to a younger tectonic event.
FIG 7 - Geological plan, Villa deposit, 1385 RL (approximate bedrock), Fig 8 is on section line A–B.
Gold mineralisation occurs in three settings within the Villa deposit (N Tate, unpublished data, 1995). In decreasing order of abundance they are: 1.
small grains, <0.1 mm in diameter, commonly associated with pyrrhotite on fractures or grain boundaries of large arsenopyrite grains;
The Villa deposit is weathered to about 100 m below surface. Within the highly weathered part of the orebody near the surface, gold mineralisation shows limited dispersion outside the host chert into the enclosing schist horizons (Fig 8). In fresh rock, ore grade mineralisation is almost exclusively within the chert units and is closely associated with arsenopyrite. The arsenopyrite is not evenly distributed throughout the chert units, but tends to be concentrated within discrete horizons from 1 to 10 cm thick. The oxidised and primary ore are free milling and recoveries of greater than 95% are achieved at The Granites CIP plant.
DEAD BULLOCK RIDGE AND TRIUMPH HILL Stratigraphy The deposits are within the Schist Hills formation of the Davidson beds. The formation is separated from the Orac formation by the Coora dolerite and can be stratigraphically divided into five units. The Dead Bullock member (DBM) is the lowest. Lithologically it is a fine grained quartz-chlorite-muscovitecarbonaceous metapelite with varying amounts of carbonaceous material which define centimetre banding to metre scale bedding. Abundant muscovite which distinguishes the DBM from the other units of the Davidson beds is variable in content and increases at the expense of chlorite. The contact with the Coora dolerite is typically defined by a narrow chlorite-rich metapelite and chert horizon known as the Boundary chert. A further substantial chert-bearing unit occurs close to the top of the DBM. Both chert units closely resemble the iron-rich units within the Orac formation. Stratigraphically overlying the DBM is the Schist Hills iron member (SHIM), the host for the Dead Bullock Ridge and Triumph Hill gold deposits. This unit, which ranges in thickness from 5 to 20 m, is composed of chert interbedded with amphibole-chlorite-magnetite-arsenopyrite rich layers. The layers are composed of coarse clusters, 5 mm diameter, of acicular cummingtonite-grunerite and up to 30% chlorite (D Mason, unpublished data, 1996). The cherts are mostly equigranular quartz less than 0.5 mm in diameter with scattered rosettes of acicular amphibole. The principal difference between the SHIM and the iron-rich units of the Orac formation is that the SHIM contains more amphibole and much more magnetite. A distinctive quartz-chlorite-muscovitecarbonaceous metapelite horizon averaging 1 m in thickness separates the SHIM into two identical subunits. The SHIM is overlain by the Colgate schist (CS), a fine grained carbonaceous metapelite similar in appearance to the carbon-rich metapelites of the DBM, but with much less muscovite. Cherts in the CS occur as thin beds or more commonly as scattered chert boudins.
FIG 8 - Cross section 10 350 E, Villa deposit, looking west.
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The Colgate schist and DBM are frequently indistinguishable in drill chips and core because they have identical constituents separated by iron-rich chert (SHIM). In places the SHIM may be faulted out, giving rise to uncertainty
Geology of Australian and Papua New Guinean Mineral Deposits
DEAD BULLOCK SOAK GOLD DEPOSITS
in unit identification. The CS is the youngest unit shown on Figs 9–12. The Manganiferous chert unit (MCH) is above the CS and consists of chlorite-quartz-muscovite-carbonaceous metapelite, fine grained amphibole-garnet and chert. It is similar in appearance to the SHIM. In the oxidised zone the weathered garnets commonly develop a black skin of manganese oxides. The youngest unit is the Seldom Seen schist (SSS) which is similar in appearance to the CS but has been rarely identified in outcrop or intersected in drilling.
Structure The SHIM of the Dead Bullock Ridge deposit dips steeply to the south and defines the overturned north limb of the Dead Bullock Soak anticlinorium. The deposit exhibits features similar to Villa, such as boudinaged chert horizons, parasitic folding and structural thickening, caused by the various phases of deformation (D1–D5). Post-mineralisation faulting (D3) subparallel to bedding, with up to 200 m of sinistral strike slip movement, has resulted in an imbricate repetition of the SHIM in plan and section (Figs 9 and 10). Consequently the Dead Bullock Ridge deposit consists of four distinct lodes of strike length 100 to 350 m. The D3 fault zones are 1 to 2 m wide and have associated drag folding. The SHIM of the Triumph Hill deposit forms the north limb and hinge zone of an overturned isoclinal fold (Figs 11 and 12) with the Colliwobble Ridge deposit emplaced in the southern limb. The Triumph Hill deposit is the most structurally complex of all the deposits at DBS due to the extent of dislocation by D6 faulting. Numerous other faults cut the deposit to form four distinct lodes. With interpreted fault displacements of up to 200 m, modelling based on pit floor mapping and structural data obtained from drill core indicates that additional fault-displaced lodes may exist at depth. The major WNW-trending faults observed at Triumph Hill, although not accurately traced under cover, are considered to be
the easterly extensions of the same faults that dislocate the Dead Bullock Ridge deposit, 200 m to the NW.
Mineralisation Mineralisation at Dead Bullock Ridge and Triumph Hill is almost exclusively confined to the SHIM horizon. The deposits are weathered to approximately 100 m vertical depth. The ore grade mineralisation within the weathered environment for these deposits shows virtually no secondary dispersion and is tightly confined to the SHIM horizons. Within both deposits, gold occurs in similar settings to those described for Villa, however the quartz-chlorite veins are considered to contain the greatest proportion of gold with disseminated mineralisation less important. Nevertheless there is still a close association between arsenopyrite abundance and gold grade. Higher grade ore shoots are commonly associated with zones of parasitic folding or structural thickening.
ORE GENESIS CALLIE According to G W Morrison (unpublished reports, 1993, 1994), paragenetic relationships at Callie suggest that prograde thermal metamorphism, quartz vein emplacement and hydrothermal alteration are essentially part of one complex event. The emplacement of an intrusion may have driven the thermal metamorphism and metasomatism which focussed a moderate temperature, moderate salinity fluid into the D5 structural corridor. The preparation of the country rock for vein emplacement and thermal metamorphism and associated metasomatism was responsible for the mineralogical zoning of the orebody. The gold mineralisation process responsible for the Callie orebody took place in the transition from D5 north–south compression to D6 east–west compression. The hydrothermal system responsible for the transport and deposition of gold was active in the removal of carbonaceous material from some sections of the sequence. There has been
FIG 9 - Geological plan, Dead Bullock Ridge deposit, 1395 RL. Fig 10 is on section line A–B.
Geology of Australian and Papua New Guinean Mineral Deposits
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M E H SMITH et al
FIG 10 - Cross section 10 830 E, Dead Bullock Ridge deposit, looking west.
FIG 12 - Cross section 11 160 E, Triumph Hill deposit, looking west.
was an important phase in the fluid mixing prior to gold precipitation. The Callie host unit and lower Blake beds were originally carbonaceous in part prior to circulation of hydrothermal fluids. Carbon was therefore removed from the Callie host unit, the lower Blake beds and parts of the overlying Magpie schist and Callie boudin chert before the fluids became carbon saturated (E Whittaker, unpublished data, 1995). The removal of carbon created a secondary porosity within the rocks affected, facilitating fluid flow. The contact between the Callie host unit and the Magpie schist is more likely to represent an alteration front than a stratigraphic boundary.
FIG 11 - Geological plan, Triumph Hill deposit, 1400 RL. Fig 12 is on section line A–B.
significant removal of carbon in hydrothermally affected parts of the Callie system. This is graphically illustrated by rock specimens showing dark graphitic bands terminating at quartz veins after which they continue as relatively pale bands with no apparent displacement across the vein. This leaching of carbon
458
A scheme of deposition of gold devised by G W Morrison (unpublished data, 1993, 1994) involves the interaction of a heated saline fluid derived from a deep source, with other fluids of differing composition, and with carbonate beds (lower Blake beds) and carbonaceous units within the sequence. Six fluid compositions have been identified from studies of inclusions. Although the details of their interactions are a matter of continuing study, a carbon dioxide-rich fluid is considered to have given rise to the quartz-feldspar veins which host mineralisation, and, by reaction with methane from the adjacent units, to have produced gold-bearing quartz-chlorite and quartz-biotite veins. Similarly, a lime-rich solution gave rise to the sulphide-epidote-carbonate veins which contain some gold. A number of mechanisms for precipitation can be shown to have occurred but the most important involve rapid reduction in pressure, at sites such as fractures and shears, at vein-banding intersections, and direct or indirect chemical reduction by carbonaceous material. Poor selvage development around the mineralised veins is evidence that the fluids forming the veins were in near equilibrium with the country rock through which they were passing.
Geology of Australian and Papua New Guinean Mineral Deposits
DEAD BULLOCK SOAK GOLD DEPOSITS
BIF-HOSTED DEPOSITS Debate exists between the exploration geologists and various researchers as to the timing of the formation of the Villa, Dead Bullock Ridge and Triumph Hill deposits. One opinion holds that all the gold in both disseminated and vein hosted settings was deposited during one mineralising event, which most closely correlates to the regional D5 deformation. The distribution of the gold requires that the D5 event be accompanied by the re-activation of major D 2 structures. The contrary opinion is that the deposits reflect a number of mineralising events with the distribution of the gold controlled by the major D2 structures and supplemented or remobilised during the development of subsequent structures and veining events. Recent studies by NFM geologists indicate that the original mineralising fluid was most likely a dilute, slightly reduced fluid of approximately neutral pH that carried the gold as the thio complex Au(HS)2. Sulphur would have been removed from the solution as it came into contact with iron-rich carbonates, forming sulphides. This would have caused the thio complex to become unstable and break down, resulting in the precipitation of gold. Such a reaction would have been; Au(HS)2 + H + /2H2O → 2H2S + Au + /2O -
+
1
1
This reaction would have been promoted by either the addition of H+, or the removal of H2S. This would have occurred as the fluid reacted further with the iron within the host rocks to form sulphides (pyrrhotite and pyrite) by the following reactions: Fe++ + H2S →FeS + 2H+ Fe++ + 2H2S + 1/2O2 → FeS2 + H2O + 2H+
MINE GEOLOGICAL METHODS In the Callie open pit the primary grade control method integrates assays of samples from RC drill holes and geological mapping of costeans on the pit floor. Grade control holes are drilled on a 3 by 5 m pattern to a depth of 6 m at an angle of 60ο using a 2 m sample interval. Shallow costeans are cut across the orebody at 10 m spacing using a specially designed blade to minimise displacement of the ore. After the costeans are blown clean with compressed air, they are marked on 1 m intervals and mapped. Zones of run of mine ore, mineralised waste and waste are marked on each flitch for mining. Blasting is carried out on 5 m high benches with selective mining by excavator on 2.5 m deep flitches. The primary grade control method for the BIF-hosted deposits integrates geological mapping and sampling along costeans on the pit floor. The costeans are cut across the ore body at 5 to 10 m spacing to a depth of 30–50 cm and cleaned and mapped as described above. Zones are selected for sampling at 1 m intervals from detailed mapping. Angled RC drilling is used to complement the costean sampling for
Geology of Australian and Papua New Guinean Mineral Deposits
definition of the orebody in more structurally disturbed zones or where the ore–waste boundaries have an irregular dip. The spacing of the costeaning and grade control drilling is dependent on the characteristics of the orebody. Mining bench and flitch heights are 5 and 2.5 m respectively as at Callie.
ACKNOWLEDGEMENTS The authors wish to acknowledge the permission and support given by North Flinders Mines Ltd in preparing this paper. Contributions have also been made by North Flinders Mines exploration geologists E Whittaker, R Webb, Ding Puquan, R Fidler and T Ireland in reviewing the text. The results of research by G Morrison of Klondyke Exploration Services and N Tate of Geomap on hydrothermal activity have been used in the preparation of the paper
REFERENCES Blake, D, Hodgson, I M and Muhling, P C, 1979. Geology of The Granites–Tanami Region, Northern Territory and Western Australia, Bureau of Mineral Resources, Geology and Geophysics Bulletin 197. Ding Puquan and Giles, C, 1993. Geological setting of gold mineralisation in the Tanami region, Northern Territory, Australia, in Gold Mining, Proceedings of the International Symposium on Gold Mining Technology, Beijing, 15–17 June 1993 (Eds: Organising Committee of ‘93 ISGMT), p 100 (International Academic Publishers). Gee, L C E, 1911. General report on Tanami Goldfield and district (Northwestern Central Australia), South Australia Parliamentary Paper 27. Hodgson, I M, 1976. The Granites, Northern Territory - 1:250 000 geological series, Bureau of Mineral Resources, Geology and Geophysics Explanatory Notes, SF 52–3. Hossfeld, P S, 1940. The gold deposits of The Granites–Tanami District, Central Australia, Aerial Geological and Geophysical Survey of Northern Australia, Northern Territory Report No 43. Ireland, T J, 1995. The discovery of the Callie gold deposit in the Tanami region, Northern Territory, Australia, in New Generation Gold Mines: Case Histories of Discovery, pp 6.1–6.10 (Australian Mineral Foundation: Adelaide). Lovett, D R, Giles, C W, Edmonds, W, Gum, J C and Webb, R J, 1993. The geology and exploration of the Dead Bullock Soak gold deposits, The Granites–Tanami Goldfield NT, in Proceedings The AusIMM Centenary Conference, pp 73–80 (The Australasian Institute of Mining and Metallurgy: Melbourne). Mayer, T E, 1990. The Granites Goldfield, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 719–724 (The Australasian Institute of Mining and Metallurgy: Melbourne). Plumb, K A, 1990. Halls Creek Province and The Granites Tanami Inlier - regional geology and mineralisation, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 681–695 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Lee, D C, Reddicliffe, T H, Scott Smith, B H, Taylor,W R and Ward, L M, 1998. Melin diamondiferous kimberlite pipes, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 461–464 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Merlin diamondiferous kimberlite pipes 1
2
3
4
by D C Lee , T H Reddicliffe , B H Scott Smith , W R Taylor and L M Ward INTRODUCTION The Merlin kimberlite field is in the Batten region of the NT about 90 km south of Borroloola at lat 135o20′E, long 16o50′S on the Bauhinia Downs (SE 53–3) 1:250 000 scale map sheet. The field comprises 13 kimberlite intrusives distributed in five discrete clusters. The two E.Mu pipes, which were discovered by CRA Exploration Pty Limited (CRAE) in 1986 are included in the Merlin field. The other pipes were discovered seven years later by the Australian Diamond Exploration Joint Venture (ADEJV), which is managed by Ashton Mining Limited. The Devonian aged pipes are similar to Group 1 kimberlites, are predominantly of diatreme facies, and some contain significant economic quantities of diamonds. Many of the pipes are filled with a mudstone and/or sandstone sequence up to 42 m thick. A more detailed description of the geochemistry, mineralogy and diamonds is provided by Lee et al (in press).
EXPLORATION HISTORY Exploration for diamonds in the NT by the ADEJV began in 1978, a year before the discovery of the Argyle pipe in the east Kimberley region of WA. The ADEJV commenced exploration in the northwestern portion of the NT, gradually working eastwards in the following years, resulting in the identification of areas with high concentrations of microdiamonds. The Coanjula microdiamond deposit (Lee et al, 1994) was discovered in 1986 as a consequence of this work. Prior to the ADEJV focussing its efforts on the Batten region in 1989, the area was explored independently by CRAE (Smith, Atkinson and Tyler, 1990) resulting in the discovery of the E.Mu pipes in 1986 (Atkinson et al, 1990). The identification by the ADEJV of several areas in this region with anomalously high numbers of microdiamonds and in some cases, chromite, led to a bulk sampling program to test local drainages for commercial diamond content. A 75 t sample, taken close to the then undiscovered Merlin field, produced 16 diamonds of diameter >1 mm, totalling 4.78 carats in weight, including a 2.44 carat near gem quality diamond. The area upstream of this sample was then subjected to high density stream and loam sampling, geophysical surveys and shallow drilling of
1.
Chief Mineralogist, Ashton Mining Limited, 21 Wynyard Street, Belmont WA 6104.
2.
Manager, Australian Exploration, Ashton Mining Limited, 21 Wynyard Street, Belmont WA 6104.
3.
Principal, Scott-Smith Petrology Inc, 2555 Edgemont Boulevard, North Vancouver BC VR7 2M9, Canada.
4.
Post Doctoral Fellow, Research School of Earth Sciences, Australian National University, Canberra ACT 2601.
5.
Mineralogist, Ashton Mining Limited, 21 Wynyard Street, Belmont WA 6104.
Geology of Australian and Papua New Guinean Mineral Deposits
5
geophysical and soil geochemical anomalies. Eleven kimberlites were discovered and the group was named the Merlin field. The E.Mu pipes located at the northern end of the field are now considered to be part of the same field.
REGIONAL GEOLOGY The Batten region is situated on the eastern side of the North Australian Craton (Fig 1). The area south of the western edge of the Gulf of Carpentaria is dominated by the Middle Proterozoic McArthur Basin which extends over an area of 180 000 km² (Pietsch et al, 1991) and forms part of the North Australian Platform. Early Proterozoic basement rocks include the Scrutton Volcanics which have been dated by U–Pb in zircon at 1857±30 Myr (Pietsch et al, 1991). Cambrian age Bukalara Sandstone, 30 to 100 m thick, overlies the McArthur Basin sediment in much of the region and frequently forms topographic plateaux. Flood basalt of Cambrian age becomes prevalent in the southern portion of the region, although generally obscured by younger sediment. The Merlin kimberlite field represents the youngest volcanic event in the region.
FIG 1 - Major crustal subdivisions of Australia.
Cretaceous sedimentation was widespread in the area but the sediments have now been largely removed over a distance of 200 km south from the Gulf of Carpentaria coastline. This stripped area is characterised by well dissected drainage interspersed with isolated, remnant, poorly drained planation surfaces. One such remnant surface is host to the Merlin kimberlite field. The southern limit of the stripping is marked by a well defined escarpment which acts as a drainage divide. Streams to the south flow southwards, while those on the north side of the scarp flow north to the Gulf of Carpentaria.
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D C LEE et al
The major structural feature of the southern McArthur Basin is the Batten Trough, also known as the Batten Fault Zone, a 70 km wide zone of extensive faulting, trending NNW. The Batten Trough, bounded on the east by the Emu Fault and obscured to the west by the Roper Group of sedimentary rocks, is a synsedimentary graben containing up to 10 km of McArthur Basin sediment. Associated with the Batten Trough are the Mallapunyah and Calvert faults, two NW-trending regional faults, some 50 km apart.
GEOLOGY OF THE KIMBERLITE FIELD The 13 kimberlite pipes in the Merlin field are located on the eastern shoulder of the Batten Trough, some 6 km east of the Emu Fault and on the projected trace of the Calvert Fault. The 11 kimberlites discovered by the ADEJV, named Excalibur, Palomides, Sacramore, Launfal, Kay, Ywain, Gawain, Tristram, Gareth, Ector and Bedevere (Fig 2), are preserved on a poorly drained remnant land surface composed of sand and laterite. All of the pipes, including the two E.Mu pipes, have intruded the Cambrian Bukalara Sandstone, which is flat lying and unconformably overlies Proterozoic sediment in this area.
FIG 3 - Cross section of Excalibur pipe, looking north.
E.Mu 1 is a 250 m diameter, 50 m deep amphitheatre which has been breached by Matheson Creek. The E.Mu 2 pipe, which is 200 m from E.Mu 1, is located within Matheson Creek which has eroded the Bukalara Sandstone and kimberlite to a depth of 50 m. Fractures, which are concentric with this pipe, occur in the sandstone adjacent to the pipe. Drill core samples from the kimberlites show that there is variation in texture, xenolith content and degree of weathering. An emplacement model or isochron age±initial ratio of 367±4 Myr has been determined for a single sample of drill core from the Excalibur pipe by Rb-Sr in mica age dating techniques (Amdel, unpublished data, 1994). The age of the E.Mu pipes is reported as 360 Myr by Atkinson et al (1990). These data suggest that the kimberlites were emplaced during the Alice Springs Orogeny (Bradshaw and Evans, 1988).
PETROGRAPHY
FIG 2 - Pipe locations, Merlin field.
Five discrete clusters of pipes are present in the elongate field, which extends over an area of 10 km by 5 km. Within each cluster the distance between the pipes varies from 100 to 400 m. The largest pipe in the field is the E.Mu 1 pipe, which is 4.5 ha in area. All the pipes are steep sided, generally close to cylindrical in shape, and maintain their surface diameter to depths exceeding 100 m. Eight of the pipes are filled with sediment to depths of up to 42 m below the land surface. In addition, two of the pipes are covered by a thin veneer of Cretaceous sediment, which is preserved within parts of the sand and laterite surface. The fill is typically fine to medium grained sandstone overlain by mudstone (Fig 3). The age relationship of the fill sediment to the age of the kimberlites is not known. Apart from the two E.Mu pipes, the pipes have no significant surface expression. The surface expression of
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The kimberlites are difficult to investigate petrographically due to the extensive alteration which includes carbonatisation, silicification, partial replacement by secondary sulphides, clay mineralisation and chloritisation. Drill core and drill chips derived from depths between 30 and 232.5 m have been examined. All of the rocks contain two generations of pseudomorphs after olivine which include anhedral and often rounded macrocrysts (to 15 mm diameter) as well as numerous smaller (<1 mm diameter) typically euhedral phenocrysts. Mica phenocrysts (to <1 mm diameter) are present in variable abundance. The mica occurs as lath-like grains which are typically partially altered to green chlorite and distorted by the secondary replacement processes. At Excalibur, some of the mica phenocrysts were formed by two phases of crystallisation. Some of the earlier clear cores have been overgrown by a late stage mica which poikilitically encloses fine grained spinel grains similar to those that occur in the adjacent groundmass. The two phases of mica have slightly different optical properties and probably have different compositions.
Geology of Australian and Papua New Guinean Mineral Deposits
MERLIN DIAMONDIFEROUS KIMBERLITE PIPES
Observed primary groundmass minerals include mica, spinel, apatite, serpentine and carbonate (the last named at Ector, Excalibur and Palomides). Possible pseudomorphs after groundmass monticellite were observed at Kay and Ector. The nature of the olivines and the mineralogy of the matrix are typical, or even diagnostic, of kimberlites and are not characteristic of other rock types such as lamproites. This feature strongly supports the classification of these rocks as kimberlites and no features were observed which suggest that these rocks are not kimberlites. Even though mica is common and ilmenite is rare or absent, the nature of the mica and the presence of abundant groundmass spinel and possible monticellite suggest that the kimberlites are similar to Group 1 kimberlites (Mitchell, 1986). The xenolith suite includes carbonates, quartzose rocks and laminated fine grained sediments as well as igneous rocks such as granite and glimmerite. Some of the xenoliths have been ‘kimberlitised’, sometimes beyond recognition (cf Scott Smith, Skinner and Clement, 1983). The Merlin pipes have complex textures typical of kimberlite. A few rocks have uniform groundmasses and appear to comprise hypabyssal kimberlites but non-uniform textures are more common. Irregular segregationary groundmasses occur in hypabyssal kimberlites at Excalibur (serpentine with minor carbonate) and Ector (carbonate). Other rocks have well developed pelletal textures which usually comprise a single olivine or mica grain surrounded by a thin rim of very fine grained material. In a few instances probable microlitic textures were observed within the rims. The interclast matrices of these rocks appear to be composed of serpentine with rare microlitic textures and carbonate is absent. These textures are a hallmark of diatreme-facies kimberlites. Such rocks can be described as pelletal tuffisitic kimberlite (±breccias). Although difficult to discern, the probable microlites appear to be composed of mica in the Launfal, Gawain and possibly Bedevere pipes. Clinopyroxene is more typical of diatreme-facies kimberlites elsewhere, so this feature seems somewhat unusual. In one sample from Gawain, pelletal textures are not developed and the rock appears to be composed of single grains of olivine, and the rock is classified as a tuffisitic kimberlite breccia. There is no evidence to suggest that any of the samples classified as tuffisitic kimberlites are crater-facies rocks. However, bedded tuffs were reported from E.Mu 1 by Smith et al (1990) down to a depth of 68 m, underlain by magmatic kimberlite with massive tuff below. These authors described an abundance of spherical nucleated lapilli as supporting their interpretation of the sequence as crater-facies infill. At Palomides, in addition to the thin-rimmed pelletal lapilli described, all the rocks examined contain common coarser (to >10 mm diameter) spherical structures giving the rocks a distinctive appearance. These structures comprise a kernel of olivine or a xenolith with a thick (up to at least 5 mm) dark grey kimberlite selvage. Other similar structures sometimes have no apparent kernel. The selvages are composed of olivine phenocrysts, mica and possible primary carbonate laths in a matrix of serpentine and carbonate. The groundmass in the selvages probably crystallised rapidly. The inter-clast areas are composed of serpentine and variable amounts of carbonate. The paragenesis of carbonate is notoriously difficult to determine and it is not clear if any of the carbonate in these rocks is primary. The larger spherical structures and any
Geology of Australian and Papua New Guinean Mineral Deposits
primary carbonate, especially in the inter-clast areas, are typical features of globular segregationary hypabyssal kimberlites while the smaller pelletal lapilli are more typical of diatreme-facies kimberlites. It is possible that the larger structures formed either during diatreme formation or earlier, during ascent in a more hypabyssal-like environment and then were carried through into the final fluidisation process during diatreme formation.
GEOCHEMISTRY Extensive alteration of the kimberlites by weathering and contamination by xenoliths has affected the composition of most samples. One sample of carbonatised rock from Palomides and a drill core sample from a depth of 248 m in Excalibur have a low Ilmenite index of <0.47 [Ilmenite index = (FeOt+TiO2)/(2 K2O+MgO)], that falls within the range of relatively unaltered kimberlite (Taylor, Tompkins and Haggerty, 1994). The trace elements of these samples should, therefore, provide a reasonable basis for comparison with other kimberlites. A useful geochemical indicator is the Nb:Zr ratio, because niobium and zirconium are relatively immobile during alteration and weathering. These elements tend to be diagnostic for a particular mantle source. For the Merlin kimberlites, Nb:Zr is very high, having a value close to that of the Aries kimberlite located in northern WA (W R Taylor, personal communication, 1995). The high Nb:Zr ratio clearly indicates that the Merlin kimberlites do not have geochemical affinities with olivine lamproites, such as Argyle, or the Group 2 kimberlites of South Africa. Other ratios such as low Ba:Rb and high Nb:La are also characteristic of the Aries kimberlite.
MINERALOGY AND MINERAL CHEMISTRY Minerals found in heavy concentrates are dominated by chrome spinel with minor amounts of pyrope garnet, chrome diopside, apatite and sulphides. Varying amounts of galena, sphalerite, iron sulphides and zircon, probably derived from local crustal rocks, are present in some pipes. Representative analyses of chrome spinels, pyrope garnets and chrome diopside are shown in Tables 1 and 2.
TABLE 1 SEM-EDS analyses of representative range of spinels from Merlin kimberlites. 1
2
3
4
5
TiO 2
0.16
0.22
0.65
0.29
1.00
Al2O3
11.6
3.40
5.37
37.6
12.8
Cr2O3
59.0
67.9
62.9
27.2
38.0
Fe2O3
1.59
1.53
3.02
3.86
20.1
V2O3
0.33
0.22
0.34
0.20
0.13
FeO
14.4
14.2
17.1
18.4
16.4
MnO
0.00
0.00
0.37
0.00
0.29
MgO
12.8
12.1
10.50
12.9
11.7
NiO
0.00
0.0
0.00
0.38
0.28
Total
99.8
99.4
99.9
100.3
98.8
Mg#
61.2
60.2
52.2
55.5
56.0
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TABLE 2 SEM - EDS analyses of a representative rangeof pyrope garnets and two clinopyroxenes from Merlin kimberlites. 1
2
3
4
5
6
7
SiO2
41.6
40.7
41.4
42.2
42.7
55.8
55.1
TiO2
0.00
0.00
0.00
0.16
0.00
0.00
0.00
Al2O3
16.6
14.5
14.3
21.1
17.3
2.14
0.86
Cr2O3
8.54
11.9
10.4
1.88
6.58
2.39
1.51
Fe2O3
1.72
0.82
3.13
1.98
3.09
0.68
1.05
FeO
5.49
7.26
4.75
7.79
4.13
1.73
1.46
MnO
0.28
0.38
0.28
0.16
0.00
0.00
0.00
MgO
22.4
18.3
20.4
18.2
21.9
16.8
17.2
CaO
2.61
6.65
5.53
7.20
4.14
18.2
21.1
2.54
1.48
Na2O Total
99.27
100.45
99.91
100.55
99.52
100.27
99.84
Mg#
87.9
81.8
88.5
80.65
90.4
94.5
95.4
Notes: 1 to 5 - pyrope garnets; 6 and 7 - chrome clinopyroxenes.
DIAMONDS Diamonds are present in all the Merlin kimberlites. Grades vary from trace amounts in the E.Mu pipes to 102 carats/100 t in Ywain based on >1.2 mm diamonds. There is an abundance of fine diamonds in the size range >0.1 mm to 1.2 mm which are not recovered during bulk sampling to establish commercial grades. Samples of 200 kg, processed to establish the microdiamond content of the kimberlites, were found to contain up to seven small diamonds per kg. The quality of the commercial sized (>1.2 mm) diamonds varies from industrial to high value gem stones and diamonds larger than 10 carats in size occur in the kimberlites. An average price of $US60 per carat (Palomides) to $US76 per carat (from the Sacramore pipe) has been established for small parcels of diamonds obtained from bulk samples of 1000 t. A decision to proceed with a feasibility study for mining at Merlin was made in November 1996. Mine planning, engineering design and final feasibility studies will be based on an initial plant throughput of 0.5 Mtpa with the ore sourced from open pits on the four pipes that have been bulk sampled to date. The resource base targeted for open pit and possible underground development within these four pipes is in the order of 5 Mt. Further deep drilling and sampling will be necessary to confirm resource numbers. Increases in plant size to 1 to 2 Mtpa, with ore sourced from underground and open cut operations from these and other pipes in the field, will be studied as a second stage development. The first stage processing plant will be used to complete bulk sampling of the remaining eight pipes in the Merlin cluster. A decision on mine development, subject to satisfactory feasibility results, is expected in May 1997, with first production possible in the first quarter of 1998.
ACKNOWLEDGEMENTS This study was supported by Ashton Mining Limited and the Australian Diamond Exploration Joint Venture, and their permission to publish is gratefully acknowledged. C B Smith reviewed the manuscript and made many helpful suggestions
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for improvement. J N Lemon and T J Barlow are thanked for their assistance in preparing the text and diagrams.
REFERENCES Atkinson, W J, Smith, C B, Danchin, R V and Janse, A J A, 1990. Diamond deposits of Australia, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 69–76 (The Australasian Institute of Mining and Metallurgy: Melbourne). Bradshaw, J D and Evans, P R, 1988. Palaeozoic tectonics, Amadeus Basin, Central Australia, APEA Journal 1988, 28(1):267–282. Lee, D C, Boyd, S R, Griffin, B J, Griffin, W L and Reddicliffe, T H, 1994. Coanjula diamonds, Northern Territory, Australia, in Proceedings of the Fifth International Kimberlite Conference, Araxa--Brazil, June 1991, Diamonds: Characterisation, Genesis and Exploration (Eds: H O A Meyer and O H Leonardos), CPRM Special Publication 1B, pp 51–68. Lee, D C, Milledge, H J, Reddicliffe T H, Scott Smith B H, Taylor W R and Ward L M, in press. The Merlin kimberlites, Northern Territory, Australia, in Proceedings of the Sixth International Kimberlite Conference, Novosibirsk, Russia, August 1995. Mitchell, R H, 1986. Kimberlites: Mineralogy, Geochemistry and Petrology (Plenum Publishing: New York). Pietsch, B A, Rawlings D J, Creaser, P M, Kruse, P D, Ahmad, M, Ferenczi, P A and Findhammer,T L R, 1991. Bauhinia Downs, Northern Territory - 1:250,000 geological series, Department of Mines and Energy, Northern Territory Geological Survey Explanatory Notes SE 53–3. Scott Smith, B H, Skinner, E M W and Clement, C R, 1983. Further data on the occurrence of pectolite in kimberlite, Mineralogical Magazine, 47:75–78. Smith, C B, Atkinson, W J, and Tyler, E W J, 1990. Diamond exploration in Western Australia, Northern Territory and South Australia, in Geological Aspects of the Discovery of Some Important Mineral Deposits in Australia (Eds: K R Glasson and J H Rattigan), pp 429–453 (The Australasian Insitute of Mining and Metallurgy: Melbourne). Taylor, W R, Tompkins, L A and Haggerty, S E, 1994. Comparative geochemistry of West African kimberlites: evidence for a micaceous kimberlite endmember of sublithospheric origin, Geochimica et Cosmochimica Acta, 58:4017–4037.
Geology of Australian and Papua New Guinean Mineral Deposits
Hills, P B, 1998. Tasmania gold deposit, Beaconsfield, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 467–472 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Tasmania gold deposit, Beaconsfield by P B Hills
1
INTRODUCTION
MINING AND EXPLORATION HISTORY
The Tasmania reef is situated within the township of Beaconsfield, 40 km NW of Launceston, Tas, at lat 41o12′S, long 146o49′E and AMG coordinates 484 000 E, 5 439 000 N, on the Launceston (SK 55–4) 1:250 000 scale and Tamar (8215) 1:100 000 scale map sheets (Fig 1). The deposit is being developed by the Beaconsfield Mine Joint Venture between Allstate Explorations NL (51.51%) who provide management and Beaconsfield Gold NL (48.49%).
Gold was discovered at Beaconsfield in 1877 and mining continued until 1914. Mine closure was due to increasingly refractory ore and difficulties in pumping large volumes of water, from what was then the wettest mine in Australia, exacerbated by labour shortages due to the First World War. At closure, the Tasmania reef had been mined to a depth of 455 m below surface. The mine was abandoned and left to flood, with water reaching the surface in 1938. Production during the life of the mine was 1.085 Mt at a recovered grade of 24.5 g/t gold to yield 854 000 oz. Records at mine closure stated that there was no indication that the Tasmania reef was diminishing at depth and indeed indicated that a substantial resource might remain. The Tasmanian Mines Department unsuccessfully attempted to create interest in the property by drilling between 1938 and 1942. A second attempt between 1964 and 1967 obtained three high grade reef intercepts from a single collar and the modern history of the mine began. Allstate Explorations NL was the first modern explorer and a number of joint ventures managed the project from 1969. Some diamond drilling was undertaken but the prime concern of all managers was to rehabilitate the collar of the Hart shaft which had collapsed and blocked the shaft, and to dewater the historical workings. Re-establishment of the collar and clearing of the blockage was completed in 1989. The current management structure commenced in 1992 and emphasised diamond drilling and proving the resource. Successful dewatering to the 375 m level followed and the establishment of underground drilling platforms for close spaced resource definition brought the project to the feasibility study stage in July 1997.
FIG 1 - Locality and regional geological map for the Tasmania gold deposit after Gee and Legge (1971), Gulline and Naqvi (1973) and Hills (1982).
A feasibility study was completed in September 1997 to assess mine viability. The Joint Venture has established a total resource of 2.095 Mt grading 19.96 g/t gold (1.344 M contained oz) including an Indicated Resource of 1.442 Mt grading 24.64 g/t gold (1.143 M contained oz) and an Inferred Resource of 653 000 t grading 9.61 g/t gold (202 000 contained oz). Within the Indicated Resource is a Probable Ore Reserve of 1.534 Mt grading 22.32 g/t gold (1.101 M contained oz), sufficient for ten years production. Production is scheduled to commence during the 1998–99 financial year.
1.
Project Geologist, Beaconsfield Mine Joint Venture, PO Box 58, Beaconsfield Tas 7270.
Geology of Australian and Papua New Guinean Mineral Deposits
PREVIOUS DESCRIPTIONS The geology of the Beaconsfield district was described by Gould (1866) and Johnson (1888). Details of the geology of the Beaconsfield mine and the Tasmania reef were provided by Thureau (1883), Montgomery (1891), Twelvetrees (1903), Cundy and Fawcett (1914), Hughes (1953), Noldart (1964, 1968) and Noldart and Threader (1979). Williams, McClenaghan and Collins (1989), Hicks and Sheppy (1990) and Russell and van Moort (1992) have published descriptions in the modern era and numerous authors have made unpublished contributions in reports to management. Of particular importance were T V C Willsteed (unpublished data, 1973), T W Middleton (unpublished data, 1974), C D F Pease (unpublished data, 1984a, 1984b, 1985) N R Sheppy (unpublished data, 1986), J D Hicks (unpublished data, 1991) and L A Newnham (unpublished data, 1994, 1995).
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REGIONAL GEOLOGY A distinct structural and stratigraphic terrane, the Beaconsfield block, was identified by Elliott, Woodward and Gray (1993) to describe the Lower and Middle Palaeozoic sequence lying between the Precambian Badger Head block and the Tamar River (Fig 1). Quartzites of the allochthonous (Powell and Baillie, 1992) Badger Head block are overlain by a fault bounded (Berry and Crawford, 1988) mafic-ultramafic complex of Cambrian age, the Andersons Creek MaficUltramafic Complex which forms the base of the Beaconsfield block. This in turn is overlain by trough-filling andesitic volcanic and marine sedimentary rocks analogous to the Mount Read Volcanics and associated sedimentary rocks of the Dundas Trough of western Tasmania. The analogy to western Tasmania continues with an upward fining retrograding fluvial to marine sequence of conglomerate, sandstone and grit, siltstone, limestone and graphitic shale of Late Cambrian to Ordovician age. This sequence has sedimentological affinities with the Denison and Gordon groups of western Tasmania and incorporates the Cabbage Tree Formation, including the Transition beds, which host the gold mineralisation at Beaconsfield. The Ordovician sequence is overlain by SiluroDevonian turbidite sediments correlated with the Mathinna beds of NE Tasmania. The entire sequence was subjected to compressional deformation and imbricate thrusting during the Late Devonian Tabberabberan Orogeny.
conglomerate (Cabbage Tree Conglomerate) overlain by sandstone with coarse grit interbeds (Lower Transition beds) which grade to the fine grained sandstone and siltstone, variably calcareous, with stylolitic limestone interbeds of the Upper Transition beds. (The term ‘Transition beds’ was coined by the early miners to describe the gradational sequence from conglomerate to limestone hosting the Tasmania reef.). The sequence then grades conformably to the Flowery Gully Formation including limestone, the Flowery Gully Limestone of Noakes, Burton and Randal (1954) and shale of the Grubb Shale or Grubb beds (Green, 1959). In the eastern Australian Tasman Fold Belt context, Beaconsfield is at the boundary between the Kanmantoo and Lachlan fold belts. At Flowery Gully, 8 km south of Beaconsfield, the Late Silurian or Devonian Corn Hill beds of Hills (1982) which have northeastern Tasmania Mathinna beds sedimentological and faunal affinities, overlie and is imbricated within thrust slices of the Ordovician succession. The Mathinna beds sequence of slate, shale and deep water greywacke turbidite represents the Lachlan Fold Belt in Tasmania. The geology of the younger rocks is not directly relevant to the mineralised province and is not described further. Figure 2 illustrates the Lower and Middle Palaeozoic stratigraphy. Details of the stratigraphy in the vicinity of the Tasmania reef are discussed in further detail in the next section.
A prograding sequence of limestone, mudstone and sandstone of Permo-Triassic age unconformably overlies but has largely been eroded off the underlying Lower Palaeozoic sequence in the immediate environs of Beaconsfield. This sequence, which occurs throughout Tasmania and generally dips to the NE at approximately 10o, is in turn intruded by a thick dolerite sill of Jurassic age. Tertiary basalts fill the Tamar River Graben formed in an extensional environment of continental break-up and Tertiary and Quaternary gravels fill all low-lying depressions.
LOCAL GEOLOGY STRATIGRAPHY The lowermost Ordovician age of the sparse fauna preserved in the Beaconsfield area (Kennedy, 1971; Laurie, 1996a, 1996b) when compared with the biostratigraphy of Banks and Burrett (1980), makes previous direct correlation with the similar Middle Ordovician succession of western Tasmania inappropriate. A localised stratigraphic succession based on the work of earlier authors has been adopted. The effective base of the sequence at Beaconsfield is a fine grained siltstone of marine origin intercalated with rare andesitic volcanic rocks of Cambrian age, the Dally’s Siltstone and Keratophyre (Green, 1959). This sequence occupies an equivalent stratigraphic position to the lowermost sedimentary rocks of the Dundas Group and the Mount Read Volcanics of western Tasmania. Disconformably, and possibly unconformably overlying the Cambrian sequence, the Ordovician sequence is an upwardfining shallow marine or tidal sequence as evidenced by sedimentary structures and a sparsely preserved fauna. The base of the sequence includes phyllitic shale and siltstone with limestone, the Blyth’s Creek Formation (Green, 1959). This is overlain, possibly disconformably, by the Cabbage Tree Formation. The formation (Gould, 1866) is a sequence of siliciclastic sedimentary rocks including medium grained
468
FIG 2 - Lower and Middle Palaeozoic stratigraphy in the Beaconsfield area compiled with reference to Gould (1866), Noakes, Burton and Randal (1954), Green (1959) and Hills (1982).
Tasmania reef host rocks The Tasmania reef is hosted by the Transition beds of the Cabbage Tree Formation. The relation of the Tasmania reef to the stratigraphic succession is illustrated in Fig 3. The Cabbage Tree Conglomerate, over a true thickness of 50 m, is a fine to medium grained unit containing well rounded subspherical quartz pebbles of 1 to 8 mm diameter in a clastrich though matrix-supported conglomerate. The conglomerate has a crude bedding consistent with the overall
Geology of Australian and Papua New Guinean Mineral Deposits
TASMANIA GOLD DEPOSIT, BEACONSFIELD
FIG 3 - Longitudinal projection of historic mine workings illustrating the relation of the Tasmania reef to the stratigraphic succession, looking NW.
stratigraphic sequence. Detrital chromite grains are a distinctive feature of this unit, indicating exposure of the underlying Andersons Creek Mafic-Ultramafic Complex at the time of deposition of the conglomerate in a supralittoral environment. The Lower Transition beds, with a true thickness of 120 m in the vicinity of the Tasmania reef, is predominantly a sandstone although conglomeratic facies persist to varying degrees throughout. An upper littoral, though retrograding depositional environment is indicated. The lowermost member of the Lower Transition beds is the Sandstone, Grit and Pebble Conglomerate member with a true thickness of around 90 m. The unit is typically a poorly bedded, occasionally bioturbated, well graded medium-grained quartz sandstone often containing calcareous fossil fragments 2–3 mm long on the bedding surface. The sandstone is interbedded with grit and pebble conglomerate horizons generally 0.5 to 3.0 m thick and comprising around 35% of the rock mass. Subrounded to rarely angular subspherical quartz pebbles to 8 mm diameter and occasional angular subspherical to elongate carbonaceous lithic fragments of 3–5 mm diameter in a supporting quartz sandstone matrix typify these horizons. The Wet Beds member, so named by the early miners from the high water inflows encountered when the unit was intersected by development, is a microconglomerate or grit with a true thickness averaging 10–15 m. Well rounded quartz pebbles of moderate to high sphericity and 3–6 mm in diameter, and rare subangular to rounded generally elongate carbonaceous lithic fragments in the range 2–4 mm long, occur in a clast-supported matrix-poor horizon with occasional fine to medium grained quartz sandstone interbeds. The unit is typically extremely porous due to the paucity of the matrix which, if present, is typically a late carbonate-rich fill.
Geology of Australian and Papua New Guinean Mineral Deposits
The upper member of the Lower Transition beds is the 20 m thick Sandstone and Pebble Beds member. This unit is typically a well bedded occasionally bioturbated fine grained quartz sandstone or siltstone, with pebble beds 0.1–0.5 m thick on a 4–6 m spacing. The beds contain well rounded highly spherical quartz pebbles of 4–7 mm diameter in an open sandstone matrix. Thin intensely cross bedded and bioturbated stylolitic limestone interbeds to 0.5 m thick occur occasionally, towards the top of the unit. The top of the Lower Transition beds is defined by the uppermost pebble bed of the Sandstone and Pebble Beds member. The Upper Transition beds is also divided into three units with a total thickness of 180 m. A lower littoral or shallow marine depositional environment is indicated by a variety of clastic and limey facies and the presence of shallow marine fossils. The lower member of the Upper Transition beds is the Siltstone, Sandstone and Stylolitic Limestone member. This unit has a true thickness of around 120 m and comprises generally well bedded fine to medium grained quartz siltstone and minor fine grained quartz sandstone interbedded with strongly bioturbated and cross bedded stylolitic limestone. The latter beds are 0.5–4.0 m thick and comprise approximately 15% of the rock mass outside a 25–30 m zone 20 m above the base of the unit where they comprise 50–60%. Marine fossils including brachiopods and trilobites occasionally occur in the limestone beds. The Calcareous Siltstone member is a 20 m thick fine grained quartz siltstone with a calcareous cement, characterised by intense colour due to hematite. The siltstone is extensively cross bedded and bioturbated and contains numerous, generally unidentifiable, fossil fragments. The uppermost member of the Upper Transition beds is the Siltstone and Limestone member.
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FIG 4 - Stereographic representation of major discontinuity sets including: 1, bedding; 2, thrust surfaces (D1); 3, σ1 parallel shears (the Tasmania reef) (D2); 4 and 5, conjugate discontinuities about 3 (D3); and 6, flat faults (D4).
It is an alternating sequence of fine grained quartz siltstone and relatively pure stylolitic limestone beds 1.0–2.5 m thick over a total thickness of 40 m. The Flowery Gully Limestone is 170 m thick in the vicinity of the Tasmania reef but is known to vary across the district. The unit is a pure stylolitic limestone of deep water origin prone to the development of karst features. Caves have been intersected in diamond drill holes and are known at other localities. Sinkholes are also common and filled sinkholes and deeply weathered clay derived from the limestone underlie much of the town of Beaconsfield. Indeed the weathered surface expression of the Flowery Gully Limestone was previously thought to be a deep lead or channel fill. The Flowery Gully Limestone is overlain by the Grubb Shale.
Cabbage Tree Formation, interspersed with valleys bottomed in the Flowery Gully Formation, but usually filled with Tertiary and later gravels. The thrust fault planes are not readily observable in outcrop but are present as milled breccia and melange over thicknesses exceeding 20 m in diamond drill core in the vicinity of the Tasmania reef. At the micro scale, the D1 thrusting event is manifest as reverse sense striations on bedding surfaces, parallel to the direction of Devonian ENE–WSW compression.
The structural geology of the Beaconsfield area is dominated by faulting through successive compressional and extensional regimes. Here the Cabbage Tree Formation outcrops as a series of imbricate thrust slices with a regional dip of around 60o and a dip direction of 046o. Faulting is dominated by thrusting (often bedding parallel) and transverse jogs.
Second order deformation, D 2, resulted in the development of through-going transverse shears parallel to the ENE–WSW σ1 direction. The D2 structures measured at 60o/131o bisect a steeply dipping conjugate discontinuity set D3. These discontinuities have dips and dip directions of 72°/291° and 64o/166o and clearly relate to the D2 event. No sense of displacement has been reliably measured on either surface although small offsets of D2 structures have been recorded. A further discontinuity set, D4, has been measured at 26o/176o. This is a less conspicuous set in development mapping than D3 but a small normal offset on a similarly oriented structure was reported by the early miners to displace the Tasmania reef by up to 15 m in one locality. The dominant discontinuity sets are illustrated in Fig 4.
Middle Devonian thrusting occurred in a compressional regime of continental accretion which affected much of eastern Australian. Imbricate thrust slices of the Ordovician sequence are readily apparent on a macro and meso scale as prominent NNW-trending strike ridges of the lower members of the
Crosscourse faults (so called by the early miners because they offset the strike of the reef) are thought to post-date all previous discontinuities but their timing is not well understood. At least two faults with a dextral displacement of 50–70 m and reverse dip slip movement of as much as 90 m dip steeply to the
STRUCTURE
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Geology of Australian and Papua New Guinean Mineral Deposits
TASMANIA GOLD DEPOSIT, BEACONSFIELD
SW. They can be mapped or inferred on the surface in several localities on Cabbage Tree Hill. The faults offset the Tasmania reef near its western margin above -200 m RL and the current resource is not affected. On a regional scale, an extensional regime during the Tertiary led to the formation of a graben which is now occupied by the Tamar River. Parallel graben structures occur at several locations across Tasmania, particularly northern Tasmania. The effect of these features on the structural evolution of the Tasmania reef, if any, is not known.
Several phases of quartz and ankerite veins occur, with a significant base metal sulphide component, and contain gold at high grade (>20 g/t) and fineness (900–950). Gold occurs both as coarse particulate free gold in quartz and ankerite and intimately associated with the sulphides, particularly pyrite and arsenopyrite. Russell and van Moort (1992) recognised eight phases of vein paragenesis within the Tasmania reef. In chronological order these are: 1.
milky coarse crystalline quartz;
2.
creamy ankerite-pyrite;
Tasmania reef
3.
quartz with auriferous pyrite;
The reef is a fairly regular sheet vein occupying a D2 shear. It has a variable though roughly predictable average true thickness of 3 m. It has an average horizontal thickness of 3.3–3.5 m. Variations in thickness along a strike length of 320–350 m are apparently related to the relative ductility of the host rocks although the detailed mechanics of the relationship requires further investigation. The reef is known historically and from recent diamond drilling to bifurcate in some areas and the resulting ‘horses’ of country rock are apparently continuous down plunge.
4.
auriferous quartz;
5.
ankerite-arsenopyrite;
6.
ankerite-chalcopyrite-sphalerite-galena;
7.
non-auriferous bluish-white quartz; and
8.
vuggy ankerite.
Although generally occurring as an anastomosing network of shears and joints in ductile rocks, at the site of the Tasmania reef, dilation of the D2 structure perpendicular to σ1 compression in the brittle Transition beds provided a focus for significant vein mineralisation. Historical reports state that the reef ‘feathers’ into the conglomerate and limestone at the western and eastern margins of the resource. The D2 structure itself is not so constrained. It is however accepted for the purposes of this study that economic mineralisation plunges to the ENE at 75o paralleling the apparent dip of the country rock and is restricted to the envelope of the Transition beds. A dextral displacement of around 40 m has been measured on the reef. Although thin sheared pug seams and movement striae are occasionally associated with the reef in diamond drill core, their relation to the D2 structures is presumed rather than demonstrated as the condition of core makes the detailed measurement of orientations unreliable.
ALTERATION AND MINERALISATION The Tasmania reef at Beaconsfield is in many respects analogous to the gold-arsenic-quartz dilational-fill mesothermal deposits of Central Victoria, particularly the famous producers at Ballarat and Bendigo (Ramsay et al, 1996). Typical alteration features of those deposits including lower greenschist facies metamorphism of the interbedded quartz-rich and carbonaceous host rocks, arsenopyrite-pyrite haloes to mineralisation and to a lesser extent, carbonate porphyroblasts, are all associated with the Tasmania reef system. Mild chlorite-sericite alteration of the country rock is also characteristic of the immediate environs of the reef. Petrological analyses of typical siltstone, sandstone and grit from throughout the stratigraphic pile by C F D Pease (unpublished data, 1985) indicate that the country rock is predominantly subrounded, incipiently recrystallised, stressed quartz grains plus interstitial sericite with minor chlorite and carbonaceous matter representing recrystallised clays. Minor detrital muscovite was also observed. Commonly observed gangue minerals are rutile, tourmaline, leucoxene and zircon. Irregular porphyroblasts of ankerite and scattered pyrite grains are also present. Alteration was observed to have little affect on limestone.
Geology of Australian and Papua New Guinean Mineral Deposits
An episode of brecciation preceding each phase was recognised in microfracturing and straining of quartz crystals. Dominant minerals are quartz, ankerite and pyrite with lesser chalcopyrite and arsenopyrite and accessory galena, sphalerite and gold. The resource is relatively silver-poor although mineragraphic studies indicate its presence with pyrite in the earliest phase of metallic mineralisation.
ORE GENESIS Studies completed by Russell and van Moort (1992) indicated that a metamorphic origin for the mineralising fluids was most likely. This finding is consistent with current models for the gold deposits of Central Victoria (Ramsay el al, 1996; Phillips and Murphy, this publication). The spatial association of the Tasmania reef and host rocks with the Andersons Creek MaficUltramafic Complex is considered significant as the complex is the most likely ultimate source of the gold in a role analogous to that suggested for the Cambrian greenstones in Victoria (Broome et al, 1996).
MINE GEOLOGICAL METHODS Production at Beaconsfield is still in the future. It is proposed that narrow vein cut and fill mining will be used to extract ore for subsequent conventional treatment on site. The resource is broadly delineated by diamond drilling which provides tight global control on volume and grade but it will be impractical to translate this to day-to-day production. The high grade nature of the mineralisation and the presence of particulate gold will necessitate tight geological controls on ore production with constant face mapping and sampling to ensure optimum recovery.
ACKNOWLEDGEMENTS The permission of the Beaconsfield Mine Joint Venture to publish is acknowledged. The author wishes to thank M Cooper and G M Bennett for their work on drafting the figures.
REFERENCES Banks, M R and Burrett, C F, 1980. A preliminary Ordovician biostratigraphy of Tasmania, Journal of the Geological Society of Australia, 26:363–376.
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Berry, R F and Crawford, A J, 1988. The tectonic significance of Cambrian allochthonous mafic-ultramafic complexes in Tasmania, Australian Journal of Earth Sciences, 35:523–534.
Laurie, J R, 1996b. Correlation of Lower-Middle Ordovician clastics in Tasmania, Australian Geological Survey Organisation Record, 1996/23 (unpublished).
Broome, J M N, Ramsay, W R H, Keays, R R, Hughes, M, Arne, D A and Reeves, S, 1996. Cambrian volcanics and interflow sediments - Colbinabbin, Victoria - their stratigraphy, petrology, geochemistry and potential as a source for turbidite-hosted gold deposits of Central Victoria, Geological Society of Australia Abstracts, 41:59.
Montgomery, A, 1891. Report on the geological structure of the Beaconsfield goldfield, Report to the Secretary of Mines Tasmania, 1890–1891, pp 43–57.
Cundy, W H and Fawcett, L, 1914. Tasmania Gold Mine, Beaconsfield, Tasmania Department of Mines Report, TCR 14–016.
Noldart, A J, 1964. Notes on auriferous deposits, Beaconsfield goldfield. Department of Mines Tasmania Technical Report No 8, pp 10–22.
Elliott, C G, Woodward, N B and Gray, D R, 1993. Complex regional fault history of the Badger Head region, northern Tasmania, Australian Journal of Earth Sciences, 40:155–168.
Noldart, A J, 1968. Exploratory diamond drilling, Technical Reports, Tasmanian Mines Department (unpublished).
Noakes, L C, Burton, G M and Randal, M A, 1954. The Flowery Gully Limestone deposit, Report to the Bureau of Mineral Resources, Australia (unpublished).
Gee, R D and Legge, P J, 1971. Geological atlas 1:63 360 series, sheet 30 (8215N), Beaconsfield (Department of Mines, Tasmania: Hobart).
Noldart, A J and Threader, V M, 1979. Economic geology, in Beaconsfield (Eds: R D Gee and P J Legge), pp 67–112 Department of Mines Tasmania Explanatory Report, Sheet 30 (8215N).
Gould, C, 1866. Report on the geology of country around Ilfracombe, Tasmania, Tasmanian House of Assembly Paper, Report No 76 (unpublished).
Powell, C McA and Baillie, P W, 1992. Tectonic affinity of the Mathinna Group in the Lachlan Fold Belt, Tectonophysics, 214:193–209.
Green, D H, 1959. Geology of the Beaconsfield region, including the Anderson’s Creek Ultrabasic Complex, Records of the Queen Victoria Museum, Launceston, New Series No 10.
Ramsay, W R H, Arne, D A, Bierlein, F P and VandenBerg, A H M, 1996. A review of turbidite-hosted gold deposits, central Victoria: Regional setting, styles of mineralisation and genetic constraints, in Sedimentary-hosted Mesothermal Gold Deposits - A Global Overview (Ed: W R H Ramsay) pp 9–18 (University of Ballarat: Ballarat).
Gulline, A B and Naqvi, I H, 1973. Geological atlas 1:63 360 series, sheet 38 (8215S), Frankford (Department of Mines, Tasmania: Hobart). Hicks, J D and Sheppy, N R, 1990. Tasmania gold deposit, Beaconsfield, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1225–1228 (The Australasian Institute of Mining and Metallurgy: Melbourne). Hills, P B, 1982. The geology of the Lower and Middle Palaeozoic rocks of Flowery Gully, Northern Tasmania, BSc Honours thesis (unpublished), University of Tasmania, Hobart. Hughes, T D, 1953. The Beaconsfield and Lefroy goldfields, in Geology of Australian Ore Deposits (Ed: A B Edwards), pp 1233–1241 (5th Empire Mining and Metallurgical Congress: Melbourne; The Australasian Institute of Mining and Metallurgy: Melbourne). Johnson, R M, 1888. A Systematic Account of the Geology of Tasmania (J Walch and Sons: Hobart). Kennedy, D J, 1971. Geology of Flowery Gully (Northern Tasmania) and conodonts from the lowermost Gordon Limestone (Ordovician), BSc Honours thesis (unpublished), University of Tasmania, Hobart.
Russell, D W and van Moort, J C, 1992. Mineralogy and stable isotope geochemistry of the Beaconsfield, Salisbury and Lefroy goldfields, in Tasmania: An Island of Potential, Geological Survey Bulletin 70 (Eds: P W Baillie, M Dix and R G Richardson), pp 208–226 (Tasmania Department of Mines: Hobart). Thureau, G, 1883. Report on the future prospects as regards productiveness and permanency of the Beaconsfield and Salisbury mining districts, Department of Mines Tasmania Report OS 40 (unpublished). Twelvetrees, W H, 1903. Report on the mineral resources of the districts of Beaconsfield and Salisbury, Department of Mines Tasmania Report OS 204 (unpublished). Williams, E, McClenaghan, M P and Collins, P L, 1989. MidPalaeozoic deformation, granitoids and ore deposits, in Geology and Mineral Resources of Tasmania (Eds: C F Burrett and E L Martin), pp 238–292 (Geological Society of Australia Special Publication 15).
Laurie, J R, 1996a. Macrofossils from the Cabbage Tree Formation, Middle Arm Gorge, near Beaconsfield, Tasmania, Australian Geological Survey Organisation Professional Opinion 1996/008 (unpublished).
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Callaghan, T, Dunham, S and Edgar, W, 1998. Henty gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 473–480 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Henty gold deposit 1
2
by T Callaghan , S Dunham and W Edgar INTRODUCTION The deposit is on the west coast of Tasmania at lat 41o50′S, long 145o40′E on the Queenstown (SK 55–5) 1:250 000 scale and Sophia (8014) 1:100 000 scale map sheets. It is at approximately 600 m above sea level in the headwaters of the Henty River near Mount Read, about 30 km north of Queenstown (Fig 1). The mine is in an environmentally sensitive area of the West Coast Ranges close to the Central Highlands World Heritage Area and two Recommended Areas for Protection (RAP zones) and is owned by Goldfields (Tasmania) Ltd. Two deposits have been defined to date, Zone 96 with a Probable Reserve of 506 000 t at 26.9 g/t gold and the Sill zone with a Measured Resource of 23 000 t at 36 g/t gold. Two other mineralised areas, the Intermediate zone and Mount Julia are currently being evaluated. Production commenced in January 1996, and 13 699 oz of gold have been recovered from 42 275 t of ore to December 1996.
EXPLORATION HISTORY In the 1960s the Mount Lyell Mining and Railway Company Limited held an exploration licence (EL9/66) which covered a vast area from just south of Mount Darwin to Mount Read. Their intention was to find Rosebery style zinc-rich volcanic hosted massive sulphide or Mount Lyell style mineralisation. The first grid to cover the Henty Fault area was established in 1968-69. The grid was geologically mapped, and geophysical prospecting, involving vertical loop electromagnetic, ground magnetic and gradient array induced polarisation surveys and a mercury vapour soil gas survey, were conducted between 1968 and 1972 (J S Donaldson, unpublished data, 1993). Numerous anomalies were detected and two old workings were discovered in the vicinity of Koonya Creek. Costeaning revealed a small base metal lens, overlying the Henty deposit area, which averaged 2.4 m at 1.8% copper, 1.76% lead, 0.2% zinc and 37.89% iron sulphide (K Wells, unpublished data, 1974). Several diamond drill holes revealed only small low grade areas of mineralisation. These results were not considered favourable for further exploration due to the small deposit size, proximity to the Henty Fault and what were then considered unfavourable Tyndall Group host rocks.
1.
Geologist, Goldfields (Tasmania) Ltd, PO Box 231, Queenstown Tas 7467.
2.
Manager Geology, Goldfields (Tasmania) Ltd, PO Box 231, Queenstown Tas 7467.
3.
Geologist, Goldfields (Kalgoorlie) Ltd, PO Box 862, Kalgoorlie WA 6430.
Geology of Australian and Papua New Guinean Mineral Deposits
3
A review of the data in 1984 as an appraisal prior to relinquishment prompted a further drilling program and reassaying of drill core for gold. The results were encouraging, with gold mineralisation identified within silicified volcanic rocks, including a best result from hole HFZ05 of 7 g/t gold over 6.7 m at a depth of 125 m. Renewed exploration identified the Sill zone resulting in the decision in 1988 to develop an exploratory decline and drive 130 m below surface. The decline indicated that the deposit was considerably more complex, higher in grade and lower in tonnage than previously thought, and the 1987 estimate of 500 000 t at 10 g/t gold was reduced to 23 000 t at 36 g/t gold in 1989. During development of the decline in 1989, drill hole HP96A intersected 7.4 m at 107.1 g/t gold at 564.8 m depth. A further Probable Reserve of 506 000 t at 26.9 g/t gold was identified by 1992 in what is now known as Zone 96. Mining lease ML 7M/91 was granted in 1991 and it was decided in 1992 to develop an exploration shaft to access this deposit. Following negotiations with the then joint venture partner, Little River Resources Pty Ltd, and the Department of Environmental Management, work began in 1993. Production drilling and mining of the Sill zone started in 1995 to provide ore for commissioning of the CIL gold plant. Production and resource drilling at Zone 96 commenced in December 1996. Further drilling of the Intermediate zone, between Zone 96 and the Sill zone, commenced in late 1996 and exploration of ML 7M/91 has continued south of the mine. Henty style mineralisation was identified at Mount Julia south of Zone 96 in December 1995 and exploration is ongoing.
REGIONAL GEOLOGY The deposit is hosted by the Cambrian Mount Read Volcanics, an arcuate belt of acid to intermediate volcanic rocks occupying the eastern margin of the Dundas Trough. They are bounded to the east by Precambrian basement rocks of the Tyennan Region (Fig 1) and younger Cambro-Ordovician siliciclastic rocks, and to the west appear to interfinger with fossiliferous volcanosedimentary rocks of the Dundas Group and Western volcanosedimentary sequences. A major NNE-striking structure, the Henty Fault, divides the Mount Read Volcanics into two parts. Within the Henty Fault zone lie rocks of the Henty Fault sequence to the south, and the Farrell Slate to the north near Tullah. The Mount Read Volcanics north and west of the Henty Fault (Fig 1) host the massive sulphide deposits of Rosebery, Hercules, Que River and Hellyer, and to the south and east they host the Henty gold deposit and the copper-gold deposits of the Mount Lyell field. The Mount Read Volcanics south and east of the Henty Fault are divided into four lithological groups:
473
T CALLAGHAN, S DUNHAM and W EDGAR
FIG 1 - Location and geological map of the central part of the Dundas Trough, showing major rock associations of the Mount Read Volcanics and associated Cambrian and Proterozoic sequences. Based on published maps of Tasmanian Geological Survey (after Corbett, 1992).
1.
the Central Volcanic Complex (CVC) consisting of mainly rhyolitic to andesitic volcanic rocks with minor sedimentary and mafic units;
3.
the Tyndall Group comprising mainly quartzphyric felsic and intermediate extrusive and volcaniclastic rocks with interbedded epiclastics; and
2.
the Eastern quartzphyric sequence of quartz porphyritic lavas and volcaniclastic rocks;
4.
the Western sedimentary sequence of volcanosedimentary siltstone, shale, quartzose and volcaniclastic turbidite and felsic porphyry intrusives.
474
Geology of Australian and Papua New Guinean Mineral Deposits
HENTY GOLD DEPOSIT
Overlying the Mount Read Volcanics is the CambroOrdovician Owen Conglomerate which has an unconformable to interdigitating relationship with the volcanics. The Cambrian to early Middle Devonian rocks in western Tasmania have been affected by widespread Devonian folding during the Tabberabberan Orogeny. The orogeny was a multiphase deformation event, with an early phase of NNW folding (D1) and a later NW to WNW (D2) trend recognised in the region (Williams, 1989). It has produced open upright folds in competent siliciclastic units but tight folding in phyllosilicate-rich volcanic rocks. Reverse faulting is common and the rocks have developed a pervasive regional foliation. Metamorphism locally reached lower greenschist facies.
LOCAL GEOLOGY STRATIGRAPHY The Henty lease contains Mount Read Volcanics and Owen Conglomerate (Fig 2). The Mount Read Volcanics are represented by the CVC, Henty Fault sequence (HFS) and the younger Tyndall Group. The major Henty Fault traverses the lease and the geology of the lease can be described as two segments, west and east of the Henty Fault. Gold mineralisation is on the east side of the Henty Fault, hosted by Tyndall Group rocks. The HFS of carbonaceous black shale, mafic and ultramafic volcanic and quartzphyric volcaniclastic rocks lies between the North and South Henty faults in the northern and southern parts of the lease area. Rocks of the CVC occur west of the Henty Fault and are dominated by pink, feldsparphyric dacitic lavas and coarse grained crystal-rich volcaniclastic rocks. The CVC is intruded by a large number of chloritised, fine grained tholeiitic dykes refered to as the Henty dyke swarm. Crawford and Berry (1992) consider the dykes representative of a period of failed back-arc rifting of the Mount Read Volcanics predating the Tyndall Group. East of the Henty Fault the sequence is dominated by quartzphyric volcanics of the Tyndall Group and siliciclastics of the Owen Conglomerate, which is locally represented by the Newton Creek Sandstone. Minor altered dacitic volcaniclastic rocks and lavas of the CVC occur east of the Henty Fault on the south of the mine lease. The CVC is strongly sericite-pyrite altered to the south alongside the Henty Fault. The Tyndall Group overlies the CVC at an apparent unconformity on the southern part of the mine lease adjacent to the South Henty fault. White and McPhie (1996) divided the Tyndall Group into the Zig Zag Hill Formation and Comstock Formation, which respectively correspond broadly to the Upper and Lower Tyndall groups of previous workers (Corbett et al, 1974; Corbett and McNeill, 1988; Corbett, 1992). White and McPhie also subdivided the Comstock Formation into a lower Lynchford Member and an upper Mount Julia Member. Although the Tyndall Group is characterised by rapid facies changes, these subdivisions can be recognised throughout the mine lease.
Comstock Formation Lynchford Member This basal unit of the Tyndall Group varies considerably from north to south, with the main defining features being minor
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 2 - Geological map of Henty gold mine area, with mine lease outline.
carbonate horizons interbedded with feldspar-crystal sandstones and their stratigraphic position relative to overlying units. In the north the basal unit comprises coarse, polymict, matrix supported volcaniclastic rocks which host mineralisation in the Zone 96 area (Fig 3). The original textures, although overprinted by hydrothermal alteration and
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T CALLAGHAN, S DUNHAM and W EDGAR
jasper fragments. The volcaniclastic rocks vary from green to red depending on the degree of alteration. The least altered rocks are weakly hematitic and chloritic with minor carbonate veining.
Mount Julia Member The base of the Mount Julia Member in the south is characterised by massive, quartz crystal–rich volcaniclastic rocks and autobrecciated and flow banded rhyolite that extend for several kilometres south of the lease. The rhyolite unit thickens rapidly to the south of the Mount Julia deposit, but is not present to the north. A few minor quartzphyric dykes and sills found in the north may be related to this event. These units are commonly intensely albite-silica altered. At Mount Julia the middle to upper parts of the Member are characterised by polymict volcaniclastic greywacke, graded mass flows, quartz crystal–rich volcaniclastic sandstone and epiclastic shale. Overlying these are distinctive, massive, crystal-rich volcaniclastic sandstones with minor matrix supported diamict breccias characterised by quartz porphyry clasts. This unit is extensive and is possibly better correlated with the basal unit of the overlying Zig Zag Hill Formation. The sequence is similar in the north of the lease with the exception of the flow banded rhyolite and massive quartzphyric volcaniclastic rocks at the Lynchford Member–Mount Julia Member boundary to the south. These rhyolites do not occur 2 km to the north and result in the package thinning from south to north.
Zig Zag Hill Formation FIG 3 - Cross section of the Zone 96 orebody on 54 900 N, looking north.
subsequent deformation, are still discernible in hand specimen and thin section. Deformation and alteration have destroyed most primary textures of the basal unit in the Sill zone area to the extent that stratigraphic correlation is very difficult. A sequence of well sorted and finely laminated grey siltstone overlies the basal unit in the centre of the lease. At the Mount Julia deposit, the Lynchford Member is frequently intensely altered and original features are often obliterated. Sericite-quartz-pyrite alteration rapidly decreases away from mineralisation to the south. The Lynchford Member is characteristically a massive, coarse grained and crystal-rich, feldsparphyric volcaniclastic sandstone with lesser siltstone and matrix supported lithic breccias with minor rhyolitic volcaniclastic and intrusive rocks. Minor interbedded chert and carbonate bodies are a feature of this unit and their relationship to mineralisation is presently unclear. These rock types are often masked by intense albite-silica to chlorite-albite-silica alteration and by the sericite-silica to sericite-silica-pyrite alteration associated with mineralisation. One or more carbonate horizons of limited lateral continuity are interbedded with chlorite-carbonatehematite altered, coarse grained and feldsparphyric, crystalrich volcaniclastic rocks. These are often moderately magnetic and are a possible correlate of the Lynchford Member volcaniclastic rocks found to the south (White and McPhie, 1996). Carbonates are cream to pink or purple, with common
476
This formation contains polymict volcaniclastic conglomerate, coarse quartz-crystal sandstone and laminated siltstone. The conglomerates are dominated by quartz-feldspar porphyritic rhyolite clasts. Localised conglomerate of mixed volcaniclastic and siliciclastic rocks of Precambrian basement derivation, and individual beds of siliciclastic rocks identical to the overlying Owen Conglomerate occur near the top of the formation. A high degree of rounding of clasts is common as are laterally extensive graded beds.
Quartz porphyritic intrusives and extrusives Abundant quartz-feldspar porphyritic to quartz porphyritic rhyolites occur throughout the Tyndall Group and possibly at the bottom of the Newton Creek Sandstone. Numerous sills, dykes and flows with peperitic and autoclastic textures have been observed in drill core, mine openings and outcrop. The rocks characteristically contain approximately 5–10% quartz and feldspar phenocrysts, 2 to 5 mm in diameter, in a fine grained matrix. The rocks are frequently altered to a weak to moderate albite-chlorite assemblage. The majority of these rhyolites occur immediately below and within the Zig Zag Hill conglomerates and above the Mount Julia Member graded mass flows. Dykes of quartz porphyry intrude siliciclastic rocks at the lower Newton Creek Sandstone–Upper Zig Zag Hill Formation gradational contact indicating active volcanism during the onset of siliciclastic sedimentation. At the shaft bottom and to the north of the mine, the Newton Creek Sandstone apparently conformably overlies resedimented autoclastic breccia and coherent quartz porphyry.
Geology of Australian and Papua New Guinean Mineral Deposits
HENTY GOLD DEPOSIT
It is apparent that this rhyolite complex intruded the Comstock Formation and formed subaqueous extrusives and lava domes with associated autoclastic deposits within the Zig Zag Hill Formation. The autoclastic deposits have been extensively reworked and contributed a large, localised sediment input to the Zig Zag Hill Formation. Contacts between coherent volcanic, autoclastic and epiclastic rocks are commonly gradational.
STRUCTURE The Henty Fault divides into North and South Henty faults in the middle of the mine lease (Fig 2) and the intersection of these faults has a shallow southerly plunge of approximately 20o although little subsurface information regarding the North Henty fault is available. The South Henty fault has a strong south-plunging inflexion in the vicinity of the Mount Julia deposit. The rock types to the east of the Henty Fault are controlled by a major, shallowly south plunging, asymmetric syncline centred on the siliciclastic rocks of the Owen Conglomerate and the Henty Fault. The western limb of this syncline is steeply east dipping in the south of the lease, but is overturned to the east in the northern and central regions where the synclinal axis trends into the Henty Fault. Numerous north striking, steeply west dipping brittle-ductile faults were logged in drill core and mapped in underground workings in the central part of the lease. Most of these structures are associated with overtightening and thrusting of the western limb of the syncline during continued east–west compression associated with the Devonian deformation. It is possible that some of these are reactivated Cambrian faults forming the western boundary of a graben filled with the siliciclastic rocks of the Owen Conglomerate. Correlation of these faults is difficult but it appears that these are locally developed structures most prevalent in the steep limb of the syncline. A dominant foliation (S2) of approximately 340o strike and vertical to steep southwesterly dip occurs throughout the lease with the exception of the immediate vicinity of the Henty Fault, where the foliation parallels the fault. Mineralisation is strongly controlled by the South Henty fault which forms the upper boundary to mineralisation (Figs 3 and 4), and away from which mineralisation and alteration decrease. Intensely foliated phyllosilicates associated with extensive hydrothermal alteration are always present in the immediate footwall of the fault irrespective of which stratigraphic unit is present. Mylonitisation and brecciation of altered and mineralised volcanic rocks close to the South Henty fault is a feature of the Henty deposit. Numerous post–ductile deformation brittle faults, with several different orientations and displacements of a few metres, disrupt the rocks of the northern mine area, particularly within the Sill zone. It is not fully understood what happens to the host sequence at depth. The few holes that have been drilled well below mineralisation have not intersected the ore host horizon but indicate that rocks from higher in the stratigraphic sequence are present. This phenomenon had been explained previously (Halley and Roberts, 1997) by the north striking faults found in the overtightened west limb of the syncline and remains the most likely explanation, although none of these faults have been identified outside the portal area.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 4 - Cross section of the Mt Julia mineralisation on 53 350 N, looking north.
ALTERATION AND MINERALISATION The mineralisation occurs in several bodies over a strike length of 2.5 km and to a depth of 1 km below surface. Alteration affects nearly all stratigraphic units of the Tyndall Group but the best developed gold mineralisation is generally confined to the Lynchford Member. This horizon is refered to as A zone. Alteration is well zoned with distinct asymmetry moving eastwards through the Tyndall Group away from the Henty Fault (Fig 4). Alteration styles are described below from the stratigraphic footwall (west) to hanging wall (east).
Sericite-quartz±pyrite±carbonate±feldspar±fuchsite alteration (MA) This style generally includes moderate to strongly sericitesilica±pyrite±carbonate±feldspar altered, mylonitic rocks enveloping the main mineralised zone, dominantly occurring in the stratigraphic footwall to mineralisation adjacent to the Henty Fault. The stratigraphic footwall is typically an intensely foliated, hydrothermally altered rock. Original rock types are generally volcaniclastic rocks in the Mount Julia area although occasional quartz phenocrysts are preserved, perhaps indicating the presence of some coherent rhyolites. The lithology differs from the Zone 96 area where the original rock is interpreted to have been coarse epiclastic mass flows and
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T CALLAGHAN, S DUNHAM and W EDGAR
sandstones as shown by the clastic textures frequently preserved in an intensely foliated, mylonitised matrix (Taheri and Green, 1991). All these footwall rocks are presumed to be part of the Lynchford Member and possibly the underlying CVC dacites.
MV is an intensely foliated, yellow-green sericite-rich (20–40%) rock with abundant silicification and silica nodules. Carbonate veining is common with lesser sphalerite, pyrite, chalcopyrite, galena and rare fluorite, with sulphides generally less than 1%.
Fuchsite-sericite altered ‘dykes’ 0.1–1.0 m wide are commonly found in these altered footwall rocks both at Mount Julia and in the mine area.
Deformation has strongly recrystallised phyllosilicate minerals into the dominant foliation. Silicified nodules demonstrate elongation through dissolution processes (Taheri and Green, 1991).
Main mineralised zone (A zone) alteration A zone is hosted by the Lynchford Member at Mount Julia (Fig 4), Zone 96 and the Sill zone (Fig 3). It appears to be a little lower in the stratigraphic sequence at Mount Julia than Zone 96, although the southern end of Zone 96 is possibly in a similar stratigraphic position. Jasper clasts are frequently found in MQ and MV altered rocks in Zone 96 indicating overprinting of carbonate-jasper beds by subsequent intense silicasericite±sulphide alteration. This has not been observed at Mount Julia but carbonate horizons do occur throughout A zone indicating a similar overprinting relationship of carbonates by silica-sericite alteration. This indicates that mineralisation was synchronous with sedimentation if the jaspers and carbonates are accepted as exhalative sediments formed during the mineralising event. Alteration styles within A zone include MQ, MV, MZ, MP, MS and CB alteration as described below.
Massive quartz-carbonate±sulphide breccia alteration (MQ) This is the main alteration type associated with gold mineralisation at the Henty deposit. It is characteristically an extensively fractured and brecciated, grey, massive silica alteration with varying quantities of carbonate, sericite and sulphide minerals including pyrite and chalcopyrite with lesser galena and sphalerite and rare gold, electrum and native bismuth. Sulphides comprise 2–5% of the rock and grades vary from <1 g/t gold to >1000 g/t. MQ forms discrete elongate lenses enveloped in sericitised volcanic rocks. The shallowest ore lens is 150 m below surface, and the lenses are from 10 to 300 m long, 20 to 300 m high and 0.2 to 7 m thick. Sulphides, gold and carbonates fill late fractures within MQ alteration as a result of remobilisation during Devonian deformation. MQ has been partially recrystallised during D2 deformation with margins of the lenses aligning with the S2 foliation. Late brittle faulting disrupts the MQ lenses.
Sericite-silica±sulphide±carbonate schist alteration (MV) MV alteration envelops MQ and appears to be an alteration end member or selvage intimately associated with silicification. Defining the boundaries between MV and MQ alteration can sometimes be difficult. Within the mine workings MV envelops MQ alteration for tens of metres along strike to the north. MV is more extensive on the stratigraphic hanging wall of MQ in the mine area and is frequently associated with elevated gold and base metal contents. Gold and base metals occur as 1–10 cm wide late silica-carbonate veins aligned with the foliation and as blebs and aggregates within sericite alteration.
478
Silica-sericite-pyrite±base metal schist alteration (MZ) MZ is a grey-green, intensely foliated, sericite-quartz-pyrite rock with varying amounts of carbonate, chalcopyrite, galena, sphalerite and chlorite. Sulphides comprise 5–15% of the alteration assemblage. Intense sericitisation, silicification and sulphidisation with intense brecciation and shearing have largely obliterated the original rock textures. Relict clasts are frequently intensely silicified. MZ alteration envelops both MQ and MV alteration and is volumetrically the most abundant alteration style within the mine area. It is intimately associated with MV and frequently extends further into the hanging wall than the other mineralised alteration styles. Information to date suggests that MZ alteration is much less abundant at Mount Julia than in the main mine area. The progressive change in metal content from MQ through to MZ alteration is shown in Table 1. TABLE 1 Average metal contents for MQ, MV and MZ alteration styles, in ppm. Alteration style
Au
Ag
Cu
Pb
Zn
MQ
55
18
1577
1557
580
MV
4
0.8
282
317
110
MZ
0.6
0.2
38
12
33
Carbonate-jasper (CB), massive galena-sphalerite (MS) and massive pyrite (MP) alteration Extensive bedded carbonates are common in the Lynchford Member and appear to be intimately associated with A zone mineralisation. Carbonate horizons occur at the stratigraphic top of A zone and extend laterally away from the alteration package to the south. Carbonates are white to pale pink and are commonly interbedded with feldsparphyric sandstone, mass flows and siltstone. Beds vary in lateral extent and thickness. Red jaspers commonly occur within carbonate beds. It is not clear if the carbonate-jasper beds at Mount Julia are the same as those that occur in a similar stratigraphic position to the south of Henty or even within the mine lease. Massive base metal lenses and massive pyrite lenses have been observed at the top and in the stratigraphic hanging wall of A zone in both the Zone 96 and Sill zone orebodies. These bodies are generally associated with, or near the stratigraphic position of, the carbonate horizons and form discontinuous lenses a few metres long by less than a metre thick. Base metal lenses frequently occur as disseminations and blebs within epiclastic sandstones. Base metal contents associated with
Geology of Australian and Papua New Guinean Mineral Deposits
HENTY GOLD DEPOSIT
veins and blebs of galena and sphalerite are generally elevated towards the top of A zone above the MQ lenses. Gold mineralisation is commonly associated with massive pyrite lenses and occasionally within carbonate lenses. Recent drilling has identified massive pyrite and bedded massive sulphide mineralisation associated with carbonates and jaspers at Mount Julia.
Albite-silica alteration (AS) Extensive AS alteration occurs in the immediate hanging wall to mineralisation and extends 20–70 m out from A zone in the Sill zone and Zone 96 bodies. At Mount Julia the zone is extensive and occurs in excess of 100 m out from the auriferous altered rocks. The alteration occurs in both coherent rhyolites and volcaniclastic rocks of the Mount Julia Member and even up into the Zig Zag Hill Formation. The altered rock varies from a massive, textureless grey rock to an orange, fine grained AS alteration of the matrix of volcaniclastic rocks to a chlorite-albite-silica overprint of the rhyolites occuring at the base of the Mount Julia Member in the south of the lease. Observations from underground workings demonstrate that the alteration is frequently irregular with completely texture-destructive, white AS alteration grading into weakly chloritised crystal sandstones over a few hundreds of millimetres, with only a reaction front of orange albite rimming the intense white AS alteration. Early AS alteration is frequently overprinted by late sericitecarbonate±pyrite±chalcopyrite veinlets. The significance of this later alteration is not understood but it generally contains only a trace of gold. It is not clear if AS alteration is associated with the mineralising event. Albite alteration is typical of late diagenetic processes acting on submarine acid volcanic rocks and is particularly common within the Tyndall Group. However the intense alteration at Henty is abnormal and appears to be spatially associated with the underlying alteration which suggests that it may be related to the mineralising process. It is possible that the albitisation process has been enhanced stratigraphically above the mineralised horizon due to higher heat flow after burial of the hydrothermal system (S Halley, personal communication, 1996).
ORE GENESIS Henty is considered to be a shallow water volcanogenic gold deposit formed under the sea floor in an actively filling basin. Hydrothermal fluids focussed on the proto–South Henty fault during and after the deposition of the Lynchford Member of the Comstock Formation. Mineralisation and alteration are strongly controlled by the South Henty fault and are zoned laterally and vertically from the fluid conduit. A series of mineralised lenses have formed adjacent to the fault.
Geology of Australian and Papua New Guinean Mineral Deposits
Intense sericitisation and silicification of the unconsolidated footwall volcanic rocks below the sea floor concentrated copper, gold, silver, lead and bismuth. Low level gold with zinc, lead and iron sulphides and possibly carbonate and jasper were deposited at or just below the sea water interface forming small massive pyrite lenses and disseminated to massive galena-sphalerite lenses associated with extensive carbonate and jasper zones. There is no barite within the alteration zones at Henty, suggesting a lack of sulphate at the time of mineralisation. Rapid burial of the hydrothermal system has resulted in areas of alteration extending well into the overlying rocks. AS alteration of the hanging wall has occurred after burial. Devonian orogenesis strongly deformed the sequence, particularly the altered volcanic rocks, resulting in repeated fracturing and veining of brittle rocks, and sericite-altered rocks were the focus of high strain and ductile deformation. Pressure solution of sulphides has enhanced local transportation of gold and base metal sulphides into late brittle fractures (Halley and Roberts, 1997).
ACKNOWLEDGEMENTS Goldfields (Tasmania) Ltd are acknowledged for permission to publish.
REFERENCES Corbett, K D, 1992. Stratigraphic-volcanic setting of massive sulphide deposits in the Cambrian Mt Read Volcanics, Tasmania, Economic Geology, 87:564–586. Corbett, K D and McNeill, A, 1988. Geological Compilation Map of the Mt. Read Volcanics and Associated Rocks, Hellyer to South Darwin Peak, 1:100 000 scale, Tasmanian Department of Mines, Mt Read Volcanics Project Map 2. Corbett, K D, Reid, K O, Corbett, E B, Green, G R, Wells, K and Sheppard, N W, 1974. The Mt. Read Volcanics and CambroOrdovician relationships at Queenstown, Tasmania, Journal of the Geological Society of Australia, 21:173–186. Crawford, A J and Berry, R F, 1992. Tectonic implications of Late Proterozoic-Early Palaeozoic igneous rock associations in Western Tasmania, Tectonophysics, 214:37–56. Halley, S W and Roberts, R H, 1997. Henty: A shallow-water, gold rich volcanogenic massive sulfide deposit in Western Tasmania, Economic Geology, 92: 438–447. Taheri, J and Green, G R, 1991. The origin of the gold mineralisation at the Henty prospect, Department of Resources and Energy, Division of Mines and Mineral Resources, Report (unpublished) White, M J and McPhie, J, 1996. Stratigraphy and palaeovolcanology of the Cambrian Tyndall Group, Mt. Read Volcanics, western Tasmania, Australian Journal of Earth Sciences, 43:147–159. Williams, E, 1989. Middle Palaeozoic deformation, in Geology and Mineral Resources of Tasmania (Eds: C F Burrett and E L Martin), pp 239–253, Geological Society of Australia Special Publication, 15.
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Berry, M V et al, 1998. The Rosebery lead-zinc-gold-silver-copper deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 481–486 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Rosebery lead–zinc–gold–silver–copper deposit 1
2
2
2
3
3
by M V Berry , P W Edwards , H T Georgi , C C Graves , C W A Carnie , R J Fare , 3 3 3 3 C T Hale , S W Helm , D J Hobby and R D Willis INTRODUCTION Rosebery is in western Tasmania at lat 41o48′S, long 145o35′E on the Queenstown (SK 55–5) 1:250 000 scale and the Sophia (8014) 1:100 000 scale map sheets. The deposit is hosted by the Cambrian Mount Read Volcanics which also host the Hercules, Que River, Hellyer, Henty and Mount Lyell sulphide deposits (Fig 1). The mine is owned by Pasminco Limited. The Identified Mineral Resources at June 1996 were 8.29 Mt at 4.3% lead, 13.1% zinc, 0.43% copper, 144g/t silver, 2.6g/t gold and 10.2% iron. Recent exploration led to the discovery of three new orebodies which are described in this paper. Base metal mineralisation was discovered at Rosebery in 1893. Mining commenced within three years and continued semicontinuously until 1913 when local smelting facilities closed. For the next 20 years mining was intermittent, with continuous operations commencing in the mid 1930s when a concentrating plant was constructed at Rosebery. Since then annual mining production has grown to 0.6 Mtpa. The geology of western Tasmania and the Rosebery district has been described by Campana and King (1963), Corbett (1986), Corbett and Lees (1987), Corbett et al (1989) and Lees et al (1990). Detailed mine scale geological descriptions have been compiled by the authors cited above and by Green, Solomon and Walshe (1981), Green (1983), Lees (1987), Allen and Cas (1990) and Berry (1991).
GEOLOGICAL SETTING The package of Cambrian rocks of the Central Volcanic Complex containing the Rosebery deposit is fault bound; on the footwall side (west) by the Rosebery Fault and on the hanging wall side (east) by the Mount Black Fault, both reverse faults dipping nominally 45o east. Within this fault block the mine footwall sequence (MFW) consists of subaqueously-deposited massive volcaniclastic pumice breccias of dacitic to rhyolitic composition. The mine host sequence (MHS) represents a transitional environment of bedded volcaniclastic siltstone and sandstone. In places this horizon is capped by a black slate which grades upward into the mine hanging wall sequence (MHW), composed primarily of dacitic and rhyolitic
FIG 1 - Simplified geological map of the Rosebery area, modified from R L Allen (unpublished data, 1991).
volcaniclastic rocks. Overall the entire stratigraphic sequence dips at 40 to 50o east, subparallel to the main bounding faults, however the cleavage dip tends to be consistently steeper than bedding by 10 to 20o. A generalised cross section of the mine illustrates the broad geological setting and distribution of ore lenses (Fig 2).
1.
Chief Geologist, Pasminco Rosebery Mine, PO Box 21, Rosebery Tas 7470.
OREBODY FEATURES
2.
Senior Geologist, Pasminco Rosebery Mine, PO Box 21, Rosebery Tas 7470.
3.
Mining Geologist, Pasminco Rosebery Mine, PO Box 21, Rosebery Tas 7470.
Lead-zinc dominant ore deposits in the Mount Read Volcanics occur as clusters of podiform to tabular bodies that individually rarely exceed 3 Mt. Rosebery, Hercules, South Hercules, Que River and Browns Tunnel are typical examples, but Hellyer is a
Geology of Australian and Papua New Guinean Mineral Deposits
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M V BERRY et al
The composition of the ore varies considerably from lens to lens in terms of economic sulphide mineralogy, gangue sulphide association and silicate mineral composition. Sphalerite and galena are ubiquitous, although the iron content of sphalerite varies substantially both within and between lenses. Volumetrically pyrite is the other main sulphide, with subordinate pyrrhotite and chalcopyrite, however there is extreme variation in the proportion of these sulphides from lens to lens. Tonnage and grade data for production and/or resources for each lens with gangue mineral associations are presented in Table 1.
GEOLOGY OF RECENT DISCOVERIES
FIG 2 - Generalised cross section of the Rosebery mine, looking north.
clear exception. McArthur and Dronseika (1990) describe Hellyer as being formed as a single sulphide accumulation, up to 800 m long, 200 m wide and with an average vertical thickness greater than 40 m, totalling some 17 Mt. At Rosebery mining operations now extend over a strike length of 2 km and to more than 1 km depth. The deposit consists of many lenses and pods which historically have been alphabetically named and vary in size from tens of thousands of tonnes (M and Q lenses) up to a maximum of some 5 Mt (F lens). Individual lenses are typically of the order of 100 to 400 m strike length, 100 to 500 m down dip length and mostly less than 20 m true thickness, often less than 10 m. In cross section ore lenses occur as single sulphide accumulations within the MHS as shown by part of D lens and most of J lens in Fig 2. Elsewhere sulphides form multiple stacked lenses and may display a tendency to migrate lower in the MHS down dip where geological relationships between lenses may be complex. This is illustrated in the central portion of Fig 2 by E, F, G and H lenses. In longitudinal projection (Fig 3) the global distribution of ore lenses takes on an incomplete ovoid shape around a ‘barren’ core. Stacking of lenses is concentrated in the upper central mine area, and the ore distribution appears to be simpler to the north and south.
Three large orebodies were found by surface and underground diamond drilling between 1989 and 1995. In addition to these major lenses a number of smaller ore zones have been discovered during the last six years which have contributed to a lesser extent to the Identified Mineral Resources. These include BP, M, Q and RS lenses and most recently T lens (which is not yet included in the resource inventory). Exploration continues to define the extent of these areas and potential exists for several of these lenses to exceed 1 Mt. The following sections summarise the geological setting of the three new orebodies, designated J, K and P lenses (Fig 3), whose Measured, Indicated and Inferred Resources totalled some 6.1 Mt as of June 1996. J lens already contributes to existing production, with P lens scheduled to commence in the 1998–99 financial year and K lens early next century. The lens descriptions highlight the diversity in both the broad geological setting of sulphide mineralisation as well as the sulphide and gangue mineral association.
J LENS Discovery J lens is at the southern end of the mine between -100 and -420 m N and from 2750 to 2500 m RL. It was discovered in June 1986 in hole R4214 during an underground drilling program testing the down dip extension of F lens. The total Measured, Indicated and Inferred Resource is currently estimated at 2.22 Mt at 3.3% lead, 12.4% zinc, 0.5% copper, 125 g/t silver, 2.0 g/t gold and 14.6% iron. Mining commenced in March 1993, with 0.84 Mt extracted to the end of June 1996.
Host rock
FIG 3 - Longitudinal projection of the Rosebery mine looking east, showing distribution of ore lenses.
482
J lens is hosted by a variably altered feldsparphyric volcaniclastic sandstone within the middle of the MHS. The MHS footwall to the ore horizon is a monotonous sequence of fine or medium grained pervasively altered, well foliated sandstone. In contrast the MHS hanging wall to the ore is a package of less strongly altered and foliated, fine to coarse grained sandstone and siltstone, commonly within upward fining sequences. The immediate area north of J lens (70 m to M lens), south of J lens and down dip of J lens (30 m to RS lens) hosts scattered uneconomic zones of pyrite and minor sphalerite often associated with tourmaline and magnetite alteration with minor pyrrhotite. A barren zone up dip for 50 m vertical distance, containing disseminated pyrite and scattered patchy sphalerite, separates J lens from the base of F lens.
Geology of Australian and Papua New Guinean Mineral Deposits
ROSEBERY LEAD-ZINC-GOLD-SILVER-COPPER DEPOSIT
TABLE 1 Rosebery mine, production plus resources statistics and mineral associations, by lens. Production plus resources1
Lens Ore (Mt)
Pb (%)
Py
Po
Mt
He
A
0.98
4.7
18.9
0.4
465
0.9
B
3.42
4.9
13.8
0.3
129
2.1
11.7
sig
–
–
11.1
sig
–
–
BP
0.30
2.2
3.9
0.1
218
3.3
2.8
tr
–
C
2.84
4.6
19.4
0.5
D
2.18
3.6
12.5
1.2
170
1.6
17.3
sig
123
2.2
22.3
sig
E
3.16
3.7
12.8
1.3
106
2.7
23.3
F
4.98
4.3
14.6
0.7
125
3.1
10.5
G
1.64
H
2.33
4.2
12.8
0.8
135
2.2
3.7
11.1
0.4
84
2.2
J K
2.22
3.3
12.4
0.5
125
2.0
2.77
6.2
16.7
0.4
193
3.4
M
0.08
4.9
15.3
0.1
88
P
1.10
4.0
15.5
0.4
Q
0.08
1.1
11.4
0.3
2.7
11.9
RS
0.21
T
N/A
Total
28.29
-
Zn (%)
4.3
14.3
Cu (%)
Mineral association2
0.2 0.6
Ag (g/t)
Au (g/t)
Fe (%)
Ba
Carb
Si
–
–
sig
sig
–
tr
sig
sig
tr
tr
sig
min
sig
tr
–
–
–
sig
sig
–
–
–
–
sig
sig
sig
–
–
–
tr
sig
sig
sig
sig
tr
tr
–
sig
min
11.5
sig
–
–
tr
min
tr
min
8.1
min
–
min
min
sig
sig
min
14.6
sig
sig
sig
tr
tr
min
min
8.6
min
–
–
–
tr
min
min
1.7
10.1
min
min
min
tr
tr
tr
tr
154
3.2
6.5
min
–
tr
tr
sig
sig
sig
37
0.6
6.9
min
min
min
tr
–
tr
sig
105
1.2
21.7
sig
sig
sig
–
–
tr
min
-
-
-
sig
sig
sig
min
–
tr
sig
145
2.4
13.5
1.
Tonnes and grade estimates are by necessity a combination of production data and resource and reserve calculations undertaken since mining records have been kept. As a result, some data are not strictly comparable.
2.
The abundance of the major gangue mineral association is summarised as follows: sig (significant), greater than 5% by volume; min (minor), greater than 1% but less than 5% by volume; tr (trace), less than 1% by volume; -, not recorded. Minerals are: Py, pyrite; Po, pyrrhotite; Mt, magnetite; He, hematite; Ba, barite; Carb, carbonate; Si, quartz.
Ore characteristics
Structure
J lens consists of semimassive and massive sulphide mineralisation dominated by sphalerite and pyrite. Sulphide banding is common at all scales particularly at millimetre to centimetre scale towards the margins of major ore pods. The bulk of J lens consists of a series of interconnected massive ore pods ranging in true width from 2 m to a maximum of 25 m and continuous for up to 150 m along strike. Anomalous copper mineralisation as disseminated chalcopyrite interspersed within semimassive to massive pyrite occurs stratigraphically below the sphalerite-pyrite mineralisation in the lower northern half of J lens. In comparison with other nearby ore lenses, J lens has a unique association with footwall copper mineralisation, is more pyrite dominant than F lens (up dip) but less pyritic than D or E lens (up dip and along strike to the north). All nearby lenses (F, D, E) by comparison are close to the MFW–MHS contact.
J lens consists of a series of interconnected continuous and semicontinuous sheet-like or tabular ore pods semiconcordant with the MHS. Numerous thinner, less continuous ore pods occur on the margins commonly on the footwall to, down dip and along strike to the south of the main sulphide ore zone, giving the appearance of a partly stacked ore lens package. In cross section the package is broadly sinuous, with the ore pods at the top and bottom of J lens fingering out at steeper dip angles in comparison to the more shallow dipping massive central ore zone (Fig 2). Mapping indicates that the MHS footwall to the main sulphide body exhibits well developed brittle fracturing, however shearing usually cannot be traced into or through massive sulphides. There is no evidence which suggests the margins of J lens are sheared off.
Alteration
K LENS
Two styles of alteration characterise the J lens area. The MHS is commonly pervasively altered by a chlorite-sericite-pyritecarbonate-silica assemblage. Chlorite is dominant in the footwall to the ore where alteration is more pervasive, and sericite is prevalent in the hanging wall to the ore. Subordinate pyrite, carbonate and silica are common within the entire asymmetric alteration envelope. Pervasive alteration of sulphide mineralisation by pyrrhotite, magnetite, tourmaline, pyrite, garnet and occasionally chalcopyrite is common on the margins of sulphide pods and lenses and is more prevalent in the lower, down dip part of J lens.
Discovery
Geology of Australian and Papua New Guinean Mineral Deposits
K lens is at the northern end of the mine and extends from 1280 to 1660 m N and from 2080 to 2680 m RL, and is the most northerly ore lens found to date. It was discovered in May 1991 by surface drill hole 120R which penetrated the lens in its middle southern area. The June 1996 Indicated and Inferred Resource is estimated at 2.77 Mt at 6.2% lead, 16.7% zinc, 0.4% copper, 193 g/t silver, 3.4 g/t gold and 8.6% iron, and production is scheduled to commence in 2000–2001.
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M V BERRY et al
Host rock
Host rock
The MFW–MHS contact in the K lens region is variable from sharp to gradational although the MHS–MHW contact is normally abrupt. Here the MFW is a sequence of medium to coarse grained quartz augen schists grading into the MHS, which is represented by interbedded sandstone and siltstone. The immediate hanging wall to K lens is a porphyry consisting of quartz and feldspar phenocrysts in a fine grained silicified groundmass, with sharp contacts with the surrounding rocks. The porphyry is currently believed to be intrusive, with the upper contact abutting the MHW, which is here a series of epiclastic sandstone and siltstone.
The MHS in the P lens area is variably altered volcaniclastic sandstone, siltstone and chert. Most of the MHS is partially mineralised, albeit only by disseminations and stringers of sphalerite or fine grained pyrite. In parts the MHS is capped by a pyritic black slate unit, which is up to 6 m thick although it is discontinuous in the central mine zone. The MFW–MHS contact is often obscured by later silicic or sericitic alteration. The MHS–MHW contact is erosional and usually readily identified, however in several holes the clearly visible rounded quartz and feldspar phenocrysts characteristic of the MHW are masked by silicification.
Ore characteristics Ore in K lens occurs as massive to semimassive sulphides within the MHS. It is dominated by sphalerite (typically a massive, pale to medium brown, fine to medium grained, weakly banded low iron variety) with common thin bands of fine grained galena and sporadic minor fine grained pyrite. Chalcopyrite occurs erratically as vein fracture fill. Drill holes to date indicate that K lens forms a consistent tabular horizon 350 m in strike length, 600 m long down dip and varying from 2 to 17 m in true thickness. Compared with the other lenses K lens is relatively depleted in iron, as pyrite, and copper, and is enriched in gold and silver. A thin hanging wall zone in the upper northern region of the lens is highly enriched in gold and silver, to 33 g/t gold and 3500 g/t silver. Relative to its nearest neighbours, P lens (200 m to the south) and B lens (150 m up dip and to the south), K lens mineralisation is thicker and has less internal host rock dilution.
Alteration The MFW is dominantly silicified, with patchy zones of chloritisation and sericitisation. The MHS sandstone and siltstone are predominantly chloritised and sericitised with increasing carbonate alteration towards the porphyry sill. The chlorite and sericite alteration in the MHS is asymmetric, with chlorite alteration dominant on the footwall side and sericite alteration dominant on the hanging wall side of the sulphide lens. The hanging wall porphyry is variably silicified with the degree of alteration diminishing into the hanging wall.
P lens is 200 m south of K lens and 300 m down dip of BP lens. The up dip limits are poorly defined and it may extend to 2700 m RL, as the series of economic to subeconomic baritic sandstone units known as BP lens. To the south mineralisation appears to be cut off sharply where the MHS becomes strongly chloritic. Between K and P lenses the few holes drilled to date indicate only a trace of mineralisation and the lower limit of P lens is not yet defined.
Ore characteristics P lens is a series of stacked, tabular to podiform, stratabound sulphide bodies separated by zones of weakly mineralised MHS. The largest individual sub-lens is a zinc-lead-gold dominated baritic sandstone, consisting of a pale brown ironpoor sphalerite intergrown with galena and pyrite. The relative proportions of metals vary in the different sub-lenses. Although the main lens is zinc-lead rich and iron poor, either as pyrite or within sphalerite, other sub-lenses contain iron-rich sphalerite and significant pyrite. The gold distribution also varies, with the highest levels often associated with baritic ironpoor sphalerite lenses, carbonate alteration zones and remobilised sulphides in quartz veins. Compared with the others, P lens most resembles H lens in composition, due to its gold-enriched baritic nature and the multiple sub-lenses nearby, however it lacks the hematite and magnetite found throughout H lens. Up dip P lens is laterally transitional into a baritic sandstone which is sporadically enriched in gold and has low levels of pyrite and iron-poor sphalerite.
Structure
Alteration
K lens is a tabular orebody concordant with the host sequence. The strong cleavage found in the MFW and MHS weakens in the porphyry and main sulphide body, and the competency of the rocks is strong throughout the whole K lens region. Bedding is parallel to subparallel to cleavage with minor parasitic folding in the MHW epiclastics. Minor shearing is found predominantly in the MHS sandstone and siltstone.
Alteration in the area is more typical of the north end of the mine with only minor evidence of the Devonian alteration assemblage (tourmaline, magnetite, pyrrhotite) found in J lens. The MHS is variably altered by sericite, silica, chlorite and barite and is similar to MHS rocks throughout the mine. There are also massive carbonate and rhodochrosite bands within the host and ore assemblage, and commonly up or down dip from an ore horizon. The current data are too limited to ascertain if the along strike ore–carbonate association found in B lens is also common in P lens.
P LENS Discovery P lens is in the north of the mine between 880 and 1070 m N and from 2165 to 2465 m RL. It was discovered in March 1993 during an underground-based exploration drilling program testing the area south of K lens. Eight holes have intersected the lens to date including one surface hole. The current Inferred Resource is estimated at 1.1 Mt containing 4.0% lead, 15.5% zinc, 0.4% copper, 154 g/t silver, 3.2 g/t gold and 6.5% iron.
484
Structure The P lens area contains a series of stacked lenses, inclusive of the barite-gold mineralisation known as BP lens. Although the MFW and MHS are generally moderately to strongly cleaved, the ore zones and immediate host rocks are usually competent. Ore–host contacts are often gradational with only the massive ore horizons having sharp, unsheared margins. Both the
Geology of Australian and Papua New Guinean Mineral Deposits
ROSEBERY LEAD-ZINC-GOLD-SILVER-COPPER DEPOSIT
MFW–MHS and MHS–MHW contacts are commonly faulted with several water-bearing faults recorded. From drill core it is evident that the bedding dip is shallower by 5 to 15o than the dominant dip of the mine cleavage.
ORE GENESIS MODELS Several genetic models have been applied to the Rosebery deposit since its discovery. Until the late 1960s the deposits were regarded as magma-associated mesothermal replacements formed in altered tuffaceous rocks and in structural traps (Hall et al, 1965). Brathwaite (1969, 1974) favoured a Cambrian exhalative origin with the ores deposited as stratiform lenses in a tuffaceous sedimentary environment. The ‘ore horizon’ was underlain by subaerial ignimbritic ash flow tuffs and overlain by quartzphyric volcaniclastic rocks deposited in a subaqueous environment (Green, Solomon and Walshe, 1981; Lees, 1987). The juxtaposition of subaerially deposited rocks against rocks of subaequeous origin was thought due to either caldera collapse (Green, 1983; Lees, 1987) or to longitudinal rifting of the Mount Read Volcanic Arc (Large, Herrmann and Corbett, 1987) with the sulphides deposited in a relatively deep water environment. Apparent repetition and stacking of the ore lenses was believed due to tight, overturned folding of the originally stratiform bodies.
mineralisation and alteration minerals, paragenetic replacement relationships between the ore components and localisation of the mineralisation in extensional structures. Berry (1992) disputes some of this evidence. Metasomatic replacement related to a post-orogenic Devonian granite intrusion is evident at the south end of the Rosebery mine. Here the ore lenses are partially replaced by assemblages of magnetite-biotite+chalcopyrite, pyrrhotitepyrite, or tourmaline-quartz+magnetite (Khin Zaw and Large, 1996; Khin Zaw, Large and Huston, in press). The diversity of current opinions on the paragenesis of the Rosebery deposit is a reflection of the geological complexity, which can provide field and laboratory scale observations to support different hypotheses. However, the divergent models summarised above require substantially different strategies in the search for additional resources, both in the immediate mine area and in the region.
ACKNOWLEDGEMENTS The authors thank Pasminco Rosebery mine for permission to publish this paper, G Bennett for drafting of the figures and L Goninon for associated activities.
REFERENCES
Berry (1991, 1992) concurred with the Cambrian seafloor origin for the sulphide mineralisation but interpreted a series of reverse faults to explain the apparent repetition and stacking of the ore lenses. Geochemical evidence was used to help locate the position of the host rock–footwall volcanic rock contact as a guide to recognising faults.
Aerden, D G A M, 1991. Foliation boudinage control on the formation of the Rosebery orebody, Tasmania, Journal of Structural Geology, 13(7):759–775.
Recent studies have favoured a replacement origin for the deposits, with two times of formation. R L Allen (unpublished data, 1991) and Khin Zaw and Large (1992) proposed a Cambrian origin, with hydrothermal fluids altering or replacing permeable horizons, depositing metals shortly after the emplacement of the host rocks. In contrast Aerden (1991, 1992, 1993) proposed a Devonian origin with alteration or replacement and veining occurring in dilational structures formed by deformation during the Tabberabberan Orogeny.
Aerden, D G A M, 1993. Formation of massive sulphide lenses by replacement of folds: The Hercules Pb-Zn mine, Tasmania, Economic Geology, 88:377–396.
R L Allen (unpublished data, 1991) considered the footwall volcanic rocks to be subaqueous mass flows derived from subaerial pyroclastic rocks, which were deposited rapidly and syneruptively in a deep, fault bounded depression, proximal to a relatively deep water pyroclastic caldera volcano with a locally emergent or subaerial vent. He concluded that a major part of the mineralisation formed by replacement of permeable strata up to 300 m below the sea floor shortly after deposition of the footwall volcaniclastic rocks. Minor exhalative sulphide is also present, particularly near the stratigraphic top of the ore host horizon. The syndepositional faults would have acted as conduits for the hydrothermal fluids. Khin Zaw and Large (1992) arrived at similar conclusions from a study of the alteration and mineralisation at the South Hercules deposit. Aerden (1991, 1992, 1993) proposed a metamorphic origin involving dissolution of volcanogenic metals from Cambrian source rocks, and fluid transfer along the Rosebery shear zone (the zone of strongly cleaved rocks between the Rosebery and Mount Black faults). The metals were then deposited in structural traps, syn- to post-Devonian deformation. Evidence presented includes overgrowth of host rock cleavages by the
Geology of Australian and Papua New Guinean Mineral Deposits
Aerden, D G A M, 1992. Macro- and micro-structural controls on the genesis of the Rosebery and Hercules massive sulphide deposits, Tasmania, PhD thesis (unpublished), University of Queensland, Brisbane.
Allen, R L and Cas, R A F, 1990. The Rosebery controversy: distinguishing prospective submarine ignimbrite-like units from true subaerial ignimbrites in the Rosebery-Hercules Zn-Cu-Pb massive sulphide district, Tasmania, in Gondwana: Terranes and Resources, Tenth Australian Geological Convention, Hobart, Geological Society of Australia Abstracts, 25:31–32. Berry, R F, 1991. Structure of the Rosebery deposit, AMIRA Project P291: Structure and mineralisation of Western Tasmania (Australian Mineral Industries Research Association Limited: Melbourne) (unpublished). Berry, R F, 1992. Geochemical evidence for the structure of the Rosebery deposit, AMIRA Project P291: Structure and mineralisation of Western Tasmania (Australian Mineral Industries Research Association Limited: Melbourne) (unpublished). Brathwaite, R L, 1969. The geology of the Rosebery ore deposit, PhD thesis (unpublished), University of Tasmania, Hobart. Brathwaite, R L, 1974. The geology and origin of the Rosebery ore deposit, Tasmania, Economic Geology, 69:1086–1101. Campana, B and King, D, 1963. Palaeozoic tectonism, sedimentation and mineralisation in west Tasmania, Journal of the Geological Society of Australia, 10:1–53. Corbett, K D, 1986. The geological setting of mineralization in the Mt. Read Volcanics, in The Mount Read Volcanics and Associated Ore Deposits (Ed: R R Large), pp 1–10 (Geological Society of Australia, Tasmanian Division: Hobart). Corbett, K D and Lees, T C, 1987. Stratigraphic and structural relationships and evidence for Cambrian deformation at the western margin of the Mt Read Volcanics, Tasmania, Australian Journal of Earth Sciences, 34:45–67.
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Corbett, K D, Solomon, M, McClenaghan, M P, Carswell, J T, Green, G R, Iliff, G, Lees, T, Howarth, J W, McArthur, G J and Wallace, D B, 1989. Cambrian Mt. Read Volcanics and associated mineral deposits, in Geology and Mineral Resources of Tasmania (Eds: C F Burrett and E L Martin), Geological Society of Australia Special Publication, 15:84–153. Green, G R, 1983. The geological setting and formation of the Rosebery volcanic-hosted massive sulphide orebody, Tasmania, PhD thesis (unpublished), University of Tasmania, Hobart. Green, G R, Solomon, M and Walshe, J L, 1981. The formation of the volcanic-hosted massive sulphide ore deposit at Rosebery, Tasmania, Economic Geology,76:304–338. Hall, G, Cottle, V M, Rosenhain, P B, McGhie, R R and Druett, J G, 1965. Lead-zinc deposits of Read-Rosebery, in Geology of Australian Ore Deposits (Ed: J McAndrew), pp 485–489 (8th Commonwealth Mining and Metallurgical, Congress: Melbourne; and The Australasian Institute of Mining and Metallurgy: Melbourne). Khin Zaw and Large, R R, 1992. The precious metal-rich South Hercules mineralisation, western Tasmania: A possible subsea floor replacement volcanic hosted massive sulphide deposit, Economic Geology, 87:931–952.
Khin Zaw, Large, R R and Huston, D L, in press. Petrological and geochemical significance of a Devonian replacement zone in the Cambrian Rosebery VHMS deposit, western Tasmania, Canadian Mineralogist. Large, R R, Herrmann, W and Corbett, K D, 1987. Base metal exploration of the Mount Read Volcanics, western Tasmania: Pt 1 Geology and exploration, Elliott Bay, Economic Geology, 76:267–290. Lees T C, 1987. Geology and mineralisation of Rosebery–Hercules area, Tasmania, MSc thesis (unpublished), University of Tasmania, Hobart. Lees, T, Khin Zaw, Large, R R and Huston, D L, 1990. Rosebery and Hercules copper-lead-zinc deposits, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1241–1247 (The Australasian Institute of Mining and Metallurgy: Melbourne). McArthur, G J and Dronseika, E V, 1990. Que River and Hellyer zinclead-silver deposits, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1229–1239 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Khin Zaw and Large, R R, 1996. Petrology and geochemistry of sphalerite from the Cambrian VHMS deposits in the RoseberyHercules district, western Tasmania: Implications for gold mineralisation and Devonian metamorphic-metasomatic processes, Journal of Mineralogy and Petrology, 57:97–118.
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McQuitty, B M, Roberts, R H, Kitto P A and Cannard, C J, 1998. Rension Bell tin deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 487–492 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Renison Bell tin deposit 1
2
3
by B M McQuitty , R H Roberts , P A Kitto and C J Cannard INTRODUCTION The Renison underground tin mine is at Renison Bell, western Tasmania at lat 41o48′S, long 145o26′E on the Queenstown (SK 55–5) 1:250 000 scale map sheet. The mine is operated by Renison Limited, a wholly owned subsidiary of RGC Pty Ltd This paper updates the significant developments in exploration, mining techniques and research into the genesis of the deposit that have occurred since the last general description (Morland, 1990). Foremost amongst the new developments has been the Rendeep project comprising the discovery of the Rendeep orebodies beneath the northern part of the operating mine (Fig 1) and construction of a 575 m hoisting shaft to exploit them. Other important developments since 1990 include: 1.
studies of structural and geochemical controls of mineralisation by Kitto (1994) and McQuitty (1995);
2.
an interpretation of the morphology of the underlying Pine Hill Granite by D Leaman (unpublished data, 1990, 1994);
3.
understanding of the detailed structure in the region of the Federal–Bassett Fault by Renison geologists; and
4.
identification, from deep drilling, of the Renison mine sequence stratigraphy in the hanging wall of the FederalBassett Fault (Fig 2).
These developments have enabled a more confident interpretation of the origin of the Renison deposit and associated fault structures and their relationship to the underlying Pine Hill Granite.
RECENT DEVELOPMENT HISTORY In 1990 Renison commenced the Rendeep project, a major exploration initiative aimed at locating sufficient reserves beneath the lowest operating levels of the mine to justify construction of a hoisting shaft. A shaft was seen to be essential to the long term viability of the mine, because the mounting cost of trucking ore to surface via the decline system placed severe limitations on mining of deeper reserves.
4
Drilling commenced in January 1990 into a 600 m by 600 m area, over 600 m below surface beneath the North Bassett orebodies in the northern part of the mine (Fig 1). This area was targeted on the basis of a few widely-spaced intersections of ‘stratabound’ (carbonate replacement-style) mineralisation in drill holes completed a decade earlier. This style of mineralisation was known, from the upper part of the mine, to be more amenable to metallurgical treatment and to have less grade variability than the ‘fault’ mineralisation (Morland, 1990) which predominates beneath the central (Federal) and southern (South Bassett) regions of the mine (Fig 1). A total of 25 km of diamond drilling was carried out from surface, utilising wedges drilled from parent drill holes up to 1100 m long. The evaluation strategy was reviewed as confidence in the resource grew and in June 1992 construction of an underground drilling drive in the orebody hanging wall commenced (Fig 2). Development comprised a crosscut of 270 m and an along-strike drive of 450 m. From these openings a further 29 km of relatively short and accurately targeted drill holes were completed, improving confidence in the geological interpretation. Drilling of the Rendeep orebodies delineated Probable Ore Reserves of 3.3 Mt at 1.96% tin which were the basis for the shaft feasibility study in June 1994 (G Thomas, unpublished data, 1994). At that time, remaining Proved and Probable Reserves for the upper mine were 6.4 Mt at 1.41% tin (G Thomas and R Roberts, unpublished data, 1994). Thus the Rendeep discovery contributed 64 680 t of tin to reserves for a total of 154 920 t of contained tin. Since the discovery of the Renison field in 1890, production to January 1994 was 131 500 t of tin metal. The Rendeep reserves are being accessed in late 1996 via an extension to the decline system from surface. Ore is brought to surface via the 575 m hoisting shaft and conveyor system, commissioned in June 1996. The shaft will enable Renison to mine over 700 000 tpa at grades above 1.7% for the next nine years based on current Proved and Probable Reserves of 6.3 Mt at 1.72% tin (R Roberts, unpublished data, 1996).
ADVANCES ON PREVIOUS DESCRIPTIONS 1.
Formerly Senior Project Geologist, Renison Limited, now Supervising Geologist, RGC Exploration Pty Ltd, PO Box 322, Victoria Park WA 6979.
2.
Formerly Chief Geologist, Renison Limited, now Supervising Geologist, RGC Exploration Pty Ltd, PO Box 322, Victoria Park WA 6979.
3.
Formerly CODES Key Centre, Geology Department, University of Tasmania, GPO Box 252C, Hobart Tas 7001. Now Principal Geologist, RGC Exploration Pty Ltd, PO Box 322, Victoria Park WA 6979.
4.
Formerly Chief Geologist, Renison Limited, now General Manager-Exploration, RGC Exploration Pty Ltd, PO Box 322, Victoria Park WA 6979.
Geology of Australian and Papua New Guinean Mineral Deposits
Since Morland (1990) described the Renison deposit, coordinated research, exploration and mine geological work have resulted in significant advances in the knowledge of the structure and ore genesis. Interpretation by D Leaman (unpublished data, 1990, 1994) of the morphology of the Pine Hill Granite stock from gravity data is central to this work (Fig 3). Research by Kitto (1994) and McQuitty (1995) has demonstrated that the Renison deposit was formed from magmatic hydrothermal fluids escaping from the underlying Pine Hill Granite via a network of extensional faults developed in the host rocks over a shoulder of the intruding granite body.
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FIG 1 - Location map and longitudinal projection, Renison mine.
The Rendeep drilling and ongoing interpretation by Renison mine geologists have resolved the detailed structure in the vicinity of the major Federal–Bassett Fault. Increased drilling densities with subsequent improved interpretation and the application of higher cutoff grades have resulted in the ‘stratafault’ orebodies of Morland (1990) being subdivided into their individual fault ore and stratabound ore components. This has allowed more selective mining than was originally intended resulting in the recovery of ore which is nearly twice the grade of the ‘stratafault’ ore reserves quoted by Morland (1990).
REGIONAL GEOLOGY The Pine Hill Granite which underlies the Renison deposit is a steep sided, relatively flat topped stock, which outcrops locally at Pine Hill (Figs 3 and 4). A NW-trending ridge in the granite approximates to the axis of a broad regional anticline. The Federal–Bassett Fault occurs above a local high point in the upper surface of the granite, close to the granite’s steeply dipping northeastern margin. The fault has a normal sense of displacement overall, with a component of dextral wrench, and is transitional to a monoclinal fold in the northern part of the deposit (Fig 3). A full thickness of the mine sequence is present on the downthrown side. Syn-intrusive, subvertical extension produced a 770 m vertical displacement of the Renison mine sequence in the North Bassett–Rendeep area (McQuitty, 1995) through a combination of brittle and ductile deformation.
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RENDEEP ORE DEPOSIT FEATURES The Rendeep orebodies occur within and adjacent to the North Bassett Fault, the northern segment of the Federal–Bassett Fault (Fig 1). The North Bassett Fault acted as the main conduit for mineralising fluids in this region. Clearly identifiable blocks of mine sequence rocks have been dragged and rotated into this fault so that the bedding is subparallel to its strike and dip. A group of subvertical faults such as faults A and Z (Fig 2) splay off the fault and truncate the mine sequence into a series of downfaulted blocks. Mine sequence dolomites are mineralised where they occur in close proximity to the fault. There are two main types of mineralisation: 1.
fault mineralisation, mainly found in the North Bassett Fault; and
2.
stratabound mineralisation, which consists of semimassive pyrrhotite with significantly more chlorite, talc and phlogopite than in the upper regions of the deposit. This reflects the general trend throughout the mine of decreasing iron and sulphur and increasing magnesium with depth.
Stratabound mineralisation occurs on both the footwall and hanging wall sides of the Federal–Bassett Fault. Rendeep ore, both fault and stratabound, is higher in average tin grade (2%) and contains coarser cassiterite (of average grain diameter 100 µm) than ore in the upper deposit. These characteristics make Rendeep ore metallurgically more
Geology of Australian and Papua New Guinean Mineral Deposits
RENISON BELL TIN DEPOSIT
FIG 2 - Stratigraphuc column and cross-section on 66 800 m N, showing North Bassett–Rendeep structure.
amenable to treatment, with recoveries up to 10% higher than for upper mine ore. Rendeep fault ore has also been shown to have recoveries equivalent to stratabound material (G Thomas, unpublished data, 1994).
ORE GENESIS AND CONTROLS Four stages of fault movement and associated phases of mineralisation have been recognised by Kitto (1994). The Federal–Bassett Fault formed by brittle-ductile, layer-parallel extension of the well bedded Renison mine sequence close to its contact with the more massive overlying Crimson Creek Formation. Strain transfer from the Federal–Bassett Fault
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 3 - Plan of Federal–Bassett Fault and granite contact contours, and schematic cross sections showing the changes in the form of the Federal–Bassett Fault.
resulted in a series of normal transverse footwall faults such as faults A and Z in the Rendeep area and L and P in the Federal region, which dissect the mine horst (Figs 2 and 3). These
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FIG 4 - Schematic cross section through Pine Hill Granite at mine northing 66 800 m N.
second order listric faults propagated from concave-east flexures in the Federal–Bassett Fault. The axial regions of the flexures, which plunge at about 70 o south, became major upflow zones for high temperature (400oC) tin-bearing hydrothermal fluids as reactivations of the Federal–Bassett Fault breached a shoulder of the cooling Pine Hill Granite carapace.
Thermodynamic models for production of cassiterite-rich pyrrhotite ore predict that dolomite dissolution was most effective in precipitating >90% of the tin in solution and that processes such as cooling, boiling or mixing with ground water were not effective mechanisms for cassiterite deposition (Kitto, 1994).
Deposition of high temperature (>400oC) Stage 1 oxidesilicate vein assemblages (quartz-arsenopyrite-cassiterite) 3000 m beneath the Devonian palaeosurface was associated with initial faulting and the introduction of reduced (CH4bearing), moderately saline (~10 eq wt % NaCl), magmatic (δ18Ofluid 9‰) NaCl-KCl-H2O brines (Kitto, Cooke and Large, 1996).
MINE GEOLOGICAL METHODS
The Stage 2 sulphide-dominated mineral assemblage is pyrrhotite±cassiterite-quartz-fluorite-stannite-chalcopyritearsenopyrite and minor base metals. It is characteristic of the stratabound carbonate replacement orebodies at Renison and resulted from fault dilation during dextral wrench reactivation and ingress of 350oC magmatic hydrothermal fluids similar to Stage 1. These fluids evolved to brines rich in CaCl2-MgCl2NaCl-H2O during fluid-rock interaction with dolomite in the upper deposit levels. Uneconomic Stage 3 base metal veins of rhodochrositegalena-sphalerite-quartz, associated with minor fault reactivations, overprint the earlier vein stages, as do Stage 4 vug-fill carbonate-quartz veins (quartzcarbonate±fluorite±pyrite). Stage 3 and 4 veins were associated with reduced (CH4-bearing), low temperature (150 to 200oC), bimodal salinity (<2 and ~10 eq wt % NaCl), NaClKCl-H2O brines formed by mixing of contemporary meteoric ground waters derived from nitrogen-bearing sediments with (CH4-bearing) magmatic hydrothermal fluids.
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Greater control of dilution, improved mining selectivity achieved through greater emphasis on drilling and geological interpretation, and higher cutoff grades have raised the grade of ore mined from 1.11% in 1989 (Morland, 1990) to 1.59% in 1995–96 (R Roberts, unpublished data, 1996). In the same period mine dilution has fallen from over 25% to 10%. Mining methods have also changed, with bench stoping introduced in 1991 to replace the traditional cut and fill method in the steeply dipping orebodies. Bench stoping now accounts for 45% of total production and is expected to grow to 60% (R Roberts, unpublished data, 1996). Shaft hoisting of ore commenced in June 1996. Shaft hoisting is expected to account for 64% of production in 1996–97 and decline haulage of ore will be phased out over the next four years. The decline system presently extends to 770 m below surface, to 1430 m RL in Rendeep.
ACKNOWLEDGEMENTS The authors acknowledge the permission of Renison Limited to publish this paper. The many contributors to the revival of the Renison mine since it reopened after temporary closure in April 1991 are also acknowledged.
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REFERENCES Kitto, P A, 1994. Structural and geochemical controls on mineralisation at Renison, Tasmania, Australia, PhD thesis (unpublished), University of Tasmania, Hobart. Kitto, P A, Cooke, D R and Large, R R, 1996. Evolution of the Renison hydrothermal system, western Tasmania, Geological Society of Australia Abstracts, 41:232.
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McQuitty, B M, 1995. Structural controls on mineralisation in the Rendeep area, Renison tin mine, Tasmania, MEconGeol thesis (unpublished), University of Tasmania, Hobart. Morland, R, 1990. Renison Bell tin deposit, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1249–1251 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Phillips, G N and Hughes, M J, 1998. Victorian gold province, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 495–506 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Victorian gold province 1
2
by G N Phillips and M J Hughes INTRODUCTION
Significant developments in our knowledge of Victorian gold deposits over the last five years contrast with the virtual neglect of this world-class gold province during the 1980s global boom in gold exploration and production. The Victorian gold province (Fig 1, Table 1) has yielded 2480 t of gold, or 2% of all world gold production to 1997, mostly between 1851 and 1920. It was Victorian gold production in the second half of the 19th century that provided the foundation for rapid development of the State of Victoria and of many cities including Melbourne, Ballarat and Bendigo. With production of 700 t of gold, the Bendigo Goldfield is of international significance, which is even more remarkable given that sustained production ended in the 1920s. There is no comparable gold province that has historically produced so much gold as Victoria, and yet received so little application of the exploration methods and ideas that have evolved in the last two decades. Understanding this anomaly adds to our appreciation of the ingredients necessary for successful exploration, and also opens up opportunities for discovery of gold deposits in Victoria and elsewhere.
This contribution summarises the major advances in Victorian gold geology, especially the global context of this province, the advances in the geology of Victoria that impact on gold metallogeny, and the rapid changes in mining and exploration activities. The discovery of at least one new economic style of gold mineralisation in Victoria in the last few years suggests that Victoria's potential may be starting to be realised.
NATURE OF THE VICTORIAN GOLD PROVINCE The province consists of several thousand workings, mostly very small, which were operated in the 19th century. They are heterogeneously distributed across Victoria, in a wide range of host rocks, and only a small fraction of them have produced 1 t or more of gold. However, virtually all the primary deposits are structurally controlled, related to quartz veining, confined to rocks older than Carboniferous in age, and dominated by gold as the economic commodity rather than silver or base metals. A recent development is the discovery of a variation on this theme, in which open-space filling, including quartz veining is far less volumetrically important. The more important past gold producers are concentrated in central Victoria, specifically in the area from Stawell to Walhalla (Fig 1). There is some correspondence between this gold distribution and subdivisions of the Victorian Palaeozoic (Gray, 1988), such that the Ballarat zone, and to a lesser extent the Stawell and Melbourne zones are more auriferous than zones to either the east or west (Fig 2).
FIG 1 - Map of the Victorian gold province showing the area of Palaeozoic outcrop, the nine major geological zones comprising the Palaeozoic succession, and two major structural boundaries in the east (Mount Wellington fault zone: MWFZ; Kiewa fault: KF), as well as the Mount Easton fault zone (MEFZ) and Wonnangatta line: WL. Twelve goldfields that have each produced 30 t or more of gold (1 Moz), and a further nine significant past producers are shown, plus the Fosterville resource. The Victorian gold province is part of the more extensive Lachlan Fold Belt of SE Australia, itself part of the Tasman Orogenic Zone which dominates much of the eastern third of Australia. A full locality list is provided by Douglas and Ferguson (1988).
1.
General Manager, Great Central Mines Ltd, c/- PO Box 3, Central Park Vic 3145.
2.
Consultant, 1034 Geelong Rd, Mt Clear, Ballarat, Vic 3350.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 2 - Map of the Victorian gold province showing for each zone the approximate total gold production, production of the largest goldfield, number of 30 t producers (production is in t; no significant production has come from the two zones each at the eastern and western extremities). Major palaeoplacer (‘deep lead’) systems are also shown highlighting their abundance in the area from which most primary gold has been won (ie Ballarat zone).
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TABLE 1 Victorian gold production, in tonnes, to 1997. Total alluvial 1
Placer 2,3
540
157
157
65
343
273
70
173
27
146
61
27
54
Major goldfields
Total production
Primary
Bendigo
697
Ballarat
408
Castlemaine Stawell
82
Creswick4
81
Walhalla
68
68
Palaeoplacer 2
21 81
Maldon
65
56
9
9
Woods Point
52
40
>12
12
Clunes
47
37
>10
10
Chiltern
46
1
>45
45
Maryborough
32
3
>29
29
Berringa
30
18
>12
Egerton
27
16
11
Harrietville
24
12
>12
12 11
Avoca
23
23
18
5
Ararat
20
<1
19
14
5
Daylesford
20
17
>3
Tarnagulla
>13
>13
major
St Arnaud
12
12
3
Beaufort
>10
major
8
Dunolly-Moliagul
>10
4
major
3
Taradale-Lauriston
>10
6
major
2480
980
1490
>27
>27
Total Victorian Province5 Fosterville 1. 2. 3. 4. 5. 6.
6
300
At several goldfields a meaningful subdivision of alluvial into placer and palaeoplacer is not possible. Placer and palaeoplacer gold production are assigned to the adjoining major ‘primary’ goldfields from which that gold is thought to be derived (Phillips and Hughes, 1995). Substantial placer production is poorly accounted for (ie over 500 t of ‘total alluvial’ is unassigned). Creswick figure has been substantially revised upwards with the recent addition of placer production not included in Phillips and Hughes (1996). The nature of source information means that totals are rounded. Fosterville figure is predominantly a resource, rather than production.
A very important source of gold was placer and palaeoplacer deposits, again with most production concentrated in central Victoria, and related to adjacent primary deposits (Fig 2).
GOLD MINING AND EXPLORATION DURING THE 1980s AND 1990s The developments in exploration and mining in the province over the last decade provide a particularly interesting study of the interplay of government, community and industry perceptions and attitudes as they impact upon what should be viewed as a highly prospective gold province. The results of a few positive changes can already be seen in the early 1990s through increased mining and renewed exploration expenditure.
PERCEPTIONS OF PROSPECTIVITY One of the reasons that Victoria did not attract the exploration attention during the 1980s that was afforded many other gold provinces was the perception that gold in Victoria was essentially exhausted (Bowen and Whiting, 1975, p 648;
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Thomas, 1988), and the belief that the types of deposits to be found in Victoria were likely to be uneconomic. Such a viewpoint is not totally disproved until major discoveries are made, but on the basis of the minimal exploration effort in Victoria since 1910 and the observation that all other major gold provinces worldwide yielded further deposits during intensive 1980s style exploration, Phillips and Hughes (1995) argued strongly against the perception of gold exhaustion in Victoria. Recent additions at Stawell (Fletcher, 1995; Fredericksen and Gane, this publication) and new mineralisation styles at Fosterville (Zurkic, this publication) and Bailieston (Sebek, this publication) may be the start of a new period of discovery.
ATTITUDES TO MINING The closure of most of Victoria's gold mines by 1915 meant that the strong mining culture of the 19th century was all but gone by the 1980s. With little community or government support for mining and some significant opposition, exploration was at a very low level through the 1980s and discoveries were negligible, having the effect of reinforcing the negative
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VICTORIAN GOLD PROVINCE
sentiment towards Victorian gold prospectivity. The Victorian Minister for Energy and Minerals referred to the 1980s decline in Victorian mining activity by noting: ‘This decline is due to several factors including government indifference, a plethora of regulatory approvals and an anti-mining ethic’ (Plowman, 1994). His department went on to rapidly address some of these shortcomings.
province in which economic gold deposits form by the same fundamental genetic process (Phillips and Hughes, 1995, 1996), rather than as a geographic region in which these deposits have formed by multiple, unrelated or vaguely related processes.
NEW INITIATIVES
Gold production in Victoria during the period 1970–1985 never exceeded 1 tpa. By 1995 production had reached 5 t. This increase can be attributed to the production from the Stawell and Fosterville mines, and some intermittent mining at Heathcote, Benambra, Nagambie, Maldon, Avoca and St Arnaud, as well as recovery from old tailings dumps.
Initiatives since the 1980s that have changed the role of exploration in Victoria can be divided into areas of policy, geological data and ideas. Policy primarily involves government decisions and action that impact on exploration and mining, geological data include geophysics and relate to new information to assist exploration, and ideas refer to conceptual inputs to exploration models.
Policy Dealing with regulatory bodies related to mining and exploration in Victoria has been greatly facilitated by reducing the number of individual inquiry points, and by speeding up the approval process for granting of tenements. In this regard Victoria is now competitive with other Australian States.
Data Areas where new data are particularly useful, such as airborne geophysics, have been addressed through a major initiative by the Victorian Government (Victorian Initiative for Minerals and Petroleum which commenced in 1994), which includes new aeromagnetic, gravity and radiometric surveys. Enormous potential exists to reprocess and re-evaluate the extensive, well documented observations of Mines Department personnel made in the past during the active period of each mining operation, and these are now becoming available in digital form.
Ideas Until recently, very little synergy had been achieved from the relationship between descriptive models and genetic models as they relate to Victorian gold deposits, compared to the achievements, say, with Archaean gold deposits. This situation has arisen from the lack of major synthesis studies of Victorian gold deposits in the 1980s, the perception that Victorian gold geology was unlike the setting in other gold provinces, and the large gap between some genetic models which had been suggested from Victoria (eg gold in granites, exhalite gold), and what can be gleaned from past mining records of all significant producers (gold in metasedimentary rocks and mafic dykes). The metamorphic model being adopted here for Victorian gold deposits represents a significant advance on this, and has arisen from greenstone gold work (Phillips and Groves, 1983), from other slate belts (Goldfarb, Snee and Pickthorn, 1993), from theoretical studies on gold deposit formation, and from work in Victoria including the Castlemaine district (Cox et al, 1995 and earlier references; Hughes, Ho and Hughes, 1996) and Stawell (Wilson et al, 1992). As a genetic model, it provides the basis for prioritising data collection (much being already available in the literature), testing ideas, revising the genetic model, and collecting further data in an iterative process capable of generating considerable synergy (Phillips, Eshuys and Hellsten, 1996). It involves viewing Victoria as a
Geology of Australian and Papua New Guinean Mineral Deposits
MINING AND EXPLORATION ACTIVITY
With new regulations and some mining successes, exploration activity has increased dramatically in Victoria from a very low base. From an expenditure on gold exploration of $8M/yr in 1990, this increased to $26M/yr in 1995, and in 1996 the non-petroleum exploration expenditure (mostly on gold exploration) was approximately $50M/yr. Most exploration has focussed on significant past producers (eg Tarnagulla, Maldon, Ballarat, Bendigo, Clunes, Creswick and Woods Point) and prospective ground of the Ballarat zone. However, there has been additional exploration in previously unproductive areas such as far NW Victoria under deep alluvial cover where major companies have taken out large tenement holdings, and in far eastern Victoria.
ADVANCES IN VICTORIAN GOLD GEOLOGY In the 1980s, substantial advances were being made in understanding the geology of Archaean greenstone gold provinces, the Witwatersrand goldfields, the Alaskan slate belt provinces, and of some gold-only epithermal systems, but no similar advances occurred for the Victorian gold province. Such synthesis has recently been achieved for the Victorian province by an integration of detailed and regional gold metallogenic studies that open up a field of possibilities for new exploration and models (Phillips and Hughes, 1995, 1996; Hughes, Phillips and Gregory, 1997). Two dimensions of integration have provided the platforms for advance over the last decade. The first is integration of all aspects of geology to provide the best possible understanding of a single goldfield. This was pioneered at Castlemaine–Chewton (several papers summarised in Cox et al, 1995), and this approach has also been effective at Stawell (Wilson et al, 1992) and Maldon (Hughes, Phillips and Gregory, 1996, 1997). By using all available geological data, these long-term projects have been able to tackle some of the key issues influencing Victorian gold distribution. Three of these integrated studies are described where the results have widespread application to Victorian gold deposits. The second dimension of integration relates to scale, and has involved considering gold mineralisation, its chemistry, and structural and tectonic setting, at several scales. This includes the scales of ore shoots, orebodies, goldfields, geological zones and the province, and the Victorian gold province in terms of other major global gold provinces. This dimension of integration has been possible by combining a knowledge of the essential features of many Victorian goldfields with that derived from extensive work on Archaean greenstone, Witwatersrand, slate belt and other gold-only provinces,
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worldwide (Phillips and Hughes, 1995). It has necessarily hinged on the belief that the Victorian gold province conforms in many ways to other major gold provinces, rather than being unique in its development. The following discussions address some of the advances relating to the geology of Victorian gold deposits.
FLUID FLOW AND STRUCTURAL CONTROL IN THE CASTLEMAINE GOLDFIELD The Castlemaine Goldfield produced 173 t of gold (Willman, 1995), mostly from very rich placer workings. Cox et al (1995) combined surface and underground mapping with microstructural, mineralogical, geochemical, fluid inclusion and stable isotope studies to elucidate many of the important controls on gold location in the quartz reefs of the Wattle Gully deposit. The deposits are in the hanging wall of a major, west-dipping reverse fault, as in several other Victorian goldfields. Auriferous quartz veins are tightly confined to an interval several hundred metres wide and several kilometres long, marking the domain of primary gold distribution, and presumably the domain of extensive fluid flow through the upper crust. The main structural control on primary mineralisation at Wattle Gully is a dilational jog on a high angle reverse fault, and continued fault movement and fluid pressure variations (as indicated by a multitude of crack-seal events) enhanced fluid interaction with what we infer were chemically favourable carbonaceous slates that led to reduction and gold precipitation. The auriferous fluid at Wattle Gully was of low salinity and had relatively non-diagnostic isotopic characteristics, consistent with a metamorphic fluid origin. Mineralising temperatures and pressures of around 300oC and 1.3 kb have been inferred.
STRUCTURAL SETTING OF THE STAWELL GOLDFIELD The Stawell field (Fredericksen and Gane, this publication) has produced 82 t of gold with indications that known resources might extend the endowment above 100 t (Fletcher, 1995). The structural setting of auriferous quartz veins in the hanging wall of a west-dipping reverse fault is similar to Castlemaine, but the host rocks include Cambro-Ordovician pelite, volcaniclastic rocks, Cambrian metabasalt, and the hornfelsed equivalents of these rocks. A combination of mapping, regional and detailed structural geology, mineralogy and metamorphic studies has provided an insight into the complexities of the Stawell mineralisation (Wilson et al, 1992; Mapani and Wilson, 1994). Detailed metamorphic petrology has suggested that depths of burial were very shallow (2 kb) at the time of D4 deformation and granite intrusion (Xu et al, 1994). Regional ductile deformation in the Stawell area pre-dated the Stawell Granite (dated around 395 Myr) in what has been inferred as D1–3 structural events (Wilson et al, 1992). Further deformation (D4) is broadly synchronous with this granite (although in detail gold introduction may pre-date the granite), and is inferred to be a Tabberabberan deformation on the basis of its NE–SW compression direction (Wilson et al, 1992). Such an interpretation might require Tabberabberan deformation to be diachronous across Victoria, as suggested by Collins (1994), and spanning a period of more than 20 Myr.
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RELATIONSHIP OF THE MALDON GOLDFIELD TO ADJACENT GRANITE The Maldon goldfield has produced 65 t of gold, and its 56 t of primary production makes it one of the largest Victorian primary gold producers. Maldon is easily the most significant source of gold from deposits adjacent to granite, and it is an ideal place to study the changes in gold mineralisation proximal to a batholith. A combination of mapping, mineralogy, geochemistry, metamorphic petrology and quartz texture work elucidates some of the geological issues of the gold field (Hughes, Phillips and Gregory, 1997). Primary gold production has come from high grade, steeplydipping quartz veins that strike north towards the contact of a major 360 Myr granite batholith (Ebsworth, de Vickerod Krokowski and Fothergill, this publication). Mineralisation is abruptly terminated at the granite contact, and the granite is unaffected by either the gold-hosting shear zones, or the goldrelated alteration. Auriferous quartz veins are recrystallised approaching the granite, and distal arsenopyrite gives way to proximal loellingite plus pyrrhotite in the potassium feldspar zone within 1.5 km of the granite. The geochemistry of gold mineralisation at Maldon is comparable to most other Victorian gold deposits in metasedimentary rocks away from granite contacts (ie elevated values for sulphur, arsenic and gold), but with an additional contribution of molybdenum, bismuth and tungsten in close proximity to the granite only. The predictable nature of the geochemistry, with the structural, textural and mineralogical relationships observed, makes it unnecessary to propose that the Maldon deposit, or the deposits near the granite, result from a fluid dramatically different to many other Victorian gold deposits, although it does suggest a contribution from the granite. In contrast to the geochemistry, the mineralogy of the Maldon deposits is quite unusual [eg maldonite (Au2Bi), loellingite, biotite selvages on quartz veins, abundant pyrrhotite] and reflects higher temperatures than at many other Victorian gold deposits, and is consistent with contact metamorphism and the effects of significant retrograde resetting. The Maldon field is interpreted as a mineralised zone more than 6 km long intruded at its northern end by a Late Devonian granite. A thermal peak after gold mineralisation caused low pressure metamorphism in a 2.5 km wide aureole resulting in modified quartz and sulphide textures, shear zone foliation, alteration and host rock mineralogy. The nature of the mineralisation, alteration and structure provide little evidence that the granite was either the cause or source of the gold.
RELATIONSHIP OF PRIMARY GOLD DEPOSITS, PLACERS AND PALAEOPLACERS The relationship of primary to alluvial gold in Victoria has considerable impact on endowment figures and economic projections, and is one of the important inputs into genetic models. In the past, placer and palaeoplacer production have commonly been treated separately from primary production (eg Woodall, 1979). However, for the purpose of understanding the distribution and scale of primary gold mineralising systems in the province, there are benefits in viewing the primary and secondary deposits together. To gain a proper appreciation of the Palaeozoic processes that led to the Victorian gold province, it is necessary to combine primary and alluvial production. The revised endowment figures highlight the fact
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VICTORIAN GOLD PROVINCE
that there are 12 goldfields of 1 Moz or more in the Victorian gold province. The area containing these 1 Moz fields indicates the region in which there will be the highest probability of finding deposits likely to be viable today; and the styles of primary mineralisation within these 1 Moz fields indicate some (but probably not all) of the styles likely to be economic today. At many fields, the early prospectors used placer and palaeoplacer gold as a direct, and very successful, pathfinder for primary deposits. At Ballarat, highly auriferous reefs along three north-trending mineralised lines, two of 100–200 m width and the third consisting of three subzones of similar width, are drained by stream systems which have very rich placers that can be traced directly into palaeoplacers where younger basalt flows have filled old valleys (Fig 3; Taylor, this publication). The primary source is less clear at other goldfields. For example, the upper reaches of the Creswick palaeoplacer systems only contain auriferous quartz veins which have produced minor gold compared to the palaeoplacer production (but extensive basalt cover in the area may conceal larger deposits). More importantly, on a broad scale, the source area of the major palaeoplacer systems, including Creswick, is in the very heartland of Victorian primary gold deposits, namely the Ballarat–Bendigo area with its many primary deposits (Fig 2). On a wider scale, the same close relationship of rich placers to primary fields is borne out by recent comparisons with some Pacific Rim placer fields (Goldfarb, Nokleberg and Phillips, 1996). On this scale, the largest alluvial gold province in the southern Pacific (Victoria) not only coincides with the largest primary field, but the largest alluvial deposits in Victoria are remarkably close to the focus of the largest primary deposits.
GOLD DISTRIBUTION RELATED TO GEOLOGICAL ZONES The gold endowment figures show a strong focus of gold mineralisation in the Ballarat zone with marked decreases in other geological zones to the east and west, as has been known for a long while. This pattern, however, holds not only for total gold production, but also for the number of 1 Moz goldfields, number of reef mines producing over 1 t of gold, and the maximum size of goldfields in any geological zone. It probably also reflects the regional quartz vein density, total shortening across geological zones and location of high strain zones, and the distribution of anomalous but subeconomic gold values. The distribution of significant gold mineralisation in areas outside the Ballarat zone can be spatially related to high strain thrust zones. Examples are the Beaufort, Avoca and St Arnaud goldfields in the hanging wall of the Avoca and Percydale faults; the Stawell and Ararat fields associated with the Stawell–Copes Hill fault system and the Woods Point and Walhalla fields associated with the more strongly deformed eastern margin of the Melbourne zone, in the vicinity of the Mount Easton and Mount Wellington fault zones. The significance of the Harrietville and Chiltern fields immediately west of the Kiewa–Kancoona fault zone in the Tabberabbera zone is less clear. This pattern has numerous implications for exploration and gold genesis. It gives credence to the adage of ‘the best place to find a million-ounce deposit is near existing deposits’ rather than any suggestion that some areas develop many small deposits at the expense of a major one. It suggests that on the scale of geological zones, certain factors ‘went right’ for gold mineralisation in the Ballarat and neighbouring zones, but did not occur in the far east and west of Victoria. The pattern
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 3 - Map of the Ballarat Goldfield showing the three parallel lines of north-trending primary mineralisation at Ballarat West, Ballarat East–Buninyong West, and Little Bendigo-Buninyong East. The alluvial deposits were rich and extensive immediately downslope from the major primary deposits, and these were traced a farther 20–30 km downstream as rich palaeoplacer deposits (‘deep leads’) beneath Quaternary basalt flows.
probably means that the volume of fluid and its gold-carrying capacity is critical on a zone scale, and on this scale (as distinct from the scale of an individual orebody) it is not the occurrence of special local structures that is of prime importance. Instead, the distribution of goldfields relative to regional scale faults suggests that the distribution of regional-scale structural ‘plumbing’ systems is important in focussing the flow of this fluid.
RELATIONSHIP OF GOLD DEPOSITS IN METASEDIMENTS, VOLCANIC ROCKS AND DYKES The three major gold mines in dykes of the Woods Point dyke swarm (the Morning Star and A1 mines near Woods Point, and the Long Tunnel mine, Walhalla) have never been integrated into the evolution of the slate-hosted deposits around Ballarat–Bendigo and the volcanic-hosted Stawell mineralisation.
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Comparison with other gold provinces indicates that there are very good reasons to expect major gold deposits in both dykes and volcanic rocks, and black slates. In the absence of conflicting genetic evidence (or of any evidence other than differing host lithologies), these deposits are most reasonably part of one province and one overall event rather than being independent in time or origin. In the slate belt province of southeast Alaska, the major Juneau goldfield is dominantly hosted by a mafic dyke (Goldfarb, Snee and Pickthorn, 1993). In the Archaean greenstone gold provinces, many major gold deposits are related to iron-rich rocks (eg basalt, dolerite, banded iron formation) or less commonly carbon-rich host rocks such as black shale. In the Carlin province of Nevada, which shows similarities to some Victorian gold deposits (Hughes, Phillips and Gregory, 1997), there is a close association between gold mineralisation and pyrobitumen. These relationships of major gold deposits to iron-rich and/or carbon-rich host rocks and possibly to pyrobitumens, are attributed to the effectiveness of interaction with iron and carbon in precipitating thio-complexed gold from the low salinity fluid. There is an urgent need for detailed orebody studies which investigate these proposed controls on ore deposition. Applying these observations to the Victorian province, the predicted favourable host rocks in the Palaeozoic succession, which consists of Cambrian metabasalt, a flysch sequence of clastic metasediment, and a range of intrusions from ultramafic to felsic in composition, would be those rich in carbon (black slate) and iron (mafic dykes, metabasalt, volcaniclastic rocks). It is particularly notable that the Woods Point dyke swarm consists of hundreds of intrusions that range from ultramafic to intermediate and felsic in composition, and yet the major gold deposits are in the more iron-rich dykes of mafic composition (‘diorite’ and ‘lamprophyre’ in some earlier literature), in which iron-titanium minerals have been replaced by pyrite and rutile (or leucoxene). The nature of the wall rock alteration around deposits in the mafic dykes (ie chlorite, carbonate, mica and pyrite) is predictable given the composition of the goldbearing fluid and the host rocks. The close association of gold with dykes might be interpreted as a genetic association between the magmatic event and the source of gold. However, the dykes vary from peridotitediorite suites in the Woods Point dyke swarm and at minor deposits in dykes which occur in three other swarms which strike northwards, east of Maryborough and west of the Landsborough and Percydale faults. At the other extreme are quartz porphyry dykes at minor deposits elsewhere (eg west of Maryborough and Diamond Creek). Gold mineralisation in these dykes occurs in fractures which are systematically oriented with respect to the dyke walls and regional deformation fields in the surrounding rocks, and gold spatially unrelated to dykes occurs in sedimentary rocks of the same areas. The role of the dykes is better seen in terms of their physical response to later deformation, and their chemistry. This recognition of the probable role of carbon and iron in major gold deposits, and the observation that mineralisation can occur in most rock types, leads to the need for caution in using the term ‘slate belt’ for these deposits, or for the province. In essence, the Palaeozoic succession of Victoria is a gold province with many of the major deposits in slates.
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GEOCHEMISTRY OF VICTORIAN GOLD DEPOSITS With some notable exceptions, most of the Victorian goldfields have a systematic and relatively simple geochemical signature comprising anomalous gold, arsenic, silver, sulphur, potassium, rubidium and carbon dioxide (Goldfarb and Phillips, 1995; Hughes, Phillips and Gregory, 1997). Minor fields in and near the Melbourne zone, of which Fosterville is the only one large enough to show on Fig 1, have anomalous to economic antimony with gold mineralisation; whereas deposits in or near granites (such as Maldon) have distinctly higher levels of bismuth, molybdenum and/or tungsten compared to all other gold deposits. This enrichment in potentially graniterelated elements is a signature additional to the main gold-only signature and generally extends less than 1 km from plutons; it does not appear to indicate a magmatic fluid origin for such gold deposits (Hughes, Phillips and Gregory, 1997). There are goldfields east of Stawell, and in the Omeo zone in eastern Victoria, that have slightly to distinctly different chemical signatures, with high silver, arsenic, copper, zinc and/or lead (Hughes, Phillips and Gregory, 1997); and from this signature a different and possibly more saline fluid is inferred, particularly for the Omeo zone, compared to most Victorian gold deposits. Sulphide ores of the Omeo zone were very small producers, and the less sulphidic ores of the eastern Stawell zone at Avoca and St Arnaud are more significant economically.
TIMING OF VICTORIAN GOLD EMPLACEMENT The timing of gold emplacement is relatively well constrained in some locations by a combination of host rock age, post-gold granites, deformation, and the essentially non-auriferous Carboniferous and younger sequences. For much of the Stawell and Ballarat zones, there is a large time range between host rock sedimentation and post-gold intrusions, with some uncertainty remaining as to the age of regional deformation. At Maldon, Early Ordovician metasedimentary rocks were mineralised before emplacement of the Late Devonian Harcourt Granite (Hughes, Phillips and Gregory, 1997), and similarly at Stawell and in the NW of the Ballarat zone (Mt Hooghly granite near Dunolly; Tarnagulla), gold mineralisation appears to predate intrusion of the 395 Myr (Early Devonian) granite. Gold emplacement appears syn- to late-deformational in the Stawell zone, and in parts of the Ballarat zone (Cox et al, 1991a) of central and western Victoria. However, the deformation is likely to be diachronous, and like the Devonian thermal event, be slightly earlier in the west. In the Stawell and Ballarat zones, economic gold mineralisation therefore pre-dates 395 and 365 Myr granites, whereas other, economically unimportant auriferous quartz veins post-date 395 Myr granites in the Stawell zone. However, in the Melbourne zone, economic gold mineralisation appears to be younger. In the Woods Point and Walhalla fields, for instance, auriferous quartz veins occur in dykes of probable Middle Devonian age which intrude the strongly deformed Early Devonian flysch sequence, whereas thick Carboniferous molasse sequences have no primary gold mineralisation, making major gold mineralisation Middle to Late Devonian in age. The widespread Late Devonian granites and acid volcanic complexes (365 Myr) appear to post-date the introduction of the major gold mineralisation mined prior to the 1980s, although some gold mineralisation may post-date this magmatic event (Hughes, Phillips and Gregory, 1997).
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On a broader scale, the evidence is compatible with a single period of gold introduction in any one field, with some diachroneity of that period across the province. Existing evidence permits the total range of time during which gold mineralisation occurred across the province to be at least 35 Myr. However, this corresponds to, and outlasts by at least 15 Myr, a minimum of 125 Myr of continuous turbidite sedimentation, and therefore corresponds to the final tectonic and thermal event at the close of this sedimentation. Remobilisation of existing gold mineralisation, including addition of some granite-related components, has occurred in the vicinity of some major plutons (eg Maldon) where postmineralisation dykes are closely associated with earlier quartz reefs which have probably acted as ideal plumbing systems for later fluids. There is little critical evidence to support the introduction of gold synchronous with sedimentation and/or Cambrian volcanism, nor to relate it to fluids directly sourced from individual plutons. There are several unresolved issues relating to timing of the mineralisation, and experience in other gold provinces suggests that their resolution might come from indirect studies involving field relationships and the dating of critical rock types, rather than direct attempts to date mineralisation and alteration.
STRUCTURAL GEOLOGY A number of structural studies (including a seismic transect in central Victoria) have led to significant advances in the understanding of the structural geology and especially how it relates to the distribution and controls of goldfields (Cox et al, 1991a, b, 1995; Gray, Wilson and Barton, 1991; Gray and Willman, 1991; Wilson et al, 1992). A major decollement surface near the boundary between the Cambrian metabasalt sequence and the Cambrian to Devonian flysch sequence is the sole to a series of west-dipping listric faults in the Stawell to Melbourne zones, and there may be a further decollement surface at the base of the Cambrian metabasalt, as well as duplexing within it. The dominance of this listric geometry rather than staircase faulting has been attributed to the preponderance of monotonous mudstone (slate, shale, siltstone) rather than sandstone, especially in the Ballarat zone. The major change from these west-dipping faults to SW vergence of structures occurs in the Tabberabbera zone and approximates to the eastern boundary of most major goldfields and thin-skinned tectonics (Scheibner, 1992). Major goldfields do occur in the Tabberabbera zone (eg Chiltern), but within rocks which could otherwise have been derived structurally from the Ballarat zone. This structural work has also indicated more complexity in the Melbourne zone than previously recognised, that included south-vergent thrusting, two periods of folding and an overprinting cleavage, all in the Devonian (Gray and Mortimer, 1996; Morand, Hughes and Jones, 1997). The understanding gained for the Archaean deposits of the relationship between far-field stress, orientation of rock units, and gold deposits (Ridley, 1993) can be translated to the Victorian province. In a flysch sequence where the chemically favourable host slates are incompetent compared to the surrounding rocks, dilational sites will occur where bedding in these units is oriented parallel to the far-field stress prior to deformation, resulting in saddle reefs. However, where the favourable host is more competent than the surrounding rocks, such as in greenstone belts and in thick mafic dykes, dilation will occur where units are oriented perpendicular to the far-
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field stress, resulting in stockworks and ladder veins. In part, this link accounts for the remarkable similarities between the Mount Charlotte deposit at Kalgoorlie and the A1 deposit near Woods Point, for example. The similarity of host rocks and auriferous fluid also contributes to the similarity of structural control, mineralisation and alteration between these two deposits. Closer investigation of the structures in major Victorian fields indicates a wide range of structures with relatively constant relationships to the regional stress field (eg Phillips and Hughes, 1995, see Fig 3), and also defines a common theme of repetition of structures in all major fields (eg stacked saddle reefs, multiple ladder veins and multiple reverse faults).
METAMORPHISM AND MAGMATISM An important advance in the knowledge of metamorphic petrology arose from the study of the Palaeozoic rocks in the vicinity of Stawell gold mine (Wilson et al, 1992). The thermodynamic modelling of reactions in mafic rocks applicable to the Stawell sequence has led to a much wider application to gold deposits of many ages (Powell, Will and Phillips, 1991). This modelling suggested that low grade metamorphism of carbonate-bearing mafic rocks should evolve a distinctive fluid containing water and carbon dioxide in proportions buffered by the mineral assemblage and modestly dependent upon pressure and temperature, and that the bulk of fluid evolution should accompany the greenschist to amphibolite facies transition. Comparison with reported natural fluids suggests that the theoretically predicted fluid is uncommon in nature and in most ore systems, but is virtually identical to that identified in a number of gold-only systems including slate belts, Archaean greenstone, Carlin and Witwatersrand gold deposits. To date this metamorphic fluid model is the only genetic model that predicts the composition of gold-only fluids. The model also underpins many of the links between slate belt gold provinces such as Victoria, and other gold-only types (Phillips and Powell, 1993). In a detailed study of the granitic rocks of the Lachlan Fold Belt (Chappell et al, 1991), the Central Victorian magmatic province stands out for its abundance of Late Devonian, posttectonic plutons. Some of these plutons are very high level (ie hot and water-undersaturated), and several are spatially and genetically associated with felsic volcanic centres, all confirming a major thermal event during the Devonian. This period was immediately preceded by the intrusion of ultramafic to intermediate dyke swarms, suggesting that the thermal event is deeper than just crustal in extent. Using this granite compilation for all of the Victorian gold province, it becomes clear that there is no positive correlation between gold production and granites (in fact, a negative correlation exists), and that gold mineralisation in or near granites is associated with all granite types, including I- and Stypes, fractionated and unfractionated, and mafic and felsic types (Hughes, Phillips and Gregory, 1997). Granites make up around 20% of the Victorian gold province yet deposits within granites have yielded less than 10 t of gold, mostly from outside the main goldfields region, suggesting that the non-granite parts of the Victorian gold province are two orders of magnitude more prospective for gold than the granites (Phillips, 1996). On the basis of what is known of Victorian gold mineralisation and of the Maldon deposit, and what is known in comparable gold-only provinces globally, this underrepresentation of granites as host rocks for gold-only deposits appears widespread and predictable.
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LACHLAN FOLD BELT IN VICTORIA DEFINING THE TECTONIC SETTING AND TECTONIC BOUNDARIES The nature of the unexposed basement and the tectonic setting of Victoria have been contentious issues for many years (Gray, 1988), and it would appear that some of this uncertainty has arisen because the Lachlan Fold Belt is most unlike many other orogenic belts (eg Coney et al, 1990). This dissimilarity with other Phanerozoic orogenic belts is based on a number of unusual features (Coney et al, 1990; Coney, 1992), including its 800 km easterly width, its 50% shortening, lack of Precambrian basement rocks, lack of evidence for any great elevation in the past, lack of deep metamorphic rocks, and high proportion of Siluro-Devonian acid magmatic rocks. Similarities have been drawn between the Lachlan Fold Belt and Archaean greenstone belts, especially regarding their tectonics (Coney et al, 1990). There is some consensus that there was an ocean floor, island arc and subduction setting during the Cambrian, with extensive continental margin, back arc or marginal sea sedimentation on thin cratonic blocks in the Ordovician, perhaps akin to the present Bengal fan of Fergusson and Coney (1992). Episodic deformation started in the Cambrian but essentially ceased in the Lachlan Fold Belt by the Early Carboniferous with a further change to more continental and rift-related sedimentation and volcanism. Three boundaries are critical to fully understanding the Palaeozoic of Victoria, these being the Mount Wellington fault zone on the east of the Melbourne zone, the Wonnangatta line within the Tabberabbera zone, and the Kiewa fault on the east of the Tabberabbera zone (Fig 1; Collins and Vernon, 1992; Glen, 1992; Glen, Scheibner and VandenBerg, 1992; Scheibner, 1992; Collins, 1994). These boundaries are also important in understanding the controls of the distribution of gold mineralisation (Fig 2). The Mount Wellington fault zone (used here in the sense of the bounding fault zone which passes through Cambrian greenstone of the Dolodrook inlier and eastern Howqua River), has been interpreted as the boundary between the allochthonous Stawell, Ballarat and Melbourne zones of the Melbourne terrane, and the Tabberabbera zone (part of the Benambra terrane). Transport of the Melbourne terrane from further south prior to 390 Myr (Glen, 1992; Glen, Scheibner and VandenBerg, 1992), or of the Tabberabbera zone from further north, have been proposed. The Mount Wellington fault zone is an important boundary with dominantly calc-alkaline and MORB Cambrian rocks associated with the Barkly River fault zone, with a Devonian age for D1 and east vergence all to the west of the Mount Wellington fault. To the east of the same fault are MORB basalts and ultramafic rocks (the latter directly associated with the Mount Wellington fault zone) which have a pre-Devonian age for D1 and SW vergence. There is a change of granite type across this boundary, with Early and Middle Devonian granites confined to the east, whereas the granites to the west are virtually all part of the Central Victorian magmatic province. The Wonnangatta line within the Tabberabbera zone is a distinctive fault zone marking the boundary between a domain of single, SW-verging deformation to the west, and more complexly deformed rocks to the NE. This has been interpreted as the boundary between thin-skinned tectonics to the west, and thick-skinned tectonics further east (Scheibner, 1992). It has
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recently been suggested that the age of some rocks in this SWverging zone may be Silurian, rather than Ordovician (Fergusson, 1996). The Kiewa fault separates low grade metamorphic rocks to the west from medium to high grade metamorphic rocks of the Omeo zone over much of its length (Morand, 1990). It is also a boundary based on the strontium isotopic composition of granite (Gray, 1990), and the eastern boundary of all significant gold-only deposits (Phillips and Hughes, 1995). Some of these boundaries have been explained in terms of tectonic processes. Collins (1994) inferred two periods of eastward migrating deformation, one east of the Kiewa fault during the Silurian, and one from the Stawell zone to the Kiewa fault in the Devonian. This pattern of deformation was explained by eastward delamination of the mantle lithosphere, but does not yet explain the pre-Devonian deformation in the Tabberabbera zone. The model, however, does help to explain the wide distribution of low pressure metamorphic and granitic rocks. Recent mapping of critical areas in Victoria has produced substantial revisions of the geology near the western margin of the Lachlan Fold Belt (Simpson and Woodfull, 1994; Cayley, 1995), and has indicated a complex relationship between gold, granites and deformation in central to western Victoria. The western margin is now considered by some workers to be the Moyston fault, east of the Grampians. Mapping of Silurian rocks of the Grampians area has indicated repetition through thrusting (Cayley, 1995; Cayley and Taylor, 1996) not recognised in earlier mapping, and significant deformation of the Grampians Group prior to intrusion of Early Devonian granites. Some of the relationships between folding, gold and granites can be rationalised if the Tabberabberan deformation, which is classically Middle Devonian in the Melbourne and Tabberabbera zones (but a second subordinate phase of deformation in the Tabberabbera zone), is diachronous across the province and slightly older in the west. This would be compatible with the slightly older magmatism in the west.
VICTORIA AND OTHER ‘GOLD-ONLY’ GOLD PROVINCES Comparison between the Victorian and the two other major synorogenic (‘mesothermal’) Phanerozoic gold provinces of the Pacific Rim (ie including Alaska and the Mother Lode of the Cordilleran of western North America, and the Russian Far East and Eastern Eurasia provinces) puts some focus on those aspects which might be essential in the generation of a major gold province, and those that may be less important. Eastern Eurasia and the North American Cordilleran provinces formed during the Phanerozoic when widespread Aand B-type thrusting and transcurrent faulting exposed a range of crustal levels and various ‘suspect terranes’ were accreted. In contrast, deformation and magmatism in the Tasman Orogenic Zone involved little amalgamation of terranes (especially for the Lachlan Fold Belt west of the Tabberabbera zone) and only a limited crustal section was exposed during ocean-directed thrusting in the Victorian province. A very different tectonic setting for Victoria, relative to the other provinces, is therefore indicated, although transcurrent faulting and reactivation of thrusts are common. It has been concluded that the thermal regime and its influence on fluid generation were more important in generating these major gold provinces
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than any particular tectonic setting (Goldfarb, Nokleberg and Phillips, 1996), although the tectonic regime may influence the plumbing system and the distribution of goldfields within a province. These more global studies also allow comparison of slate belt gold provinces with other gold types. Slate belt gold provinces have many obvious differences from greenstone, Witwatersrand and Carlin deposits, and not the least of these differences is the nature of the host sequence. However, closer inspection of these styles suggests that low salinity fluids play a role in many if not most examples, and a major thermal event is usually another key part of mineralisation. Furthermore, despite the great variation in host rocks, the role of host chemical composition can be relatively easily rationalised with the chemical behaviour of gold in low salinity fluids. It would thus appear that there are substantial differences between the major gold provinces with regard to depositional-type features, but there may be many fewer differences when considering the composition of the fluids, and in some cases the source region for the fluids and the processes which operate there.
COMMON FACTORS IN VICTORIAN GOLD GENESIS There are numerous features common to many primary Victorian gold deposits, and there are others that show great diversity. Common features include the universal structural control, the carbonate–white mica–sulphide alteration, the low salinity water–carbon dioxide fluid, the chemical composition of host rocks to major deposits, inferred relationship to a major thermal event, and a metal association dominated by gold, arsenic, antimony, potassium and sulphur with low silver, copper, zinc and lead. Great diversity exists in deposit geometry and type of structural control, the alteration minerals and their proportions, the abundance of particular granite-related components such as bismuth, molybdenum and tungsten, the type of host rock, the age of host rocks, distance to nearest granites and type of adjacent granite (if any). Importantly, the similarities can be summarised as a commonality of some deep crustal features such as fluid composition, presence of major fluid channelways and thermal event. The differences imply diversity of the upper crustal features at or near the depositional site such as host rock, host structure, alteration assemblage and spatial relationship to granites.
UNANSWERED QUESTIONS RELEVANT TO VICTORIAN GOLD GEOLOGY As understanding of the Victorian geological framework and gold improves, some aspects emerge that need resolution prior to advancing gold genetic models further. As the youngest host to major goldfields, the Woods Point dyke swarm is critical to the chronology. From stratigraphic evidence, the dykes post-date Early Devonian metasedimentary rocks, and are assumed to pre-date the weakly folded Late Devonian to Carboniferous molasse sequence which they do not intrude, and this is all compatible with their age of 380 Myr. However, this is an old K-Ar age (Marsden, 1988) and its accuracy might be dramatically improved with modern methods. Dating of the Woods Point dyke swarm together with better age constraints on the unfossiliferous, presumed Middle Devonian strata, would assist the unravelling of key gold-related processes during the Devonian.
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The age of deformation(s) in the Ballarat and especially the Stawell zones is not well reconciled with tighter constraints for the Melbourne and Tabberabbera zones. A single age for Tabberabberan deformation of Middle to Late Devonian across the province is difficult to rationalise with existing observations, so either there are distinct and separate deformation events or the Tabberabberan deformation should be viewed as markedly diachronous. The ages of dykes in these zones, some of which appear to pre-date 395 Myr granites, might further constrain deformational events. Of more importance to gold metallogeny is the major thermal event in the Lachlan Fold Belt of Victoria and this seems well constrained by the Early–Late Devonian age of granite magmatism in the Melbourne, Ballarat and Stawell zones.
GENESIS OF VICTORIAN GOLD DEPOSITS Until recently, the genesis of Victorian gold deposits has mostly relied on information from within the Victorian gold province and has not fully exploited the additional constraints placed by gold geochemistry and global gold metallogeny. Many ideas have been proposed based upon studies at isolated goldfields but until recently virtually none have covered the whole province. Almost all gold genetic models have been represented in the literature on Victorian deposits, including models which have invoked exhalative gold, Ordovician gold mineralisation, gold introduction from Cambrian to Devonian times, granite-derived, lamprophyrerelated gold, and a metamorphic fluid model for gold (Wall and Ceplecha, 1976; Cox et al, 1991b; Phillips, 1991). However, there has been little critical evaluation or testing of some of the proposals. Our preferred model for the genesis of the major Victorian gold deposits involves a metamorphic fluid derived from beneath the Early Palaeozoic flysch sequence during the Devonian thermal event. This low salinity fluid was water–carbon dioxide dominated, with a source possibly related to the pre-Ordovician mafic succession, but was emplaced along pathways that potentially incorporated minor components from higher crustal levels. Emplacement of the granites, mafic to ultramafic rocks (ie ‘diorite’ and ‘lamprophyre’ of the Woods Point dyke swarm) and felsic volcanic rocks was part of the same thermal event that generated the gold-bearing fluid, but these magmas all represent melting at considerably greater depths than the greenschist–amphibolite facies transition that sourced the auriferous fluid. Upward migration and focussing of the auriferous fluid was guided by decollement features, listric faults and, at shallow depth, smaller structures. Gold mineralisation occurred in all rock types that pre-dated the thermal event, but rocks rich in iron or carbon were most favourable for gold precipitation and hence hosted the largest fields, especially where they were fractured favourably. The most important gold-bearing region, the Ballarat zone, corresponds to the region with the greatest density of listric faults and the greatest concentration of chemically favourable host rocks. With the recognition of the close geological similarities between the Victorian gold province, other slate belt gold provinces, and many Archaean greenstone gold provinces, exploration ideas from these areas can be transferred to Victoria. For example, suggested genetic models for Victorian
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gold deposits can be evaluated and ranked by looking at the effectiveness of such models in other terrains. Such an approach will not answer all the questions about the evolution of the Victorian gold province, but will provide a means of optimising the cost effectiveness of exploration and give a better chance for major discovery in Victoria, and elsewhere.
ACKNOWLEDGEMENTS The authors wish to acknowledge support and encouragement from members of the Geological Survey of Victoria and the Victorian Chamber of Mines, especially J O Reynolds and T W Dickson. Major funding to the authors from the Australian Research Council facilitated a number of specific research projects on Victorian goldfields, and especially the synthesis of the whole province and the global comparisons. Discussions with P Arden, D Arne, G Corbett, S Cox, R Goldfarb, D Groves, J Law, C Mawer, S McKnight, V Morand, R Powell, J Vearncombe, C Wilson and other colleagues have significantly influenced our ideas on Victorian gold. We thank Alliance Gold Mines NL for support and access at Maldon, and participants of the Vicgold 95 conference in Melbourne at which many of these ideas were either first aired or discussed in some detail.
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Morand, V J, Hughes, M J and Jones G N, 1997. Discussion: Implications of overprinting deformations and fold interference patterns in the Melbourne Zone, Lachlan Fold Belt, Australian Journal of Earth Sciences, 44:145–148.
Scheibner, E, 1992. Influence of detachment-related passive margin geometry on subsequent active margin dynamics: applied to the Tasman Fold Belt System, Tectonophysics, 214:401–416.
Phillips, G N, 1991. Gold deposits of Victoria: a major province within a Palaeozoic sedimentary succession, in World Gold ‘91, pp 237–245 (The Australasian Institute of Mining and Metallurgy: Melbourne). Phillips, G N, 1996. Maldon goldfield, Victorian gold, and granites, Geological Society of Australia (Tasmanian Division), Abstracts, July 1996, 1. Phillips, G N, Eshuys, E and Hellsten, K, 1996. Integrated exploration techniques for Archaean gold, Western Australia, in Proceedings 1996 AusIMM Annual Conference, pp 289–293 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Simpson, C J and Woodfull, C J, 1994. Geological note: New field evidence resolving the relationship between the Grampians Group and the Rocklands Rhyolite, western Victoria, Australian Journal of Earth Sciences, 41:621–624. Taylor, D, 1995. Gold mineralisation controls in the Ballarat area, Geological Survey of Victoria Symposium Abstracts, pp 3–4. Thomas, D E, 1988. The Ballarat Goldfield, in Bicentennial Gold 88, Excursion No 2 Guide, Central Victorian Gold Deposits, Publication 13 (Ed: D G Jones), p 69 (The Geology Department and University Extension, The University of Western Australia: Perth).
Phillips, G N and Groves, D I, 1983. The nature of Archaean goldbearing fluids as deduced from gold deposits of Western Australia, Journal of the Geological Society of Australia, 30:25–39.
Wall, V J and Ceplecha, J C, 1976. Deformation and metamorphism in the development of gold-quartz mineralisation in slate belts, in 25th International Geological Congress, Sydney, Abstracts, 1, pp 142–143.
Phillips, G N and Hughes, M J, 1995. Victorian gold: a sleeping giant, Society of Economic Geologists Newsletter, 21:1, 9–13.
Willman, C E, 1995. Castlemaine goldfield. Castlemaine-Chewton, Fryers Creek, Geological Survey of Victoria Report 106.
Phillips, G N and Hughes, M J, 1996. The geology and gold deposits of the Victorian gold province, Ore Geology Reviews, 11:255–302.
Wilson, C J L, Will, T M, Cayley, R A and Chen, S, 1992. Geologic framework and tectonic evolution in Western Victoria, Australia, Tectonophysics, 214:93–127.
Phillips, G N and Powell, R, 1993. Link between gold provinces, Economic Geology, 88:1084–1098. Plowman, S J, 1994. Message from the minister, Australian Mining, 86:19. Powell, R, Will, T M and Phillips, G N, 1991. Metamorphism in Archaean greenstone belts: calculated fluid compositions and implications for gold mineralization, Journal of Metamorphic Geology, 9:141–150.
Woodall, R, 1979. Gold - Australia and the world, in Gold Mineralization, Publication 3 (Eds: J E Glover and D I Groves), pp 1–34 (The Geology Department and University Extension, The University of Western Australia: Perth). Xu, G, Powell, R, Wilson, C J L, Will, T M, 1994. Contact metamorphism around the Stawell granite, Victoria, Australia, Journal of Metamorphic Geology, 12:609–624.
Ramsay, W R H and Willman, C E, 1988. Gold, in Geology of Victoria, (Eds: J G Douglas and J A Ferguson), pp 454–481 (Geological Society of Australia, Victorian Division: Melbourne).
Geology of Australian and Papua New Guinean Mineral Deposits
505
506
Geology of Australian and Papua New Guinean Mineral Deposits
Zurkic, N, 1998. Fosterville gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 507–510 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Fosterville gold deposits 1
by N Zurkic
INTRODUCTION The deposits are 25 km east of Bendigo in central Victoria, at lat 36o42′S, long 144o30′E, on the Bendigo (SJ 55–1) 1:250 000 scale map sheet (Fig 1). They were developed in a historic goldfield, although the Fosterville mineralisation differs from the typical Victorian discrete quartz vein deposits. Here there is only a minor association between gold and quartz veins, and gold is predominantly impregnated in porous sandstone.
TABLE 1 Fosterville resources and reserves summary at 30 December 1996. Resources1
Measured
Indicated
Inferred
Gold ‘000 t Gold ‘000 t Gold grade grade grade g/t g/t g/t
Oxide
‘000 t
(0.5 g/t Au cutoff)
2970
1.2
1581
1.0
660
0.9
(1.2 g/t Au cutoff)
2500
3.2
2162
2.5
1770
2.2
(0.5–1.19 g/t Au cutoff)
1495
0.8
1885
0.8
2282
0.8
Sulphide
Reserves
Proved
Oxide
‘000 t
(0.5 g/t Au cutoff)
1473
1
Probable
Gold ‘000 t Gold grade grade g/t g/t 1.4
506
1.3
Resources are inclusive of Reserves.
oz per year and is expected to be lifted by an additional 50 000 oz per year with the commencement of sulphide ore production in 1997.
EXPLORATION AND MINING HISTORY The field was a low grade producer, discovered in 1894. Historic production to 1982 was 1588 kg of gold, at a recovered grade of 3 g/t, mostly from open pits and underground stopes in the oxide-sulphide transition zone, between 1894 and 1909. This included production by Bendigo Gold Limited (BGL), from 1982 to 1984, of 108 kg of gold by the CIP retreatment of battery tailings. After virtual abandonment of the field by 1910, no significant work took place until 1973. Between 1973 and 1983, Lone Star Exploration NL, Noranda Australia Ltd and Apollo International Minerals NL carried out limited exploration including some drilling. These companies concluded that the target potential did not meet their tonnage criteria.
FIG 1 - Locality map.
Production at Fosterville by Perseverance Exploration Pty Ltd (Perseverance) to 30 June 1996 was 2.594 Mt of ore at 1.5 g/t gold, at a waste:ore ratio of 3.58:1, to yield 96 000 oz of gold. All production was from oxide ore, from a series of shallow pits on the Fosterville and O’Dwyers fault systems. Proved and Probable Ore Reserves in the oxide zone are 1.98 Mt at 1.4 g/t gold at a 0.5 g/t cutoff (Table 1). Current production is 40 000
1.
Senior Geologist, Perseverance Exploration McCormicks Road, Fosterville Vic 3557.
Geology of Australian and Papua New Guinean Mineral Deposits
Pty
Ltd,
BGL drilled more than 450 reverse circulation (RC) holes by mid 1989, following mapping, sampling and orientation bedrock geochemistry by BGL geologists in the period 1985 to 1987. Bulk samples of sulphide ore for metallurgical testing were obtained from a 30 m exploration shaft. BGL reported ‘an unclosed resource of approximately 3.8 Mt at a grade of 2.5 g/t gold’. This included an oxide reserve of 1 Mt at 2.6 g/t gold (McConachy and Swensson, 1990). In 1991 Brunswick NL began production from the Central Ellesmere, Fosterville and Robbins Hill open pits, but the operations closed in mid 1991 due in part to a high level of
507
N ZURKIC
project debt. In 1992 Perseverance acquired the deposit and began small scale mining of the Fosterville open pit and completed treatment of the heap leach ore produced by Brunswick NL. When Perseverance acquired the deposit the combined Measured, Indicated and Inferred Resources of oxide ore were 2.65 Mt at 1.9 g/t gold. The Inferred Resource of sulphide ore was 1 Mt at 2.9 g/t gold. Proved and Probable Reserves of oxide ore were 974 000 t at 1.7 g/t gold. Exploration for oxide ore deposits led to the discovery of a substantial sulphide resource below the Central North oxide mineralisation (Fig 3). This resource, with the known sulphide resource at Central Ellesmere, drew attention to the potential for production from sulphide ore. In 1994 it was decided to fully explore the central 4 km length of the Fosterville Fault, from the Harrington’s Hill area in the south to the Fosterville pit area in the north (Fig 2), to evaluate the potential for sulphide ore with the results shown in Tables 1, 2 and 3.
TABLE 2 Fosterville oxide and sulphide resources, 30 December 1996.
Measured
Indicated
Inferred
‘000 t Gold ‘000 t Gold ‘000 t Gold grade grade grade g/t g/t g/t Oxide (0.5 g/t cutoff) Robbin’s Hill
109
1.2
121
1.1
325
1.0
Central North
249
1.2
226
1.0
50
1.0
Central Ellesmere
54
1.1
66
0.9
79
1.0
Sharkey’s
193
0.8
176
0.8
34
0.7
Daley’s Hill
512
1.4
314
1.3
19
1.1
Farley’s
146
1.4
41
1.3
4
0.8
Rehe’s
53
0.9
14
0.8
0
0.0
Harrington’s Hill
156
1.3
190
1.0
106
0.9
O’Dwyers
987
1.0
243
0.9
12
0.7
Fosterville
276
1.1
147
1.0
30
0.7
Read’s
235
1.5
43
1.3
1
0.9
Sulphide (1.2 g/t cutoff) Robbin’s Hill
3
1.9
2
1.8
94
2.0
Central North
932
4.2
538
3.5
398
2.8
Central Ellesmere
825
3.0
818
2.5
535
2.1
Sharkey’s
51
1.6
53
1.5
51
1.5
Daley’s Hill
278
2.4
282
1.8
253
1.7
Farley’s
40
3.0
10
2.7
4
2.5
Harrington’s Hill
228
2.3
270
2.1
268
2.2
O’Dwyers
67
2.6
109
2.2
125
2.0
Fosterville
35
1.8
25
1.6
8
1.4
Read’s
41
1.5
55
1.6
34
1.7
Sulphide (0.5–1.19 g/t) Robbin’s Hill
3
0.8
3
0.8
227
0.8
Central North
184
0.8
137
0.9
165
0.9
Central Ellesmere
279
0.8
361
0.9
352
0.8
Sharkey’s
136
0.8
226
0.8
205
0.8
Daley’s Hill
207
0.8
239
0.8
205
0.8
Rehe’s Harrington’s Hill FIG 2 - Geological plan of the Fosterville area.
A feasibility study on bacterial oxidation of sulphide ore is near completion which will allow for some 300 000 oz of gold to be added to the sulphide reserve base.
REGIONAL GEOLOGY The Fosterville goldfield is in the Ballarat Trough of the Lachlan Fold Belt. The country rock is an Ordovician (Lancefieldian) turbidite sequence of alternating mudstone, siltstone and sandstone, common throughout Victoria. The Fosterville Fault, which defines the eastern limit of the northtrending Strathfieldsaye Synclinorium, transects the sequence (Cas and VandenBerg, 1988). The generally tightly folded
508
1
0.6
0
0
1
0.5
192
0.8
307
0.8
331
0.9
O’Dwyers
279
0.7
349
0.7
594
0.7
Fosterville
186
0.7
230
0.7
190
0.7
Read’s
28
0.9
33
0.9
12
0.9
sequence is poorly exposed except for the sandstone of the Sugarloaf Range to the west. The Fosterville Fault is a major, steep, west dipping fault. The fault zone has mostly undergone west side up sinistral and reverse movement, with some late stage sinistral strike slip (S King, unpublished data, 1996). It appears that shearing was transferred in the NE direction via an en echelon set of structures (Fig 2) up to and beyond the O’Dwyers line. On the O’Dwyers line a rhyolitic porphyry dyke has intruded the main O’Dwyer’s shear structure.
Geology of Australian and Papua New Guinean Mineral Deposits
FOSTERVILLE GOLD DEPOSITS
TABLE 3 Fosterville oxide ore reserves, 30 December 1996. Proved
Probable
‘000 t
Gold grade g/t
‘000 t
Gold grade g/t
Daley’s Hill
437
1.5
256
1.4
Central North–Vanessa’s
88
1.4
26
1.5
Oxide (0.5 g/t cutoff)
Sharkey’s
16
1.0
10
1.1
Farley’s
119
1.5
25
1.5
Rehe’s
57
1.3
0
0
Fosterville
29
1.8
19
2.3
Read’s
170
1.7
26
1.5
O’Dwyers
495
1.2
102
1.0
Harrington’s Hill–Sandhurst
62
1.1
42
1.0
Mineralisation is closely related to fault and shear structures, which are generally steeply dipping (Fig 3) and strike subparallel to the fold axes but cut across these at an oblique angle. The most favourable zones for mineralisation are :1.
major faults, which provide a conduit for fluids from depth,
2.
fold axes, where a zone of weakness has been created,
3.
favourable rock types which fracture, and are porous but also provide open spaces through brecciation, and
4.
an interplay of folding, faulting and favourable rock type which produces broad areas of brecciation conducive to gold deposition.
MINERALISATION Natural oxidation to depths of 30 to 60 m has released the gold in finely particulate form, with diameter of 1 to 10 µm. The gold particles occur within iron oxide, both being formed by the breakdown of the sulphide minerals. The iron oxide forms veins and fracture fill. The mineralisation occurs as zones of dispersed iron oxides with some remobilised silica. The most prospective zones were previously believed to be concentrated around the most extensive areas of primary quartz-sulphide veining, stockworking and silicification prior to oxidation. However, growing evidence suggests that this is not crucial, as high grade gold mineralisation is also homogenously disseminated through porous sandstone and to a minor extent in siltstone, with little or no stockworking or quartz veining. The primary mineralisation is a quartz-pyrite-arsenopyrite assemblage with gold occurring in the free state in minor amounts but principally within pyrite and arsenopyrite as free grains with a diameter of 1 to 10 µm. The importance of this ‘quartz-poor’ mineralisation style is that, historically, production in Victoria has been from quartz reef and alluvial systems and past exploration efforts have almost solely targeted ‘reef’ style mineralisation.
FIG 3 - Cross section on line 9033 m N, Central North pit, looking north, showing the Fosterville Fault mineralisation displaced by secondary faulting.
ORE DEPOSIT FEATURES HOST LITHOLOGY AND STRUCTURAL CONTROLS There is some lithological control of mineralisation. Sandstone with brittle fracturing has provided a favourable host to open space filling by vein quartz. In addition some mineralisation shows a homogenous ‘impregnation’ of sulphides of the porous sandstone adjacent to shear structures, with negligible quartz veining. To the NE of the Fosterville Fault, porphyritic rhyolite dykes also provide a favourable ore host, as they have a similar competency to the sandstones.
Geology of Australian and Papua New Guinean Mineral Deposits
Ore zones generally conform in shape and trend to the broader fault zones within which they exist. Ore zones may be between 2 and 30 m wide depending on the interaction of structure and rock type. On average, ore zones are 5 m wide with strike lengths to 400 m, dip to the west and are open at depth. Secondary structures (Fig 3) produce areas of higher gold grade mineralisation which may trend up to 20o off the main north (mine grid) mineralisation trend. These high grade bands are generally narrow and can only be mined in conjunction with the low grade surrounding material.
EXPLORATION METHODS The position of the Fosterville Fault and associated O’Dwyers porphyry are well known. The Fosterville Fault can be traced from the south, where it is concealed by Tertiary basalt, northward for approximately 13 km, to the north end of the mining lease (Fig 1) where it is hidden by alluvial clay cover. The fault can be traced clearly, along topographic highs and through mine workings and in unmined areas, using bedrock geochemistry. Values in excess of 100 ppb gold and 100 ppm arsenic outline currently known deposits and can be used as target thresholds. Antimony and arsenic show a close relationship with gold.
509
N ZURKIC
The intensity of mineralisation is highly variable in the fault structures (Fig 4). This leads to the conclusion that both contacts of the fault zones need to be drilled on an initial 50 m spacing to satisfactorily evaluate mineralisation. Geophysical techniques are of limited use as the position of the structure is not the issue, but rather the intensity of mineralisation. Induced Polarisation (IP) surveys have been successful in defining areas of sulphide concentrations or shallower sulphides, although generally the sulphide content is not high enough to give a wholly reliable IP response.
collected from blast holes drilled on a 5 by 2 m pattern. Grade control is by a pit technician who directs the excavator, with the ore margins defined by a combination of visible fault boundaries, assays from blast holes and, on the deeper benches, metallurgical criteria. The technician uses mining plans prepared by the mine geologist who models the ore boundaries on the bench from kriged blocks derived from the blast hole assays. Monthly reconciliations are made with the grade control model and the ore reserve model. Reserves are periodically updated to accommodate accumulating grade control and exploration data. Ore treatment is by conventional cyanide heap leaching. Characterisation of mineralisation by its level of oxidation is important, as gold recovery changes from 85% for surface ore to 1 or 2% at the base of oxidation. The primary mineralisation is refractory and gold can not be extracted from the sulphides by leaching alone. Recent metallurgical testwork reviewed a range of possible oxidation techniques, with bacterial oxidation the preferred outcome.
CONCLUSIONS As oxide ore is depleted over the next few years, the future of the Fosterville project will depend on the mining and treatment of sulphide ore. The feasibility study to be completed by early 1997 proposes that some 2.5 Mt of sulphide ore will be mined from three open pits extending to 130 m depth on the central portion of the Fosterville Fault. The 500 000 tpa bacterial oxidation treatment plant is planned for production in late 1997. Sulphide mineralisation in the Daley’s Hill and O’Dwyers areas will probably be mined from the fourth and fifth sulphide open pits.
FIG 4 - Cross section of the Central Ellesmere deposit, on line 7635 m N, looking north, showing the Fosterville Fault mineralisation.
The high grade mineralisation in the hanging wall of the Fosterville Fault in the Central North area may be won by underground mining in the future, when production of a combination of open pit and underground sulphide ore should maintain Fosterville’s output at about 100 000 oz per year.
ACKNOWLEDGEMENTS Large diameter conventional face sampling RC holes are used for resource evaluation. RC is preferred to diamond core due to the large sample provided. This is considered more useful than structural information from diamond drilling which is less relevant for open pit mining on a relatively simple structure. Drilling patterns on the Fosterville Fault are generally 25 by 25 m, and 20 by 20 m on the O’Dwyers porphyry deposits. The RC drill hole chips are logged and 2 m composites are subsampled and assayed for gold and a variety of elements useful as geochemical indicators or for metallugical quality control. Sulphur, carbon and iron levels are determined in material from potential ore zones only, as the proposed bacterial oxidation process requires a homogenous feed with respect to these elements.
MINE GEOLOGICAL METHODS
Appreciation is extended to the management of Perseverance Corporation Ltd for permission to publish this paper. The work of the geological team associated with the project is acknowledged. Thanks are also extended to S King of ERAMAPTECH for specialist structural interpretation of the Fosterville field.
REFERENCES Cas, R A F and VandenBerg, A H M, 1988. Ordovician, in Geology of Victoria (Eds: J G Douglas and J A Ferguson), pp 63–102 (Geological Society of Australia, Victorian Division: Melbourne). McConachy, G W and Swensson, C G, 1990. Fosterville gold field, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1297–1298 (The Australasian Institute of Mining and Metallurgy: Melbourne).
A nominal mining cutoff grade of 0.5 g/t gold is used for oxide ore. The orebody is mined on 2.5 m flitches, and samples are
510
Geology of Australian and Papua New Guinean Mineral Deposits
McDermott, G J and Quigley, P W, 1998. Williams United gold deposit, Bendigo, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 511–516 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Williams United gold deposit, Bendigo 1
2
by G J McDermott and P W Quigley INTRODUCTION
The deposit is in Mining Lease 1345 at the northern end of the Bendigo Goldfield on the New Chum Anticline (Fig 1). It is 150 km NNW of Melbourne, Vic, at lat 36 o43′S, long 144o15′E on the Bendigo (SJ 55–1) 1:250 000 and Bendigo (7724) 1:100 000 scale map sheets.
Bendigo Mining NL (BMNL) has held exploration and mining licences over the Bendigo Goldfield since 1993, and has delineated a near surface open pittable deposit within the Williams United area. A Probable Ore Reserve of 1.3 Mt at 1.0 g/t for 42 000 oz of contained gold has been estimated for the deposit. This paper examines the methodology used to evaluate the complex and spatially erratic grade distribution of the deposit.
EXPLORATION AND MINING HISTORY The Bendigo Goldfield is Australia’s second largest field in terms of past production. Gold mining in Bendigo began in 1851 with the discovery of alluvial gold in Bendigo Creek. Hard rock mining commenced in 1853 and average annual production exceeded 200 000 oz until 1915. Several hundred quartz reefs were mined between 1853 and 1954, to a maximum depth of 1400 m, from 12 main parallel anticlines within a zone 4 km wide by 15 km long. Over 1300 companies held small mining leases over the goldfield and its surroundings. The closure of the Central Deborah and North Deborah mines in 1954 marked the end of over a hundred years of reef mining. The discovery of gold on the New Chum Anticline was relatively early in the mining history of Bendigo. Quartz reef mining on this anticline commenced in the mid 1850s and by 1921 all major companies had ceased operation. The most productive period was between 1860 and the late 1880s when mining occurred within 200 m of the surface. Gold was mainly mined from reefs other than traditional saddle-type reefs. More than 20 companies mined in the area of the Williams United deposit, for a total recorded gold production of 504 600 oz. Exploration and mining on the New Chum Anticline were dormant from 1921 until the 1930s when Bendigo Mines Limited carried out limited but unsuccessful exploration programs. FIG 1 - Location map for Williams United gold deposit.
Historic gold production from the Bendigo Goldfield is estimated to be 22 Moz or 684 t from discovery in 1851 until cessation of mining in 1954, ranking Bendigo as one of the most prodigious gold producing areas in Australia. Production from the New Chum Anticline is estimated to have been 2.6 Moz, ranking it as the most important line of reef in Bendigo after the Garden Gully line.
1.
