ru FranzJ.Dahlkamp W"t-;-"
ia' / 1'' J...E
tJraniumOre Deposits With 161Figuresand55Tables
Springer-Verlag Berlin HeidelbergNew York London Paris Tokyo Hong Kong Barcelona Budapest
tQi)
Prof. Dr. Franz J. Dahlkamp
A+ n ^ ?
L0 OlbergstraBe D-5307Wachtbers Germany
ISBN 3-540-532&7Springer-Verlag Berlin HeidelbergNew York New York Berlin Heidelberg ISBN 0-387-532&LSpringer-Verlag
Libran of Conpress Cataloging in Publication Data Dahlkamp, Flenr J. Uranium ore deposits/ Franz J. Dahlkamp. lncludes bibliographical references and index. ISBN 3-54G5326f 1 (Berlin HeidelbergNew York: acid-freepaper). - ISBN 0-387-53261-l(Neu York Berlin Heidelberg:acid-freepaper) 1. Uranium ores. I. Title TN490.U7D28 1991 5fi.a'%2- dc20 91-%92CIP This work is subject to coplrig:ht. All rights are reserved.u,hether the whole or part of the material is concerned.speciicallv the rights of transiatjon,reprinting, reuseof illustratrons. recitation.broadcasring,reproductionon microfi.lmor in an1 olher war. and storagein data banks.Duplication of this publicationor pans thereof is permitted onlv under the provisions of the German Copvrig:htLau of September9, 1965.in its currenl version. and permission for use must aluals be obtarnedfrom Springer-Verlag.Violations are liable for prosecution under the German Copyright l-au. @ Springer-VerlagBerlin Heidelberg 1993 Pnnted in Germanl' The use of generaidescriptivenames.registerednames,trademarks.etc. in the publication doesnor implv. even in the absenceof a specificstatement,that suchnamesare exemptfrom the relevant protective laus and regulationsand therefore free for general use. Product Liability: The publishers cannot guarantee the accuracv of any information abour dosageand application connined in this book. In everv individual case the user must cbeck such information bl consultingthe relevant literature. Typesetting:Best-setTypesetter Ltd., Hong Kong Printing: Druiklrems Beltz, Hemsbach Binding: J. Schdffer.Grinstadt 32/3145-54 3 2 1 O - Prinred on acid-free paper
Preface An important prerequisiteto the long-term use of nuclear energy is information on uranium ore deposits from which uranium can be economically expioited. Hence the basic purpose of this book is to present an overview of uranium geology. data characteristic for uranium deposits, and a synthesisof these data in the form of a typological ciassification of uranium deposits supported by more detailed descriptionsof selecteduranium districts and deposits.An additional goal is to provide accessfor the interested reader to the voluminous literature on uranium geology. Therefore a register of bibliography as global as possible,extending beyond the immediate need for this book, is provided. The volume presented here was not originally designed as a product for its own sake. It evolved as a by-product during decadesof active uranium exploration and was compiled thanks to a request by the Springer Publishing Company. Routine research work on identifying characteristicfeatures and recognition criteria of uranium deposits, combined with associatedmodeling of types of deposits for reappiication in exploration, provided the data bank. The publisher originally asked for a book on uranium deposits structured as a combined text- and reference book. The efforts to condense all the text into a single publication were soon doomed. The material grew out of all feasible proportions for a book of acceptable size and price, a wealth of data on uranium geology and related geoscienceshaving become available during the past decade, too vast for one voiume. So the original idea had to be abandoned in favor of a two-volume publication. The contents of both volumes are arranged in such a way that each volume still represents,to an optimum degree, an entity. This first volume deals primarily with geological principles of uranium deposits amended by descriptions of seiected examples of deposits and districts. The companion volume contarns presentations of individual deposits organized by different countries. For the sake of the comprehensivenessof each volume, not all the information could be distributed without some repetition. Nevertheless, the interested reader is recommended to use both books for crossreference or as a guide for his own research and deposit modeling. Finally, it was the author's intention not only ro present data and his own views on uranium geology and metailogenesis. but also theories and models of other geoscientists(Chap. 5) , in order to stimuiate and encourage further research to achieve continuous progress in the understanding of uranium deposits and their metallogenesis.
FranzJ. Dahlkamp
P.S.: After finishingand proof printing of the manuscript.the opening of the former Eastern Block countnes dunng 1990 and 1991 providednew and more preciseinformation of uranium depositsin thesecountries.The availabledatahavebeenaddedasfar asfeasable
VI
Preface
in Chapter 4. Particularly one type of deposit characterizedby structure-bounduraniummineralizationassociated strata-controlled, with black shaleswas an important uranium source in severalEast Block countries.This type wasnot consideredeconomicby Western World standardsand wastherefore not designatedasa specialtype in the original manuscript.It now hasbeenincludedasType 16 StrataControlled,Structure-Bound.
Acknowledgements The author is indebted to the efforts of many geoscientistsand colleagues in the uraniumindustry,nationaland internationalinstitutions, and universitiesfor discussionsand reading and correcting variouspartsof the manuscriptand assisting in numerousother ways. Discussionswith them date back many yearsand have contributed tremendouslyto the understandingof uranium geology, and tbe decipheringof recognition criteria of uranium mineralizationson a local (individual deposit) and regional scale (uranium province). Their ideas, observations,and data have directly or indirectly become a part of this report. The author is most appreciativeto James A. Rasmussenand Elmer Stewartfor their endeavorto read and correctmost parts of the manuscript. He wishesto thank especiallythe followingindividualsfor reviewing and improving descriptionsof individual depositsor districts. (in brackets country, district, deposit reviewed): Adamek P. (Scandinavia),Adams S.S. (general),Arnaiz de GuezalaJ. (Spain), Barthel F. (general),Bernik J. (Yugoslavia),Brynard S.A. (South Africa, Namibia), ChenowethW. (ColoradoPlateau,USA), Coste A. (Limousin,France),CuneyM. (granites, France),DardelJ.R.M. (France),EwersG. (PineCreek Geosyncline.Australia),FritscheR. (mineralogy),FohseH. (general),FuchsH. (Brazii), Grauch R. (westernUSA). Gautier A. (brecciapipe deposits,USA), Haliadal' Ch.R. (easternUSA), HarshmanE.N. (WyomingBasins,USA). Hruby J. (CSFR),Kolb S. (Bavaria.W-Germany),Krol W. (Eastern Europe).MatosDiasJ.H. (Portugal).McCarnD. (general),Ott G. (general).RamdohrP. (mineralog-v, metallogenesis), RobertsonJ.A. (Blind River-ElliotLake, Canada).RuhrmannG. (Saskatchewan. Canada),RuzickaV. (CSFR).Saucier A.E. (GrantsRegion,N.M., USA), Smith R.B. (South Texas, USA), Tan H.B. (Canada), ThammJ. (ColoradoPlateau,USA), TauchidM. (general),VelsB. (general), Voss C. (general), Wallace A.R. (Schwartzwalder, USA), WutzlerB. (Australia). The numerous manuscriptswere repeatedly typed by Ms. Bartelmus,Bonn, Reichl,Leoben,Vollmer and Weyer,Essen.and Boody, Denver. Most drafts were preparedb,vMs. Gldssner.Drafting of cartoons of the types of depositswere performedby MRRD, Leoben, for which H. Kiirzl and J. Wolfbauer deservemy gratitude.
Contents Remarks, Definitions. Units Organizationof the Volume Citing of Authors .
1 ., ,)
Bibliography
2
Geological.Mineralogical,Mining and Related Terms. . . ConversionFactors Abbreviations
J J
I Introduction 1.1
Brief History of Uranium
r.2
Typesof Uranium Depositsand Occurrencesand Their EconomicImportance . . .
8
1.3
GeographicDistributionof Uranium Deposits
10
1.1
Resources. Reserves.Gradesand Productionof Uranium
t2
1.5
World Resources of Uranium
I+
l.o
UraniumProduction
15
2 Geochemistryand Minerochemistrvof Uranium . . . . . .
17
2.1
ChemicaiPropertiesof Uranium
17
2.2
GlobalGeochemical Abundanceof Uranium
I7
2.3
MinerochemicalDistnbutionand Abundanceof Uraniumin Minerals 2 . 3 . r MinerochemistryandCrystallographyof Uranium . . . . 2 . 3 . 2 MinerogenicDisrributionof Uranium ',
18 18 2l
1
GeochemicalDistributionandAbundanceof Uranium in Rocksand Waters 2.1.1 Uraniumin Magmatic/AnatecticEnvironments . . . .. . 2.1.2 Uraniumin Sedimentary Environments. . 2.1.3 Uraniumin MetamorphicEnvironments. . 2.1.1 Uraniumin Metasomatic Environments.. 2.,1.5 Uraniumin Waters 2.1.6 Uraniumin LivingOrganisms andTheir Decay Products 2.1.7 Uraniumin ExoterrestrialRocks
33 34
2.5
34
UraniumProvincesand Districts
23
28 30 JJ
vIII
2.6
Contents
CrustalEvolution and Related Uranium Distribution
36
SelectedReferencesand FurtherReadinefor Chapters2znd3
38
3 Principal Aspectsof the Genesisof Uranium Deposits
41
3.1.
The GlobalMetallogenicCvcleof Formationof Uranium Deposirs
The Time-RelatedOccurrenceof Uranium Deposits. . Time-Stratigraphic Reiationshipof Uranium Deposits Distribution of Uranium Resources Geochronological Relationshipof Geochronologic-Metallotectonic Uranium Deposits 3.2.4 Uranium DepositGenerationsandTheir Time-Stratigraphic Ranking. . . . 3.2 3.2.I 3.2.2 3.2.3
SelectedReferencesand FurtherReadinsfor Chapter3.2... ......
rAt 11
-ll
49 50
50 51 56
4 Typologyof Uranium Deposits.
JI
4.7 Type 1: Unconformify-Contact 4.7.7 Subtype1.1 Proterozoicunconformity-related 4.I.2 Subtype1.2 Phanerozoicunconformitv-related
57 66 68
4.2 Typel: Subconformiry-Epimetamorphic 4.2.1 Subtype2.1 Not albitizedsediments. 4.2.2 Subtype2.2 r'Jbitizedsediments
69 12
4.3 Type3Vein.. 4.3.1, Subtype3.1 Granite-related 4.3.2 Subtype3.2 Not granite-related. . .
74 77 82
4.4 Type4 Sandstone 4 .4.7 Subtype4.1 Tabular/peneconcordant . 4.4.2 Subtype4.2Rollfront.....
84 88 92
4.5
Type 5 CollapseBrecciaPipe .
94
4.6 4.6.7 4.6.2 4.6.3 4.6.4
Type 6 Surficial S u b t y p e 6 .D l u r i c r u s t e d s e d i m e .n. t. s. Subtype6.2Peat-bog.. . . . Subtype6.3 Karst-cavern.. Subtype6.4 Surfcialpedogenic andstructurefill . . . . .
ts
96
100 102 103 103
4.'l
TypeT Quartz-pebble (Lower Conglomerate Proterozoic) 4.7.7 Subtype7.1 U-dominatedwith REE 4.'7.2 Subtvpe'7.2 Au overU-dominant.. 4.8
Type 8 BrecciaComplex
704 lUO
107 108
Contents
IX
1 . 9 Type 9 Intrusive 1 . 9 . 1 Subtype 9.1 Alaskite (Cu-porphyry) +.4.Subtype .. ... 9.2 Quartzmonzonite 1 . 9 . 3 Subtype9.3 Carbonatite + .v . + Subtype9.-t Peralkaline syenite 4 . 9 . 5 S u b t y p e 9 .P5e g m a t i t e . . . .
110 112 t12 113 II4 114
4 . 1 0 Type 10 Phosphorite 4 . 1 0 . 1Subtype10.1 Phosphoria. . 4 . 1 0 . 2Subtype10.2 Landpebbte
1l)
II7 1i8
4 . 1 1 T y p e1 1 V o l c a n i c 4.11.1Subtype11.1 Structure-bound 4 . 1 1 . 2S u b t v o e 1 1 S . 2t r a t a - b o u n d . . . . . .
118 I20 121
,t 11 +.IL
r22 125 176
Type i2 Metasomatite Metasomatizedgranite 1.12.2Subtvoe 12.2 Metasomatizedmetasediments. . .
1 . r 2 .Subtype r 12.1
4.13 Type 13 Synmetamorphic ... 1.71 Type14 Lignite 4.74.I Subtype14.1 Joint-fracture-related .... 4.11.2 Subtype14.2 Stratiforn. . .
129 130
4.15 Type 15 BlackShale 4.15.1Subtype15.1 Bituminous-sapropelic blackshale 4.I5.2 Subtype15.2 HumiciKolmin alumshale
131 t32
4.16 Type 16 Strata-Controlled, Structure-Bound. . . . .
141 IJI
r32 IJJ
SelectedReferencesand Further Readine for
Chapter4
Examplesof EconomicallySignificantTypes Selected of UraniumDeposits 5.1
Uranium Examples of Unconformity-Contact-Type BasinRegion. Deposits(Type 1. Chap.4): Athabasca Canada
1a/
IJ+
r37
r-1 /
5.2
Examplesof Subunconformitv-Epimetamorphic-Type 168 Uranium Deposits(Tvpe2, Chap.4) UraniumDeposits 5.2.1 Subunconformity-Epimetamorphic Alligator River in Not-Aibitzed Metasediments: 168 UraniumField. Austraiia . . . UraniumDeposits 5.2.2 Subunconformity-Epimetamorphic in Albitized Metasediments: Uranium Citv Reeion. 101 ..:........... Canada Examplesof Vein-TypeUraniumDeposits(Type3. Chap.4) 5.3.1 IntragraniticVein UraniumDeposits:Limousinlla CrouzilleDistrict. France 5.3.2 PerigraniticMonometallicVein Uranium Deposits: PifbramDistrict. CSFR . 5.3
201 201
2r8
X
Contents
5.3.3 PerigraniticPolymetallicVein Uranium Deposits: District, CSf'R St. Joachimsthal/Jachymov Deposits in Contactmetamorphic Uranium 5.3.4 Perigranitic Rocks:IberianMeseta,Portugal-Spain. . . Not Granite-RelatedVein 5.3.5 Metasediment-Hosted, Mine, Front Uranium Deposits:Schwartzwalder Range.USA. Not Granite-RelatedVein Uranium 5.3.6 Sediment-Hosted, Deposits:Shinkolobrve.KatangaCopperProvince, Zaire . Examplesof Sandstone-Type Uranium Deposits (Type4.Chap.4) Depositsin 5.4.1 ExtrinsicCarbonAlumate-Uranium PhanerozoicSandstones: GrantsUranium Reeion.
?24 232
237
246
5.4
usA . 5.4.2 5.4.3
5.4.4
5.4.5
5.4.6 5.5
5.6
......... .
Vanadium-UraniumDepositsin Phanerozoic Sandstones: ColoradoPlateau.USA . ChanneliBasalUranium Depositsin Phanerozoic Sandstones: Monument Valley-WhiteCanyon Districts.USA . Roll-TypeDetrital Carbon-UraniumDepositsin Wyoming PhanerozoicContinentalBasinSandstones: Basins,USA. Roll-T1peExtrinsicSulfide-UraniumDepositsin PhanerozoicCoastPlain Sandstones: SouthTexas CoastalPlain,USA UraniumDepositsin ProterozoicSandstones: FrancevilleBasin. Gabon
250
250 ?70
284
290
305 319
Examplesof CollapseBrecciaPipe-TypeUranium Deposits(Type 5, Chap. 4): Arizona Strip Area, USA
3Z+
Examplesof Surficial-TypeUranium Deposits (Type6, Chap.4): SurficialUranium Depositsin DuricmstedSediments: YilgarnBlock,Australia.....
334
5.7
ExamplesofQuaru-PebbleConglomerate-Type UraniumDeposits(Tvpe7. Chap.a) 5.7.1 Uranium-RareEarth ElementsDepositsin QuartzPebbleConglomerate: Blind River-ElliotLake. Canada 5.7.2 Gold-UraniumDepositsin Quartz-Pebble Wit*'atersrandBasin.SouthAfrica . . Conglomerates:
353
Examplesof Intrusive-TypeUranium Deposits (Type9, Chap.5): AlaskiteUraniumDeposits; Rrissing,Damara OrogenicBelt, Namibia . . . .
366
5.8
Appendix (Table of U-Minerals)
343 343
373
Contents
XI
Bibliography.....
JIY
Locality Index
143
SubjectIndex
Remarks,Definitions,Units economic mineralizations ranging in size from small mineralogicalshowingsto almost economic occurrences. For this reason, the classification The emphasisof this volumeis on the charac- chapter was not restricted to types of economic terizationof uraniumdeposits. deposits but was amended to include types with Chapter 1 includes an introductory note in n o t t o m a r g i n a le c o n o m i cp o t e n t i a l . the form of a brief summarvof world uranium The views offered by the author in this and resourcesand their definitionswith respectto other chapters are his own interpretations. confidenceclassesand cost categories. This was though strongly influenced by discussions with consideredjustifiedinsofaras an understanding many coileagues.of data collected in the field of an ore depositcannotbe achievedfrom purelv and from pertinent literature study, and must geologicalparameters.Economicconsiderationsnot necessarily represent the final and correct haveto be included.Demandfor the commodity version. The reader is therefore strongly encourand, in the western worid. related price/cost aged to study the literature cited as references to factors dictate and define whether a localized form his own opinion, which may be contrary to metalconcentration is a depositthat canbe profit- the one presentedhere. ably exploited presentlyor in the future, or Chapter 5 contains abbreviated descriptions of whether it is a mineraloccurrenceof only scien- seiected major uranium districts or deposits contific value. sidered to be representative examples for types Uranium geochemistry hasbeendealtwith in a of uranium deposits of established or potential more general or subordinateway in Chapter2, future economic interest. This chapter was inintendedto provideonly the sufficientbasicinfor- cluded for various reasons. (1) to support and mation needed within the scopeof the work. complement the generalized descriptions in R.W. Boyle (1982) has publisheda compre- Chapter a, (2) to provide the reader with more hensive review on uranium (and thorium) geo- detailed data on important deposits and (3) to chemistry and the reader is referred to this present data and views of geoscientists workpublication for detailedinformationon the sub- ing on these deposits which are not necessarily ject. Concerning uranium metallogenesis,its congruent with those interpretations and hypoprinciples are at presentsufficientlywell under- theses presented in Chapter 4. stood only for some types of deposits,whereas Not all deposits are well researched. Some other types are understoodto a lesserextentand data are vague, if not biased or wrong. Othen in varving degrees.For this reasonand to avoid are presented ambiguously, being easily mistoo much speculationor geofantasy, the principal interpreted. Descriptions of the same deposit aspectsof the genesisof uranium depositshave or specific features thereof by different authors been addressedonly briefly in a separatechapter are not necessarilvunanimous. Interpretation of (Chaprer3) and summarizedin the presentation certain criteria may likewise be conflicting (see of tvpes of deposits(Chapter4). Nevertheless. also introduction to Chap. 3). The attempt was Chapter5 includesmore detailedviewsbasedon made to reconcile the conflicting data and deviatpublishedinformation of the depositsor distncts ing hypothesesas far as possible; but it has to be selected as representativeexamples for the admitted that this demanding task was not always important and economictypesof deposits. satisfactorily achieved. In any event, the vanous Chapter 4 forms the main part of the book views are presented and the reader is recomand describesthe principal recognitioncritena, mended to study the original literature and to dimensions,and metallogenetic aspectsof identi- come up with his own interpretation. For confied types of deposits. venience, a tabulation of uranium minerab is In order to comprehendeconomicdepositsand added in the Appendix. their parameters,it is equallyimportantto recogGraphic presentations and tables had to be nize and understand criteria typical for sub- limited to the extent considered necessary to
Organization of the Volume
Remarks. Definitions, Units
illustrate adequately the principles of geological setting and configurationof deposits.However, quantity and quality of illustrations are variable depending on the availabilit-v and reliabiiity of data in the sourcematerial.
lished since the final revision of the manuscript have been added in the bibliographv but, for technicalreasons,could be incorporatedinto the standingmanuscriptonly in exceptionalcases. The attempt was made to provide a bibliography as completeas possible,but some papers will still be missing.This deficiency does not reflect my disregard of the respective contriCiting of Authors bution, but should rather be excusedas an lmperfectionon my side.Proceedingsof workshops, All the main chaptersinclude a referencelist of symposia,etc. were in many instancesnot pubauthors whose data have been used directly or lished until several years later. Meanwhile, indirectly or who have contributed work to the some authors had publishedthe workshop data deposit or subject describedi-n that particular elsewhereor the data had been disseminated otherwiseand hencethe material may have had chapter.This schemewas selected inffuencedand may havefound accessto publicaa) to serveas referenceindex on literature per- tions of other workersprior to the printing of the taining to the respectivedeposit or subject. original presentation.Consequently,publication The list is restricted to respective principal yearsof referencedata do not necessarilyreflect uranium papers and to contributionsto the the first presentationof results. general geology with relevant or possible implicationson the uranium geology.Special pubiications not directly related to uranium geology,e.9., age datingsof rocks, are cited Geological, Mineralogrcal, Mining, and in the text (titles of the papers can be found listed according to the author's name in the Related Terms Bibiiography); b) to credit authors who have worked on the Connotation and spelling of geological and subject; mineral terms are in principle understood as and c) to reduce the immense repetition of authors' basedon thosegivenby: Thrush and the Staff of namesto a bearableminimum within the text. the Bureauof Mines(eds.),1968,in A Dictionary In this kind of synopticalreview, often using of Mining, Mineral, and RelatedTerms US Dept. numerouspaperson one singie depositor sub- of the Interior Washington,DC. ject, a completecitationof all authorswould in many instanceshave required a list of names Exceptionsor additionsto this are: after a coupleof sentencesor a short section. Alternatively, numbers referring to authors Uraninitelpitchblende: In this book "uraninite" and their papers could have been used. My is used for the macrocrystalline,more or less preferenceis, however, to see the name of euhedralvariety of UO2** which typicallv occurs an author and not a colorlessnumber which in rocks of higher P-T metamorphic grades requiresadditionalsearchin the bibliography (amphibolite grade and higher. conractmetafor the numbered individual. Although the morphic), igneous rocks such as granite and selectedsystemmay not satisf,vall authorswho pegmatite but also in vein and veinlike-type wish to seetheir namespreciselyrepeated,for deposits."Pitchblende"is usedfor UO2*r varithe sakeof easierreadingthey may forgiveme. eties of micro- or crypto-crystalline,colloform (collomorphous, botryoidal, spherulitic) habit which typically occur in low grade metamorphic and nonmetamorphicrocks such as greenschist Bibliography faciesmetasediments and sandstone, and in most vein and veinlike-type uranium deposits. It is This sectionis organizedin alphabeticalorder of understoodthat both varietiescrystallizein the authors providing complete coverageof papers samecrystallographic system,the cubic system, cited in the text and referencelists. Paperspub- but they have certain discriminating physico-
Abbreviations Table 1. Proposed terms for the alternativeidiomatic equivalentsof uranium oxides Habit
Terminology
Idiomorphic ( macrocrystalline) Collomorphous,botryoidal (micro-, cryptocrystalline) Sooty, earthy ( amorphous)
Uraninite Pitchblende Sootypitchblende
o( or
Euhedral(subhedral)uranrnire Colloform uraninite Soow uraninite
chemicalproperties(for detailsseeFritscheet al. although manv other metalsmay be presentbut in in press;Ramdohr1980,and Sect.2.3.1). t r a c e o r s u b e c o n o m i cq u a n t i t i e s . The term pitchblendehas been maintained Granitelgranitoid, pegmatitelpegmatoid, etc.: for traditionai reasons.It was the first name the terms are used synonymouslyand not in their used for black uranium oxide mineralsback strict genetic sense. Various authors appiy both in 1565(seeChap. 1.1)and is widelyused,par- words differently and it is not always clear under ticularly in Europe. Uraninite is a term com- which connotation. monly used for all kinds of uranium oxides in Regolith: refers to saprolite/paieosol. It is not American literature. Worldwide, both terms used in the senseoften applied in Canada, where are applied by a number of authors variablv also weathered rocks are called regolith. In this and in an overlappingway. The criteriaused by case. the term regolithic rock is here preferred. various geoscientiststo differentiate between Reserveslresources,grades:calculated in metric uraniniteand pitchblendeare sometimesconflict- tons (mt or tonnes) U3Os and percent (%) U3O8 ing and can lead to confusion. (respectively in ppm U for low grade mineralizaIn order to avoidfurther misunderstanding, it is tions) except for data published by NEA/IAEA that for the variousoxidephaseseither (presented in Chap. 1) which are in tonnes U met. suggested the classicalnames are applied or alternatively Costs, expendirures: in US $ unless otherwise the term uraninite is amendedby a descriptive stated. prefix to describehabit and/or physico-chemical Terminology, definitiow and classificatioru of properties of the uranium oxide in question resourcesand oroduction: seeSect. 1.4.-1.6. (seeTable 1). Secondaryuranium minerab: this term. commonly referring to coloredU minerals,wasabandoned in favor of "hexavalentU minerals"to Conversion Factors avoid coniusion. "Secondary minerals" are in several deposits.e.g., in surficialdeposits, = 1.1023 1mt sh.t. = 2200lbs of primary origin. Both terms, primary and = 1.18mtU3Og lmtU secondary.have been restrictedin this book to = 1 . 3 0 s h . t . U : O e =2 6 0 0 l b s U 3 O g lmtU their strict genetic sensedenotingprimary or lmtU3Os = 0.848mtU secondaryorigin of a given mineral. 1 m t U 3 O 3 = 1 . l s h . t . U : O s= 2 2 0 0 l b s U 3 O g Mineralization.alteration.etc.'.thesetenns are 1 sh.t.U3Os = 0.769mt U usedin both connotations. to denotethe process 1 sh.t.U3Os = 2000lbsU3Os implied and the productof the process. Slnb U3OB = $2.6/kgU Ore: synonymouswith minablemineraiization. = $0.3824llbUrOs S1/kgU Polvmeral lic minerali zationIp olymetallic miner alogy (corcespondingto complexmineralization/ mineralogy of some authors): mineralization containingat leasttwo differentmetalsincluding Abbreviations U in economicor potentiallyeconomicamounts. Monomemllic
min erali zatio n I mine r alo gy
(simple mineraiization/mineralogy): mineraliza- a . o . tion containine U only as recoverableelement, b . v .
amongothersor and others billion years: 1000m.y.
4
EAR lb. mt m.y. RAR
Remarks, Definitions, Units
estimatedadditionalresources pound (7000grains: 16 ounces : 451 grams) metric ton(s) million years reasonablyassuredresources
redox REE sh.t. U met WOCA
reduction-oxidation(boundary) rare earth elements short ton metallicuranium or natural uranium World OutsideCentrally Planned EconomiesAreas
1 Introduction
1.1 Brief Historyof Uranium compilationof the historical A comprehensive knowledgeabouturaniumincludingan extensive listing of uranium occurrences. known pnor to about 1900has been publishedby Kirchhermer (1963) in his book Dle Geschichtedes Urans (Historyof Uranium).
chemistTo(r)bern Olof Bergman(1735-1784), wascoinedin 1786by AbrahamGottlob Werner (I749-1817), i.e., prior to the discoveryof uranium. Another strikingly colored uranium mineral, "autunite", although known from variouslocationsin the early 18th century,was named in 1852 after a discoverylocalitv near Autun in the northeasternMassifCentral.where it was discoveredin 1800 by Joseph-Franqois de Champeauxde Saucy(I775-18.15.t. Eariiest coloredpicturesof uraniummineralsdatebackto 1797.showingyellow and green crystalsfrom Cornwall.
Discovery and first publications: Uranium was discoveredin 1789by Martin Heinnch Klaproth (1743-1817), pharmacist and professor for chemistry in Berlin. Klaproth detected the element when analyzingpitchblendefrom the Early uraniummining:During the first decadesof George Wagsfort mine at Johanngeorgenstadt.the 19thcentury,uraniumore was recoveredas Erzgebirge (Saxonian Ore Moun- by-productin Saxony,Bohemia,and Cornwall. S2ichsisches tains). Uranit was the first name proposed. In about 1850, mining for uranium as a main that was changedin 1790to uranium,the name productstartedin Joachimsthal now in the CSFR. derived from the planet Uranus discoveredin First recoveryof uranium ore in North America 1781by FriedrichWilhelmHerschel(1738-1822). was reportedin 1871from the Central City area, A.lthoughKlaproth discovereduraniumhe did Colorado. In Cornwall, the South Terras mine not manage to produce the new element in its openedfor uranium production in 1873.During metailicstate.Heatingof the "gelbenUrankalks" the past century, additional discontinuousor (yeilow uranium lime) with reducingsubstances occasionalminingof uraniumore hastaken place resultedonly in blackoxide.Other uraniumcom- in Autunois/lvlassif Central.in Oberpfalz(Upper poundsproducedalreadyin 1789inciudenitrate, Palatinate)/Bavaria. and at Biilingen/Sweden. suifate,phosphate,acetateand potassium-and In Saxony,approximately110t of uranium sodium-diuranate.Klaproth established the were recoveredduring 1825-1898.mainly from important propertiesof uranium for commer- the Erzgebirge. Total sales price was about ciai application,e.g., the coioring effect on a 525000Marks or the equivalentof "t.70 Marks/ glassmelt. kg U met. In Bohemiaberween1850and 1898, Uranium mineralshad beennoticedby miners Joachimsthalproduced in excessof 620t U of tor a long time prior to their chemicalidentifica- mainly high grade ore (Joachimsthalyielded a tion. and had given rise to a variety of specu- total of about100001Ubeforeit finallyclosedin lations on their composition.Pitchblendeof 1968). the SaxonianOre Mountains(Erzgebirge)was Cornwall.England,producedat least300tU mentionedin 1565as "Bechblende",a dialect in the 19th century,most of it comins trom the version of the German "Pechblende".Other SouthTerrasmine nearSt. Stephen(ca.275t). accountsof pitchblendedate back to the years In Colorado,USA, minesof the CentralCity 1727 (Joachimsthal, Erzgebirge) and 1763 district yielded approximately 50t high grade (Wittichen,Schwarzwald/Black Forest)where it uraniumore until 1895.The Wood mine wasthe was referred to as "schwarzebleyschwereErzt- most productive one. Mining of carnotite ore Arth" (black. lead-heavyore type). Green and startedin 1898in the ParadoxValley, Montrose yellow micasand ochers,the relation of which to Counry (now Uravan Mineral Belt), yielding uraniumwasfirst suspected in I778, couldlater be annua.lly about10tU metal. establishedas uranium minerals by Klaproth. Striking discoveriesof uranium deposits in The name "torbernite". namedafter the Swedish this centurvprior to World War II were in 1913
1 Introduction o I
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Brief History of Uranium Unconformity-contacttype (I ) ; (l) AthabascaBasin. Canada 1 Kiggavik, Canada 2 Kintyre (?), Australia 3 type (2); (t) Subunconformiy,-epimetamorphic 1 UraniumCitylBeaverlodge,Canada; Alligator Rivers, Australia: 3 South Alligator River, Australia; 3 -l Rum Jungle, Australia; Kintyre (?), Australia. 5
Tonco-Amblayo,Argentina: SierraPintada,Malargue,Argenrina: Pichinan.Argentina; ParandBasin/Figuera,Brazil; Loddve. Pierre du Cantal, Lombre, Coutras, France: 1.1 Mazarete.Spain; i5 North Bohemian Basin - Elbsandsteingebirge, CSFR-Germany; 16 Zirovski Vrh, Yugoslavia; 17 llecsek Mts., Hungary: Apuseni Mts., Banat. Romania: Orranovo-SimitliBasin. West Rhodope Block/Eleshnitza, Tracian Basinillomino. NW Balkan Mts.. Bulgaria; 18 Caucasusregion/Stavropol. Russia: l9a KyzylkumskyA.iavoi region. Uzbekistan: l9b Kokchetavsky/Semizbay. Kazakhstan; l0 Tim Mersoi Basin, Niger: 2l FrancevilleBasin. Gabont 22 Beaufon-West,South Afnka: 23 Dera Ghazi Khan area, Pakistan: 21 Ngalia Basin,Australia; 25 Amadeus Basin, Australia: Westmoreland.Australia; 26 27 Lake Frome Embayment, Austra[a: l8 Manyingee.Australia: 29 Tono, Ninyo-Toge, Japan; Z-'l Zhungel-Tianshan uranium provinceiMengqiguer, Daladi. Kashi, China: Y-L Yinshan-Liaohe uranium province/Jianchangarea region, Quinlong-Daxing'aniing-North Hebei Langshanarea, China; provinceAVudanguranium Q-Q Qilian-Qinling Huaiyong Massif, China; W-Y West Yunnan uranium province.'South Lancang and Gaoligangareas,China: J-B Jingan Basin/Tunling, China. 9 10 II 12 l3
Vein type (3); ( t1 (includesvein tvpe depositsof metasomatite(m) (12) and partly of intrusivevolcanic(v) (11) tvpes) RossAdams, Bokan Mountain (m), USA; Rexspar(v), Canada; Midnite Mine, USA; USA; Colorado Front RangelSchwartzwalder, Coles Hill/Swanson.USA; 5 Port Radium, Canada; 6 Macusani (v), Peru; 7 Itataia. Brazii: 8a Espinharas(m), Brazil; 8b Lagoa Real (m), Brazil; 8c Poqosde Caldas (v), Brazil; 9 Igaliko Fjord, Greenland; 10 1 1 Arj epiog-Arvidsjaur, Sweden; 1 2 Armorican Massif, France; IJ Massif Central, France; t4 Iberian Meseta, Spain-Portugal; l) Black Forest. Germanyl 1() Bohemian Massif-Engebirge.CSFR-Gennanyi t7 Krivorozhsky-Kirovogradky (m), Ukraine; 1 8 Kokchetavsky (v ?), Kazakhstan; 1 9 Apuseni Mts., East Carpathians, Romania; East and West Balkan Mts., East and West Rhodope Mts., Bulgaria (includesv); 20 Kitongo (m), Cameroun; Collapse Breccia Pipe rype (5); ( ) l1 Hoggar region (m?), Algeria; I Anzona Strip area, USA. 22 Katanga Copper ProvinceiShinkolobwe,Zaire; Tete district. .Vozambique: Surficialtype (6); (-) 1A Jaduguda, India; la Stevens County, USA - Okanaean Valley region. 25 Maureen (v), Australia: Canada; ao Ben Lomond (v), Australial 1b Pryor Mts. (U in karst cavern), USA: 2'l Radium Hill. Australia; 2 Achaia BatholithiSchlagintweit..\rgentinal T Zhungel-Tianshan uranium province,iBalyanghe 3 Namib Desert, Namibia - nofthern Cape Province, (v), China; South Africa; Y-L Yinshan-Liaohe uranium province,/Lianshanguan .1 Dusa Mareb-El Bur, Somalia; (m), Quinlong-Daxrng'anlingvolcanic belt (v,m), 5 Yilgarn Block. Australia; Langshan (v), China; 6 Arunta/Musgrave Block, Australia. O-O Qilian-Qinling uranium province/West and East 7 Ferghana Basin/Tyuya Muyun (U in karst cavern), Longshoushanbelts (m), North Qilian belt (mt. Uzbekistan: Xi'an area. China: grantte Quartz-PebbleConglomerater."*pe(7)'. (O) S-CH SoutheastChina uranium province,t',lanling Btind River - Elliot Lake. Canada: granite belt. 1 U Jiangxi U belt. Central-southern Witwatersrand, South Africa; 2 Lower Yangzi granite U belt, Gan-HangvolcanicU Serra de Jacobina,Braal: belt (v), South Jiangxi- Nonh Guangdongvoicanic 3 .1 QuadrilateroFerrifero, Brazil. U belt (v), China.
z-
Sandstonerype (4); (t) I Blizzard. Canada; 2 Sherwood. USA; 3 Wyoming Basins. USAI -l Black Hills - Crow Butte. USA; 5 Colorado Plateau/Chinle and Salt Wash distncts, USA; 6 Colorado Plateau/GrantsUranium Region, USA; 7 South Texas. USA; 8 Burgos Basin. Mexicol
Breccia Complex type (8); (-) I Olympic Dam. Australia. Intrusive type (9); (l) Madawaska/Bancroft,Canada: I 2 Kvanefjeld/Illimaussaq,Greenland: 3 Rdssing,Namibia; 4 Phalaborwa,South Africa; Y-L Saima Massif. China; Q-Q West and East Longshoushan belts, DanfengZhuyangguan-Shangnan area. China.
1 lntroduction Phosphorite type (10h (L-) 1 Florida/Bone Valley - knd Pebble district, USA; 2 Bakouma, Central African Republic (perhaps surficial tYPe); 3 Youssoufia - Kbouribga, Morocco; 4 Zr.fa, Israel. Volcanic rype (11); ( v ) (seeako vein-rype) McDermin Caldera. USA: I Sierra de Pena Blanca, Mexico; 2 Cotaje, Bolivia; 3 4 Duobblon, Swedenl Karamazarsky, Uzbekist"n: -5:'l Pribalkhashky, Kazakhsun; 6 7 Streltsovsky,Transbaikal.Russia; Z-T Zhungel-Tianshan uranium province/Balyanghe, CNna: province/QuinlongY-L Yinshan-Liaohe urnnium Daxing'anling volcanic belt. Langshan, China: S-CH SoutheaslChina uranium province/Gan-Hangvolcanic belt. Yingtan and Xiangshan districts; South Jiangxi - Nortb Guangdong volcanic belt, China. Mewomatite rype (12); (see under vein-rype) Synmeumorphic rype (13); (/t) 1 Kitts - Makkovik area, Canada; 2 Ambindrakemba, Madagascar: 3 Mary Kath.leen.Australia. Lignile type Qa); ( t ) Willjston Basin, USA; 1 Inrermontane basins in Hercynian orogenic beit/ 2 Freital. Stockheim, Germany; Sokolov, Trutnov, Kladno.6FR: Black Shale type (15); (2) I Chatanooga, USA; Ranstad, Sweden; 2 controlled - stntcrure-bound rype (16); (+) Ronneburg, Germany; Kyzylkumsky/Dzhanruar,Koscheka.Uzbekistan; Qilian-Qinling uranium province, S. Qiniing areal Norgai, China; S-CH Southeast China uranium province,D(uefong-Jiuling regionl north and soutb side Jiangnan Block/ Guanpxi-Hunan-N. Chanziping, Pukuitang: Guangdongregion/Chengxian.China: S-G Southwest Guizhou uraruum region/Chienxinan. China. Uncommon and Uncertain rypes ( . ) Zakaspiyisky. Meiovove Peninsula/Temakskove, Tasmurun, Tavbagar, Kazakhstan (U in Cretaceous-Paleogene-Neogenesediments associated with phosphatic bone detritus of fossil fish in pyritic clays); 2 Onezhsky/Kem,Chupa area, Russia(pegmatite?) 3 Zauralsky-Chelyebinsk/Vishnevogorsk, Novogorny, Russia(U associatedwith nephelinesvenite); A Yeniseisky/Abokan, Vikhorevka, South-central Siberia. Russia (U fracnrre-bound in Archean metamorphics); Vitirnsky, SoutheastSiberia, Russia; 5 6 Central-Transbaikalsky/Cbita region, Russia; 7 Far East, Eastern Siberia, Russia; Keberovsk, SoutheasternSiberia, Russia. 8 9 Cuddapah Basin, lndia (U, V, Mo, Cu mineralization in Middle Proterozoic phosphorousdolomite). Srrua I 2 O-O
Shinkolobwe/Katanga(now Shaba Province/ Zaire) and in 1931 Port Radium. Great Bear Lake/Canada.Others include Beira Province/ Portugal, Tyuya Muyun, Ferghana/Uzbekistan, and RadiumHill/Australia. Intensiveexploration for uranium started at the end of World War II, initiatedby the manufactureof atomic bombs.With the development of nuclearreactorsfor the peacefuluse of nuciear energy,new incentivesfor searchingfor uranium arose.During three major explorationbooms, in the secondhalf of the 1950's.from about 1967 to 1977,and 1976to 1982,most of the major depositspresentlyknown were discovered.
1.2 Typesof Uranium Depositsand and Their Economic Occurrences Importance Fifteen principal types of uranium depositsand are recoglrizedin WOCA and a sixoccurrences teenthtype in former East Block countriesbased in the first instanceon host environment andior geometry.The typesare listedin Table 1..1.More than 40 subtypesand classescan be attributed to Table l.l. Types of uranium deposits and occurrences and their future and past economic 131king with respect to share of WOCA production (b = byproduct, c = coproduct; i = high (>20"/" of WOCA production), 2 : medium (>10%),3 = low (<10%), ? = subeconomic)(for explanation see text) Type of deposit
Economic ranking Future
ln producion I Unconformit_v-contact 2 Subunconformity-epimetamorphic 3 Vein 4 Sandstone 5 Collapsebreccia pipe 6 Surficial
Past
IJ
12 23 21 ?r?)
Co- or by-product 7 Quartz-pebbleconglomerate(Au) 8 Brecciacomplex(Cu + Au) 9 lntrusive (Cu) l0 Phosphorite(P2O5)
1(b) 3 (c) 3 (b) 3 (b)
Possiblefumre or past production 11 Volcanic 12 Metasomatite 13 Synmetamorphic 14 Lignite 15 Blackshale
? ? ? ? ?
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Typesof Uranium Depositsand Occurrences and Their EconomicImponance Table 1,2. WOCA. common gradesand resourcesof major rypesof deposits Type of deposir
Typical grade 9/oU3O"
i Unconformity-contact I Subunconformitvepimetamorphic 3 Vein -l Sandstone 5 Collapsebrecciapipe 6 Surficial 7 Quartz-pebble conqlomerate" 3 Brecciiacomplexb 9 Intrusive
Resources
Product
1000mtU.O'. Deposit
District
Main
Co-iBv-
<1- 170 <1-150
>.100 >300
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N i .A u Au
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<0.1-15 <5-25 <1.5 <0.1-30 <10- 100
<60 1 0 -1 5 0 <15 <1-60 >250
U
A g . C o ,N i . B i V . C u .M o .S e A u . . A . gC, u
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U U U Au Cu..A,u U Cu
Y. REE U U U.Zr
" Minable as bvproduct or due to favorable contracts. o Only one deposit is known. Olympic Dam, Australia. containingtotal proven and possibleresourcesof 360000mt U_.O, and unspecifiedadditionalresources of 8-10000mt U3O3ar 0.06% U3OB,(Batteyet al. 1987);the listed50000mtUrO, correspondto the amount recoverableas co-product to Cu-Au production over about the next l0 vears.
the 15 types. They are discussedin Chapter -1 subsequent to a reviewon principlesof uranium geochemistrvand metallogenesis.Figure 1.1 displays the worldwide distribution of major uraniumdeposits.Table 1.2 givesa summaryof the size and resourcemagnitudeof the various typesof uraniumdeposits,and Fig. 1.2showsthe order of resourcesby type of depositsknown in WOCA.
can The 15 main typesof uraniumoccurrence be groupedin order of presentand near future economicranking as set out in Table 1.1. The primary economicparametersare grade(average >0.2"/oUrOs), contained uranium resources (>10000mtU:Or, or lower tonnagebut higher grades,and the amenabilityto either conven(e.g.. in situ leaching, tional or unconventionai heap leaching)mining and milling techniques.
RAR
Type of deposil
290 -
1 Unconformity-contoct
P r o d u ct i o n -80--
--100'103mt
U:os
90
2 Subunconformity-epimelomorpnrc
F
3 Vein
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7 Q u o r t z - p e b b l ec o n g l o m e r c t e 8 Breccro complex") 9lntrusive
Fig. f .2. Approximate uranium resources(categorvRAR <$30nb U:Or) and past production by npe of deposit"(status Januarv.1990. former Eastern Block countries excluded). "U recoveredfrom phosphonc acid amount to <5000mt UjOs). bhalf of the amount recoverableas byproducr.'recoverableas coproduct,pioven resourcesj60000mtUrOg, only one deposit known (Olvmpic Dam, Austraiia)
10
I Introducrion
Locally, however,other factorsmay influencethe economicsbut theseare not consideredhere. In terms of prices, deposits requiring total recoverycostsof lessthan US$20 to 25llbU3Og are of economicinterest.Basic prerequisitesare free trade and normal marked conditions. For comparison,the economicranking of depositsin the past (until the early 1980's)is added. As can be seenfrom Tabie 1.1. for the opening of a new mi-ne,at present and in the near future only four main typesof depositsare economically attractive for uranium only (ranking 1 and 2) and consequentlyshould constituteprime targets for exploration. Four more types contain uranium recoverableas a by- or coproduct. Their resources, although huge in some deposits (>0.5 mio. mt U3O6), are economically available only in limited quantity, i.e., to the amount achievedby the production of the main commodity. i-n most cases gold, copper, or phosphorous. rao'
t&'
1.3 GeographicDistributionof Uranium Deposits Although uranium is geologicallya widespread commoditvand numerousuranium showingsand occurrences are known throughoutmany parts of the world. major uranium depositshave been discoveredin relatively few countries, as illustratedin Fig. 1.1. Figures1.3 and 1.4 document the striking accumuiationof establishedconventionaluraniumresources of the RAR <$80/kgU categoryin sevencountriesof the WOCA each containingin excessof about 100000mtU (ca. 116000mtU:Oe).Four of these countries are developed industrialized nations. Australia, Canada,SouthAfrica, and the USA. Besidesthe wealthof uraniumresources,thesefour countries account also for more than 80% of WOCA's uranium production of the past 30 to 40 years. Namibia and Niger, and to some extent Brazil, tzo
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Explsnation Canada ' t1 . 7 L i Productlon in 19e6/199O / in 1000 mt U (1 3 s.o) RAB in 1OOOmt U a Countries with producflon < 500 mt u /year
Fig. 1.3. Countries witb significanturanium production and resourcesin the <$80,&9U RAR caregon.. WOCA figures 1!o_1-theuranium production in 1986 and 1990and their declineduring this period resulting in a drop from a toial of 37,ZFmtUlyear in 1986 to about 28,000mtU/year in 1990. Shown produition data of the formei Eastern Block countriesare for 1989. In.the same vear produced (estimatedresourcesin the <$80/kgURAR caregory in brackets) Bulgariaabout 450mtU (1-s,000mtU),Hungary about 600mtU (35.000mtU) and RJmania about i30 mtU (1g.000 mt U). _Sincethen_productionceasedin Eastern Germanv and decreasedto lessrhan 800mt U/year in CSFR and ro abour 400mt U/year in Hungary. WOCA resource and production dara for 1986from OECD (NEA) IIAEA 1990.Other data calculatedfrom various sources.
Distribution of UraniumDeposits Geographic achievedtheir high levelsof uranium resourcesby the intense exploration activity of mining companies from industrializednations. To find the highest rates of uranium resources in developed countries is not fortuitous. Complementary to favorable geology and developed
11
infrastructure, stable and reliable political and economic environmentshave encouragedintense exploration activity. The high domestic demand for uranium in the USA has added to the exploration efforts in this country.
mlU a 600 -l
0
AU
U5
SA
NI
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b eoo N\S .g8o/ksU
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Fig. 1..1. Distribution of uranium resourcesamong countries.a ReasonablvAssuredResources.b EstimatedAdditional Resources- Category I. AU Australia; A/- Algena: BR Brazil: C7 Canada;DE Denmark/Greenland: FR France; GA Gabon: 1N India: rVA Namibia: ,V/ Niger; SA South Africa; SP Spain;.!l/ Sweden:US United States.[OECD (NEA)i
rAEA 19901
I
12
I Introduction
1.4 Resources,ReservesrGrades' and Production of Uranium
are based on specific sample data and measurements of the deposits and on knowledge of deposit characteristics. Reasonably Assured Resources have a high assuranceof existence. The Nuclear Energ-v Agency (NEA) of OECD, Estimated Additional Resources - Categorv I together with the International Atomic Energv (EAR-I) refers to uranium in addition to RAR Agency (IAEA), have in the past published that is expected to occur, mostly on the basis at approximately bi-annual intervals a volume of direct geological evidence. in extensions of entitled Uranium Resources. Production and well-explored deposits. and in deposits in u,hich Demand of WOCA countries. the so-called Red geological continuity has been established but Book. In order to facilitate the categorization of where specific data and measurements of the uranium resources presented in the Red Book. deposits and knowledge of the deposits' characthe NEA/IAEA have established a classification teristicsare consideredto be inadequateto classifv scheme. the resource as RAR. This scheme differs from resource classification Estimated Additional Resources - Category' Il systemsof other countries. Table 1.3 provides. (EAR-II) refers to uranium in addition to EAR-I for comparison, approximate correlations of that is expected to occur in deposits beiieved to terms and categories used i-n uranium resource exist in well-defined geological trends or areas of classification schemes by IAEA and the major mineralization with known deposits. Estimates of uranium-producingcountries. tonnage and grade are based primarill,' on knowledge of deposit characteristicsin known deposits within the respective trends or areas and on such Terminology and Definitions of NEA/IAEA sampling, geological, geophysical. or geochemical Uranium Resource Categories (from NEA/IAEA evidenceas may be available. 1988) Speculative Resources(SR) refers to uranium, in addition to Estimated Additional Resources a) Definitions of Resource Categories Category II, that is thought to exist mostly on the Reasonably Assured Resources (RAR) refers to basis of indirect evidence and geological extrauranium that occurs in known mineral deposits of polations, in deposits discoverable with existing such size, gpade, and configuration that it could exploration techniques. The location of deposits be recovered within the given production cost envisaged in this category could generallv be ranges,with currently proven mining and process- specified only as being somewhere within a given ing technologv. Estimates of ronnage and grade region or geological trend.
Table 1.3. WOCA. approximate correlation of resource classificationschemesused in major uranium producing counlries.The terms illustrated are not strictll'comparable as the criteria used in the various systemsare not identical. "Grav zones" in correlation are therefore unjvoidable. particularll'as the resourcesbecome lessassured.Nonetheless. the chart presentsa reasonableapproximation of the comparability of terms. (NEAAAEA 1988) NEANAEA
Reasonablya-s-sured
EstimatedadditionalI
Estimatedadditionalll Speculative
Austraiia
Reasonablva-ssured
Estimatedadditional
Undiscovered Prognosticated
Speculative
Estimatedadditional II
Soeculative
Canada(Energr'. Measured Mines and Resources)
ln.ijcated
Inferred
France
ReservesI
R:serves II
PersoectivesI
South Africa
Reasonabh'a-s:sured
Estimatedadditional I
United Srates (DOE)
Reasonablvalsured
Estimatedadditional I + II
Persoectives II
Decreasingconfidencein estimates
Speculative
,,
L
',. trr'
13Y1,1
Resources,Res6rves,.Cradesahd Productioo oPUr n t u
b) Cost Categories The resourcecategoriesare further separatedinto I e v e i so f e x p l o i t a b i l i t yb a s e do n t h e e s t i m a t e dc o s t of production. The cost brackets are:
r e s o u r c e s( e . g . , p h o s p h a t e sc, o a l . l i g n i t e s .b l a c k s h a l e s .e t c . ) .
llote: 1. Estimatesof resourceswithin the various catL e s s t h a n $ 8 0 / k gU ( < $ 3 0 / l bU r O s ) egories fluctuate with changes of economic S 8 0 - 1 3 0 i k gU ( $ 3 0 - 5 0 / l bU 3 O 8 ) conditions.with progressin exploration stages. S 1 3 0 - 2 6 0 / k gU ( $ 5 0 - 1 0 0 / l bU 3 0 8 ) and with improved extraction technology. When estimating the cost of production for Local production costs may !'arv in response within categories. to inflation or variations in exchange rates of resources these cost assignine been following currencies. The recoverv costs of uranium in has taken of the costs: account deposits mav consequently cross the certain - t h e d i r e c t c o s t s o f m i n i n g . t r a n s p o r t i n e ,a n d line between two cost categories. requiring a p r o c e s s i n gt h e u r a n i u m o r e : revision of the original estimatesinto a higher - the cost of associatedenvironmental and waste or lower cost class.With proeressingexploramanagement: tion there is a shift of resourcesfrom EAR-I to - the costs of maintaining nonoperatingproducRAR and as exploitation proceeds there is tion units where applicable: a corresponding depletion in RAR. Also. - in the case of ongoing projects, those capital improvements in technolog;- may result in costs which remain unamortized: better recoveryratesand hencein upgradingof - the capital cost of providing new production the resource estimate. It is not unusual that a units where applicable including the cost of particular depositcontainsresourcesof several financing; of the above listed cateeories. - indirect costs such as office overheads, taxes, 2. National resource/reservefigures published by and rovalties where applicabie; are provided in most cases by NEA/IAEA - furure exploration and development costs government agencies. Some of these national wherever required for further ore delineation authorities occasionallyunderestimate the real to the stage where it is ready to be mined. mining costs. Therefore some of the national resource figures appear in, and consequently Sunk costs were not normallv taken into inflate to a certain extent. lower cost resource consideration. categoriesof the NEA/IAEA tables. 3. Based on presentprice levelsof uranium. only c) Recoverable Resources deposits which can be mined for costs of less Resource estimates in the RAR and EAR-I catthan about $55 to 60/kg U (approximately $20 egories are expressed in terms of recoverable to 25ilb UsOs) are of economic interest for the tonnes of uranium. i.e., quantities of uranium recoverable from minable ore, as opposed to future. quantities containedin minable ore. or quantities -1. A further restriction to cost categoriesis imposed by recoverv costs of recvcled uranium in situ. (and plutonium) from reactors. which put an upper bracket of approxrmateiy S80 to d) Conventional, Unconventional. and By100/kgU (ca. $30-.10/lbU:Os) on production Product Resources c o s t so f n a t u r a lu r a n i u m . Conventional resourcesare those that have an establishedhistory of production where uranium 5. In this book, only national resource data are taken from the NEA/IAEA Red Book. is either a primary product. co-product or an gold proResource figures of individual deposits or imponant bv-product. as in the caseof discussed in later chapters do not distncts duction in South Africa's Witwatersrand or necessarily correspond to the NEAiIAEA copper (-gold) production at Olympic Dam, in nomenclature and cateqories u outlined in the Australia. The first nine deposit types listed next paragraph. resources. Tabie 1.1 are consideredconventional Verv low grade resources.which under todav's market condition (1992) are not economic or from which uranium is only recoverable as a minor bv-product, are consideredunconventional
14
1 Introduction
Kazakhstan:>90000 mt U (<$80/kgU) Romania:>10000mtU Russia:>120000mt U (<$80/kgU) The terminologysuggestedby NEAiIAEA has Ukraine: >70 000mt U (<$80/kgU) often not been applied by mining companiesor Uzbekistan:>175 000mt U (<$80/kgU) tbeir publishedfiguresare not attributedto one or the other category.Hence in many casesit Theseresourcescan be classifiedonly as minable. (e.g.. remainsunclear whether in situ (geological)or No attributionto anv westerncostcategorrv recoverable (mining) reserves/resourcesare that appiied by NEA/IAEA) is feasiblebecause given, i.e., whether mining dilution and milling other economic or economic-politicalfactors lossesare includedor not. whetherthe numbers governuraniummining in thesecountries. refer to RAR to EAR categoriesor any other (e.g., equivalentcategoryof reserves/resources proven, probable, or possiblereserves).Mining Unconventional Uranium Resources dilution can reduce recoverablereservesto less than 75"/" of the in situ tonnageand can down- NEA/IAEA (1988)reportsthat in somecountries gradethe ore mined to 85% or lessof the in situ unconventional and by-product sources of grade. uranium are currently being exploited. With the For this reasonreserve, resource,and grade possibleexceptionof marinephosphates,none of figuresgiven in Chapter5 for individualdeposits thesesourcesis expectedto supplyiarge tonnages or districtsare best estimatesof, but not alwavs, in the foreseeablefuture. The more significant in situ tonnagesand gradesbasedon various,not resourcesof this t)?e are as follows: necessaril)/published, information (particularly Uranium in marine phosphorite is the most for figures of deposits in Eastern European important source from this category. Morocco, countries).Also, the termsreservesandresources which has the largest depositsof phosphorite, are not used in this volume in their strict sense reported over 6 million tonnes of uranium in but more synonymously.This means that no deposits with an average grade of 120ppm. great distinction is made between the terms Egypt, Jordan, and Syria also report significant "resources" (except for being used rn rather resourcesof uranium in phosphorite.Presently undefined or not clearly defined cases) and uranium is being recoveredas a by-product of "resewes" (more restricted to better defined phosphoricacid productionin Florida, USA, and resources but independent of their status of from phosphoricacid produced from imported confidence. phosphatesin Belgium and Canada.The gradeof phosphorite treated ranges between 90 and 150ppmU. Uranium in marine black shalesis reported by 1.5 \Torld Resources of Uranium the Repubiic of Korea, Finland and Sweden. Swedenlists it as conventionalRAR and EAR-I recoverable at costsbetween$80-130kgU. ConventionalUraniumResources with the Unconventional resourcesassociated carbonatiteof Sokii are reported by Finland to WOCA: As graphicallydisplayedin Fig. 1.4, total 2500tU. In previousreports,SouthAfrica convendonalresourcesof uranium in WOCA estimated RAR below $80/kgU at 5500 and counrriesamountin the <S80/kgU ciassto a total with the Palabora EAR-I at 1600tU associated of about 1.5mio. mt U in the RAR categoryand complex, from which uranium is carbonatite to about 0.9 mio. mt U in the EAR-I category. recoveredas by-product from the extraction of Uranium resourcesin former Eastern Block copper and other metals. Brazil reported uncounEiesare estimatedas follows: conventionaluranium resourcesin the Araxa Bulgaria: 15000mt U carbonatiteof about 13000t. China:>160000mtU Uranium occursalso in copperdeposits,and is CsrR:>5oooomtU beingrecoveredas a by-productof copperleachEast Germany:ca. 90000mtU ing in India,whichhassignificant resources of this Hungan': >20000mrU type, totalling 23000 tonnesU, and the USA, Terminologrof Resourcesof Individual Deposits (asusedin Chap. 5)
UraniumProduction
15
2000tonnesU in Belgian Congo, now Zaire. Eleven percent was of approximately with resources of this produced in the CSFR and the remaining 8% copperores. Also Chilereportsresources 5000t U. in the USA, Portugal, and other countries. type, totallingapproximately Four production centers yielded the bulk of the ore. Port Radium on Great Bear Lake, NWT, Canada, Shinkolobwein Katanga, Congo. St. Erzgebiree, CSFR, and Joachimsthal/Jachymov, 1.6 Uranium Production the Colorado Plateau. Colorado-Utah. USA. Additional producers include the uraniumNoteworthy uranium production started at the mica mines of Portugai, which provided 35g beginnin-eof the 20th century,up to World War II r a d i u m b e t w e e n1 9 1 1a n d 1 9 3 9 .a n d t h e u r a n i u m chiefly in responseto the demand for radium. The vanadium depositsof Tvuva-Muvun in Ferghana. fiqures for radium production thus reflect the Uzbekistan. which delivered 2.1g radium from rragnitude of uranium exploitation during this approximately 1000mt uranium ore. After World : e r i o d . b a s e d o n a r e c o v e r yr a t i o o f l g R a p e r W a r I i a n d p r i o r t o 1 9 9 2 ,a l m o s t 1 0 0 0 0 0 0 m t U were produced in WOCA countries, and an 3mtU. For 1937. mining statisticsquote a world pro- estimated 350000 to 400000mtU in the former duction of 10gRa. equivalent to slightlv more Eastern Block countries. with the largest conthan 120mt U. Canada provided approximately triburions from the CIS. CSfn. and East 66% of the amount. Some 15% came from the Germanv.
2 Geochemistryand Minerochemistryof Uranium
In 1982 Boyie gave a comprehensivereview of rhe geochemistryof uranium. Garrels and Christ ( 1 9 6 5 ) , L a n g m u i r ( 1 9 7 8 )a n d R o m b e r g e r ( 1 9 8 4 ) among others provide data and diagrams of uranium solution - mineral equilibria. The other authors referenced (see end of chapter) discuss uranium behavior during processesactive in the various geoloeical environments. The reader is referred to this literature for detailed information ,nd additionai literature. More detailed informaiion on uranium activity related to ore-forming events in individual deposits or districts is provided in Chapter 5.
2.1 Chemical Properties of Uranium
p o r t a n t . T r i v a l e n t ( U 3 * ) a n d p e n t a v a l e n t( U 5 - ) u r a n i u m c o n s t i t u t e i n t e r i m s t a q e s .b e i n g p r a c tically stable only under laboratory conditions. H e x a v a l e n tu r a n i u m ( U 6 - ) h a s a n i o n i c r a d i u s o f 0 . 8 0 A . w h e r e a st h a t o f t e t r a v a l e n tu r a n i u m ( U o * ) i s e i t h e r 0 . 9 7 A o r 1 . 0 1 A . d e p e n d i n go n the coordination number 6 or 8 respectively. Uranium is a distinctlv tithophile element with a high affinity to oxygen. In nature. uranium occurs neither as native metal nor as sulfide. arsenide or telluride. In most subsurface environments uranium occursas U"*. while the hexavalentstate (U6-; is stable merelv under oxidizing conditions prevailing at and near the surface.
Abundanceof 2.2 GlobalGeochemical Uranium
Physico-chemical data of uranium are atomic number 92, atomic weight 238, density 18.9g/cm3, melting point 1405"C,oxidation stages6, 5, 4, and The average crustal abundance of uranium is 3 (Table 2.1). 2.7 ppm (Taylor 1961) but different lithologies In Mendeleev's Periodic Table, uranium is have a wide range of U tenor, as rvill be discussed placed in group VI below chromium, molybdenum, and tungsten. Uranium belongs to the actinides characterized by close chemical relationships and peculiar contraction behavior of ionic radius simriar to the lanthanides. Of the four oxidation states,stages-l (Ua*) and 6 (Uu-) are geochemicallyand mineralogicailyrmTablel.l. Phvsico-chemi.u,-Ou,u of uranium .\tomlc ;rumber
Atomic weiqht
Valencies (oxidarion staees)
U
(u'*) u'r*
(u'*) u"*
Natural lsoropes
Percent fraction
Haif-life time
tJ?34 U 235 u 23,q
0.0058% r.6 x 105 t) i29'o 7.3x iOd ()qrr"i, .{.5x 1oe
lonrt radii
Coordination numDer
1 . 1 3A 1.12A 1 . 1A 6 0 . 9 7A 1.014
+
0.80A
6
6 b
8
0
j0
20
-<J l0 "0 Crdea ot oounoonce
c0
70
3c
Fig.2.l. Relativeabundanceofuranium in the continental crust comparedto other selectedelements.(After Dodd In: Robertson et al. 1978)
18
2 Geochemisn-v and Minerochemisrry of Uranium
later- Although uranium is widely distributedin crustal rocks and minor accumulationsare common. uranium is not an abundant element, as illustrated in Fig. 2.1, that displaysthe relative abundanceof selectedelementsin the continental crust. Uranium is slightly more common than metalssuch as As, Mo, W, and Sn, but is much less abundant than Pb, Zn, Cu, or Ni, and it is about one-fourth as abundant as thorium. Table 2.2 shows a supplementarvcomparison of the order of enrichment factors of some metalsrequired for formation of economicgrade concentrations.
c) uranium phaseshydrolyzein the presenceof solutions; d) uranium forms complexes or ion paws with a great variety of anions in aqueous environments; e) uranium has to be reducedor complexedto precipitateasmineralsfrom aqueoussolutions. but alsohasa tendencyto be adsorbedon cla1,, organic. and other particles or on certain hydroxides(Fe, Zr, Ti etc. hydroxides); f) uraniumforms a greatvariety of mineralsof its own, the bulk of them basedon hexavalent uranium,and onlv a few on tetravalenturanium (dominantlyuraninite,pitchblende).
Table 2.2, Comparison of cnrstal abundance, order of payabiliry grades, and respective enrichment facton of
ielectedmerals Metal
Crustal abundance(ppm)
Payabilitl grade(%)
Enrichment factor
2.3.1 Minerochemistry and Crystallography of Uranium
In natural mineralsuraniumis presentonly in its hexavalent and tetravalent state of oxidation. 10 Tetravalenturanium is by far the most important 2m l't50 80 r25 ion although only stableunder reducing condit70 143 ions. With increasingoxidation potential U4* 16 3r25 transforms into the hexavalentstate. 1q U 2.7 uranium has in natural minerals Tetravalent Sn 2.0 1000 Au 0.004 1250 predominantlythe coordinationnumber8, i.e., it Pt 0.002 2s00 forms a centralcationsurroundedin equidistance by eight anions, normally oxygen ions. In this case,the ionic radiusof Ua* is 1.01A. In addition, Uo* occurswith the coordinationnumber 6, then havingan ionic radiusof 0.97A. 2.3 Minerochemical Distribution and In magmatic and metamorphicenvironments Abundance of Uranium in Minerals including hypogene hydrothermal conditions the tetravalent uranium prevails. Principal ore minerals formed are uraninite and pitchblende Boyie (1982),Dybek (1962),Langmuir (1978), also referred to as nasturan and uranpecherz Romberger(1984),Smith (1984),amongothers, (Ramdohr 1980t Sobolewa and Pudov Kina have discussedin much detaii the geochemical 1957).Both mineralscrystallizein the isometric processesinvolved in uranium mineral formasvstemand are of the formulaUO2*". Pure Uation. In essence,their findingsmay be briefly 02 phasesdo not existin naturedue to selfoxidasummarized as follows. tion by radioactivedecay.Uraninite commonly In nature. uranium interactsand associates containsTh. REE. Pb and but not necessarily with a number of eiementsand compoundsin other elements,and hasgenerallya higher lattice complexwavsdue to the followingpropertiesand constant(ao > ca. 5.46A), and lower oxidation tendencies: state than pitchblende(a6 5.36 to ca. 5.465A)a) variableoxidation states,mainly lJa* and U6* Pitchblende may containCa, Si, Ti, Pb. andother as mentionedearlier; impuritiesbut rarelyTh and REE (Figs.2.2,2.3) b) variablecell units; all cell units are of large The compositionof both phases,however,relies size,hencepermit substitutionfor elementsof strongly on the formational environment and similar ionic radii predominantlyin rock con- conditions(for detailsseeauthorslistedin Figs. stitutingaccessorv minerals; 2.2 and2.3\. AI Fe Cr Ni Cu Pb
81,300 50.000
35 50 35 i I 5 0.2 0.2 0.0005 0.0005
4-J
MinerochemicalDistribution and Abundanceof Uranium in llinerals
l9
d e g r e eo f o xr d o t i o n )a z.g
mefornofphi c )a z.s
----J-z
z
metosedimenis )\
r'mefqsomotic
Pegmotitic
r,/
hydcoihermot tr.
vein\
\
t. \
i^
:Protunc0nr. \
oo9
rolnlori
\_!or t
metomorphic
\^
Pegmotitic
Fig. 2.2. Correiation diagram of oxidation degree-latticeconstant for uranium oxrde phases and their tentative attribution to genetic fields. (total of 120 samples).(Fritsche et al. in press,basedon data from Brooker and Nuffield 1953(dos and extrapolatedsolid line), Xu et al. 1981,Cathelineau et al. 1981,Fritsche et al. 1988)
In addition, the uranous ion substitutesfor a The uranyi complex displays a linear dumbbell variety of cationsof similarsizeand chargein the shaped structure, 3.4A long and 1.-lA rvide, in iattice of accessoryminerais in igneousrocks. which the hexavalent uranium is flanked by two SuitabieelementsincludeTh. Ce.Zr.Ti. Nb. Ta. oxygen ions. The uranyl complex is fairly stable Mo. W. Ca. and REE. The desreeof substitution and pret'erentiaily forms minerais with layered is a function of the sizeand chargeof the replaced stmctures of the basic formulas ion. Therefore the uranium tenor in the various -'xH2O o r B ( U O ; ; 1R O + ) : - . x H u O , host minerais varies in a large range from less A ( U O T X R O a ) than 1% U in rock consrituringaccessory min- w h e r e R i s P 5 ' , V 5 = o r A s 5 * , A i s K o r N a , a n d erals,up to about l0%U in manycomplex(Ta, B i s B a . C a , C u , F e r * . M g , P b . \b. Ti) oxides. to more than 20% in thorium Additionally, the uranyl ion tbrms hydrous oxide and thorium silicate(urano-thorianireand minerals with MoO1, SO.1,CO3, SiO3. SeO3,and urano-thoriterespectively). TeO3 as anions and a variety of cations. Hexavalent uranium occurs as mineral conThe cations are not strongly bonded and can be stituent in two configurations,as an individual substituted easily. The H2O component indicates ion having the coordination number 6 and an the necessity of a hydrous environment for minionic radiusof 0.80A, and as a complexion, the eral formation and mineral instability when the uranyl complex. The latter is the more frequent water becomes expeiled, for example at eleand forms many mineral speciesin oxidizing vated temperatures. In contrast to the uranous environments. ion (U4*), a diadochicincorporation of the uranyl The uranvl complex (UOr)t* develops in complex into other crystal structures is impossible aqueoussolution accordingto the formula for two reasons: Ua* + 2H2o = (Uu* O.)t* + 4H+ * 2e
20
2 Geochemistrv and Minerochemistryof Uranium
TiO2 r uroninile o uroninile? r pitchbtende o titcttblende? r U-Si-Ti-O-phose
tii' 3o ^ '" "ri: r" " o ^ ,l' i :, l' )',';;: trl'i-il
SiO2
:s:* /-E*=*:i'
CaO
r uroninite o uroninitc? r piichblende o gitcfiblende?
a a
o
tl aa a I
I. a
Th02
a) the dumbbell-shaped morphology is incompatible *'ith the structure of other minerals constituting complex ions; b) the uranvl complex, as stated above.is instable at elevated temperatures as found in igneous a n d m e t a m o r p h i ce n v i r o n m e n t s .
UOo-
Fig.2.3. aTiO2-SiO2-CaO trianglefor uraniumoxide phasesshowing the content variationsof these compounds.Segment,l correspondsto uraninite and pitcbblendein sensustricto which typically occur in igneous,metamorphic and sedimentaryenvironments. Segment2 correspondsto Uoxide phaseswith relative high Si-content.segmentJ to Si-richU-oxide phasesand coffinite, and segment4 to Ti-rich U-oxide phasesand brannerite.Mineral phases of segments2 to 4 are often associatedwith redistributed and metasomatic mineraiizations.(Fritscheet al. in press.including data from Aikiis and Sarikkoia 19871Cathelineauet al. 1982;Feather1981;Feather and Giatthaar 1987;Fritsche et al. 1988;Hilenius et al. 1986;Li Tiangang and Huang Zhizhang 1986; Monon and Sassano1972; Pageland Ruh.lmann1985: von Pechmannand Hiirtel 1988;Ruhlmann 1985).b CaO-ThO2-UO2 triangle for U-oxide phasesof seggnent1 (a) showingthe content of thesecompoundsin pitchblendes(field 1) and uraninites(field 2). Uraninite has commonly lower Ca contents (<1.8wt % CaO'1andhigher Th contents(0.1 to several percentTh02) than pitchblende(1-5% CaO. < 1 % T h O 2 ) .E x c e p t i o n tso theseproponionsexisr. panicularlvin metasomatrre U occurrences.(Fritscheer al. in press)Open squares .and , circlestentative loenttncatlon.
Boyle (1982) points out that both uranium ions. Uat and (UOr)'* , are stronglv adsorbedby manv inorganic and organic substances supposedl)' due to their large size and high charge densitr'. Adsorbing substances include hydrous gels or h y d r o x i d e so f F e . M n , T i , Z r , S i , A l , M o . f u r t h e r
MinerochemicalDistributionand Abundanceof Uranium in Minerals
zeolites, clay particles, clav-humic complexes, and humic substances.Sorption capacitiesof both inorganic and organic matter appear to rise with i n c r e a s i n gp H v a l u e s t o a p p r o x i m a t e l y8 . 5 , w i t h rnaximum adsorption in the pH rangeof .1.5to 7.5 ior inorganic material and 3.5 to 6.0 for humic substances.A general increaseof adsorption on most substances tends to correlate also with increasing hydrolyzation and associated polymerization of uranvl ions. Adsorption processes of uranium in humic substancesare highly variable and not well understood. They tend to depend essentially on type and origin of the irumus. its degree of maturation or decomposi:ron. perhaps involving ion exchange activiries oinding uranium through oxvsen to carbonvl groupsor uranium replacinghvdrogen in carboxyl or other compounds. Both substances,inorganic and organic, may also act as active transport media for uranium rvith optimum co-precipitation in a pH range of 5.5 to 7.5. Uranium may be desorbed from Fe and Mn hydroxides by alkali carbonare or sulfare solutions. Desorption by the tbrmer is almost complete at pH greater than 7 and, by the latter at pH beiow 5 (Boyle 1982). Figure 2.4 illustrates the general cycle of Ua* and Uo- interconversionsin nafure.