Formerly Contract geologist, now Senior Project Geologist, Bendigo Mining NL, 32 Belvoir Park Road, RSD Harcourt Vic 3453.
2.
Project Geologist, Bendigo Mining NL, PO Box 2113, Bendigo Mail Centre Vic 3554.
Geology of Australian and Papua New Guinean Mineral Deposits
In 1978 WMC commenced the first modern exploration of the goldfield. They embarked on a $28 million exploration program including historic research, 64 km of drilling and 1.5 km of underground exploratory development accessed from the Williams United shaft. In 1992 BMNL acquired the Bendigo assets of WMC which, with leases already held over the Deborah line of reef (Fig 1), resulted in BMNL gaining effective control of the goldfield. Mapping, shallow reverse circulation drilling and bulk sampling programs from May 1995 to May 1997 resulted in the delineation of the Williams United deposit. An Environmental Effects Statement for a short term open pit and heap leach operation has been prepared and is currently being assessed by an independent panel appointed by the Minister for Planning and Local Government.
511
G J McDERMOTT and P W QUIGLEY
REGIONAL GEOLOGY The deposit is within the Lower Ordovician Bendigo–Ballarat zone of the Lachlan Fold Belt of eastern Australia as described by Gray (1988). The rocks of this zone are lower greenschist metamorphic grade interbedded sandstone, siltstone and shale of the Castlemaine Supergroup (Cas and VandenBerg, 1988). The sediments have been deformed into tight upright chevron folds which plunge subhorizontally either north or south. The goldfield is intruded to the south by the Devonian Harcourt Granodiorite of age 361 Myr (Richards and Singleton, 1981) and by monchiquite dykes of unknown affinity. The dykes are generally restricted to fold axial planes and some major reverse faults. Significant gold mineralisation in Bendigo is located within 12 north trending parallel anticlines. Beneath the axial trace of each anticline, sub-horizontal auriferous quartz reefs, typically 40 to 80 m high, 0.1 to 15 m wide and between 300 and 3000 m long, are developed in zones of high strain. The reefs are stacked in a vertical manner below each anticline repeating at regular depth intervals of around 200 m. Each band of mineralisation consists of at least one major controlling structure such as a fault and up to five other accessory reefs including saddles, legs and associated spur zones (Turnbull and McDermott, this publication). Quartz reef development within the goldfield is structurally controlled and is localised around fold culminations (‘domes’), reverse faults and strong rock competency contrasts.
wavelength and amplitude of 300 and 150 m respectively. The fold geometry is modified by three 150–250 m vertically spaced west-dipping transgressive reverse faults (Turnbull and McDermott, this publication) which displace the axial plane of the fold by up to 60 m in a west over east sense of transport. Four main vertically stacked auriferous quartz reef systems (St Mungo fault, Ellenborough run, Big Slate reef and Catherine reef) are associated with the three faults. The reefs were mined within the project area to a depth of 700 m prior to 1921. The uppermost of the four reefs, the St Mungo fault (SMF), has been the focus of recent exploration. The SMF is a massive quartz reef 1 to 25 m wide, which outcrops over a strike length of 850 m within the project area. The fault is inclined at 60o towards the west and is known to extend over 100 m down dip. Initiated as a bedded fault on the western limb of the New Chum Anticline, the SMF broke through the anticlinal hinge zone to pass discordantly through the opposing limb as a series of mineralised fault splays (Fig 3). On passing discordantly through the eastern limb of the fold, the SMF refracted between rock layers in a manner similar to cleavage refraction.
LOCAL GEOLOGY LITHOLOGY AND STRUCTURE The deposit outcrops along the hinge zone of the New Chum Anticline and is hosted by interbedded quartz sandstone, siltstone and shale of turbiditic origin (Fig 2). At this location o o the anticline strikes 345 , plunges north at 10 and has a
FIG 3 - Schematic cross section through the Williams United deposit, looking north.
Associated with the fault is an array of auriferous tension veins or ‘spurs’, best developed within sandstone units, that are oriented near perpendicular to the plane of the fault. Quartz spurs may be present on the hanging wall and footwall side of the fault.
MINERALISATION
FIG 2 - Geological plan of the Williams United area and proposed mine layout.
512
The gross geological controls of the location and continuity of quartz stockwork mineralisation at Williams United are relatively well understood. The principal controls of mineralisation are host rock type, location of rock competency
Geology of Australian and Papua New Guinean Mineral Deposits
WILLIAMS UNITED GOLD DEPOSIT, BENDIGO
contrasts, position and spacing of major transgressive fault sets and proximity to fold culminations (domes). All these factors are inter-related and result in the formation of dilational sites for vein development during the ductile (folding)–brittle (faulting) deformation process.
TABLE 2 Typical RC drill hole assays from Williams United. Hole No
Assays (g/t) for successive metre samples
FSR109
0.1, 0.0, 173.0, 50.1, 0.8, 0.8, 0.1, 0.7, 3.5, 0.1
The six main vein geometries identified in the Bendigo Goldfield (Turnbull and McDermott, this publication) are:
WUR001
0.1, 10.2, 1.1, 0.5, 0.0, 0.1, 0.4, 0.0, 0.2, 0.3, 1.2, 0.1
WUR002
0.3, 0.6, 0.3, 0.4, 0.2, 1.4, 0.1, 0.9, 18.7, 0.1, 1.2
1.
bedding concordant;
WUR023
0.1, 0.3, 0.2, 0.2, 0.1, 0.1, 0.1, 42.6, 0.3, 0.1
2.
transgressive;
WUR034
3.
tensional;
1.5, 0.2, 6.4, 0.5, 0.6, 0.8, 52.9, 1.6, 2.2, 0.4, 1.2, 0.5, 0.2, 1.4
4.
perpendicular;
WUR050
0.1, 319.0, 14.0, 1.3, 1.8, 0.2, 0.7
WUR065
0.1, 1.4, 0.9, 0.3, 0.4, 0.1, 0.1, 0.1, 0.1, 0.1, 7.1, 0.1, 0.5
WUR140
0.5, 0.6, 0.2, 49.4, 0.8, 0.2, 0.1, 0.2
WUR145
0.1, 0.1, 0.0, 1.2, 0.0, 0.2, 0.0, 23.1, 0.5, 0.1
WUR157
0.2, 0.9, 0.4, 0.4, 0.4, 1.6, 0.6, 0.4, 2.0, 0.1, 0.2, 1.3, 0.9, 13.8, 0.1
WUR160
0.2, 16.3, 6.8, 1.1, 0.4, 0.2, 0.2, 0.3
WUR170
0.8, 0.0, 0.5, 0.1, 0.1, 0.7, 0.1, 10.8, 0.3, 0.4, 0.2
5.
axial; and
6.
saddle.
At Williams United most of the gold mineralisation is associated with vein types 2, 3, 1, 5 and 6, in decreasing order of importance. The average vein orientations are summarised in Table 1. The type, density, dimension and continuity of veining all impact on the grade of the mineral deposit. TABLE 1 Typical Williams United vein orientations. VEIN TYPE 1 2 3
DIP
STRIKE
50–90o E or W
245o
o
50–75 W
245ο
o
296ο
o
20 NW
3
20 SE
140o
4
75–90o N or S
165o
o
COMMENTS
Conjugate set of spur veins Unmineralised AC veins
o
5
70–90 E or W
245
6
15° NW (plunge)
340o (Plunge azimuth)
Very rare vein type
The drill hole samples were analysed by a bulk cyanide leach technique, using a 1.2 kg or 3.0 kg aliquot rather than a 50 g fire assay. Orientation tests of assay methods demonstrated a significant decrease in assay variance with increasing sample size. In other words, the larger sample size tended to be more representative of grade than smaller samples for mineralisation with an erratic high nugget gold distribution such as the Williams United deposit. The difficulty in correlating high grades between cross sections warranted closer spaced RC drilling. Line spacing was decreased to a nominal 25 m, and in some instances to 6 m. Across strike drill hole spacing was maintained at a nominal interval of 25 m. Drill hole chips were logged for lithology, per cent quartz, alteration, oxidation and sample quality. All data were collated and interpreted with PC based Gemcom software.
Typical ore in the oxidised zone consists of vein quartz and variably silicified limonite-goethite (after sulphides) and psilomelane-stained sandstone and siltstone. Gold is free, occurring with colloform textured goethite and psilomelane. In the primary zone, Williams United ore consists of laminated, breccia or buck vein quartz containing visible free gold particles to 10 µm diameter, fine submicron size gold as fracture fillings in sulphide, and 2% sulphides, commonly arsenopyrite, pyrite, galena and sphalerite. The gold is generally free milling and occurs as irregular clots, filaments or stringers. The distribution of grade within the quartz stockworks is extremely erratic. Examples of typical assays, of consecutive metre samples of drill hole cuttings through the stockwork system, are shown in Table 2.
RESOURCE ESTIMATION METHODS DRILLING AND ASSAYING Following a literature search and detailed 1:1000 scale mapping and sampling, potentially economic quartz stockwork mineralisation of the SMF was tested by 50 m spaced lines of reverse circulation (RC) drill holes. This first pass drilling was directed towards defining major structures, lithology and grade distribution.
Geology of Australian and Papua New Guinean Mineral Deposits
BULK SAMPLING AND METALLURGICAL TESTING A program of bulk sampling was initiated across the SMF to provide information on metallurgical recovery, grade reconciliation against RC drill hole data and to expose quartz stockworks for mapping and sampling. Five representative 100 t bulk samples were extracted at systematic intervals across the SMF and processed through the company’s bulk sample testing plant. Five lines of 2.5 m east–west spaced grade control RC holes were drilled across the bulk sample sites prior to extraction for grade reconciliation purposes. A comparison of grade between the widely spaced exploration RC holes averaging 0.6 g/t gold, the 2.5 m spaced grade control RC holes averaging 0.7 g/t and bulk sample results averaging 1.1 g/t, suggests that the complex grade distribution in the quartz stockworks is under-sampled by drilling. The program also indicated that the SMF mineralisation is amenable to heap leaching.
GEOSTATISTICS A geostatistical approach was used to investigate the complex grade distribution pattern in the SMF and, in particular, the understatement of grade by RC drilling.
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G J McDERMOTT and P W QUIGLEY
Initial variography on RC drill hole data at the nominal 25 by 25 m drill hole spacing was unstructured. Therefore a comprehensive grade control drilling and trial mining program over a typical 40 m strike length of the SMF was proposed. The program comprised 170 RC holes on a 2.5 by 2.5 m grid, each 15 m long, inclined at 60° and drilled towards east. Samples were collected for each metre drilled. The top 2–3 m or 2386 t were extracted for bulk sampling and reconciliation against grade control drill hole data. During the program two distinct domains of mineralisation were identified; a higher grade hanging wall zone of massive buck quartz, and a lower grade footwall zone consisting of several 1–30 cm wide tension veins or spurs. Geostatistical (variogram) analysis of both domains indicates a very high nugget effect (75% assay variance) and short scale ranges. Grade reconciliation between the 25 m spaced exploration RC holes averaging 0.3 g/t gold, the 170 grade control RC holes averaging 0.7 g/t and bulk sample results averaging 1.0 g/t confirm the trend shown by the previous bulk sample program. This significant under-reporting of grade suggests wider spaced sampling techniques (ie drill holes) do not obtain representative samples of the five main mineralised vein geometries observed at Williams United. Bulk sampling, on the other hand, tests multiple vein orientations and therefore provides a more representative grade in erratic spatially distributed gold-bearing quartz stockworks.
Grade control drilling, although still under-reporting the true grade as determined from bulk sampling, has a higher probability of intersecting more mineralised vein combinations and effectively provides a larger sample per unit area than wider spaced exploration drilling. Comparison of assay data from twinned exploration and grade control RC holes within the test area reinforces this point, as in these instances the assays are similar. This suggests that the increase in average grade on a mining block scale from exploration to grade control drilling is purely a function of sample density and sample size.
RESOURCE MODELLING The erratic gold mineralisation at Williams United suggests that the discrimination of ore and waste zones on a mining block scale, based purely on assays of drill hole samples, is virtually impossible. For this reason the resource was interpreted using a number of key features. These were quartz abundance, alteration (ie sericite, limonite, psilomelane, arsenic value and ankerite), shearing, presence of sulphides, historic stoping and grade, including all values greater than 0.1 g/t gold. The interpreted envelope of mineralisation was Laplace modelled to create a 3D entity. Drill hole assays within this 3D shape were flagged in the database as ‘SMF’. Individual high values, including a 319 g/t, a 173 g/t and a 100 g/t assay, were not top cut in the database as they were surrounded by a dense pattern of drill holes. Resource estimation was then undertaken on the mineralised entity using ordinary kriging to interpolate
FIG 4 - Graph of gold assays in g/t, for bulk samples and the RC grade control drill hole samples within each bulk sample.
514
Geology of Australian and Papua New Guinean Mineral Deposits
WILLIAMS UNITED GOLD DEPOSIT, BENDIGO
grades into 25 m along strike by 8 m across strike by 10 m vertical blocks. The ordinary kriging technique was selected because it honours the nugget values from variography and declusters the data. The end result is a global resource estimate for the deposit, which can be subdivided, on a gross scale, by geological rationale. This commitment to a global resource estimation practice means that the discrimination of internal ore and waste blocks with respect to a cutoff grade is not feasible. The consequences of applying a cutoff grade in a high nugget deposit may result in misallocation of ore to the waste dump.
RESERVE ESTIMATION INPUT The establishment of open pit ore reserves is an iterative process involving the optimisation of the resource under a number of open pit design options. These options are assessed in terms of mineability, access, blasting requirements, cost, ground support, dilution, head grade and rehabilitation. Cultural influences must also be considered in mine design and project economics. This deals particularly with current State legislation, which provides residents within 100 m of the project with the power to veto establishment of a mine. For each open pit design, a design reserve tonnage and grade estimate was produced by evaluating the pit design against the global resource model. A wall rock dilution factor of 10% has been applied in the conversion of resource to reserve. The final mine design is illustrated in Fig 2.
UPGRADING OF KRIGED GRADE ESTIMATES A plot of grade reconciliation results comparing the head grade for each bulk sample with the average grade for each close spaced RC drill hole intersecting the site from which the bulk sample was taken is shown in Fig 4. With reference to a 1:1 correlation (the y=x line) this plot demonstrates the consistent under-reporting of gold grade from drill holes below a grade of approximately 1.1 g/t. Similar grade reconciliation trends have also been observed at the Birds and Carshalton prospects in Bendigo. This trend is simply a function of sample size, drill hole orientation and the erratic distribution of gold in the deposit. Based on these results kriged resource estimates, interpolated from assays obtained from widely spaced RC drill holes, will substantially misrepresent the true grade of the deposit. The grade reconciliation study has given BMNL confidence to increase the average kriged resource and reserve grade estimates by factors determined from the graphical plot of Fig 4. This modification of grade is similar to applying a mine call factor or to cutting high assays, both acceptable database modification practices within the mining industry applied for
Geology of Australian and Papua New Guinean Mineral Deposits
the purpose of arriving at the best estimation of the true average grade (P R Stephenson, unpublished data, 1997). If grade factoring had not been applied then the resource and reserve estimates would have substantially misrepresented the true situation. Graphical grade adjustment has resulted in the overall ore reserve grade increasing from 0.8 to 1.0 g/t gold.
CONCLUSIONS Careful consideration of features such as lithology, structure and grade distribution patterns at Williams United has resulted in the development of procedures appropriate for this style of mineralisation: 1.
Assay aliquot size has been increased by between 240 and 600% to achieve the representivity required for reliable resource and reserve estimation.
2.
Grade interpolation by ordinary kriging was chosen because it honours the grade distribution pattern and effectively declusters the data.
3.
Quoting of global resource estimates is preferred to local estimation at artificial cutoff grades.
4.
Adjusting overall kriged resource and reserve grades by factors determined from grade reconciliation tests accounts for the consistent under-reporting of true grade from drill hole samples.
ACKNOWLEDGEMENTS The authors would like to thank the management of Bendigo Mining NL for permission to publish this paper. The assistance of J Vann (Geoval) for guidance on geostatistical matters is gratefully acknowledged. Discussions with P R Stephenson (P R Stephenson Pty Ltd) on factoring of the final resource and reserve estimates under the JORC Code were very enlightening. P Walklate greatly enhanced the paper through tireless drafting and K Clark is thanked for her persistent typing of the text.
REFERENCES Cas, R A F and VandenBerg, A H M, 1988. Ordovician, in Geology of Victoria (Eds: J G Douglas and J A Ferguson), pp 63-102 (Geological Society of Australia, Victorian Division: Melbourne). Gray, D R, 1988. Structure and tectonics, in Geology of Victoria (Eds: J G Douglas and J A Ferguson), pp 1-36 (Geological Society of Australia, Victoria Division: Melbourne). Richards, J R and Singleton, O P, 1981. Palaeozoic Victoria, Australia: igneous rocks, ages and their interpretations, Journal of the Geological Society of Australia, 28: 395-421.
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516
Geology of Australian and Papua New Guinean Mineral Deposits
Sebek, R S, 1998. Bailieston gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 517–520 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Bailieston gold deposit by R S Sebek
1
INTRODUCTION The Bailieston gold deposit of Perseverance Mining Pty Limited (Perseverance) is within Historic Reserve One (HR1) on Mining Licence 4784, 12 km west of Nagambie and 120 km north of Melbourne, Vic, at lat 36o14′S, long 144o03′E on the Bendigo (SJ 55–1) 1:250 000 scale and the Nagambie (7924) 1:100 000 scale map sheets ( Fig 1).
The leases containing the historic Bailieston workings were acquired from Brunswick NL in September 1992. Bailieston has a Measured and Indicated Resource of 2.1 Mt at 0.72 g/t gold over a length of 350 m and depth of 70 m. Within that resource, the pit design is based on Proved and Probable Reserves of 823 000 t at a grade of 0.76 g/t gold at a cutoff of 0.3 g/t gold for 20 100 oz contained gold. It is one of the lowest grade deposits to be mined in Australia. The operation at Bailieston comprises a 1.5 Mtpa open pit with adjacent heap leach, with a targeted annual gold production in 1996 of 7000 oz, with gold produced at the Nagambie plant. The operation poured its first gold in March 1996.
EXPLORATION HISTORY Gold was discovered at Bailieston in 1864, after which mining and processing of gold and antimony ores continued periodically until 1905. It is estimated that 22 000 oz of gold was recovered during this period. The majority of historical production is attributable to hard rock mining although isolated shallow alluvial areas were successfully worked along the entire 5 km length of the field. The Bailieston area was originally taken up as Exploration Licence (EL) 1616 by Metals Exploration Ltd (Metals Ex) in 1986. Of the four Historic Reserves located on the licence, HR1 showed the most potential. A drilling program comprising 138 holes on approximately 50 m centres for a total of 9190 m was followed by excavation of 16 trenches of varying size for a total length of 2468 m. In addition, 12 holes for 458 m were drilled by Golder Associates as part of a hydrological study. The EL was transferred to Gold Mines of Kalgoorlie Ltd (GMK) in late 1988. Like Metals Ex, this company was a member of the Bond Gold Australia Pty Ltd group of companies whose aim was to find a large tonnage deposit amenable to bulk mining in Central Victoria. As well as detailed mapping and resource calculations, GMK drilled 17 reverse circulation (RC) holes for 119 m and 14 jetstream holes for 933 m for a total of 1052 m (D Larsen, unpublished data, 1989).
FIG 1 - Location map and structural trend map of the Melbourne Trough showing outcrop traces of major faults, fold axial surface traces, granite intrusives and trend lines, modified from VandenBerg and Gray (1988).
1.
Mine Geologist, Perseverance Mining Pty Ltd, PO Box 122, Nagambie Vic 3068.
Geology of Australian and Papua New Guinean Mineral Deposits
In 1990, Brunswick NL acquired an interest in EL 1616 in a joint venture through its subsidiary company Bendigo Gold Associates, but only carried out reviews of past exploration and resource estimates (G W McConachy, unpublished data, 1990). The transfer of EL 1616 to Perseverance as EL 3223 followed the acquisition of the Fosterville gold project from Brunswick NL. Perseverance then conducted three phases of RC drilling (3222 m) aimed at closing the drilling grid down to 20 m and further testing the continuity of mineralisation and grade distribution. A sample of 1500 t was extracted to define the metallurgical properties of near surface material and to confirm the validity of standard grade control practice. Finally, three
517
R S SEBEK
costeans were dug, sampled and mapped (C L Roberts, unpublished data, 1994), to refine the interpretation of mineralisation prior to the estimation of a preliminary resource within HR1. This exploration licence was later renumbered EL 3339 following the successful amalgamation of adjacent tenements, from within which the current mining licence (ML 4784) was granted (Fig 2). Mining commenced in January 1996.
facies, probably under baric type II conditions in conjunction with low temperatures, and contrasts with the impervious nature of the Ordovician rocks. The mineralising event in the Melbourne Trough is regarded as having occurred during the Tabberabberan Orogeny. Primary gold production from within the Melbourne Trough is dominated by the Woods Point–Walhalla belt (98 t). Other significant historic production came from Rushworth (3027 kg), Costerfield (2300 kg), Diamond Creek (1870 kg) and Whroo (1240 kg). More recently, the Nagambie gold mine, 12 km to the east, ceased operations in 1996 after production of 4170 kg.
LOCAL GEOLOGY LITHOLOGY The Bailieston open pit area has little or no outcrop, and is covered by a veneer of topsoil and red-brown clay ranging in thickness from 0.5 to 2.5 m. The clay grades down into a poorly sorted alluvial gravel containing abundant well rounded quartz clasts to 10 cm in diameter. The alluvium is moderately ferruginous and rests unconformably on strongly weathered sandstone of the Dargile Formation. The alluvium varies in thickness from 1 to 4 m primarily due to scouring in local palaeochannels. The sandstone has a uniformly cream-grey coloured, highly weathered profile which overlies the oxide zone of the deposit. The weathered sandstone is generally bleached with little or no ferruginous staining or quartz veining and little mineralisation. The weathered zone is usually 2 to 3 m thick under the first bedrock exposure but is as much as 5 m thick in some places. The soft nature of the weathered sandstone allows removal without blasting.
FIG 2 - Plan of EL 3339 showing the position of ML 4784 and the Bailieston deposit with respect to the Bailieston anticline.
The underlying oxidised sediments are dominantly well laminated, fine to medium grained sandstone with minor siltstone and mudstone. The sandstone is extremely porous, and in many instances individual beds have been silicified to such an extent that they are classified as quartzite. Voids after pyrite along bedding planes are a common occurrence and occur in sandstone and quartzite.
REGIONAL GEOLOGY STRUCTURE The Bailieston goldfield occurs within a folded and faulted Ordovician to Devonian sequence of sandstone and mudstone known as the Melbourne Trough (Gray, 1988; Gray and Willman, 1991). Bounded to the west by the Heathcote fault zone and to the east by the Mount Wellington Fault zone, the Melbourne Trough (Fig 1) is roughly triangular in shape. It is intruded by Late Devonian granite and overlain by a Late Devonian–Early Carboniferous sequence of rhyolite, red bed siltstone and sandstone (Gray and Mortimer, 1996). The sediments of the Bailieston area consist of dominantly marine turbidites of the Late Silurian Dargile Formation and Early Devonian Broadford Formation. The sediments have been deformed into relatively open folds (3 to 6 km apart) with poorly developed reticulate cleavage confined to fold hinges (VandenBerg and Gray, 1988). The Dargile Formation includes laminated and current bedded sandstone, interbedded with siltstone and mudstone, while the Broadford Formation includes sandstone and siltstone with minor greywacke conglomerate intervals (M I Miller, unpublished data, 1986). The porous and permeable nature of the Silurian–Devonian sediment has resulted from metamorphism to lower greenschist
518
The field occurs along and adjacent to the hinge of the regional Bailieston anticline (Fig 2). This fold is open and upright with simple, straight, steeply dipping limbs and a complex hinge zone (G W McConachy, unpublished data, 1990). The southern half of EL 3339 contains NNE-striking fold axes which change to a NW orientation in the northern half of the licence. The Mount Black granite outcrops to the west of the area within a structural zone around which the fold axes in the extreme north swing sharply to an easterly orientation. The distribution of north-striking quartz reefs within a line of WNW trending workings fits well with a model of dextral shearing in a WNW trending zone (S King, unpublished data, 1996). Detailed mapping of trial pits by Metals Ex also showed that the shear and vein pattern is consistent with a dextral strikeslip zone. The quartz reefs align obliquely to the zone as a whole, and can be interpreted as associated riedel shear and extensional orientations to this zone (Fig 3). Antithetic shears generally develop after synthetic shears, and this can be seen at Bailieston as the sinistral shears invariably offset the dextral shears. The fact that the zone does not continue significantly to
Geology of Australian and Papua New Guinean Mineral Deposits
BAILIESTON GOLD DEPOSIT
A thick sandstone unit lies immediately to the south of the mineralised zone. This unit may have provided the competency contrast between rock types which localised shearing. Refolding of the Bailieston anticline about an east trending axis is evident from regional geology maps and magnetic images of the area.
MINERALISATION FIG 3 - Model orientation and position of extensional, synthetic riedel and antithetic riedel shears within a dextral strike-slip zone, oriented subparallel to Bailieston structures.
the WNW suggests that the mineralisation in HR1 has developed as a termination vein array to the dextral shear zone in combination with some transfer zones between dextral shear strands. Pit exposure has revealed that the overall structure is very simple with low levels of deformation. Bedding dips consistently to the SSW at moderate angles except at the closure of the Bailieston anticline in the NE side of the pit. This fold has a near chevron style core. Cleavage is sporadically developed throughout the pit but increases in intensity in the fold core. The hinge zone of the Bailieston anticline is marked by a 50 m wide zone of minor folding which is common in the closure of folds involving well bedded sediments. The fold core is subparallel to the overall dextral shear zone and may have had some control on the shear history.
Three main styles of mineralisation have been identified in the Bailieston goldfield. Present mining operations exploit low grade disseminated gold mineralisation which includes isolated high grade areas of stockwork veining and narrow quartz veins, entirely within the oxidised zone. The mineralised area is 300 m long by 150 m wide (Fig 4) and extends to 50 m deep.
DISSEMINATED ORE Bailieston is a disseminated gold deposit with a moderate near surface supergene grade dispersion. The main sources of ore are deformation zones which strike WNW and comprise ferruginous fracture zones which have little or no effect on the orientation of the moderately dipping beds (S King, unpublished data, 1996). Throughgoing features are often difficult to define on the ground and the deformation zones are discontinuous along strike, but they are easily identified by strong iron staining of the entire sediment package. As a result of brecciation, these zones are reduced to an almost powdery consistency after blasting. Widths vary from zone to zone and also along strike but are typically 10 to 15 m with blowouts of up to 20 m and bottlenecks of less than 5 m (Fig 4).
FIG 4 - Ore outlines on RL 85 (15 m below surface), based on kriging and the position of the main WNW-trending mineralised deformation zone (SW dextral boundary) with well developed extensional structures and antithetic riedel shearing. The SW dextral boundary could also be interpreted as a major synthetic reidel shear within a larger scale strike-slip zone. Note low grade halo.
Geology of Australian and Papua New Guinean Mineral Deposits
519
R S SEBEK
The mineralised deformation zones are accompanied by irregular mineralised haloes which reflect stockwork veining and selective mineralisation of favourable beds. Some sandstone beds are clearly preferentially and passively mineralised. It is not uncommon for silicification and iron staining to die out along bedding away from the WNW shear zone. Sampling of individual beds has shown that only those beds that have been silicified carry economic grades, in the range of 0.2 to 1 g/t gold. Unstained sandstone beds immediately adjacent to silicified beds are barren of mineralisation. The presence of voids after pyrite is also a good indication of economic mineralisation within such haloes, regardless of the level of iron oxide, veining or fracturing in the host. Porphyry dykes only occur within the WNW trending shear zones. The porphyry is pale brown in colour and often contains strongly limonitic and brecciated quartz stockwork veining. The porphyry intrusions are generally 1 to 3 m wide with either sharp or gradational contacts with the surrounding sediment. Porphyry has assayed up to 0.4 g/t gold, suggesting the likelihood of multiple stage mineralisation. Limited testing has established a strong correlation between gold and antimony values. The historical Black Cloud mine (1865–1882) in the north of HR1 was primarily an antimony producer, and a similar gold-antimony relationship also exists at Fosterville, 55 km to the west of Bailieston.
STOCKWORK VEINS Stockwork mineralisation is intimately associated with disseminated mineralisation (M I Miller, unpublished data, 1986) and they are mined together. The structural environment of dextral shearing provided the regime for multi-directional quartz stockwork veining in the vicinity of riedel and extensional shears. The stockwork veins are typically 1 to 5 cm thick and strongly brecciated. They contain strong limonitic staining between fractures within the quartz and as a coating separating the quartz from the host rock. The grade of the stockwork veins varies enormously, and selective sampling has shown grades of thin veinlets from 0.1 to 160 g/t gold. The grade and intensity of stockwork veining is influenced by proximity to the related shear feature.
QUARTZ REEFS Historically, most of the gold won from the Bailieston area was derived from quartz vein or reef structures on which mining was only terminated due to the influx of water below 40 m. In the few operations that persevered below the water table, the grade of mineralisation was comparable with the grade at shallower depths. At least nine significant lines of reef were worked historically at Bailieston, with grades from 15 to 300 g/t gold. The dominant north strike of the reefs is oblique to the 5 km long WNW line of workings. The width of the reefs varied between 0.1 to 1 m but was commonly in the range 0.3 to 0.5 m. Although not a priority for the current mining operation, the existence of thin high grade veins is recognised. Occasional exploration and grade control drill hole intersections in the range 1 to 5 m at 35 to 50 g/t gold serve as reminders. Selective extraction is not possible due to narrow widths and the high amount of previous stoping. However, as mining progresses,
520
potential exists for abandoned lines of reef to be exposed and selectively extracted, as a supplement to the disseminated ore being mined.
GEOLOGICAL PRACTICE Resource estimation is by kriging using 5 by 10 by 5 m blocks. Grade control drilling is on a 4 by 4 m pattern to a depth of 5 m. The cuttings from 102 mm diameter blast holes are sampled at the collar, and samples weighing between 3 and 5 kg are assayed in Bendigo for gold. Assays are received less than 24 hours after sample submission, and are added to the database where kriging is initiated using 4 by 4 by 5 m blocks. Blast hole grades are then studied in conjunction with kriged ore blocks to define extractable zones of ore which are then surveyed on to the pit floor in readiness for mining. Blasting is undertaken in 5 m benches which are then removed as two 2.5 m flitches at a waste to ore stripping ratio of 1.3 : 1.
CONCLUSIONS Mineralisation in most of the major goldfields in central Victoria is associated with quartz in well defined reefs, shoots and saddles located in partings or openings in permeable slate and siltstone. Discoveries over the past ten years have shown that should these host rocks be sufficiently permeable and located within structurally favourable environments, the potential for discovering large tonnage, low grade disseminated gold deposits is very real. Operations such as Nagambie and Fosterville have proved that low grade disseminated gold mineralisation exists and can be successfully extracted at relatively low cost. The pit will be mined until late 1997 and a further two years will be required to extract the gold remaining in the heap. An intensive exploration drilling program planned for late 1996 will focus on the southern and northern strike extensions of the deposit where sporadic exploration in the past has isolated numerous targets, which may only require limited drilling to be brought to resource and reserve status.
ACKNOWLEDGEMENTS Appreciation is expressed to the management of Perseverance Mining Pty Ltd for permission to publish this paper. The work of the small team associated with the Bailieston project is acknowledged, and thanks are also extended to S King for his valuable structural insight.
REFERENCES Gray, D R, 1988. Structure and tectonics, in Geology of Victoria, 2nd ed (Eds: J G Douglas and J A Ferguson), pp 1–36 (Geological Society of Australia, Victorian Division: Melbourne). Gray, D R and Mortimer, L, 1996. Implications of overprinting deformations and fold interference patterns in the Melbourne Zone, Lachlan Fold Belt, Australian Journal of Earth Sciences, 43:103–114. Gray, D R and Willman, C E, 1991. Deformation in the Ballarat Slate Belt, Central Victoria, and implications for crustal structure across southeast Australia, Australian Journal of Earth Sciences, 38:173–210. VandenBerg A H M and Gray, D R, 1988. Melbourne zone, in Geology of Victoria, 2nd ed (Eds: J G Douglas and J A Ferguson), pp 11–14 (Geological Society of Australia, Victorian Division: Melbourne).
Geology of Australian and Papua New Guinean Mineral Deposits
Turnbull, D G and McDermott, G J, 1998. Deborah line of reef gold deposits, Bendigo, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 521–526 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Deborah line of reef gold deposits, Bendigo 1
by D G Turnbull and G J McDermott
2
INTRODUCTION The Deborah line of reef (DLR) is 150 km NNW of Melbourne, Vic, in the Bendigo Goldfield (Fig 1), at lat 36o45′S, long 144o16′E on the Bendigo (SJ 55–1) 1:250 000 scale and Bendigo (7724) 1:100 000 scale map sheets. Gold production from the Bendigo Goldfield is estimated to be 22 Moz (684 t) from 1851 to 1954 (Fig 2) ranking Bendigo as the second largest goldfield in Australia behind Kalgoorlie’s Golden Mile. Production from the DLR is reported to be 218 000 oz from 470 000 t treated, ranking it as the sixth most important structure in the Bendigo Goldfield. FIG 2 - Historic gold production figures for the Bendigo Goldfield between 1851 and 1996. Note late production from the DLR in comparison with the rest of the field.
Bendigo Mining N L (BMNL) holds exploration and mining licences over the Goldfield. Exploration by BMNL has included the rehabilitation of historic workings of the DLR accessed by the refurbished 1930s Central Deborah shaft. An Indicated Resource of 113 000 t at 9.3 g/t for 34 000 oz and an Inferred Resource of 800 000 t at 6.6 g/t for 170 000 oz of contained gold has been estimated for remnant ore along the DLR.
EXPLORATION AND MINING HISTORY The Goldfield is one of the most famous and prodigious gold producing areas in Australia. Its history extends from the discovery of alluvial gold in 1851 to the cessation of reef mining in 1954, and more recently tailings retreatment during the 1980s and 1990s. Early miners recognised the repetitive nature of quartz reef structures developed along 12 parallel anticlines within a zone 4 km wide by 15 km long. Exploration within the axes of these folds (‘centre country’) became an essential feature of gold mining in Bendigo. The DLR, although discovered in the 1860s, did not have the near-surface reef potential of the adjacent lines and remained relatively unworked for 70 years. It was only through deep shaft sinking, to 630 m, and underground development of the 1930s that the DLR was brought into prominence (Fig 2). FIG 1 - Location of the DLR within the Bendigo Goldfield. Inset shows location of the four main mines along the DLR.
1.
Project Geologist, Bendigo Mining NL, PO Box 2113, Bendigo Mail Centre Vic 3554.
During the period 1931–1954 four main mines worked the DLR to recover 214 200 oz of gold at an average ore grade of 14.9 g/t (Table 1). Rapidly escalating production costs, diminished ore reserves, lack of sufficient exploration funds and a fixed gold price led to the closure of the North Deborah and Central Deborah reef mines in November 1954.
2.
Senior Project Geologist, Bendigo Mining NL, PO Box 2113, Bendigo Mail Centre Vic 3554.
WMC commenced the first modern exploration of the Goldfield in 1977. They embarked on a major program
Geology of Australian and Papua New Guinean Mineral Deposits
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D G TURNBULL and G J McDERMOTT
TABLE 1 Historic gold production figures for the main mines of the DLR (modified after W Shywolup and J V McCarthy, unpublished data, 1988). Mine
Period
Ore (t)
Recovered grade (g/t gold)
Gold (oz)
Deborah
1932–50 144 500
11.2
51 900
North Deborah
1939–54 202 700
19.7
128 100
Central Deborah
1943–54
65 000
14.1
29 500
Monument Hill
1934–42
35 900
4.0
4700
Total
1932–54 448 100
14.9
214 200
MacGeehan (1990). On a mine scale, the DLR stratigraphy is dominated by upward-fining turbidite cycles between 10 and 50 m thick. These cyclical sedimentation packages are usually quite distinctive and can be traced along strike between the four major mines of the DLR. Individual beds, on the other hand, are less continuous and either lens out, interfinger with other beds or change facies. The North Deborah shaft section (Fig 3) provides exposure of 230 m of the locally defined stratigraphic column, from the United sandstone (bottom) to the Royal Albert shale (top).
including historical research, 64 km of drilling, and underground exploration with access via two refurbished shafts. In 1983 BMNL secured title to leases encompassing the majority of the DLR, and commenced dewatering and refurbishment of the Central Deborah shaft from which limited mapping and sampling were conducted. In 1992 BMNL acquired the remaining Bendigo assets of WMC thereby gaining control of the Bendigo Goldfield. Dewatering of the DLR workings recommenced in January 1994 and was completed to the Central Deborah 17 level in December 1995. In June 1995 a program of detailed structural mapping, underground diamond drilling, sampling and surveying of over 1200 m of accessible historic workings of the Central Deborah, North Deborah and Deborah mines was undertaken leading to resource estimation. This phase of work was completed in July 1996 and the resulting understanding of structural and stratigraphic controls of ore emplacement in the DLR has provided the geological model on which future exploration of the Bendigo Goldfield will be based.
REGIONAL GEOLOGY The DLR is within the western margin of the Bendigo-Ballarat Province of the Lachlan Fold Belt described by Gray (1988). The rocks of this province are Lower Ordovician interbedded greywacke and slate (the Castlemaine Supergroup of Cas and VandenBerg, 1988) of lower greenschist metamorphic grade. These sediments have been deformed into tight chevron folds with steeply east-dipping axial planes and subhorizontal shallow north and south plunging fold axes. The Goldfield is intruded by the Devonian Harcourt Granodiorite of 361 Myr age (Richards and Singleton, 1981) and by lamprophyre dykes of Jurassic age, dated at 150 Myr (McDougall and Wellman, 1976). The dykes were generally intruded along the fold axial planes and are associated with some major reverse faults. Major quartz reef development within the Bendigo-Ballarat Province is structurally controlled, and is intimately associated with fold axes, fold culminations (‘domes’), reverse faulting and lithological competency contrasts.
ORE DEPOSIT FEATURES STRATIGRAPHY An overview of the stratigraphy, metamorphism and deformation of the Goldfield has been presented by Sharpe and
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FIG 3 - North Deborah shaft section showing the mine stratigraphy and major mineralised structures, looking north.
FOLDING The Deborah anticline is typical of other anticlines of the Goldfield. They strike at 340o, plunge subhorizontally to the
Geology of Australian and Papua New Guinean Mineral Deposits
DEBORAH LINE OF REEF GOLD DEPOSITS, BENDIGO
north and south and have wavelength and amplitude of 300 m and 150 m respectively. Individual folds are generally upright to locally overturned and chevron in style with interlimb angles between 40o and 50o. Fold geometry does not remain constant along the strike of the Deborah anticline and can vary from simple rollover to faulted (‘ruptured’) east over west, west over east, or a more vertical (axial) shear geometry. Many of these changes persist over a short strike length. Regional fold culminations and depressions are a feature of the Goldfield fold geometry, and the wavelength of the main fold culmination along the Deborah anticline is 1.6 km. Associated with folding is an axial plane cleavage best developed within the hinge zones in the centre country. This cleavage is manifest as a divergent slaty cleavage in shale and a convergent spaced (10 to 20 mm) fracture cleavage in sandstone. Rare kink and buckle folds of centimetre scale overprint these earlier structures. Structural evidence supports a continuous phase of deformation from ductile (folding) through to brittle (faulting) strain conditions rather than two discrete episodes. The age of folding is thought to lie within the range Benambran (Late Ordovician to Early Silurian; see Spencer-Jones and VandenBerg, 1975; VandenBerg, 1978) to Tabberabbern (Mid Devonian; Thomas, 1939; Sandiford and Keays, 1986).
(pug), slickenslides and occasionally a laminated quartz vein. Veins of this type (‘backs’) are of variable width, usually less than 0.3 m, and can thicken rapidly toward the hinge to form ‘legs’ which are part of a ‘saddle reef’ structure .
Transgressive (C) veins These are reverse faults of up to 60 m displacement and contain several structural components. They initiate as bedded faults on one limb, rupturing the anticlinal hinge zone to pass discordantly through the opposing limb. On passing through the hinge the fault is manifest as a series of fault splays with widths to 20 m. These faults may be either east or west dipping. Faults of this classification have been historically called ‘leatherjackets’, ‘fissure reefs’, ‘slides’ and ‘neck reefs’.
Tensional (T) veins These are tension vein arrays which are the product of extension along the elongation axis. Tension veins are composed of buck quartz and are best developed within sandstone horizons associated with bedded and transgressive faults. Historically veins of this type have been referred to as ‘spurs’.
FAULTING AND QUARTZ VEINING
Perpendicular (P) veins
Faults and quartz veining at Bendigo are intimately associated. A study of the fault and vein geometry has defined several structural elements. These are illustrated in Fig 4 and are briefly described below.
These are the product of extension along the fold axis and strike at right angles to bedding, associated with a-c joints. This type rarely shows significant large tonnage gold mineralisation. Historically these structures have been included in the cross course classification.
Saddle reefs (S) These result from dilation about the hinge zone often at the interface between massive sandstone and shale. The geometry of these reefs is usually modified by other structures such as transgressive faults. Two types of quartz are generally present, a laminated vein within the legs of the saddle and a massive buck quartz within the hinge or cap. The cap quartz can contain brecciated inclusions of laminated leg quartz. Saddle reef mineralisation is generally continuous along strike for several kilometres, often at high grades.
Axial plane (A) veins These structures are developed subparallel to cleavage. Veins of this type are generally composed of buck quartz. Axial veins are intimately associated with saddle reefs, transgressive faults and neck reef structures. FIG 4 - Main structural elements of an idealised Bendigo ore system: axial (A); bedded (B); transgressive (C); tensional (T); perpendicular (P) veins and saddle reefs (S).
Bedded (B) veins Bedded faults developed early in the deformation history and represent layer-parallel slip during folding. Faulting is generally confined to contacts with competency contrasts, particularly between massive sandstone and less competent shale horizons. The interface often shows 1 to 2 cm of gouge
Geology of Australian and Papua New Guinean Mineral Deposits
MINERALISATION The host of gold mineralisation along the DLR is quartz veining associated with one or all of the structures defined above. Gold is typically free and within the size range 10 µm to 4 mm. It is closely associated with late stage sulphides such as sphalerite and galena, although free gold may be present within massive buck quartz. Typical ore consists of free gold and up to 2% sulphides, mainly arsenopyrite, pyrite, sphalerite, galena, minor pyrrhotite, chalcopyrite and bournonite (PbCuSbS3). Common gangue minerals include quartz, ankerite, sericite and minor chlorite. Associated with mineralisation is a pervasive phyllic alteration halo.
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D G TURNBULL and G J McDERMOTT
Inner and outer reefs These are structurally complex saddle reefs (type S in Fig 4) hosted by the Lady Fiona sandstone (Fig 3). The Inner reef has been worked intermittently over a strike length of 1400 m (Fig 5) for an estimated production of 73 500 t at 29.8 g/t gold for recovery of 70 500 oz (W Shywolup and J V McCarthy, unpublished data, 1988). The bulk of the production came from the east leg of the saddle. The Outer reef has been worked over a strike length of 500 m, between the Monument Hill and Central Deborah mines, for a production of 40 800 t at 6.6 g/t gold for recovery of 8700 oz (W Shywolup and J V McCarthy, unpublished data, 1988). The morphology of the two reefs changes along strike from classic small saddles to much larger structurally complex zones formed by the interaction of saddle reefs and transgressive faults. The Deborah, North Deborah and Monument Hill shaft sections (Fig 6) portray separate classic saddle geometries for the Inner and Outer reefs. The Upper Deborah fault does not appear to influence the geometry of these saddles in this area. However, at sections 124 610 m N and 124 690 m N (Figs 6 a, b) near the Central Deborah shaft there is a strong interaction between the Inner and Outer reefs. An east-dipping transgressive fault initiating in the east limb of the Inner reef is truncated by the west-dipping transgressive Upper Deborah fault coincident with the west leg of the Outer reef. This results in a chaotic zone of hinge dilation. A visible aspect of the alteration associated with the two reefs is the development of carbonate spotting, silicification and coarse arsenopyrite crystals within the host sandstone. An Inferred Resource of 45 000 t at 10.4 g/t gold for 15 000 contained oz has been estimated for remnant ore along the Inner and Outer reefs.
Deborah back This is a bedded fault (B) hosted by the Deborah Back sandstone on the eastern limb of the Deborah anticline. The
structure has been worked over a strike length of 1400 m between the Central Deborah and Deborah mines, for an estimated production of 274 600 t at 12.8 g/t gold for recovery of 112 750 oz (W Shywolup and J V McCarthy, unpublished data, 1988). The reef is composed of laminated quartz and massive to brecciated buck quartz. This structure differs from a normal bedded vein in that it terminates below the hinge. The quartz reef is 0.5 to 1.5 m wide and extends to 80 m down dip below the hinge. This contrasts with other bedded faults that rapidly pinch, typically within 20 m below the fold hinge, to uneconomic dimensions. An Inferred Resource of 31 000 t at 10 g/t gold for 10 000 contained oz has been estimated.
Deborah fault This structure is one of the dominant transgressive faults on the DLR hosted by the Deborah Back sandstone and sediment of Kingsley’s formation (Fig 3). The fault is interpreted to have a strike length of more than 1000 m and is still open to the south (Fig 5). An alteration halo consisting of pervasive sericite, coarse grained arsenopyrite and minor pyrite is developed in the host sediment. This halo usually extends 5 m out into the footwall of the mineralisation but is confined to within 5 cm of the hanging wall. Total production is estimated at 66 650 t at 13.6 g/t gold for recovery of 29 145 oz (W Shywolup and J V McCarthy, unpublished data, 1988). Typical Deborah fault ore comprises buck quartz, often vuggy, with irregular masses or laminations of sulphides, with gold as inclusions and fracture fillings within the sulphides. Limited petrographic investigations and current and historical field observations have highlighted a close relationship between gold, galena and sphalerite. A generalised paragenetic sequence for sulphides in the Deborah fault ore has been determined to be pyrite, arsenopyrite, pyrrhotite, sphalerite, galena, gold, bournonite and chalcopyrite. Gold appears to coprecipitate with the late stage sulphides and is present as composite gold-galena grains and intergrowths, which contain a trace of acicular bournonite (P M Ashley, unpublished data, 1996).
FIG 5 - Simplified longitudinal projection through the DLR, looking west, with the most significant mineralised structures defined, and location of cross sections shown on Fig 6.
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Geology of Australian and Papua New Guinean Mineral Deposits
DEBORAH LINE OF REEF GOLD DEPOSITS, BENDIGO
FIG 6 - Isometric diagram of the DLR workings showing the main mineralised structures at shaft cross sections. Insets show significant structural features referenced in the text. All insets (a,b,c,d and e) looking north.
Figure 6 shows the position of the Deborah fault at the various mines. In the North Deborah shaft section, the Deborah fault refracts through the hinge showing repeated splaying, and hanging wall faults become progressively weaker into the east limb and terminate (Fig 3). A strong footwall fault persists into the east limb beyond the lateral extent of the quartz mineralisation. In the North Deborah shaft area the interplay between major west- and east-dipping faults is highlighted. Here the east-dipping reverse fault splays as it passes through the hinge into the west limb, displacing the west-dipping Deborah fault in a series of jogs. Resultant dilatant sites within this zone are filled with several generations of quartz veining, consisting of early laminated hanging wall quartz, abundant spurs (T) and buck quartz. A similar mineralised transgressive fault has been described at Wattle Gully (Chewton) by Potter (1990). The apex of the Deborah anticline culmination or dome, centred on the Deborah shaft (Fig 5), appears to influence the magnitude of displacement of both the major west- and eastdipping transgressive faults. The displacement on both fault sets diminishes concentrically to the north, and presumably to the south, with increasing distance away from the apex of the dome. Similar fault systems have been described from the Caledonides of southern Norway where the faults are thought to originate as break thrusts (Morley, 1994). An Inferred Resource of 708 000 t at 6.3 g/t gold for 140 000 contained oz and an Indicated Resource of 113 000 t at 9.3 g/t gold for 34 000 contained oz has been estimated for the Deborah fault from recent work.
Geology of Australian and Papua New Guinean Mineral Deposits
Kingsley’s lode A modified saddle-style reef (S) is developed within Kingsley’s formation, a large package of medium grained sandstone containing thin (<1 m) shale interbeds (Fig 3). The west leg of Kingsley’s lode was worked over a strike length of 400 m in the North Deborah mine and prospected in the Deborah workings. A total production of 18 400 t at 13.6 g/t gold for recovery of 8050 oz (W Shywolup and J V McCarthy, unpublished data, 1988) has been estimated. As in the Inner and Outer reefs the fold geometry of Kingsley’s lode shows marked changes from north to south over a strike length of some 70 m (Figs 6 c, d, e). There is a normal fold hinge at 124 340 m N where Kingsley’s lode maintains a uniform width over the fold. The lode geometry changes significantly only 20 m to the south at 124 320 m N, where the fold hinge is sheared out creating an east over west dislocation. This coincides with a reduction in the interlimb angle from 70 to 55o. The influence of a transgressive east-dipping fault on fold geometry is enhanced at section 124 282 m N and illustrates the significant east over west faulting within the hinge. The increased dilation and quartz mineralisation coincident with the rupture represents a substantial exploration target. At the North Deborah section (Fig 6), the west leg of Kingsley’s lode is the bedded component of a major west-dipping transgressive fault, the Deborah fault. The west leg of Kingsley’s lode has well developed bedding-parallel laminations within a vein of up to 0.8 m width below the apex of the fold. The east leg shows only a narrow laminated quartz vein enclosed within a massive buck quartz vein. Associated quartz spurring is developed on the
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hanging wall of this structure creating a mineralised zone 3 m wide. An Inferred Resource of 15 500 t at 5 g/t gold for 2500 contained oz has been estimated for the portion of Kingsley’s lode un-associated with the Deborah fault.
access for geological evaluation within the old workings and to BMNL staff for their help during the internal review process.
Rowes reef
Cas, R A F and VandenBerg, A H M, 1988. Ordovician, in Geology of Victoria (Eds: J G Douglas and J A Ferguson), pp 63–102 (Geological Society of Australia, Victorian Division: Melbourne).
A saddle style reef (S) hosted by Rowes shale has been modified by an east over west sense of reverse faulting. The east limb and associated east-dipping transgressive fault are much more strongly gold mineralised than the west limb (Fig 6, Central Deborah shaft section). There are several generations of quartz veining along Rowes reef, which consist of laminated, brecciated and massive buck quartz over widths to 5 m. Arrays of tension veins (T) are also developed on the hanging wall and footwall side of the east-dipping fault as it passes through the hinge. The most significant aspect of this reef is the eastdipping transgressive fault that cuts through the fold axis at a slightly steeper angle than that of the fold plunge. The fault therefore transects the hinge in different rock types along strike, thereby affecting reef development. Production from this reef in the North Deborah and Central Deborah mines is estimated to be 27 700 t at 12.5 g/t gold for recovery of 11 150 oz (W Shywolup and J V McCarthy, 1988). No mineral resource has yet been estimated for the remnant ore of this reef.
New Formation reef This reef was intersected in the lowest level of the Deborah shaft workings (Fig 6) but has not been accessed by BMNL. Deborah mine records indicate that the New Formation reef is analogous to reefs associated with west-dipping transgressive faults such as the Upper Deborah and Deborah faults. The only record of production comes from mine managers reports stating the result of a single trial crushing of 143 t at 8.8 g/t gold for recovery of 40.5 oz.
ACKNOWLEDGEMENTS The authors would like to thank Bendigo Mining NL for permission to publish this paper, and W P Laing for his input into the classification of the structural elements of a typical Bendigo ore system. Special thanks go to the Deborah project team who persevered under difficult conditions to provide
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REFERENCES
Gray, D R, 1988. Structure and tectonics, in Geology of Victoria (Eds: J G Douglas and J A Ferguson), pp 1–36 (Geological Society of Australia, Victorian Division: Melbourne). McDougall, I and Wellman, P, 1976. Potassium-argon ages for some Australian Mesozoic igneous rocks, Journal of the Geological Society of Australia, 23:1–9. Morley, C K, 1994. Fold-generated imbricates: examples from the Caledonides of Southern Norway, Journal of Structural Geology, 6 (5):619–631. Potter, T F, 1990. Wattle Gully gold deposits, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1281–1285 (The Australasian Institute of Mining and Metallurgy: Melbourne). Richards, J R and Singleton, O P, 1981. Palaeozoic Victoria, Australia: igneous rocks, ages and their interpretations, Journal of the Geological Society of Australia, 28:395–421. Sandiford, M and Keays, R R, 1986. Structural and tectonic constraints on the origin of gold deposits in the Ballarat Slate Belt, Victoria, in Turbidite-Hosted Gold Deposits (Eds: J D Keppie, R W Boyle and S J Haynes), Geological Association of Canada Special Paper, 32:15–24. Sharpe, E N and MacGeehan, P J, 1990. Bendigo Goldfield, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1287–1296 (The Australasian Institute of Mining and Metallurgy: Melbourne). Spencer-Jones, D and VandenBerg, A H M, 1975. The Tasman Geosyncline in Victoria - regional geology, in Economic Geology of Australia and Papua New Guinea, Vol 1 Metals (Ed: C L Knight), pp 637–646 (The Australasian Institute of Mining and Metallurgy: Melbourne). Thomas, D E, 1939. The structure of Victoria with respect to the lower Palaeozoic rocks, Department of Mines Victoria Mining and Geological Journal, 1:59–64. VandenBerg, A H M, 1978. The Tasman fold belt system in Victoria, Tectonophysics, 48:267–297.
Geology of Australian and Papua New Guinean Mineral Deposits
Ebsworth , G B, de Vickerod Krokowski, J and Fothergill, J, 1998. Eaglehawk–Linscotts reef gold deposits, Maldon, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 527–534 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Eaglehawk–Linscotts reef gold deposits, Maldon by G B Ebsworth1, J de Vickerod Krokowski2 and J Fothergill3 INTRODUCTION The Eaglehawk–Linscotts reef system (ELRS) is close to Maldon, Victoria, in the Maldon Goldfield at lat 36o59′S, long 144o04′E on the Bendigo (SJ 55–1) 1:250 000 and the Bendigo (7724) 1:100 000 scale map sheets (Fig 1). Title to the ELRS is owned by Alliance Gold Mines NL (Alliance). Gold mineralisation occurs in narrow, fault-related quartz reefs which host high grade, typically south-plunging ore shoots. The reefs have associated ‘spur’ vein arrays which, with remnant reef and stope fill from 19th century mining, comprised a low grade deposit which was mined from 1988 to 1992 in the Union Hill open pit (Fig 2). Alliance has defined narrow vein deposits totalling about 120 000 oz of gold (Table 1) in the ELRS and is currently trial mining an ore shoot at Linscotts reef using decline access from the northern end of the Union Hill open pit. This mining and ongoing exploration has enabled preliminary structural analysis and description of gold mineralisation within the ELRS.
EXPLORATION HISTORY Gold was discovered in the ELRS in 1854 in Eaglehawk reef at Union Hill, about 1 km north of Maldon. Subsequent discoveries by individual miners and small syndicates who explored the reef system by sinking shallow shafts above the water table led to the establishment of several major underground mines on the Linscotts and Eaglehawk reefs. These produced some 500 000 oz of gold until the last closed in the late 1890s. In the late 1940s North Broken Hill Ltd carried out a program of mapping and literature research of the Maldon Goldfield, prepared a set of sections and plans largely based on the work of Moon (1897) and drilled a few diamond drill holes at Nuggetty reef, 1.5 km north of the Union Hill pit (Fig 1). During the period 1973 to 1982 exploration of the ELRS by several companies included geological mapping, geochemical sampling and diamond and percussion drilling programs. The most comprehensive exploration during this period was by Carpentaria Exploration Co Pty Ltd (CEC) from 1979 to 1982. This culminated in the drilling of 23 diamond drill holes totalling 4939 m throughout the goldfield. Exploration by Lone Star Exploration NL (Lone Star) from 1973 to 1975 identified a 1.
Senior Geologist, Alliance Gold Mines NL, PO Box 55, Maldon Vic 3463.
2.
Consulting Geologist, J and H Geo-Services.
3.
Geologist, Ballarat Goldfields NL, PO Box 1228, Ballarat Mail Centre Vic 3354.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Regional geological map of the Maldon Goldfield and location of Eaglehawk–Linscotts reef system, after Morgan and Woodland (1990), Gregory (1994) and Cherry and Wilkinson (1994).
low-grade gold resource in Eaglehawk reef. Open pit mining of this resource failed in 1975 due to a lack of working capital and an inadequate gravity treatment plant.
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Alliance Gold Mines NL was floated in 1994 and acquired Triad’s Maldon tenements and treatment plant. Since then exploration by Alliance has included further historical research, geological mapping and drilling programs to follow up several high grade drill intersections of Lone Star and CEC. Due to the relatively small dimensions of the high grade ore shoots at Maldon, drilling by Alliance has been carried out on 20 to 30 m spaced drill sections with a similar vertical spacing between drill intersections on reef. Results from 38 diamond and 47 reverse circulation drill holes (totalling over 7000 m) which tested the ELRS, plus trial mining of Linscotts reef, were used to estimate the resources presented in Table 1. The main limitation for resource definition was grade estimation, due to the extremely nuggetty style of gold mineralisation. For this reason the majority of resources defined to date have been classified as Inferred.
PREVIOUS DESCRIPTIONS The first comprehensive description of the Maldon Goldfield, which included the ELRS, was by Moon (1897) who mapped many of the major mines and described the host sequence lithology and the structure and the mineralogy of the reef systems. Mason and Webb (1953) described and interpreted the geology of the goldfield based on their surface mapping and available historical data. Morgan and Woodland (1990) described the Union Hill low grade gold deposit which forms part of the ELRS. Gregory (1994) studied the relationship of the Maldon reef systems to the nearby granitic intrusives and concluded that the auriferous reefs were formed prior to granitic intrusion.
REGIONAL GEOLOGY
FIG 2 - Diagrammatic plan of the Eaglehawk–Linscotts reef system.
Triad Minerals NL commenced exploration at Maldon in 1983 and carried out further percussion drilling at Union Hill. This led to definition of a resource comprising 1 Mt of remnant reef, stope fill and spur vein arrays which was mined by open cut from 1988 to 1992. About 55 000 oz of gold were produced from ore of head grade approximately 1.8 g/t.
The ELRS is one of the major gold producing reef systems of the Maldon Goldfield (Fig 1) and is within the central part of the Bendigo–Ballarat zone of the Lachlan Fold Belt (Gray 1988; Morand, 1996). The host rocks are Ordovician (Lancefieldian) turbiditic metasediments of the Castlemaine Supergroup (Cas and VandenBerg, 1988) which have undergone several structural events, reflecting a regional thin skinned style tectonic history (Fergusson, Gray and Cas, 1986; Cox et al, 1991). Together these represent the pre-Lower Devonian Benambran and Middle Devonian Tabberabberan deformations. Several predominantly brittle structural events have also been recognised which post-date the Tabberabberan deformation. The dominant regional structures are tight, isoclinal, northtrending asymmetric F1–2 folds with a pervasive axial plane cleavage and occasional small parasitic folds. The folds are chevron style with amplitudes and wavelengths of a few to
TABLE 1 Gold resources defined in the ELRS by Alliance. Prospect
Reef
Resource category
Ore (’000 t)
Gold grade (g/t)
Contained gold (oz)
Union Hill North
Linscotts
Measured Indicated
17 21
13 10
7100 6700
Union Hill North
Eaglehawk
Indicated Inferred
39 180
8 8
10 000 46 300
Alliance South
Eaglehawk
Inferred
140
11
49 500
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hundreds of metres. The Maldon Goldfield is situated within the western limb of a regional synclinorium which transfers to the east into a structural high along the Muckleford Fault (Fig 1; Harris and Thomas, 1934; Beavis, 1963; Cherry and Wilkinson, 1994). Regional greenschist facies metamorphism (M1) was contemporaneous with the F1–2 folding (Sandiford and Keays, 1986; Gray and Willman, 1991). Auriferous reef systems in the Maldon field are associated with D3–D6 structures and exhibit a complex history of quartz vein formation, faulting and alteration. The metasedimentary sequence and reef systems were intruded by the Late Devonian (Richards and Singleton, 1981; McKenzie, Nott and Bolger, 1984; Sandiford and Keays, 1986), subvolcanic Harcourt Granodiorite which comprised three separate, dominantly granodioritic intrusive phases (Cherry and Wilkinson, 1994). Potassium feldspar-cordieriteandalusite-sillimanite-biotite grade contact metamorphism (M2) formed in aureoles associated with the batholith (Fig 1). Quartz porphyry, diorite and mafic dykes also intrude the Ordovician sequence. The age of the quartz porphyry and diorite dykes is not known but they appear to have intruded one of the major reef systems at Maldon at Victoria reef, from mapping by Moon (1897). In the nearby Bendigo Goldfield, lamprophyres similar to the mafic dykes at Maldon have been dated as late Jurassic (Byrne, 1985). Evidence of post-granite deformation includes reverse and strike slip faulting, some of which is associated with the intrusion of the mafic dykes.
ORE DEPOSIT FEATURES STRATIGRAPHY The ELRS is hosted by a sequence of originally fine grained sandy, silty and muddy turbiditic depositional units which contain common calc-silicate nodules (Morand, 1994) and minor laminae to thin beds of pyritic black shale. The black shale beds and some distinctively spotted, originally argillaceous units, are useful as local stratigraphic markers but are generally difficult to trace over distances of more than a few hundred metres. The host sequence is strongly hornfelsed and lies within the biotite-cordierite-potassium feldspar zone of the Harcourt Granodiorite contact metamorphic aureole (Cherry and Wilkinson, 1994; Gregory, 1994).
STRUCTURE The ELRS consists of a NNW-trending, steeply west to east dipping structure (Morgan and Woodland, 1990) which deforms tightly folded Ordovician metasediment and is associated with the (F1–2) German anticline (Figs 2 and 3). Where the ELRS is well developed, the German anticline plunges south and displays western vergence with typical west limb dips of 80–90o east and east limb dips of 50–60o east. The reef-related faults have been influenced by the pre-existing fold limb attitudes. The ELRS varies from a few centimetres wide where it parallels bedding in the west limb of the anticline, to tens of metres wide where it cuts bedding in the east limb of the fold (Fig 3). Reverse displacements are estimated to be tens of metres. Within the structure a series of en echelon (both in plan and cross section), predominantly south-plunging vein arrays and associated auriferous ore shoots form two ‘reefs’, although the distinction into separate reefs is largely historical rather than structural.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 3 - A. Diagrammatic cross section of model for the ELRS (blackquartz veins; stipple- structural envelope of reef system). Quartz vein types and typical geometry of D, R1, T, R2 and P sets are shown as insets. B. Schmidt–Lambert equal area plot of 230 quartz veins within the ELRS in the Union Hill open pit and underground workings.
The Eaglehawk reef has a larger strike length and vertical extent and comprises at least three en echelon segments with associated ore shoots. The smaller Linscotts reef is linked to the Eaglehawk reef by a duplex-like structure at the northern end of the Union Hill open pit. North of this linking structure, Linscotts becomes the main reef and Eaglehawk dies out as series of NNE-striking, lenticular auriferous quartz vein arrays. A similar relationship is likely between segments of Eaglehawk reef (Fig 2). The component vein arrays of the ELRS are bounded by fault zones tens of centimetres thick which exhibit evidence of predominantly ductile (D3) deformation (Fig 4). Where most strongly developed the ductile faults have cataclastic textures, comprising highly cleaved and altered country rock and mylonitised quartz lenses with deformed and dismembered fragments of pre- and syn-fault quartz veins, similar to σ porphyroclasts (Passchier and Simpson, 1986; Simpson, 1986). The D3 shear zones are disrupted by D4 ductile to brittle faults which display an en echelon geometry in plan and cross section (Figs 3 and 4). Between individual fault segments, jog duplexes have commonly developed pull-apart structures filled with vein arrays of massive ‘bucky’ quartz grading to breccias of vein quartz and country rock. The pull-apart structures display a reverse, dip-slip geometry in cross section and a sinistral pattern in plan (Figs 3 and 4 inset A), which is homothetic to the west limb strike-slip component of flexural slip within the German anticline.
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Post-granodiorite deformations (D6–D8) comprise several phases of reverse, strike slip and normal faulting with associated localised cleavage development, second order fracturing, jointing and intrusion of mafic dykes. Brittle, reverse, low angle D7 faults and kinks called ‘floors’ at the Maldon field form conjugate sets which dip gently to the east and west. These faults cut and displace the ELRS with displacements ranging from centimetres to tens of metres. They typically have an associated metres wide halo of distinctive disseminated, patchy and vein style chloritecarbonate-pyrite alteration. A set of ENE- and ESE-trending strike slip faults and fractures form a system of conjugated D8 discontinuities known at the Maldon goldfield as ‘cross courses’. The ENE set shows dextral displacements whereas the ESE set is sinistral and both cut the ELRS with displacements in the range of a few to tens of metres. Several mafic dykes cut and disturb the host sequence close to the ELRS. The dykes are often subparallel to bedding but have also intruded along zones of competency contrast, commonly between quartz reef and country rock, as well as along floor faults. Joints in the host sequence, ELRS, Harcourt Granodiorite (Beavis, 1963) and mafic dykes show a similar geometry. They form a suborthogonal system of two subvertical sets which trend north and ESE. Within the ELRS and its host sequence the joints are often filled or coated with carbonate, chlorite and pyrite. The joints show mainly extensional features, however some fracture surfaces display an en echelon, strike slip pattern of tectonic ribs, indicating a more complex origin.
QUARTZ VEINS
FIG 4 - Geological map of Linscotts reef on No 2 level from the Union Hill decline. Inset A: D3–D4 strike slip component. Inset B: late D4–D6 strike slip.
D3–D4 faults are bounded and cut by more brittle D5–D6 faults. These also cut the quartz vein arrays but are largely focussed along zones of competency contrast between quartz vein arrays and country rock (Fig 4). The D5–D6 brittle faults display a dextral pattern in plan (Fig 4, inset B), typically contain clay-chlorite-carbonate gouge a few centimetres thick, and have associated 1 to 2 m wide localised zones of spaced cleavage composed of similar material. Fault surfaces have slickensides which indicate predominantly strike slip to oblique slip movement. The Harcourt Granodiorite is here considered to be contemporaneous with D5 and displays a weak, ESE-trending foliation which is at a high angle to bedding in the surrounding metasediments (Beavis, 1963; Carmichael, 1986).
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Quartz veins developed in the contractile shear zones of the ELRS display a range of syntectonic micro- to meso-structures including crack-seal, free face growth, hydraulic breccia and quartz mylonites which are typically found in folded rocks (Mawer, 1987; Vearncombe, 1993; Forde and Bell, 1994). Veins include bedded and laminated quartz veins, fault related ‘seamy’ and brecciated quartz, massive quartz veins, hydraulic breccias of quartz and country rock, and spurry quartz veins. Within these textural groups at least three generations of quartz veins have been recognised. Maximum quartz thickness typically corresponds to an abrupt change of fault strike and dip direction across bedding or to intersections of several faults. The earliest quartz veins recognised in the ELRS formed during D1–D2 flexural slip. They comprise bedding-parallel and gently NNW-dipping sets which are locally ptygmatically folded with axial planes subparallel to bedding. The hinges of these folds are subhorizontal to gently north plunging with folds commonly associated with bedding parallel, reverse movements. Quartz veins associated with D3 thrusting are generally barren. In contrast, most auriferous quartz veins are strongly associated with D4 structures and include intact failures and secondary reshear veins (Sibson, 1981). The D4 veins comprise three sets: QV1, QV2 and QV3 (Fig 3). QV1 veins are subparallel to D4 reverse faults or associated R1 riedel shears. Quartz veins of the QV2 set dip at low angles and predominantly consists of extensional (T) veins or less commonly are related to primary, compressional, intact shears or R2 conjugate riedel shears. Most of the QV1 and QV2 veins display a homothetic reverse and sinistral pattern. QV1 veins
Geology of Australian and Papua New Guinean Mineral Deposits
EAGLEHAWK–LINSCOTTS REEF GOLD DEPOSITS, MALDON
intersect bedding with a lineation which plunges gently south whereas QV2 and QV3 veins display northerly lineation plunges. Some QV2 veins, especially those affiliated to the R2 conjugate riedel shears display antithetic (normal) features. The QV1 and QV2 sets are related to the dip slip component of the D4 faults. Both the QV1 and QV2 sets are overprinted by QV3 veins. The QV3 set comprises extensional (T) and conjugate riedel (R2) veins related to late oblique-slip D4 faults which have a dextral strike slip component. The QV1, QV2 and QV3 sets are predominantly crack-seal veins (Ramsay, 1980; Cox and Etheridge, 1983) which commonly formed as shortening to extensional shear veins. Quartz within the QV1 and QV2 veins is not homogeneously distributed but often has en echelon, lensoidal and antiaxial geometries with a sinistral pattern and top towards the SW movement.
FIG 6 - Highly mineralised ‘seamy’ quartz from Linscotts reef No 2 level stope. The dark seams contain visible gold and bismuth.
MINERALISATION Gold mineralisation in the ELRS shows an association with arsenopyrite and minor amounts of other base metal sulphides, largely pyrite, galena, sphalerite, chalcopyrite, and marcasite, as at other central Victorian goldfields. However, it is also associated with native bismuth, bismuthinite (Bi2S3), maldonite (Au2Bi), joseite [Bi3Te(Se,S)], pyrrhotite, loellingite (FeAs2), scheelite, stibnite and molybdenite (Ulrich, 1869; Moon, 1897; Junner, 1921; Haupt, 1982; Morgan and Woodland, 1990; Gregory, 1994; H W Fander, unpublished data, 1995; T A P Kwak, unpublished data, 1997). The ore has an unusually high pyrrhotite to pyrite ratio. Free particles of gold, commonly associated with galena, sphalerite and arsenopyrite (Fig 5), occur in massive quartz and quartz-hornfels breccias (Morgan and Woodland, 1990). The gold-bismuth-telluride-sulphide assemblage occurs as dark seams in massive quartz, often close to chloritic D5–D6 faults. The seams commonly parallel the associated faults but may also have an irregular, more anastamosing geometry (Fig 6). Gold and bismuth form intimate intergrowths (Fig 7) with associated maldonite and joseite (T A P Kwak, unpublished data, 1997). Minor gold-bismuth mineralisation also occurs in hydraulic breccias of quartz and country rock where it overprints arsenopyrite-pyrrhotite-pyrite alteration (T A P Kwak, unpublished data, 1997). Gold also occurs in clay gouge (pug) and chlorite which fills D5–D6 faults and shears.
FIG 7 - Photomicrograph of the gold-bismuth-maldonite mineralisation in ‘seamy’ quartz of Fig 6. Gold and bismuth are intergrown. Maldonite occurs as small high relief blebs in gold.
T A P Kwak (unpublished data, 1997) has reported that auriferous quartz vein samples from Linscotts reef contain two phases of gold. One has a silver content of 8% which is similar to gold in other central Victorian deposits, whereas the other phase is very pure with <1 % silver and is closely associated with bismuth. At least two phases of host rock alteration are associated with gold mineralisation in the ELRS. Pervasive to vein style biotitecarbonate-arsenopyrite alteration in which the carbonate forms indistinct 1–5 mm spots (Bierlein et al, 1996) is strongly developed in host rock fragments within D3–D4 structures and for a few metres into the adjoining wall rocks. It also forms narrow selvages a few centimetres wide marginal to quartz veins, up to tens of metres from the main reef structure. Biotite has been interpreted as the (M2) contact-metamorphosed equivalent of muscovite-rich alteration selvages seen outside the aureole (Hughes, Phillips and Gregory, 1997). Patchy to vein style chlorite and carbonate alteration occurs in centimetres thick zones associated with D5–D6 faults. The alteration assemblages are mainly restricted to relatively narrow zones, suggesting that a subvertical fluid gradient was fairly constant during the prolonged process of reef formation.
STRUCTURAL EVOLUTION OF THE ELRS FIG 5 - Photomicrograph of gold-galena mineralisation in quartz from Eaglehawk reef, Union Hill. Reflected light.
Geology of Australian and Papua New Guinean Mineral Deposits
The D1 to D4 deformational stages were related to regional east–west compression with regional F1–F2 folding and D3 thrusting predominantly ductile in nature whereas D4 faulting
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G B EBSWORTH, J de VICKEROD KROKOWSKI and J FOTHERGILL
was more brittle. Together D1 to D4 caused horizontal shortening of the host sequence of about 75% in the Union Hill area. The attitude of regional compression changed from initially subhorizontal to plunge at 35o towards WSW during D4. After D4, the stress field further transformed from simple compression into a transpressional to transtensional regime in which δ2 was subvertical. Evidence of this transformation is also seen at the regional scale (Glen, 1992; Glen, Scheibner and VandenBerg, 1992; Collins, 1996). The evolution of the ELRS was a prolonged process which started late in D2. It reached its maximum development during D3–D4 and continued during D5–D6, overlapping with the intrusion of the Harcourt Granodiorite during D5. The process of thrusting initially formed steep, reverse, conjugated reshears (Sibson, 1981) with a sinistral strike-slip component which used the existing bedding and regional cleavage anisotropy in the west limb of the anticline. In the east limb, these reshears developed into dilational reverse faults and low angle shearcontrolled intact failures with a dextral strike-slip component. The fold- and thrust-related conjugate strike-slip components (sinistral in west limb and dextral in east limb) of the south plunging German anticline evolved into the transpressional to transtensional, regional strike-slip regime of D5–D6.
CONTROLS OF GOLD MINERALISATION In the ELRS gold mineralisation shows evidence of both structural and chemical controls. A kinematic model for the gold deposits based on a two directional shearing mechanism of reverse (oblique) slip to strike slip accounts well for the observed data. The major pathway for the mineralising fluids is interpreted to be the intersection direction of the fabrics resulting from the main reverse, dip to oblique slip and subsidiary strike slip movements. Zones of competency contrast between finer and coarser grained units were important in focussing these structures and mineralising fluids. The reef structure transgresses bedding, and early workers (Dunn, 1889; Moon, 1897) postulated that auriferous ore shoots developed where the structure came into contact with finely bedded silty, argillaceous to locally carbonaceous units. Carbonate (now calc-silicate) nodules within the sequence also probably played an important chemical role by modifying auriferous fluids. Mesostructural relationships and petrological evidence indicate that gold deposition occurred late in the mineralisation paragenesis and comprises two distinct phases. The first phase was coincident with the later part of reverse D4 faulting (Gregory, 1994; Phillips, Hughes and Gregory, 1996) and probably formed by a cyclic fault valve mechanism as proposed by Cox (1984, 1995) and Cox et al (1991). Cox (1984) and Cox et al (1991) suggested that within the Bendigo–Ballarat zone gold-bearing metamorphic fluids were focussed into individual goldfields by deep crustal listric faults splaying from a detachment zone at the base of the Ordovician succession. The concentration of auriferous reefs in the Maldon field may represent one or more such splays from the Muckleford Fault which outcrops some 4–5 km to the east (Fig 1). Recrystallisation textures of quartz veins (Gregory, 1994) and overprinting relationships of the Harcourt Granodiorite (Phillips, Hughes and Gregory, 1996) and associated dykes (Ulrich, 1869) place an upper time constraint on this first phase of gold mineralisation.
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The presence of a bismuth-telluride related gold phase suggests that magmatic fluids interacted with and probably remobilised pre-existing gold-sulphide mineralisation (Gregory, 1994; Arne et al, 1996; Phillips, Hughes and Gregory, 1996; T A P Kwak, unpublished data, 1997) during D5–D6 transpressional to transtensional reactivation of the reef structure. Structural preparation and controls within a transtensional regime have probably played an important role in localising and focussing the fluid flow.
ACKNOWLEDGEMENTS The authors gratefully acknowledge Alliance Gold Mines NL for permission to publish this paper. T A P Kwak is thanked for his review and constructive criticism of the manuscript, as are F P Bierlein, M J Hughes and W R H Ramsay for useful discussions which assisted in the formulation of the interpretations presented here.
REFERENCES Arne, D C, Bierlein, F P, McNaughton, N, Wilson, C J L, Morand, V J and Ramsay, W R H, 1996. Timing of gold mineralisation in Western and Central Victoria: New constraints from SHRIMP II analysis of zircon grains from felsic intrusive rocks, in Sedimentary-Hosted Mesothermal Gold Deposits - a Global Overview Conference Proceedings (Ed: W R H Ramsay), (University of Ballarat: Ballarat). Beavis, F C, 1963. Structural analysis of the Harcourt Batholith contact aureole, Proceedings Royal Society Victoria, 77:149–175. Bierlein, F P, Fuller, T, Stüve, K, Arne, D C and Keays, R R, 1996. Wallrock alteration associated with turbidite-hosted gold deposits - Examples from the Palaeozoic Lachlan Fold Belt in Central Victoria, Australia, in Victorian Gold - Timing Relationships and Emplacement, Quarterly Report for AMIRA Project P 478, for three months to 1.11.1996, Australian Mineral Industries Research Association, Melbourne (unpublished). Byrne, D R,1985. Explanatory notes for Campaspe 1:10 000 /2.2 geological map, Geological Survey of Victoria, Report 1985/53. Carmichael, T, 1986. The igneous rocks of the Maldon-Majorca district, Central Victoria, BSc Honours thesis (unpublished), Latrobe University, Melbourne. Cas, R A F and VandenBerg, A H M, 1988. Ordovician, in Geology of Victoria (Eds: J G Douglas and J A Ferguson), pp 63–102 (Geological Society of Australia, Victorian Division: Melbourne). Cherry, D P and Wilkinson, H E, 1994. Bendigo, and part of Mitiamo, 1:100 000 map, Geological Survey of Victoria, Report 99. Collins, W J, 1996. Tectonic setting of gold deposits in Eastern Australia, in Mesothermal Gold Deposits: a Global Overview, Publication 27 (Ed: D I Groves), pp 18–21 (The Geology Department and University Extension, The University of Western Australia: Perth). Cox, S F, 1984. Structural controls on the development of syntectonic gold-quartz veins in the Ballarat Trough, central Victoria, Geological Society of Australia Abstracts, 11:23–26. Cox, S F, 1995. Faulting processes at high fluid pressures: an example of fault valve behavior from the Wattle Gully Fault, Victoria, Australia, Journal of Geophysical Research, 100 (B7):12841–12859. Cox, S F and Etheridge, M A, 1983. Crack-seal fibre growth mechanisms and their significance in the development of oriented layer silicate microstructures, Tectonophysics, 92:147–170. Cox, S F, Etheridge, M A, Cas, R A F and Clifford, B A, 1991. Deformation style of the Castlemaine area, Bendigo-Ballarat
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Zone: Implication for evolution of crustal structure in central Victoria, Australian Journal of Earth Sciences, 38:151–170.
Mawer, C K, 1987. Mechanics of formation of gold-bearing quartz veins, Nova Scotia, Canada, Tectonophysics, 135:99–119.
Dunn, E J, 1889. Report on Tarrangower Company’s Mine, Nuggetty Gully, Maldon, Mining Registrar of Victoria Report, 31.12.1888, p 71.
McKenzie, D A, Nott, R J and Bolger, P F, 1984. Radiometric age determinations, Geological Survey of Victoria, Report 74.
Fergusson, C L, Gray, D R and Cas, R A F, 1986. Overthrust terranes in the Lachlan Fold Belt, southeastern Australia, Geology, 14:519–522. Forde, A and Bell, T H, 1994. Late structural control of mesothermal vein-hosted gold deposits in Central Victoria, Australia: Mineralisation mechanics and exploration potential, Ore Geology Reviews, 9:33–59. Glen, R A, 1992. Thrust, extensional and strike-slip tectonics in an evolving Palaeozoic orogen - a structural synthesis of the Lachlan Orogen of southeastern Australia, Tectonophysics, 214:341–380. Glen, R A, Scheibner, E and VandenBerg, A H M, 1992. Paleozoic intraplate escape tectonics in Gondwanaland and major strike-slip duplication in the Lachlan orogen of southeastern Australia, Geology, 20:795–798. Gray, D R, 1988. Structure and tectonics, in Geology of Victoria (Eds: J G Douglas and J A Ferguson), pp 63–102 (Geological Society of Australia, Victorian Division: Melbourne). Gray, D R and Willman, C E, 1991. Deformation in the Ballarat slate belt, central Victoria and implications for crustal structure across SE Australia, Australian Journal of Earth Sciences, 38:171–201. Gregory, L M, 1994. Contact metamorphism and quartz vein textures at Maldon, Victoria: Implications for the timing of gold mineralisation, BSc Honours thesis (unpublished), University of Ballarat, Ballarat. Harris, W J and Thomas, D E, 1934. The geological structure of the Lower Ordovician rocks of Eastern Talbot, Victoria, Proceedings of the Royal Society of Victoria, 46(2):153–178. Haupt, J C, 1982. The Minerals of the Maldon Goldfield, The Mineralogical Society of Victoria, Special Publication No 1. Hughes, M J, Phillips, G N and Gregory, L M, 1997. Mineralogical domains in the Victorian Gold Province, Maldon, and Carlin-style potential, in Proceedings 1997 AusIMM Annual Conference, pp 215–228 (The Australasian Institute of Mining and Metallurgy: Melbourne). Junner, N R, 1921. The geology of gold occurrences of Victoria, Australia, Economic Geology, 16(2):79–123. Mason, A A and Webb, B P, 1953. The Maldon Goldfield, in Geology of Australian Ore Deposits (Ed: A B Edwards), pp 1034–1041 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Moon, R A, 1897. The Maldon goldfield, Geological Survey of Victoria, Special Report, 1897. Morand, V J, 1994. Calc-silicate lenses in the Early Palaeozoic mud pile of the Lachlan Fold Belt, Australian Journal of Earth Sciences, 41:383–386. Morand, V J, 1996. The thin skinned tectonic model for the Palaeozoic of central Victoria, Australian Institute of Geoscientists Bulletin, 20:33–42. Morgan, B D and Woodland, J G, 1990. Union Hill gold deposit, Maldon, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1279–1280 (The Australasian Institute of Mining and Metallurgy: Melbourne). Passchier, C W and Simpson, C, 1986. Porphyroclast systems as kinematic indicators, Journal of Structural Geology, 8:831–843. Phillips, G N, Hughes, M J and Gregory, L, 1996. Victorian Gold: II: Large gold deposits and granites, Geological Society of Australia Abstracts, 41:204. Ramsay, J G, 1980. The crack–seal mechanism of rock deformation, Nature 284:135–139. Richards, J R and Singleton, O P, 1981. Palaeozoic Victoria Australia: Igneous rocks, ages and their interpretation, Journal of the Geological Society of Australia, 28:395–421. Sandiford, M and Keays, R R, 1986. Structural and tectonic constraints on the origin of gold deposits in the Ballarat slate belt, Victoria, Geological Association of Canada Special Paper, 32:15–24. Sibson, R H, 1981. Fluid flow accompanying faulting: field evidence and models, in Earthquake Prediction: An International Review (Eds: D W Simpson and P C Richards), American Geophysics Union Monograph, Maurice Ewing Series, 4: 593–603 Simpson, C, 1986. Determination of movement sense in mylonites, Journal of Geological Education, 34:246–261. Ulrich, G H F, 1869. Observations on the Nuggetty Reef, Mount Tarrangower Gold-Field, Quarterly Journal of the Geological Society of London, 25:326–335. Vearncombe, J R, 1993. Quartz vein morphology and implications for formation depth and classification of Archaean gold-vein deposits, Ore Geology Reviews, 8:407–424.
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Fredericksen, D C and Gane, M, 1998. Stawell gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 535–542 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Stawell gold deposits 1
2
by D C Fredericksen and M Gane INTRODUCTION
Stawell Gold Mines Pty Ltd operates underground decline mines at the Magdala and Wonga deposits, the two largest orebodies in the Stawell Goldfield. The centre of the deposits is at about lat 37o03′S, long 142o48′E on the Ballarat (SJ 54–8) 1:250 000 and the Ararat (7243) 1:100 000 scale map sheets. The two orebodies are within ML1219 which is along the eastern margin of the city of Stawell (Fig 1), 245 km NW of Melbourne. Production for the calendar year 1996 comprised 682 000 t of ore at 5.0 g/t gold from underground, which when supplemented with low grade surface and underground ores yielded 2724 kg of gold. At December 1996 Proved and Probable Ore Reserves were 3 775 000 t at 4.9 g/t gold with a further 2 416 000 t of Inferred Resource at 6.6 g/t gold. Production and contained gold in resources and reserves are in Table 1. Current mining methods consist of mechanised strike development, longhole open stoping and hand held machine mining. Ore widths vary significantly and lodes as narrow as 1.5 m are mined by longhole open stoping methods. Significant emphasis is placed on grade control through optimum design of stoping blocks. The shallow northerly plunge of the Magdala deposit makes detailed drilling of the orebody difficult to complete from standard development openings.
EXPLORATION AND MINING HISTORY Alluvial gold was discovered near Stawell in 1853 (Clappison, 1965) and an estimated 24 047 kg of gold was won from alluvial leads in the period 1853 to 1912. Production from the leads waned in the early 1860s and ceased in 1912 (Watchorn, 1986). Gold hosted within quartz reefs was discovered at Big Hill in 1855 (Watchorn, 1986) and early production came from the Sloanes Flat, Cross Vertical and Scotchmans Flats/Vertical reefs. As production from these systems declined in the early 1880s, ‘payable’ ore was discovered within shear zones to the east and further to the south in metamorphosed schists, which form the basis of the current production from the Magdala and Wonga mines respectively. Production ceased in 1926, mainly due to poor recovery, as much of the gold was associated with sulphide minerals. Watchorn (1986) estimated that the reef production had yielded 1.9 Moz (59 100 kg) of gold, including approximately 190 000 oz from tailings which were partly retreated to 1950. No significant production is recorded from the late 1920s to the 1980s, despite several attempts to regenerate interest in the goldfield. Gold Mines of Australia Ltd commenced exploration in 1944, drilling several diamond drill holes to test 1.
Chief Geologist, Stawell Gold Mines Pty Ltd, P.O Box 265, Stawell Vic 3380.
2.
Mine Exploration Geologist, Stawell Gold Mines Pty Ltd, P.O Box 265, Stawell Vic 3380.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Location of the Stawell Goldfield and Victorian geological zones, after Gray (1988) and Cayley and Taylor (1996).
the Magdala lodes, with mixed results. Utilisation of the old shafts to facilitate further exploration was planned but never proceeded, and exploration activities waned. In 1978, the Stawell Joint Venture was formed by Western Mining Corporation Limited (who had bought out Gold Mines of Australia) and Central Norseman Gold Corporation. This attempt to further explore the field led to the commencement of the Magdala decline in 1981.
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TABLE 1 Stawell Goldfield production, resources and reserves to December 1996. PRODUCTION Ore source
Gold produced (kg)
Period
Historical - alluvial - underground (reef)
24 047 59 100
(1853–1912) (1855–1926)
Recent (WMC–SGM) - underground - open cut
16 015 2 619
(1984–1996) (1984–1996)
TOTAL PRODUCTION
101 781
ORE RESERVES AND RESOURCES (end 1996) Ore source
Contained gold (kg)
Magdala - Proved and Probable Reserve - Inferred Resource
16 802 12 511
Wonga - Proved and Probable Reserve - Inferred Resource
1 561 3 375
TOTAL RESERVES AND RESOURCES TOTAL GOLDFIELDS
34 249 136 030
The Wonga mine was commenced in 1984, as an open cut on near-surface ore structures within the contact aureole of the Stawell Granite, approximately 2 km south of Magdala. A decline was commenced from the bottom of this pit in 1985 and both the Magdala and Wonga mines are currently active. The operations were sold in December 1992 to Mining Project Investors Gold Pty Ltd (MPI) in joint venture with Pittston Mineral Ventures of Australia Pty Ltd. A new company, Stawell Gold Mines Pty Ltd, was formed to manage the operations. Committed exploration in the ensuing years has led to the establishment of a substantial reserve and resource base for the Stawell field (Table 1). Since the carbon-in-leach mill was commissioned in 1984–85, 2619 kg of gold have been produced from open cut and 16 015 kg from underground sources, to the end of 1996.
REGIONAL GEOLOGY The Stawell Goldfield has been described as being within the Stawell zone of the Lachlan Fold Belt. Gray (1988), Gray, Wilson and Barton (1991), Wilson et al (1992) and Cayley and Taylor (1996) have discussed the position of the boundary between the Adelaide and younger Lachlan fold belts (Fig 1), and hence the relative geological position of the Stawell Goldfield. Greater intensities of deformation in the Stawell area and an older Late Proterozoic basaltic basement appear to give the Stawell area more of an affinity with the older Adelaide fold belt. However Cayley and Taylor (1996) suggest the Moyston Fault is the boundary between the Stawell zone and the Glenelg zone to the west, with the basement rocks being thrust into their current position. Stawell regional geology (Fig 2) is dominated by the large, zoned I-type Stawell Granite, dated at 396±5 Myr (Richards and Singleton, 1981). The granite has intruded strongly deformed Early Palaeozoic St Arnaud Group turbidites with encapsulated fault-bounded Cambrian volcanic rocks and volcanoclastic sediments. The Cambrian volcanic rocks
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comprise tholeiitic basalts which have been regionally metamorphosed and/or contact metamorphosed by the granite. They are overlain by submarine volcaniclastic sediment and have been thrust into the overlying Palaeozoic sediment along a number of NNW-trending, west dipping reverse faults. Mapping of the Cambrian volcanic rocks north of the Stawell Granite has been difficult owing to the increased depth of Tertiary cover and lack of significant relief. However drilling has located similar fault bounded packages of Cambrian volcanic rocks. The St Arnaud Group turbidites comprise a fault-duplicated and segmented package (Cayley and Taylor, 1996) of psammitic and pelitic sediment regionally metamorphosed and polydeformed. Cayley and Taylor (1996) noted an increasing metamorphic grade from east to west, with very high grade metamorphic rocks located in the hanging wall of the Moyston Fault. Drilling along the NNE-trending, west dipping Coongee fault has shown the sediments to the east to be only weakly metamorphosed and to contain only simple deformation structures (Fig 2). Apart from the Moyston Fault most other structures are truncated by the Stawell Granite. However the Coongee, Stawell and John Bull faults have a weak aeromagnetic signature which can be traced through the Stawell Granite. The interpreted northerly projection of the Mt Ararat Fault is likely to be the West Stawell lineament which, with the Coongee fault, form the lateral margins of the Stawell Goldfield. South of the granite, NE-trending faults have been mapped, which do not appear to be as prevalent to the north where significant WNW- and NW-trending faults offset the major structures. Swarms of Late Silurian felsic dykes and Jurassic lamprophyres intrude the sequence. To the north of the area shown in Fig 2 this sequence is overlain by Tertiary Murray Basin sediment.
LOCAL GEOLOGY MAGDALA DEPOSIT Stratigraphy The deposit is in a sequence of regionally metamorphosed (greenschist facies) turbidites and volcanogenic sediments which overlie the western margin of a structurally complex basalt sequence, the Magdala basalt. The unit (Fig 3) is Late Proterozoic, of tholeiitic character and comprises a number of flows. The central portion of the basalt is a north- to NNEtrending antiformal dome with the western margin, on which the Magdala orebody is located, showing an imbricate, SWover-NE thrust stack of basalt slices. Recent exploration drilling shows the eastern flank to have a similar character. The basalt slices form antiformal geometries and are locally referred to as basalt ‘noses’, which are thrust stacked over one another as a result of regional east–west crustal shortening, and vary from 10 to 50 m in width. Pillow structures are common, with epidote-, chlorite- and carbonate-rich margins. Occasional magnetite and small amounts of sediment occur within and around the pillow margins which exhibit significant tectonic flattening. In addition, glomeroporphyritic textures are common throughout the more massive parts of the sequence, with 3–4 mm felspathic phenocrysts in a fine to medium grained groundmass of chlorite, actinolite, epidote and plagioclase. The development of epidote, chlorite and
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STAWELL GOLD DEPOSITS
FIG 2 - Stawell–Ararat regional geological map. Position of the Pleasant Creek fault suggested by R A Cayley and D H Taylor (personal communication, 1997).
actinolite is evidence of regional metamorphism of the basalt package. Increasing amounts of magnetite can be observed within the eastern half to two-thirds of the package, resulting in a strong magnetic signature. The volcanogenic sediments which overlie the basalt (Fig 3) are best developed along the western flank of the antiform.
Geology of Australian and Papua New Guinean Mineral Deposits
Detailed mapping and core logging have defined a number of areas where the contact with the basalt is conformable, however shearing obliterates the nature of the contact in most areas. Those volcanogenic sequences trapped between two basalt noses are termed ‘Waterloo’ structures and tend to be quite narrow (<10 m wide) pinching down-dip where sediment has
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FIG 3 - Geological cross section through Magdala mine, looking NW.
The Magdala sequence also hosts quartz-feldspar porphyritic and lamprophyre dykes. The quartz-feldspar porphyries generally post-date the mineralisation and one date of 413±3 Myr was obtained from zircons within one quartz-feldspar porphyry intruded subparallel to S 3 crenulation within the Mine schist (D Arne, personal communication, 1997). A porphyry intruded along the hangingwall of Central lode on the 310 level has been dated from zircons at 420±37 Myr (P Stuart-Smith, personal communication, 1996). The lamprophyres vary between monchiquite and fourchite in composition, and were nominally given the date of 146–155±4 Myr, as a result of dating of other such dykes within Victoria (Tattam, 1976). Work in progress characterising Stawell intrusives has however shown that the lamprophyres intruded and cooled to less than 100oC by the early Permian (Gibson, 1997)
Mineralisation been removed during thrust stacking. Along the western flank of the antiform the thrusting has partially reduced the volcanogenic package thickness, which now varies from <10 to +50 m in thickness. It is within the areas of greatest volcanogenic thickness that the most favourable positions for ore development are located. As a result of diamond drilling, significant thicknesses of volcanogenic sediment have also been identified along the eastern flank of the antiform. The volcanogenic sediments contain varying proportions of chloritic, cherty, tuffaceous and carbonaceous sediment, with much of the sequence having undergone some degree of pre-ore silicification. This silicification appears to be most intense within sediments associated with the eastern flank of the antiform. Previous authors such as Watchorn (1986) and Mapani (1995) also report the presence of minor mafic flows. Narrow (<0.2 to 0.3 m) chert bands are common throughout the volcanogenic package and are generally isoclinally folded and boudinaged. Pyrrhotitic horizons, often with associated shearing and injected quartz, can vary in width from tens of millimetres to 2 m. These pyrrhotitic horizons are often subparallel to the contacts between the basalts and overlying volcanogenic sediments (dipping between 60 and 80 o to the SW), particularly where they are within 2–5 m of the contact. Those further into the hanging wall within the volcanogenic sediments vary to subvertical and have been known to strike obliquely from the basalt contact zone to the hanging wall. The carbonaceous sediments are generally most common within the vicinity of the crest of the antiform and in the hanging wall of the Scotchmans fault. Characteristically, these host fine grained disseminated euhedral pyrite which is speculated to be syngenetic with the sediment deposition. To the west and east of the antiform, and overlying the volcanogenics, are thick sequences of turbiditic sediments. Those west of the antiform are referred to as the Mine schist, and those to the east are referred to as the Eastern schist. Within the Mine schist, the sediments appear to grade from pelitic to more psammitic westwards and have a regional greenschist assemblage of quartz-actinolite-chlorite-muscovite-albite (Wilson et al, 1996). The sediments are known to contain three obvious ductile deformation events (D1–D3 resulting in F1–F2 folds and a fold crenulation S 3) that predate the mineralisation (Watchorn, 1986). The lack of obvious marker beds and disruption by later brittle events (D4 and D5) hamper detailed interpretation of this folding.
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Mineralisation within the Magdala mine is related to a regional tectonic event termed D4 by Watchorn (1986). The major structural event related to D4 other than the basalt anticlines is the development of the Central lodes shear system (striking at 330–340o and dipping 60–80o SW) which overprints the earlier D1–D3 events and predates the development of Scotchmans fault (D5) and emplacement of the quartz-feldspar porphyries. The central lode shear system is developed concordant with the contact between the Mine schist and volcanogenic sediment. It displays a complex system of steeply dipping thrusts which give rise to an array of faults and subsidiary faults, from well within the hanging wall mine schist through to the contact of the volcanogenic sediment with the basalt. Although the complete package of volcanogenic sediment is mineralised, economic mineralisation is mined from discrete shoots within the hanging wall quartz veins, Central lode shear, stockwork zones above the basalt noses and along basalt-volcanogenic sediment contacts.
Hanging wall quartz veins Current production from this historic source of ore is limited. They consist of massive and laminated quartz veins (Fig 3, Cross and Scotchmans lodes) with sheared contact margins, found almost exclusively within the Mine schist in the hanging wall of the Central lode shear. Shallowly north-plunging flat quartz lodes are formed in the footwall of both structures and form the loci for the majority of the gold mineralisation. Gold is found as native free gold within quartz and in association with an arsenopyrite-pyrite-chalcopyrite-sphalerite-galena assemblage.
Central lode shear The shear has been traced for up to 4 km along strike, and down dip for more than 1 km. It displays all of the complex structural development normally found within an idealised dextral strikeslip fault (Biddle and Christie-Blick, 1985). Mineralisation occurs as quartz veins, massive sulphides (pyrite) and quartzpyrite lodes within the shear system and can be related to structural positions and varying rock types within the volcanogenic sediment. The gold occurs free in quartz and in association with a pyrite-arsenopyrite-pyrrhotite-galenasphalerite mineral assemblage. Complex duplex structures and mylonites overprint several episodes of vein precipitation and mineralisation. Quartz augen are bounded by faults and lenses
Geology of Australian and Papua New Guinean Mineral Deposits
STAWELL GOLD DEPOSITS
of mylonitised quartz and sulphides. A strongly developed fault breccia with a fine clay gouge is common along strike and down dip and is recognised as being the footwall of the Central lode shear. Ore grade shoots to 100 m strike length are often developed and show a steep northerly plunge.
Stockworks Within the volcanogenic sediment package, zones of economic mineralisation exist in the structural embayments above the thrust-stacked basalt noses. Two flat northerly-dipping structures called the Nos 1 and 2 flats have previously been mined as narrow discrete orebodies. A system displaying similar flat structures has been discovered at depth and detailed mapping and closely spaced diamond drilling reveal a system of low-strain quartz gash veins, massive sulphide lenses, stringers and disseminated sulphides over widths to 30 m. Reinterpretation of the previously mined flats higher in the mine shows that significant stockwork mineralisation exists in the footwall and hanging wall of the flat structures. These structural traps are controlled by the position of the Central lode shear and the buttress formed by the basalt noses and exhibit complex riedel shear arrangements developed through continued reverse dextral shearing (Mapani, 1995). Contrasting rock competencies within the volcanogenic package have a significant effect on the development of brittle fracture zones. It would also appear that lithochemical variations between the carbonaceous and cherty chloritic sediments have a significant impact upon the local sulphide mineral assemblage deposited. Mineralisation is dominated by the development of low-strain quartz gash veins, massive sulphide lenses, stringers and zones of disseminated sulphides, from one to tens of millimetres thick. Gold is commonly associated with a pyrrhotite-arsenopyrite-pyrite mineral assemblage.
Basalt contact mineralisation The chloritic sediments immediately overlying the basalt contact host the fourth major style of mineralisation within the Magdala mine. Economic gold mineralisation is controlled by a narrow west-dipping shear zone 2–3 m wide, within the volcanogenic sediment but immediately overlying the basalt contact. A stockwork quartz-sulphide tension vein array has formed within this zone but is generally restricted to the volcanogenic sediment with mineralisation rarely extending into the basalt (Fig 4). Locally the shear consists of a narrow, 0.2–1.5 m wide, sheared quartz-pyrrhotite-pyrite-arsenopyrite vein which contains very high gold grades (to 30 g/t). This shear truncates and offsets the quartz-sulphide tension veins and has been shown through slickenside lineations to have a similar sense of movement to that of the Central lode shear. Native gold is often visible but is more commonly invisible and associated with the sulphides along grain boundaries and intergrown within the sulphides. Chalcopyrite is commonly found with pyrrhotite, and sphalerite and ilmenite are of microscopic size. The tension vein style of mineralisation is more extensive than the shear hosted veins and is controlled by two vein populations dipping 45o towards 115o and 20–50o towards 320o. Arsenopyrite commonly occurs as selvages of coarse euhedra along the margin of individual 10–250 mm thick quartz veins. Pyrrhotite is disseminated within the surrounding volcanogenic sediment, often replacing carbonate alteration spots. As with the shear hosted veins, gold is
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 4 - Geology of the basalt contact ore zones, 596 m RL, Magdala mine.
commonly visible within the tension veins and also within the sediment. The majority of the gold occurs intergrown with arsenopyrite and pyrrhotite.
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Within the Waterloo structures the tension vein style of mineralisation is predominant, and strongly developed carbonate-altered shears occur along the east and west margins of the basalt–volcanogenic sediment interface. Mineralisation associated with folded massive pyrite and pyrrhotite horizons to 1 m thick has been discovered associated with Waterloo structures. Gold grades are variable and range from 1 to 30 g/t. Tension vein development is variable, and when present shows similar styles to the basalt contact ore.
Other structures
Mineralisation One the mine scale, mineralisation at Wonga is controlled by two main fault systems (Fig 5). The hanging wall structure strikes at 350o and dips between 25 and 50o towards the east. A footwall structure mined in near-surface levels diminishes in significance at depth. Branching off the hanging wall structure is the link structure, which also has a range of dip angles. Orientations of this structure vary between 240 and 270o with individual ore grade shoots offset by a series of stepped faults with an orientation of 330o.
The entire Magdala sequence is truncated at depth by the South fault (Fig 3). This structure was considered by Watchorn (1986) to be a late D6 structure, separating the Magdala and Wonga packages. Investigations into the possible relationship to the mineralising events at Magdala and Wonga, and the timing of the offset along the South fault (ie post- or pre-ore) have shown the South fault has been a major conduit for major flow of fluids with similar alteration minerals to those within the mineralised zones of the mine (Morton, 1996). The sense of movement and offset along the South fault is unconfirmed and it is generally considered to be a reverse fault, with an offset in the vicinity of 500 to 2000 m. Another fault structure, the Upper South fault, has been identified by diamond drilling to have offset the Magdala ore structures by approximately 50 m. This structure has a reverse sense of movement and is approximately 50 m above the South fault (Fig 3).
WONGA DEPOSIT Stratigraphy The orebody is hosted by a sequence of regionally metamorphosed turbidites known as the Wonga schist. These metasediments have been overprinted by contact metamorphism as a result of the intrusion of the Stawell Granite, resulting in formation of biotite-corundum-spinelandalusite-garnet-cordierite schists (G Xu, unpublished data, 1994). Three phases of ductile deformation can be recognised within the schists, with the S2 foliation believed to coincide with the regional metamorphism. Bedding S0 can only be recognised by subtle composition variations from psammite to psammopelite. The true relationship of the Wonga schist to the Mine or Eastern schists at Magdala is essentially undetermined, although Watchorn (1986) suggested that the Wonga schist overlies the Mine schist. Diamond drilling in 1995 has shown that several areas of Wonga schist in the footwall of the South Fault are relatively unaffected by contact metamorphism, and are very similar in appearance to the chloritic psammopelite of the Mine schist in the hanging wall of the Cross Vertical structure (Fig 3). The Wonga schist is intruded by quartz-feldspar porphyry dykes, one of which has been dated using zircons at 423±12 Myr (P Stuart-Smith, personal communications, 1996), and by the Stawell Granite. The Stawell Granite has been dated at 396±5 Myr (Richards and Singleton, 1981), or more recently 399±3 Myr (P Stuart-Smith, personal communication, 1996). The pluton is a multiphase I-type granitoid and is mostly leucogranite and granodiorite, with minor diorite (Promnitz, 1986). Xenoliths of Wonga schist are common throughout the margin of the intrusion.
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FIG 5 - Ore geometry, plan projection, Wonga mine.
This style of mineralisation is dominated by buck and laminated quartz vein and quartz breccia, or combinations of the two, with up to 50% inclusions of country rock xenoliths. In places the quartz veining may be absent but ore grade mineralisation is still present. The ore zone contains pyrrhotite, arsenopyrite, pyrite, chalcopyrite, rutile and ilmenite. The highest ore grades are associated with quartz and quartz breccia veins with strongly developed selvages of fine needle-like arsenopyrite grains. Gold occurs as very fine free grains ( <10 µm diameter) and/or intergrowths within arsenopyrite. The original mineralisation and alteration assemblage has undergone contact metamorphism and a number of associations with the host contact metamorphic assemblages have been recognised. The main ore structures are truncated and intersected by a number of local but continuous faults. One set trending NW and dipping steeply to the NE clearly post-dates the main lode development and shows strong carbonate-sericite alteration which developed after the main mineralisation event. Locally this has caused remobilisation of the gold and weak gold grades are found in some structures. The Wonga mineralisation and some of the porhyries (G Xu, unpublished data, 1994) have been subjected to contact metamorphic effects and pre-date at least the final stages of the granite intrusion. Watchorn (1986) noted several porphyries which post-date the intrusion and crystallisation of the granitoid. As the South fault appears to truncate the contact aureole of the Stawell Granite, the activitation of the fault postdates the mineralising events at Wonga.
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TABLE 2 Timing of the major structural events for the Magdala and Wonga orebodies, modified after Watchorn and Wilson (1989). Pre-Early Permian
Lamprophyre dykes (pre 300 Myr)
Late D6 structures
Early Devonian
Stawell Granite ( 396±5 Myr)
Mineralisation at Wonga, crosscutting felsic intrusives
Late Silurian
Felsic intrusives (413±3 Myr and 420±37 Myr)
Development of D4 brittle structures which successively overprint mineralisation in quartz veins and shear zones. Intruded by felsic intrusives
Cambro-Ordovician
St Arnaud Group
Succesive D1–D3 deformation of Mine schist, Wonga schist and Eastern schist
Late Proterozoic to Cambrian
Tholeitic basalts (700±30 Myr)
Volcanogenic sediments, Magdala basalt
Scotchmans fault, South fault - intruded by felsic intrusives ? D5
ORE GENESIS Timing of the gold events at the Magdala and Wonga deposits has clearly been constrained by early ductile deformation events and later emplacement of intrusives. The Magdala mineralisation pre-dates the emplacement of porphyry dykes which have been intruded along brittle D4 structures. The Wonga mineralisation post-dates the emplacement of similar felsic intrusives, which have ages very close to those from the Magdala deposit but pre-dates the final emplacement of the Stawell Granite. Table 2 shows the current understanding of the timing relationships for the orebodies. The development of alteration haloes of stilpnomelane in the Magdala deposit provides evidence that the ore bearing fluids have been out of equilibrium with the the host rock sequences (Mapani, 1995). A deep crustal origin for the fluids is favoured, as indicated by the presence of platinum group elements with gold in pyrrhotite and the gold-pyrite-arsenopyrite-pyrrhotite association, both considered to be indicative of a mantle origin (Wilson et al, 1996) Fluid inclusions show a range of salinities indicating the influence of different fluids during the deformation events. The range of oxygen isotope values for the quartz lodes supports other evidence that the fluid source for the orebodies is externally derived and that the Magdala orebody, at least, is a mesothermal type. Overprinting due to contact metamorphic events has made similar studies of the Wonga orebody difficult, although observations would support the emplacement of the granite as being the major factor in the mineralising event for the Wonga orebody.
ACKNOWLEDGEMENTS The authors acknowledge the permission of Stawell Gold Mines Pty Ltd to publish this information. This work has summarised the knowledge of a large number of geologists who have contributed to the increasing knowledge of a very complex and challenging orebody. Special thanks to the drafting personnel of Stawell Gold Mines whose efforts in capturing ideas and mapping have made collating this information significantly easier. To the staff of Melbourne University Geology Department, particularly C J L Wilson, a big thank you in providing high quality research students and support for continued work at Stawell.
REFERENCES
Cayley, R A and Taylor, D H, 1996. Geological evolution and economic potential of theGrampians Area, Victoria, in Recent Developments in Victorian Geology and Mineralisation, Australian Institute of Geoscientists Bulletin, 20:11–18. Clappison, D J, 1965. A study of the petrology, mineralogy and geochemistry of the Stawell goldfield, Victoria, MSc thesis (unpublished), University of Melbourne, Melbourne. Gibson, H J, 1997. Apatite fission track analysis of four samples from the Stawell mine, Victoria, A short report for School of Earth Science, University of Melbourne, Geotrak International. Gray, D R, 1988. Structure and tectonics, in Geology of Victoria (Eds: J G Douglas and J A Ferguson), pp 1–36 (Geological Society of Australia, Victorian Division:Melbourne). Gray, D R, Wilson, C J L and Barton, T J, 1991. Intracrustal detachments and implications for crustal evolution within the Lachlan Fold Belt, Southeastern Australia, Geology, 19:574–577. Mapani, B S E, 1995. Structural evolution and mineralisation at the Magdala gold mine, Stawell, Western Victoria, PhD thesis (unpublished), University of Melbourne, Melbourne. Morton, S, 1996. The South fault, Magdala mine, Stawell, BSc Honours thesis (unpublished), University of Melbourne, Melbourne. Promnitz, S C, 1986. Geological setting and controls on gold mineralisation, Stawell region, western Victoria, BSc Honours thesis (unpublished), Monash University, Melbourne. Richards, J R and Singleton, O P, 1981. Palaeozoic Victoria Australia: Igneous rocks, ages and their interpretation, Journal of the Geological Society of Australia, 28:395–421. Tattam, C M, 1976. Basic dykes of central Victoria and South Gippsland, Geological Society of Australia Special Publication 5:37–39 Watchorn, R, 1986. Deformation in the Stawell Goldfield and its relationship to gold mineralisation, MSc thesis (unpublished), University of Melbourne, Melbourne. Watchorn, R B and Wilson, C J L, 1989 Structural setting of the gold mineralisation at Stawell, Victoria, Australia, in The Geology of Gold Deposits: the Perspective in 1988 (Eds: R R Keays, W R H Ramsay and D I Groves), Economic Geology Monograph 6:292–309. Wilson, C J L, Mapani, B S E and Geological Staff of Stawell Goldmines, 1996. Structural evidence for vein formation and nature of mineralised fluids: Examples from Stawell, Victoria, in Recent Developments in Victorian Geology and Mineralisation, Australian Institute of Geoscientists Bulletin 20:55–61. Wilson, C J L, Will, T M, Cayley, R A and Chen, S, 1992. Geologic framework and tectonic evolution in Western Victoria, Australia, Tectonophysics, 214:93–127.
Biddle, K T and Christie-Blick, N, 1985. Glossary - Strike-Slip Deformation, Basin Formation, and Sedimentation, SEPM Special Publication 37:375–386.
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Taylor, D H, 1998. Ballarat gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 543–548 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Ballarat gold deposits by D H Taylor
1
INTRODUCTION The centre of the deposits is approximately 120 km NW of Melbourne, Vic, at lat 37o34′S, long 143o52′E or AMG coordinates 753 300 E, 5 837 150 N on the Ballarat (SJ 54–8) 1:250 000 scale and Ballarat (7622) 1:100 000 scale map sheets (Fig 1a). The deposits are the second largest group in Victoria after Bendigo, and are similar to many of the other deposits within the Ballarat gold province. At Ballarat there are three principal, strike-parallel goldfields within an area approximately 5 km wide by 14 km long that contain mesothermal quartz veins in Lower Ordovician quartz-rich turbidites. From west to east these goldfields are: Ballarat West goldfield (BWG), Ballarat East goldfield (BEG) and Little Bendigo or Nerrena goldfield (LBG). From about 1856 until 1918 the reefs yielded approximately 65 t of gold at grades of about 2–15 g/t. Much of the area now lies beneath the City of Ballarat but renewed exploration by several companies in recent years has produced encouraging results.
EXPLORATION AND MINING HISTORY The original gold discoveries in the area were worked in extremely rich and nuggetty, shallow alluvial diggings (1850–1855). These gave way to mining of deeper alluvial deposits or ‘deep leads’ in buried palaeodrainages from 1855 to 1900, which was locally followed by hard rock mining (1856–1918) as the alluvial gold was depleted and the hard rock reef sources were located. In hard rock mines gold was usually selectively mined from high grade portions of individual quartz reefs by underground or open cut mining for average returns of about 5 to 30 g/t. The modern revival of the goldfields was started by Ballarat Goldfields NL, who explored the depth extent of the BEG in the 1980s and reported a ‘drill inferred resource’ of 400 000 t at 9 g/t gold (D'Auvergne, 1990). This company now has an Inferred Resource of 3.3 Mt at 9.5 g/t gold and is attempting to access these resources below the historic gold workings by a decline (Ballarat Goldfields, 1995). Exploration in the late 1980s by CRA Exploration Pty Ltd (CRAE) to the south of the Ballarat Goldfields site was continued in the 1990s by Valdora NL. William Australia NL conducted a pre-production open cut mining program at this site in 1996 based on ‘a proven resource’ of 306 000 t at 1.03 g/t gold and ‘a probable resource’ of 4 436 000 t at 5.05 g/t gold (William Resources Inc, 1996). Mining commenced and finished in 1997 with 400 000 t of ore mined at a strip ratio of 2.84 and a grade of 0.91 g/t to produce 364 kg of gold. Kinglake Resources Pty Ltd has a small open cut operation at an alluvial 1.
Geologist, Geological Survey of Victoria, PO Box 2145 MDC, Fitzroy Vic 3065.
Geology of Australian and Papua New Guinean Mineral Deposits
deposit south of the William Australia site, and Ballarat Consolidated Ltd has been exploring the BWG by deep drilling through basalt cover since 1992, but yet to define any resource. The modern exploration focus is on either bulk mining of reef systems in the weathered zone or underground mining of the high grade reef portions around and below the old workings. At best, only Inferred Resources have been defined by the modern exploration because of low drilling densities and the patchy and nuggetty distribution of the gold, hence the need for underground exploration to upgrade the status prior to the estimation of reserves.
PREVIOUS DESCRIPTIONS The authoritative report on the goldfields by Baragwanath (1923) is a valuable primary document which includes detailed plans and sections showing the structure of now inaccessible mines. Other detailed Geological Survey of Victoria reports produced when the mines were accessible (Murray, 1874; Lidgey, 1894; Allan, 1897; Bradford, 1904; Gregory, 1907) are complemented by later summaries, re-interpretations and records of limited investigations (Kenny, 1949; Baragwanath, 1953; Whiting, 1962; McKinstry, 1969; Patterson, 1974; Ransom and Hunt, 1984; D’Auvergne, 1990; and Finlay and Douglas, 1992). New mapping across the goldfields by the Geological Survey of Victoria (Taylor et al, 1996a, b) combined with data from ongoing company exploration and from academic research (Ramsay et al, in press; Jombwe, 1991: Forde and Ball, 1994) provide new insights into the geology of the deposits.
REGIONAL GEOLOGY STRATIGRAPHY The bedrock hosting the goldfields is the Lower Ordovician Castlemaine Supergroup, which has yielded sparse phyllocarid and sponge spicule remains in the vicinity of the mines and Lancefieldian and Bendigonian graptolites further to the east. At Ballarat the supergroup is probably several kilometres thick, and consists of interbedded greywacke, mudstone and minor black shale. It contains turbiditic quartz-rich greywacke beds to several metres thick. Younging is often indicated by scours and flame structures at the base of beds with some grading towards the top where there may be cross laminations. There are intercalated intervals of mudstone and rarer black shale to many metres thick. The depositional setting is interpreted as a deep water submarine fan. The rocks were metamorphosed to chlorite grade greenschist facies during regional deformation and there is a narrow biotite-cordierite contact aureole around the post-tectonic Mount Egerton Granodiorite. The Late Devonian Mount Egerton Granodiorite intrudes the deformed bedrock just east of the goldfields. Several felsic dykes of the same age occur in the mines and in places intrude
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FIG 1 - a. Locality diagram; b. Simplified geological map showing regional structures and distribution of the Ballarat goldfields; c. Cross section through the goldfields looking north, from Taylor et al (1996).
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BALLARAT GOLD DEPOSITS
along reefs. Lamprophyre dykes, possibly of Jurassic age, also intrude the bedrock. Basaltic dykes striking east rather than north like the other dykes may be related to widespread Quaternary volcanism. Occasionally the bedrock is overlain by Tertiary fluvial deposits containing placer gold, and historical production from these exceeds that from the primary hard rock source. There are also Quaternary Newer Volcanics basaltic lava flows which blanket the BWG (Fig 1b).
STRUCTURAL SETTING The goldfields lie within the Bendigo–Ballarat (structural) zone that was regionally deformed in the Silurian. Structure is dominated by a single generation of upright chevron folds. Mesoscopic folds on the scale of tens to hundreds of metres are obvious in the mines. The enveloping surface to these folds defines three regional scale anticlinoria across the goldfields which have an amplitude of about 2 km and a wavelength of about 4 km (Fig 1c). The eastern limb and hinge zone of the Albion Anticlinorium is exposed within the old workings of the BWG. The new mapping confirms the original interpretation (Baragwanath, 1923) of the Ballarat Anticlinorium just west of the BEG. Temporary exposures mapped during construction of the Ballarat freeway extension (Taylor et al, 1996b) showed that the westerly inclined Monte Christo Anticlinorium with tight to isoclinal fold limbs hosts the LBG. The tightness of the folds across the LBG and their strong easterly vergence suggests that the major Williamson Creek Fault (mapped further to the south) should lie just to the east, in the position now occupied by the Mount Egerton Granodiorite. The regional form surface appears to be relatively flat (Fig 1c), but unlike many other central Victorian goldfields there are no biostratigraphic data to confirm this. With the given form surface the LBG and BEG roughly occur at the same stratigraphic level. The BWG appears to occur at a much higher stratigraphic level, but the form surface data between the goldfields are rather sparse and an unknown number of the predominantly west-dipping reverse faults may have raised the level of the BWG, so that the possibility of it being at the same level as the other goldfields cannot be excluded. The mesoscopic folds that define the regional folds tend to be upright and symmetrical but may be inclined up to 30o so that one limb becomes subvertical or overturned, such as the folds hosting the BEG and across the LBG. Fold axes plunge either north or south at gentle to moderate angles, and the sense of plunge changes along strike on a scale of hundreds of metres, giving a dome and basin geometry to the regional folds. Mesoscopic folds may persist for many hundreds of metres, but they often grow from or merge into zones of monoclinal flexure over short vertical and horizontal distances, as shown by successive mine cross sections of the BWG (Baragwanath, 1923). Faults disrupt the folds and host the mineralised quartz reefs. The predominant faults are moderately to steeply west-dipping reverse faults with throws of tens of metres. The faults generally strike subparallel to bedding on the west dipping limbs but as they cross the fold hinges into east dipping limbs they become a more shallowly west dipping anastomosing fault system, known as ‘leather jacket’ faults in the BEG, until they reach the next fold hinge. This leather jacket geometry is analogous to the accessible and well studied fault system in the Wattle Gully mine of central Victoria (Cox et al, 1996).
Geology of Australian and Papua New Guinean Mineral Deposits
Late stage, unmineralised crosscourse faults are widespread and offset the folds and reefs. A dominant NE-striking set have dextral displacements of metres to tens of metres and possibly up to hundreds of metres in the BEG (Lidgey, 1894). Displacement on a sinistral NW set is generally much less, so that overall the folds and reefs are offset in a step wise fashion to the NE.
ORE DEPOSIT CHARACTERISTICS COMMON FEATURES The various reef types all occupy sites of dilatancy within faults that developed late in the deformation history, when brittle failure of the tight folds occurred. Where the reefs occur in fault segments parallel to bedding they tend to be thin and laminated structures. Where the reefs occur in the fault segments that crosscut bedding they tend to be large spurry pods of quartz veining called leather jacket reefs (see Gregory, 1907 for photographs). Large subvertical quartz reefs also occur along anticlinal axial planes in the BEG. The reef morphologies are indicative of episodic growth during multiple failure of the fault systems by fluid overpressuring. The gold is generally coarse grained, nuggetty and patchily distributed in the reefs so that exploratory drilling cannot readily define reserves. Most of the gold is free milling and was historically recovered by amalgamation after crushing and gravity separation. The gold is commonly associated with minor amounts of sulphides such as pyrite, arsenopyrite, galena, sphalerite, pyrrhotite, chalcopyrite, stibnite and marcasite (Lidgey, 1894; Baragwanath, 1923; Fuller, 1995; Besanko, 1996). Wall rock alteration consists of a weak halo of silica, carbonate, chlorite and sericite which imparts a bleached appearance around the mineralised reefs (Jombwe, 1991; Fuller, 1995; Besanko, 1996). Enriched grades and nuggetty gold were often reported where the reefs intersected indicators for which the BEG is noted (Lidgey, 1894; Gregory, 1907; Baragwanath 1923; Patterson, 1974). The exact nature of the indicators is unresolved (see Ransom and Hunt, 1984 and Finlay and Douglas, 1992 for discussions) but they are clearly stratabound within thin bedded mudstones, being either carbonaceous and pyritic beds or shear zones.
BALLARAT EAST GOLDFIELD This goldfield was the major producer and is the focus of most of the present day activity. About 47 t of gold was produced at an average grade of about 10 g/t (Baragwanath, 1923; Finlay and Douglas, 1992). Mining extended along a zone about 500 m wide and 14 km long from Black Hill in Ballarat southwards to some large mines near Buninyong. From about 1860 until 1915 the goldfield was worked by about 25 major companies to depths less than 550 m (Baragwanath, 1923; Finlay and Douglas, 1992). Poor cooperation between mining companies, an emphasis on mining the known reefs where they were enriched by indicators and on paying dividends at the expense of exploration have left potential for extensions of the ore at depth and laterally (Ransom and Hunt, 1984). Many of the mineralised structures are blind targets and much of the original confidence to develop at depth came from encouraging intersections in two deep government bores drilled in 1887 (Baragwanath, 1923).
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D H TAYLOR
The goldfield is situated principally on the eastern, subvertical limb of the mesoscopic First Chance Anticline (Fig 2). This and the adjacent folds are asymmetric, west-verging parasitic folds close to the hinge of the Ballarat Anticlinorium whose axis lies just to the west. Most of the production came from moderately west-dipping leather jacket reefs stacked 50 to 100 m apart as they cross the subvertical eastern limb of the First Chance Anticline (Lidgey, 1894; Gregory, 1907; Baragwanath, 1923, 1953).
Modern exploration by Ballarat Goldfields across the northern and central part of the goldfield below the city has found depth and lateral extensions of both leather jacket and breached anticline reefs (D'Auvergne, 1990). In the late 1980s and early 1990s diamond drilling of 12 fanned arrays, some in joint venture with North Broken Hill Peko, totalling 19 000 m, delineated an Inferred Resource of 3.3 Mt at a grade of 9.5 g/t gold in the 350–750 m deep zone below the old workings. The company is attempting to access this resource by a decline from the south which will allow mining of small parcels of ore left in the old workings, and these are being tested by crosscutting and exploratory drilling from the decline and from the surface. Immediately south of the Ballarat Goldfields operation, exploration in the late 1980s and early 1990s by CRAE and then Valdora included bulk sampling of 9051 t of weathered surface material which averaged 1.03 g/t. William Australia continued this exploration with a pre-production bulk sampling program in conjunction with resource delineation drilling to determine the viability of large scale open cut mining of the weathered zone across the top of the goldfield. Mining commenced and finished in 1997 with a 525 000 tpa plant processing 400 000 t of ore at a strip ratio of 2.84 and a head grade of 0.91 g/t to produce 364 kg of gold.
FIG 2 - Cross section looking north through the historic workings of the BEG where reefs are well developed in the eastern subvertical limbs of parasitic folds just east of the hinge of the Ballarat Anticlinorium, from Baragwanath (1923, 1953). Drilling by Ballarat Goldfields has discovered more of these reefs below the level of the old workings.
The reefs are lenticular quartz bodies to 25 m thick and hundreds of metres long and often have horizontal tension gashes extending outwards for another 30 m (Baragwanath, 1953; McKinstry, 1969). The reefs barely extend across into the adjacent west dipping limbs of the folds where the hosting faults become bedding parallel. Large subvertical spurry quartz reefs also occur along some of the anticlinal axial planes and contributed significantly to production, eg Robert's lode in the Llanberris 1 mine (Lidgey, 1894). They are called ‘breached anticline reefs’ (Ransom and Hunt, 1984) and appear to be fault complications associated with the leather jacket faults as they traverse the anticlinal hinges. The Sulieman lode is another leather jacket reef in the Western Anticline immediately to the west of the First Chance Anticline, and was less intensively worked in mines such as the North First Chance (Baragwanath, 1923). Where the leather jackets continue upwards into the Eastern anticline, to the east of the First Chance Anticline, only minor exploratory work was carried out in the northernmost mines, such as the Britannia United (Baragwanath, 1923; Ransom and Hunt, 1984). The goldfield may be regionally constrained within a dome on the Ballarat Anticlinorium. Mesoscopic fold hinges plunge moderately northwards in the north, fluctuate subhorizontally along much of the middle of the field (Lidgey, 1894; Baragwanath, 1923) and probably plunge gently south in the south, as suggested by sparse bedding–cleavage approximations of hinge orientation. The structure around Buninyong is poorly known, but a mine plan (Kenny, 1938) and recent exploratory drilling by Kinglake Resources show mesoscopic folds breached by multiple west-dipping reefs similar to the rest of the goldfield.
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South of the William Australia operation, near where the goldfield is covered by basalt, Kinglake Resources has been mining auriferous gravel overlying auriferous reefs in the bedrock since 1992. These reefs were first explored by Aloren NL in 1988 with five diamond drill holes totalling 1547 m, followed in 1992 by 40 shallow aircore holes totalling 2700 m by Kinglake Resources. No resources were delineated but the encouraging results may be followed by shallow open cut mining of the reefs once the gravel has been removed and further exploration undertaken.
BALLARAT WEST GOLDFIELD The goldfield is buried under multiple flows of Quaternary basaltic lava with an aggregate thickness to 150 m. The field was discovered when reefs were intersected during mining the deep leads, and produced about 26 t of gold at a grade of about 15 g/t (Baragwanath, 1923; Finlay and Douglas, 1992). The mines worked three discrete mineralised faults along a zone about 1 km wide and 7 km long. From about 1862 to 1918 the goldfield was worked by about 15 major companies to a maximum depth of 1050 m, the deepest in the Ballarat area. Mine cross sections (Allan, 1897; Baragwanath, 1923) show that the three separate reef systems are largely developed on the western limbs of three west-vergent anticlines (Fig 3). These folds are variably developed along strike, both in amplitude and plunge. Their form surface defines the eastern limb to the hinge zone of the Albion Anticlinorium. The three lodes are thin laminated reefs from 1 to 5 m wide that are mainly confined to and subparallel to bedding on the west dipping limbs. Descriptions suggest that the reefs are crack-seal veins developed in bedding-parallel fault zones, perhaps associated with the same stratigraphic horizon of black shale (Baragwanath, 1923). Drilling by Ballarat Consolidated confirms earlier descriptions (Allan, 1897; Baragwanath, 1923) that the fault zones also extend across the hinges and into the east dipping limbs where some spurry quartz bodies to 25 m thick occur in a similar fashion to the BEG leather jacket reefs. The goldfield is not closely associated with the hinge zone of the Albion Anticlinorium but spread across the folds in the
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The most easterly reef, the Monte Christo, occurs in the hinge zone of the anticlinorium. It is interpreted from diagrams (Whitelaw, 1901; Bradford, 1904) to be a stacked array of quartz-filled horizontal tension gashes up to 1 m thick within a reverse, dip-slip fault zone up to 25 m wide that is subparallel to the steeply west-dipping fold limbs. Up to six west-dipping reefs also occur in the western limb of the anticlinorium subparallel to bedding (Whitelaw, 1901; Bradford, 1904). Diagrams of the Dimocks reefs show them to be an array of flatlying tension gashes similar to the Monte Christo reef in places, and elsewhere to be an array of thin, subvertical quartz stringers lying parallel to the cleavage of a 30 m thick mudstone package (Bradford, 1904; Baragwanath, 1923).
FIG 3 - Cross section looking north through the historic workings of the BWG where mining concentrated on the laminated bedding parallel reefs, from Baragwanath (1923). Drilling by Ballarat Consolidated has shown that the leather jacket style extension of the Guiding Star lode continues across to the Albion Anticlinorium.
eastern limb. The richest mineralisation was reported to occur in the vicinity of a domal culmination of the folds near the Band and Albion 10 mine (Baragwanath, 1923). Modern exploration by Ballarat Consolidated commenced in 1993 with diamond drilling of six major fanned arrays totalling about 10 000 m which tested a leather jacket structure at 400 to 700 m depth, largely below the old workings. West of the BWG a further three diamond drill holes totalling 1000 m intersected a quartz reef within altered black shale that had been recorded in the Winters Freehold alluvial mine. Ballarat Consolidated has also undertaken drilling through the basalt to delineate possible extensions of the deep lead palaeodrainages.
LITTLE BENDIGO GOLDFIELD This is the smallest and least documented goldfield at Ballarat. Recorded gold production was about 2 t, excluding production from the Temperance reef, at grades between about 3 and 15 g/t (Bradford, 1904; Finlay and Douglas, 1992). Mining of multiple reefs extended northwards from the Yarrowee River near Nerrena along a zone about 300 m wide and up to 3 km long. From about 1856 to 1900 the goldfield was worked by about 10 major mines to depths less than 250 m (Whitelaw, 1901; Bradford, 1904; Baragwanath, 1923). This goldfield has received little modern exploration as the companies holding the ground are currently heavily committed to the BEG. The structure of the goldfield was previously described as simple west-dipping beds (Baragwanath, 1923). However, mapping of road cuttings temporarily exposed during construction of the Ballarat freeway extension showed that the reefs occur in the west limb and hinge zone of the Monte Christo Anticlinorium (Taylor et al, 1996b). This anticlinorium is inclined to the east, with tight to isoclinal parasitic folds whose overturned eastern limbs dip west, subparallel to the western limbs. These features are classic evidence (Gray and Willman, 1991) of the anticlinorium being in the immediate hanging wall of a major fault, in this case the northward projection of the Williamson Creek Fault which is a major west-dipping reverse fault with a throw of many hundreds of metres.
Geology of Australian and Papua New Guinean Mineral Deposits
The Temperance reef has previously been included in the LBG, however it is about 500 m to the west of Dimocks reefs and different in character. The Temperance reef is a laminated reef to 1 m thick that dips west, subparallel to some black shale, before cutting across a small syncline. Worked to depths of less than 230 m over a length of about 700 m in the 1860s, it produced over 1 t of gold at a grade of about 12 g/t (Bradford, 1904). It is best grouped with the nearby and poorly documented Yorkshire and Nil Desperandum reefs. All of these reefs occur close to the hinge zone of the synclinorium between the BEG and the LBG.
ORE GENESIS On the largest scale the Ballarat gold deposits occur in the hanging wall of a major fault. The best mineralisation often occurs in domal culminations of regional and mesoscopic anticlines. Thus the primary control on mineralisation appears to be domal culminations on anticlines (structural highs) some distance in the hanging wall above a large fault. The reefs occupy sites of dilatancy in various fault systems that developed late in the regional deformation when buckling and fold flattening could no longer accommodate continued shortening. The model for regional gold distribution at Bendigo (Willman and Wilkinson, 1992) seems applicable to Ballarat, with fluids introduced into structural highs within the hanging wall of a major fault. Gold was then precipitated when faultrelated openings developed during fluid overpressuring, and the major fault possibly acted as a pathway for mineralising fluids generated at deeper crustal levels. However the local controls of the patchy distribution of the gold within individual reefs are unknown. The source and transport mechanisms for the mineralisation have yet to be resolved. Summaries of abundant recent investigations of isotopes and fluid inclusions across west–central Victoria (Ramsay et al, in press) accord with early suggestions of a gold source outside the Castlemaine Supergroup, such as the underlying metavolcanic rocks (Gulson et al, 1988) or from even deeper levels (Sandiford and Keays, 1986), the gold being transported upwards by metamorphic fluids (Cox et al, 1983; Golding and Wilson, 1988). The fluids were of C-O-H composition, low salinity and trapped at temperatures of about 300oC (Ramsay et al, in press). Hydrothermally altered Late Devonian granitic dykes in the BWG that are probably associated with the Mount Egerton Granodiorite have elevated gold values and a similar alteration style to the reefs (Fuller, 1995). This suggests that a weak pulse of mineralisation accompanied the granite intrusion, perhaps with gold remobilised from the earlier syn-deformational reefs.
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There is no obvious stratigraphic control of mineralisation, and the reefs are structurally controlled within large fault systems that cut across the folds. Although the historic mines predominantly worked reefs in different parts of the fault systems, modern exploration and research are showing persistence of the reef types across the goldfields and at depth.
ACKNOWLEDGEMENTS This article is published with the permission of the general manager of the Geological Survey of Victoria and the manuscript was reviewed by P J O’Shea and R A Cayley. P B D’Auvergne of Ballarat Goldfields, G Rabone and L Dickinson of William Australia, L Austin of Kinglake Resources and L V Gentle and L Walker of Ballarat Consolidated are thanked for helpful discussions and information on the various exploration programs.
REFERENCES
Golding, S D and Wilson, A F, 1988. Stable isotope constraints on fluid sources for granitoid and metamorphic-hosted gold-quartz vein deposits in eastern Australia, Geological Society of Australia Abstracts, 23:495–499. Gray, D R and Willman, C E, 1991. Deformation in the Ballarat slate belt, central Victoria and implications for the crustal structure across southeast Australia, Australian Journal of Earth Sciences, 38:171–201. Gregory, J W, 1907. The Ballarat East goldfield, Geological Survey of Victoria Memoir 4. Gulson, B L, Andrew, S A, Mizon, K J, Keays, R R and Stuwe, K, 1988. Source of gold in Ballarat slate belt deposits and potential exploration applications, Geological Society of Australia Abstracts, 22:331–337. Jombwe, N F, 1991. Geochemical patterns in near surface rocks around auriferous quartz veins at Ballarat, Victoria, PhD thesis (unpublished) University of New South Wales, Sydney. Kenny, J P L, 1938. Buninyong Rand mine, Geological Survey of Victoria plan and section 2311/A/1 (unpublished). Kenny, J P L, 1949. The Ballarat South goldfields, Mining and Geological Journal, 3(5):26–28.
Allan, R, 1897. Report in connection with the underground plans of Ballarat West mines, Mines Department of Victoria Special Report.
Lidgey, E, 1894. Report on the Ballarat East goldfield, Mines Department of Victoria Special Report.
Ballarat Goldfields, 1995. Annual report to shareholders (Ballarat Goldfields NL: Melbourne).
McKinstry, H E, 1969. Ballarat, Victoria, Australia, in Ore Deposits as Related to Structural Features (Ed: W H Newhouse), pp 162–163 (Hafner: New York).
Baragwanath, W, 1923. The Ballarat goldfield, Geological Survey of Victoria Memoir 14. Baragwanath, W, 1953. The Ballarat goldfield, in Geology of Australian Ore Deposits (Ed: A B Edwards), pp 986–1002 (The Australasian Institute of Mining and Metallurgy: Melbourne). Besanko, J L, 1996. Characterisation of the ore fluids of the Ballarat East goldfields, BSc Honours thesis (unpublished), University of Ballarat, Ballarat. Bradford, W, 1904. Nerrena or Little Bendigo goldfield, Geological Survey of Victoria Bulletin 15. Cox, S F, Sun S S, Etheridge, M A, Wall, V J and Potter, T F, 1996. Structural and geochemical controls on the development of turbidite-hosted gold quartz vein deposits, Wattle Gully mine, central Victoria, Australia, Economic Geology, 90:1722–1746. Cox, S F, Wall, V J, Etheridge, M A, Sun, S S and Potter, T F, 1983. Gold-quartz mineralisation in slate belts: the CastlemaineChewton example, Geological Society of Australia Abstracts, 9:260–261.
Murray, R A F, 1874. Geology and mineral resources of Ballarat, Geological Survey of Victoria Progress Report, 1:63–88. Patterson, G W, 1974. The geology and geochemistry of some indicator beds in the Bendigo-Ballarat sub-province Victoria, Geological Survey of Victoria Report, 1974/12 (unpublished). Ramsay, W R H, Beirlein, F P and Arne, D C (Eds), in press. Mesothermal gold deposits - a global overview, Ore Geology Reviews. Ransom, D M and Hunt, F L, 1984. A reinterpretation of the Ballarat East goldfield, Geological Society of Australia Abstracts, 12:450–451. Sandiford, M and Keays, R R, 1986. Structural and tectonic constraints on the origin of gold in the Ballarat slate belt, Geological Society of Canada Special Publication, 32:15–24. Taylor, D H, Whitehead, M L, Olshina, A and Leonard, J G, 1996a. Ballarat 1:100 000 map geological report, Geological Survey of Victoria Report 101.
D'Auvergne, P B, 1990. Ballarat East gold deposits, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1277–1278 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Taylor, D H, Cayley, R A, Wolff, G A and Finlay I S, 1996b. Maps, sections and sedimentological logs of the Ballarat Freeway extension, Geological Survey of Victoria, Report 1996/6 (unpublished).
Finlay, I S and Douglas, P M, 1992. Ballarat mines and deep leads, Geological Survey of Victoria Report 94.
Whitelaw, H S, 1901. Report on the Little Bendigo or Nerrena goldfield, Ballarat (with plans and sections), Mines Department of Victoria Special Report.
Forde, A and Bell, T H, 1994. Late structural control of mesothermal vein-hosted gold deposits in central Victoria, Australia: mineralisation mechanisms and exploration potential, Ore Geology Reviews, 9:33–59. Fuller, T, 1995. The geology and geochemistry of the Ballarat West goldfield, BSc Honours thesis (unpublished), University of Ballarat, Ballarat.
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Whiting, R G, 1962. Geology and future economic prospects of the Ballarat goldfield, Chemical Engineering and Mining Review, 54(9):54–59. William Resources, 1996. Annual report to shareholders (William Resources Inc: Toronto). Willman, C E and Wilkinson, H E, 1992. Bendigo goldfield - Spring Gully, Golden Square, Eaglehawk 1:10 000 maps geological report, Geological Survey of Victoria Report 93.
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Mustard, R, Nielsen, R and Ruxton, P A, 1998. Timbarra gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 551–560 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Timbarra gold deposits by R Mustard1, R Nielsen1 and P A Ruxton2 INTRODUCTION The deposits are in the historical Timbarra Goldfield at the top of the Great Dividing Range in northern NSW, 30 km SE of Tenterfield, at lat 29o08′S, long 152o09′E, on the Grafton (SH 56–6) 1:250 000 scale and the Tenterfield (9339) 1:100 000 scale map sheets (Fig 1). The project is in final feasibility stage as a heap leach operation to treat 2 Mt of ore per year. An initial feasibility study completed in March 1995 was based on a Proved and Probable Reserve of 8.41 Mt at 0.92 g/t gold (249 000 oz of contained gold) from two deposits. Consolidation of the tenement position by Ross Mining NL for the current feasibility
1.
Project Geologist, Ross Mining NL, PO Box 1546, Milton Qld 4064.
2.
Exploration Manager, Ross Mining NL, PO Box 1546, Milton Qld 4064.
study will result in mining of four deposits, with a total Identified Mineral Resource of 13.65 Mt at 0.95 g/t gold (417 000 oz of contained gold) and a Proved and Probable Reserve of 10.06 Mt at 1.01 g/t gold (327 000 oz of contained gold). The mineralisation style at Timbarra is very unusual, with gold disseminated in the roof zone of a highly fractionated granite. Mineralisation and alteration are capped by microgranite, aplite and crystallisation layers close to the top of the granite, with negligible quartz veining and with minimal sulphides.
MINING AND EXPLORATION HISTORY Gold was recorded in the Tenterfield area in 1850 by Australia’s first geologist, the Reverend W B Clarke. The Timbarra Goldfield was discovered in 1853 some three years after Australia’s first gold rush. Working of alluvial and soft eluvial material was intense between 1859 and 1868 with 71 058 oz of gold produced (Carne, 1889). Following the discovery of ‘gold in grass roots’ at Poverty Point, extensive
FIG 1 - Location and geological map of the Timbarra area, modified after Brunker and Chesnut (1976).
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reticulation systems were set up to carry water some 8 km from Tin Swamp to the north to enable hydraulic sluicing in the late 1870s (H Burke, unpublished data, 1995). Despite this extensive system of water races sufficient water was only available to sluice for three months of the year (Wilkinson, 1980). Weathered granite was removed to an average depth of between 1.5 and 2 m and up to 10 m in places, leaving remnant granite cores and deep gullies (Kenny, 1924). Between 1907 and 1910 shafts were sunk along the Poverty Point lodes (Argentine dyke) and several stamp batteries were erected (Barnes, Henley and Henley, 1995). During the 1920s and 1930s cyanide plants were set up, the largest consisting of nine vats with total 40 t capacity. Mining ceased in 1938 with an estimated total production from the field well in excess of 100 000 oz of gold. At its peak in 1859 the field had over 2000 registered miners, a large proportion of whom were Chinese (The Clarence and Richmond Examiner, 18 July 1859). Modern exploration commenced in 1969 with surface sampling by Utah Development Company at Poverty Point. Between 1972 and 1974 Newmont Pty Ltd completed a comprehensive program of channel sampling and limited drilling. AOG Minerals Ltd between 1980 and 1982 made the first resource estimate for Poverty Point of 3.0 Mt at 1.0 g/t gold. Prospector Simon Whiley pegged leases in the Poverty Point and Big Hill areas in the mid 1980s and floated Timbarra Mines NL. Exploration within the leases was undertaken by Levu Gold NL and Auralia Resources NL, and larger regional programs were conducted by Electrolytic Zinc of Australasia Pty Limited (EZ), Saracen Minerals NL and Homestake Gold Ltd. Exploration by Auralia led to the discovery of a large low grade resource under the Big Hill workings to add to the Poverty Point body. Using a total of 456 holes for 21 474 m Auralia calculated Identified Mineral Resources of 10.94 Mt at 0.83 g/t gold at Big Hill and 2.60 Mt at 0.70 g/t gold at Poverty Point, using a 0.3 g/t cutoff. Proved and Probable Reserves of 6.70 Mt at 0.94 g/t at Big Hill and 1.71 Mt at 0.82 g/t at Poverty Point formed the basis of the feasibility study completed in March 1995. The grant of a Mining Lease over these two deposits in April 1996 gives Ross Mining the opportunity to mine these reserves. In the surrounding tenements EZ drilled the RMT and Hortons prospects, with later Homestake Gold Ltd drilling focussed on the high grade Hortons mineralisation. After subsequent infill drilling by Ross Mining, Identified Mineral Resources of 1.93 Mt at 1 g/t gold for RMT and 0.3 Mt at 2.47 g/t gold for Hortons have been defined, based on 110 holes for 4528 m and 41 holes for 2930 m respectively. The project area is now held 100% by Ross Mining NL. A full feasibility study is in progress with the objective of mining all four deposits, with the first gold pour expected in the first quarter of 1998.
PREVIOUS DESCRIPTIONS Little has been published on the Timbarra Goldfield. The unusual style of disseminated gold in granite was first recognised by EZ in the mid 1980s, and the description of deposit styles and models was expanded by Homestake Gold Ltd and Auralia Resources NL.
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Detailed studies by Taylor (1992) and Simmons (1993), and ongoing investigations by Simmons have done much to unravel the geology and provide a model of the Timbarra gold mineralisation. The only published information on the Timbarra Goldfield to incorporate this work is by Simmons et al (1996). An important study of the Tenterfield–Stanthorpe regional granitoid and mineralisation styles is presented by Blevin and Chappell (1996).
REGIONAL GEOLOGY The New England Fold Belt in NSW comprises a western Devonian–Permian fore-arc basin (the Tamworth Belt) and an eastern series of accretionary prism complexes bounded by the Peel Fault system (Gilligan and Barnes, 1990). Granites in the Tenterfield area are part of a major synorogenic suite of Carboniferous to Permian S-type and postorogenic Permian to Triassic I-type batholiths which intrude rocks of the accretionary prism complexes (Shaw and Flood, 1981; Gilligan and Barnes, 1990). High potassium I-type granites of the Tenterfield–Stanthorpe region form a distinct group called the Stanthorpe granite group (Blevin and Chappell, 1996). Three types of granite are recognised in this group, of which two crop out in the Timbarra area. The oldest is the Bungulla type dated at 245 Myr and characterised by mafic enclaves, relatively high biotite and hornblende content and a large number of potassium feldspar megacrysts. The other dominant granite is the Stanthorpe type, dated at 238 to 244 Myr, and consisting of leucogranite with equigranular biotite, few potassium feldspar megacrysts and little hornblende. Mafic enclaves are rare in the Stanthorpe type. The two granitoids, plus the Ruby Creek type, are considered to represent a differentiating magma series probably intruded at shallow crustal levels (Blevin and Chappell, 1996). In the Timbarra area the Stanthorpe Adamellite, a Stanthorpe type pluton, lies directly on top of Bungulla type granite. The Stanthorpe Adamellite forms the present day topographic high known as the Timbarra Tableland.
ORE DEPOSIT FEATURES The Stanthorpe Adamellite on the Timbarra Tableland can be divided into two types - variably porphyritic medium to fine grained hornblende-biotite granite called Monty’s granite, which is intruded by a later stage fractionated medium to coarse grained equigranular biotite granite termed the Surface Hill granite. The Surface Hill granite has a chilled contact with the Monty’s granite, with locally extensive development of a fine grained carapace. Miarolitic cavities and pegmatitic layers and segregations typical of tin granites are common towards the top of the Surface Hill granite (Simmons et al, 1996). Late stage aplite dykes and sills are generally confined to the top of the Surface Hill granite but may penetrate a short way into Monty’s. Gold mineralisation across the Tableland is invariably within the Surface Hill granite either at the contact or within 200 m of it. Mineralisation within the Surface Hill granite is related to regional structures. Structure has firstly influenced the intrusion of the Surface Hill granite into the Monty’s granite, with structurally controlled granite cusps and roof culminations providing the focus for alteration and mineralisation. The north
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trending RMT, Poverty, Peg Leg and Earl Grey line of prospects is associated with a ridge of Surface Hill granite (Fig 1). Secondly, cooling fractures, joints and faults within the roof of the granite have an influence on localising and enhancing gold mineralisation. A common structural theme has been obtained from mapping the four deposits, reflecting NNE (30o), ENE (75o) and ESE (110o) trends. All deposits show evidence of NNW-striking dextral shearing with associated joints. In addition to structural controls the mineralisation is localised beneath or within fine grained variants that form chilled carapaces or sills close to the granite roof. Mineralising fluids appear to have ponded beneath these less permeable granite units. This is particularly evident at RMT and Poverty. Mineralisation is associated with weak to moderate pervasive creamy yellow to green sericite-chlorite±albite, carbonate and clay alteration of feldspars and biotite. Dark quartz crystals are also typical, reflecting either an enhanced secondary fluid inclusion content or radiation damage to the crystal lattice.
DEPOSIT STYLES All four deposits contain disseminated granite-hosted gold mineralisation, however each exhibits a unique morphology based on the size of the host intrusive body, granite facies associations, a specific location within the upper levels of the fractionated granite and the degree of influence imposed by structural controls, including cooling joints, veins, sills, and dykes as well as faults. The Poverty Point and RMT deposits are along the strike of a north-trending granite ridge (Fig 1). Big Hill is at a lower structural level in the Surface Hill granite, 1 km to the east of Poverty Point. Hortons higher grade deposit is 8 km SW of Poverty Point in a much narrower Surface Hill granite stock and at a lower structural level (Fig 2).
POVERTY POINT At Poverty Point a north-striking, 15o west dipping 6 to 10 m thick microgranite carapace separates overlying Monty’s granite to the west from medium to coarse grained equigranular Surface Hill granite to the east. The apical portion and eastern margin of the microgranite have been very shallowly eroded exposing a small hill of Surface Hill granite, approximately 500 m wide at its base and rising 30 m high above the western plateau (Fig 3). The top of the hill is interpreted to correspond closely to the domal axis which strikes north and plunges shallowly southwards. Disseminated gold mineralisation is dominantly within the medium to coarse grained Surface Hill granite and conformably beneath the microgranite carapace. In cross section the +0.1 g/t gold contour defines a relatively tabular body up to 30 m thick. The body is flat lying near the dome axis and dips shallowly west beneath the preserved westerly dipping microgranite margin. On the western side, where the microgranite has been best preserved, mineralisation has a continuous strike length of 375 m. The central zone occurs above 975 m RL and is best developed north of the Argentine dyke at 22 957 m N either as a result of fluid ponding by the Argentine dyke or the effects of the south plunging domal axis, but perhaps both. The central zone has been eroded north of 23 125 m N (Fig 3). Several NNE-striking, 10–15o dipping pegmatitic microgranite bands <2 m thick occur internally within the main medium to coarse grained facies. The Surface Hill granite is crosscut by the narrow (2–3 m wide and 300 m long) ENEstriking and 75o north-dipping Argentine aplite dyke. Two vein sets striking ENE and ESE also crosscut the granite. Less significant disseminated mineralisation is associated with thin NNE-striking pegmatitic microgranite horizons. Localised higher grade mineralisation is associated with the Argentine dyke, and was mined historically along with the subparallel ESE-striking quartz-molybdenite veins.
FIG 2 - Diagrammatic cross section showing proposed genetic model, Timbarra deposits.
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FIG 3 - Geological plan and cross section of the Poverty Point deposit.
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RMT
layer, the lowermost of two mapped, within the pluton and several tens of metres below the eroded carapace.
In the NW of the RMT grid a NNE-striking and moderately west dipping microgranite margin or carapace forms the western boundary against Monty’s granite (Fig 4). In the centre of the RMT grid two 1 to 5 m thick north-striking and 10ο westdipping internal pegmatitic microgranite layers occur within medium to coarse grained equigranular Surface Hill granite. As at the Poverty Point deposit, disseminated gold mineralisation is localised conformably below a microgranite horizon within medium to coarse grained equigranular Surface Hill granite. However the microgranite represents an internal
In cross section the +0.1 g/t gold contour defines a shallowly west-dipping tabular body 30 to 40 m thick, generally above 940 m RL, which thins laterally to the west and is eroded to the east. Gold grades and thicknesses are best developed where a 100 m wide zone of NNE- to NE-trending chalcedonic quartz veins associated with fracturing or faulting crosscut the medium to coarse grained granite and lowest (southeasternmost) microgranite. Historical workings at Lee’s mine were focussed along the southeastern margin of this zone. Narrow zones of WNW-striking euhedral quartz veins also
FIG 4 - Generalised geological plan of the RMT deposit.
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R MUSTARD, R NIELSEN and P A RUXTON
crosscut the Surface Hill granite and microgranite horizons. A zone of higher grade mineralisation is localised by the intersection of the NE veins with a set of WNW veins within medium to coarse grained granite immediately below the microgranite horizon at Black’s mine. Mineralisation is bounded to the south and SE by the late NEstriking subvertical Lee’s fault adjacent to the Lee’s workings. Mineralisation is confined to the north of 22 050 m N where the strike of the microgranite horizon rotates clockwise to an east strike and shallow north dip as it swings around a northstriking, shallow north-plunging domal axis.
BIG HILL Here the country rock is a homogeneous undifferentiated sequence of medium to coarse grained weakly porphyritic biotitic Surface Hill granite (Fig 5). Several discontinuous microgranite bands 1 to 2 m thick are interpreted to strike NNW and dip at 20 to 30ο to the west. Extensive historical sluices to 300 m long by 25 m wide and up to 10 m deep target a NNEstriking zone of strong sericite-chlorite±hematite alteration containing minor quartz veins. Other quartz vein trends include, in order of decreasing importance: ESE-striking, steep south-dipping; ENE-striking, steep south to moderate northdipping; north-striking, steep east-dipping; and east-striking, moderate to steep north-dipping. Disseminated gold mineralisation strikes NNE and its surface exposure is well defined by the historical workings. The deposit is dominated by a 350 m long NNE-striking 65 to 75o west dipping 50 m wide zone which extends at least 140 m below surface. This zone is clearly structurally controlled and often contains secondary perpendicular structures trending NNE but dipping east. At the northern end of the deposit this subvertical zone intersects a tabular, flat to 25o dipping mineralised layer from 40 to 90 m thick. As a result of topography the subhorizontal control is predominant at the northern end of the deposit giving way to the steeper structural control as the land surface rises to the south. At the southern end of the deposit a third zone striking NNW and dipping at 20 to 40o west lies beneath two 1 to 2 m thick aplitic horizons (east and west aplites, Fig 5) at a higher structural level. The mineralisation at Big Hill is open to the north and terminates against the two aplite horizons in the south.
HORTONS This is a discrete northerly-trending elongate stock of Surface Hill granite about 500 m long by 50 to 75 m wide within Monty’s granite (Fig 6). Gold mineralisation has only been located at the northern end of the stock and is localised within the microgranite roof zone, where it forms a horizontal to shallowly north-plunging high grade pipe-like body trending north along the intrusive stock axis. A sequential downward crystallisation sequence starting adjacent to the upper contact and progressing inwards from miarolitic microgranite to granophyric granite to equigranular microgranite to medium grained granite has been postulated (Simmons, 1993). The higher grade gold mineralisation is believed to be related to the ability of such a discrete or localised intrusion with a high aspect ratio to focus exsolving hydrothermal fluids in its apex. The mineralisation is open to the north.
556
ORE GENESIS The unusual association of disseminated gold in the roof zone of a granite has not been described elsewhere. Simmons et al (1996) point out the textural similarities of the Timbarra deposits to some tin deposits, particularly Zaaiplaats in Southern Africa (Pollard et al, 1991). The Timbarra mineralisation is clearly related to highly fractionated high level granites with gold deposition one of the last events. The granites evolved in an essentially closed system in which chilled carapaces were formed and the late stage microgranite or aplite was intruded as dykes and sills in cooling joints and cracks. The presence of gaseous phases during final granitoid crystallisation is suggested by the miarolitic cavities and crystalline pegmatitic layers near the top of the Surface Hill granite. It is postulated that the granitoid phases were partially or totally crystalline when the altering and mineralising fluids moved up from below. Sericitisation and corrosion of feldspars and the chloritisation of biotite have been well documented (Simmons, 1993; personal communication, 1995). Recent unpublished work by Simmons notes gold deposition in corroded cavities in feldspar and along crystal boundaries and joints. The introduction of gold was associated with molybdenum, bismuth, antimony and arsenic (Simmons et al, 1996). Mineralising and altering fluids ‘pooled’ and ‘ponded’ beneath microgranite sills or layers and carapaces to form broad lensoid or tabular zones related to gently arched granite ridges at Poverty Point, RMT and Big Hill, and to form horizontal pipes in tight finger-like stocks at Hortons (Fig 2). Minor late stage fracturing resulted in the formation of narrow quartzmolybdenite-bismuthinite-carbonate veins enriched in gold, as well as rare mineralised dykes at Argentine dyke and Poverty Point. Internal zones of increased alteration intensity and higher gold grades correspond to areas where fracture zones intersect the Surface Hill granite cupolas.
ACKNOWLEDGEMENTS The authors gratefully acknowledge the permission of Ross Mining NL to publish this information. Particular thanks to S Morris for manuscript preparation and to T Holtz for technical drafting. The collection of data was largely by R Mustard and R Nielsen.
REFERENCES Barnes, R G, Henley, H F and Henley, J E, 1995. Exploration data package for the Tenterfield and Coaldale 1:100 000 sheet areas, Volume 1 - geology, mineral deposits, exploration and geochemistry, Geological Survey of New South Wales, Department of Mineral Resources. Blevin, P L and Chappell, B W, 1996. Internal evolution and metallogeny of Permo-Triassic high-K granites in the TenterfieldStanthorpe region, southern New England Orogen, Australia, in Proceedings of Mesozoic Geology of the Eastern Australian Plate Conference, pp 94–100 (Geological Society of Australia: Sydney). Brunker, R L and Chesnut, W S, 1976. Grafton, New South Wales 1:250 000 geological series, Geological Survey of New South Wales, Department of Mines, Sydney. Carne, J E, 1889. Notes on the mineral resources of New South Wales as presented at the Melbourne Centennial International Exhibition of 1888, NSW Geological Survey Records, 1(2):33–114.
Geology of Australian and Papua New Guinean Mineral Deposits
TIMBARRA GOLD DEPOSITS
FIG 5 - Geological plan and cross section of the Big Hill deposit.
Geology of Australian and Papua New Guinean Mineral Deposits
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R MUSTARD, R NIELSEN and P A RUXTON
FIG 6 - Geological plan and cross section of Hortons deposit.
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Geology of Australian and Papua New Guinean Mineral Deposits
TIMBARRA GOLD DEPOSITS
Gilligan, L B and Barnes, R G, 1990. New England Fold Belt, New South Wales - Regional geology and mineralisation, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1417–1423 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Simmons, H W, Pollard, P J, Steward, J L, Taylor, I A and Taylor, R G, 1996. Granite hosted disseminated gold mineralisation at Timbarra, New South Wales, in Proceedings of Mesozoic Geology of the Eastern Australian Plate Conference, pp 507–509 (Geological Society of Australia: Sydney).
Kenny, E J, 1924. Gold - New South Wales, Geological Survey of New South Wales, Bulletinm 7, p 66.
Taylor, I A, 1992. Petrology and geochemistry of the Stanthorpe Adamellite, with special reference to the associated gold mineralisation, northern New England, NSW, BSc Honours thesis (unpublished), James Cook University of North Queensland, Townsville.
Pollard, P J, Taylor, R G, Taylor, R P and Groves, D I, 1991. Petrographic and geochemical evolution of pervasively altered Bushveld granites at the Zaaiplaats tin mine, Economic Geology, 86:1401–1433. Shaw, S E and Flood, R H, 1981. The New England Batholith, geochemical variations in time and space, Journal of Geophysical Research, 86B:10530–10544.
Wilkinson, I, 1980. Forgotten Country, The Story of the Upper Clarence Gold Fields, 2nd ed (National Library of Australia: Canberra).
Simmons, H W, 1993. Textural and geochemical variations within Hortons granite, Timbarra, NSW, with respect to gold mineralisation, BSc Honours thesis (unpublished), James Cook University of North Queensland, Townsville.
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Teale, G S, 1998. Mount Terrible gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 561–566 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Mount Terrible gold deposits by G S Teale
1
INTRODUCTION The Mount Terrible volcanic complex (MTVC) is approximately 45 km SW of Tamworth, NSW, with its centre at about lat 31o20′S, long 150o40′E, on the Tamworth (SH 56–13) 1:250 000 scale map sheet (Fig 1). It occurs within a belt of underexplored Permo-Carboniferous volcanic and associated intrusive rocks which extend from Boggabri in the north to Murrurundi in the south. The volcanic rocks are associated with Late Carboniferous shallow water terrestrial sandstone and conglomerate and minor Early Permian volcanic sandstone and grit. Large areas of hydrothermal alteration within the MTVC associated with anomalous gold in stream sediment samples focussed the exploration activities of Werrie Gold Limited (Werrie), who discovered epithermal gold veins within the complex in 1990. An Inferred Resource of 132 000 t at 8 g/t gold has been identified at the Hillside deposit (Werrie Gold Limited, 1993).
EXPLORATION HISTORY The MTVC has been the principal exploration target for Werrie in the Boggabri–Murrurundi volcanic corridor. Early bulk cyanide leach stream sediment and soil sampling in 1990, with results to 1.8 and 1.4 ppm cyanide-soluble gold respectively, combined with pannable electrum grains up to 1 cm in diameter, created intense interest in this area, where no previous exploration licence had been granted. The Silicon Valley prospect exhibits many characteristics ascribed to porphyry or breccia mineralisation styles and the Hillside deposit has all the characteristics of gold–base metal sulphide–carbonate epithermal vein mineralisation as described by Leach and Corbett (1994) for Pacific Rim deposits. Airborne and ground magnetic surveys, stream sediment and soil sampling, auger, rotary air blast, reverse circulation percussion and diamond drilling and geological mapping have been carried out between 1990 and 1996.
REGIONAL GEOLOGY Early Permian volcanic rocks extend from Boggabri through Gunnedah to approximately Wingen, south of Murrurundi. In the Currabubula to Wingen region the Early Permian volcanic rocks were called the Werrie Basalt (Carey, 1934; Voisey and Williams, 1964). It is underlain by the dominantly epiclastic Temi Formation which in turn sits on the Late Carboniferous Currabubula Formation. Calc-alkaline volcanic arc–derived fluviatile sediment and calc-alkaline volcanic rocks dominate this formation (McPhie, 1987), which is underlain by the Merlewood Formation which contains thick andesitic units. 1.
Senior Geologist, Werrie Gold Limited, PO Box 740, North Adelaide SA 5006.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Locality and geological map of the Mount Terrible region, after Offenberg (1971).
561
G S TEALE
The Boggabri Volcanics (Hanlon, 1950), Werrie Basalt, Gunnedah Volcanics (Leitch and Skilbeck, 1991) and the Alum Mountain Volcanics (Jenkins and Nethery, 1992) have a similar age, and geochemical data for these are provided by Brownlow and Arculus (1993), Vickers (1991, 1993), Leitch, Morris and Hamilton (1988) and Flood et al (1988). According to Leitch (1993) the Gunnedah Volcanics are of similar composition and stratigraphic position to the Boggabri Volcanics and the two terms are considered synonomous, with precedence favouring the latter name. Volcanic rocks of the Early Permian Warrigundi Igneous Complex (Elmes, 1986; Flood et al, 1988) which outcrop predominantly in areas of Werrie Basalt are also supposedly geochemically similar to the Werrie Basalt and the Boggabri and Gunnedah volcanics (Brownlow and Arculus, 1993). Precise age relationships between the volcanic groups are difficult to determine due to the patchy outcrop and the massive character of many outcrops. The paucity of zircons in the mafic-intermediate volcanic rocks and the overall zeolitised and altered nature of these rocks makes it difficult to obtain highly constrained radiometric ages. The altered nature of the volcanic rocks also suggests that geochemical studies in some areas may be complicated by this overprint. Metamorphic grade throughout the belt is low, ranging from the presence of mordenite-clinoptilolite-heulandite zeolites in the Early Permian and Late Carboniferous to prehnite-pumpellyite facies in the stratigraphically and structurally deeper Devonian rocks (A C Purvis, personal communication, 1993). Younger monzonitic intrusives sometimes have a contact aureole in which biotite grade hornfels was developed. High level intrusives of gabbroic to rhyolitic composition are present throughout the belt and are cogenetic with associated volcanic rocks. Younger biotite-augite monzonites and biotite pyroxenites (lamprophyres) are also present. The Warrigundi Igneous Complex contains the most diverse grouping of intrusive rock types, from gabbroic through dioritic to granophyric syenodiorite (banatite) in one lineage. In addition plagioclase-hornblende porphyries, subvolcanic rhyolites, trachydacite sills, basaltic intrusives and a variety of trachyandesitic to trachydacitic dykes are also present. Detailed discussion of the tectonic framework of the Carboniferous-Permian rocks is provided by Leitch (1975), Roberts and Engel (1987), McPhie (1987), Blake and Murchey (1988), Flood et al (1988) and Vickers (1993). A low level, helicopter-borne magnetic and radiometric survey covering much of the Werrie areas was completed in mid 1994, with flight lines 200 m apart and ties 2000 m apart. The helicopter mean terrain clearance was 80 m (magnetometer sensor 60 m) and total line distance was approximately 5200 km. The observed magnetic field over the survey area is complex with anomalies due to many different types of sources (B Wyatt, unpublished data, 1995). Some of these sources include linear strike and crosscutting anomalies over the Carboniferous, anomalies reflecting possible intrusives at depth, subcircular features and zones of magnetite destruction, isolated positive and negative ‘bullseye’ anomalies and complex positive and negative anomalies over the Warrigundi Igneous Complex. High pass and directional filtering have highlighted lineaments which are interpreted to reflect faults, geological strike lines and the boundaries between major structural blocks (B Wyatt, unpublished data, 1995). Major north trending lineaments form part of the regional Mooki Thrust which extends for hundreds of kilometres.
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THE MOUNT TERRIBLE VOLCANIC COMPLEX STRUCTURE AND LITHOLOGY The MTVC was first investigated by Benson, Dun and Browne (1920), with later work by Carey (1934), who called the MTVC and its associated intrusives and extrusives the Warrigundi Igneous Complex. Offenberg (1971) considered all of the outcropping intrusive and extrusive rocks to be of Tertiary age and called them the Warrigundi Intrusives. The most recent investigation of the Warrigundi Igneous Complex was by Flood et al (1988) who reported a Rb-Sr age of 269.4±4.6 Myr. They considered the rocks to be dominated by calc-alkaline andesites and dacites although some aspects of their geochemistry were atypical. They also considered that these rocks were probably similar in composition to the Werrie Basalt but differed both geochemically and isotopically from the underlying Late Carboniferous lavas and ignimbrites. They concluded that Mount Terrible, as initially proposed by Benson, Dun and Browne (1920), is a stratovolcano of Early Permian age and that the atypical chemical composition suggests that it developed in an extensional regime similar to the Rio Grande rift. The early recognition by Werrie that the MTVC is apparently a ring structure bounded by ring faults (Fig 2) and dykes was confirmed by the aeromagnetic survey data. The MTVC forms a strong positive magnetic anomaly which provides a stark contrast to the extensive magnetic low associated with the surrounding Werrie Basalt. The MTVC circular structure is approximately 5 km in diameter. The MTVC is dominated by intermediate volcanic rocks with 52–65% SiO2 which, on a total alkali-silica diagram, lie on and adjacent to the line separating subalkaline from alkaline rocks. These rocks have been described petrographically as high potassium andesite, trachyandesite, shoshonite and latite. Flows, coarse and fine grained pyroclastics, epiclastics and possible talus breccias of these volcanic types have been recognised. High alumina basalt, trachybasalt, trachydacite and trachyte are also present. Rocks of rhyolitic composition are restricted to high level, subvolcanic domes and intrusives. As well as the intrusive rock types mentioned earlier, the complex is also cut by vent, diatreme, milled and tourmalinesulphide breccias. Most of the volcanic rocks are trachytic textured with phenocrysts of clinopyroxene, plagioclase (generally labradorite), titanomagnetite and lesser apatite and hornblende. Some magnetite phenocrysts contain inclusions of chalcopyrite. Orthopyroxene phenocrysts are virtually absent and Fe3+-rich ilmenite has only been observed in a few samples. Phenocryst phases sit in a groundmass generally dominated by plagioclase microlites and microphenocrysts of magnetite and clinopyroxene and interstitial potassium feldspar. Fine grained biotite occurs throughout the matrix of shoshonitic variants. Many rock types contain less than five modal per cent of phenocrysts but some trachyandesite (latite) flows contain up to thirty modal per cent of phenocryst phases. It should be noted that one major difference between the Late Carboniferous volcanic rocks (and the Early Permian Werrie Basalt) and the Warrigundi Igneous Complex is the absence of orthopyroxene in the Warrigundi rocks.
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT TERRIBLE GOLD DEPOSITS
Representative analyses of the more important rock types in the MTVC are presented in Table 1. TABLE 1 Representative analyses of various rock types from the Mount Terrible volcanic complex. %
1
2
3
4
5
SiO2
52.13
52.47
57.52
66.06
67.32
TiO2
1.70
1.71
1.25
0.89
0.70
Al2O3
16.26
16.07
17.05
15.82
15.20
Fe2O3*
11.14
11.21
7.49
4.02
3.08 0.10
MnO
0.16
0.18
0.31
0.15
MgO
3.70
3.38
1.58
1.02
0.82
CaO
7.73
6.27
2.44
2.22
2.01
Na2O
3.81
4.97
5.82
5.93
5.81
K2O
1.15
1.33
2.76
2.92
3.16
P2O5
0.25
0.30
0.41
0.21
0.12
H2O+
1.89
2.27
2.15
0.65
0.97
H2O-
0.91
0.19
0.32
0.16
0.21
0.68
0.00
1.24
0.04
1.31
101.51
100.35
100.34
100.09
100.81
Ba
223
238
469
508
552
Cr
29
6
6
6
3
Cu
22
19
0
2
2
Ga
22
21
22
21
18
CO2 TOTAL ppm
Nb
3
2
7
6
6
Ni
16
6
3
3
1
Pb
9
14
107
18
9
Rb
43
50
103
99
92
Sr
613
798
250
363
151
Th
8
8
20
14
17
U
1
2
3
2
3
V
260
243
73
32
27
Y
25
26
42
40
38
Zn
70
75
394
83
49
Zr
140
139
291
315
295
Source: G S Teale, unpublished data, 1995 * All Fe expressed as Fe2O3 1. Basalt, Hillside vein system area 2. Hornblende-plagioclase ‘porphyry’, Silicon Valley 3. Trachyandesite, Hillside vein system area 4. Latite, Taylor Mountain 5. Syenodiorite, Taylor Mountain
lodes). North- and NE-trending fault zones are also mineralised adjacent to the lodes, however the importance of the intersections of these structures is not fully understood. It is known that near-vertical trachyandesite dykes can be associated with bonanza grades where they intersect major footwall and hanging wall structures, as in drill hole RHDDH12 which intersected 0.3 m at 171 g/t gold and 258 g/t silver. The main hanging wall mineralised structure of the Hillside lodes has been intersected in drill holes over a lateral distance of approximately 350 m and a vertical distance of approximately 300 m with the deepest diamond drill hole to 452 m depth. Mineralisation increases in grade and possibly thickness to the NW along the 120ο trending structure and occurs as 1–5 m wide vein and/or breccia stockwork zones with no jogs or tension gash veins yet recognised. The lodes dip towards a major clayaltered breccia body. Numerous other mineralised structures in the vicinity, such as the Alpha, Beta and Gamma lodes (Fig 2), have not been fully tested. Mineralisation appears to be faulted off to the SE and abuts a premineralisation diorite stock along the northwestern extent of the structures. Immediately adjacent to the southeastern side of this stock is a mineralised NE-trending fault zone. According to G J Corbett and T M Leach (personal communication, 1995) this fault may have initially facilitated the emplacement of the diorite stock. The diorite may have aided ground preparation thereby explaining the presence of better mineralisation adjacent to it. To the NW of this diorite is a large circular clayaltered breccia body, the Kaolin Crest, which is cut by late stage unaltered mafic dykes. Gold mineralisation is typically associated with base metal sulphides and bismuth-bearing sulphosalts, with higher gold grades associated with reactivated and brecciated vein material. Galena, iron-poor sphalerite, chalcopyrite, arsenical marcasite, pyrite and matildite (AgBiS2) are common in the upper domains of the mineralised structures with chalcopyrite, arsenical pyrite, aikinite (PbCuBiS3), PbSss (a lead sulphide solid solution), and berryite [Pb2(CuAg)3Bi5S11] more common in the deeper zones. Gold contains up to approximately 65% silver and native silver is also present. Gold in upper domains contains up to 1.5% mercury and minor tellurium, with these elements diminishing with depth. Anomalous antimony, tungsten, rubidium and cesium are also present. The dominant gangue phases are carbonate, quartz, chlorite, smectite, sericite, adularia, albite and zeolites. Some of the deepest veins intersected contain epidote as inclusions in pyrite and quartz.
Silicon Valley prospect GOLD MINERALISATION Reconnaissance of the MTVC in 1990–1991 located large areas of hydrothermally altered breccia, tourmaline breccia and free gold grains (electrum) in soils and stream alluvium on the eastern, northern and western sides of Mount Terrible. The Hillside and Silicon Valley areas were identified in this early work.
Hillside deposit The deposit (Fig 2) comprises subparallel, steeply NE-dipping mineralised structures and veins which trend at 120ο (Hillside
Geology of Australian and Papua New Guinean Mineral Deposits
Silicon Valley is approximately 1300 m ENE of Hillside (Fig 2) and is a broadly defined 800 by 500 m topographic depression underlain by predominantly argillic and phyllic altered diatreme breccias, brecciated trachyandesite or latite (in part exhibiting tourmaline alteration and late adularia+ sericite+calcite alteration) and numerous intrusives including hornblende-plagioclase porphyries. Sheeted veins, dipping steeply NE and auriferous carbonate-base metal sulphide veins outcrop along the western flank of the depression. The altered breccias can be anomalous in gold and commonly contain gold values in the 0.05–0.15 ppm range. Disseminated sphalerite, pyrite and chalcopyrite are ubiquitous.
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FIG 2 - Geological plan of the Mount Terrible volcanic complex.
Low temperature gold–carbonate–base metal sulphide veins contain calcite, pyrite, low iron sphalerite, chlorite, galena (silver and bismuth free) and some quartz, with little or no chalcopyrite. High temperature quartz-magnetite+pyrite +molybdenite stockwork veins have been intersected in some drill holes and adjacent to these stockworks are primary chalcopyrite-bornite+chalcocite intergrowths. Calcite-pyritetourmaline+rutile+quartz+chalcopyrite veins, commonly with potassium feldspar selvages, cut biotite-altered, brecciated volcanic rocks and are also considered to represent early high temperature (~350oC) veins. The Silicon Valley and Hillside veins contain a large range of sulphides and sulphosalts as well as a wide variety of gangue phases, which are listed in Table 2.
ALTERATION Moderate to intense alteration occurs throughout large areas of the MTVC. The most widespread is an early propylitic alteration which causes replacement of clinopyroxene by epidote and chlorite aggregates, the introduction of pyrite, a patchy development of epidote-calcite-chlorite+actinolite and the albitisation of plagioclase phenocrysts (Table 3). Numerous alteration assemblages and alteration features have been observed in the MTVC. Propylitic, potassic, phyllic and argillic events are represented as are unusual variants on at
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least the early higher temperature alteration styles. Alteration styles and events are tabulated and summarised in Table 3. G J Corbett and T M Leach (personal communication, 1995) considered that the Silicon Valley area had similarities with the Kidston breccia system and there are also similarities with the Mount Leyshon hydrothermal system (G S Teale, unpublished data, 1987). Highly altered intrusives are spatially associated with mineralisation at Silicon Valley and Hillside. At the latter pyritic, sericitic and calcite-altered trachyandesite dykes are intimately associated with high grade and copper-rich goldsilver mineralisation. At Silicon Valley a potassic altered hornblende-plagioclase porphyry has been intersected in diamond drill hole SVPD2. Elsewhere altered intrusives are found as clasts in breccia and as intrusives into breccia and other rock types. It should be noted that comparatively fresh dykes and stocks cut highly altered rock types.
CONCLUSIONS 1.
The MTVC contains a variety of subalkaline to alkaline volcanic and intrusive rocks which probably formed in an extensional environment in a post-collision arc.
2.
The MTVC is represented by a circular, positive aeromagnetic anomaly within an extensive magnetic low associated with the Werrie Basalt.
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT TERRIBLE GOLD DEPOSITS
TABLE 2 Sulphide, oxide and gangue phases from the mineralised primary zone, Mount Terrible volcanic complex. MAJOR
MINOR
TRACE
RARE
Galena
Matildite
Arsenical pyrite
Native silver
PbSSS
Aikinite
Bismuthinite
Bornite
Sphalerite 1
Arsenical marcasite
Electrum
Chalcocite
Gold
Molybdenite
Sphalerite 2 SULPHIDES
Chalcopyrite
Gustavite
Pyrite
Berryite Pavonite Benjaminite? Neyerite? Galenobismutite?
GANGUE
Mn-calcite
Heulandite
Laumontite
Fluorite
Calcite
Stilbite
Epidote
Amethystine quartz
Quartz
Interlayered smectite-chlorite
Magnetite
Actinolite
Chlorite
Biotite
Hematite
Sphene
Smectite
Potassium feldspar
Gypsum
Barite
Tourmaline
Anatase
Mordenite
Kaolinite
Sericite
Roscoelite?
Adularia
Clinopyroxene
Albite
Rutile Pectolite
TABLE 3 Timing of alteration features, Mount Terrible volcanic complex. Early
Late
Propylitic
Replacement of clinopyroxene by epidote, chlorite aggregate and development of patchy epidote-calcitechlorite. Albitisation of plagioclase phenocrysts; minor actinolite. Occurs up to 2500 m from known mineralisation.
Tourmaline
Not known how it relates to potassic alteration. Occurs prior to biotite-potassium-feldspar alteration. Tourmaline developed in plagioclase phenocrysts and matrix in intermediate rocks and in the matrix of silicic rocks in and adjacent to Silicon Valley. Also located in topographically low areas up to 1300 m east of Silicon Valley.
Potassic
Biotite development in mafic to felsic rocks. Biotite replaces hornblende. Potassium feldspar rims on plagioclase. Stockwork veins of quartz-magnetite +pyrite+molybdenite+chalcopyrite cut biotite altered rocks in Silicon Valley. Silicon Valley also has quartz-epidote-sphene-actinolite-chalcopyrite+pyrite+ magnetite veins, calcite-tourmaline-rutile-pyrite veins; potassium feldspar selvages on quartz+calcite+pyrite veins. At depth at Hillside prospect are chalcopyrite-rich veins and epidote-quartz veins. Sinuous quartz+chalcopyrite +epidote veins; arsenical pyrite with epidote inclusions. Development of disseminated chalcopyrite containing (primary) intergrowths of bornite+ chalcocite; magnetite-potassium feldspar ‘clots’.
Breccia Event 1
Contains fragments of the above including the quartz-magnetite veins as mentioned above (G J Corbett, personal communication, 1995); phyllic and/or argillic alteration post-emplacement. Contains fragments of diorite and porphyry and is cut by later porphyry and mafic intrusives (I Hodkinson, personal communication, 1995). Clasts of sulphide present.
Breccia Event 2
Emplacement of shallowly south- SE-dipping tourmaline-pyrite-quartz breccias, often milled. Present as ‘sheets’ to 2 m thick.
Phyllic
Development of early adularia and/or albite with the two phases intergrown in the matrix of the volcanics. Adularia replaces plagioclase. Sericite-pyrite-calcite development. Sericite replaces adularia. Calcite from trace to 50 modal per cent. Pyrite often fringed by sericite. Some gold associated. Most pervasive of alteration types. Sericite veins; sericite developed on vein margins. Minor sphalerite associated in most observed specimens. Alteration associated with emplacement of gold-manganese-calcite-base metal sulphide veins. Later reactivation of veins and carbonate ‘flooding’. Introduction of bismuth sulphosalts+electrum+native silver. Arsenical marcasite rims early pyrite
Zeolite
Development of mordenite-heulandite as coarse aggregates in matrix of the volcanics in Silicon Valley. Heulandite growth in the matrix of the volcanics and as a vein phase at Hillside. Laumontite developed in brecciated, reactivated auriferous veins only.
Argillic
May predate zeolite development in some areas. Smectite development; smectite-pyrite totally replaces breccias in the immediate hanging wall of auriferous veins. Interlayered smectite-iron chlorite common adjacent to faults and in areas of high fluid flow. Late kaolinite observed (T M Leach, personal communication, 1995) Kaolinite veins cut zeolite veins.
Geology of Australian and Papua New Guinean Mineral Deposits
565
G S TEALE
3.
Similar circular aeromagnetic features occur throughout the volcanic corridor from Boggabri to south of Murrurundi, with anomalous gold values found in at least two of these areas.
Flood, R H, Craven, S J, Elmes, D C and Preston, R J, 1988. The Warrigundi Igneous Complex: Volcanic centres for the Werrie Basalt, NSW, in New England Orogen-Tectonics and Metallogenesis (Ed: J D Kleeman), pp 166–171 (University of New England: Armidale).
4.
Numerous breccia types, including late tourmalinepyrite-quartz breccias, have been mapped and recorded in the complex. Propylitic, potassic, phyllic and argillic alteration have been recognised.
Hanlon, F N, 1950. Geology of the north-western coalfield Part VII, Geology of the Boggabri District, Royal Society of New South Wales, Journal and Proceedings, 82:297–301.
5.
6.
The MTVC contains what is interpreted as porphyryrelated gold mineralisation which is predominantly concentrated in low temperature base metal–carbonate systems developed along faults. The gold mineralisation contains high to anomalous concentrations of bismuth, boron, fluorine, mercury, silver and tellurium. Mineralisation and alteration within the MTVC occurred over the temperature range of approximately 150 to 370oC as deduced from mineral assemblages, fluid inclusion studies (T M Leach, personal communication, 1995) and sulphur isotope temperature calculations (P Seccombe, personal communication., 1995). Gold mineralisation in bismuth-rich, carbonate–base metal sulphide veins developed at temperatures in the range 150 to 250oC. The mineralisation from the MTVC is very similar in lead isotopic composition to Early Permian mineralisation from Halls Peak, NSW (J Dean, personal communication, 1995).
ACKNOWLEDGEMENTS The author gratefully acknowledges the permission of Werrie Gold Limited to publish this information. R H Flood and J E Lynch are thanked for reviewing the original manuscript.
REFERENCES Benson, W N, Dun, W and Browne, W R, 1920. Geology and petrography of the Great Serpentine Belt of New South Wales, Part IX - The geology, palaeontology and petrography of the Currabubula district, with notes on adjacent regions, Proceedings of the Linnaean Society of New South Wales, 45:285–317. Blake, M C Jr and Murchey, B L, 1988. A California model for the New England Fold Belt, New South Wales Geological Survey, Quarterly Notes, 72:1–9. Brownlow, J W and Arculus, R J, 1993. The geology and geochemistry of the Boggabri Volcanics - a preliminary account, in New England Orogen, Eastern Australia (Eds: P G Flood and J C Aitchison), pp 315–322 (University of New England: Armidale). Carey, S W, 1934. The geological structure of the Werrie Basin, Proceedings of the Linnaean Society of New South Wales, 59:351–374. Elmes, D C, 1986. The geology of the Warrigundi Volcanic Centre, Werris Creek, New South Wales, BSc Honours thesis (unpublished), Macquarie University, Sydney.
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Jenkins, R B and Nethery, J E, 1992. The development of Early Permian sequences and hydrothermal alteration in the Myall Syncline, central eastern New South Wales, Australian Journal of Earth Sciences, 39:223–237. Leach, T M and Corbett, G J, 1994. Porphyry related carbonate base metal gold systems in the Southwest Pacific; characteristics, in Proceedings PNG Geological, Exploration and Mining Conference, 1994 Lae (Ed: R Rogerson), pp 84–91 (The Australasian Institute of Mining and Metallurgy: Melbourne). Leitch, E C, 1975. Plate tectonic interpretation of the Palaeozoic history of the New England Fold Belt, Geological Society of America Bulletin, 86:141–144. Leitch, E C, 1993. The floor of the Gunnedah Basin north of the Liverpool Range, in The Gunnedah Basin New South Wales (Ed: N Z Tadros), pp 335-341, Geological Survey of New South Wales Memoir Geology 12. Leitch, E C, Morris, P A and Hamilton, D S, 1988. The nature and tectonic significance of Early Permian volcanic rocks from the Gunnedah Basin and the southern part of the New England Fold Belt, in Advances in the Study of the Sydney Basin, 22nd Symposium, pp 9–15 (University of Newcastle: Newcastle). Leitch, E C and Skilbeck, C G, 1991. Early Permian volcanism and Early Permian facies belts at the base of the Gunnedah Basin, in the southern part of the New England Fold Belt, in Advances in the Study of the Sydney Basin, 25th Symposium, pp 59–66 (University of Newcastle: Newcastle). McPhie, J, 1987. Andean analogue for Late Carboniferous volcanic arc and arc flank environments of the western New England Orogen, New South Wales, Australia, Tectonophysics, 138:269–288. Offenberg, A C, 1971. Tamworth, New South Wales - 1:250 000 geological series SH 56–13, New South Wales Department of Mines. Roberts, J and Engel, B A, 1987. Depositional and tectonic history of the southern New England Orogen, Australian Journal of Earth Sciences, 34:1–20. Vickers, M D, 1991. The Werrie Volcanics, Wingen, NSW, Geology, geochemistry and tectonic significance, in Advances in the Study of the Sydney Basin, 25th Symposium, pp 52–58 (University of Newcastle: Newcastle). Vickers, M D, 1993. The Lower Permian Werrie Volcanics; facies and tectonic setting, in New England Orogen, Eastern Australia (Eds: P G Flood and J C Aitchison), pp 323–327 (University of New England: Armidale). Voisey, A H and Williams, K L, 1964. The geology of the CarrollKeepit-Rangari area of New South Wales, Royal Society of New South Wales Journal and Proceedings, 97:65–72. Werrie Gold Limited, 1993. Prospectus for Werrie Gold Limited, Brisbane.
Geology of Australian and Papua New Guinean Mineral Deposits
Elliott, S M, Bywater, A and Johnston, C, 1998. McKinnons gold deposit, Cobar, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 567–574 (The Australasian Institute of Mining and Metallurgy: Melbourne).
McKinnons gold deposit, Cobar 1
2
by S M Elliott , A Bywater and C Johnston
3
INTRODUCTION McKinnons gold mine is owned by Burdekin Resources NL (Burdekin) and is the westernmost operating mine in the Cobar Basin. It is 37 km SW of Cobar, NSW, at lat 31o47′S, long 145o41′E, on the Cobar (SH 55–14) 1:250 000 scale and Wrightville (8034) 1:100 000 scale map sheets (Fig 1). McKinnons is the first significant deposit discovered on the western margin of the Cobar Basin. Since the start of mining in January 1995, treatment to the end of September 1997 has totalled 1 226 482 t at a head grade of 2.56 g/t gold, with a 92% metallurgical recovery. To optimise profit and equipment utilisation, production rate is twice the treatment rate with mining completed within two years and treatment over four years. The Proved Ore Reserve is 2.5 Mt at 1.73 g/t gold.
EXPLORATION HISTORY The Cobar mineral field has produced more than 2.5 Moz of gold since 1880, mainly from the New Occidental, Peak, Mount Boppy, Great Cobar and New Cobar mines (Stegman and Stegman, 1996). Prospecting during a regional palaeodrainage sampling program carried out by Norgold Limited led to the discovery of the outcropping McKinnons deposit in December 1988. Three types of material were collected from each shallow auger hole, located at a stream sediment site. These were a 5 kg bulk leach extractable gold (BLEG) sample, a -180 µm sample, and a magnetic lag sample which principally consisted of maghemite. The last two samples were analysed for total copper, lead, zinc, arsenic, antimony, bismuth, iron and manganese, and the BLEG sample for cyanide-soluble gold. The deposit is within a catchment which is weakly anomalous in arsenic, antimony and lead in magnetic lag. In the -180 µm samples arsenic was the only anomalous element. A low order gold anomaly of maximum 0.45 ppb was detected in one BLEG sample, downstream from McKinnons, 700 m NE of the deposit. Norgold carried out follow up geological mapping, soil and rock geochemical surveys, geophysical surveys, rotary air blast (RAB), reverse circulation (RC) and diamond drilling. Surface rock chip geochemical sampling was carried out over a 500 m strike length, using continuous rock chip sampling over 10 m intervals in areas of outcrop, assisted by bulldozer rip lines where outcrop was sparse. The deposit is 380 m long and 200 m
1.
Director - Exploration, Burdekin Resources NL, 60 Punari Street, Townsville Qld 4812.
2.
Senior Mine Geologist, Burdekin Resources NL, c/- McKinnons Gold Mine, Cobar NSW 2835.
3.
District Geologist, Burdekin Resources NL, c/- McKinnons Gold Mine, Cobar NSW 2835.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Location map and regional geological map of the Cobar area (modified after Glen, 1987b, 1994).
wide and was outlined by larger anomalous zones of gold (to 12.8 g/t), silver (to 14 ppm), lead (to 780 ppm), arsenic (to 410 ppm) and antimony (to 180 ppm) in rock and soil samples. Areas covered by residual and transported soil north and south of the deposit were explored in 1989 and again in 1995 and 1996 by RAB drilling. Saprolitic material contained anomalous gold, lead, zinc, antimony and arsenic values over a strike length of 1300 m and a maximum width of 400 m.
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S M ELLIOTT, A BYWATER and C JOHNSTON
The deposit was not detected by airborne GEOTEM, ground magnetic, radiometric or gravity surveying. A SIROTEM survey tested several minor features from the GEOTEM survey but they were interpreted to be related to conductive overburden (G Taylor, unpublished data, 1991). Gradient array and dipole-dipole induced polarisation (IP) surveys carried out north and south of the deposit clearly defined (1.5 background chargeability) the primary mineralisation extending 400 m south and 800 m NW of the deposit. The IP response is derived from an extensive pyrite halo associated with base metal sulphide and gold mineralisation below the oxidation zone. Norgold drilled a total of 65 holes, comprising 4920 m of RC and open hole percussion and 1070 m of diamond core. Burdekin acquired the property in December 1993 and to June 1994, as part of an ore delineation and sterilisation program, Burdekin had completed a further 96 face sampling RC percussion holes totalling 5743 m, five diamond holes totalling 389 m, and 60 open percussion sterilisation holes, totalling 2877 m.
ORE RESERVE AND PRODUCTION DATA The initial mineral resource estimate was carried out using the ore delineation drilling results in a geostatistical nested median indicator kriging method on uncut 1 m intersections. The mineral resource model was then subjected to a pit optimisation study (Table 1). TABLE 1 Comparison of mineral resource and ore reserve estimates. Ore delineation June 94
Grade control January 95
Mineral Resource Measured
2.1 Mt at 1.78 g/t gold 2.88 Mt at 1.63 g/t gold
Indicated
0.6 Mt at 1.40 g/t gold 0.78 Mt at 0.99 g/t gold
Total
2.7 Mt at 1.70 g/t gold 3.66 Mt at 1.50 g/t gold
Cutoff grade
0.6 g/t gold
0.7 g/t gold
Bulk density
2.5
2.5
5 m E x 5 m N x 2.5 m RL
2.5 m E x 2.5 m N x 2.5 m RL
Block Size No of holes Drilling pattern
176
740
25 x 12.5 m
8 x 8 m average
Ore Reserve
(cutoff grade 0.7 g/t gold)
Proved
1.782 Mt at 1.91 g/t gold
Probable
0.418 Mt at 1.91 g/t gold
Total Contained gold Strip ratio
2.5 Mt at 1.73 g/t gold
2.2Mt at 1.91 g/t gold
2.5 Mt at 1.73 g/t gold
4202 kg
4325 kg
1.8:1
1.3:1
Grade control drilling of the oxide deposit, comprising 528 holes for 34 390 m, was carried out to a depth of 80 m before mining began. Data obtained were used to assist in the pit design, mine scheduling, and to reduce grade control sampling during production thus allowing larger capacity mining equipment to be used, therefore increasing mine production rates and reducing mining costs (Elliott, 1995).
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Resources and reserves were later re-estimated, using the ore delineation and grade control drilling data (A Tiver, unpublished data, 1995). A comparison of the two sets of data is given in Table 1. Table 1 demonstrates that the reserve estimate derived from the original ore delineation drilling was only 2.8% lower than the final estimate, which was based on an additional 34 390 m of drilling. Mining was completed in December 1996, with production of 1.8 Mt of ore at a grade of 1.92 g/t gold. Stockpiled ore is sufficient for treatment until mid 1998. The possible reasons for the discrepancy between the ore mined and the ore reserve estimation is discussed by Elliott et al (1997).
PREVIOUS DESCRIPTIONS Gold mineralisation in the Cobar district has been studied by many authors since about 1900. The principal references are Andrews (1913), Brooke (1975), Glen (1987a), Glen et al (1994) and Stegman and Pocock (1996). The McKinnons deposit has been described by Elliott (1995), Rugless and Elliott (1995) and Bywater et al (1996).
REGIONAL GEOLOGY STRATIGRAPHY The Early Devonian sediments of the Cobar Basin unconformably overlie or are in fault contact with turbidites and cherty sediment of the Ordovician Girilambone Group (Fig 1). Deposition commenced in Early Devonian with shallow water sedimentation in a NE–SW extensional basin (Glen, 1995). Turbidite deposition followed by minor local felsic volcanism (the main rift phase) occurred throughout the basin, during which time the Nurri Group, lower Amphitheatre Group and Biddabirra Formation were deposited. The Nurri Group occupies a narrow zone along the eastern edge of the basin. Overlying and laterally equivalent to the Nurri Group is the turbiditic lower Amphitheatre Group. After Nurri Group sedimentation the major sediment source switched from the eastern to the western margin to generate the thick upward coarsening lower Amphitheatre Group (Glen, 1987b). Thickly bedded turbidites of the Biddabirra Formation represent the end of rift activity or phase of basin formation. The post-rift or sag stage is represented by the upper Amphitheatre Group which is marked by thinly bedded turbidites, shale and mudstone. The Winduck Group, which in part overlies the upper Amphitheatre Group, consists of an upward-fining shelf sequence of sandstone, siltstone and mudstone on the western side of the basin. The Cobar Basin sediments are about 6 km thick and have been regionally metamorphosed to lower greenschist facies, as shown by quartz, sericite and chlorite assemblages in argillaceous rocks. Aeromagnetic anomalies suggest the presence of shallow intrusives in the Cobar Basin sediment along zones of weakness, parallel to the western margin of the basin (Elliott, 1995). High-level granitoids intruding lower Devonian sediments are common to the south in the Mount Hope Trough, and are associated with coeval volcanics.
STRUCTURE In the Cobar Basin structure is dominated by north- to NWtrending basin margin faults such as the Rookery and
Geology of Australian and Papua New Guinean Mineral Deposits
McKINNONS GOLD DEPOSIT, COBAR
Jackermaroo faults. Shorter intrabasinal faults trend WNW (the Nymagee Fault) to NE (eg the Buckwaroon and Sandy Creek faults) and were formed during crustal extension and then reactivated during subsequent compression. Some of the faults were active during and after sedimentation. The Cobar Basin reverted from NE–SW extension to compression in the late Early Devonian, from 395 to 400 Myr (Glen et al, 1994). The eastern margin of the basin represents a higher strain zone with a strong regional steep cleavage, termed S1 by Glen (1990), lying axial planar to commonly steep F1 folds. In contrast, on the western margin, regional folds are open (as in the Nullawarra Anticline) and formed in a low strain zone with little or no axial plane cleavage. The McKinnons deposit is interpreted to be located in the hinge zone of the NW-trending Nullawarra Anticline (Fig 1). A second folding event is displayed regionally NW of McKinnons where the Nullawarra Anticline is refolded by NE-trending folds with a steep cleavage (S2). Interpretation of deep seismic reflection profiles by Glen et al (1994) and Glen, Clare and Spencer (1996) suggests that upper crustal (eg intrabasin and basin–basement contact) and mid crustal detachment faults were active during basin formation and that a ramp-basin model applies to the upper crustal movement. A model of oblique opening and closing of the Cobar Basin with opening towards 040o and closing in the same direction is suggested by Glen, Clare and Spencer (1996).
ORE DEPOSIT FEATURES REGOLITH The primary lateritic profile formed at Cobar in the Late Cretaceous to Late Middle Miocene, from 97 to 16 Myr (Leah, 1996). The upper part of the profile has been eroded and only the lower saprolite zone remains. Since the Early Middle Miocene, water tables have been falling and weak secondary lateritisation has been taking place, resulting in oxidation and ferruginisation of the top of the stripped original profile. Weathering ranges in depth from 50 to 90 m with the major base of oxidation in the deposit occurring at an average depth of 60 m. Weathering has altered rocks by converting feldspar, biotite, chlorite and carbonates to clay minerals and sericite grains to kaolin or illite. Pyrite is converted to goethite, limonite and hematite. Soil cover is commonly less than 2 m, although a silt-choked channel north of the mine, filled with transported sand, gravel and clay, is up to 40 m thick. Continued lateritisation since the Middle Miocene has produced ferruginous lateritic material, now converted to maghemite. Maghemite pisolites have been extensively redistributed into drainages.
STRATIGRAPHY The McKinnons area has sparse outcrop and is commonly covered by shallow residual and thicker transported soil. The deposit occurs on a low ridge of silicified siltstone, fine-grained quartz-rich sandstone and quartz breccia veins within a low relief pediplain. Geological information from drill core and open pit exposures is shown in plan on Fig 2 with the associated gold grade distribution on Fig 3. A geological cross section is shown on Fig 4 with the associated gold grade block model on Fig 5.
Geology of Australian and Papua New Guinean Mineral Deposits
Five units have been distinguished in the lower Amphitheatre Group from diamond drill hole data in the McKinnons area. The sequence is dominated by siltstone with quartz 30–40% and sericite 60–70%, intercalated with subordinate mudstone and fine grained sandstone. The sandstone is well sorted and quartz-rich (60–80%), with subordinate sericite and clay, and accessory, leucoxene, tourmaline and muscovite. The lowest unit (Unit 1) consists of very thin to medium bedded grey siltstone interbedded with weakly carbonaceous fine-grained quartz sandstone and mudstone. Bedding contacts are commonly gradational and disrupted by minor bioturbation. A thin (1 m) fault-bounded cream, medium to coarse-grained, felsic rock consisting of feldspar, quartz and chlorite, which appears to be a felsic tuff, is present. Unit 2 is approximately 100 m thick and consists of interbedded mudstone to fine-grained sandstone. It is thinly bedded, contains quartz, sericite, siderite and accessory minerals, and has common bioturbation. The 40 m thick, very thinly laminated Unit 3 consists of alternating dark grey siltstone and very fine-grained quartz sandstone with sharp-edged planar beds. Minor truncated beds indicate that the sequence is the right way up. Unit 4 is approximately 300 m thick and comprises finegrained quartz sandstone, interbedded with 3 to 15 mm thick beds of siltstone. Bedding is extensively disrupted by bioturbation, along with numerous burrows and dark grey fecal pellets. In the oxide zone weathering tends to obliterate primary textures in hand specimen and to form bleached pale green to cream coloured clay-rich rocks. This unit contains the McKinnons oxide gold deposit. North of the pit diamond drill holes have intersected Unit 5, a fine-grained dark grey massive limestone, at least 20 m thick, which contains solitary rugose corals and gastropods. The submarine turbidite fan model used by Glen (1987b) for the lower Amphitheatre Group and Nurri Group does not appear to apply to the McKinnons sequence, as there are no Bouma sequences present. The laminated to thinly planarbedded sequence suggests a distal low velocity traction current depositional environment. Since bioturbated structures may form in shallow or deep water, only the limestone clearly indicates shallow water deposition, assuming that the limestone formed in situ. Shallow water sedimentation at McKinnons during the initial rift formation is now supported by Glen et al (1994) and Glen, Clare and Spencer (1996).
STRUCTURE Bedding dips between 15 and 35 o towards the NW and north, forming a broad anticlinal structure with numerous small subsidary scale folds along its axis. Local disruptions of bedding trends are common due to small scale dragging along major faults. Structure in the pit is dominated by 20 to 50 m wide brittle fault zones that dip steeply and trend 330o±20o, 010o±10o, 035o±10o and 090o±20o (Fig 2). This is also the chronological order from early to late stage faulting events. Each fault trend is mineralised to a different degree. The 330o trend controls the long axis of the deposit and internally the 035o trend controls the high grade zones. The zone of intersection of all the fault directions in the central portion of the pit is the area of the highest grade gold ore (Fig 3). Bedding-parallel fracture sets are common, related to D1 regional deformation events.
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FIG 2 - Interpreted geology, at 180 RL (20 m below surface), McKinnons deposit.
movement within the pit appears to be limited, commonly with less than 10 m displacement. This suggests that tectonism has largely produced in situ brittle failure which is typical of high level deformation. The fault zones contain varying degrees of fracturing with quartz fill veins, and with increasing vein density they form quartz stockworks grading to breccias. Hydrothermal breccias parallel to the steep faults and flat lying bedding are both present. Numerous bedding subparallel faults occur throughout the McKinnons system, and commonly contain barren sparry quartz. These faults are late stage, and have a reverse dip-slip movement related to thrusting.
ALTERATION
FIG 3 - 180 RL bench plan with block model gold grades.
Fault kinematic relationships suggest that the 035o trending faults dipping 70–80o east have undergone multiple movement with mineralisation located in the hanging wall. Fault
570
Most of the McKinnons gold ore is in the oxide zone, where weathering has overprinted the alteration minerals associated with the primary mineralisation. However, limited core from the primary zone shows that the gold-zinc-lead mineralisation is associated with extensive outer carbonate, intermediate pyrite and inner silica alteration zones, similar to those observed at Elura (Schmidt, 1990).
Geology of Australian and Papua New Guinean Mineral Deposits
McKINNONS GOLD DEPOSIT, COBAR
FIG 4 - Geological cross section, looking WNW. Location on Fig 2.
free gold grains, suggests that low temperature epithermal conditions (see Dong, Morrison and Jaireth, 1995) existed for some of the primary mineralisation. Sericite is widespread and abundant throughout the lower Amphitheatre Group sediments and has in places survived weathering. The majority of the sericite is less than 50 µm in diameter. Its distribution does not appear related to mineralisation but reflects the regional metamorphic grade. Later generations of fibrous sericite define the S1 and S2 schistosities.
FIG 5 - Block model gold grades on the same cross section as Fig 4.
In the pit the most obvious alteration associated with gold mineralisation is a pervasive weak to strong silicification. In thin section pervasive silicification is evidenced by sutured boundaries of microcrystalline quartz grains. With increasing intensity of alteration, the silica content increases by the progressive replacement of sericite and clays by quartz, and in hydrothermal breccias the sediment clasts may be completely replaced by quartz. In intensively silica-altered rocks the quartz grains become coarser, polygonal and interlocking. Evidence of alteration in the form of overgrowth textures on detrital quartz grains is rare, suggesting that the rock possibly recrystallised during the final stages of prograde metamorphism. The general microcrystalline nature of the quartz, particularly as seen in banded colloform quartz with
Geology of Australian and Papua New Guinean Mineral Deposits
PIMA and XRD studies by Marshall, Scott and Kamprad (1996) have shown that barren rock in the oxide zone contains kaolin. In the mineralised profile, illite is abundant at the expense of kaolin. The formation of illite reflects low pH conditions, probably caused by the oxidation of sulphides. Pale green iron-rich fine-grained chlorite forms in the brittle fault zones, either replacing sericite or as vein filling. In the primary zone extensive carbonate alteration is associated with mineralisation peripheral to the silica alteration. The carbonates range from calcite through ankerite to siderite with increasing iron content. Ankerite is abundant and occurs as disseminated porphyroblasts to 3 mm in diameter, and in association with quartz as veins within fault zones away from an inner silica zone. A spotty textured siltstone contains cavities to 2 mm in diameter, filled with white clay and sericite in the oxide zone (Bywater et al, 1996). These are interpreted to be weathered relics of the disseminated carbonate alteration of the primary zone, and often appear to be parallel to bedding. The carbonate alteration is extensive and up to 30 m wide on either side of the orebody in the southern end of the pit. The carbonate alteration predates silica alteration.
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S M ELLIOTT, A BYWATER and C JOHNSTON
An extensive carbonate alteration zone to 150 m wide is recognised in drill core, associated with primary zinc-lead mineralisation. Quartz±pyrite veins and breccia are substituted along the same NW-trending fault corridor to the NW of the deposit by ankerite±pyrite veins and breccia. Weak gold mineralisation is also associated with this carbonate alteration.
MINERALISATION Gold mineralisation within the open pit is both primary and supergene in origin, and each type contributes to the economic viability of the deposit. Primary mineralisation comprises gold grains of mean diameter 20 µm locked in quartz veins, silica breccia and pyrite veinlets. Supergene gold occurs primarily as 1 µm to 2 µm diameter blebs, associated with goethite accumulations and clay-rich fault zones. The silver content of the primary mineralisation ranges from 1 to 800 g/t whereas supergene gold is almost silver free. In spite of the overall low gold grade of the deposit, high grade supergene pockets are present, and recent sampling at the 145 m RL (60 m below mean surface) returned values of 2600 g/t gold in association with chalcocite and mercury. Weathering processes have effectively depleted the oxide zone of other elements associated with mineralisation. Copper, zinc and lead abundance is commonly low in the oxide zone but assays of material from the discontinuous supergene redox blanket are up to 10% copper, 7% lead, 4% zinc, 3 g/t gold and 800 g/t silver.
digenite (C S Rugless, unpublished data, 1995). The primary sulphides occur together and appear late in the paragenesis, replacing subhedral and euhedral fractured pyrite. Grain size varies from 0.3 to 2 mm. Sphalerite is honey coloured and low in iron. The base metal intersections contain low gold grades, commonly less than 1 g/t. Sphalerite, galena and gold-bearing pyrite also occur as veins or disseminations associated with ankerite in a fault breccia matrix.
ORE GENESIS The McKinnons oxide deposit formed largely by supergene enrichment weathering processes in the Tertiary. Enrichment occurred above an interpreted Early to Middle Devonian structurally controlled zinc-lead-gold mineralised system, which formed during the second deformational event. The style and structural controls of the primary mineralisation appear to be broadly similar to other Cobar Basin deposits which are situated on the eastern (CSA and Peak) and northern (Elura) basin margins. Fluid inclusions and sulphur, oxygen, hydrogen and carbon isotope studies undertaken on these deposits (Seccombe and Brill, 1989; Schmidt, 1990; Seccombe, 1990; Seccombe and Jiang, 1996) suggest that multiple ore fluids may be involved. Low salinity fluids at Elura and Peak may be derived from dehydration metamorphic reactions in basement rocks. High salinity and chloride-rich fluids in the zinc-lead mineralisation phase at Peak mine suggest a second major ore fluid source, possibly derived from the Cobar Basin marine sediment.
Gold mineralisation narrows from over 120 m wide in the oxide zone to less than 10 m in the primary zone. It is hosted within a 330o trending shear zone. Although free gold is present in the primary zone, most is refractory, locked up as cryptocrystalline gold in pyrite. Metallurgical recoveries vary from over 90% in the oxide zone to less than 49% in the primary zone, reflecting the change from free gold grains to largely cryptocrystalline gold bound in sulphide.
McKinnons is associated with a high level (less than 1–2 km depth) brittle fault zone, trending NW and associated with the western margin of the Cobar Basin. The zone has tapped the ore fluid source area via a permeable fault system and allowed focussed fluid flow into the overlying host rocks. Mineralisation is localised by NE- trending faults which formed dilational zones where they intersected NW-trending faults.
Pyrite is the most abundant sulphide and forms an extensive halo, associated with the NW-trending fault zone, as outlined by an induced polarisation survey and drilling. At least five generations of pyrite are recognised.
The primary fluids are interpreted to have been derived from the underlying Cobar Basin sediment and basement rocks. Ore fluid transport occurred during basin compression, focussed along deep crustal fault zones along the edge of the Cobar Basin in dilational locations. Lower Devonian intrusive activity along the basin margins may have promoted mineralisation by providing fluid convection cells, fluids, and possibly metals.
1.
The first is 0.01 to 0.03 mm diameter disseminated euhedral pyrite. Medium to coarse-grained euhedral pyrite is also disseminated in the sediment and contains parallel bladed recrystallised quartz in pressure shadows formed parallel to cleavage. This suggests that pyrite and quartz grew before the cleavage formed.
ACKNOWLEDGEMENTS
2.
Coarse grained pyrite with numerous overgrowths, and hexagonal and framboidal textures.
This paper is presented with the permission of Burdekin Resources NL. Review of this paper by R A Glen is gratefully acknowledged.
3.
Bladed pyrite pseudomorphs after marcasite occur in the core of some framboidal pyrite.
REFERENCES
4.
Medium to fine-grained pyrite-quartz veins to 7 cm wide, striking north and 040o with a subvertical dip. These unoxidised veins contain to 7 g/t gold and are up to 30 cm wide. They may represent the primary gold lode of the McKinnons system.
5.
Coarse-grained massive pyrite veins, parallel to steep faults, trend at 070o, and crosscut early pyrite and quartz veins. Beneath the McKinnons pit such veins contain up to 1 g/t gold.
Other sulphides present in the deposit are arsenopyrite, sphalerite, galena, chalcopyrite, tetrahedrite, covellite and
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Andrews, E C, 1913. Report on the Cobar copper and gold-field, Part 1, Geological Survey of New South Wales Mineral Resources 17. Brooke, W J L, 1975. Cobar mining field, in Economic Geology of Australia and Papua New Guinea, Vol 1 Metals (Ed: C L Knight), pp 683–694 (The Australasian Institute of Mining and Metallurgy: Melbourne). Bywater, A, Johnston, C, Hall, C R, Wallace Bell, P and Elliott, S M, 1996. Geology of McKinnons gold mine, Cobar, New South Wales, in The Cobar Mineral Field – A 1996 Perspective (Eds: W G Cook, A J H Ford, J J McDermott, P N Standish, C L Stegman and T M Stegman), pp 279–291 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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McKINNONS GOLD DEPOSIT, COBAR
Dong, G, Morrison, G and Jaireth, S, 1995. Quartz textures in epithermal veins, Queensland - Classification, origin, and implication, Economic Geology, 90:1841–1856. Elliott, S M, 1995. Discovery and development of the McKinnons gold deposit, Cobar, New South Wales, in New Generation Gold Mines: Case Histories of Discovery, 1995, pp 12.1–12.11 (Australian Mineral Foundation: Adelaide). Elliot, S M, Snowden, D V, Bywater, A, Standing, C A and Ryba, A 1997. Reconcilation of the McKinnon gold deposit, Cobar, New South Wales, in Proceedings The Third International Mining Geology Conference, pp 113–121 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Marshall, S M, Scott, K M and Kamprad, J L, 1996. Mineralogy and geochemistry of the regolith at the McKinnons gold deposit, Western NSW, CRCLEME Restricted Report 29R, CSIRO Exploration and Mining, Wembley, WA (unpublished). Rugless, C S and Elliott, S M, 1995. Multielement geochemical exploration in deeply weathered terrain: the McKinnons gold deposit near Cobar, NSW, Australia - a case study, in 17th IGES, 15/19 May 1995, Townsville, Extended Abstracts, pp 100-102. Schmidt, B L, 1990. Elura zinc-lead-silver deposit, Cobar, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1329–1336 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Glen, R A, 1987a. Copper- and gold-rich deposits in deformed turbidites at Cobar, Australia: their structural control and hydrothermal origin, Economic Geology, 82:124–140.
Seccombe, P K, 1990. Fluid inclusion and sulphur isotope evidence for syntectonic mineralisation at the Elura mine, southeastern Australia, Mineralium Deposita, 25:304-312.
Glen, R A, 1987b. Geology of the Wrightville 1:100 000 sheet 8034, Geological Survey of New South Wales, Explanatory Notes.
Seccombe, P K and Brill, B A, 1989. Fluid inclusion and S, O, H and C isotopic evidence for metamorphic Cu, Zn, Pb and Au ore formation at Cobar, New South Wales, Australia, 28th International Geological Congress, Abstracts, 30:66-67.
Glen, R A, 1990. Formation and inversion of transtensional basins in the western part of the Lachlan Folt Belt, Australia, with emphasis on the Cobar Basin, Journal of Structural Geology, 12 (5/6):601–620. Glen, R A, 1994. Geology of the Cobar 1:100 000 sheet 8035, Geological Survey of New South Wales, Explanatory Notes. Glen, R A, 1995. Thrusts and thrust-associated mineralisation in the Lachlan Orogen, Economic Geology, 90:1402–1429. Glen, R A, Clare A and Spencer, R, 1996. Extrapolating the Cobar Basin model to the regional scale: Devonian Basin-formation and inversion in Western New South Wales, in The Cobar Mineral Field - A 1996 Perspective (Eds: W G Cook, A J H Ford, J J McDermott, P N Standish, C L Stegman and T M Stegman), pp 43–83 (The Australasian Institute of Mining and Metallurgy: Melbourne). Glen, R A, Drummond, B J, Goleby, B R, Palmer, D and Wake-Dyster, K D, 1994. Structure of the Cobar Basin based on seismic reflection profiling. Australian Journal of Earth Sciences, 41:341–352 .
Seccombe, P K and Jiang, Z, 1996. Fluid evolution in shear-zone hosted Cu/Au vein deposits: Examples from the Cobar region, Lachlan Fold Belt, Australia, Pan-American Current Research on Fluid Inclusions, 30 May–1 June, 1996, Madison, Wisconsin. Stegman, C and Pocock, J, 1996. The Cobar gold field - A geological perspective, in The Cobar Mineral Field - A 1996 Perspective, (Eds: W G Cook, A J H Ford, J J McDermott, P N Standish, C L Stegman and T M Stegman), pp 229–264 (The Australasian Institute of Mining and Metallurgy: Melbourne). Stegman, C L and Stegman, T M, 1996. The history of mining in the Cobar Field, The Cobar Mineral Field - A 1996 Perspective (Eds: W G Cook, A J H Ford, J J McDermott, P N Standish, C L Stegman and T M Stegman), pp 3–42 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Leah, P A, 1996. Relict lateritic weathering profiles in the Cobar district, NSW, in The Cobar Mineral Field - A 1996 Perspective (Eds: W G Cook, A J H Ford, J J McDermott, P N Standish, C L Stegman and T M Stegman), pp 157–177 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Wilkins, C and Smart, G, 1998. Browns Creek gold-copper deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 575–580 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Browns Creek gold-copper deposit 1
by C Wilkins and G Smart
2
INTRODUCTION The deposit is 8 km west of Blayney and 220 km west of Sydney (Fig 1), at lat 33o32′S, long 149o10′E on the Bathurst (SI 55–8) 1:250 000 scale map sheet. The total Measured, Indicated and Inferred Resource at 30 June 1996 was 1.36 Mt at 6.10 g/t gold and 0.44% copper. During the 1995–96 year 248 616 t of ore at 5.87 g/t gold and 0.46 % copper was treated, to produce 44 397 oz of gold and 1040 t of contained copper in concentrate. As the result of an increase in the resources and reserves due to underground exploration drilling and accelerated mine development, 1997 production at Browns Creek is predicted to expand from 240 000 to 390 000 tpa, to yield more than 70 000 oz of gold per year.
EXPLORATION AND MINING HISTORY First worked in 1871, the Browns Creek mine was operated by M J Hickey from 1979 to 1986 as a minor open cut and underground operation, and then by BHP Gold and Newcrest Mining as an open pit mine (Creelman, Lipton and Stagg, 1990). Incomplete production records indicate that 28 233 oz of gold were recovered from treatment of 501 995 t of ore between 1982 and 1986. Fourteen diamond drill holes were also completed during this period. In 1986 a short but intensive exploration campaign by Western Mining Corporation Limited, involving the drilling of 63 diamond and percussion drill holes, defined a resource of 593 810 t at 5.9 g/t gold. BHP Minerals Limited acquired the Browns Creek mine and associated mining leases from M J Hickey in September 1986 and completed a major expansion of mining and treatment operations. Open cut mining by BHP Gold and subsequently Newcrest Mining between 1986 and early 1993 recovered 175 394 oz of gold from 1 569 191 t of ore grading 3.48 g/t gold. Mining by BHP and Newcrest developed the open cut mine to a depth of 140 m. Towards the end of the pit life, four deep holes were drilled well below the pit to test for ore potentially accessible from underground operations. Intercepts from this drilling gave reasonable confidence that the skarn body continued for 150 m beneath the pit floor and an Inferred Resource of 248 000 t at 8.2 g/t gold was estimated. Hargraves Resources NL acquired the Browns Creek mine and leases from Newcrest Mining in September 1993 and completed 12 deep diamond drill holes that defined a Measured, Indicated and Inferred Resource of 510 000 t at 8.2 g/t gold. On the basis of this resource and the high prospectivity
1.
Lecturer, Department of Geology and Geophysics, University of Sydney NSW 2006.
2.
Chief Mine Geologist, Hargraves Resources NL, Browns Creek Mine, Locked Bag 7, Blayney NSW 2799.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Location map and simplified geological map of the Carcoar–Blayney area (after Wyborn et al, 1994).
of the area, Hargraves Resources decided to construct an underground mine based on annual production of 150 000 t of ore and recovery of 33 000 oz of gold. Underground development commenced in November 1994 and the first production was in May 1995. Mining is by an uphole retreat benching method. As at June 1996 total production was 310 079 oz of gold recovered from 2 671 992 t of ore grading 3.64 g/t gold. The current Hargraves Resources underground operation has yielded 47 132 oz of gold from 265 806 t of ore grading 5.82 g/t gold.
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PREVIOUS DESCRIPTIONS The ore deposit has previously been described: 1.
prior to open pit mining by Stanton (1948), McManus and Mortimore (1974), Burnham (1976), Bowman, Richardson and Hobbs (1977) and Taylor (1983);
2.
during open pit mining by Newcrest Mining Ltd by Creelman, Lipton and Stagg (1990); and
3.
at an early stage of the Hargraves Resources underground mining operation by Meldrum (1995).
REGIONAL GEOLOGY The Browns Creek deposit is in the eastern Lachlan Fold Belt within the Early Ordovician to Early Silurian volcanosedimentary sequences of the Molong Volcanic Rise, which comprise the Adaminaby Group, Kenilworth Group, Bowan Park Group and Cabonne Group (Fig 1). To the west of the mine the Forest Reefs Volcanics represent a major eruptive centre within the Cabonne Group whereas the Blayney Volcanics (including the Cowriga Limestone Member) represent extensive outpourings of basaltic lavas and volcaniclastics to the east, in deeper water away from eruptive centres. The Blayney Volcanics range in composition from calc-alkaline, through high potassium calc-alkaline, to shoshonitic (Wyborn, 1992). Following cessation of volcanic activity along the Molong Volcanic Rise, sediment and volcanic rocks were laid down in the newly opened Cowra and Hill End troughs (Early Silurian to Early Devonian) and form extensive sequences in the surrounding area. The Browns Creek area was intruded by the Late Silurian Carcoar Granodiorite and related intrusive phases such as the Long Hill diorite, which are separated from the Forest Reefs caldera structure to the west by the Carcoar Fault. Low grade regional metamorphism is characterised by widespread prehnite-pumpellyite assembages and albitisation of calcic plagioclase in volcanics, with upper greenschist facies rocks to the south. Metamorphism in these upper crustal rocks was caused by extensive melting of the lower crust that gave rise to the Silurian–Early Devonian granites and felsic volcanics. The region was deformed initially during NNE-SSW dextral transpression related to the closure of the Hill End Trough to the east, followed by the Carboniferous Kanimblan Orogeny (Glen, 1992; Pogson and Wyborn, 1994).
LOCAL GEOLOGY The Cowriga Limestone Member of the Blayney Volcanics was recrystallised to a medium to coarse grained marble by the intrusion of the Carcoar Granodiorite. It contains minor interbedded basaltic volcaniclastic units and is at least 150–200 m thick, with the lower contact not yet encountered in deep exploration drilling due to fold-related thickening of the unit. The Cowriga Limestone is conformably overlain by up to 190 m of Blayney Volcanics. The basal unit consists of basaltic sandstone and conglomerate with rare limestone, all characterised by rapid lateral facies variations. It is followed by up to 100 m of clinopyroxene phenocryst-rich and plagioclase phenocryst-poor porphyritic basalt including horizons of hyaloclastite, pillow basalt and interpillow chert. Porphyritic basalt is overlain by 130 m of dominantly plagioclase-rich volcaniclastic rocks. The depositional environment for the
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Blayney Volcanics is consistent with a marine setting with shallow to moderate water depth on the flanks of an emergent volcanic island, related to a volcanic centre represented by the Forest Reefs volcanic complex 8 km west of Browns Creek (I Cooper, unpublished data, 1996). The deposit is near the NW corner of the Carcoar Granodiorite (Fig 1). The bulk of the intrusion is granodiorite, averaging 45% plagioclase feldspar, 10% alkali feldspar, 25% quartz, 10% biotite and 10% hornblende, with accessory apatite, zircon, pyroxene, sphene and ilmenite, and the local alteration products chlorite, clinozoisite, epidote, sericite, actinolite and calcite. The full spectrum of intrusives from diorite to pegmatite and aplite are present in the mine area. The intrusive contact with the Blayney Volcanics is characteristically steeply dipping and highly irregular (Fig 2). The Long Hill diorite is a local name for the intermediate intrusive within the Browns Creek open pit. Related aplite and feldspar-rich monzonite sills and dykes with hornblende and biotite are common in the underground workings and post-date mineralised skarns. Crosscutting Tertiary mafic dykes, 10 cm to 2 m thick, occur within the open pit and underground mine and are related to the Tertiary Canobolas shield volcano, 20 km to the north (Meldrum, 1995; I Cooper, unpublished data, 1996). Contact metamorphism by the Carcoar Granodiorite has obliterated much of the bedding and produced coarse grained marble and pyroxene-hornblende hornfels facies rocks in poorly bedded volcanic units, hence structural information is sparse. The Blayney Volcanics have undergone two phases of deformation, which formed the domal outcrop pattern of Cowriga Limestone near the mine (Fig 2). F1 folds have hinge lines plunging moderately NNE which swing into a SSE orientation to the east of the mine. F1 folds have steep eastdipping axial planes in the north and become reclined with easterly dipping axial planes to the south. They are overprinted by upright open F2 folds gently plunging to the WNW and ESE. Fold interference and intrusion-related deformation has produced steep to overturned bedding with only a thin veneer of mafic volcanics present between granodiorite and marble (I Cooper, unpublished data, 1996). The mine area is further complicated by steep north- and NNW-trending dextral faults. The most important fault is the Mount David fault zone which traverses the open pit (Fig 2) and forms the 4000 E fault zone' in the underground mine. This fault system controls the location of the gold-copper skarn-hosted mineralisation at Browns Creek. At higher levels the fault system has the form of a horsetail splay to the west of the Mount David fault zone.
ORE DEPOSIT FEATURES The following description briefly reviews the findings of Creelman, Lipton and Stagg (1990) and concentrates on data from underground mapping. Gold-copper mineralisation occurs within skarned marble and mafic volcanic rocks at their western contact with a major granodiorite intrusion. Massive garnet, pyroxene and wollastonite dominated skarns contain disseminated chalcopyrite, bornite and gold. From surface at 820 m RL, to the base of the open pit at 680 m RL, skarn ore was described as having a stratabound or vein character (Creelman, Lipton and Stagg, 1990). Stratabound, replacive skarn, developed along the east trending marble–volcanic contact adjacent to the pluton, consists of the
Geology of Australian and Papua New Guinean Mineral Deposits
BROWNS CREEK GOLD-COPPER DEPOSIT
FIG 2- Surface geological map of the Browns Creek mine area (after I Cooper, unpublished data, 1996).
assemblage calcite-garnet (grossular-andradite)-wollastonitevesuvianite-hedenbergite-quartz-epidote. Vertically continuous vein skarns of centimetres to 15 m width form northtrending subvertical alteration zones associated with faults or dyke margins. Skarn associated mineralisation consists of early veins of pyrrhotite-arsenopyrite-pyrite followed by later veins and clots of bornite-chalcopyrite. The association gold-hessite-hedleyite forms the final stage of mineralisation filling fine fractures in the copper ore. A significant part of the gold-rich mineralisation (+10 g/t gold) is associated with chalcedonic silica (‘jasper’) in three types of retrograde clay lodes: 1.
nontronite (montmorillonitic clay) zones at the margins of wollastonite skarns;
2.
massive clay zones due to in situ weathering of skarns; and
3.
clay breccia zones due to karst collapse in areas of intense retrograde skarning.
Geology of Australian and Papua New Guinean Mineral Deposits
MINERALISATION IN THE UNDERGROUND MINE In the open pit the orebody had an east trend but thinned towards the base of the pit as it swung into a southerly orientation adjacent to the Mount David fault zone which marked the eastern margin of the mineralisation. As the orebody thinned with depth, open pit mining ceased. The newly defined south-trending Cowriga lode has the overall form of a tabular body about 250 m long and locally up to 50 m wide, with a steep but variable dip and a moderate southerly plunge (Figs 3 and 4). The Cowriga lode consists of structurally controlled coppergold bearing skarns and gold-rich retrograde skarns located between multiple strands of the 4000 E fault zone which propagated at the contact between granodiorite and the country rocks (Fig 5). The structural complexity of the orebody became apparent when continuous exposures were available for mapping in underground openings. It was found that banding within marble, in the hanging wall to mineralised skarns, was
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duplex fault slices caused brittle failure and the formation of large volumes of extensional vein arrays preferentially hosted in pyroxene-garnet basalt-derived skarn units. These structurally controlled sheeted quartz vein arrays host lower temperature retrograde ore skarn assemblages (quartz-calciteepidote-prehnite-chlorite-sericite-gold-chalcopyrite-bornite) characterised by high, and locally bonanza, gold grades to 40 g/t. Post-mineralisation retrogression (calcite-quartz-chloritesericite) and monzonite sills and dykes are ubiquitous. Prominent east- and west-dipping low angle conjugate fault sets are the final phase of deformation and substantially increase the structural complexity of the deposit.
FIG 3 - Longitudinal projection of the Browns Creek orebody. Looking west, showing stoped out areas (solid fill) and outline of the resource.
Hydrothermal fluid flow was guided by the permeability of fault networks and caused heterogeneous skarn formation and mineralisation in alteration zones up to 50 m wide (Fig 5). Skarn reaction fronts, for example those affecting marble, are clearly fault controlled. The disposition of Blayney Volcanics, Cowriga Limestone and their skarn derivatives on the 577 level is controlled by dextral strike-slip duplexes at a major bend in the irregular granodiorite contact that imposed a fundamental boundary constraint during the propagation of fault strands. The extensional form of the fault duplex also controlled the intrusion of post-ore monzonite bodies that cut and invade sheeted quartz vein ore and contain rotated blocks of ore.
ORE GENESIS
FIG 4 - Sectional projection (on 24 775 m N) of the Browns Creek orebody looking north, showing the location of skarn mineralisation.
not relict bedding but an intense foliation in calcite mylonite developed within the 4000 E fault zone. Dextral transcurrent motion on the 4000 E fault zone produced vertical anastomosing fault duplex systems that controlled the location and formation of metasomatic prograde wollastonite-garnet-pyroxene-plagioclase exoskarn in both marble and hornfelsed basaltic volcanic rocks (Fig 5). Previously described fault controlled, marble-derived and basalt-derived skarns were the locus for the first stage of copper-gold mineralisation (pyrite-pyrrhotite-arsenopyritechalcopyrite-bornite-chalcocite-gold-tellurides) and retrograde gangue (chlorite-sericite-quartz-calcite-epidote-biotitehornblende) shortly after prograde skarn formation, as temperatures began to fall. Movement on the 4000 E fault zone, a dextral transcurrent fault system with gently plunging slickenside lineations on fault surfaces, controlled the prograde and copper-gold skarns. The sense of movement then later changed to dextral transtensional dip-slip, with steep slickenside lineations on fault surfaces, forming extensional duplexes at offsets and overlaps of fault strands. Transtensional movement along
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Skarn formation at the contact between the basaltic Blayney Volcanics and the Cowriga Limestone involved metamorphic reactions between calcium-aluminium-iron-magnesiumbearing minerals, forming prograde garnet-pyroxenewollastonite-anorthite-calcite assemblages in the temperature range 500–650oC (Taylor, 1983; Creelman, Lipton and Stagg, 1990). The peak temperature of skarn formation at 1 kb would have been 475–535oC (with reference to isobaric T–XCO2 diagrams for reactions producing the above minerals with low XCO2 skarn-forming fluids), and the presence of minor anorthite suggests temperatures greater than 540oC at some locations (Meldrum, 1995). Gold-copper mineralisation clearly post-dates and replaces peak metamorphic garnetpyroxene-wollastonite skarn assemblages and is associated with retrograde gangue assemblages overprinting the original skarn. In the underground mine (Meldrum, 1995), the following assemblages are prominent and formed from infiltrating hydrothermal fluids as temperatures waned: 1.
Retrograde calcite-quartz-sericite-chlorite-epidoteamphibole-biotite are intergrown with the disseminated copper-gold ore association of pyrrhotite-pyritechalcopyrite-bornite-gold-tellurides (hessite-altaite). Bornite is the most abundant sulphide and forms disseminations, replacements and vein fillings in all skarn types.
2.
Sheeted quartz veins containing calcite-quartz-serictechlorite-epidote-prehnite and gold-rich chalcopyritebornite-gold ore.
Ore genesis is currently being re-evaluated in the light of a strong structural control of mineralised skarns in the underground mine. Kjolle et al (1994) previously concluded that deformation of the skarn and granitoids was minor relative to that in the Ordovician volcanics, and indicated that the major deformation along the fault zone occurred prior to the intrusion of the Long Hill diorite. However, underground mapping indicates that fault-related deformation was long lived and
Geology of Australian and Papua New Guinean Mineral Deposits
BROWNS CREEK GOLD-COPPER DEPOSIT
FIG 5 - Geological map of the northern part of 577 level, showing the fault-controlled form of the Browns Creek orebody.
controlled skarning and mineralisation along the 4000E fault zone. The Long Hill diorite has been dated as Silurian, at 425±4.5 Myr from alteration minerals and 418.9±1.4 Myr from the magmatic rocks (Perkins, Walshe and Morrison, 1995) and hence skarning and mineralisation closely approximate these ages. The lead isotope ratios of the gold-copper mineralisation indicate significant mixing of crustal and mantle derived lead (Carr et al, 1995). Possible sources of lead were thought to be the Ordovician Blayney Volcanics and the Silurian intrusive Carcoar Granodiorite and Long Hill diorite, whereby intrusiverelated hydrothermal solutions leached copper, gold and some lead from the Blayney Volcanics to form the skarn mineralisation.
Geology of Australian and Papua New Guinean Mineral Deposits
Kjolle et al (1994) used neodymium, strontium and lead radiogenic isotopes to assess the source of ore fluids and metals, and also concluded that their source was not solely the adjacent granitoid. They thought that gold and copper were leached to a significant degree from the Blayney Volcanics during fluid flow around the cooling mid Silurian intrusion and subsequently deposited after prograde skarn formation. An alternative, more plausible, model involves a primary late stage intrusive-related hydrothermal fluid focussed into permeable fault zones that propagated adjacent to pluton margins, with fluid mixing and bimetasomatism during skarn formation tending to obscure a purely magmatic signature for the mineralisation.
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MINE GEOLOGICAL METHODS Resource definition is undertaken by BQ diamond drilling from positions in the granodiorite to yield an approximate 25 by 25 m grid of orebody intersections. Additional drilling may be required in more complex areas. All excavations are geologically mapped and sampled during development.
ACKNOWLEDGEMENTS
Kjolle, I, Walshe, J L, Surman, J A and Whitford, D J, 1994. Gold recycled from Ordovician shoshonites in the Browns Creek skarn deposit, NSW, Geological Society of Australia Abstracts, 37:221. McManus, J B and Mortimore, I R, 1974. Browns Creek copper-goldlimestone-marble prospect, Texins Development Pty Ltd unpublished company report to New South Wales Geological Survey, Open File GS 1974/462. Meldrum, E A, 1995. Multistage gold mineralising events at the Browns Creek gold-copper mine, BSc Honours thesis (unpublished), University of Sydney. Perkins, C, Walshe, J L and Morrison, G, 1995. Metallogenic episodes of the Tasman Fold Belt System, eastern Australia, Economic Geology, 90:1443–1466.
This paper is published with the permission of Hargraves Resources NL. The manuscript was improved by reviews and extensive discussions with B Cotton and A Border. P Van Lynt, I Cooper and E Meldrum have all contributed greatly to understanding the local and mine geological framework. P Ogden is thanked for his help producing some of the figures. CW is supported by a Sydney University research grant and by Hargraves Resources NL.
Stanton, R L, 1948. Geological report - Browns Creek, Blayney, NSW, Broken Hill South Ltd unpublished company report to New South Wales Geological Survey, Open File GS 1948/020.
REFERENCES
Taylor, G R, 1983. Copper and gold in skarn at Browns Creek, Blayney, NSW, Journal Geological Society of Australia, 30:431–442.
Bowman, H N, Richardson, S J and Hobbs, J J, 1977. Browns Creek disseminated gold-copper mine - a volcanogenic deposit, New South Wales Geological Survey Report 1977/086.
Walshe, J L, Heithersay, P S and Morrison, G W, 1995. Toward an understanding of the Tasman Fold Belt System, Economic Geology, 90:1382–1401.
Burnham, P, 1976. The geology of the Browns Creek gold mine and area, BSc Honours thesis (unpublished), University of Sydney.
Wyborn, D, 1992. The tectonic significance of Ordovician magmatism in the eastern Lachlan Fold Belt, Tectonophysics, 214:177–192.
Carr, G, Dean, J A, Suppel, D W and Heithersay, P S, 1995. Precise lead isotope fingerprinting of hydrothermal activity associated with Ordovician to Carboniferous metallogenic events in the Lachlan Fold Belt of New South Wales, Economic Geology, 90:1467–1505.
Wyborn, D, Stuart-Smith, P G, Henderson, G A M, Wallace, D A, Raymond, O, Krynen, J, Moffitt, B, Watkins, J, Hawley, S, Pogson, D, Meakin, S, Spackman, J, Scott, M, Warren, A and Morgan, E, 1994. Bathurst Geology 1:250 000 Scale Map Sheet SI 55–8, preliminary 2nd edition (New South Wales Department of Mineral Resources: Sydney, and Australian Geological Survey Organisation: Canberra).
Creelman, R A, Lipton, I T and Stagg, R N, 1990. Browns Creek gold deposit, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1399–1401 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Pogson, D and Wyborn, D, 1994. Excursion guide: Bathurst 1:250 000 geological sheet, New South Wales Geological Survey Report GS 1994/139.
Glen, R A, 1992. Thrust, extensional and strike slip tectonics in an evolving Palaeozoic orogen - a structural synthesis of the Lachlan orogen of southeastern Australia, Tectonophysics , 214:341–380.
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Geology of Australian and Papua New Guinean Mineral Deposits
McInnes, P, Miles, I, Radclyffe, D and Brooker, M, 1998. Endeavour 42 (E42) gold deposit, Lake Cowal, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 581–586 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Endeavour 42 (E42) gold deposit, Lake Cowal 1
2
2
by P McInnes , I Miles , D Radclyffe and M Brooker INTRODUCTION The deposit is on the western edge of Lake Cowal approximately 35 km NNE of West Wyalong in central western NSW, at lat 33o38′S, long 147o24′E and AMG coordinates 537 600 E, 6 278 000 N on the Forbes (SI 55–7) 1:250 000 scale and the Wyalong (8330) 1:100 000 scale map sheets (Fig 1). The exploration and mining rights are held by North Gold (WA) Limited, a wholly owned subsidiary of North Limited (North). Although the surrounding district has a history of gold mining
3
at the nearby West Wyalong and Forbes goldfields, the Lake Cowal gold mineralisation is a greenfields discovery. The deposit has a Measured and Indicated Resource of 62.6 Mt at 1.5 g/t gold using a 0.8 g/t gold cutoff (North Limited, 1996) and is amenable to conventional open cut mining and processing methods. In April 1996 a development application was refused by the NSW State Government.
EXPLORATION HISTORY The discovery at Lake Cowal is the result of an exploration program by Geopeko (now North Limited) that commenced in 1980. The program was designed to explore for porphyry copper-gold districts of Goonumbla type (Heithersay et al, 1990; Love, 1992) within the Lachlan Fold Belt (LFB). The Lake Cowal area was selected as a result of this program, largely based on the similarity of the aeromagnetic pattern to the Goonumbla district. The area was virtually unexplored and almost totally covered by a poorly consolidated lake bed sequence from less than 1 to greater than 100 m thick. The exploration program consequently relied almost entirely on aeromagnetic and gravity methods and reconnaissance drilling to gain geological, structural and geochemical information. This program identified a copper bedrock anomaly of area 4 by 2 km, subsequently named Endeavour 39(E39), that became the focus of exploration for a number of years. The E39 prospect is approximately 6 km south of E42 and was found to be a large low grade uneconomic porphyry copper system with no apparent higher grade zones. Again, using the Goonumbla model, exploration targeted satellite plugs and intrusives around the margins of the parent stock. A target zone to the north of E39 was identified, but exploration was impeded by the flooding of Lake Cowal and further exploration had to wait until 1988 when the lake partially dried up. Subsequent rotary air blast (RAB) and aircore drilling delineated zones of anomalous gold mineralisation within a northerly trending ‘gold corridor’ approximately 6 km long and 2 km wide. The E42 prospect within this gold corridor has been advanced to deposit status and several other prospects in the area are at the advanced exploration phase.
FIG 1 - Location map and tectonic units of the Lachlan Fold Belt (after Suppel and Scheibner, 1990).
1.
Regional Geologist, North Limited, Cnr Clarke & Alluvial Streets, Parkes NSW 2870.
2.
Geologist, North Limited, Cnr Clarke & Alluvial Streets, Parkes NSW 2870.
3.
Geologist, Inversiones North (Chile) Limitada, San Sebastian 2839, OF702, Las Condes Santiago, Chile.
Geology of Australian and Papua New Guinean Mineral Deposits
North completed a preliminary feasibility study in 1993, an extensive resource delineation drilling program in 1993–94 and a detailed feasibility study in 1995. The feasibility study used data from 23 971 m of aircore drilling, 17 303 m of reverse circulation percussion drilling and 23 019 m of diamond core drilling, dominantly in NQ size. The majority of the E42 deposit has been drilled on 25 m spaced east–west section lines with holes approximately 25 m apart along the lines, with very little drill hole information below 350 m. The proposed open pit is approximately 1000 m in diameter at surface and has a
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final depth of about 330 m. The block model estimation method of multiple indicator kriging was used in the resource estimation as this was considered the best method for handling the skewed distribution and spotty nature of the gold grade. An EIS was completed and a Commission of Inquiry then investigated the environmental aspects of mining adjacent to a significant wetland. The Commission of Inquiry found that mining could proceed under stringent environmental controls. The gold discovery at Lake Cowal is largely a story of persistence and dedication by field staff in rigorously testing and adapting the exploration model to fit field data, and by management who supported the area selection criteria and provided funding for the eight years prior to the discovery.
PREVIOUS DESCRIPTIONS This paper is largely a summary of a description of the deposit geology by Miles and Brooker (in press). Previous research on the E42 deposit has included a study of volcanic facies (Miles, 1993), a general geological report by M R Brooker, I N Miles and S E Thornett (unpublished data, 1995), extensive Ar-Ar and K-Ar dating (C Perkins, unpublished data, 1993) and studies by E N Bastrakov (unpublished data, 1996) of the gold and copper metallogenesis.
REGIONAL GEOLOGY Gold mineralisation at E42 is hosted by Ordovician volcanic rocks, known informally as the Lake Cowal volcanic complex (Miles, 1993), within the Palaeozoic LFB. The volcanic complex is on the western margin of the Girilambone Anticlinorial Zone, a structural unit of the LFB that is separated from the lower Palaeozoic Wagga–Omeo Metamorphic Belt by the Gilmore Fault zone (Fig 1). The Ordovician volcanic rocks are surrounded by and unconformably overlain by Siluro-Devonian sedimentary and volcanic rocks (Fig 2). The Siluro-Devonian Derriwong Group (Ootha beds and Manna Conglomerate) consisting of marine sediment, crop out to the north, south and west of Lake Cowal, in a series of hills that include the Booberoi Hills, and the Wamboyne and Manna mountains (Miles, 1993). The rocks of the Booberoi Hills and Wamboyne Mountains are highly deformed and define a structure known as the Booberoi Fault (Ingpen, 1995). This northerly trending structure is clearly apparent on regional aeromagnetic images on the western side of the Lake Cowal volcanic complex and is interpreted to be a splay off the Gilmore Fault zone (M R Brooker, I N Miles and S E Thornett, unpublished data, 1995) which is regionally significant in terms of gold mineralisation. The volcanic complex is oval in plan, approximately 40 km long and 15 km wide, and although not outcropping is defined by a distinct regional magnetic high anomaly. Exploration drilling indicates that the complex consists of intermediate calc-alkaline intrusive, volcanic and volcaniclastic rocks. The southern portion of the complex is dominated by a large zoned holocrystalline to porphyritic intrusive body, ranging in composition from granodiorite to tonalite. Marginal to this felsic intrusive is a more mafic body ranging from diorite to gabbro. The central and northern portions of the volcanic complex are dominated by volcaniclastic rocks which are intruded by diorite, gabbro and syenite.
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FIG 2 - Lake Cowal interpreted regional subsurface geological map.
ORE DEPOSIT FEATURES LITHOLOGY AND STRATIGRAPHY There is no outcrop of the host Ordovician rocks at E42, however minor gossanous float was discovered over the deposit. The area is covered by lake sediment, to 30 m thick, which is dominated by clay with a variable sand and gravel content. Beneath the lake sediment is an irregularly developed Tertiary laterite profile, comprising a mottled zone and underlying saprolite zone that averages 10 to 20 m thick over the central part of the deposit (M R Brooker, I N Miles and S E Thornett, unpublished data, 1995). Hence the subsurface geology of the deposit is only understood from drilling data. The deposit is hosted by volcaniclastic and coeval extrusive and intrusive magmatic rocks. The sequence dips to the NW at around 40 to 45o with local variations due to faulting or slumping. The stratigraphy of the volcanic sequence was informally subdivided by Miles (1993) into three conformable units, which are from the youngest to the oldest (Figs 2 and 3): 1.
The Great Flood unit consists of more than 250 m of monotonous beds, to 72 m thick, of vitric volcaniclastic debris in which the sedimentary structures are consistent with mass-flow type deposits. The coarser units are interbedded with intervals of less than 3 m of laminated
Geology of Australian and Papua New Guinean Mineral Deposits
ENDEAVOUR 42 (E42) GOLD DEPOSIT, LAKE COWAL
described as a diorite (Miles and Brooker, in press). It has a variable texture, from holocrystalline and equigranular to a porphyritic variety which contains up to 40% groundmass. Porphyritic examples contain 30% subhedral plagioclase and 30% hornblende and no visible quartz. Whole rock analyses and thin section evidence suggest that this intrusive has both dioritic and gabbroic phases, perhaps reflecting local magmatic differentiation. The Muddy Lake diorite has a K-Ar date of 4565±5 Myr (C Perkins, unpublished data, 1993). This date, from a sample of magmatic hornblende, is the minimum age of the intrusion and corresponds to a lower Late Ordovician age. A series of porphyritic to aphanitic dykes, of mafic to intermediate composition, intersect both the volcanic sequence and the Muddy Lake diorite. The dykes are 20 mm to greater than 20 m thick, however most are 1 to 3 m thick (Miles and Brooker, in press). It is considered that the dykes were emplaced in active fault zones as their margins often show signs of fault movement and they are commonly strongly altered by silica-sericite (Miles, 1993). Whole rock analyses indicate that the dykes can be classified as basalt and basaltic andesite (E N Bastrakov, unpublished data, 1996).
MINERALISATION
FIG 3 - Stratigraphic sequence at the E42 deposit (after Miles, 1993).
siltstone and mudstone and the unit also contains a 20 m bed of polymictic volcanogenic conglomerate. 2.
The Golden Lava unit comprises 60–110 m of porphyritic trachyandesite, interbedded with monomictic sand to breccia which consists of matrix- to clast-supported plagioclase porphyritic fragments. The unit conformably overlies and is overlain by sediments that were deposited in a deep water environment. The nonvesicular nature of the lava, the jigsaw fit of the breccia fragments and the envisaged subaqueous emplacement are consistent with the unit being a submarine lava with associated hyaloclastite and autobreccia (M R Brooker, I N Miles and S E Thornett, unpublished data, 1995).
3.
The Cowal conglomerate unit is the oldest part of the sequence and consists of about 100 m of massive to graded beds of well rounded to very angular, clastsupported polymictic volcanic debris, interbedded with laminated siltstone and mudstone (Miles and Brooker, in press). Sedimentary structures within the conglomerate beds are consistent with deposition of mass flows. The conglomerates have andesitic clasts and the degree of reworking of some clasts indicates that their provenance was an emergent volcanic edifice.
INTRUSIVES The deposit occurs within a prominent embayment in a north trending zone of high magnetic response caused by an intrusive diorite body which cuts the lower part of the sequence described above. This intrusive was previously named the Muddy Lake gabbro by Miles (1993) but it is more precisely
Geology of Australian and Papua New Guinean Mineral Deposits
The gold mineralisation occurs primarily in narrow dilatant veins of quartz-carbonate-sulphide and carbonate±quartzsulphide, and within narrow healed fault zones with a similar mineralogy to the veins (Miles and Brooker, in press). Gold is found to a lesser extent with pyrite stringers and disseminations, shear chlorite-carbonate veins and shear quartz-carbonate-sulphide veins. Gold mineralisation in the oxide zone, which comprises 20% of the deposit, has a more erratic character reflecting leaching and dispersion. The auriferous quartz-carbonate-sulphide and carbonate±quartz-sulphide veins occur throughout the deposit and have a consistent strike of 305o and dip of 35o SW. The density of veining is generally highest in the Golden Lava unit although veins within the Muddy Lake diorite tend to be thicker. Adularia is a common auxiliary mineral in the quartzcarbonate-sulphide veins (M R Brooker, I N Miles and S E Thornett, unpublished data, 1995). The vein quartz is often euhedral with well developed comb textures. The veins are typically parallel sided, from less than 1 mm to greater than 10 cm wide. Selvages are rarely developed around these veins, however where the veins contain abundant adularia it may also appear as a vein selvage. Occasionally the quartz-sulphide veins have small haloes of sericite-silica alteration (Miles and Brooker, in press). The sulphides in the quartz veins, in decreasing order of abundance, are pyrite, sphalerite, chalcopyrite, galena and pyrrhotite. The veins often contain small amounts of visible gold, and the best gold grades occur with sphalerite and to a lesser extent with adularia.
ALTERATION Four alteration styles have been identified at the deposit by Miles and Brooker (in press) and E N Bastrakov (unpublished data, 1996).
Propylitic Propylitic alteration at Lake Cowal is present in the Ordovician magmatic and volcaniclastic rocks as a broad background halo peripheral to structures. It is recognised by the total albitisation
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of plagioclase and the replacement of mafic minerals by chlorite. The alteration assemblage is characterised by quartz, chlorite, epidote, albite, potassium feldspar, hematite and/or magnetite, rutile, calcite and sporadic disseminated pyrite.
chlorite-carbonate-pyrite alteration is not accompanied by complete replacement of protolith minerals or by well developed metasomatic zoning. It is best developed within the Golden Lava unit, especially along its base, and within the Cowal conglomerate.
Quartz-sericite-carbonate
The gold-bearing dilational veins most commonly occur within two of the distinct alteration assemblages described above and have some specific mineral associations. The quartz-sericite-carbonate alteration is characterised by veins consisting predominantly of ankerite-quartz-pyrite-sphaleritechalcopyrite-galena with ankerite as the dominant gangue mineral. The chlorite-carbonate-pyrite alteration is characterised by veins consisting predominantly of quartz, potassium feldspar, pyrite, sphalerite and chalcopyrite, with quartz the dominant gangue mineral. Thus the gangue mineralogy reflects to some extent the host rock composition.
This assemblage is intimately associated with faulting and gold mineralisation, and appears to be controlled by steeply dipping fault or shear zones and dyke margins. It is also associated with tectonic and hydrothermal brecciation and strong ankerite veining away from the fault or shear zones. The near-fault alteration haloes of quartz-sericite-ankerite can reach several metres in width. This alteration style is better developed within the Golden Lava unit and Great Flood unit. Alteration intensity diminishes away from the fault zones, from quartz-sericiteankerite to quartz-sericite-chlorite to quartz-sericite-albitechlorite and then to a propylitic assemblage.
Quartz-potassium feldspar This assemblage is mainly restricted to the Golden Lava unit, forming irregular patches and zones, usually associated with zones of later chloritisation. It occasionally forms narrow haloes enveloping sulphide-bearing dilational veins. This alteration type is characterised by potassium feldspar, albite, quartz and carbonate, commonly ankerite.
Chlorite-carbonate-pyrite At hand specimen scale the assemblage is characterised by quartz, chlorite, albite, potassium feldspar, calcite and pyrite. At microscale this alteration is exhibited as chlorite clots, stringers and small shear veins. Usually this alteration type is spatially associated with patchy quartz–potassium feldspar alteration, or with adularisation of albite phenocrysts. The
STRUCTURAL CONTROLS Major structural trends have been interpreted and extrapolated from drill hole information and geophysical survey data. The deposit appears to be intimately related to a system of faults (Fig 4). The dilational veins occur predominantly as vein arrays adjacent to the north trending faults. Local fault geometry and rheology contrasts of rock types are considered responsible for preferential development of veining within the Golden Lava unit (Miles and Brooker, in press). N Archibald (unpublished data, 1991) suggested that the structure of the deposit may be modelled by a sinistral riedel model. Assuming that the primary shears are the steeply-dipping north trending structures, and taking into account the SW dip of dilational veins at approximately 45o, the system can be modelled as a sinistral oblique fault system that formed in response to NW–SE compression (M R Brooker, I N Miles and S E Thornett, unpublished data, 1995).
FIG 4 - Geological cross section on 6 277 850 N and Measured and Indicated Resource blocks, E42 deposit, looking north.
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ENDEAVOUR 42 (E42) GOLD DEPOSIT, LAKE COWAL
ORE GENESIS
REFERENCES
The intrusive rocks at Lake Cowal were dated by K-Ar (E42 diorite) and Ar-Ar (E39 granodiorite) methods giving ages of 456±5 and 465.7±1 Myr respectively (C Perkins, unpublished data, 1993). Sericite associated with mineralisation at E42 has an age using the Ar-Ar method of 439±4.5 Myr (Perkins, Walsh and Morrison, 1995), suggesting that the gold mineralisation occurred at least 15 Myr later than the granodiorite-diorite intrusive event at Lake Cowal. The gold mineralisation is broadly associated with shear zones, and has geometrical relationships and mineralisation styles typical of shear zone hosted deposits. The distribution of gold is controlled by dilational zones formed during the evolution of a fault system. These features are suggestive of a mesothermal style of mineralisation intermediate between the porphyry and epithermal levels. The narrow vein style of mineralisation, vein mineralogy, gangue association of quartz-carbonatepyrite-sphalerite-chalcopyrite, structural relationships (dilational zones proximal to major structure) and alteration assemblages, including quartz-sericite-ankerite, have many similarities to the quartz-sulphide-gold and carbonate-base metal-gold mesothermal subdivisions proposed by Leach and Corbett (1995).
Heithersay, P S, O’Neill, W J, Van der Helder, P, Moore, C R and Harbon, P G, 1990. Goonumbla porphyry copper district Endeavour 26 North, Endeavour 22 and Endeavour 27 copper-gold deposits, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1385–1398 (The Australasian Institute of Mining and Metallurgy: Melbourne).
ACKNOWLEDGEMENTS Permission to publish by North Limited is gratefully acknowledged. The authors would also like to acknowledge the invaluable contributions made by the many geoscientists and other North Limited staff who have worked on and contributed to the understanding of the Endeavour 42 deposit. The authors are particularly indebted to E N Bastrakov, M C Love, P R Balind, P S Burrell and R F Poxon.
Geology of Australian and Papua New Guinean Mineral Deposits
Ingpen, I A, 1995. Geological, structural and tectonic history of the Temora, West Wyalong, Grenfell and Forbes area, New South Wales: Implications for the structural controls on gold and copper mineralisation, MSc thesis (unpublished), Monash University, Melbourne. Leach, T M and Corbett, G J, 1995. Characteristics of low sulphidation gold-copper systems in the south-west Pacific, in Proceedings, Pacrim 95, pp 327–331 (The Australasian Institute of Mining and Metallurgy: Melbourne). Love, M C, 1992. Discovery and geology of the Lake Cowal, NSW, gold deposit, Geological Society of Australia Abstracts, 32:61–62. Miles, I N, 1993. The palaeovolcanology of the Late Ordovician Lake Cowal volcanics, Central New South Wales, BSc Honours thesis (unpublished), Monash University, Melbourne. Miles, I N and Brooker, M R, in press Endeavour 42 deposit, Lake Cowal, NSW, Australia - A structurally controlled Au deposit, Australian Journal of Earth Sciences. North Limited, 1996. Annual report to shareholders (North Limited: Melbourne). Perkins, C, Walsh, J L and Morrison, G, 1995. Metallogenic episodes of the Tasman Fold Belt system, Eastern Australia, Economic Geology, 90:1443–1466. Suppel, D W and Scheibner, E, 1990. Lachlan Fold Belt in New South Wales - Regional geology and mineral deposits, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), 1321–1327 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Webster, A E and Lutherborrow, C, 1998. Elura zinc-lead-silver deposit, Cobar, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 587–592 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Elura zinc-lead-silver deposit, Cobar 1
by A E Webster and C Lutherborrow
2
INTRODUCTION
PREVIOUS WORK
The deposit is 43 km NNW of Cobar, NSW, at lat 31o10′S, long 145o39′E, on the Cobar (SH 55–14) 1:250 000 scale and the Cobar (8035) 1:100 000 scale map sheets. It is the only exclusively zinc-lead-silver deposit to be discovered in the dominantly copper- and gold-bearing Cobar province. It originally contained in excess of 45 Mt of Identified Mineral Resource of which 13.5 Mt had been mined by July 1996. The combined Measured and Indicated Resource at 30 June 1996 was 28.8 Mt grading 8.5% zinc, 5.3% lead and 69 g/t silver of which 22.1 Mt is Proved and Probable Reserve (Pasminco Limited, 1996). As the deposit is still open at depth, it is likely that Elura contains more than 50 Mt of mineralisation. The mine is operated by Pasminco Limited and current ore production is 1.2 Mtpa.
The earlier geological studies are well documented by Schmidt (1990a) who showed that the most significant research work on the Elura orebody dates from the earliest period of exploration and underground testing. Schmidt (1980, 1990a) and de Roo (1987, 1989) characterised the deposit and provided the first documentation of the orebody, and the latter presented a detailed structural and paragenetic study of the mineralisation. This work was largely restricted to the part of the orebody above the 3 Haulage level. The most recent publication on Elura is that of Schmidt (1990a) who described the Northern ore zones for the first time.
The aim of this paper is to describe features of the Elura orebody that have been recently identified and it is designed to augment the descriptions of Schmidt (1980, 1990a) and de Roo (1989).
EXPLORATION AND DEVELOPMENT HISTORY The orebody was discovered in 1973 by Electrolytic Zinc Company of Australasia Limited as the result of an extensive regional exploration program that has been described by Schmidt (1990a, b). Three important factors contributed to the discovery of Elura. The first was the hypothesis by C O Haslam that to the north of the Mount Drysdale gold mines, the Cobar Group could trend NW under cover rather than NE, as was then thought (L W Davis, unpublished data, 1983). Airborne and ground geophysical surveys and detailed geological mapping were the other most important direct influences on the discovery of the deposit (Schmidt, 1990a, b). Production began in 1983 after a 24 hole resource definition program, and the mine produced at a rate in excess of 1 Mtpa until January 1991 when production was downsized, followed by a reduced exploration program from February 1992. Full production recommenced in 1994 when underground definition and exploration drilling was restarted, with the results reported here. Since 1994 the total number of holes drilled has increased from 298 to 555.
1.
Superintendent Geology, Pasminco Elura, PO Box 433, Cobar NSW 2835.
2.
Chief Geologist, Pasminco Broken Hill, PO Box 460, Broken Hill NSW 2880.
Geology of Australian and Papua New Guinean Mineral Deposits
REGIONAL GEOLOGY STRATIGRAPHY Elura is towards the northern margin of the Cobar Basin (Glen et al, 1996). It is currently thought to be hosted by the lithological equivalent of the CSA Siltstone (Schmidt, 1990a) which forms a part of the Early Devonian Amphitheatre Group, a monotonous sequence of turbiditic mudstone and siltstone with minor fine grained sandstone bands. The CSA Siltstone is host to several other deposits in the district including CSA (Glen et al, 1985). In the Elura mine area, the CSA Siltstone is a very dark grey to black, fine grained, finely bedded to laminated siltstone to shale with common 0.2 to 40 cm beds of pale grey siltstone and occasional sandy bands. It weathers to a pale yellow to cream coloured rock. Localised, pale green epiclastic marker units are seen within the siltstone, and fragments of crinoid stems have been found within the unit adjacent to the orebody (Yan Yan Sun, personal communication, 1996). Deformation at the margins of the orebody produces an intense but localised zone of cleavage in the siltstone.
STRUCTURE The regional structure of the Cobar Basin was described in detail by Glen et al (1996). The Elura orebody is within their Structural Zone 2 and has no spatial association with the intensely faulted and deformed margins of the basin, as do all other mineral deposits of the field. Structural Zone 2 is characterised by less intense deformation than the area to the east and contains WNW-trending folds with rare cleavage that are overprinted by NE-trending F2 folds with gentle plunges and weak axial plane cleavage (Glen et al, 1996). The area around the deposit has undergone lower greenschist facies metamorphism (Schmidt, 1990a).
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ORE DEPOSIT FEATURES SURFACE EXPRESSION The area around the mine has a paucity of outcrop so that the geology is only known from drill holes, sparse surface excavations and underground mapping. Recent surface reconnaissance mapping by B E Braes (unpublished data, 1996) has confirmed that significant outcrop and subcrop are present in the mine lease area.
GEOMETRY The recent large increase in geological information has led to a more complete knowledge of the orebody geometry, particularly in the region below existing mine development (currently the 6 Haulage level). Elura was recognised to consist of a series of discrete elliptically shaped pods by Schmidt (1990a). The most significant result of geological work since then is the increased understanding of the persistence of the geometry of the orebody at depth. This work confirmed that the deposit consists of a series of seven subvertical, pipe-like masses of mineralisation (Fig 1) which are linked by massive and vein and stringer style mineralisation.
In the upper levels of the mine, the main bodies of mineralisation were shown to have sharp contacts between massive ore and waste, although vein style mineralisation had been recognised (Schmidt, 1990a). The recent drilling has shown that this is true for the greater part of the orebody above the 6 Haulage level, however it has also shown that the orebody changes in character with depth. Although the massive to variably banded pipe-like zones persist at depth, it has now been recognised that the vein style mineralisation increases in intensity below 6 Haulage and forms an enshrouding, subvertical, northwesterly trending envelope. In some areas this envelope is massive, forming links between the main zones and in other areas it is characterised by vein style mineralisation. Thus, vein and stringer mineralisation link the massive pipes into a largely continuous body of mineralisation (C Lutherborrow and R Benton, unpublished data, 1996). Below 6 Haulage level the Northern zones are joined to the Main zone by a zone of silicified siltstone containing sulphide veins and stringers, and massive sulphides are continuous from the Main zone to the Crusher zone on 9400 m RL (C Lutherborrow and R Benton, unpublished data, 1996). Silicified siltstone containing sulphide veins and stringers is the only style of mineralisation at the currently known northern termination of the orebody (A E Webster, unpublished data, 1996).
TYPES OF MINERALISATION The ore types in the upper part of the Elura orebody have been described by Schmidt (1980, 1990a) and de Roo (1989) and studies of ore zoning have been undertaken in the horizontal plane (Figs 2 and 3). However, recent drilling has led to an increased understanding of the ore zoning and six styles of mineralisation are now recognised.
FIG 1 - Simplified longitudinal projection, Elura deposit, looking SW, as at July 1996 (modified after C Lutherborrow and R Benton, unpublished data, 1996). Plane of projection trends 330o (shown in Fig 3).
From south to north, the pipe-shaped bodies are known as the Main zone (including the northern apophysis, which is partially separated from the Main zone) and then a series of five variably linked sulphide masses known as Northern zones 1, 2, 3, 4 and 5. The Main zone also includes a NE-trending body of mineralisation known as the Crusher zone. At present, all zones within the Elura orebody are considered to have mineralogy and metallurgical characteristics similar to the parts of the orebody that have been mined, though arsenic content decreases with depth.
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FIG 2 - Plan showing distribution of major ore types within the Main zone, 6 haulage level (modified after R Benton, unpublished data, 1995).
Geology of Australian and Papua New Guinean Mineral Deposits
ELURA ZINC-LEAD-SILVER DEPOSIT, COBAR
Pyrite mineralisation (PY) PY corresponds to the massive ore type of Schmidt (1990a). It occurs at the outer margins of massive mineralisation but it does not form a simple, concentric ring around pyrrhotitic mineralisation as has been stated by previous workers (de Roo, 1989; Schmidt, 1990a). Regions of PY are now known to have a more complex geometry (Figs 2 and 3).
mineralisation is arbitrarily drawn at a 50% sulphide vein content (>50% equals SIPO, <50% equals VEIN) as the two types of mineralisation are transitional.
Vein and stringer mineralisation (VEIN) VEIN mineralisation forms the interface between massive ore and wall rock and consists of intensely stockworked siltstone. Where most intensely developed, VEIN consists of a varying percentage of rounded 3 to 15 cm clasts of variably silicified wall rock contained within an interconnected network of 1 to 5 mm thick pyrite-pyrrhotite-sphalerite veins. Galena is a minor component and is concentrated in wisps and streaks <1 cm wide. Sphalerite is chocolate brown in colour and is commonly concentrated within particular bands within veins. Some veins possess a fine foliation lying parallel to vein walls. There is no evidence of a replacement origin for this type of mineralisation, except where light grey siltstone beds are preferentially replaced by silica and sulphides.
Mineralised altered host rock (MINA) FIG 3 - Plan showing the distribution of the major ore types within the Northern zones at 9700 m RL, approximately 5 haulage level (modified after R Benton, unplublished data, 1995). Legend in Fig 2.
Pyrrhotite mineralisation (PO) PO corresponds to the pyrrhotitic ore of Schmidt (1990a). It forms the central core of all ore zones, particularly below the 5 Haulage level (Figs 2 and 3). It becomes more common with depth and is richer in sphalerite than other styles of mineralisation. It is now known that the average zinc grade of mineralisation increases with depth and that pyrrhotite is the dominant iron sulphide below 6 Haulage level. The increase in zinc grade may reflect the increasing percentage of PO mineralisation in the orebody at depth. Copper grades are also higher in PO, averaging 0.3% in the Main zone on the 9300 m RL.
Siliceous pyritic mineralisation (SIPY) SIPY corresponds to the siliceous ore described by Schmidt (1990a) and occurs at the margins of massive PY and sometimes pyritic PO mineralisation (Figs 2 and 3). The siliceous component of this ore type is highly variable but it generally consists of variably silicified clasts of siltstone which can range from one to tens of centimetres in diameter. Many areas also contain white quartz veining.
Siliceous pyrrhotitic mineralisation (SIPO) SIPO is texturally similar to SIPY but contains more pyrrhotite than pyrite. SIPO consists of a dense stockwork of pyrrhotitedominated sulphide veins developed at the contact of massive mineralisation and wall rock (Figs 2 and 3). The sulphide veins coalesce to varying degrees to form massive mineralisation in places. SIPO is most often dominated by pyrrhotite-pyritesphalerite mineralisation with varying percentages of centimetre-scale, rounded, silicified clasts of siltstone. The fabric of the mineralisation resembles breccia in places, with angular fragments of siltstone in a sulphide matrix. Bedding can remain continuous between clasts and wall rock but may also be jumbled. The distinction between SIPO and VEIN
Geology of Australian and Papua New Guinean Mineral Deposits
MINA forms the outermost mineralised halo of the northern zones and the deeper parts of the Main zone but rarely reaches ore grade. It consists of varying densities of pyrite-pyrrhotitesphalerite (±galena-chalcopyrite) stringers within sideritealtered, and often silicified, CSA Siltstone. MINA is particularly well developed to the SE and NW of the orebody, below 6 Haulage level and particularly in association with the Northern zones. MINA has gradational boundaries with VEIN mineralisation and siderite-altered, but unmineralised, wall rock. MINA is not present on all contacts and is largely absent from the edges of the Main zone above 5 Haulage level.
ZONING WITHIN THE OREBODY Previous workers have described a concentric zoning of PO and PY within the Main zone, with a 5 to 70 m wide alteration halo developed in the wall rock (Schmidt 1990a). However it is now known that this view of ore type distribution is too simplistic and only applies to the Main Zone above 5 Haulage level. The position becomes more complicated with depth (Figs 2 and 3). There is, however, a general tendency for the centres of the ore zones to be composed of PO, except in the uppermost parts of the Main zone where PY predominates. There is no PO at the top of the northern apophysis (A E Webster, unpublished data, 1996) SIPY and SIPO mineralisation have gradational contacts with massive and VEIN styles. VEIN mineralisation grades into MINA on its outer margins. There is a transition from massive sulphide cores (PO, PY), through siliceous ore types (SIPY, SIPO) to VEIN and MINA types. In the Main zone, above the 6 Haulage level, the contact of massive mineralisation is very sharp and VEIN mineralisation is weakly developed or absent. The last three ore types described above were poorly described or unrecognised by Schmidt (1990a) reflecting the greater knowledge that has been gained recently.
SULPHIDE TEXTURES The mineralisation is invariably banded by 0.2–4 mm bands of pyrrhotite or pyrite. The nature of this banding is unclear but at least three types are present:
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1.
a sphalerite-pyrrhotite-pyrite type in which sphalerite defines discrete millimetre-scale laminae to centimetrescale bands that are reminiscent of depositional layering;
2.
a layering in which sphalerite defines smears and streaks which are reminiscent of reoriented pre-existing linear inhomogeneities within the mineralisation (such as type 1 sphalerite bands); and
3.
an intense tectonic or cleavage style banding which predominates at the margins of the orebody and seems to be the result of deformation. Coarse-grained galena (1 to 3 mm diameter) occasionally forms discontinuous veinlets parallel to the dominant cleavage in the wall rocks but the crystals are always strained, suggesting deformation after the galena was formed (A E Webster, unpublished data, 1996), or during progressive cleavage development.
Although the form, origin and orientation of the sulphide banding are yet to be investigated in detail, type 1 banding seems to define irregular folding within the mineralisation. This folding is much more complex than that in the surrounding sediment (A E Webster, unpublished data, 1996). Sulphide banding may be useful in determining the internal structure and structural history of the sulphide orebodies. Truly massive mineralisation with no banded fabric only occurs in the uppermost parts of the deposit, particularly where the ore is dominated by pyrite, as at the top of the northern apophysis of the Main zone (A E Webster, unpublished data, 1996).
OREBODY STRUCTURE Two structural orientations are apparent in the orebody, a dominant NW trend at approximately 330o and a less well developed NE trend at 030° (Fig 4). The geometry of the orebody is a product of the two structural orientations, though the NE trend seems to be later and less strongly developed. The Main and Northern zones are aligned along 330o, which has been interpreted to be the orientation of a single anticlinal hinge (Schmidt, 1980, 1990a; de Roo, 1987, 1989). The true nature of the anticline has yet to be established and it may represent several en echelon anticlinal culminations and not a single anticline. Oriented core from holes drilled into the 330 o plane suggests that the traditional view of the Northern zones occupying domes caused by rapid multiple plunge reversals along the anticline (Schmidt 1990a) is very difficult to substantiate (C Lutherborrow, unpublished data, 1996). The NE structural orientation is characterised by a realignment of some ore zones and the apparent offset of others (Fig 4). This 030o trend is most obvious at the southeastern end of the orebody where it appears to re-orient the southern end of the Main zone. The Crusher zone lies within the 030o orientation and is aligned along the trend. In this case the 030o structure appears to have strongly modified the strike of the orebody. C Lutherborrow (unpublished data, 1996) equates this 030o trend with a regional lineament that is visible on regional gravity data which extends well beyond the orebody to the NE.
ALTERATION HALO The deposit is enveloped by a series of distinct types of wall rock alteration which have been described in detail by Schmidt (1980, 1990a). Several recent observations augment the earlier work and are discussed briefly. At mine scale, the most important alteration type is the variably developed silicification of CSA Siltstone at the contacts of the ore (part of the inner halo of Schmidt, 1990a). Silicified siltstone varies from a pale translucent, grey, almost cherty rock, where alteration is strongly developed, to ordinary looking siltstone in which the pale grey siltstone beds have been preferentially replaced by silica. In some zones of intense silicification, sulphide replacement of pale grey siltstone has taken place behind the siliceous alteration front. Silicification is highly variable and is closely associated with VEIN, SIPO and SIPY types of mineralisation. There is a close spatial association between the intensity of silicification and the density of sulphide veining. Silicification at Elura possibly equates to the ‘elvan’ of the Peak and CSA mines (Hinman and Scott, 1990) though it is very poorly developed compared to the other Cobar deposits. At Elura wall rock silicification is often totally absent at the orebody margins, particularly in the upper parts of the deposit. Siderite spotting of the CSA Siltstone is the most distinctive feature of the outer part of the visible alteration halo of the orebody and is a distinctive feature of the Elura mineralisation. It extends 10 to 50 m from the eastern side of the orebody but often stops abruptly 5 m from the ore on the western side. Spotting persists for several hundred metres NW of the known mineralisation, having been intersected in diamond drilling to the NE of the orebody (B Godsmark, personal communication, 1996). It also extends a similar distance to the SE.
590
FIG 4 - Plan showing the distribution of main ore zones and structural trends within the Elura deposit at the 9400 m RL (modified after R Benton, unpublished data, 1995). Cross hatched areas = mineralisation, NZ 2 = Northern zone 2.
Elsewhere in the deposit, most notably between the Northern zones, the geometry of the mineralisation is influenced by a series of 030ο trending planes (Fig 4). The 030o trend is the result of a later deformation which re-orientates the earlier 330o trending folds. It can be speculated that the dominant 330o trend in the orebody is associated with one or more D1 anticlines of the type identified by Glen et al (1996) within their Structural Zone 2. The later 030ο trend with its associated cleavage and tensional quartz veins at ore contacts may be the result of D2 refolding (Glen et al, 1996). General observations in the mine area do not show that the structure of the orebody area is uniquely complex when compared with other areas of the lease. Structure does not seem to explain the presence of the orebody. Gentle folding seen in backfill quarries to the NE and SW of the mine is similar to that seen underground adjacent to the orebody. This suggests that
Geology of Australian and Papua New Guinean Mineral Deposits
ELURA ZINC-LEAD-SILVER DEPOSIT, COBAR
the structural environment of the orebody is not different to the environment in the surrounding sediment, except for the intense cleavage and tensional veining formed at the orebody margins. If the structural environment of the orebody area is not unique, apart from the presence of the orebody itself, then it raises the possibility that the deposit was not formed as a result of syn-deformational replacement (Schmidt, 1980, 1990a; de Roo, 1989). It may have been present in the pile before deformation took place and became a focus of cleavage formation, quartz vein development and a locus for metamorphic fluids. The intense cleavage developed throughout the CSA Siltstone at the CSA deposit is not seen at Elura where intense cleavage is only a feature of some contacts.
Nature of ore–waste contacts Faulting is an important feature of the Elura orebody though distinct shear or fault zones have not been defined within the mine area to date. Deformation is focussed along the orebody margins and within fold hinge zones, particularly synclines. White quartz veining occurs at the ore–wall rock interface and may persist into the siltstone for several metres. Underground exposures of quartz veining at the contacts suggest that they formed as tension veins. The western and northwestern contact of the orebody are heavily faulted though it is unclear whether this is due to a single or several faults. The faulting is characterised by black chlorite, slickensided, polished fault surfaces and a well developed schistosity. Planes of movement often exploit preexisting bedding planes. A dilational fabric is also present, which consists of subvertical quartz veins parallel to the plane of movement truncated by subhorizontal dilational quartz veins. This contact always results in poor ground conditions and is a potential slip surface. On the southwestern and southern contacts of the orebody, the contact is characterised by subvertical quartz veins within the CSA Siltstone that lie parallel to the orebody contact. These are cut by subhorizontal quartz veins but no obvious fault surfaces are evident. This zone appears to represent brittle dilation. The southeastern contact of the orebody is a well developed fault plane which is evident on the 6 Haulage level and which is closely associated with the hinge and northwestern limb of a syncline lying to the SE of the orebody. The cleavage associated with this plane has resulted in poor ground conditions and may be responsible for poor ground conditions within the new decline (C Delaney, personal communication, 1996). The boundary of economic mineralisation does not usually reach this fault but the structure does mark the start of VEIN mineralisation. On 4 Drill level the ore contact lies close to this structure and the contact conditions in some areas are similar to the western contact. It is more usual for the eastern contact to consist of massive mineralisation, passing into sulphide veins along cleavage within the siltstone. The siltstone is silicified, with occasional cleavage-parallel quartz veins. The northeastern and eastern contacts of the orebody are similar to the southern contact, with subhorizontal quartz veins that cut quartz veins parallel to cleavage. However the degree of cleavage fabric replacement is high and sulphide-veined
Geology of Australian and Papua New Guinean Mineral Deposits
silicified siltstone extends well beyond the boundary of the economic mineralisation. This is the most competent contact surface with the only source of failure being brittle fracture on sulphide or quartz veins.
MINE GEOLOGICAL METHODS An urgent need for geological input at Elura was recognised in 1994. Drilling was initiated from within the orebody on nominal 20 m spaced sections when it was realised that the available geological mapping and drilling information were inadequate for interpolation of the orebody geometry. This led to the recognition that the margins of the orebody were not as regular between levels as had been thought but were irregular, consisting of a series of bulges and cuts. The current drilling strategy utilises the limited underground access to maximum advantage by drilling fans of holes from selected sites. No drilling is done on section.
DISCUSSION AND CONCLUSIONS Models for the genesis of the Elura orebody have been summarised by Schmidt (1990a). There are a number of aspects of the geology of Elura and the Cobar mineral field worthy of discussion. The Elura orebody is primarily an iron sulphide deposit that is rich in zinc and lead. The lack of well developed copper or gold mineralisation at Elura is a characteristic that distinguishes it from all other deposits of the field. The one zone of elevated silver-gold mineralisation that has been defined at Elura (R Benton, unpublished data, 1995) occurs on the western margin of the orebody at the 5 level within fairly typical VEIN mineralisation and strong wall rock alteration. It is not shear hosted, nor is it associated with cleavage development and is not comparable to gold mineralisation elsewhere in the Cobar field. In the Cobar Basin, only the CSA deposit contains mineralisation of a type comparable to Elura (Scott and Phillips, 1990). The Western, QTS Central and QTS South zones there all contain zinc-lead mineralisation but it is the CZ zone that seems to be most similar to Elura (Hinman and Scott, 1990). At CSA, as in most other deposits in the Cobar region, the zinc-lead mineralisation is recognised to be early, and copper mineralisation is late, though this is not the case at the Peak deposit where base metals are earlier than the gold mineralisation (Hinman and Scott, 1990). The observation that zinc-lead mineralisation is early in the paragenetic sequence at the CSA deposit may be important to the relationship between the epigenetic styles of mineralisation and Elura type mineralisation. Copper and gold deposits show a strong spatial association with the structurally complex margins of the Cobar Basin (Gilligan et al, 1994), particularly with the eastern margin, where deposits are related to faults and intense cleavage development, ie to structural conduits in which ore-forming fluids can flow. There is also a general consensus that the copper-gold mineralisation is epigenetic (Scott and Phillips, 1990; Hinman and Scott, 1990; Gilligan et al, 1994). However, it is possible that the zinc-lead-iron sulphide mineralisation was present in the sedimentary pile prior to the development of the syntectonic, epigenetic gold and copper systems. Gold and copper may have been deposited from such systems where they
591
A E WEBSTER and C LUTHERBORROW
encountered deformed syngenetic sulphide mineralisation in the sediment, eg at the CSA deposit. The likely host for any syngenetic zinc-lead-iron sulphide mineralisation in the Cobar Basin is the CSA Siltstone because there is a strong spatial association between this unit and zinc-lead mineralisation (Gilligan et al, 1994). Alternatively, such gold-copper fluids may have passed through lead-zinc mineralised siltstone as they ascended and leached the metals from it. It is possible that the CSA Siltstone, at depth, provided the source for the zinc-lead-iron sulphide mineralisation at Cobar and the copper and copper-gold mineralisation are related to epigenetic processes associated with faulting at the margins of the basin. If true, this would suggest that a bimodal distribution of zinc-lead and copper and copper-gold deposits should exist. However, while the entire outcrop and subsurface area of the CSA Siltstone would be highly prospective for Elura type orebodies it would not necessarily be prospective for copper or gold deposits. Elura may represent a relatively undeformed and weakly altered occurrence of zinc-lead mineralisation. The CSA deposit, on the other hand may represent a well developed hybrid of syngenetic and epigenetic mineralisation, containing both Elura style zinc-lead-iron sulphide mineralisation and epigenetic copper mineralisation. The size, richness and diversity of mineralisation at the CSA deposit may be the result of the fortuitous coincidence of CSA Siltstone at the intensely deformed margin of the Cobar Basin The Elura orebody may represent a CSA type zinc-lead system that was frozen in the earliest phases of deformation and prior to development of epigenetic copper and gold mineralising systems. The lack of significant copper or gold mineralisation at Elura may be due to the distance the deposit lies from the deformed margin of the Cobar Basin. If the bimodal nature of mineralisation in the Cobar Basin is confirmed it suggests that totally independent gold, copper, copper-gold and zinc-lead deposits may exist and different exploration strategies would be required for each. The Elura orebody is one of the largest deposits within the Cobar region. Its full extent is yet to be defined and it may yet prove to be the largest base metal deposit of the field. Underground development over the course of the next three years may provide the answers to major questions about the structure and genesis of this enigmatic deposit.
REFERENCES de Roo, J A, 1987. Structurally controlled ore genesis at Elura and Mount Carbine, Australia, PhD thesis (unpublished), James Cook University of North Queensland, Townsville. de Roo, J A, 1989. The Elura Ag-Pb-Zn mine in Australia - ore genesis in a slate belt by syn deformational metasomatism along hydrothermal fluid conduits, Economic Geology, 84:256–278. Gilligan, L B, Byrnes, J G, Watkins, J J and Pogson, D J, 1994. Cobar 1:250 000 metallogenic map SH 55–14, Geological Survey of New South Wales. Glen, R A, Clare, A and Spencer, R, 1996. Extrapolating the Cobar Basin model to the regional scale: Devonian basin-formation and inversion in Western New South Wales, in The Cobar Mineral Field - A 1996 Perspective (Eds: W J Cook, A J H Ford, J J McDermott, P Standish, C L Stegman and T M Stegman), pp 43–83 (The Australasian Institute of Mining and Metallurgy: Melbourne). Glen, R A, MacRae, G P, Pogson, D J, Scheibner, E, Agostini, A and Sherwin, L, 1985. Summary of the geology and controls of mineralization in the Cobar region, Geological Survey of New South Wales Report 1985/203 (unpublished). Hinman, M C and Scott, A K, 1990. The Peak gold deposit, Cobar, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1345–1351 (The Australasian Institute of Mining and Metallurgy: Melbourne). Pasminco Limited, 1996. Melbourne).
Annual Report (Pasminco Limited:
Schmidt, B L, 1980. Geology of the Elura Ag-Pb-Zn deposit, Cobar district, NSW, MSc thesis (unpublished), Australian National University, Canberra. Schmidt, B L, 1990a. Elura zinc-lead-silver deposit, Cobar, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1329–1336 (The Australasian Institute of Mining and Metallurgy: Melbourne). Schmidt B L., 1990b. Elura zinc-lead-silver mine, Cobar, NSW, in Geological Aspects of the Discovery of Some Important Mineral Deposits in Australia (Eds: K R Glasson and J H Rattigan), pp 161–170 (The Australasian Institute of Mining and Metallurgy, Melbourne). Scott, A K and Phillips, K G, 1990. CSA copper-lead-zinc deposit, Cobar, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1337–1343 (The Australasian Institute of Mining and Metallurgy: Melbourne).
ACKNOWLEDGEMENTS This paper is published with the permission of Pasminco Limited. P Standish, T Eadie and S Hunns are thanked for reviewing early versions of the manuscript. R Benton, I Kelso and S Hunns are especially acknowledged for their contribution to the work that is presented here.
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Geology of Australian and Papua New Guinean Mineral Deposits
Fogarty, J M, 1998. Girilambone district copper deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 593–600 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Girilambone district copper deposits by J M Fogarty1 INTRODUCTION Nord Pacific Limited (Nord) and Straits Resources Limited (Straits) own the Girilambone and Girilambone North copper mines which are operated by the jointly owned Girilambone Copper Company Pty Ltd (GCC). The deposits are 625 km west of Sydney and 105 km ENE of Cobar, NSW, at lat 31o16′S, long 146o53′E on the Cobar (SH 55–14) 1:250 000 scale map sheet (Fig 1). The mines currently produce 18 000 tpa of refined (99.999%) copper by heap leaching and solvent extraction-electro winning (SXEW) of copper carbonate and supergene copper sulphide ores, and at 30 June 1997 total sales exceeded 52 000 t of refined copper. In 1996, the Girilambone operation was the seventh largest copper producer, the third largest producer of refined copper and the largest producer of copper by the SXEW process in Australia.
Eight primary massive sulphide deposits with resources ranging from 1 to 10 Mt are now known in the region, of which four have been discovered since 1993.
EXPLORATION AND MINING HISTORY The Girilambone copper deposit was discovered by Thomas Hartman and Charles Campbell (co-founders of Cobar), George Gibbs and George Hunter in 1879 and was mainly mined from 1881 to 1907, to yield 58 408 t of ore at 1.96% copper (Shields, 1996). Cuprite and minor chalcocite ores at Bonnie Dundee and Budgerygar were mined from 1906 to 1908 (Carne, 1908, pp 194–196) and from 1906 to 1920 at Budgery near Hermidale. At Girilambone North, minor copper minerals are visible in prospector rock dumps beside the Hartmans, Larsens, and Hunters shafts but no significant production is recorded. Numerous additional prospector shafts occur throughout the Girilambone Exploration Joint Venture (GEJV) tenements but no copper production is recorded from these. Utah Development Company (Utah) explored the area from 1963 to 1973 and established a resource of 3.315 Mt at 2.12% copper in the Girilambone pit area. Australian Selection Pty Ltd (Seltrust) explored the region from 1974 to 1983 and discovered and defined a resource of 1.5 Mt at 1.8% copper at the Northeast massive sulphide body. Anglo Australian Resources NL, Aquitaine Australia Minerals Pty Ltd, Palifs Pty Ltd, Triako Mines NL, Buka Minerals NL, Sanidine NL, Hunter Resources Limited, Central West Gold NL and RGC Exploration Pty Ltd explored various portions of the tenement areas prior to 1993. In 1989, Nord purchased the Girilambone mine and the surrounding Exploration Licence from Hunter Resources and commenced exploration. This led to the establishment of ore
1.
Exploration Manager (Australia), Nord Pacific Limited, PO Box 12, Girilambone NSW 2831.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Location and geological map of the Girilambone area showing Exploration Licence boundaries and copper deposits.
593
J M FOGARTY
reserves within the current Girilambone pit area totalling 8 Mt at 1.4% copper, as leachable chalcocite and copper carbonate minerals, estimated to contain 83 700 t of recoverable copper. A joint venture (the MJV, Nord 40%, Straits 60%) was formed in 1991 to develop the deposit. An exploration joint venture (the GEJV, Nord 50%, Straits 50% ) was formed in 1991 and Nord was appointed as manager. Significant budgets allocated to the GEJV from 1993 resulted in the discovery of Larsens East and Hartmans leachable copper resources at Girilambone North, 4 km north of Girilambone, in September 1993. Ore reserves were estimated at Larsens East, Hartmans and Northeast prospects in 1994, the Tritton mineralisation was found in 1995, and resources were delineated at Tritton and Budgerygar in 1995–1996. Drilling of ground electromagnetic (SIROTEM) anomalies found in May 1995 discovered two massive sulphide bodies 22 km SW of Girilambone at Tritton (Fig 1), and one at the nearby historical Budgerygar copper mine. Drilling of the Tritton anomalies to 1000 m vertical depth in a program of 132 drill holes totalling 60 000 m since 1995 has defined a Measured, Indicated and Inferred Resource totalling 9.75 Mt at 3.01% copper, 0.2 g/t gold and 11 g/ t silver. Drilling is continuing and prefeasibility studies to determine the economic viability of underground mining of Tritton are in progress. Exploration for leachable copper mineralisation remains the prime target for further exploration (Fogarty, 1996) because of the existing infrastructure and low operating costs. The recent discovery of the Tritton primary pyrite-chalcopyrite mineralisation with higher grades in the upper levels, indicates that the region has potential for chalcopyrite orebodies suitable for economic mining by underground methods. Mining of the leachable copper resource in the Girilambone pit and construction of the SXEW treatment plant commenced in 1992. To 31 March 1997 a total of 6.1 Mt of ore at an average grade of 1.69% copper and 17.3 Mt of waste had been mined . Production was 51 500 t of 99.999% copper metal from of a total of 104 000 t of contained copper metal in ore placed upon the leach pads, including 1200 t of contained copper in ore mined from the Northeast deposit at Girilambone North. Mining commenced in 1996 at the Northeast deposit and 157 000 t of ore at 0.77% copper had been mined and placed upon the leach pads by 31 March 1997. Mining of overburden commenced at Hartmans in 1996 and mining at Larsens East is scheduled to commence in 1998.
REGIONAL GEOLOGY The Girilambone deposits are in the western portion of the Lachlan Fold Belt within the Tasman Geosyncline of central western NSW. Flysch type sediments of the Girilambone Group comprising quartz-chlorite-sericite schists of Ordovician age extend from Wagga Wagga to north of, and well to the east and west of Girilambone. In the Girilambone district, semipelitic and mafic schists (‘basement schists’) are unconformably overlain (M I’ons, unpublished data, 1993) by the Caro schist which comprises mafic schist and quartz greywacke, and by the Tritton formation comprising quartz wacke, sandstone and phyllite. Near Girilambone, the basement schists are intruded by syntectonic and post-tectonic granitoids, intermediate, mafic, and ultramafic Alaskan-type intrusive rocks similar to those at Fifield, dolerite sills and numerous dykes, and late stage quartz gabbro dykes. Only some of the younger intrusive rocks are known within the Caro schist and the Tritton formation (Fig 1).
594
ORE DEPOSIT FEATURES STRATIGRAPHY AND STRUCTURE Within the tenements, interpretation of photogeological and airborne geophysical data has indicated that the greywacke, shale, phyllite, semi pelitic schist, ‘quartzite’ and graphitic schist sequence unconformably overlies the basement schist sequence as shown in Fig 1. The significant massive sulphide occurrences at Girilambone, Girilambone North, Budgerygar, Tritton and Budgery occur within the Caro schist near the base of the greywacke sequence. The mineralisation is associated with zones of chloritisation, siderite and epidote alteration, thin magnetite lenses, hematite alteration and intense silicification, although these alteration features are not present in all deposits. Complexly folded, steep quartzite ridges which extend for a total strike distance of about 150 km within the tenements consist of intensely silicified greywacke, and the known massive sulphide lenses are relatively close to these ridges. Detailed lithological and stratigraphic mapping and interpretation near Girilambone were completed by T Hopwood, as reported in Smith (1971). The stratigraphy was redefined by Smith (1973) who considered that the pink quartzite was near the base of the sequence, as in Table 1. TABLE 1 Stratigraphic sequence for the Girilambone area, as defined by Smith (1973). BALLAST BEDS
WELTIE FORMATION
GIRILAMBONE BEDS
TRITTON FORMATION
Massive phyllite Budgery sandstone Quartzwacke, minor pelite
CARO SCHIST
Laminated quartz wacke Laminated quartz-mica schist Pink quartzite Basic volcanic rocks, serpentinite
The extensive highly siliceous zones including red jasper with disseminated pyrite (the pink quartzite) within mafic schists along the margins of the large ultramafic and mafic intrusive bodies at Birrimba and Kurrajong (Fig 1) indicated to Smith (1973) that the large intrusive bodies ‘were accompanied by large amounts of silica’. The presence of pink quartzite in these rocks at a variety of stratigraphic levels - in the basement schists as at Exley and Birrimba, its close association with the orebodies at Tritton and Girilambone and in quartz-sericite semipelitic graphitic schists at the Ben Hur, Dasypygal and Spartacus SIROTEM anomalies NW of Girilambone North, indicates that the pink quartzite is not a single stratigraphic horizon. Consideration of these occurrences and thin section descriptions indicate that the pink quartzite is of low temperature hydrothermal origin.
Geology of Australian and Papua New Guinean Mineral Deposits
GIRILAMBONE DISTRICT COPPER DEPOSITS
The mafic rocks and serpentinites within the Caro schist are believed to be related to the Fifield type complex intrusive bodies of Early Devonian age (Suppel and Barron, 1986), and intrusive into the Girilambone beds and Tritton formation. Suppel (1974) indicated that north of the Lachlan River, the Girilambone Group units form an older, more highly metamorphosed sequence occurring unconformably beneath less metamorphosed and deformed Ordovician rocks, including the Ballast beds and Weltie Sandstone. Although intense shearing within the Tritton formation near the major fault zones at Tritton and Girilambone has confused interpretation, the Tritton formation is now believed to be a turbidite sequence unconformably overlying (Fig 1) the more intensely folded and metamorphosed Girilambone beds near Girilambone and the stratigraphic sequence is now suggested to be as shown in Table 2. TABLE 2 Current stratigraphic sequence for the Girilambone area. BALLAST BEDS
WELTIE FORMATION
UN-NAMED
TRITTON FORMATION
zone with anomalous copper contents, known as the Rockdale anomaly, extends for 13 km westerly from the Girilambone mine to Lucknow (Fig 1). A southerly trending zone with anomalous copper content extends from Lucknow through Budgerygar, Tritton and Great Hermidale and intensive exploration is in progress throughout this zone. The possibility that these zones represent a particular rock type rather than mineralised shear zones has not been finally discounted.
MINERALISATION The primary mineralisation in the Girilambone area is polymetallic, consisting of chalcopyrite, sphalerite and galena, with gold and silver values generally less than 0.5 g/t and 20 g/t respectively. Eight significant massive sulphide deposits are known within the tenements: Girilambone and Girilambone Deeps; the Northeast, Larsens East and Double Tanks deposits at Girilambone North; the Tritton, Budgerygar and Bonnie Dundee deposits in the Bonnie Dundee project area; and the Budgery deposit 7 km west of Hermidale (Fig 1).
Girilambone pit Massive phyllite
Mineralisation here occurs in three forms. 1.
In chlorite-sericite schist with disseminated pyrite. Alteration associated with this mineralisation includes chlorite with lenses of epidote, siderite and magnetite. Copper carbonates and phosphates, supergene chalcocite and primary chalcopyrite mineralisation occur within a steeply dipping shear zone striking at 300o true and extending over a strike length of approximately 120 m and a true width to 10 m within the pit.
2.
The Main lode consists of massive chalcocite within laminations and as lenses within cryptocrystalline quartz, the pink quartzite of Hopwood, as described in Smith (1971). This setting is very similar in macroscopic appearance to the upper ore zone at Tritton, but is more complexly folded and faulted. Examination of some thin sections of this pink quartzite shows well rounded detrital quartz grains, indicating that the some of the quartzite is a silicified greywacke, but most thin sections of quartzite from Girilambone and Tritton contain only very fine grained quartz with few diagnostic textures.
3.
Within chlorite-sericite schist as pyrite-chalcopyrite stringers in a discrete zone in the footwall of the massive sulphides described in 2 above.
Budgery sandstone Quartzwacke, minor pelite Laminated quartz wacke Laminated quartzmica schist UNCONFORMITY GIRILAMBONE BEDS
BASEMENT SCHIST
Quartz-chloritesericite schist
The sequence within the Girilambone pit dips shallowly to the SE, but faulting and intense cross folding have complicated the interpretation. A major fault striking at 330o true and dipping gently to the NE is exposed in the wall of the pit. It can be traced by airborne magnetic survey and SIROTEM data NW from the pit through the western margin of the Girilambone North deposits, and for a further 18 km NW to Plough Tank (Fig 1). This structure appears to be a corridor of deep seated reverse faulting with numerous fault planes, into which serpentinite dykes have intruded at Girilambone, Girilambone North and Avoca Tank, but with apparently minimal horizontal displacement of the quartzite ridges to the south of the Girilambone pit. The serpentinites within the Caro schist are now believed to be intrusive dykes within the major fault rather than stratigraphic units. A series of late stage fractures trending 352 o true occur throughout the tenement area and contain minor gold mineralisation in narrow quartz veinlets, as at Bylong goldfields, Karingal and Turners Creek.
GEOCHEMISTRY Anomalous copper values in bedrock are associated with all of the known mineralisation except Tritton and with many of the mafic intrusive rocks. In addition, an extensive pyritic shear
Geology of Australian and Papua New Guinean Mineral Deposits
An Inferred Resource of primary chalcopyrite mineralisation, using a 0.2% copper lower cutoff, has been estimated for mineralisation below the final pit design at RL 85 down to RL 00. The resource comprises 3.4 Mt of ore at 0.96% copper or about 33 000 t of contained copper. Additional drilling is required to test the potential below RL 00.
Girilambone North area The deposits comprise the Larsens East, Northeast, Hartmans and Double Tanks massive sulphide bodies, of which two are currently being mined. A total of approximately 40 000 m of reverse circulation (RC) and diamond drilling has been completed to evaluate leachable copper carbonate and supergene sulphide copper ore reserves (Table 3).
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of 520 m beneath the surface, intersected chalcopyrite mineralisation over widths to 14 m, and the mineralisation is open below 520 m depth.
TABLE 3 Girilambone North leachable ore reserves. Area
Reserve class
Type
Larsens East
Proved
Oxide
647
0.49
Probable
Proved
Probable
Transition
190
0.85
1610
630
1.66
10 460
Oxide
194
0.50
970
Transition
78
0.92
720
Chalcocite
176
1.73
3040
Proved
1.04
19 970
443
0.65
2 880
Transition
84
0.70
590
Chalcocite
423
1.03
4360
6
0.55
30
Transition
1
0.50
10
Chalcocite
9
0.64
60
966
0.82
7930
Oxide
1708
0.58
9910
Transition
270
0.66
1780
Northeast
Pit base
00
1312
0.96
1010
Larsens East
Pit base
00
748
0.95
7110
1180
Total
2060
0.96
19 710
Oxide
207
0.49
Oxide
231
0.51
Transition
39
0.54
210
Chalcocite
47
0.47
220
Sub total
2502
0.57
14 310
Grand total
5383
0.78
42 210
Larsens East The Larsens East massive sulphide lens was discovered by reconnaissance transient electromagnetic surveys using SIROTEM techniques during orientation surveys over the known mineralisation at Northeast prospect (Fogarty, 1996). Massive pyrite-chalcopyrite lenses occur within weakly silicified semipelitic schist with minor mafic components. The sulphide lens has a strike extent of approximately 250 m and a true width to 46 m. The mineralisation strikes at 300o true, dips to the east at between 20 and 45o and is terminated at each end by NW-trending faults containing graphite, which produce a flat southerly pitch to the mineralisation. These faults are not exposed but are interpreted from the results of IP surveys which delineate a large number of trends with this orientation in the Girilambone North area. Deeper drilling is planned during 1997 to test the down dip extension of this mineralisation.
Northeast The Northeast massive sulphide deposit was discovered by Seltrust who drilled down dip from a gossan exposed in a costean dug by Utah. Orientation SIROTEM surveys by the GEJV in 1993 detected a weak response, filtered to overcome the effects of a nearby graphitic body, which coincided with the known mineralisation (Fogarty, 1996). Nine diamond drill holes completed by Seltrust and two by Nord, to vertical depths
596
A block model resource for primary chalcopyrite mineralisation at Larsens East and Northeast, to a vertical depth of 220 m, has been estimated by GCC using a 0.2% copper lower cutoff and an SG of 2.7 (Table 4). Further resources below 220 m depth may be estimated by using data from 11 deep diamond drill holes.
1915
Chalcocite Probable
The sulphide lens has a strike extent of about 200 m and a true width to 14 m. The mineralisation strikes at 330o true, dips east at 60o and is terminated at each end by NW-trending faults which produce a southerly pitch to the mineralisation. The northern of these two faults is exposed in the northern end of the current Northeast pit.
Oxide
Sub total Hartmans
3170
Chalcocite
Sub total Northeast
Ore Copper Contained (’000t) % copper %
TABLE 4 Girilambone North primary chalcopyrite Indicated Resource at 0.2% cutoff. Area
RL
RL
From
To
Ore
Copper Contained copper (’000 t) (t) % 12 600
Hartmans Several intersections of low grade copper mineralisation in diamond drill holes near Hartmans shaft were reported by Utah, and closer spaced drilling of this area by Nord has provided data for estimation of the leachable reserves shown in Table 3. Mining of waste in this pit has exposed a massive gossanous zone within a wide halo of limonite alteration. High gold values in the gossan are believed to indicate that the gossan is derived from supergene chalcocite with enrichment in gold due to supergene processes. Drilling indicates that the primary mineralisation consists of thin lenses of massive pyritechalcopyrite within a halo of disseminated pyrite. A weak and indefinite SIROTEM anomaly is aligned with the trend of the mineralisation (Fogarty, 1996). No primary resource has been estimated and deep drilling is planned.
Double Tanks The deposit consists of a thin lens of massive pyritechalcopyrite in semipelitic schist. High gold values in the upper levels of this mineralisation where chalcocite is common, and in the gossans up dip from the sulphide zone, are interpreted as being due to supergene enrichment. The mineralisation trends at 330o true along a strike distance of 150 m with a true width to 6 m, and dips easterly at 65o. A weak SIROTEM anomaly just to the SSE of Northeast is associated with the mineralisation (Fogarty, 1996). Further drilling is planned to test the extent of the mineralisation.
Bonnie Dundee area The area is 22 km SSW of Girilambone on the Girilambone to Hermidale road (Fig 1). Three lenses of massive sulphide
Geology of Australian and Papua New Guinean Mineral Deposits
GIRILAMBONE DISTRICT COPPER DEPOSITS
drilled to 1000 m vertical depth for a combined down dip distance of 1100 m, and remains open at depth. The mineralisation is lenticular in plan and is interpreted as being contained between two NW-trending sinistral faults, in which graphite is sometimes present, and which cause a SSE pitch to the mineralisation. A NNE-trending fault imposes a pitch dipping at 20o to the NNE upon the top of the lower zone causing a gap between the upper zone and the top of the lower zone. Figure 3 is a cross section through Tritton which shows that the mineralisation is associated with a major structure which dips at an average of 45o to the east, close to easterly dipping mafic intrusive rocks. It is believed that the sedimentary rocks of the Tritton formation shown on the cross section and occurring throughout the Girilambone-Hermidale area consist of a series of gently dipping and plunging synforms and antiforms with dips at various angles to the major shear zones, unconformably overlying the basement schist. Tritton upper zone is a massive pyrite-chalcopyrite lens with minor sphalerite and galena in a chloritic groundmass, and is roughly central within a 50 to 100 m thickness of cryptocrystalline quartz. This has been described in thin section (I R Pontifex, unpublished data, 1996) as low temperature epithermal quartz. In some drill holes, adjacent to the massive sulphide zones, this cryptocrystalline quartz contains magnetite and some red jasper due to hematite alteration, which has given rise to the term pink quartzite as used by T Hopwood (Smith, 1971). The pink quartzite is now interpreted as a zone of silicification that may be present at any stratigraphic horizon in the Girilambone district.
FIG 2 - Image of Decay constant for 100 m moving loop Sirotem measured at 1 uV/A.
mineralisation known as Bonnie Dundee, Budgerygar and Tritton are arranged en echelon over a total known strike distance of 2.5 km, due to displacement by a series NW trending late stage faults. The copper grades within the known Bonnie Dundee and Budgerygar deposits are generally less than 0.4% except in secondary supergene copper zones, whereas high copper grades occur in the Tritton deposit.
Tritton The Tritton deposits were discovered by reconnaissance SIROTEM surveys which were able to locate the upper sulphide zone under 180 m of barren rock and to record responses in the lower zone down to vertical depths of approximately 500 m (Fogarty, 1996). Approximately 60 000 m in percussion precollared diamond drill holes have been completed since June 1995 to test these zones, by 132 drill holes and 15 additional wedge or Navi-Drill deflections.
The mineralisation at Tritton consists of two zones each approximately 400 m long, which strike at 28o true, dip to the east at 20 to 70o and pitch towards 130o true (Fig 3). The upper zone commences approximately 180 m beneath the surface and has no surface expression or geochemical signature in soil or bedrock. The lower zone commences at about 60 m beneath the bottom of the upper zone, has been intersected in holes
Geology of Australian and Papua New Guinean Mineral Deposits
The upper zone is more than 180 m beneath the surface, has not been weathered or eroded and is zoned vertically. The top of the zone contains the highest chalcopyrite:pyrite ratios, the highest gold and silver values, minor bornite and tetrahedrite and more numerous lenses of hematite and red jasper. Typical drill hole intersections in the upper zone range from 5 to 24 m at grades of 5 to 21% copper and 0.3 to 1.5 g/t gold. Tritton lower zone mineralisation occurs as massive and banded pyrite-chalcopyrite lenses in chloritic semipelitic schist, immediately overlying carbonated mafic schist. Epidote and magnetite alteration are common within the carbonated mafic schist which has been described in thin section as a porphyritic andesite (I R Pontifex, unpublished data, 1996). Alteration associated with the mineralisation includes black (magnesian) chlorite, and yellow brown chlorite, with siderite and sericite in the hanging wall. Small lenses of cryptocrystalline hematitic jasper (similar to the occurrences in the upper zone) occur in this zone, supporting the concept of a common origin for the mineralisation of the upper and lower zones.
Zones of chalcopyrite stringers with low pyrite content occur in the footwall of the lower zone and occasionally form significant widths at 1% copper cutoff. The lower zone has now been subdivided into the central and lower zones. The central zone typically has drill hole intersections of 11 to 30 m at grades of 3.6 to 6.8% copper and 0.1g/t gold whereas the new lower zone has intersections of 8 to 31 m grading 2 to 3.4 % copper and 0.1 to 0.6 g/t gold. Resources at Tritton as shown in Table 5 were estimated in December 1996 (M Binns, unpublished data,1996).
597
J M FOGARTY
FIG 3 - Cross section at Tritton, bearing 103o true and looking NE, showing massive pyrite-chalcopyrite mineralisation.
TABLE 5 Tritton resources, December 1996. Zone
Resource
Ore (Mt)
Copper (%)
Gold (g/t)
Silver (g/t)
Cobalt (%)
Upper
Measured
0.65
6.22
0.42
21
0.045
Indicated
0.76
4.17
0.33
12
0.038
Inferred
0.24
2.74
0.30
10
0.025
Sub total
1.65
4.77
0.36
15
0.039
Central
Indicated
1.47
3.42
0.16
10
0.031
Sub total
1.47
3.42
0.16
10
0.031
Lower
Indicated
0.76
2.43
0.25
12
0.025
Inferred
5.87
2.48
0.17
10
0.028
Sub total
6.63
2.47
0.18
10
0.028
Measured +Indicated
3.64
3.87
0.26
13
0.034
Inferred
6.11
2.49
0.18
10
0.028
Grand total
9.75
3.01
0.21
11
0.030
Total
Contained metal Copper (t)
293 000
Gold (oz)
66 000
Silver (oz)
Budgerygar Massive and banded pyrite-chalcopyrite mineralisation occur within chloritic semipelitic schist. Mining commenced in 1906 and shafts were sunk to 82 m (Carne, 1908, pp 195–196).
3 400 000
(Fogarty, 1996) and the drilling of 39 RC holes and two deep diamond drill holes indicate that the sulphide lens has a strike extent of 400 m, a true thickness to 30 m, a strike of 352o true with a dip of 60o east. An Inferred Resource of 2.4 Mt at 0.63% copper containing 15 120 t of copper metal was estimated by the GEJV within the leachable chalcocite zone.
Exploration by the GEJV including SIROTEM surveys
598
Geology of Australian and Papua New Guinean Mineral Deposits
GIRILAMBONE DISTRICT COPPER DEPOSITS
Two deep diamond holes by the GEJV at the southern end of the mineralisation intersected massive and banded pyrite with minor chalcopyrite over a 30 m thickness, and further deep drilling is planned.
Bonnie Dundee This deposit was discovered in about 1906. Mining to depths of 91 m had been completed by 1907 and widths to 4.3 m were reported (Carne, 1908, pp 195–196). Two massive pyrite bodies to 14 m thick were intersected in holes drilled by the GEJV in 1997, and further drilling is planned.
Budgery Copper was discovered at Budgery, 7 km west of Hermidale, in 1906 and18 928 tons of ore were treated for a yield of 854 tons of copper. The orebody consisted of a pipe-like body of irregular pyrite content, of strike length 50 m and true width to 10 m, with an average grade of 3.5% copper. The ore is hosted by sandstone, slate, phyllite and schist of the Budgery Sandstone (Carne, 1908, p 194; McLeod, 1966). Massive pyrite-chalcopyrite mineralisation occurs within the axial plane of a parasitic fold in fine grained crushed quartz felsite, perhaps the pink quartzite. Several lenses of massive magnetite-hematite outcrop close to the mineralisation and suggest a similar ore genesis to that at Tritton.
Exley At Exley (Fig 1), dense pyrite mineralisation with a significant gold content occurs within lenses of cryptocrystalline quartz laminated with magnetite, which outcrop discontinuously over strike lengths of several hundred metres. These iron formations occur within quartz-sericite schist of the basement schist near its contacts with a large WNW-trending dolerite sill.
Budgery and at Exley indicate that these occurrences may be proximal and distal hydrothermal deposits related to the mafic intrusive bodies. The Girilambone and the Tritton settings are very similar although they are 22 km apart. The lower ore zone at Tritton is interpreted as a reverse shear zone extending to basement rocks, and a conduit for hydrothermal silica and sulphide mineralisation which was deposited in dilation zones in the shear, and in (or with) the pink quartzite closer to the surface. The mineralised shear zone with disseminated pyrite and minor chalcopyrite in the Girilambone pit is interpreted as a similar conduit for mineralising fluids and for the pink quartzite. The Tritton-Budgerygar sulphide zone is now interpreted as being emplaced in a late stage, northerly-trending major fault zone with dense banded and disseminated pyrite introduced within the planes of the shear zone over a 40 m width and along a strike length of at least 2.2 km. The high copper grades in the Tritton area are believed to be derived from copper in later stage fluids related to the adjacent mafic intrusive bodies (Fig 3) confined between two adjacent NW-trending faults approximately 400 m apart near the southern end of the known sulphide zone. Recrystallisation and remobilisation formed the massive sulphide zones and the oxidising (indicated by magnetite and hematitic jasper) copper-bearing fluids reacted with the pyrite. Chalcopyrite partially replaced the pyrite, and formed zones of chalcopyrite-rich sulphides of a massive character, quite different from the banded, disseminated pyrite further to the north. The siliceous pink quartzite comprising the upper zone was formed near the top of this system during these processes and the high grade massive pyrite-chalcopyrite of the upper zone was remobilised into brittle fractures in the pink quartzite. These concepts are supported by the proximity of all of the known economic copper mineralisation to mafic intrusive bodies.
ORE GENESIS ACKNOWLEDGEMENTS Many concepts of the origin of the mineralisation in the Girilambone district have been described previously. The deposits have been interpreted as Besshi type deposits related to thin mafic volcanic flows; mineralisation associated with exhalites; mineralisation due to replacement of favourable tuffaceous or carbonate horizons by silica and sulphides; or mineralisation arising from movement of hydrothermal fluids along fault zones extending to basement and associated with the margin of the sedimentary basin. The mineralisation at Tritton, Girilambone and Cobar is polymetallic, containing copper, zinc, lead, gold and silver, and occurs in similar rock types such as greywacke, phyllite and graphitic schist. The presence of black (magnesian) chlorite, yellow-green chlorite, sericite, siderite and tetrahedrite as well as highly silicified zones, called the pink quartzite at Girilambone and Tritton and elvan at Cobar, indicate similar alteration processes and may indicate a similar metallogenesis. All of the deposits are now interpreted as mineralisation emplaced in fault zones. The presence of siliceous hematitic red jasper with abundant disseminated pyrite and magnetite near mafic intrusive bodies along the margins of the Birrimba mafic intrusive bodies, as small lenses within the Tritton lower zone mineralisation, in the Tritton upper zone, and in the hematite-magnetite lenses at
Geology of Australian and Papua New Guinean Mineral Deposits
This paper is published with permission of Nord Pacific Limited and Straits Resources Ltd. Nord personnel and contract and consultant geologists closely involved in the recent exploration of the prospects include P Carson, R Berthelsen, M Binns, M Dodds, S Johnson, D Priest, B Thompson, M E I’ons, and staff of GCC including P Shields and K Pahlow. Consultants and contractors involved in the discovery of Tritton include S Collins and McSkimming Geophysics. Major assistance has been received from contractors Pontil Drilling Pty Ltd, D Mills Surveying, and Vacuum Drilling Pty Ltd.
REFERENCES Carne, J E, 1908. The Copper Mining Industry and the Distribution of Copper Ores in New SouthWales, Geological Survey of New South Wales, Mineral Resources Report No 6. Fogarty, J M, 1996. Exploration for leachable copper deposits Girilambone district, in The Cobar Mineral Field - A 1996 Perspective (Eds: W G Cook, A J H Ford, J J McDermott, P N Standish, C L Stegman and T M Stegman), pp 179–193 (The Australasian Institute of Mining and Metallurgy: Melbourne). McLeod, I R, 1966. Australian Mineral Industry: The Mineral Deposits, Bureau of Mineral Resources, Geology and Geophysics, Bulletin 72.
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J M FOGARTY
Shields, P, 1996. Geology of the Girilambone copper deposit, in The Cobar Mineral Field - A 1996 Perspective (Eds: W G Cook, A J H Ford, J J McDermott, P N Standish, C L Stegman and T M Stegman), pp 293–303 (The Australasian Institute of Mining and Metallurgy: Melbourne). Smith, E A, 1971. Report of the exploration of Exploration Licence 287, Nyngan, NSW, to July 31st 1971, Geological Survey of New South Wales, open file report GS 1971/178 (unpublished).
600
Smith, E A, 1973. Report of exploration of Exploration Licence 287, Nyngan, NSW, from August 1st 1971 to December 31 1972, Geological Survey of New South Wales, open file report GS 1973/201 (unpublished). Suppel, D W, 1974. Girilambone Anticlinorial Zone, in The Mineral Deposits of New South Wales (Eds: N L Markham and H Basden) pp 118–131 (Geological Survey of New South Wales: Sydney). Suppel, D W and Barron, L M, 1986. Platinum in basic to ultrabasic intrusive complexes at Fifield: a preliminary report, Geological Survey of New South Wales, Quarterly Notes, 65:1–8.
Geology of Australian and Papua New Guinean Mineral Deposits
Shi, B L and Reed, G C, 1998. CSA copper-lead-zinc deposit, Cobar, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 601–608 (The Australasian Institute of Mining and Metallurgy: Melbourne).
CSA copper-lead-zinc deposit, Cobar by B L Shi¹ and G C Reed² INTRODUCTION The CSA is an underground mine, owned by Golden Shamrock Mines Ltd (GSM), 11 km NNW of Cobar in central western NSW. It is at lat 31o25′S, long 145o48′E on the Cobar (SH 55–14) 1:250 000 scale and the Cobar (8035) 1:100 000 scale map sheets (Fig 1). Although a small amount of ore was mined before 1920, production commenced in earnest in 1965. From that time to early December 1995, CSA had processed 20 Mt of ore grading 2.11% copper, 0.62% lead, 1.98% zinc and 22 g/t silver. At the end of September 1996 the Measured, Indicated and Inferred Resources to 22 level ( 2100 m below surface)
were 27.8 Mt at 3.75% copper. The current mining rate of 1.0 Mtpa is being increased to 1.3 Mtpa following recent upgrades within the mine.
EXPLORATION AND MINING HISTORY The CSA deposit is named after the Cornish, Scottish and Australian origins of the initial proprietors. It was discovered as a surface gossan in 1871 by Tom O'Brien, an experienced prospector (Clelland, 1984) but there was little development until the early 1900s (Scott and Phillips, 1990). Operations finished in 1920 when a fire broke out in the shaft making it necessary to seal the underground workings. The mine was dewatered and drilled to a depth of 500 m in the late 1940s by Enterprise Exploration Company Pty Ltd (Scott and Phillips, 1990). All Cobar mining leases and other assets were purchased between 1957 and 1960 by Cobar Mines Pty Ltd and Cobar South Pty Ltd, both wholly owned subsidiaries of Broken Hill South Limited. In 1961 the decision was made to develop the CSA based on estimated reserves of 18 Mt at 2.7% copper, 0.6% lead and 2.0% zinc estimated from 22 drill hole intersections to a depth of 1000 m below surface (Scott and Phillips, 1990). This estimate was based on the Western and Eastern ore systems. In the early 1970s the copper-rich QTS North and South ore systems were discovered by diamond drilling in depth east of the Eastern system (Figs 2 and 3). The exploration program was reportedly managed by a Queenslander, a Tasmanian and a South Australian, hence the name QTS. CRA Limited acquired the Cobar assets of Broken Hill South Limited in 1980. In February 1993 a consortium led by GSM purchased Cobar Mines Pty Ltd from CRA Limited. The mine is served by two shafts 998 m and 1026 m deep as well as a surface decline. The crusher and other ore handling facilities are on 9 level (810 m deep) and mining is proceeding between 11 and 13 levels (1000 to 1180 m deep), by long hole open stoping.
PREVIOUS INVESTIGATIONS
FIG 1 - Regional geological map of the area north of Cobar area and location of the CSA deposit, modified from A J H Ford (unpublished data, 1996).
1.
Mine Geologist, Cobar Mines Proprietary Limited, PO Box 31, Cobar NSW 2835.
2.
Mine Geologist, Pasminco Mining-Broken Hill, PO Box 460, Broken Hill NSW 2880.
Geology of Australian and Papua New Guinean Mineral Deposits
Mineralisation in the Cobar area has been studied by many workers since copper was discovered in 1870. The main descriptions prior to 1990 are listed in Scott and Phillips (1990). Studies since have been reported by Glen (1990), Seccombe (1990), Jeffrey (1994), Glen, Clare and Spencer (1996) and McDermott, Smith and Jeffery (1996).
REGIONAL GEOLOGY The Cobar mineral field is a belt of mineralisation that occurs within the deep marine sediment of the Early Devonian Cobar Basin. This basin is one of many meridional grabens that developed in the Lachlan Fold Belt during the Siluro-Devonian (A J H Ford, unpublished data, 1996). The eastern margin of the graben is controlled by the Myrt and Rookery faults (Fig 1).
601
B L SHI and G C REED
FIG 2 - Plan view of ore lenses and systems of CSA mine from 11 to 12 level (1000 to 1100 m below surface).
The rocks which form the basement below the Cobar Basin are not known. They are assumed to be similar to the turbiditic Ballast beds of the Girilambone Group that are exposed to the east and north of the basin. The basinal stratigraphic units belong to the Cobar Supergroup which is divided into the Kopyje, Nurri and Amphitheatre groups (Pogson and Felton, 1978; Glen, 1982; Kirk, 1983). The host rock of the CSA mineralisation is the CSA Siltstone which is the basal unit of the Amphitheatre Group (McDermott, Smith and Jeffery, 1996). The Cobar Basin was opened by sinistral extensional tectonics and closed by dextral movement in the early Carboniferous (Glen, 1990; Smith, 1992; Smith and Marshall, 1992; Glen, Clare and Spencer, 1996; McDermott, Smith and Jeffery, 1996).
ORE DEPOSIT FEATURES LITHOLOGY The CSA Siltstone is dominantly siltstone, with lesser greywacke. The siltstone component forms approximately 95% of the rock mass and is fine grained (30–50 µm), dark grey and consists of angular to slightly shardy grains of quartz and sericite with some minor feldspar. Accessory minerals include zircon, tourmaline, anatase and magnetite (I D M Robertson, unpublished data, 1982). Grain size relationships within the siltstone component follow the fashion of the C, D and E divisions of the Bouma turbidite sequence. This is interpreted as a distal turbidite facies (McDermott, Smith and Jeffery, 1996). The greywacke component, comprising 5% of the rock mass, is pale grey and has slight variation in grain size and composition. Clast composition is 70–80% subrounded quartz
602
FIG 3 - Generalised cross section of CSA mine, looking north, with ore systems projected.
grains, 20–30% sericite and minor lithic fragments. Grain size varies from 650 µm to 1 mm (Whittle, 1991; DeMark, 1994). The matrix forms 10–20% of the rock estimated from DeMark’s photomicrographs. Greywacke units function as useful marker horizons because individual packages extend along strike and down dip for several hundred metres. The sequence of interbedded fine- and coarse-grained siltstone contains an abundant variety of inorganic mesoscopic primary sedimentary structures. Predepositional interbed bottom structures include flute marks and scour and fill. Syndepositional intrabed structures include graded beds and ripple marks. Interbed and intrabed structures were deformed during compaction by flame structures, ball and pillow structures and slumps (McDermott, Smith and Jeffery, 1996).
STRUCTURE Bedding is prominent and persists to within a few metres of the ore zones. It has an average dip of 80o towards 260o, although east dipping bedding occurs SW of the Western ore system. Approaching ore, the bedding is heavily overprinted by cleavage that has been intensified by reactivation of shear zones.
Geology of Australian and Papua New Guinean Mineral Deposits
CSA COPPER-LEAD-ZINC DEPOSIT, COBAR
The regional cleavage has an average dip of 85 to 90o towards 090o (Fig 3), and contains a steeply north-plunging down dip mineral lineation defined by sulphide elongation (Robertson, 1974). Cleavage is best developed in the argillaceous fine grained siltstone, and poorly developed to absent in silicified beds. Cleavage also commonly refracts through the massive greywacke units. Near ore zones, cleavage becomes intensified by shearing and is traced with quartz veins. Black chloritic shear zones greatly increase in frequency near ore zones. They are curviplanar, anastomosing, slickensided, commonly host mineralisation and define the footwall and hanging wall of the ore systems. Faults within the mine usually have a reverse sense of movement with a shallow dip towards the west (Robertson, 1974). Near ore zones faults may be expressed as shear zones with intense slickensides. Robertson (1974) asserted that the thrusts usually post-date mineralisation. However, on the QTS South 990 m sublevel, observed thrusts post-date flat dipping quartz vein hosted sulphides. The sulphides occur as massive and/or disseminated textures (Fig 4). This can indicate either synchronous faulting and mineralisation or a later stage of remobilised mineralisation.
ALTERATION Siltstone and sandstone of the host sequence have a primary mineral assemblage of quartz-muscovite-albite-carbonate (Kirk, 1983). Previous investigations by Robertson and Taylor (1987) indicated small depletion haloes of base metals immediately surrounding the metal sulphides. Iron and magnesium chloritisation and silicification are three types of alteration observed in the mine environments. Carbonate alteration is minor. The area overprinted by the three types of alteration is called the multiphase alteration zone (Figs 4 and 5). The stronger mineralisation usually occurs in or near the multiphase alteration zones. All alteration types are characterised by the degradation or destruction of white mica and plagioclase and the development of chlorite and quartz respectively (Robertson, 1974).
Chloritisation Brill (1989) identified several different types of chlorite. All types are consistently iron rich and fall within the range of clinochlores with approximately 30 wt % FeO. Some show nearly complete substitution of magnesium by ferrous iron (up to 50 wt % FeO) and Altschermak substitution [Aliv+Alvi for (Fe, Mg, Mn)vi+Siiv] and are chamosites. All varieties of iron chlorite give the rock a green hue overprinting the grey colour of the unaltered rock. Schistose chlorite is the predominant sheet silicate and is aligned in the direction of the regional cleavage. In areas of weak cleavage development, chlorite occurs as a network of fine anastomosing filaments wrapping around quartz and muscovite crystals. Where cleavage is strongly developed, in addition to the fine anastomosing network, domains of chlorite form, which are seen to displace bedding surfaces and early veins. Some domains of chlorite laths show simultaneous extinction during microscopic examination (Whittle, 1991). Other textures that are commonly observed are: 1.
iron-rich chlorite grains (average 45 wt % FeO) around which schistose chlorites are deflected, and veins that have been ptygmatically folded;
Geology of Australian and Papua New Guinean Mineral Deposits
2.
pressure fringe chlorite in pyrite pressure shadows, together with porphyroblastic and vermicular chlorite and quartz; and
3.
selvages of chloritised host rock which are common around veins on a small scale (Brill, 1989).
Shear zones that dissect the deposit, and may carry sulphide mineralisation together with calcite, dolomite and talc, are extremely well chloritised. These chlorites are magnesium rich and give the material a black colour. They are discussed further in mineralisation. The chlorite alteration halo asymmetrically encircles the zones of stronger structural deformation, mineralisation and veining. It may be quite extensive, to 150 m out from the location of most intense alteration (Robertson and Taylor, 1987). As the chlorite alteration increases, the overprinting green hue intensifies and the rock hardness decreases.
Silicification Silicification of the host rocks is a replacement of feldspar clasts (Brill, 1989). Binns (1985) and Binns and Appleyard (1986) have shown that this alteration is transgressive to bedding in the sediments. It is characterised by a typical bleached appearance of the rock due to increased quartz content. Colour varies from grey-green to greyish blue to cream. The intensity of silicification is variable from trace to extreme; the latter is referred to locally as ‘elvan’. Elvan is blue-grey to green-grey and often occurs as pods and clasts within the massive ore (Fig 6). The name elvan has been applied because of the similarity to cherts of the same name from the Cornish tin mines. Elvan is mainly composed of polygranular quartz clasts with small amounts of interstitial chlorite and minor accessories. Quartz overgrowth is common. Within silicification there is often spectacular pyrrhotite (Brill, 1989). Elvan zones are commonly bounded by black chlorite shears. Where anastomosing arrays of shears are present a brecciated texture is developed. The limits of silicification may be sharp or gradational where black chlorite shears are absent. Silicification may also be cut by a network of <2 mm wide quartz veinlets (Whittle, 1991). The chemical alteration halo of the CSA deposit extends beyond the limit of visible mineral alteration (Robertson and Taylor, 1987). Analysis by Brill (1989) demonstrated that there was elemental depletion and enhancement in altered rocks of the deposit with reference to unaltered rock away from mineralisation. The chemical data showed depletion of the alkalis and alkaline earths (aluminium, titanium, potassium, calcium, rubidium and tin), and enhancement of silica, iron, magnesium and manganese.
MINERALISATION There are four main ore systems (Figs 2 and 3) which all strike roughly north and dip steeply to the east, grossly subparallel to the cleavage orientation (Scott and Phillips, 1990). Mineralisation occurs in a number of vein complexes or submassive to massive bodies, all of which are locally called lenses (Fig 2). Most ore lenses display a steep northerly plunge parallel to an extension lineation. In general ore lenses at the CSA have a relatively short strike length, rarely exceeding 80 m, and are 6 to 20 m wide. The vertical dimension is the most continuous, to several hundreds of metres (Fig 3).
603
B L SHI and G C REED
FIG 4 - Pyrrhotite and chalcopyrite assemblage occurs as splash and disseminated textures in silicfied and magnesian chlorite altered siltstone. Eastern system, 1050 sublevel.
FIG 6 - Angular to rounded patches or fragments of black chlorite and chloritic siltstone, quartz, and elvan are enclosed by massive to submassive chalcopyrite, which is characterised by some spectacular intersections containing up to 20% copper over mineable widths. The textures show that chalcopyrite characteristically precipitates during later stages of ore formation, and elvan replaces pyrrhotite, pyrite and gangue minerals (quartz, chlorite) and fills an open fracture. QTS North, 13 level.
FIG 5 - Chalcopyrite occurs as massive and disseminated textures in multi-phase altered siltstone. Rock fragments in the orebody can be observed. QTS South, 990 sublevel.
A broad halo of pervasive chlorite alteration surrounds individual lenses for up to 50 m laterally, and in many cases this is accompanied by a pervasive silicification. The green chlorite (iron rich) alteration is predominant and may only occur as a coating on cleavage surfaces in the outermost part of the halo. Later shears containing magnesium-rich black chlorite and occasionally talc are common in all mineralised systems, and in some cases form a sharp boundary to the ore lenses.
Western system The Western system is the only outcropping system at the CSA. The mineralisation is discontinuous, resulting in relatively small lenses which average 45 m long and 7 m wide (Fig 2). The lenses strike northerly over 300 m, dip steeply to the east and consist of high grade pods of copper rich and lead-zinc rich ore. The copper lenses consist of vein type chalcopyrite, pyrrhotite, pyrite and quartz, and the lead-zinc rich lenses consist of galena, sphalerite, pyrrhotite, pyrite and chalcopyrite and are often banded. Average grades for the Western system are 6% zinc, 3% lead and 2% copper, but with depth there appears to be an increase in copper with a corresponding decrease in lead-zinc grades. Host rocks for the Western system ore consist of chloritised (magnesium rich), silicified and quartz veined rock, pervasive silicification and an intense form of elvan replacement of the siltstone.
604
FIG 7 - Veins and splashes of chalcopyrite associated with a network of fine quartz veinlets in a zone of intense cleavage in the Eastern system, 1050 sublevel.
Eastern system The Eastern system consists of at least four lenses of moderate length (50–80 m) and variable width (average 10 m), each of which comprises a number of vein systems. Average grade for the system is around 3% copper. Mineralisation consists predominantly of chalcopyrite, pyrrhotite and numerous quartz veins, as well as pyrite veins in an intensely cleaved chloritic siltstone. The Eastern system structure outcrops but no economic accumulations occur above 250 m below surface. Economic lenses are defined by sufficient concentrations of veining (Fig 7). The CZ lens, which is a lead-zinc rich orebody, lies between the Eastern and Western systems and has no surface expression (Fig 3). Average grade of the CZ ore is about 10% zinc, 1% lead and <1% copper. Pyrite content may be as much as 50%. The host siltstone is strongly chloritised (magnesium rich), sometimes talcose and often weakly silicified. The footwall is often marked by a strong black chlorite shear.
Geology of Australian and Papua New Guinean Mineral Deposits
CSA COPPER-LEAD-ZINC DEPOSIT, COBAR
QTS North
Pyrrhotite ore
The system is a blind orebody, approximately 100 m east of the Eastern system. QTS North consists of several lenses, generally less than 100 m long, with an average width of 10 m, which have a northerly strike and dip steeply to the east (Figs 2 and 3). QTS North is copper rich with no lead-zinc lenses. Average grade for the system is about 5% copper. The westernmost lens, M (formerly known as D zone), consists of chalcopyrite and pyrrhotite with grades similar to those of the Eastern system. M lens is associated with an unusual and extremely strong black chlorite alteration. The easternmost lenses, P, Q and R, consist predominantly of chalcopyrite and pyrrhotite with high grade ore (+20% copper) and only minor quartz.
Pyrrhotite ore normally contains 60 to 80% sulphide as pyrrhotite, pyrite, sphalerite and galena, and is hosted by black chloritised siltstone (Fig 5). The predominant pyrrhotite is responsible for the magnetic signature of the deposit. Schmidt (1983) studied pyrrhotite from the Elura deposit and concluded that monoclinic pyrrhotite formed in the presence of pore fluids as the deposit cooled below 260oC after regional metamorphism. Continued deep retrograde alteration at CSA formed carbonate, talc and secondary pyrite. A minor siliceous variant of pyrrhotite ore is present locally, in which pyrrhotite occurs with coarse fibrous quartz.
QTS South
PREVIOUS MODELS
QTS South, like QTS North, is a blind orebody. It is approximately 500 m south of the other ore systems and 100 m east of QTS North, making it the most easterly group of ore lenses at the CSA. QTS South consists of several subparallel lenses, to 200 m long with an average width of 8–10 m, striking north and dipping steeply to the east. The system is principally copper rich, with minor pyrrhotite, and isolated pods of galena and sphalerite at the extremities. Portions of the lens have high grade cores (+20% copper) which are intimately associated with zones of strong black chlorite alteration.
Numerous models for the Cobar deposits have been suggested since their discovery last century, which can be broadly be divided into the three categories epigenetic, syngenetic and structurally controlled.
ORE TYPES The orebody can be subdivided into four types on the basis of composition, mineralogy and density.
Massive ore Most massive ore (Fig 6) occurs in the QTS North and South orebodies. Massive ore contains 70 to 90% sulphide, dominantly chalcopyrite and pyrrhotite, with a quartz and/or chlorite gangue. The average density is 3.3. Massive ore is typically homogeneous, though numerous silica rich bands and patches are present which may grade into siliceous ore. Textural relationships are simple, hence metallurgical problems are minimal.
Siliceous ore Most siliceous ore (Fig 7) is found in the Eastern and Western systems, and typically contains 20 to 50% silica in various forms, associated with pyrrhotite and pyrite. Siliceous mineralisation is highly variable and includes brecciated, banded, cherty quartz veins and granular types. Envelopes of siliceous ore 2 to 20 m thick surround most of the lenses in the deposit, and have an average density of 3.2. Siliceous ore is very fine grained with complex textural relationships that cause metallurgical recovery problems.
Pyrite-sphalerite ore Pyrite-sphalerite ore normally contains 70 to 90% sulphide, dominantly pyrite and sphalerite with lesser pyrrhotite, galena and chalcopyrite, in a gangue dominated by magnesian chlorite. The average density is about 4.2. The ore is typically found in CZ lens. Numerous pyrrhotite-rich bands and patches are present which may grade into pyrrhotite ore.
Geology of Australian and Papua New Guinean Mineral Deposits
ORE GENESIS
Epigenetic models were first proposed by Andrews (1913) who noted that the mineralisation occurs within faults or fissure zones that crosscut sedimentary bedding, thus implying an epigenetic origin. Sullivan (1951) suggested that the mineralisation was localised by zones of intersecting cross fractures which resulted in north plunging pipe-like bodies of highly fractured rock through which hydrothermal fluids moved. Epigenetic models returned to favour in the 1980s (Kirk, 1983; Binns and Appleyard, 1986; Glen, 1987; Scott and Phillips, 1990). Syngenetic models were proposed by Robertson (1974) and Brooke (1975) who considered the mineralisation was formed close to its present position and later remobilised into structures. This partly explained Robertson’s (1974) observations of mechanically remobilised sulphides. Gilligan and Suppel (1978) and Sangster (1979) suggested that the mineralisation is of the stratabound exhalative type, with its orientation parallel to cleavage explained by strain during deformation. Structurally controlled models proposed by Glen (1987, 1988, 1990) have focussed the discussion on structural ore emplacement and basin deformation. B A Brill and P K Seccombe (unpublished data, 1990), B A Brill, P K Seccombe and A Chivas (unpublished data, 1990) and Seccombe (1990) have discussed the fluids generated by metamorphic and deformation processes.
DISCUSSION The CSA deposits have some features in common with other deposits in the Cobar field, but also a number of important differences. Features of the orebodies that must be explained by any genetic model are: 1.
the gross discordant relation of the mineralisation to the turbiditic host rock sequence;
2.
structures in the orebodies and adjacent host rock including steepening of bedding and local intense folding;
3.
extensive alteration of the host rock, with formation of quartz-(carbonate-talc) and magnesian chlorite veins in the contact zone, and a broad halo of green iron-rich
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B L SHI and G C REED
chlorite surrounding the ore zones and extending outwards from them for up to 50 m, more or less coincident with a zone of low muscovite content; 4.
the nature and relative abruptness of contacts between the orebody and host rock, and between ore types within the body;
5.
thin concordant sulphide (pyrrhotite) beds in the host rock;
6.
the presence of siltstone rock fragments in the orebody with similar bedding orientation to the adjacent host rock (Fig 3); and
7.
compositional, mineralogical and textural zoning within the deposit.
Zoning within the deposit and wall rock alteration were produced by spatial and temporal changes in ascending mineralising fluids. The solutions no longer in equilibrium with wall rock began to react and progressively replace the wall rock. Siliceous ore at the orebody margins represents an arrested early stage of the process, and pyrrhotite ore with equilibrium textures represents an advanced stage. The various ore types probably formed simultaneously as they are believed to reflect the degree of chemical and textural maturity. The mineralising fluids are assumed to have moved upwards from the NE (Fig 3). Compositional zoning within the orebody is a reflection of a contemporary cleavage-forming event. Conformable sulphides replaced particular reactive or permeable sedimentary horizons near the body. Post-mineralisation deformation caused extension and the development of the dilation bedding and other later events recorded in the orebody. Considerable work by Robertson and Taylor (1987), DeMark (1994) and Jeffrey (1994) has been done on base metal, alkali and alkali-earth depletion haloes and alteration mineralogy around the CSA deposits. It is believed that the source of the metals was widespread leaching during dewatering of the sedimentary sequence and the subsequent focussing of the leaching fluids into structural conduits (shears). However the alteration is very laterally restricted, suggesting a far more localised vertical movement of ore fluids. B A Brill, P K Seccombe and A Chivas (unpublished data, 1990) noted that the δ34S range for the CSA deposit was 4.5 to 9.8‰ with a mean of 7.5‰. The samples used to derive this were taken from a number of lenses and levels within the mine. The δ34S data show that: 1.
galena has slightly lower values than other sulphides;
2.
there is no variation in the δ34S isotopic composition between the copper and lead-zinc rich ore types in the different mineralised lenses and levels within the mine; and
3.
the results are consistent with those of Marshall et al (1983).
Temperature estimates range from 240oC for pyritepyrrhotite to >1000oC for pyrite-chalcopyrite (B A Brill, P K Seccombe and A Chivas, unpublished data, 1990). A complex paragenesis with several generations of pyrite and chalcopyrite is responsible for this isotopic disequilibrium. Fluid inclusion, chlorite compositional and mineralogical data from B A Brill and P K Seccombe (unpublished data, 1990) suggest that the temperature of ore formation was 300 to 350oC.
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Banding in the ore is parallel to lens and vein walls. Microscopic (Whittle, 1991; DeMark, 1994) and mesoscopic textures indicate recrystallisation due to deformation and at least local remobilisation. Textures indicate mostly open space filling with some replacement (Figs 4–7), especially in QTS North and South. The bulk of the orebody is spatially and texturally associated with syntectonic quartz veining. The orebodies are coplanar with and hosted by regional shears, and pitch parallel to a steeply north-plunging stretching lineation. The orebody post-dates cleavage formation.
CONCLUSION The genesis of the CSA deposits is as yet not fully resolved. The evidence above supports a syntectonic origin for the mineralisation with ore being deposited in brittle-ductile dilation zones in active shears. The metals were probably derived from hydrothermal solution (in the case of copper) or from remobilisation of syngenetic ore.
ACKNOWLEDGEMENTS Golden Shmarock Mines Limited are thanked for permission to publish.
REFERENCES Andrews, E C, 1913. Report on the Cobar copper and goldfield, Geological Survey of NSW Mineral Resources 17. Binns, R A, 1985. Age of the CSA mineralisation at Cobar, NSW, CSIRO Division of Mineralogy and Geochemistry, Research Review 1985, pp 26–27. Binns, R A and Appleyard, E C, 1986. Wallrock alteration at the Western System of the CSA mine, Cobar, New South Wales, Australia, Applied Geochemistry, 1: 211–255. Brill, B A, 1989. Geochemistry and genesis of the CSA Cu-Pb-Zn deposit, Cobar, NSW, Australia, PhD thesis (unpublished), University of Newcastle, Newcastle. Brooke, W J L, 1975. Cobar mining field, in Economic Geology of Australia and Papua New Guinea,Vol 1 Metals (Ed: C L Knight), pp 683–694 (The Australasian Institute of Mining and Metallurgy: Melbourne). Clelland, W, 1984. Cobar Founding Fathers (Macquarie Publications Pty Ltd: Dubbo). DeMark, P, 1994. Geology and mineralisation of the Western Gossan, CSA Mine, Cobar, NSW, BSc Honours thesis (unpublished), University of Technology, Sydney. Gilligan, L B and Suppel, D W, 1978. Mineral deposits in the Cobar Supergroup and their structural setting, NSW Geological Survey Quarterly Notes, 33:15–22. Glen, R A, 1982. The Ampitheatre Group, Cobar NSW. Preliminary results for the new mapping and implications for ore search, NSW Geological Survey Quarterly Notes, 49:1–14. Glen, R A, 1987. Copper- and gold-rich deposits in deformed turbidites at Cobar, Australia: their structural control and hydrothermal origin, Economic Geology, 82:124–140. Glen, R A, 1988. Basin inversion, thrusts and ore deposits in deformed turbidites at Cobar NSW, A preliminary report, NSW Geological Survey Quarterly Notes, 73:21–26. Glen, R A, 1990. Formation and inversion of transtensional basins in the western part of the Lachlan Fold Belt, Australia, with emphasis on the Cobar Basin, Journal of Structural Geology, 12 (5):601–602. Glen, R A, Clare A and Spencer, R, 1996. Extrapolating the Cobar Basin model to the regional scale: Devonian basin-formation and inversion in Western New South Wales, in The Cobar Mineral
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Field - A 1996 Perspective (Eds: W G Cook, A J H Ford, J J McDermott, P N Standish, C L Stegman and T M Stegman), pp 43–83 (The Australasian Institute of Mining and Metallurgy: Melbourne). Jeffrey, S I, 1994. A structural, geophysical, isotopic and geochemical appraisal of the CSA deposit, Cobar, Australia. Implications for the deformation of the Cobar Basin and mineral potential, MSc thesis (unpublished), University of Tasmania, Hobart. Kirk, B I, 1983. A review of the geology and ore genesis of the Cobar mining field, in Proceedings Australasian Institute of Mining and Metallurgy Conference, Broken Hill, 1983, pp 183–195 (The Australasian Institute of Mining and Metallurgy: Melbourne). Marshall, B, Sangameshwar, S R, Plibersek, P K and Kelso, I J, 1983. Cobar Supergroup deposits: polymodal genesis during protracted tectonism, in Lithosphere Dynamics and Evolution of Continental Crust, Sixth Australian Geological Convention, Canberra, Geological Society of Australia Abstracts, 9:305–306. McDermott, J J, Smith, C K and Jeffrey, S I, 1996. Geology of the CSA deposit, in The Cobar Mineral Field - A 1996 Perspective (Eds: W G Cook, A J H Ford, J J McDermott, P N Standish, C L Stegman and T M Stegman), pp 197–213 (The Australasian Institute of Mining and Metallurgy: Melbourne). Pogson, D J and Felton, E A, 1978. Reappraisal of geology, CobarCanbelego-Mineral Hill Region, Central Western NSW, NSW Geological Survey Quarterly Notes, 33:1–4. Robertson, I G, 1974. The environmental features and petrogenesis of the mineral zones of Cobar, NSW, PhD thesis (unpublished), University of New England, Armidale.
Sangster, D F, 1979. Evidence of an exhalative origin for deposits of the Cobar district, New South Wales, BMR Journal of Australian Geology and Geophysics, 4:15–24. Schmidt, B L, 1983. Aspects of mineralisation at Elura, in Lithosphere Dynamics and Evolution of Continental Crust, Sixth Australian Geological Convention, Canberra, Geological Society of Australia Abstracts, 9:313–315. Scott, A K and Phillips, K G, 1990. CSA copper-lead-zinc deposit, Cobar, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1337–1343 (The Australasian Institute of Mining and Metallurgy: Melbourne). Seccombe, P K, 1990. Fluid inclusion and sulphur isotope evidence for syntectonic mineralisation at the Elura mine, southeastern Australia, Mineralium Deposita, 25:304–312. Smith, J V, 1992: Experimental kinematic analysis of en echelon structures in relation to the Cobar Basin, Lachlan Fold Belt, Tectonophysics, 214:296–276. Smith, J V and Marshall, B, 1992. Patterns of folding and fold interference in oblique contraction of layered rocks of the inverted Cobar Basin, Australia, Tectonophysics, 215:319–334. Sullivan, C J, 1951. Geology of New Occidental, New Cobar and Chesney Mines, Cobar NSW, Bureau of Mineral Resources Geology and Geophysics, Report 6. Whittle, M A, 1991. The geology, mineralisation and metal zonation of the Western System, CSA Mine, Cobar, Central Western NSW, BSc Honours thesis (unpublished), University of Queensland, Brisbane.
Robertson, I D M and Taylor, G F, 1987. Depletion haloes in fresh rocks surrounding the Cobar orebodies, NSW, Australia: implications for exploration and ore genesis, Journal of Geochemical Exploration, 27:77–101.
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Cook, W G, Pocock, J A and Stegman, C L, 1998. Peak gold-copper-lead-zinc-silver deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 609–614 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Peak gold-copper-lead-zinc-silver deposit, Cobar by W G Cook1, J A Pocock2 and C L Stegman3 INTRODUCTION The deposit is approximately 8 km SE of Cobar, in central western NSW, at lat 31o34′S, long 145o53′E on the Cobar (SH 55–14) 1:250 000 scale and the Wrightville (8034) 1:100 000 scale map sheets (Fig 1). It is owned and operated by Peak Gold Mines Pty Limited, a wholly owned subsidiary of Rio Tinto.
gold, 0.9% copper, 1.0% lead, 0.9% zinc and 8 g/t silver. Current production is from a single underground operation with planned ore production of 570 000 t in 1996, expanding to 600 000 t in 1997.
MINING HISTORY The deposit is at the southern end of the Cobar Goldfield, which has produced approximately 1.7 Moz of gold since 1880, and includes the largest gold producer in NSW, the New Occidental mine. Historic production from the Peak area, prior to commencement of mining at Peak, amounted to a modest 20 670 oz gold and 13 585 kg silver. The exploration history and the early thoughts on the deposit were discussed by Hinman and Scott (1990).
REGIONAL GEOLOGY The Cobar Goldfield is near the eastern margin of the Early Devonian Cobar Basin within the Lachlan Orogen (Glen, 1995). Regional crustal extension of the Lachlan Orogen in the late Silurian (410 Myr; Sherwin, 1985) created a north-trending deep water basin in the Cobar region over a basement of Cambro-Ordovician rocks of the Girilambone Group (Fig 1). Basin architecture was controlled by a series of internal and bounding WNW-trending extensional faults and internal NEtrending transfer faults (Glen, 1991; Glen et al, 1994).
FIG 1 - Regional geological map of the Cobar area, with location of Peak gold mine and historic workings.
Between the commencement of the operation in October 1992 and the end of 1995, the mine produced 428 421 oz of gold, 65 616 t of copper concentrate, 12 989 t of lead concentrate and 9302 t of zinc concentrate from 1 690 659 t of ore. Production since 1992 plus Proved and Probable Ore Reserves at 31 December 1995, total 4.6 Mt grading 7.6 g/t 1.
Senior Geologist, Minproc Ltd, PO Box Z5266, Perth WA 6831, formerly Senior Mine Geologist, Peak Gold Mines Pty Ltd.
2.
Technical Specialist - Mine Geology, Peak Gold Mines Pty Ltd, PO Box 328, Cobar NSW 2835.
3.
Manager - Geology, Peak Gold Mines Pty Ltd, PO Box 328, Cobar NSW 2835.
Geology of Australian and Papua New Guinean Mineral Deposits
Glen (1991) divided the Cobar Basin fill into syn-rift and post-rift packets (Table 1). The syn-rift sediments (Nurri Group) comprise initial alluvial fan deposits and subsequent more extensive turbiditic sediments along the eastern basin margin. Volcanic units form only a small proportion of the exposed syn-rift sequence in the Cobar Basin, although volcanic units may be more prevalent at depth. Small bodies of felsic porphyry crop out near the Queen Bee mine to the south of Peak mine and bodies of intrusive flow-banded rhyolite and rhyolite breccia are exposed underground at Peak. The post-rift sag phase sediments (Amphitheatre Group) comprise much more extensive lower energy turbidites interfingering with sediments deposited on the western shelf. Together these postrift sediments obscure the original western margin of the Cobar Basin. The Basin was inverted in the late Early Devonian (395–400 Myr) in response to NE–SW compression (Glen, 1990, 1991). This compression was partitioned into a high strain zone along the eastern margin of the basin and a low strain zone in its central and western parts. Deformation of the basin sediments was controlled by the reactivation of the early basin-forming extensional faults and by the formation of new faults (Glen, 1991). Inversion of the western margin of the basin, at a
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W G COOK, J A POCOCK and C L STEGMAN
LOCAL GEOLOGY
TABLE 1 Cobar Basin stratigraphy. Age
Setting
Unit
Early Devonian
Post-rift basin
Amphitheatre Group
Early Devonian
Syn-rift basin
Nurri Group Great Cobar Slate Unnamed silicic volcanics Chesney Formation Queen Bee Conglomerate Member
CambroOrdovician
Basement
Girilambone Group Metasediment
STRUCTURE Composition Siltstone, sandstone, mudstone Siltstone, mudstone Porphyry, rhyolite Sandstone, siltstone Fanglomerate, sandstone
The Peak shear zone is approximately 3 km long and up to 300 m wide. It comprises a series of anastomosing thrust-type, subvertical, NNW-trending shears, locally referred to as the Peak (underground it is known as Polaris), Blue and Lady Greaves shears (Fig 2). These structures parallel a prominent NNW-trending regional cleavage and both the shears and the cleavage dissect a series of south-plunging folds parasitic to the Chesney–Narri Anticline (Hinman and Scott, 1990).
Sandstone, siltstone
considerably lesser intensity than that experienced by the eastern margin, occurred in the Carboniferous (Glen, 1991). Mineralisation within the Cobar Goldfield and the broader Cobar Basin demonstrates a strong structural control and an intimate association with thrust-type faulting, particularly that focussed along the eastern margin of the basin. Glen (1995) includes the Cobar mineralisation in a distinct class of thrustassociated mineral deposits within the Lachlan Orogen. The Peak orebody is located on the western limb of the Chesney–Narri Anticline (Fig 1), a moderate to tight southplunging anticline cored by sandstone of the Chesney Formation (Glen, 1987b). Mineralisation is associated with the Peak shear zone, which juxtaposes the Chesney Formation with the Great Cobar Slate and is broadly parallel to the eastern margin of the Basin. The Peak shear is interpreted to be a subvertical to west-dipping imbricate fault to the Great Chesney Fault with the latter an east-dipping back thrust on the major basin margin fault, the Rookery Fault (Glen, 1991). A strong subvertical regional white mica cleavage and a steep north-plunging mineral and extension lineation is present throughout the Peak area. This cleavage is subparallel to the Peak shear zone but transects the axial planes of the Chesney–Narri Anticline. Glen (1990) attributed this relationship to the formation of cleavage after the folding under a rotating left-lateral transpressional strain regime. Detailed thin section examinations in the Peak area by Hinman (1992) revealed that the prominent regional cleavage overprints a weaker more northerly-trending cleavage that is essentially subparallel to the axes of the folds in the Peak area. Hinman attributes this early cleavage and the folding to D1, and the prominent regional cleavage and faults to D2. Glen (1994) however, maintains that there is no unequivocal evidence for the presence of this early cleavage on a regional scale. Dating of strongly cleaved fine-grained sediment of the Chesney Formation, Great Cobar Slate and CSA Siltstone by 40 Ar-39Ar and K-Ar methods (Glen, Dallmeyer and Black, 1992) constrains the age of regional cleavage formation to 390–400 Myr. Further 40Ar-39Ar dating of alteration muscovite at the Peak mine indicates an age for regional cleavage formation of 402 Myr (Perkins, Hinman and Walshe, 1994).
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FIG 2 - Cross section through the Peak deposit on 10 530 N (mine grid), looking north.
The component shears of the Peak shear zone have a steep westerly dip, although they also locally dip steeply east. Displacement of the Great Cobar Slate–Chesney Formation contact across the Peak shear zone is west block up by 250 m. Both Environmental Resource Analysis Ltd (unpublished data, 1987) and Hinman (1992) identified a left lateral strike-slip component to the shearing in the Peak area. A prominent topographic high, caused by pervasive silicification of the underlying sediment, is developed along the eastern side of the Peak shear zone.
DISTRIBUTION OF MINERALISATION All mineralisation in the Peak area is intimately related to structures within the Peak shear zone. Although gold-silver mineralisation is evident over the length of the shear, most of the historic production came from the Brown, Conqueror and Blue lodes in the central portion of the shear, on and around the
Geology of Australian and Papua New Guinean Mineral Deposits
PEAK GOLD-COPPER-LEAD-ZINC-SILVER DEPOSIT, COBAR
Chesney Formation–Great Cobar Slate contact (Fig 1). Minor silver and gold were won from the Silver Peak and Lady Greaves workings approximately 1100 m north of the Brown, Conqueror and Blue lodes and from the Perseverance and Great Peak South workings 900 m to the south. Mining at all of these deposits was restricted to the zone of weathering (Rayner, 1961). Individual lodes exposed in these workings are 50 to 300 m long and associated with narrow, strongly defined chloritic and talcose shears and, to a lesser extent, quartz veining and silicification of the host sandstone and siltstone. Gold-silver mineralisation characteristically occurs as a series of small, discontinuous high grade pods with minor associated secondary copper and lead minerals. These pods are typically 1 to 2 m wide, steeply south plunging and extremely irregular. Some appear to plunge shallowly to the south, suggesting development along the intersection of bedding and cleavage (Sullivan, 1947; Rayner, 1961). In contrast to the other lodes associated with the Peak shear, the Big lode, located immediately west of Brown and Conqueror lodes, is a thicker more persistent lode characterised by intense silicification and quartz veining. It is approximately 250 m long and up to 15 m wide. It contains appreciable chalcopyrite, galena and sphalerite but only minor gold (Rayner, 1961). The Big lode is now known to represent the surface expression of the Western Lead Zinc lens of the Peak orebody.
ORE DEPOSIT FEATURES DEPOSIT SETTING The Peak deposit occurs vertically below the near-surface Conqueror, Brown and Big lodes. Although the near surface mineralisation occurs at or near the intersection of the Peak shear and the conformable Great Cobar Slate–Chesney Formation contact, the Peak orebody lies vertically below this contact, within the Chesney Formation (Fig 2). The deposit is localised about the apical portions of a series of blind flowbanded rhyolite and rhyolitic subvolcanic breccia bodies of uncertain affinity, known only from drill core and underground openings. These volcanic bodies are shallowest in the centre of the Peak deposit where they are 400 m from surface and are known to extend for at least 1000 m south, 500 m north and 300 m east of the deposit.
HOST ROCK TYPES The sediments of the host Chesney Formation have been described previously by numerous authors including Glen (1987a) and Rayner (1961). At least three separate disc-shaped fault-bounded bodies of rhyolite or rhyolitic breccia are present at the Peak deposit (Figs 2 and 3). The eastern and central bodies have a fine grained flow-banded core within an envelope of commonly coarse grained breccia, and the western body is predominantly breccia. Contacts between the flow banded rhyolite core and the breccia margins are transitional over distances of several metres. The core consists of potassium feldspar, chlorite, quartz and sericite-altered, devitrified flow banded rhyolite. Primary flow banding is defined by a weak layering of varying proportions of quartz, chlorite and potassium feldspar. The surrounding
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 3 - Geological plan of the Peak deposit at 600 level (9645 RL).
breccias comprise pervasive quartz, potassium feldspar, sericite and chlorite-altered, variably clast and matrix supported, poorly sorted, monomict lithic subvolcanic breccias with a fine grained quartz, potassium feldspar and sericite matrix (I R Pontifex, unpublished data, 1993). The main breccia complex has various textures indicating volcanic and subvolcanic emplacement of the rhyolite bodies approximately coeval with deposition of the Chesney Formation. The rhyolitic bodies are interpreted to have been intruded into wet sediment, with rapid quenching causing brecciation and producing in situ hyaloclastite. The hyaloclastite consists of fragments of rhyolite within an altered fine-grained sediment matrix. The clasts are angular, poorly sorted and up to 150 mm long. Dark green chlorite–quartz–potassium feldspar clasts are dominant, with subordinate pale and lesser dark green, highly siliceous, quartzchlorite-sericite altered rhyolite clasts. Some clasts show relict primary banding of quartz and chlorite. The banding in most clasts is of a common orientation and the clasts are roughly imbricate and closely packed. The rock is predominantly clast supported and matrix poor. The matrix is a pale grey, very fine grained to amorphous, intensely silica-flooded siltstone. Hyaloclastic textures occur on the northern margin of the eastern rhyolite body. The hyaloclastite–sediment contact is gradational, and rare clasts of volcanic material occur in a matrix of silicified sediment up to 4 m above the contact. The proportion of volcanic clasts varies to a maximum of 90% of the rock, grading to in situ, clast supported hyaloclastite. The hyaloclastite–volcanic breccia contact is sharp, where the matrix becomes quartz–potassium feldspar–sericite.
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W G COOK, J A POCOCK and C L STEGMAN
Silicification of the sediment at the hyaloclastite–sediment interface is intense and diminishes away from the contact. A less common breccia type, inferred to be of tectonic origin, is restricted to narrow shear zones, typically at the contact between rhyolite and surrounding sediment. This breccia comprises angular clasts of sediment and rhyolite in a quartz dominated matrix. These breccias are interpreted by Jiang (1996) to be equivalent to what has been described herein as rhyolite breccia.
MINERALISATION The Peak orebody is between 300 and 700 m below surface and extends over a strike length of 300 m along the Peak shear. The mineralised system is thickest around the volcanic rocks, where it attains a maximum horizontal width of 150 m and rapidly tapers upwards into the overlying sediment along the shear zones. In a broad sense, the deposit is vertically elongate parallel to the dominant steep west-dipping cleavage and the strike of the Peak shear zone. The majority of the gold and base metal sulphide mineralisation is spatially related to the volcanic rocks and occurs on the margins and within the rhyolite bodies. There are also two ore lenses, named Western Lead Zinc and Repulse South, peripheral to the volcanic bodies, which are entirely within pervasively silicified zones in sandstone of the Chesney Formation. Sulphide mineralisation, which consists of pyrrhotite, chalcopyrite, sphalerite, galena and pyrite, occurs as splashes, fracture fillings, veins and disseminations. Currently there are 17 discrete mineralised lenses interpreted at the Peak. The lenses vary geometrically from arcuate to planar and generally strike and dip parallel to the regional cleavage, which trends north and is subvertical to steeply westdipping. Another series of NW-trending, moderately SWdipping lenses are localised about the southern contact of the eastern rhyolitic body. The lenses consist of a series of narrow, stacked en echelon veins which are elliptical to thin lensoidal in plan. The host structures have strong vertical continuity, persisting at depth below the currently defined orebody. Economic mineralisation is not continuous within the host structures, and gold grades are highly variable in all directions within each lens. The lenses can be subdivided into four groups according to their styles of mineralisation, geometry and mineralogy.
Sediment hosted mineralisation Sheeted veins to 200 mm wide of quartz-sphaleritegalena±chalcopyrite-gold occur in zones of intensely silicified sandstone and lesser siltstone. The planar vein sets are parallel to the regional cleavage and are predominantly developed on the western side of the orebody. The Western Lead Zinc (WLZ) lens is the largest example of this style and contains 870 000 t at 6.8 g/t gold, 0.2% copper, 0.9% zinc, 1.1% lead and 3.3 g/t silver (W G Cook, unpublished data, 1995).
Contact zone hosted mineralisation The contact zones generally have an arcuate geometry and occur on the western volcanic–sediment interface and wrap around the lensoidal rhyolite bodies. The zones consist of altered sediment at the sheared contact of the rhyolite bodies. Only minor mineralisation occurs within brecciated rhyolite. The contact zones contain lenses of disseminated to
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semi-massive pyrrhotite-chalcopyrite±sphalerite±galena-gold mineralisation in zones of intense quartz veining to 25 m wide. Total sulphide content is typically greater than 10%. The Copper lens, which wraps around the western margin of the rhyolite bodies, is the largest lens of this style. It contains about 50% of Peak’s contained gold, and is estimated at 1.47 Mt grading 13.8 g/t gold, 1.3% copper, 1.9% lead, 1.5% zinc and 8.8 g/t silver. Copper lens grades along strike to the north into the Copper North lens, which contains chalcopyritepyrrhotite±pyrite-rich and gold-poor mineralisation. The Copper North lens contains 221 000 t at 5.5 g/t gold, 2.6% copper, 0.6% lead, 0.1% zinc and 11.4 g/t silver. The Hecla lens, which occupies the apical and southern contact portion of the eastern rhyolite body, contains 310 000 t at 7.1 g/t gold, 0.4% copper, 0.9% lead, 0.8% zinc and 5.0 g/t silver (W G Cook, unpublished data, 1995).
Volcanic hosted mineralisation The eastern rhyolite breccia body hosts a series of narrow lenses, 1 to 20 m wide, comprising 1 to 20 cm wide en echelon quartz veins developed in the structures parallel to the regional cleavage. These veins are developed in riedel shears and although very narrow, are sufficiently high grade (at several hundred g/t gold) to be economically mined within a minimum 4 m wide stope. The veins contain pyrrhotite-chalcopyritegalena±sphalerite-gold mineralisation. The Terror lens is the largest of this style and contains 230 000 t at 7.5 g/t gold, 0.4 % copper, 0.4 % lead, 0.1% zinc and 3.4 g/t silver (W G Cook, unpublished data, 1995).
Late stage shear-hosted mineralisation Small discontinuous pods of banded to semi-massive sphalerite-galena±chalcopyrite-pyrrhotite-pyrite carrying high grade silver and patchy gold mineralisation associated with black chlorite-talc-carbonate alteration are developed within late shears including the Polaris shear and components of the Hecla shear. Mineralisation of this style persists to surface in the Polaris shear and was exploited in the near surface Conqueror, Brown and Blue workings. It is known to extend to 1000 m below surface. Gold grades are best developed in this mineralisation where it is directly above the main Peak orebodies, between 250 and 500 m from surface. The Polaris lens contains 570 000 t at 5.3 g/t gold, 0.8% copper, 0.8% lead, 1.8% zinc and 26.2 g/t silver (W G Cook, unpublished data, 1995). This style of mineralisation has a higher silver:gold ratio (5:1) than the other three styles of gold mineralisation, which have a silver:gold ratio of the order of 1:1. The historic production from the near surface workings is characterised by a silver:gold ratio of 20:1.
STRUCTURE OF THE PEAK OREBODY AND HOST SHEARS Hinman and Scott (1990) first suggested that the unique, inhomogeneous nature of strain in the Peak area during deformation played a critical role in the alteration and mineralisation events that produced the Peak orebody. Although the folds throughout most of the Peak area demonstrate a consistent southerly plunge, Hinman (1992) mapped an area of doubly plunging folds in the central part of the Peak shear zone coincident with the up dip projection of the
Geology of Australian and Papua New Guinean Mineral Deposits
PEAK GOLD-COPPER-LEAD-ZINC-SILVER DEPOSIT, COBAR
Peak deposit. Hinman interprets this fold pattern as indicative of inhomogeneous strain within the Peak shear zone, rather than as evidence for an overprinting second deformation. At the Peak, deformation causing shearing and bulk shortening has been focussed in the sediment within zones of high competency contrast. The greatest competency contrast exists between the rigid rhyolitic bodies and the surrounding sandstone and siltstone. Lesser competency contrasts exist within the sediment package itself, between sandstone and siltstone and between zones of silicified material and lesser or non-silicified material. Of the three shears mapped at surface, only the main Peak shear is unequivocally recognised in the Peak mine, where it is referred to as the Polaris shear. The other component shears of the Peak shear zone, the Blue and Lady Greaves shears, are tentatively correlated with a series of shears on the eastern margin of the orebody. The principal shear within the deposit is the Polaris shear which separates the western and eastern volcanic bodies. All other shears appear to be cross faults and splays off the Polaris shear. At least four generations of shearing have been identified.
Syn–main stage mineralisation shearing Zones of strong to intense penetrative cleavage development and tectonic brecciation to 50 m wide are localised within variably altered sediment at the western margins of the rigid rhyolite bodies. These zones of shearing rapidly thin along strike away from the rhyolite bodies. The westernmost zone of shearing merges with the Peak shear zone above the Peak orebody. A similar zone of shearing, the Hecla shear, is developed along the southern margin of the eastern rhyolite body. Narrower, more discrete shears are also developed within the eastern rhyolite bodies; these are west-dipping and form a stacked en echelon array. The bulk of Peak mineralisation is localised within these shears. These shears, even though obviously very intense, do not appear to have appreciably displaced either the Great Cobar Slate–Chesney Formation contact or the rhyolite bodies. Although displacement is minimal, the sense of shear implied by the geometry of the dilational zones and stacking in the Eastern lenses is reverse west block up.
Post–main stage mineralisation shearing The Gecko shear is restricted to the western side of the deposit and is interpreted to be a planar dilational zone with little or no displacement. The shear appears to have merely pulled apart the two western rhyolite bodies and contact mineralisation, particularly Copper lens (Fig 3). Again, the sense of dilation and orientation of the Gecko shear relative to the Polaris shear suggests a west block up sense of movement.
Syn–late stage mineralisation thrusting The Polaris shear is interpreted to have been reactivated as a thrust fault after deposition of the main stage of mineralisation in the Peak orebody. A series of cross faults splay off this thrust. The Hecla shear also appears to have been reactivated at this time. These shears are characterised by minor to strong black chlorite alteration. Displacement on the Polaris shear is estimated to be approximately 250 m west block up with a 20 m component of left lateral strike-slip.
Geology of Australian and Papua New Guinean Mineral Deposits
The Hecla shear links to the Polaris shear and truncates the southern margin of the eastern rhyolite body and terminates the mineralised shears within the eastern volcanic lens. It is interpreted to have formed as a short cut structure along the rhyolite–sediment contact. Striae on the shear planes suggest left lateral strike-slip movement on the Hecla shear. A series of north-striking moderately west-dipping shears, linking to the Polaris shear at the base of the eastern rhyolite body, dismember and displace both the rhyolite and the contained Eastern lenses (Fig 2). The displacement on these shears is 5 to 30 m west block up.
Recent brittle faulting Late stage brittle faulting characterised by subhorizontal slickensides and minor (<5m) strike-slip displacement is present within the Polaris shear. These faults are typically open and water filled and clearly post-date all other faulting and mineralisation.
MINERALISATION PARAGENESIS AND ASSOCIATED ALTERATION Mineralisation at Peak is characterised by a complex array of overlapping alteration and mineralisation types. Based on detailed mineralogical and geochemical studies by Hinman (1992) and Jiang et al (1995) and on Peak Gold Mines’ detailed mapping, the following generalised paragenetic sequence for the mineralisation at Peak is proposed: 1.
pervasive silicification associated with muscovitesericite alteration;
2.
minor quartz-calcite-pyrite veining associated with disseminated pyrite-pyrrhotite mineralisation;
3.
quartz–minor chlorite veining and chlorite alteration;
4.
silver-poor, sphalerite-galena mineralisation;
5.
quartz-rich, pyrrhotite-chalcopyrite-lesser pyrite(sphalerite-galena)-gold-electrum-trace bismutharsenopyrite-chlorite-muscovite veining;
6.
quartz-chlorite veining;
7.
silica-absent, black chlorite-(muscovite)-talc rich, silverrich sphalerite-galena-pyrrhotite-pyrite-(chalcopyrite) mineralisation; and
8.
quartz–coarse veining.
grained euhedral potassium feldspar
Stages 1 to 5 are synchronous with regional cleavage formation and the main period of deformation (Hinman, 1992), with stages 4 and 5 being the main ore forming events. Stage 7 mineralisation crosscuts and overprints cleavage and is temporally associated with a later period of deformation. This interpretation is supported by age dating by Perkins, Hinman and Walshe (1994) who report 40Ar-39Ar dates of 401.5±1.0 Myr for alteration muscovite associated with the main stage of copper-gold mineralisation, and 384±1.4 Myr for the late stage silver-rich sphalerite-galena mineralisation. Jiang and Secombe (1995) have determined a K-Ar date of 331±11 Myr for the last stage of potassium feldspar veining.
ORE GENESIS The following genetic model for the genesis of the mineralisation associated with the Peak shear is proposed. The model attempts to synthesise previous workers’ studies and ideas with current interpretations:
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1.
2.
3.
4.
5.
6.
7.
8.
9.
The Peak shear was initiated during formation of the Cobar Basin. Environmental Resources Analysis Ltd (unpublished data, 1987) proposed that the shear originated as an extensional normal fault at the eastern margin of the Cobar Basin. Rhyolitic subvolcanic magma was intruded into unconsolidated sediment along the Peak shear. Glen (1987a, 1990) suggested that faults like the Peak shear have no early history, but the presence of the rhyolitic volcanic rocks and intrusives along the eastern margin of the shear suggests that some form of precursor structure is likely. The Cobar Basin was inverted. Deformation and bulk shortening led to the development of a prominent northto NW-trending subvertical cleavage parallel to the Peak shear, and formation of zones of intense dilational shearing and brecciation at the margins of, and to a lesser extent within, the rhyolite bodies. The Chesney Formation was pervasively silicified along the eastern margin of the Peak shear and especially along zones of penetrative cleavage developed at the western margin of the rhyolite bodies. The zones of silicification, like the rhyolite bodies, became loci for mineralisation because of their competency contrast with the adjacent sediment. This silicification extends significant distances above and below the volcanic bodies. The main stage of mineralisation was associated with multiple generations of quartz veining and chlorite alteration during the peak of deformation and cleavage formation. Mineralisation was focussed in dilations around the rhyolite bodies. Two stages of sulphide mineralisation are recognised, silver-poor galenasphalerite-minor chalcopyrite-gold mineralisation, and chalcopyrite-pyrrhotite-gold-minor galena, which grades along strike into chalcopyrite-pyrite-gold poor mineralisation. Each stage is preceded by discrete stages of quartz veining as demonstrated by Jiang et al (1995). Further shearing and dilation took place along discrete zones, including the Gecko shear, causing segmentation of both the rhyolite body and the copper-gold mineralisation concentrated at the margin of this body. Early zones of shearing were reactivated as thrust faults. These shears dismembered the rhyolite body and the main stage mineralisation. Late stage mineralisation was focussed in dilations along the reactivated thrust faults. It typically comprises silverrich sphalerite-galena-pyrrhotite-pyrite mineralisation associated with black chlorite-talc alteration. Where it overprints the main stage mineralisation, it inherits a significant gold component. Recent brittle strike-slip faulting characterised by open fractures affected the main Polaris shear.
REFERENCES Glen, R A, 1987a. Geology of the Wrightville 1:100 000 Sheet 8034, Geological Survey of New South Wales, Department of Mineral Resources. Glen, R A, 1987b. Copper- and gold-rich deposits in deformed turbidites at Cobar, Australia: their structural control and hydrothermal origin, Economic Geology, 82:124–140. Glen, R A, 1990. Formation and inversion of transtensional basins in the western part of the Lachlan Fold Belt, Australia, with emphasis on the Cobar Basin, Journal of Structural Geology, 12:601–620. Glen, R A, 1991. Inverted transtensional basin setting for gold and basemetal deposits at Cobar, New South Wales, BMR Journal of Geology and Geophysics, 120:13–24. Glen, R A, 1994. Geology of the Cobar 1:100 000 sheet 8035, Second Edition, Geological Survey of New South Wales, Department of Mineral Resources. Glen, R A, 1995. Thrusts and thrust-associated mineralisation in the Lachlan Orogen, Economic Geology, 90:1402–1429. Glen, R A, Dallmeyer, R D and Black, L P, 1992. Isotope dating of basin inversion - the Palaeozoic Cobar Basin, Lachlan Orogen, Australia, Tectonophysics, 214:249–268. Glen, R A, Drummond, B J, Goleby, B R, Palmer, D and Wake-Dyster, K D, 1994. Structure of the Cobar Basin, New South Wales, based on seismic reflection profiling, Australian Journal of Earth Sciences, 41:341–352. Hinman, M C, 1992. The structural and geochemical genesis of the Peak base and precious metal deposit, Cobar, New South Wales, Australia, PhD thesis (unpublished), James Cook University of North Queensland, Townsville. Hinman, M C and Scott, A K, 1990. The Peak gold deposit, Cobar, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1345–1351 (The Australasian Institute of Mining and Metallurgy: Melbourne). Jiang, Z, 1996. Geochemical studies of the Peak and Chesney Gold Deposits, Cobar, NSW, Australia, PhD thesis (unpublished), University of Newcastle, Newcastle. Jiang, Z and Seccombe, P K, 1995. Pb and Sr isotope characteristics of mineralisation and hostrocks and new ages for igneous activity and mineralisation Peak Mine, Cobar, NSW, Centre for Isotope Studies Research Report 1993–1994, pp 76–81, CSIRO, North Ryde, NSW. Jiang, Z, Seccombe, P K, Andrew, A S, Todd, A J and Luo, Z, 1995. Oxygen and hydrogen isotope study at the Peak mine, Cobar, NSW, Centre for Isotope Studies Research Report 1993–1994, pp 71–75, CSIRO, North Ryde, NSW. Perkins, C, Hinman, M C and Walshe, J L, 1994. Timing of mineralization and deformation, Peak Au mine, Cobar, New South Wales, Australian Journal of Earth Sciences, 41:509–522. Rayner, E O, 1961. The Peak area, Cobar mineral field, New South Wales, New South Wales Department of Mines Technical Report 6 for 1958, pp 49–62. Sherwin, L, 1985. Biostratigraphy, in Summary of the geology and controls of mineralisation in the Cobar region, Geological Survey of New South Wales, Report GS 1985/203 (unpublished). Sullivan, C J, 1947. The geology of the Cobar mineral field and its bearing on prospecting, Bureau of Mineral Resources Geology and Geophysics Record 1947/74 (unpublished).
ACKNOWLEDGEMENTS The authors gratefully acknowledge the permission of Peak Gold Mines Pty Ltd to publish the information and ideas contained within this paper.
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Morland, R and Leevers, P R, 1998. Potosi zinc-lead-silver deposti, Broken Hill, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 615–618 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Potosi zinc-lead-silver deposit, Broken Hill 1
by R Morland and P R Leevers
2
INTRODUCTION
HISTORY OF DISCOVERY
The deposit is 2 km NE of the City of Broken Hill, NSW, at lat 31o56′S, long 141o30′E on the Broken Hill (SH 54–15) 1:250 000 and the Broken Hill (7134) 1:100 000 scale map sheets (Fig 1). It was rediscovered by Pasminco Mining in the course of a mine lease exploration program in 1991, and ore is currently being mined from an open pit at a planned production rate of 200 000 tpa. Recent exploration has identified a significant underground resource which is open down plunge. The importance of the Potosi discovery is twofold. First, it is the second largest discrete orebody in the district after the mammoth Broken Hill orebody (Morland and Webster, this publication). Second, whereas both orebodies are in the Broken Hill Group of the Willyama Supergroup, the Potosi orebody is found at a lower stratigraphic level separated by a distinctive massive pelite-BIF marker horizon. This has exploration implications in the district.
1.
Formerly Manager - Potosi Project, Pasminco Mining - Broken Hill, now Ron Morland Consulting Pty Ltd, 1 Hartwell Hill Road, Camberwell Vic 3124.
2.
Formerly Senior Mine Geologist, Pasminco Mining - Broken Hill, now Superintendent - Geology, Pasminco Mining - Elura, PO Box 433, Cobar NSW 2835.
EARLY ACTIVITY The Broken Hill orebody was discovered in 1883. Since that time there has been extensive exploration for additional orebodies with very limited success. To the NE of Broken Hill activity was intense along the line of lode and there are old workings, usually based on surface gossans, at the Round Hill, Silver Peak, Silver Hill and Potosi mines (Fig 1). These are discussed in Jaquet (1894), Andrews (1922) and numerous annual reports of the New South Wales Mines Department. The Round Hill and Silver Peak mines are in the same stratigraphic position as the Broken Hill orebody. At Silver Peak about 1500 t of ore was extracted intermittently. The Silver Peak exploratory shaft was sunk by North Broken Hill Limited (NBHL) in 1973 to 240 m following a lead-rich lode which has yet to be mined. The Silver Peak resource was estimated at 90 000 t grading 13.4% lead, 4.7% zinc, 90 g/t silver and 0.14% copper (W G Widdop, unpublished data, 1973). It currently fails to meet the latest resource definition requirements. The Silver Hill and Potosi operations were based on ore from the sequence below that of the Broken Hill orebody. The Silver Hill mine, also known as Copper Blow, closed in 1927 after producing about 1000 t of ore. The Potosi operations were more substantial. BHP took over the operation
FIG 1 - Location plan, Potosi open pit.
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in 1909, sank the main shaft to 76 m and mined intermittently until 1927, extracting some 3000 t of ore (M D Garretty, unpublished data, 1947). Between 1947 and 1988 NBHL explored this area, completing several diamond drill holes and geophysical surveys.
PASMINCO EXPLORATION Pasminco Limited was formed in 1988 by the merger of the lead-zinc interests of NBHL and CRA Limited. Pasminco Mining began to actively explore the leases, consolidating the combined knowledge of both operations. The search was also expanded from large tonnage underground targets to include small, near surface orebodies which could supplement the rapidly dwindling underground ore reserves. Work at the Flying Doctor prospect and Round Hill mine was not successful in proving up an orebody. The search then turned to the old Potosi workings, and a review of the possible controls of mineralisation indicated it had potential to host economic mineralisation (Pasminco Mining, unpublished data, 1991). One of the significant breakthroughs was the recognition of sheared mineralisation as distinct from discontinuous lenses. This also directed the search to the position of the unfaulted mineralisation. The third drill hole intersected 5.2 m grading 15.1% lead, 20.6% zinc and 130 g/t silver. A mise-a-la-masse survey at this drill hole was completed with spectacular results and reinforced the potential of the mineralisation (Pasminco Mining, unpublished data, 1993). Emphasis was placed on the near surface potential and, following a major diamond drilling program, an Inferred Resource of 1.0 Mt to a depth of 200 m was estimated (Pasminco Mining, unpublished data, 1993). Further diamond drilling was undertaken on 40 m spacing and mining, metallurgical and general feasibility studies were completed. This culminated in a Proved and Probable Ore Reserve of 1.1 Mt grading 2.1% lead, 8.9% zinc and 26 g/t silver in the top 90 m (Pasminco Ltd, 1995). The project was also deemed to be financially robust. At this stage a total of 11 910 m had been drilled in 91 diamond drill holes. The first ore was sent to the Southern Operations mill in April 1996.
POST-FEASIBILITY EXPLORATION Within the pit outline Following approval to proceed with development a program of reverse circulation drilling was undertaken to better define the mineralisation within the upper part of the pit. A total of 9726 m in 306 drill holes were completed which allowed further o refinement of the resource. All holes were drilled at 60 , hole spacing approximated to a 10 by 10 m grid and hole depth ranged from 12 to 46 m. The 1996 Proved and Probable Ore Reserve (Pasminco Ltd, 1996) is 1.0 Mt grading 2.3% lead, 9.1% zinc and 28 g/t silver in a pit 400 by 220 by 90 m deep.
Down plunge In the pit design the deeper mineralisation was excluded, and the initial discovery drill hole is not within the proposed pit design. The shallow plunge of the mineralisation was noted and this, in conjunction with the mise-a-la-masse geophysical response, led to continued exploration to the NE by diamond drilling on a sectional basis at spacings between 100 and 200 m
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(Fig 2). A crosscutting normal fault, Morland’s fault, has been identified, which offsets the mineralisation and forms the limit of the geophysical response of the ore cut by the discovery drill hole. A similar survey was carried out on a mineralised intersection on the downthrow side with equally spectacular results and this has provided a focus for recent deeper testing. To the end of June 1996, 38 diamond drill holes totalling 15 000 m had demonstrated continuity of mineralisation for 1.8 km strike length, and the mineralisation is still open at depth. The 1996 Indicated and Inferred Resource outside the pit was 1.8 Mt grading 3.7% lead, 13.7% zinc, 45 g/t silver and 0.28% copper using a 7% lead plus zinc cutoff (Pasminco Mining, unpublished data, 1996). Exploration is continuing.
FIG 2 - Longitudinal projection, showing proposed Potosi pit and the adjacent underground mineralisation, looking NW. Figure 3 location shown.
REGIONAL GEOLOGY The Broken Hill and the Potosi deposits are within the upper parts of the Broken Hill Group of the Early to Middle Proterozoic Willyama Supergroup (Haydon and McConachy, 1987). The deposits are on opposite limbs of the Hanging Wall synform (Larsen, 1994). The Broken Hill deposit sits in the middle part of the Hores Gneiss (Unit 4.7) which is characterised by pelite, psammopelite, Potosi gneiss and garnet quartzite. This distinctive stratigraphic package can be followed for more than 20 km strike length. At the base of the Hores Gneiss is a distinctive local marker horizon, the 4.6 pelite with local BIF development which separates it from the underlying Freyers Metasediments (Unit 4.5), a well bedded psammite-dominant sequence of metasediment with lesser pelite and psammopelite units. The Potosi orebody is developed in the upper part of Unit 4.5. The Hanging Wall synform is faulted by two major subparallel structures, the Globe–Vauxhall shear zone and the Western shear between which the mineralisation lies. The structures are roughly subparallel to the mineralisation, dipping 60 to 70ο to the SE (Fig 3).
POTOSI-SILVER PEAK GEOLOGY AND MINERALISATION LOCAL GEOLOGY Four features characterise the geology in the Potosi–Silver Peak area (Fig 4): 1.
The Potosi shear is a strongly foliated retrogressed shear characterised by abundant sericite. Mineralisation is contained within ‘psammitic rafts’ of blue quartz lode that occur within the shear.
Geology of Australian and Papua New Guinean Mineral Deposits
POTOSI ZINC–LEAD–SILVER DEPOSIT, BROKEN HILL
MINERALISATION The Potosi mineralisation and its host rocks cut out to the SW against the Potosi shear which is a splay structure of NNE trend crossing from the Western shear to the Globe–Vauxhall shear zone. It is planned to mine mineralisation within the shear (as in the workings at the old Silver Hill and Potosi mines) and also unfaulted ore (Fig 4). Minor oxidised ore is present but is low grade. In the SW the host sequence is synformal in and near the pit, but going NE the synform becomes a west dipping flexure which steepens as it approaches the Western shear (Fig 3). The mineralisation in Unit 4.5 appears to migrate down dip and, at the most northerly drilled section, is caught up in the Western shear.
2.
3.
4.
Unit 4.5 is dominated by well bedded blue-grey psammite, with minor pelite and psammopelite. Lode rocks within 4.5 consist of blue quartz with fine to coarse grained gahnite common. Unit 4.6 is a massive sericite-rich pelite. Sericite generally occurs as knotted clots with biotite. Garnets vary from fine to very coarse grained and are generally ragged in appearance. A BIF marker horizon is common within 4.6. It is garnet quartzite–like with very fine magnetic bands occurring up or down dip from mineralisation. Unit 4.7 is characterised by interbedded pelite, psammopelite and Potosi gneiss. The Potosi gneiss is characterised by abundant medium grained garnet, a weak gneissosity and ptygmatically folded pegmatite segregations.
The mineralisation as described by P R Leevers (unpublished data, 1996) and N F Carroll and L G Reid (unpublished data, 1996) contains coarse grained, black, iron-rich sphalerite (marmatite) as the dominant sulphide. Galena, chalcopyrite and pyrrhotite are also present. Gangue minerals include gahnite, garnet and blue quartz. Mineralisation is in four main types (N F Carroll and L G Reid, unpublished data, 1996): 1.
Stringer to submassive (rarely massive) medium to coarse marmatite, with interstitial fine to medium grained galena and rarely submassive galena-dominated bands in a blue quartz, garnet and gahnite gangue. The typical grade range is 3 to 20% lead plus zinc.
2.
Massive brown (lower iron) to black marmatite with some ‘pebbly ore’ texture (durchbewegung texture) with clear or translucent quartz gangue, to 4 or 5 m true thickness, often showing sheared margins. Typical grade is greater than 30% lead plus zinc.
3.
Varying proportions of disseminated galenachalcopyrite mineralisation with very fine veinlets and stringers of predominant marmatite with subordinate
FIG 4 - Plan showing surface geology, proposed Potosi pit.
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galena, and chalcopyrite in blue quartz, garnet and gahnite gangue and in grey-green feldspar pegmatites. The typical grade is below 7% lead plus zinc. 4.
Stringer marmatite with negligible galena in blue quartz psammite. The typical grade is less than 10% lead plus zinc.
The host rocks within Unit 4.5 are either blue quartz, finegrained garnet psammite, pegmatised blue quartz psammite, or rarely a grey-green feldspar pegmatite. It is possible to link most of the mineralised intersections from one section to another based on mineralogy and stratigraphic position. Indirect confirmation of the interpretation is given by the misea-la-masse response.
EXPLORATION IMPLICATIONS As the exploration effort has progressed to the NE it is interesting to note that it appears that the better developed sections drilled so far in Unit 4.5 correspond to the higher grade intersections in the overlying Unit 4.7, like the Silver Peak ore shoot (Fig 3). A recent review of the Silver Peak ore shoot gave a better understanding of the controls of this mineralisation indicating greater potential than has been realised to date (A E Webster, unpublished data, 1996). From a small beginning the Potosi mineralisation has been demonstrated to be a significant resource. Its plunging, pipe like nature and limited response to geophysical testing made it a difficult and costly target to test.
Unit 4.5 has largely been overlooked from a regional exploration perspective with most work concentrated on the more obvious target, Unit 4.7. Time will tell if there are other Unit 4.5 orebodies and if their style of mineralisation is similar to Potosi.
ACKNOWLEDGEMENTS This paper is published with the permission of Pasminco Limited. Three people deserve particular mention — D F Larsen refound the orebody, recognised its potential and systematically explored it; J Bishop of Mitre Geophysics has been unstinting in his efforts to assist the exploration; and J R Dini, General Manager at Broken Hill at the time, is thanked for his total support of the project. In addition, the contributions of N F Carroll and L G Reid in the continuing exploration deserve mention. The help of D Rolton with word processing, and M Powell and F Sette with drafting is willingly acknowledged.
REFERENCES Andrews, E C, 1922. Geology of the Broken Hill district, Geological Survey of NSW, Memoir 8. Haydon, R C and McConachy, G W, 1987. The stratigraphic setting of Pb-Zn-Ag mineralisation at Broken Hill, Economic Geology, 82:826–856. Jaquet, J B, 1894. Geology of the Broken Hill lode and Barrier Ranges mineral field, Geological Survey of NSW, Memoir 5. Larsen, D F, 1994. The Potosi orebody — a newly discovered base metal deposit at Broken Hill, New South Wales, Geological Society of Australia Abstracts, 37:238). Pasminco Limited, 1996. Melbourne).
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Geology of Australian and Papua New Guinean Mineral Deposits
Morland, R and Webster, A E, 1998. Broken Hill lead-zinc-silver deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 619–626 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Broken Hill lead-zinc-silver deposit 1
by R Morland and A E Webster
2
INTRODUCTION The deposit is at lat 31o58′S, long 141o28′E in NSW, on the Broken Hill (SH 54–15) 1:250 000 scale and the Broken Hill (7134) 1:100 000 scale map sheets. It is one of the richest accumulations of lead, zinc and silver in the world. Mining by the Broken Hill Proprietary Company Limited began in 1883 and continues today in the Southern Operations of Pasminco Mining - Broken Hill. This paper documents results of significant research work and mine exploration that has taken place at both the North Mine and Southern Operations since the description by Mackenzie and Davies (1990).
RESOURCES AND PRODUCTION The Broken Hill deposit originally consisted of about 185 Mt of mineable ore (R Morland, unpublished data, 1995), associated with an additional 100 Mt of mineralisation exceeding 3% combined lead and zinc (Haydon and McConachy, 1987). Besides lead and zinc approximately 28.7 Mkg silver and 23 t gold have also been produced as byproducts of base metal mining since mining commenced. Pasminco Mining - Broken Hill was formed in 1988 by the merger of the North (North Broken Hill Limited) and the Zinc and NBHC Mine operations (ZC Mines Pty Limited). The North Mine produced an estimated 34 Mt of ore typically 14% lead, 230 g/t silver and 11.5% zinc to a depth of 1.7 km prior to its closure in 1993 (R Morland, unpublished data, 1993), mainly from the underground operation but with 0.7 Mt from Number 1 open cut. Between 1991 and 1993, a total of 80 000 t of Zinc lode ore grading 5.5% lead, 100 g/t silver and 9.5% zinc were mined at the North Mine from a Measured and Indicated Resource of 175 000 t (A Aitchison, unpublished data, 1993). Between 1911 and 1995, ZC Mines produced 89.6 Mt of ore (R Morland, unpublished data, 1995). Some remnant mining and mine rehabilitation work continues at the South Mine central leases currently held by the Poseidon Ltd subsidiary Minerals Mining and Metallurgy Limited (MMM), but ore is no longer treated on site. MMM and their predecessors produced 51.9 Mt of ore to the end of 1990 (R Morland, unpublished data, 1995). In 1996, 2.5 Mt of ore was produced from the Pasminco underground operation at grades of 5.5% lead, 54 g/t silver and 7.7% zinc (Pasminco Limited, 1996).