2l
2.3.2 Minerogenic Distribution of Uranium Qualitative and quantitative distribution of uranium in minerals is governed at hish temperatures such as in endogenic magmatic and metamorphic environments, by crvstallization of uranous oxides (essentiailvuraninite or uranothorianite) and silicates (coffinite, uranothorite). and substitution of adequate cations in accessory minerais. Principal rock-forming minerab (Fig- 2.5) are diadochically entered by uranium only in limited extent due to coordination restrictions.But these minerals may contain some uranium concentrated in lattice defects. adsorbed along cn'stai inhomogeneities. grain margins and/or as inclusion of uraniferous microcrystals. At low temperatures, such as in exogenic sedimentary regimes, the minerogenic behavior of uranium is dominated by changes in redox potential andior adsorption. Therefore syngenetic uranium minerals are almost missing in sediments except for detrital species. urano-organic and urano-phosphatic complexes. and uranium adsorbed on sedimentary particles. P etr ogenic-g eo ch emic aI distrib urio n of uranium is primarily associated with three groups of uraniferous minerals. each characterued by rockrelated minerai abundance and minerogenic uranium content (Fig. 2.5): - rock constituting accessorymilerals: - comolex ore minerals:
B o u n d i n m r n e r o l s s u c h o s u r o n i n r t e .p r t c h b i e n o e ,c o f f i n r r e , e t c . f i x e d o s o r e p l o c e m e n t i o n fo r Y . C e . 7 r . T h , C o . o n d 3 o i n o t h e r , porticutorlv occessory mtnerots ond odsorbed os ion cn cloy r n r n e r o t s ,n v d r o u s i r o n o x i d e s e t c . R e d u c i i o n :s o l u t i o n , odsorption, ond precipitorion
i
uo'
Solution ond mobrlizcrion os coiloids
tmmooile
0xidotion; solrrtron mobilizotion /,/ond os colloids
0 e g r o d o t i on of orgonrc \ . ^d^^r
rnrl<
\
l ' l o b r l e o s ( U O 2 ) 2 c' o m o t e x e s | 16+ lmmoDile/bounorn uronote. v uronyl mlnerols, plonts. ond qntmols
obsorptionby plonts onda/
I 1 4 + M o b r l eo s , J " c o m o l e x e s v l m m o b i l e , / f i x e crln h u m u s . plonts. onrmols,ond orgono-merqllic compounds
o c l s o fo t t o n . c h e l o t r o n
Bocteriot ond chemicol oxidotion
Fig. 2.{. Interconversioncycle of retravalent (Ua*) and hexavalent (U6*) uranium in nalure. (After Boyle 1982)
))
2 Geochemistry and Minerochemistry of Uranium
s
s
U
r -.
ao
-9
I
:J
Th !
Et--E. -o
a-o
o
K
f--
I
o
.e- =6 -o
i
E
o
Uronrum ore mrnerols Uronnrle------Prlcholenoe---Cotlinile- -----Cornotite-------Uronophon€--- -Bronnaarie-----Comolex ore minerols minerols AccessorY Alro^ite-------Euxcnile-polyc.o3c--- ----Xrnolim- ---- -- -Z;rcon-cyrololyleDovidite---- ---- -
,
o
d -
-- --- -- -- -
UronothorEnrl€ -Uronothorile---P yrochlor€-dcroht e - - - - -- -- - ----Somo.skit€----B€tofite--------Godoli^ite------5 | eens I rophe -- -Apotit e- - ----- -- Monozrte--------Titodilr {sphcnClCommon rock formina mtnerolq Hornbl€nde. biotite; :i 9r. 9d-----.----Pyrox€ne------Mqgn"tit€, in 9r. 9d-------lldedlc - - -- - - - -- Muscovite, sericil€---------Epidote---------Gornel---------0livine----------Plogrocrqsc. in grodl€-------Plogioclosc, rn gobDro. qt: dio-------Orthoclqse------ouorlz ----------
Fig. 2.5. Range and averagecontent of uranium,thorium, and potassiumof natural minerals.Bars indicate the range of concentration; ? ? possiblerange of concentration;solil triangle averageconcentration;open triangle possibleaverage concenrration.(Baied on Roylince J.G., in Van Blarimm ed. 1981,Adams et al. 1959;Bohse et al. 1974;Clark et al. 1966;Dybek 1962; Rogen and Adams 1969b)
- uranium ore minerals (predominantly Ua" sphene,rutile, and eudyalite,uranium contents minerals and subordinately Uo* minerals are generallybelow 1%. Alpha emissionof the uranium may isotropizethe mineralsby destrucwhich are mostly derivedfrom the forme(r). tion of the internal order of the originally crystalFor completeness,it should be noted that line structure (metamictization)but mostly oni!' uranium ma1'be presentadsorbedas interstitial to a limited degreeand very rarely completell'. mineralsaccountfor the bulk or intergranularfilms,alonggrain boundariesand Although accessory on crystallographicor structuralinhomogeneities of uraniumcontainedin rocks,they neverachieve enrichment to economic magnitude except for and discontinuitiesin mineralsand rocks. Rock-constitutingaccessorymineralscontain apatite in phosphoritesfrom which uranium is uranium only as a subordinatecomponen!.The recoveredas by-product. Complexore mineralsare typical constituents ionic radiusof Ua* of either 0.97or 1.01A pervariety of certainpegmatites,carbonatites,alkali svenof mits only a limited substitutionin a ites, etc. They also do not necessarilycontain abundant compoundsby replacingother far more uranium as a major component.Uranium can monazite, elements.In suchmineralsas apatite,
GeochemicalDistribution and Abundance of Uranium in Rocks and Waters
substitute for suitable elements up to 10% and more. U4+ preferentially replacesTh, REE, Ti, and Fez+ in davidite, euxenite, samarskite etc. \{ost of these minerals are isotropized and, in contrast to accessoryminerals. often to a complete metamict stage. Mineral color is mostly lustrous black and the original optical properties have been destroyed. In the order of 200 complex minerals are recorded. Some of them, like euxenite and samarskite. or (urano)thorianite and (urano)thonte, occur in appreciable amounts typicaily but not erclusivelyin pegmatites,whereasothers, such as :rose of the pvrochlore group. are more charac:eristic for carbonatites.Many complex minerals are resistiveto weathering. They can accumulate in fluvial or beach placers together with other heav-v minerals such as those of Sn, Nb, Ta, or Au with which they may be recovered as a by-product. Most complex minerals have little economic importance as a uranium resource, partially due to their refractory property. Uranium ore minerals contain uranium as the rnain metallic component. They include, in order of economic importance, the uranium (Ua") oxides uraninite and pitchblende (UO2*,, mostly given as U:Oa), the silicate coffinite (USiO+), the U-Ti-oxide phases frequently referred to as brannerite (UTi2O6), and hexavalent uranium minerals often formed successively from the "primary" varieties. The U"* mineral group oredominantly consistsof uranyl hydroxo-oxides, carbonates, phosphates, arsenates, vanadates, and silicates. which locally mav accumulate in ore-tbrming quantities.
2.4 Geochemical Distributionand -{bundanceof Uranium in Rocksand Waters Petrogenesis-relatedaffiliations and tendenciesof uranium can be outlined schematicailyin context with the three principai environments of the geological cvcle of crustal evoiution, namely:
ZJ
cursor rocks decomposedby chemical or physicalattackat the earth'ssurfaceresulting in clastic,chemicalor biogenicsediments. c) Processesinvolving metamorphosisof preexistingrocksresultingin metamorphicrocks. behaviorof uranium In summary,thegeochemical in igneous environmentsreflects its iithophile properties.It is affiiiatedwith leucocratic rocksof the alkalinesuitesrather than with mafic rocks. Igneous facies which meet this criterion are primarily leucocratic peraluminous granites, granitic-alaskitic suites,sodium-richintermediate Theiruraniumtenormay rocks,andcarbonatites. rangewidelyfrom abouttwicenormalabundance to severalhundredsof ppm. This featureapplies equally to intrusive and extrusive suites and their late-stagederivatessuchas pegmatitesand magmatichydrothermalformations. In sedimentaryenvironments.uranium distribution falls into two basic categories,(a) of syngeneticand (b) of epigeneticemplacements. accumulationsrypically occur in Synsedimentary marinephosphoritesand blackshales.Epigenetic favor continentalclasticinhomogconcentrations enous sediments, deposited in intracratonic basinssurroundedby or proximal to uraniferous crystallinerocksof dominantlyPrecambrianage. In metamorphicenvironmenrs.uranium distribution canbe twofold, (a) in a closedsystemas fine disseminations,for example in stratiform mode in metasedimentsreflecting the original endowmentof its protoiith or in more ubiquitous distribution in metamorphosedigneous rocks; and (b) in an open systemwhere uranium is redistributed and concentrated into sites of favorabledepositioncontrolledby structuresand locally in addition bv certain rock facies.Figure 2.6 providesa summaryof U contentsin the variousrock faciesamendedby tenorsof Th and K which often, althoughnot necessarilv.show a geochemical relationshipto the uranium. 2.4.1 Uranium in Magmatic/Anatectic Environments
Uranium occursin trace amountsin practically a) Processes associated with magmatism or all magmatic/anatecticrocks. Average tenor anatexisipalingenesis resulting in respective is commonly low. from fractions of l ppm to l5ppmU, but may be augmentedin certain magmatic lithologies. b) Processes associated with weathering, trans- lithologies. particularly in leucocratic facies port and redeposition of constituents of pre- to 100ppmU and more. Igneousrocks of the
24
2 Geochemistry and Minerochemistry of Uranium
s
s
U !
-e
i; -
9P
I
E
rh a-+ !
R
F
9e
o
:?3
s
K
tr
;
-o
-
-O
O
=
o
-o
Fetsic intrusives Nepheline syenite----------
Gronite--------nhyolii€-------Felsic extrusives O b s i d i o ho n d o t n e r g l o s s e s - - Hrgh potosSlum-Low potoSSrum--Glosses. generolRhyolite--------lntermediote
intrusives
Ouortz monzonile Gronodiorite,quqrtz drorit€---lntermedidte.
generoi---------
Intermedrote exlrusrves Ouorlz lolit€----Lotite----------0qcite---------- Mofic intrusives D r o r i l e .d i o b o s e - Gobbro---------M o t i c .g e n e r o l - - Mofic exlrusives A ndesile - -- - -- - Tholeiitic bosolt-Eosolt- --- ----- - oltvine bosolt---Ultromofic intrusives Serpentinite----Pefldolite --- - --- -
Dunite----------Ultrooofi c- oenerol--------S e d r m e n to r v : Morne block shole----------Mq.ne groy or green shole-----____-------T u ff o c e o u s s o n d s t o n e - - - - - - Ark0se---------Sillstone- -- - - --S o n o s l o n e- - - - - - 0olostone------L r m e s t o n-e- - - - - -
Metomorphic:o G n e r s s .e D i d o t e - o m g h i b o l i l eo c i e s - - - - - - - - - - - v e i n e d g n e i s s e s .h i g h o m g n i 5 o l i t el o c i e s - - - - - G n e i s s e s l. o w g r o n v l i t e i c c i e s - - - - - - - - - - - - - - Chornocxire oneisses. h i g h g r o n u l i t ef o q e s - - - - - - - - - - F o r p h y . o b l o s t t cm o ^ 2 o n i r € , h t g h g r o n u l i l et o c t e s - - - - - - - - - - Eclogit€ lmelomorphic ona volconrc]---------Slooy heleorit€-Chondrite (meteo.it€i-------
o
Nor*egron 5ludy oreo {onolysis of leucocrolic bonds in sugposed metosecirmenlory sequence)
F i g . 2 . 6 . R a n g e a n d a v e r a g e c o n t e n t o f u r a n i u m , t h o r i u m , a n d p o t a s s i um maojfo r r o c k s t y p e s . B a r s i n d i c a t e t h e r a n g e of concentration; ? ? possible range of concentration;solid trianglesaverage concentration;open triangles possible averageconcentration.(After RoylanceJ.G., in Van Blaricom ed. 1981)
GeochemicalDistributionand Abundanceof Uranium in Rocks and Waters
oceanic crust average 0.5 ppm U (fresh seafloor b a s a l t s0 . 0 2 t o 0 . 0 8 p p m , i s l a n da r c a n d e s i t e s0 . 2 r o 0 . 5 p p m U ) , w h e r e a st h e c o n t i n e n t a lc r u s t h a s I t o 3 p p m U ( c o n t i n e n t a lb a s a l t sa n d a n d e s i t i c 'rasalts 0 . 5 t o 1 . 9 p p m U , a n d e s i t e1 - 4 p p m U , 2 _rranite to 15ppmU). Various rock types generally have highly vanable contentsof uranium with the generaltrend of higher values in more felsic. alkatic and late-stage members of igneousdifferentiation seriesthan in mafic earlv-stage lithologies. Essential factors correlating with variations of uranium contents include:
-
25
postmagmaticprocesses(pneumatolvtic alteration. metasomatism); rock derivation from crustal or mantle sources.
Individual magma series commonlv display an increaseof uranium tenor (a) from basic to acid m e m b e r s ,i . e . , ( b ) w i t h i n c r e a s i n ed i f f e r e n t i a t i o n , expressedby increaseof silica. and potassiumor sodium or total alkali contentsin successivefacies ( s e eb e l o w ) . a n d ( c ) t o d e c r e a s i n ea g e f r o m o l d e s t to youngest facies. Rare elements such as Be. Hf. Nb, Ta. Th, Zr. and REE follow the same p a t h . b u t c o r r e l a t et o o n l y a l i m i t e d e x t e n t w i t h uranium. Reversalsin these trends. ho"vever.are - SiO:. K or Na, or total alkali contents; knou'n from some cogenetic sequencesof both - t]'pe of rock-constitutingminerals: intrusive and effusive rock suites. - relative age of facies in a cogenetic igneous Tables 2.3 to 2.5 present a compilation of suite: uranium tenors in the principal igneous rock - internai differentiation of a single batholith: tvpes. The values show an increase in uranium - orogenic or anorogenic emplacement of from gabbroid to granitic masmas bv a factor intrusion; of 3 to 4, and of more than 100 compared to
Table 2.3. Uranium contentsof isneous rocks Extrusive
Intrusive Average
Common range
ppmu
ppmu
Ultrabasic Dunite. peridotite Pvroxenite
0.02 0.02 0.70
0.003-0.05
itsic Cabbro
0.90 0.84
0.20-3.10 0.60-I .07
lntermediate Dionte I Quartz diorite J Granodiorite
2.00
r.10-3.03
2.00 2.60
0 . 5 0 -I 1 . 5 0 1.00-9.00
Felsic Granite \lonzo-granite '-euco-granite
4.60 3.50 7.50 8.00
tlrskite Peomrtit.
:.20-21.00 : . 2 0 -1 5 . 0 0 6 . 0 0 -1 8 . 0 0 6.00-21.00 10.00-500.00 r0.00-1000.00
.llkaline-rich Alkali granire Alkali svenite \epheline svenire Lujavrite .A.tbitite
Joo
0.04->20 10.00-200.00 1.00-20.00 i.00-60.00 10.00-1200.00 1.00-55.00
Others Kimberlite Carbonatite
1ro
50.00-500.00
Average
Common range
ppmu
ppmu
Basalt Trachydolerite
0.60 1.-r0
0.10-2.30
Andesrte Lamprophyre
0.90 5.00
0.80-3.00
5.00 1.00 5.00 3.00 l 1.00
i.50- 15.00 0.90-7.50 1.00-8.00 i.00-25.00 3.00-18.00 i0.00-65.00
Dacite Rhvodacite Rhsolite Bostonite Quanz bostonite Alkali trachyte Phonolite
-1.00
3.00-50.00 10.00-50.00 3.00-18.00
From data and compilationsby Belevtsev 1980;Boyle 19821Clark et al. 1966:Dvbek 1962;Goodell 1985a;Maucher 1962:Rosers and Adams 1969b:Roubault 1958:Wennch 1985:Vinoeradov 1963
26
2 Geochemistry and Minerochemistry of Uranium
ultrabasic rocks. Crust-derived felsic igneous tain in the order of 100ppmU (Phair 1.952). rocks (peraluminous leucogranites) affected by Albite-riebeckite (-pyrochlore) granites of the late magmatic processesin particular exhibit Jos Plateau, Nigeria, have contents of 100 to increaseduranium contents(10-25ppmU in St. 130ppmU (Maucher1962).ln anv event,highly Sylvestregranite, Massif Central, France (see differentiatedgranites and pegmatiteshave the Chap.5.3.1). largestamountsof uranium. Volcanic equivalents may contain 1.5 to Effusive iithologies(Tables2.3, 2.4) commonly exhibit a greater variance in uranium values. 2 times as much uranium as their plutonic But here again uranium concentrationincreases counterparts. Anorogenic alkaline intrusions of agpaitic aiialogousto progressingmagmatic differentiation. Uranium averages0.1-1ppm in basaltic compositionoften have higher uranium contents flowsand 5-10ppm in rhyolites,locallyincreas- than their orogenic equivalents.For example. ing to as high as 20-40ppmU and more. Par- the nepheiine syenite complex of Iiimaussaq/ ticularly,acidvolcanicsrich in vitreousmatrix are Greenland (Table 2.5) averagesin total about 60ppmU with concentrationsof up to 200ppmU stronguranium-coliectors. A comparison of calc-alkaline and alkaline in fine-grained black lujavrite members and igneousrock suitesshowsthat the latter containa 3000ppmU in a secondgenerationof coarsehigher uranium concentration than the corre- grained black lujavrite (Bailel' et al. 1981: spondingcalc-alkalinemembers. For exampie, Kunzendorf et al. 1982). Similarly. caldasite alkali basaltshave up to 2ppmU comparedto (baddeleyiteand zircon) veins of the alkaline tholeiiticbasalts,which average0.1-0.5ppmU. Poqosde Caldasintrusion,Brazil. containseveral Calc-alkaline granites contain 3 to 6 ppm U, hundredppmU. Analogousto uranium,Th, Zr, whereas high-alkali granites of the alkaline Nb, and REE are also enriched in nepheline suite may contain two to ten times as much and syenites.In both localitiesuranium is bound in more uranium. The K-Na-rich biotite granite refractorymineralsexceptwhere redistributedbv of Conway/White Mountain Magma Series, youngerprocesses. New Hampshire, averages about 15ppm U Carbonatitecomplexescontain concentrations (Richardson 1964). Quartz bostonite dikes of several10ppmU, mostly bound in refractory of the Central City district, Colorado, con- minerals such as pyrochlore but also in uranothorite as in Phalaborwa,South Africa. The Phalaborwacarbonatiteaverages40ppm U. Table 2.4 Uranium contents of selected volcanic Crystallophysicsand crystallochemlsrrycontrol complexesin uilnilslalized areas the selectivebehaviorof uraniumin the magmatic cycle. Magmatic differentiation progressesfrom SanJuan Mu, Colorado, USA (Pliocene) (1956) Hinsdale Lavas low silica,mafic precipitatesof the earlv crvstall-arsen et al. Basalt 1.0 lization stage through intermediate, and felsic Trachydolerite 1.4 varieties, to pneumatolvticand pegmatiticfinal Andesite-basalt 1.9 crystallizates. Uranium and other elements of Andesite 3.2 Latite 2.9 largeionic radiusor inproperchargecannotsubRhyolite 9.2 stitute in most of the ferromagnesianminerals Japan(Tertian') Heier and Rogers(1963) dominatingthe earlvstage,and in principalminTholeiitic basalt 0.15 erai constituents of the main stageof crystallizaHigh alumina tion. Consequentl-v. uranium,aswell asthe other basalt 0.2? *Alkali-olivine incompatibleelements, become progressively basalt 0.53 concentrated in the late felsiclithoiogies. of early crvstallizationand Typical minerals Canibbean Islands incipient main crystallizationsuch as olivine. ffertiary to Recent) Rogersand Donnellv (1966) pyroxene, amphibole, and basic plagioclase Spilite 0.22 resistthe incorporationof uranium.and because Extrusive uraniferousaccessorial mineralsare lacking,the keratophyre 0.29 Intrusive abundanceof uranium in melanocraticrocks is keratophyre 0.77 low. This is consistentwith the low tenor of Basalticandesite 0.79 uraniumin the mantle(0.03ppm) and in ulrra-
GeochemicalDistributionand Abundanceof Uranium in Rocks and Waters
27
T a b l e2 . 5 . U r a n i u m c o n t e n t sof selectedisneous complexesand rocks in U mineralized districts Intrusrve \,I assif Central, France fenile, (Hercynian) Leucogranite, - with assoc.U deposits - without U deposits - sterile Leucograniteof St. Sylvestre,(Herc1'nian) (peraluminous2-micagranite) Biotite granitesof Gudret Vendie, France of Mortagne( Hercynian) I-eucogranite - ,rnented.porph.r-ntic. biotitedominance - eibitized - ;nuscovitedominance
Average
ppmu
Moreauet al. ( 1966)
7 . 9 -1 0 . 8 5 . 3 -1 0 . 5 3.0-3.-r
5.0-14.6 r.9-21.9 l.?--r.6 Moreau(1977)
6.i-lt.u
Moreau ( 1977)
4 . 2 -5 . 9
Renard(1974)
12.3 lJ.+
20.4 Stuckless er aL (1971)
Granite Mountains, Wyoming, USA (Late Archean) Biotite alkali granite Leucocraticalkali granite .A.lbitizedgranite Silicifiedand epidotizedgranite Iiimaussag,Greenland(Middle Proterozoic) Pulaskite i{ererogenousfoyaite Sodaiitetbyaite Naujaite Lujavrite Kakortokite Fine-grainedluj avrite Medium-grainedlujavrite Steenstrupinelujavnte Coarse-grained black luj avnte
Rangeppm U
- 1 3 8 0 ) ( e q 'U ) 1 2 . 9 - 5 6 (. 1 ( e q .U ) 1.5-23.1 7 . 3 - 1 r . 2( - 1 0 7 . 5 )( e q .U ) (eq.U) 1s..t 9.0 10.0 14,0 -
Bohseet al. (1974) Baileyet al. (1981) Kunzendorf et al. ( 1982)
6.0-12.0 20.0-67.0 19.0-23.0 134.00-350.00 190.00-310.00
Mc Dermitt Caldera, Nevada-Oregon, USA (lvliocene) -\urora lava .\urora lava, altered Rh,volite Andesite Ash-flow tutf (rhvolitic, welded, or silicified) Opalizedshale Chalcedonybed
1 1 - 1 0( - 1 0 3 ) 3- 160 3-71(-163) 120-195 35
Sierra de Pena Blanca, .Vexico (O lieocene-Eocene) La Mesa Fm. (trachyticto rhyolitic ignimbnte) PenaBlancaFm. (rhyoliticignimbrite) ahontes Fm. (polvmict volcanicconglomerate) FscuadraFm. (rhvolitic ignimbrite) Vitrophvre Nopal Fm. (rhvoliticignimbrite) Limestone Chalcedony
mean+ SD. 3.00 1.00 1.55 1.92 2.00 0.00 i 1 .1 5 3 2 . 9 5 11.25 5.31 3.87 6.t5 3.46 3.13 5.83 1.26
mafic (0.01ppm) and mafic facies(0.75ppmU) (Nash et al. 1981).When ultrabasicor basicrocks 'Jisplay a somewhat higher uranium content, they commonly have been affected by later pneumatoiytic or hydrothermai overPrints with associateduranium introduction.
2-86(-27e1) r?_50 /-11R\
Goodell (1.985) (SD = standarddeviation)
With increasing silica content uraniferous accessorialminerals such as allanite. anatase, rutile, apatite, zircon. sphene. mon.vite. xenotime, pyrochlore. and/or uraninite begin to crystallize, resultingin an increasein uranium content in the rock. In addition, rock-formins minerals such as
28
2 Geochemistry and Minerochemistry of Uranium
quartz, feldspar,amphibole,and micasmay collect uranium. Although individual minerals accumulateonly very minor amountsof uranium (felsic minerals up to 10ppmU' mafic minerals particularlybiotite up to 60ppmU; Fig. 2-5),the large volume of principal rock constituentsmay cumulativelyadd substantiallyto any lithologrc uranium background.Another trap for uranium is as adsorptionor as minute grainsof uraninite 6n cleavageplanes. crvstal faces. grain boundaries, intergranularfilms. or, in felsic voicanics. incorporatedin the glassvmatnx. Accessorial refractorl' minerals account for from 50 to60"h ofthe uranium contentin granitic rocks.The remaindermaYbe presentin the form of uraninite and/or adsorbed along cleavage planesin biotite or in intergranularspace'The presenceof this leachable uranium fraction is particularlv qpical for differentiatedleucocratic peraluminousgranitesof the alkaiic suite' which ma,vhave in the order of 5 to 20ppm U (Table 2.5). They are consideredfertile uraniumsource rocks for uranium deposits as discussedin Chapters4 and 5. For more details on uranium in magmatici anatectic environments,the reader is referred to papersin IAEA, 1985 (Uranium Depositsin Volcanic Rocks); IAEA, 1988 (Recognitionof Uranium Provinces); IAEA, 1989 (Uranium Depositsin Magrnaticand MetamorphicRocks); Maurice (ed.), 1982 (Uranium in Granites); Mitler (ed.), 1983 (Evolution of the Damara Orogen).
2,4.2 lJranium in Sedimentary Environments
Table 2.6. Uranium contents of sedimentary rocks Average Common range ppmU ppmU
0.45-3.25 - 3.21 0.4-s 0.50-2.10 0.20-0.60
Arenites and rudites Sandstone Greywacke Orthoquartzite Bunter sandstone, argillaceous
1to
Lutites and pelites Common shale.argillite Grey & greenshalesin N-America Mancosshale(western
3.50 3.70 3.20
2 . 0 0 -5 . 9 0 1 . 0 0 -1 3 . 0 0 I .20-I 2.00
3.70
0.90-12.00
usA)
Kupferschiefer (Germanl') Black shale. continental Black shale.marine Mud. PacificOcean Mud, Indian Ocean Mud, Black Sea Carbonates Limestone.Florida Limestone.California Dolomite. dolomitic Iimestone Evaporites Anhydrite, gvpsum Halite, sylvrte
0.4-s ?.20
39.00 2.00-4.80
;J -. / 2.20 2.00
l'o 0.10 0.10 0.10
Phosphorites Marine Guano Fossilbones Others Femrginous laterite Bauxite Bentonite OceanicMn nodules
10.00-1244.00 1.50-4.00 0.20-0.50 0 . 0 i- 9 . 0 0 0.50-6.00 0.03-4.90 0.03-2.00 0.01-0.43
8.50-300.00 50.00-300.00
11.40 5.00
(- 19'o) 10.00-100.00 3.00-27.00 1.00-21.00 3.00-6.50
by Bovle 1982:Clarket al. Fromdataand compilations 1966;Dybek 1962:Rogersand Adams 1969b;Samama 1984
Uranium tenors in sedimentsaveragefrom 1 to 4ppm, but rangein individualfaciesover a wide span, from fractionsof i ppm to ore grade con. Favorable continental and marine sediments centrations(Table2.6). for uranium accumulation are: For supergeneepigenetic or syngenetlcredepositionin sediments.primarily dispersedand - sedimentsrich in organic matter: fixed uranium must be mobilized, transported - sedimentscontaining sulfides: and fixed again. Fundamentalphysico-chemical- sedimentsof phosphatic composition. are providedparticconditionsand environments Principal reactions controlling the precipitarion ularlyby climate,typeof weathering,topographic and fixing of dissolved uranium in sediments relief, hydrology, hydrochemistry,and source include: and host rock. They control release.transport, of uraniumin eithercontinental - reduction of the uranyl ion to uranous ion. and redeposition resulting in precipitation of uranium minerals or marine sedimentsand by either svn- or epiprocesses. ( p i t c h b l e n d ee t c . ) ; sedimentarv
GeochemicalDistributionand Abundanceof Uranium in Rocks and Waters
-
precipitation of insoluble uranyl compounds ( u r a n y l v a n a d a t e se c t . ) ; adsorption on clay particles, organic material a n d s e c o n d a r yo x i d e s( l i m o n i t e e t c . ) ; substitution for other elementsof similar ionic r a d i i a n d c h a r g e( f o r C a i n a p a t i t e ,e t c . ) .
The type of sediment and uranium-fixing mechanisms by themselves are not sufficient for uranium collection. Assuming that a potential uranium source is present, the other factors mentioned become to various degreesinvolved in any Drocess of uranium mobilization and distribu'ron. Chemical weathering and destruction of .,raniferousrocks result in oxidation and release ;f uranium commonly as the soluble uranyl ron. In humid climates most of the uranium is transported by rivers into the ocean (averagecontent o f s e a w a t e r 0 . 3 - a p p b U ) a n d c o n c e n t r a t e di n organic ooze sediments (sapropel. black shale, coal), in phosphorites, and to a lesser extent in argillaceous sediments. Clean sandstones and pure calcareous rocks (limestone, dolomite) are usually poor in indigenousuranium. In arid and semi-qrid climates uranium becomes concentrated in solutions particularly in groundwater (the water table of which being relatively well below surface) and to a lesser extent in surface streams. Drainage is either intermittent to the sea or centripetal into intracratonic and intermontane basins,where uranium may precipitate in favorable environments (see subtvpe 6.1 and type 4, Chap. a). Where chemical weathering is incomplete or cannot keep up with erosion, uranium may remain in rock constituents, particularly in resistate accessories,and end up in immature clastic sediment such as arkose. In environments dominated by physical rveathering,under nonoxidizingconditions,which are essentiallyrestrictedto the earlv Proterozoic ind eariier, all constituentsof the precursorrocks rncluding uraninite (unstable under oxygenated chemical weathering conditions) survive mechanical transport and form placer deposits. Such placers locaily contain up to severalhundred to a few thousand ppmU (see tlpe 7, Chap. a). Conrinental sediments containing carbonaceous matrcr have a certain capability to precipitate uranium from percolating solutions by reduction of hexavalent (uranyl) ions to the tetravalent state. Such fluvial and lacustrinesedimentsrange widely in their uranium tenor. Normally they
--.-
29
contain 0.5 to 5ppmU. In a uranium province, background values mav increase to 10 or 20 ppm U and up to few hundred ppm in the vicinity of uranium deposits. Immature clastic sediments containing vegetal organic matter andior acid volcanic components exhibit higher uranium contents to the extent of forming uranium mineralization composed in part of pitchblende. Uranium in fluvial sedimentsis partly present in resistate minerals such as monazite. zircon. apatite. sphene, and others. The remainder occurs predominantly adsorbed on clay. silt, and vegetal particles. on various gels and microcrystalline substances;andior as coprecipitate of hydrous substancessuch as limonite (10 to 7000ppm U), wad, leucoxene,humic compounds. and other gelatinousand cryptocrystallinealuminiferous, phosphatic. titaniferous. zirconiferous and siiiceoussubstances.Their uranium content varies over a large range, from 5 to 100ppm, locally up to 7000ppmU (Boyle 1982). Peqts, lignites and coab may also adsorb uranium from solutionswith concentrationsranging from 1 to about 100ppm U. although localized concentrations may be as high as IYo U (see subtype 6.2 and type 14, Chap. ,1). Sediments containing hydrocarbon residues (asphaltite) average 100 to 500ppm U but sometimes accumulate up to 4000ppm U. Sediments containing sulfides generally constitute a reducing environment. Anaerobic conditions may produce H2S, a strong reductant capable of reducing large amounts of uranium (and other metals). Under favorable conditions uranium concentrationmay attain minabie values in continental permeable. impure suifidized sandstones(see type a. Chap. -l). Biruminous or black shales and alum shales characterized by reducing conditions during deposition reflected by abundant sulfides. finely dispersed organic material and/or distillable hydrocarbons sustained synsedimentary precipitation of uranium and other metals (Cu. Ni, Co, Mo. Ag, etc.) from seawater. Of the various marine black shales. in first instance. those of bituminous sapropelic composition are uraniferous (average 20 to 60ppmU. rarelv up to 400ppmU as at Ranstad, Sweden). rvhereas those of humic composition contain lower uranium concentrations(see type 15, Chap. 4). Certain shalesmav have as much as 1000ppmU. as the graptolite shale at Ronneburg, Germany (which
30
of Uranium 2 Geochemistr-v andMinerochemistry
may have been caused,however.by overprinting bauxite formed from felsic igneous rocks may have cogenital uranium (<27 ppm U) from its processes). and of. parent rocks. Fenuginous lateite may accumulate phosphatic composition of Sedimens to about from some 10ppm 10 to 100ppmU and locallyup to I"h U. (Table range origin marine 1000ppmU locally up to several 2.6) and 300ppmU Duricrusu (calcrete,gypcrete)may accumulate as in Cabinda, Angola. Bedded phosphorites formed at tbe marginof a continentalshelf (e.g., uraniumup to severalhundredppm U and locallv PhosphoriaFormation, Idaho, USA) generally in excessof 1000ppmU (seesubtype6.1, Chap. have higher uranium contents (average100 to 4 ) . 200ppmIJ, max. 6500ppmU) than phosphorites For more details on uranium in sedimentary of shallow marine near-shoredeposition(e.g., environmentsthe reader is referred to IAEA/ Bone Vallev Formation. Florida, average20 to Nininger et al.. 1983, IAEA/Finch (ed.) 1985, 80ppmU). The latter, however,may by rework- IAEA/Grimbert et 81., 1988, and papers in ing have achievedsecondaryenrichmentsof up IAEA/Toens (ed.) 198a. to 500ppmU. In both casesit is assumedthat uranium was more or less synsedimentary,extracted from sea-water and incorporated into 2.4,3 Uranium in Metamorphic apatite grainsby substitutingfor calcium(seetype Environments 10, Chap. 4). In contrastto marinephosphorites, all continental phosphatic rocks are low in Metamorphicrocks have rather variableuranium uranium,rarely exceeding20ppmU. tenors(Table2.7), in manyinstances comparabie Soilshavevariableuraniumcontentsaveraging to thoseof their protoliths.It appearsthat uranium in the order of 1 to 5ppm dependingon the and some other trace elements are relatively piuent rock and soil composition.Higher con- insensitivero epi- and mesozonalmetamorphic centrationsof up to 100ppmlJ or more are com- processesin closed systems (Getseva 1963; mon in areasunderlain by uraniferouslithologies. Cevales1961).Under theseconditionsthe main In this caseenrichmentsare usuallyconcentrated impact of regional metamorphismup to upper in the C-horizon, whereas commonly the A- amphibolitefacies seemsto be more or less in horizon is the main uranium collector. Part of situ remobilization (range of millimeters to the uranium in soils is present in resistatessuch perhapsmeters)and, if no other effectsinterfere, as monazite, xenotime, and zircon which, under recrystall2ationof uraniumto pitchblendein low favorable conditions, can form eluvial depositsof grade and to uraninite in medium to high grade heavy minerals.Another part of the uranium is metamorphic regimes (Ramdohr 1986). Other intimatelyboundto clayey,bauxitic,Tihydroxide, than some preferentialconcentrations certain in Zr hydroxide, hydrous Fe and Mn oxide, and vegetalorganiccompoundsof the soils. Boyle (1982) designatesas dominant para- Table 2,7, Uranium contents of metamorphic rocks meters controlling uranium mobilization and Average Common range fixation in soils: climate. parent rock and assoppmU ppmU ciated soil composition,pH, coprecipitationand adsorption of uranium ions by the various soil Quartzite.meta-gle1'rvacke 1.50 2 . 8 0 -5 . 5 5 colioids,clay mineralsand hydrouscompounds Carbonaceouslgraphitic guaruite mentionedabove,further reactionswith humic 2.70 organic material and interaction of micro- Slate 1.90 organisms.Uranium appearsto be more mobile Phyllite, meta-argillite 2.W 0 .1 0 -1 0 . 0 0 in soils of arid to semi-aridzonesthan in soilsof Schist 3.50 1.00-100.00 tropicaland temperateregions,supposedly due to Graphitic schist 4.70 Biotite schist the low abundanceof organic substances in the Amphiboiite 0.50 0.30-3.50 former. Gneiss 3.00 0 .r 0 - 1 0 . 0 0 t J . 2- 2 . 5 0 1.00 Surficialalterationproductssuchas bentonites Granulite Eclogite 0 . 2 0 0.01-0.80 derived from tuffaceous material may have elevateduraniumvaluesof up to 20ppmU which From data and compilationsby Boyle 1982:Dybek 1962; they inherited from their protolith. Likewise, Pagel and Svab 1985; Rogers and Adams 1974
GeochemicalDistributionand .{bundanceof Uranium in Rocks and Waters
mafic minerals,(particularlvbiotite, amphiboles Table 2.E. Uranium content chanses durine metaerc.) and locally in fold noses, no significant morphism rransportation, introduction. or removal of Regionalmetamorphism ppmu uranium is apparent,as reflectedby the com:r'lonly almost strata-concordantdisseminated Gneiss.Longoy,Norwayl Epidote-amphibolitefacies J.-+) distribution. Hieh eradeamphibolitefacies t.22 In contrast,high gradegranulitefaclesregimes Low sradegranulitefacies 0.88 may suffer uraniumdepletioncomparedto their Higi eradegranulitefacies 0.22 igneousor sedimentary unmetamorphosed equivppm U ppm U alentsas reported by Dostal and Capredi(1978) Schistand gneiss.Aldan ShieldC . I52 amphibolite granulite andHeier (1979)(Table2.8).Thisis explainedby facies facies the possibleupwardmigrationof uraniumin the (low srade) rrust during the very high grademetamorphism +1./: 2.25 ,Heier and Adams 1965).However.this appears Schisr.gneiss i i t lrot to be valid as a rule. lnt Granitegneiss,migmatite 1.29 Adamson (1983)and Adamsonand Parslow ( 1985)investigated Green cratonicgranulitesourhof the Contact metamorphism: Hornfels schistfacies facies AthabascaBasin, Saskatchewan, and found no evidencefor depletionof uranium(and thorium) Carbonaceousquanzite -2.97 5.55 (centralAsia) or, in addition,that reworked"linear beits" are *3.-10 enhancedin both elemenrs.irrespectiveof the Carbonaceous,siliceousand 5.,18 carbonaceous,micaceous metamorphicgrade. schists(Erzgebirge. Pagel and Svab (1985) researchedgranulite Germany) lacies in the Carswell Structure within the A.lbite,carbonateschist 2.80 (Erzgebirge,Germany) Western Craton, located in Saskatchewanto the northwest of Adamson's (1983) area, and I Heier and Adams 1965;2Yermolavev 1971 documentedhigherthan averageuraniumvalues. The authorsascribedthe enhancedU tenor to the presenceof carbonaceous material(graphite)that maintained a low fO2 during granulite-grade metamorphism and as such a low mobility of 2.4.4 Uranium in MetasomaticEnyironments irranium,therebyretainingthe originaiU content isee also Cuney 1981 and Barbev and Cuney Metasomatized rocks of both contact meta1e82). provenance reIn some contrastto the above observations, somatic and autometasomatic sulted from migration of inherent or introduced Belevtsev(1980)notesthat migrationof elements becomesstrongerwith increasingmetamorphic elements which replace original rock constituents in igneous and (meta) sedimentary rocks. Naactivitv.Uranium-bearing sediments loseuranium and other metallic elements proportional to metasomatized rocks appear to be the most important with respect to uranium accumulation. the intensityof metamorphism. Dunng dynamrc They include albitite and lithologies dominated regionalmetamorphism.uranium migrateswith maximum mobility in the amphibolite-granulite by other Na-bearing minerais such as Na(aeginnites etc.). facies when practically ail uranium is removed amphibolesand Na-pvroxenes (1986)reviews albitite-related uranium Fritsche from its parentrock. and discusses nomenclature and According to Yermolayev (1971), contact mineralizations processesinvolved in metasomatism and albitizametamorphismof greenschistfaciesrocks results tion in general and of uranium albitites in parin depletionof uranium(Table2.8). ticular. He attributes the various albitites into the following genetic groups.
A B C Ca
magmatic albitites autometasomaticalbitites metasomaticalbitites late-magmaticalbitites
32
2 Geochemistryand Minerochemistryof Uranium
Cb albitites not associatedwith magmatites - metamorphic albitites - other albitites Magmatic albitites (A) are primary crystalhzed rocks derived b1' magmatic differentiation. They include intrusive magmatic bodies or parts of them, veins. and volcanics.The latter include the so-called spilites of primary origin (Amstutz r974). Automensomatic ulbitires (B) have originated from magmatic solutions and formed mainly in the roof- or contact zone of magmatic bodies. There has been no significant time interval between the intrusion of the magmatic body and the alteration. Autometasomatic albitites often grade towards depth into zones of potassium metasomatism reflected, e.9.. b1' microclinization. until the unaltered ma-ematite is reached. Beus and Zalashkova (1964) call this alteration early albitization, which is often associated with gleisens and fenites (Br6gger 1920; Dukovsky 1980; Herzberg 1977: Smirnor' 1970). This group includes in part the spilites of autohydrothermal origin (Amsfitz,1974). Late-magmatic albitites (Ca) are the product of late-magmatic alkaline metasomatism defined by Borodin and Pavlenko (7974].as the displacement of rocks by alkali feldspar, alkaline mafics. and other alkali-rich minerals. Formation temperature is below the granite- and nepheline-syeniteeutectic. The late-magmatic metasomatism is controlled bv post-intrusive faults. It is charac-
terizedby a distinctzonationof the affectedrocks (Korshinskv1965b).Beusand Zalashkova(1964) call this kind of alteration "late albitization". Late-magmatic albitites occur preferentially within alkaline provinces.Various conceptsas to the sourceof the solutionsarepresented.Derivation by magmaticdifferentiationas well as from deepseatedcrust or even mantle-derivedsolutionsare envisioned(Bonin 1982). Metamorphic albitites (Cb) are thought to have formed subsequentlvto progressivemetamorphism. Thel' typically occur within metamorphic units affected by migmatization and granitization (Tugarninov and Smevenkowa 1961;Kazanskyet al. 1978b,Omelyanenkoand Mineyeva1982;Belevtsever al. 1984)consider the Na-rich solutionsmetamorphicmobilisates. Mehnert(1960)showsthe possibleNa-enrichment dunng metamorphismand anatexis. Metamorphicalbititestypicallydisplaydistinct zonations.But, in contrastto the late-magmatic albitites, the formation of secondary minerals dependslargely on the original rock composition (Kazansky and Laverov 7977' Omelyanenko 198a).This group includesin part the spilitesof autometasomatic ongin (Amstuz 7914). (Cb) includealbititesof possibly Otheralbitites sedimentaryor diageneticorigin and the spilites of so-calledsecondaryorigin (Amstutz 19'14). Uraniferousalbitites. Based on the above presented typology of albitites and taking into account earlier classification schemes by Kazansky and Laverov
rocks Table2.9. Uraniumcontentof metasomatized Averageppm L Kitongo, Cameroun(Cambrian) Diorite Granodiorite Granire Arfvedsonite albitite L-eucocratic arfvedsoniteriebeckite albitite Melanocraticarfuedsoniteriebeckite albitite teucocratic nebeckite albitite Melanocraticriebeckitealbitite Aegirin albitite Mosquito Gulch, I,ionacho, NWT, Canada (Proterozoic) Albitite Knvoi Rog, Ukraine (Proterozoic) Biotite gneiss Granite and migmatite Microclinire (syenite) Coarsetabular albitite
Rangeppm U Fntsche(1986)
(ar'. of samples.) <5 <_5 tq
1.924 189 390 3502 a
<3-7 <3 3-E <3- 10 4-40 50- 1953C7e7) d-3tl
7-4W (-r7s6) 7-zffi (-4180) 4-24m
Fritsche(1986) Belevtsev(1980)
9 11 J)
<50 <100 <550 <550
GeochemicalDistributionand Abundanceof Uranium in Rocks and Waters
(1977) and Sarcia (1980), Fritsche (written pers. commun.) distinguishesthe following uraniferous albitite facies. o matic aI b itites especially Uranium in autometas rn association wrth alkaline intrusions. Nametasomatism and ore mineral concentration takes place simultaneously with the magmatic intrusion or shortlv afterwards. U tenors range from <10ppm to severai 100ppm and locally up r o a f e w 1 0 0 0 p p mU . ( T a b l e 2 . 9 ) Uranium in late-magmaricqlbitites. An obvious relationship to intrusion does not necessarily e.xist. Albitization and U distnbution are con.rolled by brecciatedand cataclasticfault systems ',r'ithin.at the contact with. or outside the magmatic body. The solutionscausingthe albitization are considered to be late intruding differentiates of a magmatic differentation process. U tenors are in the range of some lOppmU with local enrichments of several 1000ppm U. Uranium associatedwith metamorphic albitircs formed in areas of anatexis, metamorphism and rnigmatism. These albitites are often associated with deap-seated fault systems and. therefore the sodium-rich solutions mobilized by anatectic or metamorphic processes may be mixed with meteoric water or mantle-derived fluids or with both (Belevtsev 1980;Kazanskiy et al. 1978).The source of sodium and uranium may or may not be identical. U tenors average 10 to 50ppmU with local concentrationsof a ferv 1000ppmU.
JJ
Table 2.10. Uranium contentin natural waters
ppbu Oceans EnglishChannel Bay of Biscay Baltic Sea Black Sea CaspianSea Gulf of Mexico
0 . 3 - 3 . 3( a v e r a g1e. 3 ) 3.3 3.3 0.8-5.9 2.0 3 . 0 -1 0 . 0 3 .l 5
Rivers
0.03-5.9
Springs Thermal springs
0.20-.10.0 0.20-.r8.0
Groundwater ( average) Groundwaterrin Igneoussilicic terrane Igneousbasicintermediate
<0.10-.10.0 0 . 0 0 - 3 2 .(0a v e r a g4e. 5 ) 0.9) 0.00-9.2(average
IEITANC
Metamorphic terrane Sedimentary terTane Sand.gravel Sandstone, conglomerate Siltstone,shale Limestone, dolomite Mineralizing aquifen
(average 4..1) 0.00-37.0
(average 2.5) 0.00-74.0 (average 0.00-2100.0 2.?-26.2) (average 10.6) 0.00-69.0 (average 2.0) 0.00-33.0 10.0-400.0
by Arakel 1988;Culbertet ai. Fromdataandcompilations 1984:Dybek 1962;Maucher1962;Rogersand Adams 1969b: LScottand Barker1962
2.4.5 Uranium in Waters Sea and ocean waterscontain 0.5 to 10ppbU. Oceansaverage1.3ppb U (Tabie2.10). Terrestrial waters although normally Iow in uranium have valuesrangingwidely from lessthan 0.1ppb to inore than 1 ppm U. Surfacewaters usuallycontain little uranium(<1ppb). Uraniumin groundlvaters is commonlv one order of magnitude higher than in surfacewatersof the sameregion. Groundwatersaverageabout0.5 to 10ppbU but may carry increasedamountsup to 100ppbU or more in terranesunderiainby uraniferouslithologies hereby reflecting the lithologic uranium background.Watersrich in dissolvedsaltssuchas carbonate, chloride. sulfate. nitrate, and/or phosphate are particularly receptive for high uranium contents. Such chemically loaded groundwatersare typicallyencounteredin arid to
semi-arid regions. In the extreme case. such waters may condenseto brinescontainingextraordinary high amounts of uranium and other eiements,as for examplein the SearlesLake, California, which contains as much as several hundredppbU.
2.4.6 Uranium in Living Organisms and Their Decay Products Table2.11grvesthe contentsof uraniumin plants and animalsand their decompositionproducts.
y
2 Geocbemistry and Minerochemisrry of Uranium
Table 2.11, Uranium content in living organismsand their decomposition products (Boyle 1982). Materials
Flora Algae Aquatic plants Moss Lichen l,and plants L,andplants
Fauna Aquatic animals I.and animals Land animals
Uranium content (ppm) (range)
Remarks
0.5-50
In ln In In ln In
l-l)
0.5-5000 0.3-200+ 0.005-0.1 s-5000
dry matter dry matter the ash the ash dry maner dry matter; viciniq' of uraniferous deposits
0.2-0.5
In dry matter
0.003-0.05 0.01-0.08
In dry maner In dry maner: "icinify of uraniferous deposrts
Table 2.12. Uranium content in exoterrestrial marerial
ppbu Meteoites Iron Stonv iron Cbondrite Achondrite
0.003-21.9 0.005-19.0 8.0-240.0 1 . 5 -1 9 8 . 5
Tectites Average Range
1 . 0 - 2 . 0l.d 0 . 8 - 2 . 71. d
Glcsses
0 . 8 - 1 8 . 4l .G '
From data and compilations by Dybek 1962; Rogers and Adams 1969b
2.5 Uranium Provincesand Districts
Uranium occurrencesare not distributed randomly throughout the earth's crust. Instead, uranium concentrations,like those of other metals,are found in restrictedareasreferredto as metallogeneticprovinces. A uranium province may be defined as a broad, more or less conIn dry material; tiguous domain of the continentalcrust, where vicinity of youngerrocksof variablelithologies successively uraniferous deposits and provenancecontain uranium levels above In lignite as found; Lignite 30-7000 nonnal crustal abundanc€.When metal values vicinity of increase to concentrations of economicmagnitude uraniferous (deposit). form an province. they economic deposits Sub-bituminous 30-1000 In coal as found; The evolution of a uranium province begins vicinity of coal with an initial concentrationof uranium from uranilerous igneous differentiation in the respectivecrustal deposits section. Uranium continuesto be lithologically vicinit-v in Bituminous ln of and up to 500f) glanifglgg5 (in ash) residue, redistributionor introducenrichedby successive deposits humate, etc. tion over periodsof time in someregionsrunning Thucholite, In vicinity of and in up to 20"/. to hundreds of millions of years. This applies (in ash) ucholite, uraniferous equally to concentrations and depositsin sedietc. deposrts mentary, metamorphic, and anatectic igneous rocksforming a province.Someevidencesuggests that the episodic redistributionof the uranium may be relatedto orogenicor epeirogenicevents 2.4.7 Uranium in Exoterrestrial Rocks and associatedprocesses. The oldest unit of such a cycle constitutes Table2.72provides,for completeness, somepub- the factual initial sourcefor the uranium in the lishedanalyses on uraniumtenorsin meteorites. younger rocks. Therefore, its uranium endowtectites,and exoterrestrialglass. ment must have been present,partly at least, in labile form, so that subsequentendogenic or exogenicprocesses could extractand redistribute it into the younger rocks of the province, under favorableconditionsto gradesand tonnagesof depositsize. Decomposition producu Peat 0.05-3 Lignite 0.05-3 0.05-3 Coal Petroleum 0-5ppb av. 1 ppb Rock asphalt andbitumen av.1000ppb Peat and muck 5-40m
ln ln ln In ln
peat asfound iignite as found coal as found petroleum as found asphalt asfound
Uranium Provincesand Districts
35
In essence,an economicuranium province Brazil orosenicbelt requiresa favorableor fertile uranium-bearing - Atlantic Shield/Brazilian sourceenvironmentfrom whereuraniumcan be - ParandBasin mobilizedto form economicallyviable deposits Bolivia- Peru in younger host environments,provided that - Andeanvolcanicbelt and/or thermodynamic phvsico-chemical condit- Mexico ions were adequateto promote the required - Sierrade PenaBlanca.Chihuahua concentrationprocesses(for more details see - La Sierrita- BurgosBasin,Nuevo Leon e.g., in IAEA L988, Recognitionof Uranium Provinces,and Toens1981). Afica Uranium districts(economic)may be defined Algeria- Niger as areas within uranium provincescontaining - Hoggar- Air Massif- Agades Basin clustersof deposits,perhapswith the restriction Gabon :c depositsof the sametype. - Massifde Chaillu- FrancevilleBasin The accompanying listingof uraniumprovinces Namibia (for locationsee Fig. 1.1) marks most of them DamaraBelt bv discovered depositsor at least noteworthv Sambia- Zaire occurrences.It should be noted that the tabling - KatangaSynclinorium does not include all existing provinces but is South Africa restricted to presently known and prominent - WitwatersrandBasin- Kaapvaa! Craton uranium provinces. On a worldwide basismost of the prominent uranium provinces are associateddirectly or Austalia - Pine Creek Geosyncline,Northern Territory indirectly with Precambrian terrane, and in - Westmoreland- PandanusCreek, Northern Europe additionallywith the Hercynianorogenic Territory - Queensland belt. - Mount Isa Geosyncline,Queensland - GeorgetownInlier, Queensland North America - LachlanFold Beit, Queensiand Canada Arunta Block - Ngalia Basin - Amadeus - Athabasca Basin - Uranium City region. Basin,Northern Temtory Saskatchewan Lake Frome Embayment,South Australia - Eiliot Lake - Quirke Lake - Blind River, - Stuart Shelf. SouthAustralia Ontario - Yilgarn Block, WesternAustraiia - Bancroft, Ontario - Great Bear Lake, NWT - Baker Lake - ThelonBasin.NWT Asia - Makkovik - SealLake, Newfoundland China - Kelowna- Beaverdell.BritishColumbia - Zhungel- Tianshan,NW, China - Yinshan- Liaohe,NE China United States - Colorado Plateau - Qilian - Qinling, CentralChina - Wyoming Basins- Black Hills - WestYunnan.S. China - South Texas CoastalPlains - SouthwestGuizhou.S. China - Colorado Rocky MountainsiFront Range. - JiangnanOld Land, SE China Colorado India - Northern Rocky Mountains/Jv{ount Spokane - SinghbhumThrust Belt Region, Washington-Idaho Japan - Appalachian-Piedmont, North Carolina- - Central and western Honshu (Tono-Ningyo Virg:nia-Pennsvlvania toge) - Alaska Panhandle/BokanMountain, Alaska Kazakhstan - Pribalkhashsky Latin America Kokchetavsky Argentina Pakistan - Sub-AndeanZone - EasternBaluchistan(BaghalChur)
36
2 Geochemistryand Minerochemistryof Uranium
Russia - Streltsovsky,Transbaikal Uzbekistan - Kyzylkumsky - Karamazarsky Europe Poland - CSf'n - Germany- France- Spain Portugal Moldanubian and Saxo-Thuringian metallogeneticzone of the Hercynianorogenicbelt and enclosedand adjacent continental sedimentary basins,including - BohemianMassif,Poland- CSfn - Germany - Black Forest- Voges,Germany- France - Massif Central - Armorican Massif, France - Cornwall, Great Britain - Iberian Meseta,Spain- Portugal Bulgaria - Western and eastern Balkan metallosenetic zone Russia - Caucasusregion Sweden - Svecofennian belt (Arj eplog-ArvidsjaurSorsele) - Hotagen- Olden Window Greenland - Ketilidian Belt. south Greenland Ukraine - Krivorozhsky/Krivoj Rog region
and geosynclines and shallow water basins (after2.2b.y.). Ferguson(1988),Nashet al. (i981), and IAEA (1986)reviewedthe evolutionof the earth'scrust and related uranium distribution and arrived at the followingconclusions; lst Period- prior to about2.8b.y.: Characterized by protocrustdevelopment. The early Archean crust of the earth was dominated bv mafic and ultramafic, mainlv komatiitic rocks and minor felsic rocks as reflected by the abundanceof greenstonebelts in old cratons.The originalchemicalcompositionof the greenstonesis thought to have been comparable to modern oceanicbasalts. The earl-v Archean rocks have concentratedU and Th bi' a factor of five comparedto chondriticabundances but averagelessU, Th, and K than voungerrocks of equivaient facies. It seemsthat during this period of the earth, neither uranium concentrations nor recognizableuranium provinces developed.The first noticeableintroduction of radioelementsinto the crust occurred with the generationof sodium-richmagmas,at about 3.8 to 3.3b.y. ago (Ferguson1988).
2nd Period- ca. 2.8-2.2b.y.: Characterizedby cratonization, granitization, development of intracratonicbasins,appearance of marine organisms (algae),anoxicatmosphere. This period is characterizedby the evolution of two major regimes. (a) Development of potassium-richsalic magmasby subduction and recycling of crustal material at about 2.8 to 2.6 Crustal Evolutionand Related .y. ago, by fractionationto a first noteworthy 2.4b Uranium Distribution enrichmentof the radioelements in granitic rocks and therebvto the initiationof uranium provinces Four prominent periods of the crustal evolution, (seelater). Theseuraniferous graniticcomplexes partitioned at about 2.8,2.2. and 0.4 b.v. ago mav constitute the source for subsequenturanium be distinguished with respect to a characteristic recvcling in various geologicaienvironments. (Examples:Yilgarn Block. Australia: Superior behavior of uranium. After the initial cratonization of the earth Province. Canada: Kaapvaal/KalahariCraton. and except for magmatic processes,the mode of South Africa; 56o FranciscoCraton, Brazil). (b) riobilization and lithologic distribution of uranium Development of the first major intracratonic during these periods is pronouncedly [nked to basins became possible after prerequisirory distinct surface-related exogenic conditions and cratonization of the earth's crust during the environments, namely anoxic atmosphere (prior late Archean-early Lower Proterozoic. The to ca. 2.2b.y.) oxygenated atmosphere (afrer basinswere filled with clasticsediments.mainlv approximately 2.2b.y.), evolution of marine life arenites, including oligomictic quartz-pebble forms (since ca. 3b.y.) and larer of land plants conglomerateswhich accumulatedsignificant (since ca. a00m.y.) (Fig. 2.7), development of amountsof detrital uraniniteand other heavy early intracratonic basins (prior ro 2.2b.y .) minerals. Crucial for the mechanical trans-
Crustal Evolution and Related Uranium Distribution
0, bound
( 0 2 )= r 0 . 5 3e M , e { -1e
37
rn FerO,
{-39%l
{tr=1.16,10ey-l) O, bound rn SOa2t-56%) Rtse of O2 level in otmosohere-oceon s\6tem
T.45
1..0
Mrlestonesoi , oroonic evolutlon D i s t r i b u t i o no f contrnentol red
0, in otmosphere / oceon system l-.:./.| /
stort of sedrmentdry record
I 30 ,
a \l,J
20
100 ^ \Q
^ u \J/ C./4,
b.y ogo I
Distribution of bonded ironstone Predominent U tronsport S e d i m e ni o r y U ossocrotion
:Corboncceousisemr-loelrte orgoe. elc. denveo orgonrc
1. from prior to@morine btock shote ond phosphorite 2 . o f t e r @ c l o s t E s e d i m e n t sw i i h p l o n i f r o g m e n t s Fig.2.7. Evolution of the Eanh's atmosphereand organic life and associatedconditionsfor uranium transportand most rar.orabledepositional sedimentaryenvironments.Arrow in b indicatesthe start of sedimentaryrecord. a Evolution of rhe Earth's iotal budget of phorosyntheticoxygen, based on the terrestrial tiC balance and iurrent conceptsfor the accumulation of the stationary sedimentarymass as a function of time (Schidlowski et al. 1975). Stationaryoxygen reservoirs existing at times T are expressedas fractions of the present reservoir (O:),p. M,p is the present sedimentary masswith tp = 4.5 b.y. Column on the right sideshowspartitioningof oxygenberweenthe "bound" and "free" reservoirs in the present budget (Li 1972). with molecular oxygen accountingfor about 57o only. A disproportionationexists between bound and free reservoirs during the Precambrian and earlv Paleozoic. most probably due to a short circuit of the ancient oxygen cycle in the Precambrianseas.Tentative rise of free oxygen level in atmosphere-ocean system(r/ack field) is inferred from paleontolosicaiand geologicalevidenceshown. b Milestonesoforganic evolution indicatedare 1 appearanceof oldest algal bioherms (photoautotrophicblue-greenalgae),2 appearanceof eukaryotic biota, J appearrnce of oldest eumetazoanfaunas:.l life conqueringcontinents(Upper Silurian):5 appearanceof exuberantcontinental ,ioras (Upper Carboniferous). (After Schildlowskiet al. 1975:Schildlowski1981)
port oI uranlnlte gralns was a nonoxlozlng atmosphere, which prevailed prior to about 2.2b.v. (Examples: Huronian Blind River - Elliot Lake - Quirke Lake Basin. Canada; Witwatersrand Basin. South Africa: Serra de Jacobina rnd Quadriiatero Ferrifero. Brazil; Nullagine Conglomerate.Australia; PaiimbaConglomerate. Jenisseiprovince, Russia). The presence of contemporaneous Lower Proterozoic banded iron formations has occasionailv been cited as counter-evidence of an anoxic atmosphere at the time of uraniferous conglomerate deposition. However. as Schidlowskiet al. (1974), and others poinr our, the first oxygen produced by photosynthesis of early life forms (green algae) was in seawaters. Over a long time span the oxvgen was consumed for oxidation of reduced comDounds abundant in all oceans,
particularly ferrous iron. In restricted manne basins where orygen generationwas early or more advanced.oxidized iron couid precipitate as banded iron formation, whereas in coeval intracratonic basins uraninite placers couid accumulate. by a 3rd Period- ca.2.2-0.4b.y.: Characterized stabilizedsialiccrust,developmentof geosynclines andshallowwaterbasins.marinemicroorganisms (algae),oxygenatedatmosphere. The early stage of this period (ca. 2.2 to 1.9b.y.) is characterizedby a rigid crust and and large developmentof the first geosynclines. intracratonicshallow water or lagoonalbasins. Contemporaneously,a rapid spread of marine microorganismsis postulated coupled with a progressivelyincreasing photosyntheticoxygen
38
2 Geochemistry and Minerochemistryof Uranium
generationto present-dayoxygen levels in the Niger; Parand Basin, Brazil: North Bohemian Germanr': Kyzylkumsky, atmospherestarting sometime between2.2 and Basin, CSFR 1.9b.y. ago. Large-scaleoxidizing conditions Uzbekistan;JinganBasin,China. Uranium from uraniferousarkosesand similar evolvedand oxygenatedmeteoricwatersbeganto dissolveuranium and transportit in solutionin its rockswasfurther recycledby magmatic-anatectic hexavalentstate.mainly asuranylion. Part of the processesinto highly uraniferous leucogranites liberated uranium traveled into shallow water and relatedhydrothermaldeposits. basins, where it accumulatedin carbonaceous Example: Hercynianorogenicbelt, Europe. pelite, psammite,and chemical(carbonate)sedi-ments.Precipitationof the hexavalenturanium occurredprimarily where marine microorganism References and Further Reading for (algae)generateda reducingenvironment. Examples:Aphebian basins(Wollaston.Amer Chapters2 and 3 Lake, Hurwitz, Aillik groups). Canada: Pine (for detailsof publicationseeBibliography) Creek Geosyncline,Australia; FrancevilleBasin, Gabon. phosphoritesactedas uranium Adams 1954;Adams et al. 1959;Adamson 1983:Adamson In geosynclines and Parslo*' 1985; Adler 19771Ahmad and Wilson 1981; collectors (Example: Upper Jatulian Group, Ahrens 1965; Alekseyev et al. 1981; Andreyev and Finland). Chumachenko1964:Arribas 1986:Ashiey 1984:Aumenro These uraniferous sediments became the 1979; Avital 1983; Ball and Basham 1979: Banas and Mochnacka 1989;Barbey and Cunel' 1982; Barbier et al. source for subsequenturanium enrichmentsin 1967; Barbier and Ranchin 1969a. 1969b; Bell 1985; youngerrocks or in deposits,either by endogenic Barreto 1988; Banhel and Mehnen 1970: Basham et al. magmatic-anatectic or metamorphic,or by exo- 7987a,1982b,1982c;Belensev 1980:Belewsev et al. 1984; Berman 1957; Berthelin et al. l9&4; Berzina et al. 1980; genicincludingdiagenetic processes. Betekhtin 1959:Binns et al. 1980;Bohse et al. t974;Boyle Examples: Beaverlodge,Athabasca,Kaipokok 1982,1984;Breger 1974;'Briot 1982;Brooker and Nuffield Bay - Big River districts, Canada; Alligator 1952; Brookins 1980; Brynard and Andreo[ 1988; Burt Rivers, Rum Jungle districts, Australia; Pan- and Sheridan 1980, 1985; Burwash and Cummins 1976; African and Brazilian mobile belts, Africa- Butler 1975:Bun 1988:Calas 1979:Carlisleet aI 1978; Cathelineau 1981a,1982, 1987a;Catbelineau et al. 1982, South America; SinghbhumThrust Belt, India; 1985, 1987; Cevales 196l; Chen Thaobo 1981; Chen Arjeplog-Arvidsjaur province, Sweden; South Zhaobo et al. 1982;Chen Zhaobo and Xheng Fang 1985; Greenland province, Greenland; Krivoj Rog, Chernokov 1982; Civetta and Gasparini 19721.Clark et al. 1966; Collot 1981; Cordtunke 1969; C.onini 1984; Cuney Ukraine. 4th Period - 500/400-0m.y.: Characterizedby euxenicbasinsand developmentof land plants, and a crustalconfigurationas in the third period. During the Cambrian to Silurian uranium accumulatedin euxenicbasinsto form low grade uraniferousalum or biack shales.Examples: Billingen- Ranstad,Sweden:Chattanooga Shale, USA; Ronneburg,Germany. With the adventof landplantsin the Devonian and their incorporation into continental and marginal marine sedimentsfrom then on, the previous geochemical-metallogeneticregime changed. The vegetalorganicmatter was able to create reducing conditions in their host sediments capableof reducingand collectinguranium from circulatingground and surfacewaters. Examples: Colorado Plateau, Wvoming Basins,South TexasCoastalPlains,USA; Lake Frome Embayment,Australia; Agades Basin,
1982,1985;Cuney and Friedrich 1987; Cuney et al. 1981, 1984, 1989; Coppensand Bernard 1978; Dahlkamp 1982, 1985; Dall'Aglio et al. 1974; Darnley 1981; Darnley et al. 1977; Davidson and Cosgrove 1955; Davidson and Ponsford 1954t Dekken et ai. 1983b: Dill 1980. 1983a, 1983b, 1985; Dimroth and Kimberley 1976; Doi et al. 1975; Dostal and Capredi 1978; Dubessy et al. 1987; Dubinchuk and Siderenko1978.1980: Dvbek 1962: Dvck 1978:Dyck and Tan 1978rEthridgeet ai. 1980:Eweri er al. i984; Fehn et al. 1978:Ferguson1987. 1988;Ferguson et al. 1980a:Finch et al. 1980;Flehocet al. 1989;Fowler and Doig 19791Friedrich 1984,1986:Fnedrich et al. 1987; Fritsche et al. 1988; Frondel 1958: Frondel et al- 7967; Fuchs 1989: F,1fe 1979: Gabelman 1970, 1977; Galer and O'Nions 1985; Galloway 1978: Garrels and Chrisr 1959, 1965; Garrels and Larsen 1959; Geffrov and Sarcia 1955.1958,1960;Georgeet al. 1986:George-Anielet al. 1985; Gerasimovskiy 1957; Getseva 1963: Giere 1986; Goldhaber er al. 1979. 1987; Goodell 1985b; Goodell and Trentham 1980; Gorbunova 1974: Gosnold 1976; Grandstaff 1916; Granger 1976;Granger and Finch 1988; Grangerand Warren 1969,1974,1978,1979;Grauch 1978; Grauch and Zarinski 1976; Gruner 1956; Gruzder and Rubtsov 1972; Guineberteau 1986; Gustafson and Curris 1983; Haack 1983; Hambleton-Jones1976, 1978, 1984: Hambleton-Jonesand Smit 1984; Harlass and Schiitzel 1965; Harshman 19'14;Hedges et al. 1984; Heier 1979;
CrustalEvolution and RelatedUranium Distribution Heier and Adams 1965:Heier and Rhodes1966;Heinrich 1958; Hoffmann 1943a, 1943b; Hostetler and Garrels 1962;Hsi and Langmuir 1985;Hunter and Michie 19881 Husmann1956;Hutchinsonand Blackwell1984:Jennings 1976;Jurain and Renard 1970a,1970b;Kazanskyet al. ,')68. 1974. 1975. 1976, 1978a,1978b;Kaspar and Hejl 1970:Kazdan i978; Kimberley (ed.), 1978;Klepper and W1'ant1957; Kochenov et al. 1965,1978;Komarov and Shukolvukov 1966; Korolev and Rumvantseva 19761 Kostov 1977;Kryloy and Atrashenok1959:Kuznetsovet aI. 1975; Lambert and Heier 1967, 1968; Lambert et al. 1976;Landais i989; Landaiset al. 1987:Lang et al.1962; Langmuir 1978; Larsen et al. 1956;Larsen and Gottfried 1961:Larsen and Phair 1954: Le 1975; Lemoine 1975'. Lerov and Holliger 19841Leventhalet al. 19861Levrnson i980: Li et al. 1980;Liebenberg1955;Little 1970;Little :r al. 1972; Locardi l9'/7. i988; Lyons 1961, 1964; .Iehaderan 1988; Mahod 1983; Makarov et al. 1960: .rlaian 1972: Mann and Deutscher 1978; !{arjaniemi and Basler i97l; Marlow i983; Maruejol and Cuney 1985; Maucher1962;Maurice(ed.), 1982:McKelvevet ai. i955; McMillan 1977a. 1977b; Mclennan et al. 1980:Melgunov et al. 1975;Mel'nikov and Berzina 1973;Meunier et al. 19871Miller and Kulp 1963;Mineeva 1984:Mineeva and Tarkhanova 1964; Minter 1976; Modnikov et al. 1978: Mohagheghi 1985; Moore 1954, 1967; Moreau 1977; lloreau et al. 1966;Morton and Sassano1972:Nakashima et al. 1984;Nash et al. 1981;Naumov 1959.1961,1973, 1978;Naumov and Mironova 1965.1969a.1969b;Naumov et al. 1971;Needham and Stuart-Smith1984:Needhamet al. 1988; Negga et al. 1986; Nekrasova 1958; Nininger 1977; Nguyen-Trung 1985; Omel'yanenko and Mineeva 1979,1982;Orajaka 1981:Ostle 1982;Pagel 1979.198la, 198lb, 1982a.1982b,1983;Pageland Pironon 1986;Pagel and Ruhlmann 1985:Pearceet al. 19841Penlik et al. 1969; Petrov et al. 1972;Phair 1952, 1975:Pironon 1986;Pliler 1956; Poty et al. 1986; Poty and Nguyen Trung 1989; Pretorius 1976; Rackley 1976; Rafal'skiy 1958, 1979; Rafal'skiy et al. 1979; Rafal'skiy and Odipov 19671 Ragland and Rogers 1980: Rakotondratsimba1983; .iamdohr 1957. 1961, 1980; Ramaekers and Hartlinq
39
1979: Ranchin 1970, 1911, Renard 197-1;Revnoldsand Goldhaber 1978.1983;Revnoldset al. 1977,1982,1986; Rich et al. 1971.Richardson 196-1;Robb 19861Robenson 1989;Robertsonet al. 1978:Robinsonand Spooner198.1; Rogers1947;Rogersand Adams 1969a.i969b; Rogerset al. 1965. 1978: Rogers and Ragland 1961; Romberger 1984; Rosholt 1983: Rosholt and Banel 1969; Rosholt et al. 1971. 1973: Roubault 1958: Rouzoud et al. 1981; Rozhkovaet al. 1958:Ruhlmann 1980.1985;Rytubaand Glanzmann1978;Saagerand Strupp 1983:Samama1984: Schidlowski1976;Schmidt-Collerusl9?9: Scott and Scott 1985;Sergeyevaet al. 1972;Shatkovet al. 1970;Sibbald i988: Siderenko1958:Silveret al. 1981:Simov1988.19f19: Simpsonand Hurdler' 1988; Simpsonet al. 1976,1979, 1982a,1982b;Simpsonand Plant i98-1:Simpsonand Yu Shiqing1989:Smellie19821Smellieet al. 1978;Smithet al. 1982: Smystov 197.1:Sobolewa and Pudow Kina i957; Sorensen1970:Starinskvet al. 1982:Stevenet al. 1981: Stuckless 1979. 1987: Stuckless and Ferreira 1976: Stucklessand Nkomo 1978;Stucklesset al. 1977,1981a, 1981b; Swanson 196{1.1961; Szalav 1964; Szalay and Samsoni1969;Tarkhanov and Poluarshinov1989;Tauson i954; Theis 1979;Titov et al. 1975:Thomas 1983;Toens 1981:Toens and Andrews-Speed198-f:Trentham 1981; Treuii 1985;Treuil and Joron 1989;Tusarinov L%3,1975; Tugarinov et al. 1964:Tugarinov and Naumov 1969,1974; Turekian and Wedepohl 1961;Turpin et al. 1989;Tweedie 1979: von Bontel 1984: Voultsidis and Carsten 1978; Walton 1978; Waltoo et al. l98l; wanty et al. 1987: Waters and Granger 1953;Watson and Plant 1979;Watson er aI. 1982;Weber et al. 1985;Wedepohl 19741Weeksand Thompson 1954: Wenrich i985; Wesmrm and Gronvold 1962; White and Martin 1980; Whideld et al. 1959; Woodmansee 1976; Wyllie 1979: Xu et al. 1981; Yhao 7*nkal et al. 1986: Yeliseyeva 1977: Yermolayev 1971, 1973,1975;Yermolayev and Zhidikova 1966;Yermolayev et ai. 1965: Yeliseveva and Omel'yanenko 1976: Yevseyevaet al. 197-1:Zhuravleva et al. 1976; Ziegler 1978; Zielinski 1978. 1979, 1982, 1983- 1985;Zelinski et al. 1987;Zimmer 1986: (see aiso referencesin chapter4) (for extensivereferencesup to 1982see Boyle 1982)
3 Principal Aspectsof the Genesisof Uranium Deposits
O u r k n o w l e d g e o f t h e m e t a l l o g e n e s i so f m a n v uranium deposits, although believed to be globally or principaily understood, is in manv cases meager, dubious. or ambiguous. This becomes obvious rvhen trying to reconcile conflicting data from various geoscientific disciplines on the .ame subject. Some depositsare rvell researched. . .hers. including important ones. are not. In . ; r h e r c a s e s ,a c o m p l e x . p o l v p h a s ee v o l u t i o n o f deposits greatly hampers the deciphering of the actual origin of the mineralization. Based on theoretical and empincal deductions and interpretations of the available data. interesting concepts and sound models on the metallogenests of uranium deposits have been forwarded by many knowledgeable geologrsts.In spite of this, :he factual origrn of many deposits is still entirely or partially enigmatic. The principles of metallogenesis are presently sufficiently well understood only for some types of deposits including Lower Proterozoic quartz-pebble conglomerate, rollfront sandstone, intrusive, phosphorite, and certain surficial deposits. In addition. the genesis of the Hercvnian, Limousin-type vein deposits has been :iiltisfactorilyestablished.Ail other uranium ore ieposits are understood to a lesser extent and in varving degrees. This chapter provides a svnopsison the global and principai processesinvolved in deposit formation. Metallogenetic hypothesis on specific types of deposits and individual depositsare presented in Chapters 4 and 5 respectivelv.Selected References and Further Reading for Chapter 3 ire inciuded in those of Chapter l.
l. Primary uranium deposits formed by endog e n i c m a q m a t o q e n i c / a n a t e c tpi cr o c e s s e s . 2. Secondary uranium deposits formed by subsequent exogenic. i.e.. superqene and/or sedimentary (including diagenetic) processes from the primarv t,vpes. 3. Tertiary uranium deposits formed by endogenic metamorphic processes from primarv a n d s e c o n d a r vd e p o s i t s . This cycle closes if deposits of the secondarv and tertiary categories are retransformed by anatexisor palingenesisinto those of the primary category. On the other hand. exogenicprocesses may liberateuranium from metamorphic environment and reconcentrate it to form secondary category deposits. Figure 3.1 gives an overview of the three categoriesof deposits and their genetic relationships.Figure 3.2 illustratesdiagrammatically the geological position of uranium deposits and the presumed migration of U in deposit formation. The tbrmation of uranium deposits, however. is not restricted to these principai genedc regimes. Instead, complex poivgenetic processesmay be responsiblefor the senesisof many deposits. Some of the deposit tvpes cannot be attributed either to the one or the other category. They will be dealt with under "mixed categorv" deposits. The formation of uranium deposits and the implications involved in the various inferred metallogeneticprocessesmay be summarized as follows:
l. Primarv uranium deposis: The ongin of the uranium in primary mineralizations is reasonably well established as either juvenile magmatic, prevaiiing in Early Precambrian. or anatectic/ 3.1 The Global MetallogeneticCycleof palingenetic.prevaiiing in vounger times. Formationof Uranium Deposits To the primary deposits belong sensu stricto onlv the syngenetic (intra)intrusive deposits in a giobal sense. formation of uranium deposits (type 9. Chap. l). (Examples: Rossing,Namibia; ::an be attributed schematicallyto three genetic Paiabora, S. Afnca: Kvanetjeld. Greenland: categories. each related to one of the three domi- Bancroft, Canada) and, sensu lato. epigenetic nant regimes of the geoiogcai cycle of crustal vein deposits(subtype 3.1. Chap. -l) which comevolution. These catesoriesare: monly are of polyphase origin (see Chap. 5.3).