1.
2.
Formerly Manager - Technical Services, Pasminco Mining Broken Hill, now Ron Morland Consulting Pty Ltd, 1 Hartwell Road, Camberwell Vic 3124. Formerly Superintendent - Geology, Pasminco Mining - Broken Hill, PO Box 433, Cobar NSW 2835.
Geology of Australian and Papua New Guinean Mineral Deposits
After 113 years of mining the Broken Hill orebodies still rank as one of the largest lead-zinc deposits in Australia, with a Proved plus Probable Reserve of 28.0 Mt of ore containing 5.9% lead, 55 g/t silver and 8.5% zinc (Pasminco Mining, 1996).
DEPOSIT GEOLOGY The Broken Hill deposit lies within the Broken Hill Group of the Palaeoproterozoic Willyama Supergroup (Willis et al, 1983; Stevens et al, 1983) and consists of nine separate but closely related orebodies that are stacked within a single stratigraphic package in the Hores Gneiss of the Broken Hill Group. They are known from mine base to top as 3 lens (3L), 2 lens (2L), 1 lens lower (1LL), 1 lens upper (1LU), A lode lower (ALL), A lode upper (ALU), Southern A lode (SAL), Southern 1 lens (S1L) and B lode (BL). A tenth body of disseminated stratabound mineralisation, C lode (CL), lies above BL and has recently been well enough defined to be mined.
THE MINE SEQUENCE The deposit lies within a distinctive stratigraphic package known as the mine sequence This has been shown to be remarkably consistent having been traced for 25 km along strike and to a depth of 2 km (Carruthers and Pratten, 1961; Carruthers, 1965; Johnson and Klingner, 1975). The stratigraphic setting of the deposit has also been described in detail by Haydon and McConachy (1987) and by Wright, Haydon and McConachy (1987, 1993). The orebodies lie within a unit of the mine sequence known as the lode horizon (Johnson and Klingner, 1975) which is subdivided into four units in the mine area, named from lowermost to uppermost, the clastic and calc-silicate horizon (CCH), the garnet quartzite horizon (GQH), the C lode horizon (CLH) and the 4.5 mineralisation (4.5H) by A E Webster (unpublished data, 1995). Orebodies rich in calcite, fluorite, lead and rhodonite (2L, 1LL, 1LU and to a lesser extent 3L) are located within the CCH, a unit dominated by clastic psammopelitic to pelitic rocks with some well developed calcsilicate layers, weak amphibolite and Potosi gneiss. The orebodies that are rich in primary quartz (ALL, ALU, S1L, SAL, BL) lie within the GQH. The 3L orebody, which is also rich in primary quartz, has some garnet quartzite independent of the GQH but lies within the CCH. The CLH and elements of the 4.5H and CCH persist beyond the main deposit area and are represented on a district scale. There appears to be a link between the style of mineralisation and the host rock type. Significant mineralisation occurs in several horizons outside the main orebodies, especially in unit 4.5 H, which contains widespread zinc mineralisation, including the Potosi orebody (Larsen, 1994; Morland and Leevers, this publication).
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GEOMETRY OF THE MINERALISATION The 2L and 3L orebodies are the largest and second largest in the field respectively and are stratigraphically continuous for a strike length of 8 km. They have produced most of the ore at Broken Hill. The ALL orebody is the next most extensive mineralised position in the field, persisting for 5 km, though structural and/or stratigraphic discontinuities are present. The orebodies strike NE and are arranged in an en echelon manner which is most pronounced at the southern end of the field where the greatest number of lenses occur. 3L and 2L are structurally terminated at the northern end of the deposit by the Globe–Vauxhall Shear Zone. Their continuations NE of the shear, known as the 2K zone, have been located but their size and extent are unknown. All Broken Hill ore lenses and associated mineralisation are strongly linear, approximately parallel in strike and lenticular in cross section. The extreme stratigraphic linearity and great strike length of 2L, 3L, ALL and CL-style mineralisation and the 4.5 mineralisation produce a series of ‘ribbons’ of mineralisation within the Pasminco mining leases. The 2L and 3L orebodies describe a boomerang-shaped arch in longitudinal projection (Fig 1). This arch plunges to the north at around 40°, steepening to 70° at the northern end of the field and is southerly plunging at approximately 20° in the Southern Operations. The arch culmination occurs in the middle of the field where erosion has removed about 60 Mt of ore. The plunge variations of 2L and 3L also affect all other ore lenses and the 4.5 mineralisation. Although there are numerous local discordances each orebody shows a consistent, conformable relationship with the surrounding strata and occurs in a characteristic stratigraphic position within the mine sequence. Orebodies and associated lode rocks form strongly linear positions within otherwise essentially tabular stratigraphic units.
OREBODY TYPES Traditionally the Broken Hill orebodies have been divided into two categories; the zinc lodes and the lead lodes, based on the single criterion of lead to zinc ratio (Gustafson,1939; King and O’Driscoll, 1953). However a closer examination of all of the
geological features of the ore lenses show that this definition is too simplistic. The Broken Hill orebodies are still classifiable into two main types (Webster, 1996b), but based on a much greater number of their features. They are described below.
Calcitic orebodies The calcitic orebodies (2L, 1LL, 1LU) contain calcite, rhodonite-bustamite, apatite, garnet and/or fluorite, abundant lead and are largely hosted by clastic metasediment within the CCH. Calcitic orebodies also contain a suite of unusual gangue minerals, including knebelite [(Fe, Mn)2SiO4], wollastonite, hedenbergite and ilvaite [Ca Fe22+ Fe3+ (SiO4)2 OH]. These more unusual minerals tend to occur in zones at the contacts of calcitic and rhodonitic mineralisation.
Primary quartz orebodies The second and most common type is the primary quartz orebodies (ALL, ALU, SAL, S1L, BL, 3L) which are rich in primary quartz and garnet. They also contain gahnite and cummingtonite, have little or no calcite or calc-silicate minerals, relatively low lead, and lie in characteristic stratigraphic positions within the GQH. The 3L orebody has no physical association with the GQH but develops garnet quartzite layers along its margins in the north, central and southern parts of its length. The primary quartz orebodies mostly lie in the upper part of the southwestern end of the deposit, and the calcitic orebodies lie in the lower part of the sequence. Garnet quartzite is the dominant host of ALL and the orebodies lying above, and 1LL and 1LU merge with the GQH in the southern part of their strike length. The 2L orebody is not associated with garnet quartzite, except at the extreme south end of the deposit where its upper contact meets the footwall of the GQH. The 3L orebody lies at the base of the deposit and differs from other primary quartz orebodies by its size, great strike length, clastic metasedimentary host, weakly developed garnet quartzite association, abundant fluorite and high lead grade at the northern end. The 3L and ALL bodies are the only primary quartz orebodies to contain significant rhodonite-bustamite mineralisation.
FIG 1 - Cross section, looking SW, showing the main elements of Western A Lode at Southern Operations (from M Hudson, unpublished data, 1994).
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Geology of Australian and Papua New Guinean Mineral Deposits
BROKEN HILL LEAD-ZINC-SILVER DEPOSIT
Calcitic and quartzitic styles of mineralisation are transitional, with the major changes taking place between 1LU and ALL. There is no transition between 3L and 2L though they do share some common characteristics which suggest a close relationship. Such features include rhodonite-bustamite units, fluorite gangue (restricted in distribution in 2L), calcitic gangue (minor zones with a 2L metal ratio develop in 3L) and greater lead than zinc (only in northern 3L).
THE NATURE OF MINERALISATION The impression that the Broken Hill orebodies are all massive sulphides with little else has long been dispelled (Hodgson, 1968; Maiden, 1972; Webster, 1993, 1994a). Not only can different styles of mineralisation be identified but the information also gives valuable insights into the ways the deposit has responded to deformation and also to the predeformation characteristics of the deposit and therefore its genesis. Massive sulphides form only a minor part of each orebody and are most common in deformed regions of the lenses. Sulphide-rich mineralisation is more often coarsely banded by variations in gangue mineral and sulphide abundance. Each orebody contains a high percentage of gangue minerals, including primary calcite, calcium or manganese pyroxenoids or pyroxenes, or quartz, that are distributed through the mineralisation as discrete bands, layers and stratigraphic horizons. Sphalerite (marmatite) is the dominant sulphide constituent of all ore lenses and the percentage of galena diminishes in each orebody from 3L to BL. The zinc grade is comparable for all orebodies but lead diminishes upwards from 3L to BL. Lead content also diminishes along strike from NE to SW in 3L even though the zinc grade remains relatively constant. Lead grade diminishes from base to top in the mine as calcite decreases as a gangue component. Important accessory sulphides include pyrrhotite, chalcopyrite and loellingite. The orebodies have not been ‘scrambled’ by deformation but preserve most of their predeformational features, including internal layering and banding defined by sulphide and gangue mineral abundance, wall rock associations and their stratigraphic position within the lode horizon and mine sequence. All orebodies preserve structural fabrics in the form of fluid phase and mechanically mobilised sulphides, gangue mineral changes and zones of greatly coarsened gangue minerals with unusual calc-silicate assemblages. Characteristic structural fabrics and associated mineralogical changes were developed within the orebodies during each of the recognised structural events which affected the Willyama Supergroup in the mine area.
CLASSIFICATION OF MINERALISATION STYLES The orebodies contain a diverse group of visually distinctive styles of mineralisation. Their textures, location, gangue mineralogy and relationship with other ore types suggest several possible origins. These features vary from those which preserve the early stratification to those which suggest formation in response to deformation and/or metamorphism. The information that such relationships between styles of mineralisation preserve can be used to determine the structural and metamorphic history of the mineralisation and lode rocks. Gangue mineral assemblages show the metamorphic grade at
Geology of Australian and Papua New Guinean Mineral Deposits
which these styles of mineralisation crystallised and their textures suggest the processes that formed them. By preserving such features the mineralisation can be regarded as another metamorphic rock. This information shows the orebody to be highly structured and stratified and allows a model for the structural evolution of the mineralisation to be constructed (Webster, 1994a, b, 1996b). Compilation of existing geological mapping information and remapping in key areas have resulted in the recognition of three major mineralisation styles (Webster, 1993, 1994a, b, 1996b).
Stratiform mineralisation Stratiform mineralisation is a suite of predeformational styles that occurs within all orebodies. They predate deformation and metamorphism and comprise the bulk of all orebodies at Broken Hill. Stratiform styles of mineralisation are variably banded by relative abundances of sulphide and gangue minerals, chiefly calcite or quartz. Massive rhodonite mineralisation, the most widespread type, forms conformable marker horizons (with variable amounts of bustamite) within 2L, 3L and ALL, and distinct fluoritic horizons have been observed in 2L (south end) and 3L (north end) by A E Webster (unpublished data, 1995). Stratiform mineralisation remains conformable with the surrounding metasediment and demonstrates the primary features of the orebodies, preserving syndepositional layering, stratification, stratiform metal zoning and conformity with the surrounding strata, including lode rocks. The primary features are modified and overprinted by syndeformational textures. Such textures preserve evidence of all deformational events recognised in the Broken Hill district (Laing, Majoribanks and Rutland, 1978; Webster, 1993, 1994a, 1996b; A E Webster, unpublished data, 1995). Stratiform mineralisation shows that base metal deposition was associated with calcium carbonate, quartz and iron. Lead was preferentially deposited with calcium and possibly with fluorite. Zinc was deposited with iron and quartz. The deposition of both calcium and lead diminished up sequence from 2L to ALL as the system evolved with an increase in the deposition of quartz relative to calcium. Zinc and iron deposition was largely constant throughout the deposition of all the orebodies. All lead-zinc-silver deposition was antipathetic to manganese deposition, consequently all primary manganiferous rocks are barren of mineralisation. As a result, all orebodies have similar zinc grades but are richer in lead and calcium in the calcitic orebodies (2L, 1LL, 1LU, 3L). Lead and calcium deposition diminished with the onset of the formation of garnet quartzite.
Mobilised mineralisation Mobilised styles of mineralisation overprint the primary stratigraphic and textural features of the orebodies and most preserve prograde gangue mineral assemblages which show that the D2 deformation (Table 1) took place at granulite grade while others are retrograde in character. Mobilised sulphides are generally rich in recrystallised primary gangue components such as rhodonite and bustamite (3L, 2L, ALL), particularly in the North mine. Hedenbergite, wollastonite and knebelite are also commonly found within mobilised sulphides or calc-silicate mineralisation that is associated with them, mainly in the calcitic orebodies. The two-pyroxene mineralogy of most mobilised styles of mineralisation clearly shows that they formed at granulite
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TABLE 1 The structural evolution of the Broken Hill deposit. D1 (Olarian Orogeny) D2 (Olarian Orogeny)
D3A (Olarian)
D3B (Olarian)
D4A (Delamerian)
Pegmatite intrusion crosscuts lode horizon and at margins of psammitic units in the Northern Leases
Asymmetric, south plunging tight folding throughout the deposit with east dipping axial surfaces transects the orebodies at 20 o to their original strike
Belt of attenuation and the British shear develop 250–300 m sinistral movement, west block up (reversed)
Quartz-sericite-biotite shear zones develop early biotite-rich phase, later sericiterich phase
Brittle fault systems develop throughout the deposit. Hydrothermal activity along faults produces laminated veins and carbonate alteration of rhodonite-bustamite near fault systems
Late-stage faulting with chloritic, brittle and puggy stage of movement on D4 faults
D4B (Delamerian)
Biotite-sillimanite foliation (S1) parallel to bedding in pelitic and psammopelitic rocks adjacent to the orebodies
Western antiform, Eastern synform, WAL-WK synform, Hangingwall synform, BH synform
Attenuation of F2 folded geometry of 2L, 3L, 1LL, 1LU (Belt of Attenuation). 250 m sinistral, west block up displacement of 3L and 2L in the Browne Shaft area (Thompson shear). GQH dissected by BL Dropper shear and related structures
Final stages of Dropper shear development
Formation of ABH Consols siderite-silver vein at intersection of reactivated D3B shear and amphibolite unit. Galena -quartzsiderite Consols type vein cuts orebody in the British Mine
Reactivation or continuation of D4A
Isochemical metamorphism and grainsize coarsening in orebodies
Grainsize coarsening of gangue and sulphides. Annealing crystallisation throughout orebodies
Extensive silica metasomatism of ore and wall rocks in sheared parts of the deposit
Completion of shear offset of 2L and 3L in the North Mine area (Fitzpatrick orebody and 2K)
Intrusion of finegrained dolerite dykes into mineralisation along NW planes
Chloritic, puggy and sericitic fracture planes with deformation of D4A features
Coarse knots and bundles of fibrolite developed in pelitic rocks in the northern part of the deposit
Extensive mechanical and fluid phase sulphide mobilisation in the orebodies
Development of dropper orebodies within select shear planes and attenuated regions
Completion of the attenuation of 2L and 3L between the 29 and the 32 levels of the North Mine
Minor mechanical sulphide mobilisation in ore, dolerite dykes dismembered
Chloritic faults, shears, pug zones and milled ore
Banding in lode pegmatite?
Fluid phase sulphide migration into rhodonite-bustamite margins and wall rocks (mainly garnet sandstone and garnet quartzite)
Minor F3 folds develop in zones of transposition. Needle-like sillimanite characteristic
Offset of D3A Thompson shear by British shear in Browne Shaft area
Minor hydrothermal activity in orebodies garnet alteration of dolerite dykes
Secondary calcite lining vughs
Differentiation of sulphide constituents into Pb-Ag-Au-As-W (Cu)–rich fluid phase and Zn-Fe-Cu–rich fluid phase
Attenuation of F2 folded geometry of 2L and 3L between 29 and 32 levels in the North Mine
Minor fluid phase sulphide mobilisation (pyrrhotite, chalcopyrite and minor galena mobilisation). Mobile sulphides impregnate dyke fragment margins in ore
Jointing and faulting throughout the deposit
Structural influence on galena distribution in 3L in North Mine
Start of shear offset of 3L and 2L between 32–34 levels in the North Mine (Fitzpatrick area)
White quartz veining and silicification of some pegmatite. Wall rock bleaching silicification, sericitisation and coarse muscovite in wall rocks
Pyrite along fault planes, vughs and joints
Fracturing/brecciation in GQH Siliceous metasomatism, veining and stockworks within rocks on 3L-2L margins in North Mine
WAL offset from ALL by 250 m movement of BL Dropper shear WMIN offset from WAL?
Gentle refolding of line of lode on NW axis
Silicification and sericitisation of stratabound pegmatite Associated Mineralogy rhodonite, knebelite, fluorite, calcite, quartz
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rhodonite, bustamite, quartz, hedenbergite, wollastonite
hedenbergite, wollastonite metasomatic quartz (veining)
actinolite, cummingtonite, sturtite, sericite
garnet, manganocalcite, siderite carbonate alteration
chlorite and/or secondary calcite and/or pyrite
Geology of Australian and Papua New Guinean Mineral Deposits
BROKEN HILL LEAD-ZINC-SILVER DEPOSIT
grade and therefore the most significant sulphide mobilisation within the Broken Hill orebodies took place during D2. This conclusion is supported by the textures and lithological relationships of mobilised sulphides, primary silicates and calcite within the orebodies. Two processes of sulphide mobilisation operated during D2. The first process was dominated by the mobilisation and transport of sulphides in solution (or possibly as melts) which were then deposited in favourable sites (fluid-phase mobilisation). The second process was dominated by the in situ recrystallisation during flow of the sulphide and gangue components of the mineralisation (mechanical mobilisation).
Metasomatically-altered mineralisation Metasomatically altered mineralisation forms a small but varied category which has been altered in situ by the introduction of material from fluids, especially silica. This style overprints all stratiform and most mobilised styles of mineralisation. Siliceous metasomatism was widespread at the margins of the orebodies and within adjoining wall rocks during D2 and D3A (Webster, 1993, 1994a, b, 1996b). The interaction between siliceous fluids, carbonate and manganese silicates within the ore lenses formed a suite of distinctive styles of mineralisation containing variable amounts of hedenbergite, wollastonite, knebelite, bustamite, quartz and garnet. Silicification was also strongly developed within garnet quartzite in association with stockwork vein formation, and clastic metasediments were intensely silicified at ore–wall rock contacts, especially in the northern part of the deposit. Siliceous metasomatism was strongest in the northern part of the deposit during D2, especially in the North mine and was also strongly developed along the attenuated margins of 2L, 3L, 1LU and 1LL during D3A in the Southern Operations. Extensive zones of metasomatic mineralisation were developed within lithostructural sites in the GQH adjacent to the orebodies, particularly within F2 folds and within D3A shears. In Pasminco’s Northern Operation, 3L and 2L are associated with a well developed metasomatic alteration zone consisting of pale creamy pink garnet-quartz rock known locally as ‘GO’ rock, particularly on their up dip margins, which reflects metasomatism along the orebody margins during D2 folding (D F Larsen, personal communication, 1993; Lips, 1994; White et al, 1995; D F Larsen and A E Webster, unpublished data, 1996). Extensive metasomatic silicification and sulphide mobilisation have also strongly affected the clastic metasedimentary wall rocks along the margins of 2L and 3L and within the zone between the two orebodies. This process has formed large zones of low to high grade mineable siliceous blue quartz lode. Fluid-phase mobilised sulphides penetrated the silicified metasediment to produce zones of low to high grade mineralisation which were not originally part of the orebodies. Metasedimentary banding within adjacent clastic metasediment can be traced by mapping from unaltered metasediment into the blue quartz lode zones. Similar low grade siliceous mineralisation was also developed on the contacts of 2L and 3L in the Southern Operations but generally in association with zones of D3A transposition. In such zones, the siliceous blue quartz mineralisation is observed to overprint the banded texture of transposed metasediments.
Geology of Australian and Papua New Guinean Mineral Deposits
The styles of mineralisation formed during siliceous metasomatism lie at the contacts of the orebodies, especially where they have been highly attenuated. Saccharoidal quartz and/or calc-silicate mineralisation may be the only constituent of the orebodies in severely attenuated regions of 2L, 3L, 1LU and 1LL (Webster, 1993, 1994a, b). Significant calc-silicate components are only found within orebodies with a significant calcite and/or rhodonite content. Within the orebodies at Broken Hill, wollastonite, hedenbergite, bustamite and the suite of less common calcsilicates are the products of intense but localised syndeformational and structurally controlled metasomatism and largely post-date F2 folding.
STRUCTURAL EVOLUTION OF THE DEPOSIT The relationships of the three styles of mineralisation and the fabrics they define and preserve show that two major phases of deformation took place during a single regional metamorphic event at granulite to upper amphibolite facies in the Olarian Orogeny. A later phase of deformation, associated with the Delamerian Orogeny, also affected the deposit and is recorded in the mineralisation. The structural history of the Broken Hill deposit is summarised in Table 1. Several important observations can be made about the relationship of mineralisation and structure: 1.
All orebodies preserve gangue-defined stratification and layering which predate all deformation. Such features are concordant with the surrounding stratigraphy of the lode horizon. The relationships between the GQH, CLH and mineralisation were established before deformation.
2.
F2 folds developed throughout the deposit during granulite grade regional metamorphism, transgressing the linearity of the lode horizon. There is no relationship between the linear form of the deposit and fold structure. Folding cut across the orebody at approximately 20o to its original trend.
3.
There has been no large scale movement of mineralisation into fold hinges during fold development.
4.
Two planes of intense sinistral, west block up shearing and transposition, with 200 to 300 m of movement, developed during upper amphibolite grade retrograde metamorphism. F2 folds were severely attenuated and reoriented in the shear planes.
5.
The deposit was affected by the Delamerian Orogeny, with fault zones developing in which hydrothermal activity took place (Webster, 1994a, 1996a, b), including the development of the ABH Consols quartz-siderite vein. Gentle refolding of the orebody along a NW axis may also have taken place (Webster, 1996a).
CONCLUSIONS ABOUT THE STRUCTURAL HISTORY OF THE DEPOSIT It is concluded that there are no structural guides to the location of the Broken Hill lead-zinc-silver deposit. It is a part of the sedimentary succession in which it lies and has been deformed and metamorphosed along with the other rocks with which it is interlayered. The controls and indicators of the Broken Hill orebodies are mostly syngenetic stratigraphic features and all of the evidence preserved by the Broken Hill deposit shows that it formed subaqueously as a linear series of chemical sediment bodies within the lode horizon.
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The linearity and stratification of the orebodies and GQH predate deformation, metamorphism and pegmatite formation. The internal stratification of ore lenses, particularly ALL, 3L and 2L, are preserved throughout their strike length, even in the northern part of the deposit where D2 deformation was most intense and internal sulphide mobilisation was pervasive. The orebodies have not been moved from their stratigraphic positions on a mass scale by deformation nor were they formed as a result of syntectonic epigenetic processes. The depositional system which produced the Broken Hill deposit can be determined, in some detail, from its present form. Detailed structural reinterpretation has led to a more complete understanding of the relationship of the stratigraphy of the deposit to its structure. This work has suggested that the Western A lode (WAL) segment of ALL is truncated by west block up, sinistral shearing to the west of the D2 WAL synform. The mineralised position may continue to the NW of WAL and exploration potential exists between this position and the Western–Centenary mineralisation below the City of Broken Hill. Sinistral, west block up shearing may place the continuation of ALL to the south and above its last known position on the 19 Level of the Southern Operations, a location that would not be highlighted by previous structural models. Given the multitude of exploration models proposed and tested in the district, and the large exploration expenditures, it is interesting to note that no new major Broken Hill deposits have been found. It can only be concluded that they are not there, or at least not within the detection limits of current technology.
MINE EXPLORATION DEVELOPMENTS 1988–1996 NORTH MINE Relatively little has been published on the geology of North Mine since the 1950s, the most recent being by Leyh and Hinde (1990).
Fitzpatrick orebody and the 2K mineralisation In the late 1960s, North Broken Hill geologists realised that 2L and 3L were truncated at depth by the Globe–Vauxhall shear zone. Exploration was successful in locating a fault offset extension to the NE named the Fitzpatrick orebody (Widdop, 1983; Figs 1 and 2), which was mined from 1982 to 1993. In the lower levels the Fitzpatrick orebody cut out or was attenuated in the Western shear. Exploration for the extensions continued for several years. Mineralisation was identified on the NW side of the Western shear after a very costly diamond drilling program and named the 2K in recognition of its two kilometre depth below surface (Figs 1 and 2). Downhole electromagnetic surveys were undertaken (Bishop and Morland, 1994; J R Bishop, unpublished data, 1991, 1992) and an Inferred Resource of 0.6 Mt grading 18.0% lead, 350 g/t silver, and 17.0% zinc was estimated (Pasminco Mining, unpublished data, 1992). Work on 2K was stopped prior to mine closure due to the high cost of exploration, perceived high mining costs if exploration were successful, timing issues and the relatively small size of the mineralisation. The orebody is open at depth.
North Mine Zinc lode After an extensive exploration effort Zinc lode was mined from the Fitzpatrick area from late 1991 (Fig 2). The ore was dominated by coarse grained marmatite and galena with a
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FIG 2 - Composite cross section, looking SW, of the lower levels of North Mine, looking NE (modified from Widdop, 1983).
gangue of quartz, blue quartz, gahnite and garnet with lesser bustamite, manganhedenbergite, calcite and fluorite. The high silver content is in galena. Whereas the Zinc lode is within a similar stratigraphic position to those at the Southern Operations some 5 km away, it is debatable as to which, if any, it directly relates to. In 1992 an extensive surface diamond drilling program was undertaken to test for the development of Zinc lodes associated with 2L and 3L in the upper part of North Mine (D F Larsen and P Jackson, unpublished data, 1993), because minor Zinc lode–type mineralisation had been identified in Number 1 open cut. Eleven drill holes were completed with associated downhole geophysical measurements but no economic mineralisation was identified.
SOUTHERN OPERATIONS C lode and droppers The CL orebody is extensive in its distribution, being at least 2.6 km long with a maximum horizontal width of 130 m and a vertical extent of 250 m. It is a garnetiferous spotted psammopelite with blue quartz–gahnite lode rocks and lode pegmatites. Overall it is poorly mineralised with marmatite, galena and pyrrhotite (Mackenzie and Davies, 1990). A breakthrough in the understanding of the distribution of the higher grade parts of CL was made early in the 1990s (J Stockfeld, unpublished data, 1993; A Wilson, unpublished data, 1994) when it was recognised that the higher grade pods of mineralisation within CL are in fact BL which was mobilised into the overlying CL sequence during D3A shearing. BL sulphides were mobilised as upward droppers along a D3A shear zone that is closely associated with the hinge of the D2 Western antiform (A E Webster, unpublished data 1995). This helps to explain why the mineralisation style in the droppers is so different from most of CL, is more akin to BL, and also why
Geology of Australian and Papua New Guinean Mineral Deposits
BROKEN HILL LEAD-ZINC-SILVER DEPOSIT
the ore shoots are discordant to the stratigraphic sequence. It is characterised by sulphide-quartz-actinolite ore with the presence of coarse-grained sphalerite to 5 cm, clear vein quartz, galena, actinolite as coarse crystals or knots of needles, pyrrhotite, arsenopyrite and chalcopyrite in order of decreasing abundance. CL sensu stricto is uneconomic except when the droppers are mined in conjunction with it. In 1996 the +10% lead plus zinc Measured and Indicated Resource stood at 1.3 Mt grading 5.0% lead, 50 g/t silver and 10.5% zinc (R Morland, unpublished data, 1996). It is currently being mined and exploration is ongoing.
Previous interpretations were based on drill hole data but only recently has mapping of underground exposure validated and enhanced them. The western termination of the orebody, recently exposed, indicates a structural end of the orebody and thus offers hope in the search for additional ore.
4.5 mineralisation Exploration drilling is underway to provide more data on the 4.5 mineralisation. This hosts the Potosi orebody to the north (Morland and Leevers, this publication) but its prospectivity in the south has never been fully evaluated.
The southern termination of the orebodies Western A lode The WAL orebody is defined as the ALL lower position to the west of the hinge of the D2 Western antiform (Fig 3). The mineralisation is more discontinuous than the rest of ALL and many geologists had tried to unravel its internal geometry and lack of homogeneity without success. Recent improved understanding (J Moore, unpublished data, 1992; M Hudson, unpublished data, 1994) has led to mineralisation being defined and stopes being developed in WAL. In 1996 the resource using a +10% lead plus zinc cutoff was 3.2 Mt grading 3.9% lead, 37 g/t silver and 14.3% zinc (S O Stansfield, unpublished data, 1996).
A major underground diamond drill campaign has been under way since 1993 aimed at testing for the extension of the Broken Hill orebodies, in particular BL. Previous exploration, based on knowledge at the time, was aimed too deep and the prospective ground, the Southern Extensions (Fig 3), was untested (R Morland, unpublished data, 1992). To July 1996 a total of 13 191 m has been drilled in 21 holes in fans on sections 200 m apart. A consistent stratigraphic and structural picture has developed (L G Reid, unpublished data, 1996; S O Stansfield, unpublished data, 1996) and the southern termination of the orebody has been clearly identified. Exploration drill hole data and the reassessment of the stratification and structure of the orebodies within the Southern Operations have shown that BL, SAL and S1L diminish in size and strength to the south as GQH dies out. Mineralised positions persist for several tens of metres beyond the end of the GQH and eventually peter out into scattered mineralisation, within a linear position, which has been observed throughout the prospective sequence for over 1 km strike length. This mineralisation is thin and impersistent both along strike and down dip. Spotted psammopelite–hosted C lode–style mineralisation within the CLH dominates the lode horizon on the Southern Leases and contains common 1 to 2 m widths of mineralisation, but none of economic significance at present. Currently the only potential seen is for the mineralisation to remake within the lode horizon further down plunge to the south. This is currently being investigated.
ACKNOWLEDGEMENTS
FIG 3 - Longitudinal projection of the Broken Hill orebody, viewed from the SE.
The main body of ALL is a series of relatively thin (less than 8 m wide) lenses of marmatite and galena with a distinctive band of rhodonite, quartz and hedenbergite (Haydon and McConachy, 1987). WAL has been divided into three major mineralised horizons (lower, middle and upper) within the unmineralised garnet quartzite envelope. Primary sulphides are marmatite and galena with accessory pyrrhotite, arsenopyrite, pyrite and chalcopyrite. The principal gangue minerals are quartz, rhodonite, bustamite, hedenbergite and garnet sandstone, with minor garnet quartzite, gahnite, cummingtonite, garnet, biotite, apatite and staurolite (M Hudson, unpublished data, 1994; Bottrill, 1984). The upper limb is also enriched in pyrrhotite locally as an envelope within the surrounding rhodonite, probably replacing it.
Geology of Australian and Papua New Guinean Mineral Deposits
This paper is published with the permission of Pasminco Limited. It would not have been possible without the excellent geological mapping undertaken by mine geologists at Broken Hill for over 60 years. Their work has made Broken Hill one of the best recorded mineral deposits of any in Australia. Mine geologists only see a small part of the orebody in their time and in recognition of their combined efforts this paper is dedicated to them. The authors thank D Rolton for word processing, M Powell and F Sette for drafting.
REFERENCES Bishop, J R and Morland, R, 1994. Recognising false anomalies in drill hole EM, The AusIMM Proceedings, 299(1): 21–27. Bottrill, R S, 1984. Aspects of mineralogical variations in the WAL, NBHC Mine, Broken Hill, NSW, Australia, MSc thesis (unpublished), University of New South Wales, Sydney. Carruthers, D S, 1965. An environmental view of Broken Hill ore occurrence, in Geology of Australian Ore Deposits (Ed: J McAndrew), pp 339–351 (8th Commonwealth Mining and Metallurgical Congress and The Australasian Institute of Mining and Metallurgy: Melbourne).
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Carruthers, D S and Pratten, R D, 1961. The stratigraphic succession and structure in the Zinc Corporation Ltd and New Broken Hill Consolidated Ltd, Broken Hill, NSW, Economic Geology, 56:1088–1102.
Stevens, B P J, Willis, I L, Brown, R E and Stroud, W J, 1983. The Early Proterozoic Willyama Supergroup: definitions of stratigraphic units from the Broken Hill Block, New South Wales, Geological Survey of New South Wales Record, 21:407–442.
Gustafson, J K, 1939. Geological investigation in Broken Hill, Final Report, The Central Geological Survey (unpublished).
Webster, A E, 1993. Sulphide orebodies and structure: mapping within an orebody and what it can tell — an example from Broken Hill, NSW, in Proceedings International Mining Geology Conference, pp 133–141 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Haydon, R W and McConachy, G W, 1987. The stratigraphic setting of Pb-Zn-Ag mineralisation at Broken Hill, Economic Geology, 82:826–856. Hodgson, C J, 1968. The mineralogy and structure of the New Broken Hill Consolidated Limited Mine, Broken Hill, NSW, PhD thesis (unpublished), University of California, Berkeley. Johnson, I R and Klingner, G D, 1975. Broken Hill ore deposit and its environment, in Economic Geology of Australia and Papua New Guinea, Vol 1 Metals (Ed: C L Knight), pp 476–491 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Webster, A E, 1994a. The structure and stratification of Lead Lode, Southern Operations, Broken Hill, NSW, Australia, MSc thesis (unpublished), James Cook University of North Queensland, Townsville. Webster, A E, 1994b. A structural interpretation of the Broken Hill orebody as suggested by the internal features and macroscopic geometry of the mineralisation, Geological Society of Australia Abstracts, 37:456.
King, H F and O’Driscoll, E S, 1953. The Broken Hill lode, in Geology of Australian Ore Deposits (Ed: A B Edwards), pp 578–600 (5th Empire Mining and Metallurgical Congress and The Australasian Institute of Mining and Metallurgy: Melbourne).
Webster, A E, 1996a. Delamerian refolding of the Palaeoproterozoic Broken Hill Block, Australian Journal of Earth Sciences, 43:85–89.
Laing, W P, Marjoribanks, R W and Rutland, R W R, 1978. Structure of the Broken Hill mines area and its significance for the genesis of the orebodies, Economic Geology, 73:1112–1136.
Webster, A E, 1996b. A detailed description of the Broken Hill deposit - lessons from the ore fabrics, in New Developments in Broken Hill Type Deposits (Eds: J Pongrantz and G Davidson) pp 95–104, CODES Special Publication 1 (University of Tasmania: Hobart).
Larsen, D F, 1994. The Potosi orebody — a newly discovered base metal deposit at Broken Hill, NSW, Geological Society of Australia Abstracts, 37:238. Leyh, W R and Hinde, J S, 1990. Fitzpatrick orebody North Mine, Broken Hill — a case history, in Proceedings Mine Geologists’ Conference, pp 147–154 (The Australasian Institute of Mining and Metallurgy: Melbourne). Lips, A L W, 1994. A structural study of the North Mine open cut, Broken Hill, Australia; high temperature shearing associated with the Broken Hill ore deposit, NSW, Australia, MSc thesis (unpublished), University of Utrecht. Mackenzie, D H and Davies, R H, 1990. Broken Hill lead-silver-zinc deposit at ZC Mines, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1079–1084 (The Australasian Institute of Mining and Metallurgy: Melbourne). Maiden, K J, 1972. Studies on the effects of high grade metamorphism on the Broken Hill orebody, PhD thesis (unpublished), University of New South Wales, Sydney. Pasminco Limited, 1996. Melbourne).
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White, S H, Rothery, E, Lips, A L W and Barclay, T J R, 1995. Broken Hill area, Australia as a Proterozoic fold and thrust belt: implications for the Broken Hill base-metal deposit, Transactions of the Institution of Mining and Metallurgy (Section B: Applied Earth Science), 104:B1–B17. Widdop, W G, 1983. The geology of the Fitzpatrick area, North Broken Hill Limited, Broken Hill, NSW, in Proceedings Broken Hill Conference, pp 177–182 (The Australasian Institute of Mining and Metallurgy: Melbourne). Willis, I L, Brown, R E, Stroud, W J and Stevens, B P J, 1983. The Early Proterozoic Willyama Supergroup: stratigraphic subdivision and interpretation of high to low-grade metamorphic rocks in the Broken Hill Block, New South Wales, Journal of the Geological Society of Australia, 30:195–224. Wright, J V, Haydon, R C and McConachy, G W, 1987. Sedimentary model for the giant Broken Hill Pb-Zn deposit, Australia, Geology, 15:598–602. Wright, J V, Haydon, R C and McConachy, G W, 1993. Sedimentary analysis and implications for Pb-Zn mineralisation at Broken Hill, Australia, EGRU Contribution 48, James Cook University of North Queensland, Townsville.
Geology of Australian and Papua New Guinean Mineral Deposits
Pringle, I J and Elliot, J, 1998. Bowdens silver-lead-zinc deposit, Mudgee, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 627–634 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Bowdens silver-lead-zinc deposit, Mudgee 1
by I J Pringle and J Elliot
2
INTRODUCTION The deposit is 25 km SE of Mudgee in eastern NSW at lat 32o39′S, long 149o52′E on the Dubbo (SI 55–4) 1:250 000 scale and the Mudgee (8832) 1:100 000 scale map sheets (Fig 1). The deposit is near surface and amenable to open cut mining, and is located within a group of contiguous Exploration Licences held by Silver Standard Australia Pty Limited (SSA).
Indicated and Inferred Resources total 18.8 Mt at 99 g/t silver (3.2 oz/t), 0.32% lead and 0.37% zinc, for a contained silver content of approximately 60 Moz. The estimate is based on drilling which has not yet defined the limits of the resource at depth or along strike. The mineralisation is a low sulphidation, silver–base metal type characteristic of major silver mining districts in Mexico, USA, Russia and Chile. It is a style of mineralisation not previously recognised in Australia.
EXPLORATION HISTORY 1.
Exploration Manager, Silver Standard Australia Pty Limited, Suite 403, Level 4, 65 York Street, Sydney NSW 2000.
2.
Consultant Geologist, Anzeco Pty Ltd, 26 Casey Circuit, Bathurst NSW 2795.
During July 1988, CRA Exploration Pty Limited (CRAE) follow up of an anomalous reconnaissance -80 mesh stream sediment sample containing 2.8 ppm silver, 200 ppm lead and 290 ppm zinc and mineralised stream float within a 2.5 km2 drainage basin led to the discovery of outcropping sulphides in
FIG 1 - Location and regional geology of Bowdens deposit in the area of the Northern Capertee High (modified from Pemberton et al, 1994).
Geology of Australian and Papua New Guinean Mineral Deposits
627
I J PRINGLE and J ELLIOT
breccia, containing up to 860 ppm silver, 0.5% lead and 1% zinc (T F McConachy, unpublished data, 1993). There are several shallow prospectors’ pits at the outcrops, which are about 100 m west of Bara road (Fig 2), but the history of these workings is not known. Rock chip and grid soil sampling, geological mapping, induced polarisation (IP) surveys and reverse circulation (RC) and diamond core drilling from 1989 to 1992 defined the much larger Bowdens Gift zone of outcropping mineralisation 500 m east of the discovery outcrops (T F McConachy, unpublished data, 1989; J E Terrill, T F McConachy and A R D Doe, unpublished data,1990).
data, 1996). An optimum process route comprising fine grinding and flotation to produce a silver-rich lead concentrate was defined by this work, although other promising processing options include heap leaching and direct agitation leaching of ground ore, for both high and low grade mineralisation. For the flotation option, silver and lead recoveries of over 90% can be expected. Further processing could include cyanide leaching of the concentrate to recover most of the silver as bullion, to produce a lead concentrate with lower, but considerable remnant silver content. There is also potential for recovering a separate zinc concentrate.
Golden Shamrock Mines Limited purchased the project in August 1994 and have extended the mineralisation at Bowdens Gift prospect along a NNW trend for approximately 300 m beneath a thin cover of unmineralised Permian sandstone (I J Pringle and J Elliot, unpublished data, 1994). The Bowdens Gift prospect was renamed Main Zone South during GSM work and the northern extension was called Main Zone North
During mid 1997 the project was purchased by SSA which is a subsidiary of Silver Standard Resources Inc, a Vancouver based silver exploration company.
Further drilling by GSM some 400 m west of the Main Zone and along the NW trending, structurally controlled Blackmans Gully has defined another prominent zone of mineralisation (I J Pringle, unpublished data, 1995). This has been intersected by six RC drill holes to the north of Bundarra homestead (Bundarra North zone) and extends to the south along the valley where 12 RC drill holes have been completed (Bundarra South zone). Since discovery, 51 RC drill holes and ten diamond core holes for a total of 7039 m have been completed at the deposit. Metallurgical test work on several composite samples of drill core has recently been completed (G P Sheldon, unpublished
The Bowdens deposit is on the northeastern margin of the Lachlan Fold Belt within the Northern Capertee High, a shallow marine to subaerial topographic feature which formed between the Late Silurian and early Middle Devonian. The High has been largely preserved in the Capertee Anticlinorium together with Late Ordovician basement, Late Devonian marine sediment, Middle Carboniferous granite intrusives and the Late Carboniferous to Early Permian silicic pyroclastics of the Rylstone Volcanics (Pemberton et al, 1994). The Bowdens mineralisation is largely hosted by altered tuff and breccia of the Rylstone Volcanics. These are a suite of rhyolite and dacite pyroclastics, lavas and epiclastics which
10 000 m E
LEGEND Alluvium
BUNDARRA NORTH ZONE
RYLSTONE VOLCANICS
Sandstone - SHOALHAVEN GROUP Victric tuff Tuff breccia Crystal lithic tuff
REGIONAL GEOLOGY
10 500 m E
MAIN ZONE NORTH 10 500 m N
SECTION
Metasiltstone - LUE BEDS
BUNDARRA SOUTH ZONE
Geological boundary Fault
MAIN ZONE SOUTH
CRAE RC and diamond drill hole 10 000 m N
GSM RC and diamond drill hole Area with drill intersections of more than 10m at >80g/t Ag 50 30 20
Contours of IP chargeability Old prospecting pits
MN
FIG 2 - Geology plan of Bowdens deposit (modified from T F McConachy, unpublished data, 1993).
628
Geology of Australian and Papua New Guinean Mineral Deposits
BOWDENS SILVER-LEAD-ZINC-DEPOSIT, MUDGEE
crop out over 45 km along the eastern margin of the Capertee Anticlinorium (Fig 1). Similar epithermal silver-base metal mineralisation has been located at the Coomber prospect, 22 km SSE of the Bowdens deposit, which is also hosted by rhyolitic tuff and breccia near the base of the Rylstone Volcanics. Pemberton et al (1994) described the depositional environment of the Rylstone Volcanics as a proximal intracaldera facies with subaerial ignimbrite sheets, coignimbrite breccia, lava dome growth and subordinate airfalls and ash flows. Structures described include shallowly dipping sheets with broad, gently plunging folds, fiamme, columnar jointing, ripples, crossbeds, parallel laminations, flutes and accretionary lapilli in mudstone. Most of these structures are recognisable in the Bowdens area. The Rylstone Volcanics were deposited unconformably on the Lue beds, an Ordovician deep water sequence of finegrained basic volcanogenic sediment. They are in turn partially covered to the east and north by flat-lying conglomeratic sandstone of the Permian Shoalhaven Group, and by Permian to Triassic sediment of the Sydney Basin succession which, locally, is a sandy, transgressive shoreline sequence deposited in a fluvioglacial environment.
LOCAL GEOLOGY STRATIGRAPHY AND LITHOLOGY Sparse outcrops of fine grained sediment of the Lue beds occur in low ground to the SE of the deposit (Fig 2). Several of the deeper drill holes in the southern part of Main Zone South have intersected indurated, fine grained sandstone and mudstone of the Lue beds. Mapping and drilling by CRAE have defined a distinctive, massive ‘crystal lithic tuff’ with a porphyritic texture, usually with whitish feldspar and rarer quartz and mica phenocrysts in a pale grey matrix. The rock is commonly fragmental, and was considered to have formed from a thick pyroclastic flow at the base of the Rylstone Volcanics (J E Terrill, T F McConachy and A R D Doe, unpublished data, 1990; Marshall, 1990). The outcrop of this rock is coincident with a well defined, circular IP anomaly in the SW portion of Main Zone South (Fig 2). Similar rocks outcrop on the eastern side of Blackmans Gully and these are also close to or within IP anomalies. In places the rock is spotted with coarse disseminated pyrite, and it is generally more indurated and massive than the tuff breccia elsewhere in the Rylstone Volcanics. These outcrops are interpreted to be high-level rhyolitic plugs or lava domes which have intruded the volcanic sequence, and are enveloped by a rubbly carapace which is indistinguishable from the surrounding tuff-breccia dominated volcanic host rocks. Further work is required to confirm the nature of this rock type and to determine its relationship to the silver mineralisation. Drilling by GSM at the Bowdens deposit has shown that thick, shallowly north-dipping lithic tuff breccia is the main rock type in the Rylstone Volcanics. Diamond drill holes GSMD1, 2 and 3 intersected massive tuff and tuff breccia sequences with insufficient marker horizons to allow reliable stratigraphic correlation between drill holes. The breccia consists of subrounded volcanic clasts to 20 cm in diameter and rare exotic clasts which resemble the underlying siltstones of the Lue beds (T F McConachy, unpublished data, 1994). The volcanic clasts consist of flow banded rhyolite and ignimbrite and reworked lithic and crystal tuff. Geology of Australian and Papua New Guinean Mineral Deposits
Epiclastic breccia and secondary hydrothermal breccia and associated veining can be recognised in drill core. Both types are often silicified and contain ore minerals, carbonate and clay assemblages. Examples of mineralisation hosted by hydrothermal breccia crop out near prospectors’ pits and the collar of drill hole CRAE37 west of Bara Road (Fig 2). Unconsolidated tuff breccia may have provided favourable channelways for mineralising solutions since secondary, cherty quartz, with or without ore minerals, has filled cavities and partially replaced the breccia matrix in many of the drill hole intersections with high silver content. The original glassy selvages on some breccia fragments appear to have been preferentially altered to clay and possibly resorbed by the mineralising fluids (J Elliot, unpublished data, 1995). Vitric textures have been identified in outcrops and drill chips of layered tuffs near the collar of hole GSMD1 and north of Bundarra homestead, and were noted in the underlying lenses of sandy laminated tuff units during CRAE work (J E Terrill, T F McConachy and A R D Doe, unpublished data, 1990). Drill chips and core of probable vitric welded tuff display abundant fiamme, and fine laminations which may be flow structures. These fine grained rocks are relatively hard and silicified and have been logged as distinctive intervals in several of the drill holes (J Elliot, unpublished data, 1995). They frequently contain zones of quartz-healed brecciation. Laminated vitric tuff is common in drill holes GSM6 to GSM9, and generally lacks significant silver or base metal mineralisation. However, similar rocks in core from drill hole GSMD3 contain high silver values (I J Pringle, unpublished data, 1995). Although fragments of flow-banded rhyolite are dominant within the tuff breccia, lava flows and rhyolitic intrusive dykes within the volcanic sequence are difficult to recognise and are considered to be relatively minor within the area of GSM drilling. Marshall (1990) mapped vertical flow-banded rhyolite on a low ridge 800 m to the east of the deposit and interpreted it to be proximal to a volcanic vent. Fine grained clay, carbonate and sericite alteration is pervasive throughout the mineralised rocks but in most instances this alteration has not destroyed primary textures. The sericite varies from a very pale to a pale sea green colour, and is often distributed in patches throughout the rock. The clay is more evenly distributed as fine disseminations. Carbonate minerals readily oxidise on drill core surfaces after several weeks of exposure. Minor silicification is widespread and sometimes occurs as breccia fill.
STRUCTURE Within the Northern Capertee High major structural trends associated with the Capertee Anticlinorium trend NW and are generally parallel to the strike of the Hill End Trough strata to the west (Fig 1). At Bowdens deposit, faults with an easterly trend appear to have been offset by NNW structures which strike along the valley along Bara Road and the low ground to the east of the deposit (Fig 2). The offset along these structures is unknown and their relationship to mineralisation is unclear. An easterly trending structure forms the northern margin of the Main Zone mineralisation, and drill holes GSM5, 6 and 7 to the north of this fault intersected barren tuff breccia. Limited drilling SW of Main Zone South suggests that another easterly trending fault may have displaced the southern end of the main body of mineralisation several hundred metres to the west.
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FIG 3 - Grid east section on 10 500 N showing the outline of >40 ppm silver in the Main Zone North and Bundarra North Zone of Bowdens deposit. Assays of major drill hole intervals are in Table 1.
Numerous thin zones of shearing and faulting were noted in drill core, and the amount of shearing generally tends to increase towards the contact with the underlying Lue beds. The Rylstone Volcanics–Lue beds contact shows evidence of movement along a prominent mylonite zone in hole GSMD1. Local, preferential structural movement has most likely occurred along an angular unconformity at this contact. Within the Rylstone Volcanics sulphide-bearing breccias, veins and fractures have formed preferentially along shears. Crosscutting veins of pinkish brown sideritic carbonate are common. In some instances later movement has occurred along these shear vein zones and small gouge zones have formed.
MINERALISATION Drilling by CRAE at Main Zone South has shown that the mineralisation here occurs as a thick, near-surface zone which dips at 25 to 30° towards the NW. Mineralised intersections become thinner and of generally lower grade down dip. Within the Main Zone North deposit the mineralised units are generally of higher grade, and appear to dip at moderate angles towards the NE, beneath drill hole GSM8 (Fig 3). North of 10 500 N the Bundarra North and Main Zone North deposits appear to be a connected flat lying blanket of mineralisation, although further drilling is required to determine the depth extent of the deposit, particularly to the west. In many drill hole intersections of lithic tuff breccia, pyrite, galena and pale yellow sphalerite occur in trace to minor amounts, generally less than 3% by volume. The sulphides are usually finely disseminated but less commonly occur as small aggregates and veins. Thin vein-hosted mineralisation is common in core from hole GSMD1, drilled beneath the main
630
body of mineralisation in the northern part of Main Zone South deposit. This may indicate the presence of a zoned system consisting of a disseminated pyrite-galena-sphalerite core, surrounded or underlain by a halo of low grade mineralisation, which is typified by irregular thin veins and shears containing base metals and quartz. Drilling by CRAE beneath outcropping mineralised tuff breccia and rhyolite at Main Zone South has shown that low grade mineralisation extends below the basal unconformity into the Lue beds. Drill hole CRAE25 intersected 10 m which graded 36 ppm silver and trace to 0.11 ppm gold, within arenaceous units of the Lue beds several metres beneath the contact with Rylstone Volcanics. Diamond drill hole CRAE29 contains a 2.2 m interval of high grade polymetallic mineralisation (14.4% zinc, 4.4% lead, 196 ppm silver and 1.4 ppm gold) from a down hole depth of 204.8 m within the Lue beds. This structurally-hosted, massive sphalerite mineralisation indicates potential for auriferous high grade feeder zones beneath the known resource and further deep drilling is required to test this concept. Intervals of relatively high grade base metal mineralisation of 2 to 3% combined lead and zinc are widespread throughout the deposit, and are usually accompanied by high silver values. RC drill hole GSM10 in the southern part of the Bundarra North zone (Table 1, Figs 2 and 3) intersected massive galenasphalerite mineralisation with 15.6% combined lead and zinc between 54 and 56 m down hole depth. Other drill hole intersections contain high silver values but lower lead and negligible zinc and arsenic content, and many intervals of relatively well developed argillic alteration and disseminated pyrite contain galena-sphalerite mineralisation but little silver. In almost all instances, assays for silver are required to determine intervals of ore grade mineralisation.
Geology of Australian and Papua New Guinean Mineral Deposits
BOWDENS SILVER-LEAD-ZINC-DEPOSIT, MUDGEE
TABLE 1 Analyses of major drill hole intervals on Section 10 500 N (Fig 3), Bowdens deposit. Hole
From (m)
To (m)
Interval (m)
Ag (g/t)
Pb (%)
Zn (%)
As (%)
GSM11 GSM11
46 51
51 56
5 5
342 29
0.43 0.12
0.03 0.08
<0.01 <0.01
GSM10 including
30 54
106 56
76 2
69 1050
0.76 8.72
0.6 6.9
0.05 0.24
GSM14
56
66
10
174
0.44
0.53
0.04
GSM3
54
100
46
77
0.36
0.37
<0.01
GSMD3 GSMD3 GSMD3 GSMD3 GSMD3 GSMD3 GSMD3 GSMD3
75 93 105 121 139 166 178 189
78 99 113 127 152 172 179 190
3 6 8 6 13 6 1 2
261 509 263 98 133 76 1000 275
0.46 0.85 0.53 0.29 0.95 0.38 2.33 0.23
0.25 0.45 0.42 0.17 0.23 0.18 0.77 0.29
0.05 0.04 <0.01 <0.01 <0.01 <0.01 0.1 0.09
GSM4 GSM4 GSM4
76 94 104
82 96 146
6 2 42
130 615 111
0.34 0.98 0.33
0.18 0.39 0.14
0.02 0.01 <0.01
27 90 104 134.4 143
32 104 154 139 150
5 14 50 4.5 7
202 35 181 658 542
0.06 0.06 0.35 0.6 1.14
0.01 0.03 0.09 0.03 0.06
<0.01 <0.01 <0.01 <0.01 0.01
GSMD2 GSMD2 GSMD2 including
In the Bundarra South zone (Fig 2) several drill holes have intersected high grade, near surface silver mineralisation within down hole intervals of 6 to 22 m (CRAE34, CRAE36 and CRAE37). This zone has considerable potential to provide additional resources. Silver, lead and zinc assays show a strong log normal distribution. Correlation coefficients using assays from over 3000 drill hole samples show good correlation between silver and lead, silver and zinc, lead and zinc and silver and antimony. Using data from 240 samples, moderate correlation is displayed between lead and cadmium, zinc and cadmium and silver and cadmium. Correlation of manganese with silver and base metals is poor. Minor and trace elements were determined by neutron activation analysis of 111 samples of silver mineralisation from the GSM drill holes (Table 2). Except for silver no values of economic significance were identified by this work. Lead isotope age determinations for twelve mineralised samples from Bowdens deposit have dated the silver mineralisation at 274 ± 10 Myr (Marshall, 1990). This is only slightly younger and within the error overlap for the age of the Rylstone Volcanics, which has been determined to be 292 ± 10 Myr by Marshall (1990).
Geology of Australian and Papua New Guinean Mineral Deposits
ALTERATION Thin sections and XRD analyses of RC drill hole chips containing high grade silver mineralisation (500–1000 ppm silver) have identified two alteration assemblages. There is potassic to argillic alteration associated with disseminated pyrite and ‘sideritic’ veins, and rarer advanced argillic alteration with quartz in veins and as a matrix component of hydrothermal breccia (A C Purvis, unpublished data, 1994). Clasts within the breccia include potassic-altered rock fragments with microcrystalline adularia replacement of plagioclase phenocrysts, and are generally carbonate poor. A similar study of drill core samples has shown that secondary quartz occurs as microsparry linings to voids, rimming fragments and in veins and stockworks with ironmanganese carbonate (‘siderite’) and ore minerals (I R Pontifex and A C Purvis, unpublished data, 1995). Less commonly, secondary quartz occurs as an extremely fine pervasive mixture with clay minerals and widespread potassic alteration includes microcrystalline adularia replacement of feldspar phenocrysts. The sideritic carbonate contains several per cent zinc and 39.8% iron, 4.6% manganese and 0.4% calcium in semiquantitative spot SEM analysis. It commonly shows a direct association with ore minerals, in stockworks, veins, and locally as replacement of igneous feldspar phenocrysts.
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I J PRINGLE and J ELLIOT
TABLE 2 Minor and trace elements in 111 drill hole samples of silver mineralisation. Element
antimony arsenic barium bromine cadmium cerium caesium chromium cobalt europium gallium gold hafnium iridium lanthanum lutetium mercury
Lowest assay (ppm) 9.46 8.71 118 <2 <10 44.3 2.28 <5 <1 0.85 <100 <5 ppb 2.01 <20 ppb 23.4 <0.2 <5
Highest assay (ppm) 408 3350 1850 390 55.2 109 39.5 163 12.7 3.2 <100 128 ppb 8.93 <20 ppb 56.3 0.56 <5
Average
Element
(ppm) 51 354 824 <8 <16 78.4 7.9 <19.4 <2.5 1.8
molybdenum rubidium samarium scandium selenium silver sodium tantalum tellurium terbium thorium tin tungsten uranium ytterbium zirconium
5.6 40
Lowest assay (ppm)
Highest assay (ppm)
<5 130 3.72 1.99 <5 <5 200 <1 <5 <1 6.11 <500 <2 <2 0.55 <500
179 350 10.3 13 38.9 2800 3600 1.64 <5 1.18 18.6 <500 89.3 7.19 4.76 <500
Average (ppm) <6.9 258 6.7 5 365 1000
11 9.9 2.35 2.71
Assays by neutron activation analysis, Becquerel Laboratories.
Colloform banded carbonate also occurs within the matrix of secondary breccias and it is not clear if this formed at the same time as the more widespread pervasive clay-carbonate alteration. Pyrolusite, jarosite and clay assemblages are common in weathered outcrops of mineralisation, particularly near the Rylstone Volcanics–Shoalhaven Group contact. Scanning electron microscope (SEM) examination of weathered mineralised RC drill hole chips from 10 m below the collar of drill hole GSMD001 showed veined jarosite with small, leadrich cores and more normal potassic rims (A C Purvis, unpublished data, 1994).
ORE MINERALS The most abundant sulphide minerals (pyrite, galena, sphalerite, arsenopyrite and marcasite) are commonly fine grained (of diameter less than 5 µm) and generally comprise less than 3% of the rock. The common sulphides occur in epigenetic veins, and as breccia matrix fill with epithermal quartz and sideritic carbonate. Fine grained disseminated mineralisation is widespread within the altered groundmass of the highly mineralised samples and is sometimes selectively associated with potassium feldspar and sericite pseudomorphs of primary feldspars. Petrological work by CRAE found that very fine grained pearcite (Ag16As2S11) and argentite are the main silver minerals at Main Zone South and that 45% of the silver minerals occur as liberated grains, 19% occur in a binary association with galena, and 13% in a binary association with sphalerite (T F McConachy, unpublished data, 1989). A wide variety of trace ore minerals identified by SEM studies in both RC drill hole chips and core (Table 3) often occurs with the common sulphide phases. Silver is present as small grains of cerargyrite with trace bromine in weathered RC drill hole chips from 10 m below the collar of drill hole GSMD001 (A C Purvis, unpublished data, 1994).
632
TABLE 3 Ore and gangue minerals identified by semiquantitative SEM study.
• pyrite, galena, iron-poor sphalerite (3–4 % iron), arsenopyrite, marcasite
• silver-zinc-tetrahedrite to freibergite plus plagionite • silver-bearing tetrahedrite • pearceite-polybasite sulphosalt group which includes variable contents of antimony, arsenic, and lead with silver>>copper.
• argentite (varieties which are both stable and unstable under electron beam)
• acanthite • cerargyrite • argyrodite plus stephanite plus unaccounted copper and lead • • • • •
(with high germanium). Arsenic appears to substitute for antimony in stephanite. pyrargyrite-proustite plus stephanite alloy (silver, gold, lead, mercury) native silver native lead native gold
Source: Unpublished data of T F McConachy, 1989; A C Purvis, 1994; I R Pontifex and A C Purvis, 1995.
I R Pontifex and A C Purvis (unpublished data, 1995) described a selective distribution of finely disseminated sulphides along perlitic structures in clay-sericite altered glassy groundmass, indicating some degree of lithological control on the distribution of mineralisation. The assemblage of silver minerals indicates that the rocks were mineralised at shallow levels within a hydrothermal system. Ore formation at relatively low temperature is also supported by trace amounts of mercury in some of the silverbearing phases.
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BOWDENS SILVER-LEAD-ZINC-DEPOSIT, MUDGEE
DISCUSSION
REFERENCES
Mineralisation at the Bowdens deposit includes many of the textural and alteration characteristics of epithermal deposits. High silver to gold ratios, adularia and manganiferous carbonate in gangue, and high silver, lead and zinc values of the Bowdens mineralisation are features associated with a distinctive style of epithermal deposits termed lowsulphidation silver-gold-base metal deposits by Sillitoe (1993) and White and Poizat (1995). Many of the characteristics of the Bowdens mineralisation compare with low-sulphidation silver-gold-base metal deposits in silver-rich mining districts such as Comstock and Creede (USA), Fresnillo and Real de Angeles districts (Mexico), Potosi (Bolivia), Arcata (Peru) and the Ducat ore field of NE Russia (Graybeal, 1981; Pearson, Clark and Porter, 1988; Candiotti de Los Rios, Noble and McKee, 1990; Konstantinov, Rosenblum and Strujkov, 1993).
Candiotti de Los Rios, R, Noble, D C and McKee, E H, 1990. Geological setting and epithermal silver veins of the Arcata district, southern Peru, Economic Geology, 85:1473–1490.
ACKNOWLEDGEMENTS The authors are grateful for permission by SSA, GSM and CRAE management to publish this information, and to the CRAE staff, in particular J E Terrill and T F McConachy, who were instrumental in the discovery and early evaluation of the deposit. T F McConachy is also thanked for his review of the manuscript.
Geology of Australian and Papua New Guinean Mineral Deposits
Graybeal, F T, 1981. Characteristics of disseminated silver deposits in the western United States, in Arizona Geological Society Digest volume XIV (Eds: W R Dickinson and W D Payne), pp 271–282. Konstantinov, M M, Rosenblum, I S and Strujkov, S F, 1993. Types of epithermal silver deposits, NE Russia, Economic Geology, 88:1797–1809. Marshall, G J, 1990. Geology and mineralization of the Rylstone Volcanics near Lue, New South Wales, BSc Honours thesis (unpublished), University of Wollongong, Wollongong. Pearson, M F, Clark, K F and Porter, E W, 1988. Mineralogy, fluid characteristics and silver distribution at Real de Angeles, Zacatecas, Mexico, Economic Geology, 83:1737–1759. Pemberton, J W, Colquhoun, G P, Wright, A J, Booth, A N, Campbell, J C, Cook, A G and Millsteed, B D, 1994. Stratigraphy and depositional environments of the Northern Capertee High, Proceedings of the Linnean Society of NSW, 114(4):195–224. Sillitoe, R H, 1993. Epithermal models: genetic types, geometrical controls and shallow features, in Mineral Deposit Modelling (Eds: R V Kirkham et al), pp 403–418, Geol Assoc Canada, Spec Pap 40. White, N C and Poizat, V, 1995. Epithermal deposits: diverse styles, diverse origins?, in Proceedings 1995 PACRIM Congress (Eds: J L Mauk and J D St George), pp 623–628 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Valliant, R I and Meares, R M D, 1998. Lewis Ponds gold-silver-copper-lead-zinc deposits, in Geology of Australian and Papua New Guinean Mineral Deposits, (Eds: D A Berkman and D H Mackenzie) pp 635–640 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Lewis Ponds gold-silver-copper-leadzinc deposits 1
by R I Valliant and R M D Meares
2
INTRODUCTION The deposits are 15 km east of Orange in the eastern Lachlan Fold Belt, NSW, at lat 33o16′S, long 149o15′E on the Bathurst (SI 55–8) 1:250 000 scale map sheet (Fig 1). The property is held by Tri Origin Australia NL (Tri Origin), the Australian subsidiary of Tri Origin Exploration Ltd of Canada.
gold, 123 g/t silver, 0.2% copper, 2.6% lead and 4.2% zinc in the Central lens of the Main zone, and 1.0 Mt grading 2 g/t gold, 214 g/t silver, 0.3% copper, 5.4% lead and 7.9% zinc in Toms zone. Additional lower grade Inferred Resources totalling 4 Mt occur in hanging wall and footwall lenses adjacent to the Central lens. Lewis Ponds is an interesting member of the gold-rich volcanogenic massive sulphide (VMS) deposits of eastern Australia (Large, 1992), but differs from most of the others due to its interpreted shallow water depositional setting.
EXPLORATION HISTORY Gold was discovered at Lewis Ponds in the 1850s, and the gossans, small supergene sulphide zones and cupriferous quartz veins were mined from 1887 to 1921. A smelter at Lewis Ponds treated gold-silver-lead ore from mines in the district. Modern exploration commenced at Lewis Ponds in 1964, and since then a number of major companies including Aquitaine Australia Minerals Pty Ltd, Amax Exploration (Australia) Inc, Shell Company of Australia Ltd and Homestake Australia Ltd have conducted exploration programs, including 21 diamond drill holes, to evaluate the depth and strike extensions of the orebodies mined in the old Lewis Ponds workings. Much of this drilling was shallow and failed to intersect significant mineralisation. Tri Origin commenced exploration at Lewis Ponds in 1991. The initial strategy was to compile and interpret previous data, generate new targets through systematic geological mapping, and conduct a detailed induced polarisation (IP) survey. This work resulted in reinterpretation of the geological environment hosting the known mineralisation and identified a number of untested areas. Testing these targets by deep diamond drilling proved successful in 1992 when the third and fourth drill holes intersected the near surface projection of a new sulphide occurrence (now called the Main zone) approximately 250 m NW of the previous diamond drilling. The second deposit (Toms zone) was discovered in 1995 by step-out drilling from the Main zone. FIG 1 - Geological plan of the Lewis Ponds mineral deposits.
The project is at the advanced exploration stage. Indicated Resources in the two massive sulphide zones discovered by Tri Origin currently total 3.7 Mt, comprising 2.7 Mt grading 3.6 g/t
The subsequent exploration approach at Lewis Ponds has been to delineate the favourable host sequence by geological mapping, and to define targets using soil and rock geochemical surveys, IP surveys and down hole electromagnetic measurements. Exploration and deposit delineation diamond drilling totalling 29 000 m in 56 holes to depths of up to 700 m below surface has been completed to date.
1.
Managing Director, Tri Origin Australia NL, Suite 701, 121 Walker Street, North Sydney NSW 2060.
PREVIOUS DESCRIPTIONS
2.
Formerly Exploration Manager, Tri Origin Australia NL, now Exploration Manager, Malachite Resources NL, PO Box 42, Linfield NSW 2010.
Most of the information on Lewis Ponds is contained in unpublished company reports, however aspects of the deposits are described by Stevens (1975), Davis (1990), Glen and
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R I VALLIANT and R M D MEARES
FIG 2 - Interpretive geological cross sections on 400 N (Toms zone) and 1275 N (Main zone) looking NW. Bearing of each section plane is 32o true.
Watkins (1994), NSW Department of Mineral Resources (1994), Pogson and Wyborn (1994) and Carr et al (1995).
REGIONAL GEOLOGY The deposits are contained within the Mumbil Group, part of the shallow water Late Silurian Mumbil Shelf sequence recognised by Glen and Watkins (1994). These rocks are stratigraphically underlain to the west by the Ordovician Molong volcanic belt and overlain to the east by rocks of the Late Silurian–Early Devonian Hill End Trough. Other Late Silurian volcanic and sedimentary rock sequences in New South Wales host significant VMS-style deposits including Woodlawn (20 Mt) and Captains Flat (4 Mt) approximately 200 and 250 km south of Lewis Ponds (Davis, 1990; Large, 1992). In the Lewis Ponds district other VMS-style base metal mineralisation is associated with interpreted volcanic centres at the Icely, Mount Bulga, Mount Lindsay, Mount Shorter, Calula and Daydawn deposits. Regionally the sequence has been weakly metamorphosed to lower greenschist facies, intruded by granite bodies of the Late Carboniferous Bathurst Granite, and by narrow lamprophyre dykes first recognised by Tri Origin at Lewis Ponds.
636
The major deformation of the Mumbil Group occurred during the Middle Devonian Tabberabberan Orogeny when at least two periods of compression took place, with the dominant episode resulting in the development of NW-trending isoclinal folds and the associated axial plane slaty cleavage.
ORE DEPOSIT FEATURES HOST ROCKS At Lewis Ponds the sequence has been divided into three main rock units. From SW to NE in ascending stratigraphic order, these comprise: 1.
a thick unit of felsic (rhyodacitic) crystal tuff;
2.
a transitional unit which hosts the massive sulphide deposits and consists of tuff, siltstone, limestone and heterolithic fragmental rock interpreted to be debris flow; and
3.
a thick hanging wall siltstone unit (Fig 2).
Contacts between each of these major units are interpreted to be conformable and gradational.
Geology of Australian and Papua New Guinean Mineral Deposits
LEWIS PONDS GOLD-SILVER-COPPER-LEAD-ZINC-DEPOSITS
The footwall crystal tuff unit (also called the western crystal tuff) consists of a southward-thickening homogenous pile of felsic quartz-eye bearing pyroclastic rocks lacking primary layering but displaying a strong NW-trending cleavage. This unit is interpreted to be part of the Mullions Range Volcanics. Petrographically these rocks consist of 5–15% quartz phenocrysts and angular quartz crystal fragments (to 3 mm long) with subordinate plagioclase phenocrysts set in a quartzofeldspathic, granoblastic matrix carrying variable amounts of sericite, biotite and chlorite (D Mason, unpublished data, 1995). The foliation and moderately pervasive alteration have obscured most of the original textures, although local fragmental textures suggest a probable pyroclastic or mass flow origin. In addition, flattened pumice fragments (fiamme) have been observed and suggest an ignimbritic origin for at least part of this unit. A massive aphanitic rhyolite and fragmental equivalents intersected at depth in the footwall at Toms zone may represent a rhyolite dome and its pyroclastic apron. The transitional unit comprises generally thin ( <10 m) layers of felsic crystal tuff, felsic crystal-lithic tuff (often magnetite bearing), siltstone, massive white and grey fossiliferous limestone, and breccia of debris flow origin. These heterolithic breccias form a northward-thinning wedge and contain poorly sorted angular clasts of limestone, felsic tuff or siltstone in a reworked tuffaceous matrix. All of these rock types show rapid facies variations along and across strike, and commonly the grain size of individual beds decreases northwards from Toms zone to Main zone. The unit varies in thickness from less than 100 m at Toms zone to more than 200 m at Main zone, suggesting a deepening of the depositional basin from south to north. This transitional unit hosts the Main zone and Toms zone massive sulphide deposits (Fig 2). Main zone consists of three lenses which occur in the basal and central parts of the transitional unit, and are overlain by poorly laminated ash tuff and siltstone. Toms zone is approximately 80 m stratigraphically higher than the Main zone at, or near, the contact of the poorly laminated ash tuff and siltstone, and overlying bedded coarse- and fine-grained tuffaceous siltstone. The hanging wall siltstone unit consists of massive to thinly laminated grey to pink calcareous siltstones which form a northwesterly thickening wedge and become the dominant rock type north of Main zone. The siltstones are characterised by disseminated sulphides as small (<2 mm diameter) elongate blebs of pyrrhotite, and contain minor tuffaceous and sandstone interbeds. The hanging wall siltstone is overlain by a discontinuous jaspilite unit which averages approximately 10 m thick and consists of recrystallised quartz and minor disseminated hematite and magnetite. This iron oxide facies unit carries anomalous gold values and may be related to massive sulphide deposition. The jaspilite is stratigraphically overlain by a large body of felsic to intermediate crystal tuff, called the eastern crystal tuff, which marks the eastern limit of known sulphide occurrences. Rare rhyodacitic porphyry dykes with plagioclase > quartz > biotite > apatite phenocrysts have been observed crosscutting the cleavage in the western crystal tuffs SW of Toms zone. Rare dolomite-altered clasts of this porphyry also occur in the hanging wall sediment north of the Line 12 N fault. Porphyry clasts incorporated into the overlying sediment indicate that the porphyry dykes are subvolcanic intrusives associated with the
Geology of Australian and Papua New Guinean Mineral Deposits
western crystal tuff volcanic event. In the SE section of the property, the transitional unit is intruded by a series of dykes and larger bodies of the Carboniferous Bathurst Granite. However no evidence has been found of contact metamorphic effects on the Silurian volcanic and sedimentary rocks or the massive sulphide mineralisation.
STRUCTURE The sequence hosting the deposits lies on the east limb of the Mullions Range Anticline, and dips and faces steeply NE with average dips of 70o at Main zone. The average dips at Toms zone are near vertical, ranging from 80o NE near surface to 80o SW at depth, as interpreted from limited drilling information (Fig 2). Primary layering is poorly preserved in most rocks. The dominant foliation is a regional NW-trending subvertical cleavage, most likely a product of the regional D2 deformation event described by Glen and Watkins (1994). Locally, transposition of bedding into cleavage has been observed, as have rare small-scale isoclinal fold closures. In addition, bedding-cleavage lineations have been observed which parallel the elongation direction of pyrrhotite blebs within the hanging wall siltstone. These lineations plunge 70o NW at Main zone, and steeply SE at Toms zone. A second period of deformation is indicated by broad SWtrending corridors of kink bands which deform the earlier dominant cleavage in a right lateral sense, as observed in outcrop. Rocks at Lewis Ponds are crosscut by two sets of faults. One set strikes SW and dips NW at 60o. These faults occur at approximately 150 m intervals along strike, and the Line 12 N fault appears to bound the northern margin of the Main zone and truncate the volcanic sequence (Fig 1). Other parallel faults appear to bound blocks of similar rock types. Consequently these faults are interpreted to be both post- and possibly synmineralisation and syndepositional in age (ie growth faults). The second set of faults strikes east and dips north at 80 to 90o. The age relationships of the second set of faults remain poorly understood. Little fault displacement of rock units or faults (where crossed by other faults) has been noted.
MINERALISATION The deposits are stratabound and locally stratiform, gold-rich, massive, semi-massive and disseminated base metal sulphide lenses which occur over a stratigraphic interval of approximately 200 m and a strike length of at least 1.3 km. Southward the footwall crystal tuff unit hosts an elongate zone of stringer copper veining with dimensions of 2 km by 700 m. The Main zone and Toms zone massive sulphide deposits occur at two separate stratigraphic levels. In addition, the stratigraphically higher New Lewis Ponds mineralisation, at which some historic mining took place, occurs at the top contact of the uppermost felsic tuff. In the case of the Main and Toms zones, mineralisation has been traced by drilling along strike for at least 800 m SE of Main zone and 800 m NW of Toms zone (Fig 1).
Main zone The Main zone consists of three subparallel stratabound sulphide lenses which have a total strike length of 300 m and a
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down-plunge dimension of greater than 650 m. The deposit is generally bounded along strike on the SE side by a local ‘basement high’ limestone reef and on the NW side by the Line 12 N fault. The mineralised system is open at depth, with subeconomic mineralisation intersected in the deepest drill holes completed to date at 650 m below surface (Figs 2 and 3). Central lens contains the Indicated Resource delineated at the Main zone. It extends from 100 to 500 m below surface and averages 15 m in horizontal thickness. It consists of locally banded and stratiform massive pyrite, with lesser iron-bearing, red-brown sphalerite, galena, chalcopyrite, pyrrhotite and tetrahedrite, and native gold as grains or in association with pyrite. The sulphide grains are recrystallised, and the only marked zoning noted is the enrichment of gold in the stratigraphic top of the Central lens. Massive and some semimassive sulphide layers, ranging from tens of centimetres to several metres in thickness, alternate with layers of disseminated sulphide which dilute the base metal grades across the thickness of the lens. The Hanging wall lens averages 4 m in horizontal thickness and is geologically similar to the Central lens but of lower grade. The Footwall lens averages 18 m in horizontal thickness and consists of abundant subangular clasts of white limestone, or less often of felsic tuff, enclosed by a massive or disseminated sulphide matrix which makes up 30% of the volume on average. The Footwall lens extends from 150 to 500 m below surface and has an Inferred Resource of 3.5 Mt grading 0. 5 g/t gold, 40 g/t silver and 1. 72% zinc.
Toms zone This deposit is 900 m along strike SE from the Main zone and
has many similarities to the Main zone. It currently has a strike length of 250 m, as defined by drilling, and a further potential strike extension of 150 m to the SE is indicated by a zone of discontinuous gossans. Toms zone extends from surface to 500 m below surface and is open down dip. The Indicated Resource is contained in a single lens of massive sulphide which averages 5 m in horizontal thickness and does not exhibit the dilution of base metal grades due to internal disseminated sulphide zones as at the Central lens. Parallel narrow hanging wall and footwall massive sulphide lenses have been correlated between drill holes and may provide mineable widths outside the area currently drilled. Toms zone carries higher average base metal grades but lower gold grades than the Main zone, resulting in a 50% higher in situ average unit metal value compared with the Central lens. The Toms mineralisation is better banded and more massive than the Central lens, and the host sequence is finer grained and more competent. These benefits, and because it is open along strike and down dip, make Toms zone and the possible occurrence of nearby lenses of similar character attractive exploration targets.
Footwall stringer mineralisation Numerous copper-rich veins, veinlets, stringers and disseminations occur in the western crystal tuff for up to 2 km south of the massive sulphide deposits, over an area of 2 km by 700 m. They are characterised by quartz-chlorite-chalcopyrite veins dominantly parallel to the cleavage, although stockworks cutting across the cleavage have been noted. These vein systems have been worked by shallow pits and shafts at many locations including the Haywards, Little Bell,
FIG 3 - Composite longitudinal projection of Lewis Ponds deposits looking SW. Bearing of section plane is 302o true (parallel to baseline).
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Geology of Australian and Papua New Guinean Mineral Deposits
LEWIS PONDS GOLD-SILVER-COPPER-LEAD-ZINC-DEPOSITS
Britannia and Mount Regan mines, however these targets have received little modern exploration. This broad zone may represent a deformed footwall feeder system to the massive sulphide deposits discovered in the overlying transitional unit. In addition, individual vein systems highlight the potential for siltstone units mapped within the enclosing western crystal tuff unit to host blind massive sulphide deposits.
zone, and locally intense zones of sericitisation occur in the immediate footwall at the southern end of Toms zone.
ALTERATION
The Lewis Ponds gold-rich massive sulphide deposits are hosted by a Late Silurian volcanic-sedimentary shelf sequence. It is proposed that the deposits formed during the waning stages of Mullions Range felsic volcanism in a shallow submarine environment by hydrothermal discharge at the sea floor.
Alteration associated with the massive sulphide lenses exhibits variability in mineralogy, intensity and distribution, and a full study has not been undertaken. Alteration minerals consist of one or more of magnesian chlorite, sericite, carbonate (most commonly dolomite), talc and silica. These minerals may form the gangue to and/or enclose individual massive sulphide lenses; weakly or pervasively overprint the footwall rocks or nearby hanging wall felsic tuff or siltstone layers; or less commonly occur as foliation-parallel or vein-like alteration zones. Most massive sulphide intercepts in drill core are associated with a distinctive grey carbonate rock which commonly occurs as layers forming the gangue to sulphide minerals. It consists of intense dark green chloritic angular fragments and locally lime green talc, enclosed by disseminated pyrite, sphalerite and galena-bearing grey dolomite masses, which may be cut by late dolomite veins. Although most of the alteration occurs on the SW (footwall) side of the massive sulphide lenses, a broad zone of talc-chlorite-carbonate alteration occurs sporadically in the hanging wall to the Main zone. Chloritic alteration occurs most intensely in the western crystal tuff sequence below Toms
In addition, an apple-green mica and possibly pyrophyllite alteration have been noted in the aphanitic rhyolite at depth in the footwall to Toms zone.
DISCUSSION
Deposition of massive sulphides took place in local basins bounded by interpreted syndepositional growth faults and/or limestone reefs. The Main zone deposit is in part constrained to the SE by a limestone reef complex, containing rugose corals, which formed a ‘basement high’ at the time of massive sulphide deposition (J Pickett, unpublished data, 1995). The deposit is bounded to the NW by the Line 12 N fault (Fig 4). During the early stages of the deposition of the Main zone mineralisation, active volcanism, fault movement, and gravity slides off the limestone reef resulted in the episodic deposition of talus during the formation of the Footwall lens, which consists of limestone and felsic tuff clasts in a weakly-banded massive sulphide matrix. The Central lens was subsequently deposited during a less active phase of basin development as shown by its lower talus content. Finally, the overlying siltstone indicates a cessation of local basin development, although the thickly bedded nature of the siltstone and the thickening of the unit northward may indicate rapid fill of larger
FIG 4 - Interpretive facies relationships diagram of Lewis Ponds host rocks and mineralised zones prior to deformation.
Geology of Australian and Papua New Guinean Mineral Deposits
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R I VALLIANT and R M D MEARES
seaward basins while volcanic and hydrothermal activity were dominant. Toms zone appears to have been deposited in a basin formed at a palaeotopographic depression in the footwall volcanic rocks and, in part, adjacent to a synvolcanic dome (Fig 4). Better developed banding in the massive sulphide and lack of fragmental textures in the host rocks suggest a quieter depositional environment than at the Main zone. Evidence for a syngenetic, pre-deformation origin for the Lewis Ponds deposits includes: 1.
alternating monomineralic particularly at Toms zone;
sulphide
banding,
2.
isoclinal folding of massive sulphide layers similar to that in the enclosing host rocks;
3.
alternating sulphide and non-sulphide parallel layers within the massive sulphide lenses which parallel depositional layering in the host rocks;
4.
localised hydrothermal alteration zones predominantly in the stratigraphic footwall to the massive sulphide lenses;
5.
quartz-chlorite-chalcopyrite vein and stringer zones which are hosted by the footwall western crystal tuff and adjoin the southern end of Toms massive sulphide zone;
6.
the continuity of individual massive sulphide lenses over hundreds of metres both horizontally and vertically; and
7.
the continuity of stratabound massive, semi-massive, and disseminated sulphide mineralisation for 800 m along strike SE of the Main zone and 800 m along strike NW of the Toms zone deposit.
A recent study by Carr et al (1995) of the lead isotopic signatures of gold and base metal deposits in the Lachlan Fold Belt of NSW suggests a 435 Myr (Silurian) age for the Lewis Ponds mineralisation.
The recent discoveries at Lewis Ponds have confirmed the potential of the eastern Lachlan Fold Belt to host gold-rich VMS base metal deposits. The interpreted shallow water depositional environment at Lewis Ponds suggests that other Silurian sequences in the region require re-examination for their VMS potential.
ACKNOWLEDGEMENTS This paper is published with permission of Tri Origin Australia NL. The contributions of the many geoscientists who have been involved in this project are acknowledged. In particular, the authors wish to thank D Robinson, C Butella, B Ferris, C Spurway and V Squires for their enthusiastic efforts.
REFERENCES Carr, G R, Dean, J A, Suppel, D W and Heithersay, P S, 1995. Precise lead isotope fingerprinting of hydrothermal activity associated with Orodovician to Carboniferous metallogenic events in the Lachlan Fold Belt of New South Wales, Economic Geology, 90:1467–1505 Davis, L W, 1990. Silver-lead-zinc-copper mineralisation in the Captains Flat-Goulburn Synclinorial Zone and the Hill End Synclinorial Zone, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1375–1384 (The Australasian Institute of Mining and Metallurgy: Melbourne). Glen, R A and Watkins, J J, 1994. The Orange 1: 100 000 sheet: A preliminary account of stratigraphy, structure and tectonics, and implications for mineralisation, Geological Survey of NSW Quarterly Notes, 95:1–17. Large, R R, 1992. Australian volcanic-hosted massive sulphide deposits: features, styles, and genetic models, Economic Geology, 87:471–510 NSW Department of Mineral Resources, 1994. Lewis Ponds gold and base metals prospect, Minfo, 42:4–5. Pogson, D and Wyborn, D, 1994. Excursion guide Bathurst 1:250 000 geological sheet, Geological Survey NSW Report, GS 1994/139. Stevens, B P J, 1975. A Metallogenic Study of the Bathurst 1:250 000 Sheet, Geological Survey of NSW.
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Newcrest Mining Staff, 1998. Cadia gold-copper deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 641–646 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Cadia gold–copper deposit by Newcrest Mining Staff
1
INTRODUCTION The Cadia porphyry gold-copper deposit is about 20 km south of Orange in central NSW, at lat 33o28′S, long 149o00′E and AMG coordinates 6 296 000 m N, 685 000 m E on the Bathurst (SI 55–8) 1:250 000 scale map sheet. Current resources are 9.4 Moz of contained gold and 1.2 Mt of contained copper (Table 1) in mineralisation within adjacent bodies of sheeted quartz vein and disseminated mineralisation at Cadia Hill and Cadia East. An extensive halo of subeconomic mineralisation extends NW and SE of these bodies, significantly enhancing the metal inventory of the mineralised system, and they are flanked on the north by gold-copper-magnetite skarns at Big and Little Cadia (Fig 1). TABLE 1 Cadia resources and reserves. Mt
g/t Au
% Cu FIG 1 - Location map, Cadia deposit.
Cadia Hill Resource Measured
230
0.69
0.15
Indicated
91
0.54
0.16
Inferred
27
0.51
0.18
Cadia Hill Reserve (within the Resource above) Proved Probable
152
0.79
0.17
50
0.54
0.17
138
0.44
0.47
occurrences at Copper Hill and Cargo, on the relatively well exposed Molong volcanic belt. A much less well exposed parallel belt to the west, the Narromine–Parkes–West Wyalong–Temora belt (Fig 2), hosts the only currently producing porphyry deposit in NSW at North Parkes
Cadia East Resource Inferred
EXPLORATION HISTORY Exploration and mining at Cadia dates back more than 140 years to the discovery of copper and gold mineralisation in 1851. Small-scale mining was intermittently conducted in the district over the next 100 years, mostly at Big and Little Cadia. Significant exploration by mining companies commenced in the 1950s, culminating with the discovery of the Cadia Hill mineralisation by Newcrest geologists in late 1992 (Wood and Holliday, 1995).
REGIONAL GEOLOGY The eastern part of the Lachlan Fold Belt of NSW contains a number of Ordovician volcano-intrusive complexes which host porphyry style gold-copper mineralisation and highsulphidation epithermal gold deposits. Cadia is the largest of the known deposits and is located, with other porphyry
1.
Newcrest Mining Limited, PO Box 1367, Milton Qld 4064.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 2 - Ordovician volcano-intrusive complexes in the eastern Lachlan Fold Belt.
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NEWCREST MINING STAFF
(Goonumbla), as well as small high-sulphidation deposits at Gidginbung and Peak Hill, and volcanic-hosted gold and porphyry copper deposits at Lake Cowal.
calcareous siltstone and mudstone of the Ordovician Weemalla Formation are in contact with the volcanics, but the relative age relationship is uncertain.
The volcano-intrusive belts of the eastern Lachlan Fold Belt are relatively undeformed and despite their Ordovician age are only metamorphosed to lower greenschist facies. Apparently long-lived, mostly meridional-trending fault systems bound the belts and separate them from Ordovician flysch sequences and younger, Siluro-Devonian volcanics, sediment and granitoids. Compressional tectonism in post-Silurian time produced westerly-dipping, low-angle reverse faulting at Cadia.
The Forest Reefs Volcanics and Weemalla Formation are intruded by a relatively small (3 by 1.5 km) composite stock which on its northern flank, from Cadia Hill to Cadia Quarry, is predominantly quartz monzonite. This Cadia Hill Monzonite ranges from coarsely orthoclase-porphyritic quartz monzonite (porphyry) in the east to equigranular quartz monzonite in the west. The less investigated southwestern margin of the stock is dioritic (Tunbridge Wells Diorite) and produces the strong positive magnetic signature associated with the Cadia Hill Monzonite. Overall the composition of the Cadia stock is alkaline, and petrographical and geochemical evidence suggest that it could have developed through fractionation of a deeplysourced shoshonitic magma, which was intruded to a relatively shallow level within the crust. Elsewhere in the district other Ordovician alkaline intrusive activity is evidenced by several small stocks of syenite to diorite composition and a number of NW-trending latite and monzodiorite dykes.
The origin of the belts is controversial. Cas (1983) and Powell (1984) propose an intra-oceanic island arc setting, whilst Wyborn (1992) has developed a model of delayed melting of a previously mantle-enriched subcontinental lithosphere. Geochemical data accumulated by Wyborn (1992) support a shoshonitic parentage for the Ordovician magmatism and form the basis for a mineralisation model (Wyborn, 1994) involving fractionation of sulphur-undersaturated magmas derived from metasomatised mantle lithosphere, producing mineralisation in zoned potassic intrusive complexes.
LOCAL GEOLOGY STRATIGRAPHY Mineralisation at Cadia is hosted by units of the Forest Reefs Volcanics and the Cadia Hill Monzonite (Fig 3), both of Late Ordovician age. The volcanics contain a significant volcaniclastic component (volcanic breccia, epiclastic sandstone and conglomerate, with minor mudstone and limestone lenses), and augite-dominant (lesser plagioclase) porphyritic to phaneritic lavas and subvolcanic intrusions of basaltic andesite to shoshonite composition. Variably
The Cadia mineralisation is partly obscured by the Early to Middle Silurian Cadia Coach Shale (argillite to arenite, with minor calcarenite), basaltic and trachytic members of the Miocene Canobolas Volcanic Complex, and by Tertiary and Recent gravel, transported soil and alluvium. Apart from a barren portion of its alteration halo, the mineralisation at Cadia East is completely obscured by Cadia Coach Shale.
STRUCTURE Widespread faulting has disrupted the local geology. Emplacement of the Cadia intrusive was probably facilitated and localised by the development of a major NW-trending dilation zone which is well defined by regional magnetic data.
FIG 3 - Local geology of the Cadia deposit (amended from mapping by Newcrest staff), with location of section lines for Figs 4 and 5.
642
Geology of Australian and Papua New Guinean Mineral Deposits
CADIA GOLD-COPPER DEPOSIT
This structure also exerted influence during and after the mineralising event, as it is the predominant direction of the mineralised sheeted quartz veins in the Cadia deposit, and the direction of a number of faults, including the PC40 fault which dislocated the Big Cadia skarn. Extension occurred orthogonal to this trend during the formation of the sheeted veins. The other dominant structures at Cadia trend northerly, and these probably exerted influence prior to intrusion of the Cadia stock and attendant mineralisation, but were clearly influential post-intrusion. The northern end of the regional, meridional Wongalong fault system (as presently recognised) transects the Cadia deposit. Splays from this fault system resulted in Cadia Hill being thrust partly over Cadia East by the Gibb fault, and they separate Cadia Hill from Cadia Quarry along Cadiangullong Creek. Folding of the volcanics and sediment at Cadia is mostly relatively subdued, with broad open folds and dips up to 40o. Any intense folding is localised and directly associated with post-mineral reverse faulting.
PORPHYRY SYSTEM Mineralisation at Cadia is spatially related to the porphyritic quartz monzonite (porphyry) phase of the intrusive stock, and is interpreted to be genetically related to crystallisation of deeper portions of this porphyry. Most of the exposed porphyry is hydrothermally altered and mineralised to some extent. The presently defined porphyry alteration–mineralisation system extends over 5.5 km in a northwesterly direction, is up to 3 km wide, and has been traced by drilling to a depth of just over 1.6 km at Cadia East. Parts of the system are ‘stacked up’ by reverse faults related to the Wongalong fault system, but insufficient work has been completed to reconstruct the original configuration of the porphyry system with any degree of certainty. Cadia East is thought to represent a shallower level of the porphyry system and is totally hosted in the volcanic wall rock to the intrusive porphyry. In relatively simple terms there are five components to the Cadia porphyry system - distal skarns at Big and Little Cadia, volcanic wall rock–hosted disseminated and sheeted vein mineralisation at Cadia East, intrusion- and volcanic wall rockhosted sheeted vein mineralisation at Cadia Hill, intrusionhosted sheeted vein mineralisation at Cadia Quarry, and probable late-stage distal veins.
ORE DEPOSIT FEATURES CADIA HILL Cadia Hill is the fault-bounded western portion of the deposit and is delimited by low angle reverse faults or thrusts on the west and east, and by a steep normal fault on the north. It is unconstrained to the south, and contains the greater part of the presently defined resource. Mineralisation is mostly hosted by the porphyritic monzonite (porphyry) phase of the Cadia intrusive. Mineralogically the porphyry ranges in composition from granodiorite to monzogranite, although it is variably hydrothermally altered. The unaltered quartz monzonite is interpreted to have consisted of plagioclase, orthoclase, quartz, biotite, hornblende, pyroxene, magnetite, apatite and zircon. Narrow syenogranite aplite dykes less than 20 cm wide and small diffuse ‘felsic’ segregations of orthoclase and quartz occur within the porphyry. ‘Monzodiorite’ which is typically equigranular to slightly porphyritic in texture is present in chilled margins,
Geology of Australian and Papua New Guinean Mineral Deposits
dykes and apparently less evolved edges to the porphyry. Mafic intrusive phases of the composite stock are exposed immediately to the west of the western thrust boundary, and include ‘diorite’ of diorite to monzodiorite composition, and gabbro. Intruded wall rocks to the porphyry are part of the Forest Reefs Volcanics and occur on the eastern and southern side as major roof pendants. The volcanics here are dominated by lavas and subvolcanic intrusions. Post-intrusion and postmineral faulting have destroyed the original intrusive stock configuration. All rock types at Cadia Hill are propylitically altered with chlorite, epidote, calcite and sphene the common alteration products. The intensity of alteration across the deposit ranges from low where primary mafic minerals are partly preserved, to strong, where sericite and albite replace primary plagioclase and potassium feldspar. Fine hematite dusting of feldspar is common, especially in the porphyry, and is responsible for the characteristic reddening of the rock, which clearly post-dates the propylitic alteration. Significant potassic alteration has not been identified, although narrow potassium feldspar selvages are developed marginal to some quartz veins. Late-stage structurallycontrolled alteration is phyllic in character (sericite, clay or illite, pyrite and quartz), and is restricted to fill and selvage to some faults. Very late-stage carbonate alteration occurs as siderite or ankerite in association with calcite and quartzannealed faults and fractures. Pervasive alteration appears to be a single event, with overprinting alteration related to later faulting and fracturing. Texture destruction is rarely strongly developed, and there is no discernible increase in the strength of alteration associated with the higher grade mineralisation. Mineralisation at Cadia Hill consists predominantly of copper sulphides and gold within and disseminated about sheeted quartz veins, and occupies a broad package 100 m to 350 m wide and more than 1 km long, which is unconfined at the depths tested to date. The package has a general dip of 65o to the SW (Fig 4) and is mostly hosted by the porphyry, but locally extends into the volcanic wall rock. The term ‘sheeted’ has been used to describe the veins because of their subparallel nature and consistent orientation. The boundary of the vein package is diffuse and the core of the package is not necessarily marked by a greater density of veining, but the density is typically more consistent in the core. Individual veins are semicontinuous over tens of metres, tabular and of reasonably even width, but locally may be undulatory; the veins rarely subdivide or develop spurs and offshoots. Crosscutting veins are rare. Most veins are 1–20 mm thick, with a vein density of 2–5 per metre. They rarely occupy more than 5% of the rock mass. Vein development was probably episodic and multiphase, and appears to be have occurred within a consistent stress regime. Post-vein brecciation and faulting have significantly disrupted some portions of the vein package. Quartz dominates the vein mineralogy, with chlorite and calcite commonly centrally located within the veins and lining internal fractures. Associated minerals include potassium feldspar, sericite, chlorite, epidote and hematite. The quartz is typically drusy and crystalline, with its c axis orthogonal to the vein margin. It is typically unstrained, suggesting passive open space filling. In the vein centre the quartz can be anhedral and sutured along fractures. Only a small percentage of the quartz veins have a visible selvage, with potassium feldspar the most abundant selvage mineral.
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NEWCREST MINING STAFF
FIG 4 - Schematic cross section showing grade and mineral zoning, Cadia Hill, looking NW.
The dominant sulphide minerals are chalcopyrite, pyrite and bornite, with subordinate chalcocite and digenite. Sulphide content is low, averaging less than 1% overall. Within the sheeted quartz veins the sulphides typically occupy the centre of the vein, where they fill voids within the veins and line fractures cutting the veins. Disseminated sulphide mineralisation occurs marginal to the veins and scattered irregularly through the intervening wall rock. Sulphide mineral species are broadly zoned within the sheeted vein package as shown on the schematic section (Fig 4), progressing with increasing depth and from east to west across the package from bornite through chalcopyrite to pyrite (with, or replaced by, magnetite). The absence of a direct relationship between gold and copper grades and also with sulphide species is reflected by the different patterns of distribution for gold equivalent values and sulphide zones (Fig 4). Gold equivalent values were calculated using 1% copper = 1.8 g/t gold.
CADIA EAST The Cadia East mineralisation extends in a southeasterly direction from Cadia Hill for about 1.5 km. Disseminated and sheeted quartz vein gold-copper mineralisation is hosted by a sequence of moderately to strongly altered lavas and volcaniclastic breccias of the Forest Reefs Volcanics. A significant host is a 200–300 m thick unit of volcaniclastic breccia sandwiched between porphyritic volcanics. Rare brachiopod fossils indicate a marine depositional environment. In places the mineralisation is obscured by up to 200 m of postmineralisation Silurian sediment (Fig 5).
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Hydrothermal alteration has a vertical and lateral zoning with four assemblages, which in order of increasing intensity are: 1.
weak propylitic;
2.
weak sericite-silica-albite;
3.
moderate to strong grey silica-albite flooding with hematite and potassium feldspar; and
4.
strong sericite-silica-albite with grey silica-albite flooding±tourmaline.
Alteration intensity tends to weaken with depth. Strong to very strong alteration to the east is extensive and transgresses rock types. Alteration zoning at shallower levels tends to have a flatter geometry due to selective alteration of more permeable rocks. Gold mineralisation is localised around a core of steeply-dipping sheeted quartz-calcite±chalcopyrite±bornite± molybdenite±covellite±pyrite±magnetite veins within an envelope of disseminated chalcopyrite, bornite and pyrite which is more extensively developed at shallower levels within the deposit. The mineralised sheeted quartz veins post-date the strongest alteration. In the western part of the body disseminated copper-rich gold mineralisation dominates at shallower levels as a discrete interval above the sheeted vein mineralisation. The relative timing of the disseminated and vein-controlled mineralisation has yet to be convincingly established.
Geology of Australian and Papua New Guinean Mineral Deposits
CADIA GOLD-COPPER DEPOSIT
FIG 5 - Schematic cross section through Cadia East, looking NW.
Using a 0.3 g/t gold equivalent contour the mineralised shell occupies a steeply dipping zone up to 300 m wide which has been traced to a depth of 1.6 km in one part. Higher grade mineralisation (+1.0 g/t gold equivalent) is confined to the sheeted vein envelope. The distribution of the ore minerals is broadly concentric around the interpreted core of the alteration-mineralisation system and essentially reflects the geometry of the zones of strong and very strong alteration, and the sheeted vein envelope. The observed lateral and vertical mineral zoning comprises: Margin or top -
pyrite chalcopyrite±pyrite±magnetite chalcopyrite-molybdenite±pyrite± magnetite±gold
Centre or bottom - bornite-chalcopyrite-molybdenite± covellite±pyrite±magnetite±gold The distribution of pyrite is broader than that of all the other sulphides and mirrors the intensity of alteration. Chalcopyrite distribution reflects that of the sheeted vein envelope and at shallower levels extends outwards as a disseminated zone independent of the vein envelope. Magnetite is generally more abundant within the more weakly altered and deeper levels of the system, where it is both disseminated and vein controlled.
Geology of Australian and Papua New Guinean Mineral Deposits
Mineralisation at Cadia East is dislocated by at least two major faults. The west-dipping Gibb fault, with a reverse displacement of at least 300 m, separates Cadia Hill from Cadia East. A second reverse fault 1 km to the east of the Gibb fault displaces mineralisation by more than 100 m, with an oblique component. Late-stage faulting with focussed very strong phyllic alteration developed during the waning stages of the mineralisation.
CADIA QUARRY Immediately to the west of Cadia Hill and the Cadiangullong Creek splay of the Wongalong fault system (Fig 3), low grade sheeted quartz vein mineralisation extends for about 1 km in a package up to 200 m wide. Mineralisation is hosted by a variety of propylitically-altered rocks including microdiorite, tonalite, hornblende-biotite diorite, monzonite and quartz monzonite porphyry. The area is structurally complex with the intrusive phases strongly crushed and brecciated, with a dominant northwesterly trend to fractures and minor faults. Superimposed on this fabric are several faults related to the Wongalong fault system which have dislocated the intrusionmineralisation envelope. Widespread structurally-controlled phyllic alteration overprints the pervasive propylitic alteration and is closely associated with mineralisation, the bulk of which occurs in sheeted quartz veins.
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SKARNS The low grade copper-gold deposits at Big and Little Cadia are stratabound hematite-magnetite skarn bodies developed in the Forest Reefs Volcanics (Welsh, 1975). Skarn at both sites is localised within volcaniclastic sandstone and is developed most strongly within impure limestone interbeds in the sandstone. The hematite-magnetite zone at Big Cadia is up to 70 m thick and overlain by volcanic conglomerate which is strongly altered to an essentially propylitic assemblage of chlorite, epidote, sericite, quartz, albite and carbonate minerals. At Little Cadia the overlying volcaniclastic particles are angular and poorly sorted, but are similarly altered. Higher grade mineralisation occurs within the hematite-magnetite skarn, but can also occur within epidote-hematite-silica alteration envelopes which are most strongly developed above the hematite-magnetite zone at Big Cadia, and below it at Little Cadia. Where best developed, the hematite-magnetite skarns comprise solid masses of intergrown, fine to very coarse grained, bladed hematite, with interstitial calcite±chlorite±pyrite or chalcopyrite. Magnetite partially replaces the bladed hematite. On the margin of the hematitemagnetite zone, bands of fine grained magnetite or hematite form thin interbeds in the sandstone. Chlorite is the principal silicate mineral in the hematite-magnetite zone. Epidote and vein quartz are absent, and only minor carbonate veining occurs. Chalcopyrite is the dominant sulphide and, with pyrite and calcite, occupies interstices between hematite-magnetite blades. Within the epidote-hematite-silica alteration, mineralisation occurs as finely disseminated chalcopyrite, closely associated with epidote, but more commonly as chalcopyrite slugs with pyrite and chlorite in crosscutting quartz-calcite veins.
DISCUSSION Hypogene alteration and mineralisation at Cadia are believed to be the products of late magmatic–early hydrothermal exsolutions from crystallisation of either deeper levels of the presently exposed monzonite porphyry intrusion or of an unrecognised later intrusion. Passage of these fluids through crystalline monzonite porphyry and volcanic wall rock was largely facilitated by regional-scale fracturing with a strong northwesterly orientation. Hydrothermal alteration effects vary in intensity and mineral assemblage across the deposit. Selectively pervasive propylitic alteration is a district-wide effect. On a local scale, the marked contrast between the strong pervasive, predominantly phyllic style alteration in the upper parts of Cadia East and the less intense, predominantly propylitic with very weak potassic alteration at Cadia Hill seems to imply exposure of different levels within the porphyry system. This is supported by evidence of westward-dipping reverse faulting which has elevated Cadia Hill at least several hundreds of metres relative to Cadia East. On this basis it is speculated that Cadia East represents a shallower level than Cadia Hill within a large porphyry system. This interpretation, however, does not necessarily fit
646
comfortably with the observed patterns of sulphide distribution and gold-copper-molybdenum ratios, and a plausible alternative could be that they represent separate hydrothermal events. In the absence of direct evidence linking the observed alteration-mineralisation with cooling and crystallisation of the presently exposed monzonite porphyry, the style of mineralisation at Cadia is described as wall rock porphyry, in keeping with the simple twofold division of Titley (1972) into intrusion and wall rock types. The monzonite porphyry, along with the intruded volcanics, are considered to be wall rock to a deeper crystallising intrusion (or intrusions). Much more detailed work is required to confirm or refute this speculation.
ACKNOWLEDGEMENTS The authors gratefully acknowledge the permission of Newcrest Mining Limited to publish this paper and wish to acknowledge the support of the Board and Executive of the company, which played an important role in discovery of the deposit. This paper was originally prepared and presented at the AMF conference on Porphyry Related Copper and Gold Deposits of the Asia Pacific held in Cairns on 12–14 August 1996. The paper is the product of work by the following geoscientists over the past several years: M Baker, K Braund, A Butt, R Clark, P Creenaune, P Dunham, G Eastwood, P Harris, S Hayward, M Hazelton, J Holliday, M Hope, W Hughes, M Job, G Johansen, F Leckie, C McIntosh, C McMillan, M Nimmo, A Offenberg, D Pearson, C McIntosh, B Perry, J Rayner, P Russell, J Shelley, B Taylor, I Tedder, D Wood, A Woodgate, P Wright, and D Mason of Mason Geoscience.
REFERENCES Cas, R A F, 1983. A Review of the Palaeogeographic and Tectonic Development of the Palaeozoic Lachlan Fold Belt of South Eastern Australia, Geological Society of Australia Special Publication No 10. Powell, C McA, 1984. Ordovician to earliest Silurian: marginal sea and island arc, in Phanerozoic Earth History of Australia (Ed: J Veevers), pp 290–340 (Oxford University Press: Oxford). Titley, S R, 1972. Some geological criteria applicable to the search for southwestern North American porphyry copper deposits, MMIJAIME Joint Meeting, Tokyo, May 24–27, Preprint GI2. Welsh, T C, 1975. Cadia copper-gold deposits, in Economic Geology of Australia and Papua New Guinea, Vol 1 Metals (Ed: C L Knight), pp 711–716 (The Australasian Institute of Mining and Metallurgy: Melbourne). Wood, D G and Holliday, J R, 1995. Discovery of the Cadia gold/copper deposit in New South Wales - by refocussing the results of previous work, in New Generation Gold Mines: Case Histories of Discovery, pp 11.1–11.10 (Australian Mineral Foundation: Adelaide). Wyborn, D, 1992. The tectonic significance of Ordovician magmatism in the eastern Lachlan Fold Belt, Tectonophysics, 214:177–192. Wyborn, D, 1994. Sulphur-undersaturated magmatism: a key factor in generating magma-related copper-gold deposits, AGSO Research Newsletter, November 1994, 21:7–8.
Geology of Australian and Papua New Guinean Mineral Deposits
Mason, A J, Teakle, M and Blampain, P A, 1998. Heavy mineral sand deposits, central Murray Basin, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 647–650 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Heavy mineral sand deposits, central Murray Basin 1
2
by A J Mason , M Teakle and P A Blampain INTRODUCTION The Murray Basin project is a joint venture between RZM Pty Ltd and Aberfoyle Resources Ltd (ARL), with the primary aim of developing a long term titanium-zirconium minerals operation in the central Murray Basin of NSW and Vic. The joint venture is currently exploring an area of about 25 000 km2 in both states. In total there are 44 Exploration Licences, 17 Exploration Licence Applications and two Mining Lease Applications (Fig 1) which straddle the Anabranch (SI 54–7), Pooncarie (SI 54–8), Mildura (SI 54–11) and Balranald (SI 54–12) 1:250 000 scale map sheets. The Parilla and Loxton sands of the Murray Basin have been a target of mineral sands exploration programs for the past 25 years. Deposits within the basin differ from the traditional east and west coast deposit morphology, particularly in the presence of a finer grained offshore style of deposition that is not commonly associated with current coastal shoreline mining operations. Mineral sand exploration by ARL in the Murray Basin commenced in 1986 and has defined a series of classical beach or strandline deposits within the Pliocene Loxton Sands. In 1995 RZM entered into a joint venture with ARL and assumed the role of manager.
3
To date the joint venture has investigated three potentially economic development projects, Spring Hill and Twelve Mile in NSW and Wemen in Vic, whose identified Mineral Resources are given in Table 1. The NSW projects consist of a number of subparallel beach deposits. Additionally 20 beach deposits with indications of possible economic mineralisation have been located in Vic and NSW. Figure 1 shows the distribution of all known beach deposits in the central Murray Basin. Areas of fine grained mineralisation similar to CRA’s WIM deposits near Horsham, Vic, have also been identified.
EXPLORATION HISTORY Exploration for mineral sands in the Murray Basin commenced in the early 1970s by Reef Oil NL and Austiex who discovered beach strandline deposits at the Gredgwin and Tyrell ridges. A multicommodity exploration program of the Murray Basin was commenced by CRA Exploration Pty Limited in 1979 and resulted in the discovery of the WIM 150 deposit in 1984 (Williams, 1990).
1.
Chief Geologist, RZM Pty Ltd, 6B Hynes Court, Mildura Vic 3500.
2.
Supervising Geologist, Aberfoyle Resources Limited, 1 Altona Street, West Perth WA 6005.
ARL conducted reconnaissance exploration in the Murray Basin from 1986 to 1994. Part of the exploration was carried out in joint ventures with Balmoral Resources (later Sandhurst Mining NL) and CRA Exploration Pty Limited. In NSW the exploration became focussed on two well-defined topographic highs, the Neckarboo and Iona ridges in the late 1980s, following discovery of coarse grained mineralisation. The possible southern extension of these areas, the Robinvale Ridge in Vic was investigated in late 1995, resulting in the Wemen discovery.
3.
Project Geologist, RZM Pty Ltd, 6B Hynes Court, Mildura Vic 3500.
Various geophysical methods have been used in exploration (radar, magnetic, gravity) to help narrow the targets, however
TABLE 1 Identified Mineral Resources of heavy mineral sands, central Murray Basin, at 1% heavy mineral cutoff, June 1997. Project and deposit name
Resource category
Sand (Mt)
Heavy mineral %
Measured1
9.16
5.0
Indicated2
10.0
3.8
Inferred2
1
3.0
Spring Hill–Jacks Tank Sth
2
Inferred
41
Spring Hill–Jacks Tank Nth
Inferred2
13
Twelve Mile–Birthday Gift
Inferred2
61
Wemen
1.
Based on a 160 x 20 m drilling pattern.
2.
Based on a drilling pattern of 320 x 40 m with some closer spaced drilling.
Geology of Australian and Papua New Guinean Mineral Deposits
Heavy mineral content Rutile %
Zircon %
Ilmenite %
28
12
44
2.6
21
15
55
1.9
11
31
50
3.6
19
11
49
Leucoxene %
8
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A J MASON, M TEAKLE and P A BLAMPAIN
FIG 1 - Location map for heavy mineral deposits of the central Murray Basin and the RZM-Aberfoyle joint venture area.
they have generally been hampered by significant clays and possible maghemite (?) which lies above the host sequence. The most successful exploration method to date has been reconnaissance reverse circulation drilling for stratigraphic determination.
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REGIONAL GEOLOGY The following description is from Brown and Stephenson (1991). The Murray Basin is a low-lying saucer-shaped basin defined by flat lying Cainozoic sediment which extends over an
Geology of Australian and Papua New Guinean Mineral Deposits
HEAVY MINERAL SAND DEPOSITS, CENTRAL MURRAY BASIN
area of 320 000 km2 in NSW, Vic and SA. Cainozoic sediment unconformably overlies Proterozoic and Lower Palaeozoic basement rocks over much of the basin. Rocks of Devonian to Cretaceous age fill restricted infrabasins and occur as thin erosional remnants of platform cover. The Cainozoic sedimentary blanket is generally less than 200–300 m thick but may be up to 600 m thick in the deeper west-central part of the basin. The Tertiary succession accumulated over three major depositional events, each separated by a disconformity and all involving marine sedimentation in the centre and SE of the basin. Only those sediments of the third depositional sequence are of any importance in the present exploration for mineral sand deposits (Fig 2)
The Loxton Sands is characterised by its fine to medium grained, and pale grey to yellow sands, containing minor clay and silt, in which the sand grains are subangular to rounded. Shepparton Formation deposition continued into the Early Pliocene, and it forms the ground surface of the basin in much of the area east and SE of Fig 1. The fluvio-lacustrine Blanchetown Clay was deposited over a wide area in the west at the same time. Throughout most of the Murray Basin, but particularly in the centre and the west, the Tertiary and Early Pleistocene sediments are almost entirely concealed beneath a veneer of Late Pleistocene to Early Holocene unconsolidated sediment formed in an arid to semi-arid environment. The most extensive deposits are aeolian sands of the Woorinen Formation, and the Molineaux and Lowan Sands. There are also numerous small and widely distributed occurrences of saline lake deposits, lacustrine clay, aeolian dunes, calcrete, colluvial deposits and alluvium of relict and active river channels.
ORE DEPOSIT FEATURES A disconformity between the Loxton and overlying Parilla sequence has been identified in the joint venture drilling, which shows a near horizontal contact defined by a change in sediment type. The heavy mineral sand deposits are predominantly within the Pliocene Loxton Sands. Local reworking of the mineralisation occurs at the base of the fluvial Parilla sequence, although not in significant amounts. Within the basin there are two types of mineral sand deposits:
FIG 2 - Diagrammatic stratigraphic cross section of the Tertiary of the Murray Basin, modified from Brown and Stephenson (1991).
The third sequence, from Upper Miocene to Pliocene, is 0 to 250 m thick and formed in an environment of fluvial flood plain to the east, flanking an extensive marine strandplain of prograding beach ridges with inter-ridge fluvial and estuarine quartz sand deposits to the west and south. Initial marine transgression resulted in deposition of marine clay of the Bookpurnong beds, and marginal marine to fluvial clastics of the Loxton and equivalent Parilla sands. To the east, fluvial and lacustrine deposits of the contemporaneous Calivil Formation formed a widespread sand sheet. The subsequent Pliocene marine regression resulted in the westward migration of the shoreline and the widespread distribution of the quartz sand sheet of the Loxton Sands, characterised by the development of extensive shoreline dune ridges. In the east, sand and gravel of the Shepparton Formation were deposited in a flood plain environment. In differentiating between the Loxton and Parilla sands the authors have used the South Australian subdivision terminology recently adopted by the NSW DMR (Cameron, 1996), which describes the Loxton Sands as marginal marine, beach and estuarine sediments, and the Parilla Sands as sediment of valley fill, lagoonal and fluvio-lacustrine environments. Victorian authors have traditionally termed these the lower and upper units of the Parilla Sands.
Geology of Australian and Papua New Guinean Mineral Deposits
1.
fine to very fine grained (38 to 80 µm diameter) offshore deposits of WIM type; and
2.
medium to coarse grained (90 to 300 µm diameter) beach deposits.
The joint venture has defined a series of 30 subparallel beach deposits, with a 320 o trend. The deposits are 200 to 1000 m wide, and up to 10 km long (Fig 1). They may be relatively simple with only one beach deposit, as at Wemen, or may contain multiple beach deposits as at Jacks Tank. Each beach deposit contains a number of closely related strandlines. The deposits were formed by the regression of the shoreline towards the SW, with the heavy minerals being concentrated by the standard processes of long shore drift and wave action. The mineralogy of the strandlines and the quality of the rutile and zircon are comparable with currently producing east coast deposits. The heavy mineral assemblage varies within and between strandlines, but is generally in the range of 15–30% rutile, 8–31% zircon and 40–60% ilmenite. The ilmenite is generally of lower quality than current WA products, with a higher chromium content. RZM is confident that an upgrading process can be devised to produce a saleable ilmenite product. Heavy mineral sand deposits within the central part of the basin form lenses, which are generally flat lying to shallowly dipping toward the SW, beneath 2 to 50 m of cover, and contain varying coarse-grained heavy mineral assemblages. The offshore heavy mineral deposits like WIM 150 appear to show a very flat sheet-like geometry, and are dominated by very fine grained heavy minerals and very fine grained micaceous silt. Heavy mineral grain sizes for some of the beach deposits and offshore material in the central Murray Basin are compared with the fine grained WIM150 material and a typical east coast distribution curve from Tomago in Fig 3.
649
A J MASON, M TEAKLE and P A BLAMPAIN
Conditions of continued sediment supply, stable sea levels and low to moderate subsidence rates produced prograding sequences of beach facies within the basin. A combination of these events as well as steady climatic conditions has resulted in the deposition of strandlines within the basin which become progressively younger in a southwesterly direction. The heavy minerals are thought to be derived from the Eastern Highlands and transported by the palaeo-Darling, Lachlan and Murray rivers into the Basin and progressively concentrated along palaeoshorelines.
ACKNOWLEDGEMENTS
FIG 3 - Heavy mineral grain size analysis for Murray Basin deposits. WIM 150 data from Williams (1990).
The grain size of the strandline mineralisation allows current separation and processing techniques to be used in the treatment of this type of deposit.
ORE GENESIS Heavy mineral sand deposits of the central Murray Basin have resulted from a combination of progradational wavedominated shoreline sequences and regressive shorelines, which occurred during the Late Tertiary, and are now concealed beneath Quaternary cover.
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The authors wish to thank the management of RZM and Aberfoyle Resources for permission to publish this paper, and all the staff involved in the assembly of the information.
REFERENCES Brown, C M and Stephenson, A E, 1991. Geology of the Murray Basin, Southeastern Australia, Bureau of Mineral Resources Geology and Geophysics Bulletin 235. Cameron, R G, 1996. Pooncarie, New South Wales - 1:250 000 geological series, Geological Survey of New South Wales, Explanatory Notes, SI 54–8. Williams, V A, 1990. WIM 150 detrital heavy mineral deposit, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1609–1614 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Geology of Australian and Papua New Guinean Mineral Deposits
Martin,A R, 1998. Hillview vermiculite deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 651–654 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Hillview vermiculite deposit by A R Martin
1
INTRODUCTION
EXPLORATION HISTORY
The deposit is in central NSW at lat 32o17′S, long 147ο20′E on the Narromine (SI 53–3) 1:250 000 scale and the Tottenham (8333) 1:100 000 scale map sheets (Fig 1). It is 3 km SW of Tottenham and about 100 km west of Dubbo, and wholly owned by Helix Resources NL (Helix).
Vermiculite at Hillview was first recognised by Helix during exploration for platinum group metals (PGM). The intrusion became one of the main focuses of PGM exploration when in 1988 early rotary air blast (RAB) drilling of covered magnetic targets intersected 16 m of laterite-hosted platinum mineralisation averaging 4.3 g/t, overlying olivine–rich ultramafic rocks. Subsequent drilling found further mineralisation but the higher grade zones are not thought to be of sufficient size to be of economic significance. Several of the holes drilled during this early exploration intersected vermiculite-rich laterite surrounding the PGM mineralisation. Exploration for vermiculite commenced at Hillview in early 1991 and during the next two years 309 RAB holes were drilled for a total of 14 600 m. Initial work involved drilling the weathered biotite-rich pyroxenite on 50 by 50 m centres in an area of about 0.5 km2. This resulted in the definition of a large near surface vermiculite deposit covering about half the area of the intrusion and included a high grade (coarse grained) zone on the western margin of the intrusion which was subsequently drilled on a 20 by 25 m spacing. The drill hole data were sufficient for estimation of a Measured Resource of 2.3 Mt of 33% vermiculite in the high grade zone within an Inferred Resource of 12.4 Mt at 32% vermiculite for the whole intrusion, at a cutoff of 20% vermiculite.
REGIONAL GEOLOGY The Hillview intrusion is one of 12 Alaskan-type intrusive complexes whose areas are from <1 to about 120 km2, in the 180 km long northerly trending Fifield belt (Fig 1).
FIG 1 - Location and regional geological map of the Fifield belt of Alaskan-type intrusive complexes (after Suppel and Barron, 1986).
The deposit has an Inferred Resource of 12.4 Mt grading 32% vermiculite (Helix, 1991), within the top 40 m of the laterite profile overlying biotite-rich pyroxenite of the Hillview intrusive complex. The vermiculite is contaminant free and has high exfoliation properties and this, with the large tonnage, makes the Hillview deposit one of the more significant vermiculite deposits in the world from a commercial perspective. The deposit is not currently being mined pending more favourable marketing conditions.
1.
Exploration Manager, Helix Resources NL, PO Box 825, West Perth WA 6872.
Geology of Australian and Papua New Guinean Mineral Deposits
The complexes intrude a sedimentary flysch sequence, the Girilambone Group, which marks a period of deposition in a marginal sea to the west of the major Molong Volcanic Arc during Late Cambrian to Ordovician time (Suppel and Scheibner, 1990). The sedimentary sequence, at least in part, forms the basement on which the dominantly shoshonitic volcanic rocks of the Molong Volcanic Arc were deposited during the Ordovician. The Fifield complexes are probably related to the basin forming processes east of a major crustal lineament known as the Gilmore Suture. The suture dips east in this area and was probably reactivated as a late stage detachment fault in the Ordovician, with the intrusions emplaced from upwelled asthenosphere during the subsequent uplift. Alaskan-type intrusive complexes are described in some detail in Elliott and Martin (1991). The main general features of these types of intrusions are: 1.
They are generally circular or elliptical, with a crude concentric zoning, in plan view. The ultramafic portions
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of the intrusions generally comprise one or more dunite cores concentrically surrounded by wehrlite, olivine clinopyroxenite and clinopyroxenite zones. The ultramafic portions are in turn usually within a much larger intermediate–mafic intrusion. 2.
Diopsidic augite (clinopyroxene) is the dominant mineral in the ultramafic rocks; orthopyroxene is normally rare or absent. The olivine is forsteritic and magnetite occurs as a primary crystallising phase, reaching 20% of the clinopyroxenite content in places.
3.
Hornblendite or hornblende-rich rocks and biotite-rich rocks occur near the margins as a result of assimilation of country rock.
4.
The intrusions occur in major elongate belts.
The intrusions in the Fifield belt are consistent with these general features but they also show some important differences: 1.
2.
Biotite and potassium feldspar are far more abundant than in the typical Alaskan-type intrusions described above, indicating a more potassium-rich magma. The ultramafic portions of the intrusions are typically totally surrounded by intermediate or mafic intrusive rocks, whereas at Hillview there are no surrounding intermediate–mafic intrusions and at several other intrusions the ultramafic rocks occur on the margins. Emplacement of the ultramafic intrusives at the contact with the country rock was probably one of the important factors leading to the high biotite and subsequently high vermiculite content of the Hillview intrusion.
The topography of the region is flat to gently undulating with slight depressions over many of the intrusive complexes. Outcrop of the intrusions is very rare. Soil, eluvium, sheet wash and buried palaeochannel deposits conceal most rock types.
ORE DEPOSIT FEATURES LITHOLOGY There is no outcrop in the Hillview region hence all geological data have been obtained by drilling. The Hillview intrusion is one of the smallest in the belt and occurs as a satellite intrusion to the NE of the much larger Bulbodney Creek Complex (Fig 1). It probably represents part of the ultramafic portion of the main intrusion which was emplaced outside the primary conduit system. Hillview is a NNE-trending, 1000 by 600 m elliptical intrusion (Fig 2). It contains several small olivine-rich ultramafic zones surrounded by an extensive body of biotiterich pyroxenite which makes up 70% of the area of the intrusion. The margin of the intrusion is marked by a well developed marginal series of hornblendite and hornblende-rich clinopyroxenite. The clinopyroxenite at Hillview has an unusually high biotite content, averaging approximately 20% throughout the intrusion. In other intrusions in the Fifield belt biotite-rich clinopyroxenites are only found in small areas immediately surrounding wehrlites or adjacent to marginal series rock types. The biotite, which has a slight golden colour, has a total FeO content of 9% which suggests it is very near the biotitephlogopite compositional boundary.
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FIG 2 - Geological plan of the Hillview intrusive complex.
PETROGENESIS OF THE HILLVIEW INTRUSIVE COMPLEX A model for the emplacement of Alaskan-type intrusions in the Fifield belt, encompassing multiple intrusion and fractional crystallisation, is summarised in Elliott and Martin (1991). The model is based mostly on detailed observations at the Owendale intrusion. The magma which formed the ultramafic rocks in the Hillview intrusion was probably a potassium-rich alkali olivine basalt. Crystal settling in a basal magma chamber and the loss of crystals during ascent resulted in a differentiated magma which was intruded into the Hillview and Bulbodney Creek intrusions to form clinopyroxenite with minor primary olivine and magnetite. During quiescent periods between injection, crystallisation of the in situ magma proceeded from the margin towards the centre of conduits resulting in a weakly defined vertical stratification. The development of the marginal series hornblende-rich rocks and an adjacent zone of biotite-rich pyroxenite resulted from the assimilation of the country rock. The magma eventually reached plagioclase saturation by continued crystallisation within the pipe or underlying magma chamber. Episodic pulses of the more fractionated magma then intruded to form the diorites at the Bulbodney Creek intrusion. At about the same time a more siliceous magma was intruded, resulting in the emplacement of monzonites in the larger intrusions. The discordant olivine-rich bodies and the surrounding biotite-rich haloes within the pyroxenite of the Hillview intrusion are interpreted to be the result of late stage magmatic alteration produced by addition of water to the melt. With this increase in water fugacity the clinopyroxenite was resorbed to form the more olivine-rich bodies and the surrounding biotiterich zones. The formation of the secondary olivine-rich rocks by this mechanism is probably related to the intrusion of a differentiated monzonitic magma at depth (Fig 3). The PGM mineralisation is closely related to the late stage magmatic alteration episode.
Geology of Australian and Papua New Guinean Mineral Deposits
HILLVIEW VERMICULITE DEPOSIT
Post-lateritic alluvial quartz-rich gravels and clays dissect and overlie the laterite across the entire Hillview intrusion. The gravel varies in thickness from less than 4 m on the western side of the intrusion to over 20 m along the eastern boundary.
MINERALISATION The vermiculite occurs mostly within the saprolite and weathered basement zones of the laterite profile, directly overlying the biotite-rich clinopyroxenite, and formed by weathering of the biotite (Figs 4 and 5).
FIG 3 - Schematic section showing genetic relationships of the Hillview intrusive complex.
Although the two biotite-forming processes are evident at Hillview, the small size of the intrusion and the concentration of high grade (coarse grained) vermiculite adjacent to the best developed marginal series rocks suggests that the addition of water by assimilation of country rock is the more important factor.
WEATHERING The late Palaeozoic and Mesozoic in the region were dominated by deep erosion which exposed the intrusive complexes. A period of intense weathering during the Tertiary resulted in the development of a thick laterite profile over many of the complexes. The profile generally has a well developed ferruginous zone overlying partially developed mottled, pallid and saprolite zones. The present thickness of the laterite profiles varies from 10 to 80 m. The Tertiary weathering profile at Hillview is 40 to 60 m thick and is characterised by an upper ferruginous zone of red, occasionally pisolitic, partly silicified iron-rich clays. Ferricrete is not seen at Hillview and this layer may have been eroded. The red ferruginous zone is underlain by a mottled ferruginous zone composed of mottled brown, red, yellow, grey and green clays. Below this is a saprolitic zone which grades into weathered bedrock. Where it overlies biotite-rich clinopyroxenite the saprolite is a pale, apple green colour and composed of fine unconsolidated clay and vermiculite, whereas where it overlies more olivine-rich units the saprolitic clay is partly silicified and a much darker colour, generally dark green to black. The thickness of the ferruginous and mottled portion of the laterite varies over different rock types. It is generally much thicker over olivine-rich units with a corresponding reduction in the thickness of saprolite over clinopyroxenite. Along the western side of the intrusion the mottled and ferruginous zones overlying biotite pyroxenites have been completely removed.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 4 - Mineralisation outlines at Hillview.
The vermiculite-rich saprolite occurs as a laterally continuous zone with its top between 20 and 40 m below surface over a large portion of the intrusion. In the high grade zone on the western side of the intrusion the vermiculite zone thickens considerably, and it can be as shallow as 4 m and as deep as 60 m below surface. The vermiculite occurs as flakes which vary considerably in size. On the western side of the intrusion there is a north trending zone approximately 20 m wide of very coarse grained vermiculite where typically 40% of the vermiculite flakes are over 1 mm wide. This zone may represent a synmagmatic fault along which hydrothermal alteration of clinopyroxenite occurred. Throughout most of the remainder of the intrusion the vermiculite flakes are smaller, with the majority between 0.25 and 1.0 mm wide. In the mottled and ferruginous zones of the laterite profile the vermiculite rapidly degrades and breaks down to clays.
ORE GENESIS The laterite profile developed as a result of the loss of major elements such as silicon, calcium, magnesium, sodium and potassium during weathering. This involved a breakdown of the primary igneous minerals olivine, pyroxene and hornblende to clays, under progressively more oxidising conditions in the weathered bedrock and saprolite horizons. The biotite did not break down but was altered to vermiculite.
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FIG 5 - Cross section on local grid line 4900 N, looking NW, showing geology and mineralisation.
The vermiculitisation of biotite occurs by a slight rearrangement of the atoms within the lattice and by the replacement of interlayered K+ ions by other cations, generally Mg2+, or Mg2+ plus Ca2+ by means of base exchange. The exchange process is accomplished by the introduction of varying amounts of water molecules between the lattice layers, the amount being determined by the kind of exchangeable ion present. Oxidation of Fe2+ to Fe3+ accompanies hydration, with partial replacement of Si4+ in the silicon-oxygen tetrahedron (Hoadley, 1960). At Hillview the progressive oxidation is marked by a change in vermiculite colour from green-grey at depth to a yellow-brown higher in the profile. Although the colour variation indicates some changes in composition down the profile, exfoliation tests have shown that all of the biotite is hydrated to similar levels.
MINING AND PROCESSING Extensive scoping studies based on production of approximately 30 000 tpa of beneficiated vermiculite indicate that mining and processing of the Hillview vermiculite would be relatively straightforward. Mining would be by open cut methods utilising excavators and trucks. Blasting would not be required and ore grade control could mostly be based on visual inspection. The ore would be washed in a drum scrubber to remove the clay, leaving wet vermiculite and rock particles which can be separated in two ways. In the first method, vermiculite and rock particles would be dried and screened then passed through an air winnower. Vermiculite of high quality can be produced by this method, but high recovery rates are not easily achieved for the finer grained vermiculite. The second beneficiation method is froth flotation, which works very well for the finer flake sizes but is not as well suited to flake sizes above 1 mm.
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Each of the processes produce a crude +95% vermiculite product in five size ranges. Recovery rates in pilot plant and bench scale tests vary dramatically from 60 to 90% depending on the size fractions and beneficiation method used. It is likely that the final process selected for Hillview ore will be a combination of air winnowing and froth flotation.
ACKNOWLEDGEMENTS The author wishes to thank Helix Resources NL for permission to publish this paper. It is the result of the contribution of numerous Helix geologists over many years of work at Hillview and in the Fifield region.
REFERENCES Elliott, S J and Martin, A R 1991. Geology and mineralisation of the Fifield Platinum Province, New South Wales, in Sixth International Platinum Symposium, Guidebook for the PreSymposium Field Excursion (Eds: S J Elliott and A R Martin), pp 4–23 (Geological Society of Australia: Sydney). Helix, 1991. Sixth Annual Report (Helix Resources NL: Perth). Hoadley, J N, 1960. Mica deposits of Canada, Geological Survey of Canada, Economic Geology Series, No 19. Suppel, D W and Barron, L M, 1986. Platinum in basic to ultrabasic intrusive complexes at Fifield: a preliminary report, Geological Survey of New South Wales, Quarterly Notes, 65:1–8. Suppel, D W and Scheibner, E, 1990. Lachlan Fold belt in New South Wales – regional geology and mineral deposits, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1321–1327 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Geology of Australian and Papua New Guinean Mineral Deposits
Diemar, V A, 1998. Thuddungra magnesite deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 655–660 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Thuddungra magnesite deposits by V A Diemar
1
INTRODUCTION The deposits are centred on lat 34o10′S, long 148o03′E near Thuddungra, NSW, on the Cootamundra (SI 55–11) 1:250 000 scale map sheet. They are 300 km WSW of Sydney and 40 km NW of Young (Fig 1).
1985 successive exploration programs have greatly increased the known resource of the original Mine deposit at Thuddungra and located the Noakes and Baileys deposits of similar high grade material. The Mine and Noakes deposits contain a Measured Resource of 17.1 Mt of magnesite ore containing 42.2 wt % of cryptocrystalline magnesite and Baileys contains 25 Mt in the Inferred Resource category. Recent annual mine production has averaged 182 000 t of run-of-mine (ROM) ore which is processed to give 35 000 tpa of beneficiated magnesite kiln feed from which 20 000 tpa of caustic calcined magnesium oxide is produced, mainly for the agricultural industry.
EXPLORATION AND MINING HISTORY Magnesite was discovered at Thuddungra in 1933 and mined between1937 and 1962 by small scale operators gouging thick dense irregular masses and veins within serpentinite under thin soil cover. They produced about 360 000 t of raw magnesite for use in the agricultural industry and in refractory products for the Port Kembla steel works and foundries. In 1952 YMC acquired and amalgamated the majority of the leases and installed the Causmag calcining facility in Young (J V M Giuliano and A E Jenkins, unpublished data, 1981). Magnesite within veins generally has fewer impurities than magnesite within stratiform deposits but the difficulty of separating magnesite veins from the enclosing silicate host and their small dimensions commonly makes vein deposits uneconomic. At Thuddungra the vein deposit type has not been mined since 1972.
FIG 1 - Locality and regional geological map (after Warren, Gilligan and Raphael, 1996).
Three exploration licences and 12 mining leases covering the area are owned by Orind Australia Pty Ltd (Orind) through its mining subsidiary Young Mining Company Pty Ltd (YMC). Calcining and marketing of processed ore is carried out in Young by another subsidiary, Causmag Ore Co Pty Ltd (Causmag). The deposits are stratiform chemical accumulations of magnesite overprinted by diagenetic and weathering events within Tertiary fluvio-lacustrine sediment. Thuddungra magnesite is mainly magnesium carbonate with varying but minor amounts of silica, calcium, iron and aluminium. Since 1.
Senior Geologist, Orind Australia, 37 Ayres Road, St Ives NSW 2075.
Geology of Australian and Papua New Guinean Mineral Deposits
In 1972 Devex Limited (Devex) acquired YMC and Causmag, by which time the scale of mining and processing had gradually increased, as the westward progression of the mine encountered more uniform stratiform accumulations of magnesite in fluvio-lacustrine sediment which allowed higher and more uniform annual production. The deposit was owned and operated by Devex from 1972 to June 1997 when both YMC and Causmag were sold to Orind. Systematic exploration commenced in 1985 and by 1987 the Mine deposit was defined by rotary air blast (RAB) grid drilling. Large diameter (100 mm) core drilling was trialled without success. Later infill drilling used reverse circulation air coring (RCAC) techniques and comprehensive bulk testing using 0.75 m diameter Calweld drill holes. A total of 359 holes have been drilled including 288 RAB holes, 44 RC holes and 27 Calweld holes, over an area of 1000 by 600 m. At the western end of the deposit drill hole spacing is generally 100 m but decreases to 50 m to the east closer to the mine face. Prior to mining, drill hole spacing is decreased to 25 m. Noakes deposit, a repetition of the Main deposit, was discovered in 1987–88. It is along channel to the NW in the same palaeodrainage system and is underlain by SiluroDevonian sediment (Fig 2). It was defined by over 200 holes
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The ROM ore is a blend of magnesite nodules of variable lump size in a consolidated clay matrix. During mining the low grade sections and those heavily contaminated with surficial replacement dolomite are rejected. Ore processing is based on maximising the grade of the final product by producing clean, sized magnesite as kiln feed. Oversize material (>150 mm) is crushed and minus 150 mm ore is then screened at 100, 75, 50 and 18 mm to provide graded stockpiles for kiln feed. The minus 6 mm undersize consisting dominantly of gangue is returned to the pit as backfill (Devex Limited, unpublished data, 1991) Wet scrubbed, screened magnesite is photometrically or hand sorted, depending on size, to remove gangue and lowgrade magnesite. Beneficiated magnesite is calcined in either of two kilns at Young to supply a variety of magnesia grades to domestic and international markets. Post-calcination removal of high silica kiln dust by dry screening and of magnetic iron-rich contaminants by magnetic separation, provides final upgrading of the product to produce middling and high grade magnesia. The high chemical quality of the deposits and large resource gives potential for increased production and different processing to produce a range of higher value saleable products.
RESOURCE ESTIMATES
FIG 2 - Geological setting of Mine and Noakes deposits.
A modified polygonal method of resource estimation was applied using all drill holes. Results are in Table 1. No estimate of ore reserves was attempted as recovery factors are variable depending on the end use of the material, the kiln used, the required grade of finished product and because a visual estimate for percentage magnesite in each metre logged must be used as no empirical method exists for its measurement. Resource estimates are made for total magnesite and hard magnesite only.
including 55 RAB holes, 129 RCAC holes and 6 Calweld holes, drilled in an area more than 2.5 km long and 275 m in average width. Baileys deposit, 6 km north of the Mine deposit, was found in 1989. The discovery resulted from a regional exploration drilling program testing sites geologically similar to the earlier discoveries, in an area of about 12 km2. The magnesite formed within a thick basinal lacustrine sequence of Tertiary clay and sand and has the potential to contain a number of deposits of similar magnitude to Noakes. A total of 152 holes broadly defined the extent and continuity of mineralisation at Baileys deposit. Drill hole spacing is 500 m with a 200 m spacing in the SE quadrant of the area. Drilling has been the only successful exploration tool at Thuddungra, with a total of 871 holes completed.
PRODUCTION DETAILS Production from the Mine deposit since the 1930s has exceeded 1 Mt of raw magnesite. Open cut mining methods are employed at Thuddungra involving topsoil conservation, overburden relocation, ore extraction and processing and rehabilitation. After removal of overburden at an average rate of 215 000 tpa, ROM magnesite ore is mined in three benches and transported to a semi-mobile dry screening plant for upgrading.
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TABLE 1 Magnesite resources at Thuddungra at June 1997. Deposit
Resource category
Ore (Mt)
Contained magnesite (wt%)
Mine Noakes
Measured
4.1
47.7
Measured
13.1
40.1
1
41.9
Total Baileys
17.1 Inferred
25
1. Tonnages rounded.
Bulk densities used were 2.38 t/m3 for the hard magnesite, 2.0 for soft magnesite and 1.6 for gangue. Factors relating to the resources at the Mine and Noakes deposits are compared in Table 2. Resources are sufficient to allow mining at the current rate for more than 50 years. The chemical qualities of each of the deposits are comparable. The average grade of calcined product for recoverable magnesite from the Mine deposit was 97.8% magnesium oxide which represents 26% of the ROM ore. A higher quality product of 98.5% magnesium oxide or better can be achieved by processing various proportions and combinations of ore from the three mined benches. Calcined
Geology of Australian and Papua New Guinean Mineral Deposits
THUDDUNGRA MAGNESITE DEPOSITS
TABLE 2 Comparison of parameters for Mine and Noakes deposits. Parameter
Mine
Noakes
Av overburden thickness (m)
11.9
16.7
Av magnesite thickness (m)
7.1
8.4
Overburden: ore ratio - thickness based
1.68
1.99
- volume based
1.69
1.97
- tonnage based
1.44
1.73
% total magnesite by volume
38.7
31.7
% hard magnesite by volume
31.8
25.8
Average length between holes (m)
35.7
97.6
Average width between holes (m)
36.6
96.9
Bulk density of overburden
1.6
1.6
Bulk density of ore horizon
1.88
1.82
Bank volume of overburden (Mm3)
3.659
14.106
Bank volume of ore horizon (Mm3)
2.166
7.157
Mass overburden (Mt)
5.854
22.571
Mass of ore horizon (Mt)
4.063
13.060
Weight % total magnesite
47.7
40.1
Weight % hard magnesite
40.4
33.7
Thuddungra magnesite generally contains 97–98% MgO, 1.03–1.84% SiO2, 0.56–1.28% CaO, 0.02–0.17% Fe2O3 and 0.08–0.18% Al2O3, after removing the loss on ignition,
REGIONAL GEOLOGY In the Thuddungra area the Cambro-Ordovician Jindalee Group forms a steeply to vertically dipping NNE-trending belt 3 to 5 km wide (Fig 1), separated on its eastern side from the Late Silurian Young Granodiorite by the Thuddungra Fault, an extension of the Mooney Mooney Fault, a major regional thrust fault. The Jindalee Group was derived from a marine sequence of mafic and ultramafic igneous and sedimentary rocks within the Tumut Trough which was metamorphosed during the Ordovician Delamerian Orogeny (Warren, Gilligan and Raphael, 1995). The thrust is linear and subvertical and strikes about 030o true. Proximal to the thrust the foliation is more pronounced with local occurrences of mylonite along its inferred trace. The Young Granodiorite is a massive to weakly foliated coarse grained biotite granodiorite. The western boundary of the Jindalee Group appears to be faulted against Siluro–Devonian marine sediment exposed as steeply dipping foliated metasiltstone and micaceous phyllite. Locally the Palaeozoic rocks are overlain by Tertiary and Quaternary sequences including lateritic weathering profiles, fluvial and lacustrine sediments, eluvial deposits and soil.
ORE DEPOSIT FEATURES GEOLOGY The mafic Jindalee Group outcrops as a low ridge immediately east of the mine (Fig 2). It consists of a disrupted suite of ophiolites which were metamorphosed to biotite grade greenschist facies, which was followed by at least two episodes
Geology of Australian and Papua New Guinean Mineral Deposits
of penetrative foliation development unpublished data, 1989).
(P M Ashley,
Three types of serpentinite are recognisable. Much of the central spine of the ridge in a belt 4 km long by up to 500 m wide is a dark grey, massive to brecciated and locally schistose antigorite serpentinite grading into amphibole-rich serpentinite and metamorphosed pyroxenite. In places the serpentinite is silicified and ferruginous, and represents the preserved upper part of a lateritic weathering profile which grades downwards into saprolitic and magnesitic-rich zones above less weathered serpentinite. Amphibolites, probably metamorphosed hornblende gabbro, outcrop in a continuous belt 6 km long by 900 m wide along the northern half of the Thuddungra ridge. They range from coarse to fine grained, massive to foliated and amphibole- to feldsparrich. Two belts of massive to foliated chert separated by metabasalt occur on the western side of the ridge. The chert is interbedded with phyllite and is locally manganiferous. The southern half of the ridge contains foliated and fine to medium grained metabasalt.
MAGNESITE MINERALISATION Magnesite at Thuddungra occurs in three geological environments. It occurs as pockets of cryptocrystalline veins within serpentinite host rocks. Magnesite of the Mine and Noakes deposits occurs west of the serpentinite ridge within a Tertiary palaeochannel as a stratiform fluvio-lacustrine extension of the serpentinite-hosted vein deposits. Baileys to the north occurs in a lacustrine sediment (Schmid, 1987) within a thick Tertiary sequence of clay and sand. The composition of the carbonates within the magnesite deposits reflects the provenance of the source components and the drainage patterns developed at their time of formation. The major magnesium source was brecciated serpentinite, immediately east of the Mine deposit, with additional magnesium and calcium derived from amphibolite. The formation of the serpentinite–hosted deposits and portions of the stream channel–hosted deposits occurred simultaneously. At a later stage calcium was introduced causing the deposition of dolomite, as a minor replacement of magnesite, principally at the top of the horizon but also more extensively along major watercourses affecting the eastern side of the Noakes deposit. Ore horizon thickness is uniform at Noakes whereas overburden thickness increases in a northwesterly direction due to an increased palaeoslope of the original stream channel. The Mine deposit ore horizon consists of scree slope deposits grading from coarse lag magnesite to a magnesite-serpentinite conglomerate modified and enlarged by circulating magnesium-rich ground water. It incorporates both horizontal and vertical components of chemically precipitated magnesite with associated detritus, all modified by slumping and distortion during consolidation, dewatering and dehydration. The magnesite has a variety of forms including rounded or ‘cauliflower’ structures, load fluting, cone-in-cone and desiccation mosaics (Fig 3). The hardness and bulk density of the magnesite vary widely. Much is exceptionally dense and hard, with a conchoidal fracture and hardness to 5 on the Mohs scale. The harder the magnesite the lower the porosity. These features are functions
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FIG 3 - Typical cross section of an open pit mine face, Mine deposit, not to vertical scale.
of the degree of lithification at the time of formation (P F Howard, unpublished data, 1989). The harder magnesite was dried and lithified rapidly by aerial exposure during seasonal temperature variations either due to proximity to the surface or to open spaces created by structures. The basement at the Mine deposit and the southern portion of Noakes is weathered serpentinite (Fig 3). The northwestern portion of Noakes is underlain by Siluro–Devonian sediment (Fig 1). The matrix to the magnesite in the Mine deposit and to a lesser extent the southern portion of Noakes is derived from the serpentinite and consists of claystone, sandy claystone, grit and conglomerate, the last containing an appreciable component of eroded laterite. The gangue in the remainder of Noakes consists of paler coloured sandy clay derived from the Devonian sediment. The clays are dioctohedral smectites (P F Howard, unpublished data, 1991, 1992) which constitute approximately 20% of the Mine deposit and consist of nontronite, beidellite and montmorillonite in estimated proportions of 70:25:5, with varying but minor amounts of kaolinite. Magnesite was deposited as beds in shallow water, and as vertical columns within magnesium bicarbonate saturated clays. In the latter case the magnesite formed as nodules or vertically growing columns as the lake sediment compacted and dehydrated. Slump folds and shears developed as a result of expanding clays and gravity sliding during wet periods. Desiccation of the magnesite resulted in a decrease in volume. Shears, joints, slickensides and shrinkage cracks provided channelways along which circulating ground water promoted alteration of magnesite and the clays.
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The overburden on the Mine magnesite is a laterite-derived stream sediment comprising (from surface): 1.
a 1 to 2 m layer of red-brown clayey soil containing vertical tubes of dolomite;
2.
a unit of blue-green clays containing post-depositional interstitial carbonate (mainly dolomitic) concretions and crosscutting veins;
3.
finely laminated light brown siltstone; and
4.
a basal layer of rounded nodules of basic rocks and minor fine grained silicified magnesite (Fig 3).
ORE GENESIS MODELS AND CONTROLS The Thuddungra magnesite occurs as stratabound and vein deposits. International classification methods for cryptocrystalline magnesite ore types are mainly based on deposit type, crystal structure, texture and geological environment. Stratabound deposits are mostly chemical sedimentary rocks. Clastic deposits are generally of lower grade. Stratabound chemical deposits range from stratiform authigenic beds as at Mine and Noakes to irregular replacements of limestone or dolostone. Lacustrine magnesite is commonly attributed to either hydrothermal alteration or weathering influx of magnesium from nearby ultramafic rocks (Abu-Jaber and Kimberley, 1992), as at Baileys. The source of magnesium in ultramafic-hosted veins is the serpentinite but the carbon source remains uncertain. Carbon isotopic ratios for vein magnesite have not been determined at
Geology of Australian and Papua New Guinean Mineral Deposits
THUDDUNGRA MAGNESITE DEPOSITS
Thuddungra but it is assumed that the carbon has been metamorphically or near surface derived (high percentage of C12) rather than from an inorganic atmospheric or magmatic–volcanic source (with a low C12 content). Factors affecting the origin of a magnesite orebody include fluid source, mode of supply and cause of precipitation. Magnesite deposits are commonly a combination of stratabound chemical and clastic types. They form by erosion of a serpentinite-hosted vein deposit as is the case with the Mine and Noakes deposits. Preferential erosion of serpentinite and other phyllosilicates from the vein magnesite deposit resulted in a lag of magnesite on top of the vein deposit due to the greater specific gravity and hardness of magnesite. Eventual erosion of the magnesite lag may produce a stratabound-clastic deposit. Chemical precipitation of further magnesite can greatly enhance the size of the deposit. Alteration of serpentinite to magnesite results in phyllosilicate byproducts including sepiolite, palygorskite, nontronite (iron-rich smectite) and montmorillonite. At Thuddungra nontronite and sepiolite are gangue material (P F Howard, unpublished data, 1991). Dolomite occurs in many magnesite vein–related deposits (Frost and Matzat, 1984). The Thuddungra vein deposits contain no dolomite, however in the Mine and Noakes deposits, where paragenesis of carbonate precipitation is apparent, magnesite deposition is followed by dolomite. Calcium carbonate may follow either as calcite or aragonite but neither phase has been recognised at Thuddungra The lack of talc in magnesite vein deposits at Thuddungra can be interpreted to indicate either a temperature of formation of less than 300oC or precipitation from a fluid which contained less than 4% carbon dioxide. In vein deposits the lower temperature scenario is probable. The magnesium sources for vein deposits are magnesiumrich minerals within serpentinite such as serpentine, olivine, pyroxene or brucite. Magnesium-rich pyroxene is resistant to carbon dioxide–rich fluids and brucite produces an iron-rich magnesite, therefore serpentinite and olivine are the most likely sources at Thuddungra. Veins are formed in a cool near-surface environment. Magnesite precipitation is consistently enhanced by increasing carbon dioxide. As at Margarita Island in Venezuela the preferred magnesite-precipitating reaction involves alteration of serpentinite and magnetite by aqueous carbon dioxide to produce nontronite and magnesite thus: -
12Mg3Si2O5(OH)4 (serpentine) + 36HCO3 + 4Fe3O4 + O2→ -
12FeSi2O5(OH) (nontronite) + 36MgCO3(magnesite) + 35OH + 18H2O (Abu-Jaber and Kimberley, 1992)
Precipitation of magnesite in near surface veins is also attributable to a rise in pH as carbon dioxide is released from a fluid ascending into a lower pressure zone. The pH probably rises into the bicarbonate or carbonate stability field prior to initiation of magnesite precipitation. Near-surface faulting enhances ground water permeability within the serpentinised rock resulting in magnesite precipitation. Vein magnesite tends to be very fine grained, reflecting a swift deposition and high degree of supersaturation in the carbon dioxide–rich fluid. Variation in fluid magnesium concentration appears to have less effect on precipitation than carbon dioxide concentration.
Geology of Australian and Papua New Guinean Mineral Deposits
MINE GEOLOGICAL METHODS Drill hole data are used to calculate the ROM resource quantity, quality and recovery factors in each strip and to determine ore horizon geometry and mining sequence. Reconciliations between resource estimates and mining recoveries are used to refine mine planning, resource estimation and forward resource utilisation. Successive mine faces are mapped. Exploration results and mine production are reconciled and visual quality control is applied in the pit during ore extraction. Logging techniques have been refined to a standard procedure considered to give the most accurate and consistent result. Logging is carried out over one metre intervals. Samples are washed in sieves to remove adhering dust particles to allow more accurate visual determination of the components. When magnesite is present the general particle size and hardness are noted and used to determine the percentage of hard and soft magnesite for calculation of total and recoverable magnesite. Inclusions and coatings on the magnesite are noted to help determine the quality of the magnesite and the degree of dolomite replacement. It is relatively easy to determine the total quantity of magnesite in the ore horizon. However, due to the extreme local variability in magnesium content, hardness and surficial replacement, the determination of recoverable magnesite of a particular quality is difficult and relies to a high degree on the estimator’s experience with the orebody and in mining reconciliations. From the work done in the Mine deposit it has been possible to establish that: 1.
the total tonnage mined broadly corresponds to predicted tonnage from drill hole data;
2.
the upper 25% of the orebody contains a higher proportion of undesirable contaminants such as dolomite than the lower 75%;
3.
the percentage of soft magnesite that will not survive beneficiation increases with depth; and
4.
a direct lump size: grade relationship exists in the ore with <18 mm lumps calcining to <96.5% calcined magnesia grade and >50 mm lumps calcining to >98.5% magnesia grade (Devex Limited, unpublished data, 1991).
At Thuddungra, recoverable magnesite calculations based on visual estimates of hard magnesite in drilling correlate well with mine recoveries of +18 mm magnesite. Calcined magnesia produced from particular fractions of the three horizons in the orebody results in product grade ranges of 96.5 to 97.0%, 97.0 to 98.0%, 98.0 to 98.5%, 98.5 to 98.8%, and greater than 98.8% magnesium oxide. Greater than 98.8% grade can be produced from the calcined non-magnetic fraction of >75 mm ore derived from the lower and middle horizons (Fig 3). The successively less pure grades are produced from various combinations of smaller initial lump sizes from the three ore horizons.
ACKNOWLEDGEMENTS The author wishes to thank Devex Limited, the former owner of the Thuddungra magnesite deposits, and the current owners Orind Australia Pty Ltd for allowing the publication of this paper.
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REFERENCES Abu-Jaber, N S and Kimberley, M M, 1992. Origin of ultramafichosted vein magnesite deposits, Ore Geology Reviews, 7:155–191. Frost, M T and Matzat, H W, 1984. A further large magnesite deposit along the Savage River in northwestern Tasmania, Economic Geology, 79:404–408.
Warren, A Y E, Gilligan, L B and Raphael, N M. 1995 Cootamundra, NSW-1:250 000 geological series, Geological Survey of New South Wales Explanatory Notes, SI 55–11. Warren, A Y E, Gilligan, L B and Raphael, N M. 1996. Cootamundra 1:250 000 geological sheet SI 55–11, Geological Survey of New South Wales.
Schmid, I H, 1987. Turkey’s Salda Lake, A genetic model for Australia’s newly discovered magnesite deposits, Industrial Minerals, 239:19–31.
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Birch, J S, 1998. Atric gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 663–668 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Atric gold deposit by J S Birch
1
INTRODUCTION
EXPLORATION HISTORY
The deposit is 160 km WNW of Cairns, in north Queensland (Fig 1) at AMG coordinates 218 350 E, 8 171 550 N or lat 16o31′S, long 144o22′E, on the Mossman (SE 55–01) 1:250 000 scale and the Bellevue (7764) 1:100 000 scale map sheets. Title is held by Pan Australian Resources N L (95%) and N F Stuart (5%) under Exploration Permit for Minerals (EPM) 8689.
The first record of gold in the Hodgkinson Province was the report of payable alluvial gold in the Palmer River in September 1873 by James Venture Mulligan who later discovered the Hodgkinson Goldfield SE of Mount Mulligan. Although the boom was over by 1879 small scale mining has continued intermittently until the present day. The most recent compilation of total recorded bullion and alluvial gold production from the Hodgkinson Province, to 1995, excluding Red Dome near Chillagoe, is 54.4 t of which auriferous quartz reefs contributed 19.8 t (P Garrad, personal communication, 1996). Much of the early production was probably unrecorded. The biggest quartz reef mine, the Tyrconnel in the Hodgkinson Goldfield, yielded 1.8 t (Dash and Cranfield, 1993).
FIG 1 - Location of Atric and the Hodgkinson Province, showing simplified structural framework, modified from Geological Survey of Queensland (1975).
The Atric mineralisation is in a major shear zone in the western part of the Hodgkinson Province. The gold occurs in sulphides in altered metasediment, whereas the historic Hodgkinson mines exploited auriferous sulphides in quartz veins. There are similarities to some of the Tregoora deposits some 21 km to the SE (N F Stuart, personal communication, 1996; L W Davis, personal communication, 1997). The Inferred Resource to 25 m below present drilling, which is RL 325 or 175 m below the creek, using a 0.5 g/t lower cutoff grade, is 860 000 t at 2.2 g/t gold for 1.9 t or 61 000 oz of contained gold. Using a 1.0 g/t lower cutoff to 50 m depth and a 1.5 g/t cutoff below 50 m, the Inferred Resource is 523 000 t at 3.1 g/t for 1.6 t or 52 000 oz of contained gold (R C Pyper, unpublished data, 1995). Mineralisation is open at depth and to the south.
1.
Consultant, Jenny Birch Geological Services, 6 O’Connell Parade, Wellington Point Qld 4160.
Geology of Australian and Papua New Guinean Mineral Deposits
Modern base metal exploration began in the late 1960s, boosted by a short lived antimony boom. By 1980 gold was of interest and in 1983 Tenneco Oil and Minerals Inc reported an anomalous arsenic value in a -80 mesh stream sediment sample collected downstream of Atric. Their criterion for target selection was two or more anomalous elements so there was no follow up (W T Saunders, personal communication, 1996). In 1986 N F Stuart, exploring Authority to Prospect (A to P) 4603 M for Hawk Investments Limited, instigated bulk leach extractable gold (BLEG) drainage sampling. A bulk stream sediment sample from Bellevue East creek, near the junction with the Mitchell River, contained 5.9 ppb cyanide soluble gold. During 1987 the author traced the anomaly by more detailed bulk stream sediment sampling to the tenement boundary, where the BLEG value was 27.5 ppb (Fig 2). The adjacent area was still under application by another party when the project was sold to BHP in 1988. In October 1989 the author and C R Birch, carrying out a reconnaissance program in A to P 7124M for Axis Mining NL, found the outcrop of the Atric mineralisation. Grab and rock chip samples of the partly exposed shear zone, over intervals up to 10 m, contained an average of 0.5 g/t gold. The maximum arsenic value was 2350 ppm but the low antimony, at only 12 ppm maximum, was encouraging from the metallurgical aspect. It was clear that the shear was a major feature with potential to host a large deposit. However the Hodgkinson Province was not popular at that time, and no further work ensued. In 1992 interest in the area revived, and EPM 8689 was granted to N F Stuart. D C O'Neill inspected the area, collected surface rock samples with higher gold values and negotiated a joint venture between N F Stuart and Bruce Resources NL (now Pan Australian Resources NL). Systematic exploration began in August when four mattock trenches were dug and channel sampled. The best value was 21 m at 3.56 g/t gold, open to the east. At that time, more BLEG stream sediment sampling over a wider area gave further anomalies in what became known as the Bellevue East anomalous area. Exploration of this area continued, including drilling at eight of the anomalies, five of which are shown on Fig 2.
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The two 1994 RC programs brought the Atric total to 41 holes for 4150 m. Resource estimates and further metallurgical studies were completed during 1995–96. There is no published information on Atric or Bellevue East. All work to-date has been described in unpublished company reports to the Queensland Department of Mines and Energy (DME). These reports, which are still confidential, include as appendices all consultants reports.
GEOLOGICAL SETTING HODGKINSON PROVINCE The Atric deposit is in the SW part of the structurally complex, Palaeozoic Hodgkinson Province (Fig 1) which forms the northernmost part of the Tasman Fold Belt. In the 1980s knowledge of the geological history of this province was greatly increased by an extended study carried out by the Geological Survey of Queensland (GSQ). This outline is based on information in Bultitude et al (1993, 1996), and the maps compiled by Bultitude, Donchak and Domagala (1992), Bultitude, Mackenzie and Roberts (1995), and Bultitude, Mackenzie and Hill (1996).
FIG 2 - Geological sketch plan of the Bellevue East area showing the location of Atric and five of the Bellvie East prospects; BEsz, Bellevue East shear zone; GVF, Groganville Fault. Geology modified from Bultitude, Donchak, and Domagala (1992), and Bultitude, Mackenzie and Roberts (1995).
In 1993 costeans at Atric suggested that the surface strike length was at least 140 m, with the best result 40 m at 3.36 g/t. Drilling began in November with 12 reverse circulation (RC) holes for 837 m completed. The best result was 32 m, from 52 m to 84 m, assaying 4.29 g/t gold in drill hole BEP18 which terminated in mineralisation. The final 16 m averaged 7.43 g/t gold, with 8696 ppm arsenic and 27 ppm antimony. During the drilling a discussion arose in camp regarding the average age of the various team members who had contributed to the discovery and the fact that the zone was first recognised by a geologist using a walking stick. The word geriatric seemed appropriate and was later shortened to Atric. Exploration continued throughout 1994, and included initial metallurgical work, drilling of cored holes, ongoing petrological and structural studies, trial magnetometer and scintillometer traverses and a gradient array and dipole–dipole induced polarisation (IP) survey. The IP data showed some detail of the shear zone as well as a conductivity anomaly just east of the known mineralisation but subsequent soil sampling and drilling have not explained this anomaly. However lithological and structural information from two holes totalling 400 m of oriented NQ and PQ diamond core gave helpful insights into the characteristics and controls of mineralisation.
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The ages of formations in the province range from Early Ordovician to at least Late Devonian. Most formations are of very limited extent and restricted to the western part of the province. The most extensive unit in the west is the Chillagoe Formation of limestone, sediment and volcanic rock, generally basalt. However, the province is dominated by the Devonian Hodgkinson Formation which was mainly deposited by turbidity currents, and GSQ geologists favour a back arc setting. The predominant rock types are greywacke and mudstone with minor conglomerate, chert, metabasalt and rare limestone. The monotonous rock types, the paucity of fossils, the very limited number of marker horizons, and the extensive tectonic disruption, make subdivision of the formation virtually impossible. Most members and informal units have been identified according to the dominant rock types in particular areas. Sedimentation was halted by an extensive east-directed thrust faulting event. This was followed by a NNW-trending thrusting event, then by upright folding, and finally by locally developed north-south compression. Carbonate-sericite alteration associated with shearing is widespread. A particular feature of the Hodgkinson Formation is the widespread occurrence of a rock type informally termed ‘broken formation’ in DME publications (after Hsu, 1968). According to Raymond (1984), this is ‘dismembered formation’, ie ‘bodies of rock mappable at a scale of 1:24 000 or smaller that both lack continuity of internal contacts or strata and are characterised by the inclusion of native blocks and fragments of all sizes in a matrix of finer grained material’. Both clasts and groundmass are derived from the host formation and there is no exotic material. In the Hodgkinson Formation this texture is observable at all scales from microscopic to macroscopic.
BELLEVUE EAST AREA The main rock types in this area are greywacke, siltstone and shale with very minor chert, conglomerate, tuffaceous sediment, limestone and basalt. Carbon, mainly as
Geology of Australian and Papua New Guinean Mineral Deposits
ATRIC GOLD DEPOSIT
pyrobitumen, is reported in all fresh rocks examined, the quantity increasing with decreasing grain size. Framboidal pyrite is always associated with the carbon. Carbonate, generally remobilised, and sericite are also ubiquitous. Up to 20% iron carbonate is reported in greywacke but it is not possible to determine from work to date whether it originated from diagenetic or later alteration. Metamorphism to lower greenschist grade is directly related to the intensity of shearing. The greywackes range from quartz-rich to feldspathic. They contain detrital rutile, zircon, muscovite, tourmaline and rutilated quartz implying a granitic provenance, at least in part. Lithic fragments, commonly sericitised, include quartzite, carbonaceous siltstone, chert, sericitic phyllite, acid volcanic and possibly tuffaceous material. Rare detrital flakes of highly anisotropic graphite are reported from thin sections where the bulk of the carbon is pyrobitumen. The structural framework of the Bellevue East area is outlined in Fig 2. The Groganville Fault (GVF) is a major northerly-trending fault mapped for a distance of more than 36 km. Various combinations of gold, antimony, arsenic and mercury mineralisation are associated with this structure. About 3 to 5 km SSE of Atric it is dislocated by NW-trending cross faults including the Bellevue East shear zone (BEsz) which trends NNW and is visible on satellite images for at least 30 km. At Atric there is an abrupt transition eastwards from intensely sheared to relatively unsheared rocks, whereas to the west shearing gradually decreases over a 100 to 130 m wide zone. The sharp boundary on the east side of the shear zone, which is associated with clay gouge, is referred to as the Bellevue East fault (BEf). About 3 km west of Atric and subparallel with the BEsz there is a 5 km wide belt of strongly sheared dismembered formation clearly visible on air photos and satellite images, referred to as the Big Watson Shear Zone (BWSZ) by Bultitude, Mackenzie and Roberts (1995). In the Bellevue East area the strata between the BWSZ and the BEsz are also excessively disrupted and sheared with zones of silica alteration and few discernible trend lines whereas east of the BEsz trend lines are clear and the rocks relatively undisturbed. W P Laing (unpublished data, 1993) therefore considers the BEsz to be the eastern boundary of the BWSZ. All gold prospects located to date are in the belt between the BWSZ, as mapped, and the BEf on the east side of the BEsz. The sudden change in structural style, the asymmetric nature of the BEsz, and the results of detailed structural mapping suggest that the fault is a late structure which has juxtaposed the BWSZ package of rocks, including the Bellevue East belt, against rocks that were not involved in the BWSZ event. Three possible marker horizons trend into the BEsz. Two on the north side are discontinuous lenses of chert and a conglomerate with limestone cobbles. To the south there is a very distinctive horizon of silicified concretions in fine grained, tuffaceous sediment generally closely associated with a grit bed. The concretions, like the other rocks petrologically examined, contain pyrobitumen, framboidal and microcrystalline pyrite and iron carbonate. Similar concretions associated with grit have been observed well to the south of the EPM (W T Saunders, personal communication, 1994). None of the three horizons has been located on the opposite side of the shear which also suggests that the BEsz is a major discontinuity.
Geology of Australian and Papua New Guinean Mineral Deposits
Several areas of stockworking and silicification occur in the area of Fig 2. The only one shown is flanked by gold mineralisation throughout and is spatially associated with Atric. It is not clear at this stage whether there is a genetic link. Some relatively minor silicification is associated with anomalous gold in the Bellevue East belt but the more spectacular stockworking in coarse grained greywacke or grit appears to be barren. A small limestone outcrop with intercalated metabasalt and adjacent boulder conglomerate, with clasts predominantly of varied mafic igneous rocks, is shown in the NE corner of Fig 2. Fossils indicate the limestone to be a similar age to the surrounding clastic sediments. There are two other limestone outcrops mapped to the west of the GVF some 16 km and 22 km south, both of which are associated with metabasalt.
MINERAL DEPOSIT FEATURES LITHOLOGY AND STRUCTURE The rock assemblage flanking the BEf to the west, including the mineralised zone, is multiply deformed dismembered formation in which greywacke fragments range from microscopic to more than 10 m across. Rock types are not disposed in an orderly manner, and can not be correlated even between adjacent drill holes. Although the expression of structural rock types is affected by the competency of components, they are somewhat more easy to correlate. Four structural rock types were recognised by W P Laing (unpublished data, 1994) in the diamond core: 1.
Banded greywacke schist (BGS) is a strongly deformed assemblage of greywacke, shale, and siltstone. It is commonly banded with greywacke boudins and the only quartz veins are tension gash type.
2.
Anastomosing ribbon-veined slate (ARVS) is a strongly foliated rock with characteristic quartz±iron carbonate ribbon veins from 10–90%, generally 50–70%. It contains about 10–20% of elongate clasts of greywacke and siltstone. A characteristic is carbon-sericite stylolites and/or selvages with high lustre, petrographically described as carbonaceous, sericitic phyllite.
3.
Graphitic anastomosing ribbon-veined slate (GARVS) is similar to ARVS but with 50–90% of phyllitic shale and less (10–30%) ribbon veins.
4.
Cataclasite, interpreted as late stage, post-dating mineralisation. Laing recognised three gradational forms: (i)
Foliated cataclasite which is remnant ARVS or GARVS with strong chlorite-sericite and illitesericite alteration. With increasing intensity of deformation this grades into
(ii)
matrix supported cataclasite comprising 60–90% compact illite-sericite-pyrobitumen matrix and equant to very elongate, subangular to subrounded vein and lithic fragments. This grades into
(iii) breccia cataclasite, an intensely milled breccia injected along veins and faults. Figure 5 shows the main cataclasite and ARVS zones on cross section. The ARVS is flanked to the west by a similar width of GARVS. BGS with lesser widths of greywacke, shale, siltstone and some clay gouge complete the section to east and west. The recognition of these structural rock types from percussion chips is unreliable.
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As well as the main cataclasite zone, smaller zones of this style of deformation and alteration can be seen in the drill core. The width and intensity vary from loggable intervals of cataclasite to a few centimetres showing incipient alteration. In less altered samples chlorite occurs rather than illite. Together with the mechanical destruction of the ARVS and GARVS, the formation of cataclasite involved chemical change, including the growth of chlorite and illite and partial dissolution of vein material. Gold and sulphides may also be variously leached. The remnant vein quartz-iron carbonate is mostly highly deformed and fragments persist down to 15 µm size. The sooty black colour of the cataclasite results from the large surface area of a relatively small amount of smeared carbon. Petrographic work shows the ribbon-vein quartz commonly carries ankerite and siderite to a maximum of 50% carbonate. There are cracks in quartz healed by quartz-iron carbonate and cracks in quartz-iron carbonate healed by quartz. The degree of deformation of vein quartz in one thin section of ARVS from diamond core, varies from showing mild undulose extinction to shattering with growth mosaic structures. Apart from the ribbon veins there are at least two other types of quartz veins, both containing iron carbonate, of which one is undeformed. Sulphides are uncommon in the veins although some veins have coatings of sericite and pyrobitumen with or without fine grained sulphide including arsenopyrite. There are always veins associated with gold-bearing intervals, but the amount of quartz-iron carbonate is not in proportion to gold content. Indeed there are many highly veined intervals with no gold values.
MINERALISATION At Atric the mineralised zone comprises numerous overlapping lenses of rapidly varying gold content which make up a continuous zone of 200 m strike length, open to the south, and maximum width about 40 m. The width reduces to 17 m using
FIG 3 - Surface plan of the Atric zone showing fact geology, costeans, drill hole collars and surface contours. Geology after ERA-Maptec.
the 0.5 g/t cutoff. The zone strikes about 316o magnetic (Fig 3) and plunges at 30o to the SE and dips steeply SW (Figs 4 and 5). Virtually the only sulphides are pyrite and arsenopyrite, predominantly very fine grained. Sulphides can be readily seen in greywacke, notably clusters of arsenopyrite needles, ±1 mm in diameter, locally called 'porcupines'. However, microscopic examination shows that sulphides favour the thin smeared
FIG 4 - Longitudinal projection of the Atric deposit, looking SW, showing all drill holes and gold intersections.
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Geology of Australian and Papua New Guinean Mineral Deposits
ATRIC GOLD DEPOSIT
There is little published information on the acicular habit of arsenopyrite. It may be caused by impurities in the lattice. Most reports of needles are from high grade gold-arsenic mineralisation, generally in black shale. The style of mineralisation at Bakyrchik, Kazakhstan, closely resembles Atric with acicular arsenopyrite common (N J W Croxford, personal communication, 1996). No microscopic free gold is seen in unoxidised material. The limited metallurgical work to date, on samples from the upper levels of the deposit, suggests that the gold is refractory. These tests suggest that the gold is intimately combined with the sulphides and shows definite but variable correlation with arsenic. The percussion drill sample assays show similar goldarsenic correlation and a weak arsenic halo a few metres wide around the anomalous gold zone. Within any mineralised intersection, there is erratic correlation between gold and the low levels of antimony (<5-50 ppm, usually <30 ppm). Although the highest gold intervals often have relatively low antimony values, gold intervals invariably have detectable antimony in the vicinity. The antimony halo tends to be wider than that of arsenic. The irregular antimony correlation with gold and arsenic is partly explained by the rock type mix, with antimony deposition promoted in the low carbon greywacke and gold-arsenic in the high carbon phyllite. The mobility of antimony in more reducing conditions is supported by the known geochemistry (C Cuff, personal communication, 1993).
FIG 5 - Cross section on 9750 N Atric grid showing diamond and RC drill holes with gold intersections and simplified geology. BEf, Bellevue East fault.
pyrobitumen and are concentrated in the phyllite. Pyrite occurs as framboids and as subhedral to euhedral zoned crystals of two main generations, Pyrite I (Py I) and Pyrite II (Py II), although up to four zones are recorded in crystals. Py I is <2 µm to 40 µm in diameter, slightly more yellow than Py II, and shows framboidal texture and/or concentric zoning. N J W Croxford (unpublished data, 1994) considers it has the characteristics of sedimentary and/or very early diagenetic pyrite found in carbonaceous shale deposited in reducing conditions. The younger Py II forms cubic crystals to 75 µm in diameter or occasionally larger; and there are some aggregates up to 2 mm across. Arsenopyrite forms both lozenge-shaped crystals from <10 µm in size up to 100 µm wide by 400 µm long, and needle-like crystals from <10 µm long up to 50 µm by 800 µm. Both forms occur together in most thin sections. The arsenopyrite crystals, particularly the needles, are frequently cracked and healed with quartz±sericite. Py I crystals and framboids are commonly nuclei for both the larger Py II and the arsenopyrite crystals. All these sulphides, Py I including framboids, Py II, and arsenopyrite lozenges and needles, contain carbon particles. N J W Croxford (unpublished data, 1994) considers that the appearance of the sulphides, their lack of immediate association with the hydrothermal quartz and iron carbonate, and the scarcity of sulphides within the vein material, suggest porphyroblastic crystal growth.
Geology of Australian and Papua New Guinean Mineral Deposits
The only other sulphides seen under the microscope are traces of sphalerite and rare tennantite from hole ADD1. No stibnite has been observed. Limited multi-element analyses of drill chips also gave low levels of other metallic elements. Base metal values were usually less than 50 ppm, except zinc which has a maximum value of 184 ppm. Apart from arsenic and antimony, elements showing positive correlation with gold are sulphur, silver, iron and perhaps lead and tellurium; probably neutral are copper, zinc, cobalt, nickel, vanadium, chromium, barium and zirconium; three elements which may show negative correlation are manganese, mercury and calcium; and those below detection were molybdenum, cadmium, bismuth and selenium. Limited silver assays suggest a ratio of silver to gold of about 1:4 or lower. Microcrystalline rutile as prisms (<5 µm diameter) or needles (up to 1 µm wide by 30 µm long) occurs in the sericitic carbonaceous phyllite. So far it has only been noted in goldbearing intervals, with up to 5 volume % estimated in a thin section from an interval which assays 14.7 g/t gold.
CONTROLS ON MINERALISATION Within the mineralised zone two factors seem to influence the deposition of gold: 1.
The ratio of greywacke to shale. The preference of sulphides and therefore gold for carbonaceous phyllite is clear but the evidence for the role of greywacke is mixed. Significant gold values occur in ARVS, GARVS and cataclasite but in the diamond drill core ARVS is the main host. Strain shadows, created during deformation by the more frequent greywacke and siltstone clasts in ARVS, compared to GARVS, may have favoured sulphide deposition in the adjacent phyllite. Within the limitations of the percussion drill hole logs, there is some
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support for ARVS as the prime host but many of the highest gold assays are from intervals free of greywacke and siltstone. Moreover, high greywacke intervals are low in gold and not all ARVS is auriferous either at Atric or elsewhere along the BEsz. There are some large (about 10 m wide) greywacke bodies which probably created large strain shadows also favourable for sulphide deposition. How many of these larger greywacke bodies there are or where they are is unknown. 2.
Folding in the shear zone. Although the dip of the shear is generally subvertical, W P Laing and T Baker (unpublished data, 1994) recognised in the diamond core a central zone of shallow dips which is about 20 to 40 m wide. This is interpreted as fold related. The dilation sites created by this folding should favour fluid flow and can be seen in the core to favour sulphide deposition.
GENESIS OF MINERALISATION Throughout the Province a series of tectonic events has mobilised various fluids, at various times, as well as controlled the channels available. These fluids have caused alteration, and deposited gold, arsenic and antimony minerals either in sulphide-sediment zones or in quartz stockworks or lodes. In the Hodgkinson Formation widespread framboidal pyrite and, in places, Py I crystals have formed during earliest diagenesis in carbonaceous shale and coarser sediment deposited in a reducing environment. Iron carbonate may also be diagenetic. Possible sources of arsenic and gold are: 1.
intra-basin basaltic volcanic activity providing either source rocks or fluids to directly enrich some sediment layers, ie, metabasalt in the Larramore Metabasalt, OK, and Kitoba members and near the GVF associated with limestone;
2.
igneous debris in the sediment load; and
3.
the volcanic rocks of the Chillagoe Formation which may underlie the Hodgkinson Formation in part. They would also be a possible carbonate source.
If the source was the intra-basin volcanism, gold and arsenic would be localised rather than dispersed throughout the sediment pile. The existence of relatively gold-rich belts in the Hodgkinson Province, one of which is spatially associated with basalt, is evidence in favour of a volcanic source. At Atric the gold is associated with sulphides, particularly arsenopyrite, rather than hydrothermal quartz. Possible local sources for the gold and arsenic are suggested by sizeable outcrops of basalt to the south, west and north, smaller outcrops of basalt along the GVF trend and the tuffaceous concretion horizon. The BWSZ indicates a nearby zone of intense fluidgenerating activity. The fracturing of arsenopyrite crystals indicates some movement after the mineralising event. The absence of detectable gold from some ARVS and most GARVS in the diamond drill core, and the arsenopyrite in coatings on veins suggests that the gold-arsenic mineralisation was deposited before the main deformation. The Atric mineralisation could be synsedimentary and deposited from volcanic hot springs. If so, it has been modified by all the tectonic events including that producing the dismembered formation. Another possibility is that mesothermal fluids generated during metamorphism became enriched in gold and arsenic from one of the sources listed above. These minerals were then deposited in a highly sulphidic
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shale before or in an early stage of the main deformation. The antimony deposition may belong to a seperate event or a seperate pulse of the event. Relatively early deposition of gold and arsenic is in accord with the findings of Mumin, Fleet and Chryssoulis (1994) on the Ashanti Gold Belt, Ghana. The two types of mineralisation they studied have many features in common with the Atric and the Hodgkinson gold-quartz styles. They suggest that most primary gold was deposited in solid solution in pyrite and arsenopyrite, and that much of this was later redistributed and reconcentrated in varying degrees including into quartz veins.
ACKNOWLEDGEMENTS The author would like to thank the management of Pan Australian Resources NL for permission to publish this paper and for access to company information, and the consultants and field staff who have contributed so much to knowledge of the area. Significant input includes structural work by W P Laing and ERA-Maptec, with additional input from T Baker, G Daneel, and G Doherty; petrology by N J W Croxford; metallurgical tests by AMDEL and Hydrometallurgical Research Laboratories; and the mineral resource estimate by R C Pyper. Their work has been invaluable in the preparation of this paper. A A Brickell, R J Bultitude, N J W Croxford, L W Davis, G Doherty, W P Laing, D E O’Neill, and G S Stafford are also thanked for their comments on the manuscript and P Loch for his drafting work.
REFERENCES Bultitude, R J, Donchak, P J T and Domagala, J, 1992. Maytown, 7765, 1:100 000 map geological series, preliminary edition, Department of Resource Industries, Queensland. Bultitude, R J, Donchak, P J T, Domagala, J and Fordham, B G, 1993. The Pre-Mesozoic stratigraphy and structure of the western Hodgkinson Province and environs, Department of Minerals and Energy, Queensland Geological Record, 1993/29. Bultitude, R J, Mackenzie, D E and Hill, R B, 1996. Mossman, SE 55–01, 1:250 000 scale geological map, second edition, Geological Survey, Department of Mines and Energy, Queensland. Bultitude, R J, Mackenzie, D E and Roberts, C W, 1995. Bellevue Region, sheet 7764 and part sheet 7864, 1:100 000 scale geological map, Geological Survey, Department of Minerals and Energy, Queensland. Bultitude, R J, Rees, I D, Garrad, P D and Champion, D C, 1996. Mossman, Queensland 1:250 000 geological series, 2nd edition, Geological Survey of Queensland, Explanatory Notes, SE 55–01. Dash, P H and Cranfield, L C, 1993. Mineral occurrences - Rumula 1:100 000 sheet area, North Queensland, Department of Minerals and Energy, Queensland Geological Record 1993/17. Geological Survey of Queensland, 1975. Queensland Geology, 1:2 500 000 scale map, Department of Mines, Brisbane. Hsu, K J, 1968. Principles of melanges and their bearing on the Francisan-Knoxville paradox, Geological Society of America Bulletin, 79:1063–1074. Mumin, A H, Fleet, M E and Chryssoulis, S L, 1994. Gold mineralisation in As-rich mesothermal gold ores of the BogosuPrestea mining district of the Ashanti Gold Belt, Ghana: remobilisation of ‘invisible’ gold, Mineralium Deposita, 29:445–460. Raymond, L A, 1984. Classification of melanges, in Special Paper 198, Melanges: Their nature, origin, and significance (Ed: L A Raymond), pp 7–20 (The Geological Society of America: Boulder).
Geology of Australian and Papua New Guinean Mineral Deposits
Nethery, J E, 1998. Anastasia gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 669–674 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Anastasia gold deposit by J E Nethery
1
INTRODUCTION
EXPLORATION HISTORY
The deposit is 50 km SSW of Chillagoe, in north Qld, at lat 17o33′S, long 144o16′E, and AMG coordinates 209 200 E, 8 056 000 N on the Atherton (SE 55–5) 1:250 000 scale and Lyndbrook (7762) 1:100 000 scale map sheets (Fig 1). The deposit is notable because it highlights the regional metallogenic association of gold with tin-tungstenmolybdenum-bismuth porphyry style mineralisation. It is one of the few high sulphidation epithermal systems to be recognised in the Palaeozoic of eastern Australia. An Indicated Resource of 390 000 t at 2.5 g/t gold using a 0.8 g/t cutoff grade was estimated to a depth of 70 m (Nethery, 1989) .
Late Palaeozoic volcanic cauldron subsidence structures and a small rhyolite volcanic vent were defined by Bureau of Mineral Resources geological mapping in 1957–1962 (Best, 1962). The volcanic features were described in more detail and named the Scardons Cauldron Subsidence Area by Branch (1966). A regional exploration joint venture between AOG Minerals Ltd and Carpentaria Exploration Company Pty Ltd (CEC) culminated in the drilling of the Galala Range tin-tungstenbismuth- molybdenum ‘greisen’, about 15 km SE of Anastasia. CEC withdrew late in 1981 when this program achieved only minor tungsten intersections. Notably, trace gold was also detected. A Climax-type porphyry model was invoked for this system after re-assessment of the metal and alteration assemblage, zoning and structural controls. This showed a circular molybdenum-bearing recessive core partly rimmed by a tungsten-bearing resistant quartz-sericite annulus, then a resistant linear radial structure with disseminated sulphides zoned from anomalously high copper and lead out to high arsenic values at the extremities. An interpretation of aerial photographs and Landsat images over the remainder of the title area was prompted by this new model. Subsequent ground checking of anomalous structural patterns resulted in the discovery of the Anastasia deposit, which comprises auriferous silica-clay-sulphide breccias and amorphous silica flooding, associated with the small rhyolite dome and hydrothermal eruption breccia, in the area previously described as a vent by Branch (J E Nethery, unpublished data, 1982). AOG Minerals continued detailed and regional exploration with partners Esso Australia Ltd (1983–1984), Freeport of Australia Pty Ltd (1985–1986), and Elders Resources Ltd (1986–1991), which resulted in a cumulative 10 000 m of drilling. Changes in company structures saw control of the project in the hands of Niugini Mining Limited and Newcrest Limited in mid 1991. The Indicated Resource did not meet the target parameters of either company, and control was passed to Centamin Limited, the current titleholders.
PREVIOUS DESCRIPTIONS Original data on the deposit spanning the period of the author’s involvement from 1980 to 1989 are contained in unpublished exploration reports to the Queensland Department of Minerals and Energy on A to Ps 2155, 3435, 3686 and 4030, and in AMIRA Project 82/P 163 reports. FIG 1 - Location map and Carboniferous–Permian volcanics, intrusives and structure, Lynd River region (after Best, 1962).
1.
Principal Geologist, Nedex Pty Ltd, 1 Eastern Street, Chillagoe Qld 4871.
Geology of Australian and Papua New Guinean Mineral Deposits
REGIONAL GEOLOGY The deposit is at the NE extremity of the Middle Carboniferous to Early Permian Scardons Volcanics cauldron subsidence, at a complexly faulted boundary between the Scardons Volcanics, McDevitt Metamorphics and Carboniferous Amber granite (Figs 1 and 2).
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domes, and is therefore interpreted as valley fill, typical of ignimbrites, blanketing the lower elevation flows. This area of disagreement remains to be resolved, and clearly more detailed mapping is required. At Anastasia eutaxitic foliation in the first phase ignimbrite dips consistently subvertically towards the NW. The steep dips were previously attributed to caldera collapse, however recently this deformation was attributed to a province-wide Middle Carboniferous folding and thrusting event postdating the O’Briens Creek Supersuite intrusives and extrusives (Nethery and Barr, 1996). From a regional perspective, rocks of this suite are strongly fractured, particularly subhorizontally, and the volcanics are folded. The second and third phase volcanics are subhorizontal except for tilting adjacent to cauldron subsidence faults. Arcuate caldera collapse and intrusion of porphyry and microgranite ring dykes followed both the first and third phase ignimbrite events. The second phase flow-domes and associated alteration and mineralisation were focussed by the collapse fractures postdating the first phase ignimbrite (Fig 2). Post-mineralisation caldera-collapse faulting followed the last ignimbrite eruption.
FIG 2 - Generalised geological map of NE Scardons Volcanics cauldron subsidence.
The Metamorphics are of probable Middle Proterozoic age and are correlated on the basis of rock type and metamorphic grade with the Robertson River Subgroup of the Etheridge Group of the Georgetown region (Withnall, 1984). They comprise quartzite, quartz-muscovite schist and amphibolite with subhorizontal bedding, schistose foliation and shearing and are crosscut by NW-trending subvertical kink fold axes. A 30 m wide subhorizontal sill of foliated amphibolite was intersected in a number of drill holes, but is not exposed. The Scardons Volcanics occupy a series of overlapping calderas exposed over an area of about 800 km2 and have a maximum thickness of 400 m. These edifices are deeply eroded with almost complete stripping of the ignimbrite sheets surrounding the calderas, and are overlain to the west and SW by Jurassic to Tertiary Carpentaria Basin sediment. Three eruption phases were mapped during exploration. The first phase, which is exposed around the erratically downfaulted edges of the cauldron and in a few deeply dissected sections, comprises minor basal andesite and trachyte flows, moderately welded quartz–potassium feldspar–biotite porphyritic rhyodacitic ignimbrite and minor volcanoclastic rocks. The second phase, a relatively quiescent event, comprises resurgent rhyolite flow-domes and minor airfall tuff, with an areal extent of less than 10 km2, and is best exposed in the vicinity of the Anastasia deposit and to the immediate NW. The third phase comprises an extensive sheet of light grey to pink, quartz–potassium feldspar porphyritic, poorly welded rhyolite ignimbrite, with a fine ash matrix. It averages less than 50 m thickness over most of the cauldron subsidence area, but occurs as remnant mesas in the NE segment of the cauldron near Anastasia. This interpretation differs from later Queensland Geological Survey mapping which places the rhyolite flowdome cycle after the final ignimbrite (Cranfield, 1992). The third phase ignimbrite is at a higher elevation than the flow-
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The Amber granite, of the Middle Carboniferous O’Briens Creek Supersuite (Champion and Heinemann, 1994), rims the SE edge of the Scardon Cauldron, where it juxtaposes the first phase volcanic rocks. This leucogranite body locally exhibits high level features such as extensive stanniferous greisen, a chilled sugary textured quartz carapace, micrographic and crenulate quartz or ‘brain’ textures, and an intrusion breccia comprising clasts of chilled carapace in a medium grained granite matrix. The occurrence of cassiterite in greisenous Amber granite and in sericitic replacement alteration in the first phase ignimbrite at Anastasia indicates that this ignimbrite predates Amber granite. The timing relationships suggest that this granite is coeval with the second phase of rhyolite flowdomes. The Amber granite is intruded nearby by the Rose Creek granite, an 8 km2 stock of molybdenitewolframite–bearing high level granite (Fig 2).
DEPOSIT GEOLOGY STRUCTURAL CONTROLS Anastasia is contained within a 2 km2 complexly step-faulted triangular wedge, confined within a SSE-trending, west-block down, arcuate, first phase ring fault to the east; a west-trending, north-block down, first phase subsidence fault to the south; and a SW-trending, west-block down, third phase subsidence fault to the west (Fig 2). The wedge is divided into a northern metamorphic segment and a southern first phase ignimbrite segment by a SW-trending fault and a colinear elongate second phase rhyolite dome (Fig 3). This dome is autobrecciated and flow banded with indications of hydrothermal and possibly magmatic venting at the NE intersection with the outer arcuate ring fault. The eastern arcuate ring fault was intruded by a second phase quartz-porphyritic microgranite ring dyke (Figs 2 and 3). The western boundary fault is marked by late-stage sheeted unmineralised chalcedony veinlets. Mineralisation is largely confined to the brecciated exodome section of the rhyolite and to the northern metamorphic segment. Mineralisation within the metamorphics occupies a zone of approximate dimensions 500 by 300 m, and is controlled by the main structures as a series of tabular quartz-
Geology of Australian and Papua New Guinean Mineral Deposits
ANASTASIA GOLD DEPOSIT
FIG 5 - Cross section B–B′, Anastasia.
FIG 3 - Interpreted geological map, Anastasia deposit, with location of cross sections, Figs 4 and 5.
sulphide fracture fill and replacement zones, with two main orientations; subvertical towards the NW (Fig 3), and subhorizontal (Figs 4 and 5). The siliceous replacement bodies selectively replace the quartz-muscovite schist, as a branching
stockwork with steeply plunging milled breccia pipes, up to 10 m diameter, at the subvertical structure intersections. The breccia pipes root at depth into several apophyses of rhyolite. Individual siliceous lenses are up to 300 m long, 10 m wide and extend to a depth of at least 120 m. At the SE end of this zone the silica flooding becomes pervasive and merges into a polymictic milled breccia on the periphery of the dome, then beyond the dome a west concave arcuate zone of sporadic silicification and brecciation, hosted by first phase ignimbrite, extends south then SW for 1000 m. A branching set of NE-trending faults crosscut the mineralised zone (Fig 3), and these offset the zones of silicification with a consistent NW block down normal throw. One fault of this set forms the western boundary of the Anastasia fault wedge, where first phase ignimbrite is juxtaposed with metamorphics and a flat sheet-like body of siliceous replacement. This fault set is unaltered except for a set of sheeted unmineralised fine fibrous and chalcedonic quartz veinlets along part of the Anastasia fault wedge boundary fault. This fault set splays off an arcuate ring fracture bounding the third phase ignimbrites, postdates the mineralising event, and is related to the second stage of cauldron subsidence. The first phase ignimbrites range in thickness from 30 to 60 m in the area immediately west of the boundary fault, hence the net westerly downthrow between the current level of exposure of the Anastasia rhyolite dome and a similar rhyolite flow-dome 1 km to the west is greater than 60 m. Near surface features in the Anastasia rhyolite plug such as flow banding, autobrecciation and milled hydrothermal eruption breccias suggest that the palaeosurface was probably less than 200 m above current exposure. Outcrop of the first phase of volcanic rocks in the Anastasia fault wedge is sparse, due to a combination of pervasive alteration and deep weathering, and this minimal outcrop shows a complex distribution of the two main ignimbrite types and common changes in the orientation of eutaxitic foliation. This indicates that considerable subsidiary block faulting and tilting occurred within the boundaries of the main fault wedge. Interpretation of outcrop mapping and ground magnetics suggests the dominant block faulting trend is 290o magnetic.
FLOW-DOME AND BRECCIAS
FIG 4 - Cross section A–A′, Anastasia.
Geology of Australian and Papua New Guinean Mineral Deposits
An elongate rhyolite dome, of the second phase of volcanism, with dimensions of 750 m by average width of 100 m, trends at 225o magnetic from the outer ring dyke on the east, bisecting
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the Anastasia fault wedge (Figs 2 and 3). Beyond the fault wedge on the extrapolated 225o trend, and also on the 290o trend of the siliceous replacement and breccia zone, several small flow-banded and autobrecciated rhyolite plugs and a flow-dome intrude and overlie the first phase ignimbrite (Fig 2). The mineralised dome comprises white flow-banded and partly autobrecciated rhyolite over much of its length, which at the NE end, adjacent to the ring dyke, grades progressively into hydrothermal shatter breccia then into a milled breccia pipe with dimensions of 250 by 150 m. The milled breccia comprises subrounded to angular clasts of rhyolite and subordinate metamorphic rocks, supported by a matrix of rock flour of predominantly rhyolite composition. On the NW edge the rhyolitic milled breccia grades outwards to a rim of polymictic milled breccia composed mainly of the various metamorphic rock types, but with a minor proportion of rhyolite clasts and rock flour. Subvertical sheet-like apophyses of polymictic milled breccia extend out from this rim into the metamorphic rocks along the 290o magnetic fracture direction. Small bodies of this breccia are also exposed elsewhere around the rim of the dome and may be more extensive than observed to date, due to the negative topographic expression relative to the rhyolite and unbrecciated metamorphic rocks.
A second style of alteration is vein- and cavity-fill related, and occurs as an overprint on the zones of brecciation and intense silica replacement in the metamorphic rocks, and on the milled breccia and surrounding shatter breccia within the main rhyolite plug. The vein- and cavity-fill assemblages comprise fine drusy comb and crustiform quartz, dickite, hematite and crustiform and colloform sulphides and sulphosalts within the rhyolite and milled breccia, zoning outwards to an assemblage as above but with the addition of chlorite, calcite, and siderite.
HYDROTHERMAL ALTERATION
Sulphides comprise approximately 5% by volume of the highly silicified rocks, especially the breccias, but apart from pyrite, are not evenly distributed and tend to form semimassive, fine to medium grained aggregates and wispy lenses. They are also prominent in the vein and cavity-fill assemblage as ultrafine to medium grained colloform and crustiform laminae.
Pervasive and minor vein-related hydrothermal alteration extends over about 2 km2 of the Anastasia fault wedge, the adjacent ring dyke to the east, and first phase ignimbrites and second phase flow-domes to the west. The pervasive style is zoned from an inner advanced argillic facies to a surrounding mixed phyllic-intermediate argillic facies to an outer poorly defined propylitic facies. The pervasive advanced argillic facies is largely confined to an area of 300 m diameter covering the milled rhyolite breccia and polymictic milled breccia rim (Fig 3), but scattered small irregular zones have been defined over a length of 500 m along the SW-trending exposed rhyolite, 500 m along the NW-trending mineralised silicified zone, and 1000 m along the south- to SW-trending arcuate zone. Gold mineralisation is confined to the advanced argillic assemblage and peaks within intense silica and minor sulphide replacement along sharply defined linear and arcuate zones, predominantly within the block of metamorphic rocks, and to a lesser extent within the milled rhyolite breccia and the first phase ignimbrites. The silicified zones within the metamorphic rocks are controlled by fracturing and hydrothermal brecciation along steep kink fold axes, by subhorizontal bedding, shearing, brecciation and schistose foliation in quartz-muscovite schist bands. In the latter case a ‘ghosting’ of the schistose foliation is commonly preserved within the quartz flooding. The combination of steeply-dipping and shallowly-dipping structural control produced an unusual anastomosing network of silicified zones (Figs 4 and 5). The advanced argillic assemblage, of microcrystalline quartz, dickite, hematite, pyrite and minor barite, alunite and jarosite, overprints and grades into the phyllic to intermediate argillic assemblage of quartz, sericite, illite and montmorillonite, and minor barite, carbonate and pyrite. The outer low intensity propylitic zone, developed mainly in amphibolite, comprises quartz, chlorite, calcite, siderite and epidote, and minor prehnite, apatite, leucoxene after magnetite and actinolite after pyroxene.
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Another style of alteration, which is only a minor component of the system, is the high level equivalent of the advanced argillic-silicification facies. Lapilli tuffs, interbedded with second phase rhyolite flows, immediately west of the Anastasia fault wedge, contain opaline silica, ultrafine hematite, minor kaolinite, and traces of alunite and barite, and contain moderately high gold (0.1 ppm) and arsenic (200 ppm) values in rock chip samples. This style indicates near surface, low temperature and acid leach conditions.
MINERALISATION The sulphide, sulphosalt and precious metal assemblage, apart from pyrite which is ubiquitous throughout the alteration zone, is largely confined to zones of intense silicification.
The assemblage in approximate order of decreasing abundance is pyrite, chalcopyrite, sphalerite, galena, arsenopyrite, tennantite, tetrahedrite, covellite, enargite, chalcocite, pyrrhotite, digenite, bornite, aikinite (PbCuBiS3), emplectite (CuBiS 2), benjaminite [Pb(Cu,Ag)Bi2S4] and gold telluride. Notably, free gold was not detected. This sulphidesulphosalt and gangue assemblage is characteristic of a highsulphidation epithermal system. Cassiterite and topaz are minor components of the mineral assemblage. In the early part of the exploration program many of the surface chip samples of silicified rocks were found to contain strongly anomalous gold values, and also tin in the range 100 to 2500 ppm. Tin was not, however, analysed throughout the drilling program so that the overall distribution can not be defined. Fine-grained sugary quartz and kaolinite form pseudomorphs after arsenopyrite, which is associated with aggregates of fine grained cassiterite and topaz in fractures in the first phase ignimbrite. Topaz was also observed in the altered rhyolite. The pseudomorphing of the arsenopyrite associated with cassiterite by sugary quartz indicates that this is an early phase of mineralisation, perhaps unrelated to the later high sulphidation style. Sheeted veins of chalcedony and fluorite are developed in the microgranite ring dyke separating the first phase ignimbrites from the Amber granite along the cauldron subsidence fault, and are probably coeval with the intrusion of the second phase rhyolite flow-domes. Some indications of zoning are evident from systematic drill core and surface chip sampling of the silicified zones. Averaged values of 10 m intersections show an increase in arsenic values from 200 to 1250 ppm, and in antimony from 40 to 80 ppm towards the centre of the intense advanced argillic alteration in the milled rhyolite breccia. Other elements
Geology of Australian and Papua New Guinean Mineral Deposits
ANASTASIA GOLD DEPOSIT
including bismuth, copper, lead and zinc show a more random distribution. Drill hole sample assays show an average gold:silver ratio of 1:8. The gold content increases markedly to typically 5 to 15 ppm within small steeply-plunging multiply brecciated pipes and in close proximity to rhyolite apophyses, when compared with the typical grade within the ‘flood’ replacement of 0.5 to 1 ppm.
breccia pipes. The Anastasia rhyolite, which is fluorite-topaz bearing, also intrudes the first phase ignimbrite, and is inferred to be coeval with the Amber granite. The adjacent Rose Creek granite is molybdenite- and wolframite-bearing and intrudes the Amber granite. The common occurrence of bismuth sulphosalts in the high sulphidation pipe also suggests a genetic link.
Circumstantial evidence suggests that the gold grain size is microfine. Screen fire assaying of various size fractions down to 200 µm showed higher concentrations in the fine fraction. Extensive pan concentrate sampling of gullies draining the main silicified zone failed to detect visible gold, even though random surface rock chip samples commonly produced assays in the range 5 to 20 ppm.
Climax-type deposits characteristically lie on major mantle tapping fracture systems, and involve multiple intrusive phases (Wallace, 1995). Interpretation of satellite images defined two major intersecting fracture corridors which seem to control the position of Anastasia and many other porphyry systems in the region (Fig 1). The use of such imagery may therefore be of considerable value in targeting mineralised corridors.
DISCUSSION AND CONCLUSIONS
ACKNOWLEDGEMENTS
Mineralisation is genetically related to hydrothermal eruption associated with a rhyolite flow-dome complex which intruded along the caldera’s arcuate ring fault. Siliceous replacement zones span advanced argillic and phyllic to intermediate argillic zones. The main rhyolite plug is partly rimmed by a polymictic, milled breccia interpreted as a hydrothermal eruption breccia. The milled rhyolite breccia within the plug indicates that magmatic fluid boiled and explosive degassing occurred in a near surface environment.
Centamin Limited, the titleholders, are thanked for their permission to publish. Special acknowledgement is due to R Jenkins, G Nicol, M Castle, M Erceg and A Edwards for mapping and drill core logging, to P Ashley and I Pontifex for petrography and mineragraphy, to K Camuti for XRD and clay mineralogy, to CSIRO Division of Mineral Physics, Remote Sensing Group for experimental airborne infrared absorption mapping of clay species and surface confirmation of these species. B Oversby, D Mackenzie, M Baker and G Morrison are acknowledged for useful field discussions. I Plimer and P Pollard are thanked for discussions and reviews.
The step-like NE-oriented west block down normal faulting, which postdates alteration and mineralisation, has juxtaposed progressively higher levels of the same system towards the NW (Fig 3). Approximately 500 m west of the Anastasia fault wedge, and directly on the extrapolated 290o trend of the mineralised zone, a small autobrecciated rhyolite flow-dome has intruded through and flowed over the top of the first phase ignimbrite. Several other rhyolite domes and plugs are exposed in the immediate vicinity but none show evidence of explosive degassing. At one point some 500 m west of the deposit an advanced argillically-altered lapilli tuff is interbedded with these flows. It is suggested that this lapilli tuff may be related to the explosive degassing within the Anastasia rhyolite dome. An epithermal acid-sulphate style (Heald, Foley and Hayba, 1987) is indicated by the brecciation, flooding and replacement of a rhyolite dome, peripheral volcanic rocks and metamorphic basement by amorphous jasperoidal quartz, dickite, alunite, sulphides and sulphosalts (notably enargite). A number of features indicate that this high sulphidation system forms part of a spectrum of deposits that includes Climax-type porphyry systems (White et al, 1981; Wallace, 1995) and gold bearing porphyry systems in north Queensland such as Red Dome, Mungana and Kidston, which are characterised by an association of tungsten, molybdenum, tin, bismuth and gold. The first phase of ignimbrites, which are cassiterite-topaz bearing, are juxtaposed with, and older than, the Amber granite, which has high level features such as extensive cassiterite-rich greisen, a chilled sugary-textured quartz carapace, micrographic and crenulate quartz textures, and an intrusion breccia at Sugar Mountain (Figs 1 and 2). Another alteration zone at Mount McDevitt shows an intermediate style linking the epithermal high level Anastasia zone with the hypothermal Sugar Mountain zone (Figs 1 and 2). The Mount McDevitt zone shows pervasive mesothermal quartz-sericite-pyrite alteration of a foliated Proterozoic granite, and associated minor milled hydrothermal milled
Geology of Australian and Papua New Guinean Mineral Deposits
REFERENCES Best, J G, 1962. Atherton, Queensland - 1:250 000 geological series, Bureau of Mineral Resources Geology and Geophysics Explanatory Notes SE55–5. Branch, C D, 1966. Volcanic Cauldrons, Ring Complexes and Associated Granites of the Georgetown Inliers, Queensland, Bureau of Mineral Resources Geology and Geophysics Bulletin 76. Champion, D C and Heinemann, M A, 1994. Igneous rocks of northern Queensland: 1:500 000 map and explanatory notes, Australian Geological Survey Organisation Record 1994/11. Cranfield, L C, 1992. Geology of the Lyndbrook 1:100 000 sheet area (7762) North Queensland, Queensland Department of Resource Industries Record 1992/19. Heald, P, Foley, N K and Hayba, D O, 1987. Comparative anatomy of volcanic-hosted epithermal deposits: acid-sulphate and adulariasericite types, Economic Geology, 82:1–26. Nethery, J E, 1989. A to P 4030M, Rocky Tate, progress report for the six months to 31/12/89, Queensland Department of Mines and Energy, CR 21425 (unpublished). Nethery, J E and Barr, M, 1996. Revised late Palaeozoic tectonics of north Queensland, Geological Society of Australia Abstracts 41:313. Wallace, S R, 1995. The Climax-type molybdenite deposits: What they are, where they are, and why they are, Economic Geology, 90:1359–1380. White, W H, Bookstrom, A A, Kamilli, R J, Ganster, M W, Smith, R P, Ranta, D E and Steininger, R C, 1981. Character and origin of Climax-type molybdenum deposits, in Economic Geology, 75th Anniversary Volume (Ed: B J Skinner), pp 270–316 (The Economic Geology Publishing Company: El Paso, TX). Withnall, I W, 1984. Stratigraphy, structure, and metamorphism of the Proterozoic Etheridge and Langlovale Groups, central Georgetown Inlier, north Queensland, Geological Survey of Queensland Record 1984/59.
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Harvey, K J, 1998. Mount Wright gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 675–678 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Mount Wright gold deposit 1
by K J Harvey
INTRODUCTION
REGIONAL GEOLOGY
The deposit is 9 km NW of Ravenswood and 86 km south of Townsville, north Queensland, at lat 20o02′S, long 147o50′E and AMG coordinates 482 400 E, 7 784 100 N, on the Charters Towers (SF 55–2) 1:250 000 scale and the Ravenswood (8257) 1:100 000 scale map sheets (Fig 1). It is hosted by a PermoCarboniferous altered rhyolite and granite breccia system which intrudes Early to Middle Ordovician granite and granodiorite of the Ravenswood Batholith.
The gold mineralised system at Mount Wright is associated with a Permo-Carboniferous rhyolite which intrudes (Fig 1) the Early to Middle Ordovician Glenell Granodiorite and Millaroo Granite (Clarke, 1971) in the eastern portion of the Ravenswood Batholith (Hutton, Rienks and Wyborn, 1990; Hutton et al, 1994). The Glenell Granodiorite is a sparsely outcropping grey, medium to coarse grained, foliated hornblende-biotite granodiorite. In this area it also includes small bodies approaching adamellite in composition. The Glenell Granodiorite hosts thin quartz-sulphide-silver-gold bearing veins to the west of Mount Wright. The Millaroo Granite is a highly variable pink, fine to coarse grained, biotite granite. Abundant miarolitic cavities and an area of unmineralised quartz-flooded breccia to the north of Mount Wright indicate that the granite melt contained a high volatile content. Potassic alteration of the adjacent Glenell Granodiorite has resulted from the intrusion of the Millaroo Granite. Although the limited exposure does not allow the contact of the Millaroo Granite and the Glenell Granodiorite to be observed, the magnetic data indicate that these contacts are often faulted (Fig 1) since they correspond to magnetic linears. Magnetic data also show extensive northerly, northwesterly and northeasterly trending fracturing and faulting of the Ordovician intrusives. These fractures are emphasised in the magnetic data by alteration in the area surrounding Mount Wright. Northeasterly and northerly trending Ordovician intermediate to basic dykes intrude both the Glenell Granodiorite and Millaroo Granite (Fig 2).
FIG 1 - Location map and regional geological plan, Mount Wright area.
Historical production from the deposit is approximately 20 000 oz gold, and the Inferred Resource is 10 Mt at 3 g/t gold. Exploration of the deposit is in progress. It is held by Carpentaria Gold Pty Ltd, a wholly owned subsidiary of MIM Holdings Limited. Brief details of the exploration and mining history of Mount Wright are outlined by A-Izzeddin, Harvey and McIntosh (1995). 1.
Manager Gold Projects Eastern Australia, MIM Exploration Pty Ltd, GPO Box 1042, Brisbane Qld 4001.
Geology of Australian and Papua New Guinean Mineral Deposits
In addition to numerous thin rhyolite dykes, larger PermoCarboniferous rhyolite intrusive bodies occur at Mount Wright and to the SW and NW. All of the rhyolite bodies are sericitised and some contain brecciated areas. The rhyolite body at Mount Wright is closely associated with the gold mineralisation. It strikes northwesterly within a broad alteration zone with this strike direction. The other rhyolite bodies generally have a northeasterly strike, but only those at Mount Wright are known to be mineralised. Minor Tertiary to Quaternary sandstone and conglomerate occur to the east of Mount Wright.
ORE DEPOSIT GEOLOGY LITHOLOGY The bulk of the known gold mineralisation in the Mount Wright area is hosted by altered and brecciated rhyolite and a small amount is hosted by granite breccia. Vein mineralisation similar in style to that at Ravenswood (Collett et al, this
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K J HARVEY
FIG 3 - Geological cross section on line 10 000 N, Mount Wright, looking north, showing gold mineralisation. See Fig 2 for location.
FIG 2 - Geological plan showing brecciation and alteration, Mount Wright.
publication) is relatively insignificant at Mount Wright and probably resulted from leakage from the main mineralising rhyolite body. The central part of Mount Wright is altered and brecciated granite and rhyolite with a surrounding halo of altered granite (Figs 2 and 3). The granite breccia can be broadly subdivided into two types. The first type is well represented in outcrop and consists of angular fragments of granite with some rhyolite and andesite, in a matrix of finer granitic and rhyolitic material. It appears to have formed from explosive activity associated with the emplacement of the rhyolite. The second type consists of intensely sericitised and essentially disaggregated granite and contains fragments of granite, rhyolite and andesite. It appears to have resulted from initial intense alteration of the granite which produced a mass of remnant quartz and altered feldspar crystals in a sericite matrix. This mass apparently flowed under stress, presumably from the intruding rhyolite, and incorporated fragments of altered country rock. The proportion of this second breccia type increases at depth and becomes the dominant type below about 100 m RL or 300 m below surface. Apart from the gold mineralisation at the Mother lode and areas
676
either containing significant rhyolite or adjacent to rhyolite, the granite breccia is usually poorly mineralised. A rhyolite body comprising strongly sericitised, brecciated and flow banded rhyolite outcrops on the western flank of Mount Wright and is the host to most of the gold mineralisation. In outcrop the rhyolite breccia comprises rounded clasts of rhyolite and some granite, with rare clasts of altered andesite and metasediment, in a matrix of fine grained rhyolite and granite. Areas of flow banded rhyolite may be either very large clasts of rhyolite or small intrusive bodies. Granite clasts are more common near the margins and the body grades outwards to granite breccia containing some rhyolite clasts. Breccia clasts become more angular and the amount of clast supported breccia increases with depth, reflecting an overall decrease in the amount of matrix present. The rhyolite body is crudely elliptical in plan with dimensions of about 200 by 50 m. Its long axis strikes at 322o true and the body has a vertical dip. Diamond drilling indicates that the body maintains a similar shape and dimensions to a depth of 800 m. Other smaller areas of rhyolite breccia occur within the main body of granite breccia but have not yet been adequately defined because of a lack of detailed information.
ALTERATION All of the breccias at Mount Wright are strongly altered and the central brecciated area is enclosed by a halo of alteration of the
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT WRIGHT GOLD DEPOSIT
surrounding granite country rock. In addition, alteration extends along fractures into the country rock and along basic to intermediate dyke margins for hundreds of metres from the main breccia body. Alteration of granite and rhyolite includes sericitisation, sideritisation and silicification of feldspars, alteration of biotite to chlorite and rutile, replacement of hornblende by siderite and sulphides and replacement of sphene by siderite and anatase. More intense alteration in the mineralised areas is associated with further replacement, particularly of the breccia matrix, by sericite, siderite, quartz and sulphides. This is accompanied by emplacement of sulphides, siderite and quartz in intraclast voids and fractures. Some of the pink potassium feldspar present within highly altered granite breccia adjacent to the rhyolite body may be due to potassium alteration rather than an original potassium feldspar component of the granite.
MINERALISATION Two discrete orebodies occur within the area of brecciation and alteration. A relatively small body of gold mineralisation known as the Mother lode occurs in altered granite breccia at the glory hole on the eastern side of Mount Wright. This vertical mineralised body has a diameter of approximately 40 m and occurs on the eastern margin of the altered and brecciated granite that forms the bulk of Mount Wright. The mineralisation has been mined to about 80 m below surface, and drilling shows that only low grade mineralisation continues below the mined area. All significant gold production came from this body and includes 6000 t at 8 g/t gold mined from 1927 to 1942 and 104 000 t at 4.6 g/t gold during 1992 and 1993. Although rare clasts of altered andesite occur within the mineralised breccia, rhyolite clasts are absent.
content decreases from above 5000 ppm at 300 m RL to less than 100 ppm at -400 m RL, and lead shows a decrease from above 500 ppm to less than 5 ppm. Copper, silver, arsenic and bismuth values display similar though less pronounced trends. The mineralisation in the Mother lode is similar to the Main lode. Gold is accompanied by sulphides, sericite, quartz and siderite and largely occurs in intraclast positions in the breccias and in fractures. Sulphides, sericite, siderite and gold also occur in flow banded layers in unbrecciated rhyolite in the Main lode. In the upper parts of the Main lode, rhyolite breccia contains approximately the same gold content as flow banded rhyolite. At greater depth however, brecciated rhyolite is more strongly mineralised and gold within rhyolite breccia accounts for the higher grades of the intersections in this area. A summary of the main alteration and gangue minerals and a proposed paragenesis is shown in Fig 5. Early melnikovite pyrite is common at Mount Wright, followed by marcasite, with or without pyrite, which is braided by siderite. Sphalerite is dark brown to black in colour and contains a high iron content. Bismuth minerals include native bismuth and bismuthinite. Minor galena occurs in the ore. Gold occurs late and is associated with siderite, chalcopyrite, galena and tetrahedrite which commonly heal cracks in earlier pyrite and arsenopyrite. Gold occurs as electrum and therefore the metallurgical recovery is high.
Most of the known gold mineralisation at Mount Wright occurs within the Main lode, which is hosted by the body of rhyolite breccia and flow banded rhyolite described above. Significant gold grades in the Main lode commence about 100 m below surface (Fig 3) and increase downwards to a maximum at about 700 m below surface. The host rhyolite at surface is essentially barren. Strong vertical zoning is a feature of this lode, as shown in Fig 4. The gold content peaks at about -300 m RL, but other elements decrease markedly with increasing depth. Zinc FIG 5 - Summary of main alteration, gangue and ore minerals, and a proposed paragenesis for the Main lode and Mother lode mineralisation at Mount Wright.
ORE GENESIS
FIG 4 - Average metal abundances for drill hole samples with gold grades above 1 g/t for the Main lode at Mount Wright plotted against reduced level (RL). Samples up to 50 m above and 50 m below RL have been averaged and plotted at the RL.
Geology of Australian and Papua New Guinean Mineral Deposits
It is proposed that Mount Wright is a mineralised diatreme breccia lying above an originally volatile-charged rhyolite intrusive. Intense alteration of the granite country rock by fluid from the rhyolite and brecciation by the intruding rhyolite preceded the emplacement of the diatreme. Further explosive brecciation of the granite accompanied the diatreme event. Mineralisation in the Main lode immediately followed the emplacement of the diatreme with metal derived from the underlying volatile charged rhyolite. Gold was carried in the fluid as a bisulphide complex and was deposited by phase separation of hydrogen sulphide due to the vertical pressure drop in the diatreme, as discussed by Henley (1985). Other metals carried by chloride complexes, and not affected by the
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K J HARVEY
boiling, were deposited higher in the diatreme principally by the temperature drop of the fluid. Gold mineralisation in granite breccia at the Mother lode and in veins to the west of Mount Wright was deposited from fluid derived from the same rhyolite intrusive.
ACKNOWLEDGEMENTS The author acknowledges permission given by MIM Exploration Pty Ltd and Carpentaria Gold Pty Ltd to publish this paper and also acknowledges the significant contribution made in the investigation and discovery of the deposit by other employees of MIM Exploration, particularly D McIntosh, D AIzzeddin and S Summers. N J W Croxford carried out the petrological work. Special thanks are due to D Munt for his comments on the paper and to E Kangas for preparing the illustrations.
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REFERENCES A-Izzeddin, D, Harvey, K J and McIntosh, D C, 1995. Mount Wright gold deposits, Ravenswood district, in Mineral Deposits of Northeast Queensland: Geology and Geochemistry, Contribution 52 (Ed S D Beams), pp 109–115 (Economic Geology Research Unit, James Cook University of North Queensland: Townsville). Clarke, D E, 1971. Geology of the Ravenswood 1-Mile Sheet Area, Queensland, Geological Survey of Queensland, Report No 53. Henley, R W, 1985. The geothermal framework for epithermal deposits, in Geology and Geochemistry of Epithermal Systems, Reviews in Economic Geology, Vol 2 (Eds: B R Berger and P M Bethke), pp 1–24 (Society of Economic Geologists: El Paso). Hutton, L J, Rienks, I P and Wyborn, D, 1990. A reinterpretation of the Ravenswood Batholith, north Queensland, in Proceedings Pacific Rim Congress 90, Volume III, pp 179–185 (The Australasian Institute of Mining and Metallurgy: Melbourne). Hutton, L J, Rienks, I P, Tenison-Woods, K L, Hartley, J S and Crouch, S B S, 1994. Geology of the Ravenswood Batholith, north Queensland, Department of Minerals and Energy Queensland, Geological Record 1994/4.
Geology of Australian and Papua New Guinean Mineral Deposits
Collett, D, Green, C, McIntosh, D and Stockton, I, 1998. Ravenswood gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 679–684 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Ravenswood gold deposits 1
2
3
by D Collett , C Green , D Mcintosh and I Stockton INTRODUCTION The Ravenswood Goldfield of about 25 km2 is centred on the township of Ravenswood, approximately 90 km south of Townsville and 60 km east of Charters Towers, Qld, at lat 20ο06′S, long 146ο53′E on the Charters Towers (SF 55–2) 1:250 000 scale and the Ravenswood (8257) 1:100 000 scale map sheets (Fig 1). Early production came from alluvial and eluvial sources and oxide reef ore from 1868 to 1872, and from hard rock mines from the late 1880s until 1917. Up to 1967 the field had a recorded production of 901 000 oz of gold (Fig 2). In 1987 MIM Holdings Limited (MIM) commenced gold mining operations at Ravenswood, managed by its wholly owned
4
subsidiary Carpentaria Gold Pty Ltd (CG). Since then 294 000 oz of gold have been produced from open cut and underground mines adjacent to the township, and from Mount Wright, 10 km NW of Ravenswood (Harvey, this publication). Current open pit mining and processing operations at Nolans project area, 1 km south of Ravenswood, were commissioned in 1995, in a joint venture with Haoma Mining NL. The initial Proved Ore Reserve was 12.5 Mt of sulphide ore at 1.75 g/t gold and 2.2 Mt of oxide ore at 1.1 g/t. The mine is expected to produce 120 000 oz from sulphide ore and 30 000 oz from oxide ore during the 1996–97 financial year. A feasibility study is underway to evaluate the adjacent Sarsfield deposit, discovered in 1995 on mining leases held by CG, with an Indicated Resource of 24 Mt grading 1.4 g/t gold.
EXPLORATION AND MINING HISTORY 1.
Geology Superintendent, Carpentaria Gold Pty Ltd, PO Box 5802, Townsville Qld 4810.
2.
Senior Geologist, MIM Exploration Pty Ltd, PO Box 7037, Garbutt Qld 4814.
3.
District Geologist, MIM Exploration Pty Ltd, GPO Box 1042 Brisbane Qld 4001.
4.
Project Geologist, MIM Exploration Pty Ltd, GPO Box 1042 Brisbane Qld 4001.
HISTORICAL WORKINGS Alluvial gold was discovered near Ravenswood in 1868, and by 1871 the population had reached 900. The alluvial gold is generally fine grained and occurs with magnetite-bearing sand. The rich alluvial deposits were soon depleted and mining became focussed on oxidised ferruginous quartz veins, with
FIG 1 - Location map and geological plan of part of the Ravenswood Goldfield.
Geology of Australian and Papua New Guinean Mineral Deposits
679
D COLLETT et al
and two underground mines. Open pits were developed at MSA on a vein on the northern side of the Goldfield, at BRW, SYC, and OCA on the Buck Reef structure, and at Area 4, Area 5 and Nolans on vein stockworks (Fig 1). The underground mines were developed between the OCA and SYC pits on the Buck Reef, and on a single vein at Area 2 (Fig 1). TABLE 1 Carpentaria Gold Pty Ltd Ravenswood production, reserves and resources to 30 June1996. Reconciled Mine Production Deposit
Mine
‘000 t
Gold grade g/t
Years mined
SYC
Slaughter Yard Creek pit
526
2.7
1987–90
OCA
OCA pit
290
3.4
1987–89
BRW
Buck Reef West pit
160
2.8
1988–91
OCA
OCA underground mine
149
4.1
1991
Area 4
Area 4 pit
50
2.4
1990
Area 5
Area 5 pit
260
2.4
1988–91
MSA
Melaneur– Shelmalier pit
48
3.5
1990–91
Area 2
Area 2 underground mine
174
10.1
1992–93
Nolans
Nolans pit
4100
1.25
1993–96
5757
1.96
FIG 2 - Historical gold production, Ravenswood Goldfield.
five separate stamping mills in operation by 1871. Typical gold grades of hand picked vein ore were between 1 and 10 oz/ton. Beneath the base of oxidation (between 15 and 18 m below surface) the sulphide bearing ore was found to be refractory, and as the oxidised ore diminished much of the population left for other goldfields. Some enterprising individuals stayed on recognising that large quantities of gold were contained in the rich quartzsulphide veins and that an efficient extraction method was required. Metallurgical experiments included smelting, chlorination, cyaniding and use of advanced milling machines and ‘Wilfley’ tables. The results of this experimental work and the entreprenurial efforts of two key individuals, H H Barton and A L Wilson, initiated and sustained the second major gold production period. The Sunset underground mine dominated gold production yielding a total of 208 949 oz. This mine worked a single 45o dipping vein structure to a depth of 200 m below surface. Other mines developed on similar vein structures but which were found to be significantly less extensive than at the Sunset mine. After 1912 a number of factors combined to cause a steady decline in gold production (Fig 2), including diminishing production from the Sunset mine, the failure to discover other major veins, a major industrial dispute and finally the outbreak of World War 1. Gold mining and exploration at Ravenswood between 1917 and 1980 were limited to minor underground extensions to preexisting mine workings and a few drill holes. However during the 1950s high grade silver ore was extracted from an underground silver mine at Totley 1.6 km north of Ravenswood from a vein structure very similar in geometry to the Sunset vein. During the early 1980s several old mullock heaps and tailings dams were reprocessed in two heap leaching operations operated by The North Queensland Company. MIM began systematic exploration at Ravenswood in early 1985. Effective exploration methods for gold resources include geological mapping, ground magnetic surveys, soil sampling, trenching and drilling. Diamond and percussion drilling have been used, with face sampling of reverse circulation percussion drill holes now predominantly used for orebody delineation.
PRODUCTION AND RESOURCES SINCE 1987 Since 1987 CG has produced a total of 294 000 oz of gold from 5.7 Mt of ore of average grade 1.96 g/t from the Ravenswood Goldfield (Table 1). The ore was obtained from seven open pits
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Totals
Reserves and Resources Deposit Nolans
Sarsfield
Proved Ore Reserves, open cut Indicated Resources, low grade sulphide Indicated Resources
mt
Gold
10.5
1.7
2.7
0.7
24
1.4
The Nolans treatment plant, alongside the Nolans open pit 1 km south of Ravenswood, has a design capacity of 2.1 Mtpa. Heap leach facilities at Sandy Creek, 2 km east of Ravenswood, are expected to be expanded as the oxide ore from the Sarsfield deposit becomes available.
PREVIOUS DESCRIPTIONS The geology of the Ravenswood Goldfield was reported by Jack (1886), and early mines were described by Cameron (1917) and Reid (1934). Four university theses have been written on the Ravenswood geology and mineralisation, by Talbot (1982), Malagun (1985), Sparks (1991) and Crees (1994). Recent descriptions of mine geology and mining operations have been provided by Neindorf et al (1989) and McIntosh, Harvey and Green (1995).
REGIONAL GEOLOGY The Ravenswood goldfield is in the eastern part of the Ravenswood Batholith (Hutton, Rienks and Wyborn, 1990), the major element of the Ravenswood–Lolworth Province. The batholith covers 6000 km2 and is bounded to the north by the Broken River Province, by the Drummond and Bowen Basins to the south, and to the east by the NE part of the New England Fold Belt. It also hosts the Charters Towers, Mount Leyshon, Mount Wright and Rishton gold deposits.
Geology of Australian and Papua New Guinean Mineral Deposits
RAVENSWOOD GOLD DEPOSITS
The batholith comprises Early Ordovocian, Middle Silurian to Middle Devonian, and Middle Carboniferous to Early Permian intrusives. The basement to the batholith is the Late Cambrian to Early Ordovician volcanics and sediment of the Seventy Mile Range Group, and the Early Palaeozoic Charters Towers Metamorphics, Argentine Metamorphics and Kirk River Beds (Clarke, 1971).
1.
Major faults and shear zones which traverse the Ravenswood Batholith near Ravenswood are the north-striking Millaroo fault zone to the east, the east-striking Alex Hill shear zone to the north and the NE-striking Connolly Lineament (TenisonWoods and Reinks, 1992), immediately south of Ravenswood.
These structures have been recognised throughout the Buck Reef, Nolans and Sarsfield deposits, where they are an important control on the distribution of gold mineralisation, with the Buck Reef (Figs 1 and 3) the most prominent example. The structures are characterised by strong chloritic alteration and brecciation. Strong pyrite and pyrrhotite mineralisation is a common feature of the gold-bearing parts of these shear zones. These structures provide a significant control of the gold mineralisation but are secondary in importance to the second group of structures.
LOCAL GEOLOGY HOST ROCKS The Ravenswood gold deposits (Fig 1) are hosted by the Jessops Creek Tonalite (Rienks, Tenison-Woods and Wyborn, 1994). It is typically a grey, medium grained tonalite with abundant euhedral plagioclase (50–65%), hornblende (10–25%), biotite (5–15%), quartz (15–25%), and alkali feldspar (0–5%)and accessory magnetite, apatite and zircon (Malagun, 1985). Small bodies of unnamed mafic rocks, dominantly diorite but also gabbro and hornblendite, outcrop at Ravenswood. The relationship between the Jessop Creek Tonalite and these mafic rocks is unclear, and the diorites may be older or younger than the tonalite. Talbot (1982) observed a gradation in composition across the boundary between the diorite bodies and the Jessops Creek Tonalite. Roof pendant blocks have been mapped within the Jessops Creek Tonalite (Fig 1) including metasediment and volcanic rock which are probably remnants of the Seventy Mile Range Group. Their presence indicates that the current land surface approximates to the original upper level of the Jessops Creek Tonalite.
Subvertical brittle shear structures occur with a general east strike which divide into three subgroups with strikes of 060, 080 and 120o. They vary in width from hairline traces to about 10 m, but are most commonly 0.5 to 2 m wide. Individual structures can be traced for hundreds of metres in some instances. Similar structures also occur as shorter, subparallel shear zones, sometimes in an en echelon series.
2.
Moderately dipping joints and veins are characterised by quartz-sulphide filling and intense narrow sericitechlorite alteration selvages. These veins host most of the gold at Ravenswood, and appear to have dilated by reverse dip reactivation. At least eight structural reactivations and accompanying alteration events have been recognised in the veins as described in the alteration section below. Geometries range from single, multiphase crack-seal veins such as at Area 2, to zones of subparallel closely spaced veins as at the Keel structure at Sarsfield, to branching and anastomosing stockworks at Nolans (Fig 3).
Locally pegmatitic aplite dykes to 2 m wide are common and in a few locations have developed ‘brain rock’ textures. Minor andesitic dykes also intrude the tonalite.
STRUCTURE AND MINERALISATION Fault and shear zones in the Ravenswood area (Fig 1) which spatially appear to constrain the known gold mineralisation include the Ravenswood Lineament (locally named the Jessop Creek fault zone) which strikes north, the Connolly Lineament (locally the Plumwood fault) which has an ENE strike, and the Mosgardies Shear Zone (Heidecker, 1986) which strikes east. Subordinate structural features within the goldfield present a network of intersecting faults, shear zones and veins. These have been delineated by a combination of mapping and interpretation of airborne and ground magnetic data. Significant wall rock alteration is associated with many of these structures and has locally depleted the magnetite in the Jessops Creek Tonalite, so that the faults are revealed as magnetic low linear traces. Resolution of the structural paragenesis in the Ravenswood field is difficult due to the multiple reactivation of many of the structures and the general lack of control surfaces in an intrusive terrain. However the main structures can be broadly divided into three groups on the basis of generalised order of formation and type.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 3 - Diagrammatic cross sections through (a) Buck Reef West (b) Nolans and (c) Sarsfield gold deposits. Legend and location on Fig
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In the Area 2 underground mine the extent of selvage alteration around large veins was minimal in contrast to some narrow and irregular veins with extensive sericite and chlorite alteration zones. Abrupt changes in vein texture, sulphide content and orientation are common and occur over short distances.
1.
The earliest alteration consists of barren quartz vein filling with minor sericite and chlorite alteration, and occurs as broken segments within the first group of structures, the subvertical shears. Zones of finely layered silicification preserved as breccia clasts in the shears may also be part of this early alteration.
Four principal vein orientations are recognised across the goldfield:
2.
A broad distribution of microveins followed. Narrow, white potassium feldspar selvage alteration associated with veinlets of potassium feldspar, amphibole and magnetite is followed by generally narrow green and buff coloured epidote-albite selvage alteration associated with veinlets of epidote and quartz.
3.
Multiple zones with variable intensity potassic alteration formed. In zones of low intensity alteration, aggregates of fine-grained biotite replace primary biotite and hornblende, and also fill fine fractures. A pervasive ‘crackling’ appears to have preceded the biotite alteration in some places. Epidote, white potassium feldspar and pyrite and rare molybdenite are commonly associated with this group. Generally this alteration tends to be semipervasive around structures but in some areas of intense biotite alteration, such as zones along the Buck Reef in the SYC and OCA pits, the whole rock is replaced by biotite.
4.
A propylitic phase, with actinolite filling veins and breccias, was followed by zones of intense chlorite mostly associated with subvertical shear zones. The intensity of selvage chlorite is variable. Single and smaller shear structures tend to develop limited alteration zones whereas larger and multiple shear structures develop substantial wall rock alteration zones, such as along parts of the Buck Reef.
5.
The main gold bearing phase is characterised by phyllic alteration. Narrow selvage zones of sericite, calcite, chlorite and pyrite are associated with most of the quartzsulphide veins at Ravenswood. This selvage alteration is typically intense and is commonly zoned. This alteration
Dip (degrees)
Dip direction
Example
30–70
NNE
Sunset
30–70
East
Grant
10–40
SSW
Keel
10–40
SW
Nolans
The largest single vein at Ravenswood is the Sunset, with a length of 900 m, a down dip extent of at least 250 m and a thickness of 10 cm to 3 m. The multiveined Keel structure at Sarsfield is 15 to 25 m thick. Single veins in the Nolans and Sarsfield deposits are generally less than 0.5 m thick, and have strike and dip lengths to 250 m but generally less than 100 m. 3.
Unmineralised steeply-dipping late faults are characterised by carbonate filling and by strong claysericite wall rock alteration. Many of these are probably reactivated pre-existing shear structures. The prominent structure in this group is the Jessop Creek fault zone which strikes north, and lies to the west of Ravenswood (Fig 1). Some smaller structures of this group have a general ENE strike and minor sinistral dislocation.
ALTERATION AND MINERALISATION The rock alteration and vein filling at Ravenswood consist of multiple overprinted phases. The current understanding of the sequential order recognises 15 separate events (Table 2) which can be broadly divided into seven groups.
TABLE 2 Generalised fill and alteration sequence at Ravenswood. Group
Event name
Fill
Wall rock alteration
Structure type
1
1 Buck quartz
White quartz?
Chlorite, Sericite? layered silicification
Veins
2
2 Potassium feldspar
Magnetite, amphibole, hornblende, potassium feldspar
Potassium feldspar
Veinlets
3 Albite-epidote
Albite-epidote
Albite, epidote
Veinlets
3
4 Potassic
Biotite, chalcopyrite, pyrite, molybdenite
Biotite, white feldspar, epidote
Veins
4
5 Actinolite
Actinolite
Pale sericite
Veins and breccias
6 Chlorite breccia
Chlorite, pyrrhotite, pyrite
Chlorite
Breccia
7 Magnetite
Magnetite, calcite
Magnetite, iron-carbonate
Breccia and veinlets
8 Quartz-pyrite
Pyrite, quartz
Pyrite, sericite, chlorite, calcite
Veins
9 Main gold event
Pyrite, pyrrhotite, sphalerite, galena, sericite, chlorite, gold
Pyrite, sericite, chlorite, calcite
Veins, brecciated veins
10 Arsenopyrite
Arsenopyrite, quartz, pyrite, chalcopyrite, gold?
?
Veins, brecciated veins
11 Chalcopyrite
Chalcopyrite, pyrite, pyrrhotite
Sericite
Veins
12 Calcite breccia
Cream calcite, quartz
Calcite
Breccia
13 Mill breccia
Mill breccia rock fragments, grey quartz
?
Breccia
14 White quartz-calcite
Calcite, quartz, gypsum
Sericite, chlorite
Joints, veinlets
15 Microshear
Pyrite micoshears
?
Microshears
5
6 7
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Geology of Australian and Papua New Guinean Mineral Deposits
RAVENSWOOD GOLD DEPOSITS
replaces pre-existing minerals including primary plagioclase, hornblende and both primary and secondary biotite and magnetite. The vein fill follows the general sequence quartz-pyrite or pyrrhotite-sphaleritearsenopyrite-chalcopyrite-pyrite-pyrrhotite. Gold is free milling and has been observed on grain boundaries and in fractures in pyrite, sphalerite, arsenopyrite and quartz. The highest grade mineralisation is associated with sphalerite and arsenopyrite. 6.
7.
A vein brecciation phase occurred after the formation of the main quartz-sulphide veins. Brecciation is a feature of the Area 2 vein and has also been mapped at Nolans. Initial angular breccia with calcite and quartz fill is followed by a matrix-supported milled breccia of well rounded wall rock and vein fragments. The breccia matrix is dominantly rock flour with sericite-silicacarbonate alteration. Sericite-chlorite alteration zones have developed around some carbonate-filled vein structures and other fault structures such as the Jessop Creek fault zone. The fill is typically white and pink manganiferous calcite, and occasionally zeolite and gypsum.
The paragenesis is complicated by the reactivation of structures. Not all of the alteration types have been recognised in any one area of the goldfield and with further study this may lead to a model encompassing fluid evolution, alteration distribution and structural paragenesis. Some minor sulphide minerals, namely tetrahedrite, bismuthinite and melnikovite, have not yet been assigned to the paragenetic sequence. In general the paragenesis indicates precipitation from a successively cooler fluid or fluids as shown by the range from hot potassic (biotite) alteration, to cooler propylitic and phyllic, to carbonate alteration.
WEATHERING The weathering profile comprises an upper, completely oxidised zone and a lower, partially oxidised zone down to 18 m below surface. Complete oxidation of sulphides and decomposition of the tonalite occurs to 12 m depth. This ‘deco’ material has a bulk density of 2.0 compared to the average of 2.8 for fresh sulphide ore and is typically excavated without blasting. The lower partial or transitional oxidised zone has an average bulk density of 2.5. Supergene weathering effects are generally restricted in extent and intensity to zones between 1 and 5 m thick. Where present these zones are marked by minor (less than 1% by volume) secondary copper minerals such as chalcocite.
ORE GENESIS Dating of sericite alteration associated with the gold mineralisation at 320 Myr indicates a Lower Carboniferous age (Neindorf et al, 1989), which means the fluid source post-dated the host tonalite by approximately 100 Myr. Limited fluid inclusion studies by Malagun (1985) and Crees (1994) have identified trapping temperatures in the range 280–384oC and salinities of 7 to 17 wt % NaCl. These fluid characteristics, the vein textures and the general alteration style at Ravenswood suggest that the ore was derived from an intrusive-related fluid in a mesothermal to lower epithermal environment.
Geology of Australian and Papua New Guinean Mineral Deposits
Only minor occurences of possible fluid source rocks of Lower Carboniferous age have been mapped within the Ravenswood Goldfield. The closest Permo-Carboniferous intrusives are felsic dykes at Mount Wright 10 km to the north (Harvey, this publication), and within the Boori Igneous Complex, 15 km NW of Ravenswood. Ravenswood lies to the NW of a north trending series of Carboniferous ring dykes and granites, a continuation of which could underlie the field.
MINE GEOLOGICAL METHODS PIT MINING Excavation is generally on two 2.5 m high flitches. Boundaries between ore and waste are delineated using the assays of blast hole samples. Mapping of alteration and sulphide veining in the host tonalite provides a reliable visual confirmation of ore boundaries. In some zones the vein style mineralisation has produced a high degree of heterogeneity (nugget effect) in the gold distribution. These effects are partly overcome by a high density of sampling and by using a large primary sample (8 kg). Generally blast hole sample density equates to one sample per 80 to 135 t of material.
UNDERGROUND MINING AT OCA A subvertical tabular ore zone within the Buck Reef structure immediately below the OCA pit was mined via a decline through the east wall of the adjacent SYC pit. Forty metre deep reverse circulation percussion holes drilled for grade control were used as blastholes loaded from the OCA pit floor.
ACKNOWLEDGEMENTS The authors thank Carpentaria Gold Pty Ltd and MIM Exploration Pty Ltd for permission to publish this paper, E Kangas for drafting the figures and K Harvey for his technical advice. R Taylor from James Cook University of North Queensland is thanked for his contributions over the years to understanding the paragenesis.
REFERENCES Cameron, W, 1917. Recent mining development on the Ravenswood Goldfield, Queensland Government Mining Journal, 18:222–228. Clarke, D E, 1971. Geology of the Ravenswood 1–Mile Sheet Area, Queensland, Geological Survey of Queensland, Report No 53. Crees, S, 1994. The nature of the ore forming fluid of the Nolans gold deposit at Ravenswood, North Queensland, BSc Honours thesis (unpublished), James Cook University of North Queensland, Townsville. Heidecker, E J, 1986. Technical note - lineament guidelines to mine development: a continuing case history, northeastern Queensland, Proceedings Australasian Institute of Mining and Metallurgy, 291:67–69. Hutton, L J, Rienks, I P and Wyborn, D, 1990. A reinterpretation of the Ravenswood Batholith, north Queensland, in Proceedings Volume III, Pacific Rim Congress 90, pp 179–185 (The Australasian Institute of Mining and Metallurgy: Melbourne). Jack, R L, 1886. Progress report of the Government geologist for the year 1885, Geological Survey of Queensland, Publication 92. Malagun, S, 1985. The origin of gold veins at Ravenswood, North Queensland, BSc Honours thesis (unpublished), James Cook University of North Queensland, Townsville.
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McIntosh, D C, Harvey, K J and Green C J, 1995. Ravenswood gold deposit, in Mineral Deposits of Northeast Queensland, Geology and Geochemistry, EGRU Contribution 52 (Ed: S A Beams), pp 101–107 (James Cook University of North Queensland: Townsville). Neindorf, L, Dennis, R, Palmer, G and Clarke, S, 1989. Ravenswood gold mine, an experience of rapid exploration, development and production, in Proceedings North Queensland Gold Conference, pp 115–126 (The Australasian Institute of Mining and Metallurgy: Melbourne). Reid, J H, 1934. Some Ravenswood mines - A summary, Queensland Government Mining Journal, 35:44–45, 77–78.
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Rienks, I P, Tenison-Woods, K L and Wyborn, D, 1994. Geology of the southeast portion of the Ravenswood Batholith, Queensland Government Mining Journal, September 1994, pp 29–47. Sparks, G J, 1991. The geology of the Melaneur-Shelmalier veins, Ravenswood North Queensland, MSc thesis (unpublished), James Cook University of North Queensland, Townsville. Talbot, P, 1982. Aspects of the geology and gold mineralisation at Ravenswood, Queensland, BSc Honours thesis (unpublished), James Cook University of North Queensland, Townsville. Tenison-Woods, K L and Rienks, I P, 1992, New insights into the structure and subdivision of the Ravenswood Batholith - a geophysical perspective, Exploration Geophysics, 23:353–359.
Geology of Australian and Papua New Guinean Mineral Deposits
Richards, D R, Elliott, G J and Jones, B H, 1998. Vera North and Nancy gold deposits, Pajingo in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 685–690 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Vera North and Nancy gold deposits, Pajingo 1
2
by D R Richards , G J Elliott and B H Jones INTRODUCTION The Pajingo epithermal system is about 53 km SSE of Charters Towers, Qld, and 150 km SSW of Townsville, with its centre at lat 20o32′S, long 146ο27′E on the Charters Towers (SF 55–2) 1:250 000 scale and the Pajingo (8156) 1:100 000 scale map sheets. The system occurs within a circle of diameter about 14 km and area about 150 km2, and hosts the mined out Scott and Cindy lodes and the recently discovered Vera North and Nancy deposits (Fig 1). The gold deposits are typical low-sulphidation epithermal deposits as described by White and Hedenquist (1995).
3
lode by BMG in 1984 was the first major epithermal gold deposit recognised in the Drummond Basin. The Scott lode was mined by open pit between September 1987 and August 1992 and by underground hand-held cut and fill methods from December 1992 to December 1993. A total of 1 353 481 t of ore of calculated head grade 8.86 g/t gold and 29.4 g/t silver was milled to yield 366 500 oz of gold and 1 022 601 oz of silver. A small open pit was mined at Cindy from February 1994 until July 1994, followed by underground development and mining, using modified Avoca cut and fill, from September 1994 to April 1996. A total of 207 806 t of ore of average calculated head grade 7.28 g/t gold and 4.52 g/t silver was milled to yield 46 468 oz of gold and 25 066 oz of silver. Inferred Resources have been estimated for Vera North and Nancy based on 40 by 40 m surface drilling. At Vera North 1.68 Mt grading 14.1 g/t gold and 10.8 g/t silver have been defined while at Nancy the resource is 0.82 Mt grading 12.0 g/t gold and 15.6 g/t silver, for a combined total of 1.08 Moz of contained gold. A significant resource increase is probable once drilling is completed along strike and down dip. Detailed underground drilling on 20 by 20 m spacing is planned prior to the estimation of a mining reserve. Both deposits will be accessed by declines, and the underground mining technique will be chosen following trial mining and further geotechnical studies. Decline development commenced in July 1996 and production from development ore is planned to start in February 1997.
EXPLORATION HISTORY FIG 1 - Location and geological plan of the Pajingo epithermal system showing location of section line for Figs 4, 5 and 6.
The epithermal system is enclosed by tenements controlled by the Pajingo Joint Venture, a 50:50 joint venture between Battle Mountain Gold Company (BMG) and Posgold Operations Pty Limited, a subsidiary of the Normandy Mining Group. BMG manages the exploration and Posgold will be the mine operator at the discoveries. The discovery of the Scott 1.
Senior Geologist, Battle Mountain (Australia) Inc, 76 Jersey Street, Jolimont WA 6014.
2.
Consultant Geophysicist, Battle Mountain (Australia) Inc, 76 Jersey Street, Jolimont WA 6014.
3.
Exploration Manager Eastern Australia, Battle Mountain (Australia) Inc, 5/106 Dalrymple Road, Currajong Qld 4812.
Geology of Australian and Papua New Guinean Mineral Deposits
In mid 1983, Duval Mining, the predecessor of BMG, targeted the Permo-Carboniferous volcanics in the Pajingo area for bulk tonnage gold deposits similar to Kidston and Mount Leyshon. No modern exploration or mining had occurred in the Pajingo area prior to Duval’s work. Mapping by Duval geologist Ralph Porter identified epithermal quartz veining containing anomalous gold in the Pajingo area, and tenements were subsequently applied for in November 1983. Follow up prospecting and drilling discovered the Scott lode in July 1984. BMG used reconnaissance mapping, prospecting and stream sediment sampling combined with aeromagnetic and remote sensing techniques to identify further epithermal targets. Numerous outcropping auriferous quartz veins were discovered, however shallow reverse circulation (RC) percussion drilling failed to establish any new mineable reserves. In the late 1980s, detailed geophysical surveying using a bidirectional, low level aeromagnetic survey and a gradient array resistivity program identified major structures defined by
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coincident magnetic lows and resistive highs as the best hosts for significant quartz veins. These techniques also defined structures beneath areas of shallow Tertiary and younger cover. In 1990, with most of the outcropping targets tested to shallow depths, exploration focus shifted to concealed targets. Due to the increased cost and higher risk of this exploration, BMG joint ventured the Pajingo project to ACM Gold Ltd in 1991. ACM was subsequently taken over by Posgold who continued with the joint venture. The Cindy quartz vein was originally identified by a magnetic anomaly, and initial drill testing by BMG in 1990 intersected minor gold mineralisation. Subsequent drilling by ACM-Posgold at an adjacent resistivity anomaly intersected the main part of the Cindy orebody beneath 5 to 15 m of Tertiary sediment. In 1993 BMG undertook further drilling to define the small resource, and Posgold elected to take a royalty rather than participate in the mining operation. Following the success at Cindy, BMG recommenced exploration for epithermal gold deposits. In 1994, a reappraisal by the authors of the known mineralised trends as defined by previous resistivity and magnetic surveys and RC drilling led to further work at Vera North. The recognition that the shallower portion of the vein was similar to the upper part of a conceptual low-sulphide, epithermal gold system (Corbett and Leach, 1994), led to several deeper drilling programs that discovered the main high grade zone at Vera North in October 1995, 250 m below surface. A further review of the data identified that Nancy, immediately NW of Vera North, was similar and hosted by the same structure. The first phase of deep drilling, completed in December 1995, intersected the Nancy mineralisation. From May 1994 to July 1996, 246 holes comprising 22 298 m of HQ diamond core and 40 554 m of RC percussion drilling were completed at the two adjacent prospects.
REGIONAL GEOLOGY The regional geology of the Pajingo area is poorly understood due to limited work and extensive cover. The description below is largely from Porter (1990). The Pajingo mineralisation is in the northern portion of the dominantly continental Late Devonian to Carboniferous Drummond Basin. The rock types in this part of the basin include felsic to mafic lava and tuff, volcaniclastic sediment and fluvial to lacustrine sediment. These units were classified by Clarke and Paine (1970) as the Lower Carboniferous Star of Hope and Raymond formations. An alternative interpretation favoured by Battle Mountain is that the host volcanic and sedimentary units in the Pajingo area are equivalent to unnamed Upper Devonian to Lower Carboniferous volcanics and sediment mapped to the east by Clarke and Paine. The volcanics are lithologically similar, and their mapped extent correlates with a magnetically homogeneous unit extending over a 50 km easterly length through the Pajingo area. Also, to the south and west of Pajingo, varicoloured sediment and volcanics more typical of the Star of Hope Formation appear to overly the andesitic unit. The Drummond Basin sequence overlies the southern margin of the Ordovician to Devonian Lolworth–Ravenswood Batholith which intrudes a Cambrian to Early Ordovician volcano-sedimentary sequence named the Seventy Mile Range Group by Henderson (1986). The Lolworth–Ravenswood Batholith and Seventy Mile Range Group form part of the
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Lolworth–Ravenswood Block which hosts the +2 Moz Mount Leyshon and Ravenswood gold deposits. The Drummond Basin and Lolworth–Ravenswood Block are intruded by a series of Late Carboniferous to Permian high level felsic plugs, dykes and intrusive breccias. Felsic volcanics are locally associated with these intrusives. K-Ar dating of sericite (illite) alteration associated with the Scott lode has returned ages of 342+3 Myr indicating a Middle Carboniferous age for the mineralisation. The host rocks to the veins in the Pajingo epithermal system are andesitic lava and tuff (fine ash to block size) with minor volcaniclastic rocks. No detailed subdivision of the local volcanic units has yet been attempted. The volcanics are dominantly porphyritic, with phenocrysts of potassium feldspar, plagioclase (An50) and pyroxene or amphibole set in a fine grained matrix. The sediments are siltstone, feldspathic sandstone and quartz sandstone. Both andesitic and sedimentary units are intruded by undated flow-banded rhyolitic units in the eastern part of the Pajingo epithermal system. These rhyolitic units are also cut by veins of the Pajingo epithermal system. Lateritised Tertiary sediment (locally derived talus and colluvial material) of the Southern Cross Formation and Recent colluvium and soils form extensive areas of cover. The regional structures defined by aeromagnetic data strike dominantly NE and NW, with lesser easterly trends. Work on the Scott lode (R E Bobis, unpublished data,1990), which is hosted by a boomerang-shaped structure formed by intersecting NE and east trends, identified normal faulting with a vertical displacement (south side down) of 290 m. The NW structures are considered to be predominantly strike slip faults.
LOCAL GEOLOGY The Scott lode and Cindy deposits have been described by Porter (1990) and Cornwell and Tredinick (1995) respectively and further descriptions will not be given here.
GEOLOGICAL SETTING The Vera North and Nancy deposits are coincident with a major, 3.5 to 7 km long, NW-trending structure which is defined by resistivity and aeromagnetic data (Figs 2 and 3). This structure is considered to be predominantly a strike slip
FIG 2 - Regional aeromagnetics data for the area shown on Fig 1, showing the host structure of the Vera North and Nancy deposits. Dashed white line indicates host structure.
Geology of Australian and Papua New Guinean Mineral Deposits
VERA NORTH AND NANCY GOLD DEPOSITS, PAJINGO
very fine grained disseminations locally comprising up to 40% of the rock mass. In this zone, the feldspar phenocrysts are often altered to pale green clay which is interpreted to be predominantly illite-sericite. Sporadic pale green alteration also occurs throughout the groundmass of the phyllic zone. Moderate to strong red-brown hematitic dusting occurs on the margins of the phyllic zone and sporadically closer to the vein and appears to be overprinted by the silica-pyrite alteration. Minor argillic alteration characterised by clay minerals (illitesmectite, kaolinite-dickite) has been included in the phyllic zone.
FIG 3 - Gradient array resisitivity data for the area shown on Fig 1, showing the host structure of the Vera North and Nancy deposits. Dashed white line indicates host structure.
At least three phases of brecciation have been recorded adjacent to the veining. The first and probably dominant phase consists of variably altered andesitic fragments and lesser quartz vein fragments in a very fine grained silica-pyrite groundmass. The second and third phases are distinguished by fragments of previous breccias.
Vera North fault similar to the model of Corbett and Leach (1994) who proposed seismic pumping as the mechanism causing dilation and subsequent flow of mineralising fluids. Vera North and Nancy are interpreted to be sited on dilational jogs or flexures that developed as irregularities along a regional strike slip feature. The auriferous epithermal veins at Vera North and Nancy are predominantly hosted by porphyritic andesitic lithic tuff (fine ash to bomb size) weathered to an average of 70 m depth. No attempt has been made to distinguish the different volcanic units; however mapping by BMG suggests they dip at 5–15o to the south. The surface expressions of the ore zones are low ridges of silicified, brecciated, moderately to strongly quartzveined andesite. This material was intersected in drill holes to nearly 200 m depth, and the classic epithermal veins and textures were only encountered in the deeper holes. Detailed rock chip sampling at Vera North provided sporadic gold values, with 25% of the samples containing >1 g/t gold, to a maximum of 10.7 g/t. At Nancy, limited sampling provided a maximum value of 1.15 g/t gold with an average of 0.61 g/t. Both deposits are partially obscured by up to 80 m of lateritised Tertiary sediment.
At Vera North, the main mineralised zone, shown as V2 on Fig 4, is hosted by a massive, tabular, 1 to 10 m thick quartz vein (average width 3.4 m) which was intersected over a 500 m strike length. Dip is towards the SW and changes from 57o at the NW end to 64o at the SE end, with local variations. The associated mineralised zone is 1.5 to 17 m thick (average 5.4 m) with higher grade sections (with >5 g/t gold) hosted by the main vein, and low to medium grade parts (with 1–5 g/t gold) in minor quartz veins in the adjacent wallrock. Consistent high grade mineralisation commences at about 250 m below surface (Fig 5), with economic values intersected to 420 m depth, open at depth. At the SE end of Vera North, a narrow SE-plunging shoot extends to the surface as a siliceous, veined outcrop.
A 0.1 to 5.0 m wide, strongly foliated fault zone containing chlorite, clay and pyrite has been intersected to 15 m below the veining. The fault is subparallel to the veining and post-dates the mineralisation, alteration and brecciation. It is interpreted as a remobilisation of the original host structure. The alteration and brecciation described in the following paragraphs are restricted to the hanging wall above the fault, indicating that there has been significant displacement, the extent and direction of which have yet to be determined. The volcanics below the fault are typically propylitically altered with patchy silica-clay alteration causing local bleaching. Petrological and XRD studies (Kingston-Morrison Mineral Services Ltd, unpublished data, 1996) and field observations indicate that the alteration grades from propylitic, distal to the veining (ie 80 m distant), to phyllic adjacent to the veining. The propylitic alteration is characterised by dark green chlorite and is common throughout the andesitic volcanics. The phyllic alteration is dominated by strong silica-pyrite which becomes intense adjacent to the veining. The silica consists of grey, translucent chalcedony, and the pyrite occurs predominantly as
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 4 - Cross section B–B′ through the Vera North orebody looking NW.
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FIG 5 - Longitudinal projection A–A′ (looking NE) of the Vera North and Nancy orebodies showing metal content contours in g.m, and location of cross sections on Figs 4 and 6.
Several narrow (~1 m thick) hanging wall veins, collectively termed V1, have been intersected immediately above V2 over the eastern 300 m strike length. Bonanza grades and widths occur where these veins merge with V2 (Fig 4).
Nancy Nancy is about 200 m NW of Vera North and potentially economic quartz veining has been intersected over 550 m strike length with the mineralisation still open to the NW and down dip. Most of the economic mineralisation is hosted by a 1 to 9 m thick quartz vein, of average width 3.2 m, labelled N1 in Fig 6. In the upper and SE zones, the vein dips 70o to the SW; however, towards the deeper parts (below 150 m) of the NW end the dip becomes subvertical. Detailed resistivity data show that the central part of Nancy is disrupted by several SSEtrending faults, which combined with the limited drill data, make it difficult to complete a detailed interpretation. In contrast to Vera North, the distribution of high grades is less regular, with several shoots possibly reaching the surface. Drilling in the 200 m gap between Nancy and Vera North intersected moderate brecciation and silica alteration with minor veining and weakly anomalous gold values (in the range 0.1–0.5 g/t), similar to those adjacent to the higher grade zones further along the strike. Both ends of the Vera North mineralisation and the SE end of Nancy are coincident with strong ESE-trending resistivity lineaments. These are interpreted to be faults, which possibly control the position of the high grade zones. These faults have not been recognised in the drilling and their sense of movement is unknown. An alternative interpretation is that the zone represents a compressive portion of the NW-trending host structure.
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FIG 6 - Cross section C-C’ through the Nancy orebody looking NW.
MINERALOGY AND GEOCHEMISTRY Vein fill varies from grey, translucent chalcedony to milky white, locally amethystine microcrystalline silica. The dominant vein texture is massive with common drusy cavities
Geology of Australian and Papua New Guinean Mineral Deposits
VERA NORTH AND NANCY GOLD DEPOSITS, PAJINGO
and minor colloform and crustiform banding. Rare bladed pseudomorphs interpreted to be after carbonate crystals are present. Multiple brecciation of pre-existing vein fill is common with further deposition of similar silica. XRD analyses completed during metallurgical testing (C D Carey, unpublished data, 1996) of veins from Vera North indicated minor to trace amounts of illite, kaolinite and sulphides (predominantly pyrite), with occasional dolomite. Gold in the oxidised zone occurs as electrum and in the native form. The native gold formed by supergene leaching of the silver from electrum during weathering. Scanning electron microscope studies show that the gold in the primary zone is in two types of electrum with average compositions of Au75:Ag25 (3:1) and Au61:Ag39 (1.56:1). There is no apparent trend laterally or vertically in the Au:Ag ratios. Grain size varies from 7 to 144 µm and averages 47 µm. Trace amounts of acanthite (Ag2S), stromeyerite (CuAgS), native silver, chalcopyrite, sphalerite, galena and pyrrhotite were also observed in the primary zone (CD Carey, unpublished data, 1996). Silver is the only element to show a direct correlation with gold. Other elements determined were arsenic, antimony, mercury, copper, lead, zinc, molybdenum and manganese. Only mercury appears elevated, averaging 0.57 ppm (Kingston-Morrison Mineral Services Ltd, unpublished data, 1996). Elsewhere in the Pajingo epithermal system mercury, arsenic, antimony, barium and possibly fluorine are reported to be anomalous (Porter, 1990) and are considered to be useful regional exploration indicators. Recently completed fluid inclusion studies (KingstonMorrison Mineral Services Ltd, unpublished data,1996) are inconclusive, as the vein quartz is generally unsuitable for preservation of fluid inclusions. One sample from Vera North recorded a homogenisation temperature of 240oC. An earlier study of the nearby Vera vein (KRTA Ltd, unpublished data,1988) reported vapour-rich inclusions with highly variable homogenisation temperatures, indicative of boiling fluids. The Vera vein is coincident with the regional structure that hosts Vera North and Nancy, and is part of the same system. Freezing measurements and crushing tests from the Scott lode (Etminan et al, 1988) recorded low salinities and minor carbon dioxide. The similarities in texture between the Scott lode, Vera North and Nancy deposits suggest that fluid compositions are typical low-salinity epithermal fluids of meteoric origin.
ORE GENESIS The Vera North and Nancy veins exhibit similar textures to those in the Scott lode and are interpreted to have formed by similar epithermal processes. The depth of the high grade ore at Vera North and Nancy (>200 m) compared with the Scott Lode (<200 m) is possibly due to post-mineralisation tilting of the Drummond Basin towards the south. Subsequent movement along the host structure may have also been important. Vein breccias and vapour-rich fluid inclusions suggest that primary
Geology of Australian and Papua New Guinean Mineral Deposits
gold deposition was caused by fluid boiling. Boiling possibly occurred due to depressuring of hot fluids which were forced into dilational zones by recurring fault activity associated with seismic events. This would cause the epithermal banding and multiple brecciation seen at Vera North and Nancy. The chalcedonic vein silica and illite in the veins and adjacent alteration, combined with limited fluid inclusion data, indicate that the temperature of deposition ranged from <200oC to 250oC (Corbett and Leach, 1994). This suggests the deposits formed between 100 and 600 m depth, similar to their current setting.
ACKNOWLEDGEMENTS The authors gratefully acknowledge the permission of Battle Mountain Gold Company and Posgold Operations Pty Limited to publish this information, and particularly thank K McKay and R Evans for their support and valuable comments. Special acknowledgment is also given to the numerous geologists and support staff who have worked on the Pajingo project since 1983 and who have contributed to the understanding of the area. Staff at the Pajingo gold mine also provided valuable information.
REFERENCES Clarke, D E and Paine, A G L, 1970. Charters Towers, Queensland — 1:250 000 geological series, Bureau of Mineral Resources Geology and Geophysics, Explanatory Notes, SF 55–2. Corbett, G J and Leach, T M, 1994. SW Pacific Rim Au/Cu systems: structure, alteration and mineralisation. A workshop presented by Corbett Geological Services and CMS New Zealand at Townsville, Queensland, 28–29 November 1994 (unpublished). Cornwell, J and Tredinnick, I, 1995. Geology and geochemistry and mining of the Pajingo epithermal vein system, in Excursion Guide, Excursions 4, 5, 6 and 7, 17th IGES, Townsville, Queensland pp55–68 (The Economic Geology Research Unit, James Cook University of North Queensland, and The Association of Exploration Geochemists: Townsville). Etminan, H, Porter, R G, Hoffman, C F, Sun, S-S and Henley, R W, 1988. Initial studies of hydrothermal alteration, fluid inclusions and stable isotopes at Pajingo gold deposit, north Queensland, in Bicentennial Gold ’88, Extended Abstracts Poster Programme (Comps: A D T Goode, E L Smyth, W D Birch and L I Bosma), Geological Society of Australia Abstracts, 23(2):434–435. Henderson, R A, 1986. Geology of the Mt Windsor Subprovince - a Lower Palaeozoic volcano-sedimentary terrane in the northern Tasman Oregenic Zone, Australian Journal of Earth Sciences, 3: 343–364. Porter, R G, 1990. Pajingo gold deposits, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1483–1487 (The Australasian Institute of Mining and Metallurgy: Melbourne). White, N C and Hedenquist, J W, 1995. Epithermal gold deposits: styles, characteristics and exploration, SEG Newsletter, 23(1):9–13.
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Seed, M J and Ruxton, P A, 1998. Wirralie gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 691–694 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Wirralie gold deposit 1
by M J Seed and P A Ruxton
2
INTRODUCTION The deposit is 210 km SSE of Townsville, Qld, and 40 km north of Mount Coolon, at lat 21o07′S, long 147ο16′E on the Mount Coolon (SF 55–7) 1:250 000 and Mount Coolon (8355) 1:100 000 scale map sheets (Fig 1). The property is owned by Ross Mining NL. Wirralie is a low sulphidation, quartz-adularia epithermal vein and replacement type deposit hosted by volcanic rocks and sediment of Late Devonian age.
EXPLORATION AND MINING HISTORY The nearest historic mining to Wirralie was at Mount Coolon, between 1913 and 1939, with production of 6143 kg of gold and over 1866 kg of silver (Malone, Corbett and Jensen, 1964). Australian Consolidated Minerals Pty Ltd (ACM) discovered the Wirralie deposit by follow up of two anomalous 5 kg -6 mm bulk leach extractable gold (BLEG) stream sediment samples collected in 1986 as part of a regional stream sediment sampling program. No systematic exploration for gold had taken place prior to the ACM work. The two samples were from Sugarbag Creek, the only creek to drain the Wirralie deposit, and returned highly anomalous values of 6.79 ppb and 8.96 ppb cyanide-soluble gold. Follow up BLEG samples nearer to the deposit gave results of 16.8 ppb and 42.8 ppb cyanide-soluble gold. Recognition of an aerial photographic anomaly devoid of vegetation encouraged follow up in the anomalous drainage and outcropping mineralisation was identified, with rock chip assays to 7.14 ppm gold, 1010 ppm arsenic, 115 ppm antimony and 3370 ppb mercury. Mapping and detailed rock chip sampling defined a drilling target, the first hole intersecting 55 m at 2.57 g/t gold. ACM commenced development of the Wirralie mine in November 1987 as an open pit operation based on a reserve of 3.65 Mt of oxide ore at a grade of 2.75 g/t gold, using a cutoff grade of 1.0 g/t (Fellows and Hammond, 1990). Production between March 1988 and July 1993 was 4.84 Mt of ore at a grade of 2.45 g/t gold using the same cutoff grade. Gold recovery was generally in excess of 90%, and a total of 374 000 oz of gold was produced. Ross Mining NL purchased the Wirralie gold mine in July 1992 and treated 1 Mt of stockpiled ore grading 1.75 g/t gold to recover 56 000 oz of gold. Infill and extension drilling in 1994 west of the two pits upgraded the unmined, lower grade oxide Measured Resource to 7.61 Mt at a grade of 0.80 g/t gold using a cutoff grade of 0.5 g/t (A J Richmond, unpublished data, 1995).
FIG 1 - Drummond Basin regional geology, from Olgers (1972).
The total unmined Indicated and Measured Resource of sulphide ore is 6.63 Mt at 1.78 g/t gold using a 1.0 g/t cutoff, all of a refractory nature.
REGIONAL GEOLOGY 1.
Senior Exploration Geologist, Ross Mining NL, Yandan Gold Mine, PO Box 242, Collinsville Qld 4804.
2.
Exploration Manager, Ross Mining Nl, PO Box 1546, Milton Qld 4064.
Geology of Australian and Papua New Guinean Mineral Deposits
The Wirralie deposit is in the NE portion of the Drummond Basin, a large NNW-trending intracratonic basin which developed between the Late Devonian and Early Carboniferous. The Drummond Basin sequence crops out over
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an area of 25 000 km2, from south of Charters Towers to south of Clermont (Fig 1). In the NE, the Basin sequence is overlain by the Late Carboniferous Bulgonunna Volcanic Group, and to the SE, south and west, it is overlain by Permian sediment of the Bowen Basin, Denison Trough and Galilee Basin. The Anakie Metamorphic Group and Ukalunda beds to the north form a central NNW-trending inlier of Early Palaeozoic tightly folded quartz-mica schist, phyllite, slate, sandstone, limestone and minor volcanic rocks. The inlier divides the Drummond Basin into an eastern domain, dominated by volcanic successions, and a western domain with a greater sedimentary component. Olgers (1972) described the geology of the basin and recognised three tectonic-sedimentary cycles separated by disconformities within the basin sequence. These include a Late Devonian to Early Carboniferous volcanogenic and proximal sedimentary phase (Cycle1); an Early Carboniferous phase of fluvial sedimentation (Cycle 2); and an Early Carboniferous volcanogenic and sedimentary phase (Cycle 3). Cycle 1 comprises the Silver Hills Volcanics, Saint Anns Formation, Mount Wyatt Formation and Bimurra Volcanics. These units are intermediate to felsic volcanic rocks and proximal volcanolithic sediment and contain Late Devonian to Early Carboniferous flora (Hutton, 1989). The volcanic rocks are in separate sub-basins formed by early extensional tectonics which produced graben and half graben structures. The rocks are characterised by rapid lateral facies changes and typically have associated andesitic volcanic and volcaniclastic rocks. Cycles 2 and 3 rocks were deposited during a broad basinwide subsidence. Sedimentation ceased in the basin during the Middle Carboniferous, and the sequence was deformed soon after in the Kanimblan Orogeny. Intrusive activity occurred throughout the development of the Drummond Basin, from emplacement of Late Devonian domes and plugs associated with the volcanic activity to Late Carboniferous and Permian granitoid intrusions. Flat-lying Tertiary fluviatile and lacustrine sediment of the Suttor Formation and lateritised Drummond Basin sequences associated with Tertiary deep weathering profiles have subsequently been dissected by recent erosion to create a series of mesas that have a strong influence on the present day landscape.
ORE DEPOSIT GEOLOGY LITHOLOGY AND STRUCTURE The host rocks comprise two packages, the Bimurra Volcanics hanging wall sequence and the footwall Mount Wyatt Formation. Locally the Bimurra Volcanics consist of three distinct lithic and crystal-lithic rhyolitic tuffs (designated the Upper, Middle and Lower tuff), each separated by volcaniclastic sediment. The Bimurra Volcanics crop out north of the open pits and have a 20 to 30o northerly dip. To the south of pit B (Fig 2) they dip northeasterly at 10 to 20o. The Mount Wyatt Formation consists of locally carbonaceous well-sorted fluviatile sandstone and siltstone which dip variably to the south and SW at between 20 and 45o. The two packages are separated by a low angle fault called the Moderate Angle shear (MAS) which has a bend in the pit area and strikes east with a 45o dip to the north, and SW with a 45o dip to the NW. The Bimurra Volcanics parallel the MAS and the contact is only exposed in pit A (Fig 2).
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FIG 2 - a. Geological plan; b. cross section, looking west, of pit A; c. cross section, looking west, of pit B.
Further west the Bimurra Volcanics are offset by a series of parallel NW-striking faults possibly related to the nearby NWtrending Anakie Metamorphics–Drummond Basin contact. The change in strike of the MAS also coincides with a later NE-trending structure, the Juggler fault, which is a barren zone that separates the pit A and pit B orebodies. Apparent movement along the Juggler fault indicates that the western block is displaced downwards relative to the eastern block.
Geology of Australian and Papua New Guinean Mineral Deposits
WIRRALIE GOLD DEPOSIT
The Tertiary Suttor Formation covers the southern and eastern strike extents of the Bimurra Volcanics and Mount Wyatt Formation. Drilling indicates that the Tertiary sediments rapidly reach thicknesses of up to 45 m SW of the mining operation. In the SW corner of pit A a basal Tertiary channel of coarse immature conglomerate or scree is exposed that contains angular clasts of epithermal chalcedonic quartz derived directly from the Wirralie mineralisation. Ore grades were found in portions of the conglomerate channel.
ORE CONTROLS Gold mineralisation occurs as a tabular body that strikes east and dips north at 25 to 30o, and is crudely conformable to bedding. Mining to date has been restricted to oxidised ore above the base of oxidation, which averages 40 m below surface. The gold mineralisation in pit A dips north and is bounded by the MAS and the Upper–Middle tuff contact, which suggests some stratigraphic control on mineralisation (Fig 2). In the centre of pit A the mineralisation is characterised by intense stockworks of quartz-chalcedony veins and quartz-matrix breccia veins which include the Amethyst vein, the northern limit of high gold grades. Down dip from the Amethyst vein mineralisation occurs as a broader series of stockwork quartzchalcedony veins and pervasive silica replacement of the volcaniclastic sequence. A series of WNW-striking faults that dip steeply south within the broad stockwork zone also control higher gold grades. In pit B and further west, two parallel high-angle reverse faults that strike east and dip steeply south separate the NEstriking Lower tuff to the south from east-striking Middle tuff to the north. The northernmost east-trending fault places the Middle tuff adjacent to stratigraphically younger sediments. Gold mineralisation in pit B dips gently north, parallel to bedding within and below the Middle tuff horizon (Fig 2). The southern boundary is defined by the east-striking faults. Mineralisation is characterised by broad quartz-chalcedony vein stockworks and pervasive silica replacement of the host volcaniclastic rock as in pit A. Chalcedonic veins within the Middle tuff which strike WNW are locally prominent.
ALTERATION Hydrothermal alteration is evident in both footwall and hanging wall stratigraphic packages. The alteration in pit A is zoned about a barren silica cap which is anomalous in arsenic and antimony. The silica cap is a zone of non texturedestructive quartz-kaolinite which progressively declines in intensity towards the MAS (G Morrison, S Jaireth and H Lawrie, unpublished data, 1992). The quartz-kaolinite alteration overprints illite-pyrite and adularia alteration which coincide with the main area of gold mineralisation. Footwall alteration consists of an upper illite-quartz-pyrite type which grades downwards to illite-carbonate-quartz and possibly to an outer smectite-carbonate halo. West of pit A the silica cap is absent and illite-quartz-pyrite is the dominant alteration type. Peripheral alteration consists of pervasive smectite-carbonate and possibly celadonite.
ORE MINERALOGY In the oxidised zone argentiferous gold, with an average grain diameter of 25 µm, occurs in hematite and goethite, along
Geology of Australian and Papua New Guinean Mineral Deposits
quartz grain boundaries, and in relict pyrite (Fellows and Hammond, 1990). The gold to silver ratio is about 4:1. The pathfinder elements arsenic, antimony and mercury average 270 ppm, 11 ppm and 0.31 ppm respectively (Fellows and Hammond, 1990). Studies have shown that beneath the base of oxidation there are four main mineralisation assemblages based on mineralogy and texture (M L Fellows, unpublished data, 1995). Assemblages 1, 2 and 4 occur within the hanging wall sequence, and assemblage 3 occurs below the MAS. Assemblage 1 is characterised by pervasive silica alteration with minor disseminated fine-grained euhedral pyrite to 1 mm diameter and minor low sulphide, chalcedonic quartz veinlets. Grade is generally below 0.5 g/t gold. Assemblage 2 consists of prominent dark chalcedonic silica containing pyrite or arsenical marcasite as veinlets, veins, matrix to breccias and replacement of rock matrix. Grades vary from 0.5 to 2.5 g/t gold and this assemblage constitutes the majority of the sulphide resource. Assemblage 3 is pyrite-rich footwall shear type mineralisation, and contains traces of arsenopyrite and sphalerite with grades >2.5 g/t gold. Assemblage 4 consists of colloform banded chalcedony veins with fine laminae of goldbearing sulphides (arsenical pyrite?) that can attain very high grades (to 100 g/t gold), in which gold is locally visible. Studies of the four assemblages have identified three types of pyrite on the basis of morphology, size, habit and optical characteristics (G Dong, unpublished data, 1995). Pyrite 1 is very fine grained, ragged, porous and pseudomorphic after marcasite. No visible gold was seen, however ‘invisible gold’ was detected (G Morrison, S Jaireth and H Lawrie, unpublished data, 1990) using ion probe analyses. The invisible gold occurred preferentially in the arsenic-bearing fine grained pyrite (Pyrite 1?) as submicron sized colloidal inclusions. Pyrite 2 is characterised by anisotropic lath shaped and irregular crystals and is often arsenical. It is commonly associated with visible interstitial or intergranular gold. Pyrite 3 consists of isotropic cubic and octahedral crystals and is usually found with Pyrite 2. It is associated with chalcopyrite, arsenopyrite, sphalerite and visible gold. The possible time relationship of the pyrite types is from Pyrite 1 earliest through to Pyrite 3 last. Gold fineness varies from 670 to 841, and particle diameter ranges from 1 to 100 µm. Sulphides within assemblages 1 and 2 are predominantly of type 3 pyrite with minor type 2 and trace type 1. Assemblage 3 and 4 sulphides are dominated by type 1 pyrite.
ORE GENESIS Wirralie occurs within Cycle 1 sediments and volcanic rocks which filled graben type structures during the early formation of the Drummond Basin. Mineralisation is potentially linked to reactivation of such graben type features which manifest as listric style structures. The MAS is interpreted to represent such a structure The change in strike of the MAS from east to SW is thought to control the focus of mineralisation and probably represents a major zone of dilation. The MAS was probably the main conduit for migration of ‘hot spring’ type geothermal fluids. Epithermal silica deposition that formed the silica cap at pit A possibly sealed the system to advancing fluids, and with repeated seismic activity, a multiphase vein stockwork was produced by boiling, brecciation, silica deposition and resealing. The apparent stratigraphic controls of mineralisation are possibly due to
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competency contrasts between the interbeds of sediment and tuff. Alteration patterns and sulphide characteristics confirm the episodic and epithermal nature of mineral deposition. The interpreted sequence of structural events at Wirralie is pre-mineralisation extension, syn-mineralisation compression and relaxation, then post-mineralisation compression, which follows the regional structural sequence (G Morrison, S Jaireth and H Lawrie, unpublished data, 1992).
ACKNOWLEDGEMENTS The authors gratefully acknowledge the permission of Ross Mining NL to publish this information. Particular thanks to S Morris for manuscript preparation and T Holtz for technical drafting. The collection of data has been a combined effort from a number of ACM and Ross Mining geologists who also deserve credit.
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REFERENCES Fellows, M L and Hammond, J M, 1990. Wirralie gold deposit, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1489–1492 (The Australasian Institute of Mining and Metallurgy: Melbourne). Hutton, L J, 1989. A stratigraphic and tectonic model for the Drummond Basin and its relationship to gold mineralisation, in North Queensland Gold 1989 Conference, Oct 30–Nov 4, Australian Mineral Industries Research Association, Project 247, pp 3–12. Malone, E J, Corbett, D W P and Jensen, A R, 1964. Geology of the Mount Coolon 1:250 000 sheet area, Bureau of Mineral Resources Geology and Geophysics Australia, Report 64. Olgers, F, 1972. Geology of the Drummond Basin, Queensland, Bureau of Mineral Resources Geology and Geophysics Australia, Bulletin 132.
Geology of Australian and Papua New Guinean Mineral Deposits
Ruxton, P A and Seed, M J, 1998. Yandan gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 695–698 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Yandan gold deposit 1
by P A Ruxton and M J Seed
2
INTRODUCTION The deposit is 240 km SSE of Townsville, Qld, 40 km west of Mount Coolon and 40 km SW of the Wirralie gold deposit, at lat 21o10′S and long 149o58′E on the Buchanan (SF 55–6) 1:250 000 scale and the Scartwater (8255) 1:100 000 scale map sheets (Fig 1). The property is held by Ross Mining NL. Yandan is a low sulphidation, quartz-adularia epithermal, replacement type deposit hosted by sediment of Late Devonian age.
Reconnaissance soil and rock chip traverse lines originally focussed on East Hill, with values of 1.8 ppm and 1.06 ppm total gold at the end of two lines over Main Hill. With grid based follow up, the +61 ppb gold contour effectively defined the Yandan Main Hill ore zone, in which the maximum surface rock chip value was 9.8 ppm gold. The second WMC drillhole (YNDC2), in July 1987, contained 59 m at 3.04 g/t gold. Intense follow up drilling and engineering investigations culminated in a feasibility study presented at the end of 1988. Further drilling in 1990 resulted in a revised WMC Proved Reserve of 2.7 Mt at 2.3 g/t gold (200 000 oz of contained gold) using a 1.5 g/t gold cutoff and based on data from 310 drill holes for 31 232 m (5776 m core, 20 404 m reverse circulation and 5052 m air track). Two mining leases were granted to WMC in June 1991. The deposit was acquired by Ross Mining in mid 1992 as part of the Wirralie, Mount Coolon, Yandan mining lease and exploration tenement purchase from Poseidon Ltd and WMC. Ross Mining recalculated the Proved and Probable Reserve to be 6.42 Mt at 1.54 g/t gold (318 000 oz of contained gold) in millable ore with a cutoff of 0.7 g/t and a further 2.25 Mt at 0.55 g/t gold (40 000 oz of contained gold) in dump leach material between cutoffs of 0.2 g/t and 0.7 g/t gold. Mining commenced in September 1993 with a mill throughput of 1.2 Mtpa. With the low stripping ratio of 0.22 to 1 and the softer kaolinitic ore of higher grade (+2 g/t) near surface, initial cash operating costs were very low at $A169/oz for the first nine months of production. A mill upgrade to 1.5 Mtpa was completed in June 1996 with a corresponding increase in production of dump leach ore. For the two years and nine months of operation to June 1996 Yandan produced 221 028 oz of gold at an average cash cost of $A214/oz.
FIG 1 - Locality map and regional geological sketch map of the Drummond Basin (modified after Olgers, 1972).
EXPLORATION AND MINING HISTORY Yandan was a greenfields exploration discovery by Western Mining Corporation Ltd. (WMC) in early 1987. Following the Pajingo and Wirralie discoveries in early 1986, WMC commenced regional exploration in a large area originally designated as water development land for the southern extension of the Burdekin Dam. Yandan was identified by a value of 34 ppb total gold in a -80 mesh stream sediment sample on a black soil plain adjacent to the Suttor River.
Two other deposits have also been mined, and treated at Yandan. A total of 0.27 Mt at 5.53 g/t was mined from the Mount Coolon deposit (Fig 1) and milled to yield 47 000 oz of gold, and a further 0.94 Mt at 0.57 g/t was mined from South Hill (Fig 2) and treated by dump leaching. Current Proved and Probable Reserves at Yandan at June 1996 total 3.14 Mt at 1.28 g/t (129 000 oz of contained gold) in mill ore and 2.51 Mt at 0.52 g/t (42 000 oz of contained gold) of dump leach material. Previous published information on the Yandan deposit includes descriptions by Western Mining Corporation (1989), Tate, Morrison and Johns (1992), an MSc thesis study by Goulevitch (1992), Johnston (1994), Chenoweth (1995) and Seed (1995).
REGIONAL GEOLOGY 1.
2.
Exploration Manager, Ross Mining NL, PO Box 1546, Milton Qld 4064. Senior Exploration Geologist, Ross Mining NL, Yandan Gold Mine, PO Box 242, Collinsville Qld 4804.
Geology of Australian and Papua New Guinean Mineral Deposits
The Yandan deposit is in the northern section of the Drummond Basin, a major NNW-trending intracratonic basin developed during the Late Devonian-Early Carboniferous (Fig 1). The basin is overlain to the west by Lower Carboniferous to Triassic
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sample values draining from Drummond Basin rocks by detritus from the Tertiary deposits renders the interpretation of stream anomalies complex. Mineralisation in the Drummond Basin is dominated by high level epithermal gold-silver deposits hosted in volcanic rock or sediment of Late Devonian age. The most significant deposits, at Pajingo, Wirralie and Yandan, are hosted by Cycle 1 deposits of the Drummond Group adjacent to the basement unconformity (Tate, Morrison and Johns, 1992).
ORE DEPOSIT FEATURES STRATIGRAPHY AND STRUCTURE The Yandan mineralisation is hosted by Cycle 1 sediment of the Drummond Group near the contact with the Anakie Metamorphics (Fig 2). The sequence at Yandan is approximately 500 m thick and consists of an upward fining sequence, from a volcanic rich bottom to a carbonate and sediment rich top.
FIG 2 - Geological map of Yandan deposit.
sediment of the Galilee Basin and to the east and south by Permo-Triassic continental and shallow marine sediment of the Bowen Basin. Basement to the Drummond Basin is black phyllite and shale which characterise the Anakie Metamorphic Group of Lower Palaeozoic age. These units are complexly folded and regionally metamorphosed to lower greenschist facies. In many parts of the basin younger basement of Early Devonian sandstone with minor fine grained sediment of the Ukalunda beds unconformably overlies the Anakie Metamorphic Group. These are unconformably overlain by the Late Devonian Drummond Group which contains plant fossils. Olgers (1972) recognised three deposition cycles within the Drummond Group reflecting rifting and widening of the basin with time. Cycle 1 deposition is typified by rapid lateral facies changes in discrete tectonic sub-basins formed at the beginning of the rifting episode. Intermediate and felsic volcanic rocks predominate with coarse grained clastic, immature sediment intermixed. An association of andesitic volcanic intrusives with volcanoclastic rocks is common. Cycle 2 is dominated by fluviatile sediment with greater lateral facies continuity and more uniform unit thickness formed during broader basin sag. More mature fluviatile sediment is characteristic of Cycle 3 with associated lacustrine deposits and volcanic rocks. Large sheets of ignimbritic volcanic rocks and associated intrusive centres dominated in the Late Carboniferous as the basin emerged as a basement high. Further high level intrusion with extrusive components continued into the Permian. The Drummond Basin probably formed a basement high during deposition in the adjacent Permo-Triassic troughs. Tertiary deposits unconformably overly the Drummond Basin sequence and form large flat sheets, often dissected, leaving mesas on the present land surface. These sediments are locally termed the Suttor Formation, and contain oil shale deposits. At least two phases of lateritisation have been recorded in the Tertiary. The dilution of stream sediment
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The lowest part of the sequence is characterised by andesitic volcanic flows and associated coarse grained epiclastic rocks. These pass upwards into the lower Yandan host unit of tuffaceous sandstone and siltstone. The sediments progressively become more fine grained and carbonate rich as they pass from the lower to upper Yandan host unit. Basal marly units pass up into algal limestone at the top of the upper Yandan host. The plant fossils Leptophloeum and Protolepidodendron have been identified (Olgers, 1972) with various lacustrine and shallow marine lamellibranchs. The mineralised sequence passes up into the Yandan limestone which was probably deposited in a shallow marine environment. The Yandan host sequence generally strikes ENE and dips at 20 to 30ο to the NNW. The mine host sequence is repeated in the South Hill area but sparse outcrop prevents a detailed understanding of the relationships. The sequence is bounded by ENE-trending block faults. The faults are downthrown to the north and appear listric, hinting at extensional tectonics operating locally at least. An unusual anticlinal structure described in detail by Goulevitch (1992) has an associated sinter horizon at East Hill.
MINERALISATION Mineralisation at Main Hill is largely confined to calcareous sandstone of the upper Yandan host (Fig 2). The deposit is a roughly tabular body (Fig 3) conformable with the NNW dipping sequence. Alteration around the mineralisation is characteristically zoned. In the core of the deposit the ore host has a porcelaneous texture with fine grained quartz and adularia, probably after carbonate. Plant fossils and bivalve shells are preserved by this selective epithermal replacement. This style of alteration is associated with up to 2% disseminated pyrite, minor graphite, and traces of chalcopyrite and bladed carbonate textures (now replaced by silica). The periphery of the deposit is propylitically altered as a characteristic apple green celadoniteillite halo with carbonate, chlorite and clay. The appearance of the green celadonite alteration marks a decrease in gold grade to less than 0.3 ppm. Within the core of the deposit two dominant ore types were recognised by M J Johnston (unpublished data, 1995). At depth and particularly on the western side of the orebody a rusty
Geology of Australian and Papua New Guinean Mineral Deposits
YANDAN GOLD DEPOSIT
The East Hill prospect was extensively costeaned and drilled by WMC and formed the subject of an MSc thesis by Goulevitch (1992). The deposit is hosted by andesite and coarse grained epiclastic rocks towards the base of the Cycle 1 Drummond Group sequence exposed at Yandan. A well developed sinter associated with an arcuate fault is typical of sinters in the Drummond Basin (Cuneen and Sillitoe, 1989; Goulevitch, 1992). Distinct silica layering on the scale of 1 to 10 mm is accompanied by colloform, chalcedonic and opaline textures with evidence of several stages of deposition of gelatinous silica.
FIG 3 - Schematic gold grade distribution at Yandan: (a) plan at RL 180 m (±12 m below surface); (b) longitudinal section on 48 050 N; (c) cross section on 14 400 E.
orange-brown siliceous ore is typical. Silica is fine grained and dominant over adularia and illite. Beneath the oxide zone this rock is a grey colour, contains disseminated pyrite, and the gold is largely refractory. Along structures, chalcedonic veins and brecciation are observed. The second type is a white kaolinitic ore in which kaolin and illite dominate over adularia and quartz. This type is softer and tends to occur towards the top of the deposit, or is localised along faults at deeper levels. The main ‘feeder’ zone at the top of the deposit which trends ESE consists of kaolinised ore and formed the high grade (+2 g/t) starter in the Stage 1 pit (Fig 3). The gold is of similar grain size to the quartz-adularia alteration. The gold particle size is 92% less than 30 µm and 50% less than 4.5 µm. The fineness is unusually high at 900. Pathfinder elements are present at low levels, with negligible silver and a maximum of 45 ppm copper, 20 ppm antimony and 60 ppm arsenic.
MINOR DEPOSITS The single vertical diamond drill hole drilled by WMC intersected the low grade South Hill deposit, 300 m south of the Main Hill. Data from a further 53 RC holes for 3800 m drilled by Ross Mining, including another diamond hole, were used to define a 1 Mt orebody of dump leach grade (0.57 g/t gold) material. The deposit was mined during the 1996 calendar year. The South Hill ore is similar to the Main Hill fault-localised hydrothermal brecciated ore and consists of siliceous and porcelaneous sediment. The deposit lies in the same stratigraphic position as the main orebody in a structural repetition of the upper Yandan host sequence. The ore zone is ovoid and elongate easterly along a localised fault breccia zone. Mineralisation at the adjacent East Hill prospect contrasts geologically with the Main and South Hill deposits. Gold at East Hill is associated with quartz-adularia-calcite veins which underly a well developed sinter.
Geology of Australian and Papua New Guinean Mineral Deposits
Beneath the sinter horizon pervasive potassic-illitic hydrothermal alteration of the andesitic volcanics accompanies four types of epithermal veins (Goulevitch, 1992). The first three vein types are typically crustiform and contain quartzcalcite-chalcedony and adularia with increasing proportions of calcite and adularia, with finally the addition of pyrite and colloform textures in the latest stage. The fourth type is more massive chalcedony-pyrite±adularia. Veins vary from 1 mm to 50 cm in width and all contain gold as electrum with fineness ranging from 460 to 530. Vein mineralogy includes galena, chalcopyrite, electrum, argentiferous tetrahedrite, pyrargyrite, polybasite and eucarite.
ORE GENESIS Detailed mapping of the Stage 1 pit at the Main Hill deposit by M J Johnston (unpublished data, 1995) identified steep NEtrending structures at either end of the pit with a central ESEtrending fault. This central fault coincided with the feeder zone identified by WMC. Towards the top of the deposit hydrothermal breccias were recognised along the NE- and ESE-trending faults. The number of breccia zones diminishes with depth and they become narrower. The evidence points to the mineralisation and alteration being focussed around a fault offset or jog, which created structural space through which the hydrothermal fluids moved. Boiling probably occurred towards the top of the deposit and is evidenced by the development of widening hydrothermal breccia zones and the association of these zones with argillic and kaolinitic alteration. Boiling triggered by structural release produced a change from more alkaline conditions evidenced by adularia deposition to more acid kaolinite-illite assemblages which overprinted earlier alteration. Some concentration of gold during this event is suggested by the higher grades in the feeder zone which is highly kaolinised and probably formed a major focus of fluid flow. In addition to the structural control of mineralisation the tabular nature of the orebody (Figs 3a, b) and its association with a particular calcareous sandstone suggests a subordinate lithologic control. Selective replacement of primary calcareous grains by silica and adularia within this particular unit was the probable mechanism of ore deposition.
ACKNOWLEDGEMENTS The authors gratefully acknowledge the permission of Ross Mining NL to publish this information. Particular thanks to S Morris for manuscript preparation and T Holtz for the technical drafting. The collection of these data has been a team effort by WMC and Ross Mining geologists.
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REFERENCES Chenoweth, L M, 1995. Geochemical orientation for stream sediment and soil sampling at the Yandan gold deposit, Drummond Basin, North Queensland, poster presentation to 17th International Geochemical Exploration Symposium, Townsville, May 1995, Association of Exploration Geochemists. Cuneen, R and Sillitoe, R H, 1989. Paleozoic hot spring sinter in the Drummond Basin, Queensland, Australia, Economic Geology, 84:135–142. Goulevitch, J, 1992. Stratigraphy, structural geometry and relationship between sinters and epithermal gold mineralisation at East Hill, Yandan, Queensland, MSc thesis (unpublished), James Cook University of North Queensland, Townsville. Johnston, M D, 1994. Mineralisation style and controls: Yandan Mine area, in New Developments in Geology and Metallogeny: Northern Tasman Orogenic Zone, Conference, 21–22 February
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1994 (Eds: R A Henderson and B K Davis), p 105 (Contributions of the Economic Geology Research Unit, James Cook University, North Queensland). Olgers, F, 1972. Geology of the Drummond Basin, Bureau of Mineral Resources Geology and Geophysics, Bulletin 132. Seed, M J, 1995. Discovery history, geology and geochemistry of the Yandan gold deposit, field excursion notes for the 17th International Geochemical Exploration Symposium, Townsville, May 1995, Association of Exploration Geochemists. Tate, N M, Morrison, G W and Johns, H J, 1992. Gold mineralisation in the northern Drummond Basin, Queensland, Australian Mineral Industries Research Association Gold Metallogenic Bulletin, No 16. Western Mining Corporation, 1989. The Yandan gold project, excursion guide in Proceedings NQ Gold ’89 Conference, (Ed: C Cosstick), pp 4–11 (The Australasian Institute of Mining and Metallurgy: North Queensland Branch).
Geology of Australian and Papua New Guinean Mineral Deposits
Forrestal, P J, Pearson, P J, Coughlin, T and Schubert, C J, 1998. Tick Hill gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 699–706 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Tick Hill gold deposit 1
2
3
by P J Forrestal , P J Pearson , T Coughlin and C J Schubert INTRODUCTION The deposit is 110 km SE of Mount Isa, Queensland, and 15 km north of The Monument township, at lat 21o39′S, long 139o55′E, on the Duchess (SF 54–6) 1:250 000 scale and the Dajarra (6854) 100 000 scale map sheets (Fig 1). It was discovered by Mount Isa Mines Limited in October 1989 during regional exploration for copper-gold mineralisation.
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analysis for base metals and a bulk sediment sample for determination of cyanide soluble gold at each site. Sampling density for gold was approximately one per 4 km2. A site returning 6.95 ppb cyanide soluble gold (Fig 2) led to the Tick Hill discovery.
FIG 1 - Location map, from Forrestal et al (1995). FIG 2 - Cyanide-soluble gold values for bulk stream sediment samples, from Forrestal et al (1995).
Gold mineralisation occurs in the Corella Formation, a Middle Proterozoic unit of calc-silicate rock, quartzite and schist. The host rocks for the gold mineralisation are bodies of quartz-feldspar ‘laminite’ with a lenticular, probably mylonitic internal structure. Gold occurs as disseminations of free native metal particles with diameters ranging from 1 µm to 2 mm. Mining commenced in 1992 and gold production was completed in 1995, to yield 15 900 kg (511 000 oz) of gold from 706 000 t treated.
EXPLORATION AND MINING HISTORY Forrestal et al (1995) document the discovery of the deposit. The following paragraphs summarise the main points. Stream sediment sampling was used as a primary reconnaissance technique, collecting the -80 mesh fraction for 1.
MIM Holdings Limited, 410 Ann Street, Brisbane Qld 4000.
2.
Ore-Forming Solutions SA, Lima, Perú.
3.
Department of Earth Sciences, University of Queensland, St Lucia Qld 4076.
4.
McArthur River Mining Pty Ltd, PO Box 36821, Winnellie NT 0821.
Geology of Australian and Papua New Guinean Mineral Deposits
Additional bulk stream sediment samples were collected as follow up, taking one at the anomalous site and seven from tributaries further upstream (Fig 2). The anomalous value at the original site was not repeated, the check sample returning only 0.85 ppb. Upstream, the eastern tributary returned a weakly anomalous value of 1.20 ppb and the western tributaries showed moderately anomalous values of 6.30, 7.05 and 7.35 ppb. The two central tributaries returned background values. Attention was then focussed on the western part of the drainage. Surface soil samples collected on lines parallel to the creeks returned many gold values in the range 10 to 88.15 ppb. Soils in the headwaters of the western drainage were then sampled on a 100 m north by 25 m east grid as shown in Fig 3. A coherent >0.5 ppm anomaly, comprising six samples on three adjacent grid lines with a peak value of 7.90 ppm, was the obvious focus for immediate follow up. A drill rig was mobilised into the area in October 1989 and seven reverse circulation (RC) drill holes were completed for a total of 798 m. All of the holes were declined toward the east against the moderate west local dip (Fig 3). Significant gold intersections were made in three holes drilled under the southern half of the main soil anomaly.
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FIG 3 - Soil sample values and first phase drill hole sites, from Forrestal et al (1995).
Three diamond core holes were drilled under the RC holes (Fig 3). The first intersected a zone of strong visible gold mineralisation, returning 15 m at 39.0 g/t. The second, 60 m south, had a narrower, lower grade but still probably economic intersection, while the third, 60 m north, was subeconomic. An intensive phase of exploration and orebody delineation followed, using RC and diamond drilling from surface sites. Final definition of the deeper part of the orebody was completed by diamond drilling from hanging wall drill drives after underground access had been established. Mining started in August 1991 on Proved Ore Reserves and additional Identified Mineral Resources as in Table 1 (MIM Holdings, 1991). A 70 m deep open pit was mined by drill and blast on 5 m benches and ore was extracted by two 2.5 m flitches. Selective mining produced high grade (148 600 t at 31.4 g/t gold) and low grade (161 200 t at 5.8 g/t gold) ores. Underground development commenced in January 1992 and mining to a depth of 240 m below the surface, to produce 396 200 t at 26.0 g/t gold, was completed in January 1994. Exploration in the surrounding area has not found any additional ore.
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TABLE 1 Tick Hill resources and reserves, August 1991. Ore (t)
Gold grade (g/t)
Ore Reserves
Proved
218 000
17.1
Identified Mineral Resources
Measured
95 000
37.0
Indicated
121 000
39.9
Inferred
36 000
30.0
REGIONAL GEOLOGY Tick Hill occurs within the southern part of the Mary Kathleen Fold Belt (MKFB) of the Lower to Middle Proterozoic Mount Isa Inlier. Within the MKFB, volcanic and sedimentary rocks which were deposited between about 1870 and 1670 Myr (Cycles 1 to 3 of Blake, 1987) have been deformed, metamorphosed and intruded by granitoids and mafic rocks in a protracted tectonothermal phase between about 1740 and 1500
Geology of Australian and Papua New Guinean Mineral Deposits
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Myr. Metamorphosed rocks of Cycle 2 dominate the MKFB and comprise a coherent stratigraphic sequence of, in ascending order, mafic volcanic rocks of the Magna Lynn Metabasalt, felsic volcanic rocks of the Argylla Formation, arenites of the Ballara Quartzite, and calc-silicate and pelitic rocks of the Corella Formation. The Cycle 2 succession is thought to have been deposited during a rift-sag cycle between about 1800 and 1740 Myr (Etheridge, Rutland and Wyborn, 1987). The Corella Formation originally contained a large evaporite component, which accounts in part for the unusual range of rock types and postdepositional alteration suites (Oliver and Wall, 1987). Three regional deformational and/or thermal phases are recognised throughout the MKFB (Holcombe, Pearson and Oliver, 1992; Passchier, 1986): 1.
D1 (or De) around 1740 Myr was largely associated with extension and intrusion of the Wonga–Burstall bimodal intrusive suite.
2.
D2 around 1550 Myr involved east–west shortening and upright folding that produced much of the ‘grain’ of the fold belt.
3.
D3 around 1500 Myr involved broadly east–west shortening, faulting and intrusion of the Williams–Naraku bimodal intrusive suite to the east.
The most intense deformation, metamorphism and intrusive activity was focussed around the Wonga–Shinfield Zone (Derrick, 1980), a north-striking D2 anticlinorium which exposes an interpreted tightly folded D1 extensional décollement developed around 1740 Myr (Passchier, 1986; Pearson, Holcombe and Oliver, 1987; Passchier and Williams, 1989; Holcombe, Pearson and Oliver, 1991). This zone is essentially stratabound, up to 2 km thick, and ramps up the section from the Argylla Formation to the Corella Formation from west to east across an interpreted north–south extensional transfer zone. In the southern MKFB, the Wonga–Shinfield Zone is manifest as a linear belt of high D1 strain that is offset by several NE- and NW-striking D3 faults such as the Fountain Range and Plum Mountain faults. After strike slip displacements on these late fault structures are restored, the Wonga–Shinfield Zone can be traced southwards into the ‘Tick Hill enclave’, a fault block bounded by the Plum Mountain Fault to the north, the Mount Bruce Fault to the south, the Pilgrim Fault to the east and the Saint Mungo Granite to the west (Fig 4).
ORE DEPOSIT FEATURES Various aspects of the geology of the Tick Hill deposit and its surrounds are described in unpublished theses and internal Company reports, which are cited in the relevant parts of the text.
STRATIGRAPHY The Tick Hill enclave is regionally distinct from elsewhere in the MKFB because of the amount of pre-D2 metasomatism and associated felsic meta-intrusives (N H S Oliver, unpublished data, 1992). The nature and distribution of this metasomatism, and the possible implications for mineralisation at Tick Hill, prompted 1:10 000 scale geological mapping in the area (T J Coughlin, unpublished data, 1993).
Geology of Australian and Papua New Guinean Mineral Deposits
The enclave is dominated by calc-silicate, silicate and pelitic rocks of the Corella Formation and volcaniclastic rocks of the Argylla Formation, all metamorphosed to amphibolite facies. A laterally discontinuous quartzite <50 m thick defines the stratigraphic boundary between the Argylla and overlying Corella formations. This horizon is assumed to correlate with the Ballara Quartzite, identified by previous workers in the same stratigraphic position elsewhere along the MKFB (Stewart, 1992; Reinhardt, 1992; Holcombe, Pearson and Oliver, 1991, 1992). Felsic sills and dykes, interpreted to be correlatives of the Wonga–Burstall granite suite, intrude and metasomatise this Middle Proterozoic succession. The sequence has been affected by all three of the major regional deformation events. Younger rocks which unconformably overlie the Lower to Middle Proterozoic succession include Adelaidean sandstone, Cambrian near shore marine facies rocks, Tertiary laterite and younger fluvial sediment and palaeosoil. The sequence at Tick Hill dips moderately to the west and from the top down consists of: 1.
calc-silicate rocks of the Corella Formation;
2.
hanging wall quartzite, with a well developed magnetite zone;
3.
the ‘lodestones’ enclosing the strongly gold mineralised laminite bands;
4.
footwall quartzite; and
5.
footwall pelitic (biotite-quartz-plagioclase±sillimanite) schist, possibly of the Argylla Formation.
STRUCTURE AND METASOMATISM D1 deformation is characterised by the development of a penetrative, folded, originally subhorizontal foliation (S1), parallel zones of distributed shearing, and a discrete stratabound shear zone some 50 to 100 m in true thickness. This shear zone is known locally as the Tick Hill shear zone (THSZ) and broadly follows the boundary between Corella Formation calc-silicate and underlying volcaniclastic and pelitic rocks of the Argylla Formation. The THSZ was first recognised by V J Wall (unpublished data, 1989) and has been correlated with part of the Wonga–Shinfield Zone to the north (P J Pearson, unpublished data, 1991). D1 strain usually increases continuously towards the THSZ, and is characterised in calc-silicate rocks by the progression from highly partitioned shearing and local recumbent isoclinal folding to penetrative transposition and strongly developed metasomatic layering. In the underlying meta-intrusives and in the rocks of the Argylla Formation, a penetrative S1 foliation is cut by local small scale mylonite zones. At a local scale and particularly within the THSZ, strain is strongly inhomogeneous. A strong L1 mineral extension lineation is present in the Tick Hill mine (P J Pearson, unpublished data, 1991; C K Mawer, unpublished data, 1992) and elsewhere throughout the Tick Hill enclave, in areas of well developed S 1. Prior to the subsequent deformation events, S1 was horizontal and the L1 lineation trended approximately NW. Westward-verging asymmetric intrafolial folds, asymmetric porphyroclasts and the westward plunge of F1 fold hinges in zones of highest D1 strain collectively indicate top block west movement on the THSZ. Several WNW striking fault zones and magnetic
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FIG 4 - Geological map of the Tick Hill enclave, adapted from T J Coughlin (unpublished data, 1993).
lineaments are apparent in Corella Formation calc-silicate rocks structurally above the THSZ. These may constitute high angle D1 fault zones that have developed synchronously with deformation on the subhorizontal THSZ (Passchier and Williams, 1989). The local abundance of scapolite- and albite-rich rocks, coupled with the occurrence of quartz-diopside-magnetite ‘skarns’ and lensoidal quartz bodies, indicate strongly localised fluid and heat flow within and adjacent to the THSZ (N H S Oliver, unpublished data, 1992). Scapolite, albite and quartz are associated with zones of high D1 strain and with pre-D2 felsic meta-intrusive phases, evident particularly along the THSZ. The felsic meta-intrusive rocks are variably affected by this metasomatism and by D1 (T J Coughlin, unpublished data, 1993). Furthermore, confinement of most of the meta-intrusive rocks below the THSZ suggests that it controlled synkinematic dilation and emplacement (Pearson, Holcombe and Page, 1992).
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D2 in the Tick Hill enclave is characterised by upright, noncylindrical, doubly plunging, east verging macroscopic folds, by axial surface faults and by west block up reverse faults (Fig 4). The THSZ and its internal fabric are clearly folded by D2, such that the footwall meta-intrusives and Argylla Formation rocks are exposed in the D2 antiforms and the hanging wall Corella Formation calc-silicate rocks are exposed in D2 synforms. Penetrative S2 is poorly developed relative to elsewhere along the MKFB, and is generally represented locally by a spaced amphibole and quartz-amphibole cleavage. Complicated D2–D1 fold interference patterns are evident in the Corella Formation calc-silicate rocks throughout the Tick Hill enclave. A series of NE- to ENE-striking faults offset presumed D1 WNW-trending faults, the THSZ and the D2 fold axial traces. These D3 faults are associated with NE-striking, open, doubly plunging, inclined folds. D3 is accompanied by ‘red rock’ alteration, which in the Tick Hill enclave generally
Geology of Australian and Papua New Guinean Mineral Deposits
TICK HILL GOLD DEPOSIT
comprises hematite-epidote-sericite-potassium feldsparalbite± magnetite ± actinolite, as veining, breccia and wall rock alteration. The orebody is in the eastern limb of a tight, asymmetric D2 synform, which is subhorizontal to gently north plunging in the mine area. The broadly tabular orebody dips 55o to the west (Fig 5) and has a total strike length of 140 m at the surface, tapering to 30 m at its deepest level 240 m below surface. The orebody varies in width from 1 to 30 m with an average of 18 m and pitches steeply down the dip of its planar enveloping surface. This shoot parallels the orientation of a strong downdip L1 lineation within the plane of the dominant foliation.
FIG 5 - Cross section through the Tick Hill deposit, on 1880 N, looking north, from Schubert and Crookes (1993).
MINERALISATION High grade gold mineralisation occurs in generally stratabound, quartz-feldspar laminite bands enclosed within a broader strongly strained zone (Fig 6a). Individual goldbearing laminite bands are 0.01 to 0.50 m thick and are continuous along strike and down dip for up to 50 m. The bands commonly have sharp contacts with the surrounding rocks and are generally zones of strong quartz veining and silicification and intense sodium metasomatism. Early oligoclase is overprinted by later albite and/or scapolite. Most of the quartz lenticles are stretched, folded and recrystallised about the west dipping foliation. A few thin gold-bearing quartz veins are relatively unstrained and occur as millimetre to centimetre scale en echelon arrays overprinting the earlier veins (S P Boda, unpublished data, 1993). The mineralised envelope lies structurally below the hanging wall quartzite (Fig 5) and is hosted in ‘lodestone’, a grey to green to purple translucent quartz-rich rock with interspersed layers of chloritised biotite, altered hornblende, albite, magnetite, hematite, pyrite, sphene, scapolite and apatite and minor calcite and epidote. The lodestone appears to be the silicified and altered product of former calc-silicate rocks,
Geology of Australian and Papua New Guinean Mineral Deposits
scapolite schist and amphibolite (S P Boda, unpublished data, 1993; R A Crookes, unpublished data, 1993). The boundaries of the ore zones are sharp and gold anomalism (>0.02 ppm) does not generally extend more than 2 m laterally beyond the limits of the orebody. The orebody has a sharp northern boundary but the southern margin is more diffuse, being affected by strong silicification and polycyclic brecciation related to post-mineralisation faulting. Some 90% of the gold in the deposit is contained within the quartz-feldspar laminite bands and the remainder is hosted by the surrounding lodestones (R A Crookes, unpublished data, 1993). Gold grades within the laminite bands reach a maximum of 3209 g/t recorded over 0.50 m (Schubert and Crookes, 1993). More typically, values range between 10 and 1000 g/t. Gold grades in the surrounding lodestones are much lower, in the range 0–10 ppm (S P Boda, unpublished data, 1993). The orebody contains sporadic copper values to 1200 ppm. Although traces of euhedral pyrite and minor pyrrhotite are present, there appears to be no systematic relationship with the gold mineralisation (R A Crookes, unpublished data, 1993). S P Boda (unpublished data, 1993) noted at least two generations of pyrite. The quartz-feldspar laminite bands and their internal foliation are commonly folded about steep D2 axial planes within the deposit, and are therefore interpreted as zones of strong D1 strain partitioning, recrystallisation and fluid flow within the THSZ (P J Pearson, unpublished data, 1991; N H S Oliver, unpublished data, 1992; S P Boda, unpublished data, 1993; R A Crookes, unpublished data, 1993). Their relatively early timing is confirmed where amphibolite facies alteration assemblages (eg amphibole-scapolite-silica-muscovite) were developed as veins and diffuse patches in a low strain environment (D2) which in places overprint or ‘digest’ the earlier quartz-feldspar mylonites (P J Pearson, unpublished data, 1991; S P Boda, unpublished data, 1993). Some bands have undergone D3 brittle fracturing and shearing, recorded by oblique en echelon vein sets containing quartz and hematite fill (R A Crookes, unpublished data, 1993). This overprinting retrograde alteration and deformation are best exposed in the southern end of the orebody, where individual mylonite bands are completely brecciated and cut by late quartz-hematite. Gold at Tick Hill occurs in four or possibly five textural settings (S P Boda, unpublished data, 1993). The first texture comprises grains <0.75 mm in diameter and sometimes euhedral in form contained within the strained quartz, albite or potassium feldspar of the laminite bands (Fig 6b, c). These gold grains are included in and are in textural equilibrium with quartz grains possessing a strong grain shape and a lattice preferred orientation reflecting the D1 fabric (N H S Oliver, unpublished data, 1992). The second gold texture is a breccia featuring gold grains <0.25 mm in diameter, commonly pseudomorphs after feldspars. A third, possibly related, texture comprises coarse euhedral grains of gold (<2.00 mm diameter) which are intimately associated with albitic alteration. Overgrowths of ragged gold on earlier euhedral gold are reported by N J W Croxford (unpublished data, 1993), and are believed to be associated with a retrograde phase, (Fig 6d). An additional late stage gold assemblage is related to chlorite-hematite and breccia textures. This gold is generally coarse grained (0.5–1.0 mm diameter) and subhedral, located along joint surfaces.
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FIG 6a - A typical quartz-feldspar laminite or mylonite that hosts the high grade gold mineralisation at Tick Hill. Note the less strained lodestone at the bottom of the photo, from R A Crookes (unpublished data, 1993).
FIG 6c - Euhedral gold inclusion contained within recrystallised quartz grain in a quartz-feldspar mylonite. Note the similar extinction (blue) of many grains, suggesting an inherited axis preferred fabric through several grains (D1?), from N J W Croxford (unpublished data, 1993).
FIG 6b - Inclusions of euhedral gold crystals in recrystallised quartz grains in the quartz-feldspar mylonite. Note retrogression of much of the feldspar to albite-hematite ‘red rock’, from N J W Croxford (unpublished data, 1993).
ORE GENESIS The genesis of the Tick Hill deposit remains contentious seven years after discovery. The key issues are the timing of emplacement of the bulk of the gold with respect to regional tectonothermal events and the structural and stratigraphic controls. Gold emplacement has been proposed as occurring : 1.
Synchronous with deposition of the enclosing volcanosedimentary sequence, perhaps in a rift-hosted hot spring environment (R N England, unpublished data, 1995).
2.
Synchronous with the regional D1 extensional deformation and granitoid emplacement (Choy, 1992; N H S Oliver, unpublished data, 1992; C K Mawer, unpublished data, 1992; T J Coughlin, unpublished data, 1992; S P Boda, unpublished data, 1993; Watkins, 1993).
704
FIG 6d - A composite gold grain, with a suspected core crystal (prograde assemblage?) (1), rimmed by a younger overgrowth of dendritic gold (retrograde assemblage?) (2), from N J W Croxford (unpublished data, 1993).
Geology of Australian and Papua New Guinean Mineral Deposits
TICK HILL GOLD DEPOSIT
3.
4.
Synchronous with regional D2 upright folding and prograde metamorphism (P J Pearson, unpublished data, 1991). Synchronous with D3 late faulting, kinking and granitoid emplacement (W P Laing, unpublished data, 1992; D I Groves and N McNaughton, unpublished data, 1995).
Textural evidence is ambiguous, because the gold grains are commonly equant and rarely show deformed shapes (R A Crookes, unpublished data, 1993). However, trains of grains subparallel to the D1 foliation are present. Two interpretations are possible, the first being that gold was present before being incorporated into the equant but deformed quartz and plagioclase grains during progressive dynamic recrystallisation and grain boundary migration at high temperatures. The second possibility is that gold was introduced late in the tectonic history, forming a disseminated ‘dusting‘ subsequent to the formation of the D1 laminites (W P Laing, unpublished data, 1992). N H S Oliver (unpublished data, 1992) determined that the weight of evidence points to a large proportion of the gold in the deposit being present early, probably during D1. The critical evidence is that gold occurs mainly as inclusions in recrystallised quartz ribbons, or in albite or potassium feldspar grains, most commonly in zones of highest apparent D1 strain (D J Patterson, unpublished data, 1990, 1991). Furthermore, equant quartz grains containing gold possess a strong crystallographic preferred orientation parallel to the D1 foliation dominating the deposit (N J W Croxford, unpublished data, 1993; Fig 6c). Earlier oligoclase and scapolite with gold are commonly replaced or veined by aggregates of ‘chequerboard’ albite±hematite±sericite±potassium feldspar ±chlorite±gold (D J Patterson, unpublished data, 1990, 1991; N J W Croxford, unpublished data, 1990, 1993). The later assemblage is, however, transgressive and retrograde whereas the earlier is a prograde assemblage clearly formed prior to, or during, the process that formed the laminite bands. Spatial overlaps between the two sodic assemblages possibly led to the plethora of conflicting relationships documented by W P Laing (unpublished data, 1992) and by D I Groves and N McNaughton (unpublished data, 1995). More recent Pb-Pb ages obtained by D I Groves and N McNaughton (unpublished data, 1995) from coarse pyrite separates from Tick Hill are consistent with an age of 1530 Myr (D2–D3). This age could be interpreted as the primary gold mineralisation age, or as a ‘reset’ age related to D2–D3 retrograde alteration and pyrite growth, and therefore does not unequivocally date the deposit. The geometric and timing relationships suggest that the orebody was localised in a dilatant lateral ramp in the original extensional shear zone (C K Mawer, unpublished data, 1992). Genetic links with the pre- to syn-D1 pegmatites are argued by Watkins (1993), because of the strong sodium metasomatism (albitisation and scapolitisation) around the early intrusive bodies. The quartz-feldspar-gold laminites probably formed by quartz veining of albitic pegmatite through complex multistage fractionation of the Saint Mungo Granite in a sodium chloride rich system (Watkins, 1993). Finally, analogies for the deposit have been drawn with the Middle Proterozoic Starra and Osborne copper-gold deposits of the Cloncurry district (V J Wall, unpublished data, 1989) and with the Archaean Hemlo gold deposit of Canada (N J W Croxford, unpublished data, 1990).
Geology of Australian and Papua New Guinean Mineral Deposits
ACKNOWLEDGEMENTS The authors thank the management of MIM Exploration Pty Ltd and Carpentaria Gold Pty Ltd for permission to publish this paper. The text was typed and the paper compiled by S Fleming and the figures were prepared by H Richmond. The paper was critically reviewed by A D Munt. Many workers contributed significantly to the Tick Hill story. The contributions of the exploration team led initially by W G Perkins and later by K R Buckland, of C J Tedman-Jones at the detailed evaluation stage and of the mine geology team led by R A Crookes are particularly acknowledged.
REFERENCES Blake, D H, 1987. Geology of the Mount Isa Inlier and environs, Queensland and Northern Territory, Bureau of Mineral Resources Geology and Geophysics Bulletin 225. Derrick, G M, 1980. Marraba, Queensland - 1:100 000 geological series, Bureau of Mineral Resources Geology and Geophysics, Geological Map Commentary 6956. Etheridge, M A, Rutland, R W R and Wyborn, L A I, 1987. Orogenesis and tectonic process in the Early to Middle Proterozoic of northern Australia, in Proterozoic Lithospheric Evolution, Geodynamic Series 17 (Ed: A Kröner), pp 131–147 (American Geophysical Union; and Geological Society of America: Washington, DC). Forrestal, P J, Perkins, W G, McIntosh, D C and Buckland, K R, 1995. Discovery and evaluation of the Tick Hill gold mine, North West Queensland, in New Generation Gold Mines, pp 13.1–13.10 (Australian Mineral Foundation: Adelaide). Holcombe, R J, Pearson, P J and Oliver, N H S, 1991. Geometry of a Middle Proterozoic extensional décollement in northeastern Australia, Tectonophysics, 191:255–274. Holcombe, R J, Pearson, P J and Oliver, N H S, 1992. Structure of the Mary Kathleen Fold Belt, Bureau of Mineral Resources Geology and Geophysics Bulletin 243: 257–288. MIM Holdings, 1991. Annual Report to shareholders (MIM Holdings Limited: Brisbane). Oliver, N H S and Wall, V J, 1987. Metamorphic plumbing system in Proterozoic calc-silicates, Queensland, Australia, Geology, 15:793–796. Passchier, C W, 1986. Evidence for early extensional tectonics in the Proterozoic Mount Isa Inlier, Australia, Geology, 14:1008–1011. Passchier, C W and Williams, P R, 1989. Proterozoic extensional deformation in the Mount Isa Inlier, Queensland, Australia, Geological Magazine, 126:43–53. Pearson, P J, Holcombe, R J and Oliver, N H S, 1987. The Mary Kathleen Fold Belt, northwest Queensland: D1 - a product of crustal extension? Geological Society of Australia Abstracts, 19:37–38. Pearson, P J, Holcombe, R J and Page, R W, 1992. Synkinematic emplacement of the Middle Proterozoic Wonga batholith into a mid-crustal extensional shear zone, Mount Isa Inlier, Queensland, Australia, Bureau of Mineral Resources Geology and Geophysics Bulletin 243:289–328. Reinhardt, J, 1992. The Corella Formation of the Rosebud Syncline (central Mount Isa Inlier): deposition, deformation and metamorphism, Bureau of Mineral Resources Geology and Geophysics Bulletin 243:229–255. Schubert, C J and Crookes, R A, 1993. Tick Hill gold mine, open pit to underground, AIG Bulletin, 14:87–94. Stewart, A J, 1992. Stratigraphy, extension and contraction in the Ballara-Mount Frosty area, Mount Isa Inlier, Queensland, Bureau of Mineral Resources Geology and Geophysics Bulletin 243: 209–227. Watkins, R H, 1993. Deformation, felsic magmatism and alteration associated with the Tick Hill gold deposit, N W Queensland, BSc Honours thesis (unpublished), Monash University, Melbourne.
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Mustard, R, 1998. Belyando gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 707–714 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Belyando gold deposit by R Mustard
1
INTRODUCTION The deposit, formerly Hill 266, is 75 km NW of Clermont in central Queensland, at lat 22o17′S, long 147 o18′E; or AMG coordinates 530 400 m E, 7 535 200 m N on the Clermont (SF 55–11) 1:250 000 scale and the Frankfield (8353) 1:100 000 scale map sheets (Fig 1). The Belyando mine was operated by Ross Mining NL and consisted of an open cut with a 550 000 tpa heap leach facility. A pre-mining resource of 2.5 Mt grading 1.49 g/t gold was estimated based on a cutoff grade of 0.7 g/t gold for 119 800 oz of contained gold, to 90 m depth. Open cut mining of the deposit commenced in March 1989 and ceased in July 1993, with heap leaching continuing until the mine officially closed in November 1995. Total production was 2670 kg (85 846 oz) with average gold recovery exceeding 72%, and Belyando mine was placed in the lowest tenth percentile on the international cost curve for gold producers. Nearby developments include the Lucky Break mine, 15 km NNW of Belyando, which was operated by East-West Minerals NL between 1988 and 1989 and produced 216 kg of gold. Belyando is the most significant hard rock gold deposit hosted by rocks of the Anakie Metamorphic Group.
EXPLORATION AND MINING HISTORY Gold was discovered in the Clermont district in 1861 near Peak Downs, and the Clermont Goldfield was one of the major alluvial gold producers in Queensland. Production from 1878 to 1901 was 5.76 t or 185 400 oz. Gold was won from alluvial sources and to a lesser degree from narrow quartz reefs. Total gold production from the Anakie region to July 1996 is estimated to be about 14.9 t or 480 000 oz. An estimated total combined gold production of 2886 kg or 93 000 oz from the Lucky Break and Belyando mines between 1987 and 1995 represents approximately 20% of the total gold production for the region. The Hill 266 prospect was discovered in December 1985 by Australian Consolidated Minerals (ACM) during regional reconnaissance exploration. In May 1986 Menzies Gold NL (Menzies) acquired 50% of Authority to Prospect (ATP) 4165M, and the prospect, from ACM. Menzies Gold acquired the remaining share in February 1988, and sold the property to Ross Mining in April 1988, when the prospect was renamed ‘Belyando’. All exploratory work on the prospect was undertaken during the term of ATP 4165M, by ACM, Menzies Gold and Ross Mining. The evaluation program involved rock chip sampling; geological mapping; ground magnetic, dipole-dipole induced 1.
Exploration Geologist, Ross Mining NL, PO Box 1546, Milton Qld 4064.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Location map and simplified local geological map of the Belyando-Lucky Break district (adapted from Withnall et al, 1995).
polarisation (IP), chargeability and resistivity surveys; costeaning; and reverse circulation (RC), open hole percussion and diamond drilling. A program of 20 costeans for 887 m and
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R MUSTARD
a detailed 7000 m grid-based IP survey targeted shallowly covered extensions to mineralisation prior to drilling. By June 1988 a total of 8082 m of RC drilling in 129 holes, 915 m of diamond drilling in 8 holes, and 560 m of open hole percussion drilling in 20 holes had been completed on a 20 by 20 m spacing. A full feasibility study of the Belyando project commenced in July 1988. A Proved Reserve of 1.156 Mt at 2.19 g/t gold using a 1.2 g/t cutoff grade (81 000 oz of contained gold) to 50 m depth was established in August 1988. Acquisition of the 150 000 tpa Lucky Break carbon in pulp (CIP) plant from Gold Copper Exploration Ltd in November 1988 led to the decision to commence mining operations at Belyando, and the first gold was poured in March 1989. Between March 1989 and June 1990, 132 387 t of ore grading 2.56 g/t gold were mined and treated by the CIP plant delivering 9608 oz of gold. Mining during the period was based on selective mining of high grade (cutoff grade 1.8 g/t) and low grade (cutoff grade 0.7 g/t) ore, the former being processed through the Lucky Break CIP plant and the latter stockpiled. A second feasibility study involving the installation of a 550 000 tpa heap leach operation led to a decision to treat all Belyando ore by heap leaching of ore won by bulk mining methods using a cutoff of 0.7 g/t. The Lucky Break plant was placed on care and maintenance in July 1990, and construction of the heap leach facility was completed by the end of June 1990. A revised resource of 2.37 Mt grading 1.43 g/t gold based on a cutoff grade of 0.7 g/t gold (109 000 oz of contained gold) to 90 m depth was calculated. A third feasibility study determined that direct dump leaching of run of mine ore, without crushing the material grading 0.5 g/t to 0.7 g/t gold which was previously classified as waste, would increase profitability. For the period July 1990 to June 1995, 75 612 oz of gold were delivered from the heap leach operation. Total gold production over the life of the mine was 2670 kg (85 840 oz) from combined CIP and heap leach operations, with average gold recovery exceeding 72%.
PREVIOUS DESCRIPTIONS The only published detailed description of the geology of the Belyando deposit is that of Mustard (1989), summarised in Mustard (1990). Resource calculations and production statistics are available from company annual reports between 1988 and 1996. Mackay (1987) completed a study of the Lucky Break deposit and several nearby abandoned gold prospects, documenting features of ‘Anakie style’ deposits.
and Jensen, 1964; Malone, 1968; Veevers et al, 1964a, b; Olgers, 1968, 1969, 1972; Hutton et al, 1991). Surveys covering the Late Devonian to Early Carboniferous Drummond Basin (Olgers, 1972) and the Permian Bowen Basin (Dickins and Malone, 1973) also included portions of the intervening Anakie Inlier. These surveys defined the extent of the Anakie Metamorphic Group and recorded structural data, however the rocks were not studied in detail. The most recent detailed study and review of regional geology of the southern Anakie Inlier was by Withnall et al (1995). The Anakie Metamorphic Group was subdivided into six main mappable units, namely the Bathampton Metamorphics, Rolfe Creek Schist, Monteagle Quartzite, Wynyard Metamorphics, Hurleys Metamorphics and the Scurvy Creek Meta-arenite. The Belyando ore host rocks were placed within an undifferentiated unit of the Anakie Metamorphic Group which consists of undivided phyllite, cleaved siltstone, labile meta-arenite and quartzite. Withnall et al (1995) recognised three major deformations and subsequent minor folding events affecting the Anakie Metamorphic Group. Greenschist to amphibolite facies regional metamorphism accompanied the D1 and D2 deformations resulting in the development of muscovitebiotite-chlorite and staurolite-andalusite mineral assemblages. Minor Cambrian-Ordovician? S-type, foliated, two-mica granite, possibly sourced from the basement during Middle to Late Cambrian D1 deformation and metamorphism, has intruded the Anakie Metamorphic Group. Foliation within the granite is roughly parallel to S2 within the surrounding metasediment. Extensive intrusion of Middle to Late Devonian I-type granitoids has resulted in overprinting by contact metamorphism. The Retreat Batholith forms a composite intrusion exposed over a 1500 km2 area, and contains members which range from diorite through granodiorite to granite, and are biotite and/or hornblende bearing. The Late Devonian to Early Carboniferous Drummond Basin and Permian Bowen Basin sequences unconformably overly the Anakie Inlier to the west and east respectively. The Silver Hills Volcanics form the basal unit of the Drummond Basin sequence and consist of basement-derived sediments, epiclastic rocks, rhyolitic ignimbrites and rhyolitic to trachyandesitic lavas. Early to Middle Tertiary Suttor Formation sediment and Tertiary-Quaternary residual soil form extensive areas of cover. Deep weathering during the Tertiary has resulted in the development of a strong lateritic weathering profile associated with the Suttor Formation sediment and exposed Anakie Inlier bedrock.
REGIONAL GEOLOGY ORE DEPOSIT FEATURES The Belyando mineralisation is hosted by multiply deformed low grade metasediment of the Neoproterozoic or PreOrdovician (Early Cambrian?) Anakie Metamorphic Group (Fig 1). The Anakie Metamorphic Group forms the main component of the Anakie Inlier, a 230 km long and up to 80 km wide NNW-trending basement ridge. These basement rocks are generally referred to as part of the Thomson Fold Belt (Kirkegaard, 1974; Murray and Kirkegaard, 1978). Regional geological mapping of the Clermont, Emerald and Mount Coolon 1:250 000 sheet areas, which cover much of the inlier, was completed by joint Bureau of Mineral Resources–Geological Survey of Queensland (BMR–GSQ) efforts mostly in the early 1960s and 1970s (Malone, Corbett
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STRATIGRAPHY Prior to mining, the Belyando deposit outcropped as a prominent white quartz ‘blow’ forming a small hill rising 15 m above the surrounding soil covered plains (Fig 1). Quartz outcrop and subcrop measured 275 m long by 85 m wide, striking at 290ο magnetic. The host metasediments are not exposed in the vicinity of Belyando and were first revealed during costeaning. Local cover consists of 0.5 to 30 m of Recent residual red or green clay-rich soil, derived from weathered bedrock and containing narrow quartz-rich pebble or cobble horizons.
Geology of Australian and Papua New Guinean Mineral Deposits
BELYANDO GOLD DEPOSIT
FIG 2 - Geological plan (top), contoured assay plan (bottom) and cross section, Belyando pit.
Geology of Australian and Papua New Guinean Mineral Deposits
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The Belyando deposit is hosted by a homogenous sequence of foliated, tightly folded, pelitic to finely psammitic quartzmuscovite metasiltstone (Fig 2). Two episodes of deformation have resulted in the obliteration of all sedimentary structures. In thin section the unaltered host rock has a primary grain size of 0.005 mm and consists of subangular to subrounded, unstrained to mildly strained quartz grains to approximately 0.5 mm in diameter and detrital muscovite flakes scattered in a matrix of silt sized grains of quartz (0.03 to 0.005 mm) and sericite aligned parallel to S0 and S1 (S 0/S1). Aggregates of very fine secondary rutile occur after detrital biotite.
STRUCTURE Two major deformation events have affected the host metamorphic rocks at Belyando (Fig 3). D1 produced a strong foliation (S1) parallel to lithological layering (S0). S1 is best preserved in psammopelitic (metapelite) horizons as an intense, spaced laminar foliation with planes about 1–3 mm apart, deformed by tight assymetric F2 folds that locally have an S2 axial plane cleavage. In the metapelites S1 is generally strongly overprinted by another layer-differentiated crenulation cleavage S 2, which is axial planar to F2 folds. S1 and S0 strike between 330ο and 20o and dip both east and west. The S2 axial plane cleavage strikes at 350 ο and dips west at 45ο to 65o. The F 2 folds are tight to isoclinal, moderately inclined to the west and have a wavelength of 20 cm to 5 m. F2 fold axes plunge at 15o towards 350o. Poles to S1/S0 show a well formed girdle around the shallow NNW plunging F2 fold axes and foliation-crenulation intersection L12. The stretching (mineral elongation) lineation L22 varies between subhorizontal and a plunge at 15o towards 180o. Progressive post-D2 NW-trending sinistral shearing crosscuts earlier fabrics.
METAMORPHISM AND PLUTONIC EVENTS Post-D1, pre- to syn-(?)D2 contact metamorphism has resulted in the development of cordierite and minor andalusite porpyroblasts interpreted to be related to shallow emplacement of an intrusive beneath the pit. The intrusive may be similar to the massive to weakly foliated, medium grained, equigranular (to minor pegmatitic), quartz-plagioclase-potassium feldspartourmaline-muscovite granodiorite and associated porphyroblastic contact aureole which outcrop at abandoned gold prospects at Byjingo (Frankfield Ridge) and Frankfield Hill, 11.5 and 13.5 km NW of Belyando respectively (Fig 1). The intrusives may correspond to a group of small unnamed Middle to Late Devonian leucocratic granite and pegmatite plutons containing tourmaline, as described by Crouch et al (1994). The author suggests they may be a late stage differentiate of the Retreat Batholith. Three subparallel medium to fine grained and equigranular feldspar porphyritic trachyte dykes, 0.2 to 2 m wide, crosscut S2 and the main quartz body at Belyando, but are superseded by at least one generation of fine comb quartz veining and brecciation. The WNW striking, steeply NE dipping dykes were emplaced along joint planes, crosscutting the Hanging wall, Footwall and Central high grade ore zones.
MINERALISATION The deposit comprises a low grade ore envelope enclosing three higher grade ore shoots. The zone of low grade gold mineralisation outlined by the 0.5 g/t gold contour is sigmoidal in plan, measuring 280 m along the long axis and 160 m along the short axis (Fig 2). The low grade envelope strikes at 290o magnetic, dips at 60ο towards 020ο, and plunges at 75ο towards 310o. It has been defined to a vertical depth of 150 m and is open down dip. This envelope is directly associated with the main quartz body, and surrounds quartz stockworked metasiltstone and associated phyllic alteration. The three higher grade (>1.5 g/t gold) shoots named the Hanging wall, Central and Footwall zones are parallel to the margins of the low grade ore envelope. The Hanging wall and Footwall shoots have a strike length of approximately 200 m, trend WNW and dip moderately steeply to the NNE. These two zones coincide with the stockworked margin of the main quartz body and represent an enhanced concentration of mineralised stockworking at the contact between the main quartz body and the surrounding host rock. The Central shoot is less well developed, and strikes east with a moderately steep dip to the north. The Central zone locally coincides with an internal zone of stockworking and brecciation, and prior to mining it outcropped near the top of the hill. The Hanging wall shoot is 5 to 45 m thick, the Central zone is 5 to 10 m thick, and the Footwall zone is 10 to 20 m thick. Gold mineralisation at Belyando is associated with intense brecciation, stockworking and quartz veining and contemporaneous phyllic alteration. A sigmoidal body of quartz lodes, breccia and intense phyllic (silica dominated) alteration crosscuts the main fabric (S2) at a high angle and is surrounded by an envelope of quartz stockworking and moderate phyllic (sericite dominated) alteration (Fig 2).
FIG 3 - Summary of structural data for the Belyando gold deposit, as contoured lower hemisphere equal area projections of poles. Contour intervals are 1%, 6%, 11% and 15% per 1% area.
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The sigmoidal body is approximately 250 m long and up to 100 m wide in the central portion, with an overall WNW trend (290ο magnetic). The body dips at 50 to 70o to the NNE, plunges steeply towards the NW, and extends to at least 150 m vertical depth.
Geology of Australian and Papua New Guinean Mineral Deposits
BELYANDO GOLD DEPOSIT
A complex paragenesis of six events of brecciation, stockworking and veining and associated replacement has been recognised. In summary the main body was formed by three episodes of brecciation, cementation and replacement by fine grained silica-sericite±pyrite-arsenopyrite. Low level (<0.5 g/t) disseminated gold mineralisation was introduced during the second and third (fine and coarser grained sulphide bearing) brecciation events. The major gold bearing event is early postbrecciation and post-quartz body formation, and is associated with stockworks of later euhedral fine comb quartz-pyrite, which were focussed preferentially along zones of high competency contrast between the main quartz body and the host metasediment. Later post-mineralisation phases include sulphidic stylolites, several generations of unmineralised fine to medium grained comb quartz±carbonate veining (sulphide absent), spider veinlets and localised mill brecciation.
pervasive green sericite-silica-pyrite-arsenopyrite alteration is associated with this event (Fig 4d). Veins range in width from 0.5 to 5 cm. A mottled white-grey appearance is due to fine white euhedral quartz crystals surrounding late stage fine grained opaque fill (Fig 4e). The euhedral crystals have a maximum length of 15 mm, an a:c axis ratio between 1: 4 and 1:10 and terminate in an interstitial fine grained anhedral quartz fill 0.2 to 0.5 mm in size. Three subtypes of mineralised comb quartz vein are recognised based on sulphide distribution and grain size, including veins with either fine black sulphidic margins or coarse interstitial sulphide or both. The fine grained sulphides on the margins consist of a simple pyritearsenopyrite assemblage, whereas the coarse interstitial sulphides also include chalcopyrite, galena, sphalerite and tennantite.
Event 1. Early quartz veining (post-D1, pre-D2)
Event 4. Recrystallisation with sulphidic stylolites and fractures
Milky white, irregular, anastomosing euhedral ‘buck’ quartz veins follow and locally crosscut the S1 foliation and are boudinaged by S2. These early barren veins may have played an important role in localising later superimposed mineralised veins, and acting as an initial brittle host responsible for dilation during NW-trending shearing in which later mineralised quartz was emplaced.
Event 2. Quartz lode formation (syn or post-D2) B1 Monomictic siliceous shatter breccia An intense shatter brecciation of the host rock was accompanied by moderate to strong pervasive fine grained silicification. Off-white to pale green very angular to angular silicified metasiltstone clasts are supported by a grey-opaque quartz matrix. Totally silicified metasediment clasts form ‘ghost’ clasts barely distinguishable from the matrix in slabbed hand specimen. Partially silicified metasediment may exhibit a weak relict foliation defined by minor sericite, clay and iron oxides (Fig 4a).
All previous generations of quartz were heavily strained, partly recrystallised and locally sheared by compressive deformation (Fig 4f). Quartz grains that have been subject to strain display undulose extinction, irregular grain shapes (embayed margins), dislocations (lattice imperfections), abundant solid and liquid inclusions, abundant microfractures, recrystallisation and subgrain formation. Stylolites are distributed throughout the main body but are more abundant in the east and west ‘limbs’. The irregular interlocking seams in quartz are characterised by the dissolution of quartz and the concentration of insoluble coarse and fine grained pyrite.
Event 5. Late comb quartz veining and/or stockworking (post-D2) Barren veins of medium to fine grained vuggy comb quartz devoid of suphide mineralisation crosscut the main quartz body and locally brecciate its margins and the trachyte dykes. The veins have a dominant NE strike and steep dip, following jointing, and they crosscut S2. This late phase of quartz has not been subject to strain (Fig 4g).
B2 Monomict to polymict shatter or rotational sulphidic breccia (low grade mineralisation)
Event 6. Late stage mill or intrusion breccia
A black shatter to rotational breccia is accompanied by a pervasive fine grained silica-pyrite-arsenopyrite alteration. The polymictic breccia contains both clasts of metasediment and the previous phase of shatter brecciation, exhibiting a small degree of mixing (Fig 4b).
Mill brecciation occurred in the western region and along the footwall contact of the quartz body. Subrounded to subangular clasts of bleached, white quartz fragments, of all previous quartz phases, are supported by an uncemented rock flour matrix of fine quartz grains, kaolinite, minor sericite and smectite clay.
B3 Polymict rotational breccia (low grade mineralisation)
MINERALOGY
The breccia has undergone a moderate degree of milling and rotation and mixing, reflected by the diversity in clast type and the higher matrix to clast ratio (50:50) than that produced by previous events. The clast diameter varies from 1 mm to 3 cm and fragments have a broad range of angularity dictated by the primary clast type (Fig 4c).
Event 3. Post-brecciation comb quartz stockworking (main mineralisation phase) Mineralised fine comb veins and stockwork veins crosscut all previous breccia phases and the host metasediment. A strong
Geology of Australian and Papua New Guinean Mineral Deposits
Sulphides present within the deposit in decreasing order of abundance are pyrite, arsenopyrite, galena, chalcopyrite, sphalerite and several very minor phases, including tennantite and covellite. Primary gold occurs as inclusions within pyrite, ranging from 1 to 50 µm in diameter and averaging 10 µm, although rare grains reaching 0.35 mm have been recorded. Gold occurs as round to ovoid shapes, and less commonly as vein-like inclusions in pyrite closely associated with similar shaped galena inclusions. Multi-element analysis indicates a gold-silver-arsenic-copper-lead-zinc elemental association. Electron microprobe studies of primary ore reveal a narrow range in gold fineness, between 771 and 794.
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FIG 4 - Complex paragenetic brecciation, stockworking and veining textures at the Belyando epithermal deposit.
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ALTERATION AND WEATHERING. Hydrothermal alteration is phyllic and propylitic. Phyllic alteration has been further subdivided into intense phyllic (silica dominant) and moderate phyllic (sericite dominant) subtypes. Lateral alteration zoning grades outwards from a central zone of intense phyllic (silica±sericite-pyritearsenopyrite), to an intermediate zone of moderate phyllic (sericite±silica-pyrite) to an outer propylitic zone (chloritesiderite-sericite-pyrite). The central silica zone consists of pervasive replacement by microcrystalline silica, fine and coarse grained sulphides and minor sericite associated with multiple episodes of brecciation (Event 2). Silicification of the metasiltstone is generally texture destructive, producing a dense grey chert-like rock which exhibits occasional relict textures. The intermediate zone of moderate phyllic alteration consists of pervasive replacement by sericite associated with mineralised quartz vein stockworking (Event 3). A propylitic alteration mineral assemblage of chlorite-siderite-sericite-pyrite is best developed in the footwall and occurs syn- to postmineralisation (Events 3 and 5). Intense silicification has resulted in the preservation of primary sulphide mineralisation at surface. However, progressive deformation of the brittle quartz body resulting in intense fracturing, as well as late stage uncemented mill brecciation (Event 6) has led to enhanced permeability. Weathering has produced extensive zones of limonite along open fractures to over 150 m vertical depth. Mixed oxide and sulphide zones occur in the eastern part of the deposit, but the western part comprises only oxide ore. Supergene enrichment is best developed in the top 50 m of the orebody where goldbearing limonite has been deposited on fracture planes. Weathering has resulted in the formation of supergene kaolinite after sericite, particularly in the intermediate zone of moderate phyllic alteration.
ORE GENESIS
Quartz body development and mineralisation (syn- or postD2) is spatially and temporally associated with intrusive activity. Mineralisation post-dates (pre-D2) contact metamorphism marked by porphyroblast development in host rocks which is related to intrusive emplacement, and closely predates (post-D2) trachyte dykes. Pre-mineralisation stockwork quartz veins and silicification are inferred to be localised around the roof portions of plugs responsible for the contact metamorphism. Characteristics of the mineralising fluids (Event 3) obtained from a preliminary fluid inclusion study include a 10 wt % CO2 content, 5±2 eq wt % NaCl and a wide range of homogenisation temperatures considered to result in boiling. Estimated P-T conditions during mineralisation are 300 to 350oC at depths of 1.5 to 2 km. Gold transport via thiosulphide complexes and precipitation from boiling and sulphidation is considered most likely. Mineralising fluids may have been derived from a metamorphic source, in the surrounding metsedimentary pile by remobilisation during D2 metamorphism, or from a magmatic source, from a late stage fluid associated with granodiorite or porphyry intrusion. Based on the syn- or post- D2 timing of mineralisation, the Belyando deposit is considered to be no older than late Devonian. Epithermal gold mineralisation hosted by the overlying Devonian–Carboniferous Drummond Basin, eg Yandan (Ruxton and Seed, this publication) and Wirralie (Seed and Ruxton, this publication) has a late Devonian (350 Myr) age. It is inferred that the Belyando deposit may represent the deeper mesothermal level of a mineralised system whose uppermost epithermal portion was hosted by rocks of the Drummond Basin sequence and has subsequently been eroded.
ACKNOWLEDGEMENTS This paper is published with the permission of Ross Mining NL. The author would like to thank P Ruxton for comments on the manuscript, S Morris for formatting and T Holtz for drafting services.
REFERENCES The primary structural control of quartz development, mineralisation and subsequent deformation has been progressive sinistral strike-slip movement with a minor reverse component, along a major NW-trending and steeply east dipping brittle-ductile shear zone, active syn- or post-D2 regional deformation. Belyando is interpreted to be localised within a WNW-trending dilatant shear zone, localised where the shear intersects a zone of competency contrast produced by early post-D1 and pre-D2 quartz veining and silicification related to emplacement of a tourmaline-bearing granodiorite at depth. Progressive shearing has been documented over a prolonged period from post-D1 (pre-Event 2 and overprinted by microcrystalline silica associated with B1 brecciation, and post-Event 1 (sinistrally offsetting quartz veins) to syn- or postD2 (resulting in deformation and recrystallisation of the main quartz body associated with Event 4 and pre-emplacement of the trachyte dykes). Earlier low grade disseminated style mineralisation associated with dilational brecciation and later higher grade mineralisation with related veining, are focussed in regions of high competency contrast at the quartz body-metasediment contact.
Geology of Australian and Papua New Guinean Mineral Deposits
Crouch, S B S, Withnall, I W, Tenison Woods, K, Hayward, M A and Carr, P, 1994. New plutonic rock units in the southern Anakie Inlier, central Queensland, Queensland Government Mining Journal, July 1994, 95:12–24. Dickins, J M and Malone, E J, 1973. Geology of the Bowen Basin Queensland, Bureau of Mineral Resources Geology and Geophysics, Bulletin 130. Hutton, L J, Grimes, K G, Law, S R and Mclennan, T P T, 1991. Geology of the Mount Coolon 1:250 000 sheet area, Queensland Resource Industries Record 1991/19. Kirkegaard, A G, 1974. Structural elements of the northern part of the Tasman Geosyncline, in The Tasman Geosyncline - a Symposium (Eds: A K Denmead, G W Tweedale, and A F Wilson), pp 47–62 (Geological Society of Australia, Queensland Division: Brisbane). Mackay, C R, 1987. The genesis of gold mineralisation in the Anakie Metamorphics, central Queensland, BSc Honours thesis (unpublished), James Cook University of North Queensland, Townsville. Malone, E J, 1968. Mount Coolon, Queensland - 1:250 000 geological series, Bureau of Mineral Resources Geology and Geophysics, Explanatory Notes SF 55–7. Malone, E J, Corbett , D W P and Jensen, A R, 1964. The geology of the Mount Coolon 1:250 000 sheet area, Bureau of Mineral Resources Geology and Geophysics, Report 64.
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Murray, C G and Kirkegaard, A G, 1978. The Thomson Orogen of the Tasman Orogenic Zone, Tectonophysics, 48:299–325. Mustard, R, 1989. The geology and genesis of the Belyando gold deposit, central Queensland, BSc Honours thesis (unpublished), James Cook University of North Queensland, Townsville. Mustard, R, 1990. The geology and genesis of the Belyando gold deposit, central Queensland, in Report on Activities of the Geology Department and Economic Geology Research Unit, 1989, Contributions of the Economic Geology Research Unit 36 (Ed: R M Carter), pp 58–59 (James Cook University of North Queensland: Townsville). Olgers, F, 1968. Emerald, Queensland - 1:250 000 geological series, Bureau of Mineral Resources Geology and Geophysics Explanatory Notes, SF 55–15.
Olgers, F, 1972. Geology of the Drummond Basin, Queensland, Bureau of Mineral Resources Geology and Geophysics, Bulletin 132. Veevers, J J, Randal, M A, Mollan, R G and Patten, R J, 1964a. The geology of the Clermont 1:250 000 sheet area, Queensland, Bureau of Mineral Resources Geology and Geophysics, Report 66. Veevers, J J, Mollan, R G, Olgers, F and Kirkegaard, A G, 1964b, The geology of the Emerald 1:250 000 sheet area, Queensland, Bureau of Mineral Resources Geology and Geophysics, Report 68. Withnall, I W, Blake R N, Crouch, S B, Tenison Woods, K, Grimes, K G, Hayward, M A, Lam, J S, Garrad, P and Rees, I D, 1995. Geology of the southern part of the Anakie Inlier, Central Queensland, Queensland Department of Minerals and Energy, Queensland Geology, 7.
Olgers, F, 1969. Clermont, Queensland - 1:250 000 geological series, Bureau of Mineral Resources Geology and Geophysics Explanatory Notes, SF 55–11.
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Messenger, P R, Taube, A, Golding, S D and Hartley, J S, 1998. Mount Morgan goldcopper deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 715–722 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Mount Morgan gold-copper deposits 1
2
3
by P R Messenger , A Taube , S D Golding and J S Hartley INTRODUCTION The historic Mount Morgan mine is some 37 km SSW of Rockhampton, Qld, at lat 23o38′S, long 150o22′E on the Rockhampton (SF 56–13) 1: 250 000 and Mount Morgan (8950) 1: 100 000 scale map sheets (Fig 1). Mining and processing of about 50 Mt of ore grading approximately 5 g/t gold and 0.7% copper commenced in 1882 and continued almost uninterrupted for over 100 years until the cessation of tailings retreatment in 1989. Although it was the third largest gold producer in Australia (Woodall, 1990), it had a relatively 1.
Formerly Department of Earth Sciences, University of Queensland, now Senior Exploration Geologist, Great Central Mines Limited, 46 Kings Park Road, West Perth WA 6005.
2.
Project Manager, Mount Morgan Joint Venture, PO Box 72, Mount Morgan Qld 4714.
3.
Senior Research Fellow, Department of Earth Sciences, University of Queensland Qld 4072.
4.
Formerly Project Manager, Mount Morgan Joint Venture, now at PO Box 153, Mission Beach Qld 4852.
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unusual style and its genesis remains the subject of considerable debate. It is similar in terms of tonnage, metal content, ore mineralogy, alteration, ore deposit form and rock association to the Horne deposit in Canada. Since 1992 the deposit has been the focus of research at the University of Queensland and exploratory diamond drilling by the current managers, Perilya Mines NL. This paper outlines recent advances in understanding the stratigraphic and structural setting of the Mount Morgan gold-copper deposit and illustrates its stratigraphic relationship to the nearby, newly discovered Car Park and Slag Heap sulphide deposits. Exploration and mining history as well as resource and production details are summarised by Taube (1990). Geological descriptions and genetic discussions were presented recently by Taube (1986), Arnold and Sillitoe (1989), Taube (1990) and Golding et al (1993). Summary whole rock geochemical data for the host volcanic and associated plutonic rocks can be found in Messenger, Collerson and Golding (1996) and fuller descriptions of the volcanic host rocks will be presented by Messenger, Golding and Taube (in press).
FIG 1 - Location and geological map of Mount Morgan mine area showing location of section lines for Figs 2, 3 and 4.
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REGIONAL GEOLOGY
LOCAL GEOLOGY
The deposits are within the Calliope terrane, defined here as the package of Lower to Middle Devonian volcanic sequences and related plutonic rocks exposed in the Gladstone–Mount Morgan–Rockhampton area. This is equivalent to the Calliope Volcanic Assemblage of Morand (1993) and the northern part of the Calliope Island Arc of Day, Murray and Whitaker (1978). The Calliope terrane consists of felsic to mafic volcanic rocks and associated subvolcanic granitoids, coralline limestones and exhalites. The westernmost stratigraphic sequence of the Calliope terrane is the Capella Creek Group which comprises an upper, dacitic division and a lower, low-potassium rhyolitic division, all cut by a series of dykes and irregularly shaped intrusions ranging from basalt to rhyolite. Deposition of the Capella Creek Group occurred within a marine basin at shallow to moderate water depth.
The Mount Morgan deposits are hosted by the Mine Corridor volcanics, a rhyolitic volcanic belt measuring some 6 km long and <2 km wide. This belt is completely enclosed by the Mount Morgan Tonalite, which displays intrusive contacts, and is unconformably overlain by the Frasnian Dee Volcanics. Fordham and Taube (1994) correlated the Mine Corridor volcanics with the lower, rhyolitic part of the Capella Creek Group on the basis of lithological similarities and conodont assemblages. The geology of the Mine Corridor volcanics was described by Taube (1986, 1990) and partly re-interpreted by Arnold and Sillitoe (1989). Recent and ongoing diamond drilling immediately SE of the open cut and a re-examination of several hundred thin sections collected during the 1970s from the now flooded mine, has allowed further definition of the stratigraphy and structure of the mine sequence.
The Capella Creek Group was intruded by a protracted series of contiguous stocks known as the Mount Morgan Tonalite soon after deposition. The Mount Morgan Tonalite is dominated by trondhjemite, with subordinate tonalite, quartz diorite and quartz gabbro stocks. Together the Capella Creek Group and the Mount Morgan Tonalite form a cogenetic volcano-plutonic suite. The degree of deformation in this suite of rocks is low and metamorphism is lower greenschist facies.
STRATIGRAPHY
The preserved rock association and supporting evidence from geochemical studies indicate that the Calliope terrane represents a mature island arc as suggested by Marsden (1972), Day, Murray and Whitaker (1978) and Murray (1986). Henderson (1980), Henderson et al (1993) and Morand (1993) proposed that the Calliope terrane developed in place at the Australian continental margin because of the lack of evidence for a collision between this terrane and the Australian continent during the early Late Devonian. Trace element and isotopic data (Messenger, Collerson and Golding, 1996) preclude any contribution from evolved continental crust during the development of the Calliope terrane. It is therefore most probable that this terrane developed more or less in place at the margin of Gondwana as a continental island arc above a juvenile volcanic substrate.
The general stratigraphy of the Mine Corridor volcanics is summarised in Table 1. Much of the Upper mine sequence and Mine volcaniclastics are geochemically and lithologically indistinguishable. Distinction between these units is made only in terms of their position relative to the Banded mine sequence. They comprise a lithofacies association of thickly bedded, volcaniclastic breccia and tuffaceous sandstone of lowpotassium rhyolite composition. Individual beds range from about 10 to more than 110 m thick. The Mine Corridor volcanics are dominated by this lithofacies association which has a combined thickness of over 900 m. The distinctive Banded mine sequence conformably underlies the Upper mine sequence (Figs 1 and 2). This is a 200 m thick package of thinly bedded ribbon jasper, tuffaceous mudstone, siltstone and crystal-rich tuffaceous sandstone. Abundant pumiceous porphyry clasts up to 2 m in length occur throughout this package. Below the Banded mine sequence is a succession comprising the Mine volcaniclastics and Siltstone sequence (Figs 2, 3 and 4). The Siltstone sequence is a thick package of massive to diffusely bedded tuffaceous siltstone with a few thin interbeds of crystal-lithic lapillistone and sandstone.
TABLE 1 Stratigraphy of the Mount Morgan Mine Corridor volcanics. Unit
Thickness (m)
Description
Interpretation
Upper mine sequence
750–900
Crystal-rich (quartz+plagioclase), pumiceous breccia and tuffaceous sandstone-siltstone, peperitic quartz-plagioclase porphyry, aphanitic rhyolite, and minor carbonate lenses or blocks.
Crystal and pumice-rich volcaniclastic mass-flow deposits and synsedimentary porphyritic intrusions, aphanitic rhyolite lavas or high level sills
Banded mine sequence
120–200
Thinly bedded jasper, chert, tuffaceous mudstonesiltstone-sandstone, carbonate lenses, porphyry clasts, and peperitic quartz-plagioclase porphyries.
Interbedded exhalites, reworked volcaniclastic turbidites, water settled ashfall and pelagic deposits, locally derived vesiculated and nonvesiculated porphyritic lava blocks and synsedimentary porphyritic intrusions.
Mine volcaniclastics1
~270
Crystal-rich (quartz+plagioclase), pumiceous breccia-sandstone-siltstone.
Crystal- and pumice-rich volcaniclastic mass-flow deposits.
Siltstone sequence
~80
Massive to diffusely laminated rhyolitic tuffaceous mudstone-siltstone, minor crystal-rich sandstone, banded chert, oxide-sulphide facies and thin carbonate horizons.
Reworked volcaniclastic turbidites, water-settled ash and pelagic deposits, detrital limestone lenses, exhalites and replacement massive sulphide mineralisation.
1. This unit was divided into the Middle and Lower mine sequences by Messenger, Golding and Taube (in press).
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FIG 2 - Cross section on 20 000 N, looking NW.
FIG 3 - Cross-section on 19 550 N, looking NW.
All stratigraphic units were intruded, prior to lithification and dewatering, by a number of synvolcanic quartz-plagioclasephyric low-potassium rhyolite domes of less than 1 km diameter that typically display peperitic textures at their margins. The Mine Corridor volcanics represent a proximal, intrabasinal, cogenetic, explosive-effusive facies association that developed above a subvolcanic trondhjemite magma chamber represented by early trondhjemite stocks of the Mount Morgan Tonalite (Messenger, Golding and Taube, in press).
Geology of Australian and Papua New Guinean Mineral Deposits
The Mundic sequence (Figs 1, 2 and 3) is a package of interbedded, graded dacitic lapillistone and mudstone. It is excluded from the stratigraphy in Table 1 because its primary relationship with the rhyolitic sequence is not clear. It is tentatively correlated with the upper, dacitic division of the Capella Creek Group on the basis of similar rock type and alteration.
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FIG 4 - Longitudinal projection on 20 230 E, looking SW.
STRUCTURE The structure of the Mount Morgan Mine Corridor is complex and remains only partly understood. Nevertheless, some elements of the structural setting of the deposit have become apparent following recent diamond drilling and systematic petrographic studies of historical samples collected from the open cut benches.
Doming The structure of the open cut mine area is a faulted half dome (Fig 1). Bedding dips change progressively from 30 to 50o NE in the northern benches to 55 to 70o east in the eastern benches. Staines (1953) and Cornelius (1968) reported dips of 80o south in the southern benches. The orebody was therefore surrounded on three sides by outward dipping strata. Vertical ore shoots within the orebody first reported by Staines (1953), constrain ore formation to either syn- or post-doming. Doming was interpreted by Cornelius (1968) to be a result of porphyry intrusion and although some of Cornelius’ lithological interpretations are questionable, this model of an intrusionrelated structural dome is supported by the following lines of evidence: 1.
2.
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Many examples of porphyry-jasper and porphyrysiltstone peperite occur around the benches of the open cut suggesting the presence of a larger, synvolcanic porphyry intrusion at depth. Several synvolcanic, dome-like quartz-plagioclase porphyry intrusions to 500 m in diameter have been identified elsewhere within the Mine Corridor (Messenger, Golding and Taube, in press).
3.
Although strongly altered, a transgressive, dome-like quartz±plagioclase-phyric body underlies the Main Pipe and Sugarloaf ore bodies (Fig 1), and the core of the structural dome, suggesting that this is an altered, domelike intrusion.
Faulting Several major fault zones cut the deposits and dome, thereby complicating their geometries (Fig 1).
Arcuate faults The Footwall faults are a series of arcuate, inward-dipping listric faults which partly bounded the Main Pipe deposit (Taube, 1986). These faults are interpreted to be early structures that were active during or immediately prior to mineralisation (Golding et al, 1993). Reactivation of these faults resulted in normal dip slip of a few tens of metres prior to emplacement of a series of Triassic trachyandesite dykes. The Trough fault is a steeply dipping, slightly arcuate fault oriented east to NE. It has north block–down separation of about 100 m. This fault is cut by the Invoka fault and has not been detected east of the Linda fault. It is therefore one of the oldest faults that cut the Mine Corridor volcanics.
NE-trending faults The Slide and Ballard’s faults trend NE and cut the dome, and the Sugarloaf and Main Pipe orebodies. They are normal dipslip faults, the former having undergone oblique slip reversal following emplacement of the Triassic dykes. Net separation across both faults is less than 60 m in both dip and strike.
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT MORGAN GOLD-COPPER DEPOSITS
NW-trending faults
Car Park and Slag Heap deposits
The Linda fault (C Fraser, unpublished data, 1914; Taube, 1986) is a steep fault with considerable but indeterminable dip and strike separation. This fault juxtaposes the Banded mine sequence in the west against the Upper mine sequence in the east (Fig 1).
Recent drilling by Perilya Mines NL resulted in the discovery of another zone of mineralisation completely concealed beneath cover rocks, immediately SE of the open cut (Fig 1). Numerous styles of mineralisation were recognised including stratabound sulphide-magnetite horizons (Slag Heap mineralisation, Fig 4) and a large footwall massive pyrite body (Car Park mineralisation, Figs 3 and 4). The major sulphide and oxide ore minerals in this zone of mineralisation include pyrite, magnetite, sphalerite, chalcopyrite and pyrrhotite, with minor galena, hematite, leucoxene and tetrahedrite-tennantite. The stratabound mineralisation consists of pyrite-rich bands with anhedral pyrite aggregates, subhedral to euhedral pyrite grains and lesser magnetite and quartz, as well as siliceous pyrite-poor bands. Sphalerite is most abundant in the Slag Heap mineralisation, whereas chalcopyrite is most abundant in the Car Park mineralisation. The approximate maximum dimensions of the Car Park mineralisation are 400 by 160 by 100 m. The form, mineralogy, metal zoning and associated alteration assemblages of these newly discovered sulphide deposits were interpreted in terms of sea floor and sub-sea floor volcanogenic massive sulphide mineralisation by Hartley and Taube (1997).
The Cornelius fault was shown but not named on detailed mine plans prepared by Cornelius (1968) as a NW-trending fault separating the Main pipe and Sugarloaf. It is a poorly documented fault that appears to have a considerable dip separation and also juxtaposes a block of Upper mine sequence and Mundic sequence rocks against the Main pipe orebody (Figs 1 and 2). Although not yet fully resolved, there is some evidence to suggest that a second, parallel fault lies <100 m west of the Cornelius fault. It is postulated that the two fault planes bound a narrow, down-dropped block. The Invoka fault is a steeply dipping NW-trending fault recognised during exploratory diamond drilling SE of the open cut by Perilya Mines (Figs 1 and 3). It separates unaltered, dacitic volcaniclastics of the Mundic sequence in the west from altered rhyolitic rocks in the east. Similar, unaltered dacitic rocks have also been recognised against altered rhyolitic rocks adjacent to the southern extension of the Cornelius fault in the open cut (Messenger, 1996). If the Mundic sequence is equivalent to the dacitic part of the Capella Creek Group, then the Invoka fault displays west block-down separation (Figs 1 and 3). It is not yet clear if the Invoka fault is the southern extension of the Cornelius fault.
ORE DEPOSIT FEATURES MINERALISATION Main Pipe and Sugarloaf orebodies The mined deposit at Mount Morgan formed a boot-shaped body of massive and stringer sulphide mineralisation with maximum mineable areal dimensions of 640 by 274 m (Staines, 1953). The ore deposit comprised the Main pipe and Sugarloaf orebodies (Fig 1) as well as several smaller pipe-like bodies including the Pyrrhotite pipe. Northwest-trending faults including the Cornelius fault probably separated the Main pipe and Sugarloaf orebodies, although subsequent movement on the Slide fault has partly obscured this relationship. The original relationship between these two orebodies is not clear. The Main pipe orebody contained 70% of the ore mined and was characterised by massive pyrite mineralisation cut by anastomosing quartz veins and vertical, high grade, gold ore shoots (Cornelius, 1969; Taube, 1986; Golding et al, 1993). The Main pipe lay at or about the level of the Banded mine sequence and extended immediately below this level. The Sugarloaf orebody differed from the Main pipe in its form and mineralogy. It was characterised by disseminated and stringer mineralisation of pyrite, pyrrhotite and chalcopyrite with massive sugary quartz, all cut by anastomosing quartz veins (Taube, 1986). In contrast to the Main pipe, pyrrhotite was far more abundant, and sphalerite, gold-silver tellurides and high grade gold shoots were absent. These differences have been interpreted (Taube, 1986) in terms of a massive sulphide core of mineralisation (Main pipe) and an underlying stringer zone (Sugarloaf). It is envisaged that the Sugarloaf originally underlay the Main pipe and that movement on faults associated with the Cornelius fault subsequently juxtaposed the two.
Geology of Australian and Papua New Guinean Mineral Deposits
ALTERATION The Main Pipe and Sugarloaf orebodies lay within a pipe-like zone of intensely altered rock that extends beneath the open cut for over 700 m and is partly truncated by unaltered or weakly altered tonalite and quartz gabbro plutons of the Mount Morgan Tonalite. This alteration pipe consists of an outer zone of sericite-quartz-pyrite-chlorite, measuring about 1.5 by 1 km in area, enclosing an inner silicic zone of quartz-sericite-pyrite which immediately underlay the massive and stringer sulphide ore. Mineralisation-related alteration at Mount Morgan forms a transgressive footwall alteration pipe mineralogically similar to the inner zones of some Kuroko volcanogenic massive sulphide (VMS) deposits as described by Urabe, Scott and Hattori (1983). An aureole of quartz-albite-actinolite-biotite alteration extends for up to 100 m from the tonalite contact in the western part of the open cut and overprints the sericite-pyrite-chlorite zone (Messenger, 1996). This overprinting relationship constrains tonalite emplacement to post-mineralisation. Rocks in the footwall to the Slag Heap mineralisation were strongly altered to an assemblage of sericite-pyrite-quartzchlorite for more than several hundred metres vertically beneath the sulphide horizons. The Car Park massive sulphide body lies within this footwall alteration zone. By contrast, rocks in the hanging wall above the Slag Heap mineralisation are less intensely altered. Heavy disseminated and veined pyrite mineralisation extends for <30 m above the Slag Heap sulphide deposits, whereas weak sericite-pyrite-chlorite alteration is more widespread. Newly discovered stratabound mineralisation at Mount Morgan therefore exhibits footwall and hanging wall alteration zones typical of many VMS deposits (Large, 1992). In contrast to the relatively unaltered tonalite and gabbroic plutons, trondhjemite adjacent to and underlying the Mine Corridor volcanics was variably altered to an assemblage of albite-chlorite±sericite. The similarity in style and degree of
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alteration of the trondhjemite granitoids and Mine Corridor volcanics suggests that trondhjemite was emplaced into the chemically equivalent volcanic pile during widespread sea floor hydrothermal alteration.
TIMING RELATIONSHIPS Field and laboratory studies (Messenger, 1996) indicate that trondhjemite stocks were directly related to rhyolitic volcanism and were emplaced in the mine sequence during the latest stages of volcanism. In the NW sector of the Mine Corridor unaltered tonalite cuts altered and weakly mineralised trondhjemite and rhyolite (R H Sillitoe, unpublished data, 1987). Irregularly shaped dacite intrusions cut mineralisation and were themselves cut by tonalite in the northern benches of the open cut (Taube, 1986). Cornelius (1968) presented photographic evidence that unaltered tonalite (his quartz diorite) transgressed mineralisation in the NW of the open cut. Field evidence therefore indicates that gold-copper mineralisation predated tonalite emplacement. Lack of widespread hydrothermal alteration and generally coarser grain sizes suggest that tonalite and gabbroic stocks were emplaced at greater depth than the trondhjemite and post-dated both goldcopper mineralisation and trondhjemite emplacement. Goldcopper mineralisation at Mount Morgan was therefore broadly coeval with low-potassium rhyolitic volcanism which was related to trondhjemite magmatism.
ORE GENESIS Early workers at Mount Morgan (Newman and CampbellBrown, 1911; C Fraser, unpublished data, 1914; The Staff, Mount Morgan Limited, 1965) believed that genesis of the gold-copper ore bodies was related to tonalite emplacement. This view was prompted by the close spatial relationship between gold-copper mineralisation and the Mount Morgan Tonalite, and the abundance of relatively insoluble ore elements such as copper, gold, tellurium, molybdenum and bismuth within the deposit. Similarities of rock association, orebody form, mineralogy and alteration with well known VMS deposits led others to interpret Mount Morgan in terms of a volcanogenic model (Paltridge, 1967; Gibbons, 1974; Lawrence, 1977; Taube, 1986; Golding et al, 1993). Arnold and Sillitoe (1989) noted the strong structural control on mineralisation, ie the intersection of major structures, and localisation at the core of a structural dome with surrounding arcuate, inward dipping listric faults. They also described, but did not provide photographic evidence for, an early beddingparallel schistosity that they claimed was overprinted by mineralisation-related alteration. In addition to these structural arguments, they believed that contact metamorphic assemblages were the oldest alteration minerals and that these were overprinted by mineralisation-related alteration. They therefore concluded that the deposit was a structurally controlled, intrusion-related replacement deposit. Evidence for a structural control on mineralisation at Mount Morgan is strong; however, relationships between alteration mineral assemblages described by Arnold and Sillitoe (1989) are equivocal. Messenger (1996) provided photographic evidence illustrating tonalite-related, quartz-albite-biotiteactinolite alteration overprinting the early, mineralisationrelated quartz-sericite-pyrite alteration. This paragenesis is in agreement with previous studies by R Balde (unpublished data,
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1976), who defined an alteration aureole surrounding the tonalite and overprinting the alteration pipe. Recent detailed petrographic studies could find no evidence for the beddingparallel schistosity described by Arnold and Sillitoe (1989) in the Banded mine sequence. Furthermore, many macroscopic features described by Arnold and Sillitoe (1989) as evidence for post-lithification intraformational shearing are readily explicable in terms of soft sediment deformation, differential compaction around large porphyry clasts, and peperitic mixing between unlithified, water-saturated sediment and rhyolitic magma (Messenger, Golding and Taube, in press). There is therefore little supporting evidence for the hypothesis that mineralisation significantly post-dated volcanism. The lack of associated banded lead-zinc ore has also been used to argue against a VMS origin for the Mount Morgan deposit. However, this may not be unusual in the proximal, copper-gold rich, pipe-like, sub-sea floor class of VMS deposits such as the Horne Mine in Canada. In any case, the recent discovery of the zinc-rich Slag Heap mineralisation effectively negates this argument. Recognition of the proximal nature of rhyolitic volcanism at Mount Morgan provides a ready explanation for the magmatic signature of the ore fluids in terms of metal content, fluid salinities and some isotopic characteristics (Eadington, Smith and Wilkins, 1974; Golding et al, 1993). The submarine nature of volcanism is also consistent with a sea water component of these fluids indicated by sulphur, oxygen and carbon isotope studies (Golding et al, 1993). The form and mineralogy of the deposits, their lithological association, structural and stratigraphic setting, ore fluid composition, isotopic characteristics, relative age constraints and association with zinc-rich stratabound mineralisation are most consistent with their interpretation as sub-sea floor replacement deposits related to and broadly coeval with rhyolitic volcanism.
ACKNOWLEDGEMENTS The authors wish to thank Perilya Mines NL for their ongoing support. This work was partly funded by ARC grant A39130274 to S D Golding and A H White.
REFERENCES Arnold, G O and Sillitoe, R H, 1989. Mt Morgan gold-copper deposit, Queensland, Australia: Evidence for an intrusion-related replacement origin, Economic Geology, 84:1805–1816. Cornelius, K D, 1968. The ore deposit and general geology of the Mount Morgan area, PhD thesis (unpublished), The University of Queensland, Brisbane. Cornelius, K D, 1969. The Mt Morgan mine, Queensland a massive gold-copper pyritic replacement deposit, Economic Geology, 64:885–902. Day, R W, Murray, C G and Whitaker, W G, 1978. The eastern part of the Tasman Orogenic Zone, Tectonophysics, 48:327–364. Eadington, P J, Smith, J W and Wilkins, R W T, 1974. Fluid inclusion and sulphur isotope research, Mount Morgan, Queensland, in Proceedings of Southern and Central Queensland Conference, pp 441–444 (The Australasian Institute of Mining and Metallurgy: Melbourne). Fordham, B G and Taube, A, 1994. Application of stratigraphic correlation to exploration in equivalents of the Mine Corridor Sequence at Mt Morgan, in Queensland Department of Minerals and Energy Symposium, Queensland Exploration Potential 1994 Handbook, p 33 (Queensland Department of Minerals and Energy: Brisbane).
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MOUNT MORGAN GOLD-COPPER DEPOSITS
Gibbons, G S, 1974. Mineralogical studies at Mount Morgan Queensland, in Proceedings of Southern and Central Queensland Conference, pp 445–463 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Messenger, P R, Golding, S D and Taube, A, in press. Volcanic setting of the Mt Morgan Au-Cu deposit, central Queensland: implications for ore genesis, Geological Society of Australia Special Publication 19.
Golding, S D, Huston, D L, Dean, J A, Messenger, P R, Jones, I W O, Taube, A and White, A H, 1993. Mount Morgan gold-copper deposit: The 1992 perspective, in Proceedings AusIMM Centenary Conference, pp 95–111 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Morand, V J, 1993. Stratigraphy and tectonic setting of the Calliope Volcanic Assemblage, Rockhampton area, Queensland, Australian Journal of Earth Sciences, 40:15–30.
Hartley, J S and Taube, A, 1997. New mineralisation at Mt Morgan, Queensland: Stratabound VHMS deposits, in New Developments in Research for Ore Deposit Exploration, Abstracts, Third National Conference of the Specialist Group in Economic Geology, p 37 (Geological Society of Australia: Canberra). Henderson, R A, 1980. Structural outline and summary geological history of northeastern Australia, in The Geology and Geophysics of Northeastern Australia (Eds: R A Henderson and P J Stephenson), pp 1–26 (Geological Society of Australia, Queensland Division: Brisbane). Henderson, R A, Fergusson, C L, Leitch, E C, Morand, V J, Reinhardt, J J and Carr, P F, 1993. Tectonics of the northern New England Fold Belt, in New England Orogen, NEO '93 Conference (Eds: P G Flood and J C Aitchison), pp 505–515 (University of New England: Armidale). Large, R R, 1992. Australian volcanic-hosted massive sulphide deposits: features, styles, and genetic models, Economic Geology, 87:471–510. Lawrence, L J, 1977. The syngenetic-epigenetic transition, an Australian example, in Time and Stratabound Ore Deposits (Eds: D D Klemm and H J Schneider), pp 46–54 (Springer-Verlag: Berlin). Marsden, M A H, 1972. The Devonian history of northeastern Australia, Journal of the Geological Society of Australia, 19:125–162. Messenger, P R, 1996. Relationships between Devonian magmatism and Au-Cu mineralisation at Mount Morgan, central Queensland, PhD thesis (unpublished), University of Queensland, Brisbane. Messenger, P R, Collerson, K D and Golding, S D, 1996. Isotope and trace element geochemistry of a Middle Devonian tonalitetrondhjemite-sodic rhyolite centre: Mt Morgan, Queensland, Australia, EOS Transactions of the American Geophysical Union, 77(22):W125.
Geology of Australian and Papua New Guinean Mineral Deposits
Murray, C G, 1986. Metallogeny and tectonic development of the Tasman Fold Belt System in Queensland, Ore Geology Reviews, 1:315–400. Newman, J M and Campbell-Brown, G F, 1911. Notes on the geology of Mt Morgan, Queensland, Transactions of the Australian Institute of Mining and Metallurgy, 40:439–470. Paltridge, I M, 1967. Breccia pipe mineralisation at Mount Morgan - a discussion, Economic Geology, 62:861–862. Staines, H R E, 1953. Mount Morgan copper and gold mine, in Geology of Australian Ore Deposits (Ed: A B Edwards), pp 732–750 (5th Empire Mining and Metallurgical Congress: Melbourne; and The Australasian Institute of Mining and Metallurgy: Melbourne). Taube, A, 1986. The Mount Morgan gold-copper mine and environment, Queensland: A volcanogenic massive sulphide deposit associated with penecontemporaneous faulting, Economic Geology, 81:1322–1340. Taube, A, 1990. Mt Morgan gold-copper deposit, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1499–1504 (The Australasian Institute of Mining and Metallurgy: Melbourne). The Staff, Mount Morgan Limited, 1965. Copper-gold ore deposit of Mount Morgan, in Geology of Australian Ore Deposits (Ed: J McAndrew), pp 364–369 (8th Commonwealth Mining and Metallurgical Congress: Melbourne; and The Australasian Institute of Mining and Metallurgy: Melbourne). Urabe, T, Scott, S D and Hattori, K, 1983. A comparison of footwallrock alteration and geothermal systems beneath some Japanese and Canadian volcanogenic massive sulphide deposits, Economic Geology Monograph, 5:345–364. Woodall, R, 1990. Gold in Australia, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 45–67 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Nethery, J E and Barr, M J, 1998. Red Dome and Mungana gold-silver-copper-leadzinc deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 723–728 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Red Dome and Mungana gold–silver–copper–lead–zinc deposits 1
by J E Nethery and M J Barr
2
INTRODUCTION Red Dome and Mungana are 150 km west of Cairns, and about 15 km and 18 km WNW respectively from Chillagoe. They are at lat 17o07′S, long 144o24′E and lat 17o06′S, long 144o23′E and AMG coordinates 224 000 E, 8 105 500 N and 221 500 E, 8 107 000 N respectively on the Atherton (SE 55–5) 1:250 000 and Mungana (7763) 1:100 000 scale map sheets (Fig 1).
The Red Dome open cut mine produced 12.8 Mt at 2 g/t gold and 0.5% copper in the ten years to June 1996. Mungana contains an Indicated and Measured Resource of 1.6 Mt at 2.1 g/t gold, 106 g/t silver, 1% copper, 2.3% lead and 2.3% zinc to 240 m depth, and mineralisation extends to at least 500 m depth.
EXPLORATION HISTORY The Red Dome and Mungana gold, silver and base metal breccia and skarn deposits are within the historical Mungana group of workings. The Mungana deposit has only a few peripheral prospecting pits on oxidised manganese-lead-silver mineralisation. The main deposit, which occupies a topographic depression, was discovered by a process of gradual intensification of exploration following initial peripheral rock chip sampling, with mediocre results, by Amoco Minerals Australia Company in 1982, initial drilling by Elders Resources Limited in 1986 to 1990, then intensive drilling by Niugini Mining Limited from 1992 to 1996, involving 215 holes for 38 361 m. Detailed descriptions of the geological and structural controls of mineralisation at Red Dome have been provided by Smith (1985), Torrey (1986), Ewers and Sun (1988), F Vanderhor (unpublished data, 1989), Ewers, Torrey and Erceg (1990), Nethery, Barr and Woodbury (1994), Woodbury (1994), Holland (1994) and Barr and Nethery (1995). The geology of the Mungana deposit has been described by Halfpenny (1991) Nethery, Barr and Woodbury (1994), Woodbury (1994) and Barr and Nethery (1995). This paper covers recent advances in geological understanding since Red Dome was documented by Ewers, Torrey and Erceg (1990).
REGIONAL GEOLOGY Recent geological mapping and interpretation of remotely sensed data have advanced the understanding of the regional and local geology, and drilling, petrography and mineragraphy have changed the paragenetic model.
STRATIGRAPHY FIG 1 - Location, regional geological setting and mine geological plan at 300 m RL, Red Dome gold mine.
1.
Principal Geologist, Nedex Pty Ltd, 1 Eastern Street, Chillagoe Qld 4871.
2.
Principal Geologist, Mike Barr & Associates, 422 Kamerunga Road, Redlynch Qld 4870.
Geology of Australian and Papua New Guinean Mineral Deposits
The Middle Proterozoic Dargalong Metamorphics lie SW of the Palmerville Fault (Fig 1). They are intruded locally by the Silurian Nundah Granodiorite, which SHRIMP U-Pb zircon dating has placed at 435 Myr (Donchak and Bultitude, 1994). The Middle Palaeozoic Hodgkinson Province lies NE of the Palmerville Fault and is represented in the Chillagoe district by the Hodgkinson, Chillagoe and Mulgrave formations. The Siluro-Devonian Chillagoe Formation lies on the western margin of the Hodgkinson Province and is separated from the
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Palmerville Fault by a thin fault wedge of flysch sediment and minor basalt of the Ordovician Mulgrave Formation. The Chillagoe Formation occupies a northwesterly trending belt traceable for over 150 km with widths of 5 to 10 km. The Formation comprises chert, flysch, limestone, and subordinate mafic volcanic rock, with a cumulative thickness of less than 1000 m (Fordham, 1990). The Chillagoe Formation is overlain to the east by the Devonian Hodgkinson Formation, which comprises greywacke, siltstone, chert and mafic volcanic rocks. A widespread lenticular unit, the Quadroy Conglomerate, which lies unconformably on the Chillagoe, Mulgrave and Hodgkinson formations, is inferred to be of Early Carboniferous age. Four suites of Permo-Carboniferous volcanic rocks were then erupted.
STRUCTURAL FRAMEWORK The Palaeozoic sequence is extensively disrupted to form a major imbricate thrust package with westward younging thrust ‘horses’ ( Fawckner, 1981; Hammond, 1986; Shaw, Faulkner and Bultitude, 1987). Recent subdivision of the Chillagoe Formation in the Mungana area, constrained by conodont dating, has indicated at least 12 thrust-induced stratigraphic repetitions (Fordham, 1990). Most of the incompetent units, especially adjacent to rock unit interfaces, have some mylonitic fabric. The Proterozoic sequence is disrupted as a 2 km wide mylonite zone which has undergone multi-episodic development (Bultitude et al, 1993). The Palmerville Fault is the bounding fault between the Proterozoic and Palaeozoic rocks within a wide zone of thrusting. A regional structural model invoking substantial dextral oblique slip in the development of the imbricate thrust package, and two stages of Carboniferous thrust deformation was inferred from regional mapping assisted by an interpretation of new airborne magnetic data (Nethery and Barr, 1996). Late Devonian to Middle Carboniferous deformation is characterised by reorientation of the regional stress field. A NE-oriented principal stress axis produced dextral transpression from 370 to 325 Myr. This produced a northerlyoriented imbricate thrust package, with ductile mylonitic strain fabric, as previously recognised. A brief extensional lull followed and was marked by listric fault rotation and oversteepening of the thrust package, and intrusion and extrusion of the O’Brien’s Creek Supersuite. Sinistral transpression with a NNE-oriented principal stress axis commenced circa 315 Myr, coeval with the closing stages of the O’Brien’s Creek episode. This phase folded the extrusives into broad asymmetric structures, and fractured the plutons, producing a major oroclinal fold which reoriented the Chillagoe district section of the thrust package, from a south to a SE orientation, and overprinted the steep SE-trending ductile fabric with brittle regime, east- to ENE-trending, steep reverse faults and shallowly dipping thrust duplexes. Another stress lull extension period was followed by reorientation to NW dextral transtension, with an east-oriented principal stress axis, in the period circa 305 to 280 Myr. Displacement and interference between the major NWtrending dextral faults related to the initial thrust episode, and east- to ENE-trending steep reverse faults and shallow thrusts related to the second episode, provided important structural preparation for the focussing of mineralising intrusives at Red Dome, Mungana and other deposits in the district.
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PERMO-CARBONIFEROUS MAGMATIC EPISODES The Permo-Carboniferous Coastal Range Igneous Complex forms a NW- to west-trending belt of intrusives and extrusives from Townsville to Mornington Island (Mackenzie et al, 1994). In the Chillagoe district four supersuites were defined, based on age and whole rock geochemical character (Champion and Heinemann, 1994; Donchak and Bultitude, 1994; Woodbury, 1994).The Late Carboniferous supersuites are of I-type affinity, and span rhyolitic to andesitic composition, and those of Early Permian age are of A-type affinity and are almost entirely rhyolitic (Mackenzie, 1987). The initial O'Briens Creek Supersuite (326–303 Myr) comprises fractionated reduced I-type plutons and extrusive rocks, with associated tin, tungsten, molybdenum and gold mineralisation. The Ootann Supersuite (306–299 Myr) comprises fractionated reduced to oxidised I-type plutons and associated tungsten, molybdenum, bismuth, copper, lead, zinc and gold mineralisation. The Almaden Supersuite (303–292 Myr) comprises fractionated reduced to oxidised I-type plutons and associated tungsten, molybdenum, bismuth, copper, lead, zinc and gold mineralisation. The final Lags Supersuite (290–280 Myr) comprises A-type plutons and extrusive rocks and associated minor base metal sulphide and sulphosalt, uranium, fluorine and gold mineralisation.
LOCAL GEOLOGY STRATIGRAPHY Red Dome and Mungana lie within a NW-trending belt of disruption, informally called the Mine corridor, in which most of the rock unit boundaries with competency contrast were subjected to shearing and mylonite development in the Early Carboniferous thrust episode, and brittle faulting and brecciation in later episodes. The Chillagoe Formation sequence within the Mine corridor is less disrupted than elsewhere in the district (Fordham, 1990; Bultitude et al, 1993). The sequence is SW-younging, and comprises tholeiitic basalt, overlain by a thin massive chert unit, limestone, interbedded chert, shale and siltstone, and then by a coarse grained flysch sandstone unit which represents a disconformable or possibly unconformable erosional unit. This sequence is consistent on a regional scale.
STRUCTURAL CONTROLS A NW-trending, generally steeply SW-dipping mylonitic fabric is most apparent in incompetent units such as mafic volcanic rocks and the interbedded chert, shale and siltstone unit. Tectonic brecciation followed the ductile phase and appears to have focussed on these narrow maximum strain zones. The complex interference zones at the intersection of steep NW-trending dextral slip faults with ENE reverse faults were the focus of intrusive activity at both Mungana and Red Dome. Activity continued on these fault sets during and after intrusion and mineralisation (Figs 1, 2, 3 and 4). At Red Dome the early crackle brecciation–related gold-bearing vein stockworks in the quartz veined porphyry commonly formed an ENEtrending sheeted pattern, and then the porphyry was subsequently disrupted into a series of fault slices with the same trend.
Geology of Australian and Papua New Guinean Mineral Deposits
RED DOME AND MUNGANA GOLD-SILVER-COPPER-LEAD-ZINC DEPOSITS
FIG 2 - Geological cross section on 6900 E, looking NW, Red Dome deposit.
FIG 4 - Typical geological cross section, looking NW, Mungana deposit.
preparation, particularly in the area of dilational jogs. Tectonic brecciation was also active after mineralisation, generally producing unmineralised monomictic breccia with the same composition as the brecciated protolith. Hydrothermal fill is absent from this type of simple fault breccia.
INTRUSIVES
FIG 3 - Interpretive surface geological plan, Mungana deposit.
Shallowly-dipping thrusts with south over north sense of movement disrupted the sequence and early porphyry at Red Dome in a repetitive step-like pattern (Fig 2). These faults then became the focus for the second stage green-garnet skarn development, which commonly shows a regular subhorizontal layering within marble blocks. Post-mineralisation faulting further complicated the already complex structural picture. Dextral transtension reactivated the earlier compressional faults to produce SE- to east-oriented dextral slip, north-oriented normal faulting, and detachment sliding on the shallowly-dipping former thrusts.
TECTONIC BRECCIATION Brecciation commenced with an episode of brittle deformation post-dating the early mylonitic deformation, and was tectonic rather than hydrothermal, however intrusive and hydrothermal activity clearly exploited and overprinted this early
Geology of Australian and Papua New Guinean Mineral Deposits
A range of intrusive relationships involving small plugs is evident within the Mine corridor, indicating intrusion of two or more of the supersuites, in any individual mineralised system, and telescoped multiphase hydrothermal activity spanning 35 Myr. At Red Dome, two porphyry phases are present. There is an early quartz veined porphyry, and a later crowded porphyry which appears to have intruded along faults within the early porphyry. SHRIMP U-Pb dating of zircon from a quartz veined porphyry dyke (C Perkins, personal communication, 1994) produced a zircon crystallisation age of 325 Myr (range 330 to 310 Myr). K-Ar dating of sericite produced an argon retention age of 310 Myr (range 314 to 306 Myr), which indicates a minimum age for formation of sericite in this system. Two phases of intrusive emplacement are therefore inferred; an early O'Briens Creek Supersuite phase, and a later Almaden Supersuite phase. At Mungana the sequence is not so precisely determined. Dating of sericite by 40Ar- 39Ar (C Perkins, personal communication, 1994) produced a retention age of 308.3 Myr (range 308 to 308.6 Myr), suggesting an Almaden Supersuite alteration event. K-Ar dating of sericite produced a minimum age of 291 Myr (range 284 to 298 Myr). Whole rock geochemical data for the Mungana intrusive (Woodbury,
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1994), combined with the inferred sericite resetting at circa 290 Myr, has implicated A-type intrusive activity (Lags Supersuite) in the late stages of the paragenesis. The only recognised porphyry plug has undergone reverse faulting which suggests that it was intruded prior to the 315 Myr deformation (Fig 4). The anomalously high tin concentration also suggests the presence of an intrusive of the O’Brien’s Creek Supersuite. This age anomaly is solvable only by assuming that the retrograde sericite alteration system, coeval with the 315 Myr deformation, cooled below the argon retention temperature at about 308 Myr, and that these particular domains within the system were protected from later 290 Myr resetting. High level aphanitic flow-banded rhyolite plugs which produced hydrothermal eruption brecciation are correlated with the Lags Supersuite. An intrusive of this type is exposed at Girofla midway between Red Dome and Mungana, and a similar plug is present at depth to the east of Red Dome at Griffiths Hill.
ORE DEPOSIT FEATURES Six distinct phases of mineralisation are superimposed at Red Dome and Mungana. Planar sheets of deformed massive pyrite -chalcopyrite-sphalerite-galena were initially overprinted by minor hypothermal tin-tungsten-molybdenum as disseminated and veinlet stockwork in the apical area of a microgranite plug and in the surrounding prograde skarn halo. The hypothermal phase was overprinted by pervasive and vein-related mesothermal hydrous retrograde alteration with associated quartz-arsenopyrite-gold veining, and this in turn was crosscut by a second hypothermal skarn phase characterised by the presence of green andradite-magnetite-bornite-chalcocite (Torrey, 1986; Ewers, Torrey, and Erceg, 1990). This was overprinted by a second mesothermal hydrous retrograde alteration with associated carbonate-quartz-chalcopyritesphalerite-galena veining. The final phase ranged from minor epithermal colloform quartz-adularia veining at depth, through massive sulphide-sulphosalt, chaotic and partly subhorizontally laminated breccia pipe deposits, to highly oxidised clay-rich hydrothermal eruption breccias, and chalcedonic silica caps near surface. The deposits differ in three factors. At Red Dome the intrusive plugs are larger and calcareous rocks more prevalent, so that skarn assemblages tend to dominate and are more extensive than at Mungana. Conversely at Mungana a planar sheet of early deformed massive sulphides dominates. The chaotic and laminated massive sulphide-sulphosalt breccia material is far more prominent at Mungana.