AA
3 Principal Aspects of the Genesisof Uranium Deposits Mode of origin
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Metosediments r C o l l o p s eb r e c c i o -A r i z o n oS t r i p .U S A Breccio I I pipe ( l ot e r o l m i g r o i i o ne 1 C o m p t e xb r e c c i o { ? i- 0 l y m p i c D o m . A u s t r o l i o U n c o n f o r m i t yc o n t o c t - A t h o b o s c oB o s i n .C o no d o Sondstone,/ m et o s e d i m e n t S u b u n c o n f o r m i t{yp o r t i o l l y )A- l l i g o t o rR i v e r s . { d i og e n e t i c)
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The Global MetalloeenicCvcleof Formationof Uranium Deposits MoCe ot ortgtn
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Fig. 3.1. Schematicmetallogeneticcorrelation of uranium depositsand occurrences.Note: Some of the depositsare gradationalinto eachother. others are of polyphaseorigin, and of a numberof deposittypesthe provenanceis uncertain. These deposits are here attributed to processeswhich supposedlygeneratedthe final'stage of mineralization and/or deposit configuration
M
3 Principal Aspects of the Genesis of Uranium Deposits 3-ll
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Lignite l1/.) Block shole {15) lgneous U source rock U ' o n i n i t e ,u r o n o t h o . i o n ' t e U rmpregnotion P . p q r r m p nt r n n q n n r ?r i ; . p c t i o no ' D i k e . B = b o s i c .P : p e g m o t i t i c Pelitic sediment Psommiiic sedi:'nent Psephitic sedrment Foult with drsplocementdirection Re g o l i t h l nt r u s t v e L o t e o r p o s t - r n i r u s i v eh y d r o t h e r m o l MetomorphichyCrothermol Exogenic Diogenetic hydrothermol
Fig. 3.2. Schematicdiagram of regional structural and lithologic settingof uranium depositsand occunences and the hypothetical migration paths of uranium to form the deposits
The Global MetalloeenicCvcle of Formationof Uranium Deoosits
( E . x a m p l e sP: i i b r a m . J d c h v m o v .C S F R ; L i m o u s i n and Vendde.France). I f m a g m a t i c / a n a t e c t i ec v e n t s g e n e r a t e m e t a s o m a t i s m . s o m e m a r g i n a l o r t r a n s i t i o n a ls t v l e s i';i nrineralizationmay develop. Some auto.r.etasomatic and contact-metasomaticdeposirs rre probablyof this nature(type 12. Chap. 1). 2. Secondarv uranium deposits: These deposits owe their existence to exogenic processes.Two careqoriesof depositshave formed. distinguished b v s v n - a n d e p i g e n e t i cu r a n i u m e m p l a c e m e n t rTable 3.1). The Lower Proterozoicconglom: : : i r e . t h e b l a c k s h a l e . a n d t h e p h o s p h o r i t ed e ' . , r i L sa r e o f s v n s e d i m e n t a rnva t u r e . T h e v a r i o u s r,ineralizations in sandstone, surficial. sedimentary breccias,and lignite types are epigenetic. .\ further division can be drawn bv either detrital or chemical uranium transport. Prior to the oxygenation of the earth's atmosphere (older than about 2.2b.y.), mechanical rveathering and sedimentation formed placer ,:eposits with detntal uranium minerais in oli{omictic quartz-pebble conglomerates (type 7, Chap. 4). (Examples: Witwatersrand, S. Africa: Elliot Lake, Blind River, Canada). After the change to an oxidizing atmosphere, which occurred at the latest during the middle of the Lower Proterozoic (approximately 2.2b.y. ago) (Roscoe 1969; Frarey and Roscoe 19701 Schidlowski et al. 19'74,see also Chap. 2 and Fi_e.2.7) chemical liberation of uranium from a ..irietv of uraniferous source rocks (granites. ieisic volcanics, metasedimentsetc.) led to concentrations of uranium mineralization in suitable litho-chemical environments. Dependent on ciintatic and transport conditions uranium was (a) either transported into the sea and concentrated svnsedimentarvin black shales and phosphontes (type 15 and 10, Chap. a). (Examples: R.anstad.Sweden; Land pebble district. Florida -nd Montpeiier, Idaho, USA, respectively) or ib) concentrated epigeneticallyinto terrestrial sediments in intracratonic and intermontane basins (tvpe -1. Chap. a) (Examples: Wyoming Basinsand Colorado Plateau.USA). occasionail;tn littoral, marginal marine clastic sediments (Example: South Texas Coastal Plains) and in surficial deposits (type 6, Chap. a) (Examples: Yeeiirrie. Australia: Langer Heinrich, Namibia: Stevens Countv, USA). Part of the sedimentbound uranium, however, may also have been introduced bv geothermalsystemsassociatedwith
-
15
volcanism (subaquatic volcanic metalliferous exhalations'l) as suggestedfor the breccia complex type deposit Olympic Dam. Australia (type 8 , C h a p .4 ) . Localization and formation of second categorv depositsrequire a combination of distinct regional and local metallogeneticconditions. Uraniferous source rocks combined with adequate paleotopography.paleoclimate.paleoweathering, paleobiology (evolving of life forms). paleohydrology, and tectonicsare essentialto produce f a v o r a b l eh o s t e n v i r o n m e n t sa n d t o p u t u r a n i u m i n t o s o l u t i o n . T h e c o i n c i d e n c eo f t h e s e f a c t o r s leads to suitable hydrodynamic and chemoph1'sical regimes (solution/groundrvater florv. redox interface etc.) and uranium precipitation. Deposits of economic magnitude formed only where the host provided sufficientopen spaceto allow the precipitation of sizable concentrations of uranium. 3. Tertiary urqnium deposits:The metallogenesis of tertiary uranium mineralizationsis still vague. Relatively little is known about the behavior of uranium dunng metamorphism. In a generalized approach, two thermodynamic systems may be envisaged, the first a closed, the second an open system. A closed s))stemrefers to a crustal unit affected by a metamorphic event of more or less in situ mobilization and recrystallization without marked introduction or loss of elemental constituents. In contrast. an open system is reflected by brittle deformational ground preparation (dilation faulting) and internal andior external mobilization and redistribution of sensitive elements by metamorphic hydrothermal fluids. In a closedsystem,formation of mineralization reiies on the intrinsic uranium inventory of the protolith. For example.if sandstonetvpe deposits such as that of Okio. Gabon. which is of the Lower to Middle Proterozoic age. undergo iow to medium grade metamorphism (below upper amphibolite-granulitefacies) in a closed system. one might expect to find a stratiform synmetamorphic mineralization(type 13. Chap. -l) such as at Forstau. Austria. Uranium tends to move oniv in the range of millimeters to perhaps meters, as indicated by its stratiform disseminated distribution with preferential but not exciusive atfinities to mafic minerals and sometimes in fold noses. The only difference between greenschist facies and amphibolite facies _qradeof metamorphism tends to be in a general way the degree of crystal-
46
3 Principal Aspects of the Genesis of Uranium Deposits
o
o
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The Time-RelatedOccurrenceof Uranium Deposits
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48
3 Principal Aspects of the Genesis of Uranium Deposits
lization. Pitchblende occurs in the former and uraninite in the latter (see also Chap.2). In an open system, where metamorphic hydrogenic fluids can migrate, veinlike (metamorphic hydrothermal) deposits may be initiated such as type 2, (Chap. 4). perhaps in a late orogenic phase. The fluids may leach uranium and other elements/compounds (COr'*, SO.t* etc.) by percolating through uraniferous metasediments or other fertile rocks and transport it to favorable stmctures. Here. uranium could be precipitated from the solutions by mechanisms comparable to those of vein generation (type 3. Chap. 4) (changes in P-T, eh, and/or pH conditions). These processes ma)' or may not be associated with metasomatism, such as albitization etc., of host environments. If metasomatism is invoived. deposit types transitional to those of the metasomatic deposits mav evolve. In many cases. processes, for instance triggered by epeirogenic movements associatedwith or without dike intrusions. or diagenesisof overlying sediments may generate remobilization regimes with addition or removal of uranium. Under these conditions, recrystallization of precursor mineral assemblages will occur and camouflage initial parageneses to a degree depending on the intensity of the overprint. In the extreme case it may result in a completely new mineral assemblage.As a consequence, the archetype of the deposit cannot be identified. Instead, these deposits may appear disconnected of metamorphism and related to the younger processes. Deposits of the subunconformityepimetamorphic type (type 2, Chap. 4) belong to this category. (Examples; Beaverlodge district. Canada. Ranger One. Alligator Rivers district, Australia) "Mixed category]' deposits are of two modes. One group cannot be clearly assigned to anv one of the main regimes of the geological cvcle and constitutes a transitional category. It includes metasomatic. certain vein and volcanic type deposits (type 12, 3.2. and 11 respectivel),, Chap. 4). The other group includes deposits evolved by' ambiguous'frequently multistage processes,the individual stages of which often being difficult to decipher. This group inciudes particularly unconformity-contact, certain subunconformityepimetamorphic and collapse breccia pipe deposits (ti'pe 7, 2 and 5 respectively, Chap. 4). Mechanismssussestedfor the formation of such
deposits range from supergene concentration, diagenetic hydrothermal to hypogene hydrothermal and unknown hydrothermal processes. Either one or a combination of these processesis suggested by the various investigators, partly dependent on individual preference and observations. Finally, it should be mentioned that metallogenv as related to plate tectonics has been applied to explain the creation of uranium provinces and the formation of certain types of uranium deposits. The regional location of these deposits and of uranium provinces as viewed in their total geological context appears to be connected with plate tectonics. Fyfe (1979), Gabelman (I977a, I977b). Toens et al. (1980), and IAEA (1986) among others have reviewed this topic. Gabelman (I977b) notes that regional mineralized zones are associated with tensionai taphrogenic tectonics. He aiso expresses the opinion that a close spatial and genetic relationship exists between epigenetic uranium occurrences and circumcontinental mobile belts. Favorskaya (1977) obviously shares this view and puts forward a hypothesis according to which polystage magmatic complexes and mineral deposits had their origin at great depth.
3.2 The Time-RelatedOccurrenceof Uranium Deposits The time and formation relationship of certain metallogenetic parameters applying to the various types of uranium deposits indicates in general the following: a) Restrictionsin distribution of uranium deposits to distinct epochs in the earth's history. b) Uranium-rich source rocks prevailed during d i s t i n c tg e o l o g i c a lt i m e s . c) Uranium deposits in sediments and metasedimentsshow on numerous occasionsa distinct spatial affinit1- to uraniferous source rocks which themselvesare related to distinct geologicaltimes as indicated under (b). d) The known uranium deposits can be grouped on the basis of time-strati_qraphic and metaliogenetic parameters into five principle senerations.
The Time-RelatedOccurrenceof Uranium Deoosits 'l-ypes li urcnrum deposrtg :^ oti.-: oi ec:nomrc srgn:frccnce 2 (4.) 3
1. Ce n o z o t c
Alpine L o r o m l oe Kimmerton
I Mesozoic
tercynron
Po l e o zo t c
Coledcnron P o n - , Af r i c o n - B ra s i l i o n o
19
tt
(
v
I
(
A
I
I
1v -Tf-
--
Ph
A
900 1750 Hudsonron
r Y L --- I
2500 Archeon
!
Uncono f rmiiy-conloct deposrt{type i}
l{
C o l l o p s ab r e c c r o p i p e ( 5 )
T
S ' r b u n . o n f o r m i t y - e p i m e t o m .( 2 )
t!
Surficiol (6)
A
vein (3)
|
ouortz-oebble conglomerote i7)
E
a L
Sondstone (tl. tobulor 14.1)
S o n , r s t o n er.o l l f r o n t{ 4 . 2 ) S o n d s t o n e .t e c t o n i c - l i t h o l o g (i c4 3 )
L-
? E
Ereccio complex {8)
l n t r u s r v e{ 9 } P h o s p n o r r t(e1 0 1
I distribution of major types of uranium deposits Fig. 3,3. Prevalent geochronologic-stratigraphic
ii any event. any time-bound ore forming pro- b) During the .Vesozoic, tabular and, to a lesser -eis may constitute the final or an intermediate extent, roilfront type and tectonic-lithological 'taqe. This synthesis is based on the following sandstone deposits prevail. Some granitedata ot both qualitative and quantitative nature. unrelated veins are found in the Laramide orogenic belt. c) During the Paleozoic. tabular sandstone deposits.particuiarly in post-Devonian strata. and granite-related vein tvpe deposits re-r.2.1Time-Stratigraphic stricted to the Hercvnian Oroeeny occur. A Relationship of few unconformity-bound deposits are known at UraniumDeposits the Permian-Hercvnianunconformity. the Prorerozoic. the Upper d) During Plotting of the main types of deposits in a few deposit (intra)intrusive Rossing and geochronoloeical stratigraphic column with granite-unrelatedvein depositshave formed. stmultaneous considerations of their economic e) During the Lower n Middle Proterozoic, significance (Fig 3.3) reveals the following and subunconformity-epimetamorphic ttme-bound distribution: well the as as deposits. unconformity-contact 3 ) Dunng the Cenozoic.surficialdepositsoccur in deposits of tectonic-lithologic sandstone Tertiary to Recent sediment-duricrusts; rollprevail. Gabon iront and some tabuiar deposits occur in f ) During the Upper Archean to lowermost Lower Tertiary sandstones. Most volcanic type U Proterozoic. the uraniferous quartz-pebble occurrences have formed during this period. conglomerates have formed.
50
3 Principal Aspects of the Genesis of Uranium Deposits
Table 3.2. Geochronologic-stratigraphic distribution of uranium resources(RAR < $30ru3o8) and former production (status:January 1987)(in U3O3)
RAR
Production
1000mt
1000mt
Cenozoic-Mesozoic
3m
Paleozoic (mainly Hercynian + Permian)
100
6
70
ca. 280
l5
70
A1
160
Upper Proterozoic
740
Lower-Middle Proterozoic Lower Proterozoic
ca. 290
Total
ca.1800
Total Precambrian
c a .t J l u
3.2.2 GeochronologicalDistribution of Urenium Resources ReasonablyAssured Resourcesof the Western World recoverable for less than $30ru3os (IAEA cost category $80/kgU) amount to ca. 1.8mio.mtU3O6and productionprior to 1987 to ca. 0.99mio.mtU3O8(OECD/IAEA 1988). Of the combinedtotal of 2.79mio.mtU3Osabout 65% is in or was produced from Precarnbrian rocks (RAR: ca. 72"r", production: ca. 50"/o), and most of the remainder, ca. 25"h of the total (RAR; 75-20, production: 35-40%) is or was mined from Phanerozoiclithologiesadjacent to Precambrianbasement.Table 3.2 pro-
Total o/
o/
810
29
7
170
6
7
350
IJ
lb
900
)l
2'70
z7
560
20
100
990
100
7790
1m
72
500
50
1810
65
T
4?0
1000mt
CJ
vides respective data and a breakdown of the Precambrian-hosted resources and former production quantities.
3.2.3 Geochronologic-Metallotectonic Relationshipof Uranium Deposits The resourcenumbers(Table3.2 and Figs.3.4 and 3.5) already point to a quantitative prevalence of the uranium, and of certain types of deposits as well, to certain geological eras. More clearly, this is documented on a metallotectonic map (Fig. i.1) showing individual deposits and/or ore districts combined with selected
x 1 0 0 0m t U 3 0 g Ero
m.y
Type of deposit
l 00
300
500
700
900
L
rt u
C e n o zo t c
N
o c
Mesozotc Poleozoic Uppe. Proterczorc
'a
Middie Proterozorc
o o
Lower Proterozorc
o-
I I
-a 01
vr
-i
I
o
Archeon
Fig. 3.4. Geochronologic-stratigraphic distribution of total uranium resources(RAR < $30nbU3O8.solid line) plus past production (dashed line) by type of deposit. (former Easlern Block countries and China not included)
The Time-RelatedOccurrenceof Uranium Deposits
5l
x 1 0 0 0m t U 30 9 500 Ce n o t c
I
Alpine Lorcmida Krmmerico
:4e50ZOrC
Pon-Airrcon -Erosrlrono
-acer ):zierazac
900 Middle ?:3terDzoic
17 5 0 :4er '-crer0zotc
25 0 0 i
:::necn
concentrationof uranium and related deposit generationsredected bv distribuFig. J.5. Geochronologic-stratigraphic rion of cumulativeRAR (<$30/lbU3Oscateeory)and past production.(former EasternBlock countriesand Chinanot included) Numbers rc rhe right denote U deposit generations
_:eochronological-tectonicunits. The map illus:rates that certain types of deposits display a distinct affinity and restriction to cenain geologicai eras, namely to: a) the Upper Archean to Lower Proterozoic; b) the Hercynian; c) the Upper Proterozoic. but less prominently. In addition to these facts, epigenetic depositsin 'andstones and calcretesoccur remarkably close lo granitic complexes of the above eras, as illustrated graphically by a selectedexample displayrng the situation in the wesrern USA (Fig. 3.6) and evidencedon a worldwide basisby the followtng pairs (the first location refers to the deposit or ore district. the second to the assumed source area of the uranium): l.t"tsrraLia:Lake Frome Embayment (Tertiary) -.lount Painter and Willyama Block (Lower Prorerozoic): Yeelime (Tertiary to Recent) - Yilgarn Block (Archean to Lower Proterozoic). Africa: Agades Basin (Arlit. Akouta), Niger (Vise to Cretaceous) - Air Massif (Old Precambrian): Franceville Basin (Mounana. Oklo), Gabon iLorvsl-lv{i6dle Proterozoic) - Massif de Chaillu (Lorver Proterozoic): Langer Heinrich, Namibia (Quaternary) Damara mobile belt (Upper Proterozoic).
Europe:LoddveBasin,France(Permian)- Massif Central (Hercynian);Forstau,Austria (Permian) - Niedere Tauern (Hercynian); Miillenbach, - Schwarzwald Germany(Permo-Carboniferous) (Herrynian). Almost all crystalline uraniferous source rocks in central and westernEurope belong to Hercynian complexes which occur within or adjacent to Precambrian terrane and which in part at least are derived by anatexisfrom Precambrianrocks. Ameica: GasHills,Wyoming(Teniary)- Granite Mountains (Lower Proterozoic);likewise ail depositsin the basinsof Wyoming asweil asmost of the depositsof the ColoradoPlateauexhibit a spatial relationship to Precambrian crystalline as shownin Fig. 3.6. compiexes
3.2.4 Uranium Deposit Generationsand Their Time-Stratigraphic Ranking The abovelistedsituationsand relationshipslead to the questionof how far the environmentsand mechanismsgoverning uranium ore formation can be delineatedwith respectto time and space. based Investigations by meansof paleoeeography on paleomagnetism,continental drift, orogenic and epeirogenicevents.further by paleoclimates and paleobiology, asoutlinedearlier,assistin the
3 Principal Aspects of the Genesisof Uranium Deposits
Fig. 3.6. Western USA. spatial relationshipof uraniumdistrictsto Precambriancrvstalinebasementterrane. (Afier Mallen' et al. 1972)
Occurrence of UraniumDeposits The Time-Related delineationof specifictime related metallogenetic parameters that permit the identification of five principal generations of uranium deposits (Fig. 3.7) which fit into the overall evolution of the e a r t h ' sc r u s t . Generation one (Upper Archean to iowermost Lower Proterozoic, 2800 to 2200m.y. ago) The oldest nonmagmatic uranium deposits occur in oligomictic quartz-pebbleconglomerates. The primary mineralization is uraninite, uranorhorianire, and uranothorite of detrital origin, :is recosnized by Ramdohr (1955). This type of ':runium deposit remains restricted to the time :n .rarth historv when an oxidizing atmosphere ivas lacking, i.e., prior to the middle Lower P r o t e r o z o i c( > 2 2 0 0 m . y . o l d ) . Examples: Blind River-Elliot Lake, Canada; Witwatersrand. South Africa. Cenerationtwo (Lower-Early Middle Proterozoic. ll00 to 1900 to 1700m.y. ago) During the middle Lower Proterozoic (22C0to 1900m.y. ago), the oxidation potential of the atmosphere increased which effectively blocked the mechanicai transport of uranium. During and from this period onwards, uranium travelled essentially in solution as either ion or uranylcomplex and accumulated in shallow water or lagoonal basins. Initially, uranium precipitated either svn- or postsedimentary in peiitic to psamrritic sediments in or close to organic horizons. E.ramples: the unmetamorphosed sandstone deposits of the Franceville Basin. Gabon (Oklo, Mounana etc.) dated ca. 2050m.y.: metasediments of the Pine Creek Geosvncline.Australia. and the Tazin and Cree Lake Mobile Belt. Canada. During the Hudsonian and time equivalent orogenies (1900 to 1700m.y. ago) amphibolite to lranulite grade metamorphism affected these seciimentary units. causing a more or less in sltu crvstallization, or limited late orogenic redistribution of the uranium by metamorphichydrothermal processes resulting in either synmetumorphic stratiform mineralizations (type 13, Chap. 1) or epimetamorphic veinlike deposits (tvpe 2) respectivelv. Examples: (a) synmetamorphic stratiform deposits: Forstau. Austria; Kitts. Canada: (b) subuncontb rmitv -epimetamorphic deposits: Beaverlodge (Ace-Fay-Verna mines), Canada; and perhaps a precursor type to the Alligator
53
Rivers deposits, Australia. Other veinlike type deposits of this period are in the Singhbhum Thrust Belt, India. and at Krivoy Rog, Ukraine. Ceneration rhree (Middle Proterozoic, 1500 to 9 0 0m . y . a g o ) During the nriddle and late Middle Proterozoic deposits of the unconformiry-contact type (type 1) developed. They formed either by polygenetic evolution, including supergene preconcentration of uranium after a climate change from tropical humid to arid, followed by diagenetichydrothermal concentration of uranium to final deposit size after burial by sandstone. or alternatively by diagenesis-related hydrothermal processesalone. Similar diageneticprocessesaffected buried subunconformity-epimetamorphic deposits(tvpe 2). generatinga secondgeneration of redistributeduranium. Where the redistributed uranium was reconcentratedimmediately below the eariy Middle Proterozoic unconformity as in the Alligator Rivers area and locally in the Beaverlodge region the impression may arise of an original uncontormity-related deposit. Examples for t-vpe 1 deposits: Athabasca region (Cigar Lake, Key Lake etc.), Canada; for redistributed mineralizations in type 2: Ranger, Alligator Rivers, Australia, and Bolger, Beaverlodge, Canada. Other rypes of deposits of this time interval ate intrusive varieties formed in mobile belts. Thev include the pegmante deposits (subrype 9.5) of the Bancrott district, Canada, and the peralkaline syenite occurrence (subtype 9.a) of Kvanefjeld/Illimaussaq,Greenland. Generation four (Upper Proterozoic, 700 to 5 0 0m . y . a g o ) Deposits of generation four appear to be restncted to orogenic belts and inciude ueln (subtype 3.2) and innusive (subt-vpe 9.1) deposits. They are found in the Damara - Katanga Orogen (part of the Pan-African Orogeny) in Shaba, Zaire. and in Namibia respectivelv, and in the Brazilian mobile belt, Brazii. The uraniferous alaskitesof Rossing, Namibia. are interpreted as anatectic differentiates derived from uraniumbearing feldspar-rich clastic sediments of older units. The origin of the katathermal vein deposits of Shaba is not yet resolved. One hypothesis assumesthe derivation from an unknown granitic pluton, another speculateson the partlv uraniferous sediments as found. for examole. in the
3 Principal Aspects of the Genesisof Uranium Deposits
or zones and (b) proximity to uranium source rocks which are predominentlyUpper ArcheanLower Proterozoicand, to a lesserextent, Upper Proterozoic and Hercynian granites and/or metasediments of pelitic to psammiticorigin. Rocks of the above-mentionedprovenances and similar lithochemical character are found worldwide, whereasepigeneticuranium deposits of the sandstonetype are of restricted distribution. Obviously, additionalparametersand processesare necessar)'to concentrateuranium in economicquantitiesin thesesandstones.Certain climatic conditions appear to be an imponant prerequisite, in particular periods of arid to semi-aridclimatesprecededby a tropical humid climate. Examples: A time-related subdivision of generation five deposits displays the following distribution of selected type examples during to Recent, Devonian the Phanerozoic(number of type, subtype,class Generation five [(Cambrian) (500)400 to 0 m.y. agol refernng to Chap. 4 are in brackets). With the onset of the Phanerozoic,uranium Early Paleozoic(500 to 320m.y. ago): (a) metallogenesischangedto the formation of other black or alum shales (15): Ranstad, Sweden; types of depositspartly associatedwith the evolu- Ronneburg, Germany; Tien-Shan and Altai tion of continentalplant life. Mountains,CIS; (b) alkalinegranite/albitite(L2): In Proterozoic times more or less synsedi- RossAdams, USA. mentary and structurally controlled deposits Permo-Carboniferous(300 to 250m.y. ago): prevailed. During the Phanerozoicera, epigeneric (a) veins (3.1) associatedwith the Hercynian sandstone deposits (type 4), surfcial deposirs Orogenyin Europe: MassifCentral and Vend6e, (type 6) and, in orogenic belts, vein and some France; Erzgebirge-BohemianMassif, CSfn unconformity - related deposits (type 3 and 1 Germany; (b) sandstone (a)r Loddve Basin, respectively), and to a lesserextent volcanictype France;SierraPintada,Argentina;AgadesBasin, deposits(type 1i) becameprominent. Niger; Ngalia and Amadeusbasins,Australia; (c) Granite-related vein deposits are in Europe phosphorite(10.1):Idaho, USA. (250to 150m.y. ago): tabuiar typical for the Hercynian (:Variscan) Orogen Triassic-Jurassic wbere thev are associatedwith highly differen- sandstone: ColoradoPlateau,USA. tiated, late gganites(330 to 300m.y.old), which Cretaceous-Tertiary Q00 to 5m.v. ago): (a) occur in or adjacentto Precambriancomplexes. rollfront sandstone @.2): Wvoming Basins No economic uranium deposit of this rype has and South Texas CoastalPlains. USA: North . x^, beenfound in the CaledonianandAlpidic orogens BohemianBasin, CSFR; Konigstein.Germanl'; in Eurasia. The resourcepotential of Eisonian, (b) intrusive, carbonatite (9.3): Phalaborwa. Appalachianand Laramideorogensin Americais South Africa; alkaline (9.4): Pogosde Caldas, undetermined. A few depositsof the volcanic Brazil; (c) phosphorite(10.2): Florida, USA; type have been discoveredin the WestAmerican Bakouma,CAR. Cordillera. These mineralizationsare associated Tertiary-Recent:(a) surficial duricrust (6.1): witb rhyolitic extrusivesand volcaniclastics. Yilgarn Block, Australia; Namib Desen, Epigenetic sandstonedepositsare commonly Namibia:Mudugh,Somalia;(b) surficialorganic relatively uniform with respect to host rock, (6.2):Stevens County,USA. geotectonic-morphologicposition and metallogenesis.Their main geneticand ore-controlling In summary, the various types of uranium deparametersare (a) continentalclasticsediments, posits reveal a distinct direct or indirect affinoften containing carbonaceousmaterial, with ity to certain geochronological intervalsin earth changingredox conditionsalongchemicalfronts history. While the Upper Archean-Lou'er "S6rie des Mines" as a source of the uranlum mobilized during the Pan-African Orogeny and redepositedin favorable structural traps. Metallogenetically this fourth generationis, in a limited sense,comparable to t}te BeaverlodgeAliigator Rivers (Ranger) depositsof the second generation,except that depositformation seems to have been more synorogenicand in response to higher grade metamorphic processesof the phase. In a "closed syspalingenetic-anatectic tem" these processesformed intrusive alaskite depositsand led in an "open system"to locally distinct mobilization and transponationof U and other metals, such as Co, Ni, etc., into Lugher levelsresultingin vein deposits. Examples: Shinkolobwe, Shaba. Z,aue; Mindola, Sambia; Rossing, Namibia; Lag6a Real, Itatia, Espinharas,Brazil.
55
The Time-Related Occurrenceof Uranium Deposits
I
a i ' q ^- a [: l t a
n?Q-d uaec^rlox,.i-
^ v
otvORr^N
i
[,f +
0o^ "e^
rtrou5
---------r C 15
L
'oar'
,z
I ,f con tr ocn
.u;
'4" 'Qe^
tqt
ptont trtc
'p-
+\
, ole.o?oic
[.+
ul 2 . 2 b .y . o x y o t m o v c r s i o n
Fig. 3.7, Schemadc diagram of geochronologicdistribution and deposit generationsof major uranium deposits. For .e:end see Fig. -1.2
Proterozoic is favored by deposits of the quartzpebble conglomerate type, the subunconformitvepimetamorphic deposits prevail in the eariy \liddle Proterozoic and unconformity-contact Jepositsin the middle to late Middle Proterozoic. ,Jranite-related veins demonstrate a preference for the Hercynian. The position of the unconformity-epimetamorphic and vein deposits, however. mav not be so much a time-bound restriction as a style of orogeny/metamorphismrelated phenomenon. Furthermore. certain uraniferous granites or granite-relared intrusives and extrusives and netasediments of the mentioned geological time tntervals represent prime uranium source rocks. Terrestrial sediments, especially of Phanerozoic age, enveloping these source terranes offer
optimum locationsfor the formationof sandstone and surficialtvpe deposits.However,provenance and time stratigraphicposition of favorablehost rocks in conjunctionwith fertile sourcerocks do not provide, on their own, the necessarymetallogeneticenvironmentand processesto produce substantiai ore bodies.Epigeneticnon-magmatic and non-metamorphicuranium deposits originated oniy in regrons where distinct climatic conditionsgeneratedand controlled surfaceand groundwaterregimesand therebythe qualitative and quantitativemobilization.rangeof transport, and concentrationof the uranium. Where uraniferous (meta)-sedimentshave been incorporated into magmatic/anatectic processes, recyclingof the uranium could result in vein or intra-intrusivetypesof deposits.
56
3 Principal Aspects of the Genesisof Uranium Deposits
SelectedReferences and Further Reading for Chapter 3.2 (for detailsof publicationseeBibliography) Anhaeusser 1973: Barbier 1974; Bowie 1979; Dahlkamp 1977.1980; Fergnrson1987.1988;Fiebiger1976;Grandstaff 1973;Morosenlio 1965: Robertson 1974;Robertson et al. 1978; Schidlowski 1976; Schidlowski et al. 1975; Seyfert and Sirkin 1973; Simov 1979: Toens 1981: Toens and Andrews-Speed1984;Toens et al. 1985;\'evstrakhin 1967
4 Typologyof UraniumDeposits
of A number of globai and regional classifications uranium deposits have been proposed in the past by Heinrich (1958), Ruzicka Q,971), Ziegler (1971), Kazansky and Laverov (1917), Mickie and Mathews (1978), Mathews et al. (1979). D a h l k a m p ( 1 9 8 0 ) , N a s h e t a l . ( 1 9 8 1 ) ,B a r t h e l e t r l . ( 1 9 8 6 ) ,a n d o t h e r s . A l t h o u g h t h e y r e m a i n i n rrinciple valid. recently pubtished descriptions ,,i uranium deposits discovered during the past erploration boom, and new research data on earlier estabiishedand defined types of uranium deposits justifu a rearrangement and refinement of the ciassificationscheme.Many open questions still exist with respect to specific provenance of the ore forming uranium and soiutions, the conditions of uranium mobilization. transport and redeposition, and repetitive redistribution in many cases. Therefore a classification scheme based purely on metallogenetic criteria does not appear feasible. Instead, for all practical purposes, a typology based more on descriptive data has been given preference here. The pnncipal recognition criteria used for the identification and definirion of the individual tvpes and subtypes of deposits include particuiarly lithologic and structural relationships,alterr t i o n . m i n e r a l o g y . p a r a g e n e s i sa. g e c o n s t r a i n t s . and spatial geotectonic distribution of deposits. According to the chosen system. the terminology selected tor types and subtvpes refers primarily to the geotectonicsettingor host environmentof the discussedtype. On this basissixteenprincipal types of uranlum deposits are distinguished. They include almost :crtv subtvpes and classes.All classeshave cer:rin principal parameters in common permitting their attribution to a specific type. but they also exhibit distincrivefeaturesjusti$ing an individual status. Each tvpe is introduced bv a type Defnition, foilowed bv Principal Recognirion Citeria and .Venllogeneric Aspectsbefore presenting characteristics of subtypes. Inevitably, this kind ot organization involves overlap and repetition, rvhich is considered minor in order to achievea better tvpe- and subtype-related comprehension and precision and avoidance of confusion.
Complementing the selectedtype examples,a list of examplesis added. The attribution of these examples is in many casesonlv tentative and the example may beiong to another type, subtvpe or classthan listed, or it may have characteristicsof several subtvpesor classes. The description also includes selected references though not exclusivelv of authors who either reviewed comprehensiveiythe given type of deposit or districts thereof and list extensive bibliographiesfor further reference,or of authors describingin detail a specificdeposit taken as type example. Those types of generally subeconomic order which are not complemented by a detailed deposit/district description in Chapter 5 are more extensively reviewed here to provide sufficient background for their understanding. Cartoons (Figs. 1.1 to -1.i5) are added to furnish a schematized presentation of the geological setting of the various fvpes, subtypes, and ciassesof deposits. Division and numbering of the presented typology correspondsto that of Table 1.1, which gives an overview and economic ranking of the types of deposits. Table 4.1 summarizesthe significant geologrcal recognition criteria of the various types, subtypes, and classesof deposits.
4.1 Typel: Unconformity-Contact ( F i g .+ . 1 ) Definition Unconformitv-contact deposits are associated rvith and occur immediatelv below and above an unconformable contact that separates a crystalline basement intensely altered by lateritic weathering from overlying clastic red bed type sediments of either Proterozoic or Phanerozoic ageThe unconformitv-contact twe includes the following subtypes and classes:
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lwe SubtYPe Clogs
1. U N C O N F O R M I T Y - C O N T A C T (ossociotcd with loteriticolly weothered crystolline bosement) '1.1 Middle Proterozoic unconformity-reloted 1.1.1 frocture-bound 1 . ' l. 2 c l o y - b o u n d
65
'l .2 Phonerozoic unccnformity-reloted
MDOLE PROTEROZOIC . unconfcmity rcaolith E N
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Fig.{.1
Subtype l. 1: Proteroanic unconformity related Class1.1.1: Fracture-bound Type Example: Rabbit Lake, Eagie Point, Athabasca Basin, Canada
unconformity-contact depositsare Phanerozoic of similar settingas Middle Proterozoicdeposits but are small and of low srade.
Class1.1.2:Clay-bound Type Example: Cigar Lake, AthabascaBasin. Canada
Principal RecognitionCriteria
Subtype 1.2: Phanerozoicunconformity related Tvpe Example: Benholdne, Avevron, France
Host Environment
R.eferences:Dahlkamp and Adams 1981; Evans (ed.) i986: Fogwrll 1985; IAEA/Ferguson (ed.) 198a:Laind et al. (eds.)1985;Sibbaidand Petruck(eds.)1985;Tremblay 1982
- Basementis similarto type 2 subunconformityepimetamorphic deposits and consists of middle to upper Lower Proterozoicmetasediments includinggraphitic horizons and strata with elevatedU contentsurroundingArchean graniticdomes,cut bv variouspegmatites - Cover of middle Middle Proterozoic continental red bed faciessandstone(Athabasca Group ca. 1500m.y.) above markedly weathered crvstalline rocks below an unconformity - Diabasedikes,and sills cuttine both basement and cover - Ancient lineaments and myionite zones in basement - Tensional repeatedly reactivated structures in the basementwith enensions into the sandstone
The rwo classesof Proterozoicunconformitycontactdepositsare definedby their settingwith iesDect to the unconformity, host rock, paragenesrs.and ore grade.Fracrure-bounddeposis lciass1.1.1)occurin metasediments immediately below the unconformitv.Mineralizationis commonly monomerallicand of medium grade(0.31% U3O8). Clay-bound.deposix (class I.1.2) occur associatedwith clay at the base of the sedimentary cover directly above the unconformity. Mineralizationis commonlypolymetallic (U + Ni + Co + As) andassociated with bitumen of high to very high grade (1-14% U3Os). Alteration 3! rnnctpal uranium phases in both classesare pitchblendeand uraninite. - Intenseregionalpaleoweatheringof basement
6
4 Typologyof UraniumDeposia
- Intense alteration ha-losaround deposits in basementand sandstone
b) Although all large unconformitytype deposits known are restricted to the above listed environmentand belongto subtype1.L, deposits of equivalent type also occur in comparable Mineralization geological environmentsof younger age (sub- U present dominantly as pitchblende or type 1.2, Phanerozoicunconformity related) uraninite in disseminated to often massive but areof smallermagnitude(seealsoRemarks accumulations in Section4.1.2 subtype1..2). - Mineralization of class 1.1.1 mostly monoFor more detailsseeChapter5.1. mslallig,of class1.1.2polymetallic - Mostly sharp ore-wasteboundaries - Redistribution of uranium and sulfidesinto 4.1.1 Subtype l.l: Proterozoic-unconformity fracturesin the sandstonewithin the alteration related halo abovethe deposits Age Constraints
Class1.1.1: Fracture-bound
- Formation of major depositsof subtype 1.1 Type Example: Rabbit Lake and Eagle Point, tends to be restricted to middle Middle Pro- Saskatchewan, Canada terozoic (seefollowing Remarks) References: Heine1986;Eldorado Resources Ltd. 1987 - U-Pb minimum ages of first generationpitchblendeof Athabascadeposits;1200to 1400m.y. H ost RoclcsI Stntctures MetallogeneticAspects Processesleading to mineralization and related alteration are only partially established.The most obvious are products of diagenetic-hydrothermal activity indicating the critical role of these processes as the essential step in the formation of the deposits as they exist today. It remains unclear,however,whether or not polyphaseprocesseshave developed this type of deposit. The open question is whether (a) the diagenetic hydrothermshave also introduced the uranium (and other metals) or (b) have only finally concentratedand recrystallized (and redated) preexistingmineralizationformed by supergene surficial (paleoweathering) or perhaps early diagenetic processes.A surficial type deposit (type 6) may be envisionedas a precursorstage transformed by diagenetic-hydrothermaloverprinting to a Proterozoic unconformity type deposit.
Altered gneisses,schists,with intercalationsof graphitic horizons, metacarbonates(dolomitic marble etc.) and calc-silicaterocks originally metamorphosedto amphibolite-granulitefacies. Locally migmatites.Often abundantpegmatitic, micrograniticand/or graniticdikes or segregations (pegmatitic and granitic segregationsconstitute ).5to 20Y"of host rocksat Eagle Point). Diabase dikes.Host rocks are stronglyfractured, sheared and/or brecciated along repeatedly reactivated faults including old mylonite zones. Deposits tend to occur in subsidiarystructures in the hangingwall of major thrust faults. Unmetamorphosed red bed arenites mav overlie the basement. Alteration Three principal distinguished:
types of
alteration are
- early Na- and/or Mg-metasomatism(albitization, dolomitization)of the basement; - pre-cover sandstone (Meso-Helikian in Remarks Saskatchewan)paleoweatheringextending in excess of 50m below the unconformity a) Middle Proterozoic unconformity deposits (hematitization, chloritization, sericitization, commonly are high-grade and have large argillitization); resourcesconstitutingabout one third of the - post-sandstone early diagenetic,and later orewesternworld's low-clst uranium reserves. related diagenetic hydrothermal alteration
Type 1: Unconformiry-Conract
67
overprintingthe former alterationzones by Remarks illitization. chloritization, carbonatization, tourmalinization(dravite), silicification,sul- Fracture-bound depositsof the Rabbit Lake area fidization,destructionof graphite,and cor- (Rabbit Lake, Eagle Point) display peculiar rosion of quartz. This alterationis intensely geological features resembling those of subundevelopedwithin a depositbut forms only a conformity-epimetamorphic deposits such as found in the Beaverlodgedistrict (subtype 2.2), narrowaureolearoundit.
e.g., albitization, migmatitization. and essentiaily monometallic mineraiization. An explanation Ore and AssociatedMinerals may be that the class 1.1.1 deposits originated from preexisting Beaverlodge-type deposits by and alteration products Uraninite/pitchblende thereof. Quartz and carbonates in minor unconformity-type overprinting.
:rmounts.Fe-chlorite,Mg-chlorite,illite-sericite. '.,:iciinite. dravite and hematite. Base metal Examples of Middle Proterozoic [Jnconforminr;nerais in trace amounts.Ore and associated contact, Classl. I. I Fracture-bound Depositsl mineralsare presentin severalgenerations. Occurrences .Vode of M ineralization
Australia: ? Nabarlek/Alligator Rivers, Northern Territory, ? Kintyre, West Australia Canada: Dominique-Janine, Dominique-Peter, Horseshoe, Raven/Athabasca Basin, Saskatchewan, Kiggavik/Thelon Basin, NWT Guvana-Venezuela:Roraima reeion
Uraniummineraisoccurdisseminated to massive and fairly continuous in moderate to steep dipping fracturesand brecciazonesin the baserrent. Thesezonesare commonlybest developed proximal below the unconformify but may extend to a depth exceeding400m. Mineralization may persist upward along fracturesinto the overlying sandstone.These deposits are Class1.1.2:Clay-bound commonly monometallicwith moderateto high uranium gradesand containonly tracesof other Type Example: Cigar Lake and Key Lake, metallicelements. Saskatchewan. Canada References: Fouques et al. 1986; Ruhrmann 1986 "ige Constrains
Host RockslSffuctures Class1.1.1depositsarerestricted to areasaffected by early Middle Proterozoicorogeny(Hudsonian Argillaceousfacies(clay,mudstone,argillicsandin Canada), foilowed by a warm-humidclimate stone), locally containingcarbonaceousmaterial changing into an arid climate as reflected by (bitumen) resting along the unconformity upon latentic paleosol (regolith) development.then paieoweathered crystallinebasement.The clay is succeededby sedimentationof red bed arenites mostlyconcentratedat slightbasementridgesand (Athabasca Group ca. 1500m.y.). Minimum grades upwards and laterally into ubiquitously UrPb agesof earliesturanium oxide generation oxidized sandstone.Basement under or near are 1,100m.y. at EaglePoint(EldoradoResources overiving deposits is disturbed by faulting inLtd. 1987) and ca. 1280m.y.ar Rabbit Lake cluding ancient lineaments (mvlonite zones). (Cummingand Rimsaite7977). The clay may have originated from pedogenic or lacustrine deposits of ourwashed lateritic paieosol,or from fault gougeor diageneticalterDimensionslResources ations.or from a combinationof severalof these Individual depositsmay be up to 1000mlong, a features. few metersto several10m wide and several10m to more than 400m deep. containing up to Alteraion >50000mtU3O3at gradesrangingfrom 0.3 to 1.07o,rarely a few percentU3O8.Districts may Paieoweathering, diageneticand mineral-related containup to 100000mtU3O8. alterationcorrespondsto that of class1.1.i, but
68
4 Typologyof Uranium Deposits
the latter forms a far larger halo. In contrastto subtype 1.1.1, albitization of basementrocks is practicallyabsent.
4.1.2 Subtype 1.2: Phanerozoic unconformity-related
Ore and AssociatedMinerak
Type Example; Bertholdne and Le Roubei BrousseBroquies,Aveyron, France References:Schmitt et al. 1984:Georee 1985
Uraninite/pitchblende and alteration products thereof.Al-, Mg- and Fe-chlorite,illite, kaolinite, dravite, quartz and hematite.Veinletsof quartz, carbonates. and dravite. Associated metallic mineralsmay includesulfidesand arsenidesof Ni, Co, Cu, Pb, Zn, Mo, Bi, Sb, and V, locallyTe. Se, Au, Ag, and Pt-group elements,some in appreciableamounts(up to severalpercent).Ore and associatedminerais are present in several generations.
Permo-Carboniferous clasticsedimentswith some volcanic components resting along the postHercynian unconformity on altered Upper Proterozoic to Lower Paleozoic schists and gneissescontaining minor graphitic horizons. Faults transectbasementand sediments.Redox conditionsin the coversedimentsare variable.
Mode of Mineralizarion
Alteration
U milerals occur in the clay envelop as disseminatedto often massive,continuousmineralizationforming linear, pipe or cigar-shaped, more or less horizontal deposits.composedof a high grade core surrounded b1' a lower grade halo. The depositsexhibit a rather sharpore-wallrock boundary. Mineralization may extend along cataclasticzones higher up into sandstoneand downward into the basement. These deposits consist almost always of polymetallic mineralizationof high to very high grades(>1% U3O8).Ni is commonlythe mostnotabiyenrichedassociated metal.
Two typesof alterationare distinguished:
Age Constrains Correspondto thoseof class1.1.1. Dimensions/Resources Individualdepositsare a few 10m to ca. 1500m long. a few meters to ca. 100m wide and a few meters to 25m thick. containinga few 100 to >i25000mtU3Os at grades ranging from ca. 1 to I41LU3Os. Districts may contain up to 200000mtU3Os and more. Examplesof Middle Proterozoic Unconformirycontact,Class1.1.2 Clay-BoundDepositsl Occurrences
H ost Rocl<sI Structures
- pre-mineralization: albitization, Fe-chlontization, sideritization; - syn-to post-mineralization: silicification,argillitization (smectite, illite, minor kaolinite), carbonatization,pyritization,hematitzation. Ore and AssociatedMinerab Coffinite, minor pitchblende and tetragonal c-U3O7,and alterationproductsthereof (uranyl vanadates). Gangue or similar minerals: Illite, smectite. dolomite,calcite,siderite. Metaliic minerals; Pyrite, minor marcasite. galena,sphalerite.hematitereplacingpyrite. Mode of Mineralization U minerals occur as disseminationsand fine veinletsforming smallore bodies.The ore bodies straddlealong the post-Hercynianunconformin' most commonlywhere the unconformityis transectedby faults. Mineralizationoccurs either in the basement(Bertholdne)or cover sandstone (Bennac)or in both (Le Roube). Age Constraints
Mineralization in the type examplesformed in with intrusion Jurassictime, contemporaneously Canada:Cluff Lake "D", Collins Bay, Dawn of basicdikesand approximately time-equivalent Lake, Maurice Bay, Midwest, McClean, P2- to globaltectonicsmarkedby the incipientstaee NorthlAthabasca Basin,Saskatchewan of tbe openingof the North Atlantic ocean.
Type2:Subconformity-Epimetamorphic 69
lResources Dimensions
4.2 Type2: Subunconformity(Fig.a.2) Individualdepositsmay be up ro 1000mlong, a Epimetamorphic iew metersto 50m wideand may extendup to ca. 30m aboveand ca. 30m belowthe unconformity. Definition Resources are a few tensto ca.2000mtU3Os at gradesof 0.1 to 0.15%U:Oe. raverage) Subunconformitv-epimetamorphic deposits
are strata-structure-boundin metasediments below an unconformity on which clastic sedimentsresr. Remarks Type example deposits occur below an early Differencesberween subtype 1.1 depositsin Middle Proterozoic unconformity that is overlajn Saskatchewanand those of subtype I.2 in by early Middle Proterozoic clasticsedimentsand .{vevron, France include (a) well-developed volcanics. :lbitizationin Avevronartributedto diagenesis of Deposits consist of peneconcordantlenses or :over sediments. Albitizationis practicallyabsent tabular mineralizationsemplacedjn fracturesand in class1.1.2depositsin Saskatchewan. (b) Clav breccias within distinct stratigraphic units. Host minerals associated with mineralization are strata are predominantly pelitic (subt,r-pe2.2 dominentlyillite and smectitein Aveyron, and or carbonatic (2.1) sediments with intercalated illite and Al-, Mg-chloritein Saskatchewan. (c) carbonaceoushorizons of Iate Lower Proterozoic Metal associarionis more simplein Aveyron than age metamorphosed to amphibolite grade facies in Saskatchewan. (d) Cover sedimentshave con- and superimposed by retrograde (greenschist) trastingredox environmentsin Aveyron, whereas metamorphism. Granitic-migmatitic complexes thosein Saskatchewan are uniformlyoxidized.(e) occur discordantly in the metasediments. paleoGraphite horizonsare of only minor importance weathering of the crystalline rocks was oniy mild in Aveyron. (Pagel1989). (in contrast to areas of "unconformitv-contact"
TyPc
- EPTMETA}'ORPHIC 2. SUBUNCONFORUITY (stsoto-stsucturc bound in lotc Lorcr Protsozoic mctorcdimcnta)
Subtypc
2.1
not olbitizcd mctss.dimontl
Typc cxomplcr(cloar)
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olbitzcd mctosGdimqntt
o) Jobiluko, b) Rongcr I, c) Foy, Koongoro Rum Junglc Vcrno
d) Gunnor
EARLY MIDDLE PROTEROZOIC
ls
LATELOWER PROTEROZOIC inqrcosc of No-mctqcomoticm ? incrcoacd dcrrclopmcnt of pcAmoUtce ond onotaritcc ?
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El
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A
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E E
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ond volconics
migmotitc
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70
4 Typology of Uranium Deposits
deposits). Principal uranium phases are pitchsarily developed adjacent to and at intersectionsof major faults (particularly evident blende and uraninite. Intense and extensivehost in Beaverlodgedistrict) rock alteration surroundsmineralization. Two settings of mineralization are recognized - Host structuresare faults, fractures. breccias, stockworksarranged more or less peneconon the base of Na-metasomatismof the host and other features: cordantto attitude of strata metasediments Aheration Subtype 2.1: not dbitized metasediments Type Example: a) Jabiiuka, Koongara, b) Ranger, Alligator Rivers district. Australia Subtype 2.2: dbitized metasediments Type Example: c) Fay-Verna, d) Gunnar. Beaverlodge district, Canada References: Beck 1986; Dahlkamp and Adams 1981; Ferguson and Goleby (eds.) 1980; lAEAiFerguson (ed.) 1984;Needham et al. 1988;Tremblay 1978 a), b), c), and d) refer to type examplesshowninFig. 4.?.
- Mild paleoweathering-relatedalteration of basement (in Beaverlodgedistrict of more physicalthan chemicalnature) - Pervasivediageneticalterationincluding Mg-. Li- and B-metasomatism of severalgenerations imposed on both basement and overlying sedimentsand reflected by tourmalinization (dravite), carbonatization(calcite, dolomite). argillitization.sericitizationetc. - Intense mineral-relatedwall rock alteration including argillitization, chloritization, desilicification, silicification, sulfidization and hematitization - Widespreadpre-ore albitization of basement rocks hostingsubtype2.2 deposits
Subtype 2.1 does not display albitization but of both Mineralization extensiveMg-, Li-, and B-metasomatism host metasediments and overlying sediments. Principal uranium minerals are pitchblende, Subtype 2.2 exhibits albitization of host rocks partly of high intensity up to albitite formation uraninite and alteration products thereof (coffinite, etc.). For associatedminerals and and also shows a stronger structural control and contains relative abundant gangueminerals as metals,seeTable 4.2 comparedto subtype2.1. Gangue minerals are associatedwith subtype 2.2but rare or absentin subtype2.1 Most deposits,particularlyall large ones are Principal RecognitionCriteria monometallic except for some containing locally Au; some smaller deposits are poivmetallic(Rum Jungle:Cu, Pb, Zn) Host Environment Several generationsof mineralization exist, - Orogenicbelts mainly derived by redistribution of primary - Merasediments (schist, gneiss) with intermineralization(locallyinto cover sandstone) bedded graphitic horizons of pelitic and Ore distribution consistsof U disseminations psammiticeugeosvnclinal or lacustrineongin or massiveveinletswithin host structures - Regional metamorphosed to amphibolite Mineralizedstructuresare arranged + penegrade locally up to granulitegradefacies concordantto attitude of host strata - Retrograde greenschistfacies metamorphic Type example (a) Jabiluka and Koongara overprint . display the most pronounced strata-bound - Presenceof granitic-migmatiticcomplexesand structurecorrelation,whereasgoing from type pegmatitedikes in basement examples(b) to (c) and (d) the strala-structure - Cover by continental clastic sedimentswith relationshipbecomeslessevident intercalatedand transgressive basicvolcanics Mineralizationis fairly continuous - Post-sedimentary diabase/dolerite dikes Depth persistenceis variable but can extend - Brittle deformation of host strata by intense to great depth (>1600m in Beaverlodge faultins and brecciationoften but not necesdistrict)
T ype 2: Subconformity-Epimetamorphic 71 Table 4.2. Associatedmineralsand metals Gangueminerals
Others(replacement, authiqenic.etc.)
Vein fillings
Basement
Overlyingsandstone Locally Mg-chlorite, tourmaline (in Kombolgiesandstone and basement)
c)
Calcite,dolomite,quartz, chlorite(?)
Chlorite Sericite/illite Kaolinite Silica Quartz Carbonate Hematite Hematite Epidote
d)
Quartz (?) Calcite(?) Chlonte (?)
b)
Metals
Cu, Pb, Zn Co, Ni, Pb C u . P t ,A u , A g A s , S e ,S
Albite Calcite
ua to d referto typeexamples in Fig..1.2
Age Constrains -
No age constraint except limitation to orogenies younger than late Lower Proterozoic
-
All major deposits known are associatedwith upper Lower Proterozoic sediments metamorphosed during the Hudsonian or time equivalent orogenies (ca. 1900 to 1700m.y. ago)
Metallogenetic Aspects
had opened within the uraniferous strata. In contrast,in areasof closedsystems,i.e., those that lack the above listed criteria, uranium remainedin situ and formed stratiformsynmetamorphic mineralizations(Type 13). The position of more structurally dominated mineralization [type examples(c) and (d) at Beaverlodge]or more lithologicallycontrolledmineralization[(a) and (b) in the Alligator Rivers district] may be relatedto zonesinfluencedby Na-metasomatism, In migmatization and/or palingenesis/anatexis. regions where subunconformity-epimetamorphic depositswerecoveredby earlyMiddle Proterozoic continental sediments,the cover protected the depositsagainst weatheringand erosion. More or lessintensediageneticallyinducedmagnesium and boron metasomatismoccurredin both crystalline basementand overlyingsandstoneas displayed in the Alligator Rivers distnct. These processes very probably created diagenetic modificationsin the depositsincluding redistribution. partly into the overlyingsandstoneas at of the uranium Beaverlodge,and recrystallization and intense host rock alteration. Still younger processesled to further modifications,as reflectedby severaigenerationsand isotopeagesof uranium and associatedminerais.
Subunconformitv-epimetamorphic uranium deposits have a complex, polyphase evolution. Deposits in the Alligator Rivers and Beaverlodge districts most likely have their roots in late Lower Proterozoic time when peiitic-psammitic sediments interbedded with carbonaceous horizons collected anomalous amounts of uranium and other metals. It is possiblethat pre-metamorphic relocation of uranium led to the formation of sandstone-type deposits such as those in the Franceville Basin, Gabon. During Lower to Middle Proterozoic times these sediments were regionally metamorphosed to amphibolite and locally to granulite facies. In regions of open systems, i.e., in tectonically active terrane characterized by anatexis, migrnatism, metasomatism, acidic and mafic intrusions. and brittle defor- Remarks mation, uranium was locally mobilized by late metamorphic and/or metasomatic hydrothermal a) Monometallic subunconformitlr-epimetamorphic U deposits particularly those of Lower to processesand reconcentrated in structures which
72
4 Typology of Uranium Deposits
Middle Proterozoicage,often havemediumto Alteration large resources(up to >200000mtU-rOs)at wallrock alteration and extensive metalow to medium grade (0.2-0.4%U3Os) but Strong (Mg. B, Li) of several generations, somatism may also contain sectionsof very high grade both basement and sandstone latter affected The (several % U3O8). In contrast,polymetallic tourmalinization (dravite), and includes cover depositsexceptthosewith gold (e.g.,Jabiluka) (Mg andior Fe-rich), carbonatichloritization have small resources. (calcite. dolomite), sericitization. argiliitb) Although all large depositsof this category zation desilicification, and silicification and ization, known are associatedwith Lower to Middle Alteration related to mineralihematitization. Proterozoic rocks. similar depositsmay also very intense and may extend is zation commonly occur in comparablegeologicalenvironments (m variable distance to i0'sm) into wall rocks. to with youngerorogenic-metamorphic associated events. Ore and Associated Minerals
4.2.1 Subtyp2.l: metasdiments
Not albitized
Australia TypeExample:PineCreekGeosyncline, Monometallic (except for local Au concentration): a) Jabiluka, Koongara,Austraiia; b) Ranger One, Australia Polymetallic:Rum Jungle (a) and b) refer to Fig. 4.2) References: Ewen et al. 1984; Ferguson and Goleby (eds.) 1980; IAEAlFerguson (ed.) 1984;Needham et al. 1988
H ost Ro cl<sI Structures
Principal uranium minerals are pitchblende, rarelv uraninite and alteration products thereof (coffinite, brannerite, sooty pitchblende). Associated minerals/metalssee under main headins.
Type2. Mode of Mineralization or massiveveinsfilling U occursasdisseminations fractures.brecciasand stockworks, + peneconcordantto attitudeof the strata.Mineralizationis more or lesscontinuousand may extendto more than 500m below the unconformity. Most ore is monometallicexceptfor some small deposits (Rum Jungle) and locally payable gold values (Jabiluka, Koongara). Several generations of uranium mainly derived by redistribution and recrystall2ation (rejuvenated U/Pb ages) of primary mineralization partly by diagenetic processes.Mineralizationis controlledby both structure and lithology as reflectedby emplacementin cataclasticzones * peneconcordantto folded metamorphosedstrata and often adjacent to graphitic horizons. Other recognition criteria inciude intense, partly pervasivechloritization, sericitization.argillitization,andhematitizationof wallrocks;wide-spread halosinto coversediments of tourmalinization,carbonatization, silicification, etc.; (former) presenceof continentalcover sedimentsand volcanics;minor paleosoldeveiopment prior to sedimentation and numerouspegmatite and diabase(dolerite)dikes.
Metasediments(schist,gneiss)with intercalated graphitic layers and carbonatic horizons derived from pelites and psammitesof eugeosynclinal or lacustrine origin, regionally metamorphosedto amphibolite-granulitef acies. Migmatitic/anatectic complexes occur near deposits. Pegmatite and diabase/doleritedikes intrude host sequences.Continental sandstone with intercalatedand discordantbasicvolcanics. if not eroded. overlie mineralizedareas. Host rocks are deformedby intensivefaulting and brecciation. Structurescontainingmineralization consist of fractures, brecciasand stockworks arranged+ peneconcordant to attitude of strata.Type examples(a) Jabilukaand Koongara display the most distinct strata-boundstmcture correlation whereasthat of examples(b), Age Constraints Ranger One and Rum Jungle is more structure All examplesknown are associatedwith upper prominent. [,ower Proterozoic sediments metamorphosed duringan orogenyat about1700to 1900m.v. ago.
Type3: Subconformitv-Epimeramorphic 73 DimensionslResources Individual deposits may be up to >1000m long, several tens to more than 400m wide a n d i n e x c e s so f 5 0 0 m d e e p . c o n t a i n i n gu p t o 200000 mt U3Os at (averase) gradesrangingfrom 0 . 1 t o 0 . 4 % U j O 8 o c c a s i o n a l l yt o > 1 % U r O s . Districts may contain up to -100000mtU3O5. Remarks F o r m o r e d e t a i l ss e e C h a p t e r 5 . 2 . 1
E.ramp les of Subunconformi n'-Ep imenmo rp hic, Subvpe2.1 Nor Albitized fulerasedimenu lOccurrences Deposits
consist of fractures. breccias, and stockworks arranged + peneconcordant to the attitude of strata. Structural control is more prominent as in s u b t y p e2 . 1 . Alteration Extensive and locallv intense Na-metasomatism and strong wall rock alteration of severai generations modified the basement.Na-metasomatism is a pre-ore phenomena and locally achieved albitite formation. Host rock alteration of preto svn-mineralization age includes pervasive hematitization. chloritization. epidotization, carbonatization.and silicification.
Australia:Alligator Rivers district.Rum Jungle Ore and Associated Minerab district, ? South Aliigator River district/Pine Principle uranium minerals are pitchblende, Creek Geosyncline,Northern Territory, ? Kinrarely uraninite and alteration products thereof tyre, West Australia (coffinite. brannerite, sooty pitchblende). Associated mineralsimetals (see under main heading, Tvpe 2).
4.2.2 SubtyP 2.22Albitized metasediments
Mode of Mineralizarion
U occurs as disseminations or massive veins filling fractures, breccias and stockworks, + peneconcordant to attitude of the strata. Mineralization is more or less continuous and may extend to a considerable depth (>1600m). Most ore (ed.) References: 1986:Tremblay1978: is monometallic except for some small deposits Beck 1986;Evans Ward 198.1 (e.g., Nicholson). Severalgenerationsof uranium mainly derived by redistribution (iocally into catacover sandstone) and recrystallization (reclastic Host RockslStructures juvenated U/Pb ages) of primary mineralization. Metasediments (schist, gneiss) with intercalated Mineralization is controlled by both structure graphitic layers derived from pelites and psam- and lithology as reflected by emplacement in strucmites of eugeosynclinal or lacustrine origin, tures + peneconcordant to folded metamorphosed regionallv metamorphosed to amphibolite- strata and often adjacent to graphitic honzons. 3ranulite facies and partlv Na-metasomatizedto Other recognition criteria include intense, partly eibititerocks. pervasive chioritization, sericitization, argillitiMigmatiticranatecticcompiexesand/or granitic zation, hematitization. carbonatization, siiicifiintrusions occur near deposits. Pegmatite and cation. etc. of the host rocks; (former) presence diabaseidolente dikes cut the host rocks. Con- of continentai cover sediments and volcanics; tinental sandstone with intercalated and dis- little or no paleosoi (regolith) development prior cordant basic volcanics. if not eroded. overlie to sedimentation and numerous pegmatite and mineralized areas and may carry (redistributed) diabase(dolerite) dikes. mineraiization. Host rocks are deformed by intensive faulting Age Corctrains and brecciation. often but not necessarily developed adjacent to and at the intersection of All major deposits known are associated with major faults. Structures containing mineralization upper Lower Proterozoic sediments metamor-
Type Example: Beaverlodge, Uranium City region,Canada monometallic:c) Fay - Verna; d) Gunnar polvmetallic:c) Nicholson (c) and d) refer to Fig. 4.2)
4 Typologyof Uranium Deposirs
phosed during the Hudsonian Orogeny(ca. 1700 to 1900m.y-).
Subtype 3.1: granite-related (Fig. 4.3a)
D irnensio nsIResour ces
Class3.1.1: intragranitic (Limousin t1'pe) Type Examples:3.1.1. 1 veinsin granite: Fanal'. France 3.1.1.2disseminations in episyenite pipes:Pierresplant6es.France
Individual depositsmay be metersto 100m long, several meters to more than 100m wide and in excess of 1600m deep, containing up to >15000mtU3O, at (average)gradesranging from 0.1 to 0-4ohU:Os. Districtsmay containup to 25000mtUrOa. Remarl<s For more detailssee Chapter5.2.2. Examplesof Subunconformity-Ep imetamorphi c, Subrype2.2 Albitized MetasedimensDepositsl Occurrences Canada: Bolger, Dubyna, Hab, Lake Cinchi Beaverlodge,Saskatchewan
4.3 Type 3: Vein (Figs.4.3a,b)
Class3. 1.2: perigSanitic Type Examples:3.1.2.1 veinsin (meta)-sediments: monometallic (Bohemiantype) Pifbram. CSFR 3.1.2.2veinsin metasediments: polymetallic (Erzgebirgetype) St. Joachimsthal/ Jachymov.CSFR 3.1.2.3in contact-metamorptucs : (Iberian type) Alto Aientejo. ponugal Subtype 3.2: not granite-related (Fig. 4.3b) Class3.2.1: in metamorphicrocks Type Example: Schwartzwalder,USA Class3.2.2: in sediments(polymetallic) Type Example: Shinkolobwe,Zaire References:IAEA/Fuchs (ed.) 1986;Basham and Matos Dias 1986; Derriks and Vaes 1956; Fnedrich er al. l9g7; Kolektiv CSSR tS84; Poty et al. t9g6; Ricb et al. lgTl: Ruzicka 1971;Wallact 1986
Definition Vein deposits consist of uranium mineralization in lensesor sheetsor disseminations filling joints, fissures,brecciasand stockworksin deformedand fracnrred rocks. Size and complexity of vein sets are variable. Distribution and intensityof mineralization are irregular. Principal uranium phases are pitchblende.uraninite and coffinite.Gangue mineralsare alwayspresent.Uranium may form monometallic mineralizations or polymetallic mineralizations.Associated metals include Co, Ni, Bi, Ag. Cu, Pb, Zn, Mo and/orFe in form of sulfides, arsenidesor sulfarsenides.Wall rock alteration is commonly restricted to a narrow margin(<1m). Two principal subtypes are recognized,veins spatially and genetically(?) related to granites (subtype 3.1) and veins not related to granites (subrype3.2) which are further subdivid-ed into the following classes:
Granite-relateddeposiuare associatedwith highly differentiated peraluminous leucogranites and form veinseitherwithin (intragranitic, class3.1.1) or around(perigranitic, class3.1.2)the intrusion. Intragranitic deposits are commoniy monometallic and occur either as (a) linear ore bodies in form of distinctveinsor stockworksemolaced in fractured granite or (b) disseminationsin pipes or chimneysof episyenite,a dequartzified, micaceousvuggy alteration product of granite. Depth extensionof intragranitic veins is commonl;"lessthan 300m. Perigraniticdeposis emplacedin (meta-)sediments are either monometallicconsistineessentially of pitchblende andganguemineralse.t.Z.t) or polymeta[ic(3.7.2.2)containingboth U and Co, Ni, Bi and Ag mineralsin economicquantities. The U and the other elementsare nor geneticallyrelated.Both monometallicand polymetallicveinscanpersistasmuch as2000m deep. Ore occupancyof host structuresis generallylow (in the order of 5 to 30%). Perigraniticdepositsemplacedin the contactmetamorphicaureoleof the intrusion (3.L.2.3)
Type 3: Vein
Typc
J.
Subtypc
at
Closs J. 1.1
VEtN
(1)
gronitc-reloted
inbogronitic
3.1.2 pcrigronitic
3 . 1 . 1 . 13 . 1 . 1 . 2
3.1.2.1
3.1.2.2
vcin
in (mcto)scdimcnts monomctollic
in mctoscdimants polymotollic
cpisycnite
o 0
q
a a E
vcin U minerclizotion cpisycnite U mincrolizotion
lcucoctotic diffcrcnUotqd gronitc
a
contoctmctomomhic oureolc
N
scdimcntg
ld-l t-4-)
mctorcdimcnts
Type
a
vd
3. vEtN
lomprophyrc ond othcr dikcs foult
(z)
Subtype Closs
J.Z not-gronite-reloted 3.2.1 in metosedimentsl) J.2.2 in sediments2)
.A f .71
metosediment
7...,f1 pegmotite
ff|
sondy dolomitic shole
ffil
silicious dolomite
7-n
foult (borren)
f---=
dolomitic+csbonoccous shole
l-T--t
horsetoil frocture
R
stongly frocturcd rock
,_l
cover rocks
l--l
principol foutt
F----1
1) ideolized ofter Schworkwolder, USArAfollocc1986 Fig. 4.3a, b
yRRn
2) ideolizcd oftcr Shinkotobwe,Zoirc/Daiks
& Oostcrbosch 195g
76
4 Typologyof UraniumDeposits
3.1 and 3.2) Alteration(Subrype have monometallic mineralization in form of Ore-Related fractured in intensely veinletsand disseminations - Spatially restrictedwall rock alteration comschistand hornfels,speckledandalusite-cordierite monlv persistingfor lessthan 0.5m, rarely to similar rocks up to approximately2km wide 3 m, from veinsinto wall rock around the granite. Host rocks are severely Type of alterationdependsto somedegreeson altered. host rock composition Not granite-relateddeposits(subtype 3.2) are Alteration types may include carbonatization, similar in mineral compositionand wall rock alterchloritization, argillitization. silicification, ation to perigranitic veins in (meta-)sediments hematitization,K-feldspath2ation but do not reveal any apparentlink to granitic intrusions. 3.1 and3.2) Mineralizarion(Subrype - Principal uranium minerals: pitchblende. locall;-uraninite, coffinite,and alteration productsthereof Host Environment of Granite-relatedDeposits - Associated metallic minerals: dominantll' - Orogenicbelt pyrite, marcasite,and minor Cu, Pb, Zn, Mo - Presence of highly differentiated igneous sulfides complex including peraluminousleucocratic - In somedepositseither arsenidesor sulfidesof pitchblendebut not Ag, Co, Ni, Bi accompany granite of crustal origin denved by multistage paragenetically magmaticand deutericprocesses - Uranium content in granite above Clark - Gangueminerals:quartz,chalcedony,carbonates,fluorite, baryte(in class3.1.1 presentin standard(>5 ppm) - Uranium fixed in leachablephases(uraninite) minor amountsand sometimesalmost absent. - Sufficient siz^eof pluton (outcrop extensionat in class3.1.2abundant) - U and associatedmineralsare mostly present least 100km') - Presenceof leucocratic(pegmatite,aplite)and in severalgenerations - U and ganguemineralsform irregularlyshaped mafic (lamprophyre)dikes - Limited depth of erosion level (indicatedby and discontinuousore shoots separated by barren intervals abundanceof roof pendantsin pluton, dikes, - Low frequencyof uranium ore occupancyof etc.) - Structuralvein control by commonlyone, + host structurescommonlyin the range from 5 to 30oh in subtype 3.1. more continuous in parallel oriented dilationalfracturesystem subtype3.2 Host Environment of Not Granite-relatedDeposiu - U mineralizationis often associatedwith inof veins(changein trend. thickhomogeneities - Orogenicbelt ness)and wali rock lithologies - Absence of nearby granitesor other igneous uraniumsourcerocks - Host rocks are folded sedimentsor metasedi- Age Consrraints(Subrype 3.1 and3.2) ments, the latter often of mafic composition No stratigraphic age constraint and metamorphosedup to amphibolitefacies Restriction to late orogenic stages - Intense brittie deformation by major faults In Europe the dominant association of subtvpe - associatedwith abundant subsidiaryfractures 3.1 is with the waning stage of the Hercynian and brecciaswhich may form stockworksor Orogeny horsetailpatterns - Emplacementof mineralizationpreferentially in subsidiarystructures Principal RecognitionCriteria
Metallogenetic Aspects
RegionalAlrcration (Subtype3.1) - Albitization,muscovitization - Feldsparand mica episyenitization
Poty et al. (1986) suggest f.or granite-related pitchblende-vein deposits in the Hercynian orogenic belt in France (class 3.1.1) the following
m e t a l l o g e n e t i cm o d e l . A l t h o u g h l a r g e l y b a s e d on research of intragranitic veins, the model appears, at least to some extent. to be also applicable to perigranitic vein deposits except for tectonic processes,creating the structural frame i o r v e i n e m p l a c e m e n t ,a n d . p e r h a p s , i n c o r p o r ation of uranium from metasediments. Evolution of granite-related vein uranium depositsstarted dunng the waning episodeof the Hercynian Orogeny marked by generaluplift, and intrusion of granite and a suite of leucocratic to mafic dikes. Certain peraluminous leucogranites contain uraninite as a primary magmatic rock :onstituent distributed in two modes (a) homo,:enously throughout the granite and (b) acc u m u l a t e d( u p t o 1 0 0 p p m U ) a l o n g s y n m a g m a t r c shear zones. In zones of intense faulting, high heat flow. in response to tectonism (?) generated deep reachingconvectivecirculationof mixed connatemeteoric waters in late to post-magmatic time. The fluids leached the uraninite and transported uranium as uranyl-carbonate complexes to the marginal zones of the pluton. Precipitation of pitchblende occurred in either dilational fractures (vein mineralization) or vesicular, vuggy episyenite bodies (disseminatedmineralization). Precipitation supposedly resulted as a responseto boiling of the hypogene convective hydrothermal fluids triggered by either a pressure drop, and/or by reactions with mafic rocks. Pitchblende crystailization was accompaniedby deposition of pyrite and gangue minerals (mainlv quartz, carbonate) and by K-metasomatism and muscovitization of wall rocks adjacent to veins. In several subsequent stages, hematite, coffinite, marcasite, and additional gangue minerals formed. most of them on account of the orieinal ore generation. A supergene overprint redistnbuted part of the uranium. Petro5 et al. (1986) postuiate a metasedimentary uranium origin rvith subsequentenrichment durine magma differentiation and final hydrothermal redistribution into dilational structures, possiblvwith some contribution of uranium from residual magmatic fluids for perigranitic (Bohemian tvpe) veins (class 3.1.2.1) in the Piibram district. CSf'n. Metallogenetic hypotheses of lberian type deposits (3.1.2.3) are contentious (perhaps due to limited exploration in depth) and include concepts such as supergene.lateral secretlonary and hvpogene origins.
lvletallogeneticconsiderationsfor subtype 3.2. not granirc-relatedveins. face in manv casesthe problem of an adequateuranium source and a mechanismfor propelling the mineralizing fluids. Wallace (1986) suggests for the metamorphite hosted Schwartzwalcierveins (class3.2.1) that the source of uranium and all other vein components was the metamorphic terrane surrounding the deposit. Leaching of the elements occurred by evolved connate hvdrothermal fluids. The fluids probably derived originally from meteoric waters and resided in deep cataclastic zones. Mobilization of the fluids was generated during the incipient uplift of the crvstailineblock of the Front Range in the course of the Laramide Orosenv. lvlieration occurred along permeable fault zones created bv repeated deformation of brittle rocks. The connate hydrothermai solutions were carbonate-rich and produced successive wall rock alterationsaround fractures and several generations of vein mineralization during repeated major movement along faults. Mineral deposition resulted durine episodic brecciation that reduced the confining pressurewhich simultaneouslyincreasedthe pH and decreasedf(COz) and f(O2). Reduced sulfur species in solution reduced the uranium carried in solution. Similar processes may have formed the sediment hosted Shinkoiobwe deposit (class3.2.2). In this case sediments rvith anomaious contents of uranium and other metals (Sdrie de Mine) probably provided the vein forming elements.
Remarks Vein depositsare otten composed of smail lodes and almost ahvavsoccur in grouPs which cumulativeiy yield small to large resources.Grades are highiv variable and many known depositscontain sectionsof verv high grade ore.
{.3. I Subtype 3.1: Granite'related Class 3.1.1: lntragranitic (Limousin type) 3 . 1 . 1 . 1v:e i n si n g t a n i t e Type Example:Fanav.Limousin.France pipes in episvenite i.l. i.l: disseminations Margeride.France PierresPlantees. Tvpe E.xample: Cariou 196{: Cathelineau1985:Friedrichet Reierences: 1982: LerovandCathelineau 1978b: al. 1987:Lerov1978a.
78
4 Typology of Uranium Deposits
Host Rocks/Structures
M ode of M ineralization/ D imensio ru
Ziegler and Dardel (1984) define ore-hosting Veins in Granite. Veins, veinlets or stringers of fertile granite as a highly differentiated two-mica pitchblende and associatedminerals occur in leucogranite with an average composition of ca. simple linear configurationbut more commonly 36o/o quartz, 27% orthoclase, 27"h albite, 10"/" occupy a complex network or stockwork of muscovite and biotite, enriched in Be, Li. F, Sn, fracturesor brecciasparticularlyin proximity to W. Th and U. The main facies is medium to lamprophyre dikes, roof pendants, and other coarse-grained and has intercalations of a fine- inhomogeneitiesin the host granite. Individual grained facies. The granite is of peraluminous veinlets/veins vary in thicknessfrom a few mm to composition derived from crustal material rarelv more than 5 m. Lateral and vertical con(Friedrich et al. 1987). Lamprophvre, pee[natite, tinuity is variable, averagingfrom less than 1m aplite and micro-granite dikes transect the to some 10m, but may be as long as 1000m complex. is generallylessthan and more. Depth extension Mineralized structures are dominantly' con- 300m. Mineral distributionalso varies considertrolled by a dilational systemof a distinct direction ably. Reservesrange betweenless than 1mt to (NW-SE in Limousin). Major faults are rareiv some100mtU-rOsand gradesberweenlessthan or not mineralized. The most favorabie host 0.1'h and up to 40%U3Osin some ore shoots structures are those of secondary order, forming (the Henriette deposit,France, vielded 120mt + parallel fractures or complicated interwoven U3Os at an averagemining grade of 37"/"U3OB stockwork systems. from a vein averaging5m long, 0.4m thick and
250m deep). D isseminations in Episyenite. Mjner alization is hosted in irregularlypipe-or lense-shaped bodies Aheration of mica episyenitedevelopedat intersectionsof Pre-mineralization alteration consists of albititwo structuresystems.Vertical extensionsrange zation and episyenitization. Episyenitization is between 30 and 200m, and diameters between reflected by dissolution of quartz with destruction few meters and several10m. Grades are generand neoformation of minerals leading to feldspar- ally high, up to 1% U3O6and more. Where minepisyenite (commonly barren of U) and mica- eralized veins intersect episyenitepipes grades episyenite (Leroy and Cathelineau 1982). may increase to between 1 and 10% U3O8. Ore-related alteration extends usually for less Reservesvary betweena few tonnes and several than 0.5 m into wall rocks adjacent to veins and 100mtU:Oe, rarely 1000mtU3Osor more. The includes + total muscovitization (phengitization) type example PierresPlant6esin the Margeride of K-feldspar, chloritization of mafic minerals district, France, is an episyenitepipe measuring and pyritization accompanying pitchblende em- 35 to 50m in diameter,175m deep and yielded placement. A second phase of alteration asso- 1300mt U3Oswith an averagemining gradeof 0.3 ciated with coffrnite formation after pitchblende to 0.5%UsOe.Mining districts may coveran area consists of hematitization, montmorillonita few km to some10km iongand *'ide (Limousin ization, adularia formation, and silicification districtFrance:ca. 15km long, 5km wide). partly bi/ replacement of first stage alteration products.
Resources Individual vein systemscontain reserves that range from some L0mt to several100mtU3Og Pitchblende, coffinite and the alteration products but may be as large as 4000mtU3O6(La Comthereof are usually associated with pyrite, mar- manderie,Vend6e, France),at mining grades casite, melnicovite, quartz, chalcedony,fluorite, rangingfrom 0.15%U3O6(La Commanderie)to baryte, and calcite. Gangue minerals occur in 37"/"U3O8 (Henriette). Episyenite-hosted ore minor amounts and locally are absent. Other bodies have reserves of a few tonnes to sulfides (galena, bismuthinite, a.o.) are scarce, 1300mtU3Os(PierrePlant€es)averaging0.3 to but occur in some deposits in significant amounts I"/"U3Os. Districtshave resourcesin the order (e.g., Bois Noirs, Forez, France). of 5000mt U3O6(Morvan.France,averagegrade Ore and Associated Minerals
Tvpe 3: Vein to at least 35 000 mt U3Og 0.16% U:Os) (Limousin, average grade0.2"/"U:Os). Remarks F o r m o r e d e t a i l s s e e C h a p t e r5 . 3 . 1 . Examples of Vein, Class3.1.1 Intragranitic DepositslOccunences Brazll: ? Itataial Cearit Canada: Millet Brook/Nova Scotia. ? Gunnari Saskatchewan China: Xiazhuang/Guidong Massif, Ruijin/ Jiangxi. SE China, Xian/Centrai China France: Limousin, Marche, Margeride, lvlilievaches, Morvan districts/MassifCentral, Vend6e district/Armorican Massif Germany: Menzenschwand/BlackForest. Grosschloppen/Fichtel gebirge Portugal: Alto Alentejo, Beiras districts Spain: Los Ratones/Caceres
Class3.1.2:Perigranitic 3.1.2.1:Veinsin (meta)sediments, monometallic (Bohemiantype)
79
Alteration Pre-uranium host rock alteration includes silicrfication, sericitization, and carbonarization. Uranium mineralization related alteration commonly extend from 0.1 to 3m. locally to severaltens of meters (in zonesof closely spaced or bifurcating veins.; into the wall rocks and include sericitization. chloritization. and hvdrohematituation. Ore and Associated .\[inerals Princrpal uranium minerals are pirchblende, coffinite. uranium-anrhraxolite (hieh polymeric bitumen containing pitchblende, cotfinire. calcite and other minerals). and the alteration products thereof. Associated metallic minerals include sulfidesof Fe. Pb. Zn. Cu. a.o., which are present in minor to trace amounts. Gan_rueis abundant, and includes carbonates (dominantly calcite), quartz. and chlorite. Certain ore and gangue minerals form a characteristic paragenesisdeveloped during consecutivestages. M o de of M ineraliz ario n I D imensioru
Prominent types of mineralizarion are pitchblende-calciteveins. emplaced both inside and immediately outside of the intrusive contact and Type Example: Piibram, Central Bohemian mixed uranium-anthraxolite-pitchblende veins Pluton.CSfn occurnng at some distance from the contact. Reference: Petro5et al. 1986 Uranium and gangue minerals form stringers, veiniets. coatings. reniform accrerionsand pods, that range from mm to several 100cm wide. This Host RockslStructures assemblage aqgresares to irrezularlv shaped Geologv (at Pifbram) consistsof weakiy meta- tabular or lense-likeore shootsthat range from 1 morphosed Upper Proterozoic schists and to severaltens of m long and deep. and a few cm Cambrian conglomeratesand sandstonesfoided to 5m wide. interrupted by barren. gangue, or into a large NE-SW trending anticline. The SE gouge-filled inten'ais within mineralized struclimb of the anticline is occupiedby granitic rocks tures. Ore shoots commonlv develop at inhomoof the differentiated Central Bohemian Pluron. geneities in the veins (change in strike or dip, Three dominant fault systemstransectthe region. thinning or widening, change of wall rock Regional faults subparallelingthe intrusive con- lithoiogies. spiaying of structures). Ore-bearing tact and anticlinal axis partition the anticline into structures mav persist to as much as ?000 m deep longitudinai segments. The segment closest to containing irreguiarlv distributed ore shoots at the pluton hosts the mineralized veins. These variable depth. Ore shoots usuallv occupy beveins mostly occupy secondor third order faults tween 1 and 50% (averageabout l0 to i5%) of a trending preferentiallv perpendicular to oblique structure rvith the remainder being barren. to both the anticlinal axis and intrusive contact. A Mining districts mav cover areas a few to some wide varietv of magmatic dikes occurs, including 10 km long and rvide and mav contain up to aplitic granite. granite-porphyry, apiite. pegma- several tens of mineable veins (Pfibram I to 2 km tlte and lamprophvre. rvide. 15 km long. about 30 shafts).
80
4 Typology of Uranium DePosits
granite. Three dominant fault sets transect the district. Mineralized veins preferentially follow Individualvein systemscontainfrom a few tonnes minor faults oriented perpendicular and oblique to a few 1000mtU3Osat gradesvaryingbetween to the anticiinal axis. ca. 0.7"/" to some 10%U3Os. Districts may A wide variety of magmatic dikes occurs, accountfor a few 100mt to several10000mt U3Og including aplitic granite, granitic porphyry, (Pifbram presumably yielded in the order of pegmatite, and lamprophyre.
Resources
50000to 60000mtUsOe,at gradesof about 1 to 2%U30). Remarks For more detailsseeChapter5.3.2. Examplesof Vein, Class3.1.2.1,Perigranitic M onometallic D ep ositsI Occurrences
China;seeclass3.1.1 CSFR: Damdtice/CentralBohemianPluton. Brezinka/ZelezneHory France:? Bois Noirs/Forez.LesBondonsll,oz6re, Massif Central. La Dorgissidre, Le Rousset/ Vendde, Pennaran, Materie Neuve/Gu6rande Peninsula,Armorican Massif Greenland: ? Puissagtaq/IgalikoFjord, SE Greenland USA: ? MidniteAilashington [may belong to srrata-structuretype (16)]
Class3. 1.2: Perigranitic polymetallic 3.1.2.2: In metasediments, (Erzgebirgetype)
Alteration Pre-uranium host rock alteration is reflected by chioritization, phlogopitization.pvritization. and calcitization associatedwith destruction of mafic minerals. These alterations are overprinted by intense silicification with replacement of earlier formed minerals. Uranium mineraiization related alteration rarely extendsmore than 10 cm into the wall rocks and is reflected by' hematitization of pvrite, dolomitization of calcite and recrvstallization of albite and adularia in calcite veins. Ore and AssociatedMinerals Principal uranium minerals are pitchblende, coffinite and alteration products thereof. Associated metallic minerals present in minable quantity are primarily sulfides and sulfarsenides of Ag, Co, Ni and Bi. Sulfidesof other metals and hematite also occur. Gangue is abundant and includes carbonates (dominantly dolomite), quartz, albite, adularia, fluorite, and baryte. Certain ore and gangue minerals form characteristic parageneses developed during successive stages. Mineral phasesoccur in severalgenerations.
Type Example: St. Joachimsthal/Jdchymov, M o de of M ineraliz ation / D imensio ns Massif.CSnn Karlovv Vary/Eibenstock Two principal varieties of vein forming mineral assemblagesare distinguished. "Slzple veins" are composed of pitchblende, carbonate, and gouge Host RockslStructures with mylonitized rocks, often selvaged by quartz, consists intensely of adularia, albite, and fluorite. These veins occup,v Geology at Joachimsthal folded Upper Proterozoic to Lower Paleozoic, structures of second or third order. Length partly pyritic or graphitic mica schists, phvllites commonly is 150 to 400m and width 3 to 25cm, and calc-silicateunits, folded into an asymmetric rarelv 50cm. "Complex veins" are composed of anticline. The metasediments are intruded by several generations of pitchblende, carbonate. the highly differentiated Eibenstock Massif which and younger carbonate-arsenideand quartz-sulfide includes a younger deuterically (?) altered ieuco- assemblages, intermixed with brecciasand gouge. These veins occupy higher order structures. cratic granite facies characteized by high U/Th ratios. This facies underlies and partly surrounds Length of the veins may exceed 1000m (max. the mineralized district in a form akin to a half- 22Nm). Width is from 10 to 60cm. bowl at a depth of as much as 1000m deep. Mineralization occurs as erraticallv distributed A contact-metamorphicaureole (hornfels, more ore shoots (a few m2 to rarelr' 1000m2 in size) massive texture) up to 60 m wide surrounds the seoarated bv barren intervals down to a denth Reference:Kominek and Vesel(' 1986
Tlpe 3: Vein
of about 700 m below surface. Cumulative occupancy of a structure by ore, although highly v a r i a b l e ,a v e r a g e sl e s st h a n i 0 % . A verticai mineral zonation is reflectedby predominance of arsenides and native Ag in the upper levels (down to 400m), arsenides and native Bi in the medium to lower levels down to about 700m below surface.The bulk of uranrum occurs in the lower level. Near and within the contact-metamorphosed zone enveloping the granite, at distances of ca. l0 to 120m from the contact ore structures tend to lose uranium mineralization except for a verv few caseswhere '.r'ideveins contain some uranium down to the rranite. Structural ore control and distribution corr e s p o n d st o t h a t d e s c r i b e du n d e r c l a s s3 . 1 . 2 . 1 . Resources individual vein systemscontain from a few tonnes to several hundred tonnes U:Os. Grades range from <0.1% to severalpercent UrOa. In addition. high grades of Ag, Co, and Ni may occur. Known districts account for a few 100mt to 10000 mt U3Og and more (St. Joachimsthal presumably yielded 12000mtU:Oa, ranging in grade from 0.1 to 1% U3O8).
81
Host RockslStrucrures Hosts rocks are pelitic-psammiticsedimentsthat exhibit little or no effect of resional metamorphism (schist-grerrvackecomplex of Proterozoic to Lower Paleozoicage on the Iberian Peninsula) but is contact-metamorphosed into hornfels, speckled andalusite-cordieriteschisr etc. up ro approximatelv 2km. from the sranite intrusion. Granites and vein composition are similar to those in Limousin (see class3.1.1). Zones of intense fracturins. brecciation and shearing disrupt the metasediments. Faults displace the granite contact. Dikes of microgranite and lamprophyre occur. Alteration Cordierite megacn'stsmay be completely altered to aggregatesof sericite and muscovite. Chloriterich bands crystallized. Considerable hematitization occurs around mineralized zones. Ore and Associated Minerals Principal uranium minerals are hexavalent U species.Rare pitchblende and coffinite associated with quartz, pyrite. and chalcopyrite occur at depth.
Remarks Mo de of Minera li : arion I D imensions
For more detailsseeChapter5.3.3. Uranium minerals occur as disseminationsmainly in the weathered or oxidized zone from 0 to 40 m Examples of Vein, Class3.1.2.2. Perigraniric deep. within intenselv tractured slivers. The P o ly metal lic D epos itsI O ccurrences minerals impregnate and coat fracture and Canada: ? Port Radium. Echo BayiGreat Bear schistosity surtaces. Slivers extend laterally for tens to hundreds of meters. lvlineralized zones Lake. NWT CSf'R: Javornik. Bila Voda. Rychlebske Hory composed of slivers with discontinuous mineralization have rvidths up to 2 km and may extend (Glatzer Schneeberg) Sermanv: Alberode-Niederschlema. Pohia- for a length up to several km along the granite Tellerhauser, Schneeberg, Johanngeorgenstadt. contact (Nisa deposit Portueal: 5 km long, 1 to 1 . 5k m w i d e ) . Erzgebirge. Wittichen/Black Forest Great Britain; Cornwail region Poland: Kowarv-Schmiedeberg/Riesengebirge. Resources Kletno/Snieznik Klodzki (Glauer Schneeberg) Individual deposits contain some l0mt to about USA: Black HawkA.{ew Mexico 2500mt U3Os (Nisa). Districts have a t'ew 1000mt to some 10000mt U;Os. Average grade is commonly low, about 0.1% UjOs or less. ClassJ.1.2: Perigranitic 3. l. 2.3 : In contact-metamorphics(Iberian type) Type Example: Alto Alentejo. Portugal Reference: BashamandMatosDias(1986)
Remarks For more details see Chaoter 5.3.-1.
82
4 Typology o; g161lrrm Deposits
mostly of Fe, Cu. Pb, Zn, Mo and trace amounts of Ag, Co, Ni, Hg, Sb, and others. Gangue minerals may be carbonates(calcite, dolomite, Italy: Alm Bos/AdamelloMassif ankerite, rhodochrosite), quartz, chalcedony, Poland: Kowary-Schmiedeberg/Sudetes adularia, albite, fluorite, baryte and chlorite. Pornrgal: Beiras district All minerals mav be present in one or several Spain: Don BenitofBadaj6z, Ciudad Rodrigo generations. district/Salamanca USA: Little Man MineAilvomins Mode of Mineralization Examplesof Vein, Class3-1.2.3, Perigranitic Dep ositsI Occurrencesin Conuct-metamophtcs
4.3.2 Subtype 3.2: Not granite-related Class3.2.1: In metarnorphicrocks Tlpe Example: Schwartz*'alder,Colorado,USA Reference:Wallace1986 Host Rocks/Stucture Host rocks include a variety of mafic metasediments often, but not necessarily,metamorphosed to amphibolite grade facies. Schwartzwalder ore is associated with large tensional structures (Illinois and Rogers faults) and their branching horsetail fractures that discordantly transect strata. They contain ore atmost exclusively where they cut horizons of garnet-biotite-gneiss (protoiith: iron and sulfur-rich pelitic sediment) and quartzite adjacent to hornblende gneiss (mafic volcanite). Regionally, the Schwartzwalder deposit is situated within the mobile belt of the Laramide Orogen a few tens of km away from the Colorado Mineral Belt. Alteration Several types of alteration ma1' affect the host rocks. These are commonly restricted to a zone of less than 2m into the walis from veins. Schwartzwalder veins are selvaged by earlv carbonatization and sericitization of the mafic wall rock constituents and later hematitization and K-feldspathization (adularia) replacing the earlier alteration products immediately proximal to a vein. Ore and Associated Minerqls Principal uranium minerals are uraninite, pitchblende, coffinite, and in oxidized zones hexavalent U minerals. Associated minerals can include a variety of sulfides, arsenides, selenides
Uranium and gangue minerals form stringers, veinlets,and veins within larger tensionalstructures and particularlyin horsetailfractureswhich branchawayfrom the main lodes.Mineralization is relativeivcontinuousalthoughgradesare highly variable.Abrupt changesin strike and dip of host structuresare associated with differencesin width of veins and ore quality. Commonly wider structures correspond with lower grades. Sites of branchingveins are often marked by high grade ore accumulation. Age Constraints Thereare no ageconstraintsexceptthe restriction to orogenicbelts. DimensionslResources Individual veins (depositsin brackets)may be a few meters to 150m (<200m) long, severalmm (50m) to more than 15m (150m) wide and m to 150m (in excessof 1000m) deep, containingup to >10000mtU3O8at (average)gradesranging from 0.2 to 0.4"t'"U3Os occasionallyto several % UrOa. Districtsmay containseveral10000mt U:oe. Remarl<s Class3.2.1of vein depositsresemblesvery much perigraniticvein deposits(3.1.2.1)exceprfor the noticeableabsenceof a granitic intrusion, and relative continuity of mineraiization. Whether the continuity of mineralization is fortuitous and typical only for the Schwartzwalderdeposit or whether it is a generalrule has not been established. For more detailsseeChapter5.3.5. Examplesof Vein,Class3.2.1 Not GranireRelated(M onometallic)Deposits/ Occurrencesin MetamorohicRocks
Ty'pe3: Vein Algeria: ? Hoggar region CSf'R: RoZnii. Ol5f/Moravia, Okrouhl:i Radoui, S. Bohemia,Zadni Chodov,Dylen/ W. Bohemia France: Retail/Vend6e \{ozambique: ?Tete district Romania: Apuseni Mts., East Carpathians Sweden: Arjeplog-Arvidsjaur districts USA: Ascension, Mena/Front Range, Colorado, Coles Hill/Virginia Russia: ?VikhorevkalLake Bavkal resion
Class3.2.2: [n sediments(pol],metallic) Tvpe Example: Shinkolobwe. Shaba, Zaire Demks andVaes1956 Reference: H ost Ro cks I Structures IAlteration Host rocks may consist of various sedimentary lithologies. At Shinkolobwe host rocks are dominantly siliceous dolomite and dolomitic and carbonaceous shales, partly affected by Mgmetasomatism with magnesite replacing dolomite. Regionaily, Shinkolobwe is iocated within a fold belt close to the hinge where the fold axis changes direction and the style of folding changes from open to tight overturned folds associated with thrusting. No granites or other intrusives are exposed in the area. At the deposit. host rocks are heaviiy fractured within a tectonic zone bound bv two major faults about 200m apart ar the surface. Due to complex tectonics, flat-lying rvedgesof impermeable strata formed. They are underlain by more incompetent rocks in which numerous fissures. shears and joints opened, hosting the bulk of the mineralization.
fl3
matrix, replacement masses,nodules and as disseminated particles and aegregatesin the host rocks. Ma;or faults are barren. lv{ineralizationis highly variable in grade and distribution bur in generai fairlv continuous. At Shinkolobwe and other depositsin the Katanga copper belt of Zaire and Zambia. uranium veins alwavs occur in beds underiying the cupriferous strata- which locally contain anomaiouslv U tenors, at the base of a thick pile of sediments of shallow marine origin. Age Constraints No age constraints appear to be valid except a verv likeiv time correlarion with regional tectonism associatedrvirh an oroqeny (in the case of the Shaba copper district rhe core zone of the Lufiiian Orogenv. reflected bv granite intrusions and up to amphibolite srade metamorphism runs about 150km S of Shinkoiobwe). Dimensions I Resources Individual veins (deposits in brackets) may be m to 10m (>200m) long, mm to lm (more than 100m) wide and m to several 10m (>a50m) deep. Reserves are up to ca. 25000mtU3Os at (average)grades raneing from 0.1 to >1% U3Os. Districts may contain up to several 10000mt
U:os. Remarks
Shinkolobwe and other vein deposits of class 3.2.2 display a vein to stockrvork qvpeore distribution discordanr to strata. resembling in many aspects. notablv in its ore and eangue mineral association and strucrural controi eranite-related vein deposits of class 3.1.2.2. Major differences Ore and Associated Minerals include absence of granitic intrusions, a relative ?rincipal uranium minerais are uraninite (at continuity of mineraiization. and in the case of Sirinkolobwe) or pitchblende and alteration pro- the Katanga copper belt. anomalously uraniferous ducts thereof. Associated minerals may include sediments which mav have provided the source Co-Ni sulfides and selenides.sulfidesof Fe, Cu. for uranium and the associated elements. For more details see Chaoter 5.3.6. Mo, Pb, Zn, a.o. and trace amounts of precious metals, phosphates (monazite) etc. Gangue minerals are carbonates (magnesite, dolomite), quartz, chlorite etc. Examples of Vein, Class 3.2.2 Not Granite.\t ode of M ineralizstion
ReIated (P o ly merallic ) D epo s itsI O ccurrencesin Sedimens
Uranium and gangue minerals occur in veins, USA: ? Pitch MinerMarshall Pass. Colorado stockworks, along bedding planes, as breccia Zair e: Kalongwe. SwambolShaba
U
4 Typologyof Uranium Deposits Class4.1.2: vanadium-uranium(a, b) Type Example: a) Uravan Mineral Belt, USA, (Salt Wash type) b) Mounana, Gabon (Franceville rype)
4.4 Type 4: Sandstone (Fig. 4.a) Definition Sandstoneuranium deposits occur in reduced continentalfluvial and less commonly in mixed fluvial-marine(arkosic)sandstonesthat contain, are interbeddedwith and bounded by less permeable horizons. Primary uranium phases are generallyof tetravalentstateuranium and consist dominantlyof pitchblendeand coffinite. Associated organic material in Phanerozoic (post-Devonian)deposits(type 4a) is of terrestrial plant origin as distinct to marine, algae (?) derived material in Proterozoic deposits (type 4b). Based on configuration, spatial relation to the depositionaland structuraienvironmentand/ or elemental associations,sandstoneuranium depositsmay be divided into three overall subtypes and further into classesthat can be gradational into eachother:
Subt"vpe4.1: tabular/peneconcordant( (a) Phanerozoic. (b) Proterozoic)
Class4.1.3: basalchannel(a) (Chinle type) Type Example: Monument Valley, USA Subtype4.2: rollfront (or roll-type) (Phanerozoic) Class4.2.1: continentalbasinassoc.witb detrital carbon (Wvoming rype) Type Example: Wyoming Basins,USA Class4.2.2: mixed fluvial marine assoc.with extrinsic sulfide (South Texas type) Type Example: South TexasCoastalPlains, USA Subtype4.3: tectonic-lithologic Type Example: a) Grants Uranium Region, USA b) Mikouloungou, Gabon [Descriptionof subtlpe 4.3 is includedin sections4. 1.1 and a.1.2(b)l References: (a) Adams and Saucier 1981; Adams and Smith 1981; Boyle 1986; Chenoweth and Malan 1973; Crew 1981; Crawley 1983; Grutt 192: Galloway 1985; Granger and Finch 1988; Grutt 1972; Harshman and Adams 1981; Rackley 1976; Thamm et al. 1981; IAEA/ Finch (ed.) 1985; Turner-Petersonet al. (eds) 1986 (b) Diouly-Osso and Chauvet 1979;Gauthier-l-afaye 1986
Class4.1..1:ertrinsic carbon (a) (Westwater Canyon rype) Type Example: Grants Uranium Region, USA
4. Sondstone
Type
{o/Phonerozoic: ossocioted with orgonic moteriol of terrestriol ptont origin) { b , / P r o t e r o z o i c :o s s o c i o t e o w i t h o r g o n i c m o i e r i o l d e r i v e d f r o m o l g o e )
S u b t y p e 4 . 1i o b u l o r Closs 1.1.1
4.2 rollfront
4 . 3 t e c t o n i c - l i t h o l o g i(co ,b )
r,.'1.3
humote-uronium (o) chonnel./bosol {o) L t.t
1.2.2 introcrotonic/ coost-oloin,/mixed continenlol bosin lo) fluviol-morine io)
v o n o d i u m - u r o n i u m ( c .b )
Fl
Fig. 4.4
U minerolrzotion
l.l
s o n os l o n e
lv
t:
s i l i s lo n e
I wwY l.Ig
volconic flows {bosolt)
l -l
mudstone
l{ .+l |5
bosemenl {gronitic)
w-q
v o l c o n i c l os t i c s
A ,*0,
Type 4: Sandstone
Tabular deposits also referred to as peneconcordant deposits consist of uranium matrix impregnations that form irregularly shaped frequently elongated lenticular masses within selectivelv r:duced sediments. The mineraiized zones are, 'rn a large scale, oriented parallel to the depositional trend but, on a small scale, they crosscut sedimentary features of the host fluvial sandstone. Further subdivision into classes is based on uranium fixing agentssuch as amorphous organic material of extrinsic origin (e.g., humate), or detrital plant debris of intrinsic origin, or metallic ::sociations (vanadium) that occur in fluvial ;)stems. Deposits in sandstonechannels incised into unconformably underiying sediments or crystalline rocks are referred to as basai channel deposits. The primary mineralization may be redistri''stack" deposits in buted into secondary uranium the host sandstone. Rollfront deposits consist of arcuate zones of uranium matrix impregnations that crosscut sandstone bedding extending from overlying to underlying less-permeable horizons. The zones are convex down the hydrologic gradient. They display diffuse boundaries with reduced sandstone on the down-gradient side and sharp contacts with oxidized sandstone on the up-gradient side. The normally oxidized up-gradient sandstone can also be in a reduced state if it has been ;e-reduced through the influx or re-introduction of reductants as found in some deposits of class .1.2.2.The mineralized zones are eiongate and sinuous approximately parallel to the strike, and perpendicular to the direction of deposition and groundwater flew. Further subdivision of rollfront deposits is based on emplacement either in inracratonic basins fiiled with continental fluviai sediments end containing detrital carbon as a potential reductant.or in mixed ffuviai-marinesedimentsof coastal plains containing pynte and marcasite as potentiai reductant that originated from influx of H2S into the host sands. Tectonic-lithologic deposis are discordant to strata. They occur along permeabie fault zones with iinguiform impregnation ,of the adjacent clastic sediments where uranium may form rather thick ore bodies which are also referred to as stack deposits when denved from redistribution of uranium.
Principal
Recogaition
85
Criteria
Host Environment -
Immature permeable sandstone.feldspathicor arkosic, more rarely quartzose and cherty sandstone, pebble conglomeratesor marginal marile or eolean siltstone and sandstone - Dominantlv medium to coarse grain size. rarelv fine or verv coarse (pebble)-grained - Mostly cross-stratified and with lenticular bedding - Coetficient of permeabiiity 75 to 350llm2lday (Austin and D'Andrea 7978) - Abundance of U precipitants/reductants, panicularly carbonaceousmaterial (fragments of woody material + coaiified. humic components, amorphous humate). hydrocarbons (petroleum. "dead oil") and/or sulfides(H2S, pyrite) - Tuffaceous material may be present as volcanic debris within host sands. interbedded tuff-rich layers or overlying bentonitic mudstone derived from tuffs - Often interbedded with impermeable horizons (mudstone) Minerulization -
Multiple mineralized horizons may exist Roilfront deposits are crescent-shapedin crosssection, transgressive to stratification of host sands, in pianview they resemble an irregularly laid pipe; roll fronts can occur in multiple superjacent horizons - Boundaries of mineralized bodies are in some deposits sharp and continuous whereas in other deposits they are highly irregular and diffuse - Tectonic-lithoioeic ore bodies of stack type are muiti-shaped. sometimes Chnstmas-tree like, depending on smlctural distribution and impregnation of permeabie horizons adjacent to host structure - Configuration. size and composition of subtypes seeminglv are a function of (a) rype and permissivity of aquifer/host sandstone, (b) stratigraphic interbedding of permeable with impermeable beds. (c) kind, mode, and distribution of reductants and/or complexing agents, (d) derivation. hydro-chemistrv and flow rate of eroundwater svstem
86
4 Typologyof UraniumDeposits
ated to keep uranium in solution for transport, and limited to the point that reduction can take - Principal distribution in sedimentsof middle placein order to precipitateuranium in ore grade Paleozoicto Tertiary age, i.e., after develop- quality and quantity. Complexingagentssuch as ment of lush terrestrial vegetation carbonateions are highly capable of enhancing - Minor in Precambriansandstones,particularly the solubilityand mobility of the uranyl ion in the in those containing carbonaceous material form of carbonateor other complexesin groundsupposedlyof algae origin (e.g., Franceville water that is neutral or alkaline and that may Basin, Gabon) be oxidizingor reducing(Hostetler and Garrels 7962\. For precipitation,the hexavalenturanium in solutionmust be reducedto tbe tetravalentstate MetallogeneticAspects to form pitchblende or coffinite, the principal uranium minerals in most reduced sandstone Generallyit is acceptedthat sandstoneuranium low-tem- deposits. Under certain conditions uranium deposits are of diagenetic-epigenetic perature origin. Groundwater chemistry and mineralsmay also crystallizein an oxidizing enmigration are instrumentalin uranium leaching vironmentwhen complexingagentsare present, from source rocks and its transportationto a for example vanadium compounds, to fix the chemicalinterfacecommonly provided by reduc- uranyl-ionin form of uranyl vanadateswhich are ing or complexing agents where uranium is fairly stable in oxidized rocks. To reduce the deposited.Essentialparameterscontrollingthese hexavalent uranium a reductant is required. processesand localization of uranium mineral- Many substanceshave been invoked as uranyl ization are depositionalenvironment,host rock reductantsincluding + coalified vegetal, woody' lithology, permeabiiiry. adsorptiveireducing fragments(coalificationnot higher than subbiorganicmatter (humate), agents, adequatesolutions, a uranium source, tuminous),structureless petroleum,"dead" oil, "sour" natural gas.hydroand apparentlyan arid to semi-aridclimate. Fluvial, first cycle feldspathic or arkosic sand- gen sulfide,and pyrite or other sulfides.Bacterial stonesof limited thickness(<10m) interbedded activityis consideredby someauthorsan importwith layers of fine-clasticsedimentsdepositedin ant factor in producinga reducingenvironment. intracratonic basins provide the most favorable The formation of either tabular or rollfront host for large and relatively high-gradeuranium type deposits occurred in response to specific deposits.A marginalmarine environmentis also processes. prospective,but to a lesserdegree.The presence Tabulardepositspresumablyformed stationary of uraniferoustuffaceousmaterialeitheras a con- in locally reduced zones within oxidized sandstituentof the host sandstoneor in the overlying stones.Transportof uraniumsupposedlyoccurred strata may enhancethe favorability, due to its bv weakly alkaline, mildly reducing to oxidizing potentialas a uranium sourcerock. The feldspar groundwatercapableof keepinguranium in solucomponentof the host rock is probabl,vof no tion as uranvl-carbonateuntil it was localll' direct importance in the mineralizingprocess, precipitatedby concentratedreductants,such as but indicatesa gTaniticsource from which the humate,plant remainsor H2S, and for chelation uraniummayhaveoriginatedand an environment or complexingby which organic matter accreted of rapid erosionand sedimentationprovidingthe uraniumfrom the groundwater(Schmidt-Collerus required hydro-physicalconditions, particularly 1979).Granger (1976)proposesthar peneconpermeabilityfor adequategroundwatermigration. cordant depositsremained stationary and accu- Impermeableor less permeablestrata or other mulated their uranium from waters flowing barriersmay be instrumentalin channelingverti- mainly outsideand not through the ore zonesas cally and laterallyuraniferousfluids to favorable in rollfront deposits.To permit such a mechansitesof uraniumdeposition.at the sametime pro- ism, the authorsuggests diffusionof uraniumions hibitingwidespread flushingand dilution of fluids. in the groundwaterto the site of depositionalong Uranium is soluble in large quantities only a concentrationgradient.The gradient develops in its hexavalent state. Therefore. uranium- by the extractionof uranium from the groundtransportingfluidshave to be suffrcientlyoxygen- waterflowingmarginalto the deposit. Age Corutrainx
Type .l: Sandsrone
Unlike tabular deposits, rollfront depositsare of dynamic nature. They form at the down-dip migration boundary of an active but short-lived oxidation interface ahead of a pervasive alteration tongue within originally reduced. pyritebearing sandstone.The spatial distribution of the altered tongue and unaltered sandstonesuggests an oxidizing uraniferous groundwater of neutral to slightly alkaiine nature that moved downdip in a confined aquifer in response to a hydraulic gradient. As the oxygenated solution moved downdip in the permeable horizon it penetrates the reduced facies. Organic material and sulfides. lainly pyrite, were destroyed.and ferric iron was produced until the water lost its potential for oxidation. In zones of abundant reductants,such as detrital carbon (commonlv plant debris, class 4.2.1) or extrinsicsulfides(H2S. pyrite, marcasite, class 4.2.2) a distinct geochemical front developed with an abrupt redox interface. At this site and for a short distance ahead of the ferrous/ ferric iron interface, the uranium transported in the oxygenated solution is reduced and deposited as pitchblende. Since the geochemical alteration system was of dynamic nature. the influx of oxidizing groundwater continued. The alteration front "rolled" downflow and spread laterally towards the boundaries of the transmissive host bed. Previously crystallized pitchblende became decomposed and the uranium redistributed across the iront for renewed deposition as long as the system was operative. The crescent shape of a rollfront ore body (in cross-section)is considered evidence for the dynamic movement of the front which migrated more rapidly in the more permeable units mostly in the middle part of the sandstone laver.
87
stack wpe depositsare rn Tectonic-lithologic part consideredthe product of redistributionof primary uranium inro permissivehosts such as structuresby youngerprocesses astvpicallyfound in the Grants lllanigm Region. but likewise they may have originatedby the introductionof primaryuranium as interpretedfor somedeposits in Gabon(Mikoulouneou). The sourceof uranium and associatedmetals in sandstonedepositsis consideredto be either of uraniferousgranitic provenance,or clastic granitic material or felsic volcaniclastics in the (e.g., host sandstone.or felsic volcaniclastics in rhyoiitic tuff) overlvins andlor underlying beds.
Remarks Sandstoneuranium depositsare commonlyof low to mediumgrade(0.05-0.4%U3Os)and of small to large size, up to 50000mtU3O3rarelv more, but districts may have substantialresourcesof several100000mtU:Os.
ExamplesofSandstone,Subtype4.1, Tabular (Not Differentiated) DepositsiOccurrences
(C : Cenozoic.M = Mesozoic.P : Paieozoic, PC = Precambrian) Argentina: Tonco-Amblayo (M)/Jujuy-Bolivian Basin,Los Colorados(M)/PaganzoBasin. Sierra Pintada (P)/San Rafael Basin. Malargue (M)/ NeuqudnBasin.PichinaniChubut(M) Australia:Angeia (P)/AmadeusBasin,Malbooma (P)/Drummond Basin. Mulga Rock (P)/Officer (PC)/Queensiand, ManyinBasin.Westmoreland Most of the rollfront depositsare considered geeW.A. the resulrof a singlemajorcvcleof uplift, erosion, Brazil:Figueira(P)ParanriBasin by fluidsmov- Buigaria:Orranovo-SimitliBasin(C), Eleshniza ,edimentation,and mineralization ing in responsero the samehydraulicgradients district(C), W. Balkan Mts. (P) which have been responsiblefor the sedimen- China:Mengqiguer.Daladi. Kashi(M)/ZhungelAnother varietyof TianshanUprov..Jianchang,Quinlong,Langshan tation of the hosr sandstone. rollfront deposits, although verv similar to the areas (M)IYinshan-Liaohe U prov., Wudangtvpe describedabove, are related only to the HuafvongMassif (M)/Qilian-QiniingU prov., physicalandchemicalnarureof the hostsediments SouthLancangRiver. Gaoligong(C)/W. Yunnan but not ro rhe hydrologicsystemresponsiblefor V p.ou., Tunling (M)/Jingan Basin the host rock deposition.Insteadthey originated CSFR: Hamr-Striiz(M)A{orth BohemianBasin iongafterdepositionof the hostrocksby a second Germany: Konigstein (M)/Elbsandsteingeb., cycleof uplift, erosion,anddeposition(Harshman Mtillenbach(P)/Black Forest Basin Gabon:Oklo (PC)/Franceville and Adams 1981).
88
4 Typology of Uranium Deposits
France: Coutras (C)/Aquitaine Basin, Lombre (P)/Cerilly'Basin Hungary: Pecs(P)/TvtecsekMountains Madagascar:Antsirabe Basin (C) Niger: Akouta, Arlit. Ebala, Madaouela. (P), Azelik, Imouraren,Takardait, (M) /AgadesBasin Pakistan:Dera Ghazi Khan area (C) Romania:Apuseni Mts (P), Banat (P) SouthAfrica: Beaufort West (P)/KarooBasin ospain: Mazarete(M) USA: ColoradoPlateau(P + M), Sherwood(C)/ Washington, TallahasseeCreek (C)/Colorado, Anderson(C)1DateCreek Basin. Arizona Slovenia:Zirovski wh (P) Russia: Stavropolskl'.P-vatigorsk,Krslovodsky, Lermontovsky(P)/Caucasus
Examplesof Sandstone,Subtype4.2 RoMront (liot Differentiated) Deposits/Occurrences Australia: Beverlev (C)/I-ake Frome Basin, Manyingee(M)/Carnarvon Basin Bulgaria:TracienBasin (C) Mexico: La Sierrita (C)/BurgosBasin Niger: Akouta (P)/AgadesBasin USA: Wyoming Basins (C), Black Hills (C + M)/South Dakota, Crow Butte (M),Nebraska, Weld County (M)/Colorado, Texas Coastal Plain (C) Uzbekistan: Uchkuduk (M), Sugralv (M), Lyavlyakan (C), Beshkak (C), Bukinay (M), Kanimekh (M)A.{avoiregion, Kyzylkumsky
Examplesof Sandstone, Subtype4.3 TectonicLithologicDeposits/Occurrences Australia: Red Tree (PC)Ailestmoreland France:Mas Lavayre (P),4-oddveBasin Gabon: Mikouioungou. Mounana (PC)/ FrancevilleBasin USA: Ambrosia Lake district (M)/Grantsresion
References:Adams and Saucier 1981;Granger and Finch 1988;Turner-Petersonet al. (eds) 1986
The subsequentsummaryalso addressesclass 4.3.1, tectonic-lithologicstackdeposits,as found in the Grants Uranium Region spatially associatedwith class4.1.1 deoosits. Host Rocks/Structures Type example is uranium mineralizationin the WestwaterCanyon Member of the Late Jurassic Morrison Formation. Grants Uranium Region. New Mexico.USA. Host rock is fine- to coarse-grained,poorly' sorted.cross-bedded mostlylight vellow-brownto gray arkosicsandstone,containingsmall pebbles and cobblesand thin seamsor bedsof gray mudstone and siltstone. Sandstoneconsistsof 50 to 90"/oquarv.- 5 to 35"h feldspar, up to 30% chert andlessthan 0.5% heavyminerals.White clusters of kaolinite are common. Other matrix materials are calcite. iron oxides, and clav. Amorphous carbonaceous materialin the form of humatefills intersticesand coats sand grains. Permeability rangesfrom low to high. Thicknessof the host unit rangesfrom one to severaltens of meters. The WestwaterCanyon Member is overlain by a greenish-gray bentonitic (tuffaceous) siltmudstoneof the Brushy Basin Member. Sandto mud/shaleratiosare L:1 to 2:I for the Morrison Formation withil the mineral belt. Depositional environmentof the host sequences is an extensive coalescing alluvial fan system developed by aggradingbraided streamswithin a continental basin (San Juan Basin). Sourcearea of the host rocks is granitic terrane and volcanic air-fall ash and tuffaceousmaterial. Regional tilting associatedwith major N-S faultingdevelopedduring the Laramide Orogeny, and caused redistribution of primary uranium mineralization into roll-typeand, nearstructures. into class4.3.1 "stack" deoosits. Alteration
Both oxidation and either a single or multiple episodesof reductionhavealteredthe host rocks. Hematitization and weak limonitization characClass4.1.1:Extrinsiccarbon(WestwaterCanyon terize oxidized facies. Reduced facies displav type)(Phanerozoic) destructionof detrital feldspar, volcanic ash, magnetiteand ilmenite associatedwith sulfidiType Example: Grants Uranium Region, San zation (mainly pyrite), kaolinitization,chloririJuanBasin,USA zation, albitization.montmorillonitization,and 4.4. I Subtype 4. 1: Tabular/peneconcordant
Type .l: Sandsrone
ti9
carbon (humate) formation. Calcitization and minor silicification cemenred the units. The carbon with which the uranium is associatedwas introduced into the sandsronefrom adjacent silt' n u d s t o n e sd u n n g c o m p a c t i o n .
and other prospective districts. It constitutesthe largestsingleknown sandstoneuranium region in the western *'orld. F o r m o r e d e t a i l ss e e C h a p t e r 5 . . 1 . 1 .
Ore and Associated Minerals
Examples of Sandsnne, Class1.1.I Tabular, Exrrinsic Carbon DepositslOccurrences
Principal ore minerals are pitchblende, coffinite and uraniferous humate present in reduced ore. Locally hexavalent U minerals occur in oxidized o r e . A s s o c i a t e dm i n e r a l so f A s . B a . F e , M o . S b . V. and others are commonlv present in trace jinounts. Pyrite is more abundant. .ll o de of ;\I in er ali z atio n Class4.1.1 mineralization is characterizedby the associationof uranium with humate in isolated and stacked peneconcordant lenses. Uranium minerals occur as disseminations coating sand srains,fiiling small intersticesand partly repiacing ieldspar in the host sandstone.Pnmary mineraiization occurs in multiple horizons commonlv concomitant with humate. Mineralized zones are lenticular or tabular, peneconcordant elongated parallel with the paleochannel systems which are termed bianked or trend ore. Boundaries between mineralization and host rocks are irregularlv shaped. Redistnbuted mineralization is ,.vithin structures and also displays features of rollfront character deveioped proximal to fault ::nd fracture zones that cut primary peneconcordant bodies. Redistributed uranium forms much thicker ore bodies than those of primary tabular mineralization and is therefore referred to as a stackdeposit.
France: Loddve Basin (P) ?, Aquitaine Basin
(c) ?
South Africa: Beaufon West (P)iKaroo Basin
Class4.1.2: (a) Vanadium-uranium (Salt Wash tlpe,) (Phanerozoic) Tvpe Example: Uravan Mineral Belt, Colorado P l a t e a u .U S A Reference: Thammet al. 1981 Host RockslStntctures
Individual tabular deposits ("stack" deposirs in -rackets) mav be severaltens of meters to 2000m , < 1 0 0 m ) l o n q . s e v e r a lm e t e r sr o s e v e r a l1 0 0 m ( a few m to 40m) wide, a ferv centimerers to 5m ( f e w m t o 5 0 m ) a n d i n e x c e s so f 1 5 0 0 m d e e p , containilg severai tens and up to 50000mtU3Og at (average)gradesrangingfrom 0.1 to 0.4% U3Og occasionallyto >1% U3Os. Districts may contain up to 300000mtU3Os.
Tvpe exampie is V-U mineralization in the Salt Wash Member of the Late Jurassic Morrison Formation as present in the Uravan Mineral Belt, Utah-Colorado. USA. Host rock is a fluvial fine- to medium- and coarse-grained feldspathic to quartzose sandstone. This unit ranses from 50 to 120m thick, is red. brown or gray in color. and contains 5 to 15% tuffaceous material and abundant organic debris in the form of logs and frasments of vegetal trash. These materials accumulated particularlv at sites where paleochannels change direction, in mud bars or changes in stream load carrving ability. Sandstone lavers are interbedded with reddish and _gravsiltstones and bentonitic mudstones. The Salt Wash Member is pan of a coaiescing alluvial fan formed by a svstem of aegrading braided streams. The Uravan Mineral Belt occupies a transversal zone rvhere grain size of the sandstoneis transitional from medium to fine. Sourcearea for Salt Wash sedimentswas the sranitic Mogollon Highland. Positive areas forned by sait diapirs and associated anticlines within the basement controlled river orientation and facies pinchouts during Salt Wash time.
Remarks
Alrcrailon
The Grants Uranium Region is 175km long and up to 80 km wide and includesfive mining districts
Reduction alteration is recognized chielty by color changes and is most obvious in mudstones
DimensionsI Resources( Grants U ranium Region)
90
4 Typologyof Uranium Deposits
turning from purplish or reddish to gray-greenat contacts with mineraiized sandstoneswhich are stainedtight yellow-brown. Common authigenic transformations include pyritization, calcitization, and argillitization. Detrital ilmenite and magnetiteare corroded.
sandstonehad resourcesof a few hundred to severalthousandtonnesuranium. For more detailsseeChapter5.4.2. Examplesof Sandstone,Class4.1.2(a),Tabular, Vanadium-Uranium Deposiu/ Occurrences
Argentina: Rodolfo (C)/Cosqindistrict Australia: Bigrlyi (P)AJgaliaBasin *Principalore mineralsin reducedzonesare pitch- USA: La Sal-La Sal Creek (M). Lukachukaiblende, coffinite, and vanadium minerals. Carrno (M), Henry Mountains (M)/Colorado Oxidized zones are dominated by uranyl Plateau vanadates.commonlycarnotiteand tvuyamunite. Associatedminerals are pyrite and marcasite. Class4.1.2:(b) Vanadium-uranium(Franceville Mo, Cu and Se mineralsoccur in trace amounts. type) (Proterozoic) Vanadium-uraniumratio is 5: 1 to 10:1 in the Uravan Mineral Belt and in other Salt Wash Type Examples:FrancevilleBasin. Gabon districts1 :1 to 15:1. 4.1.2.1tabular:Oklo. Gabon 4.l.2.2tectoniclithologic: Mounana and Mikouloungou,Gabon Mode of Mineralization Ore and AssociatedMinerals
Class 4.1.2 minerelization is characterizedby vanadium-uraniumassociatedwith plant debris. Mineralization occurs as disseminationsfilling pore spaces, coating sand grains and replacing interstitialclay, organic substances, and cementing material. U-V minerals have accumulatedin commonly small pods which locally are highly mineralizedtree tmnks. The pods may display shapes ranging from tabular, concordant to bedding to roll-typ€. Deposits consist of clusters of pod-like bodies aligned parallel to the paleochannel,where thev may occur in severalsuperjacent sand horizons. Distribution of depositsis rather erratic. DimensionsIResources( Uravan Mineral Beh) Individual deposits(ore trends in brackets)may be up to 200m (up to a few km) long, 3"/"V2(J^s.
References:Diouly-Osso and Chauvet 1979; GauthierLafaye et al. 1980
Both type examplesoccur in the same stratigraphic horizon and show a number of similar featuresexcept for the structural componentof mineralizationand respectiveconfigurationof ore bodies.The subsequentsynopsisaddressesboth subtypestogether. H ost RocksIAh erationIStructures Continental sandstone and arkose containing redistributedmarine carbonaceousmatter. The uraniumhostingunit is overlain by carbonaceous shalesand restson early Proterozoicgranite and gneiss.Folding and faulting distorted the sediments. No apparent alteration except some silicification. Ore and AssociatedMinerals Pitchblendeand coffinitein reducedzones.and uranyl vanadatesand phosphatesin oxidzed zones.Associatedmineralsinclude in reduced karelianite, montroseite, mineralization roscoelite,and minor amountsof pyrite, marcasite, meinicovite, galena, sphalerite, chalcopyrite,baryte, and calciteas fissurefillings.
Remarks Modeof Mineralization The UravanMineral Belt is the largestknownVU district in the USA. Most other districtswith Uranium is in tabular deposits(Oklo) concensimilar mineralizationassociatedwith Salt Wash tratedoredominantlvin fluvial sandsin the form
Type .l: Sandstone
of microcrystallinepitchblende within the organic matrix and occurs preferentiallv on the flanks or rims of paleochannelsor where an intermediate thicknessof sandstoneis combined with high con:entrations of organics. N{ineralization of the :ectonic-lithologic subtype occurs in the same organic-rich sedimentological environment but displays a strong affinity to cataclasticzones adjacent to uplifted crystalline basement (Mounana) or to an overthrust of carbonaceous shales (Mikouloungou). 1ge Constaints .'^ certain age constraint is imposed by the required development of organic creatures(algae) in a continental environment. DimensionslResources Tabular deposits are up to 900m long, 600m rvide, and several meters thick and contain as nuch as 13 000 mt U3O6 at a grade of ca. i).4% U306. Tectonic-lithologicdepositsrange up ro 100m long, 40m wide and 120m deep, containing up to 6000mtU3Os at a grade of 0.2 to 0.6'h U:Oa. The Franceviile distnct, Gabon, accounts for approximateiy 25 000 mt U3Os. Remarks {lthough uranium occurrences are not uncom:non in Proterozoic sandstone displaying the above-listedfeatures. the Franceviile Basin is the only one known to contain minable U deposits. F o r m o r e d e t a i l ss e e C h a p t e r 5 . 4 . 8 .
9l
Formadon, as exposed in the Monument Valley, Utah. USA. Host rock is a fluvial mediumto coarse-grained conglomeratic, crossbedded, poorly' sorted. lenticular arkosic sandstone.This unit ranqesfrom 3 to 75 m thick, is eray-vellowish and red to brown in color and contains interbedded greenish-eray mudstone and siltstone. Carbonaceous materiai. logs, and fragments of vegetal trash are locally abundant, especially at bends. meanders of the paleochannels,in mud bars or where changes in stream load carrying abilitl' occurred. The sediments deposited by deeradrnganastomosingstreamsincising distinct, relatively narrow channels into flood plains consistins of red siltstone of the lloenkopi Formation in the Monument Valley. In other resions, deposits may occur in extensive blanket sands formed in braided fluvial systemsthat either unconformably overlie or are eroded into underlving sedimentary or crystalline rocks. Local areas of moderate relief controlled river flow and pinchouts during deposition of the Shinarump Member. Alteradon Reduction reflected by bleaching of the originally red and brown sandstone to light gray, cream to yello*rsh and greenish colors is the most obvious aiterarion. Caicitization caused cementing of sandstone(up to 15% CaCO3). Ore and AssociatedMinerab
Principal ore minerals in reduced zones are pitchbiende. coffirute and locally vanadium minerals (montroseite. corvr.rsite). Oxidized zones are Examples of Sandstone,Class1.1.2(b), dominated by hexavaient U minerals and uranyl P rotero z o ic D ep o sitsI O ccurrences vanadates. Associated minerais can include and minerals -{ustralia: Westmoreland/Queensland,Pandanus sulfides of Cu. Fe. Mo, Pb. Zn. containing Ag. Cd. Cr, Co. Ni. Se. Sr. In several I reek/Northern Territorv districts V,O_. contents exceed U3O8 contents ( V : U r a t i o1 : 2 t o 7 : 1 ) . Class4.1.3: Basal-channel(Chinle type) Type Example: Monument Valley, Colorado Plateau. USA Reference: Chenowethand Malan 1973 :lost RockslStrucures Type example is uranium mineralization in the Shinarump Member of the Late Triassic Chinle
-Vo de of M inerali z ario n Class -1.1.3 mineralization is characterized by uranium associatedwith plant debris deposited in distinct channelspreferentially in their basal parts or scours cut into underlying formations. This situation is in principle also valid for mineralization in all other memben of the Chinle Formation. Uranium minerals imDregnate sandstone
Y2
4 Typology of Uranium Deposits
voids,coatsandgrains,replacequartzgrains,clay particles and particularly fossil plant debris, and fill vertical fissuresbeneath the scour base.Most depositsare of small size.They consistof closely spacedlenticular pods which are generallyconcordantwith the bedding.I-ocalizationof deposits within channelsis controlledby the abundanceof trashwhich otherwiseaccumulated carbonaceous of paleochannels. at inhomogeneities
DimensionslResources
4.4.2 Subtype 4.22Rollfront Class4.2.1: Continentalbasin.associatedwith detrital carbon(Wyomingtype) Type Example:Wvoming Basins'USA References:Crew 1981;Harshman and Adams 1981
Host RockslStructures
Host rocks are Upper Cretaceousand Tertiarv Individual deposits(ore trends in brackets)mav poorlv consolidated medium-to coarse-grained, be a few meters to 300m (a feu' kilometers) poorly sorted arkosesto feldspathicsandstones long, <1m to more than 100m (several100m) that range from a fraction of 1m to 100m thick wide and <0.2 to 3m (<15m) thick, containing (ar'. lessthan 30m). Thin discontinuous beds of L to several i00mtU3O6 at (average)grades mudstone.some pebblesof crvstaliinerocks or ranging from 0.1 to 0.35%U3Os occasionalll' mudstone,and tuffaceousdebris mav be interto 0.5%U:Oe. Districts may contain up to stratified.Pyrite and carbonaceous matter in the 5000mt U:Os. form of woody remains and massesof humrc Remarks Class4.1.3depositsresemblethoseof class4.1.2 except that the vanadium content is commonly' lower and that the depositsare restrictedto disand generallyto tinctly developedpaleochannels only one horizon. For more detailsseeChapter5.4.3. In some countries. e.g. Okanagan region (Blizzard deposit), British Columbia, Canada, and CentralHonshu (Tono. Ninyo-Toge),Japan, U mineralizationoccurs in sand filled channels incisedinto a basementwhichincludesuraniferous granites.U is thoughtto have been leachedfrom the granitesand redepositedin the channelsands. (For details see Boyle 7982b. 1985. 1986, Katayamaet al. 7974.Katavamaand Kamiyama 1977)
Examplesof Sandstone,Class4.1.3 Tabular, BagglChannelDepositsI Occurrences Australia: ? Mulga Rock (P)/Officer Basin, Yarramba (C)/Lake Frome Basin, Manyingee (C)/CarnarvonBasin Canada: Blizzard, Tyee (C)/OkanaganHighlands France:St. Pierre du Cantal (C)/MassifCentral Japan:Ningyo-Toge(C), Tono (C) USA: White Canyon, Lisbon Valley (M)/ ColoradoPlateau
componentsare abundant.In somebasinsnatural gasand methaneare present.The host sandsare regionally transmissive,providing conduits for groundwater migration downdip. Mineralized sands are part of a stratigraphicsequenceof alternating sandstone,siltstone and bentonitic mudstone depositedpredominantly by braided coalescingalluvial fans streamsin wedge-shaped with overbank flood plains. Sand/shaleratios range from 1:1 to 4:1 in the host formations. Provenanceof the host sedimentsare granitic highlandssurroundingthe basins,and reworked pre-existingsediments.Volcanic air-fall ash and pile andoccurs tuff contributedto the sedimentary either as constituentof the mineralized sands. interbeddedtuffaceousrich layers or in superjacentbentoniticmudstone. The attitude of the mineralizedbeds is commonlv verv shallow,usuallvdippingiessthan 5'. Steeperdips up to 25" existlocalll,'.Minor faults dissectthe mineralizedhorizons.Tiiting of some basinssubsequent to ore formation(ShirleyBasin and Gas Hills) causeda reversalin the groundwater flow resulting in incipient destruction of mineralizedzones. Regional controis for the mineralized paleodrainages)'stem included regional tectonic movementswhich were instrumentalin intracratonicdownfaultingof basinsof restricteddimensions in the forelandof fold belts. Grabenstructureshavecontrolledpaieodrainiage svstems.Intraformationalunconformitiesappear to be influentialon groundwatermigration.
Type 4: Sandstone
Alteratton .A,lteration effects vary to some degree from b a s i n t o b a s i n d e p e n d i n go n m i n e r a l c o n s t i t u e n t s r:r host sands. The most conspicuousis hematitization and limonitization associatedivith decomposition of pvrite in some basins.In other basins pyrite corrosion, argillitization of feldspars and montmorillonitization of tuffaceous components may be more dominant. Normally. gray. tan. or pink sandstonesare bleachedby mineral forming solutions to cream or light grav hues. Formation ,tf Fe-rich montmorillonite imparts greenishcilowish colors to the rock. Ore and Associated MineraLs Principal ore minerals are pitchblende and coffinite with associated pyrite, marcasite, hematite. ferroselite. native seleniumand calcite. Minerals of Cu, As, P. and others may be present. Mode of Mineralization Uranium mineralization follows the redox front separating nonoxidized from oxidized sandstone. The mineralization transects the stratification of the host beds and is thus discordant with the strata. In cross-section,the form of a deposit resembles a crescent. In plan view, a deposit :ppears like a sinuous. irreguiarly laid pipe ioilowing the geochemical front of invading oxidation. The uranium minerals impregnate intersticesof the sandstoneand coat sand grains. Pitchblendeand coffinite are concentratedin the front part of the redox zone, ferroselite has formed on the concave side. whereas native selenium. jordisite, and calcite typically occur on ihe convex side of the rollfront.
93
up Io 0.5%U3Oe or more. Districtscontain several10000mtU3O8(65000mtU3Osin Gas Hills. WindriverBasin) Remark F o r m o r e d e t a i l ss e e C h a p t e r5 . 4 . - 1 . Examples of Sandsrone, Class4.2.1 Rollfront, Conrinental Basin .lssociated with Detrital Carbon D epositsI Occurrences Australia: Honevmoon. Beverley (C)ilake Frome Basin. Manvingee (M)/Carnarvon Basin USA: Gas Hills (C), Powder River Basin (C), Shirlev basin (C). Red Desert (C)AVyoming, Black Hills (M)iSouth Dakota. Weld County (M)/Denver-Julesburg Basin
Class -1.2.2:Mixed fluvial-marine, associatedwith extrinsic sulfide (South Texas type) Type Example: South Texas Coastal Plain. USA a) Fluvial-marine: Panna-Maria; b) Fluvial: Benavides References: Adamsand Smith1981:Gailowav1985 Host RockslStrucrures
Host rocks consist of a variety of fluvial to marginal marine, poorly consolidated sandstones that range from 5 to 20m thick. These units are interbedded with or overlain by voicanic ash or tuffaceous beds of several formations. Host rocks include a) fluvial-marine fine-grained carbonaceous, tuffaceous. quartzose sandstone and greywacke interbedded with blackish shale and lignite deposited as lagoonal. deltaic and barrier bar sediments under shallow water, nearshore marine or b r a c k i s hw a t e r c o n d i t i o n s : DimensionslResources b) fluvial fine- to coane-grained sands interIn the apex segmentof the rollfront the thickness bedded with bentonitic mudstone of an aggrading cr height of mineralizationrangesfrom a few tens stream svstem and fine-grained teidspathic sands of cm to 10m and rarely 15m. decreasingin the containing onlv minor vegetal debris, deposited in tails to zero. The width normal to strike (parallel a fluvial-ailuvial fan. stream and interchannel to cross-section)variesbetween.afew centimeters fl oodplain environment. and several 100'sm. The tength along the strike The sandbeds are locally along faults, invaded rnav extend up to several kilometers. by hvdrocarbons, methane and H2S. Distribution Resources range from a t'ew tonnes to several of mineral trends is controlled by broad scale 1000'sof mt U3Os in individual depositsat grades sedimentary features such as megachannel averaging between 0.05 and 0.25% U3O3, rarely svstems.
94
4 Typology of Uranium Deposits
Examplesof Sandstone,Class4.2.2 Rollfront, M ixedFluviol-maine Associatedwith Extrinsic SulfideD eposis/ Occurrences
Faulting is extensive in the South Texas CoastalPlain. A seriesof growth faults trending parallel to the coast line permined influx of hydrocarbons,H2S and methane into the host sands.Likewiseactivesalt domes,while creating favorableenvironmentsfor petroleumreservoirs, also permitted migration of gases. On a regionaltectonicscale,all knownuranium occurrencesare confined to the Rio Grande Embayment.
Karnes,Live Oak, Duval and Webb Counties districts(C)/TexasCoastalPlains,(?) Crow Butte (M)/Great Plains,Nebraska
Aheration
4.5 Type 5: Collapse Breccia Pipe (Fig.a.s)
Severalstagesof reduction, oxidation, and rereductionaffectedthe host rocks, in part similar to the alterationsin the Wyoming Basins (class 4.2.1). Processesinclude pvritization. marcasitization, hematitization,corrosion, and argillitization of feldspars, silicffication. calcification. destruction of heav,vminerals and montmorillonitizationof pyroclastics.H2Sintroducedalong faultsbeforeore formationreducedthe hostsands andpreparedthe hostsfor rollfront development.
Definition
CollapseBrecciaPipe depositsoccur in more or lesscircular(i0-300m in diameter)and vertical (up to 1000mdeep)collapsestmcturesfilledwith down-droppedcoarsefragmentsand fine matrix of the penetratedsediments.Uranium mineralization is predominantlt' confined to particular intervalsof permeable(+ sandy)matrix material and to arcuateboundingfractures(annularring). Principaluranium phaseis pitchblende,which is Ore and AssociatedMinerals commonlyaccompaniedby a variety of ore (Cu, Principalore mineralsarepitchblendeandcoffinite Ag, etc.) and gangueminerals. commonly presentas poorly crystalline,minute particles.AssociatedelementsincludeMo, Se,V, Type Example:Orphan Lode and Hack Canyon, andP. In oxidizedzonesa varietyofhexavalentU Arizona Strip area, USA mineralshave formedReferences: GSA/Wenricband Billingsle.v(eds.) 1986; Wenrichand Sutphin1989
Mode of Mineralization Uranium has accumulated in both penecon- Principal Recognition Criteria cordantblankettype, and rollfront depositsalong paleochannels.Roll-rype deposits greatly re- Host Environment semblethosein the Wyoming Basins. -
D imensionsI Resources Rollfrontdepositsvary from 0.5to 7.5m in thickness,10 to 100m in width and are up to 10km Iong. Individual depositscontain several100 to 5000mtUrOs,rarely up to 10000mt,at grades between 0.05 and 0.2"/"U3O8. Total resources of the South Texas Coastal Plain district are estimatedat up to 80000mtU3Osat an average gradeof about0.1 to 0.2"/"UrOs. Remarl<s For more detailsseeChapter5.4.5.
-
Mineralized structures are roughly circular and vertical, extending from karst caverns in a basal limestone upward through flat-lying beds of alternating shales, siltstones, sandstones. and impure limestones occasionaliy as high as into uraniferous sandstones Pipes are filled with highly brecciated material composed of fragments and matrix material of the various penetrated lithologies Uranium mineralized pipes appear to be restricted to an area covered or formerly covered by sandstones that locally contain uranium deposits (Chinle Formation) Pipes are bounded by a set of concentric. circular fractures termed annular rins
T1'pe ,i: Collapse Breccia Pipe
Type
5.
COLLAPSE
BRECCIA
PIPE
F_
silty sediment corbonotic sediment porty sondy
t...t
l'.'. . 1
sondy sediment
E
froc!.lre
m
E
95
breccio pipc filled with frogments ond motix
o I
|r)
50-200 m Fig. .1.5
U minerolizoUon in pipe*onnulorring
Alteration
.A
,RRD
a sulfide cap (3-15m thick. up to 80% sulfide)
Bleachingof red sediments,carbonatization, above the uppennost U mineralization silicification, and some sulfidization of pipe infill and immediatelyadjacentwall rocks Age Constrains Alteration is commonlyof weak intensityand little extensionfrom pipe into wall rocks No principal time constraint except that the type Vineralization - Principal U phasesare pitchblendeand coffinite and alteration products thereof - Associatedmineralsinclude sulfides.arsenides. and locaily oxides. carbonatesand sulfatesof Fe, Cu. Ni. Co. Mo. Pb, Zn, As and trace a m o u n t so f A g , A u . H g , S b a n d V - Gangue minerals include calcite. doiomite. siderite. baryte, collophane, chalcedony, quartz, gypsum. and anhydrite - Uranium minerals occur (a) as veinlets and stringers in tractures within and surrounding the pipe, (b) as stratiform disseminationsin porous sandstoneblocks and irregular impregnations of the matrix within the pipe interior - U mineralization is particularly associatedwith sand dominated pipe segments - Sulfides are disseminated through most of the prpe - Pyrite is enriched up to 15% in mineral2ed zones and often forms together with marcasite
of sedimentation. hvdraulic groundwater systems (karst development) and provision and transport of sufficient uranium require a coincidence of certain climatic conditions that prevail only during restricted periods in earth history.