MUNGANA At Mungana there is a sheet of massive to disseminated sphalerite-chalcopyrite-galena-pyrite mineralisation averaging 4 m thick with a strike length of 400 m and a depth extent exceeding 500 m (Fig 4). This sheet-style sulphide zone occurs at the sheared and mylonitic interface between Late Llandoverian mafic lava and overlying massive chert. Texture of the sulphide varies from deformed, with durchbewegung texture (balling and boudinage of minor silicate gangue in a sheared massive sulphide matrix) very evident, to a coarse grained annealed polygonal texture adjacent to the intrusive plug. Annealing textures adjacent to the intrusive indicate that this phase predates the O’Brien’s Creek Supersuite, and the stratabound, albeit sheared, morphology suggests a volcanogenic massive sulphide (VMS) Besshi-style deposit.
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Sulphur isotope ratios in the range 3.1 to 4.7 (Woodbury, 1994) are consistent with age trends determined from other Palaeozoic Eastern Australian VMS deposits (Large, 1992). Prograde skarns (garnet-wollastonite-hedenbergite) and retrograde overprint (quartz-chlorite-sericite-sulphide), and extensive pervasive biotite and subordinate orthoclase alteration are apparent in the deeper parts of the Mungana resource. They also occur at shallower levels in the east end of the deposit, away from the influence of the later phase acid leaching. Wide zones of gold-bearing quartz-arsenopyritemolybdenite vein stockwork mineralisation occur within the apical region of the intrusive, as at Red Dome. This is a mesothermal, laminated, crack-seal style, related to the first retrograde alteration phase. A large vein of this type, with estimated maximum width of 10 m, occurs within an interpreted tension gash, but the general style is as a random to sheeted stockwork of veinlets. Quartz veining also occurs as a halo around the intrusive at Mungana, and is best developed within silicified sandstone. The vein stockwork has a preferred orientation parallel to the plane of a major ENE-trending reverse fault, the Mundara fault. In non-carbonate rocks, quartz-sericite-pyrite alteration occurs as selvages to these veins. The mesothermal quartz vein phase is overprinted by the second garnet skarn phase. Quartz-carbonate-base metal sulphide-silver-gold disseminated and vein mineralisation overprints the gold in the quartz stockwork veinlet phase, and is related to the second retrograde phase. The assemblage comprises coarse grained iron-rich sphalerite, chalcopyrite, galena, pyrite, arsenopyrite, tetrahedrite, and traces of native silver, bismuth and electrum, in decreasing order of abundance. Gangue minerals include quartz, calcite, sericite, chlorite and actinolite. Typically this material assays 1.5 g/t gold, 175 g/t silver, 3.5% copper, 0.3% lead and 10% zinc. The upper portion of the deposit is dominated by oxidised breccia (Fig 4). Brecciated silica sinters, mud pot sediments, fluidisation and milling textures and hydrothermal kaolinite were recognised in breccia columns during re-evaluation of diamond drill core in 1992, and phreatic brecciation was proposed (Nethery, Barr and Woodbury, 1994). At depth within the breccia column the acid leach oxidation facies grades into a high-sulphidation breccia. This zoning occurs at both Red Dome and Mungana, and at other deposits in the district. At Mungana, rhythmically-graded massive fine-grained sulphides and sulphosalts, rock flour and minor fine grained carbonaceous material with subhorizontal bedding are evident to depths of 200 m. The finely bedded nature of these indicates deposition in a hot spring setting. The bedded sulphides are commonly slump folded, contain sporadic ‘drop clasts’ of altered aplitic intrusive, and are crosscut by fluidised milled polymictic, sulphide clast, sulphide matrix breccias, which indicate upflow. The matrix comprises comminuted rock flour and dark grey to black, fine-grained sooty sulphides, sulphosalts and carbonaceous material, enclosing clasts of aplite, spongy silica, earlier sulphide phases and sediment. The sulphide-sulphosalt assemblage includes iron-rich sphalerite, chalcocite, chalcopyrite, galena, arsenopyrite, pyrite, digenite, bornite, covellite, tennantite, tetrahedrite and electrum. The oxidised assemblage comprises native copper, gold and silver, smithsonite, cerussite, malachite, azurite, cuprite, copper arsenates, copper phosphates and chrysocolla. The alteration
Geology of Australian and Papua New Guinean Mineral Deposits
RED DOME AND MUNGANA GOLD-SILVER-COPPER-LEAD-ZINC DEPOSITS
assemblage comprises nontronite-berthierine [(Fe,Al)3(Si,Al) 2 O5(OH)4]-kaolinite-alunite-jarosite-plumbogummite [PbAl3 (PO4)2 (OH)5.H2O]-hematite-goethite with spongy, amorphous and jasperoidal silica.
RED DOME Planar sheet-like deformed and annealed massive sulphides were not observed at Red Dome, however clasts of this style occur in the later phase upflow breccias, which indicates the presence of such a body at depth below the current level of exploration. Lead isotope ratios from Red Dome (J Dean and G Carr, unpublished data, 1989) show trends consistent with derivation from the Precambrian Georgetown Block, and are similar to those for Siluro-Devonian granites in the region. Their position on the isotope growth curves is consistent with a Silurian model age. Previous reports referred to a two-phase prograde skarn development at Red Dome, with an intervening retrograde hydrous overprint (Ewers, Torrey and Erceg, 1990). The skarn phases were called ‘brown garnet skarn’ and ‘green garnet skarn’ after the characteristic colour of the andradite. This was confirmed during mining. The first brown garnet skarn phase also has a widespread associated pervasive potassic alteration, as fine-grained secondary biotite and orthoclase growth in the non-calcareous rocks. The first retrograde phase is associated with shallow thrusting and steep reverse faulting, which disrupted the quartz veined porphyry and brown garnet skarn, prior to the second, green garnet skarn phase. This retrograde phase produced the auriferous mesothermal quartz veins as discussed below. Unusual breccias comprising milled brown garnet skarn and retrograde altered porphyry clasts, in a matrix of milled and partly recrystallised marble, and overprinting green garnet skarn show the timing difference between the two skarn phases. Wide zones of gold-bearing quartz-arsenopyritemolybdenite vein stockwork mineralisation occur within the apical region of the intrusive. This is a mesothermal, laminated, crack-seal style, related to the first retrograde alteration phase. The general style is as a random to sheeted stockwork of veinlets. The vein stockwork has a preferred orientation parallel to the plane of a major ENE-trending reverse fault. In non-carbonate rocks, quartz-sericite-pyrite alteration occurs as selvages to these veins for distances of several times the vein width. The mesothermal quartz vein phase is overprinted by an auriferous green andradite, magnetite, bornite, chalcocite skarn (Ewers, Torrey and Erceg, 1990). This green garnet skarn is very prominent as a reaction rim along shallowly dipping thrust planes, particularly in marble. Quartz-carbonate–base metal sulphide-silver-gold disseminated and vein mineralisation, similar to Mungana, overprints the gold in the quartz stockwork veinlet phase, and is related to the second retrograde phase. The upper portions of the Red Dome deposit were dominated by oxidised breccia, regarded by Smith (1985) as being formed by solution (karstic) collapse. This breccia formation mechanism remained unquestioned until brecciated silica sinters, mud pot sediments, fluidisation and milling textures and hydrothermal kaolinite were recognised in 1992, and phreatic brecciation was proposed (Nethery, Barr and Woodbury, 1994). High-sulphidation breccia, identical to that at Mungana, was observed only as large clasts, to a maximum
Geology of Australian and Papua New Guinean Mineral Deposits
diameter of 10 m, within the oxidised breccia at the lower levels of the western end of the Red Dome pit. The oxidised assemblage appears identical to that at Mungana.
DISCUSSION AND CONCLUSIONS The Red Dome and Mungana mineralisation and alteration are ‘telescoped’ overprinting systems, ranging in style from VMS through hypothermal to epithermal, and reflect the major changes that developed in the Palaeozoic regional setting. The Nundah Granodiorite (435 Myr) intruded the basement craton, creating a retrograde greenschist halo in the gneissic metamorphics, and anomalous ‘protore’ concentrations of gold, uranium, and thorium. Coevally, clusters of Besshi-type VMS deposits developed regionally in the Hodgkinson Province, at spacings of around 25 km, at the spacing of submarine Silurian tholeiitic volcanic centres. These sulphide lenses were deposited on lava flows, at the interface with massive exhalative chert. Late Devonian to Middle Carboniferous compression produced overthrusting, and slivering of the Palaeozoic sequence into a melange of competent horses, bounded by narrow zones of mylonite and cataclasite. The shape and orientation of the horses indicate NW-oriented dextral strike slip movement, in addition to north-directed overthrusting. Strain was commonly focussed on the VMS horizons, and deformation and subsequent annealing are apparent. Extension and oversteepening of the thrust package and considerable erosion ensued. Initial I-type plutons (circa 320 Myr) of the O’Brien’s Creek Supersuite, containing disseminated and vein-style low grade tin-tungsten-molybdenum, produced widespread prograde high P-T potassic alteration and prograde skarn assemblages. The late stage crystallisation of these intrusives was coeval with further thin-skinned brittle deformation which produced oroclinal folding and north- to NE-directed overthrusting on east-oriented axes, producing steep reverse faults, shallowlydipping brittle thrusts, and asymmetric, long-wavelength concentric folding in the coeval O’Briens Creek Supersuite volcanic units (Nethery and Barr, 1996). Dilational jogs and interference zones controlled the maximum development of cataclasites, which in turn focussed acid intrusives. The first hydrous retrograde alteration and main gold-bearing quartz veins developed at this time. Fractionated I-type plugs (circa 300 Myr) of the Almaden Supersuite followed. These were more oxidised and iron-rich, and consequently produced the prograde anhydrous green andradite skarn of Torrey (1986), with major copper mineralisation. The release of carbon dioxide and steam induced brecciation, a second phase of retrograde alteration and carbonate-quartz-base metal vein mineralisation. Considerable erosion preceded the A-type intrusive activity (at circa 290 Myr) of the Lags Supersuite, so that the early mesothermal porphyry and hypothermal skarn were exposed at the Lower Permian palaeosurface (which is also the current surface). Boiling of fluid from these high level intrusives produced acid fluids, which attacked limestone and marble producing explosive release of carbon dioxide and steam to the palaeosurface, forming phreatic eruption craters. High sulphidation and advanced argillic assemblages formed within outflow and upflow zones within the breccia pipes at Red Dome and Mungana. This hydrothermal leaching is similar to that described from deposits such as Cerro de Pasco, Peru (Lacy, 1991).
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The oxidised mineral assemblage of copper silicates, arsenates and phosphates suggests that in addition to hydrothermal leaching, deep long-term weathering prevailed. Periods of weathering are inferred in the Early Permian, Late Jurassic and Tertiary to Recent. Interpretation of regional airborne magnetic data shows a correlation of high magnetic intensity with the 2 km2 exposure of a multiple phase intrusive, the Sentinel Range Igneous Complex, some 5 km NW of Mungana. This is the peak intensity area within a regional 50 km2 anomaly, interpreted as an original elliptical shape, but with a superimposed system of anastomosing strike-slip faults which underlie Red Dome and Mungana. It has been difficult to rationalise the wide distribution in the Mine corridor area of pervasive potassic biotite alteration and widespread pervasive quartz-sericite alteration in terms of the size of the known intrusives. It is proposed therefore that the entire area is underlain by a large pluton, and that the small intrusive plugs at Red Dome and Mungana were pipe-like apophyses of that pluton, focussed on the strike-slip faults. A combination of geological mapping and interpretation of airborne magnetic patterns was used to predict relative strike-slip movement on the major faults throughout the mineralising period from 320 Myr to 280 Myr. This model predicts 5 km of dextral slip since intrusion of the O’Brien’s Creek Supersuite circa 320 Myr, and 2 km since 280 Myr, and therefore suggests that Red Dome and Mungana, and deposits in between, such as Girofla and Lady Jane, may all have been part of the same porphyry system at the time of the main gold deposition.
ACKNOWLEDGEMENTS The authors thank Niugini Mining Australia Pty Ltd for permission to publish this paper. This paper has benefited from the knowledge of numerous colleagues, particularly the field work of K Lawrie, D Shatwell and G Lo Grasso, petrology and XRF by K Camuti, recognition and discussions on the oxidised mineral assemblage by P Williams, and discussions and review of this paper by I Plimer. Author M Barr is currently involved in studies for an MSc degree at James Cook University of North Queensland, with Mungana as his major research topic, and acknowledges discussions with and guidance from staff at that institution.
REFERENCES Barr, M J and Nethery, J E, 1995. Gold, silver, and base metals of the Chillagoe district, Australia: hypothermal to epithermal overprinting, in Proceedings PACRIM 1995 Congress (Eds: J L Mauk and J D St George), pp 31–36 (The Australasian Institute of Mining and Metallurgy: Melbourne). Bultitude, R J, Donchak, P J T, Domagala, J and Fordham, B G, 1993. The pre-Mesozoic stratigraphy and structure of the Western Hodgkinson Province and environs, Queensland Geological Survey Record 1993/29. Champion, D C 1991. Petrogenesis of the felsic granitoids of far north Queensland, PhD thesis (unpublished) Australian National University, Canberra. Champion, D C and Heinemann, M A, 1994. Igneous rocks of northern Queensland: 1:500 000 map and explanatory notes, Australian Geological Survey Organisation Record 1994/11 (unpublished).
R Keays, W R H Ramsay and D I Groves), pp 218–232 (The Economic Geology Publishing Company: El Paso, TX). Ewers, G R, Torrey, C E and Erceg, M M, 1990. Red Dome gold deposit, in Geology of the Mineral Deposits of Australia and New Guinea (Ed: F E Hughes), pp 1455–1460 (The Australasian Institute of Mining and Metallurgy: Melbourne). Fawckner, J F, 1981. Structural and stratigraphic relations and a tectonic interpretation of the western Hodgkinson province, northeastern Australia, PhD thesis (unpublished), James Cook University of North Queensland, Townsville. Fordham, B G, 1990. Microfossils and gross structure and stratigraphy of the Silurian–Devonian Chillagoe formation, western Hodgkinson province northeast Australia, in Gondwana: Terranes and Resources, Proceedings of the 10th AGC, Hobart 1990, Geological Society of Australia Abstracts, 25:48–49. Halfpenny, R W, 1991. Porphyry related gold and base metal mineralisation at northwest Mungana, north Queensland, MSc thesis (unpublished), James Cook University of North Queensland, Townsville. Hammond, R L, 1986. Large scale structural relationships in the Palaeozoic of northeastern Queensland: melange and mylonite development, and the regional distribution of strain, PhD thesis (unpublished), James Cook University of North Queensland, Townsville. Holland, C L, 1994. Physical and chemical processes involved in the bleaching of basalt at the Red Dome gold skarn deposit, BSc Honours thesis (unpublished), James Cook University of North Queensland, Townsville. Lacy, W C, 1991. Hydrothermal leaching in the epithermal environment, in Proceedings World Gold ‘91, pp 247–250 (The Australasian Institute of Mining and Metallurgy: Melbourne). Large, R R, 1992. Australian volcanic-hosted massive sulphide deposits: features, styles, and genetic models, Economic Geology, 87:471–510. Mackenzie, D E, 1987. Geology, petrology and mineralisation of the Permo-Carboniferous Featherbed Volcanics Complex, Northeastern Queensland, in Proceedings Pacific Rim Congress 1987, pp 297–301 (The Australasian Institute of Mining and Metallurgy: Melbourne). Mackenzie, D E, Champion D C and the AGSO-GSQ project team, 1994. Permian–Carboniferous magmatism and metallogeny in North Queensland - a new perspective, in New Developments in Geology and Metallogeny: Northern Tasman Orogenic Zone (Eds: R A Henderson and B K Davis), pp 67–68, EGRU, James Cook University of North Queensland, Contribution 50. Nethery, J E and Barr, M, 1996. Revised late Palaeozoic tectonics of north Queensland. Geological Society of Australia Abstracts, 41:313. Nethery, J E, Barr, M J and Woodbury, M J, 1994. Chillagoe District gold, silver, base metal deposits - hypothermal to epithermal overprinting, in New Developments in Geology and Metallogeny: Northern Tasman Orogenic Zone (Eds: R A Henderson and B K Davis) pp 71–74, EGRU, James Cook University of North Queensland, Contribution 50. Shaw, R D, Fawckner, J F and Bultitude, R J, 1987. The Palmerville Fault system: a major imbricate thrust system in the northern Tasmanides, North Queensland, Australian Journal of Earth Sciences, 34: 69–98. Smith, J T, 1985. A mineralised solution collapse breccia, Red Dome, Mungana north Queensland, BSc Honours thesis (unpublished), James Cook University of North Queensland, Townsville.
Donchak, P J T and Bultitude, R J, 1994. Geology of the Atherton 1:250 000 sheet area. Queensland Geological Survey Record 1994/5.
Torrey, C E, 1986. The geology and genesis of the Red Dome (Mungana) gold skarn deposit, north Queensland, MSc thesis (unpublished), James Cook University of North Queensland, Townsville.
Ewers, G R and Sun, S-S, 1988. The genesis of the Red Dome Au skarn deposit, northeast Queensland, in The Geology of Gold Deposits: The Perspective in 1988, Economic Geology Monograph 6 (Eds: R
Woodbury, M J, 1994. Red Dome and Mungana porphyry Cu-Au and base metal skarns of North East Queensland, BSc Honours thesis (unpublished), Australian National University, Canberra.
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Broadbent, G C and Waltho, A E, 1998. Century zinc-lead-silver deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 729–736 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Century zinc-lead-silver deposit by G C Broadbent1 and A E Waltho2 INTRODUCTION
Inferred Resources about the margins of the deposit and associated with faulting within the deposit area.
The Century zinc-lead-silver deposit is approximately 250 km NNW of Mount Isa, Qld, at lat 18o43′S, long 138o36′E on the Lawn Hill (SE 54–09) 1:250 000 and Lawn Hill (6660) 1:100 000 scale map sheets (Fig 1). Stratiform, carbonaceous shale-hosted mineralisation is developed within shale and siltstone of the Lawn Hill Formation, the youngest member of the McNamara Group of the Lawn Hill Platform, which forms the NW exposed portion of the Mount Isa Inlier.
Feasibility studies clearly favour development of a large scale, selective, truck and shovel based open pit mining operation producing concentrate for export via a slurry pipeline linking the mine site with a port facility at Karumba on the southern Gulf of Carpentaria, 330 km E of the mine site. Development of the project awaits completion of cultural clearances and the granting of mining leases for the slurry pipeline route. RTZ-CRA Limited reached an agreement with Pasminco Limited covering the sale of the Century and Dugald River zinc-lead-silver deposits to Pasminco in early 1997. This will enable Pasminco to proceed with development of the Century project in time for ore production to commence during 1999, with initial plans to produce up to 450 000 tpa of zinc in concentrate.
EXPLORATION AND MINING HISTORY PREVIOUS EXPLORATION The Lawn Hill area, in the Burketown mineral field, has a long history of exploration and mining from 1887 when the first leases were pegged by F H Hann, the lessee of Lawn Hill Station. Early activity focussed on mining lead and silver from discordant, NE-trending quartz-siderite lodes, zinc being of little economic interest at the time and a minor constituent of these deposits. Total production of 6174 t of lead and 174 000 oz of silver was recorded from up to 47 small mines.
FIG 1 - Location map, Century deposit and major zinc-lead-silver deposits of the Mount Isa Inlier and adjacent McArthur Basin.
The deposit is within the Burketown mineral field, a centre of sporadic, small scale lead and silver mining between 1887 and the late 1970s. The Century deposit derives its name from the 100 years between pegging of the first mining lease on the Burketown mineral field in 1887, and the granting of exploration tenement covering the deposit to CRAE in 1987. Evaluation drilling since discovery of the deposit by CRA Exploration Pty Limited (CRAE) in April 1990 has resulted in definition of a total in situ resource of 167.5 Mt grading 8.24% zinc, 1.23% lead and 33 g/t silver, including a high grade resource of 105.1 Mt grading 12.10% zinc, 1.69% lead and 46 g/t silver at a cutoff grade of 3.50% zinc. The majority of the resource is classified as Measured, with some Indicated and 1.
Principal Geologist, Rio Tinto Exploration Pty Limited, PO Box 175, Belmont WA 6104.
2.
Principal Geologist, Mining and Resource Technology Pty Ltd, GPO Box 2579, Brisbane Qld 4001.
Geology of Australian and Papua New Guinean Mineral Deposits
Modern exploration of the field commenced in 1930 with an active campaign of prospecting and drilling by The Mining Trust Limited (Blanchard, 1938). Regional geological mapping of the area was completed by the Aerial, Geological and Geophysical Survey of Northern Australia (Jensen, 1941). The only two surface exposures of the Century mineralisation were visited during the course of this survey but their significance was not recognised. The fact that a major mineral deposit remained undiscovered for 100 years in an actively worked mining district highlights the importance of the mining industry retaining access to land for mineral exploration purposes.
CENTURY DEPOSIT DISCOVERY Nine groups held exploration tenements over the Century deposit and conducted geological mapping, geochemical and geophysical prospecting and drilling throughout the area without success, prior to CRAE being granted title in 1987. CRAE’s attention was drawn to the Lawn Hill area in 1986 by a study of the base metal potential of the Mount Isa Inlier (J V Wright, unpublished data, 1986), in which the Lawn Hill Formation was identified as being prospective for stratiform zinc-lead mineralisation. Favourable metallogenic indicators included the occurrence of discordant lode mineralisation in
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close proximity to the Termite Range fault, part of a continental-scale structure interpreted to have been active during basin evolution. Initial work in the immediate vicinity of Century, and Watson’s lode, one of the larger discordant lode systems in the area approximately 10 km to the south, revealed areas of significant lead and zinc values in soil where there were no visible indications of mineralisation as prominent gossans. Reverse circulation (RC) drilling of geochemical targets followed recognition that surface exposures need not appear immediately prospective, leading to the discovery of Century mineralisation in April 1990 (Waltho and Andrews, 1993; Broadbent, 1995).
RESOURCE EVALUATION Evaluation of the Century resource commenced in April 1990. Initial work comprised widely spaced RC drill holes to rapidly assess the tenor of the resource, followed by diamond drilling (mainly in HQ size) during the remainder of 1990 and throughout 1991 (Table 1). This resulted in an average drill hole spacing of approximately 150 m, and a level of confidence in mineralisation continuity required to classify the resource as indicated. It became apparent at a relatively early stage that grade continuity was much less of an issue than structural continuity due to the mineralised sequence being affected by intense small scale faulting. Subsequent drilling focussed on this aspect. Evaluation of the resource and initial feasibility studies proceeded between 1992 and late 1995. The detailed (Type III) feasibility study for the project commenced in August 1994, was completed during late 1995, and was followed by a detailed, external, technical audit of geological and mining studies at that time.
REGIONAL GEOLOGY Mid-Proterozoic rocks of the Mount Isa Inlier and adjacent McArthur Basin outcrop over an area exceeding 50 000 km2 in NW Qld and the NT (Blake et al, 1990). They host the major zinc-lead-silver deposits at Mount Isa, Hilton and George Fisher (Hilton North), Cannington, Dugald River, Lady Loretta, Century and McArthur River (Fig 1) in what has become referred to as a ‘zinc belt’. The combined resources of these deposits led to the mid-Proterozoic basins of NW Qld being recognised as a world class zinc-lead province. The number of documented zinc-lead occurrences, however, is surprisingly small in comparison with the number of major deposits. Only about 80 occurrences are known throughout the Mount Isa Inlier and more than 40 of these occur within a 20 km radius of Century.
Units of the McNamara Group in the NW portion of the Inlier, represent a 5000 to 10 700 m thick column of fine grained clastic sediment with subordinate dolomitic clastic sediment, carbonate, volcanic rocks and chert (Hutton et al, 1981). The Lawn Hill Formation, host to Century mineralisation, is the youngest preserved unit of the McNamara Group (1595±6 Myr), comprising between 1800 and 2200 m of shale, siltstone, tuff, tuffaceous siltstone and sandstone. It is subdivided into six mappable members, Pmh1–6, (Fig 2), of which two units Pmh3, Bulmung Sandstone and Pmh5, Widdallion Sandstone, are formally named (Sweet and Hutton, 1982). The Century mineralisation occurs over a 45 m interval, 80 to 100 m below the conformable boundary between unit Pmh4 and the overlying Widdallion Sandstone.
ORE DEPOSIT FEATURES STRATIGRAPHY The Pmh4 host unit comprises an 850 m thick sequence of siltstone, shale, carbonaceous shale and sandstone. Interbedded siltstone, sideritic siltstone, shale and minor fine quartz-lithic sandstone occur in the hanging wall and footwall of the mineralised sequence, forming a 300 m thick, slightly coarsening upward package, overlying 300 to 400 m of carbonaceous, pyritic shale. The thick shale has a fining upwards trend from the conformable boundary with the underlying Bulmung Sandstone (Fig 2). Establishing detailed stratigraphic correlations in Pmh4, above and below the mineralised sequence is difficult, due to the lack of lithologically or geophysically distinct markers across the deposit area. Reworked tuff horizons in the hanging wall siltstone-shale sequence, up to 30 m above the top of the mineralised sequence, can be used for local stratigraphic correlations. The Lawn Hill Formation, in the immediate area of the deposit, is overlain by Cambrian carbonates which are intensely faulted and folded fragments from a considerable interval of the Georgina Basin succession (Szulc, 1993).
STRUCTURE The deposit is preserved within the core of the Page Creek syncline, one of a series of large scale fold structures developed in the McNamara Group in the Lawn Hill area. The sediments hosting the mineralisation are affected by relatively gentle, open folding, demonstrated by structure contours (Fig 3) and cross sections (Fig 4). The mineralised sequence dips at between 5 and 25o over most of the deposit area, with dips to 70o at the margins of the deposit.
TABLE 1 Resource definition drilling, Century deposit, 1990–1995. Year
Holes
1990
67
1991
181
1992
Purpose
Metres drilled
Initial resource definition
13 118
Resource definition
34 592
83
Investigation of deposit structure (folding and small-scale faulting)
19 858
1993
30
Structural and geotechnical investigations
1994
140
1995
71
730
6589
Structural, geotechnical and groundwater investigations
26 402
Geotechnical and dewatering studies, collection of additional detailed mine design data
15 424
Geology of Australian and Papua New Guinean Mineral Deposits
CENTURY ZINC-LEAD-SILVER DEPOSIT
FIG 2 - Stratigraphy and lithology of the Lawn Hill Formation, Century deposit area.
smaller scale faults, and a second, possibly conjugate, set of faults is developed approximately perpendicular to these structures. These faults exhibit a combination of reverse and normal character, are generally steeply inclined, and have a significant impact on the detailed distribution of mineralisation. In excess of 30 have throws large enough for them to be regarded as significant from a mine planning standpoint. The more significant NE-trending faults also control the location of the quartz-siderite lead-silver lodes mined historically in the Burketown mineral field.
FIG 3 - Structural map of the Century deposit. Dashed lines show the positions of cross sections in Fig 4.
The Termite Range fault, a major NW-trending, long lived fault system, is immediately adjacent to the NE margin of the deposit (Fig 3). It corresponds with a regional scale geophysical linear that can be traced from south of Mount Isa to McArthur River in the north (Fig 1). Sedimentological evidence and separation analysis demonstrate that the fault was active during mid-Proterozoic sedimentation (Andrews, in press), throughout the later Isa Orogeny, and in a minor way at the end of Cambrian time. The fault is paralleled by numerous
Geology of Australian and Papua New Guinean Mineral Deposits
The chaotic fault and fold pattern in the overlying Cambrian carbonates is considerably more complex than that developed in the Lawn Hill Formation, implying the existence of a thrust or extensional decollement surface at the base of the Cambrian sequence for which current evidence is equivocal. A series of low angle, SW-dipping faults are developed within the Cambrian sequence. These are responsible for emplacement of blocks of Lawn Hill Formation from all parts of Pmh4 and Pmh5 as megaclasts between dismembered blocks of carbonate. The thrust hypothesis raises the possibility of further significant low angle faulting lower in the mid-Proterozoic sequence and thus structural repetition of the Century stratigraphic position elsewhere in the Lawn Hill area. Currently defined mineralisation occurs within three distinct blocks. The Eastern block is approximately 100 m east of the NE limit of the Southern block. The Southern and Northern blocks contain essentially all of the economically significant resource (Fig 3). The deposit has an areal extent of about 1200 m from east to west and 1400 m from north to south. Mineralisation is closed off in all directions by east trending normal faults, the
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FIG 4 - Cross sections, showing the geology of the Century deposit, and the relationships between mineralisation and structures forming deposit boundaries.
unconformity between the mid-Proterozoic and overlying Cambrian sequences, or the present day land surface. The southern boundary of the Southern block is the Magazine Hill fault, a shallowly north dipping, listric normal fault with a throw of at least 250 m. The Pandoras fault, a second northdipping listric fault separating the Southern and Northern blocks, is interpreted to be a splay developed off the Magazine Hill fault at a relatively shallow depth beneath the Northern block. Rotational movement is evident across the Pandoras fault, with throws varying from nothing in the east, where the Northern and Southern blocks effectively join, to over 150 m on
732
the west margin of the Northern block. Nikki’s fault, which partly controls the northern margin of the Northern block, dips steeply south and has a normal throw of at least 70 m. The eastern boundary of the Southern block and all other boundaries of the Northern block occur at the MidProterozoic–Cambrian unconformity. The western boundary of the Southern block occurs at the present day land surface where the upper portion of the mineralised sequence formed a prominent, low ridge prior to excavation of a trial pit at this site in 1994.
Geology of Australian and Papua New Guinean Mineral Deposits
CENTURY ZINC-LEAD-SILVER DEPOSIT
Numerous discordant, irregular intrusive bodies of carbonate breccia occur throughout the deposit, preferentially developed along pre-existing fractures within both the Cambrian and MidProterozoic sequences. The breccia bodies have clasts of Cambrian limestone and dolostone in a carbonate matrix and are 0.1 to 34 m thick.
characteristic stylolitic bedding surfaces in siltstone beds, and thicker and more abundant black shale beds within the mineralised envelope. The stylolites appear to be the product of silicate dissolution from the matrix of the siltstone, and in many instances are filled with trace sphalerite and pyrobitumen. Their development is intimately linked to the mineralising process.
MINERALISATION
Although most sulphide mineralisation occurs as bedding parallel lamellae, mineralisation overall transgresses stratigraphic layering, with the position of the most intense mineralisation moving upward within the mineralised sequence from SE to NW. This manifests itself as a gradual, systematic grade variation within each mineralised shale unit along this vector. In addition, zinc grades generally increase across the deposit from SW to NE with grades in all units generally increasing towards the Termite Range fault. This grade variation occurs without appreciable change in the thickness of the host shale. Additionally, the host shales show no lateral chemical or textural changes indicative of exhalative facies within the preserved portions of the deposit. Localised areas of high lead and silver grades occur in the southeast portion of the deposit in association with discordant but stratabound galena veining. Plans illustrating zinc and lead grade variation within the deposit are presented by Waltho et al (1993).
Mineralogy and geometry Century differs from the well documented McArthur River, Hilton and Mount Isa orebodies in that it is hosted by siliciclastic rather than carbonate-rich sediment. Mineralisation is sphalerite dominated. Diagenetic pyrite, regarded as characteristic of other shale-hosted deposits in the region, is much less abundant and appears to be distributed as a halo around the main zinc-lead mineralisation. Sideritic carbonate, with approximately 70% iron, is the principal ironbearing gangue phase. Sphalerite in the deposit is particularly pure (with +62% zinc) and is in part associated with significant quantities of pyrobitumen. Economic grade mineralisation consists of fine grained sphalerite, galena and minor pyrite which occur as delicate, bedding parallel lamellae in carbonaceous shale units. These are separated by distinctly less mineralised sideritic siltstone, or carbonaceous sideritic mudstone marker horizons within the 40 to 50 m thick mineralised interval within Pmh4, as shown by a typical drill hole (Fig 5). This interval is sedimentologically similar to the siltstone and shale immediately above and below. The important differences between the mineralised and unmineralised portions of the sequence are the development of
Ore textures Two principal varieties of stratiform sphalerite are recognised. ‘Porous’ and ‘non-porous’ sphalerite are distinguished on the basis of their high and low pyrobitumen content respectively. The two varieties appear to be cogenetic. Both forms are grey to white in colour and appear highly replacive, the porous variety
FIG 5 - Mineralised sequence stratigraphy, Century deposit, hole DD91LH231. Histograms representing zinc, lead and silver assay data demonstrate the preferential development of sulphides in carbonaceous shale, as opposed to sideritic siltstone beds within the mineralised interval of the deposit sequence. Average grades of zinc, lead and silver are given for the two main mineralised zones.
Geology of Australian and Papua New Guinean Mineral Deposits
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G C BROADBENT and A E WALTHO
having a preference for organic-rich microlaminae, and the non-porous for siliceous microlaminae. Silica replacement is interpreted to be linked in genetic process terms to the dissolution of silica during stylolite development. Porous sphalerite is slightly more common than its non-porous counterpart, and much more common than non-porous sphalerite in lower grade portions of the deposit. Non-porous sphalerite also appears to be more abundant in the upper portion of the mineralised sequence than porous sphalerite, although both types exist in the same hand specimen. Two further types of sphalerite occur within the deposit. Coarser grained, yellow to honey-coloured sphalerite occurs as fracture fill stratabound within the overall mineralised sequence. Coarse grained, red-brown, higher iron sphalerite occurs within brecciated carbonate veins developed along NEtrending faults. This form of mineralisation is not commonly developed where these faults intersect stratiform mineralisation forming the Century deposit (only rare examples have been observed in drill core) but is common about the intersection of the Nikki’s, Page Creek (a relatively large NE-trending fault bisecting the Northern block), and Termite Range faults (Fig 3). Galena occurs, primarily, as two textural forms: 1.
2.
Fine, frequently zoned euhedra formed in bands within, and concordant to sphalerite lamellae (Figs 22 and 23 in Waltho and Andrews, 1993), most commonly developed in the upper portion of the mineralised sequence. Coarse grained, irregular, stratabound veins are developed throughout the mineralised sequence.
Thin bands of galena euhedra are distributed throughout the deposit whereas discordant, irregular galena veining is most common in the SE corner of the deposit where it appears to be associated with NE-trending faults. Numerous similar faults control minor lode mineralisation throughout the Lawn Hill region. Galena veins are mostly stratabound within carbonaceous shale beds, and appear to overprint stratiform sphalerite lamellae in hand specimen, or may be surrounded by a narrow band of shale in which no other sulphides are developed. Thin fracture coatings of galena, in both shale and siltstone, are frequently associated with this form of mineralisation. Siderite within the siltstone is the main iron-bearing gangue phase. Petrographically, siderite development appears to postdate much of the compaction of the host siltstone, and is in part synchronous with stylolite development and sphalerite deposition. Microprobe analyses of siderite reveal very marked changes in siderite composition with time. The earliest formed siderites contain up to 20% manganese and 5% zinc, but later generations are more iron- and magnesium-rich with correspondingly less manganese and no zinc. This chemical change appears to coincide with the introduction of mobile carbon phases (pyrobitumen interpreted to have then been liquid hydrocarbon) and the onset of major sulphide deposition.
Relative timing of deposition, diagenesis and mineralisation The timing of mineralisation relative to sediment compaction is constrained by the development of nodular patches of silica cement in the siltstone which pre-date the deposition of the earliest siderite. The deformation of bedding around these patches indicates that significant compaction took place during
734
and after the silica cementation but before the deposition of siderite. Some compaction clearly continued after the formation of nodular siderite cement within the mineralised sequence, constraining subsequent sphalerite deposition to quite late in the diagenetic history of the host sediment.
Sulphur isotope geochemistry The sulphur isotopic composition of sphalerite in the Century deposit appears to evolve with time to progressively heavier values. Earliest porous and non-porous forms have values of between 5 and 10 per mil, but the later fracture filling styles evolve to values of between 10 and 20 per mil. This isotopic evolution appears to follow through into more widespread vein-style lodes in the 50–100 km2 area surrounding the deposit. Early sphalerite in these lodes has a sulphur composition of 20 to 25 per mil and later sphalerite generations have progressively heavier values reaching a maximum of 25 to 30 per mil in the final stages of vein mineralisation (Bresser, 1992).
METAL DISTRIBUTION Restriction of significant zinc and lead mineralisation to carbonaceous shale horizons within the mineralised portion of the sequence is clearly evident in assays for all drill holes, typified by DD91LH231 in Fig 5. Within the shale horizons, mineralisation exhibits remarkable lateral continuity. Individual sulphide bands, even when only several millimetres thick, may be traced over tens of metres in underground and open pit exposures. This is reflected in variography based on zinc assay data in particular, where ranges between 170 and 525 m along a major axis trending between 295o and 335o (approximately parallel to the Termite Range fault) have been modelled in individual shale horizons. A gradual decrease in zinc grade from NE to SW occurs across the deposit, with increasing distance from the Termite Range fault (Waltho et al, 1993). A similar pattern is evident in relatively low lead grades attributed to the distribution of galena euhedra in sphalerite lamellae throughout the deposit, referred to previously. High grade lead areas, however, are associated with the occurrence of discordant, coarse grained galena veins. Average lead grades decrease markedly with depth in the mineralised sequence. The distribution of silver in shales is not readily accounted for by geological features influencing the distribution of zinc and lead. Silver occurs as a solid solution replacement in sphalerite (Waltho, Allnutt and Radojkovic, 1993) rather than as a distinct silver mineral phase, but the distribution of high grade silver tends to flank areas of discordant, stratabound vein galena mineralisation although this association is far from definite and requires further examination. Spatial variation in zinc:lead ratios has been cited as a means of determining the position of feeder systems for mineralising fluids in shale-hosted zinc deposits, including Sullivan (Hamilton et al, 1982), Lady Loretta (Hancock and Purvis, 1990) and McArthur River (Logan and Dennis, 1981). An increasing trend in zinc:lead ratios away from the SE corner of the Century deposit was noted by Waltho et al (1993). This trend, however, becomes indistinct when the interpreted contribution of vein-type galena mineralisation to the trend is taken into account, leaving an impression of a relatively constant zinc:lead ratio throughout the deposit. Slightly elevated copper and elevated thallium and mercury levels in the
Geology of Australian and Papua New Guinean Mineral Deposits
CENTURY ZINC-LEAD-SILVER DEPOSIT
eastern portion of the Southern block, along the Pandoras and Magazine Hill faults, however, lends weight to proposals that these structures, and the Termite Range fault, played an important role in fluid transport during mineralisation (Waltho, Allnutt and Radojkovic, 1993). Lack of a distinct zinc:lead ratio trend may also point to a different mode of origin for the Century deposit from the ‘sedex’ model advanced for those cited above, adding weight to petrographic evidence for diagenetic replacement.
ORE GENESIS Conventional ‘sedex’ models for ore emplacement at Century are considered inadequate to explain: 1.
the overall mineral zoning, and the transgressive nature of mineralisation at deposit scale;
2.
the development of stylolites and petrographic evidence for silica dissolution and mobility associated with sulphide deposition;
3.
the lack of any relationship between ore host shale horizon thickness and total sulphide content or zinc and lead grades;
4.
the intimate association of sulphides, siderite, and pyrobitumen;
5.
timing relationships between sulphides and siderite, and compactional features; and
6.
the lack of complex silicate or barite gangue phases indicative of exhalite facies.
Mineral zoning observed within the deposit could possibly be explained by sulphides being derived from more than one exhalative source at the time of deposition (1 above). This, however, fails to account for petrographic and other evidence obtained from mineralised sequence shale and siltstone (2–6 above). Century shares some of the key geological features of other shale-hosted zinc deposits but does not exhibit petrographic and geochemical characteristics that enable its genesis to be explained in terms of the sedimentary exhalative deposit model used for most other large, sediment hosted zinc-lead-silver deposits. Timing relationships between stylolite development, sulphide deposition and compaction in particular point to a syndiagenetic replacive rather than a syn-depositional mode of mineralisation.
ACKNOWLEDGEMENTS The authors wish to thank CRA Exploration Pty Limited and Century Zinc Limited for permission to publish this paper. Research work by G C Broadbent in relation to the genesis of the Century deposit was supported by a Haddon King Scholarship, awarded by CRA Exploration Pty Limited.
Geology of Australian and Papua New Guinean Mineral Deposits
REFERENCES Andrews, S J, in press. Stratigraphy and depositional setting of the upper McNamara Group, Lawn Hill region, Economic Geology. Blake, D H, Etheridge, M A, Page, R W, Stewart, A J, Williams, P R and Wyborn, L A I, 1990. Mt Isa Inlier: regional geology and mineralisation, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 915–926 (The Australasian Institute of Mining and Metallurgy: Melbourne). Blanchard, R, 1938. Lawn Hill concession, ATP100M, Reports to The Mining Trust Limited, Queensland Department of Mines and Energy Open File Report CR525 (unpublished). Bresser, H A, 1992. Origin of base metal vein mineralisation in the Lawn Hill mineral field, north western Queensland, BSc Honours thesis (unpublished), James Cook University of North Queensland, Townsville. Broadbent, G C, 1995. The Century discovery, northwest Queensland is exploration ever complete?, in Proceedings Pacific Rim Congress ’95, pp 81–86 (The Australasian Institute of Mining and Metallurgy: Melbourne). Dundas, D and Orridge, G, 1974. Authority to Prospect 1163M (Lawn Hill) final report, Newmont Pty Ltd, Queensland Department of Mines and Energy Open File Report CR5033 (unpublished). Hamilton, J M, Bishop, D T, Morris, H C and Owens, O E, 1982. Geology of the Sullivan orebody, Kimberley BC, Canada, in PreCambrian Sulphide Deposits, H S Robinson Memorial Volume, Special Paper 25, pp 597–665 (Geological Association of Canada: Toronto). Hancock, M C and Purvis, A H, 1990. Lady Loretta silver-lead-zinc deposit, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 943–948 (The Australasian Institute of Mining and Metallurgy: Melbourne). Hutton, L J, Caveney, R J and Sweet, I P, 1981. New and revised stratigraphic units, Lawn Hill Platform, northwest Queensland, Queensland Government Mining Journal, September 1981, pp 423–434. Jensen, H I, 1941. The Lawn Hill silver-zinc-lead field, Lawn Hill Wollogorang district, Aerial, Geological and Geophysical Survey of Northern Australia, Report Queensland 46. Logan, R G and Dennis, R W, 1981. Pb-Cu-Ag mineralisation in the HYC deposit, McArthur River, Northern Territory, in Sediments Through the Ages, Fifth Australian Geological Convention, Sydney, Geological Society of Australia Abstracts, 3:8–9. Sweet, I P and Hutton, L J, 1982. 1:100 000 geological map commentary, Lawn Hill region, Queensland, Bureau of Mineral Resources, Geology and Geophysics and Geological Survey of Queensland. Szulc, S A, 1993. The stratigraphic reconstruction of a mega-breccia; a sedimentological study of the south-western corner of the Lawn Hill outlier, BSc Honours thesis (unpublished) James Cook University of North Queensland, Townsville. Waltho, A E and Andrews, S J, 1993. The Century zinc-lead deposit, northwest Queensland, in Proceedings AusIMM Centenary Conference, pp 41–61 (The Australasian Institute of Mining and Metallurgy: Melbourne). Waltho, A E, Allnutt, S L and Radojkovic, A M, 1993. Geology of the Century zinc deposit, northwest Queensland, in Proceedings World Zinc ‘93, pp 111–129 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Rea, P S and Close, R J, 1998. Surveyor 1 copper-lead-zinc-silver-gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 737–742 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Surveyor 1 copper-lead-zinc-silver-gold deposit 1
by P S Rea and R J Close
2
INTRODUCTION The deposit is 230 km NW of Townsville and 35 km NW of Greenvale, Qld, at lat 18o46′S, long 144o43′ on the Einasleigh (SE 55–9) 1:250 000 scale and the Conjuboy (7860) 1:100 000 scale map sheets (Fig 1). It is one of four volcanic-hosted massive sulphide (VHMS) deposits within an 8 km long belt at the northern end of the Balcooma Metavolcanics (Withnall, Black and Harvey, 1991). These have been correlated with the Cambro–Ordovician Mount Windsor Volcanics (Henderson,
1986) which host similar VHMS deposits in the Charters Towers region 200 km to the SE. The Balcooma district has about 33 km strike length of prospective metavolcanic rocks and associated metasediments on which exploration is still in progress. Title to all deposits is held by Lachlan Resources NL (Lachlan).
EXPLORATION HISTORY The earliest work in the area was at the nearby Balcooma Goldfield, a small field that achieved its heyday between 1890 and 1896. Modern exploration commenced in 1960, but no discoveries were recorded until October 1975, when Geopeko geologist Warwick Maehl mapped and sampled the Surveyor 1 gossans with results to 0.21% copper, 0.58% lead, 0.14% zinc, 28 ppm silver and 0.4 ppm gold. Mining Lease 1393 was pegged on 1 November 1975 (Fig 2). In 1978, Carpentaria Exploration Company Pty Limited (CEC) took up Authority to Prospect 2036 M surrounding the Surveyor 1 lease, and in late 1978 discovered the Balcooma deposit 2 km to the NE (Huston and Taylor, 1990). Geopeko drilled a hole in December 1980 to test induced polarisation and geochemical anomalies at the eastern gossan zone and intersected some near surface high grade oxide lead (31%) and silver (109 g/t) values in a collapsed gossan. Further drilling in March 1981 intersected disseminated base metal mineralisation with a best grade of about 3.3% combined lead and zinc over 19 m in hole PDH3. Although not known at the time, this intersection is about 50 m up dip west of the main deposit in pyritic tuff of the massive sulphide horizon. Shortly afterwards Geopeko entered into a joint venture by which the Conjuboy Associates (80% Noranda Australia Limited, now Plutonic Operations Limited, and 20% Jones Mining Limited) could earn a 50% interest in the Surveyor 1 lease and other nearby Geopeko tenements. The first major intersection of massive high grade lead-zinc sulphide mineralisation at the Surveyor 1 lease was made in hole PDH11 at 86.5 m in December 1982. This hole was continued as diamond core hole SD13 (Fig 3) in 1983 and intersected 26 m at 1.0% copper, 8.8% lead, 23.6% zinc, 202 g/t silver and 1.05 g/t gold.
FIG 1 - Location and regional geological map of Balcooma area.
1.
Senior Geologist, Lachlan Resources NL, Level 37, 100 Miller Street, North Sydney NSW 2060.
2.
District Geologist Base Metals, Lachlan Resources NL, Level 37, 100 Miller Street, North Sydney NSW 2060.
Geology of Australian and Papua New Guinean Mineral Deposits
By October 1983 the shallowly plunging massive sulphides were outlined over 200 m strike length, being 40 m wide at the south end with a maximum depth of 115 m to the top of the sulphides. After four diamond holes failed to intersect mineralisation at the south end of the deposit a cross fault, now known as the Andesite fault (Figs 2 and 4), was postulated (S Roderick, unpublished data, 1985). The only obvious target that remained for thorough testing was a geochemical anomaly
737
P S REA and R J CLOSE
FIG 2 - Geological map of environs of the Balcooma, Surveyor 1 and Dry River South deposits (after Huston and Taylor, 1990), showing position of long section Fig 4.
associated with gossans in the southwestern corner of ML 1393. Hole PDH20 was drilled to test this in September 1984, and intersected 2 m of massive sulphide grading 0.66% copper, 2% lead, 3.1% zinc, 48 g/t silver and 0.44 g/t gold. A coincident pulse electromagnetic (EM) conductor suggested continuity of sulphides to the south outside the mining lease (S Roderick, unpublished data, 1987). In 1985 CEC reinterpreted data from a SIROTEM survey and located a conductor 200 m south from PDH20. This was drilled in October 1985 by a hole which intersected three massive sulphide lenses totalling 10 m in thickness between 191 and 210 m depth, including 6 m at 1% copper, 3.5% lead and 11% zinc. This was named Dry River South deposit and is now known to be the faulted extension of the Surveyor 1 deposit (Fig 2).
738
In June 1991, Lachlan purchased the Plutonic equity to become sole operator and owner of the Balcooma tenements (Lachlan Resources NL, 1991). Lachlan has since completed a major review of all exploration data and reinterpreted the geology and resources on the basis of detailed drilling. The current Surveyor 1 resource estimate is in Table 1. In 1993 the focus of exploration shifted from the known deposits to regional prospects, resulting in the discovery of a new massive sulphide system 5 km south of Surveyor 1 at Boyds 5 deposit. Systematic exploration, with a focus on EM surveys and drilling, has outlined a VHMS deposit 1.5 km long, 300 m wide and 5 to 30 m thick. The pyritic massive sulphides extend to within 150 m of the surface and occur within altered sodium-depleted felsic Dry River volcanics near the contact
Geology of Australian and Papua New Guinean Mineral Deposits
SURVEYOR 1 COPPER-LEAD-ZINC-SILVER-GOLD DEPOSIT
with overlying metagreywacke of the Clayhole Creek beds. To date drill intersections are subeconomic with a best value of 5 m at 1.46% copper, 1.53% lead, 3.71% zinc, 57 g/t silver and 1.09 g/t gold at 250 m depth. Given the size of the system and the textural similarities to the Dry River South deposit, further drilling may define an economically viable resource at Boyds 5.
REGIONAL GEOLOGY The Balcooma Metavolcanics (Withnall, Black and Harvey, 1991) are a NNE-trending belt of strongly deformed Cambro–Ordovician metavolcanic rocks and metasediment some 33 km long and 8 km wide (Fig 1). The northern part of the belt, which contains the VHMS deposits, is metamorphosed to middle amphibolite facies. The Balcooma Mylonite Zone is a major structure which forms the contact between the Balcooma Metavolcanics and the Late Proterozoic Einasleigh Metamorphics to the west. To the east the Balcooma Metavolcanics were intruded by the Silurian Dido Tonalite and to the north the belt is covered by Cainozoic basalt (Withnall, 1982).
FIG 3 - Cross section on line 7010 N, Surveyor 1 deposit, looking NE.
Lachlan geologists have adopted in part the informal nomenclature of Harvey (1984) although his stratigraphic relationships are not supported. Resolution of the geology at a number of prospects has demonstrated that the Clayhole Creek beds, previously interpreted by Harvey (1984) to be the
FIG 4 - Longitudinal projections of the Balcooma, Surveyor 1 and Dry River South deposits. For location of section lines see Fig 2.
TABLE 1 Surveyor 1 mineral resource estimate 1992. Mineralisation type Oxide and supergene
Ore (’000 t)
Cu %
Pb %
Zn %
Ag g/t
Au g/t
Resource category
95
1.50
11.32
0.59
213
2.00
Indicated
Primary low grade
338
0.70
1.11
4.16
25
0.42
Indicated
Primary high grade
538
0.89
7.34
19.49
152
1.09
Indicated
Geology of Australian and Papua New Guinean Mineral Deposits
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P S REA and R J CLOSE
overturned basal unit, are upright and overlie the Dry River volcanics. Despite complex folding and deformation the vast majority of drill core facings and surface mapping observations in the Balcooma–Dry River South deposit area, and at Boyds 5 deposit, indicate an upward younging and east dipping sequence. To the west of the Dry River volcanics the relationships are unclear, but westward facings and steep easterly dips in the West Branch Creek beds indicate that overturning has occurred. The Dry River volcanics consist of volcaniclastic units, massive quartz-feldspar porphyry, massive aphyric rhyolite and minor metasediment. To the west of these are the metasediments of the West Branch Creek beds which are reported to be overlain by intermediate volcanics of the Golden Creek volcanics and rhyodacitic rocks of the Lochlea volcanics. To the west interbedded metatuff and metavolcanic rocks of the Highway beds overlie the West Branch Creek beds. The sequence was intruded by the Cambro–Ordovician Ringwood Park Microgranite in the central portion of the belt immediately east of the massive sulphide deposits (Huston et al, 1992). Withnall, Black and Harvey (1991) dated an intrusive quartzfeldspar porphyry dyke from the Balcooma deposit using SHRIMP ion probe analysis. They obtained two different ages using different isotopic ratios; an age of 471±4 Myr using Pb207/U238 ratios, and an age of 507±22 Myr using Pb207/Pb206 ratios. Both ages are consistent with the correlation of the Balcooma metamorphic belt with the Mount Windsor Subprovince to the south.
ORE DEPOSIT FEATURES STRATIGRAPHY The deposit area is bounded on the east by the Ringwood Park Microgranite in probable fault contact with metagreywacke of the Clayhole Creek beds (Fig 2). On the west side of ML 1393 the deposit host rocks are in partial fault contact with quartzfeldspar porphyry of the Dry River volcanics. The Surveyor 1 deposit is synformal and plunges south at 20o over a strike length of 250 m with a width of 80 m and a maximum true thickness of 23 m (Figs 2 and 3). A smaller oxide and supergene lens, the Surveyor East deposit, of approximately 50 000 t, lies about 200 m to the east. These deposits are interpreted to be part of the same folded horizon and are truncated down plunge to the south by the NE-trending Andesite fault (Fig 4). This fault has dextrally displaced the Dry River South deposit, the southern extension of Surveyor 1. The turbiditic metagreywacke of the Clayhole Creek beds occurs east of this fault and stratigraphically above the deposits. Massive sulphide mineralisation occurs at a regional scale hiatus in volcanic activity marked by the contact between felsic Dry River volcanics and metagreywacke of the Clayhole Creek beds. At Surveyor 1 the mineralisation occurs concurrent with the ‘last phase’ of volcanism and correlates stratigraphically with the lowest polymetallic lens (Zone 4) at Balcooma and with the Dry River South deposit. At Balcooma the Zone 4 lens (Fig 4) is, in contrast, underlain by the lower metagreywacke of Huston (1990) which probably represents time equivalent sedimentation on the flanks of the volcanic centre in a more quiescent environment. The stratigraphic sequence at Surveyor 1, with the uppermost unit first, is: 1.
upper metagreywacke;
2.
orbicular textured dacite;
740
3.
rhyolite fragmental;
4.
massive sulphide;
5.
pyritic biotite-spotted quartz-sericite schist; and
6.
Surveyor rhyolite.
The massive sulphide lens has a sharp conformable hanging wall contact with the rhyolite fragmental unit and a diffuse footwall contact with semi-massive and stringer sulphides. The lower portion of the massive sulphide unit contains siliceous rounded fragments interpreted as silicified and brecciated footwall rocks. Fragments of siliceous exhalite also occur throughout the massive sulphide lens, many being finely banded. The lens exhibits rapid grade reduction up dip to the west associated with interfingering of fine lapilli tuffs and chloritic shale. A keel of stringer and disseminated sulphides is situated below the core of the massive sulphide and attenuates down the sequence (Fig 3). The Surveyor 1 deposit is underlain by intensely altered, pyritic, biotite-spotted quartz-sericite schist which is interpreted to be rhyolite lapilli tuff and coarse fragmental rocks. The Surveyor rhyolite, the lowest recognised unit in the Surveyor 1 sequence, is typically a massive glassy aphanitic rhyolite that probably represents a ‘Kuroko style’ rhyolite dome. Its upper margins are often indistinct, marked by silicification that intensifies down sequence, and by sericite alteration that increases up sequence and masks the contact with the footwall quartz-sericite schist. The immediate Surveyor 1 hanging wall is a rhyolite fragmental rock which consists of subrounded to angular tightly packed rhyolite fragments in a sericitic matrix with occasional disseminated pyrite and small sulphide clasts. It is conformably overlain by the orbicular textured dacite, a massive lava flow that is characterised by red hematite-altered ovate patches which often contain elongate dark fragments or ovate quartz-carbonate filled vesicles. Some of the fragments in the orbicular patches appear to be derived from the underlying rhyolite fragmental unit. The ovate shape of the unit in cross section suggests that it was extruded and confined to a narrow topographic depression, which is in keeping with the model that the underlying sulphide accumulated in a trough with a faulted margin. The orbicular textured dacite unit and the underlying rhyolite fragmental unit also occur in the hanging wall of the Dry River South deposit directly south of the Andesite fault. The uppermost stratigraphic unit in the lease area is the upper metagreywacke which also occurs in the hanging wall of the Dry River South and Balcooma deposits.
INTRUSIVES Five types of intrusive are recognised. The largest by volume is the Ringwood Park Microgranite to the east of the Balcooma, Surveyor 1 and Dry River South deposits. Quartz-feldspar porphyries are the most significant intrusive close to these deposits, although at Surveyor 1 they do not appear to have stoped out significant quantities of massive sulphide mineralisation as is the case at Balcooma. Three phases of quartz-feldspar porphyry dykes are present and all crosscut bedding and mineralisation. The third phase dykes are parallel to the S2 foliation and were injected along thrust faults during or slightly after peak D2 activity. The third type of intrusive is mafic hornblendic dykes which occur within foliation-parallel thrust faults and often occupy the same faults as late quartz-feldspar porphyry sheet dykes.
Geology of Australian and Papua New Guinean Mineral Deposits
SURVEYOR 1 COPPER-LEAD-ZINC-SILVER-GOLD DEPOSIT
The last two groups of intrusives are basaltic and occur as dykes within brittle late faults or reactivated older faults such as the Andesite fault. The most recent of these are undeformed and vesicular, and are probably the same age as the Tertiary flood basalts.
STRUCTURE AND METAMORPHISM Previous workers (Harvey, 1984; Huston, 1988) concluded that the deformation around Balcooma is dominated by folding. Four cleavages, which indicate four separate folding events, have been recognised. Of these only two, D2 and D3, have had a marked effect on the deposit. The first three folding events are coaxial, with axes plunging 20o to the SSW (Huston and Taylor, 1990). It is currently argued that the structure is dominated by D2 foliation-parallel shearing which has ‘plastically’ deformed the sulphide lens. The lens is penetrated by a number of shears parallel to D2, some of which host quartzfeldspar porphyry and mafic dykes. Recent studies of mineralogical and textural relationships indicate that the deposit progressed through a cycle from peak metamorphic temperature of 600oC at 4 kb during D2 to 318oC at 5 kb during D3. The pervasive S2 cleavage is commonly crenulated by an S3 cleavage which has a NNE strike and a near vertical dip (Withnall, 1982). The Andesite fault is interpreted to be of D3 age with a dextral sense of movement. Two later deformations were recognised in the area by Huston (1988); a D4 cleavage crenulating S2 and S3, and a number of NE-trending faults interpreted to be D5. The dominant D2 foliation has a variable strike, between 030 and 055o magnetic. All sheared quartz-feldspar porphyry contacts strike within this range. The degree of deformation is locally intense with massive quartz-feldspar porphyry often sheared to sericite schist with rare quartz ‘eyes’ over widths of 1 to 3 m. Both the east and west sides of the deposit are in fault contact with quartz-feldspar porphyry intrusives, and the eastern fault has east block up displacement of about 70 m.
MINERALISATION The massive sulphides are coarsely recrystallised and consist of euhedral sphalerite with up to 15% interstitial galena, ‘buckshot’ pyrite and 1–5% chalcopyrite. Minor to rare arsenopyrite, pyrrhotite and tetrahedrite are also present interstitially. Gold is associated with copper and both have highest grades at the core of the deposit. These features and an apparent grading of the sulphide grain size, from coarse in the core to fine at the margins, may reflect the zoning of a primary mound in a VHMS style deposit (Eldridge, Barton and Ohmoto, 1983). Massive magnetite is absent from the deposit, unlike Balcooma where massive primary magnetite is a common ore component. However disseminated magnetite is present in the overlying orbicular textured dacite flow and in pyrite-chlorite altered quartz-sericite schists from deep in the footwall. No siliceous or baritic cap is present at Surveyor 1. The Balcooma deposits are unusual for Phanerozoic deposits in that they lack barite gangue (Huston et al, 1992).
Geology of Australian and Papua New Guinean Mineral Deposits
ALTERATION At Surveyor 1 footwall alteration comprises proximal sericitepyrite alteration and distal silicification. The proximal alteration is characterised by relative losses in silica, calcium, sodium and strontium, and relative gains in titanium, aluminium, iron, manganese, potassium and in nearly all trace elements. Strong potassium enrichment is associated with sericitisation, probably after an original kaolinitic alteration. Strong relative gains in iron, copper, lead, zinc and arsenic are all related to sulphide mineralisation. The distal silicified footwall has the same immobile trace element signature as the altered proximal quartz-sericite schist and was probably altered by fluids from the same magmatic source. The unit is characterised by relative gains in silica, titanium and aluminium with depletion of manganese, magnesium, calcium and potassium. However sodium is not depleted in this distal alteration halo. Hanging wall alteration is present in the orbicular textured dacite where strong hematite staining, abundant magnetite, biotite, ferrohastingsite and carbonate in the orbicular alteration patches indicate that iron-calcium metasomatism has occurred, probably from late fluids passing up through the deposit.
ORE GENESIS Surveyor 1 is a high grade and metamorphosed, recrystallised polymetallic volcanic-hosted massive sulphide deposit. Classical VHMS zoning is apparent, with the highest copper grade occurring with high grade lead-zinc mineralisation. Copper and gold are well focussed in footwall stringer mineralisation that has its highest grades in the immediate footwall to the massive sulphide, and appear to outline fluid feeders to the deposit. The metal zoning has been partially reoriented by the D2 deformation with lead zoning oriented parallel to the foliation, but with copper apparently still defining some primary zoning.
ACKNOWLEDGEMENTS The authors thank the management of Lachlan Resources NL for permission to publish this paper and recognise the important contributions made by other company geologists both past and present who have worked on the Surveyor 1 deposit and the Balcooma area.
REFERENCES Eldridge, C S, Barton, P B and Ohmoto, H, 1983. Mineral textures and their bearing on the formation of the Kuroko ore bodies, in The Kuroko and Related Volcanogenic Massive Sulfide Deposits (Eds: H Ohmoto and B J Skinner), pp 241–281 (Economic Geology Publishing Company: El Paso, TX). Harvey, K J, 1984. The geology of the Balcooma massive sulphide deposit, north-east Queensland, MSc thesis (unpublished), James Cook University of North Queensland, Townsville. Henderson, R A, 1986. Geology of the Mount Windsor Subprovince, a lower Palaeozoic volcano–sedimentary terrane in the northern Tasman Orogenic Zone, Australian Journal of Earth Sciences, 33:343–364. Huston, D L, 1988. Aspects of the geology of massive sulphide deposits from the Balcooma district, north Queensland and Rosebery, Tasmania: Implications for ore genesis, PhD thesis (unpublished), University of Tasmania, Hobart.
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Huston, D L, 1990. The stratigraphic and structural setting of the Balcooma volcanogenic massive sulphide lenses, northern Queensland, Australian Journal of Earth Sciences, 37:423–440. Huston, D L and Taylor, T W, 1990. Dry River copper and lead zinccopper deposits, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1519–1526 (The Australasian Institute of Mining and Metallurgy: Melbourne). Huston, D L, Taylor, T, Fabray, J and Patterson, D J, 1992. A comparison of the geology and mineralisation of the Balcooma and Dry River South volcanic–hosted massive sulfide deposits, Northern Queensland, Economic Geology, 87:785–811.
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Lachlan Resources NL, 1991. Annual Report 1991 (Lachlan Resources NL: Sydney). Withnall, I W, 1982. The geology of the Greenvale–Balcooma area, in Handbook for the 1982 Field Conference, Charters Towers–Greenvale Area (Ed: I W Withnall), pp 31–47 (Geological Society of Australia, Queensland Branch: Brisbane). Withnall, I W, Black, L P and Harvey, K J, 1991. Geology and geochronology of the Balcooma area: Part of an early Palaeozoic magmatic belt in north Queensland, Australian Journal of Earth Sciences, 38:15–29.
Geology of Australian and Papua New Guinean Mineral Deposits
Richardson, S M and Moy, A D, 1998. Gunpowder copper deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 743–752 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Gunpowder copper deposits 1
2
by S M Richardson and A D Moy INTRODUCTION
The deposits are 120 km north of Mount Isa, Queensland, at approximately lat 19o20′S, long 139o22′E on the Camooweal (SE 54–13) 1:250 000 scale map sheet. They comprise the structurally controlled Esperanza and Mammoth orebodies and several smaller prospects within the Western Fold Belt of the Mount Isa Inlier (Fig 1). Together they form the basis of the Gunpowder operation of Aberfoyle Limited.
Resource and production data are shown in Tables 1 and 2 respectively. TABLE 1 Measured,Indicated and Inferred Resources for the Gunpowder deposits at 30 June 1997. Deposit
Ore type
Ore (Mt)
Copper grade (%)
Mammoth
Chalcocite Chalcopyrite
5.3 2.1
3.0 2.9
Esperanza
Chalcocite Chalcopyrite
3.8 0.8
8.5 4.0
Subtotal
Chalcocite Chalcopyrite
9.1 2.9
5.3 3.2
12.0
4.8
TOTAL
TABLE 2 Mammoth mine production to June 1997. Period
Producer
Ore (t)
FIG 1 - Location of the Gunpowder deposits. Tectonic units after Blake et al (1990).
Until recently ore was mined from the Mammoth orebodies by open stoping with decline haulage to surface leach pads. Leaching of broken ore (in place) and of surface heaps, followed by solvent extraction and electrowinning (SX-EW), produced copper metal on site. In March 1997 mining was suspended at Mammoth and the focus of the Gunpowder operation switched to development of the Esperanza orebody, which is expected to begin open cut production in mid 1998. SX-EW methods will produce 44 000 tpa of copper from an expanded plant.
1.
Senior Geologist, Aberfoyle Limited, Exploration Division, PO Box 952, Burnie Tas 7320.
2.
Mine Geologist, Aberfoyle Limited, Gunpowder Division, PO Box 2543, Mount Isa Qld 4825.
Geology of Australian and Papua New Guinean Mineral Deposits
1927–1959
Shah/Foschi
1959–1969
Foschi
1969–1972
Copper (%)
Copper metal (t)
745
14.6
5974
9.8
Surveys and Mining Ltd
171 307
2.3
1972–1977
Gunpowder Copper Ltd
1 900 000
2.7
1979–1982
Gunpowder Copper Ltd
n/a
n/a
3700
1989–1996
Adelaide Brighton Ltd
n/a
n/a
35 654
1996–1997
Aberfoyle Ltd
n/a
n/a
8247
n/a - figure not available
EXPLORATION AND MINING HISTORY Copper mineralisation was discovered near Gunpowder Creek in 1923 by the Shah brothers, two Afghan cameleers, who worked a small open cut on the Mammoth No 1 orebody from 1927. Between 1948 and 1969 Mammoth was worked as a small underground operation by Italo Foschi, who sank two shafts with three sublevels. In 1969 Surveys and Mining Pty Ltd took control and embarked on large scale development, including an open cut, a 330 m exploration decline and a copper flotation plant. A joint venture between Consolidated Goldfields Limited and Mitsubishi Limited took control in 1971 and further developed the operation to carry out sublevel overhead bench mining, with
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S M RICHARDSON and A D MOY
decline access. Operation was intermittent over the next ten years until mining was terminated in 1982. Exploration drilling in 1969 beneath gossanous outcrops, 1 km west of Mammoth mine, led to discovery of the Esperanza deposit. A decline to investigate the mineralisation was commenced in 1972 after completion of approximately 25 drill holes but was abandoned in 1974 after several months battling poor ground conditions and high water flows. Adelaide Brighton Limited took control in 1989. Production at Mammoth mine resumed with in place and heap leaching using SX-EW methods. In 1996 Aberfoyle Limited acquired the Gunpowder operation from Adelaide Brighton Limited. From August 1996 to April 1997 resource drilling was carried out at Esperanza to provide further information on the known deposit, within 150 m of surface, on a nominal 25 by 25 m pattern. Including the earlier exploration drilling, a total of 128 holes for 24 000 m were completed. In early 1997, a footwall drive and crosscut were developed from the old exploration decline, 12 m into ore, to obtain a 300 t bulk sample for an onsite pilot leach plant. Following feasibility studies it was decided to proceed with open cut mining of the deposit in July 1997 and ore production is expected to begin in mid 1998. Mining was suspended at Mammoth in March 1997 after resource drilling at Esperanza indicated sufficient reserves to establish an open cut operation. Underground mining is planned to resume at Mammoth when the Esperanza reserves are exhausted.
REGIONAL GEOLOGY The deposits occur within the Western Fold Belt of the Mount Isa Inlier (Fig 1). They are associated with faults related to the Mount Gordon Fault Zone, which varies between the single Mount Gordon Fault and a broad, braided fault array. In general, the Mount Gordon Fault Zone separates gently folded McNamara Group of the Lawn Hill Platform to the west, from older, tightly folded rocks of the Haslingden Group of the Leichhardt River Fault Trough (LRFT) to the east. Mineralisation at Gunpowder is hosted by Proterozoic units at various stratigraphic levels, ranging from Haslingden Group at Mammoth to McNamara Group units at Esperanza (Fig 2). Deformation is related to four phases (D1-D4) of the 1620–1550 Myr Isan Orogeny (Blake et al, 1990). In the Gunpowder area D1 is reflected as an east-trending lineation. Regional and mesoscopic, upright, north-trending folds and greenschist facies metamorphism relate to deformation during D2. Major faulting occurred during D3, although overprinting relationships suggest many faults were active during the later stages of north oriented folding during D2. D4 is expressed as a number of mutually overprinting fracture cleavages. Primary copper mineralisation is associated with faulting during D3 (van Dijk, 1991). Regional faults associated with the Mount Gordon Fault Zone trend dominantly NNE, eg the Esperanza fault (EF). These are associated with ENE-trending cross faults, such as the Mammoth and Mammoth Extended faults (MF and MEF). The EF, MF and MEF all intersect near the southern termination of the Esperanza orebody (Fig 3). Recent structural studies indicate that the cross faults formed, or were reactivated, as a consequence of movement on the regional faults and that all were active at essentially the same D3 time (R L Askew and K A Connors, unpublished data, 1992).
744
FIG 2 - Stratigraphic column for the Gunpowder area, modified after Moore and Stoakes (1975).
Various models have been proposed for the observed fault geometry. A north to south directed thrust duplex system (D1) was proposed by Bell (1983) in which the EF was thought to be a roof thrust, folded into its current orientation during D2. The MF and MEF were interpreted as subordinate duplex structures. Changes in the thickness of some units in the LRFT across east-trending structures led Derrick (1982) to propose that these structures represent reactivated growth faults. The original syndepositional structures were interpreted to relate to development of the Mount Gordon Arch, a north-trending palaeogeographic element within the LRFT. Various sequences are characteristically thinner on top of the Mount Gordon Arch than to the east or west. Major thickness changes of some post-Myally Subgroup units, across the MF and MEF, indicate that these structures formed by reactivation of syndepositional faults. Askew (1992) proposed that the Mount Gordon Fault Zone is the main structure within a NNE-trending D3 dextral strike slip system in which the MF and MEF form a trailing extensional imbricate fan splaying off the EF. The Portal fault (PF) is seen as a subordinate structure paralleling the Esperanza fault. Flat ridge tops in the Gunpowder area are the remnants of a Mesozoic land surface. This surface has been lateritised, presumably during the Tertiary.
Geology of Australian and Papua New Guinean Mineral Deposits
GUNPOWDER COPPER DEPOSITS
ESPERANZA DEPOSIT STRATIGRAPHY The oldest stratigraphic units in the Esperanza area are the Lena Quartzite Member and intervals of metabasalt of the Eastern Creek Volcanics which outcrop on the western side of the EF (Fig 3). The Paradise Creek Formation is east of the EF and stratigraphically at least 2 km higher than the Eastern Creek Volcanics. It comprises grey to cream, laminated, dolomitic siltstone around 150 m thick which outcrops east and south of the orebody. Dips are consistently west at about 50–60o. Conformably overlying the Paradise Creek Formation is a sequence of well bedded to locally massive, black carbonaceous to locally grey or grey-green, weakly dolomitic siltstone and shale. Interpreted to be around 200 m thick and correlated with the Esperanza Formation, this unit is host to the Esperanza orebody. Carbonaceous rocks are dominant, especially in the vicinity of mineralisation. Included within the Esperanza Formation are bands, usually 1 to 5 m thick, of laminated chert and bodies of chert breccia. The largest chert body (Esperanza chert) immediately overlies ore and locally hosts ore grade supergene mineralisation. At surface north of the MF, this unit of pale grey to pink laminated chert and chert breccia is at least 100 m thick and apparently conformably overlies the siltstone-shale sequence. The chert abuts the MEF along its entire length but south of 3475 N (Esperanza mine grid) the chert is bounded by the MEF, the top of the orebody and possibly the MF (Fig 4).
Faulting has resulted in extensive brecciation of the chert. Brecciation is clearly dilational as there is a large amount of void space visible on surface and in drill core. Introduction of silica and iron followed brecciation, as silica±iron oxides line voids and cement the breccia. Brecciation and recementing appear to be most intense for 20–30 m above the ore. The Esperanza chert is part of a relatively continuous, conformable unit with a strike length of at least 15 km. It has a close association with the MEF and EF and is brecciated along its entire length. In addition, at Esperanza the chert is not continuous at depth but generally terminates abruptly, usually with a flat contact, on top of the orebody, at around 5200 RL (Fig 5). Exceptions to this are north of 3575 N, where supergene ore is hosted by chert and around 3425 N where chert extends down the eastern side of the orebody to 5125 RL. Leaching of outcrop is common and is related to lateritisation. Ferruginisation is common adjacent to faults and mineralisation, extending for at least 300 m down permeable structures. The most intense ferruginisation occurs along and on the southern side of the MF where hematite and limonite are well developed. At surface a broad halo of ferruginisation, north of the MEF and south of the MF, is thought to relate to recent weathering and erosion of gossanous outcrops (G F Taylor, unpublished data, 1997).
STRUCTURE There are three major faults in the Esperanza area.
Mammoth Extended fault (MEF) The NE-trending, steeply south dipping MEF is the main structure controlling mineralisation at Esperanza. Map
FIG 3 - Simplified geology of the Gunpowder area, modified after Richardson and Moy (in press).
Geology of Australian and Papua New Guinean Mineral Deposits
745
S M RICHARDSON and A D MOY
FIG 4 - Geological plan of the Esperanza area, after Richardson and Moy (in press).
relationships suggest a dextral offset but interpretation suggests that the MEF is a reverse fault with a lesser component of sinistral movement. At surface the MEF is expressed by strong shearing of ferruginous siltstone for several metres from the Esperanza chert boundary. Below surface the northern edge of the chert, where diamond drilled, is also seen to be a fault, interpreted to be the MEF. Often the edge of the orebody is apparently unfaulted, presumably because the sulphides represent fissure filling of the fault zone. Drilling beneath the orebody indicates a zone, 10–30 m across, of fracturing with quartz and pyrite±chalcopyrite veining, which is interpreted to represent the MEF at depth.
Mammoth fault (MF) The ENE-trending MF is mapped at surface striking into the Esperanza area at around 3475 N but its location and the nature of its termination west of this point are poorly constrained. A subvertical dip to the north is inferred in the Esperanza area, contrasting with the 70o southerly dip of the fault in the Mammoth mine area. South of 3475 N the MF is interpreted to mark the southern margin of the Esperanza chert, although the area between the MF and MEF effectively represents a single broad zone of fault breccia. At depth it is interpreted to merge
746
FIG 5 - Cross section through the Esperanza orebody on line 3450 N, looking grid north, after Richardson and Moy (in press).
with the MEF beneath the main sulphide body. At surface the MEF and MF are interpreted to merge at around 3275 N, although outcrop is very sparse. The MF is interpreted to be a sinistral reverse fault.
Esperanza fault (EF) The EF is a NNE-trending regional structure dipping around 55–70o west. West side up apparent displacement of greater than 2 km is evident. The MEF and MF splay off the EF in the poorly understood area just south of the orebody.
Geology of Australian and Papua New Guinean Mineral Deposits
GUNPOWDER COPPER DEPOSITS
Movement along the fault during D3 is interpreted to have been dextral reverse. Haslingden Group rocks form a pop-up structure between the east dipping Mount Gordon Fault and the west dipping EF within the regional dextral strike slip system associated with the Mount Gordon Fault Zone (Askew, 1992).
In the primary zone colloform textures are not seen and a correlate of this phase is uncertain but is presumably an as yet unrecognised early generation of pyrite veining. This phase of mineralisation may correlate with an early hydrothermal pyrite recognised at Mammoth (Scott, 1985).
MINERALISATION
Primary copper mineralisation
Introduction
Primary copper mineralisation occurs as a kernel within the surrounding supergene zone. Mineralisation is hosted by carbonaceous shale and occurs as:
The Esperanza orebody is a structurally controlled coppercobalt orebody, hosted by carbonaceous shale and chert breccia near the convergence of the Mammoth and Mammoth Extended faults. The orebody trends NE with a strike of 450 m and is subvertical. It comprises a primary zone of pyritechalcopyrite-cobaltite mineralisation as veins, disseminations and minor massive sulphide. Primary mineralisation is enveloped above, below and to the NE by supergene mineralisation of variable style and considerably enhanced grade. Supergene copper minerals include digenite (Cu1.8S), djurleite (Cu1.96S), covellite, anilite (Cu1.36 S) and yarrowite (Cu1.13S) with minor enargite (Cu3AsS4), chalcocite, spionkopite (Cu1.4S) and geerite (Cu1.6S). In overall setting, Esperanza displays characteristics similar to the nearby Mammoth orebodies but differs in being a more iron-rich system containing abundant massive sulphide. Esperanza also features a higher copper grade and porosity, and primary mineralisation is closer to the surface than at Mammoth. The Esperanza orebody is described in more detail by Richardson and Moy (in press).
Primary mineralisation Syngenetic or diagenetic pyrite Fine grained framboidal pyrite, locally forming conformable bands of bedded pyrite, occurs throughout the carbonaceous shale but is especially well developed within shales associated with and adjacent to copper mineralisation. This style of mineralisation is most noticeable on the western side of the orebody where supergene enrichment has not obscured primary textures. Bedding to core axis angles show that bedded pyrite in these shales cannot be west dipping as is generally seen at surface but is consistent with a SE dip, parallel to the western orebody boundary. Bedded pyrite is often disrupted by soft sediment deformation to the point of forming pyrite clasts in a contorted shale matrix. At depth and on the eastern side of the orebody, deformed ‘ptygmatic’ pyrite veins at a low angle to bedding are often associated with bedded pyrite. The abundance of syngenetic pyrite associated with the Esperanza orebody indicates that copper mineralisation may overprint the remnants of a sediment-hosted exhalative pyrite body.
Colloform pyrite mineralisation Pre-dating the introduction of copper but post-dating syngenetic pyrite is a second phase of pyrite mineralisation. This phase is inferred from relict colloform pyrite fragments within the supergene zone. Colloform textures clearly indicate open space filling. Intense brecciation and veining by a later phase containing relict chalcopyrite is evidence of the overprinting nature of the copper mineralisation.
Geology of Australian and Papua New Guinean Mineral Deposits
1.
chalcopyrite-pyrite veinlets and disseminations overprinting and apparently replacing bedded syngenetic pyrite;
2.
veins 1–2 m thick of chalcopyrite-pyrite±chlorite; and
3.
massive sulphide bands to several metres thick containing up to 60% chalcopyrite.
Green chlorite is often associated with chalcopyrite veining. Carbonaceous fragments of uncertain origin, generally 1–3 mm in diameter, occur within veins and massive sulphides.
Cobalt mineralisation The nature of cobalt mineralisation at Esperanza is not well understood. Elevated cobalt values are only associated with the primary zone and the transition to supergene mineralisation. The supergene zone is depleted in cobalt. Within the orebody cobaltite and siegenite [(CoNi)3S4] have been observed associated with chlorite. Peripheral to the orebody, zones of cobalt mineralisation, typically to 0.2% over several metres, are associated with pyrite veins in carbonaceous shale.
Supergene mineralisation The upper, northern and less well defined lower parts of the orebody comprise supergene mineralisation. The base of supergene mineralisation consistently dips towards the SE at around 20o in the north to about 60o in the south (Fig 5). In longitudinal projection (Fig 6) this boundary closely parallels the base of the chert until 3600 N, where it plunges steeply to the north. This surface can be modelled using ferric-soluble copper assays, as the frequency distribution of the soluble copper to total copper ratio defines two populations corresponding to primary and secondary mineralisation. Supergene mineralisation is characterised by having more than 50% of the total copper content available as ferric-soluble copper. Below around 5000 RL a second zone of sparsely drilled supergene mineralisation is present where ground water has penetrated down the MEF. The character of secondary mineralisation is variable, of massive, vein and disseminated types. In general the upper and central parts of the orebody, from the southern end to 3600 N, contain a core of massive sulphide. This grades west, over a few metres, into semi-massive and disseminated mineralisation containing primary chalcopyrite, where bedding within pyritic mudstones becomes very prominent. The eastern margin is more variable, sometimes gradational as on the western side, but often showing a very sharp contact. North of 3600 N mineralisation is dominantly vein and disseminated in style,
747
S M RICHARDSON and A D MOY
MAMMOTH DEPOSIT INTRODUCTION
FIG 6 - Longitudinal projection of the Esperanza orebody, after Richardson and Moy (in press).
hosted by a broad fault zone (MEF). A very sharp top to the orebody occurs around 5200 RL, presumably reflecting the water table during secondary enrichment. A pipe like kernel of unmineralised intensely hematitic siltstone, to 20 m across, occurs near the top of the massive sulphide from 3350 N to 3400 N. The origin of this kernel is uncertain. Opaline silica, which unlike the Esperanza chert is clearly related to supergene processes, is locally found as bands up to a metre across on the top edges of the orebody in this area. Mineralogy is variable but dominated by digenite, djurleite and covellite. Microprobe analyses from the central part of the orebody indicate an overall vertical zoning from digenitecovellite-anilite at the base to digenite-djurleite-covellite and minor chalcocite toward the top. Minor enargite occurs throughout. Pyrite and marcasite are the dominant gangue minerals, with minor quartz, chlorite, hematite, carbonaceous matter and opaline silica.
Gossan and chert Although it is uncertain to what extent primary mineralisation occurred within the Esperanza chert, gossanous chert breccia, locally containing massive hematite, crops out south of around 3450 N. The copper content of this material and throughout the Esperanza chert is generally less than 1000 ppm. The origin of the Esperanza chert and its relationship to mineralisation are uncertain. Although its base appears to largely coincide with the water table during secondary enrichment, it is difficult to attribute major changes seen across this boundary solely to supergene processes. Primary mineralisation within what is now the Esperanza chert was presumably the matrix component of a siltstone fragment–rich breccia. Below the boundary there is very little evidence of siltstone or chert fragments; only massive sulphide, much of which appears to predate the supergene event. It is suggested the base of the chert may be a primary geological boundary and the top of the supergene mineralisation marks the water table during secondary enrichment. The two positions coincide over much but not all of the orebody.
748
Much of the current knowledge of the Mammoth orebodies is from work of mine geologists during the 1970s, such as Moore (1974), Mitchell and Moore (1975), Moore and Stoakes (1975), and G P Moore (unpublished data, 1977), and researchers during the late 1970s to mid 1980s, such as Scott and Taylor (1977) and Scott (1985). Mine development at that time was restricted to the upper levels and although much new exposure on lower levels was developed during the 1990s, very little information from this part of the ore system has been documented. In place leaching and collapses have now rendered much development in the middle and upper levels unsafe and drill core from these areas has deteriorated. Consequently, recent workers have been unable to access much of the orebody. The following description of the Mammoth mineralisation is based on the work of early workers and current knowledge of the lower parts of the ore system.
STRATIGRAPHY Mineralisation at Mammoth mine is hosted by upper units of the Myally Subgroup, which strike north and dip steeply west at 65–85°. The lowermost unit exposed within the mine is a pink massive to weakly bedded quartzite which abuts the Portal fault and is at least 60 m thick. Stratigraphically overlying the quartzite is a complex sequence of interbedded laminated sandstone, arkose, siltstone and minor quartzite 60 m thick near surface and increasing in thickness with depth. Texturally these rocks are poorly sorted with subangular to subrounded quartz, feldspar or lithic grains. Magnesian chlorite is a common matrix component. Unconformably overlying the Myally Subgroup are conglomerate, sandstone and siltstone of the Surprise Creek Formation. These units also strike north but their overall dip is around 10–15° shallower than the dip of the underlying Myally Subgroup, due to the angular unconformity. The Surprise Creek Formation is approximately 300 m thick north of the Mammoth fault and comprises a basal sequence of about 80 m of well bedded conglomerate, interbedded with graphitic, chloritic, ferruginous and siliceous siltstone. The conglomerate contains subrounded to rounded pebbles of quartzite, arkose, sandstone and siltstone. Average diameter is 1–2 cm but near the base cobbles up to 10 cm are present. The remainder of the formation overlying the conglomerates is graphitic, micaceous and locally ferruginous siltstone. Outside the mine area the Surprise Creek Formation is overlain unconformably by the McNamara Group.
STRUCTURE There are three major faults at Mammoth mine.
Mammoth fault (MF) The MF strikes ENE and dips at around 70o south. Movement is interpreted as sinistral reverse with an unknown displacement. The fault is expressed as a zone of fracturing, shearing and quartz veining with silica and hematite alteration over a width of 2 to 15 m. Occasionally a 30 cm wide chloritic, sericitic, puggy shear is present on the footwall side of the quartz veining.
Geology of Australian and Papua New Guinean Mineral Deposits
GUNPOWDER COPPER DEPOSITS
Portal fault (PF) The PF is a north-striking reverse fault dipping 55o west. It terminates against the MEF and the MF terminates against the PF. Conflicting crosscutting relationships between the PF and the subparallel MF and MEF are interpreted by Askew (1992) to be the result of mutual overprinting during a single deformational event. A 2–5 cm laminated hematite and limonite-rich pug zone characterises the PF and often occurs in the centre of a zone of shearing, brecciation and hematite veining to 30 m across.
Mammoth Extended fault (MEF) Although not intersected by underground development, the MEF is known from drilling to be associated with minor mineralisation near its intersection with the PF. Expressed as a zone of silicification, brecciation and numerous small shears a few metres across, it dips about 75o south and is interpreted as a sinistral reverse fault.
MINERALISATION Introduction Mineralisation at Mammoth comprises bodies of sulphidematrix dilational breccia, largely developed within a block bounded by the MF, PF, MEF and the unconformity at the top of the Myally Subgroup. This block plunges at approximately 60o to the SW and from current drilling is known to contain mineralisation to at least 1000 m below surface. Economic mineralisation is confined near the junction of the MF and PF, in the footwall of the MF and hanging wall of the PF (Figs 7 and 8). Within the block individual orebodies comprise subvertical to steeply west dipping mineralised zones with a strike length of less than 150 m. Away from the controlling faults they are subparallel to bedding but as individual lenses approach the MF their strike swings parallel to that structure. The faults are not mineralised and the orebodies display a crude en echelon pattern towards the SW (Fig 8). The style of mineralisation is variable and reflects the extent of host rock fracturing. Mineralisation texture furthest from the controlling faults varies from crackle to jigsaw breccia, where individual fractures pick out bedding and joint planes resulting in a stockwork of veinlets. As the faults are approached intensity of brecciation increases to fragment and matrix supported, locally milled breccia, comprising a matrix of copper and iron sulphides enclosing fragments of wall rock. Adjacent to the MF or PF wall rock fragments are often rare, resulting in zones of massive sulphide to several metres across. Only local small scale replacement of host rock is observed. The mineralised system as a whole is zoned. Mineralisation occurs as pyrite and chalcopyrite at depth, with a top at around 500 m below surface. Above this depth chalcopyrite gives way to bornite and at higher RL more gradationally to ‘chalcocite’ group minerals. In the following discussion the term chalcocite is used to refer to undifferentiated copper-sulphur species including digenite, djurleite and chalcocite. Bornite and chalcocite mineralisation also extends downward, adjacent to the main faults, to an unknown lower limit. Currently the chalcopyrite zone is regarded as primary, and the bornite and chalcocite zones are considered to be supergene mineralisation.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 7 - Cross section through the Mammoth B orebody, on line 20 800 N, looking north.
FIG 8 - Geological plan on Mammoth 4850 RL showing B, C, and D orebodies.
Orebody details Four closely related orebodies (No 1, B, C, D) and several smaller subeconomic lenses have been delineated to date (Fig 9). All have diffuse boundaries defined by a grade cutoff and in detail lobes of mineralisation sometimes connect one orebody to another. At low cutoff grades much of the block defined above is mineralised, but at a higher cutoff grade ore is confined closer to the controlling structures.
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minerals. These may represent breccias that originally contained primary mineralisation but have been leached of copper during the supergene process. D orebody is currently the deepest defined. Mineralisation is hosted by interbedded massive and laminated siltstones. At depth D orebody strikes north, subparallel to bedding, but at higher RLs mineralisation trends away to the SW and becomes confined to breccias associated with the MF. It is uncertain whether the SW-trending top of D orebody (Fig 8) is hosted by a fault bounded sliver of Myally Subgroup or lies within the hanging wall of the MF. Mineralisation is dominantly chalcocite-bornite-pyrite above the base of supergene mineralisation and chalcopyrite-pyrite below that boundary.
FIG 9 - Longitudinal projection of the Mammoth orebodies, looking west.
The extent of stratigraphic control of mineralisation is uncertain. G P Moore (unpublished data, 1977) stated that in the upper parts of the mine the most intense brecciation and highest copper grades were associated with favourable horizons richer in clastic feldspar. This observation has not been confirmed by mine geologists for the deeper orebodies. However, at least parts of some orebodies have boundaries parallel to bedding suggesting that some units may be more favourable for brecciation or precipitation of metals. No 1 orebody originally extended from surface to 5050 RL but is now largely mined out. What remains is currently being leached in place. Mineralisation is oriented ENE along the footwall of the MF and dilation against this structure is clearly the primary control. Near surface the orebody consisted of a number of shoots of economic mineralisation which merged to become a single body at depth. Chalcocite is the dominant copper mineral, often displaying eutectic intergrowths with small quantities of bornite (Moore and Stoakes, 1975). Minor chalcopyrite, pyrite and covellite are also reported. Sooty chalcocite was common to about 70 m below surface. B orebody occurs down plunge from No 1 orebody (Fig 9). Mineralisation occurs within a breccia zone hosted by argillaceous sandstone and siltstone. Unlike No 1 orebody, B orebody strikes subparallel to bedding. In detail mineralisation crosses the strike of bedding at a low angle, with mineralisation to the north sometimes hosted by stratigraphically higher units than ore adjacent to the MF. The large vertical extent of B orebody results in the deeper parts of the lens comprising chalcopyrite-pyrite, but at higher levels mineralogy is predominantly chalcocite-bornite and some pyrite. Chalcocitebornite is more abundant relative to pyrite closer to the MF. C orebody is hosted by brecciated, massive, siliceous sandstone or quartzite about 30 m stratigraphically below B orebody, adjacent to the PF. Mineralogy is entirely chalcocitebornite-pyrite. Unmineralised matrix and clast supported breccias have been mapped between C and B orebodies. The matrix appears to be hematitic rock flour with no copper
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Subeconomic mineralisation is also known from several adjacent localities (Fig 3). No 4 orebody is a small lens of chalcocite-bornite mineralisation, developed along strike from B orebody, against the MEF. North Portal mineralisation comprises quartz-chalcocite or bornite-hematite veins, with minor chalcopyrite-pyrite. It occurs within a dilational zone in the footwall of the PF, near its intersection with the MEF. About 500 m ENE of the mine, weak mineralisation has been intersected in the footwall of the MEF and is known as the Mammoth Extended mineralisation. Weak mineralisation is also reported from the hanging wall of the MF at South Mammoth I and South Mammoth II.
Primary mineralisation Early workers considered bornite and chalcocite mineralisation, comprising the bulk of the orebodies, to be primary and part of an overall primary zoning. Currently mine geologists regard the chalcopyrite-pyrite zone, in the lower part of B and D orebodies, as the only primary hydrothermal mineralisation.
Syngenetic or diagenetic pyrite The earliest recognised sulphide at Mammoth is fine grained, disseminated, framboidal or euhedral pyrite, which is best developed within the more silty units and often associated with soft sediment deformation (Moore, 1974).
Hydrothermal mineralisation Two stages of hydrothermal mineralisation are recognised at Mammoth. The first is evidenced by pyrite-chlorite-quartz veins, often containing abundant host rock fragments. Diffuse zones of pyrite, totally or partially replacing the host rock, sometimes occur as vein selvages. van Dijk (1991) considered disseminated pyrite throughout the host rocks to be the result of replacement, as he could make no microstructural distinction between disseminated and vein pyrite. Copper mineralisation is related to the second stage. Chalcopyrite, quartz and chlorite were introduced into preexisting veins and replaced pyrite. Disseminated chalcopyrite also replaced disseminated syngenetic or early hydrothermal pyrite.
Cobalt mineralisation In the past Mammoth has not been considered a cobalt-rich system as is the case at Esperanza. Until recently cobalt has not been routinely determined but recent data suggest cobalt values greater than 0.2% are common. The mineralogy and distribution of cobalt are not known.
Geology of Australian and Papua New Guinean Mineral Deposits
GUNPOWDER COPPER DEPOSITS
Supergene mineralisation As stated above, workers at Mammoth have been divided over the extent of supergene mineralisation. Early workers eg Mitchell and Moore (1975) regarded chalcocite-bornite mineralisation as primary and only sooty chalcocite, extending to 70 m below surface in No 1 orebody, was considered supergene. Currently all chalcocite-bornite mineralisation is regarded as supergene. This is based largely on the observation that the bornite and chalcocite zones extend downward, between the major faults and the chalcopyrite zone. This morphology is taken as evidence of very deep circulation of groundwater, down the MF, MEF and PF. However, other evidence, such as petrography, is not presently available and primary bornite and chalcocite mineralisation is not ruled out. The character of mineralisation within the supergene zone is similar to the primary zone, with chalcocite group minerals and bornite, rather than chalcopyrite, present within breccia veins or disseminated within the host rock.
ALTERATION Alteration associated with the Mammoth orebodies has been documented by Scott and Taylor (1977) and Scott (1985). In general the stratigraphic hanging wall to mineralisation is effectively unaltered, containing primary potassium feldspar and magnesian chlorite. An exception is C orebody, which occurs in the footwall to B orebody, resulting in its hanging wall displaying some features of typical footwall alteration. Host rocks to ore are strongly altered, and depending on the host rock type, contain muscovite, hematite, pyrite and iron chlorite. B and C orebodies contain kaolinite±dickite in the top 100 m, where the host rocks are least siliceous. Footwall rocks and their lateral equivalents display similar assemblages to the ore zones, except that sulphides and clays are absent and hematite is more abundant, especially close to the MF and PF.
SUNDRY MINERALISATION Minor copper deposits in the Gunpowder area are shown on Fig 3. Esperanza South mineralisation occurs along the Esperanza fault, 800 m SW of the Esperanza deposit. Supergene copper mineralisation is hosted by shale and chert of the Esperanza Formation. Two mineralised zones are present, parallel to and footwall to the EF. The Pluto prospect is on the MEF, 500 m NE of Esperanza. The geology is very poorly understood here with several narrow supergene copper intersections displaying poor continuity.
ORE GENESIS Similar geological and geochemical features suggest a common origin for the Gunpowder deposits. Several phases of mineralisation are inferred. Correlation of individual phases between deposits is not possible at present. The first phase is represented by syngenetic or diagenetic pyrite disseminations and bedded pyrite within and adjacent to the orebodies. At Esperanza the restricted distribution of semimassive to massive bedded pyrite indicates that later mineralisation may overprint the structurally preserved remnants of an exhalative pyrite accumulation.
Geology of Australian and Papua New Guinean Mineral Deposits
Later mineralising events are interpreted to relate to multiphase faulting during D3 of the Isan Orogeny. Compression at this time is interpreted to be ENE–WSW, allowing dilation and focussing of hydrothermal solutions into structurally favourable sites. At Esperanza this was the area near the intersection of the MEF and MF, with fluids primarily channelled along the MEF. At Mammoth sinistral reverse movement on the MF and dextral reverse movement on the PF caused a dilational zone to develop in the footwall of the MF and hanging wall of the PF. Fluids were focussed into dilational breccias developed around this intersection. Elsewhere fluids were focussed into minor zones of dilation resulting in small subeconomic bodies. Initially these solutions appear to have deposited pyrite only. This is evidenced by the large body of massive, locally colloform pyrite (now brecciated) in the supergene zone at Esperanza. At Mammoth the first stage of hydrothermal mineralisation is pyrite veining. Copper mineralisation at Esperanza is related to a later event in which continued faulting has extensively shattered preexisting massive pyrite and allowed influx of copper-rich solutions. Precipitation occurred where oxidised fluids encountered reducing conditions in the carbonaceous and pyrite-rich Esperanza Formation. Copper mineralisation at Mammoth is also related to a second phase of the hydrothermal event. Precipitation may have been caused by a combination of rapid pressure reduction within dilational breccias and/or interaction with reduced fluids derived from pyritic Myally Subgroup rocks or overlying graphitic and pyritic rocks of the Surprise Creek Formation. Copper is interpreted to originate from nearby Eastern Creek Volcanics (Scott and Taylor, 1982). At the time of deposition these units were brought stratigraphically closer to the host rocks by development of the Mount Gordon Arch. The MF and perhaps the MEF may result from D3 reactivation of syndepositional faults formed during development of the Mount Gordon Arch. Secondary enrichment processes during the Tertiary(?) have considerably modified primary mineralisation and increased copper grade in the upper parts of both orebodies.
ACKNOWLEDGEMENTS Aberfoyle Limited is thanked for permission to publish this paper. The authors would also like to thank G J McArthur, Chief Mine Geologist and A Miller, Mine Geologist for their contributions and review of this paper.
REFERENCES Askew, R L, 1992. Structural setting of E P 8297 M, Gunpowder, Qld, and implications for further work, Victorian Institute of Earth and Planetary Sciences, Australian Crustal Research Centre, Technical Publication No 4 (unpublished). Bell, T H, 1983. Thrusting and duplex formation at Mt Isa, Queensland, Australia, Nature, 304:493–497. Blake, D H, Etheridge, M A, Page, R W, Stewart, A J, Williams, P R and Wyborn, L A,1990. Mount Isa Inlier - regional geology and mineralisation, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 915–925 (The Australasian Institute of Mining and Metallurgy: Melbourne). Derrick, G, 1982. A Proterozoic rift zone at Mt Isa, Qld and implications for mineralisation. BMR Journal of Australian Geology and Geophysics, 7:81–92.
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Mitchell, J W and Moore, G P, 1975. Mammoth copper deposit, in Economic Geology of Australia and Papua New Guinea, Vol 1 Metals (Ed: C L Knight), pp 383–389 (The Australasian Institute of Mining and Metallurgy: Melbourne). Moore, G P, 1974. Geology and mineralisation of the Mammoth copper deposit, in Recent Technical and Social Advances in the North Australian Minerals Industry, North Queensland Regional Meeting, August 1974, pp 17–25 (The Australasian Institute of Mining and Metallurgy: Melbourne). Moore, G P and Stoakes, C A, 1975. The geology and mineralisation of the Mammoth copper deposit, in Queensland Division Field Conference, Mt. Isa Region, 1975, pp 34–40 (Geological Society of Australia, Queensland Division: Brisbane).
Scott, K M, 1985. Sulphide geochemistry and wall rock alteration as a guide to mineralisation, Mammoth area, NW Queensland, Australia, Journal of Geochemical Exploration, 25:283–308. Scott, K M and Taylor, G F, 1977. Geochemistry of the Mammoth copper deposit, northwest Queensland, Australia, Journal of Geochemical Exploration, 8:153–168. Scott, K M and Taylor, G F, 1982. Eastern Creek Volcanics as the source of copper at the Mammoth mine, Northwest Queensland, BMR Journal of Australian Geology and Geophysics, 7:93–98. van Dijk, P M, 1991. Regional syndeformational copper mineralisation in the western Mount Isa Block, Australia, Economic Geology, 86:278–301.
Richardson, S M and Moy, A D, 1997. The geology of the Esperanza copper deposit, in Proceedings Third International Mining Geology Conference, pp 51–57 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Geology of Australian and Papua New Guinean Mineral Deposits
Jenkins, D R, Laurie, J P and Beams, S D, 1998. Grevillea zinc-lead-silver deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 753–758 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Grevillea zinc-lead-silver deposit 1
2
by D R Jenkins , J P Laurie and S D Beams INTRODUCTION The deposit is approximately 200 km NNW of Mount Isa, Qld, and 10 km SE of Riversleigh Station, at lat 19o34′S, long 138o18′ on the Camooweal (SE 54–13) 1:250 000 scale and the Riversleigh (6659) 1:100 000 scale map sheets (Fig 1). Title is held by Coolgardie Gold NL. No resource estimates for the deposit have been made at this time.
3
edges of the current tenement. The only prior work within the tenement involved regional soil and rock chip sampling traverses in 1991 which identified the Lawn Hill Formation, the stratigraphic equivalent of the Century zinc deposit host (Thomas, Stolz and Mutton, 1992) as the most prospective geological unit. Exploration at the Riversleigh project commenced in March 1993 when Coolgardie Gold NL entered into a joint venture with Diversified Mineral Resources NL over Exploration Permit Minerals (EPM) 7797. Coolgardie Gold later became outright owners of the project. In April–May 1993 Coolgardie Gold engaged Terra Search to conduct a regional drainage geochemical sampling program in conjunction with regional mapping. Several anomalous areas were identified, including a catchment in the SW corner of the tenement with 100 ppm zinc. This area was also targeted for its structural complexity, comprising a prominent domal feature combined with strong faulting. Follow up of the stream sediment anomaly in September 1993 resulted in discovery of the Grevillea gossan, a zone of strongly ferruginous, jarositic and baritic outcrop 80 m wide and 200 m long, within a siltstone sequence. A vegetation anomaly characterised by a lack of spinifex occurs over the northern part of the gossan. Initial rock chip results were anomalous in lead, arsenic and silver (Table 1), but zinc values were notably low. Soil sampling in the gossan area gave a strong response with anomalous lead, arsenic, silver and cadmium, but zinc was once again low (Table 1). TABLE 1 Summary of maximum surface sampling results, Grevillea gossan area.
FIG 1 - Location map of Riversleigh project area and Grevillea deposit.
The pyritic accumulations intersected to-date are between 80 and 140 m thick with zones of zinc-lead-silver mineralisation to 25 m thick. The accumulations are stratabound sedimenthosted massive sulphides with many similarities to the large sediment-hosted deposits of the Mount Isa Inlier.
EXPLORATION HISTORY Exploration by previous companies in the area was limited to minor surface geochemical sampling programs around the 1.
Director and Senior Geologist, Terra Search Pty Ltd, PO Box 981, Castletown Hyde Park Qld 4812.
2.
Exploration Manager and Director, Coolgardie Gold NL, PO Box 1026, West Perth WA 6005.
3.
Director and Principal Geologist, Terra Search Pty Ltd, PO Box 981, Castletown Hyde Park Qld 4812.
Geology of Australian and Papua New Guinean Mineral Deposits
Sample type
Cu (ppm)
Pb (ppm)
22
85
28
Rock chip
123
3560
101
Soil (-80#)
119
973
99
Stream sediment
Zn Ag As Cd (ppm) (ppm) (ppm) (ppm) 55
<0.1
16
650
2.0
5.9
487
1.7
0.6
In September 1994 reverse circulation drill hole RVC001, the first hole of the drilling program, intersected 25 m at 5.2% zinc, 1.1% lead and 29 g/t silver from 88 m depth. Subsequent drilling programs have confirmed the potential large size of the system with pyrite-dominant lenses greater than 120 m true thickness intersected in the most northerly holes drilled to date. The deposit is open down dip and to the north. A recent airborne electromagnetic (EM) survey delineated a trend of several kilometres strike length to the north, interpreted to be the host horizon under Cainozoic alluvium.
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Drilling into the southern fault zone which truncates the mineralisation has shown values to 16 m at 2% copper. This zone is yet to be tested.
REGIONAL GEOLOGY The deposit is within the Middle Proterozoic Lawn Hill Platform sequence in the NW part of the Mount Isa Inlier. It is hosted by the lower units of the Riversleigh Siltstone, part of the McNamara Group. Detailed descriptions of the Lawn Hill Platform and the McNamara Group are given by Sweet and Hutton (1982) and Blake et al (1990). The McNamara Group contains the sediment-hosted sulphide deposits at Lady Loretta and Century, and the Lady Annie and Mammoth copper deposits. Table 2 summarises the main stratigraphic units for the region. The McNamara Group is an equivalent of the Mount Isa Group which hosts the Mount Isa and Hilton zinclead bodies and the Mount Isa copper deposit. TABLE 2 Regional stratigraphic units, Lawn Hill Platform.
FIG 2 - Regional geological map, Riversleigh project area (after Sweet and Hutton, 1982).
ORE DEPOSIT FEATURES STRATIGRAPHY
The oldest rocks in the area form the carbonate-rich Lady Loretta Formation. West of Grevillea lower units of the Lawn Hill Formation are the youngest Proterozoic rocks in the area, and are overlain unconformably by Cambrian limestone of the Georgina Basin (Fig 2). Regional deformation formed a series of broad basin and dome structures and open folds within the Proterozoic units. There is strong faulting in some areas, with the most prominent structure the NW-trending Termite Range Fault 9 km to the NE of Grevillea. It has been inferred to have been subject to several periods of movement, the earlier of which were critical to the formation of the Proterozoic sedimentary basins in the area (Andrews, 1996). Sets of NE-trending faults, possibly conjugate to the Termite Range Fault, are prominent within several corridors on either side of the main structure. These faults commonly delineate changes in facies and thickness of stratigraphic units. The edge of the Cambrian limestone in the area follows a NW trend parallel to the Termite Range Fault and may be attributable to a similar structure. Southwest of the Termite Range Fault the Proterozoic sequence is less deformed.
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The Riversleigh Siltstone is a sequence of shale, siltstone and sandstone dominated by fine to medium grained sandstone in the area surrounding the Grevillea deposit. It is underlain by the Shady Bore Quartzite which forms a prominent ridge to the NE (Fig 2) and also cores the domal structure to the east of the deposit (Fig 3). A stratigraphic column showing the units determined in mapping of the Riversleigh Siltstone in the Grevillea area is given in Fig 4, with a detailed subdivision of the host horizon. The sediments are only moderately folded and are basically unmetamorphosed. Close to the Shady Bore Quartzite bedding dips steeply west becoming less steep towards the west and south. The basal unit 1 of the Riversleigh Siltstone is an interbedded siltstone and fine grained sandstone with local dolomitic zones which are laterally discontinuous. The unit is siltstone dominant with a thickness of at least 80 m. Overlying this unit is a sandstone-dominant sequence with minor siltstone interbeds (unit 2). The sequence above the second unit differs on either side of the large NE-trending Southern Bounding fault (Fig 3). To the south of this fault there is a thick sandstone-dominated unit with interbedded siltstone, and north of the fault is a thick siltstone or shale unit of at least 250 m true thickness. This dolomitic and partly carbonaceous unit is the host unit for the mineralisation. South of the fault the only possible equivalent is a jarositic siltstone bed less than 30 m thick which lenses out to the SE. Overlying the host unit is another sequence of interbedded siltstone and fine grained sandstone which comprises units 4
Geology of Australian and Papua New Guinean Mineral Deposits
GREVILLEA ZINC-LEAD-SILVER DEPOSIT
and 5. These units have few diagnostic features and it is difficult to distinguish one from another. The host horizon is the only consistently recognisable bed both geologically and geophysically. The host horizon has a gradational contact with unit 4 of the hanging wall sequence by an increase in sandstone up sequence. Unit 4 is a fine grained, mostly massive sandstone unit more than 300 m thick. Its upper contact relations are unknown but it may be unconformably overlain by younger Proterozoic black shale of the Upper Riversleigh Siltstone or Lower Lawn Hill Formation. The change in rock type across the Southern Bounding fault is interpreted to indicate that the fault was important in the formation of a local depositional basin. The host horizon is interpreted to have been deposited within the sub-basin bounded by this fault. Interpretation of airborne EM data indicates that this unit extends for at least 3 km along strike to the north and 1 km down dip (B Craven, unpublished data, 1996).
STRUCTURE The NE-trending Southern Bounding fault is one of a series of faults with a northerly dip of between 60 and 80o. The faults
have an apparent dextral and normal (north block down) movement. The proportion of dip-slip to strike-slip movement is unknown. Two other faults, the Mid fault and the North Gossan fault, displace and truncate the gossan north of the Southern Bounding fault. The Mid fault is steep and displaces the gossan approximately 20 m to the NE and may be a splay off the Southern Bounding fault. The effect of the North Gossan fault is more dramatic, with a NE horizontal displacement of approximately 100 m (Fig 3). This places a sandstone unit from the hanging wall sequence directly north of the gossan. The combination of the sandstone hills to the north and south of the gossan, juxtaposed by the North Gossan fault and the Southern Bounding fault respectively, has ensured its preservation. North of the North Gossan fault the mineralisation does not outcrop.
HOST LITHOLOGY The host pyritic shale sequence is at least 300 m thick and contains massive to semi-massive sulphide lenses to 140 m thick. The sulphide accumulation is within the upper half of the sequence whereas the lower units consist of dolomitic and calcareous siltstones with minor disseminated pyrite (Fig 4). The siltstones are grey to black in colour and massive to finely laminated. The paler beds are more dolomitic and stain brown
FIG 3 - Geological plan of Grevillea deposit area. Note that gossan outcrop lies between the Southern Bounding and North Gossan faults.
Geology of Australian and Papua New Guinean Mineral Deposits
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FIG 4 - Units of the Lower Riversleigh Siltstone in the Grevillea area in relation to the stratigraphy described by Andrews (1996). Detailed stratigraphy of the host horizon (unit 3) and the sulphide body on the right.
where exposed at the surface indicating a ferroan dolomite composition. Some brecciated zones are present, commonly with an increased pyrite content. Minor coarser grained beds are present in the sequence with an increased frequency down sequence. Petrological work revealed that the siltstone consists mainly of dolomite and subordinate flaky white mica (R N England, unpublished data, 1994). Kerogen is present in most samples as fine interstitial grains and microbial mats along with disseminated octahedral pyrite. The siltstone beds may also contain potassium feldspar particularly in the more pyritic zones. Thin very fine grained red or pink beds within the sulphide lens are potassium feldspar–rich volcaniclastic rocks. There are no glass shards typical of the tuff marker beds in the similar Mount Isa and Century deposits, and correlation of these beds between holes has been unsuccessful.
MINERALISATION Within the sulphide-rich zone the siltstone is well laminated with interbeds 20 µm to 2 mm thick. Where these beds are thicker and more massive the pyrite content decreases. The overall pyrite content of the high sulphide lens is estimated to be 40 to 50 %. There are two different types of pyrite present; a fine grained octahedral form which occurs as massive conformable pyrite laminations within the sequence, and coarse grained, cubic pyrite associated with barite, carbonate, galena and sphalerite. The finer pyrite forms thin laminae which are planar or irregular. Microscopically the fine pyrite may be filamentous with fine dendritic networks suggesting algal replacement. The coarser pyrite forms bands subparallel to bedding with some associated breccia and replacive textures associated. Barite and dolomite are consistently associated with the coarse grained pyrite. These coarse grained pyrite bands are present throughout the sulphidic zone but form higher concentrations at particular stratigraphic levels. Strong barite and coarse pyrite breccias and bands mark the top of the sulphide zone. Lead and zinc minerals are closely associated with the coarse grained pyrite and barite in the lower parts of the sulphide lens (Fig 4). Sphalerite is generally pale honey coloured, perhaps indicating a low iron content. It is usually coarse grained and massive and associated with coarse grained barite and pyrite. Galena occurs in wispy masses and stringers with a very close association with barite. The coarse grained zones contain all
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the significant zinc-lead mineralisation and the interbedded fine grained massive pyrite and dolomitic siltstones contain a few hundred ppm of zinc and lead. Silver values, although showing some correlation with lead and zinc, remain highly anomalous in all zones including the low lead-zinc fine grained pyrite zone. Coarse grained bands often replace and broadly crosscut the fine grained beds. There are abundant later barite and/or dolomite veins in a multitude of orientations. The dolomite veins are generally planar, and the barite veins are irregular to planar. There are many crosscutting fractures and shears. Stylolitisation of early faults during compaction has been observed in several areas. The major faults identified have a variable nature depending on their position. The Southern Bounding fault is a 50 m wide zone of several faults with sandstone and siltstone blocks or beds throughout the zone. Oxidation to at least 200 m below surface has been observed and the fluids associated with this may have leached lead and zinc from these areas. Silver appears to be less vulnerable to this leaching. Later crosscutting sphalerite and galena have been observed in both the Southern Bounding and North Gossan faults. The faulthosted sphalerite is much darker and more iron-rich and often rims the remobilised galena where both are present. Primary mineralisation tends to have much higher zinc values than lead but the fault-hosted mineralisation shows the opposite trend. Faults in the area generally contain elevated copper values, of the order of 100 to 1000 ppm. The Southern Bounding fault contains copper as chalcocite and minor chalcopyrite. Grades to 1% over 2 m have been recorded from most of the holes that intersect this fault zone. Hole RVC036 was drilled into the fault zone adjacent to the sulphide accumulation and intersected 16 m at 2.05% copper. This intersected partially oxidised brecciated sandstone with pyrite and chalcocite (after chalcopyrite?) veins throughout. The controls on copper deposition are not yet understood, and further diamond drilling is required to characterise the mineralisation. The NE-trending faults are the main control of the geometry of the sulphide lens and in turn the zinc-lead zone. The north dipping, normal nature of the faults controls the position of the mineralised horizons (Fig 5). The Southern Bounding fault appears to form one edge of the sulphide body with the slightly steeper North Gossan fault separating the outcropping Gossan block from the Northern block. Other faults further complicate the geometry. The throw of the North Gossan fault has formed a blank zone in plan view between the two blocks (Fig 5). None
Geology of Australian and Papua New Guinean Mineral Deposits
GREVILLEA ZINC-LEAD-SILVER DEPOSIT
FIG 5 - Plan projection of sulphidic zones and drill holes, with surface trace of faults, Grevillea deposit.
of the most northerly holes have tested the full thickness of the northern block (Fig 7). The Gossan block sulphide mineralisation has a true thickness of over 80 m where a full sequence is preserved above the Southern Bounding fault (Fig 6). This block contains a zinc-lead-silver rich subzone of 20 to 25 m true thickness which is pinched out at depth between the Southern Bounding and North Gossan faults (Fig 5).
FIG 7 - Cross section on grid line 15 400 m N, Northern block, looking NW.
The Northern block contains a much thicker sulphide accumulation, more than 140 m thick, although there are zones of sulphide poor (10–30% pyrite) material within the body which are absent from the Gossan block. These appear to be areas with greater shale content. All of the drill holes in the Northern block have either finished in the sulphide body or hit the North Gossan fault (Fig 7). Hole RVD047 intersected 6 m of primary mineralisation before it was truncated by the fault. The fault itself contains sphalerite and galena which may be remobilised primary mineralisation. Further drilling is required to unravel the structure and stratigraphic complexities and to test continuity and grade of the zinc-lead-silver and copper mineralisation.
ORE GENESIS Work on the Grevillea deposit is still at an early stage, however some indications of its genesis are apparent. The body is a stratabound and possibly stratiform body, hosted by dolomitic siltstone. The Southern Bounding fault and the other faults of similar orientation were active during sedimentation and played a key role, first in the formation of the basin in which the host horizon was deposited, and secondly as a conduit for the mineralising fluid, which was possibly basinal brine. The fine grained pyrite and associated silver bearing minerals are interpreted as syngenetic with deposition at the sediment–water interface.
FIG 6 - Cross section on grid line 15 200 m N, Gossan block, looking NW.
Geology of Australian and Papua New Guinean Mineral Deposits
Barite, coarse grained pyrite, sphalerite and galena were deposited as an early diagenetic phase, replacing the more dolomitic siltstones interbedded in the pyritic sequence. Stylolitisation of fractures that have displaced this baritic phase of mineralisation places the mineralisation event prior to final compaction. The variation in thickness of units observed between the Gossan and Northern blocks may be an indication that deposition within the basin was at times highly localised with complex faulting causing sub-basin formation and rapid facies variation.
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Copper found in the Southern Bounding fault may be related to the deeper and hotter parts of the mineralising system.
ACKNOWLEDGEMENTS The authors would like to thank Coolgardie Gold NL for permission to publish this paper. R N England is also thanked for his work on petrography, B Craven for his interpretation of geophysics and W P Laing for work on structural interpretation.
REFERENCES
Blake, D H, Etheridge, M A, Page, R W, Stewart, A J, Williams, P R and Wyborn, L A I, 1990. Mount Isa Inlier - regional geology and mineralisation, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 915–925 (The Australasian Institute of Mining and Metallurgy: Melbourne). Sweet, I P and Hutton, L J, 1982. Lawn Hill region, Queensland, Bureau of Mineral Resources Geology and Geophysics, 1:100 000 Geological Map Commentary. Thomas, G, Stolz, E M and Mutton, A J, 1992. Geophysics of the Century zinc-lead-silver deposit, northwest Queensland, Exploration Geophysics, 23:361–366.
Andrews, S, 1996. Stratigraphy and depositional setting of the Upper McNamara Group, Lawn Hill Region, in MIC ’96 Extended Conference Abstracts (Eds: T Baker, J Rotherham, J Richmond, G Mark and P Williams), pp 5–9 (James Cook University of North Queensland: Townsville).
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Geology of Australian and Papua New Guinean Mineral Deposits
Ryan, A J, 1998. Ernest Henry copper-gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 759–768 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Ernest Henry copper-gold deposit by A J Ryan1 INTRODUCTION The deposit is approximately 35 km NE of Cloncurry in the Mount Isa–Cloncurry mineral district of NW Queensland at AMG coordinates 469 200 m E, 7 739 100 m N or lat 20o27′S, long 140o42′E on the Cloncurry (SF 54–2) 1:250 000 and the Clonagh (7057) 1:100 000 scale map sheets (Fig 1). The property is owned by Ernest Henry Mining Pty Ltd, an incorporated joint venture between MIM Holdings Limited (51%) and Savage Resources Limited (49%). The deposit is named after Mr Ernest Henry (1837–1919), a pioneer, pastoralist, early prospector and subsequent mining entrepreneur who first discovered copper in the Cloncurry region (Great Australia mine) and established the town of Cloncurry in 1867. The discovery of the deposit was the result of a geophysically and geologically driven exploration program guided by a simple empirical model for Selwyn and Osborne style coppergold deposits (Webb and Rowston, 1995). The Identified Mineral Resource at July 1997 is 166 Mt at 1.1% copper and 0.54 g/t gold. This includes a combined Proved and Probable Ore Reserve of 127 Mt at 1.1% copper and 0.55 g/t gold that will support an anticipated mine life of 15 years. Mining will be by open cut methods, though there is potential to mine extensions to the orebody by underground methods further extending the mine life. Mineralisation is open at depth. Pre-strip mining commenced in December 1995 with concentrate production scheduled for the second half of 1997. The nominal ore production rate of 9 Mtpa will give an annual metal production in concentrate of about 95 000 t of copper and 120 000 oz of gold from a conventional, large scale mill and flotation circuit. The final pit has a designed surface diameter of about 1300 m and a depth of 570 m.
EXPLORATION HISTORY During 1974 Savage Exploration Pty Ltd (SEL), while exploring for magnetite, secured mining leases over several aeromagnetic anomalies to the north and NE of Cloncurry. Previous exploration had been limited by a combination of depth of cover and lack of exposure. The anomalies were identified using existing regional airborne magnetic data from a survey by the Bureau of Mineral Resources (BMR), currently the Australian Geological Survey Organisation (AGSO). Exploration activity was limited until the area was targeted by Western Mining Corporation Limited (WMC) in the late 1980s. Initially searching for stratabound iron formation–hosted gold deposits and copper-gold skarns 1.
Formerly Chief Geologist, Ernest Henry Mining Pty Ltd, now Principal Adviser Investor Relations, MIM Holdings Limited, GPO Box 1433, Brisbane Qld 4001.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Location and regional geological map, modified from Blake and Stewart (1992).
(Craske, 1995), the exploration strategy was reappraised in 1989 to focus on potential Selwyn and Osborne style coppergold deposits, both associated with strong magnetic anomalies (Collins, 1987; Gidley, 1988; Anderson and Logan, 1992). Prior application for much of the target area by Hunter Resources Ltd (HRL) led to WMC forming a joint venture with HRL in 1990. Transient electromagnetic (TEM) techniques were used in 1990 and 1991 to filter the magnetic target areas, based on the theory that any economic mineralisation should have enough associated sulphides for it to be a conductor and hence responsive to TEM methods (Webb and Rowston, 1995).
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In October 1991 drilling of a very discrete and moderate amplitude anomaly resulted in the discovery of copper and gold mineralisation (Webb and Rowston, 1995). Subsequent down hole TEM surveys demonstrated that, at least in part, the initial surface TEM target was due to a native copper–rich zone within the supergene mineralisation and also demonstrated that the primary ore did not produce a TEM anomaly (Webb and Rowston, 1995). Ground magnetic, gravity and induced polarisation methods were subsequently used to help guide the early delineation drilling phase and assist in reducing the amount of sterilisation drilling required prior to the mine site layout being finalised. In 1993 the deposit was determined to lie within Mining Lease 2671, owned by SEL. Under an agreement with Savage Resources Limited (SRL), MIM Holdings Limited purchased 51% of SEL, which thereby became a joint venture company and was renamed Ernest Henry Mining Pty Ltd. Evaluation of the deposit was completed during several phases of diamond drilling. The July 1997 Identified Mineral Resource is based on 173 diamond drill holes totalling 53 600 m. Drilling in the shallower supergene ore zone is on 40 m centres, sufficient to allow estimation of a Measured Resource. The deeper, primary zone has generally been drilled on 80 m centres to Indicated Resource status with some infill drilling on 40 m centres on 80 m east-west sections to Measured Resource status.
PREVIOUS DESCRIPTIONS No geological description has previously been published. Several unpublished company reports give geological information, the most significant of which is a detailed description by J M A Hronsky for WMC in 1993.
REGIONAL GEOLOGY The following is largely from Blake and Stewart (1992) and references therein. It concentrates on rocks generally younger than cover sequence 1 within the Eastern Succession.
about 1760 to 1720 Myr. During this time the Wonga Granite, Burstall Granite, Lunch Creek Gabbro, further volcanic rocks of the Argylla Formation, Marraba Volcanics, Toole Creek Volcanics and numerous small felsic and mafic sills and dykes were intruded or extruded. The Mount Fort Constantine metavolcanics were extruded during this time. The period from 1540 to 1450 Myr is also interpreted as a time of significant granitoid intrusion in the Eastern Fold Belt and includes emplacement of the Williams and Naraku batholiths at around 1500 Myr (Blake et al, 1990). It has been suggested that the deposit is in the roof zone of the younger Naraku Granite, and that the mineralisation is related to the emplacement of the Williams and Naraku batholiths and associated plutons at around 1500 Myr. C Perkins (personal communication, 1994) calculated an age of approximately 1480 Myr for the deposit from alteration of biotite, suggesting a broad association with the late phases of these batholiths. The possibility of a Williams Batholith–related alteration event at about 1530 Myr was noted from hornblende but Perkins concluded that much more work needed to be completed on the paragenesis of the deposit in order to better interpret all the dates.
LOCAL GEOLOGY PHANEROZOIC SEQUENCE The Proterozoic rocks hosting the deposit are concealed by a flat lying sequence of Phanerozoic sediment of the Carpentaria Basin which overlies rocks of the Mount Isa Inlier to the NE of Cloncurry. In the immediate deposit area the sequence thickness is controlled by basement topography and is generally between 35 and 60 m thick, increasing towards the south. The top of the orebody forms a basement topographic high and a localised thinning of the cover sequence to 25 m (Fig 2).
The Ernest Henry deposit is east of the Cloncurry Overthrust, in the Cloncurry–Selwyn zone within the Eastern Fold Belt of the Mount Isa Inlier (Fig 1). Situated NE of the outcropping Proterozoic sequence, the deposit is in an extensive area of Phanerozoic cover and has no outcrop. The stratigraphic position of the volcanic rocks which host the Ernest Henry deposit is unknown, though several authors have speculated that they are equivalent to the felsic metavolcanic rocks which outcrop at Mount Fort Constantine, 11 km SW of Ernest Henry (J M A Hronsky, unpublished data, 1993). The Mount Fort Constantine metavolcanics have been dated at about 1730±10 Myr (Page, 1993). This would give a position towards the top of cover sequence 2 for the Ernest Henry host rocks. The only other outcrop in the area is the Mount Margaret granite, dated at 1480 Myr, which is about 12 km east of Ernest Henry. The Mount Isa Inlier in general and the Eastern Fold Belt in particular have a long history of igneous intrusion and extrusion. Widespread igneous activity took place during the deposition of cover sequence 2, with bimodal vulcanism occurring from about 1790 to 1780 Myr (Magna Lynn Metabasalt and the felsic volcanic rocks of the Argylla Formation). Significant felsic plutonism, minor mafic plutonism and some felsic and mafic extrusion occurred from
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FIG 2 - Schematic north–south section looking west, showing geology and distribution of mineralisation. Some faults omitted for clarity.
Geology of Australian and Papua New Guinean Mineral Deposits
ERNEST HENRY COPPER-GOLD DEPOSIT
The following units are recognised in the cover sequence (from top to base): 1.
Black soil
2.
Orange clay-rich sediment
PROTEROZOIC SEQUENCE Understanding of the Proterozoic geology of the deposit is incomplete, hampered by the lack of local exposure. Schematic sections are presented in Figs 2 and 3.
Unconformity 3.
Quartz pebble conglomerate Tertiary–Mesozoic unconformity
4.
Mesozoic sediment Mesozoic–Proterozoic unconformity
Black soil The uppermost unit comprises grey, clay-rich soils, typically 2 to 3 m thick (average 2.3 m) with abundant platy gypsum crystals of <5 mm diameter below a depth of 0.5 m and infrequent carbonate nodules of <3 mm diameter. This unit grades downwards to:
Orange clay-rich sediment The clay-rich sediment, 1 to 27 m thick (average 10 m), has a higher iron content than the black soil and a low level of organic matter. It commonly contains areas of manganese oxide–coated cracks within the clays giving the appearance of coarse angular gravels.
FIG 3 - Schematic east–west section looking north, showing geology and distribution of mineralisation.
Quartz pebble conglomerate This unit, named from samples taken during open hole drilling, ranges from 0 to 24 m thick (average 11 m) and is now known to be a sequence of cross-bedded river sand and gravel. The base of the unit is generally coarser, consisting of poorly sorted gravels grading upwards to include clay-rich zones. There is localised carbonate cement especially towards the lower contact.
Mesozoic sediment This unit is 0 to 40 m thick (average 16 m) and consists of dark grey, carbonaceous and pyritic clay-shale with green to creamyellow coloured sand units towards the base. Plate-like growths of gypsum are noted on some joint surfaces. The sediments pinch out against topographic highs. The upper 2 to 3 m are weathered to a pale claystone and displacements of this contact highlight a number of steeply dipping normal faults with displacements of the order of metres. Several laterallyextensive sand units are present within the shale and range from thin, centimetre-thick lenses to horizons metres thick. The sand units increase in thickness corresponding with the increasing thickness of the unit towards the south away from the basement high. Significant water flows have been encountered from the sand units towards the base of the sequence which are interpreted as belonging to the Wallumbilla Formation. Further to the south where the lower sand units lie directly on the Proterozoic contact, they may belong to the upper sediments of the Gilbert River Formation (C Hill, personal communication, 1997). Assays of resource delineation samples returned significant but isolated gold values in sediment immediately above the Proterozoic unconformity. Follow up drilling of the potential palaeo-alluvial deposit failed to identify a resource.
Geology of Australian and Papua New Guinean Mineral Deposits
The general lithological sequence from hanging wall to footwall is: 1.
diorite intrusive;
2.
felsic volcanic rocks;
3.
brecciated volcanic rocks, hosting the orebody sequence;
4.
footwall siltstone;
5.
marble matrix breccia (MMB); and
6.
felsic volcanic rocks.
Note that the main MMB intersections, to the footwall of the orebody, and the footwall siltstone occupy similar positions in the sequence and to a limited degree may be laterally transgressive.
Diorite A diorite intrusive was intersected immediately to the south of the deposit. The contact, which strikes approximately ENE, can be identified from high resolution aeromagnetic data which show a sharp decrease in magnetic response to the south. This line of magnetic contrast extends to the known position of the Cloncurry Overthrust. The contact with the felsic volcanic rocks dips towards the SE at angles as low as 30o and is often marked by a brecciated zone. A Barber (personal communication, 1997) noted extensive veining and alteration of the volcanic sequence in the contact zone intersected by drill hole EH261, implying an intrusive contact. The true nature of this contact remains inconclusive.
Felsic volcanic rocks The dominant rock type at Ernest Henry is a variably foliated, brecciated and altered felsic volcanic rock, although, within the Ernest Henry sequence, widespread feldspathic alteration has
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made identification of primary rock composition by modal analysis unreliable (J M A Hronsky, unpublished data, 1993). Available data suggest that the majority of the Proterozoic rocks within the Mount Fort Constantine area have a felsic volcanic rock protolith (J M A Hronsky, unpublished data, 1993). Mineralogically these rocks are dominated by feldspar with minor quartz and ferromagnesian minerals and are characterised by pervasive fine grained magnetite-biotite alteration. Primary features preserved include porphyritic (feldspar phenocrysts) and amygdaloidal textures and rare localised flow banding. The amygdaloidal units are largely concentrated in the hanging wall of the orebody and may reflect preservation of primary textures away from the mineralisation.
Siltstone units The Footwall siltstone is a finely laminated grey carbonaceous unit, generally in the order of 10 to 30 m thick. Some folding and soft-sediment deformation textures are noted. The unit dips towards the SE at between 40 and 50o and occurs in the footwall of the mineralised felsic sequence towards the shallower, northern parts of the deposit (Figs 2 and 4). It is progressively more altered to the south to a point where identification of the siltstone becomes difficult due to the intensity of alteration. At this stage the siltstone appears as a banded biotite-chlorite rich rock, almost indistinguishable from intensely sheared and altered volcanic rock.
Two red rock alteration events are recognised within the volcanic sequence. The earliest, a regional sodium metasomatism event related to regional metamorphism, characterised by albite with hematite dusting, can be seen most clearly away from the mineralised zone. The second, potassium feldspar with hematite dusting, is spatially associated with the mineralisation and increases in intensity towards the orebody. The hematite is fine grained and occurs in the groundmass of the rock and in the feldspar giving the rock a distinctive pink to red colour.
Brecciated volcanic rocks The host for economic mineralisation is almost exclusively brecciated felsic volcanic rock. Evidence from drill core suggests that mineralising fluids, at least initially, were focussed by pre-existing fracture structures, and that the current textures are the result of a combination of fracture vein filling followed by successive replacement and alteration of the host. A complete gradation exists from unbrecciated volcanic rocks through crackle fracture veining to clast-supported and matrixsupported breccias to zones of total clast digestion (massive matrix). The matrix is dominantly a magnetite-carbonate-sulphide assemblage. The copper and magnetite-carbonate minerals are considered to be coeval. Two distinctive, end member, matrix assemblages are noted; a magnetite-biotite-chlorite assemblage, and a more carbonate-rich (±barite) assemblage. Both styles can be strongly mineralised, although typically the carbonate-rich breccias have a more uniform and generally higher sulphide content. The two assemblages are not mutually exclusive and a complete gradation is observed. The bulk of the economic mineralisation is restricted to breccia zones with a significant (>10%) matrix component. A gross zoning is exhibited from altered unbrecciated felsic volcanic rocks on the margins of the deposit, progressing inwards to clast and matrix supported breccias as the proportion of matrix increases. The progression between breccia types, though gradational, tends to be rapid. The hanging wall and footwall breccia contacts are commonly planar whereas the eastern and western limits of brecciation are characteristically irregular. Gradation from matrix supported breccia to crackle breccia is rapid at both contacts and allows the ore–waste limit to be readily defined. S Coates (personal communication, 1997) noted that breccias similar to those within the ore zone, but some distance from the deposit, were intersected during sterilisation drilling. These have a dominantly carbonate-rich matrix (actinolite was also noted) but do not have the characteristic magnetite matrix component and do not carry copper mineralisation.
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FIG 4 - Simplified plan of geology and mineralisation at 100 m below surface.
Other siltstone units of lesser importance have been noted within the main breccia body and, where mineralised, are typically zones of lower grade.
Marble matrix breccia The marble matrix breccia (MMB) is a distinctive rock type. It occurs dominantly in the footwall of the orebody (Fig 2) where it is linked to a relatively persistent footwall zone of strong foliation, though its location is often variable and in places it is absent. The MMB is a granular carbonate-rich rock containing volcanic and rare sedimentary clasts. The volcanic clasts are commonly rimmed by distinctive dark reaction haloes of a biotite±magnetite±chlorite assemblage. MMB is characterised by banded swirling matrix textures and local folding indicating that ductile deformation continued during its formation. In shallower footwall intersections, this unit is frequently closely associated with the Footwall siltstone (Fig 2). Limited evidence from drill hole intersections suggests that the MMB can locally grade into a carbonate-rich sediment.
Geology of Australian and Papua New Guinean Mineral Deposits
ERNEST HENRY COPPER-GOLD DEPOSIT
between the magnetite-rich mineralisation and the unmineralised sequence during a deformation event. Neither case fully explains the distribution of foliation and the answer may only become clear as mining progresses. In many places there is also a weak to moderately developed extension lineation on the main foliation (C Mawer, unpublished data, 1994). The foliation dips moderately to the SE, and the extension lineation plunges moderately to the SE within this foliation. The orebody is roughly parallel to the extension lineation. Mawer concluded that this strongly suggests a causal link between the strong foliation and lineation, and formation of the deposit.
Faulting A parallel series of post-mineralisation brittle zones which influence the geometry of the orebody have been identified (Figs 3 and 6). Dipping at 50o towards 145o, they have an apparent dip subparallel to the orebody in north-south sections. They have a true spacing of about 100 m and estimates of displacements of up to 80 m are noted, with a reverse movement. A single, later fault dipping at 45o towards 065o has been identified and offsets the earlier set at depth (Fig 2).
FIG 5 - Typical orebody breccia texture. Note the digestion of relict clasts. The white mineral is calcite, grey is sulphide.
Carbonate alteration in the footwall sequence is extensive and not restricted to the MMB unit, which may only represent a zone where structural weaknesses have focussed and increased the intensity of alteration. Carbonate alteration is noted throughout drill hole EH 249 which intersected over 600 m of variably carbonate-altered volcanic rocks in the footwall sequence of the orebody. Where the adjacent felsic volcanic rocks are mineralised, the MMB locally contains significant, but low grade (0.2 to 0.8% copper) mineralisation.
STRUCTURAL FEATURES Foliation The entire felsic volcanic rock sequence is variably foliated. Although there is generally an increase in intensity towards the hanging wall of the orebody, the distribution of strong foliation is complex. Earlier suggestions of continuous hanging wall and footwall shear zones (J M A Hronsky, unpublished data, 1993) have not been supported by subsequent drill hole evidence. The foliation generally dips at approximately 50o towards 165o, similar to the regional foliation trend. Similarities with some aspects of the orebody orientation support an influential link between the foliation and orebody geometry. However, foliation is generally absent from the mineralised breccia sequence. To date, insufficient work has been completed to determine the timing of the foliation. If it is pre-mineralisation, it could represent a remnant foliation not destroyed by the mineralisation processes. If post-mineralisation, it could represent a stress focus caused by the competency contrast
Geology of Australian and Papua New Guinean Mineral Deposits
The faults are marked by 2 to 30 m zones of interlocking but loose fragments which report as rubble during drilling. The actual fault position can be identified by narrow zones of clay fault gouge within the rubble. Late carbonate cementing of the fragments has been noted in some intersections. Where these zones cut a block of mineralisation, the fault zone may be only weakly mineralised or totally unmineralised. However, only occasional and localised leaching is noted and these zones probably represent short intervals of unmineralised volcanic rocks caught up in the faulting.
Textures Although the original focus for the mineralising event was most likely the result of a brittle event, possibly related to the cooling of the Naraku Batholith, the overall progression from massive volcanic rock to altered and brecciated volcanic rock is largely a chemical, ie, metasomatic effect. Significant clast rotation is rare. Hairline fractures would have acted as conduits for mineralising fluids, which in turn altered and digested the clasts to the varying degrees seen in core. The degree of digestion is important because areas of increasing matrix to clast ratio generally correspond to higher grades of copper and gold. Rare polymict breccias are observed which contain clasts of volcanic rock and sediment or clasts of volcanic rock with very different alteration overprints. These clasts are often well rounded indicating considerable tectonic working.
MINERALISATION Mineralisation at Ernest Henry is structurally controlled and is developed in a SE dipping, south- to SSE-plunging body of altered and variably brecciated felsic volcanic rock. The ore zone is elongated in the plunge direction. The combined thickness of the mineralised sequence is approximately 250 m, the width averages 300 m and the down dip length is at least 1000 m. The orebody is open down dip and to the SW. Drilling of the deeper intersections has been limited to that needed to provide resource data for the current pit design and there is no indication of a reduction in cross sectional area or grade of mineralisation at the drilled limit (Fig 6).
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and porous rock with a frequent strong overprinting of chlorite, hematite and limonite. Less frequent occurrences of a kaolinite-rich pale grey to white clay are known. A sequence of weathering zones has been established for the supergene zone. However, complex overprinting relationships and rapid lateral changes mean that a simple 'layer cake' model is not appropriate for the supergene profile. The zoning is presented below but it should be noted that the chlorite and iron oxide zones, in particular, may be laterally as well as vertically transgressive:
FIG 6 - Schematic east–west section looking north, showing the impact of faulting on orebody geometry, as shown at the southern limit of mineralisation.
Within the mineralised zone there are upper and lower mineralised breccias which merge to the SW. The interval which separates the upper and lower lens ranges from barren unbrecciated volcanic sequence to a low grade interval of brecciated volcanics. The geometry of the two lenses is influenced by faulting and it is possible that they represent a stacked fault repeat of a single mineralised sequence. The development of a low grade footwall zone of mineralisation at depth gives an apparent steepening to the ore zone. In addition to the main ore zones, a separate ore zone has been located approximately 100 m to the east. The style of mineralisation in this eastern lens is similar to that of the main ore zone but there is a significant difference in the copper to gold ratio, with the relative proportion of gold generally higher. The extent of this zone at depth is unknown and it remains a target for underground extraction. Mineralisation can be broadly subdivided into two zones, supergene and primary. The supergene zone shows various degrees of alteration as a result of weathering which can extend to a depth of 150 m below surface (Fig 3). This zone can be further subdivided into two zones; an upper zone which lies above the base of complete oxidation (BOCO) and a lower zone which lies between the BOCO and the base of partial oxidation (BOPO). The primary zone lies below the BOPO contact and shows no weathering effects.
Supergene zone The supergene zone which constitutes approximately 12% of the Identified Mineral Resource is mineralogically complex. Although some of the highest copper and gold grades occur within this zone, indicating local enrichment, the overall resource grade is similar to that of the primary zone. Above the BOCO there is an almost complete loss of primary textural features. The zone is characterised by strongly leached
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1.
Phanerozoic sequence
2.
Proterozoic unconformity
3.
Clay zone - a pale grey to white, kaolinite-rich leached zone immediately beneath the Mesozoic unconformity; better developed towards the north. Rare relict fabric, and frequently gold enriched.
4.
Chlorite zone - with dark to medium green chlorite and septochlorites; frequently leached and vuggy with remnant felsic volcanic clasts. Commonly with lenses of hematite-altered volcanic rock.
5.
Iron oxide zone - yellow-brown to purple-red in colour, frequently leached and vuggy, with remnant felsic volcanic clasts. Grossly hematitic in composition, but with complex subzones of limonite, goethite and/or chlorite.
6.
BOCO
7.
Silica pitted zone - a grey siliceous zone at the base of the weathering profile, characterised by pitting and vuggy cavities to 5 cm in diameter.
8.
BOPO
9.
Unweathered Proterozoic sequence
Material lying between the BOCO and BOPO is characterised by leached and porous rock but in contrast to the upper zone, primary textures are generally retained and frequently drill core appears fresh in casual observation. The depth to the BOCO and BOPO is influenced by the major brittle features which intersect the Proterozoic unconformity within the mineralised zone (Fig 3). A combination of the presence of mineralisation and ready access for fluids provided a focus for weathering. Contacts can appear to be highly variable between adjacent intersections due to the presence of remnant blocks of relatively fresh rock which have survived within areas of strong weathering. Weathering propagates along fractures, joints and other discontinuities leaving fresh blocks of incomplete weathering. This has led to a difficulty in defining a consistent base of oxidation which, for orebody modelling purposes, is smoothed using indicator kriging. The influence of the major brittle fracture zones on the depth of weathering is clearly seen by contouring these smoothed BOCO and BOPO surfaces. It highlights a trough-like feature parallel to the strike of and coincident with the intersection of a brittle fault zone with the Proterozoic unconformity. Away from these structures and the mineralisation, the depth of weathering is generally limited and can be within centimetres of the unconformity. To produce the complex supergene mineralisation patterns observed, the local Eh-pH controls of the oxidation process must have fluctuated rapidly both laterally and vertically. Primary features of the host rock are variably preserved within the supergene profile, and porphyritic bands and amygdales are rarely preserved.
Geology of Australian and Papua New Guinean Mineral Deposits
ERNEST HENRY COPPER-GOLD DEPOSIT
Supergene mineralogy The dominant copper species in the supergene profile are chalcocite, bornite, secondary chalcopyrite and native copper (J Knights, unpublished data, 1993). Chalcocite is used here as a copper sulphide group mineral term inclusive of lesser but undifferentiated non-stoichiometric djurleite (Cu1.96S) and digenite (Cu9S5) which accompany chalcocite (Cu2S). Bornite is closely associated with the chalcocite. The average supergene mineral abundances above (and below) the BOCO in terms of percentage of copper bearing minerals are chalcocite 55% (43%), bornite 18% (17%), secondary chalcopyrite 22% (32%) and native copper 5% (8%). Native copper occurs as two populations, a very fine grained disseminated distribution not readily identified in drill core and a coarse grained population. Copper arsenides, chiefly domeykite (Cu3As) occur as close associates with native copper as separate grain aggregates and also with algodonite (Cu6As) as rimming intergrowths to native copper (J Knights, personal communication, 1997). Trace covellite is also ubiquitous but abundances have not been determined. No copper carbonates and rare oxides, as cuprite associated with native copper, are noted. Mineral species distribution is complex with little correlation between adjacent drill holes on 40 m spacing. Petrographic studies indicate that a series of oxidation–reduction events have affected the supergene profile leading to a complex overprinting pattern of sulphides (J D A Clarke, unpublished data, 1993; J Knights, unpublished data, 1993; V Landmark, unpublished data, 1993, 1994). Assay data show that, in the supergene zone, unlike the primary mineralisation, gold is largely decoupled from copper suggesting that the weathering of the copper sulphides liberated gold. Locally gold can occur with negligible copper and vice versa. The similarity in copper and gold ratios between primary and supergene ores indicates that little, if any, metal has been removed from the system. Work by J Knights (personal communication, 1994) suggests that gold is usually extremely fine grained (1 to 2 µm) and has been noted in interstitial gangue sites and the sulphides. This fine grained distribution is supported by the gold variography. The iron oxides in the supergene profile display similar complexity to the copper sulphides. The primary magnetite in the orebody is largely oxidised to hematite and/or goethite through a series of phases. Occasionally primary magnetite remains above the BOCO. The average iron grade for both the supergene and primary zones remains fairly consistent at approximately 20%. Secondary carbonates as calcite and siderite are ubiquitous throughout the supergene altered rocks and locally act as favoured sites for the precipitation of chalcocite and bornite. Carbonates occur as yellow filamentous webbing throughout the chlorite and iron oxide gangue zones. The siderite is largely a manganoan variety.
Evolution of the supergene zone The development of the supergene profile at Ernest Henry is the result of several chemical alteration events affecting the orebody. J D A Clarke (unpublished data, 1993) described a postulated evolution of the supergene weathering profile and his work is drawn on here, as follows:
Geology of Australian and Papua New Guinean Mineral Deposits
1.
Exposure of the Proterozoic contact prior to the Cretaceous transgression, with erosion and oxidation of the deposit. The primary chalcopyrite was oxidised, producing copper sulphate–bearing solutions which precipitated chalcocite and other copper sulphide species. There is evidence of some reaction rims with primary pyrite. Further oxidation formed native copper.
2.
Burial during the Cretaceous transgression with the deposition of reducing sediment (Mesozoic shale). Phreatic fluids were able to pass through the sediment and into the orebody along fracture zones, in turn becoming reduced and further modifying the sulphides. There was local reduction of chalcocite to secondary chalcopyrite, and of native copper to chalcocite.
3.
Marine regression and Cainozoic uplift. Oxidation along faults and joints resulted in the development of broad bands of iron oxides, principally precipitating as hematite and goethite. This phase of oxidation also resulted in the formation of native copper from secondary copper sulphides within the iron oxide–rich gangue.
Primary zone In comparison to the supergene zone, the mineralogy of the primary zone is simple. The ore assemblage is dominated by chalcopyrite within a magnetite-carbonate gangue. No other sulphides or oxides of economic importance have been noted and pyrite is locally abundant. The average sulphide content of the primary ore is about 9%. Copper grades show a bimodal distribution which relates to matrix volumes. The low grade population with <0.7% copper is predominantly associated with fracture filled crackle breccia which has minimal clast digestion. Gold is chiefly contained within chalcopyrite, although detailed microprobe analyses have indicated that pyrite and hematite may locally be important sites of gold mineralisation (J D A Clarke, unpublished data, 1993). There is a strong correlation (95%) between copper and gold in the main primary zone and samples with 1% copper contain 0.5 g/t gold. However, the mineralisation of the eastern lens does not show this relationship, and here copper grades are generally lower and gold grades are more erratic, tending towards higher concentrations than in the main orebody. The average magnetite content of the primary ore ranges from 20 to 25%. In the MMB, magnetite also occurs as a minor component with low grade chalcopyrite mineralisation. Three stages of magnetite alteration have been identified. From the earliest to the latest these are a pervasive, fine grained ‘dark rock’ alteration, a medium grained magnetite associated with the main mineralising event and commonly associated with varying amounts of carbonate, and a coarse grained magnetite which is associated with coarse grained quartz and carbonate in tension veins. Pyrite is present throughout the orebody but decreases markedly in proportion to chalcopyrite in the higher grade zones. Pyrite and chalcopyrite often occur as intergrowths, but also occur separately. A halo of pyrite alteration extends approximately 50 m into the surrounding sequence. Locally coarse grained globular pyrite forms intervals of massive sulphide to 0.5 m wide.
Significant minor elements Anomalous amounts of cobalt, molybdenum, uranium, rare earth elements (REE), arsenic, fluorine and barium are associated with the copper mineralisation.
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Cobalt averages 500 ppm, and in the primary ore occurs largely in solid solution with pyrite and as isolated grains of cobaltite [(Co,Fe)AsS] totally enclosed within pyrite. Rare cobaltian pentlandite and carrollite [Cu(Co,Ni)2S4] are also noted. In the supergene ore, cobalt occurs predominantly as secondary cattierite (CoS2) in which some copper, nickel and iron are substituted for cobalt in the lattice, accompanied by residual cobaltite and minor carrollite (J Knights, personal communication, 1997). Molybdenum averages 180 ppm, as molybdenite, and shows a good correlation with copper grade. Due to the low average uranium concentration of 50 ppm, the overall uranium mineralogy has not been determined conclusively. Grains of uraninite have been identified in association with carbonate alteration fronts and in particular intergrown and enveloped by brannerite [U,Ca,Ce)(Ti,Fe)2O6] and accompanied by rutile (J Knights, personal communication, 1997). There is a moderate correlation between copper and uranium grades. REE are present to 1200 ppm in 2 m drill core samples. Only a limited number of samples have been analysed for REE and total values range from 400 to 1200 ppm. REE are present in grains of bastnaesite [(Ce,La)CO3(F,OH)] and rare monazite [(Ce,La,Nd,Th)PO4.SiO 4)], both associated with apatite. Arsenic averages 300 ppm in the supergene zone and 350 ppm in the primary zone. Arsenic accompanies cobalt as solid solution As-Co in pyrite lattices, occurs in residual arsenopyrite and cobaltite and as copper arsenides in the supergene ore (J Knights, personal communication, 1997). Additional arsenic-bearing minerals associated with chalcocite and chalcopyrite in decreasing order of abundance include loellingite (FeAs2), tennantite [(Cu,Fe)12As4S13], in which copper>>silver, iron>>zinc and arsenic>>antimony, and rare safflorite (CoAs2). Loellingite, tennantite and safflorite are more prevalent towards an ill-defined transition zone connecting the primary and supergene ores. Arsenic in the primary ore chiefly occurs as a variable solid solution of arsenic and cobalt in pyrite lattice sites, as primary cobaltite and as arsenopyrite. There is no significant correlation between copper and arsenic grades, however there is a correlation between arsenic and cobalt grades.
anomalous copper, gold, carbonate, barium as barite, fluorine as fluorite and within apatite, uranium and REE (possibly associated with apatite). The focus for mineralisation at Ernest Henry was a preexisting fracture or fault zone. Anomalous copper and gold values are associated with several NNW-trending interpreted brittle features. The nature of these features is unknown as they have not been directly intersected but there could be a link with the cooling of the Naraku Batholith which may underlie the deposit at depth. The host breccia developed primarily by fluids permeating along fractures or faults. Reaction with the primary volcanic rock resulted in replacement by magnetite, carbonate and copper-gold mineralisation. These fluids could have derived from the cooling of the Naraku Batholith which is known to be enriched in uranium and has extensive associated iron-rich alteration. The timing of the mineralisation at 1480 Myr (C Perkins, personal communication, 1994) and the batholiths at 1500 Myr (Blake et al, 1990) also supports the link. Localisation of this reaction would have been influenced by variations in temperature and pressure, host rock chemical composition and possibly by mixing and reaction with a second fluid. S Coates (personal communication, 1997) notes the local existence of similar breccias barren of magnetite, copper and gold mineralisation with primarily a carbonate±actinolite matrix. He also notes extensive local carbonate alteration and suggests that two separate alteration events gave rise to the localisation of the orebody. An earlier brecciation is proposed with carbonate±actinolite replacement of clasts, and a later alteration with the introduction of magnetite and associated copper-gold mineralisation. The focus was pre-existing zones of structural weakness with possible localisation by variations in the chemical composition of the earlier breccia matrix. The author supports the concept of a separate and widespread carbonate±actinolite alteration event but favours a single, localised mineralising event giving rise to the Ernest Henry deposit.
ACKNOWLEDGEMENTS
Barium from a limited data set consistently exceeds 200 ppm and is often present between 0.1 and 0.8% in supergene and primary ore. Barite is commonly noted as an accessory mineral.
The author gratefully acknowledges the permission of Ernest Henry Mining Pty Ltd to publish this geological description and thanks the geological staff for their input and assistance. Particular thanks are offered to S Coates and A Barber for their critical review, ideas and assistance in the compilation of this manuscript, and J Knights for his assistance with the mineralogical description.
ORE GENESIS
REFERENCES
A limited data set shows fluorine values between 100 and 3000 ppm. Fluorite and apatite are noted as accessory minerals.
The Ernest Henry deposit has many characteristics of the group of orebodies described as Proterozoic iron oxide (Cu-U-AuREE) deposits by Hitzman, Oreskes and Einaudi (1992). Related deposits include the Olympic Dam copper-uraniumgold-silver deposit in South Australia, the Wernecke Mountain breccias of the Yukon, the Kiruna iron ore district of Sweden and the SE Missouri iron ore district. Hitzman, Oreskes and Einaudi (1992) suggested that they be referred to as ‘Kirunatype’ and that they formed primarily by shallow hydrothermal processes, probably related to deep seated magmatism. As with other examples of this type, the Ernest Henry deposit has the characteristic association of dominant iron oxide with
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Anderson, C G and Logan, K J 1992. The history and current status of geophysical exploration at the Osborne Cu & Au deposit, Mt Isa, Exploration Geophysics, 23:1–7. Blake, D H, Etheridge, M A, Page, R W, Stewart, A J, Williams, P R and Wyborn, L A I, 1990. Mount Isa Inlier - regional geology and mineralisation, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 915–925 (The Australasian Institute of Mining and Metallurgy: Melbourne). Blake, D H and Stewart, A J, 1992. Stratigraphic and tectonic framework, Mount Isa Inlier, in Detailed Studies of the Mount Isa Inlier (Eds: A J Stewart and D H Blake), pp 1–11, Australian Geological Survey Organisation Bulletin 243. Collins, S, 1987. The geophysics of the Starra gold/copper deposits, Exploration Geophysics, 18:20–22.
Geology of Australian and Papua New Guinean Mineral Deposits
ERNEST HENRY COPPER-GOLD DEPOSIT
Craske, T E, 1995. Geological aspects of the discovery of the Ernest Henry Cu-Au deposit, Northwest Queensland, in Recent Developments in Base Metal Geology and Exploration, pp 95–109, Australian Institute of Geoscientists Bulletin 16.
Page, R, 1993. Geochronological results from the Eastern Fold Belt, Mount Isa Inlier, AGSO Research Newsletter, 19:4–5. Webb, M and Rowston, P, 1995. The geophysics of the Ernest Henry Cu-Au deposit (NW) Qld, Exploration Geophysics, 26:51–59.
Gidley, P R, 1988. The geophysics of the Trough Tank gold-copper prospect, Exploration Geophysics, 19:76–78. Hitzman, M W, Oreskes, N and Einaudi, M T, 1992. Geological characteristics and tectonic setting of Proterozoic iron oxide (CuU-Au-REE) deposits, in Precambrian Metallogeny Related to Plate Tectonics (Eds: G Gaál and K Schulz), Precambrian Resources, 58: 241–287
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Hodgson, G D, 1998. Greenmount copper-cobalt-gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 769–774 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Greenmount copper-cobalt-gold deposit by G D Hodgson1 INTRODUCTION
EXPLORATION HISTORY
The deposit is about 35 km south of Cloncurry in NW Queensland, at AMG coordinates 451 200 m E, 7 674 500 m N, and lat 21°02′S, long 140°32′E, on the Duchess (SF 54–6) 1:250 000 scale and the Mount Angelay (7055) 1:100 000 scale map sheets (Fig 1). In 1995 Majestic Resources NL (Majestic) acquired a 75% share of the Greenmount deposit, and became manager and operator of the project. William Resources Inc owns the remaining 25%.
At Greenmount green copper stain occurs on shale and sandstone outcrops along a strike length of 2.4 km. Historical workings amount to a few shallow scrapes and one small partly filled shaft, and vague tracks from limited exploration in the early 1950s and 1960s (Ivanac and Branagan, 1960). In the mid 1980s the area was acquired by Valdora Minerals Ltd, who identified anomalous gold values in heavy mineral and bulk cyanide leach stream sediment samples from creeks draining the Greenmount area (G D Hodgson, A J B Thompson and I M Hart, unpublished data, 1988). The results encouraged Homestake Gold of Australia Limited to enter into a joint venture with Valdora.
1 40 °20 ’ E
1 4 0°40 ’E
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Pb r L 140°30’E
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Ernest Henry Cu Au Cloncurry EASTERN FOLD BELT Greenmount Cu Co Au
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Selwyn Au Cu Cannington Pb Ag Zn Osborne Cu Au
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As manager and operator of the joint venture, Homestake initially located anomalous gold values in soils at Greenmount. Follow-up work between 1988 and 1995 included 50 reverse circulation percussion (RC) and a dozen percussion precollared diamond drill holes (M J Cussen, R L Krcmarov, K McKenna, C C Medina, P L Paull, S Omotosho, G Rabone and J I Stewart, unpublished data, 1988–1995). An Inferred Resource of 3.6 Mt grading 0.78 g/t gold, 1.5% copper and 420 g/t cobalt was calculated (R L Krcmarov, unpublished data, 1995) before Homestake’s share of the deposit was sold to Majestic. In 1996 Majestic completed a 65 hole, 7000 m RC drill program targeting copper and cobalt mineralisation. Using block modelling (inverse power distance method) and a 0.5% copper metal equivalent cutoff, Majestic calculated an Inferred and Indicated Resource of 23.8 Mt grading 0.47% copper and 493 g/t cobalt (Majestic Resources, 1996). The gold grade has not been determined for the whole Resource but is expected to average less than 1.0 g/t.
ul a n
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Undifferentiated
Mary Kathleen Group >1740 - 1790 Myr L Ppr
Ro mere uart ite
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FIG 1 - Location and regional geological map of the Greenmount region, after BMR Cloncurry, Marraba and Kuridala Special 1:100 000 geological map sheets.
1.
Consultant, PO Box 137, South Johnstone Qld 4859.
Geology of Australian and Papua New Guinean Mineral Deposits
Because mineralisation comes close to surface and is covered by only 1 to 3 m of soil and alluvium, the top 50 to 70 m of ore could be extracted from an open pit. Trial leach tests on bulk samples of oxide material have begun. The first results suggest that the oxide mineralisation is amenable to heap leaching, solvent extraction and electrowinning. No detailed feasibility studies have been undertaken to date and no work has been done on the sulphide mineralisation at depth.
REGIONAL GEOLOGY STRATIGRAPHY The deposit lies within the Quamby–Malbon zone of the Eastern Fold Belt of the Mount Isa Inlier. The rocks of the Greenmount area, Staveley Formation arenite and Marimo Slate black shale, are part of the Middle Proterozoic Mary Kathleen Group of cover sequence 2 which ranges in age from 1790 to 1760 Myr (Blake et al, 1990).
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450000 m E
E END 00
Dolerite gabbro
E
Six hundred metres SE of the old Greenmount shaft a sandy brecciated rock is exposed in a low NW-trending ridge. The ridge area has not been drilled in detail, but mineralisation is exposed at surface and has been intersected by drilling in two zones, one either side of the ridge (Fig 2). The breccia clasts comprise potassium feldspar–altered and hematite-dusted arenite and are very similar to some Staveley Formation rock types. However, the unit has Marimo black shale above and below, and occupies a tight, gently north-plunging and slightly inclined anticline–syncline couple. The surrounding soft shales have cleaved and the competent sandstone band has brecciated due to the tight folding. The Marimo Basin lies between two 1500 Myr granites, the Williams Batholith to the south and the Naraku Granite to the north. Irregular amphibolite bodies occur from place to place along the Staveley–Marimo contact, and a narrow fine-grained diorite dyke striking subparallel to the Staveley–Marimo contact is exposed along the southern extension of the Greenmount mineralisation. Medium grained granite is exposed in a small isolated outcrop 1 km south of Greenmount. The granite–country rock relationships are concealed.
REGIONAL METAMORPHISM AND METASOMATISM Peak regional metamorphism locally achieved potassium feldspar-sillimanite grade, but the rocks of the Eastern Fold Belt were altered subsequently by metasomatism on a regional scale. Profoundly reconstituted rocks are exposed over hundreds of square kilometres (Williams and Blake, 1993). During several tectonic events, hot hypersaline fluids were derived from the widespread evaporitic units within the various sedimentary sequences. Scapolite and/or remnant evaporite
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Blac slate siltstone
Marimo Slate
Arenite siltstone phyllite breccia Staveley Formation
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Calcarenite phyllite BIF Limestone limestone breccia
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Trend of mineralisation near surface
7675000 m N
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The Staveley Formation is a variable unit. Locally the rocks comprise calcareous, ferruginous, feldspathic and siliceous arenite, siltstone and phyllite, and also limestone and banded iron formation. On the southern flank of the Marimo Basin the Formation is an arenite or calcarenite, and though carbonate units are present it is more sandy locally than further south where, in the Selwyn area for example, it is predominantly a calcareous unit (M J Cussen, personal communication, 1996). Individual carbonate beds are separated by thin layers of silvery phyllite. In strained zones the phyllite displays a distinctive, open, pull-apart cleavage (for which the author has coined the field term ‘mackerel texture’). In the Greenmount area, facing evidence is inconclusive but it is generally considered that the Staveley Formation is overlain by the Marimo Slate. Black, variably carbonaceous and pyritic slate, siltstone and phyllite with subordinate arenite and rare limestone comprise the Marimo Slate. It is a generally recessive unit which weathers relatively easily and only forms extensive ridges around the margins of the Marimo Basin. In the Greenmount area, where the Mesozoic weathering cap has been removed, upstanding outcrops of Marimo Slate are generally bleached and silicified. Where exposed in creek beds the unit has generally weathered to a soft powdery shale.
452000 m E
50
In the area south of Cloncurry (Fig 1) the Mary Kathleen Group occupies a tectonic feature known as the Marimo Basin, between rocks of the Malbon Group to the west and rocks of the informally defined ‘Maronan Supergroup’ (Beardsmore, Newbery and Laing, 1988) to the east. Several units within the Mary Kathleen Group are not well defined and relationships between the various units and rock types are not everywhere clear.
N
unt
ua
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a t
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FIG 2 - Geological plan of the Greenmount deposit.
textures are common in outcrops of carbonate rock throughout the Cloncurry area.
REGIONAL STRUCTURE Williams and Blake (1993) summarised the regional tectonic, metamorphic and metasomatic history: 1.
D1 large scale nappes, thrusts, penetrative fabrics, and prograde regional metamorphism, at about 1625 Myr;
2.
Early D2 upright to overturned NW- to ENE-trending folds, S2 crenulation, and peak metamorphism, at about 1545 Myr;
3.
Late D2 shear zones subparallel to S2, start of retrograde metamorphism, and sodium metasomatism, of uncertain age;
4.
D3 ductile deformation comprising NW- to NE-trending folds and shear zones, second crenulation, retrograde metamorphism except in cordierite-garnet bearing aureoles of batholithic granitoids, at 1510–1480 Myr; and
5.
D3 brittle deformation involving faults and fractures, possible continued granite emplacement, calcium-iron skarns, alkali metasomatism (albite, potassium feldspar and phyllosilicate alteration), silicification and mineralisation, of uncertain age.
ORE DEPOSIT FEATURES LOCAL STRUCTURE On a local scale, the Greenmount deposit is controlled by structures associated with the NW-striking Staveley
Geology of Australian and Papua New Guinean Mineral Deposits
GREENMOUNT COPPER-COBALT-GOLD DEPOSIT
Formation–Marimo Slate contact (Fig 2). The author has measured numerous small scale upright tight to isoclinal folds most of which plunge gently northwards. This folding has affected both the Marimo Slate and the Staveley Formation. Prominent joints strike easterly. The evidence suggests that the Greenmount deposit occupies a position within a NW-trending sinistral shear zone several kilometres wide, and possibly related to late D2 structures identified elsewhere.
5000 m E
The Staveley–Marimo contact is not exposed, but it is probably occupied by a fault zone localised by the ductility contrast between the more massive sandy rocks of the Staveley Formation and the fissile shales of the Marimo Slate. S King (unpublished data, 1994) interpreted a steeply NE-dipping reverse fault along the contact and suggested that the dip of the fault surface is locally flattened (Fig 3). S
NE
GREENMOUNT S
NE
FIG 4 - Schematic cross section showing the Greenmount deposit located on the SW flank of a positive flower structure, modified from Woodcock and Schubert (1994).
Cover
GRCM1
GRCM47
structure where structures dip northeastwards; to the east of Greenmount structures dip steeply southwestwards.
GRCM17
CM 4 GR
ed ch lea ne yb o ye tion Cla ltera a
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d he ac ble one ilty r s ion we rat Lo alte
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>0 Copper metal e uivalent in drill hole Bleached one
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25
L Pks Staveley Formation sandstone
FIG 3 - Cross section on local grid line 9900 m N, looking NW.
The contact zone is complicated by late D3 north- and easttrending joints, and north- and NE-trending faults. The faults offset the contact and vertically displace segments of the mineralised zone. The north-trending structures are parallel to the strike of the axial surfaces of the small scale folds noted above. The Greenmount deposit may be situated on the western flank of a NW-trending ‘positive flower structure’ (Fig 4). Seismic work from the oil industry shows that vertical cross sections through strike-slip zones commonly show a fan-like pattern of upwardly diverging faults (eg Woodcock and Schubert, 1994). Positive flower structures are dominated by reverse oblique faults. In the Greenmount area sinistral transpression has caused a series of high angle reverse faults to develop along a NW-trending shear zone. The Greenmount mineralisation occurs on the western flank of the flower
Geology of Australian and Papua New Guinean Mineral Deposits
At Greenmount the vast bulk of known copper-cobalt-gold mineralisation is hosted by altered black shale within the basal units of the Marimo Slate, with some mineralisation along the faulted Staveley–Marimo contact. The mineralised envelope strikes NW, subparallel to the Staveley–Marimo contact, and dips moderately to steeply northeastwards. The deposit occurs beneath cover, mostly 1 to 3 m thick, along a strike length of about 500 m and drilling has defined cobalt-mineralised intercepts of altered shale, commonly 50–60 m wide but locally to 120 m. Geochemical data show that there is a general spatial relationship between copper and gold mineralisation but that their distribution is very irregular. The cobalt mineralisation forms a much more widespread and coherent envelope and appears to be related to the distribution of iron oxides on fault and joint surfaces. Local high grade patches with >1% cobalt are associated with manganese wad. Work has yet to be done on the nature of the gold and its relationship to the base metal mineralisation. Krcmarov (1995) identified the sulphide species as three varieties of pyrite, two modes of chalcopyrite, and also chalcocite and covellite, with lesser marcasite, cobaltite and rare sphalerite and pyrrhotite. Euhedral disseminated pyrite cubes occur in feldspathised rock, ragged disseminated pyrite occurs in brecciated argillically altered rock, and the vein pyrite occurs as anhedral to euhedral cubes and aggregates. In the alteration zones chalcopyrite occurs as sparsely disseminated grains, whereas in the veins it forms ragged grains or aggregates which often enclose subhedral pyrite (Krcmarov, 1995). Majestic has undertaken electron micoprobe and X-ray diffraction work on samples of mineralised drill chips and has identified the acid-soluble copper and cobalt minerals as mainly malachite and sphaerocobaltite [(Cu,Co)CO3]. From a metallurgical standpoint cobalt mineralisation manifests itself as three distinct types, namely (i) a manganese wad which leaches rapidly with SO2; (ii) copper and cobalt carbonate mineralisation; and (iii) cobalt in arsenical sulphides and pyrite,
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which will be extracted by longer term bacterial leaching. Majestic expects to use an acid/bacterial leach regime from the start because the oxides and secondary sulphides of both copper and cobalt are mixed from surface to depth. To enhance the rate of bacterial activity in the heap pads the company is considering the use of forced air (Majestic Resources, 1997a, b). Drilling has shown that oxidation has affected the altered rocks in the Staveley–Marimo contact zone to depths of at least 150 m. All sulphide species have been modified, and malachite and chrysocolla are common near surface. Pseudomalachite [Cu5(PO4)2(OH)4], secondary pyrite and chalcopyrite are exposed in one of the shallow pits near the Greenmount shaft. Yellow-brown iron carbonates are common in drill core and percussion chips from holes intersecting the fault zone at the Staveley–Marimo contact. Within the altered black shale, but away from the immediate contact zone, red-brown iron oxides have stained fractures and joint surfaces. Geochemical analyses indicate that at least one of the pyrite species is relatively cobalt rich. Fine grained steel-grey cobaltite has been observed in drill chips but it is rare. Erythrite has not been positively identified. In the southeastern part of the deposit mineralisation occurs on both flanks of the breccia ridge (Fig 2), and the mineralised zones appear to dip more steeply here. Sulphide mineralisation is more common and fluid inclusion work indicates higher temperatures (Krcmarov, 1995).
Within the Marimo Basin at any contact where calcareous and sandy units occur adjacent to carbonaceous black shale, and wherever black shale margins are exposed, there is a strong spatial association between the black shale contacts, copper stain and elevated gold values. The contact zone is an obvious chemical trap (‘protore’?) and is usually also a zone of structural dislocation due to ductility differences between the incompetent black shale and the adjacent arenite or limestone. Stewart (1991) and Williams and Blake (1993) noted the importance of evaporites within the Staveley Formation throughout the Cloncurry–Selwyn area, and suggested an association of evaporites with gold and base metal mineralisation. This association is described in detail by Stewart (1991, 1994) and Krcmarov and Stewart (in press). At Greenmount, D2 transpression produced a positive flower structure which proved suitable for emplacement of the diorite dyke and the granite 1 km to the south. Pervasive alteration and metasomatism preceded veining. The quartz-microcline veins and albite alteration indicate that there was local hydrothermal activity focussed on structures associated with the Staveley–Marimo contact. The copper-gold mineralisation was introduced with microcline-quartz veins during late D2 or ductile D3 shearing. Joints and cross faults, formed during brittle D3 deformation, provided the locus for an apparent separate mineralising event involving cobaltiferous pyrite and/or sphaerocobaltite.
ACKNOWLEDGEMENTS ALTERATION In the Greenmount area the Staveley–Marimo contact zone and the diorite dyke have been altered by alkali metasomatism. Krcmarov (1995) recognised that near the contact the zones of bleaching, commonly tens of metres wide, are due to albite alteration. He also noted the presence of subordinate sericite after microcline and lesser amounts of hematite, rutile, tourmaline and dolomite. Patches of black manganese wad also occur from place to place in the bleached zone and manganese values are always elevated along the Staveley–Marimo contact. Black wad is less obvious within unbleached black shale. Krcmarov (1995) described the veins in detail. Quartzmicrocline veins are common and locally contain pyrite and rare chalcopyrite. The vein arrays are sporadically distributed. The structural control is unclear, though S King (unpublished data, 1994) suggested that vein arrays occur in dilation zones above flat ramps on high angle reverse faults. The veins occur at the Staveley–Marimo contact, irregularly within the bleached zones, and in apparently unaltered black shale. Oxidation occurs to depths exceeding 150 m below surface in the Staveley–Marimo contact area, notably along the steeply dipping reverse fault. Mustard coloured clay and glassy chert dominate in the fault zone.
ORE GENESIS Throughout the Cloncurry area mineralisation appears to have accompanied regional defluidisation of the crust during an early post-kinematic episode (D2). The association of economic grades of copper, cobalt and gold with black shale is somewhat unusual and the ultimate source of the metals is debatable.
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The author thanks the directors of Majestic Resources NL for permission to publish this paper, especially G Button for his encouragement to do so, and acknowledges the contributions of the various Valdora, Homestake and Majestic geologists with whom he has worked from time to time at Greenmount since 1986. All have contributed in some way to the discovery, recognition and evaluation of the Greenmount deposit. Also thanked are R E Gould, G J Dickie and J I Stewart who reviewed drafts of the paper, and A Nieuwenburg and R McShea who drafted the figures.
REFERENCES Beardsmore, T J, Newbery, S P and Laing, W P, 1988. The Maronan Supergroup: an inferred early volcanosedimentary rift sequence in the Mount Isa Inlier and its implications for ensialic rifting in the Middle Proterozoic of northwest Queensland, Precambrian Research, 40/41:487–507. Blake, D H, Etheridge, M A, Page, R W, Stewart, A J, Williams, P R and Wyborn, L A I, 1990. Mount Isa Inlier - regional geology and mineralisation, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 915–925 (The Australasian Institute of Mining and Metallurgy: Melbourne). Ivanac, J F and Branagan, D F, 1960. A case history of geochemistry and prospecting in North-West Queensland, Proceedings Australasian Institute of Mining and Metallurgy, 195:25–35. Krcmarov, R L, 1995. Proterozoic geology and mineralisation of the Greenmount Cu-Au-Co deposit, Cloncurry district, MSc thesis (unpublished), University of Tasmania, Hobart. Krcmarov, R L and Stewart, J I, in press. The geology and mineralisation of the south-eastern Marimo Basin, Australian Journal of Earth Science. Majestic Resources, 1996. Annual report to shareholders (Majestic Resources NL: Perth). Majestic Resources, 1997a. March quarterly report to shareholders (Majestic Resources NL: Perth).
Geology of Australian and Papua New Guinean Mineral Deposits
GREENMOUNT COPPER-COBALT-GOLD DEPOSIT
Majestic Resources, 1997b. June quarterly report to shareholders (Majestic Resources NL: Perth). Stewart, J I, 1991. Proterozoic geology and gold geochemistry of the Marimo Basin area, Cloncurry, NW Queensland, MSc thesis (unpublished), James Cook University of North Queensland, Townsville. Stewart, J I, 1994. The role of evaporitic-shale sediment packages in the localisation of copper-gold deposits: Copper Canyon area, Cloncurry, in Proceedings 1994 AusIMM Annual Conference, pp 207–214 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Geology of Australian and Papua New Guinean Mineral Deposits
Williams, P J and Blake, K L, 1993. Alteration in the Cloncurry District: roles of recognition and interpretation in exploration for Cu-Au and Pb-Zn-Ag deposits, James Cook University of North Queensland, EGRU Contribution No 49. Woodcock, N H and Schubert, C, 1994. Continental strike-slip tectonics, in Continental Deformation (Ed: P L Hancock), pp 251–263 (Pergamon Press: Oxford).
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Fortowski, D B and McCracken, S J A, 1998. Mount Elliott copper-gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 775–782 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Mount Elliott copper-gold deposit 1
by D B Fortowski and S J A McCracken INTRODUCTION The deposit is owned by Arimco Mining Pty Limited, a wholly owned subsidiary of Australian Resources Limited. It is at lat 21o32′S, long 140o30′E on the Duchess (SF 54–6) 1:250 000 and the Mount Merlin (6954) and Selwyn (7054) 1:100 000 scale map sheets, 140 km SE of Mount Isa and 16 km north of Arimco's Starra gold-copper deposits and the Selwyn treatment plant (Fig 1).
2
Mining is by open stoping with subsequent pillar extraction by mass blast, using development sublevels at approximately 30 m intervals. Extraction is currently at a rate of 60 000 t per month. Ore is stockpiled on surface, then trucked to the Selwyn mill where it is blended with Starra ore and treated to produce a copper-gold concentrate.
MINING AND EXPLORATION HISTORY The deposit was discovered by John Elliott in 1899 and Mount Elliott Limited was floated on the Melbourne Stock Exchange in 1906 (J Knight, unpublished data, 1992). Mine production from underground workings commenced in 1906 and the ore was initially transported by camels 400 km to the railhead at Richmond (Cherry, 1906). Open cut mining commenced in 1910 (Linedale, 1910). Ore was smelted on site between 1909 (Linedale, 1909) and 1919, and following completion of the rail link in 1911 blister copper was railed to Townsville via Cloncurry (Hishon, 1911). Recorded production from the Mount Elliott smelter totals 24 862 t of copper and 34 000 oz of gold (Blake et al, 1984) from 268 000 t of ore for an average recovered grade of 9.3% copper and 3.9 g/t gold. As some ore was derived from other mines in the area such as the Hampden Consols to the north at Kuridala, exact production figures for Mount Elliott mine are unavailable. Modern exploration commenced in 1952 with drilling and other testing by various companies including Broken Hill South Ltd (seven holes), Mount Isa Mines Ltd (three holes), Rio Tinto Southern Ltd (two holes), Anaconda Australia Inc–Union Miniere Development and Mining Corporation Limited Joint Venture (two holes) and CRA Exploration Pty Ltd (airborne surveys). Most of the holes were drilled under existing workings, and the best intercepts include 18.8 m at 4.0% copper, 2.2 g/t gold in hole BHS-2, 7 m at 3.71% copper, gold unknown in MIM-1 and 17.7 m at 2.9% copper, gold unknown in RTS-2.
FIG 1 - Geological map of the Eastern Succession of the Mount Isa Inlier, showing Mount Elliott and other major mines and deposits in the area, modified from Davidson (1989).
Production recommenced in 1994 after 75 years of dormancy and by the end of September 1996 more than 1 Mt of ore had been produced at a grade of 3.45% copper and 1.64 g/t gold.
1. 2.
Senior Exploration Geologist, Australian Resources Limited, PO Box 1929, Mount Isa Qld 4825. Formerly Senior Development Geologist, Australian Resources Limited, now Senior Computer Geologist, BHP World Minerals, Cannington Project, PO Box 5874 TMC Townsville Qld 4810.
Geology of Australian and Papua New Guinean Mineral Deposits
In 1988 the Selwyn Mining Project partners (Cyprus Gold Australia Corporation, Elders Resources NL and Arimco NL) acquired the area and commenced detailed resource definition drilling. During the period 1988 to 1993, 13 500 m of reverse circulation percussion and 8000 m of diamond drilling were completed. In May 1993 Australian Resources Ltd acquired 100% of Mount Elliott by purchasing Cyprus Gold Australia Corporation's 66.67% equity. Decline development commenced in July 1993 and the first ore was produced in July 1994. In September 1995 exploration drilling to the west of the Mount Elliott orebody discovered a separate mineralised body known as the Corbould zone. This zone has not been closed off and exploration drilling here and elsewhere at Mount Elliott continues.
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RESOURCES AND RESERVES At commencement of the feasibility study for the Mount Elliott development in 1993, the combined Upper and Lower zone resource based on a block model was 2.9 Mt at 3.33% copper and 1.47 g/t gold in the Indicated and Measured categories. An additional 20 000 t at 2.42% copper and 1.37 g/t gold remained as an Inferred Resource. The Measured and Indicated Resources were subsequently converted into Probable Reserves and at December 1993 were 1.83 Mt at 3.00% copper and 1.32 g/t gold, with additional Indicated Resources of 1.003 Mt at 3.60% copper and 1.60 g/t gold and 82 000 t Inferred Resource at 2.6% copper and 1.1 g/t gold. At December 1996 the Corbould zone was estimated to contain 555 000 t at 3.35% copper and 1.50 g/t gold in the Measured and Indicated categories and a further 205 000 t at 3.70 % copper and 1.47 g/t gold in the Inferred category.
REGIONAL GEOLOGY The deposit occurs within the Eastern Succession of the Mount Isa Inlier (Fig 1) and is hosted by the Kuridala Formation. This is a Mid Proterozoic metasedimentary sequence of the Mary Kathleen Group, which is a part of Cover Sequence 2 and of 1790–1760 Myr or younger age, as described by Blake (1987). It contains several thick sill-like bodies of amphibolite (metadolerite and metabasalt) in the Mount Elliott area. The Kuridala Formation also hosts the historic Hampden group of copper-gold deposits at Kuridala, 30 km north of Mount Elliott. The area was affected by the second major period of regional deformation of the Mount Isa Inlier. The deformation consists of three phases with D1 and D2 associated with major regional metamorphism. D1 is dated at about 1610 Myr, D2 at about 1550 Myr, before emplacement of post-tectonic granites, and D3 at about 1480 Myr (Blake, 1987). The Kuridala Formation with the Staveley Formation to the west, occurs in a corridor flanked by the Gin Creek Granite to the SW, the Mount Dore Granite to the south and the Squirrel Hills Granite to the east. The Mount Dore Granite, most of the Squirrel Hills Granite and part of the Gin Creek Granite are described as post- tectonic, non-foliated, uranium-rich, A-type granites emplaced between 1500 and 1550 Myr (Blake et al, 1984). The foliated part of the Gin Creek Granite is pretectonic.
ORE DEPOSIT FEATURES LITHOLOGY AND STRATIGRAPHY
FIG 2 - Geological plan of the Mount Elliott mine area. Orebody shapes are generalised and have been projected to surface. Alphanumeric codes for rock types are explained in Table 1.
Metadolerite occurs as a sill up to 100 m thick adjacent to the hanging wall contact. Metabasalt is common in the western part of the mine area (Fig 2) and plunges east above the footwall contact. Although stratabound by the Elliott beds, observed contact relationships in outcrop and drill holes indicate that the metabasalt is intrusive and replaces host rock. The metabasalt forms the southern end of an extensive zone of magnetic metadolerite that can be traced northwards for 20 km. The Town beds, or footwall schist, consist of quartz-mica schists which may be significantly altered and in places replaced by subore grade mineralised skarn. The entire sequence is overturned (Dredge,1992) so that it is underlain by younger psammitic and calc-silicate metasediment of the Staveley Formation to the south and west of the deposit. Mineralisation is also present within this unit in the SWAN (SouthWest Anomaly) and SWELL (SouthWest Elliott) zones (Fig 2).
The Selwyn beds, at the NE end of the mine area, consist of quartzite, sandstone, schist and metasiltstone and are unmineralised.
Microdiorite dykes to several metres thick have been intersected in drill holes. These crosscut the Elliott beds but their relationship with the Town beds and Staveley Formation has not been observed. The dykes appear to follow zones of weakness including faults, and are very late stage intrusive events. They crop out in the NW of the mine area (Fig 2). Petrographic work (A Joyce, unpublished data, 1995) indicates that they are albite- and calcite-rich.
The Elliott beds host the orebody and consist mainly of carbonaceous phyllite, metasiltstone and minor schist where unaltered. Alteration progressively intensifies towards the footwall and at its peak shows an assemblage of coarse grained minerals typical of skarn type alteration. Conformable sills and dykes of metabasalt and metadolerite are common at the contacts of the Elliott beds.
The mine sequence, which includes the Elliott beds, Town beds and part of the Staveley Formation, has been subdivided by mine geologists and given alphanumeric codes. The sequence from NE (hanging wall) to SW (footwall) is shown in Table 1 and in Fig 3. All units have been metamorphosed to amphibolite facies. Units 1 to 5 in Fig 3 comprise the Kuridala Formation and unit 6 is part of the Staveley Formation.
Within the mine area the Kuridala Formation, which dips steeply NE, is informally subdivided into the Selwyn beds, Elliott beds and Town beds (Dimo, 1975) as shown on Fig 2.
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Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT ELLIOT COPPER-GOLD DEPOSIT
TABLE 1 Mount Elliott mine sequence lithology and mineralogy. Unit
Rock type
Mineralogy
Mineralisation
Comment
1A, 1B
Amphibolite-metadolerite
hbl, pl, bi, mt
py
1A sill, to 100 m thick; 1B is thin dykes
2A
Carbonaceous phyllite, metasiltstone
bi, pl, qz, gr, and
py,po
Unaltered. Occasional schist zones.
2B
Altered phyllite
pl, bi, qz, ksp, ca, fl
py
Outermost alteration halo.
3
Altered phyllite
hab, qz, sca, ca, cpx
py, po, cp, chr, ml, at, cup, tn, cu, cc
Outer skarn carapace. Hosts Upper zone orebody and part Lower zone.
4B
Undifferentiated skarn
hab, cpx, ca
py
4BX
Massive skarn pseudobreccia
cpx, hab, ca, ksp, trm, mt
py, po, cp
Host rock completely altered. Part host of Lower zone orebody.
4BM
Massive skarn pseudobreccia, magnetite rich
mt, hab, sca, cpx, ca
py, cp
Common in deeper levels below the Lower zone.
4AS
Coarse massive skarn + sulphide
cpx, ca, mt, ap, sca
cp, po, py,bn, cc
High grade crosscutting veins.
4AI
Coarse zoned skarn
cpx, ca, ap, mt, qz, amt, gyp
py
Vugs often lined with cpx, ca, qz, amt or gyp.
1C
Metabasalt
hlb, pl mt, bi, sca
Often altered to scapolitebiotite rock.
5A
Altered schist
hab, bi, qz, ksp
Intense silicification, feldspathisation.
5
Schist
bi, qz, mu and se, pl (stl, tml, gn)
Quatz boudins common. Stl, tml and gn reported.
1D
Microdiorite
hab, ca, bi, qz
Crosscuts all the above units as dikes.
6
Calc-silicate
cpx, hab, ca mt
py, cp, cu, cc, chr, ml
SWAN prospect.
6M
Calc-silicate, magnetite rich
mt, cpx, ca, hab
py, cp
Often zones with >30% mt. SWELL prospect.
amt -amethyst, and-andalusite, ap-apatite, at-atacamite, bi-biotite, bn-bornite, ca-calcite, cc-chalcocite, chr-chrysocolla, cp-chalcopyrite, cpxclinopyroxene, cu-native copper, fl-fluorite, gn-garnet, gr-graphite, gyp-gypsum, hab-hemitite-dusted albite, hbl- hornblende, ksp-potassiumfeldspar, ml-malachite, mt-magnetite, mu-muscovite, pl-plagioclase, po-pyrrhotite, qz-quartz, sca-scapolite, se-sericite, stl-staurolite, tmltourmaline, tn-tenorite, trm-tremolite.
The term pseudobreccia of Garrett (1992) has been retained for units 4BX and 4BM. It describes a rock containing ‘clasts’ that are thought to be of replacement rather than tectonic origin, ie they have less altered cores and are not of fragmental shape. Altered clasts of the original milled schist-phyllite breccia are occasionally preserved, often near the unit 3–unit 4BX contact. Rock types previously referred to as fine grained skarn (Garrett, 1992) are now referred to as metabasalt (unit 1C).
STRUCTURE The following observations are synthesised from Dredge (1992), Garrett (1992) and McLean and Benjamin (1993). The deposit occurs within the NW-trending composite Mount Elliott fault zone. The mineralisation is bounded by steeply dipping reverse faults on the hanging wall (upper crush zone) and footwall (faulted footwall schist contact). At least three phases of deformation have affected the deposit. Axial plane cleavage overprints bedding (S0) of the Elliott beds, strikes NW to NNW and dips at 50ο to 80o NE. Bedding is generally parallel to cleavage. The S1 cleavage is often tightly folded at all scales by a D2 event with amplitudes up to 100 m. The dominant SE-trending folds are doubly plunging, at 30 to
Geology of Australian and Papua New Guinean Mineral Deposits
50o to the NW and SE, and are accompanied by a very steeply dipping axial plane cleavage and associated crenulation lineation near the hinges. Folds have strong ‘z’ asymmetry, implying either asymmetric shearing during folding or that they lie on the eastern limb of a larger SE-plunging antiform. F2 folds within the Elliott beds are truncated at their faulted contact with the Town beds. There is a strong textural contrast despite a similar metamorphic grade on both sides of the fault. At depth the fault may be overprinted by alteration or intruded by microdiorite dykes. The Town beds and adjacent Elliott beds, to the west of the open cut, appear to have been folded into open flexures arranged anticlinally about a NNE axis during D3. The Upper zone of mineralisation occurs adjacent to a 15 m wide crush zone, subparallel to bedding strike and dipping at 80o to the east. Evidence of low angle, easterly dipping, dipslip reverse faulting is also apparent. Numerous easterly dipping faults at 45–60o have been observed in underground mapping in the western part of the Lower zone and Corbould zone. Faults dipping at approximately 80o to the west or east are present in the eastern part of the Lower zone. A shear zone known as ‘Jock’s fault’ which dips at 30–40o north bisects the Lower zone.
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introduced with the metasomatising fluid. Massive skarn (units 4B, 4BX, 4BM) develops with potassium feldspar, calcite and clinopyroxene occupying interstices within hematite-dusted albite and eventually replacing it. This results in a final prograde assemblage dominated by clinopyroxene with minor calcite, scapolite and sphene and rare apatite. Clinopyroxenes plot on the diopside-hedenbergite line on a magnesium-iron-manganese ternary plot with slightly more magnesium than iron and virtually no manganese (Garrett, 1992). These plots are typical of prograde anhydrous alteration in calcic copper-iron-gold skarn deposits with scapolite alteration being a coeval metasomatic event. Rare garnets occur in 4AI skarn in the L1D stope on 1130 m RL. Recent analyses (D Mylrea, unpublished data, 1996) indicate that they plot on the grossularite-andradite line on an aluminium-ironchromium ternary plot of calcic garnets. The andradite component is 78%. Massive skarn is cut by coarse grained clinopyroxenescapolite-calcite-magnetite (unit 4AI) often containing chalcopyrite (unit 4AS) which replaces previously formed calc-silicates and calcite. Chlorite, epidote, calcite, sulphides and magnetite are part of the late stage retrograde alteration. The interstitial chalcopyrite and pyrite in the massive skarn (units 4BX, 4BM) were also deposited at a retrograde stage.
MINERALISATION
FIG 3 - Cross section on line 5050 E through the Mount Elliott deposit, looking grid west (299o magnetic). Shaded areas denote copper-gold mineralisation at 3% copper equivalent cutoff. The section location is shown on Fig 2.
ALTERATION Brittle fracturing and brecciation of phyllite facilitated the introduction of hydrothermal fluids and is considered to be the primary control of alteration intensity (Garrett, 1992). Evidence suggests that at least three phases of alteration have occurred. The following description of alteration is largely taken from Garrett (1992) and McLean and Benjamin (1993). Alteration of unit 2A to unit 2B represents the outermost alteration envelope and appears as a pale bleaching due to the loss of biotite and graphite with the growth of quartz-albitesericite-calcite±pyrite, pyrrhotite and rare fluorite around veinlets or along foliation. Bleaching of phyllite progressively increases with overprinting of hematite-dusted albite and minor potassium feldspar along fractures and foliation planes. With further increase in the intensity of alteration fine grained greenish clinopyroxene or amphibole occurs as veins and pervasive replacement, typical of unit 3. Other silicates such as sphene and scapolite also occur but fluorite is notably lacking. The phyllitic texture of unit 3 is progressively destroyed with depth as fracturing and brecciation increase which can lead to massive crystalline hematite-dusted albite- clinopyroxene veins to 2 m wide, particularly near the skarn contact. The deposit is not a typical skarn, as the host rocks were not carbonates or calcareous. Calcium and carbonate were
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Several mineralised zones have been recognised over a strike length of 200 m and to a depth of 300 m below the surface (Figs 3 and 4). Mine RLs are referenced to mean sea level plus 1000 m, and the surface at Mount Elliott is at approximate RL1380 m. The Upper zone, generally above 1235 m RL, was partly mined prior to 1919. The zone contains oxidised supergene sulphides and secondary oxides (chalcocite, covellite, cuprite, malachite, chrysocolla) above 1300 m RL and primary sulphides (chalcopyrite, pyrite, pyrrhotite) below, with pyrrhotite predominant. Sulphides occur as breccia matrix with a clinopyroxene-scapolite-calcite gangue. All mineralisation occurs in the outer skarn carapace (unit 3) in irregular steeply dipping shoots. This zone extends to the surface, becoming narrower and having a conical shape with a malachite-cuprite-chrysocollalimonite-jasper gossan at the apex which was mined in the open cut. The zone is located within a NNW-trending, steeply dipping crush zone. Approximately 50 m below the Upper zone is the top of the Lower zone which is a more tabular, 35o NNE dipping zone containing ore veins to 4 m wide. The veins consist of chalcopyrite-pyrrhotite±magnetite, pyrite, clinopyroxene and calcite. Distribution of the veins is completely random with width and grade varying considerably. In contrast with the Upper zone, this mineralisation crosscuts both the skarn (unit 4BX) and hanging wall outer skarn carapace (unit 3) altered phyllite in a generally coherent dilationary zone. Peripheral zones of mineralisation form minor ‘appendages’ to the Upper and Lower zones. In a number of instances, where individual peripheral zones have been delineated in detail for stope development, they have been named separately, such as the Footwall and Wart zones. The former occurs on the footwall contact of the Lower zone, and the latter occurs to the west of the Lower zone hanging wall mineralisation and appears to be connected to it.
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT ELLIOT COPPER-GOLD DEPOSIT
The Corbould zone (Fig 2) consists of two or possibly three subparallel composite lenses. It is the most recently discovered zone at Mount Elliott and requires further definition. The zone is immediately west of, but separated from, the Lower zone. It strikes east, dips moderately at 30–60o north, and thus has a significantly different orientation from the other mineralised zones at Mount Elliott. Individual high grade lenses within the zone dip 20–30o north. The Corbould Main zone occurs as several pods and overall appears to plunge east at about 40o (Fig 4). The Corbould Footwall zone, localised along the metabasalt–footwall schist contact, is still open in a number of directions.
pyrrhotite. A zoning also exists from the outer edge of the mineralising system through to the deeper inner core as follows: (outer) po→ po-cp → cp-po→ cp-mt→ (inner), with pyrite occurring throughout.
mt-cp→ mt
Exceptions to this zoning occur. Recent mining in the lower levels (1090 m RL) of the Lower zone has shown that areas rich in pyrrhotite predominate. Sulphides have also been observed to replace clinopyroxene and calcite. Low grade chalcopyrite occurs as veins and disseminations within the outer carapace and as coarse blebby disseminations within the massive skarn.