Metallogenetic .{spects The metallogenesisof uranium mineralization in collapse breccia pipes of the Arizona Strip type is stiil enigmatic- Uranium is thought to have entered a pipe either through sandstone aquifers interbedded rvith the sedimentary sequence or through structures enending upwards into overlying uraniferous sandstone horizons or perhaps through both. The pipe structure formed by stoping from a karst cavern upwards through the flat-lving sediments possibly along joint or fracture systems. Sulfur. organic material, and other elements may have derived from surrounding sediments or mav have been introduced from basal lithologies by upward migration.
4 Typology of Uranium Deposits
96
4.6 Type 6: Surficial (Fig. a.6)
Dimensions/Resouroes Pipe diameters range from 10m to more than 300m, averaginglessthan 100m. Within the pipe. mineralizationis restrictedto variablywide sandy sections, and fissures. Vertical extension of mineralizedintervals(commonly one rarely two) ranges from few meters to more than 100m. Resourcesof individual pipes are few tonnesto aboqt 2500mtU3Osat gradesbetween0.3 and 1% U3O8. Klown reservesof the Arizona Strip area are about 15000mtU:Oa.Au. Ag, and/or Cu have been recoveredfrom some pipes.
Remarks Although of small size, collapse breccia pipe depositsrepresent low cost uranium resources due to their high grades. For more detailsseeChapter 5.5.
Definition Surficialuranium mineralizationmay be defined as young near-surfaceuranium concentrations either stratabound in predominantlv unconproximal solidatedsurficialformations/sediments or superjacentto uraniferous source rocks or structure-boundwithin source rocks. Uranrum occurs in mineral form almost exclusively as uranylspeciesor otherwiseadsorbedon hostconstituents.Thorium is absent. Based on host environment.four principal subtypesand someclassesare recognized:
Subtype6. l: duricrusted sediments Class6. 1.1: fluvialivalley-fill Type Example : Yeelirrie, Australia Class6.1.2: lacustrine/plava Type Example: Lake Maitland. Australia
Examplesof Type 5: CollapseBrecciaPipe Deposits/Occrurences
Subfype6.2: peat-boC
USA: Canyon, Easy 1, Kanab North, Mohawk, Pigeon,Pine Nut/Aiuona Strip area
Subtype 6,3 : karst-cavern
Tlpc
Type Example: StevensCo., Washingtonstate, USA
Type Example: Pryor Mts, USA
6. SURFICIAL
Srbtlgc
6.1
duicrugt
6.2
d ccdimcntc
pcot-bog
Closr 6.1.1
6.12 f,wiol vollcy fill
6.3 korut
6.1
cq\rct?l
pcdogcnic, ond ctructurc fill
locuatrinc,/ ployo
" Y l : / * +* - * 1 * r
++++rl+ + + + ++++.t*
+
.+
-
'l@ b
t@tr
ffi
Fig. 4.6
U mincrolizotion
pt
non-pcdogcniccolcrctc
ff|
fiwiot or lqcuetrinc
rcd bcda l.e€t
acdimcnts with evoporitca
korct cqvcrn in fimcctonc
E
wcoticrcd gronitc
LYYJ uronifcrous volconics
r-1 I .l
boacmonl dominonUy gronitic
E
E
pcot-bog
ru
.G
uronifcroue gronitc uronifcroug mctoacdimcnts
,RRD
Type 6: Surficial
elements(often but not necessarilyuraniferous granitic complexes) Adequate humid climatic conditions for chemical weathering and liberation of oreforming elements and arid enough to deposit t h e e l e m e n t si n i n t e r m o n t a n eo r i n t r a c r a t o n i c environments and militating asainst flushing out of the newiv formed minerais and as such preserving the mineralization (particularly important for subtype 6.1)
Subtype6.4: surficial pedogenicand structure fill T;-peExample:Davbreak,USA: Cotaje,Bolivia
-
References:Bell 1963;Butt et al. 1984;Cameron 1984; Cavaney 1984; Carlisle 1983; Hambleton-Jones1984; IAEA/Toens (ed) 1984;Johnsonet al. 1987;Mann and Deutscher1978;Pardo-Levton1985:U.S.-AEC 1959
The four subtypes are characterized by 1. Duricrusted shallow sediments in arid to semiarid climates consisting of fluvial, alluvial and eoiian channel fills and associatedplayamarginal deltas, evaporative lacustrinebasins, or alkaline and saline playas, cemented or replaced along the groundwater table by carbonates,silica and sulfatesin form ofnonpedogenic calcrete, dolocrete. silcrete, gypcrete. Organic matter is absent or occurs in insignificant amounts (subtype 6.1). 1. Highly vegetal organic and clay-riclr shallow depressionsin humid climates (6.2). 3. Karst cavelr$ in limestone having a floor cover of fallen blocks, chert fragments and other insoluble residues of limestone embedded in a matrix of reddish-brown sand, silt and clay that may be loosely consolidated or cemented by s i l i c a( 6 . 3 ) . 1. Surface-bound minerqlization (a) in pedogenic formations including soils and crustations such as pedogeruc calcrete, silcrete, gypcrete' laterite/ferncrete etc. and (b) as joint and fracture filling (6.a).
Mineralization -
-
For details on mineralogy and petrography of surficial uranium occurrences see Pagel (i984). Age Corutrains Restricted to recent times (estimated at less than 1m.v. old).
Metallogenetic -{spects Surficialuranium concentrationscan generatein a wide range of geoloeical and climatic environments provided -
Host Environment
-
Variety of surface-boundsettings(see above), capable of precipitatinq uranium and associated elements by reduction, complexing, adsorption. etc. from ground- or surfacewaters - Hydrogeochemistry of ground- or surface waters enriched in U and other elements - Zones of fluctuating groundwater table' and/or lateral groundwater migration within the host rocks .- Internal drainage, i.e., + closed basins for s u b t y p e6 . I - Absence of cover rocks - Adequate nearby source for uranium and other
Almost exciusively uranyl minerals. dominantly uranvi vanadates in subtypes 6.1 and 6.3; uranyi phosphatesin subtype 6.4 lfuch of the uranium adsorbed on host soil constituents(organic substances.clay minerals, etc. particulariy in subtype 6.2)
Remarks
Principal Recognition Criteria
-
97
-
-
an adequate source of ore forming elements is present. the ciimate promotes uranium mobilization and redeposition. adequate hvdroloeic and geomorphologic regimes exist for mobiiization and transport of the ore-formins eiements and their trapping in potentiai sites ior ore formation such as ancient and recent vallevs. basins. and structure zones. the litholog-v of potential hosts warrants precipitation and accumulation of uranium.
The source of uranium and other elemens is only empirically deduced. Since essentially all surficial uranium deposits occur in regions containing uraniferous intermediate to felsic intrusive or extrusive rocks or uraniferous (meta) sedimentsit
98
4 Typologyof Uranium Deposis
is thought that these rocks provided the com- - areasof lateral groundwatermigration or dismodities required. For example, all duricrusttype charge such as linear depressionsin form of occurrencesin the Yilgarn Block, Australia, occur fluvial drainagesystems; in terrane dominated by late Archean granite - regionsof internal drainage,i.e., closed or containing partly 8-10ppmu with maxima of partially closed basinssuch as alkaline lakes, playas,deltas, swamps,bogsetc. suppliedby 80pp-U (Gamble 1984). A similar situationis given for the depositsin the Namib Desert that surfaceor groundwaterswhich have traversed occur in regions with outcrops of Proterozoic a source zrrea; granites,pegmatites,migmatites,many appreci- - zoneswithin sourcerock terranecharacterized ably anomalousin uranium. Structure-controlled by faulting and fracturingand verticalgroundmineralizationson the Altiplano, Bolivia, are water movementand water table fluctuations. hosted by Tertiary rhyolitic ignimbritescontaining up to 95ppmU (Leroy et al. 1985).It is Within the first two given areas localization of for this spatial relationship of surficial uranium mineralization is often governed by local moroccurrences to uraniferous complexes and the phological features. (a) In linear depressions general high U content in groundwatersof the uranium precipitationmay occur at constrictions catchment3ps2q,,ithinthesecomplexesthat most laterallyby narrowingof the channelor verticall)' authors consider them as the source of uranium by a basementrise. Throughflowingwaters will and most other calcrete related elements. stagnateor will suffer reducedflow ratesat these Vanadium posesa problem. No conclusiveevi- barriers providing time for increaseduranium dencefor a sourcehas been establishedas yet. precipitation as will be discussedlater. (b) At Mafic minerals of greenstonebelts or granitic internal drainage regimes, absenceof outflow rocks such as pyroxene,hornblende,and biotite exits is a salient criteria for uranium accumulation have been suggestedas a vanadium sourcefor in semiarid and arid climates. In contrast, the Western Australian deposits by Mann and organic-rich depressionsin humid environments Deutscher (1978). Hambleton-Jones (1976) require for concentrating uranium a relatively reports certain schistsof the Damara Belt to be continuouspercolation of groundwaterthrough enriched in vanadium and a possible source for the organicmaterial. the Namib Desert surficial deposits. Lithology may exert a physico-chemicalcontrol Climate influence on the generation of the on ore localization in various ways. Argillaceous various rypes of surficial uranium depositsis re- layerscommonlyact as aquiclude.As such they flectedby the localizaliooof (a) duricrust-related may constitute both a barrier for groundwater uranium deposits such as calcrete, dolocrete, flow, thereby enhancingthe conditions for insilcrete,gypcretein channelsand playas,and of creasedprecipitationof uranium or, if presentin uranium accumulationsin closed alkaline and multiple horizons, they may channelthe waters salinebasinsaswell in hot arid to semi-ariddesert into areas optimal for ore accumulation, for zones, and (b) organic matter-hosteddeposits example in deltas, at bog-sedimentinterfaces, such as bogs and swampsin cold to moderate or in marl/clay'-siltisand-carbonaceous suites. climates. Between these two extreme climatic Porosityand permeabilityare an additionallithocasesthere are a large number of organic and logicprerequisitefor mineralformationproviding inorganic environmentsfavorable for accumu- both, an aquiferfor the mineralizingfluidsand an lation of uranium but, as mentionedearlier, ore mineral host such as the valley and delta almost all of them are of subordinatemagnitude calcretes-dolocretes in arid regions. and of no economicimportance. Boyle (1984) Mobilization, transport and redeposition of. pointsout that the economicpotentialfor surficial uranium and associatedelements,particularlyV uranium deposits, relative to climatic zones, for carnotitecrystallization,and Mg, Ca, Ba, Sr, desreasesfrom hot arid to semi-arid, cold- CO2,S04 etc. for associated mineralsin duricrust temperate, temperate and to tropical. This and karstdeposits(subtype6.1 and 6.3) and POo sequencealso correlateswith increasingdilutjon for structure fill mineralizations(subtype 6.4), and precipitationof uranium in the hydrosphere. demand a coincidence of specific conditions, Geomorphologicenvironmentsand hydrologic additionallyto the above-discussed criteria. They regimesrequiredfor ore formation include are
Type 6: Surficial
99
- accessto the requiredelements; most importantfor the generationof subtype - adequateweatheringconditions; 6.1 (duricrusttype) depositssuchas Yeelirrie, - hydrochemistry and complexing transport Australia,and LangerHeinrich,Namibia. - Waterswith pH 4.5 to 7.5 providestabilityfor agents; - physico-chemical precipitationmechanisms. complexesthat may have uranyl-phosphate beeninstrumentalin the formadonof subtype Access to elementsand adequateweathering 6.4 depositssuch as Daybreak, USA, and is requiredto liberate,at leastpartially, conditions Cotaje,Bolivia. the ore-formingelementsfrom the sourcerocks. - Waterswith pH < 5 supporturanylsuifateand To achievethis purpose,the sourceterranemust fluoridecomplexeswhich are importantduring be sufficientlyexposedto and undergointense the weathering of sulfide deposits such as chemicalweatheringunder oxidizingconditions Bondon, France. where pre-Liassicpaleoperiod. Suchweathenng in a pre-mineralization weatheringprocessescontributedto mineral effectsare reflected,in the Yilgarn Block for formation as reponed by Euirv and Vargas .:xample.by deep-reaching regolithicprofiles.For (i980). lne formation of duricrust-typedeposits,subsequent to the chemical weatheringperiod a Physico-chemicaI mechanrsrnsleading to precipichangeinto a hot. semi-andor arid climateis tation of the ore-forming minerals from the mandatoryto retain the uraniumin an internal transportingsurfaceor groundr*'aterincludeparand drainagesystemor. alternatively,to prohibitits ticularly destabilizationof uran-v-l-complexes flushingby streamsand riversinto the seas,which sorptionof uraniumand. in somecases,reduction is typicalfor humid moderateto tropicalciimates. of uranium. Processesinvolved. individuallyor Further on, under hot, arid conditions,the combined,are sporadic and mostly minor rain fall causes - decreasein hydrostatic parriai pressuredue retardment of the groundwater flow, so that to ioss of CO2, for exampie by evaporation migration of the groundwaterstransportingthe in near surface environment leading to disore-formingelementsfrom the sourceto the site sociationof uranyl-carbonatecomplexes; of deposition is slow. This situation, combined - changein pH either by lossof CO2, oxidation with a high rate of evaporation,providesalsothe of sulfides.interaction with soluble humic or hvdrodvnamicconditions favorablefor the prefulvic parciclesor mixing of different solutions, cipitation of the uraniferous nonpedogeniccalaffectingthe solubility and stability of uranylcretemineralssuchascaicite,dolomite,carnotite, complexes: :tc. Another prerequisitefor the availabilityof - changein redox conditions by carbonaceous uranium in hot climatesis the absenceof certain matter, gasessuch as hvdrocarbonsand H2S, saprolitessuch as pedogeniccalcrete,ferricrete, anaerobicbacterial activity and oxidation of ferralitic laterite, Fe-Mn oxides,and claysin the sulfides reducrng hexavalent to tetravalent source areas. These surface constituentsmay uranlum; seriouslyhamper the transportof uraniumby its - sorption processesresuiting in adsorptionof fixationin the regolith. both uranousand uranyl-ionson carbonaceous Hvdrochemistry and complexingagentsgovern particles. clavs. phosphates.zeolites and the mode of transport of the ore-formingelhydrousoxidesof Fe, Mn, A1. Ti and Si. r:mentS.Uranium is commonly dissolvedin iratural waters as the uranyl ion which may com- Other processesthat also mav have influenced plex as bi- or tri-carbonate,phosphate,sulfate, directly or indirectly precipitarion of U and/or fluonde, chloride, or hvdroxide.The natureof U-V mineralsinciude the complex is a function sf the pH domain. evaporationof surfaceand gloundwaters,for Therefore, certain uranyl complexesare specific example in arid climates by upwelling of for certain environmentsof acidity. groundwater in calcrete-dolocretedrainage Boyle (1984)lists the followingpH rangesfor channelsthat resultsin both increaseof U, V, the formation of varioustypesof surficialdeposits. and K contents and release of CO2 or, in - In waterswith pH > 7 uranvl bi-carbonateand alkaline and saline playas. leads to accumulation of U in final residues:evaporationof tri-carbonate comDlexesare dominant and
-.-__
100
4 Typologyof UraniumDeposits
soil moisture associatedwith capillary rise of groundwatermay be the mechanismfor U enrichments in pedogeniccalcretesor ferricretes in hot climatesand perhapsof the structurally controlled mineralizationsin moderateclimates such as the autunite depositsof Daybreak, USA; - interaction of two or more groundwaters,for example mixing of separate uraniferous and vanadiferous groundwaters resulting in deposition of carnotite (Mann and Deutscher 1978). For extensivediscussions of the abovesummarized mineralizationprocesses,the reader is referred. among others, to Arakel (1988), Boyle (1982, 1984), Briot (1978), Carlisle et al. (1978), Hambleton-Jones(1976), Mann and Deutscher (1978),Samama(1984). For more details on the menllogenesis of subrype6.1 depositsseeChapter5.6. Additional detailson the formationof subtype 6.2,6.3, and6.4 mineralizations aregivenbelow. Subtype 6.2, peat - bog, metallogeneticaspecsi Noteworthy mineralization has formed in glaciated terrane of cold to cool temperateclimatesof the northern hemisphere,and in recenttimes as indicated by strong radioactive disequilibrium. Prerequisite for optimal U accumulation are organic-rich channels or basins through which a relatively constant filtering of uraniferous surface or groundwaters orcurs. The waters collect the uranium by weathering and leaching from uraniferous granites, volcanics,or other lithologies.For example,in the FlodelleCreek area, probable source rocks are provided by the CretaceousPhillipsLake Granodioritecontaining 4 to 80ppmU (av. 16ppmU), part of whichmust be presentin leachableform asreflectedby sheargradingas controllednear-surfacemineralizations muchas500ppm U. Uranium-transporting waters appearto be neutralto slightlyacidandmay carry several hundred ppmu. Otton and Zielinski (1985)report for the Flodelle Creek headwaters a pH-rangefrom 5.85to 7.55,and contentsof 17 to 318ppmU associatedwith high covariationof Ca2*,Na*, Mg+, and HCO3- ions. A mechanismthat is essentialfor the fixation of uranium is not so much reduction but ion exchangeand adsorptionon organicmaterial as reflectedby the high correlationcoefficientof up to 0.8 for U to organicmatter.
Subrype6.3, karst-cavern,metallogeneticaspectsi The most likely sourceof uranium is tuffaceous sediments formerly overlying the Madison Limestone. Uranium is thought to have been leachedfrom the pyroclasticsduring the present erosioncycleby groundwaterswhich transported it downwardsto be redepositedin the karst openings as uranyl vanadates. Subrype6.4, surficial pedogenicand structurefill, metallogenicaspects:The most appealing hypothesison mineral formation at Daybreak,USA is supergene leachingof uraniumfrom a uraniferous quartz-monzonite during a period of deep weatheringsince Tertiary times and subsequent redepositionof the uraniumasopen spacefillings along a fluctuatingwater table. The actual cause for precipitation of the relative high grade uranium remainsopen for speculation.A similar type of near-surfacestructure-controlled deposit, with uranium derived from uraniferousrhyolitic volcanicsand supposedlyprecipitatedby H2S or hydrocarbons,occursat Mina Cotaje, Bolivia (ca. 100mtUsOs,0.1%U3O8).
Remarks Surficial uranium deposits are commonly relatively small and of low grade except those depositsusedastype examples.From the latter only Yeelirrie, Australia, is of large and potentially economicsize.Mining of the small depositsmentioned was possibleunder exceptionalfavorable conditions (nearby mill, amenability to heap leaching,etc.).
4.6.1 Subtype6.1: Duricrustedsediments (1984); References: Butt et al. (1984);Hambieton-Jones Mann andDeutscher(1978)
Class6.1.1: Fluvial valley-fill (also referred to as calcrete, groundwater-calcrete or valley-calcrete type) Type Example: Yeelirrie, Australia Reference: Cameron1984
Yilgarn
Block,
T;"pe 6: Surficial
H ost Ro cks IA lte ratio n I Structures Mineralization is hosted by nonpedogenicearthy or porcellaneous, porous calcrete or highly ,:arbonatizedfluvial and alluvial sedimentscom,osed of dolomite, calcite, clays, feldspar and locally gypsum and celestite.The calcretegrades laterally and downward into ciav-quartzand argillaceous grit sediments from rvhich it has been derived by groundwater-related aiteration. Calcretizedsedimentsoccupy the axial portion of shallow valleys up to a few km wide and up to 200km long. Basement topography under valleys ;raslocally strong relief with abrupt drops of up to jome tens of meters. For example, at Yeelirrie a basement high chokes the channel section. Calcrete forms near-surface. semi-continuous highiy elongated, tabular lenseswhich may be 20 to 150km [ong,0.5 to 4km wide and 2 to 15m thick. Longitudinal topographic gradient of channelsis rather gentle, between0.05 and 0.1%. Channel-filling alluvium and calcrete commonly oroaden into wide flood plains and deltas in their iower course prior to terminating in playas. In other regions (e.g., Namibia) insteadof, or in addition to calcrete, channel sediments may have been transformed into silcrete or gypcrete providing the host for mineraiization.
l0l
from a few deposits. Sepiolite occurs often and a t t a p u l g i t ei s o c c a s i o n a l l vp r e s e n r .B a r b i e r e t a l . (1980) estimate that in the Mudueh occurrence. Somalia. berween 5 and 20",'" of mineralized samplescontain attapuleite and sepiolite. This is also the case in the Hammadas. Ain Ben Tili area, Mauritania. from where Braun (1914) reports a clay minerai associationschange within the sedimentarv profrle from bortom to top as follows: the basalcongiomerateconrainsmontmorillonite and iilite. the mid section. montmorillonite and attapulgite: and in the top calcareouscap. montmorillonite decreasesand sepiolite appears. Smectite is present in several deposits. Briot (1978) found in Yilgarn Block deposits an interlayered illite,'smectite clay and suggests that smectite denved bv alteration of illite. Smectite and illite are aiso present in the Tumas River occurrence. Namibia. Kaolinite and illite exist often in surficial uranium occurrences. but their relationships to mineralization is unclear in some cases whereas in others they are the only clay minerals present. At a shon distance away from deposits other clav mineral assemblages may occur. Briot (1978) noticed chlorites with traces of illite and talc along the margins of the Yeelirrie deposit. Dimensions lResources
Ore and Associated MineraklMode of Mineralization Principal ore mineral is carnotite and rarely other uranvl-minerais. Carnotite occurs as stringers, seams and disseminations in earthy calcrete, as fracture coating and vug lining in porcellaneous calcrete, and as grain coating in clay-quartz sediments immediately beiow the calcrete. Mineral distribution. although highlv irregular in detail, displaysa general continuity in form of flat-lying .;rallow elongated lenses up to a few meters :hick. Most of the mineralization is emplaced immediatelv beneath the present water table and is best developed in the transition zone below the calcrete. Pagel (1984) notes that in valley-flll and lacustrine/playadepositsin various countries.the uranium minerais are commonly associatedwith a variety of other minerals. Celestite is known from :t number of deposits but is largely erratically distributed and confined to specific parts of a deposit. Its presence is reflected by Sr contents of as much as 6"/". Fluorite and baryte are reported
Nonpedogenic vallev calcrete U deposits may extent several km long, a few tens of meters to ca. 2km wide and <1 to 15m thick. commonly containing from some <10 to about 5000mtU3Os at gradesof 0.03 to 0.08% U;Os. Yeelirrie. Austraiia. is an exception. It m e a s u r e s9 k m l o n g . 0 . 5 t o 1 . 5 k m w i d e a n d 1 to 15m. average 3 m thick, containing 52 500 mt U3Os at an ore srade averaging 0.15% U3Os. Total resourcesof a district such as the northern Yilgarn Block mav be in the order of 6 0 , 0 0 0t o 7 0 0 0 0 m t U 3 O s . Examples of Surfcial. Class 6.1.1 Vallev Fill Duricrwt D ep ositsI O ccurrences Australia: Lake Wav, Lake Raeside. HinklerCentipedeiYii garn Block Namibia: Langer Heinrich, Aussinanis, Tumasi Namib Desen Somalia: Dusa Mareb-El Bur region/Mudugh South Africa: Brulkolk/Bushmanland Plateau. Caoe Province
IUZ
4 Typology of Uranium Deposits
Class6. 1.2: Lacustrine/Plava
South Africa : Abikwaskolk,Dirkskop/Bushmanland Plateau,Cape Province
Type Example: Lake Maitland, Yilgarn Block, Australia Reference: Cavaney 1984
4.6.2 Subtype 6.2: Peat-bog H ost Ro cksIAkeration/ Structures Type Example: Flodelle Creek, Washington Playa sediments are dominantly fine-grained state,USA clays and silts containing glpsum, halite. and Reference: Johnson et al. 1987 carbonates,and are associatedwith hJpersaline waters.Dolomite and calcire.togetherwith cla,v H ost Rocks/AherationIStructures mineralsand quartz, form a discontinuoushorizon, some dm to m thick. of calcrete/dolocrete. Swamps,bogs,muskegs,swampymeadowsat the Intervening material consists of carbonatized edge of lakes or ponds in flood plains or cutoff clays. The calcreteis a major aquifer and the meanderscomposedof vegetal organic matter principaluranium host. It lies a few metersbelow (up to 65"/"of rock), often peat mixed with and the surface under a sequenceof almost imper- embeddedin alluvial clay, marl, silt, and sand meabledolomitizedclaysand silt-claysediments. constitutethe most favorablehost environment. Silts and sandsunderlie the calcreteand may be The organic sediments have accumulated in interbeddedwith more calcretehorizons. blanket-likelenses,1 to 10m thick, elongated along the axis of valleys tens to 100m wide. Mineralized bogs or swampscommoniy occur in Ore and AssociatedMineralslModeof clusterscontrolledby topographyalongdrainages Mineralization underlain by anomalouslyuraniferous granitic Principal ore mineral is carnotite, presentas dis- rocks (at Flodelle Creek, Cretaceous Phillips seminations,coating cracksand filling vugswithin Lake Granodioritecontaining4 to 80ppm, averthe harder calcrete beneath the dolomitized clay ageca. 16ppmU). horizon. Most of the mineralization occurs in calcrete but extends into underlying silts and sands.Distribution of the mineralizationis ir- Ore MineralsIM ode of Mineralization regular although in overall continuity. No discrete U minerals occur. tJ is probably present as urano-organiccomplexesand/or adsorpt on organicmaterial (correlationcoefficient DimensionslResources U-organicmatter often 0.6 to 0.8), but also on Individualdepositsmay be <1km to Skm long, clay, marl and gray to black silty'sandparticles. <100m to 1.5km wide and 0.1 to 3m thick con- U distribution and tenors are highly variable taining <10 to ca. 4000mtU3Osat (average) apparently dependingon mode of waters (surgradesranging from 0.03 to 0.09%U3Os occa- face, ground. upwelling waters), direction of sionallyto 0.15%U:Oe. Districtsmay containup water flortr',and transmissivitvof the peat and to some10000mtUrOa. interbedded sediments.The highest concentrations(up to some1000ppmU,locallv17oover as much as 1m) may occur in either the upper or Remarks lower part, at the upstreamsectionof a peat lense For more details on subtype 6.1 depositssee but also as spotty mineralizationthroughout the Chapter5.6.' unit. Someuraniumoccurrences of this subtypecontain molybdenum and selenium and/or other Examplesof Surficial, Class6.1.2 Lacustrineor (1982) elements. Boyle reports local concentraPlaya Duricrust D epositsI Occunences tions of, and positive correlationsbetween Mo, Australia: Lake Maitland, I-ake Austin/Yilgarn Se, Cu, V, As, REE, and U in recentbogsand Block marshes.
Type6: Surficial
103
Age Constrains
limestone blocks. calcite crystals. chert nodules, as fissure and vus fillings, and as disseminations is of Holoceneage.All uraniumis Mineralization in the matrix of the cavern fill. Distribution in disequilibrium.Daughterproductsare often is restricted to major drainage routes cutting almostabsent,reflectinga recentageof uranium t h r o u g h t h e u p l i f t e d l i m e s t o n eu n i r .
:recipitation.
lResources Dimensions
Age Constraints
Individualdepositsmay be tensof metersto some 100mlong, few metersto more than 100mwide <1 to and <1m to few metersthick containing ca. 50mt U3O3 at (averaee)grades of some i00ppmU, rarelyup to 0.2%U3Os.Districtsmav ,.:ntainup to some100mtU:Oa.
Radiometric disequilibrium indicates a relatively recent ore formation. DimensionslResources
Uranium-hosting caverns varv rridely in size and configuration benveen small cavities. a few metersor lessacrossto openingsup to 10 m high. Examplesof Surficial,Subrype6.2 Peatand Bog 25 m wide and 50 m long. Mineraiized floor fill DepositslOccurrences ranges from a few centimeters to almost 3 m thick. The magnitude of ore bodies mined was Canada:Prairie Flats/OkanaganValley region South Africa: Henkries. Kannikwa/northern highly variable containing mostly a few tonnes U3Os with exceptionsas large as 80 mt U3O3. Ore CapeProvince grade Flodell mined averaged 0.5 to 0.8% U3Os and Washinston USA: CreekAIE 0.6 to I"/"VzOs- Some lodes were as rich as 1.27% U3O8 (Old Glory Mine). Resourcesof the district are estimated at several hundred tonnes 4.6.3 Subtype 6.3: Karst-cavern U:Oa.
Type Example:Pryor-LittleMountains,USA Reference:Bell 1963
H ost Roc ks IA lteration / Strucrures .{arst caverns in limestone (Mississippian \ladison Limestone) having a floor cover of fallen blocks, chert lragments and other insoluble residues of limestone embedded in a matrix of reddish-brown sand, silt and clay, that may be loosely consolidated or cemented bv siiica, constitute the host for this type of U deposit. The caverns developed in limestone units mainly in Tertiary time atter uplift dunng the Laramide Srogeny. Ore and Associated Minerab Principal uranium minerab include tyuyamunite and metatyuvamunite associated with calcite. hematite, barvte. gypsum. opal. and locally fluorite and celestite .v{ode of M ineralization Uranium minerals occur as fine-powdery coatings of fractures and solution voids. crusts on
Examples of Surfcial, Subrype6.3 Karsr Cavern DepositslOccurrences Central African Republic: ? Bakouma Uzbekistan: Tyuva-MyuluniFerghana Basin
4.6.4 Subtype 6.{: Surficial pedogenic and
structure fill Type Example: Davbreak mine. Washington state,USA Reference:US-AEC 1959
This category comprises surface-bound mineralizations occurrins widespread in pedogenic formations, including soils, pedogenic crustations such as lateritelferricrete. calcrete, silcrete, gypcrete, etc.. and stmcture-bound mineralizations. All occurrencesare associatedwith uraniferous source rocks comparable to those associated with the other types of surficial deposits. All occurrencesare of minute to small size and commonly of low grade. Only a very t'ew have been mined. The Daybreak mine, Washington state,
l&
4 Typology of Uranium Deposits
USA, will be describedas the type exampleof this style of minablemineralization.
the Pena Blanca district, Mexico, and at Cotaje. Bolivia.
H ost RocksIA herationIStntcrures
Dimensions lResources
At the Davbreak mine, quartz monzoniteis the host rock for a shallowdipping mineralizedshear zone. The shearis surroundedby intensebleaching and alteration including kaolinitization of feldspars. sericitization,and decompositionof biotite.
At the Daybreak mine, mineralization is hosted in a shear zone in which it extends discontinuously for 600m long, up to 25m wide and 30 to 40m deep with mineable ore limited to a section slightly below the water table. Total production was 2 5 m t U 3 O s a t a n o r e g r a d eo f 0 . 3 % U ? O 8 .
Ore Minerab /Mode of Mineralization
Examples of Surficial, Subrype 6.4 Pedogenic and Strucrur e Fill D epos itsI O ccune nces
The only uranium mineral at the Daybreak bathoiith. Schlagintweit/Achala mine is meta-autunitewithout any sulfides or Argentina: gangue minerals. Uranium minerals, mainly Sierras Pampeanas uranyl phosphatesand silicates, locally sooty Canada: Summerland arealBritish Columbia pitchblende and/or coffinite. are reported from Greece: Archontovouni/Paranesti other occurrences. USA: Copper MountainAilyoming At the Daybreak mine, meta-autunitelines open fractures and voids within a shear zone proximal to the water table, ageregatingto pods 4.7 Type7: Quartz-pebble Conglomerate and vuggy massesasmuch as 10cm thick and 1 m (LowerProterozoic)(Fig.4.7) long. The best mineralizationoccurs at intersectionsof the main shear with cross-fractures wherelodesup to 1.5m long, 1 m wideand 0.3m (also referred to as oligomictic conglomerate Proterozoic conglomerate Lower or thick may accumulate.Comparablemineraliza- or tions are known in rhyoliticenvironments,e.g., in paleoconglomerate type)
TyPe Subtypc
T. OUARTZ-PEBBLE CONG (Lowcr Proterozoic) 7 . 1 U - R E E c o n g t o m e r o t eb e d s r n b o s o l - s t r o t i o r o o h i cu n r t
LOMERATE 7.2
Au-U conglomerotc bcds in multi-gtrqtigrophic unita
E o o
o I
FE F
E
E
N
"l ;frF
ARCHEAN
ca. 2km
ffi]
U-beoringconglomcrotc
Archeon boscment (gronites, grcenstonee)
Fig.4.7
LowermogtProtcrozoic- Uooer Archeon clostic sequGncos (2E50-2200 m.y.)
^G
'RRD
l.vPe7: Q u a n z - p e b b l e C o n s i o m e r a t e ( L o w e r P r o r e r o z o 1 i0c5) Definition Quartz-pebble conglomerate deposits consist of uranium and/or other metallic elements of synsedimentarydetrital origin, modified by diagenetic ::ocesses (modified placers). The principal pri:-nary uranium phase is detntal uraninite. The conglomeratesare interbedded with silicic clastic sequences containing layers of quartzite and argillite. Quartz-pebble congiomerate deposits are restricted to basal Lower Proterozoic units unconformably overlying Archean rocks which include granites. Two varieties of mineralization can be in the ,;nglomerates:
Subtype7.1: U-dominantwith REE Type Example: Blind fuver-Elliol Lake. Canada Subtype7.2: Lu > U-dominant Type Example: Witwatersrand, South Africa 3.eferences:Button and Adams 1981; Hallbauer 1986: ,'retorius 1976. 1981; IAEA/Pretorius (ed.) 1987; R.obertson1989,Roscoe 1969; Ruzicka 1988
-
Thin and lenticuiar conglomerate beds often coalescinginto thin sheets of fluvial to deltaic sediments in braided-stream sedimentary' regimes - At or near the baseof a clasticsequencervithin depressionspossibly'paleochannelsincised into A r c h e a n b a s e m e n t( s u b t y p e7 . I ) o r i n r e p e a t ing conglomeratic beds (reefs; at or near (intraformationai) unconformities. within overlving several stratieraphic units. the beds partly originating bl' rervorking of precursor c o n g l o m e r a t e s( s u b t v p e 7 . 2 ) - Lateral extension of pavable conglomerate beds from a :ew km: to as much as -100km: - Interbeds of terrestrial to marginal marine quanzite. arkose. siltstone, argillite, and pol,""mict conslomerate. partlv arranged in cyclic sequencesof variable. sometimes great thickness(severai 1000'sof m): iron formations may occur higher up in the sequence - High K/Na (10 to 100) and K,Ca ratios (>40) of host-reiated arenites indicating provenance from granitic source - Basins flanked bv Archean granites and greenstone belts - no ore-related alteration, but physical weathering of Archean basement. - no structural controi except for proximity to maior uncont'ormities.
In subtype 7.2 deposits, gold is the main product and uranium is by-product. The time bracket "Lower Proterozoic" has been added to avoid confusion with younger. Mineralization Dost-oxyatmoversionoligomictic conglomerates .i hich also contain uranium but of epigenetic - Primary minerals: uraninite U Dartlv ongin. Mineralization of both kinds requires difthoriferous.uranothorite. perhapsU-Ti oxide ferent depositional environments. the first that phases,associatedwith secondaryU-Ti oxide for detrital ore mineral accumulation,the second phases(brannerite).coffinite,thucholite,a.o. that with reductantsfor precipitation of uranium - Associatedminerals: wide variety of detrital dissolved in solutionsl the latter correspondsto heavy minerals. inciuding locally native Au; sandstone-typedeposits. and diageneticauthisenic minerals. locally
carbon - Associatedtrace elementspossiblypresent: Au. Ag, REE (1 to 100ppm):As, Co. Cr, Cu, .\incipal Recognition Criteria Ni, Pb, Zn. Zr (10to a ferv100ppm): TiO2(0.I to2"/.);S (>19/').andtracesofPt, Os. Ir, etc. Host Environment - Subrype7.i is monometallicexcept for occa- Oligomictic quartz-pebble congiomerate comsionallyrecoveredREE minerais,subtype7.2 polymetallic.gold being the main product posed of well-rounded and weil-sortedpebbles (mean diameter 5 to 7 cm) of predominantly - UiTh ratio of conglomeratematrix: 1 to 15 quartz and lesser chert (10 to 20%) in highly - Stratiform- matri-x-bounddisseminationof U and associatedminerals pyritiferous siliceous matrix containing minor f'eldspar,sericite, chlonte, and heavy minerals; - Ore localization and concentrationcorrelate with well-sorted and densely packed quartzeither submetamorphic or metamorphosed to pebbles reflecting control by hydrodynamic lower greenschist facies
106
4 Typologyof Uranium Deposits
processes,abundance of pyrite and its predecessors,presence of internal parrings of pyrite-rich arenite and occasionally argillite lenticular in shape and srructured by trough or tabular crossbed sets, proximity to unconformitiesor disconformities. - Geometry of uraniferous conglomerate ore bodies is highly variable, dependingon the topographyof pre-conglomerateunconformit1,, commonlyby far longer than wide and rather thin Age Constraints
Remarks Lower Proterozoicconglomeratedepositsare of very low to low grades(0.01to 0.L5%U3Os)but containlargeresources.Depositsmined primarily for gold yieldinguraniumasby-productappearto have a longer renn economicperspectivecomparedwith strictly uranium producers.
Examplesof Type 7 Quartz-PebbleCongtomerate (Undifferentiated) Deposits/Occrurences
Brazil: Serro do Corrego/Jacobina, Bahia, - Restricted to earll' Lower Proterozoic time QuadrilateroFerrifero prior to oryatmoversion,(ca. 2200m.1'.ago) Canada: Agnew Lake/Ontario, Sakami Lakel Quebec India: westernKarnataka USA: Phantom Lake/Jv{edicineBow - Sierra MetallogeneticAspects Madre, Wyoming, Black Hills/SouthDakota A modified placer metalloeenesisis commonly Russia:? Noril'sk/northernSiberia forwarded for U mineralization in Lower Proterozoic quartz-pebble conglomerate deposits. The origin of uranium is generally accepted as synsedimentarydetrital, uraninite, 4.7.1 SubtypeT.l: U-dominant with REE and other heavymineralsdepositedas placersin fluvial or deltaic environments.The source of Type Example: Elliot Lake - Quirke Lake disuranium is thought to be Archean granite. The trict, Canada other heavy minerals are considered to have References: Robertson 1989; Ruzicka 1988 derived from either granite or greenstones. Liberationof ore mineralsoccurredessentiallyby Host RockslStructures physicalweatheringand erosion.Prerequisitefor such a proposition is an anaerobicatmosphere Pvritic oligomictic quartz-pebbleconglomerate permitting fluvial transport of uraninite and (termed "reef') of fluvial origin, deposited in other minerals which are instable and easily braided-streamchannels,with lateral migration weathered in an oxidizing environment. An coalescingconglomeratesinto thin crossbedded anaerobicatmosphereprevailedon earth during sheets,interbeddedwith quartzitebanks, chiefl1' the Archeen and eariy Lower Proterozoicuntil in paleovalleys scouredinto Archean greenstones oxyatmoversionin the middle Lower Proterozoic mainly but also into granite. Three different @a. 2240m.y. ago). Since then, the oxygenated channelsystems(termed"trend") are recognized atmosphereprohibits longer transport of uran- in the Elliot Lake-Quirke Lake district. inite,andhenceexcludes formationof thistypeof No structuralcontroi, exceptfor concentration conglomeratedeposit. in channeisand their abutmentagainstbasement in the highs,and post-oredisplacements. llhe diverseheav-vmineralsassociations various districts primarily reflect petrologically different sources. Aheration Post-depositionalredistribution and mineral crystallizationmainly by diageneticprocessesIed No ore-relatedalteration,but deep essentiallv to the formation of a suite of authigenicore and physicalweatheringand erosion of the Archean associated mineralssuchas brannerite.rutile and paleosurface. Granitesare weatheredto a quartzanataseby reactionof U with ilmenite, pyrite by microclinerock displayinga relic granitetexture sulfidizationof magnetite,the sulfidesupposedly containing sericite derived from plagioclase derivedfrom volcanism. destruction,and somechlorite.Ferromagnesian
Type 7: Quanz-pebbleConglomerate(Lower Proterozoic)
minerals and ferric iron have been removed whereas ferrous iron minerais (pyrite) and tetravalent uranium (uraninite) are still present, reflectingweathering under anaerobicconditions. The weathering is geochemicallyexpressedby Ca ieachingindicated by high KiCa ratios (>a0) due to almost complete decav of plagioclase.
107
4.7.2 SubtypeT.2:Au over U dominant SouthAfrica Type Example:Witwatersrand, Anhaeusserand Maske (edsl 1986:Pretorius References: 1976:Hallbauer 1986
H ost Ro cks IA lrcr atio n I Structures
Pyritic, oligomictic quartz-pebble conglomerate of fluvial ongin interbedded with quartzite, Uraninite. U-Ti oxyde phases (brannerite), cof- arkose. shale and volcanics.Carbonaceousmatef i n i t e . t h u c h o l i t e . u r a n o t h o r i t e .u r a n o t h o r i a n i t e . rial occurs in several horizons. Deposition of r n o n a z i t e x. e n o t i m e , l o c a l l v g u m m i t e . conglomerates in several separate stratigraphic P y r i t e ( 5 t o 2 0 w t . % ) . a l l a n i t e . i l m e n i t e , cycleswithin six large fluvial fans on the north and i h r o m i t e , c a s s i t e r i t e ,m a g n e t i t e , r u t i l e , z i r c o n . west side of the Witwatersrand Basin. sarnet, spinel. tourmaline. titanite, apatite, DVroxene. Ore and Associated MineraklMetak Ore and Associated MineraLs
Uraninite (2 to 6% Th). uranothonte. brannerite, thucholite, native gold and platinoid (Os. Ir, Ru, Uranium is the primary commodity produced Pt) minerals. with occasionalrecoveryof Th and someREE, Pyrite. arsenopyrite, pyrrhotite, cobaltite, particularlyY. gersdorffite, galena, chromite, zircon in relative Several generations of ore and associated abundance and about 60 more minerals in extreminerals occur as disseminateddetrital and mely small quantities.
,VIode of M ineralization
redistnbuted matrix componentsin at least seven congiomeratichorizons that range from Mode of Mineralization 0.5 to >3.5 m thick. Three of the thickerreefs (1.5 to 3.6m) have uraniumgradesof 0.05 to Gold is the main product (<1 to several 100ppm 0.15%UrOa. The conglomeratebedsseparated Au, average 5 to l2ppm Au) and uranium byby quartzitebanks(0.5 to 6 m thick) arewithin a product (av. 0.015 to 0.03% U:Os) except in liequence about50m thick immediatelyoverlying a few mines (e.g., Africander/Vaal Reefs: the Archean unconformity.They coverareas,up 0 . 1 5 %U r O r , 1 . 1 p p mA u ) . to 2,0km2in lareralextension.within threeNWSeveral generations of ore and associated SE orientedfluvial systems. minerals occur as detrital and redistributed matrix .1geConstrains Restrictedto the early Lower Proterozoic(ca. l l 0 0 t o 1 5 0 0 m . va. g o ) . Dimensions lResources The Nordic Trend is 5700m long and 1300to 1800mwide. The Quirke Trend is 9600miong and i800 to 2700mwide.Minablereefsare 1.5to 3.5m thick. Total resourcesare 400000to 500000 mt U3Os.Averagegradesare0.07to 0.1%U3Os. Remarks For more detailsseeChapter5.7.1.
components in several conglomerate horizons within large fluvial fans. Mineralized reefs occur in all four stratigraphic systems. but the most productive is the Bird Reef Stage in the Upper Witwatersrand System. which yielded ca. 80o/oof past production. Important concentrations of ore are restricted to distinct narrow zones occupying ca. 2",L of the entire Upper Witwatenrand Svstem within a belt running parallel to the former coastline. On a more locai scale. ore minerals tend to have preferentially concentrated (Von Backstrom 1975. 1976\: -
in conglomerate beds less than 30cm thick, immediateiy above stratigraphic and intraformational local disconformities or unconformities, particularly where conglomeratesfill
108
4 Tlpology of Uranium Deposits
depressionsor scours in the footwall rocks. or about a slight rise or swell of underlying strata. - in shallow depressions with gentle slopes, - in peripheral zones of deep depressions with steep slopes (axial center zones are weaklv mineralized probably due to poorly deveioped conglomerate), - near the base of conglomerate beds along distinct, partly carbonaceous argillaceous/shaly footwall boundaries, - in pay-streaks running generally parallel to each other characterized by denselv packed, well-rounded and well-sorted pebbles predominantly of quarz, and larger accumulations of other heaq' milerals, - rare extensions into adjacent arenaceous sediments except bv redistribution. Age Constraints
rounded by alkaline granite. Mineralization consistsof a polymetallicCu-U-Au assemblage. Geologicaldistribution, sulfideassemblage.rock association,and stratigraphic position permit a distinction between strata-bound and stratamineralizations. transgressive Type Example: Olympic Dam. South Australia Robens1988;Robertsand Hudson1984 References:
Principal Recognition Criteria Host Environment -
-
Restricted to the late Archean - early Lower Proterozoic (ca. 2300 to 2800m.y. ago). Dimersiors lResources The ore-hosting fluvial fan may be up to 40km long and 90km wide, across the distal fan base, and several 1000m thick. Individual deposits are several 100m to several 1000m long, several 10m to several 100m wide and 5 to 200cm thick containing up to several 10000mtU3O6 at (average) grades ranging from 0.015 to 0.03% U3Os occasionally to 0.L5% U:Or. Resources can be >600000mtUrOe. Gold values may range from <1 to 25ppm.
-
-
Remarks
-
For more details see Chapter 5.7.2.
-
4.8 Type 8: Breccia Complex (Fig.a.8) Defnition
Host rocks are a thick pile (ca. 800m) of matrix-nch polymict breccias (Olympic Dam Formation) with some intercalated finergrained layers Fragments consist of altered granite; felsic, intermediate, mafic volcanics: banded iron formation, hematitic siltstone, carbonate, arenite; fluorite, baryte, sulfides (diameter mostlv 1 to 3cm, but up to 12m) Matrix of several phases of hematite, sericite, chlorite. siderite and arkosic material Mineralized polymict breccias overlain and underlain by barren monomict granitic breccias Position within a large graben-like structure (>7km long, <4km wide) Surrounding rock is an anorogenic, A-type, Kfeldspar rich biotite granite containing 25 to 40ppm U, 30-50ppm Th and locally as much as 500-1000ppm REE Structural arches associatedwith dispiacement faults cut the graben Deposition of breccias in hi_ehenergy environment under arid sub-aerial conditions Rapid facies changes, disconformabie contacts between individual breccia units Intruded dolerite dikes
Aheration Two episodes of alteration reflected by: -
Weak pervasive alteration of all lithologies by
hematitization, sericitization. chloritizarion The breccia complex deposit occurs in coarse and locally silicification and carbonatization, brecciascomprisedlargely of polymict granitefragmentsin a hematiticreplacing particularly feldspars and mafic andhematite-dominated chloritic-sericitic-silicic matrix. Finer-grained minerals of granite clasts; layered rocks occur within the breccias. The - Intense vertically zoned alteration by brecciasfill a large graben-like structure surhematitization and chloritization. dominant in
Tvpe 8: BrecciaComplex
8.
TyP"
BRECCIA
109
OMPLE lu
lo 19 llo
,
o o
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-
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Minerqlizotio
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stroto-bonsgressivecc-bn, bn, cp, U
r:] l-l
stoto-fonsgressave vein cc-bn, U upgroded sbotobound
y
ffi
ferruginous discordont body
F:]
discordont polymict breccios
E
gronitic ond polymict breccios
l--l
K-feldspor-rich biotite gronite
E
unconformity
n
foult
sboto-bound hem-bn-cp-py-U stoto-bound sid-chl-py
t::l l:':::l
oxidotion
bn=bornite. cc=cholcocitei cp=cholcopyrite.py=pyrite, chl=chlorite,hem=hemotite,sid-siderite r) ideolizedofter OlympicDom, Austolio/Roberts 1988 L AT YRRD Fig. 4.8
the lower section of the mineralizedsequence (Olympic Dam Formation), and silicification and sericitization in the upper section. -',{ineralization -
Association of Fe. Cu. U. REE, Au, Ag, with trace amounts of other metais. abundant hematite. and a varietv of gangue minerals dominantlv quartz and fluorite - Principal U minerals are pitchblende. subordinate coffinire. locallv minor brannerite:Cu occurs mainll" as sulfides,in basal part of the deposit associatedrvith abundant pyrite - Sulfide and associated minerals form two modes of mineraiization: older strata-bound assemblagesof dominantly Fe- and Cu-sulfides and vounger strata-transgressivemineralization of predominantly Cu-sulfides - Strata-bound mineral assemblageis bornite (chalcocite)-chalcopyrite-pvrite associatedwith subordinate to trace amounrsof mineralsof U, Au, Ag, REE. Co, Ni, and abundanthematite. Principal gangue minerals are quartz. serrcite. fluorite, and minor sidente and barvte. Verti-
cal zonation is reflected by sulfur-rich, copperpoor assemblage (pyrite-chalcopyrite) at the base gradine upwards to a relative Cu-rich, S-poor assemblage(bornite-chalcopyrite).Ore and eangue minerals impregnate the hematiterich matrix of polvmict brecciasbut also occur in rock fragments.They constitute5 to 20 vol. % of the matrlx in the form of disseminations, replacements.void-fiIl, and occasionallyas thin stratiform lavers. The concentration of suifides displaysa consistentrelationshipto the amount of matrix Strata-transgressivemineral assemblage is chalcocite-bornite associatedwith subordinate U, minor amounts of other sulfides.arsenides/ arsenatesof Cu, Ni, Co. native Au, Ag, and Cu, and abundant hematite. Principal gangue minerals are fluorite, quartz. and subordinate sericite. chlonte. Ore and gangue minerals form veinlets. veins. irreguiar lensesrestricted to structurallv prepared linear zones paralleling the long axis of the graben Strata-bound mineralization characteristically displavssimultaneousrhythmic precipitationof hematite and sulfides
110
4 Typology of Uranium Deposits
- Strata-bound miner:lization commonly is mineralization,but below strata-transgressive spatialoverlappingexists
named "Breccia Complex" type. Although of hugesize,accessable U amountsare restrictedto the quantitiesrecoverableas coproductto Cu-Au mining.
Age Constraints Not established(host rocks at Olympic Dam are Middle Proterozoic,not oider than ca. 1600m.y. and sericitealterationis about i320m.y. old). Metallogenetic Aspects
4.9 Type 9: Intrusive (Fig.a.9) Definition
primary, Intrusivedepositsconsistof disseminated Roberts (1988) suggestsfor the Olympic Dam non-refractory uranium minerals, dominantlv deposit that it originated from a large evolving uraninite.uranothorianiteand/or uranothoritein hydrothermal system in an extensional con- rocks of intrusivemagmaticor anatecticorigin. tinental environment.During an early, more or Deposits are of low to very low grade (20lesssynsedinentaryepisode,hydrothermalfluids 500ppmU) but may contain substantialrecontaining ferrous iron are thought to have sources.Further subdivisionis basedon host rock entered sulfate-rich sedimentary environments petrology: forming by rhythmic precipitation of sulfidesand hematite strata-boundmineralizationin coarse clasticsediments.Continuousmodfficationof the Subtype9.1: alaskite mineralizationoccurred. In a final stage,extenType Example: Rcissing,Namibin sive high level intrusive activity of probably alkalinenature interactedwith and superimposed Subgpe 9.2: quartz-monzonite Cu- Type Example: Bingbam, USA widespreadstructurecontrolledtransgressive on older strata-bound the U-Au-minera[zation Subtype9.3: carbonatite mineralization. Type Example: Phalaborwa,S. Africa
Dimensions/Resources
Subtl'pe 9.4: perdkdine syenite Type Example: Kvanefjeld. Greenland
Olympic Dam is large in size, extendingover an Subtype9.5: pegmatite area of up to 7km long, 4km wide and 300m thick. Within this area. structurally controlled Type Example: Madawaska,Canada mineralizationoccursin linear zones,ereaterthan References:Alexander 1986;Brynard'and Andreoli 1988; 6km long, up to 0.7km u'ide, and more than Berning 1986; Camisani-Calzolari et al. 1985; Maurice (ed) 1982;Ssrensener al. 19'71 300m thick. Total resourcesof the deposit amount to ca. 32mio.mtCu, I.2mio.mt U3O6and 1200mtAu. Averagegrades are I.6"h Cu, 0.06%U3Osand Subrype9./ is in medium-to very coarse-grained 0.6glmtAu. This amount includeshighergrade alaskitebodies ranging in size from large stocks sectionswith probable reservesof 450mio.mt and domesto tabuiardikesand smalllensesdiscontaining ca. 11mio. mt Cu, 360000mtU_1Os cordantly to concordantly within isoclinally and 270mtAu, at grades averaging2.5"/oCu, folded highll' metamorphosedand migmatized No uraniumrelatedalterationis 0.08%U:Os, 0.6glmtAu and 6glmtAg (Roberts metasediments. present. 1e88). Subrype9.2 consistsof very low gradeuranium disseminationsin highly differentiated granitic to (cupriferous)quartz-monzonitic(copper porRemarks phyries)complexes. prospect Dam Olympic and the nearbyAcropolis Subrype9.3 is associatedwith differentiated are the only known depositsof the tentatively (cupriferous)carbonatitecomplexes.
Type 9: Intrusive
9.
TyP. SubtYPe
INTRUSIVE
9.1
oloskite
9.3
corbonotite
9.2
gronite/ monzonite
9.4
perolkoline syenite
9.5
pegmotite
rr
E
+l
'.pl tl I
+
lttl
tJ minerolizotion
r.-l
anatectic intrusive
tt +' - t- l
magmatic intrusive
g
pegmatite
w
metasediment
l_
100 to 200m
Fig.{.9
lil
.'A
Subnpe 9.-l is in peralkaline syenitic domes or stocks. Uranium phases are commoniy of more refractory nature. Strbtype9.5 is in dikes of granitic. rarely syenitic unzoned pegmatite of silcious and mafic tendency (aegirine-augite).
'RRD
|
Age Constrain* No ageconstraintsexceptfor restrictionto mobile belts. Minerali:ation
- Principal U milerals: t thoriferousuraninite, uranothorianite,uranothoriteand/or uraniferPrincipal Recognition Criteria ous refractory minerals (typical for subtype 9.-l). iocally in weatheredzoneshexavalentU Host Environment minerals - Si'n- to post-orogenic intrusions within intra- - U-minerals finely dispersed ubiquitously throushouthostrock in subwpes9.i to 9.4: in cratonic mobiie belts. pockets and clustersirregularly distributedin - Commonly sharp contacts and narrow consubnpe 9.5 pegmatite tactmetamorphic aureoles around intrusions - Subtypes 9.1 and 9.1 are late leucocratic - ThrU ratio generally<1 to 3 peraluminous Si. Al and alkali nch facies in highly differentiated granitic or granitoid comp l e x e s . s u b t v p e s9 . 3 a n d 9 . { a r e o f b a s i ca n d peralkaline tendencv and occur in distinct ,:ccks and domes - .{bundance of pegmatitic. aplitic and lamprophyric dikes - Abundance of xenoliths and roof pendants - Enrichmenc of volatiie (F. Li. Be). RE and/or metallic elemenrs (Cu. Mo. etc.) .4lterqdon - No primary ore related alteration except hematitization in peematite - Subtype 9.2 (granite) often affected by Nametasomarism ( aibitization)
\{etq llogeneticAspects granite Uranium-richalaskite.quartz-monzonite. and associatedpermatites are supposedlythe product of granitization of uraniferouscrustal materialas indicatedby Sr isotopes.Subrype9.1 deposits.suchas Rossing,are attributedmore to ultrametamorphic-anatecticprocesseswhereas f.or subwpe 9.2 magmatic differentiation rvith uranium retainedin late-stagephasesis favored. The content of U. Th. REE. and other metalsin the various granitic faciespresumablyis a function of their onginal abundancein the precursor (meta)sediments.Prerequisitefor the elemental retainment is a dry granitization processin a closedsvslem.
ll2
4 Typology of Uranium Deposits
The origin of both carbonatitesubtype9.3 and throughout the alaskite, in interstices and in peralkalinesyenitesubrype9.4 relateddepositsis microfractures. U in mineralized alaskite has particularly concentrated in zones rich in biotite, at interpretedas of orthomagmaticdifferention. Remarks Intrusive subtype 9.1 to 9.4 depositsare all of very low grade (ca. 20 to 400ppmU) but may contain large resources.Subtype9.5 pegmatite depositsma-vaverageup to 0.1%U3O8but resourcesare generalll'low (a few tonnesto a few hundred tonnes UrOs). Uraniferous intrusive depositsin most cases provide good uranium sourcerocksrather than viabledeposits.The onlv exceptions are the Rossing alaskite deposit and pegmatite deposits of the Bancroft district (Madawaskadeposit). Uranium is extractedas by-productfrom subtype 9.2, e.g., from the with Binghamporphyry copperdepositassociated quaru-morzonite.and from subtype9.3, e.g.. from the Phalaborwacarbonatite.
4.9.f Subtype 9.1: Alaskite
constrictions of wide alaskite bodies passing into dikes or apophyses, in alaskites intruded along axial planes into metasediments and in alaskites replacing amphibolite. Age Constraints Although Rossing formed during the Damara OrogenvlPan African Orogenl'. 500 to 600m.1. ago, similar deposits may have formed during any orogenic event of equivalent thermodvnamic conditions provided adequate source lithologies were available. DimensionslResources Individual deposits (in brackets Rossing deposit) may be several tens of meters to severai hundred meters (ca. 700 m) long, severaltens to more than 500m (ca. 600m) wide and in excess of 700m deep, containing up to ca. 125000 mt U3O6 (Rcissing) at (average) grades ranging from 0.002 to 0.04% U:Oa, occasionallyto 0.1% U:Os.
Type Example: R6ssing,Namibia Reference:Berning 1986; Berning et al. 1976:.Brynard and Andreoli 1988
Remarks For more detailssee Chapter5.9
H ost Roc l<s/Alteratio n I Structures Syntectonic medium- to very coarse-grained alaskite emplaced as bodies ranging from large stocks and domes to tabular dikes and small lenses discordantly to concordantly within isoclinallv folded highly metamorphosed and migmatized metasediments. No uranium-related alteration.
Examplesof Intntsive, Subtype9.1 Alaskite DepositsI Occurrences Canada: Johan BeetzlQuebec,? Charlebois Lake/Saskatchewan Namibia: Goanikontes, Ida Dome. ValenciaTrekkopje.SJ Claims/Damara OrogenicBelt
Ore and Associated Minerals Uraninite (U/Th ratio : 9), betafite and alteration products thereof . predominantll' Puranophane in near-surface zone. Monazite. zircon. apatite. titanite, pyrite. chalcopyrite. bornite. molybdenite. arsenopyrite, magnetite, hematite, ilmenite, fluorite as accessorial rock constituents.
4.9.2 Subtype9.2: Quartz monzonite (Cu-porphyry) Type Example:Bingham,Utah, USA References: John1978:Lanieret al. 1978 H ost Roc ks IA heration I Structures
Epizonal intrusion of several igneous phasesranging in composition from monzonitic to quartz Uranium minerals of minute size occur as inclu- monzonotic. In the Bingham stock six major sionsin quartz, feldspar, and biotite disseminated igneous phases are recognized, from oldest to Mode of Mineralization
T."-pe9: Intrusive
y o u n g e s t : Q u a r t z - p o o r ( < 1 0v o l . % q u a r t z ) equigranular to porphyritic monzonite, porphyritic quartz monzonite and recrystallized nronzonite; quartz-rich (>20vol. % quartz), p,-rrphyritic latite, quartz monzonite porphyry and hybrid quartz monzonite porphyry; latite porphyry dikes and quartz latite porphyry dikes. Hydrothermal alteration in and around the ore controlline quartz monzonite porphyry is reflected by Mg and K metasomatismforming an inner zone of quartz-orthoclase-phlogopite,an outer zone of actinolite-chionte-epidote,and a iare sericitic and argillic (montmorillonite mainly) ,^i erprint.
l13
l e a c h c o n t a i n i n g 8 t o 1 2p p m U . U r a n i u m r e sources recoverable as bv-product to the year 2 0 1 0 a m o u n t t o c a . 1 5 0 0m t U r O B ( O E C D / I A E A
1e86). Examplesof Intrusive,Subrype9.2. Granitel M onzoniteD eposis I Occurrences Australia:? CrockersWell/OlaryProvince(REE -!
TI\
Norte (Cu - U) Chiie: Chuquicamata USA; Twin ButtesiArizona.YenngtonrNevada (Cu -r U) CIS: ? Akbabay, Aksuyek. Kivakhn'. Koktas/S of Lake Balkash(Cu - U)
Ore and Associated Mineralst'Vode of tVineralization
4.9.3 Subtype9.3: Carbonatite
The ore minerals form overlapping sulfide mineral zones containing from the interior low grade core outward: molybdenite, bornite-chalcopyrite, chalcopyrite-pyrite,pyrite, and galena-sphaierite. Gold and silver is present in significantamounts. Pt, Pd, Re, Se, Bi, and U occur in recoverable traces. No uranium mineral is listed, but U may be present as uraninite or uranothorianite. Molybdenite and Cu-sulfides occur as disseminations. and galena-sphaleritewith part of the pyrite as veins. The mineralization is concentrated on and drapes around and through the quartz monzonite porphyry facies. Distribution and :,:ning of mineralization and alteration is controlled by the location reiative to the quartz monzonite porphyry facies within the intrusive complex. rock type, degree of fracturing, and permeability.
Carbonatite emplaced into the core of a differentiated alkaline intrusive complex. The carbonatite consists of a younger rransgressive carbonatite core intruded into banded carbonatite. Fault systems apparentlv control the position of the transgressive carbonatite. Both facies are petrographicailv almost identical and consist of a magnetite-rich soevite. A rim facies of phoscorite has developed. It consists of serpentinized olivine, magnetire (ca. 30"/"), apatite (ea. 15'/"), phlogopite and e-alcite.
Age Constraints
Ore and Associated Minerals
Although the Bingham stock is of Eocene age. ',:ere should be no principal age constraintother : n a n t o p e r i o d s o f m a g m a t i ca c t i v i t v .
Type Example:Phalaborwa.South Atrica References:Camisani-Calzolarier al. 198,61 IAEA i986a: PhalaborwaMining Co, 1976
H ost Roc ksIAlteration I Strucrures
Recoverable ore minerais are chalcopyrite. bornite, titaniferous maqnetite. baddelevite. and uranothorianite. Other minerals and eiements present in minor to trace amounts are pyrrhotite, pyrite. marcasite. pentlandite. millerite, DimensionslResources bravoite. violarite, linnaeite. tetrahedrite, Cu- and Mo-mineralization at Bingham with sphalerite,galena.covellite. chalcocite.cubanite. which the recoverable U is associatedextends vaileriite, as well as gold. silver. platinum, and over an area of about 1200m by 2100m around palladium. and through the quartz monzonite porphyry phasewhich measureslaterally 420mby 1000m. ,V o de of Mineraliz atio n Persistence of mineralization into depths is at least 1500m. Uranium content in the Cu-Mo The transgressivecarbonatite contains a high conore is 20 to 50 ppm. Uranium is extracted from tent of copper sulfides. mainlv chalcopyrite
714
4 Typology of Uranium Deposits
and less bornite, and accessorvminerals.often in veinlets, up to 2cm wide and less than 1m long, horizontally and vertically. The banded carbonatite contains copper sulfides, mainly bornite as discreet grains widely disseminated throughoutthe rock. All ore mineralsextendfrom the carbonatiteinto the phoscoritewith bornite alsoprevailingin the phoscorite.Uranothorianite and baddeleyiteare equivaientin all facies. Age Constraints
Mode of Mineralization U-Th minerals are disseminatedin lujavrite. Highest but heterogeneouslydistributed U-Th concentrationsoccur at or near the contact of intrusive apophysesand sheet-like bodies of medium- to coarse-grained lujavrite in strongly deformed and metasomaticallyaltered lavas. Analcimeveinletsare often abundantin the contact zone. Mineralizgdzonescontainingin excess of 400ppmU are rather small.
No age constraintsexceptto periodsof magmatic Age Constrainu activiry. The Illimaussaqsyenitesuiteis about 1150m.y. old but similar typesof uraniumoccurrencesmav DimensionslResources exist in comparablelithologic facies of anv age Mineralized carbonatite and phoscorite at from late Archean to Recent. Phalaborwa form an ellipticai. almost vertical dipping pipe. Diametersare 800m and 1400m. Drilled depth exceeds1000m. Averagegrade is ca. 0.004%U:Oe (0.51% Cu). Recoverable resources amount to several thousand tonnes uranium. Examplesof Intrusive, Subtype9.3 Carbonatite Deposix/ Occurrences Brazil: Araxa Finland: Sokli India: Sevathur
Dimensioru/Resources The Kvanefjeld depositcoversan area of about 700m by 1500mandextendsfor more than 100m deep (investigateddepth). Averagegjade is 0.03 to 0.04"/oU:Or. Resourcesamount to more than 30000mtU3Os. Examplesof Intrusive,Subrype9.4 Peralkaline Sy eniteD epositsI Occurrences
Brazil: Catal6o Cameroun:Lolodorf China: Saima Massif/i.IE China Greenland:Motzfeldt Center/SEGreenland 4.9.4 Subtype 9.4: Peralkaline syenite South Africa: Pilanesberg/Bophuthatswana Vishnevogorsk, Novogornv/Ural Russia: Type Example: Kvanefjeld, Illimaussaq, Mountains Greenland References: Bohseet al. 1974'. Sorensen et al. 7974 H o st Ro cks IAlteration / Structures A differentiated agpaitic-peralkaline nepheline..syenite complex includes as latest member lujavritic facies. The svenites have intruded into altered volcanic rocks and continental sediments. Ore and Associated Minerals I Elements Principal U-Th-bearing minerals are of refractory nature and include steenstrupine,eudialwe, and monazite. Associatedelementsinclude F, Li, Be, Zn, Zr, Y, Nb, and other REE.
4.9.5 Subtype9.5:Pegmatite Type Example: MadawaskalBancroft.Canada Reference: Aiexander1986 H ost Ro cks IAlteratio n I Structur es Granitic, rarely syenitic unzoned pegmatite of SiO2 and mafic tendency (aegirine-augite). stained pink to brick-red by hematite, fine- to extremely coarse-grainedemplacedas en-echelon dikes of tabular to flat lenticular shape with swells, offshoots, and apophysestransgressiveor
Type 10: Phosphorite
oaralleling metasediments and igneous rocks metamorphosed to amphibolite facies. Postmetamorphic deformation and metasomatismare common. Hematite is a characteristicalteration product.
Ore and Associated Minerals
115
4.10 Type 10: Phosphorite(Fig.4.10) Definition Phosphorite-relateduranium mineralization consists of marine phosphontes of continental-shelf origin containing synsedimentary stratiform. disseminated uranium. The dominant uranrum mineral is cryptocrystalline fluor-carbonateapatite containing svngeneticuranium substituting for calcium. Two subtypesare recognized:
Uraninite, uranothorite, and alteration products thereof mainly uranophane. Average U/Th ratio is 2. Thorite, allanite, euxenite. davidite, spencite, masnetite. pyrite, marcasite, pyrrhotite, chalcof , i l t e , r n o l v b d e n i t e ,z i r c o n , s p h e n e , f l u o r i t e i n Subtyp€l0.l: phosphoria(Idaho Pbosphonatvpel Type Example:-\{ontpelier,Idaho. US.{, ic;essorial amounts, and abundant hematite. Subtype10.2:land pebble(Florida type t
Mode of Mineralizqtion
Type Example: Land Pebbledistrict, central Florida, USA
U is concentrated in discontinuous pods, shoots and bands, commonly controlled by mafic minerals and particularly by hematite and magnetite. Some mineralization is associatedwith late fraclures. Although mineralization occurs randomly in the pegmatite in larger bodies, it is frequently proximal to the footwall and hanging wall side. Depth continuation is often more persistent than lateral continuity. lJ-tenors range from traces to 0.06% and rarely to 4ohU3Os over limited extent, changing within fractions of 1 m.
References: Altschuler et al. 1958: Heinrich 1958; McKelvey et al. 1956
.\ge Constraints The Bancroft pegmatites occur within the Grenville orogenic belt (950-970m.y. old) but similar deposits may occur in any orogenic belt.
DimensionslResources lndividual deposits may be less than 1m to :cveral 100m long, severalcentimetersto several i0 m wide and in excessof 400m deep. containing up to 200mtU3Os at (average) grades ranging from 0.05 to 0.15% U3Os. Distncts may contain up to 2000mt U3Os.
Examples of Intrtrsive, Subtype 9,5 Pegmatite DepositslOccurrences Canada: Campbell Island/Ontario Finland: PalmottuA.{ummi-Pusula Russia: 'l Kem. Chupa area/Onezhsky
Principal RecognitionCriteria Host Environment - Phosphoritesof marine origin depositedon condnentalshelf - Subrype10.1;beddedphosphontewith oolitic, pisolitic, pelletal and laminated textures associated with fine-grained miogeosynclinal facies (black shale, mudstone. chert' lesser carbonatebeds) but noticeableabsenceof carbonateswithin uraniferousphosphontes - Subtype 10.2: nodular phosphonte locally reworkedto apatitepebblesinterbeddedwith shallowmarinefacies fine-to medium-grained (sand.clay)and carbonatebeds - Widespreadextensionof phosphoritehorizons.up to several1000'sof km- Beddedphosphorites containa higheruranium content and are tormed distal to shore-line (subtype10.1)as comparedto nodularphosphorites which are proximal to shore line formation, exceptfor the land pebble subtype (10.2)(partlydue to reworkingand secondary U enrichment) Mineralization - Either none or only rare discrete pnmary uranium minerals
116
4 Typologyof Uranium Deposits
TyPe
10. P H O S P H O R I T E
Subtypc
10.'l phosphorio
1O.2 lond pebblc
lcvcl E o |r) chert
I
o
rO
mony Km block shole
Fig. 4.r0
I urqniferous phosphorite, dork phosphotic shole
corbonotes (limestone, dolomite)
Principal uranium bearing mineral is cryptocrvstallinefluor-carbonateapatite Unfavorable mineralsfor uranium accumulation are Al and Ca-Al phosphates(or other non-Ca phosphates) Positivecorrelationof U and P when concentration of both elementsis relativelyhigh Negative correlation of U and carbonate contents U exhibits a rather uniform distribution throughouta given bed No ore-relatedalterationexceptweathering No age constraints
,RRD
\\-
evoporita, light to rcd coloured shole,silt
uroniferous lond pebblc phosphorite
clostics, shollow woter cqrbonotes
higher U enrichment (up to 6500ppm. average60 to 200 ppm U) is attributed to the sedimentary environment of bedded phosphorite formation at the margin of a continental shelf, where (a) upwelling of P saturated deep marine waters provided a renewable P source, (b) slow rate of sedimentation caused longer exposure of apatite grains permitting extraction of U from the seawater to replace Ca in apatite, (c) absence of COr2* ions in the waters which allows U to remain in solution. Variations in U content relative to P are interpreted to have resulted from either one or a combination of items (a) to (c) cited above. The second environment is a shallow marine Mer^llogenetic Aspects near-shore platform represented by lower grade phosphatic and uraniferous nodular phosphorites Only phosphates of shallow marine origin contain (average I0 to 20"h P2Os, 20 to 80 ppm U) exappreciable amounts of uranium. Uranium cept for the land pebble mineralization. In this accumulation is thought to be due to synsedi- environment. fine- to medium-grained clastics mentary extraction of U from seawater and (clay, sand. glauconite) and shallow-water carincorporation into phosphateminerals. bonates precipitated contemporaneouslv with Two favorabie environmentsof sedimentation phosphate. are recognized. The land pebble phosphorite (subtvpe 10.2) as The first, represented by' the Phosphoria found in the Pliocene Bone Valley Formation. Formation, Idaho (subtype 10.1) has formed at Land Pebble district. central Florida. contains the outer or distal shelf margin where thick layers pebbles and sand size grains of fluor-carbonate of bedded phosphonte developed within a pile apatite ennched with up to 35"h P2O5 and 500 of very fine-grained miogeosynclinal sediments ppm U and locally' more. The enrichment is including biack shale, more or less carbonaceous attributed to reworking and corresponding mudstone, chert beds, and only minor carbon- re-exposure of the apatite particles to uranium ates. Principal U-P mineral is cryptocrystalline bearing seawater during repeated marine transfluor-carbonate apatite. Uranium is considered to gressions.Associated with this evolution is an have been syngeneticallyincorporated into the irregular, up to a few meters thick, leached apatite lattice by replacingthe Ca ion. The higher horizon developed within the Bone Vallel' P (25 to 35% PzO-o i v e r 1 t o 3 m ) a n d c o r r e l a t e d Formation by weathering. Its upper section,
Type 10: Phosphorite
composed of Al-phosphates (wavellite), and its middle section. dominated by Ca-Al-phosphates (millisite, crandallite, referred to as aluminum p h o s p h a t e z o n e ) . h a v e r e l e a s e du r a n i u m s u p rosedlv by acid solution leaching. Altschuler e t a l . ( 1 9 5 8 )s u g g e s tU r e c o n c e n t r a t i o na t o r n e a r the base of the leached zone composed of incipientlv leached residual apatite formed in responseto neutralizationof the acid fluids by the Ca-phosphate. Fixing of uranium resulted by adsorption on the porous. partiaily leached, residual apatite leading locallv to concentrations ,tf as much as several thousand ppm U. Principal '-'collectors a r e C a - p h o s p h a t e(sa p a t i t e )a n d , t o a extent, Ca-Al-phosphates (crandallite), .3sser rvhereasAl-phosphates (wavellite) are unfavorable U collectors. Distribution of these minerals also reflect the U-grade zonation within the leached profile.
Remarks Although uraniferous marine phosphorites constitute large U resources, their commonly very low average grade (<20 to 300ppmU) and difficult metallurgical U extraction excludes them from being a primary U source. U is recovered, however, as a bproduct of phosphateproduction. In contrast to marine phosphorites, all other ohosphatic rocks contain lower concentrations ,rl uranium. They include residual phosphorites derived by weathering of marine phosphatic iimestone, phosphatized rocks resulting from solutioning of phosphates from rocks higher in the sequence and its redeposition as interstitial fiilings or replacementsin subjacentrocks. guano mainly composed of bird and bat excrements. and detrital weathering products thereof and ,tf marine phosphorites such as fluvial pebble r:posits.
Examples of Type 10 Phosphorite Deposits/ Occurrences (not Differentiated) Central African Republic: ? Bakouma Israel: ZefalNesev \'Iorocco: Ganntour-BahiralYoussoufia, Oulad .\bdouni Khounbga USA: Bone ValleyiFlorida. Green River/UtahWyoming
lll
4.10.1 Subtype10.1: Phosphoria(Idaho type) Type Example: N{ontpelier,Idaho. USR; PermianPhosphoriaFormation Reference: McKelveyet al. 1956 H ost Roc ksIA lteratio n I Structures Phosphatic shales composed of bedded fluorcarbonateapatite as fine-grainedoolites. pisolites. pelletsor laminae and phosphaticfossil fragments (brachipod shells. fish scales, etc.) mi-red with variable amounts of finest-grained detritus. mainly clay particles, carbonaceousmarrer and locally carbonate. Sedimentarv environment is the distal part of a continentai sheif. where uranium-bearingdeep seawatercould ascendand flood the phosphorite and where conrribution of detrital clay and silt was minimal and accumulation rate was strongly retarded. Associated sediments include over- or underlying, often pyritic cherts, mudstone. black shale which laterally interfinger with sandy, carbonatic. red bed and evaporitic shallow water sediments. Ore and Associated ElemenslMode of Mineralization U occurs in rather uniform dissemination in phosphatic beds in dominantly bedded crvptocrvstalline fluor-carbonate apatite of peiletal, oolitic etc. texture. Although present in almost all phosphorite beds U grades varv considerably from 10 to 6500ppmU and to a larse extent. but not necessarily,are correlative to the phosphate content. P2O5 enrichment is greatest in the top and/or basal segments of phosphadc lavers as examplified by the Mead Peak Phosphatic Shale Member, the main phosphate member of the Phosphona Formation. This member is 60 to 150m thick and averages 11 to I29/o P:Os, whereasthe upper and lowermost 1 to 3 m contain from 25 to 35'/' P2O5and also contain the highest uranium tenors. DimeruionslResources The Phosphoria Formation stretches over many thousandsof km:, its phosphaticmember ranges from 60 to 150m thick. The best mineralizations occur over a thickness of I to 3 m. Total resources are several million tonnes U3Os. Uranium grades average60 to 200ppm.
118
4 Typology of Uranium Deposits
4.10.2 Subtype10.2:Land Pebble(Florida type)
Ore andAssocistedMinerak/Mode of Mineralization U occursin stratiform disseminationbound to Ca-
Type Example: land Pebble district, central phosphate(principallyfluor-carbonateapatite)in Florida,USA; PlioceneBoneValleyFormation the form of sandsizeparticlesand nodules in beds Reference:Altschuler et aI. 1958
mixed with quartz and clay minerals. Although apatitegrainsand nodulesare often enrichedup H ost Rocl<sIAlteration/Stntcrures to 500ppmU and locally up to several1000ppm at bottom of leachedzone,the mineralizedlower Phosphaticpebbly argillaceoussandstonesare Valley Formation averagesca. 150ppm U: Bone crudelygradedbedded,reworked and composed due to the quartz and clay matrix. of nodulesand sandsizegrainsof fluor-carbonate apatite mixed with clay minerals (smectite, montmorillonite) and quartz grains of shallow Dimensioru/Resources marine near-shorecontinental shelf origin. The Lateral extension of the lower Bone Valler, phosphoriteis interbeddedwith marinesandsand Formationis about 2500km2 in which depositsoi shallow water limestone and dolomite. Black the Land Pebbledistrict occur. Mineralizedbeds shalesand cherts are noticeableabsent.Where range in thicknessfrom lessthan a meter to 10m affectedby weathering,apatiteis replacedin the '7 averaging 5 to m. Total IJ resourcesare in the top zone by Al-phosphate(wavellite)and in the order of 500000mt U3Osor more. Gradesof land middle zone by Ca-AJ-phosphate(crandallite, pebblemineraiization averageca. 150ppmU. millisite). Smectiteis transformed to kaoiinite. Incipiently or incompietely leached residual apatite of the bottom zone is highly porous and enrichedin U, whereasthe secondaryAi- and Ca- 4.11. Type 11 :Volcanic (Fig.a.11) Al-phosphatescontain only minor U. (also referred to as volcanogenictype, or U-Mo type in Russianliterature)
11. V O L C A N I C
T}?c 11.'l
Subtypc Co f se
11.2 etroto-bound
stucbJrFbound
1 1. 1. 2 surficiol frocturc fill
't1.1.1
11.2.1
11.2.2
inbusive vein
intocoldcro
exocoldero
<+, tOO - mnr
@} m
Si-Al-rich rhyolite inbugion
m
rhyolitic outflow fociea of osh-flow tuff (welded tuff, vibophy'e, lopillituff, ignimbrite)
M
Fig.4.ll
^^,
EA,
mofic flow breccio
..rA
U minerolizotion
E
-i
,RRD
inbocolderp volconiclostic ond loke (moot) focies
m
volconiclostic -
n
boscmcnt
locusbine focies
Type 11: Volcanic
119
- Occurrence of volcanicdomes,subvolcanicintrusives,ring dike intrusives,ourtlow faciesof Volcanic deposits are associatedwith felsic to ash-flowtuffs, lavas,ignimbrites,pvroclastics. intermediate volcanicrocks and their sedimentary intracalderavolcaniclasticand lacustrineor ,-ierivates.Uranium occurs as structure-bound moat facies or strata-bound concentrations which can be - Facieswith major permeabiiityvariationsdue to weldingor fracturing(vesicularflow zones, separatedinto several classes: flow breccias.contactsbetweenthick rhyolitic units and mafic lavasor limestones,ring dike Subtype11.I : structure-bound faults, fault intersections,grabens. fracture C l a s s1 1 . 1 . 1i:n t r u s i v ev e i n s zones proximal to domes, subvolcanicinType Example:Nopal I, Mexico trusionsor major dikes) - U sourceand U host rocks are often identical C l a s s1 1 . 1 . 2s: u r f i c i a l llpe Example:Cotaje,Bolivia - Glassy and unwelded volcanics are better sourcesthan crystallineor weided rocks Subtype11.2:strata-bound Definition
Class11.2.i : intracaldera Type Example:Aurora. USA ClassI 1.2.2:exocaldera Type Example: Margaritas.Mexico References: Chen Zaobo 1981; Dayvault et ai. 1985; George-Aniel et al. 1985; Goodell 1985; Goodell and 'raters 1981;IAEA 1985;Leroy et al. 1985.1987;PardoLevton i985: Sherborne et al. 1979
Structure-bound mineralization include intrusive veins associated with volcanic intrusions, diatremes. flows or bedded pyroclastic units (class 11.1.1) and surficial fracture fills (class II.1.2) in similar lithologies. Strata-bound mineralization ,:onsists of disseminations and impregnations in permeable and/or reactive flows. flow breccias. tuffs and other lithologies. Distinction of stratabound classesis based on their intracaldera (class 11.2.1) or exocaldera(11.2.2) host environments, the latter is mixed with nonvolcanic clastic sediments.
?rincipal Recognition Criteria Host Environment
Alteration -
Feldspathization Zeobtuation Devitrification Advancedto completeargillitization Stronghematitization Intensesilicification
Mineralization - Principal uranium minerals are pitchblende, rarely coffinite, in many deposits uranyl minerals - U adsorbedon or incorporatedin varioushost minerals(U bearingclay minerals.uraniferous opal etc.) - Associatedmetallic minerais include mainly pyrite, minor to tracesof Mo, Pb. Sn- W, Li, Hg, Sb, W and other minerais - Associatedgangueminerals:fluorite. quartz, carbonates. baryte.jarosite - Occurrenceof U minerals predominantlv as disseminations,very rarely as more massive (stringers,veiniets.pods) concentrations - Higher grade mineraiizationalwavsof erratic distnbution
- Volcanogenic intrusive and extrusive rocks of Age Constrainu felsic to intermediate magmatism - Preferentially leucocratic. rhyolitic facies high No restrictions except to taphrogenic episodes in silica and alumina. low in iron and calcium. rich in volatiles - Abundance of fluorite, tourmaline, topaz re- Met^llogenetic Aspects flecting volatile enrichments of F and B - Enrichment in trace elements particularly U, Prerequisite for mineralization are volcanic facies containing U in excess of average crustal Mo, Sn, W, Hg, As. Sb, Li
L20
4 Typologyof UraniumDeposits
abundanceand in leachable form. These parametersare provided mainly by rocks of rhyolitic compositionthat are unweldedand have a glassy groundmass.Matrix-bound uranium is easily liberated by devitrification in reaction to both volcanic hydrothermal (hot springs,fumaroles) and meteoric supergene waters, which then transportedthe U to sitesof depositioneither in porous volcanic beds or fractures. In addition, U may be introduced by volcanic hypogene fluids.U-transportingsolutionsare thoughtto be oxidized and slightll' acidic, containingF. CO2 or other ions, which facilitate transport of U. Suggestedprocessesfor fixing U include reduction and adsorption.associatedwith neutralization by wall rock interaction and boiiing or evaporationof fluids. In near-surfacedepositsU precipitation often occurs along groundwater tables.
Romania:ApuseniMts Russia:Streltsovskyregion/Transbaikal Uzbekistan:Karamazarskvregion
4.11.1Subtype11.1:Structure-bound Reference: Leroyet al. (1987) Classll.1.l:
Intrusive Veins
Type Examples: Nopal I, Pefla Blanca, Mexico Reference: George-Anielet al. 1985; Mooniight. Mc Dermitt, USA Reference: Dawault et al. 1985 H ost Ro cl<sIAheration I Structures
Host lithologies are variable and inciude rhyolitic ignimbrite. rhyolitic breccias, dacite flows, granodiorite, a. o. Hydrothermal and/or diaRemarks genetic alteration is intense and may consist (kaolinite, Volcanic uranium occurrenceshave often been of devitrification, argillitization overprintedby stronghydrothermalor diagenetic montmorillonite), zeolitization and silicification. processes, making it dfficult to attribute them to At Nopal I, host rocks are completely altered. Alteration increases with intensity and density of a distinctclass. Most of the volcanicuranium occurrencesalso fracture systemsand decreaseswith distance from comply with criteria defining other tlpes of the ore body (60 to 200m away). Deposits occur depositsparticularly of vein, surficial, fracture in intensely fractured and brecciated zones caused fill and tabular sandstonetypes except for their either by crosscutting fault sets (Nopal I) or along crucial volcanogenicrelationship and subecon- ring fracture systems (Moonlight).
omic magnitude. Grades are low to very low (0.02to 0.1% U3O8)and resourcesare commonly small (<1000mtU3O6). Volcanic occurrences may be more important as potential sourcesfor other types of deposits,particularly for those of sandstonetype.
Examplesof Tlpe 1l Yolcanic Deposits/ Occurrences(not Differentiated) Australia:Maureen/Queensiand Braf,l: OsamuUrsumi/Pogosde Caldas Bolivia: Cotaje/Altiplano Bulgaria:East Rhodope Mts, East Balkan Mts Canada:Michelinll-abrador China: Quinlong-Daxing'anling volcanic belt, LangshanareaA.{EChina, BalyangheAIWChina, volcanic Gan-Hangand S. Jiangxi-N.Guangdong belts/SEChina Kazakhstan:Pribalkhashkvresion
Ore and Associated Minerals Principal uranium mineral in an unoxidzed environment is pitchblende associated with minor pyrite, molvbdenite. and traces of other sulfides. Gangue minerals may be fluorite, quartz, locall-v minor adularia. carbonate. apatite. jarosite. baryte, and Ti-minerals. Most mineraiization is oxidized containing mainly uranyl-phosphates and -silicatesand limonite. Mineraiization commonl,vincludesconcentrationsof Pb (<2000 ppm) , Mo (<500ppm) and anomalous amounts of As, Sb, Ag, Cu, Hg, Sn, W, Zr. M o de of M inerali z atio n I D imensio nsI R eso ur ces Uranium minerais occur as disseminations or coatings in a pipe-shaped deposit filled with silicified tectonic breccia at Nopal I (20 x 40 m wide, >100m deep) or as disseminations,stringers
T;,pe11: \'olcanic and veinlets ivithin tectonic breccia and siliceous cement rvhichform veins.1 to 5m wide, at least 50 m deep as at the Mooniight mine. Ore distribu: r o n i s i r r e g u l a r c h a n g i n gf r o m s e c t i o n sw i t h l i t t l e : r a n i u m t o g r a d e sw h i c h l o c a l l yr e a c h 1 0 % U 3 O s . Deposits contain from less than 1 mt to a ierv 100mt U3Os at ore grades averaging from < 0 . 1 % t o 0 . 5 9 1U" 3 O s ( N o p a l I c a . 3 5 0 m t U 3 O s ; 0.i% u3o8). E.tamplesof Volcanic, Classll.l.l StructureSound Intrusive Vein DepositsI Occurrences - - u s t r a l i a :? B e n L o m o n d / Q u e e n s l a n d ,- :nada: Rexspar/British Columbia Kazakhstan: Pribalkhashskvregion Peru: lvlacusani USA: llarysvale/Utah. White King Lakevierv, Oregon : StreltsovskyiTransbaikal R.ussia
1?1
DimensionslResources Nlineralizedstructuresare commonil'' 10 to 60m. r a r e l y f e w 1 0 0m l o n g , 0 . 1 t o 1 0m w i d e a n d p e r s i s t f r o m s u r f a c et o a b o u t 3 0 m d e e p . ( C o t a j e : 3 5 0 m long, 1 to 10m wide,30m deep). D e p o s i t sc o n t a i n f r o m l e s s t h a n 1 m t t o s o m e 10 mt U-3Os.Grades average0.05 to r).07"hU:Os, but may be as high as 2.5"h U:Os. Cotaje accounts for ca. '10mtU3Os, at an ore grade of 0.07% U3O3. Some districts may contain a ferv h u n d r e d , p e r h a p sa f e w t h o u s a n dt o n n e s U : O a . Examples of Volcanic, Class 11.L2 SrructureB o und Surficial D ep ositsI O ccurrenc es
Minel
CSf'R: Novoveska Huta-Muran/Slovakia USA: Lucky Lass Mine/Lakevierv. Oreson
4.71.2 Subtype ll.2: Strata-bound Orajaka(1981) Reference: Ciass11.1.2: Surficialveinlike Ty'pe Example: Cotaje, Sevaruyo. Bolivia i985 References: Lerovet al. i985:Pardo-Leyton H ost R o cksIAl teradonI Strucilres F{t)st lithologies are rhyolitic to rhyodacitic rcnimbnte and tuff layers with intercalated . ricanic breccias and conglomerates.The tuffs - - - : i i ei n c r e a s e d U . T h , L i . S r , B a , a n d R E E tenors. .\lteration is reflectedbv marked bleachint. argillitization, dominantly kaolinitizationand :iiicificetion. Deposits occur near surfacein zones rrf intense fracturing and brecciation.
Class 11.2.1: Intracaldera Tvpe Example: Aurora-Cottonwood Creek. Mc Dermitt. USA Dayvaultet al. 1985 Ret'erence: H o st Ro cksIA lterati o n I Strucrures
Host rocks are mafic to intermediate tuffs. flows. and flow breccias associated with domes and overlain by tuffaceous lacustrine (moat) sediments. Individual flows have massive centrai zones bordered by + strongly altered vesicular to scoriaceous flow tops aiong inraformationai unconformities.Alteration and mineralization in , ) re ottd .lssociarcd Minerals lavas is restrictedto porous zonesaiong florv tops. . ' : i n c i p a l U m i n e r a l s a r e ( s o o t v ) p i t c h b l e n d e . brecciatedlayersand fracture zones. Mineraiized ''.'itinite and uranvl phosphatesassociatedwith facies are almost compietely altered to pyritic snrttkv quartz. baryte. siderite. gypsum, and montmorillonite, chlorite, ciinoprilolite. opal. minor Fe. Pb. Zn sulfides. Local enrichment of leucoxene rocks with a groundmass of K-t'eldspar. clav. and quartz. Alteration of lacustrine sedi\lo irnd V also occur. ments is reflectedby sporadiczeolites.ubiquitous feidspar. and smectite, calcite and quartz .\ I o d e o f .V in ero liz atio n cn'stailization. L-ranium minerals are irregularly distributed in .'nen iractures tbrming narrow veins and occur Ore and Associated Minerak .iisseminated throughout zones of intense :heltring or brecciation which are commonlv Principal uranium minerals are pitchblende. coffinite. and locally hexavalent U minerals assooridizcd.
122
4 Typology of Uranium Deposits
ciated with pynte. Anomalous concentrations of Ore and AssociatedMinerak Hg, Li, As, Sb. Mo, Zn, W, and F are associated Principal uranium minerals are hexavalent U with the mineralization. species(uranyl silicates,vanadates.phosphates) associated with molybdenite, powellite, and Mode of MineralizationlDimensionsIResources locallywith jarosite, pvrite, calciteand fluorite. Uranium mineralsoccur as fine coatingsof pyrite and leucoxene mainly concentratedin highly M ode of MineralizationlDimensionsIResources alteredporouslayersalongflow topsandin breccia layers up to a few meters thick. Tuffaceous Uranium minerals occur as disseminations. laminaoccupvingfractures.joints. lacustrinesedimentscontain minor amountsof U encrustations. voids. distribution is erratic. and Mineralization (up to 200ppm U) predominantlvconcentrated in At Marearitas. higher-grade sections ranging intercalatedlavers 0.5 to 2m thick, composed 0.1 and from to 0.4'/. U3O6 0.1 to 37oMo are of thinl,"-bedded (mm to cm) opal. pyrite, ash, grade separated intervals. by low Mineralization diatomsand minor carbonaceous matter. occurs ranging from over intervals 1 to 7m thick. Mineralization extends laterally for several to wide 100 and about 1-50m 300m long. In penetrate 100m ro few 1000m. and mav upwards cases. most deposits of this type are smaller in and downwardsinto fracture zones. dimensions. Deposits may contain a few tens and up to Depositsmay contain a few tonnes to some several1000mt U:Os, at gradesrangingfrom 0.02 1000mtU-.Os, at gradesranging from 0.02 to (Aurora: to rarely0.1"21, ca. U3O6. 7500mtUrOs, rarely 0.2"/"U:Oa. (Margaritas:ca.2000mt U3Os, 0.05YoU:Os). Districts, including other types 0.i%U3Os). Districts may have resourcesof of volcanogenicU deposits,may have potential several 1000mtU3Os(Pena Blanca district: ca. resourcesof some 10000mtU3O6.(Mc Dermitt 4000mt UrOs). ca. 30000mtU3O8,0.03%UrOe). No attempts are known to recoverU as by-productof Hg, Li, or other metal mining. Examplesof Volcanic,Class11.2.2Strata-bound ExocalderaD ep o sisI Occurrences Examplesof Volcanic, Class11.2.1Strata-bound Australia: ? Maureen/Queensland IntracalderaD eposisI Occurrences Canada:Michelin/Labrador Italy: Novazza/ValSenana Italy: Sabatini,Vulsini/Latium Russia:Yubileynoye,Novogodneve/Streltsovsky,Sweden:Duobblon/Sorsele USA: Date Creek Basin/Arizona Transbailcal
Class11.2.2:Exocaldera Tlpe Exampie:Margarites,PenaBlanca.Mexico Reference: Goodell1985 H ost R oc ks IA lteraion I Structures Host rocks are facies of ash-ffow tuff (vitroclastic alkdhne rhvolitic tuffs. etc..;. At Margariras the tuffs fill a graben, 500m wide, bound on both sides b1' step faults with displacementsof up to 100m and more. Alteration inciudes extensive devitrification. partial argillitization, intense silicification and locally abundant hematitization. If tbese volcanoclasticsare interbedded with lacustrine sediments they form deposits such as those in the Date Creek Basin, Arizona.
4.12 Type 12: Metasomatite (Fig.a.I2) Definition Metasomatitedepositsconsistof unevenly disseminated deformedrocks uraniumin structuralll, that were affected by alkali metasomatismof dominantlysodiumtendencv.Metasomatic facies includealbitites,aegirinites and alkaii-amphibole rocks. Principal uranium phasesare uraninite + Th rich and U-Th oxidesand silicates. Two subtypesare definedon the basisof the rock facies: Drecursor
Type l2: Metasomatite
TyPe 5 ub tY P e
METASO m e t o s o m o ti z e d gronrte
MATITE 1 2 . 2m e t o s o m o t i z e d m e t o s e di m e n t
E O
a
U minerolizotion
rl E_t
gronite
m
metosediment
tL-:-:J ..l
No-metosomotized
F;]
olbitite
1... I
.A
r/.)
,RRD
Fig.{.12
granite :ubtype 12.I : metasomatized T''pe Example:RossAdamslBokanMountain.USA
Mineralization -
metasediments Subtype12.2: metasomatized Type Example: Krivorozhsky-ZhelryeVody, Ukraine
References: AvrashoviBendix (ed.) 19801 Belevtsev er ai. 1984;Collot 1981;Fritsche1986;Fuchset al. 1981; Kazanzky and Laverov 1977:Mineeva 1984
Principal Recognition Criteria Host Environment -
-
Albitites and granites or metasediments with metasomatic albite-aegirine, albitearfuedsonite-aegirine or other Na-silicates Distribution of metasomatitesalong cataclastic zones in parent rocks Transition margins between metasomatized facies and parent rocks are commonly narrow Emplacement of mineralization along microand macro-fractures confined to albitized facies Parent rocks have often but not necessarilv anomalous uranium background values Intrusive granites are enriched in alkalis. Al2O3, U, Th, Nb. and REE Albitized metasedimentsare mostly found in middle to high rank, parti-'"dynamothermally, metamorphosed rocks adjacent to leucocratic granite intrusions in mobile belt terranes
Alteration -
Na-metasomatism reflected by enrichment of Na2O and depletion of SiO2 and K2O Argillitization (commonly rveak) Hematitization (locally intense) Locally carbonatization
Principal uranium minerals: uraninite + Th rich, uranothorianite. uranothorite, thorite, nenadkevichite, U-Ti oxide phases (brannerite), pitchblende and coffinite Associated minerals: ailanite. bastnaesite, monazite, xenotime. zircon, apatite. _salena, pyrite, magnetite, and/or calcite in accessorial amounts; locally abundant hematite Occurrence of U minerals as disseminated anhedral to euhedral srains and in thin veinlets.
Belevtsev et al. (1984) provide the following additional feat.ures for Proterozoic albitite uranium deposits, presumably referring to the ZheLtye Vody district, Ukraine: -
-
-
U mineralized albitites occur within sediments and volcanogenic rocks that have been metamorphosed to amphibolite ,srade facies or sranitized and which contain elevated U backgrounds U hosting albitite occurs along cataclastic and myionitic zones within metasomatic albitite U hosting albitite differs from ordinary U barren albitite by blastoclastic strucrures, better permeability and presenceof hematite. aegirine,alkaiine amphibole. chiorite, epidote, and carbonate U minerals occur as fine-grained impregnations associatedwith dark-colored minerals amons albite grains
Kazanzky and Laverov (1977) und Kazanzky et al. (1978) distinguish iron-uranium deposits associated with Na- and CO3--metasomatites (Krivoi Rog) and uranium deposits associated with Na-metasomatites.For both types, a distinct zoning and the association with deepseated muiti-activated fault svstems combined with
1aA
4 Ty'pology of Uranium Deposits
brecciation and mylonitization are said to be tvpical. Age Constraints No age constraints except restriction to periods of late or post-orogenicmagmatic activity.
Metallogenetic Aspects Metasomatite-bound uranium mineralizations appear to be the final result of a (Na-)metasomatic evolution during the waning stagesof an orogen]'. In the caseof subrype12.-1deposits, metasomatism transformed during an early stage previousiy crystallized leucocratic igneous rocks, essentiallv granite. into Na-rich facies culminating in albitite in structurally deformed ground perhaps in the apex part of a pluton. Cathelineau (1986) points out that (a) the fluids responsible for albitization and associated desilicification are in composition similar to those of meteoric origin, (b) metasomatic alteration takes place severaltens of m.y. after granite crystallization. and (c) there is no relation between late magmatic, pneumatolytic to deuteric processes which also can cause alkali enrichment and albitite-forming metasomatism. Subsequent to albitization, introduction of U associated with Th and REE occurred on the same reactivated structures along which the zlSifizing fluids permeated the host granite. This spatial relationship also suggests a metasomatic origin of the U, Th, REE mineraiization. Similar processes are assumed for the formation of subtype 12.2 deposits. But instead of autometasomatism as in granite. metasedimentswere altered presumabl-v by fluids derived from or mobilized bf intruding granites or bv late metamorphic events. Mineeva (1984) comments on the metallogenesisof uranium albitites associatedwith deep rooted-lineaments which can be attributed to subtype I2.2 and states that: -
-
display only primary lateral and vertical zoning; - post-albitite events superimposed a secondary zoning reflected by a hydrothermal mineral paragenesis of various phases which typicalll' are associated with economic deposits. Thel' include apatite, chlorite, carbonate. baryte, quartz. sulfides and hematite; - spatialextensionof U mineralizationin albitites correlates with extension of carbonatization. This reflects the importance of CO2. CO3 controls the pH values in the fluid system and influencescomplexingof U and other elements in soiution and their redeposition; - U redistribution proceeds downwards and affects ubiquitously the host albitites therebv achieving U enrichments. According to Belevtsev et al. (1984), uranium mineralization resulted from residual metamorphogenic fluids contained within cataclastic and mvlonitic zones of protoactivation. The mineralizing activity progressedthrough the final stage of metamorphism. Fluid inclusion studies indicate that invoived solutions contained carbonate. chloride. and sulfide with concentrations of 10 to 40"h. Temperatures ranged during the albitization stage from 350 to 150'C and during the mineralizing stage from 200 to I20"C at pressures of 600 to 900 bar. According to Batashov et al. (1984), the temperature of mineralization was 300 to 200'C. The source of the uranium for both subtypes remains debatabie. Both lateral secretion bv the metasomatic solutions leaching U from either uraniferous host granite or host (meta-) sediments or other rocks, or U concentrationsin the residual magma are speculated.Size and grade of a deposit may be a function of availabiiit-v of uranium, ground preparation and the duration of the metasomaticprocess.Pervasivemetasomatism mav causewider distribution associatedwith diiution of the uranium throughout a large volume of rock. In contrast. narrow zones of metasomatism mav have concentratedthe uranium to form higher grademineralization.
albitization and mineralization evolvedthrough several stages ranging from ultrametamorphism to post-albitite events; polyphase processes imprinted primary and Remarks secondary lateral and vertical zoning on host rocks reflected by mineralogy and geo- The processes involved in metasomatism and chemistry. In contrast, unmineralizedalbitites related mineral formation are to some extent are not zoned and REE mineralized albitites comparable with those leading to vein deposits.
Tvpe 12: Metasomatite
The distinction between rhe two may be difn c u l t . O n t h e o t h e r h a n d , m e t a s o m a t i cd e p o s i t s o i s u b t y p e1 2 . 1c o m m o n l ya r e w i t h i n a n o m a l o u s l v uraniferous granites which may be attributed to : n e i n t r u s i v et y p e o f d e p o s i t s . Both subtvpes generalll' contain resources of smail quantities (<1000mtUjO8) and lo*, _srade (<0.2% U:,Oa). Known exceptions are rhe deposits of Ross Adams. USA. Gunnar. Canada and Zhelty Vody (: Yellow Waters). Ukraine.
According to Kalyaev (1980), aibitizationis found in almost every rock tvpe. Mostlv it occurs in masmatic rocks but likewise in metamorphic and sedimentary rocks and in porphyry copper deposits. In most casesthese autometasomatic, metamorphic or diageneticalbitization processes do not causea complete alterationof the rock and do not lead to the formation of albitites. Sarcia (1980) gives a summarv concernins uranium in albitites and classifiesthese deposits AS:
\ ote Uraniferous Na-metasomatizedlithologies have drawn much attention in the former USSR for many decades but have been largely negiectedin rhe Western World until recently. The term albirite ,ieposi* in connection with uranium mineralization was used first by Russian authors. Thoroueh investigations of this type of uranium :eposits were carried out by many of them, e.g., Kazanzkv et al. (1968) Smirnov (1977), Kaiyaev (1980). Early work in Canadaby Robinson (1955) hints to a uranium paragenesis within albitic vein rocks in the Goldfields-region (Beaverlodge district). Saskatchewan. Christie (1953) and Dawson (1951, 1956) describe there veins consisting of microcrvstalline aggregatesof albite and quartz, stained with hematite associated with ;:lcite and chiorite. Fritsche (1986) has reviervedpertinent literarure and defined types of aibitites and related uraniferous albitites (see Chap. 2). Up to the present, albitites are defined as primarv magmatic rock types in most geologicai nomenclatures. although Goldschmidt (1911) already explained albitite veins as of pneunatolvtic contact metasomatic origin. Sorensen -971) defined aibitites as rocks of plutonic :laracter with mainlv albite. about 10 vol. 96 of nore calcic plagioclase,5 vol. % of quartz and *'ith olivine, augite. amphibole, and biotite as minor components. Sorensen (1971) considered the possibility of a secondarymetasomaticorigin for albitites in addition to a magmatic genesrs. Albitization. as defined by Schieferdecker(1959) universailv, is ''the production, in a rock of aibite, as a secondarvproduct". Other definitions restrict aibitization to the autometasomaticalteration of plagiociasesin magmaticrocks only (e.g., Hoimes 1928).
125
-
-
anorogenic deposits associated with deep faults. without direct relation to magmatic rocks. Example: Deposits of the Ukrainian Shield (Smirnov 1977)or Alio Ghelle, Somalia ( C a m e r o n1 9 7 0 ) ; deposits in orogenic zones associated with faults accompaniedby granitization and magPleutajokk/Sweden matism. Examples: (Adamek and Wilson 1977).Espinharas/Brasil (Ballhorn et al. 1981), KitongoiCameroon (Fritsche 1986); deposits bound to postorogenic fault systems within the basement and the metamorphosed cover. Example: Brousse-Broquids,Aveyron Rouerque, France (Yerle 1978).
Uranium occurrences associated with alkaline bodies especially in relation to autometasomaticaUv albitized or hydrothermallv altered zones or late magmatic fractures where later, postmasmatic hydrothermal processes have been in operation are describedby Mackevett (1963) and Thompson et al. ( 1980.1982)from the peralkaiine intrusion of Bokan Mountain/Alaska and by Sorensen (1970) from Ilimaussaq (Greeniand). Similar zones are known from the intrusive bodies of Pogosde Caldas/Braziland Seal Lakei Labrador (Group 1970)or from the CIS (Kozlova and Gurvich 1979),from China and the Mongolian Peoples Republic (Saima Deposit Research G r o u o i 9 7 6 : B o s u s l a v s k ve t a l . 1 9 6 6 ) .
1.72.1 Subtype 12.1: Metasomatizedgranite Type Example:Ross AdamslBokanMountain, USA Ret-erences: Collot 1981;Thompson et al. 1980, 1982
126
4 Typology of Uranium Deposits
Host Rocks Albitic aegirine granite, albitic arfuedsoniteaegirine granite, albitite derived by epigenetic Na-metasomatism along cataclastic zones in anomalously uraniferous granite. Transitional aureoles between metasomatizedfaciesand parent rocks are commonly narow. Spatial distribution of metasomatism appears to be restricted to structurally prepared zones in the apical part of a postorogenic or anorogenic pluton. Pluton-internal shear. fracture and breccia zones confined to albitized facies commonly provide the site for mineralization. Alteration Two principal types of alteration can be distinguished: - Na-metasomatism is characterized by albitization associated with desilicification and in the case of insufficient Al to form albite. b1' the formation of aegirine and/or arfuedsonite. Geochemical changes are enrichment of Na2O and depletion of K2O and SiO2 (replacement of K-feldspar, Ca-rich feldspar and quartz by albite). - Weak argillitization of feldspars including secondary albite, and locally intense hematititization. At Ross Adams, hematite imposes a pervasive red staining on microfractured albite and coats uraninite grains indicating a postalbite, post-uranium formation.
portions of the Bokan Mountain granite geochemically enriched. in Na3O, Al2O1, U, Th, Nb and REE. Age Constraints No age constraints except restriction to periods of late- or post-orogenicmasmatic activitv. DimensionslResources Individual deposits (in brackets Ross Adams deposit) mav be several meters to >100m (ca. 1 1 0 m ) I o n g . s e v e r a lm e t e r s t o m o r e t h a n 2 0 m (7.5-22m) wide. and in excessof 100m deep. containing few tens and up ro ca. 1000mt (850mt)U3Os at (average) grades raneing from <0.1 to >1% U-3Os occasionallv to 3o/oU:Os. Districts mav contain up to few 1000mtUrOr. Examples of Metasomatite, Subrype 12.1 Metasomatized Granite Deposi* I Occurrences Australia: ? Radium HilliTVillyama Biock. South Australia Canada: ? Gunnar/Saskatchewan Brazil: Lagoa Real/Bahia Cameroun: Kitongo
4.12.2. Subtype 12.2: Metasomatized metasediments
Ore and Associated Minerab
Type Example: Krivorozhskl,-Zheltye Vody district/KrivoyRog. Ukraine
uranothorianite, Th rich. Uraninite ! uranothorite. thorite. some brannerite. and coffinite. Allanite, bastnaesite, monazite. xenotime. galena,pvrite, magnetite,zircon, apatite.fluorite. and calcite in accessorial amounts. Locallv abundant hematite.
References:Batashov et al. 1984;Kazanskv and Laverov
Mode of Min.eralization U minerals occur as disseminated anhedral to euhedral grains and in thin veinlets, with or without accessorial pyrite, galena. fluorite. and galena within zones of closely spaced jointing and shearing and some brecciation in a metasomatized albite and aegirine granite. At Ross Adams. the mineralized rocks are related to
rg't1 Host Rocl<slSrucrures Archean amphiboiite and biotite gneissintruded by quartz diorite and granodiorite form basement to Lower Proterozoic metasediments in the Zheltye Vody district. The Prorerozoic suite includes three units, a lower schist-quartziteunit composed of micaceousquartzite. meta-arkose. uraniferous meta-conglomerate. schists and leptites; a middle taconire unit of amphibolemagnetite schist, biotite schist and Fe-rich (magnetite. hematire) quartzite; and an upper dolomite-leptite unit of dolomitic marble, quartzite. graphite-biotite and actinoiite schist,
Tvpe12: Metasomatite and metasomatictalc-carbonaterocks. Stocksand dikes of microcline granite have intruded along lineaments. Host rocks are folded into large isoclrnal folds with steep axes and have repeatedly been stronglv fractured.
127
Age Constrains Age of U mineralizationat Zheltye Vodv is about 1 7 7 0 m . 1 ' .b u t i t c a n b e a s s u m e dt h a t m i n e r a l ization can take place at anv time of magmatic or m e t a m o r p h i c( ? ) a c t i v i r y .
^llrcradon DimensionsrResources Intense metasomatismhas altered metasedimenrs and granites along lineaments.Four main phases of alteration are distinguished: Early Fe-Mgmetasomatism that generated stratiform iron ,.tre bodies, Na-metasomatism.carbonate-mera-,)i.i1atism.and finally silicification that formed )-;cndary quartzites. Products of Na- and carf,onate-metasomatismare listed beiow. Ore and Associated Minerals U minerals inciude uraninite, pitchblende, coffinite, brannerite and nenadkevite. Four principal U-associated mineral assemblagesare discerned: (1) apatite-malacon(uraniferous aparite, zircon/malacon, sphene); (2) amphibolenenadkevite (nenadkevite, brannerite); (3) carbonate-uraninite (uraninite I) and amphiboleaegirine (regenerateduraninitei); and (4) sulfidepitchbiende (pitchblende, coffinite, uraninite II) (Zhukova 1980). \Iode of Mineralization !:onomic U mineralization is restricted to Na.ind carbonate-metasomatitezones in the middle and upper Proterozoic units. Mineralization occurs in fractured sections at flexures in limbs and in the axial zone of folds in form of lenses. shoots. stratiform bodiesin which U minerals are finelv disseminated and associated with darkcolored minerals. and as veins. Batashov et al. i98-l) distinguish four lithologic mineral settings ,sed on parentalheritage. ,:t albitites with alkaline amphibole. aegirine and uraninite, brannerite, and pitchbiende derived from biotite. graphite-biotite schists, and granite dikes. b) magnetite-riebeckite rocks, aegirinites. martite-carbonate-metasomatiteswith uraninite derived from iron ores. amphibole-magnetite schistsand ferruginousquartzites. c ) talc-carbonate rocks containing uraniferous apatite-malaconwith Th and REE. d ) p i t c h b l e n d e - c o fnf ii t e v e i n s .
U mineralization at Zheltorechenskoe;Zheltye Vodv extends laterallv along a laree. steeply dipping fold for i500m on rhe W limb. 900m on the E limb and 700m in the keel of the fold. It persists to a depth of ca. 2000m. U hosting metasomatites are tens of meters wide. They decreaseat depth below about i000 m associated rvith disappearanceof uraninite which gives way t o u r a n i f e r o u sm a l a c o n - a D a t i t e . Examples of Metasomatite, Subrype 12.2 tV e taso mati z ed M etasediments D ep os itsI Occurrences Canada: Mosquito GulchA.{onacho, NWT. parts of Pronto/Blind fuver-Elliot Lake, Ontario Brazil: EspinharasiParaiba Finland: Kouvervaara/Kuusamo Schist Belt
4.13 Type 13: Synmetamorphic (Fig.a.13) Definition consists uraniummineraiization Svnmetamorphic of disseminated uranium distributed strataconcordantin more or lesscontinuouslensesor erratically scatteredpatcheswithin metasediPrincipaluranium mentsand metavolcaniclastics. phases are uraninite typical for higher grade prevailingin metamorphicfaciesand pitchblende low-grade(greenschist) facies. Type Exampie: Forstau,Austria Ret'erence: Dahlkampand Scivetti1981
Principal Recognition Criteria Host Environment -
Metasediments: phyllites, schists, gneisses panly carbonaceous (graphitic), phosphatic or
t28
4 Typology of Uranium Deposits
T)tpe
13. SYNMETAMORPHIC
(abotiform U disseminotiong in unoltered metosedimsnts ond mctovolconiclostics)
E
basement comprex
@
metasediments metavolcanics
.E
l-l
) I
U m i n e r a l i z a t t ol n l e n s el o r m
[]
scattered U mineralization (uraninite)
t I
!
o
I
t
1OO-1O0Om +
.c
'RRD
Fig. 4.13
-
pvroxenitic. derived from pelites, psammites and associated sediments with or without tuffaceous interbeds of eugeosynclinal or lacustrine origin, metamorphosed to from greenschistto amphibolite grade Metavolcaniclastics; metamorphosed felsic to intermediate, dominantly rhyolitic volcaniclastic horizons which mav be interbedded with clastic sediments
Alteration -
No or only very minor alteration related to mineralization No or only minor metasomatism of host rocks
Mineralization -
-
-
-
Principal uranium minerals: uraninite and/or pitchblende Some U in lattice or on cleavage pianes of various rock constituents such as apatite, biotite, amphibole Associatedminerals: localiy some Mo, Ni, Pb, Zn, a.o. as sulfides. sulfarsenides, etc.: mptavolcaniclasticsoften with Mo enrichments U minerals occur disseminated in lenses or in minute to.small pockets scattered in favorable Iithologies Only lithologic, no structural control
Age Constraints No age constraints other than to orogenies of Lower Proterozoic to recent age.
MetallogeneticAspects Synmetamorphic uraniumoccurrences appearto resultfrom regionalmetamorphismof uraniferous sedimentsor volcanicswithin a closed system, remote from intrusions,anatectic centers, or metasomaticfronts. Dependingon crystallization conditions, the sedimentary uranium became transformedil situ either to pitchblende,which is prevalentin lower grade metamorphicenvironments or uraninite prevailing in higher grade metamorphiclithologies.
Dimensions/R€sources Synmetamorphicuranium occurrencesare most commonlyof low grade(0.001-0J% U3Os)and contain small resourcesof subeconomicmagnitudes(kg to 1000mtU3O8). The most common occurrencesare spottv mineralizations. These mineralizationsare discontinuously spreadover manv km: in favorable horizons.Thev locally achievehigh grade. The mineralizationsare small in dimensionsranging from mm to m in lateral and venical extension and exhibit erratic distribution.
Remarks Synmetamorphic mineralizations make favorable uranium sourcerocks or protore for other types of deposits rather than feasible exploration tarqets.
T,vpel4: Lignite
Examplesof Type 13 Synmetamorphic DepositsiOccurrences
t29
Principal Recognition Criteria Host Environment
.A.ustralia:? iv{ary Kathleen/Queensland Canada: Duddridge Lake/Saskatchewan,Amer Like NWT. KittsiMakkovik. Labrador, Mont L a u r i e r iQ u e b e c Finland: Nuottijdrvi/Paltamo. Lampinsaan/Vihanti (Proterozoic metamo{phosedphosphorite) Germany: Hohenstein/Bavaria !ladagascar: AmbindrakembarFort Dauphin Toso : Lama Kara/l',iiamtougou U S A : K e t t l e F a l l s G n e i s sD o m e / O r i e n t , W a s h i n c t o ns t a t e .
-
-
.1.14.Type 14: Lignite (Fig.4.14)
Lignitic sedimentsof paludal, limnic or paralic ori grn Generally thin lignitic beds Coalification less than bituminous _qrade Pyrite and high ash content Intraformational unconformities Certain degree of jointing and fracturing Lignitic seamsinterbeddedor overlain by acid pyroclastics (rhyolitic tuffs, etc.) containing anomalous contents of U Basins surrounded by granites or other rocks with anomalous U contents
Mineralizarion -
No discrete primary U minerals (locally secondary hexavalent U minerals) - U adsorbed on carbonaceous matter or as consists uraniummineralization of Lisnire-related uranyi-humate paiudal material composedof a high percentage - Syngenetic U stratiform and persistent but of iand plant debris mixed with detrituscoalified commonly of low values (few 10ppm) to not higher than subbituminousgradeand con- - Spotty and irregular epigenetic U mineraltaining either trace to minor amountsof synization of better grade (up to 0.4"/" U:Oa) uranium or sedimentary,uniformly disseminated controlled by joints, fracture systems epigenetic, irregularly distributed structurally - U often bound to upper section of the lignite controlleduranium, the uranium beingadsorbed below intraformational unconformities in both caseson carbonaceousmatter. Accord- - Host beds of mixed vegetal carbonaceous and inslv two subtvpesare distineuished: minerai detritus (silt. clay) yielding high ash content - Thin lignitic host beds (<2m) Subtype14.1:joint-fracture-related - Often associated with a varietv of metallic trace elements Subt-vpeI 4. 2 : stratiform
Definition
Tvpe Example: sourhwesternWilliston Basin,N and S Dakota.USA
Age Constaints -
Reference:Denson and Gill 1965
TyPe
Time constraint by emergence of land plants, i.e.. younger than Devonian
14. L I G N I T E
Subtlpc
14.1
1+.2 joint/frocturc contollcd
sbotiform
o
r{' I rl1
Fig.4.l4
.G
grv. km
@t
uroniferour lignitc
Eg
gondctonc
[S]
borrcn lignita
ffl
mudotonc,/shotc
E
vR&D
volconiclosticr (U-bcoring)
130
4 Typolog5rof Uranium Deposits
- Ligaites, particularly those of Late Cretaceous and younger origin are more favorable; older lignites may have suffered a higher degreeof coalification, an app:uently negative process hampering epigeneticU mineralization
MetallogeneticAspects Ligryte, coal, and carbonaceous mud-siltstone are favorable sitesfor U accumulationwhen they originated from paludal, low-lying pooriy drained shallow depressionswith abundant vegetation located either on coastalplains (paralic lignite) or within landlocked basins (limnic lignite). U supposedlyoriginated from either granites, volcanicsor other rocks that border the depressions or from interbedded or overlying pyroclasticscontaininganomalousamountsof U. U is thoughtto havebeenleachedfrom thesesources. U transportinto the basinshas been(a) eitherby surficial waters with precipitation by synsedimentaryadsorptionon organicmaterial(astoday found in swampsand bogs in adequateenvironment, e.g., StevensCounty, Washington)to form syngenetic mineralizations, or (b) postsedimentary by groundwater migration into buried basinsto form epigeneticmineralization. Infiltration in the latter caseoccurred along permeable zones, dominantly joint and fracture systems, hence the irregular U distribution. Precipitation of U resultedfrom adsorption on organicmaterial along dishomogeneities, mainly joints, fractures, more silty intercalations and intraformational unconformities.
Remarks Uraniferous lignites constitute only potential U resources.Deposits are commonly either too small or of too low grade. The latter is particularly true for stratiform syngeneticdeposits(type 10.1) whereas t)?e 10.2 deposits may locally containnoteworthyU tenors. In addition, uranium extraction is hampered by its bindirig in organic compounds crearing an almost refractory nature for metallurgical extraction.
Examplesof Type 14 Lignite Deposits (Not Differentiated) CSf'R: Sokolovand Trutnov basins/Bohemia Greece:SerresBasin, Kotili Basin Germany:Stockheim/Bavaria,Freital/Saxony South Africa: Springbok Flats-Karoo Subbasin/ Transvaal
4.14.1 Subtype 14.I : Joint-fracture-related Type Example: Slim Buttes, southwestern Williston Basin, S-Dakota,USA Reference:Denson and Gill 1965
H ost Ro cksIAheraion IStructures Pyritic, high-ashlignite, subbituminouscoal and nonmarine carbonaceous mud-siltstone. The latter containsabundant plant debris, scattered throughout the rock but preferentially concentrated alongbeddingplanes.Sulfides,dominantly pynte, are typical, as is a high ash content. The carbonaceous sedimentsare often interbeddedor overlainby uraniferousacidpyroclasticsediments (rhyolitic tuffs etc.) or sandstones.Jointing and fracturing is common. No alteration. Someof the mineralized lignites are in basins bordered by granites or other rocks containing anomalous amountsof U. Ore and AssociatedElementslModeof MineralizationI D imensioruI Resources No discreteprimary U minerals have formed. Locally secondaryhexavalentU minerals occur in joints and fractures. U is adsorbed on carbonaceous matter or boundin organiccompounds (e.g., uranyl humate) particularly in cataclastic zones. Other metals present in anomalous amountsbut not necessarilycorrelativewith U mayincludeAs, Be, Cd. Co, Cu, Ge, Mo, Ni, Pb, Se, Ti, V, Y, Zr andlor REE. Mineralization is spotty and irregularly distributed with strong variationsof tenor. Gradesare commonlya few hundredppmU and rarely 0.i% U3O8or more. Thinner seams (crn to less than 1m) may be mineralized ubiquitously, whereas in thicker seamsonly parts thereof are mineralized,often restrictedto their top. Intraformationunconformities often control U accumulations.Resources vary between(1mtU3Os to few 100mtU:Or.
Type 15: Black Shale
1.14.2 Subtype 14.2: Stratiform
131
black shale Subtypel5.l: bituminous/sapropelic Type Example:Chattanooga.USA
Type Example: Slim Buttes, southwestern WillistonBasin,S-Dakota,USA Subtype15.2:humic/kolm in alum shale Denson and Gill 1965 .Leference:
Type Example:Ranstad.Sweden References: Bell 1978;Carlssonand Nojd 1977;Kim 1988; Mutschler et al. 19'76
Host RockslModeof Mineralization
setringare moreor less Host rocksand geological identicalwith thoseof subty-pe14.1exceptthat or sand- PrincipalRecognitionCriteria interbeddeduraniferousvolcaniciastics stonesmav not be present.Uraniumis adsorbed -.n carbonaceousand clavey material and/or Host Environment round in organicionic compounds,e.g., uranyli-iumate, and occurs stratiform disseminated - Very fine-grained black shales with high contents of organic matter, pyrite and/or horizon. throughout the carbonaceousilignite marcasite, evenly laminated, dense. may Associatedelementsare similar to thoseof subcontainthin coalified.phosphaticand/or silty type 14.1.Uranium tenor is generallyvery low, intercalations amountingrarelyto more than 50ppmU. - Interbedded with limestone,sand/siltstone and shalebeds - Fairly uniform shale thickness.commonly a few metersand up to some10m -1.15. Type 15: Black Shale (Fig.a.1s) - Widespread extension of black shale over several100'sto 10000'skm2 Definition No ore-relatedalteration Black shaie-related uranium mineralization Mineralization consistsof marine organic-richshale or humic/ coal-richpyritic shale,containingsynsedimentary - No discreteprimary U minerals(exceptlocally stratiform, uniformly disseminateduranium adsecondaryhexavalentU minerals),insteadU is :orbed on organic material and clay minerals. adsorbedon organicand clay particles Very low grademineralizationrangingfrom 50 Basedon the organicsubstances two subtypesare distinguished: to 400ppmU
15. I L A C K
Type
15.1 bituminous,/sopropclic
Subtypc
SHALE 15.2 kolm,/olum sholc
E C.t
o ()
mony k m
.c ffi| =l
uronifcroussopropclic
Fr.t!l L'.I,I
block sholc siltstonc
limcstonc ffil @
Fig.4.15
F=
cloystonor/shotc
urqnifcroua kolm
F
---{
bituminous limcstonc olum sholc
vRRn
732
4 Typologyof UraniumDeposits
- U disseminated uniformly throughout and coextensivewith individual beds over large areas, with different beds commonly having different U tenors - Better gradesin certain beds (dm to m thick) either rich in organics,particularlyof vegetal provenance, and/or sediments very finegrained, or finely laminated or containing phosphaticminerals(e.g., kolm: 380ppmU) - Plesenceof small quantities of other metais (Cu, Cr, Mo, Mn, REE, V and P)
4.15.1 Subtype 15.l : Bituminous-sapropelic black shale Type Example: Chattanooga,USA References:Mutschler et al. 1976'.MSR & D 1978
H ost Rocks/A berationIStructures
The GassawayMember, the upper unit of the Devonian-Mississippian ChattanoogaShaleis the main host for uranium. It consistsof massive. grayish-blackshale with paper-thin partings of Age Corutraints silt and fllms or bands of pyrite and marcasite. No age constraints but deposits occur domi- About 20wt. "to of the shaleis organic material nantly in Phanerozoicformations particularlyof (saprolite, vitrain, bituminous coal) derived Paleozoicage. mainly from planktonic marine algae and less from land plant debris.The organicsfill interstices and coat the minute rock grainsof the shale.No MetallogeneticAspects structuresor alterationare Dresent. Black shales of marine origin were deposited in shallow, partially closedepicontinentalbasins within continentai terrane tectonicallystablefor a long period. The depositional environment is charzcterued by a low rate of sedimentation, brackishto normal marine salinities,anaerobic, strongly reducing conditions, formation of sapropelic-bituminous, and humic, coaly matter from planktonic marine algae and land plant (wood spores)debris. Synsedimentarydeposition of U from seawater by adsorption dominantlv on organic matter with particularconcentrations in humic-coalymaterial. If phosphatenodulesare present, they normally collected more uranium than the surroundins shale.
Ore and AssociatedElementslModeof M ineralizationID imensionsI Resources U distribution is stratiform, adsorbedmainly on organic and to a lesserextent on clay particles. MSR & D (i978) report from a test area in Dekalb County, Tennessee: 55-65 ppm U, 230ppmCo, 530ppmNi, 200ppmMo, 1360ppm VzOs, 8600ppmSand 9 gallsh.t.of syncrudeoil. Lateral extensionof ChattanoogaShale is in the order of 80000km2, averagsngabout 10m thick and a mean grade of 57ppmU. Estimated resourcesare at least 4 to 5 million tonnes U (calculatedfor a restrictedareaof 12 countiesin centralTennesseebv Mutschleret al. (1976\.
Remarks Black shales,althoughwidespreadin the world 4.15.2 Subtype15.2: Humic/kolm in alum and hosting enormous quantities of uranium shale (many million tonnes) constitute only potential U resources. U is perhaps extractable as a Type Example: Ranstad,Sweden by-pioductof oil, or other metalsrecovery. Reference:Carlssonand Noid 1977
Examplesof Type 15 Black ShaleDeposits(not differentiated) China:JiangnanBlock/SEChina SouthKorea: OgcheonGroup Russia:Lake Oneearesion
H ost RocksIStrucruresIA lteration PrincipalU-host is the kolm horizonwithin black bituminousalum shale of Cambrian age. The flat-lyingblackshaleunit restson Cambriansandstoneand is coveredby OrdovicianLimestone.
Tvpel6:Strata-Controlled.Structure-Bounci 133
of Ore and AssociatedElementslMode .VineralizationI D imensionsI Reso urces -
U distributionis stratiform,adsorbedon organic matter and in particularon kolm nodules.The .:olm contains about 30% ash. Nodules may ;ontain up to 80% organicsand up to 0.1"/"U. Average valuesfor the kolm horizon are: 300 ro 380ppmU (alum shaleca. 200ppmU),22% organicmaterial,13%pyrite,6.5%S, 0.065%V, 0 . 0 3 %M o , 0 . 0 2 %N i , 0 . 4 %M g , 0 . 7 %P , a n dh i g h Al and K contents. Lateral extensionof the kolm-bearingalum :hale is about 500km:. Thicknessof the alum ;laieis 15m and the kolm horizonvariesfrom 2.5 .s-,-1m. Assuming an averagegrade of 200 to -:00ppm U3Os the Ranstad occurrenceis estimated to contain 300000mt U3Os, including about 350ppmU3Os. 75000mtU3Osaveraging
-
-
dolomite (Ockerkalk) overlie the main black shale horizon Pre-orogenic diabase dikes and sills cut the sedimentarysequence Marked tectonic imprint reflected bv complex folding. pronounced faulting and thrustinq, and high densitv of minifractures t-fissures, joints. cleavages) rveaklv not or onlv Sediments are metamorphosed
Alterarion -
-
J.16 Type16:Strata-Controlled, Structure-Bound
S i l i c i f i c a t i o nc. h l o n t i z a t i o n .d o l o m i t i z a t i o n Narrow aureoles of bieachine and ;rematitization (few centimeterswide) around mineralized structures Carbonatization and suifatization in iorm of narrow fissure fillings Post-orogenic(post-Hercynian at Ronneburg) supergene oxygenation (reddenine of rocks due to Fe-oxide. Fe-hydroxide tormation) associated with partial mobilization of trace elements in subsurface (below Permian paleosurface) and funher downwards along major faults Formation of cementation zone beiow the oxidation zone
Type Example: Ronneburg District, Thuringia. Germany Lange, Schuster,weise. Gradowskipers. L.jT:X:., Mineralization Definition Strata-controlled. structure-bound uranium deposits consist of uranium concentrations in minifractures which form stockworks within or immediatelv adjacent to carbonaceous. pyntic black shales.
-
:lincipal Recognition Criteria Host environment -
-
-
Argillaceous and siliceous black shales with intercalations of dolomitic and phosphonte nodule beds (Graptolitenschiefer) High organic carbon (up to 9"/" C), sulfide (up to 3.5% S) and anomalous trace element (U, Mo, Ni, V. As. Sb) contentsin the black shale Carbonaceous sandy and carbonatic shales (Lederschiei'er) underlie. and limestone-
Older pitchblende associated with Mg-Fecarbonates. chlorite. subordinate sulfides, rare coffinite, tennantite Younger pitchblende, bravoite. marcasite,NiCo-arsenides,hematite. calcite. dolomrte (Sooty) pitchblende associatedwith p1'rite (no gangue minerals) U minerals predominantly fill. coat and impregnate minifractures (joints. cieavages) which are arranged in a stockwork-like pattern within distinct lithologic units Porous wall rocks may contain disseminated mineralization for shon distances trom mineralized fractures Distribution and concentration of U in the fractures is highly variable Mineraiized fractures group to stockrvork-tvpe ore bodies, which occur irregularly distnbuted within weakly mineralized zones Position, size and shape of ore bodies are controlled by faults, particularlv by intersections of faults
l34
-
4 Typology of Uranium Deposits
U mineralization occurs below the lower boundary of the oxidation zone from where mineralization can be found over a vertical interval of about 400m
ward along permeablestructuresinto reduced sediments,and - precipitationof uraniumin form of pitchblende mainly in subsidiaryminifractureswithin the cementationzone.
Age corutrains
After this ore forming event,supergeneprocesses (at - No principal constraints with respectto the age may have remobilized the uranium again Ronneburg at about 210-190m.y. and 120of the host strata but known deposits occur in 90m.y. which are consistentwith redistribution Eocambrian and Paleozoic sediments - Geological evidence suggeststhat the principal or introduction episodesof uranium in other depositsin central and westernEurope). ore formation coincided *'ith periods of marked oxidizing ciimatic conditions (Rotliegend/ Permian at Ronneburs. Cretaceousto Tertiarv in South China)
Typical Dimensions/Resources
At Ronneburg, mineralized zones are up to severalhundred meterslong and wide and up to Metallogenetic Aspects severalten meters,rarely in excessof 100m thick. They encloseirregularly shapedore bodies that structure-bound uranium Strata-controlied. vary from 20 to 500m long, from few meters to deposits tend to be of polyphase origin involving few hundred meterswide and from iessthan 2 m the following stages: to 100m thick. In situ U contents range from are in the order 1. Synsedimentary accumulation of uranium and 0.085to 0.17%U:Os. Resources contained in an area of of 200000mtU3O6 other metals in favorabie sediments such as 164 km2. black shales (at Ronneburg Middle Devonian Tentaculitenschiefer and Silurian Upper and Lower Graptolitenschiefer). 2. Perhaps but not necessarily limited redistri- Examplesof Type 16 Strata-Controlled, bution of uranium by metamorphic processes Structure-BoundDeposits/Occurrences (at Ronneburg during the Hercynian Orogeny reflected by elevated U concentrations of ca. China: Chienxinan/SWGuizhou; ChanzipingA,l 100ppmU in certain anticlinal structures as Guangxi; PukuitanglHunan,Chengxian/Guangxi Hunan-Guangdong region, SE China; Norgai/S compared to a synsedimentary inventory of Qinling area, Central China about 50 to 60 ppmU). These uraniferous (meta-) sediments provided the primary U source. At Ronneburg, a supplement U source is present in form of pitchblende veinlets of supposedll' late to post-orogenic hvdrothermal origin. 3. Ore grade uranium concentration by a combination of - structural ground preparation. - deep reaching weathering in a semiarid to arid climate adsociated with liberation of uranium by infiltration of oxygenated meteoric waters into the U source rocks, - fluid mobilization supposedly in response to tectonic activit)' and/or intrusion of igneous dikes (lamprophyres, porphyries) resulting in circulation of the U pregnant solutions down-
Uzbekistan:Koscheka,Dzhantuar/Kyzvlkumsky, Navoi region
Selected References and Further Reading for Chapter 4 (for detailsof publicationsee Bibliography) Adams et al. 1989; Adams and Saucier1981; Adams and Smith 1981;Alexander 1986;Altschuleret al. 1958, 1980a: Andrade 1989; Aniel 1983; Antropov et al. 1978; Austin and D'Andrea 1978; Avrashov 1980;Bailey and Childers 1977;Bailey et al. 1981; Barthel et al. 1986; Barthel and Hahn 1986; Basham and Matos Dias 1986; Battey et al. 1984; Beck 1986; Belevtsev 1980;Belevtsev et al. 1984a. 1984b;Bell 1963,1978,1982;Berning 1986;Boitrov and lrgierski 1917;Bowie1979;Boyle 1984,1985,1986,1989; Breit and Goldhaber 1987;Briot 1983;Butt et al. 19841 Button and Adams 1981; Bvers 1978; Cameron 19841
Type16: Strata-Controlled.Structure-Bound 115 Canou 1964;Carlisle1983,1984;Carlssonand Nojd 1977; Cathelineau198a.1986;Cavaney1984;Chen Zhaoboand Fang Xibeng 1985; Chenowethand Malan 1973;Collot 1 9 8 1 ;C r a w l e y 1 9 8 3 ; C r a w l e ye t a l . 1 9 8 4 1C r e w 1 9 8 1 ; Cumming and Rimsaite 1977; Dahlkamp i980, 1989r Dahlkamp and Adams 1981; Danchev and Strelyanov iar8; Dayvault et al. 1985;de Kun 1965;Densonand Gill 1y65; Dernck 1977; Derricks and Vaes 1956; de Voto 19781de Voto and Stevens1979;Evans(ed.) 1986;Ewers and Ferguson 1980: Fergusonand Golebv (eds.) 1980; Finch 1967; Fischer 1968; Fogwill 1985: Forman and Fraser1980:Friedrich Angeiras1981;Fouqueset al. 19861 and Cuney 1989t Friedrich et al. 1987; Fritsche 1986; Fuchs and Hilger 1989;Fuchset al. 1981;Gandhi 1986; Garrels and Larsen 1959;Gangloif 1970: George-Aniel et al. 1985;Goodell 1985a,1985b;Goodell and Waters. ':ds., 1981; Cosnold 1976;Granger 1976: Granger and :'rch 19881Griffith 19861Grun 19721GSA;Wenrichand 3:ilingsley (eds) 19861Halladay 1989; Hallbauer 1986: Hambieton-Jones1984; Harshman and Adams 1981: Heine 1986; Hostetler and Garrels 1962; IAEA 1979. 1980.1981,1982, 1983,1984,1985,1986a.1986b,1988a. 1988b,1988c.1989a.1989b;IAEA/Ferguson (ed.) 1984; IAEA/Finch (ed.) 1985;IAEA/Fuchs (ed.) 1986rIAEA/ 1978; Pretorius(ed.) 1987;IAEA/Toens (ed.) 198a1Jooes Jiashuand Zehong 1984;Jiashuet al. 1989;Johnsonet al.
1987;Kazanskvet al. 1978;Kazansk,vand Laverov 1977; Kazdan. 1978; Kolektiv (CSSR) 198-1:Komenik and Vesel.'i1986;Kotlyar 1961;K.vseret al. 1989;Landaiset al. 1987: Leroy 1978; Leroy and Cathelineau1981: Leroy et al. 19871Li Tiangangand Huang Zhizhang1986:Mann and Deutscher1978:\lathews et al. 1979;l\IcKelveyet al. 1956; Mellinger et al. i987; Mickle and Mathe*'s 1978; Mineeva 1984:Modnikov et al. 1978:VSR and D 1978: Mursk-"*1973;Mutschieret al. 1976:)'Iakoman1979;Nash et al. 1981;Needham1985:Nevskiyand Filonenko 1976: Orasaka1981;Otton and Zielinski Nishimori et al. 1977:1985;Page 198.3:Pagel 1984;Pardo-Leyton1985:Petro5 et al. 1986;Petrov et al. 1969;Piirainen1968:Pory et al. 1986:Poty and Pagel1987:Pretorius1976.1981:Rackley 1976.1980:Reeve et al. 1989:Rich et aI. 19r-:: Roberts 1988:Robertsand Hudson 198-l:Robenson 1989:Rundkwist and Nesheskij1977:Ruzicka1971.1981.i9E-1.1988, 19891SaimaDeposit ResearchGroup 1976:Sakar 1982; Schmidt-Collerus1979: Sherborneet al. 1919: Shiqing Yu 1989;Shtshurovand Timot-eiev1965; Sibbaid 1988; Sibbaid and Petruk (eds) 19851Skinner 1981: Thamm et al. 1981;Tremblav 1978.1982;Turner-Petersonet al. (eds)1986;US-AEC 1959:Virag and Vincze1967:Volfson et al. 1967;von Backstrom 1975, I9i6: Wallace 1986; Wennch 1986: Wennch and Sutphin i989: Yu 1986: Zieeler 1974:Zielinski et al. 1987.
5 SelectedExamplesof EconomicallySignificantTypes
of Uranium Deposits
This chapter contains abbreviateddescriptionsof selected major uranium deposits or districts including metallogenetic concepts proposed by geoscientistswho have worked on these deposits. T h e g i v e n d e p o s i t s a n d d i s t r i c t sa r e c o n s i d e r e d i'epresentativeexamplesfor the types of uranlum -rtposits of established or potential future economic value. To provide an idea of the magnitude of the deposit or district describedin the respective section, some figures on the resources and grades have been included in the introduction to this section. Related details on dimensions are provided under section lvlineraloey in the deposit descriptions. For additional comments see Paragraph Remarks, Definitions. Units.
5.1 Examples of Unconformity-ContactType Uranium Deposits(Type I, Chap.4): Athabasca Basin region, Canada fhe Athabasca Basin region is south of Lake .l.thabasca in northern Saskatchewan. Three major districts characterizedby large and highgrade uranium depositsmay be distinguished.the East Athabasca. Southeast Athabasca. and Carswell Structure districts. These districts are located in the eastern. southeastern.and central western part of the AthabascaBasin respectively. Small deposits occur at the northern edge of the .'asin in the North Rim and Northwest Rim \ t h a b a s c a d i s t r i c t s( F i g . 5 . 1 ) . Total resourcesof the Athabasca Basin region are in e.\cess ol l40000mtU.rOn. Average ore grades of deposits vary between 0.1.1and 14.1"/" U:Os, but most major deposits have grades of more than l% UrOa. The district includes with Key Lake (89 000 mt U3Os, averagegrade ca. 37" UrO*) the largest high-grade. low cost uranium nine (open pit) in the world. and with Cigar Lake { 128000 mt UrOs, average -erade 14.4% U:Oe) the largest high-grade deposit discoveredin the world.
The uranium deoosits of the Athabasca Basin region are associated with a distinct lv{iddle Proterozoic unconformity and are therefore defined as Proterozoic unconlormitv-contact deposits. Two classesof this tvpe have been dist i n g u i s h e dr v h i c ha r e f r a c t u r e - b o u n d( c l a s s1 . 1 . 1 , Chap. .1) and clar'-bound(class 1.1.2). In add i t i o n . B e a v e r l o d g et v p e m i n e r a l i z a t i o no f H u d sonian age (subtrpe l.l. Chap. l. and Chap. 5.2.3) has been identified. Both unconformitycontact classesexhibit manv similarities, but also some distinct ditferences. The following description of the regionai geology is largelv based on the comprehensive work by Lewrv. Sibbald and Ramaekers.Ruzicka ( 1 9 8 9 ) .S i b b a l d( 1 9 8 8 ) ,A r t r u e t a l . ( 1 9 8 6 ) ,L a i n 6 (1986). Fogwill (1985) Tremblav (1982), Dahikamp and Adams (1981). Hoeve et al. (1980), Pagel et al. (1980) and Voultsidis et al. (1980) provided concise reviews with conceptual genetic models. Evans (1986) and Lain6 (1985) edited major volumes on the uranium depositsof the Athabasca Region. Information from these publications together with that from a great number of other authors who contributed extensive information on a regional or local (deposit) scaie have been taken for the presentation of mineralization and alteration. All these authors cannot be listed here but are mentioned in the texr and found in the referencelist at the end of this chapter.
GeologicalSetting of Mineralization The Athabasca Basin region lies near the southwestern edge of the Churchill structural province of the Canadian Precambrian Shield. Archean and Aphebian (lvfiddle to Upper Proterozoic) crvstalline rocks constitute a basement which is overlain by dominantly continental sediments of the Meso-Helikian (Middle Proterozoic) AthabascaGroup. The Archean and Aphebian rocks are arranged in distinct lithostructural domains. each charactererized by structural styie and metamorphic
r38
5 Selected Examples of Economically Significant Types of Uranium Deposits I100
1080
1020
I060
600
(
*ro1cD ,o
4
Lo Ronge
c Fig.5.l. AthabascaBasinregion, DorrhernSaskatchewan. generalized map of Ithostrucnrral subdivision of the Canadian Precambrian Shield and Dosition of Uranium districts. (After Lewrv et al. 1978)
'l50km (/o
Uramium districts: E.A. East Athabascadistrict; S.E.A.SoutheaslAthabascad.; C.S. CarswellStructured.: N.W.R.A,.Northwest Rim Athabascad.; N,R.A.North Rim Athabascad,; U.C.Uranium City region; Lithostructuraldomains: WC Western Cration; FD FirebagDomain; CD Clearwaterd.; WGD WesrernGranulited.; VRDVirgin River d.; MD Mudjatic d.; WD Wollastond.; PLD Peter Laked.; BCRottenstoneComplex; SFCsoutheasternCoirplex; L.A.Lake Athabasca; l4l.L.wollaston Lake; c.L. cree Lake
grade (Fig.5.1). Archean rocks are dominantly of granitic and minor morzonitic composition and form NE-SW elongated domes surrounded by Aphebian metasediments. The Aphebian metasediments include a variety of partly graphitic biotite to feldsphathic gneisses and schisrs, quartzites, calc-silicaterocks and impure marbles derived from a successionof pelitic to psammitic facies interbedded with carbonaceous layers (black shale) and calcareous to evaporitic unirs
deposited in a miogeosynclinal shelf to lagoonal sedimentary environment in middle to upper Lower Proterozoic time. The metamorohic srade is predominantly middle to upper amphiSolite and locally granulite facies with larer retrogression to greenschistfacies. Migmatites and anatecticsegregationsare common features. Late granitic and pegmatitic intrusions occur locally. Table 5.1 gives a summary of the Archean and Aphebian Lithologies.