GEOCHEMICAL TRENDS Major and minor element geochemical trends within the deposit are directly related to alteration overprinting of the phyllite-schist host sequence (Garrett, 1992). Initial bleaching of the carbonaceous phyllite of unit 2A to 2B is associated with an increase in SiO2, CaO and F, the last to 9400 ppm, and a decrease in K2O and C, as quartz and minor calcite and fluorite replace biotite and graphite. There is a strong negative correlation with F and Fe2O3, with a fluorine halo enveloping the deposit (Garrett, 1992). The introduction of albite as pervasive replacement occurs early in the alteration of unit 2B to form unit 3 with an increase in Na2O and Al2O3 and a decrease in SiO2 content. Later alteration of unit 3 occurs deeper within the system, as Na2O, SiO2 and Al2O3 content becomes lower and Fe2O3, MgO, CaO and TiO2 increase, as albite is replaced by calc-silicates, mainly clinopyroxene (Garrett, 1992). Some Na2O is retained in scapolite. The alteration of unit 3 to produce units 4B, 4BX and 4BM is similar to that which changed unit 2B to 3 but the degree of alteration is much more intense.
FIG 4 - Longitudinal projection looking 359o magnetic, through the Corbould Main zone at Mount Elliott showing resources at 3% copper equivalent cutoff, as at December 1996. The section location is shown on Fig 2.
Late stage retrograde alteration is responsible for all the economic sulphide mineralisation and its associated calcite and magnetite. At this stage high levels of the trace elements copper, cobalt, nickel and gold were introduced with sulphides. Assays of a single sample of relatively pure chalcopyrite indicate that gold, at 1.97 ppm and possibly hosted in the chalcopyrite lattice, has a strong positive correlation with copper. Chalcopyrite has elevated values for zinc at 460 ppm, silver at 7 ppm, arsenic at 8 ppm, tin at 25 ppm and mercury at 150 ppb (Garrett, 1992). Selenium values to 33 500 ppb and tellurium to 4800 ppb in massive skarn are associated with higher copper values.
In addition to its different orientation, the Corbould zone has a number of other differences from the Mount Elliott Lower zone. These include metabasalt as the dominant rock type, particularly towards the west, slightly higher average copper and gold grades, pyrite rather than pyrrhotite as the main sulphide gangue mineral, and a low magnetite content. Mineralisation is hosted almost entirely in massive skarn as apparently conformable high grade veins. The Footwall zone at Corbould also contains appreciable chalcocite and bornite which may be attributed to faulting and associated oxidation.
Assays of a single sample of pyrrhotite, with 1350 ppm cobalt and 2000 ppm nickel, show that these elements correlate strongly with Fe2O3. Selenium and tellurium levels are also high in pyrrhotite, with 55 ppm selenium and 5 ppm tellurium but gold is low at 0.04 ppm.
SULPHIDE PARAGENESIS
1.
Elements with a strong positive correlation with gold and copper are iron, cobalt, mercury, tin, selenium, silver, tellurium and bismuth.
2.
Elements with a strong negative correlation with gold and copper are sodium, potassium, rubidium, barium, chromium, yttrium, europium, samarium, ytterbium and lutetium.
Studies of sulphide paragenesis of the Mount Elliott Lower zone (Garrett, 1992) indicate that pyrite is initially replaced by chalcopyrite which is subsequently replaced by pyrrhotite. Rarely chalcopyrite replaces pyrrhotite, and pyrrhotite replaces both pyrite and chalcopyrite. Magnetite forms at the expense of
Geology of Australian and Papua New Guinean Mineral Deposits
Electron microprobe analyses of eight gold grains averaged 87–90 wt % gold and 6 –10 wt % silver (Garrett, 1992). Samples of drill core from the Corbould zone were analysed by neutron activation for a suite of 34 elements, and results (M J Cussen, unpublished data, 1996) show:
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D B FORTOWSKI and S J A McCRACKEN
3.
Elements with a strong positive correlation with copper only are cerium, caesium, lanthanum and thorium.
ORE GENESIS AND CONTROLS Copper-gold mineralisation has clearly formed late in the deformational history and is postulated to post-date the D3 open flexure folding (McLean and Benjamin, 1993) and skarn formation. The massive skarn is a product of brecciation and metasomatic alteration of the hanging wall phyllite, footwall schist and minor amphibolite (metabasalt). The low grade interstitial chalcopyrite and pyrite in both the massive skarn (units 4BM, 4BX) and in veins in the outer carapace (unit 3) may represent late syn-skarn mineralisation. The breccia and later dilationary structures have controlled the gross geometry of the deposit, acting as ideal repositories for mineralised fluid. The source of this fluid is unknown. The timing and mechanisms for structural events have always been problematical. Brecciation occurred after the bleaching and induration stage of alteration (unit 2A to 2B). S2 cleavage displays variable orientations from clast to clast indicating that brecciation was at least post D2 (Garrett, 1992). Brittle fracturing and brecciation were probably initiated by reverse thrusting on the crush zone and footwall faults and later, or at the same time, associated with hydrothermal brecciation by skarn type fluids. These fluids were introduced along fractures, the interstices of breccia clasts and along bedding and cleavage planes. This provides an explanation for the large (2–3 m) diameter rotated blocks well away from the faults. In contrast, the very late stage coarse grained clinopyroxenecalcite-sulphide veins occupy apparent dilationary structures produced by ductile deformation. Possibly three thermochemically different fluids used the same conduits. An albite producing fluid and a later clinopyroxene-producing fluid both deposited prograde skarn mineral assemblages. A final retrograde fluid was the source of the calcite, magnetite and sulphides which were deposited as vugh fill and as replacements of calc-silicates. Studies by Garrett (1992) indicate that an initial low temperature, halogen-rich oxidising fluid produced bleaching and calcium-fluorine metasomatism. Then a hot, oxidising, sodium-rich fluid produced albitisation as well as some quartzcalcite-potassium-feldspar. Breccia zones were invaded by a very hot (450–650oC), high salinity, relatively oxidised, weakly acidic, magmatic (granitic) fluid. This fluid was CO2- and calcium-rich with a low total sulphur content (σ34S = 0 to 20/00), with the calcium possibly derived from the underlying Staveley Formation through which the fluid passed. This fluid provided the components for the final prograde calc-silicate dominated skarn mineral assemblage. Sulphur isotope determinations (Garrett, 1992) indicate that late stage sulphide values (σ34S = -5.7 to -3.6 0/00) are possibly the result of a mixing of magmatic sulphur with that leached from diagenetic (biogenic) sulphides in carbonaceous phyllites (σ34S = -12 to -10 0/00). Therefore the hot magmatic fluid may have homogeneously mixed with a low temperature, reducing, sulphur-rich, metal bearing fluid producing a hybrid hot fluid (250–350o). This fluid interacted with the prograde skarn and precipitated sulphides in response to decreasing fO2 and increasing pH. In this reducing environment magnetite was
780
precipitated in preference to hematite, and copper and gold were transported as chloride complexes. Fluid pressures were greater than lithostatic pressures in order for dilation zones to be mineralised. The presence of fluorine, as fluorite and apatite, and boron in rare tourmaline, add credence to a granitic origin for mineralising fluids (Garrett, 1992).
MINE GEOLOGICAL METHODS In the last seven years, resources at Mount Elliott and Corbould have been estimated several times, with the initial estimates based on a sectional polygonal method. The highly skewed and mixed sample distributions for gold and copper have led to geostatistical methods being used to estimate resources, and indicator kriging was selected as a distribution-free method for estimating block grades. For the Upper and Lower zones four block model estimates have now been completed, and two estimates have been completed for the Corbould zone. Detailed stope definition drilling has been used to define the search ellipsoid and variographic parameters. Indicator variography has shown that a high nugget effect (variance = 0.48–0.68 for gold and 0.4–0.6 for copper) exists for both Mount Elliott and Corbould. Most of the non-nugget variation for both gold and copper is related to short range structures with ranges from 6 to 12 m. The remaining variation can be attributed to relatively long range structures of about 50 m. An equivalence formula has been established for each of the major ore zones to reflect their metallurgical recoveries and likely economic constraints. For the Upper and Lower zones the equivalence formula is copper equivalent % = Cu% + (Au g/t x 0.82) whereas for the Corbould zone the formula is copper equivalent % = Cu% + (Au g/t x 0.65). A 3% copper equivalent lower cutoff has been used to define the perimeter of the resource in Figs 3 and 4.
ACKNOWLEDGEMENTS The authors acknowledge Australian Resources Limited for permission to publish this paper and for the support they were given by the company. In particular they wish to thank the many geologists who worked on the Mount Elliott project, past and present, including those from Cyprus Gold Australia Corporation, particularly G McLean and S Garrett. The efforts of P Goldner and R Singer in reviewing this paper are also gratefully acknowledged.
REFERENCES Blake, D H, 1987. Geology of the Mount Isa Inlier and environs, Queensland and Northern Territory, Bureau of Mineral Resources Geology and Geophysics Bulletin 225. Blake, D H, Bultitude, R J, Donchak, P J T, Wyborn, L A I and Hone, I G, 1984. Geology of the Duchess-Urandangi region, Mount Isa Inlier, Queensland, Bureau of Mineral Resources Geology and Geophysics Bulletin 219. Cherry, F J, 1906. Cloncurry. Warden's report for December, 6 January 1906, Queensland Government Mining Journal, 7(69):88. Davidson, G J, 1989. Starra and Trough Tank: iron-formation-hosted gold-copper deposits of north-west Queensland, Australia, PhD thesis (unpublished), University of Tasmania, Hobart. Dimo, G, 1975. Precambrian geology and copper mineralisation of the Mount Elliott area, MSc thesis (unpublished), James Cook University of North Queensland, Townsville.
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT ELLIOT COPPER-GOLD DEPOSIT
Dredge, C P, 1992. EPM 3370-Selwyn. Report for the twelve months ended 24th November, 1991, Cyprus Gold Australia Corporation Report No 770, Queensland Department of Minerals and Energy, C R No 23585 (unpublished). Garrett, S J M, 1992. The geology and geochemistry of the Mount Elliott copper-gold deposit, Northwest Queensland, MSc thesis (unpublished), University of Tasmania, Hobart. Hishon, P M, 1911. Cloncurry. Warden's monthly report, 4 February 1911, Queensland Government Mining Journal, 12(131):188.
Geology of Australian and Papua New Guinean Mineral Deposits
Linedale, J C, 1909. Cloncurry. Warden's monthly report, 30 June 1909, Queensland Government Mining Journal, 10(110):357. Linedale, J C, 1910. Cloncurry. Warden's monthly report, 30 September 1910, Queensland Government Mining Journal, 11(125):515. McLean, G and Benjamin, P, 1993. The geology and development of the Mount Elliott copper-gold deposit, in Symposium on Recent Advances in the Mount Isa Block, Bulletin 13 (Ed: K Williams), pp 47–54 (Australian Institute of Geoscientists: Sydney).
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Bailey, A, 1998. Cannington silver-lead-zinc deposit, in Geology of Australian and Papua New Guinean Mineral Deposits, (Eds: D A Berkman and D H Mackenzie) pp 783–792 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Cannington silver-lead-zinc deposit 1
by A Bailey
INTRODUCTION The deposit is 135 km SE of Cloncurry in NW Queensland, within the Proterozoic Mount Isa Inlier. It is at lat 21o52′S and long 140ο55′E on the Duchess (SF 54–6) 1:250 000 scale and the Selwyn (7054) 1:100 000 scale map sheets (Fig 1). Since the discovery of Cannington in 1990 by BHP Minerals Pty Ltd an intensive feasibility study program has been completed incorporating surface delineation drilling, underground mapping and drilling from a 1 in 8 exploration access decline, bench scale and pilot plant metallurgical test work and a series of mining and infrastructure studies. The total Identified Mineral Resource is 43.8 Mt at 11.6% lead, 4.4% zinc and538g/t silver (Table 1). When full production of 1.5 Mtpa commences from the Southern zone in 1998, Cannington is expected to be the world’s largest silver producer. Annual production will be approximately 220 000 t of lead concentrate and 100 000 t of zinc concentrate. TABLE 1 Cannington resource, May 1997. Zone
Category
Pb %
Zn %
Ag g/t
Southern
Measured
11.9
Mt
13.5
5.4
626
Indicated
18.4
11.5
4.2
544
Inferred
4.4
13.4
6.2
620
Total Southern Zone
34.7
12.4
4.9
582
Northern
Indicated
6.4
10.2
2.9
422
Inferred
2.7
5.9
3.4
251
Total Northern Zone
9.1
8.9
3.0
371
Total Southern and Northern Zones
43.8
11.6
4.4
538
EXPLORATION AND DEVELOPMENT HISTORY The discovery of the Cannington deposit was the culmination of several years of exploration for Broken Hill style deposits in Australia (Skrzeczynski, 1993). Part of this effort had been focussed on the Soldiers Cap Group of Mesoproterozoic age within the Eastern succession of the Mount Isa Inlier, which was considered to have similar characteristics to the Willyama Supergroup of the Broken Hill Block (S G Walters, unpublished data, 1994). Following initial exploration of tenements some 60 km to the SE of Cloncurry, which included the discovery and delineation of the Eloise copper-gold deposit, a further group of tenements was pegged to the south in areas 1.
Chief Geologist, Project Development and Technical Services Australia, BHP World Minerals, The Broken Hill Pty Co Ltd, PO Box 6062, East Perth WA 6892.
Geology of Australian and Papua New Guinean Mineral Deposits
covered by Phanerozoic sedimentary rocks, adjacent to outcrop of the Eastern succession. A regional aerial magnetic survey over the tenements in 1989 followed by interpretation of the results defined a series of anomalies. The Cannington deposit was discovered in 1990 by drill testing of one of these anomalies. Hole ANP03 intersected 20 m of mineralisation averaging 12.1% lead, 0.6% zinc and 870 g/t silver, and diamond drilling during 1990 and 1991 indicated a significant silver-lead-zinc resource. The feasibility study included development of a 1 in 8 exploration decline into the Southern zone to provide sites for closely spaced core drilling, to gain further information on the geotechnical and hydrological conditions and to provide sites in the lode horizons from which bulk samples could be taken for pilot plant metallurgical testwork (Bailey and Thomas, 1993; Roche, 1994). The study was completed in 1995 and underground mine development and surface construction commenced in early 1996. Total drilling to July 1996 comprised 281 cored surface drill holes (total 78 419 m) plus a further 221 cored underground holes for 25 896 m. The majority of the exploration and evaluation drilling has been carried out on the higher grade Southern zone of the deposit. For the feasibility study surface drill hole spacing in the Southern zone was on an approximate 50 by 50 m grid with some holes at closer spacing. Underground mining methods proposed include primarysecondary stoping in the thicker ore sections and longitudinal benching in the thinner areas. Stope voids will be paste filled. The ore process circuit will consist of sequential silver, lead and zinc flotation with the silver flotation product being combined with the lead flotation product to yield a final lead concentrate with high silver values. The expected metallurgical recoveries are 90% lead to lead concentrate with an average concentrate grade of 75% lead; 80% zinc to zinc concentrate with an average concentrate grade of 52% zinc, and silver recoveries of 85% and 5% to the lead and zinc concentrates respectively. An innovative approach to reducing the fluorine content of concentrates from Cannington has been the addition to the metallurgical circuit of a low temperature (50oC) fluorine leach process for both zinc and lead concentrates.
REGIONAL GEOLOGY The Cannington deposit occurs beneath 10 to 60 m of Cretaceous and Recent sediment, in the SE corner of the Eastern succession of the Proterozoic Mount Isa Inlier (Fig 1). The dominant lithological packages recognised within the Eastern succession are the Mary Kathleen and Malbon groups to the west of the Cloncurry Overthrust (Blake and Stewart, 1992) and the Maronan Supergroup, proposed by Beardsmore, Newbery and Laing (1988), which contains the Soldiers Cap and Fullarton River groups to the east of the Overthrust. Age relationships between these groups are uncertain due to the complex faulted and intruded contacts. The Maronan
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FIG 1 - Geological map of the southeastern part of the Mount Isa Inlier (after Walters and Bailey, in press).
Supergroup is dominated by migmatitic gneiss, schist, psammite, feldspathic psammite, calc-silicate breccia, amphibolite, pegmatite and thin banded iron formation with associated small base metal deposits. This package was derived (Blake and Stewart, 1992) from a premetamorphic sequence of predominantly immature clastic rocks with intervals of interbedded basic volcanic and minor carbonate rock horizons. Age determinations of a garnetiferous felsic gneiss from the Fullarton River Group (Page, 1993) gave a minimum age of 1677±9 Myr. It is not clear whether the zircon dated represents the precursor of the volcaniclastic component or the detrital component age. Laing (1990) postulated that the Soldiers Cap Group may have been rafted from the east on the Cloncurry Overthrust on to the Mount Isa Block during the Diamantina Orogeny which also affected the Georgetown and Broken Hill terranes.
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Sedimentation was terminated by the Isan Orogeny, from 1520 to 1620 Myr. Deformation consisted of an early phase of NNW-verging thrust and nappe structures (Looseveld, 1989), followed by a dominant phase (D2) of east-west compression resulting in north-aligned tight, upright folds. Regional metamorphism accompanied the deformation and peaked preor syn-D2, and has been dated at approximately 1550 Myr (Page and Bell, 1986). In the Eastern succession a general trend in peak metamorphic grade exists, from upper greenschist facies in the areas around Cloncurry to upper amphibolite facies to the south, where widespread sillimanite–potassium feldspar schist and migmatitic gneiss are present. The Eastern succession has been intruded by the extensive Williams and Naraku granites (1560 to 1480 Myr), which are distinctive, fractionated I-type granite suites (Wyborn, 1992).
Geology of Australian and Papua New Guinean Mineral Deposits
CANNINGTON SILVER-LEAD-ZINC DEPOSIT
The regional magnetic data indicate that the higher metamorphic grade sequences within the Soldiers Cap and Fullerton River groups extend for considerable distances under cover to the east and SE. However the lack of outcrop in the Cannington area, high metamorphic grade and complex deformation history make detailed lithostratigraphic correlation difficult.
DEPOSIT GEOLOGY The deposit is hosted by a sequence of garnetiferous psammite within a migmatitic quartzo-feldspathic gneiss terrain. The sequence strikes north and is cut by two major NW-trending structures, the Trepell fault which separates the Northern and Southern zones of the deposit and the Hamilton fault which forms the southern limit of the deposit (Fig 2).
STRUCTURE Four deformation phases (D R Gray, unpublished data, 1992) have been recognised at Cannington: 1.
D1 - related to the early regional thrust event, produced a local schistosity and rare, minor rootless fold hinges in foliated rocks; 2. D2 - the major structural event, represented by tight, upright north-aligned folds with a well developed axial surface schistosity (S2) and a poorly represented southerly plunging lineation (L2); 3. D3 - open folds with minor crenulation of D2 fabrics; and 4. D4 - late stage brittle structures. Peak metamorphism occurred during or shortly prior to D2 deformation and reached almandine amphibolite grade. The geometry of the Southern zone sequence is controlled by a complex tight to isoclinal recumbent D2 synform which strikes north, dips 40 to 70ο to the east and plunges to the south (Fig 3). An amphibolite body (‘core amphibolite’) within the core of the fold structure separates the footwall and hanging wall mineralised sequences. The sequence is thickest in the hanging wall and has been thinned along the footwall where more intense S2 fabrics and local truncation of the sequence suggest the presence of a high-strain shear zone. Minor D3 structures have been mapped in the exploration decline and show generally ESE plunging (60o towards 120o) open folds. Open folds and occasional crenulation of S2 foliation by D3 structures are also observed in drill core. The isoclinal fold structure is displaced by a sequence of late stage faults. The interpreted Brolga fault is a zone of northstriking minor faults which cut and displace the easterly hinge zone of the isoclinal fold. More prominent are a set of NEstriking brittle-style faults which dip steeply to the NW, show predominantly dextral strike-slip displacement and have associated silica, carbonate and pyrite alteration. This set is present throughout the Southern zone and displaces the succession and lode horizons and the Brolga fault structures. These NE-trending faults are interpreted to have developed as a conjugate set to the major bounding Trepell and Hamilton faults, which in turn are characterised by intense development of breccia, clay-chlorite gouge and associated wide fracture zones. Displacement on the Trepell and Hamilton faults is interpreted by D R Gray (unpublished data, 1993) from a conjugate fracture system related to the Trepell fault as subhorizontal sinistral.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 2 - Interpreted geology and economic lodes on 900 m mine RL (350 m below surface).
The Northern zone rocks and in particular the schistose horizons are typically less deformed than their Southern zone counterparts. D R Gray (unpublished data, 1993) has interpreted the Northern zone to lie on the eastern limb of an F1 antiform structure. There are at least two stages of local D3 open folding that develop interference patterns with folds which gently plunge both to the south and to the west to NW. Near surface the Northern zone rocks generally dip to the east but at depth they dip steeply back towards the west, forming an open fold structure (Fig 4).
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FIG 3 - Diagrammatic cross section of the Southern zone on line 4700 m N, looking north.
(+200 m) sequence of quartz-garnet-sillimanite schist and foliated garnet psammite, characterised by disseminated fine grained pink almandine garnet, is developed in the hanging wall and can be traced through the synform hinge into the structural footwall sequence where it is thinned and truncated by the footwall shear and Hamilton fault system. End members of this suite are foliated sillimanite schist with garnet and massive or banded and locally foliated garnet psammite. The hanging wall schist also contains thin (0.5 to 3.0 m), finely banded pyroxmangite-hedenbergite-fayalite±quartz-garnet horizons with accessory apatite±graphite (Bodon, 1995) and locally, low grade galena-sphalerite mineralisation.
FIG 4 - Diagrammatic cross section of the Northern zone on line 5300 m N, looking north.
HOST LITHOLOGY The host migmatitic gneiss contains intercalated (0.1 to 0.5 m thick) fine grained, schistose biotite-sillimanite-quartz bands and pegmatitic quartz-feldspar bands. S G Walters (unpublished data, 1994) considers that a feldspathic sandstone with shaly bands is the likely precursor. A relatively thick
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Microprobe analysis of a suite of garnets (Richmond, Chapman and Williams, 1996) has shown two distinct compositional groups; type A garnet with calcium garnet <12 mol % and type B garnet with calcium-garnet >22 mol %. Type B garnets are later and restricted to alteration associated with the mineralised lode horizons, the alteration selvage to pegmatite and the thin hedenbergite-pyroxmangite-fayalitequartz bands within the hanging wall sequence. The overall sequence becomes more psammitic and silicified and less schistose towards the lode horizons. In the Southern zone, within this envelope of psammitic and schistose rocks, the lodes form footwall and hanging wall sequences about a tear-shaped, south-plunging core amphibolite horizon. The amphibolite has a maximum thickness of 100 m and is a fine to medium grained, equigranular rock of hornblende and plagioclase with scattered minor quartz and accessory apatite,
Geology of Australian and Papua New Guinean Mineral Deposits
CANNINGTON SILVER-LEAD-ZINC DEPOSIT
and a weakly developed single foliation (Sheehan, 1994). Semiconformable pegmatite horizons occur throughout the deposit sequence, and were formed predominantly as a result of ‘local’ partial melts (Mark, 1993). The pegmatites were ‘emplaced’ during or prior to D2 deformation, as they are folded and show boudinage within the D2 foliation. Compositionally, the pegmatites are predominantly coarse grained potassium feldspar, quartz and plagioclase with accessory muscovite, biotite and garnet spotting, with a coarse garnet-amphibole alteration selvage. Individual horizons can be traced over considerable distances down dip and along strike. Pegmatites show partial assimilation of lode material with inclusion of sulphides and occurrences of the lead-bearing green feldspar amazonite.
SOUTHERN ZONE MINERALISATION The silver-lead-zinc mineralisation at Cannington is associated with a diverse package of siliceous and mafic rocks with extensive retrogression and alteration. A zoning of base metals is evident within the Southern zone which is consistent with the interpreted isoclinal fold structure. The lode horizons are defined by the base metal distribution. Within this semi-tabular geometry a sequence of gangue–ore geochemical and textural associations is recognised, and the lode and mineralisation types (Table 2) describe the geometry, economic, geochemical and textural relationships within the deposit. The mafic host rocks are generally defined by an overall iron content greater than 15%, but end members can contain above 30%. The package consists of moderate to coarse grains of equigranular pyroxene, pyroxenoid and olivine with local codominant or accessory magnetite and fluorspar; minor quartz and amphibole may also be present. Magnetite grains can be intensely fractured, with the fractures filled by silicates and sulphides (French, Ramsden and Walters, 1994). A clear zoning exists in the mafic assemblage with the pyroxenoid, pyroxmangite [(Fe,Mn,Ca)SiO3] associated with the outer envelope of lead-silver mineralisation, and the pyroxene,
hedenbergite [Ca(Fe,Mn)Si2O6] associated with the inner zinc mineralisation (Figs 2 and 3). Manganese-rich fayalite is associated with both phases, as are magnetite and fluorspar, although both can vary on a local scale. Widespread retrograde alteration phases occur throughout the mafic suite. These can be characterised (Bodon, 1995) as high temperature anhydrous (hedenbergite, associated with the zinc horizons and garnet), hydrous (quartz, ilvaite and amphibole), and lower temperature (pyrosmalite, chlorite, greenalite, talc and carbonate) phases. The siliceous ore hosts are mineralogically more simple than the mafic, with anhedral interlocking quartz the dominant gangue, with accessory feldspar, biotite and muscovite. The quartz is amorphous, chert-like, blue and exhibits some conchoidal fracture. Minor garnet is also present. Apatite with fluorine to 5% (French, Ramsden and Walters, 1994) is another accessory mineral, and is responsible for average phosphorus values within the siliceous package of 2000 to 3000 ppm (Fig 5). Late stage silicification is present throughout the sequence as an alteration phase and also in association with the late stage brittle-style faulting. There are local gahnite horizons within the siliceous mineralised rocks and in the hanging wall schist horizons. Magnetite and fluorspar are absent from the siliceous package. The Cannington sulphide assemblage is dominated by galena and sphalerite, with minor pyrrhotite, marcasite, arsenopyrite and chalcopyrite. The silver is contained predominantly in freibergite [Cu6(Ag,Fe)6Sb4S13], the silver-rich member of the tetrahedrite-tennantite sulphosalt series or fahlores, and in solid solution, to 1300 ppm, within galena. Other silver phases include pyrargyrite (Ag3SbS3), acanthite (Ag2S), allargentum (Ag6Sb), dyscrasite (Ag 3Sb), native and antimonial silver and a previously undescribed sulphosalt (French, Ramsden and Walters, 1994) informally called ‘canningtonite’ (4PbS.3Ag2S.3Sb2S3). Traces of proustite (Ag3AsS3) and jamesonite (4PbS.FeS.Ag.3Sb2S3) have also been recorded. In the Footwall lead lode (Nithsdale mineralisation) the silver minerals stephanite (Ag5SbS4), sternbergite (AgFeS3) and
TABLE 2 Southern zone lode and mineralisation types with gangue associations. Lode horizon (% of Southern zone resource)
Mineralisation type
Gangue–ore association
Gangue association
Footwall lead (12.3%)
Nithsdale (NS)
Mafic Pb, Ag
Pyroxmangite, magnetite, manganese-fayalite and fluorite
Warenda (WA)
Siliceous mafic Pb, Ag (low grade)
Quartz, pyroxmangite, manganese-fayalite and fluorapatite
Footwall zinc (18.9%)
Cukadoo (CK)
Siliceous Zn
Quartz, feldspar (minor) and fluorapatite
Colwell (CW)
Mafic Zn
Hedenbergite, manganese-fayalite, magnetite, pyrrhotite and fluorite
Glenholme (GH)
Siliceous Pb, Zn, Ag
Quartz, feldspar (minor) and fluorapatite
Glenholme breccia (10.9%)
Glenholme breccia (GHB)
Siliceous Pb, Zn, Ag
Quartz, feldspar (minor), carbonate and fluorapatite
Hanging wall zinc (1.4%)
Kheri (KH)
Mafic Zn (low grade)
Hedenbergite, manganese-fayalite, magnetite, pyrrhotite, fluorite and arsenopyrite
Kheri-Colwell
Mafic Zn (low grade)
Hedenbergite, manganese-fayalite, magnetite, pyrrhotite and fluorite
Burnham (BM)
Mafic Pb, Ag
Pyroxmangite, magnetite, manganese-fayalite and fluorite
Broadlands (BL)
Siliceous mafic Pb, Ag
Pyroxmangite, quartz, manganese-fayalite and fluorapatite
Hanging wall lead (56.5%)
Geology of Australian and Papua New Guinean Mineral Deposits
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FIG 5 - Median values for major and minor elements in the Southern zone lode and mineralisation types.
stetafeldite (of uncertain composition, perhaps Ag,Cu,Fe,Sb,S,H2O) have also been identified within late stage alteration (M Dugmore, personal communication, 1996). Other minor sulphides associated with the mineralisation are loellingite (FeAsS2), gudmundite (FeSbS), veenite (Pb2Sb2S5), launyaite (Pb 22Sb26S61) and bismuthinite (Bi2S3).
Footwall lead lode Two mineralisation styles are associated with this lode horizon, which is at the structural base of the deposit. These are a mafichosted high grade silver-lead mineralisation (Nithsdale style) and a down dip, lower grade more siliceous mineralisation, the Warenda type. In vertical longitudinal projection (Fig 6a), the grade.thickness product for lead (%lead x thickness) is greatest in the Nithsdale mineralisation and the grade.thickness contours define a southerly plunging shoot that is slightly to the south of, and subparallel with, the southern limit of the core amphibolite. The Nithsdale gangue assemblage is typically pyroxmangite, magnetite, fayalite and fluorite with minor hedenbergite. Pyroxmangite and fayalite have been replaced along fractures and grain boundaries by pyrosmalite and greenalite. This assemblage has a series of textures including massive equigranular pyroxmangite-fayalite and galena; coarse-banded magnetite and fluorite-galena-pyroxmangite-fayalite; and intense ductile-style milled breccia with rounded clasts of pyroxmangite, fayalite and magnetite in a matrix of sulphide and fluorite. The dominant sulphide is galena with associated silver phases, and minor sphalerite, pyrrhotite, freibergite, arsenopyrite, chalcopyrite and loellingite. French, Ramsden and Walters (1994) recorded an across-dip zoning in the Nithsdale mineralisation with pyrrhotite more abundant and coarser grained towards the hanging wall and a corresponding increase in pyroxmangite and galena towards the base. The iron content of the mineralisation has a median value of 19% (Fig 5) but can locally reach 40%, and fluorine values vary between 2 and 10% with a median of 4.4%. The transition of Nithsdale to Warenda style mineralisation down dip and along strike to the north is marked by an increasing silica content and an associated decrease in mafic
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minerals, magnetite and fluorite. The Warenda mineralisation typically comprises irregular bands of more mafic composition, with pyroxmangite, fayalite and hedenbergite intercalated with more siliceous bands. There is only a moderate galena content with associated silver phases in this mineralisation, and it is predominantly associated with the more siliceous horizons. Minor sphalerite, pyrrhotite, pyrite and arsenopyrite are also present, and trace chalcopyrite. Magnetite and fluorite are only present in the transition areas.
Footwall zinc lode This horizon is host to approximately 20% of the Southern zone resource. It has a complex zoning of ore and gangue minerals from mafic rocks with zinc mineralisation (Colwell type) to siliceous rocks with zinc mineralisation (Cukadoo type), and siliceous rocks hosting zinc-lead mineralisation (Glenholme type). However, throughout this tabular horizon the grade of the zinc mineralisation cuts across these mineralisation styles, and when plotted in vertical longitudinal projection (Fig 6b) the zinc grade-thickness product (% zinc x thickness) defines a southerly plunging shoot which is subparallel to the southern limit of the core amphibolite. The lode is structurally above the Footwall lead lode (Fig 2) and is separated from it by a zone, approximately 15 m thick, of predominantly low grade zinc mineralisation with magnetite and minor fluorite hosted by mafic rock. Down dip and to the north where the Warenda mineralisation forms the Footwall lead lode the interburden between the lead and zinc horizons is more siliceous, and magnetite and fluorite are absent. The Cukadoo mineralisation is characterised by massive, milky white to bluish strained quartz with minor potassium feldspar, muscovite and fluorapatite, and stringers of sphalerite. Galena, pyrrhotite, arsenopyrite and chalcopyrite occur as minor veinlets and disseminations. There are small zones of intensely deformed breccia containing rounded siliceous clasts in a matrix of sphalerite-pyrrhotite. The Cukadoo mineralisation forms the up dip, southplunging segment of the Footwall zinc lode, wheras down dip it is transitional into the Colwell mafic-hosted zinc
Geology of Australian and Papua New Guinean Mineral Deposits
CANNINGTON SILVER-LEAD-ZINC DEPOSIT
FIG 6 - Vertical north-south longitudinal projections of the lodes with grade.thickness contours, distribution of mineralisation types, the extent of the core amphibolite and the intersection trace of the NE-trending faults: (a) Footwall lead lode - % lead x thickness; (b) Footwall zinc lode - % zinc x thickness; (c) Hanging wall lead lode - % lead x thickness.
mineralisation. The Colwell mineral assemblage is hedenbergite±magnetite and fluorite with sphalerite, pyrrhotite and minor galena, arsenopyrite and chalcopyrite. Milled breccia textures and wispy sphalerite-pyrrhotite-fluorite flame textures are present in some areas. Olivine, partly replaced by pyrosmalite or ilvaite, is also present. Pyroxmangite is absent or at trace levels in the Colwell mineralisation as reflected in the elemental abundance (Fig 5), where manganese is associated with the lead-silver dominant mineralisation but not with the zinc mineralisation. Further down dip and towards the hinge zone of the
Geology of Australian and Papua New Guinean Mineral Deposits
synformal structure the mineralisation changes again to the siliceous Glenholme style, and contains zinc and lead in approximately equal proportions. The Glenholme and Glenholme breccia are the only mineralisation types at Cannington with elevated levels of both zinc and lead. Both mineralisation styles are characterised by a breccia texture with silica clasts and a matrix of sphalerite, galena and minor carbonate. Minor muscovite, potassium feldspar and fluorapatite are present, with some retrograde sericite, chlorite and illite. Pyrrhotite, magnetite and fluorite are absent, except in zones transitional to the Colwell mineralisation.
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Glenholme breccia lode This mineralisation is in the hinge area of the D2 synform and is the down dip extension of the Glenholme mineralisation in the Footwall zinc lode. There are no significant mineralogical differences between the Glenholme and Glenholme breccia mineralisation types. The distinction is made on the basis of texture, the Glenholme breccia having a greater development of breccia mineralisation with small areas of highly deformed and disaggregated quartz veins and segregations, and a minor increase in antimony-bearing sulphosalts thus giving a higher antimony:silver ratio (Fig 5).
Hanging wall zinc lode Unlike its footwall equivalent this lode contains only maficrock hosted zinc (Kheri style) mineralisation. The mineralogy and elemental abundance of this mineralisation are, with only minor exceptions, identical to the footwall Colwell mineralisation. These minor exceptions are a lower zinc grade, elevated iron values, and an increase in arsenopyrite, chalcopyrite and coarse grained pyrrhotite. As in the Colwell mineralisation the textural styles vary from a common granular texture with hedenbergite, magnetite, pyrrhotite, arsenopyrite, sphalerite and minor chalcopyrite, to a more intensely milled breccia, and flame textures. In the south where the core amphibolite is absent the distinction between hanging wall and footwall lodes in the synform hinge is more problematic, and in these areas no distinction is made between the Kheri and Colwell mineralisation types and the term Kheri–Colwell is used.
Hanging wall lead lode This lode constitutes more than 50% of the Southern zone resource and comprises the mafic hosted lead-silver Burnham mineralisation and the overlying siliceous-mafic hosted lower grade lead-silver Broadlands mineralisation. Together these mineralisation types form sequences up to 100 m thick in the Southern zone and extend from the south where they are truncated by the Hamilton fault, to more localised intercepts in the north. In vertical longitudinal projection (Fig 6c) the maximum grade-thickness product for lead for the Hanging wall lead lode is positioned to the south of the core amphibolite and with the outline of the Glenholme breccia mineralisation defines an envelope enclosing the southern and down dip limits of the core amphibolite. The Burnham mineralisation is the hanging wall equivalent of the Nithsdale mineralisation, and like the Nithsdale mineralisation is characterised by a gangue of pyroxmangite, magnetite, fluorite and fayalite and a sulphide mineralogy of galena, pyrrhotite, sphalerite, arsenopyrite, freibergite and associated silver-sulphosalt phases. As with the other mafic hosted units widespread hydrous retrogression is common. Similar textural associations to those in the Nithsdale mineralisation are also present, although there is a greater degree of coarsely banded magnetite and fluorite-galenapyroxmangite mineralisation. Small scale, tight folding of banded magnetite-pyroxmangite with pyrrhotite formed in the axial plane cleavage is observed in drill core and it is possible that the folding is a contributing factor to the increased thickness of the horizon.
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The Broadlands style mineralisation is principally a banded pyroxmangite-hedenbergite quartzite, equivalent to the Warenda mineralisation of the footwall sequence. This moderate to low grade galena with minor sphalerite mineralisation is usually associated with the more siliceous units. The form is typically lenses of predominantly pyroxmangite-hedenbergite-fayalite interbanded with lenses dominated by quartz, on a scale of 1 m or less. Other minor to trace sulphide phases include pyrrhotite, chalcopyrite, arsenopyrite and freibergite. Fluorapatite and graphite are also present in the siliceous units.
NORTHERN ZONE MINERALISATION The majority of the mineralisation in this zone lies above and is partially draped about an amphibolite horizon (Fig 4) in an equivalent position to the Hanging wall lead lode in the Southern zone, and it may therefore represent a simple faultedoff segment of this lode. Minor Glenholme lead-zinc mineralisation is also present, again in a similar position to its Southern zone equivalent but of lower grade and extent. These mineralised horizons are similar in mineralogy and texture to their Southern zone counterparts. The Northern zone also contains a consistent, steeply dipping sequence of mineralised horizons on its eastern flank. This mineralisation, termed Inveravon, is finely banded pyroxmangite-olivine±hedenbergite, apatite and quartzgalena±graphite, and is very similar in style to the horizons previously described in the Hanging wall schist in the Southern zone. Minor sphalerite is present, although grades of lead or zinc are usually low and only a very few samples reach economic grade. The Inveravon style mineralisation is also present above the Broadland mineralisation in the Northern zone (Fig 4).
GENETIC MODELS The Cannington deposit has many similarities with the Broken Hill deposit in NSW including its setting in a Middle Proterozoic sequence with both lithological and temporal affinities to the Broken Hill Block, high grade metamorphism, complex deformation and a distinct geochemical, mineralogical and economic mineral zoning. Research on genetic models for the deposit has suggested two alternatives: 1.
a late stage metasomatic mineralising event (skarn model), post-dating the major metamorphic and deformation events (Williams et al, 1996); and 2. a synsedimentary or early diagenetic mineralising event with subsequent modification and remobilisation during metamorphic and deformation episodes, followed by further modification by a late metasomatic event (S G Walters, unpublished data, 1994; Bodon, 1996; Walters and Bailey, 1996). The skarn model (Williams et al, 1996) can be summarised in the following paragenetic sequence: 1.
An original metasedimentary package which consisted of an iron-manganese-(calcium)–rich fraction representing the present lodes; and an outer iron- and manganese-rich peraluminous metasediment derived from quartz–pelite mixtures with local feldspathic fractions.
Geology of Australian and Papua New Guinean Mineral Deposits
CANNINGTON SILVER-LEAD-ZINC DEPOSIT
2.
Regional deformation (D1 and D2) with peak metamorphism reaching upper amphibolite facies pre- or syn-D2. Peak metamorphic minerals were quartz, sillimanite, potassium feldspar, biotite, type A garnet and graphite. 3. Peraluminous anhydrous iron-rich alteration with a similar mineral suite to that associated with peak metamorphism. 4. Anhydrous calcium-rich alteration with quartz, apatite, pyroxmangite, hedenbergite, fayalite, hornblende and type B garnet. 5. Hydrous iron-calcium-potassium alteration, with hornblende, biotite, pyrosmalite and dannemorite. 6. A mineralising phase with sphalerite, galena, pyrrhotite, chalcopyrite, arsenopyrite, pyrite, tetrahedrite (freibergite), magnetite and fluorite. The alternative ‘synsedimentary’ model (S G Walters, unpublished data, 1994; Walters and Bailey, in press; Bodon, 1996) can be summarised by the following paragenetic sequence: 1(i). Initial introduction and zoning of base metal and silver mineralisation with zinc dominant and lead-silver dominant horizons. By analogy with other base metal deposits the siliceous lead-silver style (Broadland and Warenda types) represents a capping to the mineralised sequence (S G Walters, unpublished data, 1994). This premetamorphic zoning could have been developed by processes associated with a volcanogenic sulphide system or a basin dewatering diagenetic system with the mineralisation controlled by primary porosity or matrix replacement. No evidence is currently available that would indicate which of the above should be the preferred model. (ii). Emplacement into the sequence of a series of tholeiitic basic sills (Mark, 1993). 2. Initial regional deformation (D1) with the resulting development of an S 1 regional schistosity. 3(i). Development of the major isoclinal south-plunging synform (D2) in the Southern zone accompanied by axial plane sillimanite schistosity (S2). Peak metamorphism is considered to be pre- or syn-D2. The prograde iron-rich metamorphic assemblage includes fayalite, type A garnet, pyroxmangite and magnetite. (ii). On a more local scale the intense D2 event caused extensive boudinage and the development of a preferred lineation (L2) on the S2 schistosity. Within the mineralised horizons remobilisation and boudinage caused thickening and the development of higher grade ‘shoots’ subparallel to the plunge of the L2 lineation and within the pressure shadow developed about the pinchout of the core amphibolite. A similar geometry is also recognised in the Northern zone with mineralisation wrapping around the closure of an amphibolite body. 4. A sequence of widespread alteration events followed (Bodon, 1996): (i) anhydrous high temperature alteration with hedenbergite–type B garnet–quartz dominant; (ii) hydrous high temperature quartz-ilvaite-hornblende alteration of olivine, hedenbergite and pyroxmangite. This includes a silicification event associated with and overprinting the Cukadoo and Glenholme mineralisation types which produced a siliceous breccia texture and destroyed the iron-manganese precursors. The
Geology of Australian and Papua New Guinean Mineral Deposits
introduction, upgrading and redistribution of silver and base metals may also be associated with both the retrograde alteration and silicification events. Fluorite may have been introduced in the waning stages of the high temperature retrogression (S G Walters, unpublished data, 1994); and (iii) hydrous low temperature alteration with pyrosmalitegreenalite dominant. 5. A sequence of brittle-style faulting episodes including the Trepell–Hamilton system, Brolga fault zone and NEtrending structures, accompanied by low temperature chlorite alteration and the introduction of silicacarbonate-pyrite alteration zones.
ACKNOWLEDGEMENTS The author would like to thank BHP Co Ltd for permission to publish this paper and in particular the management and staff of the Cannington project for their assistance. Special thanks go to P C Muhling and T J Roberts for reviewing the paper and to G A Yeates, P A Fell and A R Veale for assistance in preparing the illustrations. Many geologists have worked on the project and are acknowledged for their contribution, in particular: S G Walters, B P Grant, G A Yeates, M A Dugmore, S Konecny, M T Roche, T A Paterson, P A Fell, M J Pascoe and K C McGuckin. S B Bodon and G Davidson of CODES Key Centre, University of Tasmania, P J Williams, R G Taylor and P Pollard of James Cook University of North Queensland, and D R Gray of Monash University are also thanked for their contributions to the current understanding of the deposit.
REFERENCES Bailey, A and Thomas, M, 1993. The Cannington deposit - its discovery, geology and evaluation, in Proceedings Carpentaria and Mount Isa Regional Development Forum, pp 59–67 (The Australasian Institute of Mining and Metallurgy: Melbourne). Beardsmore, T J, Newbery, S P and Laing, W P, 1988. The Maronan Supergroup: An inferred early volcanosedimentary rift sequence in the Mount Isa Inlier, and its implications for ensalic rifting in the Middle Proterozoic of Northwest Queensland, Precambrian Research, 40/41:487–507. Blake, D H and Stewart, A J, 1992. Stratigraphic and tectonic framework, Mount Isa Inlier, AGSO Bulletin, 243:1-11. Bodon, S B, 1995. 1994 annual Ph D research report on the Cannington Ag-Pb-Zn deposit, Mt Isa Inlier, Northwest Queensland, CODES Key Centre, University of Tasmania, Hobart (unpublished). Bodon, S B, 1996. Genetic implications of the paragenesis and rareearth element geochemistry at the Cannington Ag-Pb-Zn deposit, Mt Isa Inlier, northwest Queensland, in New Developments in Broken Hill Type Deposits, Special Publication 1 (Eds: J Pongratz and G Davidson), pp 133–144, CODES Key Centre, University of Tasmania, Hobart. French, D, Ramsden, A R and Walters, S G, 1994. Mineralogical characterisation of the southern portion (line 4800N) of the Cannington lead-zinc-silver deposit, Queensland, CSIRO Division of Exploration and Mining, restricted investigation report 253R (unpublished). Laing, W P, 1990. The Cloncurry Terrane: an allochthon of the Diamantina Orogen rafted on to the Mt Isa Orogen, with its own distinctive metallogenic signature, Abstracts of Mount Isa Inlier Geology Conference, pp 19–22, Monash University, Melbourne. Looseveld, R J H, 1989. The synchronism of crustal thickening and high T/low P metamorphism in the Mount Isa Inlier, Australia. An example, the central Soldiers Cap belt, Tectonophysics, 158:173–190.
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Mark, G, 1993. Pegmatites and partial melting at the Cannington Ag-PbZn deposit, BSc Honours thesis (unpublished), James Cook University of North Queensland, Townsville. Page, R, 1993. Geochronological results from the Eastern Fold Belt, Mount Isa Inlier, AGSO Research Newsletter, 19:4–5. Page, R and Bell, T H, 1986. Isotopic and structural responses of granite to successive deformation and metamorphism, Journal of Geology, 94:365–379. Richmond, J M, Chapman, L H and Williams P J, 1996. Two phases of garnet alteration at the Cannington Ag-Pb-Zn deposit, NW Queensland, in New Developments in Metallogenic Research: The McArthur, Mt Isa, Cloncurry Minerals Province (Eds: T Baker, J F Rotherham, J M Richmond, G Mark and P J Williams), pp 113–117; Extended Abstracts, EGRU Contribution 55, James Cook University of North Queensland, Townsville. Roche, M T, 1994, The Cannington silver-lead-zinc deposit - at feasibility, in Proceedings 1994 AusIMM Annual Conference (Ed C P Hallenstein), pp 193-197 (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Sheehan, P, 1994. The structural geology of the host rocks to Ag-Pb-Zn mineralisation at BHP’s Cannington Deposit, Eastern fold belt, NW Queensland, BSc Honours thesis (unpublished), Monash University, Melbourne. Skrzeczynski, R H, 1993. From concept to Cannington: a decade of exploration in the Eastern Succession, in Symposium on Recent Advances in the Mount Isa Block, AIG Bulletin, 13:35–38. Walters, S G and Bailey, A, in press. Geology and mineralisation at the Cannington Ag-Pb-Zn deposit - an example of Broken Hill type mineralisation in the Eastern Succession of the Mount Isa Inlier, NW Queensland, Australia, Economic Geology. Williams, P J, Chapman, L H, Richmond, J, Baker, T, Heinemann, M and Pendergast, W J, 1996. Significance of late orogenic metasomatism in the Broken Hill-type deposits of the Cloncurry district, NW Queensland, in New Developments in Broken Hill Type Deposits, Special Publication 1 (Eds: J Pongratz and G Davidson), pp 119–132, CODES Key Centre, University of Tasmania, Hobart. Wyborn, L A I, 1992. The Williams and Naraku batholiths, Mt Isa Inlier: an analogue of the Olympic Dam Granites? BMR Research Newsletter, 16:13–14.
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Adshead, N D, Voulgaris, P and Muscio, V N, 1998. Osborne copper-gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 793–800 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Osborne copper-gold deposit 1
2
2
by N D Adshead , P Voulgaris and V N Muscio INTRODUCTION The deposit, formerly known as Trough Tank, is 195 km SE of Mount Isa, Qld, at lat 22o04′S, long 140o34′E on the Boulia (SF 54–10) 1:250 000 and Toolebuc (7053) 1:100 000 scale map sheets (Fig 1). Placer Pacific Limited owns the deposit and the surrounding Exploration Permit Minerals 9624.
mineralisation has a close spatial relationship with quartzmagnetite ironstone and, like the Starra and Ernest Henry copper-gold deposits of the Eastern Fold Belt, is an important example of this increasingly significant style of mineralisation in the Mount Isa Inlier.
EXPLORATION AND DEVELOPMENT HISTORY The long established Cloncurry mining district experienced a renewed phase of exploration following the discovery of the Pegmont lead-zinc-silver deposit in 1971. A consortium of Newmont Proprietary Limited, ICI Australia Limited and Dampier Mining Company Limited explored the southern reaches of the Eastern Fold Belt of the Mount Isa Inlier seeking comparable base metal mineralisation. An Authority to Prospect was granted in 1975 and aeromagnetic and electromagnetic surveys were flown as part of this exploration effort. Several geophysical anomalies were identified, including a strong magnetic high at Trough Tank. These anomalies were investigated by geological mapping, geophysical and geochemical surveys, plus shallow percussion drilling in areas with no basement outcrop. During 1976, seven percussion holes were drilled into two large magnetic anomalies in the Trough Tank area and the best assay from intervals of base metal–poor banded ironstone was 2 m at 0.13 g/t gold and 0.023% copper. Consequently the consortium regarded Trough Tank as unfavourable for Pegmont style mineralisation and the tenement was relinquished in late 1976.
FIG 1 - Simplified geological map of the southern part of the Eastern Fold Belt, Mount Isa Inlier. Modified from Blake (1987) and Beardsmore, Newbery and Laing (1988).
The deposit is hosted by a multiply deformed and complex sequence of metamorphic, igneous and metasomatic rocks of mid Proterozoic age that is concealed beneath 20 to 40 m of Mesozoic sediment. The epigenetic and largely hypogene 1.
Geologist, Misima Mines, PO Box 5418, Cairns Qld 4870.
2.
Mine Geologist, Osborne Mines, PO Box 5170, Townsville Qld 4810.
Geology of Australian and Papua New Guinean Mineral Deposits
In 1985, Billiton Limited and CSR Limited formed a joint venture to search the region for ironstone hosted copper-gold deposits similar to the recently identified Starra orebodies, 50 km to the NNW of Trough Tank (Fig 1). In the first year of exploration at Trough Tank, 11 reverse circulation (RC) drill holes intersected magnetite-quartz ironstone units with anomalous but subeconomic concentrations of copper and gold. Airborne and ground magnetic surveys combined with induced polarisation surveys between 1985 and 1987 outlined four distinct anomalies within the Trough Tank area. The NE anomaly, which is the largest of the four, received the most attention and was subsequently shown to identify the deposit. In June 1988, Placer Pacific Exploration Limited acquired the CSR Limited Mineral Exploration Development Group and became manager of the joint venture. Between 1985 and mid 1989, a total of 36 diamond drill holes (for 3310 m) and 80 RC holes (for 9811 m) outlined an 800 m strike length of ironstone with low grade copper-gold mineralisation. Placer persevered with the drilling program and was rewarded in late 1989 when four holes drilled into the northern portion of the NE anomaly returned high grade copper-gold mineralisation, including the ‘discovery hole’ (TTHQ029) that intersected 32 m averaging 5.8% copper and 3.2 g/t gold from 98 m. Around the same time
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that it was becoming evident that the project was economically significant, Robert Osborne, the project geologist, suddenly passed away, and the NE anomaly was renamed ‘Osborne’ in recognition of his efforts. By June 1993 Placer was the sole owner of the deposit and surrounding tenements. Step out and delineation drilling were carried out between 1990 and 1993 as part of prefeasibility and feasibility studies. A total of 475 holes for 59 272 m of RC and 30 335 m of diamond core were drilled during this period and defined a total Measured and Indicated Mineral Resource of 11.2 Mt at 3.51% copper and 1.49 g/t gold. Project approval was granted in June 1994 and site construction commenced in August. Approximately 260 000 t of oxide and minor supergene copper ore were mined from the open pit between December 1994 and October 1995. The open pit was completed in February 1996 and all of the 968 000 t of hypogene, high grade ore from the pit had been treated by August 1996. The first ore was encountered in underground development in December 1995, with production from stopes beginning in April 1996. The current mining rate from the underground operation is 1.2 Mtpa and planned annual production rates are 29 000 t of copper and 37 000 oz of gold. At the beginning of 1996, Osborne had a total Measured and Indicated Mineral Resource of 11.3 Mt at 2.9% copper and 1.18 g/t gold, which included a Proved and Probable Reserve of 10.8 Mt at 2.96% copper and 1.21 g/t gold.
PREVIOUS DESCRIPTIONS Davidson (1989) and Davidson et al (1989) provided the first published accounts of the host rock and mineralisation characteristics at Trough Tank. Davidson focussed on documenting and understanding the geochemistry, setting and genesis of the ironstone hosted gold-copper mineralisation at Starra but a limited field season at Trough Tank convinced him that the two deposits are genetically related. In addition, sulphur isotope data obtained from Osborne samples collected by Davidson were included in a comparative study of mineral deposits from across the Mount Isa Inlier (Davidson and Dixon, 1992). Subsequent geological research on the deposit focussed on the stratigraphy and structure of the banded ironstone units (Williams, 1995) and on the host rock geology, alteration paragenesis, mineralisation characteristics and hydrothermal fluid geochemistry (Adshead, 1995).
REGIONAL GEOLOGY The predominantly metasedimentary and meta-igneous host rock sequence at Osborne is concealed beneath 20 to 40 m of sediment of the Mesozoic Eromanga Basin (Figs 1 and 3). The undeformed cover rocks onlap on to the mid Proterozoic Eastern Fold Belt of the Mount Isa Inlier and, although direct correlation has not been possible, the similarity of rock types and metamorphic grade indicate that the Osborne sequence is part of the Eastern Fold Belt. Regional mapping by the Bureau of Mineral Resources and Geological Survey of Queensland between 1950 and 1983 was compiled by Blake (1987) but field coverage of the Proterozoic outcrop to the north of Osborne, south of approximately 21o45′S and to the SW of the Squirrel Hills pluton, was limited (Fig 1). Parts of the area have been remapped by Beardsmore, Newbery and Laing (1988), the JCU Cloncurry Mapping Project 1991 (P J Williams and G N Phillips, unpublished data,
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1991) and Pocock (1992) but the stratigraphy of the area remains problematical. Beardsmore, Newbery and Laing (1988) suggested that basement rocks to the north of Osborne can be correlated with the Soldiers Cap Group to the NNE based on lithostratigraphy (Fig 1). Furthermore, W P Laing (unpublished data, 1990) suggested that the psammitic and ironstone-bearing host rock sequence at Osborne is similar to the Mount Norna Quartzite of the Soldiers Cap Group, but the lithostratigraphic basis for this link is tenuous and the precise stratigraphic position remains uncertain. The current understanding of the stratigraphy of the Eastern Fold Belt of the Mount Isa Inlier is succinctly summarised in Blake et al (1990). The thick sequences of mid Proterozoic sedimentary and volcanic rocks experienced regional metamorphism and polyphase deformation before 1520 Myr and were subsequently intruded by numerous plutons of the largely anorogenic Williams Batholith (Fig 1). The predominantly metasedimentary package that crops out to the north of Osborne has the strong northerly-trending structural fabric that is widely developed across the Mount Isa Inlier, and is intruded by several large plutons of the Williams Batholith (Fig 1). The maximum P-T conditions attained during regional metamorphism vary across the Eastern Fold Belt but the rocks in the south and east preserve mineralogical evidence of uppermost amphibolite facies. Copper was first located in the Eastern Fold Belt in 1867 and there have been several subsequent periods with a high level of exploration and mining activity in the region. Small copper oxide±gold deposits are common to the south and east of Cloncurry but the future production of these metals in the region will be dominated by the ironstone and/or shear zone hosted deposits of Ernest Henry, Osborne, Starra and Eloise (Fig 1).
DEPOSIT GEOLOGY HOST ROCK CHARACTERISTICS The Osborne deposit can be subdivided into western and eastern domains based on significant differences in the host rock sequence and mineralisation characteristics. The nature of the Awesome fault separating the domains has been difficult to determine, but geometrical relationships noted during underground development suggest that the youngest significant movement on the fault was at a high angle and compressional (Figs 2, 3 and 4). Feldspathic psammite±thin layers of pelite is the dominant host rock type enveloping the lodes in each domain. The occurrence of pre-metamorphic banded ironstone units and associated schist define the extent of the western domain whereas a fault-bounded body of meta-ultramafic rock only occurs above the mineralisation in the northern part of the eastern domain (Fig 2). Sheet intrusions of amphibolite and post-metamorphic pegmatite and lamprophyre occur in both domains but they are more common in the eastern domain (Figs 2 and 3).
Feldspathic psammite and pelite The host sequence is dominated by poorly differentiated, pale pink to grey, sodic plagioclase-rich feldspathic psammite with locally developed pelitic bands and stromatitic migmatite. The majority of the feldspathic psammite comprises >95% Σ(albite and/or sodic oligoclase+quartz) with a complete spectrum
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OSBORNE COPPER-GOLD DEPOSIT
FIG 3 - Cross section on local grid line 21 360 N, looking north, across the middle of the Osborne deposit illustrating the distribution of the dominant host rock types. The location of the section is on Fig 2.
FIG 2 - Simplified level plan of the Osborne deposit (1200 RL) illustrating the distribution of the dominant host rock types. Modified from A-K Appleby, M Heinemann and N D Adshead (unpublished data, 1993).
between plagioclase- and quartz-rich examples, though plagioclase-rich rocks are much more abundant. Foliated biotite is invariably present in the granular, plagioclase-quartz mosaics but rarely comprises >5% of the total rock. Granular calcite is also locally common and other minor to accessory peak metamorphic minerals include magnetite, actinolitic hornblende, sillimanite, cobaltian pyrite, microcline, apatite, rutile, titanite, zircon, monazite and tourmaline. Accessory retrograde chlorite (particularly replacing biotite) and carbonate are common, locally accompanied by accessory muscovite, biotite, magnetite, quartz, hematite and epidote. Pelitic bands are rare in the feldspathic psammite sequence and have only been observed more than 200 m above the banded ironstone units and spatially associated metasomatic assemblages. Diagnostic pelitic minerals include almandine garnet, cordierite, sillimanite and microcline, with the typical feldspathic psammite assemblage of sodic plagioclase, quartz and biotite. Stromatitic migmatite is common and geothermometric and geobarometric studies incorporating the stability of the pelite mineral assemblages and evidence of melting suggest that the peak of metamorphism at Osborne occurred at 650–700oC and ~3–7 kb (Adshead, 1995).
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 4 - Cross section on local grid line 21 360 N, looking north, across the middle of the Osborne deposit illustrating the distribution of the high grade copper-gold mineralisation. The location of the section is on Fig 2.
The source of the sodium enrichment in the plagioclase-rich host rocks at Osborne is contentious but the rare preservation of flame structures, ripple marks and graded bedding and absence of volcanic textures suggests that the majority of the sequence was metasedimentary rather than metavolcanic. The intensity of the sodium enrichment indicates that it is probably not a sedimentary feature but, at the current level of understanding, it cannot be determined if the metamorphosed sodic alteration is
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due to diagenetic and/or synsedimentary hydrothermal fluids, or to metasomatism following burial and lithification (Adshead, 1995).
Banded ironstone and associated schist Banded magnetite-quartz-apatite ironstone is a distinctive host rock type at Osborne and has a strike length of at least 1.3 km (Fig 2). It occurs as two major stratiform units that strike NW and dip at 25 to 55ο to the NE in the northern part of the deposit, whereas towards the south the dip steepens to about 60o. The upper ironstone unit is 10 to 45 m thick and is separated from the much more continuous and 8 to 15 m thick lower ironstone unit by 6 to 40 m of feldspathic psammite and sporadic peraluminous schist (Fig 3). Subparallel but much thinner and discontinuous bands of mineralogically and texturally similar ironstone locally occur above and below the upper and lower units. The stratiform ironstone units have well developed and commonly folded internal banding but there are also intervals with more massive or breccia textures. The 0.2 to 10 mm wide bands are planar to lensoid and are defined by differences in the relative proportions of magnetite, quartz and apatite. Dark grey, relatively magnetite-rich layers have 25 to 60% quartz and a greater concentration of apatite, whereas the paler grey, quartz-rich layers may contain up to 30% magnetite. Magnetite, quartz and apatite invariably occur as equant crystals sharing simple grain boundaries and 120o triple point junctions. Magnetite and quartz crystals are generally inclusion free but the former locally contain very fine grained, ovoid inclusions of chalcopyrite and pyrrhotite. Magnetite does not contain exsolved lamellae of ilmenite, spinel or hematite and electron microprobe analyses indicate that it is pure iron oxide (Adshead, 1995). Apatite is a ubiquitous primary component (<1 to 4%) whereas specular hematite has an irregular distribution throughout the ironstone units and appears to be in equilibrium with the granular magnetite and subsequent alteration. Accessory chlorite, siderite and rare poikiloblasts of pyrite also appear to form part of the peak metamorphic assemblage. Grunerite, almandine and sodic oligoclase have not been recorded in the upper and lower units but occur in a discontinuous, 2 m wide silicate-rich ironstone in the footwall of the western domain. The contacts of the banded ironstone units are commonly obscured by alteration associated with the copper-gold mineralisation but in the least altered areas the contact zones of country rock with the upper ironstone and at the base of the lower ironstone consist of several metres of interleaved magnetite-quartz and feldspathic psammite. The upper contact of the lower banded ironstone is marked by a distinctive, pale grey and strongly foliated and lineated quartz-anthophyllite schist with minor actinolite plus retrograde talc, magnetite, chlorite, hematite and dolomite. Lineated anthophyllite is also common beneath the lower ironstone unit, where it occurs with sodic plagioclase in a distinctive schist that is subparallel to the ironstone units. Typical feldspathic psammite is the dominant host rock type between the two main ironstone units but there is also a locally continuous band of peraluminous schist containing variable amounts of sodic plagioclase, quartz, biotite, muscovite, sillimanite and corundum. Davidson (1989, 1992) and Williams (1995) argued that the ironstone units at Osborne are metamorphosed banded iron
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formation, primarily based on their stratiform habit and banded texture, and on regional comparisons. The complex folding, annealed texture and amphibolite facies silicate minerals confirm that the banded ironstone units pre-date the peak of regional metamorphism but, as with the widespread sodium enrichment in the feldspathic psammite sequence, an origin by post-lithification metasomatism cannot be discounted.
Meta-intrusives Parallel sheets of tholeiitic amphibolite and a discrete podiform body of amphibolitic peridotite (meta-ultramafic rock) are distinctive igneous rock types in the Osborne host rock sequence that predate the amphibolite facies regional metamorphism (Fig 2). The metatholeiitic dykes are <9m thick, dip at 45 to 65o towards the NE and have generally sharp and discontinuous contacts (Fig 3). They comprise strongly lineated mosaics dominated by the metamorphic assemblage oligoclase-edenitic hornblende-titanite-quartz-magnetitediopside. Igneous textures were obliterated during regional metamorphism and the peak metamorphic assemblage is in turn locally overprinted by retrograde epidote, actinolite, albite, chlorite, calcite, hematite and pyrite. The 180 m long, 100 m wide and <65 m thick amphibolitic peridotite body occurs structurally above weakly mineralised silicification in the NE portion of the Osborne deposit and is completely bound by a retrograde, phlogopite-rich shear zone (Fig 2). The least retrogressed core of the body comprises variable amounts of metamorphic olivine, orthopyroxene, hercynite, magnesiohornblende, anthophyllite, chromian magnetite and chlorite. Mean concentrations of 4000 ppm chromium, 750 ppm nickel and 25 wt % MgO (Adshead, 1995) confirm that it is a metamorphosed ultramafic rock, and is significant because similar rock types are extremely uncommon in the Eastern Fold Belt. Relative age relationships between the metatholeiite and amphibolitic peridotite have not been determined but the geochemical characteristics of each rock type indicate that they are not related to a single magmatic episode (Adshead, 1995).
Post peak metamorphic intrusives Irregular pegmatite sheet intrusions and rare lamprophyre dykes post-date the amphibolite facies regional metamorphism that affected the majority of the Osborne host rocks. Three mineralogically and texturally different types of pegmatite crosscut feldspathic psammite, banded ironstone and metatholeiite, but relative age relationships between each type have not been observed and they may be variants from a single magmatic event (Figs 2 and 3). All three pegmatite types are dominated by sodic plagioclase, microcline and quartz but the ‘porphyritic syenite’ is relatively quartz-poor, distinctly porphyritic and contains blue manganapatite; the ‘alkali feldspar granite’ is non-porphyritic and biotite-poor; and the ‘pegmatite’ is very coarse grained with perthite, graphic textures and vermicular intergrowths between quartz and tourmaline (Adshead, 1995). Three lamprophyre dykes have been recorded in the SE portion of the deposit. They are <1 m wide, have very sharp and chilled contacts and dip moderately towards the ENE. These melanocratic and texturally distinctive rocks comprise nonaligned and zoned phenocrysts of phlogopite in a fine grained groundmass of altered plagioclase, phlogopite, magnetite and apatite. Secondary hydrothermal minerals
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OSBORNE COPPER-GOLD DEPOSIT
include carbonate, chlorite, rutile and hematite and the alteration appears to be related to the crystallisation of the magma rather than a subsequent hydrothermal event. Relative age relationships between the lamprophyre dykes and different pegmatite types have not been established.
STRUCTURE The host rocks at Osborne have a complex structural history but the lack of outcrop and paucity of oriented drill core preclude the construction of a geometrically and temporally constrained structural framework for the deposit. Curved inclusion trails in cordierite and almandine porphyroblasts in pelite indicate that the dominant schistosity in micaceous feldspathic psammite across the deposit is S2 rather than S1. F1 and F2 folds have not been conclusively identified but the ironstone banding is commonly contorted into tight to isoclinal folds that may be related to the earliest periods of deformation. Quartz in the banded ironstones is rarely strained suggesting that the metamorphic annealing post-dated the main period of ductile deformation. Micaceous and chloritic shears at the margin of the amphibolitic peridotite, cutting through the banded ironstone and overprinted by copper-gold mineralisation in the eastern part of the deposit, post-date the regional metamorphism but are invariably kinked around F3 crenulations. Quartz gangue in the copper-gold mineralisation also shows evidence of strain and may be at least partly coeval with D3. Syn- to post-mineralisation brittle faulting is common. Narrow, calcite-cemented breccia zones may contain minor chalcopyrite but the majority of the late, steep and variably oriented faults are barren.
MINERALISATION AND ASSOCIATED ALTERATION OREBODY DISTRIBUTION AND AGE RELATIONSHIPS The majority of the high grade copper-gold mineralisation at Osborne is focussed along the contacts of the upper banded ironstone with feldspathic psammite in the western domain, but there is also a discrete body of mineralisation in the eastern domain that is not associated with precursor ironstone (Fig 4). Both the upper and lower ironstone units contain disseminated copper-gold mineralisation along their strike length and the gross distribution of the orebodies in the western domain mirrors the shape of the ironstones (Fig 4). The prolate pod of high grade mineralisation in the eastern domain (Eastern High Grade lode or 3E orebody) occurs about 150 m to the NE of the western domain mineralisation (Fig 4). The long axis of the lode is subhorizontal and strikes NW, and thin ‘tails’ on the roughly 350 m long, 50 m wide and 25 m thick lens-shaped body suggest that it dips at 35 to 50o towards the NE (Fig 4). Common host rocks for the 3E orebody include pegmatite, feldspathic psammite, metatholeiite and strongly foliated amphibole- and biotite-bearing schist. Banded ironstone, feldspathic psammite and metatholeiite are common host rock relicts within the main orebodies suggesting that the copper-gold mineralisation was deposited after the peak of regional metamorphism. Furthermore, 40Ar39 Ar radiometric dating confirms that the age of formation of the secondary hornblende that paragenetically predates but is closely associated with the deposition of copper-gold, at 1538±2 Myr, is significantly younger than metamorphic
Geology of Australian and Papua New Guinean Mineral Deposits
actinolitic hornblende at 1595±2 Myr and biotite at 1568±3 Myr (C Perkins, unpublished data, 1994; Perkins and Wyborn, 1996). Underground exposures in the 3E orebody indicate that the copper-gold mineralisation pre-dated or was synchronous with D3 and that pegmatite dykes both predate and crosscut the mineralisation. However, the relative age of the mineralisation with respect to the different pegmatite types has not been established.
LODE MINERALOGY, PARAGENESIS AND GEOCHEMISTRY The majority of the high grade copper-gold mineralisation at Osborne is hypogene and chalcopyrite is the only copperbearing sulphide phase in the primary ore. Chalcocite was the dominant copper phase in the now mined and weakly developed supergene ore whereas the copper in the oxide ore was present as malachite, chrysocolla, azurite, atacamite and native copper. Argentiferous gold with a fineness of 850–950 is the only auriferous phase identified in the hypogene ore (Adshead, 1995). Zones of massive, coarse grained silicification (‘silica flooding’) with abundant wall rock relicts and rare fill textures host the bulk of the copper-gold mineralisation, but textural evidence indicates that the majority of the silica flooding predates the main phase of copper-gold deposition. The early quartz is temporally associated with pyritemagnetite±siderite±talc and with small amounts of chlorinebearing silicates such as ferrohornblende, biotite and ferropyrosmalite [(Fe,Mg,Mn)8Si6O15(OH,Cl)10]. The lode margins are locally delineated by secondary biotite schist, and relict clasts of early quartz and magnetite are particularly evident in sulphide rich portions of the ore. Common gangue phases coeval with the slightly later copper-gold mineralisation include quartz, calcite, chlorite, muscovite, magnetite, pyrite and/or pyrrhotite, iron-cobalt sulphides, apatite, molybdenite and tourmaline, plus bismuth sulphides and sulphosalts (Adshead, 1995). Iron oxide and sulphide phases are common in the coppergold mineralisation and have a zoned distribution across the deposit. Disseminated mineralisation throughout the banded ironstone units is associated with secondary hematite-pyritemagnetite whereas the adjacent and well mineralised silicaflooded zone contains only magnetite-pyrite. A characteristic feature of the 3E orebody is the presence of abundant pyrrhotite and pyrite-magnetite. Relative concentrations of copper and gold vary systematically across the deposit and can be related to the ambient oxygen fugacity and pH conditions during metal deposition. The more oxidised, hematite-magnetite-pyrite altered ironstone of the western domain has relatively low copper:gold ratios whereas the more reduced, pyrrhotitemagnetite±pyrite associations in the 3E orebody have higher copper:gold ratios (Adshead, 1995). This relationship is consistent with the gross paragenesis and metal ratio in other iron-rich, copper-gold hydrothermal systems of the Eastern Fold Belt, eg Eloise (copper-rich, minor gold, abundant pyrrhotite) and Starra (gold-rich, minor copper, secondary hematite). Uneconomic elements at consistently anomalous concentrations in the high grade copper-gold mineralisation include iron, cobalt, molybdenum, silver, selenium, bismuth, mercury, tellurium, tin, fluorine and chlorine (Adshead, 1995).
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Lead and zinc values are invariably below average crustal levels. Mass balance calculations indicate that altered wall rocks are commonly enriched in potassium, rubidium, sulphur, fluorine, chlorine, carbon dioxide and water. LREE are also commonly elevated in the wall rocks, whereas the higher grade copper-gold mineralisation is relatively enriched in HREE, yttrium and niobium, and has the lowest (La:Yb)N and Zr:Nb ratios of all metasomatic rock types (Adshead, 1995).
HYDROTHERMAL FLUID CHARACTERISTICS AND METAL DEPOSITION Primary fluid inclusions in the largely pre-mineralisation silica flooding either preserve a hypersaline brine or carbonic fluid. The hypersaline inclusions contain two to six daughter phases and qualitative SEM scans on opened inclusions identified simple and complex chlorides of sodium, potassium, iron, calcium, manganese and barium (Adshead, 1995). Salinity estimates based on phase volume measurements suggest that there was between 60 to 70 wt % dissolved salts in the early hydrothermal fluid and homogenisation measurements indicate trapping temperatures of >450οC. Microthermometric measurements indicate that the carbonic inclusions are dominated by carbon dioxide with minor amounts of another volatile, possibly methane. Thus these fluid inclusion data suggest that the pre-mineralisation hydrothermal fluid was hot and chemically complex and had largely separated into immiscible hypersaline brine and carbon dioxide-rich components. Copper, gold and many other metals have a high solubility as chlorocomplexes under these fluid conditions and thus the primary inclusions probably preserve the physicochemical character of the metalliferous hydrothermal fluid prior to mineralisation (Adshead, 1995).
reduction. The combined action of all these physicochemical changes in the metalliferous hydrothermal fluid produced a significant destabilisation of copper and gold chlorocomplexes and resulted in the lodes of high grade copper-gold mineralisation at Osborne.
ORE GENESIS Early investigations of the origin of the Osborne deposit concluded that the copper-gold mineralisation and host ironstone units were deposited during a single hydrothermal event, and concentrating on the textural and environmental features of the ironstone units rather than the mineralisation, inferred that the mineralisation is syngenetic-exhalative (Davidson et al, 1989; Davidson, 1989, 1992). Although a syngenetic-exhalative origin for the banded ironstone units cannot be discounted, crosscutting relationships with the metamorphic and igneous host rocks combined with the radiometric age data confirm that the copper-gold mineralisation is unequivocally epigenetic and postdates the peak of regional metamorphism. The available field and analytical data cannot conclusively differentiate between a predominantly magmatic or retrograde metamorphic origin for the metalliferous hydrothermal system. However, the hydrothermal fluid characteristics, mineralisation geochemistry and temporal relationship with pegmatite emplacement and a late, ductile deformation event suggest that both felsic magmatism and retrograde metamorphism may have been important in generating and focussing the hydrothermal fluids (Adshead, 1995).
ACKNOWLEDGEMENTS
Trails of secondary fluid inclusions in the silica flooding commonly crosscut quartz grain boundaries and locally radiate from chalcopyrite crystals. These paragenetic relationships suggest that the largely aqueous inclusions were trapped close to the main period of copper-gold mineralisation. Microthermometric measurements on these inclusions and thermodynamic studies on the composition of the associated wall rock alteration indicate that the main phase of copper-gold deposition occurred at ~300οC from fluids with a range in salinity of 20 to 37 eq wt % NaCl (Adshead, 1995).
The majority of the research work was completed as part of a PhD project by the senior author at James Cook University of North Queensland and was supported by an OPRA scholarship from the Australian Government Department of Employment Education and Training, a W C Lacy scholarship from the JCUNQ Department of Earth Sciences, and a grant from Placer Exploration Limited. The permission of Osborne Mines and Placer Pacific Limited to publish this information is acknowledged, as are the endeavours of numerous Placer geoscientists who have worked on the deposit since 1985.
The most significant physicochemical mechanisms in depositing copper and gold from their respective chlorocomplexes include a temperature decrease, a salinity decrease and a rise in fluid pH, and there is evidence that all three were important in the formation of economic mineralisation at Osborne. Homogenisation temperature data and thermodynamic calculations on wall rock alteration phases record a cooling of ≥150oC to ~300oC in the hydrothermal system preceding copper-gold deposition but there is also evidence for coeval and significant changes in fluid pH and salinity. The continuing separation of a carbon dioxide–rich phase from the high salinity fluid would have raised fluid pH, as would the localised phyllosilicate alteration of the wall rocks.
REFERENCES
The microthermometric data indicate a decrease in salinity of ~30 eq wt % NaCl between entrapment of the primary and secondary inclusions, but the limited oxygen isotope data suggest that there is little evidence for dilution by fluid mixing (Adshead, 1995). However, the large concentration of chlorine in wall rock ferropyrosmalite, ferrohornblende and biotite could account for a significant proportion of the fluid salinity
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Adshead, N D, 1995. Geology, alteration and geochemistry of the Osborne Cu-Au deposit, Cloncurry district, northwest Queensland, Australia, PhD thesis (unpublished), James Cook University of North Queensland, Townsville. Beardsmore, T J, Newbery, S P and Laing, W P, 1988. The Maronan Supergroup: an inferred early volcanosedimentary rift sequence in the Mount Isa Inlier, and its implications for ensialic rifting in the Middle-Proterozoic of northwest Queensland, Precambrian Research, 40/41:487–507. Blake, D H, 1987. Geology of the Mount Isa Inlier and Environs, Queensland and Northern Territory, Bureau of Mineral Resources Geology and Geophysics Bulletin 225. Blake, D H, Etheridge M A, Page R W, Stewart A J, Williams P R and Wyborn L A I, 1990. Mount Isa Inlier - regional geology and mineralisation, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 915–925 (The Australasian Institute of Mining and Metallurgy: Melbourne). Davidson, G J, 1989. Starra and Trough Tank: Iron-formation-hosted gold-copper deposits of northwest Queensland, Australia, PhD thesis (unpublished), University of Tasmania, Hobart.
Geology of Australian and Papua New Guinean Mineral Deposits
OSBORNE COPPER-GOLD DEPOSIT
Davidson, G J, 1992. Hydrothermal geochemistry and ore genesis of seafloor volcanogenic copper-bearing oxide ores, Economic Geology, 87:889–912. Davidson, G J and Dixon, G H, 1992. Two sulphur isotope provinces deduced from ores in the Mount Isa Eastern Succession, Australia, Mineralium Deposita, 27:30–41. Davidson, G J, Large, R R, Kary, G and Osborne, R, 1989. The deformed iron-formation-hosted Starra and Trough Tank Au-Cu mineralization: A new association from the Proterozoic eastern succession of Mount Isa, Australia, Economic Geology Monograph 6, 135–150.
Pocock, J A, 1992. Structural evolution of an area 100 km south of Cloncurry, northwest Queensland, BSc Honours thesis (unpublished), James Cook University of North Queensland, Townsville. Williams, J K, 1995. The petrography, stratigraphy and structure of the Osborne mine sequence: Evidence for the origin of the banded iron-formation-hosted Cu-Au deposits in the Soldiers Cap Group, northwest Queensland, MSc thesis (unpublished), Macquarie University, Sydney.
Perkins, C and Wyborn, L, 1996. The age of Cu-Au mineralisation, Cloncurry district, Mount Isa Inlier, as determined by 40Ar/39Ar dating, Australian Geological Survey Organisation Research Newsletter, 25:8–9.
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Parianos, J M, Morwood, N F and Cook, J, 1998. Brolga nickel-cobalt deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 801–806 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Brolga nickel-cobalt deposit 1
2
by J M Parianos , N F Morwood and J Cook INTRODUCTION The deposit is about 45 km NNW of Rockhampton, and about 12 km NNW of Yaamba, in NE Queensland. It is at AMG coordinates 245 505, and lat 23 o02′S, long 150o19′E, on the Rockhampton (SF 56–13) 1:250 000 scale and Ridgelands (8951) 1:100 000 scale map sheets (Fig 1). It is the only nickelcobalt laterite orebody that has been mined in the region.
3
December 1986. Exploration typically involved procedures developed at the QNPL Greenvale mine. Initial drilling was on a 40 m grid, using vertical reverse circulation (RC) holes (1 m samples, Wallis Aircore System). Progressive infill drilling in mineralised areas was on a 5 m grid. After initial phases of drilling and bulk sampling, a 30 000 t trial sample was extracted in 1992, for treatment at the Yabulu ammoniacal leach refinery near Townsville. Development of the mine involved construction of a rail siding stockpile area and train loading platform, a haul road to the mine, communications, water and mains power supply, offices and other facilities, including a screening plant. Full scale mining commenced in April 1993. Mining was by back-acting excavators (Caterpillar 245, 3 m3 bucket). Dump trucks of 30 and 50 t capacity hauled waste to dumps and ore to a 200 t/hr screening plant that separated out a minus 1 cm product. When used, the screening operation beneficiated ore grades by an average of 14% for nickel and 20% for cobalt. Articulated haul trucks of 20 t capacity carried beneficiated material 6.5 km to the Glen Geddes siding, and all ore was railed approximately 680 km for treatment at Yabulu. Mine grade control was primarily directed by analyses of cuttings from the 5 m spaced drill holes, using intensive survey and data collation to produce dig sheets for use by excavator operators.
FIG 1 - Regional extent of Malborough Block, and overlying laterite (modified after INAL Staff, 1975). Inset (a) is an approximate distribution plan of the three main lateritic association at Brolga.
From April 1993, mining of the deposit took about two and a half years, with closure in August 1995. Screened ore production totalled 620 000 wet t, averaging 1.59% nickel, 0.142% cobalt, 15.7% iron and 19.1% moisture.
REGIONAL GEOLOGY EXPLORATION AND MINING HISTORY Brolga was first explored in the mid 1960s by Broken Hill Proprietary Limited (BHP), and was described by their subsequent joint venture partners, International Nickel Australia Limited (INAL Staff, 1975). The BHP and INAL work involved testing the deposit with vertical rotary air blast (RAB) drill holes on a 250 foot (~80 m) spacing, mining of a bulk sample which was used for screen beneficiation testing, and laboratory extraction testing. Tenure over the deposit was acquired by Metals Exploration Qld Pty Ltd for Queensland Nickel Pty Ltd (QNPL) in
1.
Chief Geologist, QNI Limited, 1 Eagle Street, Brisbane Qld 4000.
2.
General Manger Ore Supply, QNI Resources, 1 Eagle Street, Brisbane Qld 4000.
3.
Ore Sourcing Superintendent, QNI Resources, Greenvale Street, Yabulu Townsville Qld 4818.
Geology of Australian and Papua New Guinean Mineral Deposits
Nickel-cobalt enriched laterites are found in two areas of the Rockhampton region: as a discontinuous NW striking belt, south of Marlborough; and as cover on a group of small isolated hills, immediately north of Yaamba (Fig 1). The Brolga deposit was the largest and richest of the latter group. Basement rocks to the laterites of the Rockhampton region are an unnamed part of the Marlborough Block, which is largely serpentinised peridotite and lesser amphibolite. The Block has been described as a ‘thin-skinned thrust sheet’, and interpreted as part of an ophiolite succession that was obducted westward into the region during the Permian (Murray, 1974; Murray and Cranfield, 1989; Fergusson, 1991; Fergusson, Henderson, and Leitch, 1994). Lateritisation is thought to have been ongoing in the region after the Mid-Cretaceous regional uplift, caused by continental breakup of the eastern side of the Australian Plate (Lister, Etheridge, and Symonds, 1991). Distinct episodes of deep weathering between the Oligocene and Pliocene have been identified by Grimes (1988).
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It is envisaged that climatic conditions varied widely during the Cainozoic, with ensuing changes in the rate and type of lateritisation. The current dry tropical climate is not especially conducive to laterite formation, and the laterites in the Rockhampton region are thought to be erosional remnants of an originally more extensive mantle (INAL Staff, 1975) that first developed at least as early as the Oligocene (Burger, 1989).
although it is always serpentinised and oxidised to some degree. Olivine is invariably altered to antigorite and lizardite, and only spinels (usually picrochromites) are preserved. These often occur as distinct bands reflecting original intrusive layering. The bands are often vertical and strike roughly north, but can be in any orientation. 2.
Gabbro is a minor rock type in the western and central parts of the deposit. It occurs as a single dyke, or a parallel swarm of dykes, that are subvertical, northerly trending, and up to 10 m thick. The gabbro is normally altered, with albitised plagioclase, primary pargasitic amphiboles and chlorite. Some dykes have marginal shears affecting both the dyke and the host dunite.
3.
In addition to the prevalent serpentinisation within the dunite, massive serpentinite occurs as shear zones to 4 m wide. These are often erratic in orientation, meandering over the course of metres, however their most common strike is between NW and NE. The serpentinites are almost entirely lizardite, but include occasional minor amounts of chlorite.
4.
Harzburgite is rare at Brolga, in contrast to most other peridotite-based laterite deposits in the SW Pacific. Only one single strongly serpentinised example has been found on the northeastern margin of the deposit. There is no significant laterite developed on this example, and its relationship with the dunite is unknown. Harzburgite is common however, elsewhere within the Marlborough Block.
ORE DEPOSIT FEATURES FORMATION OF NICKEL-COBALT LATERITES The basic genesis of nickel-cobalt enriched laterites is well understood (Trescases, 1973, 1986; Troly et al, 1979; Golightly, 1979), and a brief outline here will help in presenting some of the features of the Brolga deposit. Note that the term laterite is used here for the entire weathering profile, including rock types such as saprolite, ferralite (essentially the equivalent of the mottled zone) and ferricrete. Most nickel-cobalt laterites of the ‘groundwater type’ (McFarlane, 1986), essentially comprise vertical ‘profiles’, comprising four progressively more weathered rock types (Fig 2a): 1.
basement of ultramafic composition, typically harzburgitic or dunitic peridotite that has been serpentinised to some degree;
2.
saprolite that is largely isovolumetric to the underlying basement, and contains abundant remnants and pseudomorphs after original minerals;
3.
ferralite (or limonite) that is variably compacted compared to the basement and saprolite, but which is essentially in situ, although composed entirely of reprecipitated weathering minerals; and
4.
surficial ferricrete (or iron crust) that has always had some measure of reworking.
These rock types develop by groundwater solution weathering (leaching), with meteoric waters percolating downward and outward through the rock, under largely vadose conditions. The leaching affects all elements and minerals in the profile, however solubilities of different elements vary not only amongst themselves, but they vary in different parts of the profile, due to different chemical, mineralogical and physical conditions. In essence, two factors are needed to form a nickel-cobalt laterite deposit-surface exposure of ultramafic rocks, and frequent abundant rain. Ultramafic rocks are a requirement, as they are inherently high in nickel and cobalt, averaging about 0.2% nickel and 0.015% cobalt (Levinson, 1974), and they provide a suitable source for nickel enrichment. Consequently a large proportion of the world’s nickel-cobalt laterite deposits are found near the edge of oceanic plates (developed on ophiolite) in equatorial and wet tropical regions. In regions with lower and perhaps more periodic rainfall (eg Brolga), nickel-cobalt laterites still occur, but morphological features (such as a relatively high incidence of escarpment) indicate that these are relicts of laterites that probably originated in wetter climates.
BROLGA BASEMENT There are four basement rock types at Brolga: 1.
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Dunite is by far the most common basement rock type,
LATERITIC ROCKS The laterite at Brolga is divided into four main rock types, which are subdivided into 17 varieties (Table 1). Three distinctly different combinations of these varieties have been encountered to-date, defining the three lateritic associations of the Brolga deposit (Fig 2b, c, d). The lateritic rock types are, in order of progressive weathering: 1.
lower saprolite,
2.
upper saprolite,
3.
ferralite, and
4.
ferricrete.
Table 1 lists the varieties first under their basement protolith, and then under their lateritic rock type. As variably serpentinised dunite is the most common basement rock type, lateritic rock types of unknown origin are assumed to have a dunite source. There are no varieties with a harzburgite source, due to the rarity of that basement rock type. These varieties are the product of progressive weathering and many grade into each other. The exact boundaries between them are somewhat arbitrary.
LATERITIC ASSOCIATIONS The lateritic rock types and varieties listed above are normally found in one of three main associations (Fig 2b, c, d). The concept of associations is used in this case instead of profiles. This is first because profiles assume that there is subhorizontal homogeneity, and they fail to illustrate the frequent complexity of exposure; and secondly because more than one basement rock type is usually involved in any particular locality, with differing and interacting weathering products.
Geology of Australian and Papua New Guinean Mineral Deposits
BROLGA NICKEL-COBALT DEPOSIT
TABLE 1 Lateritic rock type varieties of the Brolga deposit. Dunite and Derivatives ‘Fresh’ Dunite Lower Saprolite Varieties1
Upper Saprolite Varieties
Fresh dunite has not been found, basement dunites being invariably serpentinised and weathered to some degree Dog (H,S,D) 2
Grey±pink±cream±brown±green granular fine grained rock, typically with lizardite±magnesite±silica network veining. Groundmass olivine is altered to antigorite. Disseminated to massive spinel (picrochromite) bands are common. Subvarieties from the soft association often contain 2–5 mm ‘blebs’ of smectite3. A sheared and brecciated subvariety is marginal to serpentinite shears. Due to regional alteration and deep weathering, this rock type extends to an unknown depth beneath the deposit.
Hip (H)
Green±pink and/or brown granular fine grained rock, typically with lizardite±silica±magnesite±willemseite±reevesite (Ni6Fe2(OH)16CO34H2O) network veining. Groundmass olivine is altered to antigorite and minor goethite and/or smectite. Disseminated to massive spinel bands are common. Most brown subvarieties are silicified and indurated compared to dog and biscuit saprolite.
Waxy (S)
Green or khaki, soft, friable rock, with abundant to common smectite (normally Fe-Ni rich nontronite-pimelite), subordinate to absent serpentine and goethite, common todorokite coating joint surfaces, occasional spinel, chalcedonic silica, tridymite, and magnesite.
Biscuit (H)
Brown or red-brown friable rock typically with silica boxwork veining. Goethite predominates, with residual serpentine and spinel, and occasional todorokite and magnesite.
Earthy (H,D)
Ferralite (Limonite) Varieties
Ferricrete Varieties
Brown or orange-brown soft friable rock. Goethite predominates, with some residual serpentine and spinel, and minor todorokite and occasional eskolaite.
Waxy-earthy (S)
Similar to earthy saprolite, but with abundant smectite (normally Fe-Ni rich nontronitepimelite).
Light (H)
Brown, yellow, or khaki, very soft and light rock. Composed of goethite, with variable minor amounts of todorokite, eskolaite, hematite, kaolinite, and residual quartz and spinel. Some serpentinite texture may be preserved.
Compact (H,S)
A denser variety of light ferralite. Quartz and todorokite are much less common. No serpentinite textures are preserved.
Pisolitic (H,S)
Dark brown soil-like material with disseminated pisolites (concretions of relatively crystalline goethite±hematite±spinel). This variety marks the transition between ferralite and iron crust.
Sedentary (H,S) Sedimentary (H,S)
Dark red-brown and black rock, consisting of pisolites±spinel cemented together with ferricrete (goethite-hematite) and occasionally kaolinite. Dark red-brown, black and yellow rock, typically consisting of clasts of saprolite, ferralite (ex-saprolite clasts?) and pisolites cemented with ferricrete.
Gabbro and Derivatives ‘Fresh’ Gabbro Saprolite Varieties
Found only in drill core, it is mostly composed of medium grained, granular, variably chloritised amphibole (?pargasite) and albitised plagioclase. An altered subvariety contains disseminated quartz and minor pyrite. DSAP1 (D)
Olive-green and grey medium to coarse grained granular rock, composed of varying amounts of pargasitic amphibole, chlorite and smectite-vermiculite. On more weathered examples, asbolite and kaolinite form on joint surfaces and in veins.
DSAP2 (D)
Khaki to purple soft rock, composed of varying amounts of smectite-vermiculite and kaolinite, with trace amounts of asbolite.
Smectite Shear (D)
‘Ferralite’ Variety
DSAP3 (D)
Green-black strongly foliated friable material, comprising smectite-vermiculite and chlorite,±kaolinite, asbolite and dolomite. Indistinguishable in hand specimen from the serpentinite-smectite shear below. Pale green clayey solution pipes or mottles of almost pure kaolinite±smectitevermiculite, formed within DSAP2.
Serpentinite and Derivatives Fresh Serpentinite
Serpentinite Shear (H,S,D)
Light grey colour, orange and purple tinted subvarieties occur, often relatively unweathered (can persist into host ferralite), typically with foliated sinusoidal fabric, polished shear surfaces and slickensides. Found as narrow (<4 m) zones throughout the deposit, erratic and meandering orientation to a scale of metres.
Saprolite Variety
SerpentiniteSmectite Shear (S)
Green-black strongly foliated friable material, comprising lizardite, nontronitic smectite and todorokite,±minor magnesite
1.
Varieties are listed in approximate order of increased weathering.
2.
Associations in which a particular variety is common are listed in brackets. (H) = hard association; (S) = soft association; (D) = dyke association. Note that the names assigned to varieties are field names only, and should be referred to with caution.
3.
Most mineral identifications are by J E Fittock or B Francis, QNI Resources, using powder XRD analysis.
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With increasing weathering of the dunite, formation of the hard association involves a multitude of mineral transformations and replacements. Groundmass serpentines (antigorite-lizardite) are typically modified, and then replaced by limonites. An important modification of the serpentines is the replacement of magnesium with nickel, which can reach ore grade. Partial replacement of serpentines can also involve smectite or silica. Concurrently there is network vein precipitation of serpentine (lizardite), silica (quartz, chalcedonic, opaline and tridymite), magnesite and the nickeltalc willemseite [(Mg,Ni)6(Si8O20)(OH)4] throughout the rock. The network veins change in mineralogy with increased weathering. Initially comprising lizardite±silica±willemseite, they are often progressively replaced by magnesite and then by quartz. The quartz is then removed at the saprolite-ferralite transition and only rarely persists into the ferralite. The most resistant minerals are the picrochromites, with grains often persisting to the surface, where they are removed by mechanical erosion. However, these are still affected, with grain surfaces weathering to magnetite and ultimately hematite (Kyriazis, 1994). It seems likely that some of the liberated chromium is reprecipitated within the ferralite as eskolaite (chromium hematite).
Soft association This association occurs primarily in the centre of the deposit, but also in small quantities on the eastern margin (Fig 1a). Its important features are: FIG 2 - Schematic sections of a simplified nickel-cobalt laterite profile and the three main lateritic associations at Brolga. See Table 1 and the text for details.
Hard association This is the most commonly encountered association at Brolga. Its important features are a dunite±serpentinite source, an abundance of free silica throughout the saprolite varieties (often as network or boxwork veining), and the corresponding relative rarity of smectite. The distribution of rock types in this association is typically very irregular (Fig 2b). Pinnacles and core stones of lower saprolite in upper saprolite and ferralite are commonplace. Dunite sourced dog, hip and biscuit saprolites are the most common varieties, and earthy saprolite is rare. Where present, ferralite can be over 20 m thick, and is mostly of the compact variety. Ferricrete is erratically distributed due to ongoing surface erosion. It can be more than 10 m thick, and is normally weakly cemented. Sedimentary iron crust is found as small channel fills at the top of the deposit. Serpentinite shears form distinct, often subvertical units, that can persist unweathered up into the dunite sourced ferralite. Iron-nickel rich smectites (nontronite-pimelite) are a minor phase in some hip saprolites, and are occasionally found in a relatively pure form at the transition with ferralite. Manganese oxides or asbolites, mainly todorokite [(Na,Ca,K) (Mn,Mg)6 O12.nH2O)] are most common at the transition zone, but are found throughout the profile, as traces along joints in the saprolite, and coating voids and solution pipes in the light ferralite. Commonly high cobalt grades accompany the asbolites.
804
1.
a dunite±serpentinite source;
2.
abundance of iron-nickel smectite (nontronite-pimelite);
3.
absence of free silica except as discrete zones of thick chalcedonic veins; and
4.
much higher moisture content of the soft association compared to the hard association, being about 25% as opposed to about 19%. Note that comparable New Caledonian saprolite contains around 30% moisture.
Distribution of rock types within this association is much more regular or homogenous than in the hard association (Fig 2c). Dog saprolite (often bearing smectite), waxy saprolite, and waxy-earthy saprolite are all common These are overlain by ferralite, again mainly compact, which can be more than 10 m thick. Like the hard association, replacement of serpentine by ironnickel smectite is found throughout the saprolite and reaches maximum concentrations at the contact or transition with the ferralite. However, smectite is invariably much more common than in the hard association, and the ‘transition zone’ is much thicker. Abundant manganese oxides are associated with the smectite in the transition zone, often forming coatings on joints and shear surfaces, which may be related to compaction. Unlike the hard association, serpentinite shears are not resistant to weathering, being replaced by iron-nickel smectite. Thick zones of opaline-chalcedonic silica veining are found within the lower saprolites, including green-orange ‘moss agates’ containing asbolite dendrite growths. These often persist into the ferralite as degraded and compacted silica breccias. Gabbroic dykes occur in some parts of the soft association, but do not play an integral role in the definition of this association, and are not discussed in detail here. However, these dykes have distinct differences in mineralogy and setting when compared to the dyke association.
Geology of Australian and Papua New Guinean Mineral Deposits
BROLGA NICKEL-COBALT DEPOSIT
Dyke association This association is centred on a NNW striking gabbro dyke or dykes through the western side of the deposit (Fig 1a). With three basement rock types it is a complex system that is described only briefly here. Its important features are: 1.
a gabbro + serpentinite + dunite origin;
2.
restriction of quartz to dunite sourced saprolite and the dunite-serpentinite contact, ie exclusive of the gabbro dyke;
3.
the relatively unweathered nature serpentinite shears to the gabbro;
4.
an essentially vertical or subvertical structure of the association (Fig 2d); and
5.
the abundance of aluminium-smectite (montmorillonite), vermiculite and kaolinite in the gabbro-sourced saprolites.
of
marginal
The gabbro forms a distinctive series of weathering products, with residual amphibole and plagioclase being progressively replaced by smectite (montmorillonite-nontronite) and vermiculite. More advanced weathering commences on joint planes with kaolinite and asbolite, and culminates with massive kaolinite replacement in mottled zones. Although ferricrete has never been directly related to the gabbro, nearby sedentary ferricrete (capping dunite-sourced laterite) is unusual in containing kaolinite in its matrix. The dunite host has mostly weathered to an indurated dog saprolite. This often grades into the marginal serpentinite shears, but in some areas develops a thin contact layer of earthy saprolite. The serpentinite shears are very resistant to weathering, and remain almost totally unweathered to the surface. There are differences between the gabbro of the dyke association and the gabbro seen in parts of the soft association. The gabbro in the latter is characterised by high magnesium chlorite (clinochlore) rather than vermiculite. Where present, the marginal serpentinite shears to these soft association gabbros have been at least partially weathered to smectite.
ORE GENESIS Lateritisation is still active in the region, albeit at a slow rate. Evidence for this is plentiful at the Brolga deposit, and includes the ferruginisation of small channel fills on the surface of the laterite, and the presence of laterite mineral concentrations (asbolites and eskolaite) around live tree roots. More circumstantial is the mineralogy of the entire deposit, which is characteristic of a relatively dry climate, compared to other wet tropical examples in the SW Pacific. The presence of quartz and smectite within laterites has often been cited as due to relatively dry climates and/or poor ground water drainage (eg Trescases, 1973, 1986; Golightly, 1979; Burger, 1989). A relatively dry climate appears to be consistent with the mineralogy of all of the three associations at Brolga, particularly considering the current dry tropical environment. However, smectite is also often associated with poorly drained areas (eg basal shear/plain type laterites) in relatively wet tropical New Caledonia (Troly et al, 1979; Trescases, 1986), and relatively poor drainage is taken as the reason for the existence of hard (silica) versus soft (smectite) associations at
Geology of Australian and Papua New Guinean Mineral Deposits
Brolga. A combination of relatively poor drainage and radically different basement is thought to be the reason for the different mineralogy of the dyke association. In the hard association, vadose water flow is interpreted to be relatively unhindered, although much lower than in wet tropical laterites. The major fluid pathways are the network veining discussed above. In the soft and dyke associations, vadose water flow is much more restricted, and the locally entrapped water favours the formation of hydrous smectites. This is supported by a much higher water content in the soft association compared to the hard association. The reason for the initial formation of the soft association at Brolga is not clear. Certainly there is no consistent explanation in terms of current topography or the position of the water table. However, it should be noted that the swelling smectites discourage water flow, and that once established in laterite, the soft association may be difficult to remove by leaching. In the dyke association, vadose water flow is suspected to have been further restricted by the subvertical, unweathered and relatively impermeable boundary serpentinite shears. The formation of vermiculite and kaolinite in the gabbro-sourced saprolites is easy to envisage in terms of the original high aluminium content of the gabbro. In contrast, the gabbro dykes that occur within parts of the soft association do not have impermeable boundary shears, and the higher magnesium clinochlore was probably partially sourced from the surrounding dunite.
MINE GEOLOGICAL FACTORS In the process of mining at Brolga each of the three associations required the use of a special mining approach because of their geological characteristics.
Hard association Saprolite from the hard association provided almost all of the ore mined at Brolga, and one of its most significant characteristics was its relatively dry nature (19% moisture compared with New Caledonian saprolites at about 30%). The relative dryness of the ore, coupled with the presence of barren silica boxwork in the upper saprolite, made it practical to screen and beneficiate the material. Screening was not always done for ore that included a high proportion of hip lower saprolite, as boxworks were rare. Limited visual control in mining was possible in areas that contained a green subvariety of hip saprolite, which was almost always of high nickel grade. Visual control was also used to exclude nontronitic smectite ‘pods’ from ore, as recovery of nickel from this material is poor in the QNPL Yabulu refinery. Although the relative dryness of the hard association enabled beneficiation by screening, screening also tended to produce abundant dust, which was particularly of concern as chrysotile asbestos occasionally occurs in the saprolite network veining. Dust monitoring programs were thus necessary, and a dust suppression system was developed by the drilling contractor. The dusty nature of the hard association is also probably responsible for a bias in the cobalt content of the BHP samples from RAB holes, compared to the QNPL RC samples. Excessive winnowing of asbolite fines by the RAB technique is suspected.
805
J M PARIANOS, N F MORWOOD and J COOK
Importantly, the erratic distribution of rock types in the hard association is also often accompanied by erratic nickel mineralisation. Despite drilling to a 5 m grid (New Caledonian mines often only drill to a 25 m spacing), high dilution of nickel grades during mining was frequent.
Soft association Relatively little ore was produced from the soft association due to the prevalence of nontronitic smectites as a major nickel bearing phase. Ore produced from the soft association was high in cobalt compared with that from the hard association (~0.21% versus ~0.13%), and was also about 6% wetter. Compared to the hard association, nickel mineralisation was much more consistent in the portions of the soft association mined, and ore grade dilution consequently much lower.
Grimes, K G, 1988. Cainozoic geology, Southeast Queensland, in Field Excursions Handbook for the Ninth Australian Geological Convention (Ed: L H Hamilton), pp 53–80 (Geological Society of Australia, Queensland Division: Brisbane). INAL Staff, 1975. Nickeliferous laterite deposits of the Rockhampton area, Q, in Economic Geology of Australia and Papua New Guinea, Volume 1 Metals (Ed: C L Knight), pp 1001–1006 (The Australasian Institute of Mining and Metallurgy: Melbourne). Kyriazis, N, 1994. Nickel laterite profiles in the Marlborough Region, central eastern Queensland, BSc Honours thesis (unpublished), University of Queensland, Brisbane. Levinson, A A, 1974. Introduction to Exploration Geochemistry (Applied Publishing: Calgary).
Dyke association This association provided very little ore. Mineralisation was closely associated with the marginal earthy saprolite and smectite shears. This typically subvertical assemblage was also susceptible to nonrepresentative down-dip drilling effects.
ACKNOWLEDGEMENTS This paper was published with the permission of the QNI Limited. M Stenning, W Newton, J Holloway, S Holloway, T Wyland and T Hick contributed to technical development, and C J Rivers, S Black and S Rogan extended valuable support in manuscript preparation and criticism.
REFERENCES Burger, P A, 1989. Ni/Co laterites, Marlborough, central Queensland, in 1989 Field Conference, Rockhampton Region (Ed: W G Whitaker), pp 79–84 (Geological Society of Australia Inc, Queensland Division: Brisbane). Fergusson, C L, 1991. Thin-skinned thrusting in the northern New England Orogen, central Queensland, Australia, Tectonics, 10:797–806. Fergusson, C L, Henderson, R A and Leitch, E C, 1994. Tectonics of the New England Fold Belt in the Rockhampton Gladstone Region, central Queensland, in 1994 Field Conference, Rockhampton Region (Eds: R J Holcombe, C J Stephens, C R Fielding), pp 1–16 (Geological Society of Australia, Queensland Division: Brisbane).
806
Golightly, J P, 1979. Nickeliferous laterites: a general description, in Proceedings of the International Laterite Symposium, New Orleans, Louisiana (Eds: D J I Evans, R S Shoemaker and H Veltman), pp 3–23 (Society of Mining Engineers of the American Institute of Mining, Metallurgical, and Petroleum Engineer: New York).
Lister, G S, Etheridge, M A and Symonds, P A, 1991. Detachment models for the formation of passive continental margins, Tectonics, 10(5):1083–1064. McFarlane, M J, 1986. Geomorphological analysis of laterites and its role in prospecting, Geological Society of India, Memoir, 120:III. Murray, C G, 1974. Alpine-type ultramafics in the northern part of the Tasman Geosyncline - Possible remnants on Palaeozoic ocean floor, in The Tasman Geosyncline - A Symposium (Eds: A K Denmead, G W Tweedale and A F Wilson), pp 161–181 (Geological Society of Australia, Queensland Division: Brisbane). Murray, C G and Cranfield, L C, 1989. Geology of the Rockhampton Region, in 1989 Field Conference, Rockhampton Region (Ed: W G Whitaker), pp 1–19 (Geological Society of Australia, Queensland Division: Brisbane). Trescases, J J, 1973. Weathering and geochemical behaviour of the elements of ultramafic rocks in New Caledonia, Bureau of Mineral Resources Geology and Geophysics Bulletin, 141:149–161. Trescases, J J, 1986. Nickeliferous laterites: A review on the contributions of the last ten years, Geological Society of India, Memoir, 120:V. Troly, G, Esterle, M, Pelletier, B G and Reibell, W, 1979. Nickel deposits in New Caledonia - some factors influencing their formation, in Proceedings of the International Laterite Symposium, New Orleans, Louisiana (Eds: D J I Evans, R S Shoemaker and H Veltman), pp 85–119 (Society of Mining Engineers of the American Institute of Mining, Metallurgical, and Petroleum Engineers: New York).
Geology of Australian and Papua New Guinean Mineral Deposits
Rheinberger, G M, Hallenstein, C and Stegman, C L, 1998. Westmoreland uranium deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 807–814 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Westmoreland uranium deposits by G M Rheinberger1, C Hallenstein2 and C L Stegman3 INTRODUCTION TABLE 1 Inferred Resources, Westmoreland uranium deposits at 1997.
The deposits are approximately 400 km NNW of Mount Isa, Queensland (Fig 1) in the McArthur Basin and centred at AMG coordinates 194 000 m E, 8 063 000 m N or lat 17o30′S, long 138ο05′E on the Westmoreland (SE 54–5) 1:250 000 scale map sheet. The project is owned by Rio Tinto Pty Limited. Inferred Resources for the three main deposits are detailed in Table 1.
Deposit
Ore (’000 t)
Redtree
EXPLORATION HISTORY
1.26
12 600
5400
0.98
5300
Huarabagoo Total
Uranium was discovered at Westmoreland in 1956 by Mount Isa Mines Limited (MIM), when following up airborne radiometric anomalies detected by a Bureau of Mineral
2.
Manager, Queensland Mines Pty Limited, GPO Box 2142, Darwin NT 0801.
3.
Principal Geologist, Peak Gold Mines, PO Box 328, Cobar NSW 2835.
1.69
3000
1.20
20 900
Resources survey. Initial drilling failed to intersect significant mineralisation, however three mining leases were pegged at Redtree to cover outcropping mineralisation on behalf of a 50:50 MIM-Consolidated Zinc Pty Limited joint venture in 1959. In 1967 Queensland Mines Ltd (QML) embarked on an extensive drilling campaign that resulted in the discovery of significant flat lying uranium mineralisation extending beyond
Lawn Hill Platform
Sediment
N
QLD
NT
Sediment South Nicholson Basin Fickling Group
Tawallah Group
NT
South Nicholson Group
N E dy est e mo on rel e an d
e
on e
Murphy Inlier Nicholson Granite Cliffdale Volcanics Murphy Metamorphics
QLD Mount Isa
Uranium occurrence 20 km
dy
McArthur Group
0
1800 17 400
R ed tre e
McArthur Basin
Early Protero oic
Middle Protero oic Phanero oic
Principal Geologist, Rio Tinto Exploration Pty Limited, 2 Kilroe Street, Milton Qld 4064.
Contained U3O8 ( t)
10 200
Junnagunna
1.
Grade U3O8 (kg/t)
-17º30'S
Bas r rthu McA
in
Ca lve rt
Area of Fig.2 fau lt
r Inlie
South Nicholson Basin
138°30’E
138°E
137°30’E
phy Mur
FIG 1 - Locality and regional geology map for the Westmoreland uranium deposits, after Ahmad and Wygralak (1989).
Geology of Australian and Papua New Guinean Mineral Deposits
807
G M RHEINBERGER, C HALLENSTEIN and C L STEGMAN
REGIONAL GEOLOGY
but essentially contiguous with mineralisation known at the Redtree leases. Additional lenses of vertically disposed higher grade mineralisation associated with the northeasterly trending Redtree dolerite dyke zone were also outlined at Redtree and Huarabagoo. Following the discovery of the Nabarlek deposit in 1971, QML ceased exploration at Westmoreland to concentrate their efforts in the Alligator Rivers area of the NT.
The deposits are within the Tawallah Group, a sequence of Middle Proterozoic sedimentary and volcanic rocks at the extreme southeastern margin of the McArthur Basin, which flanks the northern margin of the Murphy Inlier. This inlier separates the McArthur Basin from the South Nicholson Basin of the Mount Isa Block to the south (Fig 1).
In 1975, Mines Administration Pty Limited (Minad) and Urangesellschaft entered into a joint venture with QML. Urangesellschaft, as operator of the joint venture, undertook appreciable diamond drilling along the strike of the Redtree dyke zone which resulted in the discovery of significant mineralisation at Junnagunna under a thin cover of Seigal Volcanics. Further details of exploration between 1959 and 1981 are contained in Fuchs and Schindlmayr (1981).
The Murphy Metamorphics, Cliffdale Volcanics and Nicholson Granite Complex constitute the Murphy Inlier (Ahmad and Wygralak, 1989). The Murphy Metamorphics are the oldest, and comprise isoclinally folded greenschist facies metasediment, typically quartz-feldspar-mica schist and gneiss. This unit forms the core of the Murphy Tectonic Ridge, but is only exposed on the NT side of the ridge along the southern margin of the Inlier.
In 1982, Omega Mines Limited entered into the joint venture and completed a program of drilling and reassay of core for gold at Huarabagoo. Results confirmed some erratic high gold values to a maximum of 85.8 g/t but potential was limited to a very restricted gold resource.
Unconformably overlying the Murphy Metamorphics are the Cliffdale Volcanics, a series of felsic volcanic and volcaniclastic rocks. These are intruded by granites or adamellites of the Nicholson Granite Complex, which, with the Cliffdale Volcanics, form the exposed portion of the Murphy Tectonic Ridge in the Westmoreland area. A detailed description of units within the Nicholson Granite Complex and Cliffdale Volcanics is provided by Sweet, Mock and Mitchell (1981) and Ahmad and Wygralak (1989).
CRA Limited entered the joint venture in 1990 and completed detailed metallurgical testing on Redtree, Huarabagoo and Junnagunna, infill drilling at Junnagunna and Huarabagoo and detailed resource estimates. The mineralisation was demonstrated to be readily amenable to acid leaching with low acid consumption and high uranium recoveries. CRA (now Rio Tinto Pty Limited) acquired full ownership of the deposits in 1997.
The Westmoreland Conglomerate unconformably overlies the igneous and metamorphic rocks of the Murphy Inlier. It comprises up to 1800 m of locally derived fluvial arkosic conglomerate and quartz arenite and is the oldest unit within the Tawallah Group, of age 1700 to 1710 Myr (Pietsch et al, 1994). This unit generally has a shallow dip and represents braided river and alluvial fan deposits. Four subunits of the Westmoreland Conglomerate are recognised (Fig 2) and are briefly described below.
Long Pocket Black Hills Sue
5
Sediment
fdal
e fa
Pts Seigal Volcanics Ptd, dolerite dykes
Middle Proterozoic
HUARABAGOO
Redtree dyke zone
Alluvium
Outcamp
Clif
TN
ult
Ptw4 Ptw3 Ptw2 Ptw1
Early Proterozoic
REDTREE 8 060 000 m N
Nam a lan
M I
gi f ault
Westmoreland Conglomerate
JUNNAGUNNA
Mesozoic Quaternary
MA B
200 000 m E
190 000 m E
The three main deposits Redtree, Huarabagoo and Junnagunna have been tested by 699 percussion and reverse circulation holes totalling 32 396 m and 644 diamond drill holes totalling 54 292 m. A total of approximately 857 holes for 66 296 m have been drilled at regional prospects largely within the Westmoreland Conglomerate.
Cliffdale Volcanics
Uranium deposit Fault
Moogooma
Fold 0
5 km
FIG 2 - Geological plan of the Westmoreland uranium deposits.
808
Geology of Australian and Papua New Guinean Mineral Deposits
WESTMORELAND URANIUM DEPOSITS
REDTREE v v
HUARABAGOO
LONG POCKET
v v v v
v v
v
v v v v v v
v v
v v
v
v
v
v
v
Ptw4
v
80-90 m
v v
v
Geology of Australian and Papua New Guinean Mineral Deposits
v v v
v
at the reverse-faulted contact between Cliffdale Volcanics and Westmoreland Conglomerate;
v
v
3.
Pts v v
v
in shear zones in the Cliffdale Volcanics near the Westmoreland Conglomerate unconformity;
max 600 m
JUNNAGUNNA
v
2.
REDTREE DYKE ZONE
v
associated with faults and fractures in Murphy Metamorphics;
SE
NW
v
1.
Mineralisation in the principal deposits is present as horizontal, vertical or hybrid styles (Fig 3).
v
Uranium mineralisation has been recognised in the Westmoreland region in numerous structural and stratigraphic positions, specifically:
MINERALISATION STYLES
v
STRUCTURAL AND STRATIGRAPHIC CONTROLS
The principal uranium deposits at Redtree, Huarabagoo and Junnagunna are along the NE-trending Redtree dyke zone. Other significant mineralisation is known from the Long Pocket area approximately 8 km east of Junnagunna. Minor mineralisation is also known in the Moogooma area, 5 km SW of Redtree (Fig 2).
v
ORE DEPOSIT FEATURES
in shear zones within the Seigal Volcanics.
The main Westmoreland deposits occur within the Ptw4 unit in association with mafic dykes, as types 6 and 7. The deposits represent thicker and higher grade concentrations of trace uranium mineralisation than is regionally common beneath the Seigal Volcanics–Westmoreland Conglomerate contact and along the flanks of the Redtree dyke zone. Mineralisation in other settings is only present in trace amounts.
v v
The Proterozoic sequence has undergone gentle flexuring and fault reactivation. Minor Cambrian and Cretaceous marine transgressions deposited thin veneers of sediment. Soil, sand and ferruginous detritus form Tertiary and Quaternary cover.
8.
v
There are many NW-trending joints orthogonal to the Redtree dyke zone within the Westmoreland Conglomerate and Seigal Volcanics. The joints are recessive fracture or joint zones within the sandstone and upstanding zones of silicification and quartz veining within the basalt. The sandstones along strike are only weakly quartz veined. Virtually all the outcrop of the Seigal Volcanics in the Westmoreland area contains zones of silicification and quartz veining.
in association with mafic dykes and sills; and
v
Numerous NE-trending fractures crosscut the Westmoreland Conglomerate. Some are filled by dolerite dykes which are considered cogenetic with the Seigal Volcanics. Of most economic significance is the Redtree dyke zone, which persists for 15 km as a complex series of en echelon dykes. Individual dykes are generally less than 20 m thick and 1000 m long.
7.
v
Conformably overlying the Westmoreland Conglomerate are the Seigal Volcanics. They consist of generally massive but locally amygdaloidal tholeiitic basaltic lava with minor siltstone and sandstone interbeds. A thin shale band to 5 m thick is commonly present at the base, marking the hiatus between conglomerate deposition and volcanic activity.
in Ptw4 especially in close proximity to the overlying Seigal Volcanics;
v
The Ptw4 unit is dominantly a porous, coarse grained quartz sandstone and is crossbedded and conglomeratic in part. It is brown coloured in outcrop and white to pale grey when fresh. Within the deposit area this unit is approximately 80 m thick with a discontinuous tuffaceous fine-grained laminated siltstone in the lower portion. This unit contains the bulk of the identified uranium mineralisation at Westmoreland.
6.
v
The Ptw3 unit conformably overlies Ptw2 or is reverse faulted over the Cliffdale Volcanics. It consists of coarse polymictic conglomerate and lesser pebbly sandstone. Clasts of Cliffdale Volcanics are common and the unit is 200–500 m thick.
within Ptw3 conglomerate about 50 m above its base; at the contact between Ptw2 and Ptw3;
v
The Ptw2 unit consists of 200–500 m of an upward fining sequence of coarse to medium grained ferruginous sandstone, characterised by its recessive outcrop.
4. 5.
v v
The Ptw1 unit comprises a basal volcanic derived conglomerate-breccia which grades up through pebbly quartz sandstone into an upper orthoquartzite. The unit is 60–240 m thick and is sourced almost exclusively from the underlying Cliffdale Volcanics.
Ptw3
Ptd
Horizontal mineralisation Vertical mineralisation Hybrid mineralisation
FIG 3. - Schematic cross section, looking NE, showing mineralisation geometry, after Schindlmayr and Beerbaum (1984).
Horizontal style mineralisation is relatively extensive and sheet like, up to 20 m (generally 5–10 m) thick, within the uppermost portion of unit Ptw4 and close to the Seigal Volcanics contact. This style of mineralisation flanks the NEtrending Redtree dyke and is generally best developed immediately adjacent to and on one side of the dyke only. Vertical style mineralisation forms subvertically dipping, relatively irregular lenses to 30 m (generally 10–20 m) thick, hosted by Ptw4 sandstone, although some mineralisation extends into the dolerite dykes. These lenses are adjacent to the Redtree dykes and their geometry closely mimics that of the dyke–joint system. Hybrid mineralisation is developed in the overlap zone between the horizontal and vertical styles of mineralisation and is, in detail, a combination of both styles. The overlap zone can be up to 50 m (generally 20–25 m) thick.
809
G M RHEINBERGER, C HALLENSTEIN and C L STEGMAN
GRADE CONTINUITY Grade continuity varies depending on the style of the mineralisation. The vertical style mineralisation is characterised by relatively sharp grade boundaries with uranium grades dropping off rapidly at the edges of the mineralised lenses. The flat lying mineralisation, in contrast, has diffuse grade boundaries but appears to be more laterally continuous. The hybrid mineralisation is typically intermediate between these two end members.
The deposit is on the flank of a ridge which essentially represents the dip slope of an upper Ptw4 sandstone outcrop. Projection of the shallow north-dipping Seigal Volcanic contact back up to the Redtree deposit area suggests that the basalt contact would have been less than 20 m vertically above the current land surface.
U O3 0 2 0 5
The uranium grade distribution characteristics are interpreted to reflect the strong structural control of the vertical style mineralisation and a progressively lessening structural control of the mineralisation at increasing distances from the dyke zone. The more diffuse nature of the mineralisation in the flat lying style reflects a more dominant sedimentological (porosity) control of the distribution of the uranium.
0 5
0
0 2 0
8500 m N
2 0 R
D 8000 m N
ORE MINERALS The uranium mineralisation in the upper weathered parts of the mineralised systems mainly occurs as uraninite (UO2,UO3) which provides 15–70% of the uranium minerals, torbernite [Cu(UO2)2(PO4)2.8–12H20] as 1–40% and carnotite [K2(UO2)2(VO4)2.3H2O] as 10–30%. There are also traces of autunite [Ca(UO2)2(PO4)2.10–12H2O], bassetite [Fe(UO2)2 (PO4)2.8H2O], ningyoite [(Ce,Ca,U)2(PO4)2.1–2H2O] and coffinite [U(SiO4)1-X(OH)4X]. The ore minerals either coat fractures or are interstitial to sand grains or pebbles. In the deeper and unweathered portions of the deposits, adjacent to the Redtree dykes and beneath the Seigal Volcanics contact, the uranium occurs as uraninite which provides 30–50% of the uranium minerals, autunite as 5–25%, ningyoite as 20–40%, bassetite as 3–5% and coffinite as 2–20%. As in the weathered zone, the uranium minerals are either interstitial to sand grains or pebbles, or coat fractures. Minor brannerite [(U,Ca,Ce)(Ti,Fe)2O6] is occasionally present.
ALTERATION The uranium mineralisation in the immediate vicinity of the Redtree dyke zone at Huarabagoo and Redtree appears to be associated with a quartz-sericite±kaolinite alteration assemblage. The mineralised sandstone is weakly silicified and bleached, although some of this alteration has been interpreted to be contact metamorphism associated with the emplacement of the dyke. Hematite is a local component. Within the dyke the uranium mineralisation is associated with hematite-quartz veins and hematite-quartz alteration of the dolerite wall rock. Peripheral uranium mineralisation, eg the flat lying mineralisation at Redtree and Junnagunna, is associated with a chlorite–minor hematite alteration gangue. The hematite associated with the uranium mineralisation in the upper weathered portions of the deposits is considered to be partly derived from the weathering and oxidation of primary chlorite but some of the hematite may be primary.
REDTREE The deposit consists of horizontal mineralisation in the Jack, Garee and Langi lenses and vertical mineralisation in the Namalangi lens. The deposit is centred on the southwestern end of the Redtree dyke zone immediately north of its intersection with the NW-trending Namalangi fault (Fig 4).
810
7500 m N
N
0
11 000 m E
10 000 m E
TN
500 metres
Nam
ala
ngi
fau
7000 m N
lt
FIG 4 - Grade x thickness plan for the Redtree deposit.
The Redtree dyke zone comprises four or five individual, en echelon, steep to subvertical NW dipping dykes. However, only two dykes are normally present in any one section and these are each 10–20 m thick and separated by up to 50 m of sandstone. The dyke zone is marked at the surface by a steep sided canyon some 20–30 m deep as the dolerite dyke material is preferentially weathered relative to the flanking sandstone. The dykes are strongly chlorite-calcite altered at their margins whereas the Westmoreland Conglomerate is generally chloritised and locally silicified adjacent to the dykes. Drill hole data reveal a 5–10 m vertical displacement of the Ptw3Ptw4 contact across the dyke zone, as east block down. Vertical style uranium mineralisation at the Namalangi lens is present in the dyke zone, particularly within the sandstone wedge isolated by the two dykes. It is best developed at the southern and northern ends of the deposit and remains open in both directions. The sandstone contains approximately 75% of the vertical mineralisation and the dykes 25%. The mineralisation has been relatively sparsely drilled and continuity of mineralisation is difficult to assess. Extensive flat lying mineralisation in the Langi and Jack lenses is entirely hosted by Ptw4 sandstone and is developed on the NW side of the Redtree dyke zone. This mineralisation forms an extensive sheet, 0–10 m below surface, of mineralisation 0.5–15 m thick and up to 500 m wide. The
Geology of Australian and Papua New Guinean Mineral Deposits
WESTMORELAND URANIUM DEPOSITS
mineralisation thickens appreciably near the dyke zone where it is 30–40 m thick within the hybrid zone. Grades are relatively uniform at about 0.1% U3O8 with locally higher values to 1.5%.
15 000 m N
A second, NE-trending elongate zone up to 150 m wide, of largely hybrid style uranium mineralisation in the Garee lens, is present in Ptw4 sandstone on the eastern side of the Redtree dyke zone. This mineralisation is 5–30 m below surface, up to 30 m thick adjacent to the dyke zone and rapidly thins to the east. No significant flat-lying uranium mineralisation is known on the eastern side of the dyke zone at Redtree.
lt
Cliffdale fau
Rare and erratic gold mineralisation has been noted at Redtree, however insufficient data are available to draw firm conclusions about its distribution.
TN 14 000 m N
HUARABAGOO U
The deposit is approximately 3 km NE of Redtree along the Redtree dyke zone and straddles the Seigal Volcanics–Westmoreland Conglomerate contact (Fig 2). The mineralisation outcrops at its southern end and is concealed to the north under a thin veneer of sandy alluvium. At the northern extent of the deposit the cover is only 10 m thick and comprises 2–5 m of sand and 5–8 m of weathered basalt.
Detailed drilling in the central part of the deposit has revealed a very complex dyke geometry comprising multiple vertical and horizontal branches that link two of the principal dykes in their overlap zone. North and south of this area the dykes appear to have a more simple, essentially subvertical form, although this may merely reflect the sparse drill hole density. Uranium mineralisation closely mimics the dyke geometry with discrete lenses associated with individual branches of the dyke. There is, however, no relationship between dyke thickness and mineralisation thickness. In excess of 75% of the mineralisation is within the flanking Ptw4 sandstone, with the remainder in the dykes. Individual lenses are 0.5 to 20 m thick, 100 to 500 m long and extend from surface to a depth of approximately 80 m. The mineralisation rarely extends into the underlying Ptw3 conglomerate.
0.3-0.5 m% 0.5-0.8 m% 0.8-1.0 m%
Drill hole
0
Redtree dyke zone
13 000 m N
11 000 m E
>1.0 m% 10 000 m E
The Huarabagoo deposit comprises a 2 km long zone of exclusively vertical style mineralisation disposed about a structurally complex area in the Redtree dyke zone. This structurally complex area appears to represent the overlap zone between as many as six staggered NE to east trending dyke segments each up to 1 km long.
3O 0.1-0.3 m%
500 metres
FIG 5 - Grade x thickness plan for the Junnagunna deposit.
Seigal Volcanics. The trace of the Cliffdale fault is marked by ridges of silicified and quartz veined basalt of the Seigal Volcanics. The Redtree dyke zone is poorly understood in the Junnagunna area due to a paucity of inclined drill holes across the dyke zone. The approximate surface trace is marked by linear drainage patterns subsidiary to Blackfella Creek. Extensive flat lying mineralisation entirely hosted by Ptw4 sandstone is developed on either side of the Redtree dyke. This mineralisation forms an extensive sheet, 20–30 m below surface, of mineralisation 0.5–10 m thick immediately beneath the Seigal Volcanics contact. Less than 1% of the mineralisation occurs within the basalt.
Narrow and erratic zones of relatively high grade gold mineralisation have been detected within the main uranium mineralisation eg 0.5 m at 85.8 g/t. Petrological examination indicates that the gold is present as free grains of diameter 40–100 µm. From the limited information available, gold appears to be best developed within Ptw4 sandstone immediately flanking the Redtree dyke within the central fracture-joint zones on which the broader uranium mineralised zones are centred. However, limited statistical analysis suggests that the gold and uranium are not directly related in detail (Russell, 1995).
Extensive hybrid style mineralisation like that at Redtree is lacking at this deposit, however the flat lying mineralisation again extends further to the NW (700 m) than to the NE (300 m) of the dyke zone and is open ended to the north and south.
JUNNAGUNNA
LONG POCKET
The deposit is centred on the northeastern end of the Redtree dyke zone and is largely south of its intersection with the NWtrending Cliffdale fault (Fig 5). The deposit is completely blind and obscured by up to 30 m of cover which comprises 3–10 m of alluvial sand over 5–20 m of weathered and fresh basalt of the
The deposits include the Outcamp, Sue and Black Hills deposits, which are approximately 8 km west of the Junnagunna deposit within the Long Pocket syncline, a gentle west-plunging downwarp cored by Seigal Volcanics (Fig 2).
Geology of Australian and Papua New Guinean Mineral Deposits
A parallel structure 400 m west of the Redtree dyke zone is associated with higher grade and greater thicknesses of mineralisation. The Seigal Volcanics contact has been displaced 10 m, grid west block down, by this fault. Drilling has failed to locate any dyke material in the fault zone. The mineralisation has gradational boundaries for thickness and grade in all directions.
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G M RHEINBERGER, C HALLENSTEIN and C L STEGMAN
The Outcamp and Sue deposits are within Ptw4 sandstone near the contact with the overlying Seigal Volcanics on the southern side of the Long Pocket syncline. Some mineralisation extends northwards beneath the basalt cover towards the axis of the syncline. The mineralisation is bounded to the SW by the NW-trending Red Hill fault and to the east by the north-trending Darlona joint. The Outcamp and Sue orebodies are separated by a NE-trending fault. Displacement of the Outcamp sill across this structure is of the order of 15 m, SE side down. No drilling has been conducted across this structure. The uranium mineralisation occurs as a series of flat lying layers 0.5–5 m thick over an area of approximately 1 km2, with the depth to mineralisation 0–30 m. Mineralisation appears related to the Outcamp sill, a persistent subhorizontal dolerite sill approximately 5 m thick. The flanking Ptw4 sandstone contains 90% of the uranium mineralisation, either immediately above or below the sill. Fuchs and Schindlmayr (1981) estimated resources at Sue and Outcamp of 675 t of U3O8 in ore of grade 0.16% U3O8 and 945 t of U3O8 in ore of grade 0.16% U3O8 respectively. The Black Hills deposit is within Ptw4 sandstone near its contact with the overlying Seigal Volcanics on the northeastern side of the Long Pocket syncline. Some mineralisation also extends westwards beneath the basalt cover towards the axis of the syncline. The mineralisation is centred on the east-trending Black Hills dyke, which is manifested in the central part of the prospect area as a silicified and quartz veined basalt ridge. The mineralisation is bounded to the north by the west-trending Jinjaree fault. Both flat lying and vertical mineralisation occur in this area. Insufficient drilling has been completed at Black Hills to allow any precise resource estimate, however the mineralisation appears to be quite discontinuous.
MOOGOOMA The Moogooma mineralisation is 5 km SW of Redtree along the Redtree dyke zone (Fig 2) and is present up to 500 m west of this zone. The mineralisation is associated with a 2–3 m thick conglomerate bed, stratigraphically close to the top of the Ptw3 unit of the Westmoreland Conglomerate. Data from drill holes and trenches by QML in the 1960s have been lost and only very limited data are available for this prospect.
ORE GENESIS Ahmad (1987) discussed possible origins of uranium mineralisation at Westmoreland and concluded that the uranium was detrital, leached from the Westmoreland Conglomerate and precipitated where suitable reducing conditions existed. An alternative hypogene model first proposed by Schindlmayr and Beerbaum (1984), involves hot oxidised uranium-bearing fluids derived from the underlying uraniferous granitoids and volcanic rocks in the Murphy Inlier ascending major structures like the Redtree dyke system. Uranium was precipitated against geochemical barriers of reducing character such as the Redtree dyke dolerite–Seigal Volcanics basalt contact or by fluid mixing with cool, relatively reduced locally-circulating ground water or against permeability barriers.
812
The primary focussing conduit for ore fluids is considered to be major structures such as the Redtree dyke zone, especially where offset by northerly trending fault relay sites. The dispersal of the mineralised fluids away from these structures is primarily controlled by the sedimentary architecture of unit Ptw4. Scour and fill structures which are hollow elements with overlying laterally extensive gravelly facies which provide fifth order bounding surfaces are likely sedimentary architecture controls (Croaker, 1996). The mechanism of uranium oxide precipitation by mafic rocks occurs by the reaction of oxidising fluid with hornblende and biotite, releasing Fe2+ into solution, reducing the uranium in solution and precipitating uraninite and hematite. Chloride ions released by uraninite precipitation are used in chlorite formation (Komninou and Sverjensky, 1996). This mechanism may also explain the hematite-chlorite alteration assembly. The extensive uranium mineralisation associated with the dyke zone may preclude the formation of significant peripheral flat-lying uranium mineralisation. This suggests that if the ascending ore fluids encounter a large reservoir of reduced or neutral fluids circulating in the Redtree dyke zone, then most, if not all, of the uranium was precipitated in and about the dyke zone. Alternatively, if the dyke zone is relatively tight and is host to only a small reservoir of reduced or neutral fluids, then the oxidised ore fluids are able to disperse outwards from the dyke zone beneath the basalt contact and form the laterally extensive flat lying style of mineralisation (C L Stegman, J A Pocock, W J Robertson and J Duke, unpublished data, 1995). The initial stages of fluid mixing in the deeper parts of the system are associated with a higher temperature, more oxidised alteration and a mineralisation assemblage comprising quartzsericite±kaolinite-hematite-uranium(gold). As the ore fluids progressively moved upwards and outwards from the dyke zone and were progressively cooled and neutralised, the alteration-mineralisation assemblage changed to a lower temperature less oxidised assemblage consisting of chloriteuranium with presumably only minor attendant gold (C L Stegman, J A Pocock, W J Robertson and J Duke, unpublished data, 1995). Subsequent erosion and exposure of the uranium mineralisation, particularly at Redtree and the southern portion of Huarabagoo, has superimposed a weathering profile on the alteration-mineralisation assemblages. The chlorite alteration has weathered to a mixture of iron oxides and clays, mimicking the deeper primary alteration assemblages. Mobilisation of the uranium mineralisation is also probable. It is suggested that the flat lying mineralisation at Redtree formed immediately underneath the Seigal Volcanic contact and that subsequent erosion of the overlying basalt and weathering of the uranium mineralisation has resulted in the downward mobilisation of the uranium into deeper portions of the Ptw4 sandstone. At most, only the top 20 m of the Ptw4 sandstone has been eroded in the western part of the Redtree deposit.
ACKNOWLEDGEMENTS Rio Tinto Pty Limited is acknowledged for permission to publish this paper.
REFERENCES Ahmad, M, 1987. Uranium occurrences of the Murphy Inlier and surrounding region, Northern Territory Geological Survey, Technical Report 87/2 (unpublished).
Geology of Australian and Papua New Guinean Mineral Deposits
WESTMORELAND URANIUM DEPOSITS
Ahmad, M and Wygralak, A S, 1989. Calvert Hills, Northern Territory - 1:250 000 metallogenic map series, Northern Territory Geological Survey Explanatory Notes and Mineral Deposit Data Sheets SE 53–8.
Pietsch, B A, Plumb, K A, Page, R W, Haines, P W, Rawlings, D J and Sweet, I P, 1994. A revised stratigraphic framework for the McArthur Basin, NT, in Proceedings 1994 AusIMM Annual Conference, pp 135–138 (The Australasian Institute of Mining and Metallurgy : Melbourne).
Croaker, M R D, 1996. The sedimentology of the Middle Proterozoic Westmoreland Conglomerate Formation and the relationship to uranium mineralisation within Unit 4 at the Redtree uranium prospect, BSc Honours thesis (unpublished), Queensland University of Technology, Brisbane.
Russell, S A J, 1995. Copper-gold-uranium in the western succession of the Mount Isa Inlier and environs, Northwest Queensland Australia, MSc thesis (unpublished), Camborne School of Mines, Camborne.
Fuchs, H D and Schindlmayr, W E, 1981. The Westmoreland Uranium Deposit, Queensland, Australia, in Uranium Exploration Case Histories, IAEA-AG-250/3 (International Atomic Energy Agency : Vienna).
Schindlmayr, W E and Beerbaum, B, 1984. Structure related uranium mineralisation in the Westmoreland District, Northern Australia, in Proceedings 27th International Geological Congress, Moscow, IX(1).
Komninou, A and Sverjensky, D A, 1996. Geochemical modeling of the formation of an unconformity-type uranium deposit, Economic Geology, 91:590–606.
Sweet, I P, Mock, C M and Mitchell, J E, 1981. Seigal, Northern Territory and Hedleys Creek, Queensland - 1 : 100 000 geological series, Bureau of Mineral Resources Geology and Geophysics Map Commentaries, 6462 and 6562.
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Geology of Australian and Papua New Guinean Mineral Deposits
Milburn, D and Wilcock, S, 1998. Kunwarara magnesite deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 815–818 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Kunwarara magnesite deposit 1
by D Milburn and S Wilcock
2
INTRODUCTION
EXPLORATION AND MINING HISTORY
The deposit is centred on lat 22o55′S, long 150o13′E, some 60 km NW of Rockhampton in central Queensland (Fig 1), on the Port Clinton (SF 56–9) 1:250 000 scale and the Princhester (8952) 1:100 000 scale map sheets. It extends over 60 km in a sinuous zone, from near Yaamba to the southern end of Broad Sound.
Magnesite occurrences have been known in the Kunwarara district since the early 1900s and early references are listed in Jones (1995). Invariably the early discoveries were of magnesite veins in the hilly belt of ultramafic rocks which stretches from the mouth of the Fitzroy River to Marlborough (Fig 1). Magnesite was mined on a small scale intermittently from the 1920s to the 1950s from some of these vein deposits. Local graziers had however previously noted magnesite rocks on the surface and in fence post holes on the black soil plains adjacent to the belt of serpentinised ultramafic rocks. QMC discovered the deposit beneath the plains at Kunwarara in 1985 (Burban, 1990). Evaluation of the deposit to date has involved drilling some 6700 rotary open holes for approximately 110 000 m, 77 large diameter Calweld holes, and excavation of pits to provide bulk samples for metallurgical testwork. Since discovery, QMC has pursued a multifaceted development strategy for the deposit. The first project was developed through its subsidiary company, Queensland Magnesia (Operations) Pty Ltd (QMAG) established primarily to produce various grades of magnesia (MgO) as feedstock for the world refractory brick market. Currently some 90% of the sintered and electrofused magnesia production is exported. QMAG has mined in excess of 14 Mt of ore from commencement in 1991 to mid 1997. QMC and associated groups are also advancing additional magnesia-based projects (Enviromag, Flamemag, Cemag) and the establishment in early 1997 of the Australian Magnesium Corporation Pty Ltd (QMC 50%) signals the development of Australia’s first commercial magnesium metal smelter, based on the Kunwarara resource.
FIG 1 - Location and sketch geological map, Kunwarara magnesite deposit.
The Kunwarara portion of the deposit is held by Queensland Metals Corporation Limited (QMC), and contains a Measured Resource of 1200 Mt of cryptocrystalline nodular magnesitebearing sediment (QMC,1997), constituting the largest known magnesite resource in Australia, and the largest cryptocrystalline deposit in the world. Mining commenced at Kunwarara in 1991 within the Queensland Magnesia Project KG1 lease area.
PREVIOUS DESCRIPTIONS The first published description of the Kunwarara deposit was by Schmid (1987). Burban (1990) described the Kunwarara deposits in more detail with the benefit of exploration data collected in the period leading to the feasibility study for the QMAG Project. Milburn and Wilcock (1994) described the geology and operations at Kunwarara and first introduced a fluvial model of deposit formation. Wilcock (in press) expanded on details of the fluvial model, based on extensive new exploration data and pit exposures at the Kunwarara minesite.
REGIONAL GEOLOGY 1.
Chief Geologist, Queensland Metals Corporation Limited, PO Box 445, Toowong Qld 4066.
2.
Senior Mine Geologist, Queensland Magnesia (Operations) Pty Ltd, Box 5798, Central Qld Mail Centre Qld 4702.
Geology of Australian and Papua New Guinean Mineral Deposits
The magnesite deposits are in Late Tertiary (post-Eocene) to Quaternary sediment deposited in a fluvial environment. They are associated with a Mid Palaeozoic serpentinised ultramafic complex which is the source of the magnesium. The ultramafic complex is part of the Marlborough Block, a component of the
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D MILBURN and S WILCOCK
northern New England Fold Belt with probable ophiolitic affinities that separates the Carboniferous Yarrol Block from the Devonian-Carboniferous Coastal Block. The emplacement of these basement rocks is considered to relate to complex Permian thrusting (Holcombe et al, in press). During the late Tertiary and Quaternary, a sinuous fluvial system developed adjacent to the ultramafic complex and the resulting sediment is the host for the magnesite deposits.
ORE DEPOSIT FEATURES LOCAL GEOLOGY The magnesite was derived from the weathering and erosion of the adjacent serpentinised ultramafic complex during the Tertiary and Quaternary. The complex partially adjoins and underlies the magnesite deposit, and is characterised by hills rising 40 to 200 m above the surrounding plains. The serpentinites also host lateritic nickel-cobalt deposits, and numerous podiform chromite and vein or stockwork magnesite occurrences. The Tertiary–Quaternary fluvial sequence is up to 40 m thick, and generally fines upwards from gravel and coarse unconsolidated sand at the base through finer grained weakly cemented sandstone to siltstone and mudstone. The magnesite nodules are generally restricted to the upper half of the sequence, within the weakly cemented sandstone and siltstone (Fig 2). The sediments are overlain by 1 to 6 m of black clay, deposited by periodic post-Holocene sheet flooding.
FIG 2 - Typical ore horizon cross section, Kunwarara magnesite deposit.
The host sediment is weakly cemented but is sufficiently competent to provide few geotechnical problems during mining. Ore thickness at the Kunwarara mine has averaged 12 m during the five years of mining operations to date, and the average recovered magnesite grade is around 35%.
MAGNESITE TYPES The magnesite is cryptocrystalline and creamy white to pure white in colour. Scanning electron microscope (SEM) studies show crystal sizes ranging from 1–10 µm. Inclusions include amorphous silica, clays, iron and manganese oxides and trace pyrite. The magnesite occurs as distinctive concretionary nodules, from a few millimetres to 60 cm in diameter, as intergrown veins and sheets and as cemented aggregates to a metre in diameter. Magnesite texture ranges from a hard, pure, porcellaneous ‘bone’ type with a conchoidal fracture through to softer, less dense, porous and chalky varieties. Bone magnesite commonly forms the most distinctive nodules, and the porous and chalky types are more common as veins and sheets. Most nodules have a skin of amorphous silica which forms a rough crusty surface. In bone magnesite, the interior of the nodules contains up to 99.5% MgO (LOI free). In the more porous types, amorphous silica penetrates deeply into the nodules along cracks and around pores, leading to a higher silica content. In the upper parts of the ore horizon, localised silicification has occurred and the nodules have deep desiccation cracks and higher levels of silica. Lime occurs in solid solution in magnesite, and ranges from around 1% to 2.5% CaO (LOI free). The main source of lime is dolomite which ranges in abundance from nil to 100% of the carbonate assemblage. Dolomite often occurs as separate nodules although intergrowths with magnesite are locally common. Dolomite nodules are generally smaller and more irregularly shaped than magnesite nodules, and also differ in that they are characterised by inclusions of quartz grains, and have higher amounts of iron and manganese oxides. As a result, dolomite has higher silica values than magnesite (around 10% SiO2 LOI free) and higher Al2O3, Fe2O3 and MnO, with lime content ranging from 35 to 45% CaO (LOI free). Within the KG1 lease dolomite is generally more abundant in the lower parts of the ore zone and in some places forms a pure dolomite layer within the host sediment at the base of the deposit. Variations in the host sequence are related to variations in magnesite quality. Bone magnesite is more abundant in the lease area within a red-brown fine grained sandstone, whereas more porous types of magnesite are more common in the grey siltstone facies. It appears that this distribution is related to higher permeability in sandstone allowing greater circulation of magnesium-rich waters. Typical unbeneficiated magnesite qualities from bulk samples are shown in Table 1.
The deposits are in topographically low areas, and do not outcrop. No fossils have been found in the sediments to date, and the only indication of the age of the deposit is a basement of Eocene oil shale near Yaamba and partial onlap by Holocene alluvial fan deposits. Sedimentary features exposed in open pit walls and interpreted from extensive exploration drilling include gravel bars, erosional sandy channel deposits, and clayfilled abandoned channels. These features are indicative of a fluvial rather than a lacustrine depositional environment.
816
ORE GENESIS The close spatial association of the magnesite deposits with a large ultramafic complex indicates an obvious source of the magnesium. No studies of magnesium dissolution from the Marlborough Block serpentinite have been undertaken to date, but studies overseas suggest likely mechanisms for this class of deposit (Zachmann and Johannes, 1989). Cryptocrystalline
Geology of Australian and Papua New Guinean Mineral Deposits
KUNWARARA MAGNESITE DEPOSIT
TABLE 1 Typical carbonate analyses, Kunwarara deposit. 1
2
3
4
MgO%
46.80
94.40
97.60
98.20
CaO%
42.50
2.04
1.40
0.82
SiO2%
7.76
2.76
0.72
0.69
Fe2O3%
0.39
0.29
0.10
0.17
Al2O3%
0.83
0.29
0.15
0.08
MnO%
0.08
0.20
0.06
0.06
1. 2.
Cream dolomite with silica inclusions, grain size 10–30 mm. White, porous magnesite with outer layer impurities, grain size 10–30 mm. 3. White, dense magnesite, grain size 20–80 mm, SG +2.75. 4. White, very dense magnesite, grain size 20–80 mm, SG above 2.85. Note: All analyses on % LOI-free basis.
magnesite formation has been linked with selective dissolution of magnesium from serpentinite under the influence of waters rich in atmospheric and biogenic CO2. Hydrated magnesium carbonates precipitate in suitable environments given an appropriate trigger mechanism, such as mixing with high pH waters or by concentration through cyclic evaporation. The hydrated magnesium carbonates are transformed to magnesite by diagenetic processes (Fig 3). A modern day analogy for this type of deposit, albeit in a lacustrine environment, has been documented at Salda Lake in Turkey (Schmid, 1987). The lake is flanked by serpentinite hills which shed magnesium-rich waters and particulate matter into the lake. Magnesite is currently being deposited around the lake shore and occurs in rubbly dunes to 10 m high. Cryptocrystalline nodules and lumps of magnesium carbonate and hydroxide are forming at the mud–water interface under the influence of seasonally varying water levels. Field evidence shows that magnesite crystallisation can occur very rapidly given an adequate source of magnesium (Schmid, 1987). Chemically precipitated nodules from Salda Lake are very similar to Kunwarara magnesite.
At Kunwarara the magnesite precipitated in situ very soon after the deposition of the host sediment. Evidence for this is available in mine pit exposures which show that channels have eroded nodule aggregates and formed channel-floor deposits of pebbly magnesite. Many original sedimentary structures are visible in pit exposures, but they are heavily disrupted and distorted by the growth of nodules and the penetration by veins. The magnesite nodules do not incorporate any of the host sediment, and field observations show that they displace the host sediment. Conversely, dolomite appears to be at least partly replacive. Field observations show gradations from fine sandstone into dolomite, and SEM studies show abundant entrained quartz grains, indicating that dolomite has replaced the clay cement in sandstone. Veins of magnesite are locally very common within the surrounding serpentinite. This type of magnesite occurrence is related to weathering of serpentinite and deposition of magnesite through interaction of weathering products with descending meteoric waters loaded with atmospheric and biogenic carbon dioxide (Zachmann and Johannes, 1989).
MINE GEOLOGICAL METHODS Mining is by conventional open cut excavation with ore production around 3 Mtpa. Primary beneficiation of the magnesite at the mine site involves crushing, scrubbing, screening, heavy media separation and optical sorting to separate the magnesite from the sedimentary host and dolomite. Conversion of the magnesite to various grades of magnesia occurs at the QMAG processing plant near Rockhampton, 70 km south of the mine. The geology of the deposit is complex, with rapid vertical and lateral variations in ore quality and abundance related to differences in the sedimentary environment. Geological techniques used to assess the orebody have been developed to quantify this variability. Primarily this involves drilling numerous holes through the potential ore zone to differentiate mineable areas. Data from drilling give a clear indication of overall magnesite grade and the relative abundance of magnesite and dolomite. However drilling cannot provide quantitative data on
FIG 3 - Genetic model, Kunwarara magnesite deposit, not to scale.
Geology of Australian and Papua New Guinean Mineral Deposits
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D MILBURN and S WILCOCK
final product grade and yield, as these are related to primary nodule size as well as abundance. Grade and yield data have been collected by excavating test pits throughout the deposit, with bulk samples processed to simulate actual beneficiation at the mine site. In addition, reconciliation with historical production data can be utilised for predictive modelling.
Holcombe, R J, Stephens, C J, Fielding, C R, Gust, D, Little, T A, Silwa, R, Kassan, J, McPhie, J and Ewart, A, in press. Tectonic evolution of the northern New England Fold Belt: The Permian-Triassic Hunter-Bowen event, Australian Journal of Earth Sciences.
ACKNOWLEDGEMENTS
Milburn, D and Wilcock, S, 1994. The Kunwarara magnesite deposit, central Queensland, in Field Conference 94, Capricorn Region, pp 99–107 (Geological Society of Australia: Brisbane).
The authors wish to thank Queensland Metals Corporation Limited for permission to publish this paper and acknowledge the contribution of past and present staff and consultants to the evolving understanding of the deposit.
REFERENCES Burban, B, 1990. Kunwarara magnesite deposit, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1675–677 (The Australasian Institute of Mining and Metallurgy: Melbourne).
818
Jones, M R, 1995. Magnesite in review, Queensland Government Mining Journal, April 1995, pp 11–20, Department of Mines and Energy, Queensland.
QMC, 1997. Annual Report (Queensland Metals Corporation: Brisbane) Schmid, I H, 1987. Turkey's Salda Lake: A genetic model for Australia's newly discovered magnesite deposits, Industrial Minerals, 239:19–31. Wilcock, S, in press. Sediment hosted magnesite deposits, AGSO Journal of Australian Geology and Geophysics. Zachmann, D W and Johannes, W, 1989. Cryptocrystalline magnesite, in Monograph Series on Mineral Deposits, 28, Magnesite, pp 15–28 (Gebruder Borntraeger: Berlin).
Geology of Australian and Papua New Guinean Mineral Deposits
Lindley, I D, 1998. Mount Sinivit gold deposits, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 821–826 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Mount Sinivit gold deposits 1
by I D Lindley
INTRODUCTION The deposits are approximately 50 km SSW of Rabaul in the Baining Mountains of the Gazelle Peninsula, East New Britain Province, PNG. They are on the Gazelle (SB 56–2) 1:250 000 and Merai (9388) 1:100 000 scale map sheets, and are centred on lat 4o37′S, long 152o03′E. Title is held jointly by Macmin (PNG) Pty Limited and Gold Mines of Niugini Holdings Pty Limited. The deposits include oxide mineralisation in the Northern, Central and Southern oxide zones of the Wild Dog and Kavursuki veins, and sulphide mineralisation in the Northern sulphide zone of the Wild Dog vein. The deposits form part of a larger 26 km long, north-trending zone of variably mineralised quartz veining in the central Gazelle Peninsula, known as the Nengmutka vein system (Fig 1). The oxide resources to October 1995 for the Mount Sinivit deposits comprise a Measured Resource of 172 000 t at 4.23 g/t gold, an Indicated Resource of 173 900 t at 4.13 g/t gold and an Inferred Resource of 246 000 t at 2.36 g/t gold. A Probable Ore Reserve of 306 449 t at 4.00 g/t gold has been established and processing of oxide ore at the rate of 100 000 tpa is scheduled to commence during late 1997. The Measured, Indicated and Inferred Resources are inclusive of those Mineral Resources modified to produce the Ore Reserves. The sulphide Indicated Resource is 201 600 t at 9.43 g/t gold and Inferred Resource is 16 700 t at 9.97 g/t gold.
EXPLORATION HISTORY Although the presence of alluvial gold and platinum in streams draining the central Gazelle Peninsula has long been known (Stanley, 1923; Fisher and Noakes, 1942), there are no surface indications or records of any production. The Wild Dog veins were discovered during July 1983 and the extent of the Nengmutka vein system was defined by exploration completed by 1987. The initial five drill holes at the Wild Dog deposit during 1984 failed to achieve any significant intersections. From 1985 to 1987 extensive bulldozer trenching, followed by diamond and reverse circulation (RC) drilling, was completed at the deposit, defining the sulphide and oxide resources. A further phase of trenching and drilling from February 1990 to April 1991 was concentrated on a search for additional oxide mineralisation in the Wild Dog and nearby veins. Significant trench and drill hole indications of oxide mineralisation were obtained from the Kavursuki vein, 900 m along strike to the north from Wild Dog. Prior to the completion of the present resource and reserve estimates a final program of closely spaced RC drilling was completed at the Southern oxide zone of the Wild Dog deposit from November 1993 to January 1994. 1.
Chief Geologist, Macmin (PNG) Pty Limited, PO Box 536, Rabaul, Papua New Guinea.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 1 - Locality map and plan showing the Nengmutka vein system, central Gazelle Peninsula.
To January 1994, 129 diamond drill holes for a total of 18 516 m and 71 RC holes for a total of 3572 m have been completed, primarily at the Mount Sinivit deposits and several veins in the southern Nengmutka vein system.
PREVIOUS DESCRIPTIONS The delineation of the Nengmutka vein system has been described by Lindley (1988b). Early descriptions of the exploration and geology of the Wild Dog deposit were provided by Lindley (1987a, b). Detailed accounts of the geology and mineralisation of the Wild Dog deposit were provided by McCulla and Wangu (1989), Noviello (1989) and Lindley (1990). Noviello (1989) focussed on fluid inclusion and microprobe studies and the geochemistry of the goldtelluride mineralising fluids. Determinations of K-Ar ages for the Wild Dog hydrothermal assemblages (S Taguchi, unpublished data, 1988) and further telluride ore petrology studies (Shiga and Higashi, 1993) followed from field work completed by visiting Japanese geoscientists during 1987. The geophysical response of the veins was investigated by a German–PNG geoscientific team during 1987 (Aruai, 1988).
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I D LINDLEY
REGIONAL GEOLOGY The island of New Britain is located along the presently active convergent plate boundary separating the South Bismarck and Solomon Sea plates. Basement rocks throughout New Britain comprise Late Eocene and Late Oligocene volcaniclastic formations (Baining Volcanics and Merai Volcanics, respectively), rocks typical of an embryonic island arc, intruded by Late Oligocene–Early Miocene dioritic plutons. Late Oligocene to Early Miocene igneous activity on New Britain is intimately associated with porphyry copper-gold and epithermal gold mineralisation. During the Early to Middle Miocene the thick carbonate sequence of the Yalam Limestone accumulated on a platform that extended across most of New Britain and New Ireland. On the eastern Gazelle Peninsula, however, volcanism was continuous from Early Miocene through to Pliocene, and was restricted to a narrow belt termed the Baining Mountain Horst and Graben Zone. Andesitic and rhyodacitic pyroclastic and epiclastic units covering a 600 km2 area, the Nengmutka Volcanics, within this zone were sourced from at least three caldera complexes (Lindley, 1988a). Fluvial and marine rocks of the Miocene–Pliocene Sinewit Formation intercalate with the Nengmutka Volcanics.
ORE DEPOSIT FEATURES STRATIGRAPHY The Nengmutka vein system is hosted by a highly faulted, essentially flat-lying epiclastic sequence of volcanic sandstone and conglomerate with interbedded air fall, vitric and crystal tuff and rhyodacitic flows named the Nengmutka Volcanics (Lindley, 1988a). The sequence is intruded by multiphase stocks of quartz diorite and micromonzodiorite and monzonite and by related dykes (Fig 2).
Epiclastic and related rocks The volcanic sandstones are typically fine to medium grained, well sorted, and occasionally contain thin interbedded lenses of rhyodacitic pebbles. Sandstone dominates the NE of the area (Fig 2). Volcanic conglomerate is coarse to very coarse and typically matrix supported, and is best developed in the catchment of the Nengmutka River west of the deposits. Clasts are generally subangular to subrounded and are typically of rhyodacite. The epiclastic sequence contains occasional interbeds of andesitic and rhyodacitic flows and air fall tuff. The thin (to 5 m) air fall and vitric tuff units may contain accretionary lapilli indicative of their air fall origin. Rhyodacitic and andesitic flows and partially welded ash flow tuff generally form a minor component of the sequence. However, a thick, massive sequence of dacitic crystal tuff occurs on Kanagas Mountain, west and NW of the Mount Sinivit deposits (Fig 2). The dacitic tuff is even grained and typically contains crystals of plagioclase, with or without quartz and biotite, in a fine grained ash matrix. The tuff contains occasional lapilli-sized clasts of dacite. The relationship of the tuff units to the enclosing epiclastic sequence is unclear, and both units appear to intercalate around the southern and eastern base of Kanagas suggesting that the mountain may have once been a small tuff cone. The Nengmutka Volcanics were considered on stratigraphic grounds to be of Miocene–Pliocene age (Lindley, 1988a).
822
They host the Mount Sinivit deposits, from which sericitic alteration has been dated at 22–23 Myr (S Taguchi, unpublished data, 1988), suggesting an earliest Miocene date for the commencement of subaerial volcanism and related epiclastic sedimentation.
Intrusive and related rocks A porphyry body intrusive into the Nengmutka Volcanics is present in the Magiabe valley to the west of the Mount Sinivit deposits (Fig 2). This intrusion appears to be multiphase. Only the altered eastern portion of the porphyry has been mapped in detail, however, aeromagnetic data suggests a small equidimensional stock approximately 1000 by 700 m. Efforts to date the intrusive rocks have been unsuccessful because of the pervasive alteration. Emplacement of the porphyry is considered coeval with the the alteration and mineralisation in the Nengmutka vein system, ie Late Oligocene to Early Miocene. Evidence from related dykes supports this conclusion. This date is similar to that for the emplacement of the subeconomic porphyry copper mineralisation at Plesyumi and Kulu River in west New Britain. The main intrusion comprises an unaltered equigranular quartz diorite, which may be locally porphyritic with quartz and feldspar phenocrysts. Several small (300 by 200 m) bodies of altered micromonzodiorite and monzonite have been mapped at the NW extremity of the porphyry body. Within the main intrusion a circular intensely altered pebble breccia body of 250 m diameter is present in Vaream Creek, extending east into Magiabe Creek. The main intrusion is locally cut by several narrow bodies of andesitic breccia and feldspar porphyry. The sequence east of the Vudal fault is noteworthy for the extensive occurrence of dykes (Fig 2). The abundance of dykes in this block is considered to indicate downfaulting relative to the block west of the Vudal fault which hosts the presumed comagmatic Magiabe Valley stock. Alteration studies of the Wild Dog deposit (T Leach, unpublished data, 1991) indicate that the age of the dykes is important in gaining an overall picture of the structural development of the district. Section 10 300 N at Wild Dog contains a shallow level quartz-diorite porphyry to monzonite dyke emplaced along a NE-trending vertical structure and cutting massive fracture-filling silicification and veining. Studies indicate it is likely that the emplacement of the diorite dyke post-dated the structure-filling episode of veining and silicification, but pre-dated the deposition of gold-copper mineralisation.
STRUCTURAL SETTING Two contrasting structural regimes are recognised: that which existed at the time of mineralisation (Late Oligocene–Early Miocene), and a subsequent period of extensional tectonism which has persisted from Early Miocene to the present.
Late Oligocene–Early Miocene structure The structural pattern during the Late Oligocene–Early Miocene is reflected by the vein filled and silicified structures known as the Nengmutka vein system (Fig 1). The district was dominated by two NNE-trending subparallel structures which are variously (hydrothermal) clay covered or contain outcropping veining and silicification. The Wild Dog structure is predominant in terms of its persistence, with a strike of 10 km. The Gunsap Mountain structure is approximately 1 km
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT SINIVIT GOLD DEPOSITS
FIG 2 - Geological map and distribution of veining, cross fractures and hydrothermal clay capping, Mount Sinivit district deposits.
west of the Wild Dog structure and is predominantly clay covered, although outcropping veins are present at its southern end. A fault jog, which trends 330–340 o, connects the partially exhumed northern end of the Wild Dog structure with the southern end of the Gunsap Mountain structure, and indicates the operation of a sinistral shear duplex during the Late Oligocene–Early Miocene.
Geology of Australian and Papua New Guinean Mineral Deposits
Early Miocene–Recent structure The Gazelle Peninsula since the Early Miocene has had a long history of extensional tectonism (Madsen and Lindley, 1994). The Baining Mountain Horst and Graben Zone, consisting of a series of NNW-trending normal faults, corresponds to a zone of crustal thinning, implying that the zone is a deep seated structure which penetrates the crust. The present day
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I D LINDLEY
topography of the Baining Mountains is dominated by vertical movements along structures such as the Mediva and Vudal faults. Considerable vertical movements, to 220 m, are documented across these graben faults. Extensive SE-oriented post-mineralisation cross fracturing of the Mount Sinivit deposits (Fig 2) may be riedel shears related to the NNW-trending horst and graben extensional tectonism.
HYDROTHERMAL ALTERATION ZONING Seventy per cent of the Nengmutka vein system within a 3 km radius of the Wild Dog deposit is non-outcropping, being capped by hydrothermal clays (Figs 1 and 2). Clay alteration studies have been completed for much of the clay covered Wild Dog structure and provide a valuable insight into the nature of hydrothermal fluids and their likely flow paths. Vein nomenclature for the Mount Sinivit deposits remains unchanged from Lindley (1990) and the reader is referred to this work. Minerals identified from XRD analyses of clay separates collected from the Wild Dog deposit are sericite, illite, interlayered illite-smectite, smectite, plagioclase, pyrophyllite, chlorite, kaolinite, alunite, halloysite and gypsum. Typical clay species encountered in the clay cap overlying vein mineralisation are sericite, illite and interlayered illitesmectite, implying that acid fluid conditions prevailed within the hydrothermal system at this level. In cross section the zone of cooler temperature illite-smectite is funnel shaped, and is considered to be related to the invasion of meteoric water into the upper levels of the system during the waning stages. The presence of alunite, kaolinite, pyrophyllite and sericite indicates localised hot, lower pH (<4) conditions, suggesting the preferential channelling of ascending hot fluids. At depth beneath the broad hydrothermal clay cap the steeply dipping, unmineralised Type I quartz-filled and silicified Wild Dog structure is accompanied by illite or sericite (Fig 3). Noviello (1989) noted up to 5% adularia. Crosscutting mineralised Type II–IV veins are associated with low pH illitic clay alteration. Host rocks to these veins and silicified zones are characterised by chlorite-plagioclase alteration, indicative of neutral to alkaline fluid conditions.
MINERALISATION Numerous ore petrological studies have been completed on the Type II–IV vein hosted gold-copper mineralisation at Wild Dog, Kavursuki, Kargalio, Mengmut and Magiabe veins (I R Pontifex, unpublished data, 1984, 1987; Noviello, 1989; N J W Croxford, unpublished data, 1990; Shiga and Higashi, 1993). Primary mineralisation in the Mount Sinivit veins consists of an association in order of decreasing abundance of chalcopyrite, pyrite, bornite, tetrahedrite, chalcocite, and telluride minerals principally native tellurium, rickardite (Cu4Te3), hessite (Ag2Te), calaverite (AuTe2), petzite (Ag3AuTe2), sylvanite (AuAgTe4), altaite (PbTe) and tellurobismuthite (Bi2Te3). An SEM modal analysis of mineralisation from the Wild Dog deposit shows that copper sulphides represent 1.29% and pyrite 0.02% by volume of the sample. Gold is generally restricted to calaverite, petzite and sylvanite, but free gold of about 1 µm diameter occasionally occurs as apparently exsolved particles surrounding large composite grains of telluride. No enargite or luzonite, typical of high sulphidation systems, has been recorded from the Mount Sinivit deposits. There is no apparent zoning of metals in the 3 km of strike represented by these occurrences and the similar mineral assemblages implies they are contemporaneous and related to a common fluid flow path.
FLUID INCLUSIONS Homogenisation temperatures of inclusions in quartz from the mineralised Type II–IV veins at Wild Dog and Kavursuki at the north end of the Wild Dog structure are relatively high, with a range of 260–320 oC and an average of 285oC. Veins and veinlets from within the clay cap above the Wild Dog mineralisation homogenised at 240–260oC. Fluid inclusions from unmineralised Type I vein quartz from beneath the clay caps at Lulai Hill and Keamgi Hill, south of Wild Dog and Kavursuki (Fig 2), homogenised over a temperature range of 240–300oC with an average 260oC. Inclusions within quartz veins collected from within the Lulai Hill and Keamgi Hill clay caps homogenised around 220–245oC. These temperature ranges are substantially lower than those for the Wild Dog deposit, indicating a decreasing fluid temperature gradient from north to south. Inclusions in the mineralised Wild Dog, Kavursuki, Magiabe and Kargalio veins are saline with 2.4–13.4 eq wt % NaCl. Inclusions in quartz from the unmineralised Lulai Hill and Keamgi Hill veins are relatively dilute, with 0.4–1.2 eq wt % NaCl. Vapour (CO2)-rich fluid inclusions are common from quartz of the Mount Sinivit deposits, indicating that fluids were also gas rich.
ORE GENESIS Potassium-argon ages of sericitic wall rock alteration indicate that the Mount Sinivit deposits formed at 22–23 Myr (S Taguchi, unpublished data, 1988). The following sequence of events is proposed for the formation of the deposits.
EARLY LOW SULPHIDATION STAGE FIG 3 - Representative section, Northern sulphide zone, Wild Dog vein cross section 10 300 N, looking north, showing relationship of vein types to overlying cap of hydrothermal alteration.
824
All available data indicate that low temperature, neutral chloride fluids were responsible for the deposition of the
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT SINIVIT GOLD DEPOSITS
weakly gold mineralised Type I veining and silicification along the steeply dipping Wild Dog and Gunsap Mountain structures. These fluids were predominantly meteoric and circulated to depths of 5 to 10 km, became heated by a magmatic body, and convected to the surface (Hedenquist, 1987). As the heated water circulated through the host rock it acquired many of its constituents through fluid–rock interaction, leaving behind at shallower depths a characteristic fingerprint of propylitic alteration. The distribution of the interlayered illite-smectite clay capping suggests that the region of fluid upflow was centred on the jog structure between the Wild Dog and Gunsap Mountain structures (Fig 1). The development of argillic zones above the upflow zone is typical of low sulphidation deposits as at Creede, Colorado (Hedenquist, 1987). Crustiform quartz, typical in low sulphidation systems and indicative of multiple boiling episodes, is relatively common in streams draining the jog structure. The outflow zone of the Nengmutka low sulphidation system was characterised by lateral fluid flow and extended up to 10 km (Fig 1). Several localised occurrences of interlayered clays in the south Nengmutka vein system may be the result of neutral pH chloride discharge, greatly displaced from the region of upflow and first boiling.
INTERMEDIATE STAGE An intermediate stage in the evolution of the Nengmutka hydrothermal system coincided with a pulse in magmatic activity possibly originating from the deep magmatic heat source responsible for driving the convecting waters of the early low sulphidation stage. The magmatic pulse resulted in the high-level emplacement of dioritic and monzodioritic dykes and stocks in the Mount Sinivit district. Some of the high level dykes tapped pre-existing structures, particularly in the vicinity of the jog, implying that sinistral tectonism accompanied magmatism. A stock intruded into epiclastic rocks in the Magiabe valley contains indications of porphyry copper style mineralisation. Tectonic activity during this stage was critical in developing a subvertical to vertical brittle fracturing in the Type I filled Wild Dog and Gunsap Mountain structures, necessary for the creation of permeable zones of mixing for the subsequent high sulphidation overprint.
LATE HIGH SULPHIDATION OVERPRINT Although enargite is not present and alunite is of limited occurrence, the intimate association of gold and copper in Type II–IV veining is overriding evidence for the existence of a late stage high sulphidation environment. This contrasts with the low sulphidation environment where copper is not associated with gold mineralisation (White, 1990). The predominance of gold-copper mineralisation in the Kavursuki and Wild Dog deposits suggests that magmatic fluids ascended upwards along the same jog fractures tapped by the upflowing early low sulphidation fluids. Evidence from the intermediate stage indicates that sinistral tectonism was responsible for a pulse of magmatic activity and the high level emplacement of dioritic and monzodioritic stocks. It is therefore likely that this tectonism triggered the upward movement of a dense, high salinity, metal-bearing liquid.
Geology of Australian and Papua New Guinean Mineral Deposits
There is no evidence for the formation of a porous leached or advanced argillic zone, typical of most high sulphidation deposits, in the region of upflow in the northern Nengmutka vein system. Pre-existing interlayered clay capping was advanced to illite-sericite grade alteration, with only localised evidence of low pH leaching indicated by limited occurrences of pyrophyllite and alunite. Residual vuggy silica typical of high sulphidation systems was not developed. The predominance of depositional quartz over residual silica and the presence of hypogene hematite demonstrates that mixing with meteoric water must have been sufficient to raise the solution pH, thereby overcoming the inhibiting effect exerted by low pH on quartz deposition. As the metal-bearing solution ascended from depth, brittle fractures developed during the intermediate stage sinistral tectonism (in Type I veining and silicification) served as suitable receptor sites for high sulphidation mineralisation, overprinting early low sulphidation veining and silicification.
ACKNOWLEDGEMENTS The author gratefully acknowledges the permission of Gold Mines of Niugini Holdings Pty Limited and Macmin (PNG) Pty Limited to publish this paper. An early draft of the manuscript was read by W Tamu.
REFERENCES Aruai, G, 1988. Test geophysical survey conducted over the Wild Dog prospect, East New Britain, Geological Survey of PNG, Technical Note 34/88. Fisher, N H and Noakes, L C, 1942. Geological reports on New Britain, Territory of New Guinea Bulletin 3. Hedenquist, J W, 1987. Mineralization associated with volcanicrelated hydrothermal systems in the circum-Pacific basin, in Transactions, Circum-Pacific Energy and Mineral Resources Conference, Singapore, August 17–22, 1986, pp 513–524 (American Association of Petroleum Geologists: Tulsa, Oklahoma). Lindley, I D, 1987a. Wild Dog telluride occurrence, east New Britain, in Papua New Guinea Mineral Development Symposium, pp 71–75 (The Australasian Institute of Mining and Metallurgy: Melbourne). Lindley, I D, 1987b. The discovery and exploration of the Wild Dog gold-silver-copper deposit, East New Britain, PNG, in Proceedings Pacific Rim Congress 87, pp 283–286 (The Australasian Institute of Mining and Metallurgy: Melbourne). Lindley, D, 1988a. Early Cainozoic stratigraphy and structure of the Gazelle Peninsula, east New Britain: an example of extensional tectonics in the New Britain arc-trench complex, Australian Journal of Earth Sciences, 35:231–244. Lindley, I D, 1988b. The discovery of the Wild Dog gold deposit, Papua New Guinea - A case study, in Gold Mining 88, Vancouver 1988 (Ed: C O Brawner), pp 509–522 (Society of Mining Engineers: Littleton, Colorado). Lindley, I D, 1990. Wild Dog gold deposit, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1789–1792 (The Australasian Institute of Mining and Metallurgy: Melbourne). Madsen, J A and Lindley, I D, 1994. Large-scale structure on Gazelle Peninsula, New Britain: Implications for the evolution of the New Britain Arc, Australian Journal of Earth Sciences, 41:561–569. McCulla, M S and Wangu, A, 1989. Geology and mineralisation of the Wild Dog (Nengmutka) Prospect, Gazelle Peninsula, East New Britain, Geological Survey of PNG, Report 89/6.
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Noviello, S P, 1989. Paragenesis, fluid inclusion study, microprobe analysis and geochemistry of the gold-telluride mineralizing fluids at the Wild Dog prospect, PNG, BSc Honours thesis (unpublished), Monash University, Melbourne. Shiga, Y and Higashi S. 1993. Epithermal gold quartz veins at Wild Dog, East New Britain, Papua New Guinea, with reference to the hydrothermal activity and ore mineralogy, Resource Geology Special Issue, 16:107–127.
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Stanley, E R, 1923. Report on the Salient Geological Features and Natural Resources of the New Guinea Territory, Including Notes on Dialectics and Ethnology, Report on the Terrritory of New Guinea, 1921–1922, Appendix B (Commonwealth of Australia Parliamentary Paper No 18 of 1923). White, N C, 1990. High sulphidation epithermal gold deposits: characteristics, and a model for their origin, in Symposium on High-Temperature Acid Fluids and Associated Alteration and Mineralization, pp 5–15 (Geological Survey of Japan: Tokyo).
Geology of Australian and Papua New Guinean Mineral Deposits
Tau-Loi, D and Andrew, R L, 1998. Wafi copper-gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 827–832 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Wafi copper-gold deposit by D Tau-Loi1 and R L Andrew2 INTRODUCTION
EXPLORATION HISTORY
The deposit is adjacent to Mount Golpu in Morobe Province, PNG, at lat 7o24′S, long 146o27′E on the Markham (SB 55–10) 1:250 000 scale map sheet, and 60 km SW of the port of Lae (Fig 1). Previous publications describing the Wafi deposit have concentrated on the gold mineralisation at zone A (Funnell, 1990; Leach and Erceg, 1990; Erceg et al, 1991). This paper describes porphyry copper-gold mineralisation at Wafi and briefly discusses some exploration implications. An Indicated plus Inferred Resource of 100 Mt at 1.3% copper and 0.6 g/t gold has been estimated for the Wafi porphyry at a 0.5% copper cutoff. Title to the deposit is held 100% by wholly owned subsidiaries of Rio Tinto Exploration.
Exploration began with a porphyry copper search and led to the discovery of gold mineralisation at zone A. Thirteen years later, porphyry copper-gold mineralisation was discovered by drilling 800 m NE of zone A. The history is summarised as follows: 1.
1977. Reconnaissance drainage sampling led to the discovery of the Wamum copper mineralisation (Shedden, 1990; B M Nichols, I D Lindley, P Rosengren and J Zerwick, unpublished data, 1978).
2.
1977. Follow-up work by CRAE yielded the first significant gold grade of 22 g/t in pyritic float in the lower reaches of Wafi River (Funnell, 1990)
3.
1980–82. PA440 Mount Wanion was granted in 1980 and follow up stream sediment sampling, ridge and spur soil sampling and geological mapping located the zone A gold mineralisation (Cuthbertson, A S unpublished data, 1982).
4.
1983–86. First exploration drilling (5789 m) commenced in 1983 at zone A (Fig 2). Second phase drilling (2800 m) and benching in 1984–86 outlined zone A and identified other surface gold zones peripheral to the Wafi diatreme.
FIG 1 - Schematic geological plan and drill hole locations, Wafi intrusive complex.
1.
Geologist, CRA Exploration Pty Limited, PO Box 804, Madang, Papua New Guinea.
2.
General Manager-Pacific, CRA Exploration Pty Limited, Private Bag 3, Bundoora MDC Vic 3083.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 2 - Oblique aerial photograph of Wafi area with annotated mineral zones, some of which are shown in plan view on Fig 1.
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D TAU-LOI and R L ANDREW
5.
6.
1988–90. CRAE farmed out the project to Elders Resources who undertook a third phase of drilling at zone A, comprising 3364 m of diamond and 1099 m of reverse circulation drilling, identifying an Indicated Resource of 18 Mt at 2.5 g/t gold (Erceg et al, 1991) 1990. At the closing stages of the Elders joint venture, CRAE recommended drilling a structure NE of zone A for high grade epithermal gold mineralisation. After two drill holes which passed close to mineralised intrusive, hole DDH WR95 intersected the first significant mineralisation of porphyry style (Fig 3) with 263 m at 1.86% copper and 0.27 g/t gold. CRAE reacquired Elders’ equity in the Wafi project and resumed management.
REGIONAL GEOLOGY The regional host rocks to the Wafi deposits are metasediments of the Jurassic to Cretaceous Owen Stanley Metamorphics (Tingey and Grainger, 1976). These are intruded by Miocene to Pliocene intrusives of dioritic to dacitic composition. Folding and faulting since the Oligocene has affected these rocks (Pigram and Davies, 1987; Rogerson et al, 1987). The regional geology has been described in greater detail in previous publications (Funnell, 1990; Leach and Erceg, 1990; Erceg et al, 1991). The Wafi porphyry was originally thought to have formed during the same intrusive event as the Edie Porphyry in the Wau district (Page and McDougall, 1972), occurring along a prominent NE-trending regional structure. However more recent K-Ar age dating (Tau-Loi, 1996) indicates that the Wafi intrusive complex, at 14 Myr, is older than the Edie Porphyry.
LOCAL GEOLOGY Interbedded conglomerate, sandstone, siltstone and shale of the Owen Stanley Metamorphics were intruded by a suite of diorite to dacite porphyry stocks and by a later diatreme or breccia pipe which forms the core of the Wafi intrusive complex (Fig 1). The Wafi porphyry is a diorite which is host to copper-gold mineralisation of porphyry style, carrying an epithermal overprint in its upper part (R H Sillitoe, unpublished data, 1990). Folds trend NE and dominantly plunge in that direction (Funnell, 1990; M K Noyce and P S Licence, unpublished data, 1992). The two principal fault strike directions are NW and NE. A WNW-striking fault (Fig 1) may have exerted control on the location of the Wafi porphyry and the Wafi diatreme. FIG 3 - Schematic cross section on 20 750 N showing alteration zones and geology, looking north, Wafi intrusive complex.
7.
1990–95. Transient EM data were used to guide the initial diamond drilling of the Wafi porphyry copper zone (Figs 2 and 3). Resource and evaluation drilling of 16 570 m was completed.
8.
1996. Stepout and infill diamond drilling recommenced in and around the zone A gold deposit.
To date 152 holes for a total of 46 430 m have been drilled at Wafi. Other exploration techniques employed include aeromagnetic, ground magnetic, SP, IP, and CSAMT surveys, shallow bedrock geochemical sampling, surface lithochemical sampling, soil geochemical sampling and geological mapping. The Wafi porphyry copper zone (Fig 2) is expressed as a -80 mesh stream sediment anomaly in one creek which drains eastwards from the surface lithocap. The anomalous copper, molybdenum and lead values in stream sediment correspond to the residual soil signature found in drilling of the oxide cap above the porphyry. The porphyry system is expressed at surface as silicified, granular to massive quartz-alunite altered metasediment with some hematite staining. This represents a hypogene lithocap which has been subjected to surface weathering. Compared to the conventional ‘leached outcrop’ of porphyry copper deposits (Blanchard, 1968), this form of weathered and silicified lithocap at surface, although it is indicative of pyrite, gives very little direct or visible indication of the style of porphyry stockwork mineralisation at depth.
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The Wafi porphyry (Fig 2) lies between two NE striking faults, the Dokaton fault in the west and Rafferty’s fault in the east (Fig 1). Both of the faults contain clay gouge and offset the NW-striking faults, host rock sequence, alteration zoning and mineralisation. These faults are interpreted to be postmineralisation. There is possibly some displacement of the Wafi porphyry at depth along the Dokaton fault.
ORE DEPOSIT FEATURES The Wafi deposit differs in character from classical porphyry copper deposits in having the upper part of the Wafi porphyry stock overprinted by an epithermal, high sulphidation stage which modified the original alteration and sulphide zoning.
GEOMETRY The Wafi porphyry is a near-vertical diorite stock with a diameter of 300 m at 350 m RL in its upper part, and apparently necks at depth to a diameter of 150 m at -300 m RL. Drilling indicates a vertical extent exceeding 900 m. Contacts with the metasediment are generally abrupt with stockwork veining diminishing outwards into the host rock (Fig 3). One or two examples of faulted contacts exist at depth on the western side of the stock. Pre- and late- mineralisation dykes or pods of dioritic composition and some brecciated zones display irregular contacts within the Wafi porphyry. The latemineralisation phases contain low grade copper-gold mineralisation.
Geology of Australian and Papua New Guinean Mineral Deposits
WAFI COPPER-GOLD DEPOSIT
TABLE 1 Vertical alteration and sulphide zoning, Wafi porphyry.
Zone
Basal contact
Silicate–oxide minerals
Sulphide minerals
Vertical interval (m)
Oxide
Sharp
silica-alunite-goethite-hematite(jarosite)1
Nil
100
Transition
silica-alunite
pyrite-chalcocite-enargitedigenite
10
Sharp
silica-alunite
pyrite-enargite-covellite(tennantite-tetrahedrite)
200
Phyllic
Transition
silica-sericite
pyrite-covellite
300
Potassic
Transition
silica-biotite-magnetitepotassium feldspar
pyrite-chalcopyrite-(bornite)
200+
Supergene Advanced argillic
1.
Minor phases are in brackets.
ALTERATION AND MINERALISATION The Wafi porphyry displays marked vertical zoning of alteration and sulphide mineralisation (Table 1). These zones comprise a combination of a primary zoning associated with the mineralising porphyry event and later epithermal (high sulphidation) overprinting. The alteration halo is cylindrical at depth and flares upwards to form a mushroom shape (Fig 3). This shape may have been influenced by the high sulphidation overprint. Beneath the oxide zone cap rocks the obvious supergene zone in the porphyry is only several metres thick. In drill hole WR95, this zone is represented by 4 m at 7% copper but supergene chalcocite, digenite and trace idaite (Cu3FeS4) occur in a transitional oxide–sulphide zone for tens of metres beneath the sharp oxide–sulphide interface. The host rock in the advanced argillic or enargite zone is essentially relict quartz stockwork veining. All silicate minerals appear to have been leached under hypogene conditions by hydrothermal, highly acidic sulphate solutions and the assemblage enargite-covellite-(tennantite-tetrahedrite) subsequently deposited at the close of acid leaching. Grades of 2.0–3.5% copper with 0.5–1.5 g/t gold are common within this zone. The very sharp base of the advanced argillic zone coincides with a similar abrupt decrease in arsenic content from +0.1% to below 0.01%. Reverse hypogene zoning of copper sulphides in the advanced argillic or enargite zone is interpreted to be a result of epithermal overprinting at low pH and moderate
Eh. Chalcopyrite rimming hypogene covellite (Fig 4) is interpreted as hypogene overprinting associated with high sulphidation alteration. Kaolinite and dickite form an intermediate argillic margin to the advanced argillic alteration zone, which is a characteristic of all high sulphidation systems. In the phyllic zone, hypogene covellite is the principal copper sulphide, replacing chalcopyrite as a stable phase under high sulphidation conditions. Grades of 1.0–2.0% copper and 0.3–0.8 g/t gold are typical. The phyllic alteration zone shows a transitional contact with the deeper potassic zone (Table 1) marked by progressively less hypogene replacement of chalcopyrite by covellite. Grades in the potassic zone are 1.0–3.0% copper and 1.0–2.5 g/t gold. At depth an actinolite-magnetite-hematite shell (Fig 3) occurs with low grade copper-gold mineralisation within the potassic zone and appears to pre-date that zone (TauLoi, 1996). Immediately outboard of the intrusive contact, the metasediment carries a shell, 5–30 m wide, of porphyry-related quartz stockwork. Maximum grades of 1.0% copper and 2 g/t gold in the shell decrease outwards as the density of stockworking decreases. The outer part of the enclosing metasediment is characterised by propylitic alteration dominated by green chlorite. Molybdenite mineralisation at levels of 0.01–0.03% occurs on the western flank of the porphyry within outer quartz stockwork in metasediment. This mineralisation also appears to wrap over the western part of the stock below the oxide interface forming a partial molybdenum halo and cap. Lithochemical data from rock chip and shallow bedrock samples display a marked zinc halo around the Wafi complex (Fig 5).
VEINING
FIG 4 - Photomicrograph, Wafi porphyry drill hole WR102, 500 m depth. Massive quartz-alunite rock with coarse hypogene covellite (maroon-purplish), variably rimmed by, and showing advanced pervasive replacement by chalcopyrite.
Geology of Australian and Papua New Guinean Mineral Deposits
Porphyry-style mineralisation is characterised by a sulphidebearing quartz stockwork which can exceed 90% of the rock volume. Early stage A veins and later stage D veins are the main elements of the stockwork, which is also cut by later pyrite veining at millimetre to centimetre scale. This pyrite veining is interpreted to be associated with a later epithermal overprint. In general, all zones from the deep potassic to the supergene carry pyrite in concentrations far exceeding normal levels for a porphyry copper deposit. Preliminary metallurgical testwork indicates that approximately 50% of the gold in the porphyry may be associated with this later pyrite.
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From K-Ar age determinations on potassium feldspar (14 Myr) in the potassic zone and alunite (13 Myr) in the advanced argillic zone, the events which formed these minerals are almost coeval. With a totally different set of physico-chemical conditions, the interpreted epithermal overprint appears to have occurred soon after the porphyry mineralisation formed. Restricted enargite mineralisation within the Wafi diatreme would place this diatreme event and its associated dacitic porphyries very soon after the epithermal stage.
EXPLORATION IMPLICATIONS Exploration drilling in and around the Wafi diatreme has intersected several intrusive bodies of dioritic to dacitic porphyry which are essentially barren. One such porphyry, flanking the western side of the diatreme (Fig 1), carries D stage veins of quartz-sericite-pyrite but no earlier copper-bearing vein sets. This ‘failed porphyry’ indicates that the later stage of the porphyry mineralising process is present in at least one other intrusive stock.
FIG 5 - Lithochemical zoning of molybdenum and zinc, Wafi intrusive complex.
ORE GENESIS The Wafi porphyry is interpreted to have formed during a subvolcanic event in a period of tectonism and volcanism postdating the Oligocene accretion of the Papua terrane and predating the Late Miocene accretion of the Finisterre terrane (Pigram and Davies, 1987). Based on K-Ar data (Tau-Loi, 1996), arc volcanism during the early Miocene resulted in the emplacement of the Wafi intrusive complex around 14 Myr. This age is consistent with ages determined by Cambray and Cadet (1994), and regional deformation events described by Pigram and Davies (1987) and Rogerson et al (1987). Previous K-Ar data indicating an age of 9 Myr for alunite (Funnell, 1990) may have reflected partial argon loss. The earliest mineralising event of the Wafi porphyry is interpreted to be potassic alteration, accompanied by hypogene chalcopyrite and bornite with trace molybdenite and gold. The interpreted epithermal stage of the hydrothermal event appears to have reconstituted the copper sulphide minerals within the low pH–moderate Eh environment of a high sulphidation overprint. In such an Eh-pH regime, sulphide stability fields are tightly grouped and small changes in pH particularly can cause forward and reverse zoning of sulphides (Fig 4). Such an environment has been described for the Summitville gold deposit in Colorado (Stoffregen, 1987) where hypogene covellite was recorded. In the context of a subvolcanic intrusive, one possible process to explain such a major overprint is sector collapse within a volcanic edifice, superimposing the near surface, epithermal environment on to the porphyry stock (R H Sillitoe, unpublished data, 1991). This ‘telescoping’ process (Sillitoe, 1994) provides an explanation of hypogene, high sulphidation assemblages overprinting the conventional porphyry mineralogical zoning.
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An elongate, semi-coincident copper-molybdenum lithochemical anomaly exists to the west and NW of the Wafi diatreme (Fig 5). This anomaly has a geochemical expression similar to that of the Wafi porphyry. Exploration drilling to date has located only the porphyry mentioned in the previous paragraph without fully explaining the source of the coppermolybdenum anomalies. The size of the zinc lithochemical halo (Fig 5) is not consistent with the scale of the Wafi porphyry alone and the halo is centred on the Wafi diatreme rather than the Wafi porphyry. These observations suggest that a larger porphyry source may exist below the Wafi diatreme. In and around the Wafi intrusive complex, several other geochemical and geophysical features which may represent porphyry mineralisation at depth remain to be tested. Several apparently barren intrusive dioritic bodies have been recognised to the north of Wafi and the surface lithocap expression of a further mineralised stock may be as subtle as that of the Wafi porphyry.
ACKNOWLEDGEMENTS The authors wish to thank Rio Tinto Exploration Pty Limited for permission to publish this paper. The contribution of many Rio Tinto geologists who have worked on the Wafi project over the last six years is acknowledged. The ongoing contribution to the project by R H Sillitoe is also acknowledged. Mineralogical work by I R Pontifex, the Rio Tinto Research and Technical Development group and, in particular, T M Leach has made a significant contribution to the understanding of the Wafi system. S Ryan and D Smith are thanked for commenting on the manuscript.
REFERENCES Blanchard, R, 1968. Interpretation of leached outcrops, Nevada State Bureau of Mines, Bulletin 66. Cambray, M and Cadet, J P, 1994. Testing global synchronism in periPacific arc volcanism, Journal of Volcanological and Geothermal Research, 63:154–164. Erceg, M, Craighead, G A, Halfpenny R and Lewis, P J, 1991. The exploration history, geology and metallurgy of a high sulphidation epithermal gold deposit at Wafi River, in Proceedings Exploration and Mining Conference, (Ed: R Rogerson), pp 58–65 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Geology of Australian and Papua New Guinean Mineral Deposits
WAFI COPPER-GOLD DEPOSIT
Funnell, F R, 1990. Wafi River gold prospect, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1731–1733 (The Australasian Institute of Mining and Metallurgy : Melbourne).
]Shedden, S H, 1990. Wamum copper-gold prospect, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1759–1761 (The Australasian Institute of Mining and Metallurgy: Melbourne)
Leach, T M and Erceg, M M, 1990. The Wafi high sulphur epithermal gold deposit, Papua New Guinea, in Proceedings of the Pacific Rim Congress, 1990, pp 451–456 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Sillitoe, R H, 1994. Erosion and collapse: causes of telescoping in intrusion-centred ore deposits, Geology, 22:294–948.
Page, R W and McDougall, I, 1972. Ages of mineralisation of gold and porphyry copper deposits in the New Guinea Highlands, Economic Geology, 67:1034–1048. Pigram, C J and Davies, H L, 1987. Terranes and the accretionary history of New Guinea Orogen, AGSO Journal of Australian Geology and Geophysics, 10:193–211. Rogerson, R, Hilyard, D B, Francis, G and Finlayson, E J, 1987. The foreland thrust belt of Papua New Guinea, in Proceedings of the Pacific Rim Congress, 1987 pp 1689–1701 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Geology of Australian and Papua New Guinean Mineral Deposits
Stoffregen, R, 1987. Genesis of high sulphidation alteration and AuCu-Ag mineralisation at Summitville, Colorado, Economic Geology, 82:1575–1591. Tau-Loi, D, 1996. Geology and genesis of the Wafi porphyry and high sulphidation epithermal Cu-Au system, MSc thesis (unpublished), The University of Western Australia, Perth. Tingey, R J and Grainger, O J, 1976. Markham, Papua New Guinea 1:250 000 geological series, Bureau of Mineral Resources Geology and Geophysics, Explanatory Notes SB 55–10.
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Geology of Australian and Papua New Guinean Mineral Deposits
Denwer, K P and Mowat, B A, 1998. Hamata gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 833–836 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Hamata gold deposit 1
by K P Denwer and B A Mowat
2
INTRODUCTION
EXPLORATION HISTORY
Hamata is within the Morobe Goldfield at lat 7o25′S, long 14o39′E on the Wau (SB 55–14) 1:250 000 scale map sheet (Fig 1). The deposit is 13 km SW of Wau, PNG, within the headwaters of the Upper Watut River, at an average elevation of 2000 m. The steep mountainous terrain is covered by dense tropical rainforest and access is by helicopter or foot track. The deposit has been described previously by Wells and Young (1991) and Denwer, Leach and Mowat (1995).
Gold was discovered in the Wau district in about 1922 (Lowenstein, 1982). The discovery started a gold rush and many small scale miners worked the field. A total production of 124 000 kg of gold has been reported, with 100 000 kg of gold and 95 000 kg of silver produced from alluvial sources to 1977 (Lowenstein, 1982) and approximately 24 000 kg of gold and 35 000 kg of silver from hard rock sources to 1993 (Carswell, 1990). An unknown amount of gold has been won by Papua New Guinean miners in recent years. Hamata is within exploration licence (EL) 497. It was discovered in July 1987 by values of 28.5 g/t and 5.5 g/t gold in 80 mesh stream sediment samples collected during a regional stream sediment survey. Work completed to 1996 includes geological mapping, soil sampling, trenching, ground magnetic surveys, and 63 diamond drill holes for a total of 10 368 m.
REGIONAL GEOLOGY The Morobe Goldfield occurs within the Owen Stanley Foreland Thrust Belt in which the basement is the Jurassic–Cretaceous slate and chloritoid schist locally called the Kaindi metamorphics. The Kaindi metamorphics are intruded by the Middle Miocene Morobe Granodiorite (Fig 1). During the Pliocene, dacitic igneous activity resulted in intrusion of porphyry bodies of the Edie Porphyry and extrusion of the Bulolo Volcanics. The sedimentary Otibanda Formation unconformably overlies these units. The igneous activity was confined to a NW-trending structural corridor termed the Wau Graben. The graben is bounded to the east by the Wandumi Fault, to the west by the Upper Watut Fault, and is truncated to the north and south by the Snake and Kemeranga transfer structures (Fig 1).