Examplesof Unconformitv-Contacr-Tspe Uranium Deposits
t39
A-thabasca Table 5.1. Northern Saskatchewan. Basin region.generalizedArchean-.A.chebian stratisraphv-lithology of rhe Cree Lake zone. ICompiledby E . S r e w a r(rp e r s .c o m m u n . )a f t e rL e w r y a n d S i b b a i c1 9 7 8 .1 9 8 0 R 1 e i ' 1 9 z z lR a y a n d \\'anless1980] Lithology
E,tn
Pegmatite:manvgenerations. threetvpes:quanz-feldsparr.\p, ) biotite-quartz-ieldspar (Ap.) and rvithcalc-silicare minerals(Ap.1 Granite: leucocraticnonfoliated(Aq1)or foliated(.\q,,1form smallintrusicrns lessthan l km tn srze Maficintrusiverock: fine-grained to medium-grained. seneiallvfoliated.Composirion variesfrom diabase(Adr) or gabbro(.\
i.:
(Quartzite Metamorphosedsandstone,arkose.congiomerate.r'olcanicsand volcanoctastics and Lower Arkosic Units). Mafic gneiss:medium-grained,moderatelyfoliated. Composedof hornblende-biotiteplagioclasegneisswith hvpersthene(Am) Felsicgneiss:medium-grained,pink. moderatelyto stronglr-tb[ated. Svenosranitemonzogranitecomposrtion(Af). Lineation due ro rodding of quartz often well develooed
Regional predominantlv NE-SW-trendingpreAthabasca fault and mylonite zones transect the Archean and Aphebian rocks. North of Lake Athabasca.clasticsedimentsof the Paleo-Heiikian Martin Formation deposited o n t h e b a s e m e n t( s e eC h a p .5 . 2 . 2 ) .S o u t ho f L a k e . { t h a b a s c a . a t h i c k ( < 2 5 0 0 m ) s e q u e n c eo f r e d :ed type arenites overlain bv marine sediments ,rf the Meso-Helikian Athabasca Group rests on the crystalline basement. Table 5.2 shows the formations and respective lithologies of the *group.A pronounced unconformity with gentle r e l i e f s e p a r a t e st h e t w o u n i t s . Intense rveathering of lateritic nature has altered the basement prior to the deposition of the Athabasca sediments. Multiple episodesof both reverseand normal post-Athabasca faulting caused vertical displacements of the unconformity of up to severaltens of meters. Diabase dikes and sills intruded both the basement and cover sediments(Fig. 5.2).
Host Rock Alterations Since the rvaning phase of the Hudsonian Orogeny, seven principal episodes and types of aiteration have impnnted their traces on rocks of northern Saskatche$'an: 1. Late to post-Hudsonian Na-metasomatism rvith local development of albitites. Albitization is particularlv npical for the Uranium City region(Chap. 5.:.:). 2. Late to post-Hudsonian rerograde metamorphism of the crvstalline basement. 3. Pre-Martin Formation phvsical weathering of the basementsurtace. 4. Post-Martin Formation and pre-Athabasca Group lateritic weatherine (regolithization) particularly evident and preserved on crystalline basement rocks below the Athabasca Group. 5. Syn-Athabasca diagenesis associated with deposition of the Meso-Helikian Athabasca Grouo.
140
5 Selected Examples of Economically Significant Types of Uranium Deposits
Table 5.2. Northern Saskatchewan,generalized stratigraphy-lithologyof the Athabasca Group. (Compiled by E Srewartafter Hoeve et al. 1981,1982,1985;Ramaekers1979,1980.1981;Tapaninen19T6-Hendry and Wheatley 1985) Eon Era Group
Formation
Lithology
Carswell
Bedded dolomite; variablethicknessof beds;stromatoiitic. oolitic, and brecciated.more than 300m thick
Douglas
Interbeddedsiltstone,shale,and fine-grainedsandstone
(William River Subgroup)
Sandstoneswithin CarswellStructurenow correlatedrrith other sandv formatronsof AthabascaGroup
Tuma Lake
Quartz-richpebbly sandstone;maximum pebblesize3-1mm. variable amoun!sof interstltialclay, planar and trough cross-beddingcommon. maximum thickness:80m
Otherside
Quanz-nch sandstone;maximum grain size2 mm. well sorted. variable amount of interstitialclay siltstonebedsand interclasts common, planar, trough. low-anglecross-beddingand horizontal beddingcommon. typicalclay: predominantlf illite. maximum thickness:350m
Locker Lake
Pebbly sandstone;maximumgrain size50mm, pebblesof quartz, quanzite, and chert abundant,trough and planar cross-bedding. typical clay: predominantlyillite. maximum thickness:120m
Wolverine Point (Upper and I-ower)
Interbeddedsandstone.siltstoneand mudstone;eenerallyquartz-rich but arkosicsectionscommon,clay-richsectionpresent.siltstoneand mudstonebeds5 to 150cm thick form up to 10% of unit. patchy' carbonatecementvariablypresent,in upper portions zoneswith severalpercentP2O5containinglocally 200ppm U, abundant and diverse sedimentary structures, typical clay: illite-chlorite, maximum thickness:700m
Lazenby Lake
Marine sandstone: typical clay: predominantlyillite, maximum thickness: 600m
Manitou Fall.
Quanz sandstone;poorly sorted.variabl. urnoun,, of interstitial clay. trough and planar cross-bedding common, thin (1 cm) silt or clay lensesa few metersin lengthcommon, locally interbeddedwith conglomerateat the baseof unit, typical ciay: 60% illite,30% kaolinite, 10% chlorite. maximum thickness:1400m
Fair Point
Conglomeraticquartz-richsandstone;abundantclav matrix, quartz pebblesup to 10cm abundant,planar, trough, hummocky, and low anglecross-beddingcommon, typical clay: chlorite, ilirte, kao[rute. maximum thickness:300m
6. Late Athabasca diagenetic hydrothermal alteration locally associated u'ith mineraluationl recrystallization of ore. 7. Post Athabasca alterations associated with uranium redistribution. Alterations which may or may not be related to ore formation or uranium redistribution include a variety of syn- to post-Athabasca processes.Early diagenesis followed by diagenetic hydrothermal events have modified both the Athabasca sediments and the upper part of the crystalline basement. Early, pre-mineralization alteration includes
siiicification, kaolinitization, tourmalinization. and finally illitization. aherarionsassociated Diagenetic-hydrothermal with or postdating the unconformity-bound mineralizationare more complex, and different at the variousdeposits.The alteration products includeseveralgenerationsof dominantlyphylloquartz,sulfides,tourmaline, silicates, carbonates, hematite, and extensivebleaching (Hubregtse and Sopuck1987).Illitization is always found. chloritizationis very frequentbut more restricted spatially, and sideritizationis aiso common although varying in extent from one deposit to another.A later argillitizationphase produced
ExamplesofUnconformitl'-Contact-TvpeUraniumDeposits
NE
SW
5 1 0m q . s t. 90m of retrei
>6000 m.y
l.ll
iJl a)
0 io 52m
A t ho b o s c o GrouP 0 t o> 2 0 0 m upper:120m
:=--
4+ z
mrddte3 to 75m l o w e r ! 3 0m
c r y s t o L l r n eb o s e m e n t ? 7 0 0 t o 1 ? 0 0m . y .
I
,' /' ,r'
i/,1
unconto4
u r o nt um deposit > 13 0 0 . n . y .
Fig. 5.2. East Athabascadistrict.schematicNE-SW sectionshowingthe principalstratieraphicunits rvirhrhicknessand approximate age, the pre-Athabascaunconformity relief, the zone of preservedreeolithizarion. and the position of a clay-boundunconformity-contacttvpe uranium deposit(exampleMcClean). (After \\-allis et al. 1986.age referencessee Table 5.9) (reprinted rvith permissionfrom The CanadianInstitute of Mining and \letallure\')
chlorite, as reflected for example bv an extensive overprint at Maurice Bay and kaolinite as observed at Key Lake. Hoeve (1984) remarks that argillitization (illitization and chloritization) and bleaching are largely congruent featuresbut display crosscutting relationships, suggestingthat argillitization predatesthe large-scaiebleaching. The various syn- to post-Athabasca Group alterationsare diasrammaticallvillustratedin Fig. 5.3. and summarizedwith other data in Table 5.3 (see also Figs. 5.6. 5.7. 5.10. 5.11.and Table 5 .1 2 ).
monometallic compared to a polvmetallic mineralization, and often reiarivelv narrow in contrast to a more extensive and pervasive alteration halo and moderate to high (0.3 to 1.5% U3O8) vs. very high averaee grades (I to 14% U:Oa) respectively.Mutual parameters are an intimate link to the Meso-Helikian unconformitv, association with fault srstems. otten if not always reiated to old mylonite zones. alteration halos of simiiar mineralogical composition. and identical geochronological mineralization stages. Most ore zones consist of a high grade core (> 1% U3O8) surrounded by a halo of lower grade mineralization which can extend for a few meters to as much as ca. -<0m around the deposits(Figs. Principal C haracteristicsof Mineralization 5.4 to 5.7). Locallv these halos persist in form of The unconformitv-contacttvpe uranium deposits U coatings on quartz grains. as disseminated :f the Athabasca Basin region include two impregnations in the clav matrlr and as veinlets for greater distances (>i50m) above the unprincipal classes: Monometallic fracnre-bound mineralization in the basement (class 1. 1. 1. conformity in grev to black and multicolored Chap. a) (Fig. 5.a) and polymetallic clav-bound Athabasca sandstones and shales (Fig. 5 3). mineralizaion at the base of the Athabasca Below the unconlormitv. the mineralization of G r o u p ( c l a s s1 . 1 . 2 )( F i g s .5 . 5 . 5 . 6 ) . B o t h v a r i e t i e s clay-bound deposits extends downward for rarely have manv features in common but also exhibit more than 150m as fracture fillings and disseminations in crystalline basement rocks. whereas distinct differences. The principal discriminating features include mineralization of the fracture-bound class may ( c l a s sl . l . l f i r s t ) e m p l a c e m e n o t f t h e b u l k o f t h e persist in excessof 300 m below the unconformity ore in either the crystalline basementbelow the as at Eagle Point. All deposits may contain a unconformity or in a zone of argillic material varietv of metallic elements but quantitatively in capping the unconformity, quantitatively a more highlv different proportions (Fig. 5.8) and with
l4z
5 Selecred Examples of Economicaily significanr Types of Uranium Deposirs
oboveunconf. m 50->300
,Lc-
,; 1 /
il /',,
'A,/,ji
t . . .. . . . .
ooo
r0-50
,l'
/
/^'r
:,-tti{-
oo
5-20 rt 2 0 - 6 0t 2 / / /
u n c on f -
-
t---
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ltZl
Biotit" gn"i==
WTZ orapnite-biotleg n e r s s Archeon
lFlrl
Gronit"9n",==
l77l
FoutLon"
Fig' 5'3' Atbabasca Basin.region- diagramatic section of alteration features surrounding '^'6 vru"! clav-bound vvu'r unconformitvcontacltype (class1.1.2)U deposirs. Sandstone(ArhabascaGroup)' a "Normal" pink sandston.,iltn.". clal'f1n6116pcomposed of ca.509'.illite i 50% kaolinite; b clav alterarion halo with illite > kaolinite: c fractured, friable sandstone,+ bleached; sulfides.carbonates.chiorite. dravite, clav.alongfractures, containsfracture controlled weak U mineralization. commonlv 1p.i.l.J u ,-".i"hr"ii"n1: d silicified inten'als, quartz overgrowth. euhedral quanz along fractures; 5l{.pqlu e clay-filledfractures. dominantly iliite, minor kaolinjte. Mg-chlorite. Fe-chlorite, locall-vsecondarl' hemarlre. disseminarei cu.bon-,,butrorrr"; / -uriiu.- cl"y en*,.lope ,ur_ roundingand penetratingmassivemineralization llasemenl(Aphebian and Archean)
basement; r' red(hematitic) g,S.'.Xt"Thrt"ljff:'#^ir'51.*T::f:' r regolithic zone,12sreen (chroritic) zoneih compositional variations from one district to the other. For example, mineralizations in the eastern Athabasca Basin are dominated by a U-Ni-Co-Mo-As assemblage(Table 5.4) whereas in the Carswell Structure district a U-Te_Se_Bi_ Au assemblage(Table 5.5) is found. These ele_
mental inhomogeneities are thought to reflecl local variations in geochemistry and metal endow_ ment of Iithostratigraphicunits. Principal uranium minerak in both classes of deposits are pitchblende, uraninite, coffinite.
Examplesof Unconformity..-Contact-Tvpe Uranium Deposits
143
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144
5 Selected Examples of Economically Significant Types of Uranium Deposits
B' SE
B NW 4 0 0 mo s . [
*\\N s\\ \ \\
:N N
'180m o . s . t-
\r\
100rn
\1\\
X\T \
\
N \
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"\
Fig. 5,4. Eagle Point, NW-SE cross-sectionacross the Eagle South ore zone u,ith distribution and grades of U mineralization and extensionof associatedalteration zones. The section demonsrratesthe persistenceof pttchblende veins into grear,depthof at least 400m belou' the surface.(After Eldorado ResourcesLtd. 1987)
minor brannerite, and some amorphous uranrum- are described from several deposits. e.g., Ke1' carbon material (carburan) and in weathered Lake (Voultsidis et al. 7982). Von Pechmann zones hexavalent U minerals. Most of the (1985) argues against this determination, and uranium and associatedminerals are present in contends that the mineral phase is uraninite, at several senerations.Euhedral crvstals of aU.O, Key Lake at least. The misleading X-rav peaks
Uranium Deposits Examplesof Unconformity-Contact-Type a
1,15
N ' s e c ti o n
:J ' ihlckness )n ma' :.:ce __.n-.n ' t>,uu -:
l-ic
:1-lc0
f
'soltne of unconio'-rti, elevolron lrn m,
f
=oult wilh dip dr:e:lron
(in sradeIhickness. doned\combinedwith Fig. 5,5. East Athabascadistrict.Cigar Lake. isolinemap of U accumulations a isolinesof unconformltvelevationancib lnrerpreredstrucluresystemsat the uncontormitvdisplavingthe correlation betweenthe three parameters.{.\fter Fouqueset al. 1986)(repnnted with permissionof The CanadianInstituteof llining and Metallurgy)
o s.l m 100
s
N
-
\ unconformrty co. r30 m b€tow surfdce
0 +
3
S'
N' o.s.l. m
b
50-
3 e l i k r o nA t h o b o s c o G r o u p l-:---:-:t i ". . .l :Fresh secrments
I
:-----1 Grey olte.ed sedrmentg
N
, m i n e r o t i z o t i o' 3no o p p m u
t--=-1 r"l -)l
N
, m r n e r o l i z o t i o,n1 z s o h l J
lone Ji renemotrlizotlon
A o h e b i o nW o l i o s t o n G r o u o l M o r n t y g r o p h i t e - o i o t i t e - c o r d t e r r tgan e i s s c u t c y o e g m o t o r d
through the easternpan of the depositshowingdistribution Fig. 5.6. East Athabascadistrict. Cigar L-rke, cross-sections of low and high-grade U mineralization. and halos in the AthabascaGroup of grav alteration (associatedwith Fesulfidization) and rehematitization. Note the relatively sharp boundariesof the high-grade U mineralization. (After Fouques et al. l9E6) (repnnted wirh permissiontrom The CanadianInstitute of llining and lvletallurgy)
146
5 Selected Examples of Economically Significant Types of Uranium Deposits U contents Pod
N-l
% u3or mt U30s
o.7g 65
N-3 0.69 85
N-2
SW 210 900
N-l
1t1
1180
2.56 31 3 0
>E 073 830
Condy Loke 0.35 17
I
3
+
Condy Loke Pod N-Pod I
z2
McCleon Loke
9.6 y'N-eoat
N - P o d2
tz t,
Condy'Loke ti::\
.f,9
[email protected]
6.2
o.77@
SE -Po d
€
U
Robbrl-Eors SW-Pod M r n e r o lr 2 ( r 1 l o n
+ ry M cC t e o n Loke
Arsenrdes P y r rt e Brovoite Siderite Hemotite
'-t-
60
42 A
Condy'Loke Pod
N-Pod 1
N-Pod2
'Condy'Loke
y'N-eoaszzt ,frft
a6 \=/
fAza
\;4V
w\-
SE-Pod
R o b b i t - E o r s SW-Pod Minerotizolion
I
Fe-chlo. te Mc-chio. l€ Koolrnl€ l l l ri e
c
Gnu
a
ffi
C i r c l e o r e o = l o t o l 7 o o f m r n e r o l i z o i i o nl o ) C i r c l e o r e o = t o l o l 7. ol cicy conleni {b) broDnrlrc ololrttc g n e i s s ( m e t o - p e l i t e ) AnomolousU {'200oom U/4.3m)
@
U Ore pod
-:l
U M i n e r o l i z o i i o no b o v e u n c o n f o r m i t y
mm
U M i n e r o l i z o i r o nb e l o w u n c o n f o r m i t y
Fig. 5.7. East Athabascadistrict, McClean. Correlation of U mineralizationwith graphitic gneissand rhe unconformitv is displayedin the plans (graphitic units interpreted from EM and drill hole daa)l Arcks iirow the per.entugesof ,0 and other metallic minerals and b clay minerals in the-various.orepods (numbers at circles give total percentage of roct fraction). (After Wallis et al. 1985, 1986) (reprinted with permissionfrom Th-e Canadian institute of Je,spective Mining and Metallurgy)
Examplesof Unconformitv-Contact-TypeUranium Deposits
Fig. 5.8. Eastern Athabasca Basin region, range of contents of selectedelementsin unconformity-contact deposits in the Wollaston Belt. (Heavy line gives contents for both classesof deposits, dashed line only for fracnrre-bound type mineraiizationclass1.l. l, dots representbulk and sqrarespit grab samplesfrom the Gaertner ore body, Key Lake, only sampleswith >0.1% U3Os). (After G. Ruhrmann wrinen pers. commun. basedon data from Eldorado ResourcesLtd. 1986;Fouques et ai. 1986: Ruzicka 1986. 19881Wray et ai. 1SAS)
Table 5.4, Key Lalie, Gaertner ore body, parageneticassemblagesof mineralization. (After Ruhrmann and von Pechmann1989) Assemblage U
Ni. Fe, Cu. Co. U
U and Gangue
Age m.y.
ca- 14O0
ca. 900
a. i00
Minerals
Uraninitcl
Ra-mmelsbergtel Niccolite I Gersdorffire Bravoite I Covellite Hauchocornite Breilhauptite lrrite Borrutc I ChalcopyriteI Coffinire I
Calcite Siderite Sericite Dolomite Rutile Apatrte
Uraninite II Uraninite III UranirureIV Uraoinite V
N i . F e .C u .P b ,Z n . U
Anatase RammelsbergteII Kerogen Marcasite Bomite II Gesdorffite BravoiteII
Maucherite Milterite I Bismuthinite
Galena I Vaeyle Miuerire II Niccolite II
Sohalenre Millerire lll Coffinire II Galena II Digenire Chalcopyrite II
148
5 SelectedExamplesof EconomicallySignificantTwes of Uranium Deposits
Table 5.5. Cars*'ell Structuredistrict, summaryof mineral parageneses,geologicalsettingand age of mineralization in basement and overlving sediments.(After Ruhlmann l9&i and l-aind 1986. reprinted with permission from The Canadian lnstiture of Mining & Metallurgy and the Geologrcal Associationof Canada) Subeconomic/pre-ore
Stage
Redisribution/l-ate ore
Matn ore II
Assemblage
Monaziteul:rmnr te
Pitchblende Uraninite-selenideuraninite sulfide
Uraninite sulfide
Pitchblende- Pitchblendehematite c:rbonatc
Coffinitesulfidc
Uraniferous phase
Monazile (th) Uraninite
Pitchblende Brannerite Uraninite Uraninite
Uraninite
Pitchblende
Coffinite
Nonuraniferous phase
Molvbdenite Prrite Galena
P.vrite Altaite - paraguanaiuatite Chalmpwite Guanajuatite - ciausrbalrte Galena Calavente - trogralire Freboldite - gersoorffi te Nickeline- skutterudle Galena- chalcolrrrtc Molvbdenum sulides Gold - bismuth
Gangue
Garnet
Albite Chlorite
Chlorite (Mg/Al)
Quanz
Host rock
Garnet - rich peg:rratoid
Feldsparrock
Basement& saadstone
Basement
Sandstone Basement
Basement
Sandstone Basemenr
Textural characreristics
Disseminated
Fractures
Coatings around the "zone d boules"
Fractures
Fractures
Fractures
Fractures
Characteristic element assoc,
u-lll
U-Mo-Bi-Se-S (Te-Ni-Co)
U-Mo-PbZn-Cu-S
U-Fe
U-Co-Ptr Cu
U.Si
I T
Molybdenumsulphide Galena- pynte Chalcopvnte Sphalerite Tennantite
Hematire Magnetite Limonite Goethite Gold
Pitchblende Chaicopvnte Galena Pvnte
Pl nte Galena
Caicite
Age. m.1'-'
19
1330?
1150- 1050
820-890
380
AJI ages
All ages
Types of occulTences
Sophie
Numac
Claude- D-N Dominique- Peter
OP - ClaudeDomin - Peter
Donna D-N
All n'pes
All types
" Reference see Geochronologv.
Table 5.6. Eastern AthabascaBasil region, published analvsesof selectedelementsof U mineralizerion in fracturebound deposits (class1.1.1). The ore typically containshiCh U gradesand low valuesof the otber metals except for Pb which is largell'radiogenic. (valuesare in ppm exceptwhere noted). (After a Eldorado ResourcesLtd. 1987.composite sample from 02 zone Eagle North; b Ruzicka 1986,nine drill core samples;c Ruzicka 1986.five grab samples from variouspans of the deposit) Eagle Point
Ag As Au B Ba Be Bi Co Cr Cu Fe
5 60 006
Rabbit Lake
b n.a. <1000
n . f. n.f.
i50-700 200-500
<10-500 20-500
1.22"/'.
Mo
Mn Mo
750 620
Nd Ni
90
50-2000 20
Rabbit Lake b
<2-,50 60 60 120 370
EaglePoint
<10-70 50-70 15-500 0.5-159'. n . f. - 5 0 0 0 . 5 -1 0 9 . n . f .- 7 0 n.f.-700 50- 200
P Pb Pt
200 1570 <0.03
Sc Sb Se Sn
<1; <20 <10
Te Th Ti I;
\, Yb Zn
500-50 m0
c n.f . - <100tt 70- 15000
3- 150
2.2
:oo-zooo 1.865% 400
t.69-43.3",'o 300-700 30- 1000
<50
n.f. 0 .1 3 - 1 7 . 9 0 , o 100-500 30-700 3- 50 _s0-700
Examples of Unconformity-Contact-TypeUranium Deposits
leading to the cU3O7 identificationare caused by coexisting uraninite and poorly crystallized "sooty" pitchblende.Ramdohr(pers.commun.) considersthe aU3O7 possibly a distorted uraninite recrystallizedafter colloform pitchblende. Additional featuresof the rwo classes(seealso Chap. Ore Control) include: a) Fracture-boundclassmineratization (Fig. 5.a) is emplacedin highly fractured and altered
+ T '-..<..<
Jx-* \
Xx
x
XXX XXXX xxx X.X X
X
crystallinebasementrocks often associatedwith graphitic metasedimentsor calc-silicaterocks. U is accompaniedby mainly Fe and pb sulfides, and minor quantitiesof mainly Cu, Mo, V and tracesof other elements(Table5.6). Local variations in accessorialelements are apparently related to host rock lithologies.principal gangue mineralsare quartz,calcite,dolomite,minor siderite and ankerite.The depositsoccur in an en-
X
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r I I
\
'.
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,,l..z'{, I
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-t -
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x x x X x x x x x X x x x x
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c
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z
d.2
=
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Archeon-Aphebion
[llll
Il"l
l---l
B i o t i t i . g n e i s s ( m e t o - s e m i p e l i toen) d f e i d s p o t h i c gneiss F;T t---l--l
Gronitoid of Torwolt Dome (below metosedimentsl
72
gio1i1t.gneiss {meto-petrte. -semipetite)
Mojor fouit
IFI
Groprriticaneiss
Z [I @
apn"Oionfeldspothrcgnerss or Archeon gronite gners
Eostern limit of Athobosco Group U deposits Isize exoggeroted)
U deposits: C B = C o l l i nB s o y { A . B . D . E .zFo n e s ) .D L = D o w nL o k e ( 1 1 , 1 1 A . 1 1z8o. 1n1e,s } . E P = E o n 1P " o i n t .H = H o r s e s h o e t , C . M cC l e o n . I E. B = J E Bt." l = M i d w e sM R L = R o b b iL t oke. Rv=Roven Fig. 5.9. Athabasca Basin region, East Athabasca U district. genernti"ed geologyof the crptalline basement interpreted from geophysical and drill hole data, and position of major uranium depos'its.Liieposits: CB Collins Bay (A, S, D, g, f 1-ne1)if.LDawn[^ake(ll, 1lA, llB, 14zones);EPEaglePoint:I/ilorseshoe:innrcB;MMidwesi;MCMcClean; RZ Rabbit Lake; Rv Raven (After McNutt 1980In: Clarke and Fogwill 1986;Eldorado ResourcesLtd. 1987;Sibbald 1983; Wallis et al. 1986)
150
5 Selected Examples of Economically Significant Types of Uranium Deposits
vironmentaffectedby markedNa-metasomatism, Peter, Claude, OP, N) represent this class of migmatitization, and the presence of often mineralization. abundant pegmatite and microgranitedikes or b) Polymetallic clay-bound mineralization segregations. The depositsoften occur in subsidiarystruc- (Figs. 5.3 and 5.5 to 5.7) occursassociatedwith massesat the baseof the arenaceous tureslocatedin the hangingwall of major reverse argillaceous faults (e.g., Collins Bay Thrust Fault at Eagle sedimentarycover (Manitou Falls Formation) Point, Rabbit Lake Fault at Rabbit Lake, Fig. directlyabovethe unconformityand in the upper 5.9). The Rabbit Lake, Eagle Point and other few tensof metersof highly altered basement. Mineralizationin thesedepositsexhibit a great depositsof the East Athabascadistrict locatedto the eastof the CollinsBay ShearZone and some chemicaldiversity. A suite of anomalous eledepositsin the Carswell Structure (Dominigue- mentsis commonlypresentin the form of sulfides. Table 5.7. Eastem AthabascaBasin region, pubiished analysesof selectedelementsof U -inet"li"ation in clav-bound deposits(class1,1.2).The ore is characterizedby high to very high U gradesand high contentsof other meials as well. Nr and As are particularly high. Vaiues of almost all other metals are significantlyhigher than those in class 1.1. I deposits. After a Ruzicka 1984,six selecteddrill core samples;b Ruzicka 1984.basedon Ruzicka and Linlejohn 1982.and Canada Wide Mines Ltd. 1980,eight drili core samples;c Wrav et al. 1985,five compositedrill core samples:d Fouques et al. 1986,averagemetal contents of deposit; e Ruzicka 1984. sevenselecteddrill core samples;/ Ruzicka 1984. three drill core samples; g Ruzicka 1984, three drill core samples Key b.ke Gaertner ore body
Cigar Lake
Midwest
Collins Bay A-Znne
B-Znne
D-Znne
Ref.: Ag As Au B Ba Be Bi Co Cr Cu Fe La Mg Mn Mo Nd Ni P Pb Pt S Sc Sb Se Sn
l0-30 0.7->10%
U
v
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tr-68.3 r.68-9.62% 0-0.34
3.77-1349.8 <5-150 t.670/. 0.03-33.46 ,to:
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tzo-tsoo 700-3000 <10-300 20n-20fn
<0.05-1.0% 30-330 14-1500
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:
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5-68 0.23->10%
1500-2000 20.4-32.r% 700- 1500
<1000
: 0.05-?.0%
10-20 <50-330
0 . 1 4 - 7 7 . 2 % 0.25-71.8% 0.01-0/9% 0.17-0.340h
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ri:on
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_ 100-2000 z:3-&.8% 10.9-2r.4% 0.09-32.2% 100-500 150-3000 -
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151
Examplesof Unconformiry-Contact-TypeUranium Deposits
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5 Seleeed Examples of Economically Significant Types of Uranium Deposits
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Examplesof Unconformity-Contact-TypeUranium Deposits
arsenides,sulfarsenides,locally selenidesand tellurides(the latter mainly in the Carswelldistrict) (Table 5.7). However,the proportionsof and in mostcases the elementsvary considerably show no economic concentrations.Principal ganguemineralsincludequartzand carbonatesin minor quantities.Key Lake and most depositsof the EastAthabscadistrictthat are locatedto the westof and at the CollinsBay ShearZone (Fig. 5.9), and somedepositsof the CarswellStructure distnct(D-ore body) belongto this ciass. Dimensionsof the known depositsin the AthabascaBasinregionare givenin Table5.8. Isotope ages of ores from the Athabasca uraniumdepositsare providedin Table 5.9. The oldesturanium generationis dated about 1400to 1250m.y. Subsequentmineralization or remobilization events occurredabout 1250to 900 m.y. and 300to 100m.y.ago.
StableIsotopesand Fluid Inclusions Stableisotope (D, tto, ttc, -S) and fluid inclusion data from different phasesof phyllosilicates, quartz,carbonates,and sulfidesreportedby Bray et al. (1982,1984,1988)(McClean),Halter et al. (1987)(CarswellStrucrure),Pagel( 1975a,1975b, 1977),PageIet al. (1980)(CarswellStructureand Rabbit Lake), Wallis et al. (1986) (McClean), Wilson (1987), Wilson and Kyser (1987) (Key Lake) amongothersdocumentthat severalfluids of different composition had been repeatedly activein the AthabascaGroup and the basement along the unconformity. Fluid inclusions in siliceousovergrowthon detrital quartz grainsof Athabascasandstoneindicate that they derived from brines under oxidizing conditions during diagenesis(Pagel 1975a). At McClean. hydrogen isotopes show two distinct populations in diagenetic illites from and regolith(D = -60 to -65) and in sandstone ore-relatedalterationillites(D : -100 to -155) demandine two different fluid compositionsor sources(Wallis et al. 1986).Similarsulfurisotope sulfides(6vS valuessuggestthat synmetamorphic : -32 to +21) providedthe sulfur for ore sulfides (6345: -24 ro +17) (Bray et al. 1982). At Rabbit Lake. stableisotopes(Pagelet al. 1980) and fluid inclusions(Pagel and Jaffrezic 1977) indicate that pale green chlorites and euhedralquartz veins,dolomite, and calcitehave formed from different and not coeval fluids. At
153
leasttwo varietiesof vein quartzcan be identified by fluid inclusions.The first has inclusionswith high and variable salinitiesand formed at a temperatureof ca. 160'C. The other has low andindicatesa T of ca. 191'C.Dolomite salinities crystallizedat 130'Cand later calciteat 120'Cand ftom 127to 150"C.Someof the vein quartz has isotopic compositionssimilar to dolomite and calcite, suggestingan origin from similar fluids. The Cl/Br ratios(55) in the fluid inclusionsof vein quartz appear to be consistent with either a marine evaporiticenvironmentor perhapsone with thermal degradationof organic matter or both. At Key Lake, Wilson and Kyser (1987) interpret their data to indicate the presenceof two fluids.One is indicatedby Mg-chloritein the basementand the other, a basinalbrine, by the compositionof illite. Mixing of these two fluids caused the concentration of uranium at the unconformityat temperaturespostulatedto be between190and 230"C(co-existingquartz-illite). Younger processesinclude kaolinitization by meteoricwatersat low temperatures(50'C) and recentalterationof hydrothermal illite by modern meteoric waters. The latter is associatedwith hydration(increaseof water content from ca.4"/" to 7.7o/"in alteredillites), lowering of 6D values to as low as -170 and partial resettingof K-Ar ages(rangefrom 1493to 414m.y.).
PotentialSourcesof Uranium Certain metasediments,particularly more mafic gneisses,calc-silicate rocks, migmatites and pegmatites,and some granitescontain elevated backgroundvalues and localizedconcentrations uranium,often in form of uraninite, of syngenetic and other metallic elements in anomalous amounts. These uranium occulTenceslargely belongto the synmetamorphictype (for example, Yurchison Lake, Karin Lake, and Duddridge Lake, Wollaston Domain) but some also to the subunconformity-epimetamorphic/Beaverlogde type (Sophie and Laure, Carswell Structure, perhapsWay Lake, Wollaston Belt). They are documentedfor the Wollaston Group in the Cree Lake Mobile Belt by Dahlkamp 1978, Dahlkampand Adams 1981,Kirchner et al- 1979, Lewry and Sibbald 1978, Parslow and Thomas 1982.Ruzicka1984,Thomas 1979,Voultsidiset al. 1980and others.Elevateduranium valuesare
154
-5 SelecredExamplesof EconomicallySignificantTypes of Uranium Deposits
likewise reported from the Carswell Structure area by Laini 1986 and Pagel and Svab 1985. The distribution of uranium deposits in the Athabasca region compared with the data supplied by the listed authors suggeststhat the uranium deposits are located in a region with seemingly more abundant anomalous uranium background values and local uranium concentrations and that a large part of this uranium was present in leachable form. This supports the hypothesis that the Aphebian metasediments in parricular constituted a valid source of uranium for the formation of the unconformitrtype deposits.
Table 5.10. Reconstruction of geological events in the Athabasca Basin region
Geochronologr'
4. Hudsonian Orogenv 1900-1700m.y
Significant ages of the Athabasca Basin regron based on isotope datings of rocks in the Western Craton and in the Cree Lake Mobile Znne are presented in Table 5.1i. and of uranium mineralization and associated alteration products in the main districts in Tabie 5.9. Figure 5.2 shows the geological setting of a deposit and approximate ages of ore-hosting units. A reconstruction of the geological events recorded by these data in the Athabasca Basin region can be found in Table 5.10 (see also Chap. 5.2.2, Uranium Ciry region).
Ore Controls and Recognition Criteria Significant ore controlling or recognition criteria of uranium deposits in the Athabasca Basin region include: Host Environment
1. Archean ending with Kenoran Orogen.v >2500m.y. 2. Earl;- Aphebian Blezardian Orogen-">2100m.1'. 3. Late Aphebian >1900m.r'.
5. Paleo-Helikian >1500m.y.
Formation of granitesrgranitic gneisses
Depositionof sediments. graniticand pelitic gneisses Depositionof WollastonGroup sediments.formation of Peter River series,calc-alkaline magmalism.crvstallizationof uraninite and monazite
Amphibolite to granulite grade metamorphism. migmatization. doming, generationof deep thrust faults and mvlonite zones,late orogenicretrogradegreenschist faciesmetamorphism.intrusion of granite and granodiorite and associateddikes, formation of subunconformitvBeaverlodge-type U deposits Lateritic weathering
6. Meso-Helikian >1500-1300m.y. Depositionof AthabascaGroup. diageneticand diagenetichydrothermalprocesses, crystallizationof uraninite/ pitchblendeof unconformitvcontact-typedeposits 7. Neo-Helikian 1310-900m.y.
8. Hadrvnianand younger 700-400m.v.
Intrusion of diabasedikes, second generationof U mineralization. polymetallicmineralization
Redistributionof uranium CarswellEvent
- All major deposits are grouped in three main 9 . 3 0 0 m . v .t o Recent Erosionof Athabascasedimen!s. districts, the East Athabasca. Southeast late remobilizarion of uranium Athabasca- and Carswell Structure (Fig. 5.1). - All three districts are characterized by a basement containing abundant Aphebian lower Wollaston Group partl)' graphitic metasedi- Aheration ments wrapping around Archean granitic domes (deposit-type related details see next - Localized structurally controlled late paragraphs). Hudsonian (?) albitization (Na-metasomatism) - Red bed type arenites of the Athabasca and dolomitization (Mg-metasomatism) are Group rest unconformably on the crystalline noticed in the area of fracture-bound class basement. deoosits.
ExamplesofUnconformity-Contact-TypeUraniumDeposits
155
Table 5.11, Northern Saskatchewan,southwesternChurchill Province, selectedgeochronologicdata Age rnm.y.
WesternCraton
Agein m.v.
CreeLakeZone
Archean-KenoranOrogeny(>2500m.y.)
3070(zircon) 3010(zircon) 2864(zircon) 2650(Sm-Nd) 2630(Rb-Sr) 2580 2510(zircon)
- Basementbelow Murmac Bay Group, Beaverlodgearea(22a). - Quartz-feldspargneiss/metaarkose, Fond-du-Lac. - Mountain Rapids granodiorite, TaltsonMagmatic Zone, W of WesternCraton (7) - Hypersthenegneiss, NevinsLake Block (2b) - Megacrysticsranite, Nolan Lake (7). - Foot Bay gneiss, Beaverlodgearea(20).
2680 2613(Rb-Sr) 2600(zircon) 2600(Rb-Sr) 27fxl(Sm-Nd) 2490(zircon)
Rhyolite,EnnadaiGroup, Hattle Lake (8). Westerngranitic gneiss, Midwest(25). Granite gneiss, Zimmer Lake (13b). Granitegneiss. K e y L a k e( 1 2 b ) . JohnsonRiver granite, BaileyLake ( l5).
Early Aphebian-BlezardianOrogeny (2500-2140m.y.)
ca:22U)-2100
2360 zJ3)
2180 2315 2320(zircon) 2130(zircon) 2179(ztrcon) 2155(Rb-Sr) 1999(zircon)
- Depositionof middle and upper Tazin Group sedimentsand volcanics including Fay Complex and perhaps Donaldson[-ake gneissin Beaverlodgearea (Tremblay 1978). - Mclntosh granite, - Anthona Mine granite, - Gunnar granite, Beaverfodge area (22a). - Garnetiferous felsic gneiss, Nevins Lake Block (2b). - Earl fuver Compiex. CarswellStructure (1). - Donaldson Lake gneiss, area(20.3). Beaverlodge - Box, Frontier gtanites, Beaverlodgearea (3). - Box granite (22a).
>2100(?) 2338(Rb-Sr) 2136(Rb-Sr)
- Deposition of Needle Falls sediments. - Eastern granitic gneiss, - Pelitic gneiss, Midwest (25).
Late Aphebian-HudsonianOrogeny (2100- 1700m.y.)
ca.2100-1900 1990 1920 1975(Rb-Sr) r928(U-Pb) ca.1880(Rb-Sr) ca.1750(Rb-Sr) 1e30(u-Pb)
1835(K-Ar) 1815(K-Ar) 1780(U-Pb)
- Depositionof Peter fuver Gneiss sedimentsin Carswellarea (Laind 1986) - Slavemonzogranite, - Konth syenogranite. Taltzon Magmatic Zone, W of westernCraton (5). - Pegmatite. Beaverlodgearea (17). - Uraninite-monazite/Sophie, - Feldspathicgneiss, - MetapeliticPeter River Gneiss, CarswellStrucrure(1). - Uraninite in pegmatite, Beaverlodgearea (Koeppel 1968, recalculatedat 1860m.y. by Beil 1981). - Gabbro dike, Beaverlodge area (2,{c). - Pegmatite, Gunnar, Beaverlodgearea (14) - Pitchblendeveins. Beaverlodgearea (Koeppel 1968, recalulated at l74Om.y. by Bell 1981)
>i900 1890 1830(Rb-Sr) 1820(Rb-Sr) 1765(Rb-Sr) 1730(K-Ar)
1711(Rb-Sr) 1735(U-Pb)
- Deposition of Wollaston Group sediments. - Junction granite (postdates Virgin River ShearZone (4). - Gneiss. - Granodiorite. Key Lake (12a). - Biotite granite (3) - Biotite and hornblendern Archean gneiss, dated2600m.y., Key Lake (12b). - Biotite in gneiss, Midwest (25). - Uraninite vein, Way Lake (12c).
156
-5 Selected Examples of Economically Significant Types of Uranium Deposits
Table 5.11. Continued Helikian-Hadrvnian (< 1700m.v. ) ca. 1740->1490 1630(K-Ar)
- Deposition of Martin Forrnation. - Basalt sill in Martin Formation. Beaverlodgearea(24a).
? >1500 1632(Rb-Sr) 1541(K-Ar) 1523(K-Ar) 1487(K-tu) ? >150G1300 1700(U-Pb) 1550 (Rb-Sr) 1-513 1430(Rb-Sr) 1310-900 1310(Rb-Sr) 1230(K-Ar) 109a(K-Ar) 949(K-Ar) 515-365(Ar-Ar)
References: 0 Armstrong and Ramaekers1985 1 Bell 1985 2a Bell and Blenkinsop 1980 2b Bell and Blenkinsop 1982 3 Bell and McDonald 1982 4 Bickford et al. 1986 5 Bostock 1987 6 Burwash etal.1962 7 Burwash et al. 1985 8 Chiaranzelliand McDonald 1986 9 Ciaueret al. 1985
10 Cumming et al. 1987 lla Fahriget al. 1987 1 l b F a h r i s e t a l .1 9 7 8 12a Hoehndorfet al. 1985b 12b Hoehndorf et al. 1989 12c Hoehndorf and Carl 1987 13a Homeniuk and Clark 1986 13b Krogh and Clark 1987 14 l"owdon 1961 15 Rav and Wanless1980 17 Sassanoer al. 1972
-
Intense paleoweathering reflected by argillitization. chloritization. hematitization, etc. altered the basement to great depth (>50m). - Diagenesis of the Athabasca Group affected both the sediments and upper part of the weathered basement by siiicification, kaolinitization, chloritization, illitization, bleaching, etc. - Diagenetic hydrothermal alteration related to the various stages of mineralization and mineral redistribution generatedhalos of illite/ sericite, chlorite, carbonate, sulfide, tourmaline, hematite, kaolinite, phosphate, bitumen/hydrocarbon, quartz dissolution, and often, but not ever)'where, destruction of
-
- lateritic weathering. - Deeplv *'eathered basement (11b). - Muscovite. highly altered basement.MauriceBay (18). - Least altered basement, - Moderatelv aiteredbasement. McClean (33 |. - Deposition of Athabasca Group. - Apatite. Nonbern Albena (10). - Paleomagmetic data. R u m p l el - a k e ( l 1 a ) . - Wolverine Point Fm. (2a). - Wolverine Point Fm. (0). - Intrusionof diabasedikes. - Diabasedike (0). - Hornblende. Cree Lake (6t - Diabase. Midwest (?-5). - Grab sample. Cluff L,ake (24c) - Cluff Breccias, CarswellStructure (1)
18 20 2l 22a 22b 23 24a 24b 24c 25
Stevenset al. 1982 Tremblay et al. 1981 van Bremen et al. 1987 van Schmuset al. 1986 van Schmuset al. 1987 Wallis et al. 1986 Wanlesset al. 1966 Wanlesset al. 1968 Wanlesset al.1979 Worden et al. 1981
graphite around deposits(Fig. 5.3; for deposittype related alteration features see next paragraph). Diabase dikes and sills affected b-v this alteration are serpentinized. Geochemical processes associated with the hydrothermal alteration include Mg and B metasomatismand/or mobilit)' of Mg, K, Na, C a , A l . S i , B . C O 2 , l o c a l l yF a n d P a n d l o c a l t o widespread dispersion of trace amounts (few ppm) of metals including U.
Mineralization Fracture-bound mineralization basement hosted ( c l a s s1 . 1 . 1 )( F i g . 5 . 4 ) :
Examplesof Unconformitv-Contact-Tvpe Uranium Deposits
The primary ore control is by structure. Lithology plavs a subordinate role in localizat i o n o f t h e s ed e p o s i t s . Mineralization occurs as breccia and fracture fillings partly as massive veins, veinlets, or stringers, and disseminations in adjacent cataclasticwall rocks. Preferential direction of mineralizedstructures fluctuatesaround NE-SW trending subparallel to the foliation and attitude of the host rocks. Host structures consist of a network of subparallel and anastomosing fractures often located in the hanging wall of major reverse and thrust fault zones. Position is at or adjacent to old. regional mylonite zones of supposedly Hudsonian origin that were reactivatedin Athabascatime. Bulk of the ore is mainiy within about 150m below the Meso-Helikian unconformitv (Rabbit Lake, Claude, Dominique-Peter) but can extend to a depth in excessof -100m (Eagle Point). Lithoiogicallv and stratigraphically, the deposits in the eastern Athabasca Basin occur preferentially in Aphebian metasediments, but are apparently not restricted to particular rock facies or specific stratigraphic leveis except for an overall association with graphitic horizons. The relationship of graphite to mineralization tends to be more spatial, since U minerals mostly occur along the margins of graphitic units and not within them. Deposits in the Carswell Structure district are positioned at or proximal above the Archean (?) Earl River Complex and Aphebian Peter River Gneiss contact (Aphebian unconformity ?). This contact correlateswith the boundary of the anatexis that affected the Earl River Complex. A major myionite zone follows the contact on the NE flank of the Dominique dome and apparentlv controls the lccation of several deposits (Tona et al. i985). Host rock environments are marked by late Hudsonian (?) Na-metasomatism(aibitization), Mg-metasomatism (dolomitization at Rabbit Lake), migmatization, and pegmatite and granite intrusions or segregations. Deposits are enveloped in a conspicuous alteration aureole characterized by moderately to strong decomposition of plagioclase, cordierite, sillimanite, ferromagnesian minerals, ilmenite, magnetite, and corrosion of quartz and replacement of these minerals by, or
157
a u t h i g e n i c d e p o s i t i o no f i l l i t e . c h l o r i t e , a n d locally mixed-layer clays of distinct crystalc h e m i s t r y 'd . r a v i t e . c a r b o n a t e .q u a r t z . s u l f i d e . h e m a t i t e . a n d l o c a l l yk a o l i n i t e . - The dimension of the ore-related alteration halo is variable but mostly narrow (at Eagle North restricted to within a few meters of mineralized structure systems.at Eaele South somewhat larger. supposedly due to more pervasivecataclasis). - Phvilosilicate minerals ma-v be zonally arranged. At Eagle Point (Fischer in Eldorado Resources 1986), magnesianchlorite (MgiFe ratio 7 to 20) apparentlvprevails in the upper 75m below bedrock surface. whereas MgFechiorites (Mg/Fe ratio ca. 1) dominate further into depth. The highest grade sectionsof ore shoots contain predominantly illite rvith mrnor or no chlorite. Lower grade mineralization is associated with chlorite in comparable proportions rvith illite. In depositsof the Carswell deposits (Pagel and Svab 1985), peraluminous chlorites dominate over Al-lvlg-chlorites within mineralization whereas marginal to these ore zones Al-Mg-chlorites prevail. The illite/ sericite phases display a similar pattern. - Later alteration or authigenic phases (dravite etc.) occur. preferentially peripheral to mineralization. - U is the dominant ore element. Associated elementsare Pb and Fe, minor amounts of Cu, Mo and V. and tracesof Ni. Co. etc. (Fig. 5.8, Table 5.6). - Ore and associatedminerals are present in severalgenerations,including an early stageof mineral introduction and at least two stagesof redistribution. - The first stage of mineralization in the eastern Athabasca region is essentiallva monometallic uranium-sulfide assemblagecontaining uraniniteipitchblende(Table 5.9) dated between ca. 1400 and 1300m.y. A second stage of pitchblende is dated between ca. 1100 to 900m.y. - Deposits in the Carswell Structure are slightly ditferent displaying a primary uraniniteselenide-sulfidestagefollowed bv a uraninitesulfide assemblage(Ruhlmann 1985). - Subsequent rejuvenation episodes modified the earlier generations, at least between 300 and 100m.v. aeo.
Clay-boundmineralizationtopping unconformity (class1.1.2)(Figs.5.3and5.5 to 5.7):
5 SelectedExamples of EconomicallySignificantTypes of Uranium Deposits
158
The boundary between high grade ore and low grade mineralization or wall rock respectively is generally abrupt. The high grade core zone is often located at basement ridges that range from a few to as much as 30m high (Cigar Lake). These are particularly prominent in the western part of the East Athabasca distnct (Cigar Lake, Midwest, McClean), whereasat other deposits they are presumably masked by later deformation (Key Lake). Host to the mineralization is soft or indurated siitv or sandy clav/mudstoneerading outwards into argillic sand of pre- or earlv Athabasca as.e.
Ore controls are by lithology and structure. The bulk of the ore occurs in an elongated core of high grade pods, pockets, aggregates,and veins enveloped in lower grade material straddling the Meso-Helikian unconformity. Stringers, veinlets, and disseminations of mineralization penetrate from the high grade core along cataclastic/permeablezones upward into sandstones and downward into basement eommonly for not more than ca. 30 m. Localll', low grade appendicesmay persist for ca. 1-50m into the basement and upward into the sandstone for up to 200m. Some mineralization can occur as perched pockets in the sandstone along fractures above deposits.
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Examplesof Unconformitl-Contact-Tvpe Uranium Deposits
Host sediments from the Carsrvell Structure are reported to include mudstones and silts t o n e s t h a t c o n t a i n v o l c a n i cc o m p o n e n t sa n d a p p a r e n t l v h a v e b e e n d e p o s i t e di n s c o u r so r depressionsincised into the basement(Pacqet and Namara 1985). Mineralized Athabasca sandstonecan be dark grav to black mainly caused by sulfides and some carbonaceousmaterial (Key Lake, north of Main Zone of Midwest Lake. Cigar Lake), or multicolored. ranging from dark green to brown and buff with shadesof pink, grav and white (Collins Bay B, Main Zone at Midwest. Mcclean North Zone. Maurice Bav) (Tremblay 1982).
159
Host material can consistof as much as 70"/" cla.v-composed of illite/sericiteand chlorite as matrix to corroded quartz grains and rounded s a n d s t o n ef r a g m e n t s . R a t i o s o f t h e p h y l l o siiicates can be highlv variable, as shorvn in F i g s . 5 . 7a n d 5 . 1 0 . The phvllosilicates may display chemical zonation. In depositsof the CarswellStructure, Al-chlorite is dominant in the high grade and MgAl-chlorite occurs at the margins of the m i n e r a l i z a t i o n( P a g e l a n dS v a b1 9 8 5 ) .A t C i g a r Lake Fe-bearing phvllosilicatesprevail in high g r a d e m i n e r a l i z a t i o n( s e eb e l o w ) . Carbon buttons (bitumen etc.), later tourm a l i n e ( d r a v i t e l . c a r b o n a t e sq. u a r t z .h e m a t i t e ,
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160
5 Selected Examplesof EconomicallySignificantTyres of Uranium Deposits
Table 5.12. Key Lake, Gaertner ore body, zonation and extensionof alteration featuresin the Athabasca Group at a level of approximatelv 10m above the unconformity (see also Fig. 5.11). (Ruhrmann i986) Alteration feature
Zone with hematite relicts(more than 80m from ore)
Quartz overgrowth Hematite Illite > (kaolinite and chlorite) Iilite < (kaolinite and chlorite) Kaolinite corroding quanz Strons chloritization Dravite corrodes quartz and impregnateskaolinitematnx Microbrecciation Pyrite Carbonatesreplace kaolinite. quartz and dravite Hematite on siderire and Fe-hvdroxidesin pores Illite replaceskaolinite and dravite Uranium (ppm)
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and other alteration products are present in varying amounts, filling veinlets and as impregnations. Late kaolinitization overprinted locally the earlier clay assemblage(Key Lake, Ruhrmann 1986; Candy Lake Pod at McClean, Wallis e t a l . 1 9 8 6 ) ( F i g s .5 . 7 , 5 . 1 0 ,5 . 1 1 ) . The alteration halo around deposits extends for several kilometers along strike of minerabzed structures and laterally and vertically for tens to hundreds of meters distant within the Athabasca Group (500m wide and 200 m high at Cigar Lake and Midwest). In the basement the alteration halo is less extensive (ca. 100m deep at Cigar Lake). The geometrv of an alteration halo exhibits in a generalized mode a zonation as shown for Key Lake in Fig. 5.11 and Table 5.12 and a dispersion pattern as documented for the Cigar Lake deposit (Fouques et al. 1986) as foliows (from top to bottom):
-
-
Quartz zone: irregular, diffuse silicification and lining of fractures lined with euhedral quartz. CIay altered zone: inqease in clay content of up to 30% of the rock. friabilization of sandstone resulting locally in loose sand. Massive clay zone at the unconformity that surrounds the main zone-of mineralization: three distinct illitic facies are discerned, (a) soft gray to white clay dominantll' composed of Mg-illite locally associated with siderite, (b) soft gray clav consistingof FeMg-illite and chlorite, and (c) indurated dark green clal' composed of Fe-rich illite, FeMg-chlorite and Fe-rrch kaolinite with siderite. Facies (c) forms the core and is surrounded bv facies (a) and (b).
Crystalline Basement -
Athabasca S.ediments -
Pvrite-carbonate zone (lessthan 40m from ore)
Gray altered zonei gray coloration of the normally pink to white sandstone due to reduction of ferric iron and formation of finely dispersed Fe-sulfides associatedwith dissolution of the siliceous matrix and simultaneous increasein clay' content.
Massive ciay' zone (unconformit-vclay): graygreenishMgAl-chlorite (sudoite)and Mg-illite. locally abundant calcite, sidente. and globuies of organic material, occasionallydravite and phosphate (goyazite). All metamorphic textures have been destroyed. Altered basement with rock textures preserved: decreaseof alteration with depth; near unconformitv intense argilitization with illite/sericite-chlorite,destruction of graphite
Examplesof Unconformity-Contact-TypeUranium Deposits
tot
within the upper few metersbelow the unwith pitchblendeand/orcoffinite.Depositsin conformity. widespreadcorrosion of quartz, the CarswellStructurecontain in addition to zircon,and monazite.From 50 to 100mbelow of the EastAthabasca the mineralassemblages the unconformitythe alterationis mainly redistnctTe, Se,Bi, and someAu and Ag. This flectedby more or lesssericitization/illitization second stage supposedlyformed between of feldsparsand other amenablerock conabout 1250and 900m.y.ago. - Episodesof rejuvenationbetweenca. 300and stituents. Basement immediately below the deposits 100m.y.aeo modifiedthe earliergenerations consists essentially of Aphebian metaand redistributed metallic elements along sedimentscontaininggraphiticintercalations cataclastic zonesinto Athabascaarenites. but other than that no preferenceto specific lithologic facies or particular stratigraphic levelsis obvious. Metallogenetic Concepts Most depositsare positionedaboveor near, and trend parailel to the intersectionof a PreviousModels prominent basementfault, often a reverse fault, with the Meso-Helikianunconformity. Debateon the ongin, nature,andchannelways of These faults apparently follow ancient metalliferoussolutions,concentration processes. (Hudsonian?)mylonitezones. and their respectiveimportance for the huge varyaround metal concentration in Orientationof the mainstructures the Athabasca NE-SW. unconformity-contactdepositshas ranged from Oblique fauits dissectand displacethe main supergeneconceptsto hypogeneconsiderations. structures. Three principal modelshave beenforwardedfor Major ore bodies are cigar-shaped(Cigar the initial U inrroductionand the formationof the Lake, Kev Lake) elongatedalong NE-SW mrneralization structures. Other deposits occur in similar diagenetichydrothermal, shapes or form pods aligned along such hypogenehydrothermal,and structures(McClean,Dawn Lake). - supergenewith diageneticoverprint. Mineralizarionis polymetallicand may include U, Ni, Co, Cu, Pb,Zn, Fe,Mn, Mo, Bi, Sb,V, Diagenetic hydrothermal model: The evidence As, Te, Se. Au, Ag, and Pt-groupelementsin supportingthis model is the significanthost rock varying but mostly subeconomicamounts. alterations surrounding depositsthat are conSome elements.notablyNi, Pb, and As are sidered to have been derived from solutions locally as abundantas U as at Key Lake. Co within the Athabasca Group based on fluid and Ni is frequentat Midwest.Ni, Pb, Ag, and inclusionsand stable isotope data. The salient Au in the CoilinsBay A andD zones,Ni, Co, componentsof this model are late diagenetic Pb, and lv{o in Cigar Lake and Au, Te, Se in processesthat have generatedcirculation of at the D ore body, CarswellStructure(all except leastfwo fluids.One fluid wasorygenated.saline, the preciousmetalsin the rangeof permilleto and camed metailiferousions, and migrated at a t'ewpercent)(Fig.5.8.Table5.7). the baseof the AthabascaGroup. The other fluid Uranium and associatedmineraisare present wasreducingand ascendedalongfault zonesfrom including the basement. Precipitation of mineralization in several parageneticassemblages two early stagesof mineralintroductionand at occurredwhere thesefwo solutionsinteractedat leasttwo more stagesof redistribution(Tables the Meso-Helikianunconformity. Pagel (1975) and Pagel and Jaffrezic (1977), 5.4,5.s). The first stage is essentiallya monometallic based on fluid inclusion studiesof gangueand containingurani- associated minerais from Rabbit Lake and uranium-sulfideassemblage nite/pitchblendedatedbetween1400and 1250 CarswellStructuredeposits.assignto particular diageneticsolutions heated to temperaturesof m.y. (ca. 1150m.y.in CarswellStructure). The secondstageis polymetallic.In the eastern about 200"Cthe major role of depositformation. Athabascaregion, mineralizationis composed Theseauthorsconsiderthe basementrocks to be of arsenides. sulfarsenides,and sulfides of the uraniumsource.Pagelet al. (1980)proposea variousmetals but dominatedby Ni associated polyphase evolution for the mineralizing pro-
162
5 Selected Examples of Economically Significant Types of Uranium Deposits
cesses.During an interim stage uranium must period to be a major impetusin the formation of and mineralization.Dahlkamp assumesa multi-stage have been mobilizedby pedogenicprocesses part of it was trapped in organicmatter in lagoons metallogenetic evolution starting with synsedienrichedin evaporiticelements(Mg, B, and Li). mentary U accumulation in Aphebian time, Subsequentdiagenetic hydrothermal processes succeededby remobilizationwith local upgrading produced the deposits, deriving the uranium during the Hudsonian Orogeny, followed by as a prominentinterim stage either from the lagoonal sediments or directly supergeneprocesses (i978) to protore or ore grade. and metal either in collection Sibbald from the basement.Hoeve (1980), work events subsequently reon at diagenetic based their Certain and Hoeve et al. presumably further concentrated mobilized and Rabbit Lake and basin-widestudiesof alteration the mineral aspropose a diagenetic and finally recrystallized earlv hydrothermal features.also present primarv to now Athabasca to the but consider the Group semblage model they be the principal sourceof metals(seediscussion uraninite/pitchblendegeneration. Critical paramodel are (a) the presence metersof a supergene further below). containinganomalousamounts Hypogene hydrothermal model: Little (1974), of metasediments Morton (1977),Kirchner et al. (1980),and more of not refractorybound U and other metals, (b) recentlyvon Pechmann(1985)advocatea hypo- a climate changingfrom subtropicalto arid. as gene(magmaticor metamorphic?) hydrothermal reflected by lateritic weathering profi.les and (c) liberation by red bed sequences, hypothesis.Von Pechmann(1985),basedon his transgressive extensivemineralogical studies of the Key Lake weatheringof leachablefractionsof U, Ni, etc. deposit, arguesthat ore mineralogy,associated and (d) collection,at leastpart thereof in surface gangue minerals, and their intergrowth are depressionsfilled with argillaceous,evaporitic identical to those of classicalhypogenehydro- sediments or in near-surfacestructures perhaps thermal U-Co-Ni-Ag-Bi vein deposits,that the comparableto surficial duricrust deposits such as ganguein the depositsconsistsof true gangue Yeelirrie. Australia. and surficial structure fi.lls minerals, and that the temperatureof ore for- such as Daybreak,USA, respectively.The prinmation is relatively high. Consequently von cipal problem with the supergeneconcept is the Pechmann (1985) proposes that hydrothermal practicallyimpossibleproof for a pre-Athabasca fluids producedthe intensealterationsupposedly preconcentrationof U, Ni, and other metals. during a period of cataclasis.Ore-forming ele- Only conditionalevidenceby analogy with prements were introduced by hypogene fluids at sently known surficial depositssuch as Yeelirrie, the time of alteration with deposition of collo- Australia, and the apparentlack of unconformitymorphousbotryoidalpitchblendefollowedby Ni- contactdeposits,elsewherein the world in areas arsenidesand perhaps Ni-sulfarsenides.About of geologysimilarto the AthabascaBasin region 1400to 1250m.y.ago the original U-oxide phase but devoid of lateritic paleoweatheringof the recrvstallZedto uraninite and Ni was redistri- basementcan support a supergeDehypothesis. buted or perhaps introduced at this stage in The later overprint by diageneticor other proresponseto either elevatedtemperaturesdue to cesses related to the Athabasca Group has the thick pile of Athabascasedimentsor intrusion masked all earlier signaturesof paleo-surficial of diabase.Later eventscausedmodificationand mineralizationin the Athabascaregion. In adredistributionof the original ore minerals. dition, the apparent radiometric ages of ore The major shortcomingof a hypogenemodel is formation(1400to 1250m.y.)whichpost-datethe the lack of any magmatic or regional meta- AthabascaGroup (ca. 1500m.y.) can also be morphic event to provide a sourceof metalsand taken as an argumentapparentlycontradictinga generation of hypogenehvdrothermal fluids at supergene pre-Athabasca metallogenetic event. the time of suspectedoriginal ore emplacement. Diabase intrusions may constitute a driving ProposedModel a PolyphaseEvolution of power, but cannot be considereda source of Unco rmity- for nf o contact Uranium D ep osis uranium. Supergenemodel: Knipping (1974),Langford Interpretation and synthesisof pertinent data (1977),Dahlkamp (1978,1982),and Dahlkamp leadsto a state-of-the-art metallogeneticconcept. andAdams(1981)amongothersconsidersurficial This concept includes a complex, polyphase processesduring the pre-Athabascaweathering evolution of the geologic environment and its
Examplesof Unconformity-Contact-TypeUranium Deposits
uranium and metal endowment which ultimately culminated in the formation of unconformitycontact depositsin the AthabascaBasin region. A condensedversion of the model with the principal uncertainties, ambiguities and remaining problems is discussedbelow. Critical factors that constrain such modelling include: -
source of uranium and associatedmetals; interim stagesinvolved in mineralization; source, chemistry and redox potential of solutions for liberation and transport of U; - conditions for precipitation of U; - cause for restriction of deposits - to the Meso-Helikian unconformity; - to the intersection of basement faultsi mylonite zones with the unconformity; - to seiectedsites at these structuraifeatures; - with respect to the clay-bound deposits their intimate association with argillaceous material topping the unconformity; - source and/or nature of reducing agents or solutions; - channelways for metalliferous fluids; - energy resources for generating circulation of fluids involved in mineralizing processes; - origin of the argiilaceousmasshosting the claybound deposits; - importance of evaporitic sediments and their constituents particularly K, Mg, B and Li; - role of metasomatism of Na (albitization), Mg (doiomitization and phyllosilicate modification) and B (tourmalinzation); - role of paleoweathering; - interrelationship of diagenetic, specific alteration and mineralization Drocesses.
IOJ
heritage.Anatexisor granitizationconcentrated uraninite and monazite aggregatesin certain pegmatiticandgraniticintrusionsor segregations. (seesectionPotentialSourcesof Uranium). Where late to post-orogeniccataclasticground preparationopenedpathwaysuraniumand other metals may have been mobilizedand expelled along with hvdrothermalvolatile fluids to form epimetamorphicveinlike depositsof the Beaverlodge type (seeSubtype2.2, Cbap. 4 and next paragraph)whichnow mayconstitutethe rootsof the fracture-boundciassof unconformity-contact type depositsas, for example,EaglePoint. The Hudsonianevent possiblyended in the formationof extensive,locallyin excessof 50m wtde mylonite zones. Becausemany Athabasca depositsare locatedat or nearthesemylonites,it is speculatedthat the mylonite zonesmay have acted as channelwaysfor metalliferousfluids or even as a preconcentrationtrap for uranium mobiiizedbv retrogrademetamorphicprocesses.
Pre-AthabascaWeatheringand Surficial Preconcentration of Uranium
During Paleo- to Meso-Helikian time, approx. 1700 to 1500m.y. ago, an extensiv€period of weatheringand erosion affected the crystalline basement. During this period the region was located berweenlatitudes 20oand 25'N (Fahrig and Jones 1969, Seyfert and Sirkin 1973) suggesting a semitropical climate. An extensive chemical weathering profile of lateritic nature more than 50m deepin solidrocksand asmuchas 150m along faults was imposedon the rocks as indicated by the present regolithic profile. Uranium and other metals must have been U ranium M o b iliz anon D uring Ap hebian during this episode.Although the bulk mobilized Sedimentation and Metamorp hbm of the liberateduraniumwasflushedinto the sea, granites Certain Archean in Northern some of the uranium was probably caught and Saskatchewancontain elevated U contents.It is concentratedin pedogenicmaterial or in catapostulated that in Middle to Upper Aphebian clasticzonessimilarto that in presentday surficial time part of this U was leached and redeposited uranium occurrences(see Chap. 4.6).This in pelitic to psammitic sediments which now concentrationcould have occurred particularly during the final stage of this episodewhen the constitute the Wollaston Group. Metamorphism of the Aphebian strata perhaps climatechangedto more arid conditions. Such pedogenicpreconcentrarionsof uranium during the Blezardian but definitely during the Hudsonian Orogeny recrystallized and locally are reported by Tremblay (1982). He analysed further concentrated the sedimentary uranium. uranium concentrations ranging from close to Disseminations and narrow stringers of uraninite zero to 0.05% U (average0.033%U) in brown to oriented more or less parallel to the bedding/ red regolithic clay but noted that it is unknown schistosity of the metasediments, reflect this how much of this uranium precipitated with the
1U
5 Selected Examples of Economically Significant Types of Uranium Deposits
regolithand how muchwaslater. Similaranomalous lJ, Ni, Co, As etc. concentrationshave been drill intersected in clayey material atop the unconformity in other properties distant from deposits.(E. Stewart,pers. commun.) The common associationof U with Ni and the often high gradesof Ni in the clay-bounddeposits posesthe questionfor Ni preconcentrationsby the existing Meso-Helikian ciimatic conditions. Cireumstantial evidencefor this is provided by the garnierite minerelizationin New Caledonia where a subtropicalclimate like that postulated for the pre-Athabascatime il Saskatchewan has formed extensivesupergenenickel deposits.This indicatesthat similar processescould also have period in beenactiveduringthe paleo-weathenng northern Saskatchewan. Although circumstantialevidencessuggesta surficial preconcentration of uranium and possiblynickel dunng the pre-Athabascapaleoweathering period it remains unceftain whether such an interim stageis required in the metallogenesisof unconformity-contact deposits.Again, circumstantialconsiderations suggesttheir necessity, becauseelsewherein the worid, important type depositshavenot been unconformity-contact discovered in terranes of comparable geology which are devoid of the regolithization typical for the Athabascaregion.
900m.y. ago, an early stage of true diagenesis succeeded by a later stage of hydrothermal activity. The latter coincided with the main mineralizing event. The two alteration systems involve oscillatingoxydizing and reducing processesactivein the AthabascaGroup and in the crystalline basement along the unconformity. Both systemsmay have been interconnected. a) Early diageneticstage:Distinct clay mineral assemblages in each formation indicate that the flou' of diagenetic solutions was confined to the various formations. Temperatures reached between200-250"Cat the baseof the Athabasca Group. Physico-chemical conditions were oxydizing, as reflected by widespread specular hematite. Fluid inclusions diagnose brines as fluids responsiblefor the diagenetic alteration (Pagel1975a,1975b). Fluids of this physico-chemicalnature are capableof liberatinguranium from uraninite or from other leachablepositionsin host minerals (U adsorbedon clays, etc.). As such, these early diageneticsolutionsor descendantsthereof mav constitute a salient prerequisite for ore formation. b) Hydrothermal event and associated main stageof ore formation (approx. >1400 to 1100 rn.y.): Concurrentgeologicaleventsthat characterue this episode are brittle deformatiom, hydrothermal alteration, intrusion of diabase dikes(afterca. 1300m.y.).and the main stageof Diagenesisand RelatedOre Forming Processes in mineralization. Meso-Helikian Time lmportant channelways for the solutions During Meso-Helikian time, approx. 1500- responsiblefor alterationand mineralizationwere 1300m.y. ago,the climatechangedto a more arid extensivefault zonesin the basementand perone. Continental,arenaceousred bed sediments meable arenite beds, in particular the locally of the AthabascaGroup were depositedon this presentbasalconglomeratic unit of the Athabasca surface. In some areas, for example in the Group. Other migration paths proposed are CarswellStructure,an argillaceousfacies,up to either the unconformity-regolithinterface or 50m thick, of pre- or early'Athabascaage was paleotopographicfeatures such as scours or laid down in small, local depressions(Pacquet depressions incisedinto the basementalong fault and McNamara1985). zones. Uranium content is ver_r'iow in Athabasca Strong developmentof Mg- and Al-chlontes sandstonesin the order of l ppm U, and that and iliites, and Mg-tourmalinewithin and around appcars to be contained largely in refractory mineralization attests to intense hydrothermal heavy minerals.(Earle and Sopuck 1987,report activity by Mg-, Al-, and B-bearing solutions. for the easternAthabascaBasin an averageof Stable isotope and fluid inclusion data (see 0.37ppmU determinedby partial extraction= section Stable Isotopes and Fluid Inclusions) fluorimetry following HN03/HCl extraction). suggestinvolvementof severalfluids of varying Accordingly, the Athabasca sandstones are composition.One fluid was a very salinebut not rejectedas a potentialsourceof uranium. always saturatedbrine (30% NaCl equiv.) of Two principal alteration systemsaffected tbe oxidizing nature, as indicated by hematite Athabasca Group, between approx. 1500 to plateletsin the inclusions(Pagel et al. 1980).
Examplesof Unconformity-Contact-TypeUranium Deposits
Evidence for circulation of reducing fluids is provided by destruction of hematite, bleaching, and sulfidizationin the Athabasca Group. Consequently a reducing front must have invaded an o t h e r w i s eo x y g e n a t e de n v i r o n m e n t . Mellinger et al. (1987) address the phvsicochemical environment that prevailed during phyllosiiicate alteration in the eastern Athabasca Basin. They noticed that the geochemicalsignature of this aiteration is very similar at all unconformity deposits. Illitization is always observed. A chlorite trend occurs at Midwest, McClean and Dawn Lake. This indicates that illite or chlonte were stable during uranium mineraiizationwhich attests to a pronounced activity of K- and/or Mg-ions in the solutions. The solutionsmust also have contained C02 becausesideriteis a common constituent in the alteration suite. Ferrous iron was available from decompositionof hematite. A later overprint of this alteration facies by chlorite is noticed at Maurice Bay (Mellinger et al. 1987) and by kaolinite at Key Lake (Ruhrmann 1986) and in the Candy Lake Pod at McClean (Wallis et al. 1986). The above criteria and processesare consistent with the physico-chemical conditions required for uranium mobilization and redeposition within the given environment. Because the phyllosilicate alteration initially resuited from oxidizing solutions, as indicated by Pagel'swork, it must have started prior to ore formation. On the other hand, these fluids were capable of leaching uranium from any accessiblesource. Two alternatives for possible uranium collection by fluids residing in Athabasca sediments proximal to the unconformitv can be contemplated. Uranium was either already collected by solutions during the earlv diagenetic stage when the environment was clearlv oxygenated, as mentioned earlier. or accumulatedat the onset of the later hydrothermal stage. Constraintson both scenarios are as follows. Scenario 1: Access of fluids to a uranium source in the basement is thought to have been very limited due to insufficient structurai pathways. Therefore solutions could coilect uranium only from sites near the unconformity, i.e.. essentiaily from uraniferous regolith. This conclusion is based on the assumption that relative stable tectonic conditions prevailed during this period and that therefore access to permeable structures required for water percolation in the basementwas limited.
165
Scenario 2: With the change of this situation which occurred just before and at the time of the intrusion of diabasedikes, a prerequisite for the second scenario was accomplished. Structural preparation of the basement provided accessfor enough oxygenated fluids to buffer the reducing effect of the basementlithoiogies and to maintain sufficient oxidation potentiai to leach uranium f r o m t h e s el i t h o l o g e s . Potential basement uranium sources could have been uraniferous rock constituents, for example synmetamorphicstrata-bound uraninite. U - e n n c h e d m y l o n i t e z o n e s .a n a t e c t i c - p e g m a t i t i c mineralizations, and Beaverlodqe n'pe pitchblende veins. Uraniferous regolith could have also been a good complementary source of uranium. In the caseof an antecedantsurficialdeposit,as assumedin a supergenemodel. this depositwould have been largely destroyed by higily oxygenated fluids. but not if the fluids had an oxidation potential below the destructionlevel. In this case. modification of all pre-existing uranium mineralizations independent of r_voe (surficial or basement hosted veins) was probably restricted to localized remobilization, disequilibration and recrystallization of the original uranium associated with a resetting of the U-Pb time clock. The next step in the mineral accumulation requires a reducing media to reduce the hexavalent uranium for precipitation of pitchblende. Again, two scenarios and perhaps a third one can be envisaged- The first two are based on reductants provided from the basement and the third on posrulated organic material contained in localized pre- or early Athabasca lacustrine (?) sediments. Basement reductants rvere very likely sulfides, as can be deduced from suifur isotope ratios which are identical in both basement sulfides and ore assemblages(Wailis et al. 1986). Although often proposed. graphite does not tend to be a vaiid reductant. Thermodynamic considerations prohibit a direct involvement. Destruction of graphite to tbrm bitumen or other hydrocarbons which constitute effective reductants apparentlv post-dated the mineralization according to Landais and Dereppe (1985) (see later). The importance of graphite is most obviously on the structural side. Graphitic horizons provided the least competent rocks for structural stress hence becoming the site for faulting. The three possibilities of reducing agents and their impact on uranium precipitation are:
.';, 'a-:.'