DEPOSIT GEOLOGY LITHOLOGY Morobe Granodiorite FIG 1 - Location and geological map of the Morobe Goldfield.
Data collected to late 1996 have been used to estimate an Inferred Resource of 9.2 Mt at 3.1 g/t gold using a cutoff of 0.5 g/t. This contains a high grade portion for which an Inferred Resource of 1.2 Mt at 9.7 g/t gold has been estimated using a 5.0 g/t cutoff. 1.
Supervising Geologist, RGC Exploration, PO Box 62, Zeehan Tas 7469.
2.
Supervising Geologist, Goldfields Exploration, PO Box 2403, Orange NSW 2800.
Geology of Australian and Papua New Guinean Mineral Deposits
The principal rock type at Hamata is the Morobe Granodiorite, which hosts all the gold mineralisation. It is a medium grained hypidiomorphic granodiorite and contains plagioclase, potassium feldspar, quartz, biotite and hornblende. Throughout Hamata the granodiorite is remarkably uniform in composition and grain size. Dating of the Morobe Granodiorite pluton has yielded an age of around 14.3 Myr, placing it in the Middle Miocene (Lowenstein, 1982). The pluton is oxidised as shown by the presence of primary magnetite, apatite and sphene.
Morobe Granodiorite–related porphyries Narrow andesitic dykes of two main types cut the Morobe
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K P DENWER and B A MOWAT
Granodiorite at Hamata. The first is a coarse grained feldspar porphyry, an andesitic porphyry with plagioclase phenocrysts to 15 mm diameter in a groundmass of plagioclase, orthoclase, hornblende, biotite and quartz. The second is an andesite porphyry which has phenocrysts of hornblende to 5 mm diameter in a groundmass of plagioclase, orthoclase, biotite, hornblende and quartz. The porphyries are interpreted to be related to the Morobe Granodiorite as late and more mafic fractionates, and they pre-date all stages of mineralisation.
Edie Porphyry Dacite porphyry occurs as a large body to the west of the main body of mineralisation (Fig 2) and as narrow dykes throughout the deposit. Rare trachyte porphyry dykes intrude the central
portions of the mineralisation. The dacite and trachyte porphyries are similar in composition to the Edie Porphyry suite found throughout the Wau area which has been dated at 2.4 to 3.8 Myr The dacite porphyry at Hamata is fine grained, with quartz, plagioclase and hornblende phenocrysts in a groundmass of these minerals plus potassium feldspar. Trachyte porphyry dykes are composed of potassium feldspar (mainly sanidine), hornblende and biotite phenocrysts in a groundmass of feldspar and quartz. The narrow Edie Porphyry dykes within the deposit are altered, whereas the main porphyry body to the west is unaltered. Intrusion of the Edie Porphyry occurred both synand post-mineralisation.
FIG 2 - Geological plan of the Hamata gold deposit.
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Geology of Australian and Papua New Guinean Mineral Deposits
HAMATA GOLD DEPOSIT
FIG 3 - Geological cross section on AMG line 9 179 900 N, looking north. See Fig 2 for section location and fault names.
STRUCTURE The Hamata reefs are truncated by faults of three dominant orientations (Fig 2). 1.
Faults subparallel to the Masi and Lower reefs trend NNE and dip easterly at about 50o. The best example of this type is the Western fault which is 5–15 m wide with a pug core, and adjacent irregular displacement zones represented by chloritic shears. A major dacite porphyry is closely associated with the Western fault, and although the porphyry is insignificant at surface it balloons out at depth.
reefs are dragged into the plane of the Cross fault 30 to 40 m either side of the fault plane. It is suspected that there are a series of easterly-trending parallel faults that truncate mineralisation at the extremities of the deposit and although evidence is sparse, these are probably at approximately 80 350 N and 79 800 N.
MINERALISATION
2.
Reverse faults with 5–50 m offsets trend NNE and dip westerly at 40–70o. This type is represented by the Camp Creek fault (CCF), CCF Subsidiary fault (CCF’), Eastern Creek fault (ECF) and the Humbug fault (HF). The faults are orthogonal to the Masi reef and progressively downthrow the reef to the east. The faults form as complex shear zones, typically puggy and up to 10 m wide. They have variable strike and dip, reflected by the CCF and CCF’ merging to the north and the ECF shallowing considerably as it approaches the Cross fault.
Primary gold mineralisation within the Morobe Goldfield is related to the intrusion of dacitic to andesitic Edie Porphyry, often as associated diatremes which have intruded along major regional structures. A full spectrum of deposit styles from proximal (mesothermal) to distal (epithermal) to the porphyry source is recognised (Denwer, Leach and Mowat, 1995). The transition between the proximal and distal styles is recognised both between deposits and within individual deposits. The deposit styles should be termed porphyry-related gold mineralisation rather than given the generic epithermal and mesothermal labels. Hamata is an example of mineralisation proximal to the mineralising porphyry. Gold is not refractory and occurs as blebs of diameter 20 to 30 µm within the pyrite.
3.
The Cross fault strikes ESE, dips at 70–80o north and is a 50 m wide complex fault. It is the only definite example of this type and is an oblique normal dextral fault which downthrows the northern half of the Hamata deposit. There is significant rotation of the reefs and the west dipping reverse faults across it. The Masi and Lower
Mineralisation at Hamata occurs in at least three subparallel zones (Masi, Lower and Eastern) which strike NE and dip at 4550o SE (Figs 2 and 3). They are up to 50 m thick and comprise quartz-pyrite veined and potassium feldspar–sericite altered granodiorite. Veining within the zones is of low density and diffuse except at the upper contacts where 3–4 m wide ‘reefs’
Geology of Australian and Papua New Guinean Mineral Deposits
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K P DENWER and B A MOWAT
(Masi and Lower) of pyrite-hematite±magnetite-quartz veins are well developed (Figs 2 and 3). The Lower reef only outcrops north of the Cross fault in the hanging wall of the Humbug fault. The Lower reef is of similar dimensions to the Masi reef but differs in that the alteration selvages to the reef are typically tight and the reef does not overlie an area of extensive alteration. The Eastern reef is typically 30–50 cm wide and of similar composition to the Masi and Lower reefs. It dips subparallel to the slope and subsequently has a wide apparent thickness in Fig 2. Three main stages of mineralisation are recognised at Hamata. Stage 1 consists of early semi-regional thin magnetite, hematite and pyrite veinlets with associated potassium feldspar and sericite alteration selvages. Stage 2 consists of coarse grained pyrite, hematite, magnetite and quartz fracture-fill veins with sericite alteration. The majority of the gold is associated with Stage 2 veins. Stage 3 veins are shear veins of quartz, pyrite, arsenopyrite and marcasite with sericite and clay alteration. Only low levels of refractory gold were introduced during this phase. Stage 3 is similar to the main gold mineralising stage at the Kerimenge deposit (Hutton et al, 1990). It is interpreted that all three stages of mineralisation are transitional and porphyry related. The changes in mineralisation style from Stage 1 to Stage 3 reflect a fluid evolution from magmatic-dominated hot mineralising fluids to meteoric-dominated cooler fluids. Two types of fluid inclusions are recognised in Stage 2 quartz at Hamata: 1.
Single phase predominantly liquid-rich fluid inclusions homogenise over a range of 292–344oC (average 318oC) with an associated salinity range of 3.2 to 7.7 eq wt % NaCl.
2.
Fluid inclusions containing halite indicate salinities in excess of 26 eq wt % NaCl (Roedder, 1984).
The dual salinities imply trapping of two generations of fluids, a hypersaline (magmatic) fluid and a more dilute (mixed) fluid. It is proposed that hot magmatic fluids have mixed with meteoric water to deposit the Stage 2 veins.
ALTERATION Regional scale weak propylitic alteration, probably of deuteric origin, comprising chlorite, calcite, epidote and sericite, is seen throughout the Morobe Granodiorite. Adjacent to the Hamata deposit the propylitic alteration assemblage is essentially the same but of greater intensity. Early high temperature alteration is seen in Morobe Granodiorite–related andesite porphyry dykes in which secondary biotite is commonly associated with early Stage 1 magnetite-hematite-pyrite veins. Secondary biotite is not seen in granodiorite alteration assemblages. Alteration associated with the first two stages of mineralised veins within the granodiorite is essentially the same, with an assemblage of potassium feldspar, sericite, calcite and pyrite (KSCP) which overprints the pre-existing propylitic alteration. The KSCP assemblage forms selvages 2 to 100 cm wide on Stage 1 and 2 veins. In stockwork zones the selvages coalesce to form massive texture-destructive alteration.
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Secondary potassium feldspar occurs as both orthoclase and adularia, principally as narrow rims to Stage 1 and 2 veins, and in some cases replaces primary orthoclase. Sericite-pyritecalcite alteration is more pervasive and of varying intensity, and in some cases is texture destructive. The selvages on Lower reef Stage 2 veins are narrow, perhaps due to a lack of fracturing and primary porosity. Stage 3 vein alteration selvages show a different assemblage, of sericite-illite and kaolinite, which imparts a distinctive grey-white bleaching to the rock. A late phase of patchy overprinting illite-kaolinite alteration obliterates the previous alteration assemblages in the centre of the deposit. This phase is erratically distributed and unrelated to veining, however in some cases the assemblage rims late unmineralised chert veins. The dacite and trachyte porphyries show very similar alteration mineralogy to that in the granodiorite, leading to the suggestion that the fluids which formed the mineralisation were driven by these intrusions.
ORE GENESIS Denwer, Leach and Mowat (1995) propose that all of the mineralisation within the Morobe Goldfield is related to intrusion of the Edie Porphyry. The process involves magmatic fluids which evolved from the parent melts of the high level Edie Porphyry stocks, which mixed with convecting meteoric fluids and were channelled along major structure and diatreme contacts. At Hamata mineralisation occurs proximal to the intrusives and gold mineralisation is associated with the deposition of pyrite-hematite-quartz±magnetite-gold. The mineralising fluids were hot, and periodically saline due to pulses of magmatic fluid.
ACKNOWLEDGEMENTS The authors wish to thank Goldfields Exploration for permission to publish this paper.
REFERENCES Carswell, J T, 1990. Wau gold deposits, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1763–1767 (The Australasian Institute of Mining and Metallurgy: Melbourne). Denwer, K P, Leach, T M, and Mowat, B A., 1995. Mineralisation of the Morobe Goldfield, Morobe Province, Papua New Guinea, in Proceedings Pacific Rim 95 Congress, pp 181–185 (The Australasian Institute of Mining and Metallurgy: Melbourne). Hutton, M J, Akiro, A K, Cannard, C J, and Syka, M C, 1990. Kerimenge gold prospect, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1769–1772 (The Australasian Institute of Mining and Metallurgy: Melbourne). Lowenstein, P L, 1982. Economic Geology of the Morobe Goldfield, Papua New Guinea, Geological Survey of Papua New Guinea Memoir 9. Roedder, E, 1984. Fluid Inclusions, Reviews in Mineralogy, 12 (Mineralogical Society of America: Washington, DC). Wells, K, and Young D J, 1991. Geology and exploration of the Hamata deposit, in Proceedings PNG Geology, Exploration and Mining Conference, pp 66–68 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Geology of Australian and Papua New Guinean Mineral Deposits
Semple, D G, Corbett, G J and Leach, T M, 1998. Tolukuma gold-silver deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 837–842 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Tolukuma gold-silver deposit 1
2
by D G Semple , G J Corbett and T M Leach INTRODUCTION The deposit is within the Central Province, PNG, 100 km north of Port Moresby at lat 8o34′S, long 147ο08′E, on the Buna (SC 55–3) 1:250 000 scale and Wasa (8380) 1:100 000 scale map sheets. The mountainous terrain restricts road access to Kubuna, 50 km from the mine, and access to the mine is by helicopter. There are airstrips suitable for STOL aircraft at Fane and Woitape, respectively 6 km and 14 km from Tolukuma (Fig 1).
3
identified a Proved Reserve of 432 000 t at 18.1 g/t gold and 46 g/t silver to be extracted from a combination of open pit and underground mining (Semple, Corbett and Leach, 1995). This forms part of the Inferred Resource estimated by Newmont Proprietary Limited for the entire Tolukuma vein system as 1.478 Mt at 13.77 g/t gold using a 4 g/t gold cutoff (Langmead and McLeod, 1991). Further drilling at the Gulbadi portion of the Tolukuma vein system during 1996 (Fig 2) outlined an Indicated Resource of 270 000 t at 26.5 g/t gold in an area formerly known as Zone A. This grade is 50% higher than Zone C. Zone B will be drill tested in 1997.
FIG 1 - Location and regional geological map, Tolukuma area.
Mining of the Tolukuma vein system commenced at Tolukuma Hill, designated as Zone C, in 1995. Additional drilling by Dome Resources NL from mid 1993 to mid 1995 1.
Semple Geological Services, PO Box 1440, Broadbeach Qld 4218.
2.
Corbett Geological Services, 29 Carr Street, North Sydney NSW 2060.
3.
Terry Leach and Co, PO Box 47295, Ponsonby Auckland, New Zealand.
Geology of Australian and Papua New Guinean Mineral Deposits
FIG 2 - Geological plan of the Tolukuma vein system, and location of cross sections (Figs 5 and 6).
EXPLORATION AND MINING HISTORY Tolukuma was discovered by Newmont during a regional ‘grass roots’ exploration program targeting epithermal gold mineralisation within volcanic rocks which had previously
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D G SEMPLE, G J CORBETT and T M LEACH
been explored in the 1960s and 1970s for porphyry copper mineralisation (Langmead and McLeod, 1990, 1991). An initial helicopter-supported stream sediment survey using bulk leach extractable gold sampling in July 1985 produced anomalies to 20 ppb cyanide soluble gold. Epithermal quartz float was identified during follow up sampling to July 1986, and led to identification of outcropping quartz veins in the vicinity of a vegetation anomaly at Tolukuma Hill (Langmead and McLeod, 1991). Geological mapping and trenching led to drill testing from 1987. The resource fell short of Newmont corporate requirements and the project was offered for sale in late 1989. In 1993 Dome Resources entered into an option agreement to purchase the Tolukuma project and the surrounding tenements, and began to upgrade the resource. Dome sought to define sufficient reserves at Tolukuma Hill to begin production prior to detailed drill definition of the vein extensions such as Gulbadi. After completion of the purchase and receipt of permits, from May 1995 a prefabricated mill and other mine facilities were airlifted from Kubuna into Tolukuma using Russian Mil 26 (17 t lift), Mil 8 and Kamov (each 4 t lift) helicopters. A Chinook (10 t lift) and Bell Longranger helicopters were also used to transport all mine equipment and personnel to the mine site which operates on a fly-in/fly-out basis. The first gold pour was on 24 December 1995. During the first 12 months of mining (December 1995–November 1996) 115 000 t of ore were extracted at 16.5 g/t gold and 48.5 g/t silver, with a waste to ore ratio in the initial open pit operation of 10:1.
The Tolukuma vein system is presently the most prospective of a number of epithermal quartz vein–style gold prospects in the Tolukuma district which are discussed in more detail by Langmead and McLeod (1990, 1991).
ORE DEPOSIT FEATURES HOST ROCK GEOLOGY Although the metamorphic basement rocks at Tolukuma are broadly similar to those throughout the region, variations are apparent in the volcanic rocks. Recent drilling at Gulbadi has demonstrated that the basement contains a significant proportion of high level diorite porphyry intrusions. Overprinting relationships are apparent in drill core and include variations in grain size, chilled margins, breccias and xenoliths. Many contacts display strong propylitic (chlorite+magnetite) alteration, locally overprinted by phyllic (quartz+illite +sericite+pyrite±chlorite) alteration. Milled matrix fluidised breccias identified in drill core (Figs 3 and 4) are likened to phreatomagmatic breccias (Sillitoe, 1985) typical of diatreme breccias in other SW Pacific rim intrusion-related gold deposits (Corbett, Semple and Leach, 1994; Corbett and Leach, in press). These typically comprise a milled matrix of finely comminuted basement shale and commonly subangular propylitised and argillic altered intrusive rock as well as basement shale. Breccias exhibit extreme variation in the relative abundance of phyllite and intrusive fragments (Fig 3). Longer drill hole intersections
REGIONAL GEOLOGY The Tolukuma vein system is localised near the intersection of the graben-like structural contact between Cretaceous basement (the Owen Stanley Metamorphics) and the overlying Pliocene Mount Davidson Volcanics, and is associated with a regional circular feature. Other vein systems are distributed about the southern rim of the circular feature (Langmead and McLeod, 1991), which may represent a caldera collapse ring fracture. Strongly folded locally carbonaceous phyllites containing metamorphic ‘sweat-out’ quartz veins predominate in the basement sequence. A 20 km wide graben is filled with volcanic rocks and described by Langmead and McLeod (1991) as intermediate lava, agglomerate and tuff into which dykes and stocks have been emplaced. Some intrusions in the region have been explored for porphyry and skarn style coppergold mineralisation, as at Etasi Creek (Fig 1). The regional structure of the Tolukuma district reflects the geological setting within the eastern segment of the New Guinea Orogen. Northwest trending structures formed during plate collision provide the pre-mineralisation fracture fabric to the region, evident at outcrop and Landsat scale. Northtrending graben structures are regionally significant as rock type boundaries and as Landsat or air photo linears, and appear to have localised the Mount Cameron Volcanic Complex some 30 km south of Tolukuma, as well as the circular feature immediately north of the deposit. Landsat and air photo linears trending NNW in the Tolukuma area correspond to fault scarps and are parallel to similar structures identified by St John (1990) from seismic data as reverse faults, no doubt related to collision associated with the Owen Stanley thrust 50 km to the NE.
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FIG 3 - Milled matrix fluidised breccias showing the variety of fragment types from volcanic-(including intrusion) to phyllite-rich. The cut core contains bedded tuffisite with accretionary lapilli. See pencil for scale.
Geology of Australian and Papua New Guinean Mineral Deposits
TOLUKUMA GOLD-SILVER DEPOSIT
contain bedded tuffisite layers of very finely milled material commonly with accretionary lapilli (Fig 4). However, most contacts are clearly intrusive and small fluidised dykes are common and locally grade into smaller fluidised crackle breccias. The milled matrix fluidised breccias are inferred to be formed by phreatomagmatic eruptions resulting from the superheating of ground water to form steam by a rapidly rising and depressurised intrusion. There is a similarity in appearance and geological setting, along a graben fault in association with high level intrusions, to the Namie Breccia (Sillitoe, Baker and Brooks, 1984) and other fault-bounded breccias in the Wau district. Although barren, these pre-mineralisation breccias are inferred to tap the degassing intrusion at depth which has been the source for the mineralisation described below.
FIG 5 - Cross section on 22 400 N, looking north, showing graben fault, hanging wall split and bonanza gold grades in the flexure. Note also the milled matrix fluidised (phreatomagmatic) breccias identified in drill core.
Three structural elements predominate at the Tolukuma deposit.
Structures trending NW to NNW developed in response to plate collision and impart a structural grain to the district. These parallel the Markham and Owen Stanley faults respectively. Sinistral north-south transpression is inferred to have dilated the Gulbadi and 120 ore-hosting NW structures. Many NW- to WNW-trending small splay veins are also present and locally host high grade gold mineralisation adjacent to the main Tolukuma vein. A pre-mineralisation puggy shear forms the margin of part of the 120 vein and many NW fractures are filled with early cockscomb quartz. Late NW-trending banded veins exploit fractures and cut the more northerly trending Gufinis veins (Fig 2).
The roughly north-trending graben structure has been reactivated during brittle deformation and mineralisation and now represents a complex fault zone. The dip varies from steep in the northern segment to moderately east in the Tolukuma Hill area, such that the contact dips below Tolukuma Hill (Figs 2 and 5). During structural reactivation of the graben, the steeply dipping portion has continued to the south as a fracture to form a hanging wall split above the dipping graben structure in the Tolukuma Hill area (Figs 2 and 5). Sinistral transpression on the Tolukuma graben fault and another parallel structure, termed the Eastern Rhomb fault, formed the dilational orehosting environment in NW-trending structures.
Faults trending NE are evident as pre-, syn- and postmineralisation structures. Mapping by Newmont suggests that the Tolimi fault becomes the graben contact south of Tolukuma Hill and so represents a deep basement structure. This moderately SE dipping and associated more steeply dipping structure (Fig 2) may have been instrumental in localising the magmatic source for the phreatomagmatic breccias and fluid upflow zone at the intersection with the graben fault. Several NE-trending structures transect the Gulbadi veins and may have created local high grade ore zones in flexures. The Tolukuma vein terminates against a series of NE faults which offset the graben structure in the Gufinis area (Fig 2).
FIG 4 - Close up of milled matrix fluidised breccias showing volcanic fragments and milled phyllite matrix. The bedded tuff at the top contains accretionary lapilli. See pencil for scale.
LOCAL STRUCTURE
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TOLUKUMA VEIN SYSTEM The Tolukuma vein system includes vein mineralisation at Tolukuma Hill, Gulbadi and Gufinis, the first two as a roughly continuous zone of about 1000 m length which has been traced by drilling to a depth of 300 m. The main Tolukuma vein in the Tolukuma Hill (Zone C) area extends for about 500 m as a laterally continuous banded fissure quartz vein which exploits the graben fault and hanging wall split (Fig 5). The flexure at the contact between the hanging wall split and the graben structure in the Tolukuma Hill area plunges south towards much of the milled matrix fluidised breccias. It therefore forms a natural fluid conduit and locus of bonanza grade gold mineralisation (Fig 5). Cross faults trending NE mark the northern termination of the main vein (Fig 2). In the Gufinis area smaller veins exploit parallel faults within the volcanic rocks and imbricate metamorphic–volcanic rock contacts. Early drilling by Newmont suggested that these veins display poor depth continuity. South of Tolukuma Hill, at Tolimi, a moderately dipping fault partly obscures the continuation of the vein which changes to a series of roughly parallel thin stockwork veins (Zone B). The Gulbadi and Tolukuma veins are inferred to represent essentially the same vein system which changed orientation in response to the varying structural domains on either side of the Tolimi fault (Fig 2). This structure has been active pre- to postmineralisation. The Gulbadi vein exploits pre-existing structures which have been dilated during mineralisation by the sinistral transpression on the north-trending structures. Other parallel vein systems in that area, which include the West Gulbadi and Ilivie veins, require further exploration. Recent drilling has demonstrated that mostly dacitic intrusions and contact breccias host the vein system at Gulbadi. Although the vein has been traced to a depth of 300 m, the lower portions are characterised by carbonate–base metal and local quartzsulphide styles of mineralisation (Leach and Corbett, 1995) at a much lower gold grade and a different gold:silver ratio than the epithermal style gold-silver mineralisation at higher elevations (Fig 6). Carbonate–base metal and quartz-sulphide gold systems generally display higher fineness gold and lack silver sulphosalts, as is the case at Tolukuma (Leach and Corbett, 1994; Corbett and Leach, in press). The 120 vein represents a pre-mineralisation NW-trending structure which has been dilated in a manner similar to the Gulbadi vein. Lower temperature crustiform-banded opaline quartz crops out at the northern end, and contrasts with the pervasive silicification and low temperature smectite clays at an elevation 200 m higher to the south, suggesting that the system is only slightly eroded. An inferred ore shoot yielded surface assays including 13 m at 15.85 g/t gold and 2 m at 97.5 g/t silver.
FIG 6 - Cross section on Gulbadi 270 N, looking NW, showing recent drilling by Dome Resources through the Gulbadi vein. Note the intrusion and contact breccia host rocks and decline in gold grade and fineness with depth.
to the veins at Tolukuma Hill. Interlayered illite-smectite near surface progressively changes to more crystalline illite at deeper levels. In the Gulbadi–Tolimi zone kaolinite overprints secondary feldspars at shallow levels, with gypsum and interlayered clays partially overprinting feldspar phases at depth. This zoning in clay mineralogy is indicative of latestage cool, and local moderately low pH fluids moving down the Gulbadi–Tolimi vein structures.
VEINS AND BRECCIAS
WALL ROCK ALTERATION
Four stages of fracturing and breccia vein development are identified at Tolukuma.
The host rocks at Tolukuma have undergone local early propylitic and phyllic alteration in the vicinity of the intrusions and a more widespread intermediate argillic alteration comprising quartz-albite-illite-chlorite±carbonate. Adularia occurs as an alteration of primary feldspar immediately adjacent to the veins at shallow levels in the Tolukuma Hill and 120 veins and at deeper levels in the Gulbadi, Tolimi and Gufinis veins. Illitic clay, which commonly postdates adularia alteration, is slightly crystalline and vertically zoned adjacent
Stage I intrusion-related breccias comprise the milled matrix fluidised breccias which exhibit weak clay-pyrite and local chlorite alteration. Stage II is the earliest of two stages of vein development and comprises colloform to crustiform banded quartz-adularia which locally alternates with wide bands of quartz pseudomorphs after bladed carbonate. A series of banded quartz±illite is transitional to later Stage III colloform banded quartz-clay and quartz-clay-carbonate veins which
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Geology of Australian and Papua New Guinean Mineral Deposits
TOLUKUMA GOLD-SILVER DEPOSIT
occur in fractures and thin breccia zones cutting the earlier quartz-adularia-bladed quartz after carbonate bands. A final Stage IV alteration comprising kaolinite-siderite-pyrite or marcasite as veins and cavity filling is accompanied by crustiform and colloform banded quartz-chalcedony in all but the Gulbadi–Tolimi veins. Banding in all the veins typically comprises alternating coarse and fine grained mineral phases, indicative of alternating slow and rapid mineral deposition respectively. Fluid inclusion analyses of coarse grained quartz indicate epithermal temperatures of deposition, with an average trapping temperature of 230–240oC. Trapping temperatures between individual bands in a single vein are indicative of a cyclic sequence of heating and cooling, generally with a temperature change of 5–15oC, although variations up to 55–60oC have been identified. The salinity of the vein-forming fluids at Tolukuma was generally very dilute, typically around 0–2 eq wt % NaCl. Plots of fluid inclusion homogenisation (Th) and freezing (Tm) temperatures are indicative of a predominantly boiling regime during Stage II, which changed to a mixing regime during Stage III. Hotter (>250oC) and more saline (>2 eq wt % NaCl) conditions were present at depth along the vein system, grading into cooler, dilute conditions at shallow levels. Anomalously high saline (>2 eq wt % NaCl) fluids at shallow levels in Tolukuma Hill are indicative of the access of a deep fluid to high levels along the flexure in the hanging wall split. Reversals in homogenisation temperatures in fluid inclusions at shallow levels in the northern extensions of the 120 vein are indicative of proximity to an outflow point of hot mineralised fluids sourced from depth to the south. This sequence of alteration and veining is indicative of an association with the emplacement of high level intrusions (Stage I), followed by early boiling of a circulating hydrothermal system (Stage II) which has progressively mixed during Stage III with descending cool ground water to form the clay bands and gas condensate, and later local lower pH fluids (Stage IV).
MINERALISATION The paragenetic sequence of sulphides and other ore phases is illustrated in Fig 7. Sulphides make up a relatively small (<2–3%) proportion of the veins, generally as very fine grained, thin, dark bands and locally in crosscutting fracture or breccia
zones. Iron sulphides are ubiquitous throughout the stages of vein development and brecciation, with pyrite then arsenopyrite dominating during Stage I brecciation and early Stage II vein development, and marcasite with pyrite in late Stage IV veins. Fibrous and radiating stibnite is a common phase in Stage IV quartz-chalcedony bands in the northern 120 and Gulbadi veins. Sphalerite decreases in iron content from red-yellow sphalerite intergrown with Stage II adularia-quartz and banded quartz, to a pale yellow to colourless phase in the later stages. Galena occurs in only trace amounts, generally in Stage III quartz-clay-carbonate bands. Chalcopyrite is only encountered as trace grains in Stage II veins, but is relatively common with later quartz-carbonate and to a lesser degree quartz-clay bands. Where sulphides have been deposited within very fine grained quartz bands overgrown on early coarse drusy quartz, the typical zoning of pyrite to sphalerite to galena to chalcopyrite to tennantite is interpreted to represent the deposition of sulphides from solution. A wide range of silver phases is encountered at Tolukuma. Silver sulphosalts and sulphides occur in most stages of veins, although they are most commonly encountered in Stage III quartz-carbonate or clay bands, overgrowing sphalerite, intergrown with galena and generally overgrown by chalcopyrite. The silver sulphosalts in order of abundance are: billingsleyite proustite-pyrargyrite [Ag3(As,Sb)S3], [Ag7(As,Sb)S6], the silver tetrahedrite freibergite [(Cu,Ag,Zn,Fe)12 Sb4S13] and polybasite-pearceite [(Ag,Cu)16 (AsSb)2S11]. The silver sulphosalts are generally copper- and antimony-rich in Stage II and early Stage III bands, becoming progressively more silver- and arsenic-rich. Argentite is encountered in late Stage IV quartz-kaolinite bands, whereas late-stage native silver is intergrown with chlorite-smectite in fine grained quartz bands in the Tolukuma Hill area. Other silver phases include stromeyerite (AgCuS), and a number of lead-silver sulphosalts, the latter locally containing appreciable tellurium (to 3%), especially within the Gulbadi veins. Gold occurs almost exclusively as electrum, although a few gold-silver sulphides such as uytenbogaardtite (AgAuS2) occur as transitions to argentite. Although gold has been encountered in association with early Stage II quartz-adularia and possibly quartz after bladed carbonate, the vast majority of electrum is associated with thin, fine grained sulphide layers in late Stage II and Stage III banded quartz and Stage III quartz-carbonate and quartz-clay veins. The electrum occurs as anhedral grains ranging from to <1 to >200 µm, mainly as inclusions in silver sulphosalts, but locally as inclusions in pyrite, intergrown with silver sulphosalts, chalcopyrite and rarely galena, and as free grains in quartz, carbonate or clay gangue phases. Tolukuma electrum has a fineness of 597–771, with an average of 686. The silver content increases from relatively high fineness electrum associated with Stage II quartz-adularia or bladed carbonate (average 730), to low fineness electrum within Stage III carbonate-clay bands (average 671). This suggests that there has been cooling between the development of the Stage II to Stage III veins and associated mineralisation.
ORE GENESIS
FIG 7 - Paragenetic sequence of mineral deposition at Tolukuma.
Geology of Australian and Papua New Guinean Mineral Deposits
The Tolukuma vein system exhibits aspects of both epithermal style gold-silver deposits in which meteoric fluids dominate
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(Henley and Ellis, 1983), and porphyry-related carbonate–base metal–gold systems as described by Leach and Corbett (1995). Evidence for the magmatic input is provided by the presence of the diatreme-like breccias, and the association of gold deposition with carbonates+rare base metal phases. The regional Tolimi and Graben faults have allowed meteoric waters to become heated by contact with an inferred buried intrusion, promoting intrusive phreatomagmatic eruptions which display clay-pyrite alteration and are inferred to exploit pre-existing structures as dyke-like bodies. Circulating dilute meteoric waters have become heated by the intrusion at depth, and under the influence of the dilational structural environment, deposited the banded quartz, adularia and bladed carbonate (later replaced by quartz) veins. These veins are characteristic of boiling conditions within an epithermal environment. Only minor gold deposition occurred at this stage. A later more saline fluid with a significant porphyry signature mixed with descending ground water and lower pH bicarbonate condensate fluid to form banded quartz-carbonate and clay veins. Fluids are inferred to have been channelled laterally from a fluid upflow in the vicinity of the phreatomagmatic breccias in the Tolimi area (Corbett, Semple and Leach, 1994). This mixing style of mineral deposition is best developed in the Tolukuma Hill area where the flexure in the graben structure plunges to intersect the milled matrix fluidised breccias, focussing the upflow to form high and local bonanza gold and silver grades. Outflow settings are evident at the northern and southern extensions of the vein system at Gulbadi and Gufinis where gold grades decrease away from the fluid source. The carbonate–base metal and deeper quartz–sulphide style mineralisation (Leach and Corbett, 1995) in the recent drill holes at Gulbadi are further indications of the zoned intrusionrelated nature of the Tolukuma epithermal vein system.
MINE GEOLOGICAL METHODS Mining at Tolukuma since December 1995 has shown some serious errors in the original reserve calculations. Mine planning, geological mapping and grade control facilitated a reconciliation which identified a progressive marked reduction from the original reserves to ore mined on each bench. The inaccurate reserve estimate appears to stem from the modelling technique used by the consultant company in which blocks 10 m high by 5 m long by 5 m wide overstated the reserves for an orebody which in most cases is 2 to 3 m wide. A recent in-house reassessment of the reserve used 1 m and locally 0.5 m wide blocks, indicating that more accurate ore reserve calculations can be achieved by use of the true vein thicknesses and small ore block sizes. Nevertheless the loss of reserve has been more than compensated for by the success of continued drilling at Gulbadi and the previously unrecognised underground resource at Tolukuma Hill (Zone C).
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ACKNOWLEDGEMENTS The authors wish to acknowledge permission by Dome Resources NL to publish this paper and contributions from the mine manager and mine geologists.
REFERENCES Corbett, G J and Leach, T M, in press. Southwest Pacific Rim GoldCopper Systems: Structure, Alteration, and Mineralisation, Short Course Manual, Society of Economic Geologists Special Publication Series (Ed: M Coveney Jr) (Society of Economic Geologists: Littleton, Colorado). Corbett, G J, Semple, D G and Leach, T M, 1994. The Tolukuma goldsilver vein system, Papua New Guinea, in PNG Geology Exploration and Mining Conference (Ed: R Rogerson), pp 230–238 (The Australasian Institute of Mining and Metallurgy: Melbourne). Henley, R W and Ellis, A J, 1983. Geothermal systems ancient and modern: a geochemical review, Earth Science Reviews, 19:1–50. Langmead, R P and McLeod, R L, 1990. Tolukuma gold deposit, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1777–1781 (The Australasian Institute of Mining and Metallurgy: Melbourne). Langmead, R P and McLeod, R L, 1991. Characteristics of the Tolukuma Au-Ag deposit, in PNG Geology Exploration and Mining Conference (Ed: R Rogerson), pp 77–81 (The Australasian Institute of Mining and Metallurgy: Melbourne). Leach, T M and Corbett, G J, 1994. Porphyry-related carbonate base metal gold systems: characteristics, in PNG Geology Exploration and Mining Conference (Ed: R Rogerson), pp 84–91 (The Australasian Institute of Mining and Metallurgy: Melbourne). Leach, T M and Corbett, G J, 1995. Characteristics of low sulphidation gold-copper systems in the southwest Pacific, in Proceedings Pacrim ’95 Congress, Auckland 1995 (Eds: J L Mauk and J D St George), pp 327–332 (The Australasian Institute of Mining and Metallurgy: Melbourne). Semple, D G, Corbett, G J and Leach T M, 1995. The Tolukuma gold silver-vein system, Papua New Guinea, in Proceedings Pacrim ’95Congress, Auckland 1995 (Eds: J L Mauk and J D St George), pp 509–514 (The Australasian Institute of Mining and Metallurgy: Melbourne). Sillitoe, R H, 1985. Ore breccias in volcanoplutonic arcs, Economic Geology, 80:1467–1514. Sillitoe, R H, Baker, E M and Brook, W A, 1984, Gold deposits and hydrothermal eruption breccias associated with a maar volcano at Wau, Papua New Guinea, Economic Geology, 79:638–655. St John, V P, 1990. Regional gravity and structure of the Eastern Papuan Fold Belt, in Petroleum Exploration in Papua New Guinea: Proceedings of the First Petroleum Conference (Eds G J Carman and Z Carman), pp 311–318 (PNG Chamber of Mines: Port Moresby).
Geology of Australian and Papua New Guinean Mineral Deposits
Dugmore, M A and Leaman, P W, 1998. Mount Bini copper-gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 843–848 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Mount Bini copper-gold deposit 1
by M A Dugmore and P W Leaman
2
INTRODUCTION The deposit is 50 km ENE of Port Moresby in the Central Province of PNG at lat 9ο18′S, long 147o35′E, on the Port Moresby (SC 55–7) 1:250 000 scale map sheet (Fig 1). The deposit is on the west side of the Owen Stanley Range at an elevation of 1200 m ASL, in steep terrain covered by dense tropical rainforest. Access to the prospect is by helicopter or on foot from Owers Corner on the Kokoda Track. An Inferred Resource of 85 Mt at 0.4% copper and 0.6 g/t gold has been estimated, with the deposit remaining open at depth. Title is held by BHP Minerals.
EXPLORATION HISTORY The deposit was discovered during helicopter-supported drainage geochemical sampling by a BHP Minerals
1.
Senior Project Geologist, BHP Minerals Exploration, QCL House, 40 McDougall Street, Milton Qld 4064.
2.
Exploration Manager Copper/Gold – Asia, BHP Minerals Asia Pacific Pty, 1 Garden Road, Central, Hong Kong.
Exploration team in May 1992 (Dugmore, Leaman and Philip, 1996). An anomalous value of 157 ppm gold in a panned concentrate sample and pyritic silicified float were recorded from Ofi Creek about 2 km downstream from the deposit. A sample of this float assayed 20.7 g/t gold, 463 g/t silver, 0.14% copper and 0.6% lead. Follow-up detailed drainage sampling results defined anomalous drainages. A systematic program of soil and rock chip sampling along ridge and spur lines proved effective in delineating an area of anomalous gold (>0.2 g/t) of plan dimensions 2000 by 200 m. Within this anomalous zone, copper (>150 ppm) and molybdenum (>18 ppm) define an area of 650 by 350 m, which is broadly coincident with the mineralised stock. Lead is anomalous in the periphery over an area of 1800 by 1300 m. Porphyry-style mineralisation was identified during geological mapping and trenching, following the soil sampling program. Diamond drilling to test the anomalous rock and trench sampling results commenced in September 1993, and hole BND001 intersected 0.31% copper and 0.47 g/t gold from 104 to 339.2 m depth. Seven diamond drill holes in two drilling phases (total 2421 m) have been completed at the prospect.
FIG 1 - Regional geological map of the Mount Bini area (modified after Pieters, 1978).
Geology of Australian and Papua New Guinean Mineral Deposits
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REGIONAL GEOLOGY The regional geological setting of the area has been described by Pieters (1978), and the tectonic setting has been outlined by Leaman (1996). The deposit occurs along a mineralised trend extending at least 400 km SE from the gold deposits of the Wau District. This trend also contains the porphyry copper-gold system at Wafi River and the epithermal gold systems at Kerimenge, Hidden Valley and Tolukuma. The oldest rocks in the area are phyllites and slates of the Cretaceous to Eocene Kagi Metamorphics, part of the Jurassic to Cretaceous Owen Stanley Metamorphic Complex (Fig 1). Deformation and metamorphism to lower greenschist facies probably occurred in the Middle Miocene (Rogerson and McKee, 1990), when the Papuan Ultramafic Belt collided with, and was thrust over, the leading edge of the Australian Plate (Papuan Plateau). The Mount Bini deposit occurs within a 15 to 20 km wide NNE-trending interpreted extensional zone in which basement metamorphic rocks are unconformably overlain by subaerial volcanic rocks and intruded by high level plutonic rocks. The Pliocene basic and minor intermediate volcaniclastic rocks of the Astrolabe Agglomerate predominate to the south where they appear to have been deposited in a local extensional basin. Immediately north of this formation is the newly discovered calc-alkaline Bavu igneous complex within which the Mount Bini deposit occurs. The complex may possibly be the coeval intrusive source of the Astrolabe Agglomerate. Porphyritic, vesicular basalt and andesite lava and minor Pleistocene eruptive centres occur to the north of the Bavu igneous complex. Isolated areas of Late Miocene to Pliocene basic to intermediate subaerial volcanic rock, sediment and shallow intrusions of the Mount Cameron Volcanic Complex are related to a partially eroded stratavolcano to the west. The low sulphidation epithermal gold mineralisation at Tolukuma, 100 km NW of Mount Bini, is hosted by this sequence (Langmead and McLeod, 1990; Semple, Corbett and Leach, this publication).
A number of intrusive phases are evident, with P 1 and P2 being the two oldest. P1 seems to be locally controlled by a NEto ENE-trending structure, which may also control the local drainage. This phase is a fine-grained phenocryst-rich porphyritic quartz diorite with phenocrysts of plagioclase within a microcrystalline mosaic groundmass of quartz, orthoclase and biotite. Phase P2 forms the southern portion of the stock and is centred on the peak of Mount Bini. This phase is a quartz diorite porphyry with abundant plagioclase, biotite, and clinoamphibole phenocrysts embedded in a fine grained matrix of orthoclase, plagioclase, and secondary biotite. Accessory minerals include magnetite, sphene, apatite and zircon. Patches of tremolite-actinolite-magnetite may represent altered calc-silicate xenolithic material. Several phases of late dykes, to several tens of metres wide, transgress the stock and metamorphic rocks (Fig 2). The mineralogical composition of these dykes is similar to that of the P1 and P2 intrusions but the dykes are considerably less altered and mineralised. Xenoliths of potassic-altered phyllite and mineralised diorite are common in the dykes, and flow alignment of plagioclase phenocrysts indicates a high level subvolcanic intrusive setting. In general, the intensity of hydrothermal alteration decreases in the younger intrusive phases. The Sirimu dyke, one of the larger dykes, has a similar composition to the P2 intrusion and occurs 350 m SW of the stock. The dyke has a ENE strike, parallel to the airphoto structural trend, and is 350 m long and 45 m wide. The Track diorite is an unaltered pyroxene diorite occurring 500 m north of the Bini Porphyry stock. This diorite is relatively potassium-rich and strongly magnetic. A helicopter-borne aeromagnetic survey has shown that the Bini Porphyry stock and Track diorite are two of many plutons which comprise the Bavu igneous complex. The Bini Porphyry occurs on the SE periphery of this complex, which is 10 km long by 4 km wide and trends NW. It is likely that porphyry emplacement was controlled by high angle structures bounding the margin of the Bavu igneous complex.
STRUCTURE ORE DEPOSIT FEATURES LITHOLOGY Metasediments The basement rocks at Mount Bini are predominantly greenschist facies pelitic metasediments. Quartz-mica slate and banded, locally carbonaceous phyllite are common. A well developed crenulation cleavage is evident with metamorphic quartz veins and boudins frequent along foliation planes.
Intrusives A variety of potassium-rich calc-alkaline rocks occurs as stocks and dykes in the Mount Bini area. The Bini Porphyry of age 4.42±0.04 Myr is a composite intrusive stock (Fig 2) with a circular topographic expression rising to 1200 m ASL, some 140 m above the surrounding stream base. The stock is approximately 650 by 275 m in plan dimensions and occupies the NW side of Mount Bini. It is a dyke-like body which appears to have been emplaced at the intersection of ENE- and NNE-trending lineaments.
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The main structural feature and control of mineralisation at Mount Bini is the intersection of linear ENE and NNE structures interpreted from Landsat imagery and aerial photographs. These structures do not appear to have any displacement along them post-emplacement.
ALTERATION Hydrothermal alteration The earliest phase of hydrothermal alteration is potassic (Fig 3). It is pervasive, generally of moderate intensity, and affects the composite intrusion and the Sirimu dyke. Phyllite is potassically altered where it is in contact with the intrusion. The most intense potassic alteration is associated with the P1 phase of the Bini Porphyry. Secondary biotite is the dominant mineral and occurs as pervasive flakes replacing earlier ferromagnesian phenocrysts and also in rare veinlets. Orthoclase post-dates early pervasive biotite alteration. Orthoclase also occurs as a fine micromosaic groundmass and as narrow and diffuse margins to quartz-sulphide±magnetite veinlets. Anhydrite is a minor component.
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT BINI COPPER-GOLD DEPOSIT
FIG 2 - Geology and alteration map of the Mount Bini deposit.
Phyllic alteration is widespread and forms a halo of some 1500 by 1500 m around the Bini Porphyry and Sirimu dyke. The alteration assemblage comprises sericite, quartz and pyrite with minor rutile-leucoxene and base metal sulphides. It postdates the potassic alteration event and is moderately to strongly developed. The phyllic alteration is best developed in the phyllite and is restricted to the outer part of the composite intrusion. Intermediate argillic alteration is present as veins and zones crosscutting mainly potassic alteration within the porphyry intrusive. This alteration is characterised (Sillitoe and Gappe, 1984) by an assemblage of sericite and chlorite with clay (illite), and replacement of magnetite by hypogene hematite. Propylitic alteration overprints the potassic, phyllic and intermediate argillic, alteration zones. It is best developed over the Bini Porphyry stock but is evident throughout the phyllic alteration zone. The mineral assemblage is characterised by magnetite, chlorite, tremolite-actinolite, quartz and sulphides (±carbonate± epidote±anhydrite) close to the central and most mineralised part of the area. This alteration style changes outward through pervasive disseminated and vein chlorite to quartz-carbonate veinlets with pyrite and epidote (±magnetite±chalcopyrite). Chlorite is generally pervasive as retrogressive alteration of secondary biotite.
Geology of Australian and Papua New Guinean Mineral Deposits
Tourmaline is developed in narrow breccias within the phyllic alteration zone and occurs in an annulus around the stock. The breccias are commonly less than 2 m wide and are usually composed of coarse fragments of cherty, fine-grained quartz and recrystallised vein quartz in a matrix of green (ironrich, titanium-poor) tourmaline, quartz, pyrite and sericite. The mineral assemblages for each alteration type are summarised in Table 1 and a cross section showing the distribution of alteration styles is shown in Fig 3.
Supergene alteration Supergene alteration resulting from weathering is locally well developed. The depth of weathering is irregular and varies from 70 m beneath the top of Mount Bini to outcropping fresh rock in watercourses. Weathering of silicate minerals, especially plagioclase and sericite, by downward percolation of acidic, oxidising waters has resulted in surface argillic alteration (kaolinite) overprinting earlier hydrothermal alteration types. Sulphides have been almost completely oxidised to goethite and limonite.
MINERALISATION Porphyry Hypogene mineralisation has occurred in a complex series of
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TABLE 1 Alteration assemblages, Mount Bini deposit. Alteration type
Definitive minerals
Sulphides and oxides
Potassic
Biotite, orthoclase, quartz±anhydrite
Pyrite, chalcopyrite, magnetite, molybdenite, rutile
Intermediate argillic
Sericite, chlorite, clay (illite)
Hematite
Phyllic
Sericite, quartz
Pyrite
Propylitic
Actinolite, quartz, chlorite±carbonate±epidote±anhydrite
Pyrite, chalcopyrite, fahlore, magnetite, galena, sphalerite
Tourmaline
Tourmaline, quartz, sericite
Pyrite
FIG 3 - Cross section A-B showing distribution of alteration styles, looking ENE.
events and is hosted by both the porphyry complex and the altered phyllite. The earliest phase is associated with pervasive biotite (potassic) alteration. Sulphides are commonly fine grained and comprise pyrite and chalcopyrite, associated with minor rutile. Sulphides are both fracture controlled and disseminated throughout the intrusions, and also occur within patches of magnetite and secondary biotite and anhydrite. The early phase of mineralisation is cut by a stockwork of quartz stringers and veins containing pyrite, chalcopyrite, magnetite and molybdenite. Gold mainly occurs as inclusions in chalcopyrite. The stockwork is locally well developed within the brittle phyllite country rock and often coincides with strong overprinting of phyllic alteration. Thin veinlets of orthoclase-biotite-quartz±chlorite±carbonate containing pyrite and chalcopyrite cut the stockwork mineralisation. Coarse grained sulphides are related to the propylitic alteration
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event. Minor fahlore (tennantite-tetrahedrite) is associated with propylitisation. The tourmaline breccias are unmineralised except for pyrite. Galena- and sphalerite-bearing veinlets and disseminations occur within the metasediment immediately peripheral to the Bini Porphyry. Galena-sphalerite-carbonate-gold veins outcrop in Ofi Creek 800 m SW of and several hundred metres below the top of Mount Bini. These veins are associated with propylitic alteration. Figure 4 shows the metal and mineral zoning of the deposit in cross section. Secondary mineralisation is minor. A small leached cap has minor malachite and occurs on ridges and in higher levels of water courses. Copper and gold concentrations in the cap increase from 500 ppm copper and 0.2 g/t gold in the upper 15 to 20 m to 0.18% copper and 0.5 g/t gold in the lower 50 m.
Geology of Australian and Papua New Guinean Mineral Deposits
MOUNT BINI COPPER-GOLD DEPOSIT
FIG 4 - Cross section A-B showing metal and mineral zoning, looking ENE.
Supergene copper mineralisation occurs in outcrops in watercourses, and has been noted in drill holes to extend for about 20 m below the lower level of oxidation. Fine grained chalcocite and covellite replace and rim chalcopyrite along fractures, locally elevating copper grades.
Epithermal Epithermal mineralisation overprints the porphyry copper-gold mineralisation. Veins to a metre wide occur over a strike length of 1400 m and in an ENE- to NE-trending zone parallel to the major lineament trends. Within this zone some veins strike NW. Minor float of jasper and opal occurs on ridges and probably represents the higher part of the epithermal system. Chalcedony veins and stockworks with elevated gold values (eg 20 m averaging 0.56 g/t) crop out immediately west of the Bini Porphyry stock. Quartz veins with crustiform and colloform banding, lattice textures, and manganoan calcite have only been intersected in drill holes and contain elevated silver values, to 8 m averaging 19 g/t. Stibnite is associated with these quartz veins. Veins of prismatic quartz-pyrite (±chlorite±sericite± smectite±barite±base metal sulphides) occur as prominent ridges 750 m SW of the Bini Porphyry stock and have associated gold and silver mineralisation. Tourmaline is spatially related to the epithermal mineralisation occurring coincidentally and immediately north of this mineralisation.
Geology of Australian and Papua New Guinean Mineral Deposits
ORE GENESIS Emplacement of the Bini Porphyry can be related to partial melting of the Australian Continental Plate as the Solomon Sea Plate was subducted beneath it during the Pliocene (Leaman, 1996). Porphyry copper mineralisation is centred on the Bini Porphyry, the probable progenitor to mineralisation. Early stage biotite alteration is of late magmatic origin and coincides with the Bini Porphyry intrusion. Minor copper and gold mineralisation was emplaced at this time. The bulk of the copper and gold mineralisation was emplaced in the outer part of the Bini Porphyry associated with fracturing of the stock, quartz stringer veining and orthoclase alteration, also of magmatic origin. Mixing of the hot magmatic fluid with cooler meteoric waters peripheral to (and probably above) the stock produced pervasive sericitic alteration and minor mineralisation. As the hydrothermal system waned and fluid temperatures cooled, propylitic alteration overprinted the entire complex. Uplift and exhumation of approximately 1 km of the roof zone exposed the porphyry system near to its current level of exposure. An overprinting low sulphidation epithermal quartz vein system resulted from circulating meteoric waters (neutral chloride) which were focussed along structures which controlled the emplacement of the porphyry system.
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ACKNOWLEDGEMENTS The authors thank BHP Minerals for permission to publish this paper and acknowledge the contributions of management and support staff, particularly those staff who worked on the project including R Philip, D Stephens, C Handley, L Pile, M Koibua, P Wanye and C Towill.
REFERENCES Dugmore, M A, Leaman, P W and Philip, R, 1996. Discovery of the Mt Bini porphyry copper-gold-molybdenum deposit in the Owen Stanley Ranges, Papua New Guinea - a geochemical case history, Journal of Geochemical Exploration, 57(1-3):84–100. Langmead, R P and McLeod, R L, 1990. Tolukuma gold deposit, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1777–1781, (The Australasian Institute of Mining and Metallurgy: Melbourne).
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Leaman, P W, 1996. The Mt Bini porphyry copper-gold deposit and its tectonic setting, Papua New Guinea, in Porphyry Related Copper and Gold deposits of the Asia Pacific Region Conference Proceedings, Cairns, 12-13 August 1996, pp 13.1–13.10 (Australian Mineral Foundation: Adelaide). Pieters, P E, 1978. Port Moresby-Kalo-Aroa, Papua New Guinea 1:250 000 geological series, Geological Survey of Papua New Guinea, Geological Map and Explanatory Notes SC 55–6,7,11. Rogerson, R and McKee, C, 1990. Geology, volcanism and mineral deposits of Papua New Guinea, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1689–1707 (The Australasian Institute of Mining and Metallurgy: Melbourne). Sillitoe, R H and Gappe, I R Jr, 1984. Philippine porphyry copper deposits: geologic setting and characteristics, United Nations Economic and Social Commission for Asia and the Pacific (ESCAP), Committee for Co-ordination of Offshore Prospecting (CCOP), Technical Publication No 14.
Geology of Australian and Papua New Guinean Mineral Deposits
Chapple, K G and Ibil, S, 1998. Gameta gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 849–854 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Gameta gold deposit 1
by K G Chapple and S Ibil
2
INTRODUCTION
EXPLORATION HISTORY
The deposit is on the NE corner of Fergusson Island in the D'Entrecasteaux Islands of Milne Bay Province, PNG, at lat 9o25′S, long 150ο47′E or AMG coordinates 256 740 E, 8 959 000 N, on the Fergusson (SC56–5) 1:250 000 scale and the Koitabu (9079) 1:100 000 scale map sheets (Fig 1). The property is owned by Union Mining (PNG) NL (UMG) and is 105 km NNE of Alotau and 33 km ESE of the now closed UMG Wapolu gold mine.
Exploration in the D'Entrecasteaux Islands prior to 1983 was directed towards nickel, chromium, base metal or porphyry copper mineralisation. Within the Gameta area, exploration for gold deposits was undertaken by Esso Papua New Guinea Inc (Esso) from 1983 to 1986, City Resources NL from 1987 to 1988, Ingold Holdings NL from 1989 to 1990 and UMG from 1995.
ESSO Helicopter-supported regional stream sediment, pan concentrate and rock chip sampling in 1983 defined five drainages anomalous in gold in which several rock chip samples assayed +1 g/t gold. Detailed stream sediment, rock chip and soil sampling in 1984 to 1986 identified the Cape Vinall, Bilu Bilu (Gameta) and Niyaila Creek prospects. Further exploration was recommended at the Cape Vinall and Bilu Bilu prospects, but had not been carried out when Esso withdrew from exploration in PNG in 1986.
CITY RESOURCES AND INGOLD HOLDINGS After acquiring the area from Esso, City Resources in 1987 carried out detailed soil sampling, geological mapping and trenching in the Niyaila Creek area, to the SE of Gameta. Five diamond core holes for 824 m were drilled with negative result, and the tenement was joint ventured to Ingold Holdings in 1989. Ingold undertook limited stream sediment and rock sampling in 1989, but could not repeat the gold anomalies during follow up sampling in 1990.
UNION MINING
FIG 1 - Location map, Gameta gold deposit.
The Inferred Resource from surface results and 130 reverse circulation (RC) drill holes is 2.30 Mt at 2.3 g/t gold, at a 1.0 g/t gold composite cutoff, or 170 000 oz of contained gold. Evaluation of the property is proceeding and is currently being funded by farm-in partner Yamana Resources Inc, a North American based, Vancouver listed company.
1.
Exploration Manager, Union Mining NL, PO Box 728, Spring Hill Qld 4004.
2.
Supervising Geologist-PNG, Union Mining (PNG) NL, PO Box 591 Alotau MBP, Papua New Guinea.
Geology of Australian and Papua New Guinean Mineral Deposits
When UMG acquired the Gameta area in 1994, the exploration effort was directed towards the delineation of open pittable, oxidised gold resources that could be economically mined, transported to, and processed at the then proposed Wapolu gold processing plant. Reconnaissance visits were conducted by UMG geologist Sumun Ibil in February and April 1995. Very encouraging anomalous gold values were obtained in the area from Wadelei, through Cape Vinall, to just outside the SE corner of EL 1070 near Gameta village, a distance of 8 km. These included minus 80 mesh stream sediment gold values to 0.11 ppm, pan concentrate results to 5.88 ppm gold, bulk cyanide leachable values in stream sediment to 1090 ppb gold and rock sample results to 7.46 g/t gold. In particular, 12 of the 23 rock samples collected were reported to contain in excess of 1.0 g/t gold. In Gaimayuisa Creek, a major landslide in April 1993 exposed a dip slope of pyritic, altered and brecciated basement metamorphic rock which extends up dip, to the SW, along the creek bed for at least 500 m. In this exposure the zone is up to 15
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K G CHAPPLE and S IBIL
m thick and is considered, at least in part, to represent a dome detachment fault. The exposure clearly shows an original mylonitic texture which has been overprinted by later brittle fracturing. Similar pyritic rock float to that exposed within the zone has been observed throughout the anomalous area. Following an appraisal visit by the authors in May 1995, a detailed evaluation was commenced in the Bilu Bilu area. The average gold value for the 254 rock outcrop, rock float and trench channel samples collected during this program was 1.94 g/t gold. Drilling in the Bilu Bilu grid and the southern part of the Cape Vinall grid areas commenced in December 1995. To date a total of 130 RC holes have been drilled for 5410 m. Future work will be directed towards investigation and drilling of the up slope extensions of the mineralisation in an attempt to increase the resource size.
REGIONAL GEOLOGY The regional geology of the D'Entrecasteaux Islands is characterised by several prominent domed, Cretaceous or older, medium to high grade metamorphosed basement highs or metamorphic core complexes, and Pliocene intrusives. These are covered, in part, by an unmetamorphosed sequence of Cretaceous or older ultramafic rocks, Tertiary and Quaternary volcanic rocks and Recent raised coral reefs, alluvium and talus. Previous descriptions of the regional geology have been provided by Davies and Ives (1965), Davies (1973) and Hill (1989, 1991). The regional geology of NE Fergusson Island is shown on Fig 2.
BASEMENT METAMORPHIC ROCKS The basement rocks represent emergent domes of metamorphosed, layered to highly deformed, sialic continental crust belonging to the leading edge of the Australian Plate. The metamorphic rocks are thought to be at least Cretaceous in age but could be Upper Palaeozoic or older as ages as old as Cretaceous have been obtained by Baldwin et al (1993) using U-Pb content of zircons. Rock types present include quartzofeldspathic schist and gneiss, metabasalt, basic schist, calcic schist, amphibolite, contorted laminated limestone, eclogite, granulite and migmatite. On Goodenough and Fergusson islands the basement metamorphic rocks have been subdivided into an outer zone of regularly layered, commonly mylonitic, gneiss and a high grade, complexly deformed core (Hill, 1991). The outer zone consists of greenschist to amphibolite grade gneiss of felsic to mafic composition, with some pelitic and calcareous units and narrow zones of lower amphibolite to greenschist grade biotiterich schist. The core zone is dominantly migmatitic with bands and blocks of mafic granulite and eclogite. The outer core rocks are considered to represent retrogressed equivalents of the core zone metamorphic rocks.
ULTRAMAFIC ROCKS The ultramafic rocks are unmetamorphosed, overlie the basement metamorphic rocks in faulted contact, and are considered to be at least Cretaceous in age. They comprise dunite, harzburgite wehrlite, enstatite pyroxenite and websterite and form rounded convex hills with sparse forest or bracken cover. Compositionally they are olivineorthopyroxene-chromite-clinopyroxene rocks that are variably serpentinised. They are considered to represent a sea floor plate of typical upper mantle rocks.
PLIOCENE INTRUSIVE ROCKS Intrusives crop out mainly within the central core zones of the metamorphic core complexes. They are primarily granodiorite with minor granite and diorite and intrude the basement metamorphic rocks and, in places, the overlying ultramafic rocks.
PLIOCENE TO HOLOCENE VOLCANIC ROCKS Pliocene to Holocene calc-alkaline or andesitic arc–trench type volcanic rocks in the D'Entrecasteaux Islands decrease in age from east to west, with K-Ar and Rb-Sr ages of volcanic rocks in SW Fergusson Island of 1.2 to 0.4 Myr, and a Rb-Sr age for the volcanic rocks on Normanby Island, to the south, of 3.2 Myr (Hill, 1990). The volcanic rocks unconformably overlie the ultramafic and the basement metamorphic rocks (Davies, 1973). Several active hot spring systems in the area appear to be associated with the Quaternary volcanism and/or waning stages of the Pliocene volcanism, and are controlled by the major core complex bounding faults and/or later crosscutting graben structures. Epithermal style gold mineralisation is present and is considered to be associated with the younger volcanic activity.
LOCAL GEOLOGY FIG 2 - Regional geological map of NE Fergusson Island (after Davies and Ives, 1965; Hill, 1991).
850
The deposit lies along the detachment fault zone (DFZ) developed on the eastern margin of the core complex referred to
Geology of Australian and Papua New Guinean Mineral Deposits
GAMETA GOLD DEPOSIT
here as the Oiatabu Dome (Fig 2). This structure corresponds to the Elologea and Gameta faults of Davies and Ives (1965). Very little of the original ultramafic upper plate remains, as erosion has now exposed the DFZ and the underlying metamorphic rocks in most areas.
BASEMENT METAMORPHIC ROCKS The metamorphic rocks in the Gameta area belong to the outer core zone. They consist of interbanded quartzo-feldspathic schist and gneiss, amphibolite and mafic schist and gneiss of amphibolite facies. There is a prominent dome-conformable layering which dips at about 40o to the NE.
Two metamorphic units or layers have been recognised from drill holes (Fig 3). Unit 1 is the upper and consists of felsicdominant gneiss, with minor interbanded amphibolite, mafic gneiss and minor felsic and mafic schist. The felsic members consist of quartz, muscovite and feldspar with minor chlorite, biotite, epidote and rutile. The unit is up to 35 m thick perpendicular to layering and is present throughout the Bilu Bilu grid area. In some areas the uppermost portion coincides with the DFZ. The unit hosts most of the known gold mineralisation developed within the basement metamorphic rocks.
FIG 3 - Geological plan of the Gameta gold deposit.
Geology of Australian and Papua New Guinean Mineral Deposits
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Unit 1 grades into the underlying unit 2, which consists of mafic-dominant gneiss and lesser amphibolite and schist with minor interbanded felsic gneiss. The mafic members consist of plagioclase, biotite, hornblende and chlorite with minor rutile or ilmenite. Unit 2 crops out in the Cape Vinall area, is of unknown thickness, and has been intersected in drill holes at depth in the Bilu Bilu grid area.
ULTRAMAFIC ROCKS Ultramafic rocks mainly crop out in the southern part of the Gameta area in the line 1900 to 2100 SE area. In most other areas they have been recently eroded, but limited areas of scattered scree are evident in some areas (eg in the line 150 SE area). They are unmetamorphosed but often extensively altered. In thin section they usually display characteristic mesh-textured serpentinite structures.
INTRUSIVE ROCKS Intrusives crop out in the Gameta area and have been intersected in drill holes. They are mainly granodiorite to diorite in composition and vary from fine to medium grained to porphyritic. Drill intersections on lines 400 SE, 700 SE, 2000 SE and 2100 SE are dyke-like occurrences up to 10 m thick. Intersections on lines 900 SE and 1000 SE are thicker and indicate the presence of sills in excess of 60 m thick. It is not known whether they are original granitic intrusives within the basement or younger intrusives associated with the diapiric rise of the core complex, similar to the intrusives near the central part of the Oiatabu Dome to the west of, and up slope of, the deposit. RC drill chip thin section work shows that the granodiorite is composed of quartz, albite and potassium feldspar with calcite and blue-green amphibole. The amphibolite is thought to have originated from earlier amphibole or pyroxene, with the strong blue colour indicating a high sodium content. The dioritic intrusives are biotite-plagioclase-hornblende microdiorites. They contain plagioclase, hornblende and biotite and minor sphene, calcite, ilmenite or magnetite and traces of disseminated sulphide. The plagioclase phenocrysts are strongly zoned. Finer grained fragments of a similar diorite in some drill hole intersections indicate chilled margins, suggesting the presence of dykes or sills. Pegmatitic dykes have been noted in some areas.
EPITHERMAL-TEXTURED QUARTZ VEINS Epithermal-textured quartz veins have been identified within the basement metamorphic rocks in the deposit area. They are colloform banded, vuggy and locally brecciated; thin section examination indicates that the silica is mainly chalcedony with various bands varying in style from cherty to fibrous. Much of the fibrous chalcedony has recrystallised to anhedral quartz. Silica bands vary in colour from very pale to medium grey with the darker colour due to fine grained sulphide. Fine grained arsenopyrite, to 4%, occurs within the cherty chalcedonic bands, while pyrite at less than 1% is more closely associated with quartz. A fine grained green mica that is closely associated with the quartz has been tentatively identified as roscoelite.
ALLUVIUM AND LANDSLIDE DEBRIS Recent surficial cover of talus and alluvium is present
852
throughout most of the deposit area. It is locally more than 50 m thick between the base of the dip slope and the coast, and in places has hindered drill testing of the prospective basement horizons. Major landsliding in early 1993 exposed dip slopes of mineralised basement in Gaimayuisa and Moaduwe creeks (Fig 3). Extensive boulder trains, probably more than 10 m thick, have been developed from the base of the dip slopes to the coast, and have significantly altered the courses of the two creeks. The basement mineralisation was therefore not exposed when the area was explored by Esso between 1983 and 1986.
RECENT SILICEOUS MATERIAL Siliceous, epithermal-textured, sinter-like material has been locally accumulating near the base of the dip slope. These occurrences often contain crystalline stibnite, and are considered to be related to the waning stages of epithermal mineralisation. They have a widespread but patchy distribution, and are usually less than 1 m thick, although at Gaimayuisa Creek an accumulation to 4 m thick has been noted. They are usually poorly sorted and vary from massive to well banded. Thin siliceous conglomeratic horizons to 1 m have been noted, containing well rounded clastic fragments up to 10 mm in diameter. Similar units have been noted at Wapolu. They usually contain gold grades in excess of 1 g/t, but provide less than 5% of the Inferred Resource.
STRUCTURE REGIONAL STRUCTURE The D'Entrecasteaux Islands are located in a structurally complex zone where a number of major tectonic features have been superimposed. The earliest major structural event is considered to be an oblique collision resulting in the obduction of a WNW-moving sea floor plate over the leading edge of the NNE- moving Australian Plate in the Lower Eocene (Milsom and Smith, 1975; Hill, 1987). The time span of this compressional tectonic event is unknown, but it must have ended before a north–south extensional regime which is believed to have commenced in the Trobriand Trench from approximately the Mid Miocene (Hill and Baldwin, 1993). North–south extension associated with the westerly trending Woodlark Basin sea floor spreading centre commenced in the Early Miocene, but this system did not propagate into the D'Entrecasteaux Islands area until the Pliocene (Milsom and Smith, 1975). A number of major basement domes emerged from the Solomon Sea in the late Pliocene to form the mountainous, NW-aligned D'Entrecasteaux Islands. These domal features are considered to be metamorphic core complexes (Davies and Warren, 1988; Lister, 1990; Lister and Baldwin, 1993) and similar to those of western North America (Coney, 1980). They are interpreted to have resulted from a combination of isostatic readjustment of the subducted portion of the sialic Australian Plate and magmatic upwelling caused by major crustal extension, much along the lines of the surficial gneiss dome model proposed by Ollier and Pain (1980). Uplift of the complexes appears to still be active. The metamorphic core complexes are bounded by shallowlydipping detachment faults that represent crustal scale ductile
Geology of Australian and Papua New Guinean Mineral Deposits
GAMETA GOLD DEPOSIT
mylonitic shear zones. The upper boundaries of these shear zones are significantly affected near surface where they were subjected to complex brittle shear deformation. This deformation is described in detail by Hill (1991). Late Pliocene to Holocene, NE-trending, normal transverse faulting associated with the Woodlark Basin spreading system has displaced the metamorphic core complexes. Extension is also associated with these structures, resulting in the development of NE-trending grabens and associated calcalkaline volcanism. Recent peralkaline volcanism is developed at the point where the Woodlark Basin spreading system propagates into the D'Entrecasteaux Islands between Fergusson and Normanby islands.
LOCAL STRUCTURE The dome-conformable layering in the metamorphic rocks is very well developed and is gently monoclinally warped, with amplitudes to 1 m on a scale of 5 to 20 m. Although no detailed structural mapping has been undertaken, ground magnetic data indicate the presence of prominent NE-striking linears. These structures have been observed in road exposures and appear to mainly dip steeply to the SE.
FIG 4 - Cross section on grid line 700 SE, looking NW.
dip and along strike (Fig 4). The gold mineralisation is, in current order of importance: 1.
pervasively developed within intense type A and C alteration;
2.
disseminated and vein controlled within weak to moderate type A and C alteration;
3.
vein controlled in fractured, but essentially unaltered host;
4.
disseminated within type E alteration;
5.
vein controlled in type D alteration; and
6.
weak to absent in type B alteration.
ALTERATION Relatively extensive alteration has been identified by surface mapping and drilling. This alteration does not appear to be restricted to the dome-bounding detachment fault and extends into the overlying ultramafic rocks, where present, and into the underlying metamorphic basement. Five main types of alteration are present, which are essentially controlled by the host rock type. 1.
Type A: Sericite-pyrite±silica±carbonate±chlorite (phyllic) alteration is dominant and developed mainly within quartzo-feldspathic schist and gneiss and to a lesser extent within ultramafic rock and is quite intense in places. Carbonate content, locally to 30%, is associated with altered ultramafic rock which is characteristically a creamy white to mid green colour.
2.
Type B: Chlorite-pyrite±silica±carbonate±sericite (propylitic) alteration is usually less intense and is developed within the more mafic schist and gneiss, within some of the intrusives and also as vein selvages within both the felsic and mafic dominant units.
3.
Type C: Serpentine-talc-opaline silica-carbonate-pyrite alteration is associated with altered ultramafic rock.
4.
Type D: Silica (quartz±chalcedony)-pyrite and roscoelite(?) alteration is developed within epithermaltextured quartz veins hosted by the basement metamorphic rocks.
5.
Type E: Clay-pyrite±silica±carbonate (argillic) alteration is associated with the recent epithermaltextured surface material.
MINERALISATION Most of the mineralisation at Gameta is developed in the DFZ in the overlying ultramafic rock and in the underlying basement metamorphic rocks within zones oriented subparallel to the dome layering which dips at 35 to 40o NE. Very little of the current resource is contained within the siliceous sinter-like material. The mineralisation is typically lensoidal up and down
Geology of Australian and Papua New Guinean Mineral Deposits
Some transported gold mineralisation is also present in the two main areas of landslide debris in Gaimayuisa and Moaduwe creeks. This is associated with fragments, to boulder size, of altered and mineralised basement metamorphic rocks. The vein type mineralisation appears to be associated with steep structures developed approximately perpendicular to the dome layering. The gold is associated with pyrite and arsenopyrite with a high correlation between gold and sulphur content. Very little oxide resource is present with sulphides occurring at or near surface in most areas. In the siliceous sinter-like occurrences, gold is often associated with crystalline stibnite, and relatively high molybdenum values occur in some areas. In many of the drill sections, two distinct mineralised horizons have been identified, but in other sections they merge into one (Fig 4). A significant amount of the resource is shallow, within 20 m of the surface. The currently known extent of the mineralisation is shown projected vertically to surface in Fig 3. This zone is open ended to the SE and up slope to the SW. Results to-date indicate that the gold mineralisation is persistent, albeit lensoidal, along strike for at least 1000 m between lines 100 SE and 1100 SE. Within this zone the mineralisation is open in all directions except towards the NW. To the NE it is covered by more than 20 m of alluvium and landslide debris and only limited drill testing has been undertaken. To the NW there has been some drilling within the Cape Vinall grid area but no anomalous intersections have been encountered. To the SW it is open up slope for at least 2000 m,
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K G CHAPPLE and S IBIL
and if mineralisation persists for this distance, only some 20% of the potential area within this zone has been tested to date. Reconnaissance rock and stream sediment sampling to date indicates that the mineralisation may extend up slope for at least 1000 m past the area currently drilled. To the SW reconnaissance sampling indicates that the mineralisation may extend for at least another 1000 m.
METALLURGICAL FEATURES The gold appears to be associated with pyrite and arsenopyrite and is not readily extractable by conventional cyanide leaching. However, preliminary metallurgical testing has indicated that 80–85% of the gold can be recovered in a flotation concentrate. Gravity concentration test work has indicated that there is negligible free gold present. Scanning electron microscope work has failed to reveal the nature of the gold and its location. Bacterial oxidation testwork is in progress.
ORE GENESIS
REFERENCES Appleby, A-K, 1995. A new model for controls on gold-silver mineralization on Misima Island, Papua New Guinea, paper presented to 101st Annual Northwest Mining Association Convention, Spokane, Washington, 6–8 December 1995. Baldwin, S L, Lister, G S, Hill, E J, Foster, D A and McDougall, I, 1993. Thermochronologic constraints on the tectonic evolution of active metamorphic core complexes, D'Entrecasteaux Islands, Papua New Guinea, Tectonics, 12 (3):611–628. Coney, P J, 1980. Cordilleran metamorphic core complexes: An overview, Geological Society of America, Memoir 153. Davies, H L, 1973. Fergusson Island, Papua New Guinea - 1:250 000 geological series, Bureau of Mineral Resources, Geology and Geophysics Australia, Explanatory Notes, SC 56–5. Davies, H L and Ives, D J, 1965. The Geology of Fergusson and Goodenough Islands, Papua, Bureau of Mineral Resources, Geology and Geophysics Report No 82. Davies, H L and Warren, R G, 1988. Origin of eclogite-bearing, domed, layered metamorphic complexes (‘core complexes’) in the D'Entrecasteaux Islands, Papua New Guinea, Tectonics, 7(1):1–21.
The D'Entrecasteaux Islands are dominated by Upper Palaeozoic to Mesozoic medium to high grade metamorphic rocks derived from the Australian Plate that have been diapirically thrust as metamorphic core complexes through an overlying ultramafic sea floor plate. The core complexes are still actively rising as a result of density disequilibrium. Late graben development has displaced the complexes and localised the emplacement of a younger volcanic suite. Gold mineralisation is epithermal in style and hosted within the DFZ, the overlying ultramafic rocks and the underlying basement metamorphic rocks. The dome-bounding DFZ is thought to have provided the channelway for geothermal systems driven by volcanic ‘heat engines’ emplaced at shallow depth within the structure. Extensive host-influenced alteration has taken place and is associated with structurally-controlled epithermal style gold mineralisation. The gold is thought to have been derived locally by leaching of the host rocks.
Hill, E J, 1987. Active extension in the D'Entrecasteaux Islands, Papua New Guinea, 16th BMR Research Symposium, BMR Record 1987/51 (unpublished)
Similar gold mineralisation has been identified at Wapolu (McNeil, 1990) and at Misima, although in the latter case the mineralisation is developed in the upper rather than the lower plate (Appleby, 1995).
.Lister, G S, 1990. Metamorphic core complexes - “hot spots” in the continental crust? Proceedings Pacific Rim Congress 90, pp 37–47 (The Australasian Institute of Mining and Metallurgy: Melbourne).
ACKNOWLEDGEMENTS The authors wish to thank the management of Union Mining NL for permission for this paper to be published. The work and support of geologists C Palaulo, E Atase and W Bawasu is acknowledged. We also wish to acknowledge assistance of H Davies, University of Paua New Guinea, who provided petrological descriptions of samples from the project area.
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Hill, E J, 1989. Extensional deformation on Fergusson and Goodenough Islands, Papua New Guinea, in Australian Tectonics Conference, Kingscote, Kangaroo Island, 6–10 Feb, Geological Society of Australia Abstracts, 24:69–70. Hill, E J, 1990. The nature of shear zones formed during extension in Eastern Papua New Guinea, Proceedings Pacific Rim Congress 90, pp 537–548 (The Australasian Institute of Mining and Metallurgy: Melbourne). Hill, E J, 1991. The formation of metamorphic core complexes in the D'Entrecasteaux Islands, Eastern Papua New Guinea, PhD thesis (unpublished), Monash University, Melbourne. Hill, E J and Baldwin, S L, 1993. Exhumation of high-pressure metamorphic rocks during crustal extension in the D'Entrecasteaux region, Papua New Guinea, Journal of Metamorphic Geology, 11:261–277.
Lister, G S and Baldwin, S L, 1993. Plutonism and the origin of metamorphic core complexes. Geology, 21(7):607–610. McNeil, P A, 1990. Wapolu gold deposit, Fergusson Island, in Geology of the Mineral Deposits of Australia and Papua New Guinea (Ed: F E Hughes), pp 1783–1788 (The Australasian Institute of Mining and Metallurgy: Melbourne). Milsom, J and Smith, I E, 1975. Southeastern Papua: generation of thick crust in a tensional environment?, Geology, 3:117–120. Ollier, C D and Pain, C F, 1980. Actively rising surficial gneiss domes in Papua New Guinea, Journal of the Geological Society of Australia, 27:33–44.
Geology of Australian and Papua New Guinean Mineral Deposits
Bainbridge, A L, Hitchman, S P and DeRoss, G J, 1998. Nena copper-gold deposit, in Geology of Australian and Papua New Guinean Mineral Deposits (Eds: D A Berkman and D H Mackenzie), pp 855–862 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Nena copper–gold deposit by A L Bainbridge1, S P Hitchman2 and G J DeRoss3 INTRODUCTION The deposit is in the foothills of the Star Mountains of western PNG, approximately 580 km upstream from the mouth of the Sepik River, at lat 4o37′S, long 141o55′E on the Mianmin (SB 54–3) 1:250 000 scale map sheet (Fig 1).
FIG 1 - Location and regional geology of the Frieda River Intrusive Complex.
Nena has a Measured and Indicated Resource of 51 Mt at 2.2% copper and 0.6 g/t gold at a 0.5% copper cutoff, plus an oxide cap of 18 Mt at 1.4 g/t gold using a 0.6 g/t gold cutoff. The deposit is hosted by an andesitic volcanic rock altered to an advanced argillic assemblage, with the copper sulphide mineralisation forming a horizontal breccia pipe. Characteristics of the deposit are a marked zoning of mineral species, alteration type and copper grade that show a strong structural and lithological control. Nena has many features that categorise the deposit as a high sulphidation style as defined by White (1990). Highlands Pacific Limited (Highlands) manages the project and is the major partner in a joint venture with a Japanese Consortium (OMRD). The project is currently undergoing feasibility studies.
EXPLORATION HISTORY The Bureau of Mineral Resources discovered porphyry copper mineralisation in the Frieda River Intrusive Complex (FRIC) in
1.
Chief Geologist, Highlands Pacific Limited, PO 1486 Port Moresby, Papua New Guinea.
2.
Supervising Geologist, Highlands Pacific Limited, PO 1486 Port Moresby, Papua New Guinea.
3.
General Manager - Exploration, Highlands Pacific Limited, GPO Box 3086, Brisbane Qld 4001.
Geology of Australian and Papua New Guinean Mineral Deposits
1966 while mapping the Mianmin 1:250 000 scale geological sheet (Asami and Britten, 1980). Carpentaria Exploration Company Pty Ltd applied for an exploration licence over the Frieda area in 1967, and commenced a vigorous exploration program that continued until 1984. The Nena deposit and several sizeable porphyry copper deposits including HorseIvaal (Fig 1) were discovered in this period and the combined total resource estimated for the FRIC exceeded 1000 Mt at 0.5% copper and 0.3 g/t gold. Nena was discovered in 1977 during regional reconnaissance around the porphyry complex by following a copper float trail to its source. Sampling of the two main tertiary drainages downstream from Nena returned values of 435 and 85 ppm copper in minus 80 mesh stream sediment samples. Grid soil and trench sampling outlined a 1200 by 200 m area of anomalous copper, gold, silver and arsenic values along a steep sided ridge of resistant silica-alunite altered volcanic rock. A ‘discovery’ outcrop of massive luzonite (Cu3AsS4) breccia was found high on one of the cliff faces and it was this discovery that led to drilling at Nena in early 1978. The first hole was drilled horizontally beneath the discovery outcrop and intersected 120 m at 2.0% copper. Between 1979 and 1982, 27 diamond drill holes for 6268 m and 5 shallow percussion holes for 320 m were drilled to define an ’inferred resource‘ of 32 Mt at 2.3% copper and 0.58 g/t gold (Hall, Britten and Henry, 1990). Mineralogical studies of Nena mineralisation indicated that the ore is a complex mixture of copper sulphosalts overprinting massive pyrite. Tests undertaken in 1982 revealed that the Nena mineralisation was not suited to conventional flotation technology since it provided poor recovery and a low grade (12–13% copper) concentrate that had an unacceptably high arsenic content. In this form the Nena concentrate had little market value and a decision was made in 1984 to cease exploration. In 1992 Highlands assessed the potential of the Frieda district and in particular the Nena resource. As part of this review detailed surface mapping and relogging of the Nena core revealed a set of post-mineralisation, complex block faults that sinistrally offset the mineralisation at regular intervals. The recognition of this style of faulting suggested that large areas of Nena remained untested. Detailed metallurgical and process studies showed that an acceptable product could be produced using leach technology. It was highlighted that a solvent extraction electrowin finish could produce copper cathode on site and have the benefit of overcoming the arsenic problem by producing a stable non toxic ferro-arsenate mineral. Commencing in February 1993, Highlands undertook a major diamond drilling program that continued for two and a half years. A total of 36 402 m in 160 holes was drilled on a 50 by 50 m grid pattern. The entire program was helicopter supported from a barging depot on the Frieda River 23 km to the ENE of Nena.
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A L BAINBRIDGE, S P HITCHMAN and G J DeROSS
The drilling undertaken by Highlands increased the resource by 65% and the confidence level to the point where more than 80% falls in the Measured Resource category. Detailed core logging showed clear alteration and mineralisation patterns that have proved useful tools when exploring other parts of the Frieda system. A wide range of geophysical techniques was used at Nena with generally excellent results. Dipole-dipole gradient array induced polarisation produces the best response with the mineralised region reporting as a conductor with high chargeability. The adjacent clay alteration zones are also conductive but have low chargeability. A ground electromagnetic (EM) survey was conducted with good response in the 7200 Hz range. EM tends to focus mainly on the zones of massive pyrite that may or may not be mineralised. Due to the destructive effect of the acid fluids on the host volcanic rock the Nena deposit and associated alteration zones provide a distinct magnetic low.
portion of the complex. Phases III and IV of this intrusive activity can be genetically linked to porphyry copper formation. The northern part of the complex is less eroded and has thick sequences of coeval subaerial andesitic volcanic rock. Nena is hosted within these volcanic rocks and is some 5 km from the nearest known porphyry copper body at Horse-IvaalKoki. Large tilted blocks of volcanic detritus and shallow marine sediment conformably overlie the volcanic rocks of the complex and reflect the waning period of volcanism and uplift at Frieda. These sediments show dislocation related to late low angle faulting that is poorly understood. A stratigraphic column for the major units in the Frieda area is shown in Fig 2.
REGIONAL GEOLOGY The FRIC formed during oblique Tertiary plate convergence between the north moving Australian Plate and the westerly moving Pacific Plate. The Pacific Plate was subducted beneath the Australian Plate to form a continental arc. In the early phase of this collision in the Late Palaeocene, prior to subduction, significant portions of the Pacific Plate were obducted into the foreland of the Australian Plate and regional metamorphism occurred (Page, 1975). These events are reflected in the Frieda basement rocks which comprise faulted blocks of schist, phyllite and ultramafic rock and are described by Rogerson et al (1987b) as being part of the Foreland thrusting. The dominant structural elements within the basement also reflect this collision period with large NW-trending arc-parallel structures clearly visible on Landsat imagery. More subtle NE-trending features have been suggested by a number of authors (eg Davies, 1991; Corbett, 1994; Rogerson et al, 1987a) to represent arc-normal transfer structures that have played a role in focussing later intrusive activity. The alignment of the Ok Tedi intrusives, Nong River intrusive complex and the FRIC may represent such a transfer structure showing a fractionation time trend, with the more primordial dioritic magmas on the plate edge at Frieda and more evolved fractionated monzonitic intrusives at Ok Tedi. The FRIC represents the eroded remnants of a large andesitic stratovolcano, formed in the Middle Miocene at 13–17 Myr as part of the Maramuni Arc volcanism (Page and McDougall, 1972). This volcanism occurred at a similar time to the collision between a south-moving island arc, known as the Melanansian Island Arc, and the Australian Continental Arc. This collision was the end of subduction on the continental arc and corresponds to a change from largely compressional forces to major extensional forces in the western part of PNG (P Uttley, unpublished data, 1994). The manifestation of this change in stress appears, on the intrusive complex scale, to be a set of east-trending strike slip faults termed linkage faults which connect subparallel NW-trending thrust faults. Dilational NNW-trending alteration zones have formed as a result of movement on the linkage and thrust faults and are important features for mineralisation in the Frieda area. The FRIC covers an area 15 by 20 km (Fig 1) and has five recognised phases of intrusive activity (Britten, 1981). All of these intrusives are exposed in the deeply eroded southern
856
FIG 2 - Stratigraphic columns for the Frieda River Intrusive Complex.
NENA GEOLOGY LITHOLOGY The Frieda copper-gold deposits are hosted within the Wogamush Formation of Middle Miocene age. The Wogamush Formation contains byproducts of the Maramuni Arc volcanism and comprises three units. The Lower Wogamush unit is dominated by deep water carbonaceous turbidite. Poorly sorted conglomerate beds, locally present at the base of this unit, mark an unconformity between the Lower Wogamush unit and basement metamorphic rock. Interbedded deep water calcareous units are locally present towards the top of this unit. The Middle Wogamush unit, also called the Debom volcanics, represents the main extrusive volcanic phase of the FRIC. A maximum thickness of 1500 m has been recorded for this unit and it consists, in order of abundance, of andesitic lava, pyroclastic rock and volcanoclastic sediment and calcareous mudstone. The quantity of pyroclastic rock and sediment increases towards the top of the unit indicating waning volcanic activity. The andesitic lava units are porphyritic, with large andesine plagioclase phenocrysts to 5 mm diameter and minor hornblende phenocrysts to 2 mm diameter within a fine grained dark green aphanitic groundmass. Autobreccia flow tops are observed in creek outcrops 1 km west of Nena.
Geology of Australian and Papua New Guinean Mineral Deposits
NENA COPPER-GOLD DEPOSIT
FIG 3 - a. Regional alteration plan at surface, Nena deposit, b. Alteration level plan at 600 m RL, Nena deposit.
The Upper Wogamush unit comprises a thick sequence of volcanoclastic and calcareous sandstone interbedded with irregular limestone and conglomerates. No mineralised porphyry clasts have been observed in the conglomerate, however 30–50% of the cobbles are of andesitic volcanic origin. The FRIC has undergone extensive alteration, including 30 km2 of porphyry style alteration and 52 km2 of advanced argillic alteration covering more than one third of its total surface area. The Nena mineralisation is contained within a 300 m thick sequence of andesitic lapilli tuff of the Middle Wogamush unit. Interbedded volcanoclastic sediments have been recorded in the SE part of Nena. Impermeable andesitic lavas cap the deposit and typically form an upper limit to alteration and mineralisation. Permeable, coarse pyroclastic units have been preferentially altered and mineralised. The horizontal pipe-like shape of the orebody is due to the intersection of an inferred subvertical structure with the pyroclastic rocks. Bedding at Nena has an overall westerly dip at 30–45o except in the SW of the area where the beds have domed around an andesitic porphyry intrusive stock, producing easterly dips.
ALTERATION FIG 4 - Cross sections on 4700 m N and 5200 m N, Nena deposit.
Plans and typical cross sections of the alteration pattern for the southern (line 4700 N) and northern (line 5200 N) parts of Nena are shown in Figs 3 and 4. Alteration assemblages were determined from XRD analysis of 354 samples and from the detailed logging of core from 160 diamond drill holes.
Geology of Australian and Papua New Guinean Mineral Deposits
The alteration pattern shows a pronounced elongate concentric zoning that is symmetrical about a central inferred fault structure. The zoning is typical of high sulphidation
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A L BAINBRIDGE, S P HITCHMAN AND G J DeROSS
deposits as described by White (1990) in studies of the Nansatsu deposits in Japan and can be subdivided into three mappable units. The inner zone is a highly leached silica core comprising interlaced silica with corrosion cavities referred to as the ‘residual silica zone’. Cavities are a result of leaching by acid fluids and show a progression from weak leaching where only the feldspar phenocrysts have been destroyed, to intense leaching where void spaces interlink to make a pumice-like rock that has up to 50% cavities and a silica skeleton. The leaching becomes more intense towards a prominent vertical structure that forms a central axis to the alteration. This is also characterised by late brecciation and shearing. In the southern part of Nena the residual silica zone is shaped like a subhorizontal pipe, approximately 250 m in diameter and extending for 800–900 m along strike. In the north the residual silica zone changes from a pipe shape to a series of separate subvertical lenses 10–20 m wide. White (1990) suggested that this type of porous silica forms in a highly acidic environment of pH 1–2 and moderate temperatures of 350–400οC. T M Leach (unpublished data, 1993) argued that the mixing of magmatic volatiles (SO2, CO2, Cl, F, etc) and ground waters produced hot corrosive acidic fluids that have used the central structure as a conduit to leach the surrounding volcanic rock. The intermediate zone is a broad zone of silica±alunite alteration that surrounds and encloses the residual silica core. This zone has also been referred to as the ‘replacement silica zone’ owing to its intense massive overprinting silicification. It is the most extensive and competent of the alteration zones at Nena and has been a major factor in confining the late cupriferous fluids to the porous residual silica zone. Primary rock characteristics are only occasionally visible and include lapilli texture. Alunite occurs as a replacement of plagioclase phenocrysts on the margins of this zone changing inwards to pervasive replacement and late stockwork alunite veins and veinlets of brown, cream and pink colour. The alteration zone reaches a maximum width of 450 m in the northern area (5300 N) in a structurally complex area that at surface is the convergence point of several silica-alunite ridges. The aluniterich zone in the near surface environment weathers to form white powdery patches. The term ‘intermediate zone’ refers to the position of this alteration type between the residual silica core and the peripheral advanced argillic clay zones. The conditions for alunite formation are not as severe as for residual style silica, requiring a pH of 3–5 and temperatures of 250–350οC (Knight, 1977). The outer zone is dominated by clay minerals±weak silicification and marks the peripheral zone to the Nena alteration pattern. It is typically 20–30 m wide and has a gradational contact with the outer margin of the silica-alunite intermediate zone. Clay minerals are distributed in regular patterns that grade from pyrophyllite clay on the margins of the silica-alunite alteration zone out to dickite clays then kaolinitic clay zones. On the outer edges of this advanced argillic clay zone is an argillic clay zone generally 5–10 m wide, dominated by greasy illite-smectite clays. The clay pattern has been useful in locating proximity to the margins of the system. All three alteration zones are sometimes brecciated or stockworked by later sulphides suggesting that the advanced argillic alteration predates the main period of sulphide mineralisation. Allied with the main sulphide phase is a
858
distinctive association of vein alunite that cuts earlier pervasive alteration styles. This late vein alunite has been used as a marker of proximity to the mineralisation since it forms a halo up to 30 m away from the main ore zone. The presence of leached silica fragments within a second phase of leached silica or replacement silica-alunite indicates that multiple phases of acid alteration have occurred. Rutile is ubiquitous through all silica and alunite vein phases as radiating needles 0.5–1 mm long. Analysis of the rutile crystals shows abnormally high vanadium, copper and chromium contents.
MINERALISATION Primary mineralisation Hypogene copper mineralisation occurs as a complex intergrowth of covellite (CuS), stibnoluzonite (Cu3SbS4) and the polymorph luzonite-enargite (Cu3AsS4). Secondary enrichment is extensive and approximately 30–40% of the mineralisation is supergene chalcocite (Cu2S), covellite and digenite (Cu9S5). Copper-arsenic sulphides (luzonite-enargite) account for around 27% of the contained copper at Nena. The remaining 73% of the copper occurs as covellite and chalcocite, of both primary and secondary origin. Luzonite is the lower temperature polymorph of the luzonite–enargite series with a temperature inversion at 320oC (Cornelius and Cornelius, 1985). A more irregular lattice structure in the luzonite allows substitution of arsenic for antimony, to form a solid solution series between arsenic-rich luzonite and antimony-rich stibnoluzonite. With increasing antimony content the colour changes from metallic pinkish brown to iridescent pink and this change has been used as a guide to the distribution of the luzonite varieties. Hypogene copper minerals most commonly occur as coarse crystalline matrix to open spaced breccias. Stibnoluzonite forms crystals that may reach centimetres in diameter and often fill large voids 5–10 cm across. Enargite commonly forms as small elongate resinous black crystals clumped in radiating aggregates. Occasional banded sulphides such as melnikovite (Fe3S4) are observed, emphasising the open space filling nature of the copper sulphide mineralisation. In those few areas where brecciation has not occurred the copper minerals fill the leached vughs of the residual silica core to give a disseminated appearance. Copper mineralisation in the silica-alunite halo zone is rarer and occurs either as a vein stockwork or filling crackle breccia zones. Copper grade is directly proportional to the intensity of brecciation. A major pyrite±marcasite depositional event occurred prior to the copper mineralising event, depositing 30–40% of the pyrite in the residual silica core and 5–10% within the silicaalunite halo. This pyrite is extremely fine grained and fills a large percentage of the voids in the residual silica zone. Like the host rock, the massive pyrite is commonly brecciated by later copper mineralisation. Analysis of the massive pyritic material away from the copper zone shows that it is low in copper and gold, inferring that it was not derived from fluids responsible for the main copper mineralising event. Gangue minerals are subordinate to copper minerals and occur randomly throughout the deposit. Barite is the most common, has a well bladed habit and is opaque white in colour.
Geology of Australian and Papua New Guinean Mineral Deposits
NENA COPPER-GOLD DEPOSIT
In the central portion of the deposit barite is pervasive, comprising 3–4% of the matrix of the breccia. The higher grade zones of copper mineralisation generally have a higher barium content. Minor coarse grained brassy pyrite is intermixed within the stibnoluzonite areas. This pyrite is quite distinct from the very dark fine-grained massive pyrite discussed above and rarely exceeds 3–4% of the rock volume. Deposition of native sulphur is the only recognised hypogene event to appear later than the copper mineralisation and sulphur occurs as well formed disseminated crystals 2–3 mm in diameter. In the footwall to mineralisation in the northern area of Nena the native sulphur content may be 8–10% by volume of the rock.
Secondary mineralisation A well developed secondary enrichment blanket occurs over the entire Nena deposit reaching a maximum thickness of 70 m (average 40 m). This blanket is regular on its upper surface, sitting just below the totally oxidised zone, but is highly variable on its lower contact, extending down the many fractures and shear zones to depths in excess of 300 m. Secondary sulphide assemblages are dominated by black sooty chalcocite forming dendritic fracture coatings and as massive wad within alunite-rich patches. At microscopic scale the chalcocite is observed to nucleate on pyrite grains forming a thin skin several µm thick. Lesser amounts of brown earthy digenite and sooty blue covellite have also been noted. Secondary chalcocite can be found over the entire vertical extent of mineralisation at Nena but is dominant in strongly fractured zones on the margin of the copper zone. In the southern zone between 4450 and 4700 N, grey metallic crystalline chalcocite occurs, which contrasts sharply with the more typical sooty black varieties observed elsewhere in the blanket. This southern zone is characterised by some high grade zones (eg 60 m at 5% copper) and is the topographic low point for the surface drainage. It was originally thought that the crystalline texture of the chalcocite could indicate a primary origin but the flat blanket-like shape of the copper mineralisation in this area may suggest it is a variant of secondary crystallisation. A flat 2 m thick zone, containing up to 30% by volume of secondary native copper in veinlets from 3 to 5 mm wide, is located within this chalcocite zone. Virtually no copper oxide minerals have been observed at Nena.
Gold mineralisation Gold mineralisation is mainly confined to the southern half of the deposit in close association with luzonite. The gold is very fine grained and several attempts using a scanning electron microscope on copper concentrate failed to locate any visible gold. Only one instance of visual confirmation showed gold occurring within pyritic grains associated with luzonite (K Lawrie, personnel communication, 1995). The luzonite is commonly associated with minor amounts of coarse brassy pyrite and the gold may be associated with the pyrite. Analysis of the flotation tails from metallurgical testwork indicates that some gold is lost with pyrite. A close statistical association occurs between gold and tellurium, which averages 80 ppm at Nena. Scanning electron microscope work on copper mineral concentrates, although failing to locate gold, showed that tellurium substitutes for sulphur in luzonite crystals.
Geology of Australian and Papua New Guinean Mineral Deposits
A near surface oxide gold cap is present in the southern part of Nena, and shows some evidence of supergene effects. The gold is very fine grained but has on several occasions been observed with the naked eye, as crystals associated with goethite on fracture surfaces. The enrichment factor is low (<10 %) compared to the primary grade beneath the cap. Copper however is strongly leached from this zone.
Zoning and paragenesis A distinct zoning is present in the primary copper species at Nena, and is highlighted in Fig 5. North of 5200 N the zone is dominated by a gold-poor primary covellite zone, changing to an arsenic-rich enargite dominated zone in the central area (4700–5200 N), then to a gold-rich luzonite-stibnoluzonite zone south of 4700 N. Fluid inclusion studies on barite intermixed with the copper minerals suggests that this zoning formed in response to lower fluid temperatures in the south than in the north. The temperature estimates range from 172oC for stibnoluzonite in the south to 330οC for covellite in the north. Fluid salinities have a relatively restricted range from 10 eq wt % NaCl in the south to around 15 eq wt % NaCl in the north. This may suggest a higher magmatic component to the mineralising fluids issuing from probable source areas in the north. Hypogene covellite has been reported to form in a highly oxidising environment at temperatures in excess of 350oC (Titley, 1982). As such the occurrence of primary covellite represents the highest temperature copper species observed at Nena. Covellite has been observed in association with other porphyry copper deposits and is usually associated with late acid alteration as at Butte (Montana) and Lepanto (Philippines). The occurrence of gold at the southern end of Nena, in association with brassy pyrite and the lower temperature polymorph, luzonite, suggests a primary deposition of gold in response to falling fluid temperatures. Figure 6 is modified from T Leach, (unpublished data, 1993) and shows the paragenetic relationships observed at Nena
ORE GENESIS The moderate temperatures and high salinity of the fluid inclusions in high sulphidation copper systems suggest a significant magmatic component to the mineralising fluids (Sillitoe, 1983). Garcia (1990) suggested that the copper within the Lepanto high sulphidation deposit in the Philippines may have been remobilised by late acid fluids from the Far South East porphyry copper deposit located 300 m below Lepanto. At Wafi, PNG, similar connections are inferred between the Wafi gold-enargite deposit formed by late acid fluids remobilising metals from the nearby Rafferty’s porphyry copper body (Erceg et al, 1991). At Rafferty’s a mineral zoning is observed away from the porphyry, going outwards from chalcopyrite to covellite to enargite to a gold halo. At Nena a single 780 m hole was drilled beneath the covellite zone to investigate a possible feeder system to the mineralisation. This hole drilled through the base of the advanced argillic alteration at 480 m and entered directly into a porphyry copper alteration assemblage. Sericite-chloriteepidote-quartz altered intrusive rock was encountered with alteration increasing in intensity with depth to the end of the hole. Purple anhydrite vein stockwork was present at depth associated with chalcopyrite-bornite mineralisation. Pyrite
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A L BAINBRIDGE, S P HITCHMAN and G J DeROSS
FIG 5 - Metal zoning at Nena.
content of 3–5% and the interfingered phyllic-propylitic alteration may suggest that the alteration encountered represents the peripheral pyritic halo of a porphyry body. Fluid inclusion work and further drilling are required to determine the link between this porphyry and the Nena high sulphidation deposit, however clear crosscutting relationships indicate that the acid sulphate system developed later and overprinted the porphyry alteration event. Remobilisation of porphyry copper mineralisation by acidic fluids to form high sulphidation copper may have occurred.
ACKNOWLEDGEMENTS
FIG 6 - Alteration and mineralisation paragenesis at Nena, modified from T M Leach (unpublished data, 1993).
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The authors wish to thank Highlands Pacific Limited for permission to publish these data and for the courage to drill conceptual holes. All the geologists who have worked at Frieda
Geology of Australian and Papua New Guinean Mineral Deposits
NENA COPPER-GOLD DEPOSIT
over the last 20 years are acknowledged for their hand in the compilation of the stratigraphy and regional datasets. T M Leach and G J Corbett have contributed significantly to the understanding of the system at Nena. Special thanks go to the Frieda people who have made field work in this remote area a pleasant and rewarding experience.
REFERENCES Asami, N and Britten, R M, 1980. The porphyry copper deposits at the Frieda River prospect, PNG, in Granitic Magmatism and Related Mineralisation, Tokyo, Mining Geology Special Issue No 8 (Eds: S Ishihara and S Takenouchi), pp 117–139 (The Society of Mining Geologists of Japan: Tokyo). Britten, R M, 1981. The geology of the Frieda River copper prospect, Papua New Guinea, PhD thesis (unpublished), The Australian National University, Canberra. Corbett, G J, 1994. Regional structural control of selected Cu /Au occurrences in Papua New Guinea, in Proceedings, PNG Geology, Exploration and Mining Conference, Lae 1994 (Ed: R Rogerson), pp 57–70 (The Australasian Institute of Mining and Metallurgy : Melbourne). Cornelius, K and Cornelius, S H Jr, 1985. Manual of Mineralogy, 21st ed (John Wiley: New York). Davies, H L, 1982. Mianmin, PNG – 1:250 000 series, Papua New Guinea Geological Survey Explanatory Notes SB 54–3 . Davies, H L, 1991. Regional geological setting of some mineral deposits of the New Guinea region, in Proceedings, PNG Geology, Exploration and Mining Conference (Ed: R Rogerson), pp 49–57 (The Australasian Institute of Mining and Metallurgy : Melbourne). Erceg, M N, Craighead, G A, Halfpenny, R and Lewis, P J, 1991. The exploration history, geology and metallurgy of a high sulphidation epithermal gold deposit at Wafi River, Papua New Guinea, in Proceedings PNG Geology, Exploration and Mining Conference (Ed: R Rogerson), pp 58–65 (The Australasian Institute of Mining and Metallurgy: Melbourne).
Geology of Australian and Papua New Guinean Mineral Deposits
Garcia, J Jr, 1990. Geology and mineralisation characteristics of the Mankayan Mineral District, Benguet, Philippines, in Third Symposium on Deep-crust Fluids, High Temperature Acid Fluids and Associated Alteration and Mineralisation Fluids (Eds: Y Matsuhisa, M Aoki and J W Hedenquist), pp 17–25 (Geological Survey of Japan: Tokyo). Hall, R J, Britten, R M and Henry, D D, 1990. Frieda River copper-gold deposits, in Geology of Mineral Deposits of Australia and Papua New Guinea (Ed F E Hughes), pp 1709–1715 (The Australasian Institute of Mining and Metallurgy: Melbourne). Knight, J E, 1977. A thermochemical study of alunite, enargite, luzonite, and tennantite deposits, Economic Geology, 72:1321–1336. Page, R W, 1975. Geochronology of Late Tertiary and Quaternary mineralised intrusive porphyries in the Star Mountains of Papua New Guinea and Irian Jaya, Economic Geology, 70:928–936. Page, R W and McDougall, I, 1972. Ages of mineralisation of gold and porphyry copper deposits in the New Guinea Highlands, Economic Geology, 67:1034–1048. Rogerson, R, Hilyard, D.B, Finlayson, E J, Holland, D J, Nion, S T S, Sumaiang, R. S, Duguman, J and Loxton, C D, 1987a. Geology and mineral resources of the Sepik headwaters region, Papua New Guinea, Geological Survey Papua New Guinea Memoir12. Rogerson, R, Hilyard, D, Francis, G and Finlayson, E J, 1987b, The foreland thrust belt of Papua New Guinea, in Proceedings Pacific Rim Congress 87, pp 579–583 (The Australasian Institute of Mining and Metallurgy: Melbourne). Sillitoe, R H, 1983. Enargite bearing massive sulphide deposits high in porphyry copper systems, Economic Geology, 78:348–352. Titley, S R, 1982. Advances in Geology of the Porphyry Copper Deposits, Southwestern North America (The University of Arizona Press: Tucson). White, N, 1990. High sulphidation epithermal gold deposits: characteristics, and a model for their origin, in Third Symposium on Deep-crust Fluids, High Temperature Acid Fluids and Associated Alteration and Mineralisation Fluids (Eds Y Matsuhisa, M Aoki and J W Hedenquist), pp 5–15 (Geological Survey of Japan: Tokyo).
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Geology of Australian and Papua New Guinean Mineral Deposits
Subject Index This index contains the names of mineral deposits which are described in the text, appear on a figure or table, or have a quoted resource estimate. In addition, key stratigraphic and structural units are listed and a few locality terms which either appear in the title of papers or refer to a mineral province or field. The deposits are cross indexed by the relevant metal, mineral or rock name.
A
Astrolabe Agglomerate, PNG
Adelaide gold, WA
Atric gold, Qld
105
Agnew gold, WA
Agnew (Perseverance) nickel, WA
321
Agnew–Wiluna greenstone belt, WA 157, 297, 307, 308, 315, 318, 321, 322, 369 Aladdin fault, WA
113
Aladdin gold, WA
111, 112, 113, 115
191
Avoca gold, Vic Avon gold, NT
Alligator gold, NT
669, 670
243, 249 655, 656 111,
Anakie Metamorphic Group, Qld 696, 707, 708
111,
Anastasia gold, Qld
Bailieston anticline, Vic
137
Anomaly 45 zinc-copper, WA Antrim Plateau Volcanics, WA Apollo gold, WA
287 388
243, 247, 250, 251
Ararat goldfield, Vic
518
Balcooma copper, Qld 740
737
680
Area 4 gold, Qld
680
Ballarat gold, Vic
Area 3 gold, WA
73
Area 4 gold, WA
72, 73, 75
Area 5 gold, Qld
680 233, 243, 244, 247,
Argylla Formation, Qld
701, 702
737,
495, 496, 543, 544
Ballarat Anticlinorium, Vic 546
544, 545,
Ballarat East goldfield, Vic 545
543, 544,
Ballarat Goldfield, Vic
665
Belyando (Hill 226) gold, Qld 695, 707
691,
216
511, 521 495, 496
Bendigo Goldfield, Vic 513, 521
495, 496, 511,
Berringa gold, Vic Beta nickel, WA
287, 288
495, 496 347
Biddy Well gold, WA
127
Big Cadia gold-copper, NSW
499, 544
Ballarat West goldfield, Vic 543, 544, 546
641
Big Hill gold, NSW 557
551, 552, 556,
Big John gold, WA
149
Big Slate reef gold, Vic Bills Find gold, WA
Geology of Australian and Papua New Guinean Deposits
665
Bernts zinc-copper, WA
495, 496
Area 2 gold, Qld
462
Bellevue East shear zone, Qld
Bendigo gold, Vic
737, 738, 739,
Balcooma Goldfield, Qld
365, 366
216
Bellevue East fault, Qld
Bendigo, Vic
518
Balcooma Metavolcanics, Qld 739
Argo gold, WA 250, 251
Beautiful Sunday nickel, WA
Ben Hur gold, WA
496, 517
Bailieston goldfield, Vic
495, 496
Bedevere kimberlite, NT
Baileys Island South gold, WA 112, 113, 115
Anomaly 45 gold, WA
Beaufort goldfield, Vic
111,
587, 602
Bailieston gold, Vic
843, 844
467
Beaver gold, WA Baileys Island East gold, WA 112, 113, 115
111,
669, 671
427, 428, 429
Beaconsfield, Tas
Baileys Island North gold, WA 113, 115
Amphitheatre Group, NSW
102
Bavu igneous complex, PNG
409, 412, 413
Amber granite, Qld
98, 100, 102
Batman gold, NT
B
Baileys Island Central gold, WA 113, 115 724, 727
71
Barton Deeps gold, WA
544, 545,
Almaden Supersuite, Qld
255
Barton gold, WA
449, 451
Baileys magnesite, NSW Albion Anticlinorium, Vic 547
255
Barramundi gold, WA
495, 496
Bahama gold, WA
Banker gold, WA Banker saddle, WA
663
Aurora gold, WA
161
843, 844
512
127
863
Bimurra Volcanics, Qld Binduli gold, WA
692
Broadford Formation, Vic
215
Broads Dam gold, WA
Binduli mining centre, WA Bini Porphyry, PNG 847
215
844, 845, 846,
Birthday Gift heavy mineral sand, NSW 647 bismuth deposits, 669
Black Flag beds, WA Black Flag Group, WA 244, 245 Black Flag unit, WA
202, 216, 218,
Brockman Syncline, WA
375, 376
Black Hills uranium, Qld Black Magic gold, WA
808
Black Rock lead-zinc-silver, Qld Black Swan komatiite, WA Black Swan nickel, WA
784
340, 341
111, 112,
347, 348, 350
Blayney Volcanics, NSW
575, 576
244
Brolga nickel-cobalt, Qld
129,
Browns Creek gold-copper, NSW
575
Boulder–Lefroy Fault, WA 347
Bukalara Sandstone, NT
715, 717, 719
Caro schist, NSW Cashman gold, WA
594, 595 64
Castlemaine Goldfield, Vic 498
Bullabulling shear, WA
651,
Castlemaine Supergroup, Vic 522, 528, 543
246
365, 366
Burgan gold, NT
Bowdens silver-lead-zinc, NSW
627
Boyds 5 copper-lead-zinc-silver-gold, Qld 737, 738
512,
512
211 512
211 Caustons gold, WA
Bullen gold, WA
495, 496,
461
Catherine sandstone, Vic
233, 240,
576 347, 348,
Catherine reef gold, Vic 273
716
389
Car Park copper, Qld
593, 597,
149, 151
265, 266, 267, 268
Boulder–Lefroy Shear Zone, WA
105 335,
Caustons North gold, WA
149
Caustons South gold, WA
149
Cave Rocks gold, WA 250, 251
409
243, 244, 247,
Cawse nickel-cobalt, WA
Burketown mineral field, Qld
729
Burrell Creek Formation, NT 428
418, 419,
335
Cawse Central nickel-cobalt, WA
335
Cawse Extended nickel-cobalt, WA 335
287
243, 244, 247,
Broad Arrow mineral field, WA
864
Yungal, WA
Bulbodney Creek Complex, NSW 652 Bullabulling gold, WA
775,
carbonatite deposits,
593, 599
Bunyip Dam nickel-cobalt, WA 337
Britannia gold, WA 248, 249
Capella Creek Group, Qld
680, 681
Bulletin gold, WA
Breakers zinc-copper, WA
462
Carnilya Hill nickel, WA 349, 352
Boulder–Lefroy fault zone (shear zone), WA 219, 220, 223
Bounty gold, WA
449, 450, 451, 452,
63, 81, 83
Budgerygar copper, NSW 598
Boorarra mining centre, WA
452, 453
Bryah Group, WA
265, 266, 268
645
142
Carcoar Granodiorite, NSW
Bluebird link gold, WA
182
112, 113
63
Budgery copper, NSW
Boogardie breaks, WA
Caledonian gold, WA
Bryah Basin, WA
111, 112, 114
582
113, 115
Cannington silver-lead-zinc, Qld 783, 793
129,
Bronzewing Discovery gold, WA 131, 133
Bluebird East gold, WA
642
Caledonian fault, WA
Calvert Fault, NT
97, 127, 137
Buck Reef West gold, Qld
Booberoi fault, NSW
641, 643
Callie gold, NT 457
801
111, 112, 113
593,
Cadia Hill gold-copper, NSW
Callie anticline, NT
615, 619
Bluebird goldfield, WA
Bonnie Dundee copper, NSW 596, 597, 599
641,
Calista gold, WA
615
Bronzewing Central gold, WA 131, 133
339, 344
Black Swan South gold, WA 116
Blue Lode gold, WA
Brocks Creek–Zapopan anticline, NT 412
Bronzewing gold, WA
Cadia East gold-copper, NSW 644
Cadia Quarry gold-copper, NSW
412, 414
Broken Hill lead-zinc-silver, NSW 619, 790
182, 183
641
Cadia Hill Monzonite, NSW
409, 417,
Broken Hill Group, NSW
468
575
Cadia gold-copper, NSW
375,
Brockman No 2 detritals (B2D) iron ore, WA 375
Broken Hill, NSW 240
Cabonne Group, NSW
Brockman Iron Formation, WA 376, 377
Brocks Creek shear zone, NT
220
Cabbage Tree Formation, Tas
197
Brocks Creek gold, NT
Galala Range, Qld
Blair nickel, WA
518
Celebration gold, WA
C
Centenary gold, WA 335
C lode lead-zinc-silver, NSW
619, 624
220, 223 255
Central Ellesmere gold, Vic
508, 510
Geology of Australian and Papua New Guinean Deposits
Central Murray Basin heavy mineral sand, NSW–Vic 647
Cobar mineral field, NSW Cockburn gold, WA
Central North gold, Vic
137, 139
508, 509 Cock-eyed Bob gold, WA
Centurion gold, WA
225, 226
215, 216, 217 Colliwobble Ridge gold, NT
Century zinc-lead-silver, Qld Chalice gold, WA
Comet gold, WA
601, 610, 723, 724
495, 496
685, 686
Claudius Creek gold, WA 165
161, 162,
Condenser Dolerite, WA
Clayhole Creek beds, Qld
738, 740
207
627,
287
Bernts, WA
737, 738, 739, 740
322 Bonnie Dundee, NSW 597, 599 Boyds 5, Qld
593, 596,
737, 738
759, 760 Breakers, WA
Browns Creek, NSW
575
461 Budgery, NSW
593, 599
cobalt deposits, Budgerygar, NSW Brolga, Qld
Cadia, NSW Cawse, WA
593, 597, 598
801
Bunyip Dam, WA
641
335, 337
Car Park, Qld
715, 717, 719
Chesney, NSW
609
Murrin Murrin, WA
567, 591, 601
329 167, 169
335 Double Tanks, NSW
Cobar, NSW
775, 776, 779
335 Deflector, WA
SM7, WA
593, 596
335 567, 587, 601, 609
Cobar Goldfield, NSW
609
Geology of Australian and Papua New Guinean Deposits
K lens, Tas
287
482, 483
Kangaroo Caves, WA 290
Dry River South, Qld 739
737, 738,
287, 288,
855
Kuridala, Qld
775, 784
Larsens East, NSW
596
Lewis Ponds, NSW
635
Little Cadia, NSW
641, 642
Mammoth, Qld
717, 718, 719 635, 636, 637, 743, 745, 748
Man O'War, WA
287
Mount Bini, PNG
769 CSA, NSW
Siberia, WA
645
63, 64, 65,
482, 483
Main zone, NSW 638
335
855
Horseshoe Lights, WA 66, 67
641, 643
Corbould zone, Qld Linger and Die, WA
596
Cadia Hill, NSW
743, 745
Greenmount, Qld
Hartmans, NSW
Main pipe, Qld
Cadia Quarry, NSW Esperanza, Qld
743
641, 644
335
Cawse Extended, WA
Gunpowder, Qld
Cadia East, NSW 335
Cawse Central, WA
769
Koki, PNG
287
495, 496
Coanjula diamond, NT
609
Greenmount, Qld
Jamesons, WA
641
307,
244, 246
Clunes gold, Vic
Great Cobar, NSW
J lens, Tas
287, 288
593, 595
759
Horse-Ivaal, PNG
Anomaly 45, WA Balcooma, Qld
707
Cloncurry Overthrust, Qld
593, 595
Great Australia, Qld
197,
536, 537
Big Cadia, NSW
Clifton gold, WA
439
102
copper deposits,
179
Cliffs-Mount Keith nickel, WA 315, 321
855
246
Coomber silver-lead-zinc, NSW 629
372, 373
Cliff-Charterhall nickel, WA
837
Girilambone, NSW
Coongee fault, Vic
Clermont Goldfield, Qld
Etasi Creek, PNG
745
475
Coolgardie mineral field, WA 211
Cleaverville Formation, WA
743, 745
Girilambone North, NSW
279
581,
759, 775, 784
Esperanza South, Qld
Frieda, PNG Comstock Formation, Tas
Cook-Reid gold, WA
chromium deposits,
Cleo gold, WA
149
Coolgardie Goldfield, WA
Weld Range, WA
Ernest Henry, Qld
Gecko, NT
Chillagoe Formation, Qld Chiltern goldfield, Vic
Endeavour 39 (E39), NSW 582
Esperanza, Qld
Comet–White Well Shear Zone, WA 150
609
Chesney Formation, NSW 611
149
Comet North gold, WA
137, 145
Chesney copper, NSW
449, 451
729, 753
261
Challenger gold, WA
Cindy gold, Qld
Eloise, Qld 759, 775, 783, 784, 793, 797
591
843
Mount Elliott, Qld Mount Isa, Qld Mount Lyell, Tas
775, 778, 784
743, 753, 759 473, 474
Mount Morgan, Qld Mungana, Qld Nena, PNG
715
673, 723, 726 855, 857, 860
865
Northeast, NSW
596
Olympic Dam, SA
Cox-Crusader gold, WA 164, 165
766
Osborne (Trough Tank), Qld 775, 784, 793, 794, 795 P lens, Tas
Peak, NSW
Crocodile gold, NT
567, 609 745
QTS North, NSW
601, 602, 605
QTS South, NSW
601, 602, 605
Red Dome, Qld
673, 723, 727
Roadmaster, WA Rosebery, Tas
Silver Peak, NSW Slag Heap, Qld Starra, Qld
715, 718, 719
784, 793, 797
Sugarloaf, Qld
Surveyor I, Qld
265, 266
CSA copper-lead-zinc, NSW 601 CSA Siltstone, NSW
587, 601, 602 111,
417, 418
Cumberland gold, WA
418
Trough Tank, Qld
371
Corbould zone copper-gold, Qld 776, 779 Corboys gold, WA
461
Merlin kimberlite field, NT 464 Diemals gold, WA
191
Digger Rocks nickel, WA
191, 192
Dingo lead-zinc-silver, Qld
784
443, 446, 447 161
508, 509
Dargile Formation, Vic
518
219, 220 495, 496
Dead Bullock Ridge gold, NT 451, 456, 457
449,
593, 596
137, 145, 146
Driffield gold, NT
427
Drummond Group, Qld
187
696
Dry River South copper-lead-zinc, Qld 737, 738, 739 Dry River volcanics, Qld 740 Dugald zinc-lead, Qld
737, 738, 753
Dead Bullock Soak gold, NT Deakin gold, WA
699, 701, 702
449
98, 100, 102
Debom volcanics, PNG
856
Durkin nickel, WA
495, 496 347
E
347, 348, 349,
Deborah back gold, Vic
524
11 Mile Well nickel, WA
Deborah fault gold, Vic
524
E.Mu kimberlite, NT
Cosmic Boy nickel, WA
365, 366, 367
Cosmo Howley gold, NT
409, 417
Cowriga Limestone Member, NSW 576
Deborah line of reef gold, Vic 521 Defiance Dolerite, WA
517
Cowal Conglomerate unit, NSW
521, 522
Dunolly gold, Vic
Coronet nickel, WA 352
583
Defiance gold, WA
235, 246, 247
243, 244, 247
Deflector gold-copper, WA Deliverer gold, WA Delta gold, WA
511,
167, 169
322
461, 462
Eaglehawk reef gold, Vic
527
Eaglehawk-Linscotts reef gold, Vic 527, 528, 529, 530, 531, 532 Earaheedy Group, WA
293, 294
East Alpha nickel, WA
348
156, 161, 162, 165 East Murchison mineral field, WA 119, 123, 155
233 East Perseverance fault, WA
866
434
Dragon gold, WA
Deborah anticline, Vic
156
255 388, 389,
297, 302
Cox gold, WA
365, 366
Dimer lag shear zone, WA
Corella nickel, WA
Costerfield gold, Vic
461,
775,
127
Corella Formation, Qld
Coanjula, NT
Double Tanks copper, NSW
Dawns Hope gold, WA
287, 291
diamond deposits,
Doon Doon breccia, WA 391
339, 343
Daylesford gold, Vic
Wheal of Fortune, WA
517
Dorothy Volcanic Member, NT
Davyhurst, WA
827, 859
235, 244,
Donovans Find gold, WA
98, 100, 102
Daley’s Hill gold, Vic
793
243
Donegal gold, WA
D
593, 597, 598
Delta South gold, WA
Dogbolter gold, NT
143
Cundaline iron ore, WA
Cygnet nickel, WA
635, 636, 638
244
567, 591,
287, 288,
64, 66
Toms zone, NSW
Wafi, PNG
Crown reef gold, WA
737, 738, 739
Thaduna, WA
Tritton, NSW
418, 419, 424
Curtin gold, WA
717, 718, 719
Sulphur Springs, WA 289
Crosscourse gold, NT
Cullen mineral field, NT
615, 617, 618
Delta Island gold, WA
Diamond Creek gold, Vic
409
Cullen Batholith, NT
759, 775
236
Devon Consols Basalt, WA 246
489
Cuddingwarra Goldfield, WA 113, 116
287, 288 474, 481
Selwyn, Qld
495, 496, 499
Crimson Creek Formation, Tas
287
Delta Island anticline, WA
225, 226
Creswick gold, Vic
482, 483, 484
Panorama, WA
Pluto, Qld
759,
Craze gold, WA
161, 162,
323, 324
Geology of Australian and Papua New Guinean Deposits
East Pit gold, WA
105
Faded Lily gold, NT
Eclipse gold, WA
149
Fairmile lead-zinc-silver, Qld
Ector kimberlite, NT
462
Fairplay gold, WA
Edie Porphyry, PNG
833, 834
Farleys gold, Vic
Edwin nickel, WA
Federal tin, Tas
347
Eel Creek Formation, WA Egerton gold, Vic
372, 373
409, 412, 413
261
488
Federal–Bassett Fault, Tas 489, 490
487, 488,
Fisher gold, WA
850
Gameta gold, PNG
849, 850
Gandys gold, NT
427
418, 428
Gareth kimberlite, NT
462
98, 102
Gawain kimberlite, NT
462
512 Fisher nickel, WA
347
Gawler Craton, SA
396, 401
776
Eloise copper-gold, Qld 784, 793, 797 Elologea fault, PNG
759, 775, 783,
Empire gold, WA
Fitzpatrick lead-zinc-silver, NSW 624 Fitzroy shear zone, WA
850 567, 587
462
Emu gold, WA
161
Speewah, WA
Emu (Waroonga) gold, WA
155, 156
Endeavour 42 (E42) gold, NSW 582
581,
81, 82, 85
261
Ernest Henry copper-gold, Qld 775, 784
759, 743, 745
744, 745
105
Etasi Creek copper-gold, PNG Evanston gold, WA
837
Excalibur kimberlite, NT
Gidgee, WA
410, 411 348
119, 123 119, 123, 124
Giffen Well iron ore, SA
63, 64, 65
401, 403, 404
Gindalbie Formation, WA
512
Girilambone beds, NSW
347
202, 340 594, 595 593, 595
Fosterville gold, Vic 508, 509
Girilambone Group, NSW 651
594, 595,
495, 496, 507,
Frankenia granite, NT
Girilambone North copper, NSW 595
508 443, 444, 446 616
149
Frieda copper-gold, PNG
Glasgow Lass gold, WA Glen Eva gold, Qld
Frieda River Intrusive Complex, PNG 855, 856 783, 784,
449, 451
Glengarry Basin, WA
63
Glengarry Group, WA
63
Golconda Formation, WA
675
112, 149
Gold Creek fault, NT
433, 434
Gold Creek gold, NT
433
Adelaide, WA 112, 149,
Agnew, WA Aladdin, WA
Geology of Australian and Papua New Guinean Deposits
691, 695
gold deposits,
G Gabanintha Formation, WA 279, 280
593,
156
Glenell Granodiorite, Qld 855
593, 599
397
528, 529
Girilambone copper, NSW
Fumarole gold, NT
Fabian Quartzite Member, SA
German anticline, Vic
Gidgee gold, WA
462
F
155, 157, 161
Gibb nickel, WA 365,
365
Fullerton River Group, Qld 785
191
Genesis gold, WA
Fosterville fault, Vic 508
Friars gold, WA
745
211, 212
Gerowie Tuff, NT
Freyers Metasediments, NSW
744
Esperanza South copper, Qld
642,
244, 252
Fosterville goldfield, Vic
Esperanza copper-cobalt, Qld
Esperanza Formation, Qld
365, 366, 368
Forrestania nickel, WA
Foster nickel, WA
167, 168,
Gellatly nickel, WA 347
Forty Foot sandstone, Vic
417, 427
Esperanza fault, Qld
Formidable gold, WA
Fortnum gold, WA
81, 82
Enigma structural zone, WA Enterprise gold, NT
387
Forrestania greenstone belt, WA 366
Endeavour 39 (E39) copper, NSW 581, 582
Enigma North gold, WA
Gecko copper-gold, NT 439
Forest Reefs Volcanics, NSW 646
161
401
82
Geko gold, WA
Flying Fox nickel, WA
Emu shear zone, WA
Gawler Craton iron ore, SA Gearless Well intrusion, WA 169
fluorite deposits,
127
Emu Fault, NT
620,
203, 204
Fiveways Deeps gold, WA
Elura zinc-lead-silver, NSW
Exley gold, NSW
Gameta fault, PNG
434
Ellenborough Run gold, Vic
Essex gold, WA
449, 453
Galala Range tin-tungsten-bismuthmolybdenum, Qld 669
508, 509
Finniss River Group, NT
Erin gold, WA
Gahn gold, NT
113
495, 496
El Sherana Group, NT
Elliott beds, Qld
784
Gabanintha shear zone, WA
105 161 111, 112, 113, 115
867
Alligator, NT
409, 412, 413
Anastasia, Qld
669, 671
Anomaly 45, WA Apollo, WA
137
Area 4, Qld
680
Area 4, WA
72, 73, 75
Area 5, Qld Argo, WA 251
233, 243, 244, 247, 250,
Atric, Qld
663
Avoca, Vic Avon, NT
495, 496
Bahama, WA
Baileys Island Central, WA 113, 115 Baileys Island East, WA 113, 115
111,
111, 112,
Baileys Island North, WA 113, 115
111,
Baileys Island South, WA 112, 113, 115
111,
Bailieston, Vic
Banker, WA
255
Barramundi, WA Barton, WA
71
98, 100, 102
Barton Deeps, WA
Beaver, WA
216 691, 695,
Clunes, Vic
495, 496 137, 139
Cock-Eyed Bob, WA 225, 226 Colliwobble Ridge, NT
Brocks Creek, NT
409, 417
Comet, WA
129, 131, 129,
Browns Creek, NSW
575
Buck Reef West, Qld
680, 681
Bullen, WA
105 409
Cadia, NSW
641 641, 644
Cadia Hill, NSW
641, 643 645
112, 113
142
64
216 495, 496
Caustons, WA
Costerfield, Vic
149, 151
409, 417
517
156
Cox-Crusader, WA 165
161, 162, 164,
225, 226
Creswick, Vic
495, 496, 499
Crocodile, NT
409
Crosscourse, NT
418, 419, 424
Crown reef, WA
265, 266
Cumberland, WA 143 Curtin, WA
98, 100, 102 508, 509
Dawns Hope, WA Daylesford, Vic
512
775, 776, 779
127
Daley’s Hill, Vic
449, 450, 451, 452, 457
Cashman, WA
102
Cosmo Howley, NT
Craze, WA
Cadia East, NSW
Calista, WA
Corboys, WA
Cox, WA
265, 266, 267, 268
Burgan, NT
Cook-Reid, WA
149
Corbould zone, Qld
Bronzewing Discovery, WA 131, 133
219, 220 495, 496
Dead Bullock Ridge, NT 456, 457
495, 496
Caustons North, WA
149
Dead Bullock Soak, NT
Biddy Well, WA
127
Caustons South, WA
149
Deakin, WA
Big Cadia, NSW
641
Cave Rocks, WA 250, 251
243, 244, 247,
Celebration, WA
220, 223
Berringa, WA
Big Hill, NSW
551, 552, 556, 557
Big John, WA
149
Big Slate reef, Vic
Centenary, WA 512
255
Central Ellesmere, Vic
508, 510
449, 451
149
Comet North, WA
97, 127, 137
Catherine reef, Vic
Ben Hur, WA
244, 246
197
Callie, NT
Belyando (Hill 226), Qld 707
Clifton, WA
Broads Dam, WA
Caledonian, WA
427, 428, 429
161, 162, 165
179
Cockburn, WA
243, 244, 247, 248,
Cadia Quarry, NSW
102
Batman, NT
Bendigo, Vic
737, 738
Bulletin, WA
495, 496, 543, 544
137, 145
685, 686
Cleo, WA
365, 366
Bullabulling, WA 211
496, 517
Ballarat, Vic
265, 266, 268
Bronzewing Central, WA 133
243, 249
Challenger, WA
Claudius Creek, WA
Bluebird link, WA
Boyds 5, Qld
261
Cindy, Qld
111, 112, 114
Bronzewing, WA
449, 451
111, 112,
508, 509
215, 216, 217
Chalice, WA
244
Britannia, WA 249
191
182, 183
Bluebird East, WA
Bounty, WA
680
Aurora, WA
Centurion, WA
Black Swan South, WA 116 Blue Lode, WA
73
Central North, Vic
215
Black Magic, WA
680
Area 3, WA
127
Binduli, WA
243, 247, 250, 251
Area 2, Qld
868
Bills Find, WA
449, 451, 449
98, 100, 102
Deborah back, Vic
524
Deborah fault, Vic
524
Deborah line of reef, Vic
511, 521
Defiance, WA
243, 244, 247
Deflector, WA
167, 169
Geology of Australian and Papua New Guinean Deposits
Deliverer, WA Delta, WA
156, 161, 162, 165
233
Delta Island, WA
244
Delta South, WA
243
Diamond Creek, Vic Diemals, WA
Friars, WA
Gahn, NT
Donovans Find, WA
255
Dragon, WA
137, 145, 146
Driffield, NT
427
Dunolly, Vic
495, 496
Eaglehawk reef, Vic
Eaglehawk-Linscotts reef, Vic 528, 529, 530, 531, 532
527,
849, 850
Horseshoe Lights, WA 66, 67
Genesis, WA
155, 157, 161
Gidgee, WA
119, 123, 124
Glen Eva, Qld
433
Golden Eagle, WA
187
Golden Hope, WA
220
Endeavour 42 (E42), NSW 582 Enigma North, WA Enterprise, NT Erin, WA
581,
81, 82
417, 427
261
105
Evanston, WA
191
Farleys, Vic Fisher, WA
Great Eastern (Lawlers), WA 156, 161 Great Ophir, WA
Gulbadi, PNG
409, 412, 413 261
255
Formidable gold, WA
244, 252
63, 64, 65
Geology of Australian and Papua New Guinean Deposits
482, 483
Jaffas, WA
149
Jaurdi, WA
105
833 149
Jim’s Find, NT John Bull, NT
443 409 427
105 174, 177, 189
265, 266, 267, 269 63, 64,
149
224, 233, 243, 244,
Jundee, WA
89, 97, 128
Jupiter, WA
180
K lens, Tas 219, 220
Harmony (Contact), WA 81, 82, 83, 85
149, 152
211
Junction, WA 247, 249
Hampton-Boulder, WA
Harlequin, WA 82
J lens, Tas
Julies Reward, WA
105
Gunbarrel North, WA
233, 243, 247
Jubilee, WA 63, 64, 81, 82, 84, 219, 220, 223
769
Harbour Lights, WA
98, 102
Ives Reward, WA
Jones Brothers, NT
837, 840
Happy Jack, WA
508, 509
155,
187
Great Victoria, WA
Hamlet, WA
Fiveways Deeps, WA
Fortnum, WA
177, 180, 181
233,
127
Jasper Queen, WA
97, 98, 127
Hamata, PNG
593, 599
Faded Lily, NT Fairplay, WA
837
Ives, WA
192, 193, 194
Granny Smith, WA
522, 524
Intrepide (Entrepide), WA 243, 247, 251
427, 431
Gunbarrel, WA
Etasi Creek, PNG
Exley, NSW
759, 775, 784
180
Golden Slipper, WA
105
Inner and Outer reef, Vic
219, 223, 253
Greenmount, Qld
Ernest Henry, Qld Essex, WA
Golden Mile, WA
Gourdis, WA 155, 156
105
212
243, 244 , 247
Indigo, WA
495, 496
Emu (Waroonga), WA
Hunt, WA
63, 64, 65,
551, 552, 553, 556,
Humpback, WA
691, 695
Gold Creek, NT
Golf, NT
Hortons, NSW 558
156
Egerton, Vic
161
64
211, 212
Golden Delicious, WA
Emu, WA
427
Geko, WA
149
127
Horseshoe, NT Horseshoe, WA
Eclipse, WA
Empire, WA
563, 564
439
Golden Age, WA
Eloise, Qld 759, 775, 783, 784, 793, 797
156
Gecko, NT
105
512
473, 481
Hillside, NSW
427
East Pit, WA
Ellenborough Run, Vic
255
Hidden Secret, WA
Glasgow Lass, WA
527
Henty, Tas
449, 453
Gandys, NT
161
Harris Find, WA
449, 451
Gameta, PNG
Harmony Southwest laterite, WA 82 Harringtons Hill, Vic 508, 509
855
Fumarole, NT
517
443, 446, 447
Donegal, WA
495, 496, 507,
149
Frieda, PNG
191
Dogbolter, NT
Fosterville, Vic 508, 509
482, 483
Kambalda-St Ives, WA Kanowna Belle, WA 340 Kargalio, PNG Katies, WA
201, 202, 339,
821, 824
149
Kavursuki, PNG Keillor 1, WA
243
821, 823, 824 71
869
Keillor 2, WA
71
Marvel Loch, WA
Keringal, WA
180, 181
Maryborough, Vic
King Of The Hills
173
Masi reef, PNG
King Of The Hills Extended, WA 173 King Of The Hills West, WA Kingfisher, WA
522, 525
Kundana, WA
207
Kuridala, Qld
775, 784
Lφ1, WA
192
Lφ2, WA
192
Lφ3, WA
192
495, 496
Maxwells, WA
McKinnons, NSW
Last Resource, SA Lauriston, Vic
495, 496 105
Lewis Ponds, NSW Lifeboat, WA
635
243, 244
Lights of Israel, WA
187
Linscotts reef, Vic
527, 528, 530
Little Cadia, NSW
641, 642
Nim 1–2, WA 102
98
90, 91, 92, 98, 100, 102
Moonlight, WA
105
Nim 4, WA
90, 91, 92, 98, 102
Nim 6, WA
90, 102
Nim 7, WA
90, 102
161 843 691, 695
Nimary, WA
89, 90, 97
253
Nolans, Qld
679, 680, 681
Mount Charlotte, WA Mount Dimer, WA
191
Norseman, WA
Mount Elliott, Qld
775, 778, 784
North Orchin, WA
Mount Joel, WA
127 473, 477
Northwest, WA
Mount Lyell, Tas
473, 474
Nuggetty reef, Vic
Mount McClure, WA 137
97, 127, 128, 715
Mount Pleasant, WA
64, 81, 82, 83
535, 536, 538, 539
Mount Todd, NT
O’Dwyers, Vic
561
417, 427 675
Magiabe, PNG
823, 824
Mungana, Qld
673, 723, 726
Main pipe, Qld
715, 717, 718, 719
Mutooroo, WA
220
635, 636, 637,
Nagambie, Vic
517, 518
Main zone, NSW 638 Makai, WA Maldon, Vic
Nancy, Qld
187, 188 495, 496
Mandilla Well, WA Mararoa reef, WA
127 265, 266
Navsix, NT
216
191
Nelson’s Fleet, WA
Marion, WA
149
Nena, PNG
239
766
Omega, WA
119
Orchin, WA
243, 244, 247
Orelia, WA
143
Orion, WA
244, 247 439
P lens, Tas
63, 64
Paddington, WA Pajingo, Qld
Peak, NSW
759,
482, 483, 484 219, 223
685, 686, 691, 695
Parmelia, WA
439, 440, 441
Marda, WA
265, 266, 267, 269
Osborne (Trough Tank), Qld 775, 784, 793, 794, 795
112, 115
Nathans-Deep South, WA Navajo, WA
OK, WA
Orlando, NT
685, 688
Nannine Reef, WA
72, 73
508, 509
Olympic Dam, SA
821
Mount Wright, Qld
527
OCA, Qld 679, 680, 683
691
Mount Terrible, NSW
98, 100
NW extensions, WA
Lone Sister, Qld
Mount Sinivit, PNG
244, 247
105
Mount Julia, Tas
180
Magdala, Vic
261, 265
North Pit, WA
Mount Morgans, WA
98, 100, 102
90, 91, 92, 98, 100,
Nim 3, WA
105
Lyons, WA
609
119, 123
Lone Hand, WA
707
155, 157
Montague, WA
Mount Morgan, Qld
Lucky Break, Qld
155, 156, 157,
New Occidental, NSW
64
149
137, 139, 143
526
New Holland South, WA
Little John, WA
Lotus, WA
609
New Holland, WA 161
98, 102
Mount Coolon, Qld
395, 396
680
821, 824
Moilers Find, WA
219
New Formation reef, Vic
Melaneur-Shelmalier, Qld
Menzies, WA
255
New Cobar, NSW
567
Mengmut, PNG
821, 822
New Celebration, WA
225, 226
Mount Bini, PNG
180
Nevoria, WA
834, 835
Mikhaburra, WA
64
Lancefield, WA
Lawless, WA
Nengmutka, PNG
Mosquito Well, WA
Labouchere, WA
870
173
123, 124, 125
Kingsley’s lode, Vic
255
137, 145 567, 609
Peak Hill, WA
63, 64, 81
855, 857, 860
Geology of Australian and Papua New Guinean Deposits
Peak Hill-Fiveways, WA 83, 84 Perch, WA
81, 82,
Pilgrim, WA
395, 396
161, 162, 165
Pink Lady, WA
182, 183
Pinnacles, WA
149
Pinter, NT
Plutonic, WA
261
Poverty Point, NSW 554
551, 552, 553,
QED, WA
Thunderer, WA 243
Santa-Craze, WA
225, 226, 227
Tick Hill, Qld
Santa North, WA
225, 226
Timbarra, NSW
Sill zone, Tas
Randalls, WA
225
Sirius, WA
Ravenswood, Qld
679
Read’s, Vic 508, 509 Red Dome, Qld
673, 723, 727 233, 243, 247
Redback Rise, NT Redeemer, WA
443, 446, 447 156, 161, 162, 163
Redoutable, WA 252, 253
243, 244, 247,
508, 509
Reptile Dam, WA Republic, WA
211
105
Republic North, WA
RMT, NSW
247,
233, 239, 243, 244,
Rising Tide, NT
409, 414
551, 553, 555
Robbins Hill, Vic Rosebery, Tas
508
474, 481
Rowes reef, Vic
526
Royal reef, WA
265, 266
Ruby Well, WA
64
Rumbles, WA Rushworth, Vic
563, 564
473, 475
225, 226 517
Geology of Australian and Papua New Guinean Deposits
679,
449, 451
Sons of Gwalia, WA
174, 177, 189
South Junction, WA
111, 112
Squib, WA
105
St Arnaud, Vic St Ives, WA
495, 496
224, 239, 243
St Mungo, Vic
Tolukuma (Tolukuma Hill), PNG 837 635, 636, 638
512
255
71
Triumph Hill, NT 458
449, 451, 456,
Trough Tank, Qld
793
Trout, WA
243, 244, 246, 247
71, 72, 76
Tucka Mining Centre, WA Tuckabianna, WA
Tuckabianna West, WA Twin Hills, Qld Two Boys, WA
261, 262
Two Hills, WA
98
Union Hill, Vic
528
Union Reefs, NT 417, 418, 419, 420, 421, 423, 508, 509
784, 793, 797
Venus, WA
Stawell, Vic
495, 496, 535
Vera North, Qld
Success, WA
137, 144
Victorian province, Vic
Sundowner, WA Sunrise, WA
127
Sunrise-Cleo, WA 182, 183 Sunset, Qld
179, 180, 181,
737, 738, 739
Tanami corridor, NT Tanami Mine, NT
Tarmoola, WA Tarnagulla, Vic
443, 444
443
Tarcoola Blocks, SA
395
173, 174, 175 495, 496
Tasmania reef, Tas 471
Victory, WA 261
685, 686, 687
467, 468, 469,
495
224, 237, 243, 244, 246
Victory-Defiance, WA 233, 235 Villa, NT
679, 680
Surveyor 1, Qld
149
Victory Flames, WA
179
149, 150
691, 695
Vanessa’s, Vic
715, 717, 718, 719
149
149
Starra, Qld
Sugarloaf, Qld
105
Repulse footwall lode, WA 249 Revenge, WA 247
149, 153
Sleepy Hollow, NT
427, 432
Triple P, WA
508, 509
Slaughter Yard Creek, Qld 680
114, 116, 149
551
Toomey Hills, WA
105
Silicon Valley, NSW
Tollis, NT
699, 700
Toms zone, NSW
759, 775
Sherwood, WA
427
Rehe’s, Vic
685
Semaphore, WA
Quigleys, NT
Red Hill, WA
265, 266, 267, 269
Sharkey’s, Vic
201
Rapier, WA
679, 680, 681
Selwyn, Qld
Poseidon South, WA
105
225, 226
Scott lode, Qld
71, 72, 73, 75
The Gap, WA
449
Santa Claus, WA
Scotia, WA
216
The Granites, NT
243, 247, 249, 250
Sarsfield, Qld
439, 440, 441
Pitman, WA
71, 72, 75
Santa Ana, WA
71, 72, 73, 75
Perseverance, SA
Salmon, WA
219, 223,
449, 451, 455, 456
Vivien, WA
161
Wafi, PNG
827, 859
Waihi, WA
187
Walhalla, Vic
517
Wapolu, PNG
849
Waroonga Laterites, WA Wattle Gully, Vic Wembley, WA
155, 157
498
64
871
West Pit, WA
105
White Devil, NT
Gorge Creek Group, WA
439
White Feather, WA
201
White Hope, WA
149, 152
517
Widgiemooltha, WA
261
Graphite fault, WA
177, 180, 181
106
Great Australia copper, Qld
759
Great Cobar copper, NSW
609
Great Cobar Slate, NSW
Harrier nickel, WA
610, 611
821, 822, 823, 824 Great Eastern (Lawlers) gold, WA 155, 156, 161 Williams United, Vic 511, 512 Great Flood unit, NSW 582 Wilthorpe, WA 64 Great Ophir gold, WA 187 Wiluna, WA 105 Great Victoria gold, WA 255 Wirralie, Qld 691, 695 Greenmount copper-cobalt-gold, Qld Wondergraph, SA 395, 396 769 Wonga, Vic 535, 536, 540 Greenvale Fault, WA 387 Woods Point, Vic 495, 496, 499, Grevillea zinc-lead-silver, Qld 753 517 Gulbadi gold, PNG
427
Woolwonga, NT
409, 417
Woorana, WA Yandan, Qld
691, 695
Yilgarn Star, WA Zapopan, NT
Gunbarrel gold, WA
255
Gunpowder copper, Qld
Zone 2, WA
161, 162, 163
Zone 3, WA
161, 162, 163
Zone 019, WA
72, 73, 76
Zone 061, WA
72, 73
Zone 96, Tas
473, 475, 476
Hamersley Group, WA
Zone 124, WA
72, 73, 75
Zone 550, WA
72, 73, 75
Hamersley surface, WA 379
105
Golden Delicious gold, WA
180
376
647, 648
647, 648 649, 650 474, 481, 482 347, 349
Henty Fault, Tas
474, 473, 477, 481
Henty gold, Tas
473, 481
Higginsville, WA
Hampton-Boulder fault zone, WA 222, 224
220,
583
Happy Jack gold, WA
105
219, 220
Happy Jack–Bulletin fault zone, WA 105, 107
474, 481
Hillside gold, NSW
156
261 563, 564
Hillview intrusive complex, NSW 653 Hillview vermiculite, NSW Hiltaba Granite Suite, SA
652,
651 397
Hodgkinson Formation, Qld Hodgkinson Goldfield, Qld Honeymoon Well nickel, WA 308, 315, 321, 322
297
Harbour Lights gold, WA 189 Harcourt Batholith, Vic
647, 648
376, 377, 378, Hidden Secret gold, WA
Golden Lava unit, NSW
872
647, 648
Hercules lead-zinc, Tas
297, 299, 300
427, 431
Jacks Tank South, NSW
Hellyer zinc-lead, Tas
Hannibals nickel, WA
Golf gold, NT
647, 648
WIM 150, Vic
821,
Hampton-Boulder gold, WA
192, 193,
Jacks Tank North, NSW
Wemen, Vic
743
149
Harakka nickel, WA
401, 402,
647
Twelve Mile, NSW
220
Golden Slipper gold, WA 194
744
Birthday Gift, NSW
105
Golden Hope gold, WA
Golden Ridge–Carnilya Hill belt, WA 347, 348
Haslingden Group, Qld
Spring Hill, NSW
187
219, 223, 253
596
119,
Golden Eagle gold, WA
Golden Mile gold, WA
388
heavy mineral sand deposits,
833
73
255
Hartmans copper, NSW
Helmut nickel, WA
Zone 114, WA
Golden Age gold, WA
508, 509
Hawks Nest iron ore, SA 403
H
Hamlet gold, WA
Harringtons Hill gold, Vic
167
Gunsap Mountain structure, PNG 822, 823
Hamata gold, PNG
495, 496
Hart Dolerite, WA
105
Gunbarrel North gold, WA
409, 413
Harrietville goldfield, Vic
Central Murray Basin, NSW–Vic 647
Gum Creek greenstone belt, WA 120, 123, 124
127
63, 64,
297, 300, 301
Harris Find gold, WA
837, 840
Gullewa greenstone belt, WA
Harmony (Contact) gold, WA 81, 82, 83, 85
Harmony Southwest laterite gold, WA 82
Wild Dog, PNG
Woolgni, NT
265, 266, 267,
97, 98, 127
Granny Smith gold, WA
219, 220
White Well, WA Whroo, Vic
Gourdis gold, WA
Harlequin gold, WA 269
372
664 663 297,
174, 177, Honeymoon Well ultramafic complex, WA 297 529, 530
Geology of Australian and Papua New Guinean Deposits
Honman Formation, WA
358
Hope Downs iron ore, WA
381
Hope North iron ore, WA
381
Hope South iron ore, WA
381
Hores Gneiss, NSW
Paraburdoo, WA Peculiar Knob, SA 405
Sequoia, SA
427
Horseshoe gold, WA
64
Horseshoe manganese, WA
Hortons gold, NSW 556, 558
551, 552, 553,
Huarabagoo uranium, Qld 811 Humpback gold, WA Hunt gold, WA
807, 808,
Jupiter gold, WA
Wilgena Hill, SA
401
Jupiter II anticline, NT
Yarrie, WA
371
Iroquois lead-zinc, WA
180 452, 453
Ives gold, WA
K lens lead-zinc-copper-silver-gold, Tas 482, 483
293
Kagi Metamorphics, PNG
127 233, 243, 247
Kalgoorlie Group, WA
Jacks Tank North heavy mineral sand, NSW 647, 648
844 244, 245
219, 233
Kambalda Dome, WA
J lens lead-zinc-copper-silver-gold, Tas 482, 483
188
807, 808,
K
371
J
347
97, 98,
371
Kambalda, WA
212
I Ida Fault, WA
Jundee-Nimary goldfield, WA 127 Junnagunna uranium, Qld 811
371
Ives Reward gold, WA
243, 244, 247
Hunt nickel, WA
64, 66, 67
89, 97, 128
371
Y10, WA
Horseshoe Lights gold-copper-silver, WA 63, 64, 65, 66, 67
224, 233, 243, 244,
Sunrise Hill, WA
Y2–3, WA
64, 66
Junction gold, WA 247, 249 Jundee gold, WA
401, 403, 405, 406
Shay Gap, WA
855
Horseshoe gold, NT
401, 403, 404,
Robinson Range, WA
616, 619
Horse-Ivaal copper, PNG
381
347, 348, 349
Kambalda Komatiite, WA 246, 348 Kambalda nickel, WA
235, 244,
347, 363, 369
Kambalda-St Ives gold, WA
243
Jacks Tank South heavy mineral sand, NSW 647, 648
Kangaroo Caves zinc-copper, WA 287, 288, 290
Inner and Outer reef gold, Vic 522, 524
Jaffas gold, WA
Kanowna mining centre, WA
Intrepide (Entrepide) gold, WA 243, 247, 251
Jamesons zinc-copper, WA
Indigo gold, WA
105
Iona Ridge, NSW
233,
647, 648
Jaurdi gold, WA
Brockman No 2 detritals (B2D), WA 375 Cundaline, WA
Gawler Craton, SA Giffen Well, SA
401
401, 403, 404
Hawks Nest, SA 401, 402, 403 Hope Downs, WA
381
Hope South, WA
381
Middleback Range, SA Mount Christie, SA
Jessops Creek Tonalite, Qld
401
401
Mount Tom Price, WA 381
Nimingarra, WA
371
Orebody 29, WA
385
Jindalee Group, NSW
409
Jones anticline, NT
431
381, 385
427
221
63, 64, 81, 82, 84,
Juggler fault, Qld
692
Julies Reward gold, WA
Geology of Australian and Papua New Guinean Deposits
821, 824
149
Kavursuki gold, PNG
821, 823, 824
Kay kimberlite, NT
462
Keillor 1 gold, WA
71
Keillor 2 gold, WA
71
Ken nickel, WA
Jubilee gold, WA 219, 220, 223
Junction Dolerite, WA
Kargalio gold, PNG
307
Keith–Kilkenny tectonic zone, WA 174, 330
347
Jubilee Dolerite, WA
203
235, 244, 246
Keith–Kilkenny lineament, WA
45
201
201, 202,
Kanowna Belle porphyry, WA
Katies gold, WA
655, 657
John Bull gold, NT
Juan nickel, WA
679, 681
443
JORC Code review
Kanowna Belle gold, WA 339, 340
Kapai Slate, WA
Jones Brothers gold, NT
381
Hope North, WA
149, 152
211
Jim’s Find gold, NT
371
287
347
Jasper Queen gold, WA
iron ore deposits,
Newman, WA
Jan nickel, WA
149
347
Keringal gold, WA
180, 181
kimberlite deposits, 149 245, 246
Bedevere, NT
462
E.Mu, NT
461, 462
Ector, NT
462
873
Excalibur, NT Gareth, NT
462 462
Gawain, NT Kay, NT
Lanfranchi nickel, WA Lansdowne Arkose, WA
462
Lantin anticline, NT
462
Launfal, NT
Merlin field, NT
461, 462, 464
395, 396
Launfal kimberlite, NT
462
Sacramore, NT
462
Laverton Domain, WA
173
King Of The Hills Extended gold, WA 173 King Of The Hills West gold, WA Kingfisher fault, WA
173
123, 124, 125
Kingsley’s lode gold, Vic
522, 525
Kundana gold, WA
Lawn Hill Formation, Qld 731
Black Rock, Qld Bowdens, NSW
775, 776
L
64
Lady Alice anticline, NT 422
729, 753
724
Lake Violet greenstone belt, WA 180
Lights of Israel gold, WA
187
Linger and Die Group, WA
336
Linger and Die nickel-cobalt, WA Linscotts reef gold, Vic
Liquid Acrobat nickel, WA
365, 366
567, 591, 601
Dingo, Qld
784
Little Cadia gold-copper, NSW 642
737, 738,
149
Livingstone talc, WA
66, 67
753
Lone Hand gold, WA
105
Elura, NSW
567, 587
Lone Sister gold, Qld
691
Fitzpatrick, NSW
Hellyer, Tas
Long nickel, WA
620, 624 753
474, 481, 482
811
Long Pocket uranium, Qld
808, 811
Loreto nickel, WA
Iroquois, WA
293
Lotus gold, WA
K lens, Tas
482, 483
Lewis Ponds, NSW Magellan, WA 295
729, 753 635
64, 66, 293, 294,
576
Long Pocket syncline, Qld
474, 481
482, 483
641,
347, 348
Long Hill diorite, NSW
Hercules, Tas
J lens, Tas
543, 547
Little John gold, WA
Dugald, Qld
784
336
527, 528, 530
CSA, NSW
Lady Loretta, Qld 89
243, 244
Little Bendigo goldfield, Vic
581
Lake Johnston greenstone belt, WA 357, 358
635, 636, 638
Coomber, NSW 627, 629
Grevillea, Qld
Lake Cowal volcanic complex, NSW 582
874
775, 783, 793
729, 753
Fairmile, Qld 418, 419,
422
Lady Loretta zinc-lead, Qld
Lancefield gold, WA
619, 790
Dry River South, Qld 739
Labouchere gold, WA
620, 624
Lifeboat gold, WA
619, 624
Century, Qld
737, 738, 739
Lewis Ponds gold-silver-copper-leadzinc, NSW 635
627
775, 784
2K, NSW
615, 617, 618
Toms zone, NSW
737, 738
C lode, NSW
207
Kuridala Formation, Qld
729, 730,
784
Broken Hill, NSW
410, 411
474, 481
Surveyor I, Qld
105
Kuridala copper-gold, Qld
Lake Cowal, NSW
474, 481
Lawless gold, WA
Cannington, Qld
Lags Supersuite, Qld
615, 625
181
Silver Peak, NSW
815
Lady Alice shear, NT
775, 784, 793
Lawlers Greenstone formation, WA 162
Kunwarara magnesite, Qld
192
567, 609
155
Boyds 5, Qld
855
Koolpin Formation, NT
Lφ3 gold, WA
Peak, NSW 495, 496
lead deposits,
124, 125
Kingfisher gold, WA
192
482, 483, 484
Rosebery, Tas
King Of The Hills gold, WA
Lφ2 gold, WA
P lens, Tas
Que River, Tas Lawlers, WA
462
192
673, 723, 726
Potosi, NSW
462
Lφ1 gold, WA
Mungana, Qld
Pegmont, Qld Lauriston gold, Vic
Koki copper, PNG
743, 753, 759
596
Last Resource gold, SA
462
Ywain, NT
Mount Isa, Qld 452, 453
Palomides, NT
Tristram, NT
635, 636, 637,
388, 389
Larsens East copper, NSW 462
Main zone, NSW 638
347
348 137, 139, 143
Lower Amphitheatre Group, NSW 568, 569 Lower Transition beds, Tas Loxton Sands, NSW-Vic Lucky Break gold, Qld Luke Creek Group, WA 280
468 647, 649
707 112, 149, 279,
Geology of Australian and Papua New Guinean Deposits
Lunnon Basalt, WA
244, 246, 348
Marda Complex, WA
Lunnon nickel, WA
347, 348
Marda gold, WA
Lynchford Member, Tas Lyons gold, WA
475, 479
Marion gold, WA MacLeod Member, WA
Moilers Find gold, WA
Marda greenstone belt, WA
535, 536, 538, 539
Magellan lead, WA 295
64, 66, 293, 294,
Maggie Hays nickel, WA 361, 362
Marvel Loch gold, WA
Maryborough gold, Vic Masi reef gold, PNG
823, 824
815
McCloy nickel, WA
348
McKinnons gold, NSW
655, 656 655, 656 715, 717,
Main zone gold-silver-copper-lead-zinc, NSW 635, 636, 637, 638
Maldon, Vic
187, 188
Maldon Goldfield, Vic 527, 528
495, 496, 498,
Mammoth copper, Qld
743, 745, 748 744, 745, 749
Mammoth Extended fault, Qld 745
744,
Horseshoe, WA
64, 66
729, 730, 744,
Mount Christie iron ore, SA Mount Coolon gold, Qld
Mengmut gold, PNG
680
821, 824
Miitel nickel, WA
233 401
Mikhaburra gold, WA
64
Millaroo Granite, Qld
675
Mandilla Well gold, WA Mararoa reef gold, WA
64, 67 89 716
Mine magnesite, NSW
127 265, 266
577
Mount Dimer gold, WA
837,
191
Mount Edwards nickel, WA
347
Mount Elliott copper-gold, Qld 778, 784
775, 777
Mount Fraser manganese, WA
64, 66
Mount Gordon Fault Zone, Qld 744
743,
Mount Isa copper-lead-zinc, Qld 753, 759
Mineral exploration research review, Australia 53 3
Mineral industry review, Papua New Guinea 33 Mineral Resources and Ore Reserves, Australian reporting standards 45
Geology of Australian and Papua New Guinean Deposits
Mount David fault zone, NSW
743,
655, 656
Mineral industry review, Australia 287
691, 695
Mount Fort Constantine metavolcanics, Qld 760
Millrose greenstone belt, WA
64, 66
Man O’War zinc-copper, WA
401
Mount Elliott fault zone, Qld
347
Mount Fraser, WA
64, 66, 67
253
Mount Davidson Volcanics, PNG 838
98, 102
Merougil Fault, WA
410,
445, 450
McNamara Group, Qld 754
Melaneur-Shelmalier gold, Qld
843
481
Mount Charlotte gold, WA
Mine corridor volcanics, Qld
Ravelstone, WA
225, 226
347
64, 67
64, 66
Mount Belches beds, WA
Mount Charles beds, NT
567
Millidie (Elsa), WA
Mount Padbury, WA
161
Mount Bonnie Formation, NT 411
669
Millidie (Elsa) manganese, WA
manganese deposits,
Mosquito Well gold, WA
Mount Black Fault, Tas
Middleback Range iron ore, SA
Mammoth fault, Qld
833
Mount Bini copper-gold, PNG
Merlin diamond (kimberlite field), NT 461, 462, 464
495, 496
833
McMahon nickel, WA
Menzies gold, WA
527
Maldon gold, Vic
433
225, 226
Main pipe copper-gold, Qld 718, 719
Makai gold, WA
834, 835
McDevitt Metamorphics, Qld
105
Morobe Granodiorite, PNG
495, 496
Maxwells gold, WA
655, 656
808, 812
Morobe Goldfield, PNG
769
655, 656
Thuddungra, NSW
Moonlight gold, WA
255
Maud Creek Goldfield, NT
Noakes, NSW
544,
552, 553
Moogooma uranium, Qld
Marra Mamba Iron Formation, WA 376, 381, 382, 383
357, 359,
magnesite deposits,
Mine, NSW
119, 123
Monty’s granite, NSW
801, 815
Mary Kathleen Group, Qld
Kunwarara, Qld
Montague gold, WA
669
382
Magdala gold, Vic
Baileys, NSW
molybdenum deposits,
Monte Christo Anticlinorium, Vic 545, 547
149
536
Magiabe gold, PNG
692
98
Galala Range, Qld
347, 349, 350,
Marlborough Block, Qld Magdala Basalt, Vic
191, 195
769, 770
Mariners nickel, WA 351
M
Moderate Angle shear, Qld
191
Marimo Slate, Qld
98, 100, 102
191
Mount Joel gold, WA
127
Mount Julia gold, Tas
473, 477
Mount Julia Member, Tas 477
475, 476,
Mount Keith nickel, WA 297, 307, 308, 309, 315, 321, 322, 323
875
Mount Lyell copper-gold, Tas 474
473,
Mount Magnet–Meekatharra Shear Zone, WA 150
Cawse Central, WA
N Nagambie gold, Vic
517, 518
Nammuldi Member, WA Mount McClure fault, WA
137, 138
Mount McClure gold, WA 128, 137
97, 127,
Mount Morgan copper-gold, Qld Mount Morgan Tonalite, Qld
715
716, 717
Mount Morgans gold, WA
180
Mount Newman Member, WA
382
Mount Norna Quartzite, Qld
Mount Pleasant gold, WA 83
Mount Seabrook talc, WA Mount Sinivit gold, PNG
64,
685, 688
473, 474,
Mount Terrible gold, NSW
417, 427
Mount Todd Goldfield, NT
Coronet, WA
347, 348, 349, 352
Naraku Granite, Qld
Cosmic Boy, WA
760
Mount Tom Price iron ore, WA 385
83, 85
Cygnet, WA
Narracoota Volcanics, WA
81
Digger Rocks, WA
Navajo gold, WA
Mount Wright gold, Qld
Mount Wyatt Formation, Qld
692
Mudgee, NSW
Edwin, WA
347
Nelson’s Fleet gold, WA
239
Fisher, WA
347
855, 857, 860 821, 822 822
511, 512 609 526
155, 156, 157, 155,
Harrier, WA
297, 300, 301
Helmut, WA
347, 349
Honeymoon Well, WA 315, 321, 322 Hunt, WA
347
365, 366 347
609 347, 363, 369
381 347
Lanfranchi, WA Agnew (Perseverance), WA Beautiful Sunday, WA Beta, WA
Blair, WA 329
347
321 Linger and Die, WA
335
Liquid Acrobat, WA
365, 366
365, 366
347 347, 348
339, 344 348
347, 348, 350 Lunnon, WA
Brolga, Qld
347, 348
801
Bunyip Dam, WA Carnilya Hill, WA 352
335, 337 347, 348, 349,
Maggie Hays, WA 362 Mariners, WA McCloy, WA
Cawse, WA
876
297, 308,
347
Loreto, WA
149
Murrin Murrin nickel-cobalt, WA
744
297
Kambalda, WA Newman iron ore, WA
Black Swan, WA
220
297, 299, 300
Harakka, WA
Long, WA
111
Myally Subgroup, Qld
348
nickel deposits,
Munni Munni platinum group element, WA 285
Murchison mineral field, WA
347
Hannibals, WA
219
Ken, WA
636
365
347
Juan, WA
Mungana gold-silver-copper-lead-zinc, Qld 673, 723, 726
Mutooroo gold, WA
Foster, WA
Jan, WA
637
365, 366, 368
Forrestania, WA
Gibb, WA
255
New Occidental gold, NSW
Mullions Range Volcanics, NSW
Murchison, WA
Flying Fox, WA
Gellatly, WA
543
New Formation reef gold, Vic
322 348
647, 648
Nevoria gold, WA
365, 366
347
Neckarboo Ridge, NSW
New Morning nickel, WA
627
Mumbil Group, NSW
Durkin, WA
East Alpha, WA
439, 440, 441
New Holland South gold, WA 157
Muddy Lake diorite (gabbro), NSW 583
339, 343
11 Mile Well, WA
New Holland gold, WA 161
675
63, 64
216
New Cobar gold, NSW 381,
365, 366, 367
Narracoota Formation, WA
New Chum anticline, Vic
427
307, 315,
112, 115
New Celebration gold, WA Mount Todd gold, NT
Cliffs-Mount Keith, WA 321
Nannine Reef gold, WA
Nerrena goldfield, Vic
Mount Terrible volcanic complex, NSW 561
322
297, 302
Nengmutka Volcanics, PNG
561
Cliff-Charterhall, WA
Corella, WA
Nengmutka gold, PNG
821
335
111, 113, 115
Nena copper-gold, PNG 66, 67
Cawse Extended, WA
Nannine Goldfield, WA
Navsix gold, NT
64, 81, 82,
Mount Read Volcanics, Tas 475, 481
Nancy gold-silver, Qld
Nathans-Deep South gold, WA
794
Mount Padbury manganese, WA 66
382
335
357, 359, 361,
347, 349, 350, 351 348
335
Geology of Australian and Papua New Guinean Deposits
McMahon, WA Miitel, WA
Norseman–Wiluna greenstone belt, WA 89, 106, 157, 162, 173, 197, 216, 219, 233, 244, 262, 321, 329, 336, 348
347
347
Mount Edwards, WA
347
Mount Keith, WA 297, 307, 308, 309, 315, 321, 322, 323 Murrin Murrin, WA
329
New Morning, WA Otter, WA
365, 366
487, 488
North Pit gold, WA
244, 247
347 315, 321,
347
Seagull, WA
365, 366
Siberia, WA
335
1A structure, WA
72, 73
316, 319, 326, 327
O’Briens Creek Supersuite, Qld 727 OCA gold, Qld
South Digger Rocks, WA
724,
679, 680, 683
365, 366 O’Dwyers shear, Vic
508, 509
Wannaway, WA 347 O’Dwyers porphyry, Vic
Olympic Dam copper-uranium-goldsilver, SA 766
348
Nim 1–2 gold, WA 102
Nim 4 gold, WA
265, 266, 267, 269
347
Yakabindie, WA 322
297, 315, 321, 90, 91, 92, 98, 100,
Omega gold, WA
90, 91, 92, 98, 100,
724
243, 244, 247
Orebody 29 iron ore, WA
385
Nim 7 gold, WA
90, 102
143
Orion gold, WA
244, 247
Nimary gold, WA
Orlando gold, NT
89, 90, 97
Nimary–Jundee goldfield, WA Nimbus silver-zinc, WA
89
273
Nimingarra iron ore, WA
371
Noakes magnesite, NSW
655, 656
Nolans gold, Qld Norseman gold, WA
679, 680, 681
381 647, 649
235, 245, 246
Parmelia gold, WA
137, 145
Peak gold-copper-lead-zinc-silver, NSW 567, 609 Peak shear zone, NSW
610, 612
81
Peak Hill gold, WA
63, 64
Peak Hill area gold, WA
81
Peak Hill Schist, WA
81
63, 82, 83
Peak Hill - Fiveways gold, WA 83, 84
Peela Conglomerate, SA
439
81, 82,
401, 403, 397
Pegmont lead-zinc-silver, Qld 784, 793
775,
71, 72, 73, 75
Perseverance (Agnew) nickel, WA 297, 304, 308, 315, 316, 319, 321 Perseverance fault, WA
316, 323
Osborne (Trough Tank) copper-gold, Qld Perseverance gold, SA 395, 396 759, 775, 784, 793, 794, 795 Perseverance ultramafic complex, WA Otter nickel, WA 347 316, 323 Outcamp uranium, Qld
808
Owen Stanley Metamorphic Complex, PNG 843, 844 Owen Stanley Metamorphics, PNG 828, 838
261, 265
Norseman Goldfield, WA
287
Paringa Basalt, WA
Perch gold, WA
Orelia gold, WA 90, 91, 92, 98, 102 90, 102
462
Peculiar Knob iron ore, SA 404, 405
119
Ootann Supersuite, Qld Orchin gold, WA
Nim 6 gold, WA
723, 724
Peak Hill mining centre, WA
850
322 OK gold, WA
Nim 3 gold, WA 102
509
297, 303 Oiatabu Dome, PNG
Wroth, WA
685, 686, 691, 695
Paraburdoo iron ore, WA
Peak Hill, WA
508
347 O’Dwyers gold, Vic
Widgie, WA
685
Panorama zinc-copper, WA
609, 610
O
322
Weebo Bore, WA
219, 223
Parilla Sand, NSW-Vic
339, 340, 342
Wedgetail, WA
63
Palomides kimberlite, NT
569
335
Victor, WA
Padbury Group, WA
Palmerville Fault, Qld
527
NW extensions gold, WA
Schmitz, WA
Sir Samuel, WA
Nurri Group, NSW
63
Pajingo gold, Qld
98, 100
Nuggetty reef gold, Vic
Padbury Basin, WA
Pajingo, Qld
596
Nullawarra Anticline, NSW
Rocky’s Reward, WA 322, 323
Silver Swan, WA
Northwest gold, WA
P lens lead-zinc-copper-silver-gold, Tas 482, 483, 484
Paddington gold, WA
105
Northeast copper, NSW
Perseverance (Agnew), WA 297, 304, 308, 315, 316, 319, 321
SM7, WA
North Bassett tin, Tas
488, 489
North Orchin gold, WA
347
Redross, WA
North Bassett Fault, Tas
P
Pilbara mineral field, WA Pilgrim gold, WA
161, 162, 165
Pilgrim shear zone, WA Pine Creek, NT
287
162
409
Pine Creek shear zone, NT
417, 418
265
Geology of Australian and Papua New Guinean Deposits
877
Pine Hill Granite, Tas
487, 488
Pink Lady gold, WA
182, 183
Pinnacles gold, WA Pinter gold, NT
285
235
71, 72, 73, 75 71, 72
90
Poseidon South gold, WA
261
Potosi zinc-lead-silver, NSW Poverty Point gold, NSW 553, 554
615, 625
551, 552,
Q
Santa Claus gold, WA
225, 226
Rendeep tin, Tas
487, 488
Santa North gold, WA
225, 226
Santa–Craze gold, WA
225, 226, 227
601, 602,
QTS South copper, NSW 605
601, 602,
474
474, 481
Republic gold, WA
Rapier gold, WA
427
105
114, 116, 149 81, 83, 85
Ravelstone manganese, WA
64, 66, 67
Ravenswood gold, Qld
679
Ravenswood Goldfield, Qld Raymond Formation, Qld
Schmitz nickel, WA 247,
679, 680
Seagull nickel, WA
Resources and Reserves, Australian reporting standards 45
Rosebery Fault, Tas
Selwyn copper-gold, Qld 759, 775 105
Sequoia iron ore, SA 406
754
551, 553, 555
Robbins Hill gold, Vic
808, 809
Semaphore gold, WA
409, 414
508
401, 403, 405,
Sharkey’s gold, Vic
287, 288
508, 509
Shay Gap iron ore, WA Sherwood gold, WA
64, 66,
315, 321,
371 149, 153
Siberia nickel-cobalt, WA
335
Silicon Valley gold, NSW
563, 564
Sill zone gold, Tas
473, 475
silver deposits,
481, 482
Rosebery lead-zinc-gold-silver-copper, Tas 474, 481
Black Rock, Qld
526
Royal reef gold, WA
265, 266
Ruby Well gold, WA
64
784
Bowdens, NSW 627 Boyds 5, Qld
Rowes reef gold, Vic
737, 738
C lode, NSW
233, 243, 247
775, 783, 793
225, 226 Century, Qld
673, 723,
619, 790
619, 624
Cannington, Qld Rushworth gold, Vic
157, 162
365, 366
Seigal Volcanics, Qld
233, 239, 243, 244,
Roadmaster zinc-copper, WA
685
Scotty Creek Formation, WA
729, 753
517 Coomber, NSW
Rylstone Volcanics, NSW 630
627, 629
628, 629, Dingo, Qld
784
Elura, NSW
567, 587
443, 446, 447 Fairmile, Qld
878
265, 266, 267, 269
Scott lode gold, Qld Research and development review, Australia 53
Rumbles gold, WA
508, 509
Redback Rise gold, NT
Scotia gold, WA
450
347
Broken Hill, NSW
686
Red Dome gold-copper, Qld 727 Red Hill gold, WA
105
Repulse footwall lode gold, WA 249
Riversleigh Siltstone, Qld
669, 670
Schist Hills iron member, NT
Rocky’s Reward nickel, WA 322, 323
225
679, 680, 681
Scardons Volcanics, Qld
Robinvale Ridge, Vic 647, 648
Ravelstone Formation, WA
Read’s gold, Vic
211
Robinson Range iron ore, WA 67
R Randalls gold, WA
Sarsfield gold, Qld
Reptile Dam gold, WA
RMT gold, NSW
Que River lead-zinc, Tas Quigleys gold, NT
487
Rising Tide gold, NT
QTS North copper, NSW 605
243, 247, 249,
508, 509
Revenge gold, WA 247 201
Santa Ana gold, WA 250
807, 808, 810
Republic North gold, WA
749
167
808, 809
Rehe’s gold, Vic
Renison tin, Tas
Plutonic Well greenstone belt
QED gold, WA
Redtree uranium, Qld
Renison Bell tin, Tas
745
Portal fault, Qld
71, 72, 75
Salt River lineament, WA
Weld Range, WA 279
Pope Well fault, WA
462
347
Redtree dyke zone, Qld
Munni Munni, WA
Plutonic gold, WA
Sacramore kimberlite, NT Salmon gold, WA
platinum group element deposits,
Pluto copper, Qld
S
243, 244, 247,
Redross nickel, WA
216
Playa shear zone, WA
156, 161, 162,
Redoutable gold, WA 252, 253
149
439, 440, 441
Pitman gold, WA
Redeemer gold, WA 163
784
Geology of Australian and Papua New Guinean Deposits
Fitzpatrick, NSW Grevillea, Qld
J lens, Tas
482, 483
K lens, Tas
482, 483
Lewis Ponds, NSW
Mungana, Qld
South Junction gold, WA
Nimbus, WA
273
482, 483, 484
Peak, NSW
567, 609
St Ives, WA
775, 784, 793 615, 625
615, 617, 618 737, 738, 739,
620, 624
Vera North, Qld
635, 636, 638 685, 686, 687
Silver Lake Member, WA
348
Silver Peak lead-zinc-silver-copper, NSW 615, 617, 618 Silver Swan nickel, WA Sir Samuel nickel, WA
339, 340, 342
243, 244, 246, 247
Slag Heap zinc-copper, Qld 719
715, 718, 679,
449, 451
SM7 nickel-cobalt, WA
335
Sons of Gwalia gold, WA 189
783, 784,
512
Stanthorpe Adamellite, NSW
551, 552
Star of Hope Formation, Qld
686
Starra gold-copper, Qld
174, 177, 174
450, 451
Staveley Formation, Qld
769, 770
Stawell fault, Vic
537
Stawell gold, Vic
495, 496, 535
395
397
Tarmoola antiform, WA
175
173, 174, 175
Tarmoola granodiorite, WA Tarnagulla gold, Vic
495, 496, 498,
287, 288
Strelley succession, WA
467
Tasmania reef gold, Tas 471
467, 468, 469, 808
Tennant Creek mineral province, NT 439 Tennyson Leucogranite, NT 432
287, 288
The Granites gold, NT 287,
127
182, 183 179, 180, 181,
449
The Granites-Tanami Goldfield, NT 449 Thuddungra magnesite, NSW Thunderer gold, WA
179
Sunrise-Cleo gold, WA 182, 183
105
715, 717,
Sulphur Springs zinc-copper, WA 288, 289
Geology of Australian and Papua New Guinean Deposits
64, 66
808
Sugarloaf copper-gold, Qld 718, 719
Sunrise shear, WA
731, 754
489, 490 Thaduna copper, WA
Sunrise gold, WA
427, 428,
137, 144 Termite Range Fault, Qld
Sue uranium, Qld
175
495, 496
Tawallah Group, Qld 536, 537, 540
395
Tarcoola Formation, SA
Tasmania gold, Tas
Stawell Goldfield, Vic 535, 536
443, 444
443
Tarmoola gold, WA
784, 793, 797
Sundowner gold, WA
Sons of Gwalia shear zone, WA 537, 540
St Mungo gold, Vic
Tarcoola, SA
The Gap gold, WA
Sleepy Hollow gold, NT
66, 67
Tarcoola Blocks gold, SA
Success Creek Group, Tas
Slaughter Yard Creek gold, Qld 680
Soldiers Cap Group, Qld 785, 794
Tanami Mine gold
512
Success gold, WA
66, 67
Tanami corridor gold, NT
St Mungo fault, Vic
Strelley Granite, WA
322
talc deposits,
Tanami Complex, NT
224, 239, 243
Stawell Granite, Vic
620, 624
Mount Seabrook, WA
536
347
St Ives gold, WA
474, 481
Toms zone, NSW
495, 496
239, 243
St Ives belt, WA
2K lead-zinc-silver, NSW
Livingstone, WA
St Arnaud Group, Vic
Tolukuma (Tolukuma Hill), PNG 837
South fault, Vic
552, 553,
T
105
St Arnaud gold, Vic
P lens, Tas
Sirius gold, WA
Surface Hill granite, NSW 555, 556
388
Spring Hill heavy mineral sand, NSW 647, 648
766
Silver Peak, NSW
387
Speewah Group, WA
Squib gold, WA
Olympic Dam, SA
2K, NSW
679, 680
754, 755 Surprise Creek Formation, Qld 744, 748 Southern Cross greenstone belt, WA 256 Surveyor I copper-lead-zinc-silver-gold, Qld 737, 738, 739 Speewah Dome, WA 387 Speewah fluorite, WA
685, 688
Surveyor 1, Qld
111, 112
Sunset gold, Qld
635, 636, 637,
Nancy, Qld
Rosebery, Tas
365,
371
Southern Bounding fault, Qld
673, 723, 726
Pegmont, Qld
Sunrise Hill iron ore, WA
63, 64, 65,
635
Main zone, NSW 638
410
South Digger Rocks nickel, WA 366
753
Horseshoe Lights, WA 66, 67
Potosi, NSW
South Alligator Group, NT
620, 624
Tick Hill gold, Qld
243 699, 700
Tick Hill shear zone, Qld Timbarra gold, NSW
655, 656
701
551
879
Timbarra Goldfield, NSW
551, 552
Walhalla gold, Vic
U
tin deposits,
Walhalla goldfield, Vic
Federal, Tas
Union Hill gold, Vic
488
528
Galala Range, Qld
669
Union Reefs gold, NT 420, 421, 423,
North Bassett, Tas
487, 488
Upper Transition beds, Tas
417, 418, 419,
487, 488
Upper Watut Fault, PNG
Renison , Tas
474
uranium deposits,
Renison Bell, Tas
157
Waroonga Laterites gold, WA 157
155,
Junnagunna, Qld
807, 808, 811
Long Pocket, Qld
808, 811
Warrigundi Igneous Complex, NSW 561, 562
808, 812
Olympic Dam, SA
255
Outcamp, Qld Redtree, Qld
Transition beds, Tas
468
Sue, Qld
71
766
807, 808, 810
593, 597, 598
Tritton formation, NSW
594, 595 449, 451, 456,
Trough Tank copper-gold, Qld
793
71, 72, 76
Tuckabianna gold, WA
149
149
Tuckabianna Shear Zone, WA Tuckabianna West gold, WA
150 149, 150
Tunbridge Wells Diorite, NSW
642
Valentine Siltstone, WA Vanessa’s gold, Vic Venus gold, WA
388, 389
669
Twin Hills gold, Qld
691, 695
Two Boys gold, WA
261, 262
Two Boys shear zone, WA 98 475
381, 382 280 280
Weld Range ultramafic-mafic complex, WA 279 64
Wemen heavy mineral sand, Vic 648
149
Vera North gold-silver, Qld 687
West Angela Member, WA
382
105
Hillview, NSW
651
West Stawell lineament, Vic
Victor nickel, WA
347
Westmoreland Conglomerate, Qld
Victorian gold province, Vic Victory gold, WA 261
Villa gold, NT Vivien gold, WA
495
224, 237, 243, 244, 219, 223,
161
White Devil gold, NT
439 201 219, 220
White Well gold, WA
149, 152
517 347
Widgiemooltha dome, WA 827, 859
Wafi intrusive complex, PNG Wafi porphyry, PNG Waihi gold, WA
828, 829
828
807
White Hope gold, WA
Widgie nickel, WA
262, 263
808
Wheal of Fortune copper-zinc, WA 287, 291
Whroo gold, Vic
W
536, 537
Westmoreland uranium, Qld
White Feather gold, WA 246
449, 451, 455, 456
Wafi copper-gold, PNG
647,
685, 686, West Pit gold, WA
Victory Flames gold, WA
Twelve Mile heavy mineral sand, NSW 647, 648
322
Weeli Wolli Anticline, WA
Wembley gold, WA
508, 509
Victory-Defiance gold, WA 233, 235
tungsten deposits,
297, 303
Weld Range platinum group element, WA 279
vermiculite deposits,
Tucka Mining Centre gold, WA
Wedgetail nickel, WA
Weld Range greenstone belt, WA
807
V
462
498
Weld Range chromium, WA
808
348
Tristram kimberlite, NT
439
Wattle Gully gold, Vic
Weebo Bore nickel, WA
808
Westmoreland, Qld
Tripod Hill Member, WA
Tritton copper, NSW
808
Moogooma, Qld
347
880
Waroonga shear zone, WA
Warramunga Formation, NT
Tramways belt, WA
Tyndall Group, Tas
849
807, 808, 811
Toomey Hills gold, WA
Galala Range, Qld
Wapolu gold, PNG
Huarabagoo, Qld
Toms zone gold-silver-copper-lead-zinc, NSW 635, 636, 638
Two Hills gold, WA
833
347
433, 434
Tolukuma (Tolukuma Hill) gold-silver, PNG 837
Trout gold, WA
468
335,
Black Hills, Qld
427, 432
Triumph Hill gold, NT 458
Walter Williams Formation, WA 336, 337
487
Tollis Formation, NT
Triple P gold, WA
495, 496
Wannaway nickel, WA
Rendeep, Tas
Tollis gold, NT
517
Widgiemooltha gold, WA Wild Dog gold, PNG 824
347 261
821, 822, 823,
187
Geology of Australian and Papua New Guinean Deposits
Wild Dog structure, PNG 823
821, 822,
Wilgena Hill iron ore, SA
401
Williams Batholith, Qld
760
Williams United gold, Vic Wilthorpe gold, WA Wiluna gold, WA
511, 512
64
Yelma Formation, WA
293, 294, 295 427, 428
K lens, Tas
Yendilberin shear zone, WA
191, 195
Kangaroo Caves, WA 290
Yerrida Basin, WA
63, 293
Yerrida Group, WA
63
Yilgarn Star gold, WA
Wiluna Fault system, WA 107 Wiluna Goldfield, WA
105, 106, 105, 106, 107
Yilingbun Granophyre, WA
Wirralie gold, Qld
Ywain kimberlite, NT
Wittenoom Dolomite Formation, WA 382, 383 Wogamush Formation, PNG Wondergraph gold, SA Wonga gold, Vic
856
395, 396
535, 536, 540
Wonga schist, Vic
Woods Point gold, Vic 517 Woolgni gold, NT
Woorana gold, WA Wroth nickel, WA
Zig Zag Hill Formation, Tas
643,
Bernts, WA
427 409, 417 265, 268
127
Bowdens, NSW
348
737, 738
Broken Hill, NSW C lode, NSW
619, 790
619, 624 775, 783, 793
729, 753
Wyman Formation, WA
Dingo, Qld
784
288
Y
627, 629
Dry River South, Qld 739 371 371 297, 315, 321, 89, 97,
691, 695
371
Yarrie plateau, WA
371
273
Geology of Australian and Papua New Guinean Deposits
482, 483, 484
753
Elura, NSW
567, 587
737, 738,
784
Fitzpatrick, NSW Grevillea, Qld Hellyer, Tas
620, 624 753
Pegmont, Qld
775, 784, 793 615, 625 474, 481 287, 288 474, 481
Slag Heap, Qld
615, 617, 618 715, 718, 719
Surveyor I, Qld 2K, NSW
287, 288,
737, 738, 739
620, 624
Toms zone, NSW
635, 636, 638
Wheal of Fortune, WA
287, 291
Zone 2 gold, WA
161, 162, 163
Zone 3 gold, WA
161, 162, 163
Zone 019 gold, WA
72, 73, 76
Zone 061 gold, WA
72, 73
Zone 114, WA
473, 475, 476
73
Zone 124 gold, WA
72, 73, 75
Zone 550 gold, WA
72, 73, 75
Zuleika Shear Zone, WA 187, 188, 198, 200, 207, 246, 261, 262, 263
474, 481, 482
Hercules, Tas
474, 481
Iroquois, WA
293
J lens, Tas
567, 609
Zone 96 gold, Tas
Dugald, Qld
Fairmile, Qld
287
Sulphur Springs, WA 289
287
Century, Qld
371
673, 723, 726
Silver Peak, NSW
627
Cannington, Qld
Yarrie iron ore, WA
Mungana, Qld
Rosebery, Tas
784
567, 591, 601
Yarrie, WA
743, 753, 759
Roadmaster, WA
CSA, NSW
Yandan gold, Qld
Mount Isa, Qld
Que River, Tas
287
Black Rock, Qld
Breakers, WA
Yandal greenstone belt, WA 98, 127, 137, 138
475, 476
287, 288
Boyds 5, Qld
Yakabindie nickel, WA 322
287
Potosi, NSW
Coomber, NSW
Y10 iron ore, WA
Man O'War, WA
Peak, NSW
zinc deposits,
Wydgee–Meekatharra greenstone belt, WA 112
Y2–3 iron ore, WA
635, 636, 637,
Nimbus, WA
409, 413
635
Main zone, NSW 638
Panorama, WA
Zapopan gold, NT
701
Woolyeenyer Formation, WA
389, 391
Z
495, 496, 499,
Woolwonga gold, NT
388, 391
462
Anomaly 45, WA
Wongalong fault system, NSW 645
256, 257,
287, 288,
729, 753
Lewis Ponds, NSW
P lens, Tas
540
Wonga-Shinfield Zone, Qld
191, 255
Yilgarn Star shear zone, WA 258, 259
Yungal carbonatite, WA
482, 483
Lady Loretta, Qld
255
WIM 150 heavy mineral sand, Vic 649, 650 691, 695
287
Yenberrie Leucogranite, NT
Yilgarn mineral field, WA
105
Jamesons, WA
482, 483
881
882
Geology of Australian and Papua New Guinean Deposits
Author Index A
Bywater, A
Adshead, N D
793
Andrew, R L
827
Andrews, D L
297
Archer, N R
265
B Bailey, A
Bainbridge, A L
855
161
Donaldson, J S
C Caia, G P
Drew, G J
439
Callaghan, T
473
Cannard, C J
487
Carnie, C W A 783
Dodunski, N
567
297
Dugmore, M A Dunham, S Dunnet, D
481
255
Cervoj, K M
321
E
Chanter, S C
105
Ebsworth, G B Edgar, W
Chapple, K G
Barker, A J
215
Close, R J
737
Edwards, P W
Barnes, S J
357
Collett, D
679
Eilu, P
Cook, J
Beams, S D
753
Cook, W G
Beckett, T S
201
Copeland, I K
Belcher, R
417
Berry, M V Birch, J S
481 663
Blampain, P A Blucher, I Bolger, C
647
127 225
Bosel, C A
439
Broadbent, G C Brooker, M Broome, J
627
Elliott, G J
219
685
Corbett, G J
837
Elliott, S M
Coughlin, T
699
Erickson, M E
Cowden, A
273
F
Crookes, R A
255
Cucuzza, J
53
Czerw, A
191
567
Fahey, G J
105
201
Fairclough, M C Fare, R J
D
173
481
Fazakerley, V W
329
729 Davies, M B
401
Fogarty, J M
593
DeRoss, G J
855
Forrestal, P J
699
161 de Vickerod Krokowski, J
527
Fortowski, D B
775
173 De Vitry, C
161, 315
Fothergill, J
527
Fowler, M F
215
357 Denn, S
Buckley, P M Byass, A P
233
581
Brown, J C Buck, P S
Elliot, J
609
481
105
Ellery, S G
801
527
473
339
723
849
843
473
Balfe, G D
Barr, M J
233
335
71 Denwer, K P
833
Fredericksen, D C
Diemar, V A
655
Frost, K M
535
89
Geology of Australian and Papua New Guinean Mineral Deposits
365
883
Fuller, T A
427
Hibberd, T J
G
111
Hicks, J D
Gane, M
Hill, J
535
Gent, P G
Gibbs, D
481
179
Hills, M G
81
Hills, P B
467
197
Hitchman, S P
Golding, S D
715
Hobby, D J
Goode, A D T
53
Graves, C C Green, C
Hopf, S
679 273
179
H Hale, C T
807
Hamilton, G
409
Harmsworth, R A Harper, M A Harris, J L
81
Harvey, K J
675
Hellsten, K
Ion, J C
215
Jenkins, D R
615 335
Libby, J W
315, 321
Lindley, I D
821
Lovett, D R
449 587
Maclean, D R
89
443
Martin, A R
651
Mason, A J
647 347
753 McCracken, S J A
Johnston, C
McDermott, G J Jones, B H
685
Jones C M
179
McInnes, P
Jones, G F P
105
Journeaux, T
161
307 335
197
K Kellett, R J
511, 521
581
McIntosh, D
679
McIntyre, J R
191
McKenna, D M
375
McQuitty, B M
293, 487
Meares, R M D
635
Messenger, P R
715
187
481 211
775
567
123
Henderson, R G
461
Lewis, C R
Marsh, S
167
Hemming, G R
884
155
273
Joyce, R M Helm, S W
843
Masterman, E E
273
Head, D L
Leaman, P W
M
849
J
715
Hazard, N J
Ibil, S
837
Lutherborrow, C
Ivey, M E
137
Hayden, P
495
Inwood, N A 375
Hartley, J S
Hay, I P
395
I
481
Hallenstein, C
409
161
Hughes, M J
207
Leevers, P R
307
Hughes, F J
Lea, J R
315, 321
753
Lee, D C
225
Hosken, J
239
Leach, T M
769
Horsburgh, J R
Greenaway, A L Grove, A
Laurie, J P
481
Holmes, R
481
487
Langworthy, P J
855
Hodgson, G D
297
Kitto, P A
L
357
Glasson, M J
Gole, M J
409
Kriewaldt, M
417
Hill, R E
215
Georgi, H T
339
Kirk, C M
71
Geology of Australian and Papua New Guinean Deposits
Mikucki, E
105
Milburn, D
815
Miles, I
Rogerson, R
P Paquay, R D
581
Parianos, J M Parks, J
Miskelly, N
45
Pascoe, D J
293
Pearson, P J
699
Morant, P
287
279
Perkin, D J
615, 619
Perring, C S
Morris, B J
401
Phillips, G N
Morrison, I J
433
Morwood, N F
801
Mowat, B A Moy, A D Muir, M R K
Mulholland, I R Murphy, R Muscio, V N Mustard, R
551, 707
Ness, P K
63
Sebek, R S 365
609 3, 63
449
Rak, D
669, 723
Newcrest Mining Staff Newton, A W
401
Newton, P G N Nguyen, P T
641
179, 225, 417 233
551
581
Ranford, L C
Reddicliffe, T H Reed, G C
601
Renton, J I
81
461
807
685
Richardson, S M
Orton, J E
161
Smart, G
575
Smith, B
225
Smith, D W
119
Smith, M E
149 449 167 609, 807
Stephenson, P R Stockman, P R
427
Ormsby, W R
601
Stegman, C L
737
Richards, D R Olzard, K L
261
Steemson, G
3
Rheinberger, G M
O
Shedden, S H
Smith, M E H
127
Rea, P S
837
Simpson, C
511
Radclyffe, D
381
691, 695
Semple, D G
Shi, B L
627
461
517
Seed, M J
R
Nethery, J E
Nielsen, R
Pirajno, F
Quigley, P W
N
699
Scott Smith, B H
Q
793
Schubert, C J
161
Pringle, I J
97
449
Pilapil, L
Pring, P I
273
201
Sando, B G
97, 127, 495
Preston, W A
321
S
357
Pocock, J A
743
759
Sage, P W
Pitkäjärvi, J T
833
179
Ryan, A J
3
Morland, R
551, 691, 695
Ryall, A W
409
329
119
Ruxton, P A
801
Miller, G C
Monti, R
Ross, D I
381
33
743
45 321
Stockton, I
679
Stone, W E
347
Switzer, C
417
T
427
427
Roberts, R H
487
Rogers, K A
387
Geology of Australian and Papua New Guinean Mineral Deposits
Tangney, G
417
885
Taube, A
715
Tau-Loi, D
827
Wilkins, C
575
Willis, R D
481
Taylor, D H
543
Wilson, G M
201
Taylor, W R
461
Winnall, N J
111
Teakle, M
647
Woodhouse, M
Teale, G S
561
Woodhouse, W K
The geological staff of New Hampton Goldfields NL Thornett, S E
81
Thynne, D S
111
Tin, M
187
Z Zurkic, N
Treacy, J A
187
Y Young, C H
197
Tunks, A
219
297, 365
507
433 443
Turnbull, D G Turner, B J
521 265
V Vallance, S A
357
Valliant, R I
635
Vearncombe, J R Vickery, N M
97, 127
71
Voulgaris, P
793
W Wahdan, E
111
Waltho, A E Ward, L M
729 461
Watchorn, R B Waters, P J
371
Webster, A E Whittle, M
243
587, 619 321
Whitworth, D J Wilcock, S
886
427
815
Geology of Australian and Papua New Guinean Deposits
Geology of Australian and Papua New Guinean Mineral Deposits
887
888
Geology of Australian and Papua New Guinean Deposits
Geology of Australian and Papua New Guinean Mineral Deposits
889
890
Geology of Australian and Papua New Guinean Deposits
Geology of Australian and Papua New Guinean Mineral Deposits
891
892
Geology of Australian and Papua New Guinean Deposits
Geology of Australian and Papua New Guinean Mineral Deposits
893
894
Geology of Australian and Papua New Guinean Deposits