166
5 Selected Examples of Economically Significant Types of Uranium Deposits
a) Reduction by ftxed host rock cottstituents (mainly sulfides):Uraniferous ffuids must have percolatedalong permeablestructuresbelow the sulfide-depletedregolithic section of the basement to interactwith reductantssuchas sulfides. A mechanismlike this cannot account for the formation of clay-bounddepositspositioned at the unconformity.But it mav at leasttheoretically explainthe generationof fracture-bounddeposits of class1.1.1.A reasonhasto be found,however, for restriction of these depositsto a few sites when it has to be assumedthat formation of the deposit should have taken place wherever adequate reducing conditions were provided by host lithologiesunlesssuch conditionswere limited to the sitesof presentlyknown deposits, whichis very unlikell'. b) Reductionby reducingsolutions:Solutions of reducing capacitycould have evolved from watersor from Athabasca eitherconnatebasement waters invading reducing basement environments. In the latter case the solutions should not have been enriched in uranium. because the uranium would have already precipitatedin earlier. Whateverthe the basement,as discussed origin of the reducing fluids may have been, position, configurationand mineral paragenesis of the clay-bounddeposits straddling the preAthabascaunconformitydemandsolutionseither and most likely, upwellingalong fault zonesfrom the basement,or migrating along channel-like at the unconformity.Only morphologies/features under these preconditionscould the solutions arrive at the given sites to interact with uraniferous fluids. The uraniferousfluids were either stagnantin basalAthabascaarenitebeds,or more probably, were migrating along the intersection of the unconformity with the structuresfrom where reducingsolutionswere expelled. c) Reductionby sedimenthostedorganics:A final possibility for a reductant is provided by organic matter that may have originated from algaeformed in smalllagoons,ponds,etc. which containedevaporiticand argillaceoussediments presumably derived from lateritic outwash material of the Meso-Helikian pediment. Sediments of this nature are reported, for example, from the CarswellStructure (Pagel et al. 1980, Lain6 1986). Suchan origin of the organic matter found in the argillaceoussedimentsis rejectedby Landais and Dereppe(1985).They considerthe bitumens found at the depositsto be derivativeof graphite
from the underlying basement. Perhaps new support for a nongraphitic origin has emerged from Leventhal et al. (1987). These authors report an "unusual uranium-organic matter association"in the Claude deposit, Carswell district,which enclosesfragmentsof U minerals that crystallizedprior to their incorporation in the organicmaterial.The authorsreject graphiteas a source. In summary,the abovelistedgeochemicaland mineralogicalcritena combined with contemporaneousregional geologicalevents may have contributed to the evolution of the deposits as follows. A regional thermal/tectonicepisode initiated perhaps bi' broad epeirogenic uplift partially coincidentwith the Grenville Orogeny coupled with late low-grade burial metamorphism/ alterationin the basementand/or diagenesisin the Athabasca strata were fundamental for generatingthe processes for the formation of the depositsas we know them to day. Steepeningof the geothermalgradientbv regional intrusion of diabase dikes (Hoeve and Sibbald 1978) or, alternatively,by radiogenicheat generatedfrom concentrationof uranium in the basement(Fehn et al. 1978,Tilsley 1980),may have induced a large-scale convection of diagenetic pore solutionswithin the overlying Athabasca sedimentsand to some extent in the basement.Significant K- and Mg-ion activify and oxidation potential cbaracterued these solutions. The elementsin thesesolutionsare assumedto have beenderivedfrom decompositionof evaporitesat the baseor within the AthabascaGroup. Concurrenttectonismdue to isostaticreadjustmentin periodicuplifting gaverise responseto successive to thrust faulting and reactivationof old shear and mylonite zones which ma1'have contained preconcentrations of uranium. This faulting increasedsecondarypermeabilityand, consequently, allowedthe orygenateddiageneticsolutions accessto a number of sourcesof uranium and other metalsin basementand regolith. The crucial criterion for the next step of deposit formation is the oscillation of groundwatersbefweenoxidizingand reducingconditions along the unconformity. During an initial stage when the solutionswere sufficientlyoxygenated, they leached and collected uranium, as mentionedabove.Due to convectivecirculationthese uraniferoussolutionsascended,at least partially to the unconformitv and into basal oermeable
Examplesof Unconformity-Contact-Type Uranium Deposits
Athabascabeds forming a reservoirof uranium in solution. Dunng the course of this process the oxidation potential of later invading fluids diminished by reaction with basementreductantsand finally turned into reducing. It is debatable whether the solutions achieved their reducing nature in the basementlevel of previouslyuranium leached rocks due to renewed cataclasis permitting accessto virgin ground. or at specificsites of significant enrichments of reductants such as sulfides, or bv reaction with organic material in lacustrine sediments resting locally on the unconformity. or a combination of the one with the other. In a scenario for the formation of cla1,-bound depositswhere the reducing fluids originated in the basement and then migrated upward along fault zones to the unconformity, these fluids generated a redox front at the interface with the postulated oxyeenated uranium-bearing fluids residing in basal Athabasca beds. Under these conditions. uranium should have been reduced and pitchblende precipitated along this redox interface at the intersection of the cataclastic channelway and the unconformit-v. In such a model, the ore-hosting clay masses may have derived from fault gouge sqeezed out at the unconformity. Such a clay origrn poses a problem, however. Gouge normally seals fracture zones and hence would prohibit the postulated migration of the reducing fluids. Under the assumption that lacustrine sediments containing organic material existed. precipitation of pitchblende took place where ascending or lateraily migrating uraniferous soiutions invaded these environments. Such a reducing matter combined with the adsorptive capability of the clays provided a highly favorable trap to capture and fix the uranium. This could explain the restriction of the clay-bound class of deposits to the argillaceoushost rocks at the unconformity, and at the same time the orign of the clay masses without the nesative impact of gouge derived clays. With respect to the formation of the fracturebound deposirs which occur in the basement proximal to but below the unconformity, two scenarios of uranium precipitation can be envisioned. The same processesmay have been active, as discussed earlier, but due to a lack of argillaceous beds topping the unconibrmity, the basement strucrures provided the only suitable site of uranium deposition. Or alternatively, if
t67
it is assumed that these deposits derived from deeper-seatedBeaverlodge-typeveins, the fluids involved caused only local remobilization and redistribution of the uranium along these fractures. Fluid inclusionstudiessuggestthat the original mineral formation of unconformity-contact depositstook place at temperaturesof about 200'C. Stable isotope dating of phyllosilicatesrelated to mineral accumulationyields agesof about 1,400to 11ffim.y. These ages concur with U-Pb ages of the oldest identifiable pitchblende/uraninite generation in all multimetallic cla;"-bound deposits in the Athabasca Basin. Although the multimetallic mineral assemblage and associatedphyllosilicatesin clay-bound deposits tend to have formed by the same processesas proposed by Pagelet al. (1985), precipitation of uranium and other metals did not occur simuitaneouslyas documentedby Ruhrmann and von Pechmann(1987) for the Kev Lake mineralization, and by Fouques et al. (1986) for Cigar Lake. Ruhrmann and von Pechmann (1987) note that the oldest pitchblende/uraninite generation formed at about 1255m.y. ago (corrected to ca. 1400m.y. by Carl et al. 1988),whereas the introduction of arsenides and sulfoarsenides occurred at about 900m.y. ago. Pagel et al. (1985) also report another episode at about 900m.y. ago which introduced a multimetallic minerai assemblage different to the preceding one, and the formation of a distinct Mg-tourmaline. The nature and origin of the ffuids involved in this second stage of mineralization remain open to debate. But in anv case,they must have been of reducing character because otherwise they would have destroyed the older urantum ore.
Redisrribution of Lranium in Hadn,nian to Recent Time Subsequent to the important Meso-Helikian event. penodic mild disturbancesand reactivation of structures permitted influx of meteoric waters that causedlocal redistribution of the mineralization. This is reflected in younger ages of modified and new ore and gangue mineral assemblages, and the introduction of uranium up into the Athabasca sediments. Three principal stages are documented by isotope dating: At about 900m.y.. as mentioned
168
5 Selected Examples of Economically Significant Tlpes of Uranium Deposits
above,at about 300m.y. and at about 100m.y. and younger. At about 300m.y., i.e. duringthe time of the HercynianOrogenyelsewhere,it is proposedthat renewedcataclasisof the older mineral assemblages occurred as a result of uplift of the Athabascaregion. It was accompaniedby multiple remobilization and redistribution of metals into wall rocks and up into sedimentsof the Ath-abasca Group, combined with hydrothermal carbonateand clay alteration.Ruhrmannandvon Pechmann(1987)noticedthat sphaleriteappears during this stage, suggestingthe introductionof new material.at leastzinc. Ruhrmann (1986)ascribesthe kaolinite developmentwithin the Key Lake depositalso to the 300m.y. event, which he attributes to a basrnwide event that overprinted the earlier illite aureole. Stableisotope data are interpretedby Wilson and Kvser (1987) to indicate that the kaolinitization occurred at temperatures of about 50"C. In recent times, less than 100m.y. ago, shallowstructural adjustmentresultedin vertical displacement of deposits. Processesdue to modern meteoric waters causedhematitization, limonitization and altered and hydrated preexistinghydrothermal illites and partially reset their K-Ar ages(Wilson and Kyser 1987).
Referencesand Further Reading for Chapter 5.1 (for detailsof publicationsseeBibliography) Alcock 1936: Artru et al. 1986; Beck 1986: Clark et al. 1982; Clarke and Fogsdll 1985; Cramer and Vilks 1987; Dahlkamp 1978; Dahlkamp and Adams 198i; Dubessy et ai. 1989: Earle and Soouck 1989a. 1989b: Eldorado ResourcesLld 1986, 198?: Farstad and Avers 1986; Fogwill 1985;Fouquet et al. 1986: Gatzweiler et a|.7979; Glackmever K.. person. commun; Halter et al. 1989; Harper 1978; Harper et al. 1986;Heine 1986:Hoehndorf et al. 1987; Hoeve 1984: Hoeve and Quirt 1986. 1987; Hoeve and Sibbald 1978; Homeniuk and Clark 19861 Hubregtse and Sopuck 1989; Ibrahim and Woo 1985; Jone31980;Knipping 1974'.1-ain6.1985, 1986;Lain6 et al. (eds.), 1985; l-angford 1978, 1986; Lehnert-Thiel K., person.commur;, Lewrv and Sibbald 1980;Mac Donald 1980,1985;Mazimhaka and Hendrv 198'1 ,1989; McMillan 1977;Mellinger 1980, 1987;Mellinger et al. 1987; Money 1968; Money et al. 7970; Munday 1979; Pagel 1975a, 1975b;Pagel et al. 1980, 1985; Pagel and laffrezic 1977; Pageland Svab 19851Parslow 1989;Parslowand Adamson 1982; Parslow and Thomas 1982: Philippe and Lancelot 1988;Quirt 1985; Ramaekers1983;Ramaekersand Dunn
1977; Ramaeken and Hartling 1978; Ruhrmann, G., person.commun; Ruhrmann 1987;Ruhrmann et al. 1987; Ruhrmann and von Pechmann 1989; Ruzicka 1984, 1985, 1986.1987.1988;Sibbald1985.1988:Sibbaldet al.1976. 1.977.l98L; Sibbald and Quirt 1987; Sopuk et al. 1983; Stewart E., penon. commun; Tan 8., person. commun; Tremblay 1978. 1982, 1983; Wallis et al. 1986; Wilson and Kyser 1987; Wra-v 1985; References of workers on individual deposits see Evans (ed.), 1986; Laind et al. (eds.).1985;Tremblay 1982.
5.2 Examples of SubunconformityEpimetamorphic-Type Uranium Deposits (Type2,Chap.4) 5.2.I Subunconformity-Epimetamorphic Uranium Depositsin Not Albitized Metasediments:Alligator Rivers Uranium Field, Australia TheAlligatorRiversUraniumField(A.R.U.F.) liesalongthecentralstretchof the EastAlligator ArnhemLand, N.T. The Riverin northwestern districtcoversan areaof about200km long in NE-SWdirectionandis about100kmwide(Fig.
s.r2).
The major deposits in the district are Ranger One, Jabiluka, Koongarra, and Nabarlek. Jabiluka, Koongara, and Ranger One are composedof two or more ore bodies.Jabilukais the largestsingle uranium deposit of the world by tonnage.It contains2O4000mtU3Osat an ore gradeof 0.39% U:Or. Total reservesof the Alligator Rivers Uranium Field includingproductionamount to ca. 370000 mtU3O6(Batteyet al. 1987). All depositsare strataand structurecontrolled emplaced in metasedimentsbelou' a distinct Middle Proterozoicunconformity. It is for this setting that they are here defined as subunconformity-epimetamorphic type urantum deposits(subtype2.7, Chap.4). Fergusonand Goleby (1980)edited a major volume on Uranium in the Pine Creek Geosynclinewith articlescoveringalmostall geoscientific fieldsand giving a stateof the art presentationof the ongoingresearchwork. Ewers et al. (1984) published a concise and comprehensivecompilation and synthesison the Pine Creek Geosynclineand its uranium mineralization.Other
Examplesof Subunconformitv-Epimetamorphic-Type Uranium Deposits 1320
l3lo I
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169
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B.T 3othurst Terroce D.R.3 Doly RiverEosin McA.A. Mc Arthur 3osrn
Fig. 5.12. Pine Creek Ceosvncline. generalizedgeotogicalmap with situation of the three pnncipal uranium districts (surrounded by tlashed line\ and minor uranium deposits. Former U mines: 1 Rockhole; 2 El Sherana and Palette: J Coronation Hill: J Sleisbecktj Fleur de Lys: 6 Adelaide River and George Creek: 7 Rum Jungle Embavment area: 8 Rum Juneie Creek South (After Battev et al. 1987)
reviews have been provided by Battey et al. lowing descriptionis primarily based upon the ( 1 9 8 7 ) , N e e d h a m ( 1 9 8 5 ) ,N e e d h a m e t a l . ( 1 9 8 0 . papersof theseaurhorsamendedby data of the 1988), and Stuart-Smith et al. (1980). The fol- other authorslisted.
170
5 Selected Examples of Economically Significant Types of Uranium Deposits
GeologicalSetting of Mineralization
orogenic event affected the whole of the Pine Creek Geosyncline. Metamorphism associated The Alligator Rivers Uranium Field is situatedin with the orogeny was strongestin the Alligator the northeastern portion of the Pine Creek Rivers region. Here it reached the stauroliteGeosyncline(Fig. 5.12). Principal stratigraphic almandinegrade of the amphibolite facies and units(Fig. 5.13,Table 5.13)are, from bottomto resulted in polyphasedeformationimpnnting a top, the Kakadu Group, the Cahill Formation, regional NW-SE-orientedschistositv.The intenthe lower member of which hosts all important sity of metamorphismdecreasedwestwardsto uraniumdeposits,and the NourlangieSchist.This greenschistfacies. The amphibolite/greenschist sequOnceof Lower Proterozoic metasediments gradefaciesboundaryroughly coincideswith the and intercalatedmetavolcanics occupiesa folded, western limit of the Aliigator Rivers Uranium north to northwest-trendingsynclinoriumlving Field. between two prominent granitic and migmatitic The orogenywas accompaniedby a period of highs. To the west is the Nanambu Compiexof igneous activity stretching from about 1870 to Archean/Lower Proterozoic age (ca. 2470m. -v.. 1690m.y.During this periodpre-orogenic (Zamu all age dates from Page et ai. 1980)and to the Dolerite) and post-orogenictholeiitic dolerites northeast is the Lower Proterozoic or older (OenpelliDolerite).and numeroussyn-and postNimbuwah Complex. orogenicplutons of biotite granite, adamellite, At about 1870 to 1800m.y. ago, a regional granodiorite,and syenite such as the Nabarlek,
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Examplesof Subunconformirv-Epimetamorphic-Type Uranium Deposits
r71
Table 5.13. Allieator Rivers Uranium Field. synopsisof stratrgraphy(for spatial correlationsee Fig.5.13) (After Needham 1985amendedbv data from Ewers et al. 1984.reDrinredwith permissionfrom The Canadianinstituteof Mining & Metallurgy) Isotopicdates ( P a g ee t a t . 1 9 8 0 )
Unit
Thickness \lain rock tvpes (m)
Relationships
Vlarinesediments
Clay. silt. mud. sand and coqulna
Intenidalmuds.coastal and estuannealluvium and beachridges
Continentalsediments
Silt. sand, clav, sandv srltstone.black and brorvn humic soils
Creek and river alluvium transitionalinto estuarine deposits.outwashand colluvium.abandoned channeldepositsin estuanneplains OverliesEarlv Teritarv latente
Sandand colluvium
<75
sand. Unconsolidared ferruginousand clavevin places
Latente
<3
\odular, concretionary, Developedmainly on pisoliticand vermrcular deeplyweathered Cretaceousand Early Proterozoicrocks.Minor pedogenicand transported rypes
Marligur Member
<25
Poorly consolidated, rvellsorted and rounded fine to very coarse quartz sandstone. mrnor siltstoneand conglomerate
r -
0
Neocomian to Aptran
Interfingeringmarginal marine members,north of Oenpelli
O
Late Jurassicto Earlv Cretaceous
Darwin Member
<35
Fine argillaceous sandstone. siltstone, micaceousmudstone, radiolarianshale.basal conglomerate
PetrelFormation
<10
Sandstone. siltsrone
Contemporaneouswith and higher level continentalequivalentsof BathurstIsland Formation
Resionalunconformitv
5 2 2m . y .
Minor basalticdikes
<0.i
Basalticdike
IntrudesKombolgie Formationvolcanicseast of Ranger I
1 3 1m 6 .v.
Mudginbem Phonolite
<1
Phonolitedikes
IntrudesNanambu Complexaround Mudginbem homestead
1 3 1m 6 .y.
Maningkorrirr Phonolite
<1
Phonolitedikes
IntrudesNimbuwah Complexin northeast
1370-1200m.v
Dolente dikes
<2
Quartz dolerite dikes
Widespread.poorly exposed.steep- occupy major fracturesin Kombolgie Formation
N
!
€
172
5 Selected Examples of Economically Significant Types of Uranium Deposits
Table 5.13. Conrinued lsotopic dates (Pageet al. 1980)
Z;Z vE OL
Unit
Thickness Main rock types (m)
Upper sandstone
350
Sandstone.minor conglomerateand siltstone
Gilruth Volcanic Member
<5
Tuffaceoussiltstone, vesicularbasalt
Nungbalgarri Volcanic Member
0.170
Basalt,trachybasalt; rhvolite and ignimbrite at top; rare intercaiated sedrment
Lower sandstone
<300
Sandstone,pebbly and conglomeratic,locally ferruginous
Oenpelli Doierite
<250
Porphlritic and ophitic oiivine dolerite, quartz dolerite, grranophyric dolerite. minor serpentinite
Lopoliths forming basementridges to Kombolgie Formation
Edith River Group
l}W
Tuff, tuffaceous siltstone,altered rhyolite, microgranite
Valley fill volcaniclastics
>c E30
Relationships
Markedly unconformable on older units except Edith River Volcanics: membersgeneralll conformablebut upper sandstoneunconformable on lower sandstonenear Deaf Adder Gorge. with NungbalgarriVolcanics absent
Major unconformitl' 1688m.v.
G c
U
Unconforrniry
N
ca.1780m.y.
Nabarlek Granite
Altered biotite gfanite cut by numerous quartz-filled shears
0
ca.1755m.y.
Tin Camp Granite
Altered biotite granite cut by numerous quanz-fi.lledshears
ca.1730m.1'.
Jim Jim Granite
Altered biotite granite
El SheranaGroup
Rhyolite, greywacke, siltstone,sandstone, basalt
z
NourlangieSchist
v?
Unknown
Quartz mica schist, quanz schisr,cornmonly with garnet
>O
ca. 1800m.1'.
Marginsmostly faulted, post-orogenic
Probablemetamorphic equrvalent of Wildman Siltstone+ parts ofSouth AliigatorGroup and FisherCreek Siltstone
TransitionalZone
Quartz and quanz mica schistwith minor quartzfeldsparbands
Lit-par-Lit Gneiss Znne
Quaru, biotite,garnet, and amphiboleschistand gneisswith quartzofeldspathiclayers and pods
Contain resistersof Cahill Formationand Kakadu Group - probable metamorphicequivalent of thoseformations
Nimbuwah Comolex
Graniticto tonalitic gneissand migmatite, gTanite,pegmatite
Partly metamorphosed (granulitefacies) 1E70m.y.granite
Examplesof Subunconformity-Epimetamorphic-Tvpe Uranium Deposits
t73
Table 5.13. Continued Isotopicdares (Pageet al. 1980) ca.1800m.v
Unit
Thickness Main rock types (m)
Relationships
Regionalmetamorphismand deformation
ZamuDolerite
<300
Ortho-amphibolite, tholeiiticbasait
Sillsfoldeciand metamorphosed*,ith enclosinssediments
BurrellCreek Formation
1500 -5m0
Siltstone.shale.siate. grel'wacke. arkosoquartzite.schist. minor interbedded volcanics
Conglomeratebeds containclastsof South .{lligator fuver Group and MassonFormation
KapalgaFormation
<,5000
Chert-banded ferruqrnous siitstone. carbonaceous shaie
Southwesronlv: conformableon Koolpin Formation
;: .^
M o u n t B o n n i eF m . Gerowie Turf
Grervacke.shaie. In centerand rvestof siltstone : felsicvolcanics geosvncline volcanolithicgre1"wacke
Koolpin Formation
1000
Chert-banded ferruginous siltstone. shaleand carbonaceous dolomite in southwest. Bandedfemrgrnous quartziteand garnet graphiteschistin CannonHill area
Unconfbrmablein southwest.contaclsnot seenin Cannon Hiil area
Wildman Siltstone
2000
Bandedcarbonaceous siltstone,minor sandstone,grervacke. phyllite
Conformableon Mundogie Sandstone; westonlv
MundogieSandstone
<5000
Sandstone.quanzite. siltstone.phyilite. conglomerate
Probablvunconformable on MassonFormationand StagCreek Volcarucs. Coaner and less metamorphosed equivalentof Cahill Formation upper member. Southwestoniy
StagCreek Volcanics
1000
Mafic volcanicbreccra. Conformableon -Vasson hawaiite.tuff. tuffaceous Formation. southwestonly grevwackeand shale
MassonFormation
3000
siltstone. Carbonaceous sandstone. dolomite
! v D
2. r)
ca. 1870-2-100
z
Lowest exposed Proterozoicunit in southwest.Less metamorphosed.open marineeouivalentof Cahill Formation lower member
174
5 Selected Examples of Economically Significant Types of Uranium Deposits
Table 5.13. Conrinucd Isotopic dates (Pageet al. 1980)
N
1800-2500m.y rrl t9 -N oO
/:'
Unit
Thickness (m)
Main rock types
Relationships
Cahill Formation
3000
Mica and quartz feldspar schist,quanzite; lower member (600m) a.lso containscarbonate, calcsilicategaleissand carbonaceousschrst
Apparently conformable on Kakadu Group, may overiap onto Archaean of Nanambu Complex
Kudjumarndi Quartzite
1-50
Orthoquartzite+ mica, feldsparand hornblende; quartzgnelss
Confinedto northeast. Equivalent of Munmarlan, Quartzite. Conformable on Mount Howship Gneiss
Mount Howship Gneiss
2W0
Quanz feldspargneiss
Confinedto Mvra Falls Inlier. Equivalentof Mount Basedow Gneiss. Lowest known Proterozoic unit in east
Munmarlan' Quartzite
2000
Muscoviteand feldspar quartzite
Equivalent of Kudjumarndi Quartzite, transitionalinto Proterozoiccomponent ol Nanambu Complex
Mouni Basedo'*' Gneiss
>1500
Meta-arkose,gneiss. minor schist
Equivalent to Mount Howship Gneissimarginal to and transitional with Proterozoiccomponent of Nanambu Complex
Nanambu Comolex
Pegrnatoidgneiss, Mantled gneissdome Archaean partly muscovite-biotite granite,schist,pegmatite metamorphosed granite with accreted leucocratic Proterozoic metasediments(mainly from Kakadu Group)
Tin Camp, Jim Jim granites. and pegmatites were Host Rock Alterations intruded. Deposition of earl!' Carpentarian/lr{iddle Several episodes of alteration have altered the Proterozoic platform cover sediments. mainll, crystallinehost rocks of the uranium depositsand sandstone (<650 m thick) intercalated with to some extent the overlying Kombolgie Forbasalt, trachybasalt,and rhyolite (ca. i648m.r,.) m a t i o n ( F i g s . 5 . 1 4 t o 5 . 1 9 ) . P r o m i a e n t p r o c e s s e s of the Katherine River Group/Komboigie For- are chloritization, sericitization. hematitization, mation marked the beginning of the tectonic sibcification, minor carbonatization, desilicificastability whictr has persisted to the present day tion, and carbonate dissolution. Disruption and except for the intrusion of some phonolite and brecciation of certain strata accompanied some of guartz dolerite dikes in Adelaidean/eariy Upper the alteration stagesand/or mineralization. P r o t e r o z o i c t i m e ( 1 3 7 0 - 1 2 0 0 m . 1 ' . ) .A m a ; o r Ewers et al. (1984) recognized the following weathering period eroded 1 to 2km of the base- characteristicfeatures of alteration associatedwith ment forming a prominent unconformity prior to deposiu. the sedimentationof the Katherine River Grouo Chloritization is most prominent and occurs in (Ewers et al. 1984; see also Table 5.16). multiple generations in all deposits. It forms a
Examplesof Subunconformitl--Epimetamorphic-Tvpe Uranium Deposits
sw
175 NE
Reverse foutt brecc,o Secondory U ore Prrmory u are
Fig. 5.15. Nabarlek. SW-NE cross-sectionthrough the southernend of the ore body displayingthe distribution of primary (uraninite/pitchblende) and secondary (dominantly hexavalentU minerals) ore and alteration faciesin the inner halo. (After Wilde and Wall 1987)(reproduced from Economic Geology, 1987,v. 82. p. 1161)
removed
Fffi
Ort", holo.quortz deposiied
l-l-Tl tnn"r.hoto.quortzremoved Fouli
Ei:=':T:,.:lU o r e b o d y uiillriiLilil titli!iiii:t
Fig. 5.14. Nabarlek. map showingthe ore body (pnmary mineralization proiected to surface) and distribution of associatedalteratron. (Strike of metasedimentsis ca. NESW) (After Wilde and Wall 1987) (reproduced from Economic Geoloqv.1987.v. 82. p. 1160)
"x
I Outer holo
Inner holo
IIi
IV
0re
Post-ore veins
LOre olterotion
0uor tz C h l o r i te
Fig. 5.f6. Nabarlek. parageneticschemeof ore and gangue/alteration mineralsand their distribution in the outer and inner halos.(After Wilde and Wall 1987)(reproduced from Economic Geology, 1 9 8 7v. . 8 2 ,p . 1 1 6 5 )
White-mrco Rutile Uroninite C o ff i n i t e Hemoiite Pyrite CholcoPYr. Goleno Dolomile Colcite
r
-
176
5 Selected Examples of Economically significant Types of Uranium Deposits
o.s.i. m '100-
S
N
Ouortz w. hemotite dusting
Drusy quortz veins
No detriol oxrdes
Minor bleoching hemotite veins Alluvrum
^=-_sssi
E
LL
o
E ! E
ia:,';i| : :.,:'.':'i
-,/i
t,
-200 -
JY
(
J E]
Sondstone with hemoiite {:25%)
[]
Sondstonenot chloritized
tm
m t\\l a
E
\\ 0
S i l i c i f r e oz o n e s Corbonote veining
M e t o s e d r m e n t sw r t h h e m o t i t e lz1"hl
t-.F1 lj.-..,j,:l
depth m
Zone of mossive cnlorite {:157o)
U n c o n fo r m r t y
t""l
l00m
l-....,-.....r--J
Sondstone with motrrx chlorite (:5%)
w
Zone of grophite removol Metosedrments
F i g .5 . I 7 . J a b i l u k a c. r o s s sectionthrough the Kombolgie Formation and subjacent unconformir-vabove the Jabiluka II deposit. The scheme displavsin a simplified manner tbe vanousalterationsignatures in the sandstone.(After-Curtis 1985)(reprinted with permission from The Canadian Institute of Mining and Metallurgy)
""""1
low
135
. i-
Hemotitic
sonosIone
Chlorite vein wrth bleoched holo
sonostone
M g . Fe 2 ' / F e 1 ' . U . Z l P . Y . L oC u . N i . C o . \ '
tn
C h l o r i t i z e d p e 9 m or r l e low U.8e. high Rb.Co,Ni.Y
nr9n z
M o g n e s r t e- d o l o m r t ev e r n h r g h U P b
Cu
ssive chlorite ol unconf ormity
U.Pb.Zr.Cu. N i . V . Yo. L. 8 e
S e r i c i t r z e dp e g m ot r t e h r g h U . 8 e . l o w R b C o . N yr
Fig. 5.18. Jabiluka,schemeof alterationoroductsand associated chemicalchanqesin metasedimentsal the unconformitl an-doverlying sandstone.(After Gustafson and Curtis tlA3) i t e { c h l o r i l e ) s c h i s t (reproduced from Economic Geology, 19g3, v. 78, p. 40)
Chloritized schrst
Examplesof Subunconformir.v-Epimetamorphic-Type Uranrum Deposits D r o g e n r ce v e n t s
Mrnerol
Red 1 0 " , . . , o ,b e d
ovefgrowlhl I rortz .I der orl- -
Jlu>9ul
..t
- lM o o n e t r t e , / r l m e n r t e -l - A m p h r b o l ee t c
r".ot'te >ercrre
I
I
I I
^-"lo'"" ,3(omrre
I
'roenesite
I
Srrfiae ipy)
j
S u rf i c j o l
C o r b o n ot e R e t r o weothering Ouort z v e r n s ond grode ve i n s r e p r o ce m
dolom.
l---
-
I
P o s t d i o g e n e i i ce v e n t s
A l t e ro t r o n 0 u o rt z C h l o r ret cf S o m p o ct r o r o v e r - v e i n s o n d 3efirtols Qrowll replocem
?
?_?
I
Kooirnrtr: -tmonlte
I I
-
Formotion
Per sistonc e
, Destruction
Fig. 5.19. Jabiluka,paragenetic schemeo f a l t e r a t i o n . ( A f t e r G u s t a f s o n and Curtis 1983) (reproduced from Economic G e o l o g y , 1 9 8 3v, . 7 8 . p . + 0 )
pervasive halo but of variable intensity and spatial extent around the various deposits. Composition of chlorites within ore zones and adjacent host rocks overlap and are highly variable with respect to Al, Fe. Mg, and to a lesserextent Si, although in general Mg-rich phasesprevail. Sericitization (white mica. illite) is common in all deposits and replacesparticuiarly feldspars. At Nabarlek, sericite is the dominant phyllosilicate in the ore zone derived by overpnnting chlorite. Hemaridzarion is widespread and generally associated with chloritization particularly of ferromagnesian minerals. Hematite occurs disseminated,in veinlets.and as pseudomorphsafter magnetlte. Silicification is a common feature in and around ore bodies. It produced chert replacing carbonates and euhedral quartz filling fractures and voids. At Nabarlek, silicificationalong fault pianes. together with chloritization and sericitization, forms an outer haio around the deposit, whereas the ore zone itself was subjected to desilicification (Wilde and Wail 1987). Carbonatization is a minor feature and retlected by locai veinlets of dolomite, calcite and magnesite. Decarbonatization is indicated by the presumed removal of bedded carbonatesfrom mtn-
eralized zones. Because this decarbonatization rs commonly associated with silicification, Eupene (1980) has postulated that silicification of carbonate resulted in reduction in volume and caused collapse and disruption, whiie Ferguson et al. (1980b) consider a soiution of the massive carbonate, which led to excavation and collapse. Age datings indicate that most pronounced intervals of alteration occurred at about 1650 to 1 5 0 0 m . y . . 1 2 6 0r o 8 4 0 m . y . a n d 5 0 0 m . y ' .( T a b l e 5.14). Fission track dating of zircon from basement rocks yields an additional age of ca. 1420 m.y. and a respectiveannealing temperature of ca. 175"C(Koul et al. 1984. 1987).
Principal Characteristics of N{ineralization Pnncipal ore minerab are uraninite, pitchblende. minor coffinite, brannerite, locaily thucholite. and in weathering zones hexavalent uranium minerals. Jabiluka II and Koongara contain economic gold and Ranger subeconomicgold. Associated minerals and elemenlr generally presentin amountsof lessthan 1% include pyrite, pvrrhotite. chalcopvrite,and galena. Locally and mostly in traces occur bornite, chaicocite, covellite. digenite, arsenopyrite, cobaltite, marcasite.
178
5 Selected Examples of Fronomically Significant Types of Uranium Deposia
cuprite, native copper, native gold, tellurides, mercurlr, and palladium. Hematite is relatively conrmon. Principal gangue minerals include chlorite, quartz, white mica/sericiteand lesser amounts of dolomite, magnesite,calcite, kaolinite, montmorillonite, apatite, tourmaline, sphene, anatase, leucoxene, and rutile. The T4!.le 5.14. Alligator Rivers Uranium Field, selected apparent agesof altered rocks from the major deposits. Rock
Apparent age (m y.)
Method
Reference
1600+80
Rb-Sr
?
+ 90 Rb-Sr 153.1 to 140(l 1380+ 110 Pb-Pb
3
Jabiluka Chloritic-sericitic schist Altered metasediments Pyrite in retrogressed graphitic schist few meters away from ore
1
Nabarlek lnner alteration haio: sericite-chlorite schist Outer alteration halo: sericitechlorite schist - muscovite separate * biotite separate Altered sericitechlorite schist (meanmodel age) Altered Nabarlek Granite Mineralized sericitechlorite schist Sericite/illite
1648 to1512
Rb-Sr
1781+ 35
K-Ar
2
1583+ 32 1610+ 40
K-Ar Rb-Sr
1
1 5 6 1+ 1 7
Rb-Sr
1260 to 842 450 842
Rb-Sr
?
Rb-Sr Rb-Sr
2
r&8 - 17
K-Ar
2
1630 to 1600
K-Ar
2
1630
K-Ar
?
1420
Fission- 2 track
') ')
4
Jabiluka.Ranser One. Nabarle[ areas Altered dikes (amphibolite) Altered Oenpelli Dolerite (clinopyroxene) Severalaltered granites(average) Zircon from basement
Data from (1) Gulson and Mizon 1980;(2) Page et al. 1980; (3) tuley et al. 1980; (4) Wilde and Wall 1987; (5) Koul et al. 1984
variousore and associated mineralsare presentin severalgenerations. Uraninitelpitchblende forms fine stringers, coats foliation planes,and occurs interstitial to, and in complex intergrofih with the phyllosilicates,particularlyMg-chlorite,and at Nabarlek, secondarysericite. Jabiluka contains up to 100ppm palladium. with values up to Economicgold concentrations 695ppm (average 10ppm) as found localiy in Jabiluka 2 (Fig. 5.20) occur independent of lithology and stratigraphicposition. Ali observed gold accumulationsare confinedwithin uraninite veins. A few tinl' gold grains(<3 pm) have been noticed in chlorite replacinguraninite, but there is no significantcorrelationbetween amounts of gold and uranium(Grauch1984). Sulfidesare presentin all depositsbut in minor quantities(
r79
Uranium Deposits Examplesof Subunconformity-EPimetamorphic-Type
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5 Selected Saamples of Economically Significant Types of Uranium DePosits
180
'-1{ ' o 1Nro . 4
h- *\r ts
I
0
Sdlr
-
rr.rl
{'
ffi-i"J' KombolgieFm.
ffi ffi W
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StableIsotopesand Fluid Indusions
N
ffi r--l
ffi
Footwoll sheor zone
; I
Dolerite Pegmotite. pegmotoid Honging Woll s c h i sl Upper Mine s chrst Lower Mrne chert R e c r y st o l l i z e d corbonote
1
Footwoll schist ond gneiss S u rt o c e e x p r e s s | o n of U ore
( (
O no.2Tnomotv
ffiffi-':I
I
F
a
No.9 onomoty
Fig. 5.21. Rahger One, generalizedgeologicalmap with localization and surface expressionof the two main ore bodies, No. I and No. 2 and adjacent uranium showings (anomalies). The Foor*'all schist and gneiss are sup posedly part of the contact zone of the Nanambu Complex. (After Ewers et al. 1984. based on Eupene et al. 1975,Hegge et al. 1980)
Fuzikawa(1982)and Ypma and Fuzikawa (1980) establishedfwo types ot fluid inclusions in quartz and carbonatemainly of late stage veins of the Jabilukaand Nabarlekdeposits'Type 1 contairs a salinebrine with dominant CaCI2,MgCI2, and NaCl, and yields homogenizationtemperatures between100and 160"C.Type2 has low salinities and CO2in the vaporphase.Ypma and Fuzikawa a mixing (1980)interpretthesedataassuggesting fluid. COz-rich of a very salinebrine with a more possiblv uranium, causing the deposition of involvingreductionby methane. Witde et al. (1988)confirmedthesefindings'In quartz from veins, breccias,and country rocks from Jabiluka. Koongarra, and Nabariek. and from post-ore dolomite cementing breccias at sevendifferent typesof Jabiluka.they established be attributed to four can inclusions which display an The inciusions populations. cogenetic associated and in composition variability extreme interpreted is variability The properties. physical by the authors as resulting from trapping of varyingproportionsof two discreteCa- and Mgenrichedend membersolutions. The first fluid is consistentwith a hypersaline brine (salirutyca. 40wt."hCaCl2 equivalent) of relatively reduced nature which was trapped at high pressureof as much as 1.6kbar and temperatures of.24trC or more. This fluid is assumed to have been saturated with respect to early Fe-richchloriteand white mica in the alteredhost rocks. Solid inclusionsof Fe-rich chlorite suggest that the fluid can be relatedto the early alteration stagepresentin the outerzoneat Nabarlek,while soiid inciusionsof dolomite probably attest to a persistenceof this fluid to the post-ore dolomite veining stage, or alternativelvthat minor doiomite formed during the early alteration stages. The secondfluid is a more dilute brine (ca. 7.5wt."hCaCi2equivalent),more oxidized and CO2 gassaturated.Recordedpressureis only 500 bar. Minimum temperaturemust have been at least 95'C accordingto the lowest vapor disappearancetemperature. Wilde et al' (1988) postulate that this fluid may have caused the overprinting alteration halo including desilicificationof the inner zoneat Nabarlekbecausethe chloriteof this zoneis markedlymagnesian,and hematiteand anataseare the stableoxide phases ascomparedto magnetiteandilmenite,whichare stablein the outer halo.
Examplesof Subunconformiry-Epimetamorphic-Type Uranium Deposits
w
181
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ooo^oo o;9X r.19"o od 9 o
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::fi^ r;r,r
il
rlrlr
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uoo"r mineschist vFPv. Lower mine chert C o h i l lF o r m o t i o n C h l o r i t i cc o r b o n o t e LenticieschiEt
pesmotoid e.e.otite, oceoxidotionlnffiffifiIffi FTI ,on"of surf FiTI
NtxHxHix -
c rl y s i o l l r z e d c o r o o n o t e u 1 l l l 1 l l l lR l 1el 1 Foorworl scnist ond gnerss
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%:r';"::lx'o"oi''"^"
D
g o r " o f w e o t h e r e oz o n e
D o l e r iet
Fig, 5.22. Ranser One, No. 1 ore body, W-E cross-sectionshowing distribution of mineraiizationand host lithologtes. (After Ewers et al. 1984,basedon Eupene et al. 1975,Hegge et at. 1980)
Ewerset ai. (1984)reviewstablebotopestudies performed on sulfides,carbonates.and organic matter from Jabiluka, Ranger One and Koongarra. Bedded sulfides.mainly pyrite, occurring in graphitic metasedimentsof the lower Cahill Formation adjacent to the depositsyield 6yS values of.+2 + 1Y*,i.e.. they are in accordwith
those of mantle suifur and thereforepresumably derived from volcanogenichydrothermalfluids. Vein and vug filling sulfidesfrom the deposits have a wide range of 6345values of between -6 and +I4y*. This range indicatesa probable metai sulfide crystallization in an organic-rich environmentfrom sulfatesbv H,S. The hvdroeen
r82
5 Selected Examples of Economically Significant Types of Uranium Deposits
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Examplesof Subunconformiry-Epimetamorphic-Type Uranium Deposits
sulfide is thou_eht to have been generated by anaerobic sulfate-reducingbacteria. Donnelly and Ferguson (1980) applied sulfur isotope geothermometry and figured out that bedded and ore zone coexisting sulfides record rhe same temperature of (re-)crystallizationof about 270'C. Binns et al. (1980b) registereda similar temperature for one sulfide pair from Jabiluka and conclude that initial temperatures of the main mineralizing event were about 300 to 350"C. Vein and rug carbonarc from mineralized zones have variable 613Cand d18Ovaluesranging from -20 to 0-* and +7 to +20%" respectivelv. The 613C values suggest crystallization of the carbonate from organically derived CO2, in part at least, whereas the 6180 values indicate rnvolvement of groundwater. The 613Cand 618O values of vein and vug carbonates from Jabiluka and Koongarra plot on rwo reasonably defined correlation lines. Ewers et al. (1984) interpret these correlations as supporting the hypothesis calling for in siru reactions caused by influx of oxidizing groundwater whereby carbonate has been recrystallized and organic material oxidized. A greater groundwater involvement is indicated for the intenselv altered ore zone at Koongarra by various studies and supported by 6I3C values of organic material. Stable isotope studies of carbonates from the Nabarlek mineralization by Ewers et al. (1983) show 18O enrichment which suggestsa low fluid/rock ratio. The isotope compositions of these carbonates also indicate higher temperature reactions than established for the 13Cdepletion rn other deposits mentioned above. = the Nabarlek carbonates 1613C -25 to -15) are interpreted by the authors to indicate incorporation of CO2 derived from organic matter. The low fluidirock ratio is supported by 6180 data from Oenpelli Dolerite at Nabarlek reported by Ypma and Fuzikawa (1980).
Potential Sources of Uranium Data provided bv Ewers et al. (1985),Ewers and Higgins (1985). and Needham (1985)indicatethat Archean granite of the Nanambu Complex contains an average of 9ppmU with uranium enrichments of up to 30ppm. which is reported by McAndrew and Finlay (1980) to be readily leachable. Middle Proterozoic granites average 8 to 25ppmU with values of as much as 47ppmU.
183
According to Ewers and Higgins (1985), all unaltered metasediments away from uranrum occurrences contain U tenors which more or less compare with the average abundance for the given lithologies on a worldwide basis. For these reasons, these authors favor the Archean granitic rocks as the most likely source for uranium or perhaps feisic volcanicsintercalaied within the Lower Proterozoic metasedimentary sequence. It has to be kept in mind. however. that Fersuson and Winer (1980) and Binns et ai. (1980a) establishedsignificant U concentrations of up to 16ppm U in graphite schists"awav from t h e o r e z o n e s " . I t r e m a i n st o b e s e e n w h e t h e r these concentrations represent onlv a distal uranium halo around a deposit, i.e.. the oreforming processdistributed uranium far beyond the actual zone of mineralization,or the elevated values are perhaps associated with ancient uraniferous mylonite zones comparabieto those found associated with unconformity-bound uranium deposits in the Athabasca Basin and vein deposits in the Hercynian Massifs in France. (see respectivesectionsin Chaps. 5.1 and 5.3.1)
Geochronology Table 5.16 provides a synopsisof geoiogical and metallogenetic events in geochronological order as proposed by Needham (1985). Mineralizationiremobilization occurred during several penods but not all generations have been isotopically and/or mineralogicallv recorded in all deposits. Pnncipal post-metamorphic mineralizationiremobilization events are dated at a b o u t 1 7 3 0 m . y . ,1 6 5 0r o 1 6 0 0( t o 1 4 4 0 )m . y . , 9 2 4 to 800m.y. and 500m.y. Apparent agesof alterationproducts are given in Table 5.14. Reponed agesof uranium minerals are as follows: Hills and Richards (1976) identified by U/Pb isorope dating of pitchblende-uraninitean age of ca. 1855m.y. corresponding to the time of the metamorphic event, an age of ca. 1700m.y. for the oldest uranium mineralization at Ranger One and an age of 920 to 800m.y. as the best-defined event of mineralization. Ewers et al. (1984) report isotope ages of 1600m.y., 900m.y., and 500m.y. for uraninite and galena, which reflect a time of mineralization and subsequent remobilization post-dating re-
184
5 Sctected Examples of Economically Signfficant Types of Uranium Deposits
Table 5.16. Alligator Rivers Uranium Field, synopsisof the geologicalevolution and metallogenetic events proposed by Needham (1985). (Ages of U mineralization after Hills and Richards 1976, other ages after Page et al. 19801for additional ages see text). (Needham 1985) Igneous events
Tectonic events
Metamorphism
Age
Deposition
Recent
Aggradation ofswales, alluviation of maior valleys
Coastalemergence-7m Dissecdon of Koolpinyah Surface
Pleistocene .
Aggradation of major vdeys, formation of sandplains. minql dggP weathering
Wide fluctuations of sealevel. dissection of Koolpinyah Surface
MiocenePhocene
Deposition of unconsolidated sands over iateritized lowlands to f orm Koolpinl'ah Surf ace
Conrinued oxidation of orebodies near surface; minor precipitation of uranium in organicallv-rich black soiisand possiblyin pyritic black muds
Conunued oxidation of orebodiesnear-surface
l-ateritiz'tion of lowiand areas, mechanical weathering. redeposition of sandsfrom Kombolsie Formation
Cretaceous
Events relevant to ore genesls
Deposition of Bathurst lsland Formation: sandstoneand siltstone in Dorth
Funher relative uplift and retreat of seasto beyond present coastline
Removal of Mesozoic to largely re-exhumepreKombolgie surface
Retreat of Mesozoic seassuggesting relative uplift in south
Burial of pre-Kombolgie surface
Regional unconforrrity Intrusion of minor dolerite (522m.y.) Minor alreration at Nabarlek (ca. 920n.y.)
Intrusion of phonolites ( 1 3 1 6r n . y . ) , dolerites(1370 and1200m.y.)
Adelaidean
l-ow-grade metamorphism and metasomatismofthe uraniumdeposits and sunounding rocks
Carpentarian 1 6 1 0m . y .
r650-
Uplifi. Extensive faulting and jointing of Kombolgie Formationand reactivationof some basementfaults, displacements <600m
1 6 1 0m . y
1659m.y.
Deposition of Kombolgie .'Formation: quartz sandstone, minor conglomerate and siltstone: extrusioo of basalt memb€rs
Erosion of Kombolgie Formation mainly by scarp rereal to expose preKombolgie surface, followed by partial oxidation oforebodiesnear surface.Apparent ageof U mtneralization (ca. 900 m.y and ca. 500m.y.) Main mineralizing event, probablv svnchronous with faultilg. Possiblebydratron ofanhvdnte
Examplesof SubunconformityUranium Deposits Epimetamorphic-Type
185
Tabfe5.16. Continued Deposition
lgneousevents
Tectonicevens
Metamorphism
Eventsrelevantto ore genesrs
R e g i o n aul n c o n f o r m i t v 5 8 8m . v .
1 1 t { )m . y
Depositronof volcani. c l a s r i c st.h i c k n e s s u n k n o w n( < 2 0 m ' l )
'--3()-
Intrusionof differentiated olivine dolerite Iopolithsof Oenpelli Dolente
Higher heat flow c o n d i t i o n sC . onrinued erosionof Early Proterozoicrocks. Apparent ageof U mrneralizationca1 7 U ) m . v .a t R a n g e rI
Effusiveactivity to S (Edith fuver Volcanics)
Possiblerntroductionof U, cnrichedrocks. Erosion of Earlv Proterozoicrocks
Intrusronof P e n o do f c o m p l e x preoroeenic de[ormation granlte(ca. i n v o l v r n ga t l e a s t- { 1 8 7 0 m . y .i )n fold eprsodes NE. Postincluding2 rsoclinai orogenlc even6. Late near(r'78O-1730 v e n i c a l .m a i n l v m . y . ) i n E a n d S northerlv faulting
i l r 7 0m . v
Early Proterozoic
Intrusionof quanz-dolerite sills and minor dykesof Zamu Dolente
Possibleperiod of non-depositron
Koolpin Formation: siltstone. shale,pyntic carDonaceous cneil. banded siltstone
Regronalmetamorphlsm ro staurolite-almandine subiacies. Formation of Nimbu*'ah Complex by migmatizatlonof 1870m.v. granite. Formation of NanambuComplex by metamorphsm and accretion of Archaean granite and Kakadu Group
Possibletransporrof U bv metamorphichydrothermal fluids.Possible developmentof fault brecciazones.Penodof major connateand groundwater movements. Apparent ageof U mrneradzatlon- 1800m.y. I mlnor occurTence)
His,herheat flow conditions
Possibletransport of U and precipitationin black shales
Unconformrtv Nourlansie Schist: sandstone.siltstoneand shale
Probabiemild tblding at close of deposition Possibielocal uncontornutv
Cahr-llFormation : clavel' sandstone.srltstonewith hmestoneand carbonaceousshalenear base
Stableshelf surrounding moderatelvmature h i n t e r l a n d q: u a n z nch upper member representseradual transgression over shelf
Transportof U and precrpitatronin reduced environmentsand possibly in evaporites.Possible diageneticalterationof claysto chlonre
Possible disconlbrmitv Kakadu Group: arkose. sandstone.minor shale
Juvenilehinterland
Possiblerransport of U - no known precrpitation
186
5 Selected Exampies of Economically Significant Types of Uranium Deposits
Table 5.16. Continued Age
Deposition
lgneous events
Tedonic events
Metamorphism
Events relevant to ore genesls
Unconformity
nffi12500m.y.
Erosion of granite basement,removal of Archaean cover rocks
Possiblemechanical transport of U-reduced species
Uplifi AilJraean 2500m.1 .
Crvstallizatronof granrtebasemenl
Introciuctronof U-enriched rocks into crusr
gional metamorphismand the depositionof the from the site of ore formation. This implies thar KombolgieFormation.The 900m.y. eventwas reworking in situ can be envisionedonlv for the associatedwith widespread sericitization that casethat the original uranium accumulationwas may have erasedevidenceof an earlier uranium of very low grade. generation,as suggested By far the most significantdisturbanceof the by Ewers et al. (1983) for the Nabarlekdeposit. U-Pb isotopesystemoccurredin a poorl,vdefined Maas and McCulloch (1988) provide SmNd interval about 600 to 400m.1'.ago. During this isotope data giving identical ages for prima+' period,most of the post-1.4b.1'. radiogenicPb of uranium rnineralizationat Jabiluka II (1614 + the high-gradeores of Jabiluka were removed. 132m.y.)andNabarlek(1616+ S0m.y.).SmAld The ore at Rangerwassimilariyaffected,but not data for uraninites from Koongzra are scattered, asstrongly.Someof the oresof both depositsalso but indicate a model age of 1650 to 1600m.y. experienceda net lossof uraniumor alternativelv Uraninitesfiom Ranger do not form an isochron a gain in radiogeniclead during this interval. of SmA{d values, but indicate a possibleage rangeof 1750to 1600m.y. Ludwig et al. (i987) identified apparentU/Pb Ore Controlsand RecognitionCriteria agesof 7732+ 20m.y. and 1437+ 40m.y. for the Rangerand Jabilukamineralizationrespectively, Significantore controllingor recognitioncriteria and an episode600 to 400m.y. ago. The authors of the uranium depositsin the Alligator Rivers conclude from these data that a first (pre- Uranium Field include: KombolgieFormation) higfr-gradeconcentration of uranium as estabiishedfor Ranger occurred Host Environment after the climaxof regionalmetamorphismbut at piutonswere - All depositsoccurwithin the lower memberof a time whenearlvpost-metamorphic (post-Kombolgie) . still intruded Later high-grade the Cahili Formationor a supposedcorrelative mineralization as typicalll' found in Jabiiuka, thereof. Koongara,and Nabarlek.probablyoriginatedby - Host rocks are predominantlv highlr redistributionof the earlier uranium concentrabrecciated quartz-muscovite/sericite-chlonre tions. althoughtheseeariier concentrations may schistswhich grade laterally into carbonate. have been of very low grade. 207Pbl26Pb ages Graphite is a constituantof these schistsor older than ca. 1400m.y.were not established at adjacentlayersat Jabiluka,RangerOne, and Jabiluka, which argues against an in situ reKoongarrabut Ewerset ai. (1984)do not find mcibilizationin this deposit of high-gradepreanyapparentcorrela(ionbetweenthe presence Kombolgie ore. The presenceof relativelyhigh of graphiteand the concentration of uranium. contentsof commonPb in many of the Jabiluka - The stratigraphichorizon hosting Jabiluka. samplessuggeststhat the 7431-m.y.-old minerRanger One, and Koongarrais regionally'a alization event was not sufficiently effectiveto carbonateunit but almostdevoidof carbonate removelead suchas the radiogeniclead generat the deposits.Positionof the ore hosting ated from the 7731or 1437-m.v.-oldsenerations breccias-schists suggests,however, a close
Examplesof Subunconformity-Epimetamorphic-T.vpe Uranium Deposits
-
-
correlation with the presumed original distribution of the bedded carbonate units. Most deposits occur in the proximity of an Archean complex (Nanambu) affected by vounger migmatization. All large deposits are located immediately below the Middle Proterozoic unconformity covered by red bed type sandstonewith intercalated volcanics (Kombolge Fm.).
.llteration
187
-
IVlineralizationis practically absent from the cover rocks. - Formation temperaturesduring mineralization are bracketed between 300 and 95'C. The upper limit is defined by relict metamorphic muscovite in altered host rock, retaining peak metamorphic K-Ar and Rb-Sr signatures characteristicfor temperatures not exceeding 300'C (Page et al. 1980). The lower limit is concluded from CO2-rich fluid inciusions(see beiow) demanding temperatures of at least 95"C (Wilde et al. 1988).
-
Alteration products include chlorite, white m i c a r s e r i c i t e , i l l i t e ,h e m a t i t e , q u a r t z , c h e r t , d o l o m i t e , m a g n e s i t e ,a n d c a l c i t e . - Mg-rich chiorites prevail in the immediate vicinitv of the ore zones. suggesting Mgmetasomatism. whereas Fe-chlonte together with white mica is apparently more dominant in the outer halo (Wilde et al. 1988). - Decarbonatization and locally variable desilicification modified the host lithologies, resulting in volume reduction. - Extension and intensity of alteration halos around individual deposits is variable, ranging from few ten meters as at Ranger One and Koonsarra to more than 200 m at Jabiluka and ca. 1000m at Nabarlek. - An inner alteration halo measuring a few meters to some tens of meters is overprinted on an outer halo (Fig. 5.14). - Alteration follows primarily structures and is commonlv most intense within the brecciated ore zones.
MetallogeneticConcepts A variety of models have been put forward for the ongin, transport, and fixation of rhe primary uranium generation. The two most recently publishedmodels are provided by Needham et al. (1988) and Wiide et al. (1988). The two models represent two end members of favored hypotheses with respect to the time of ore formation, viz. an origin in pre-Kombolgie Formation versus a post-Kombolgie Formation time. While these two models involve supergene and d,iagenetic processesand refute a hypogene genesis of the uranium deposits. Binns et al. (1980b) are proponents of a hypogene origin of the ore. Model I: Pre-Kombolgie Ore Formarion
Needham's et al. (1988) model follows to some extent that of Ferguson et al. (1980b) and Ewers et al. (1984). The authors state that subsequent to the regional metamorphism a penod of Minerali:ation weathering imposed a saprolitic profile as much - Mineralization is monometallic exceptfor local as 100m deep and peneplaned the Lower Proterozoic rocks. During this period, solution of e c o n o m i c c o n t e n t so f g o l d . - Principal U phases are uraninite and pitch- the carbonate horizons resulted in solution blende. coilapse structures and brecciated zones which - Associated metallic sulfide minerals, domi- became the hosts for mineralization at Jabiiuka, nantlv Fe. Cu, and Pb-sulfidesare present in Ranger and Koongarra. Insoluble residue of minor amounts only, as opposed to hematite, breccia fragments accumulated in these breccia which is a common component in ore zones. zones together with washed-in material such as - Dominant sangue mineral is Mg-rich chlonte clays, soil constituents, and perhaps organic material now represented by thucholite. The and. to a lesser extent, white mica. - Ore and associatedminerals cement breccias presence of biogenic carbonate and the postuand form veins and veiniets within tabular, lated existenceof sulfate-reducing bacteria within lenticular or bowl-shaped zones localized in the breccia zones support the presenceof organic more ore less strata-concordant breccia zones substances(Donneily and Ferguson 1980). The great depth of solution cavity develop(Figs. 5.20 to 5.22), exceptfor Nabarlek, which ment to 600 m below the Middle Proterozoic to the strata. trends oblique host
188
5 Selected pxamples of Economically Significant Types of Uranium Deposits
unconformify ar Jabiluka is attributed by magmatism of the pre-Kombolgie Oenpelli McArthur, G.A. (in Needham et al. 1988)to a Dolerite and flows intercalatedin the Kombolgie paleotopographichigh of at least 250m, which Formation. presumablyfacilitatedthe percolationof waters. Later igneous intrusions probably produced the temperatureof about 270'C documentedbv In contrast to the depositsmentionedabove, Nabarlek and depositsin other districtsof the stable isotope data by Donnelly and Ferguson Pine Creek Geosyncline (Rum Jungie, South (1980).During theseeventsamorphousuranium Alligator Valley) occur in tectonicallygenerated oxidesmay have recrystallizedto euhedralhabits (uraninite) in disseminateddistribution. fracturesand breccias. Very mild conditions.as forwarded by Binns main mineralizing event after The occurred et al. (1980b)and Ewers and Ferguson(1980). prior fracturing and to the brecciation and uraninite and depositionof the Kombolgie Formation,which is probably remobilizeddisseminated deducedfrom the lack of mineralizationin the redepositedthe uranium as stringersand collocover rocks. The nature of the ore bodies form aggregatesof pitchblendesubsequenth. indicates that they formed from downward e.g., during the ca. 900m.y.and ca. 500m.1'. percolation of mineralizing solutions. Critena episodeswhich may have been generated by supporting such a mechanisminclude ponding epeirogeneticmovements.The 900m.y'.event is effectscausinguranium enrichmentson the upper contemporaneouswith Upper Proterozoic sediside of steeply dipping rmpermeablebarriersin mentation southwest of the Pine Creek Geothe mineralized zones, bottoming out of min- svncline and the 500m.y. event correlateswith eralizationagainstimpermeablemafic intrusives the developmentof the Daly River Basin to the asat JabilukaI and Nabarlek. and lirnitedvertical south (and with intrusion of basicdikes). Preservationof the ore bodieswas achievedb1 continuity of the deposits. During the episode when clays and organic matter accumulated rapid deposition of cover rocks. The Middle within the breccia zones, the plumbing system Proterozoic Kombolgie Formation apparently within these structures permitted migration of fulfilled this protection request(Needhamet al. €foundwaterwhich wasof low temperature,CO:- 1e88). rich, and contained uranium as uranyl carbonate complexes.These waters are thought to have probably also transportedother elementscharac- Model II: Post-KombolgieOre Formation teristically enriched in the ore zones, viz. As, Wilde et al. (1988) studiedfluid inclusionsfrom Au, B, Co, Cu, Li, Mo, Nb, W, and REE. Jabiluka. Koongarra, and Nabarlek. The)' Alternatively, the ore zones might have partly endorse a post-KombolgieFormation origin of inherited tbeseelementsfrom the carbonaceousthe uranium mineralization and arrive at the rich residue produced during formation of the following summarizedhypothesisbasedon these breccias. metallogeneticp arameters: Precipitation of uranium occurredin response to reduction in the anaerobic organic-richen- - All depositsoccurwithin or adjacentto reverse vironmentin the breccias.Reductionmay have fault zones which are of post-Kombolgie further been facilitated bv redox Drocesses Formation age, as reported by Johnston (1984). (Note: Accepting this statementas involving Fez* contained in phyllosilicatesand valid does not exclude reactivation of preother ferrous iron-bearingmineralsas proposed earlier by Ewers and Ferguson(1980).EnrichKombolgiestructures.) ment of uraniumwaspossiblyalsoenhancedby U - Quartz, pyrite, and/or hematiteoccur in preadsorptionon particulatesubstances. ore veinsand breccias. -Oncethe depositswere sealedeither by com- - Alteration associatedwith mineralization at paction of the postulated regolith over topoJabiluka postdates deposition of the graphiclows or by loading and dewateringof clay KombolgieFormation(seeremarksbeiow). within the breccias, thereby reducing perme- - Mineralization is associated with more ability, all subsequentprocessesrecorded in restricted pen,asive alteration manifestedby the deposits occurred as in situ modificarioru. growthof white mica,Mg-chlorite,anataseand Thesemodificationsmav have beentriggeredby locallyhematiteor apatitein the basementand high heat flow conditions generatedby mafic alone the sub-KombolsieFormation uncon-
Examplesof Subunconformity-Epimetamorphic-Type Uranium Deposits
formity. This alteration overprinted earlier alterations. -
Ore formation postdates Mg-chlorite, the principle gangue mineral, and white mica. It is accompanied by desilicificationwhich can be substantial,as documentedby Wilde and Wall (1987) at Nabarlek, where up to 4O"hof SiO2 were removed from the vicinitv of the ore zone. - Native gold is presentas inclusionsand veinlets in uraninite, reflecting gold deposition overlappingthat of uraninite. - Veinlets of dolomite and occasionallyquartz, often with minor chalcopynte and galena, represent the last stage of mineralization. - An incongruent dissolution event overprinted the primary ore, resulting in formation of coffinite. - Fluid inclusion data outlined earlier indicate two prominent fluids which were active during the time span of alteration and mineralization. Wilde et al. (1988) conclude from these data that mineralization resulted from interaction of two fluids distinct in terms of oxidation state and salinity which derived from overlying sediments. The authors propose faulting as the mechanism to permit access of the fluids into the relatively impermeable basement and to facilitate mixing of the two fluids. They also attribute the extreme differences in pressure determined from halite Cissolution to reflect variation from lithostatic to hydrostatic pressures at a depth of ca. 6 km due to faulting. The alteration and assumed ore-forming solutions supposedly evoived (successively)from overlying terrestrial. evaporite-bearing sediments (McArthur Basin) during deposition and compaction of the cover sequence. The variations in salinity between the two fluids are explained as sedimentarv bnnes tapped from various levels within the sedimentary pile. An early invading hypersaline brine is thought to have become reduced by interaction with the basement rocks. Wilde et al. infer that the fluid. at a temperature in excess of 240'C, achieved saturation with respect to the relatively reduced Fe-chlorite and white mica in the alteration zone preserved in the outer aiteration halo at Nabarlek. It developed into a high pressure (1.6kbar) "resident" brine. The high pressure presumably reflects deep burial under a cover about 6500m thick.
189
Continued brittle fracturing permitted influx of an originally cooler. more oxidized, less saline, low pressure CO2-bearing solution from higher up, or laterally in the sedimentarvsequence.The solution was very probably in equilibrium with hematite and possibly anhydnte in its provenance environment. Ingressof this solution to the site of ore precipitation was associatedwith heating to the ambient temperature at a depth of ca. 6km. After the fluid achieved thermal equilibrium with the surroundingrock. it was consequentlycapable of dissolving and removing silica, and mixing of the two brines produced the alteration as observed, for example, in the inner halo at Nabarlek. Mixing of the two fluids is also proposed by Wilde et al. (1988) as the possible mechanismfor ore formation. The problem with applying fluid inclusion data as evidence for processes involved in actual uranium deposition is the lack of suitable gangue minerals which are unequivocallv in paragenesrs with uraninite. In the Alligator Rivers district, Mg-chlorite is the main gangue mineral, while quartz and carbonates appear to be earlier or later than the earliest ore mineral stage. The above discussedfluid inclusion data may thus not necessarilymanifest ore formation but rather an early alteration and/or later modification/ remobilization event. Wilde et al. (1988) address this point. The analyzed hydrothermal quartz from veins and breccia cement predates mineralization. Most fluid inclusions are secondary, however, and thus may have trapped fluids which were coeval with or later than uranium precipitation. Another criterion is put forward by these authors in support of the metallogenetic validity of the presented fluid inciusion data. Accidental solid inclusions of primary alteration minerals have been found in fluid inclusions. Thev are believed to evidence a link between these fluid inclusions and ore formation. Fluid inclusions in post-ore dolomite are likely to be primarv and represent a fluid present after mineral2ation. i.e.. they are the final product of the mineralizing event. Gustafson and Curtis (1983), based on their work at Jabiluka, arrive at the tbllowing genetic concept. which also postulates a post-Kombolgie Formation ore genesis. Intense chioritization in and above the ore zones is the result of a metasomatic event(s) which followed diagenesis of the Kombolgie Formation. The fluids necessarvtbr the alteration
190
5 Selected Examples of Economically Sigtrificant Types of Uranium Deposits
were probably meteoric-di.agenetic groundwaters derived from the Kombolgie Formation. Their movement, it can be speculated, was driven by a regional heating event which was also manifested by intrustion of the phonolite and/or diabase dikes. The major channelways for movement of metasomatic fluids below the unconformity were bedded carbonates in the lower member of the Cahill Formation. The initial permeability. which led to the concentration of subsequent soiution flow within these units, was probably the result of minor karst development at the profound preKombolgie unconformity. The subsequent dissolution of magnesite and dolomite marbles by hvdrothermal fluids produced large volumes of collapse breccias and markedly increasedthe Mgcontent of the fluids. Highly fractured graphitic schists were stoped into the open spaces which were also channelways of major solution flow. In theory, this produced an opportunit)' for the interaction between uranium-bearing fluids and carbonaceous material that would have resulted in the reduction and fixation of uranium. (Since graphite itself is chemically inert, it remains unknown what actually caused the reduction except for the speculation of a reactive alteration product of graphite.) In the overlying sandstones, practically no pitchblende was deposited, because no organic carbon was available for reducfion, even thoug} solutions may not have been exhausted of uranium. While reduction was essential for uranium fixation, this alone was not an adequate mechanism for the uranium precipitation from solutions which themselves were relatively reducing, as suggested by the Fez* lFe3n ratio of chlorite in the Kombolgie Formation. The formation of chlorite. at least some of it apparently coprecipitated with minute inclusions of pitchblende, is a feature of the mineralization processclosell'linked to uranium frxation. But it can only be speculated on the processesinvolved in the paragenetic formation of pitchblende and chlorite. One of the critical parameters is the rather close correlation of mineralization with chlorite. more oxidized than chlorite in the surrounding metasedimentsand sandstone.Ferguson et al. (1980b) suggestthat the oxidation of iron in the chlorites was completed with the precipitation of uranium, whereas Gustafsonand Curtis (1983) suggest that this oxidation took place long after the mineralization. Based on these and other
research data, Gustafson and Curtis (1983) proposethat the high-gradeuranium ore forma/ion resulted from a relatively low-temperature (100-200"C) hydrogenicsystemof groundwater activated by a regional thermal event contemporaneous with the pbonolite and/or diabase intrusions. The circulation of the fluids rx'as largely through permeable carbonate units of the lower member of the Cahill Formation close to the unconformity and through faultsolution collapse brecciasin the surrounding schistsand overlyingsandstone.The ore mineral depositionmay have been coupledwith precipitation of chlorite and a resultantdrop of pH from alkaline approachingneutral. The drop would decomplexU6* compoundsand allow its reduction where organicmaterialwas available.Reduction could not occur in sandstonebarren of organicmaterial. Sandstones of the KombolgieFormation were an essentialpart of the ore-forming hydrologic svstemafter the lithification of the lower thick clastic section, but possibly contemporaneous with diageneticeventsil the upper part. Gustafsonand Curtis's(1983)model concun in many respectswith thoseof Eupene (1980) and Hegge et al. (1980), who ascribe the mineraiization to post-Kombolgie metasomatism, guided largely by collapsebrecciation following dissolutionof carbonatesequencesof the Cahill Formation. They both. however, stress the probability of lower grade pre-Kombolgie uranium concentrationspossiblyrelated to latestagemetamorphicevents. Differences between the above model and thoseof other geoscientists include: Fergusonet al. (1980b),Ewers et ai. (1984) and Needhamet al. (1988)call primarily for a pre-KombolgieFormationtime of ore formation rather than post-Kombolgie. Ypma and Fuzika (1980)and Wilde et al. (1988)considera mxing of ven' salinebrineswith a more CO2-richmeteoricfluid as a mechanism for mineraiization. Crick and Muir (1980) propose a sabkha environmentin which uranium r,r'asenriched in evaporiticsediments.Expulsionof uranium-rich brines took place during diagenesisand deposition of uranium in depositsis postulatedto have occurred during post-metamorphicreplacement of evaporitesbv magnesite. In contrastto the abovemodels,Binns et al. (1980b)suggest that stableisotopedataindicatea
Examples of Subunconformity-Epimetamorphic-Type Uranium Deposits
deep-seated,hypogenesource for the initial mineralizirrgfluids which were possibly modified by a mixture with surface water. They point out that the Jabiluka ores are enrichedin elementssuch as B e , S c , Y , N b , S n , M o , B i , a n d p o s s i b l yB ( B i n n s et ai. 1980a;Fergusonand Winer 1980),as well as in Hg (Ryall and Binns 1980) and totai REE (Mclennan and Taylor 1980). Lithium is enriched in both ores and the retrogressedaureole, and thorium is low. Granitic gneisseswhich carry partly altered primary uraninite within the reactivated Archean Nanambu Complex underiying Jabiluka are considereda favorabie source. Binns :t al. postulate that Jabiluka is located in the ,,ipwelling discharge zone of a large fluid convection cell penetrating these source rocks. Fracturing due to epeirogenic movements associated with deposition of the Kombolgie Formation, or with its subsequentfauiting, is suggested as a reason whv the mineralization episode was deferred until the 1550-1440-m.y. age period indicated by geochronology.
191
Field. They are listed in Battey et al. (1987) and the reader is referred to this publication.
5.2.2 Subunconformity-Epimetamorphic Uranium Depositsin Albitized Metasediments:Uranium City Region, Canada
The UraniumCity region,locatednonh of Lake (Fig. 5.1), Athabascain northernSaskatchervan distnct around comprises the Beaverlodge and the mine situated Gunnar Uranium City about25km SW of Uranium City. All deposits are in an area some 25km wide and -10km long. Mineralizationis of two varieties.monometallic and polymetallic.All major depositsare monometallic.They have yielded more than 90"/' of the uraniumproductionof the region.The largest amountof uranium(I9200mtU3Os,averageore grade0.25'/"U:Os) came from the 1660m deep Ace-Fay-Verna mining compiex. Remaining Referencesand Further Readingfor Chapter 5.2.1 reservesin this complexare estimatedat 8600mt (for detailsof publicationsseeBibliography) U3O8. All other mines produced cumulatively 11400mtU:Oa,grvinga total of almost40000mt includingpastproduction. Battey et al. 1987: Berkman 1968; Berkman and Fraser U3Osresources 1980; Bone 1983; Compston and Arriens 1968; Condon All major depositsof the Beaverlodgedistrict and Waipole 1955:Crick and Muir 1980;Crick et al. 1980; are essentiallyof veinlike configurationand may Curtis 1985; Dodson 1972; Dodson et al. L974: Dodson be best defined as structure-stratacontrolled and Prichard 1975: Donnelly and Ferguson 1980; Dunn 1962; Durak 1983: Durak et al. 1983; Eupene 1980: uranium deposits of the subunconformityEupene et al. 1975:Ewers et al. 1983,1984.19851Ewers epimetamorphictype (subtype 2.2. Chap. a) G, pers commun.: Ewers and Ferguson 1980; Ewers and or lessalbitizedcrystallinerocks Higgins 1985;Ferguson 1980.1984:Fergusonet al. 1980a. hostedby more 1980b; Ferguson and Goleby (eds.) 1980: Ferguson and below a distinct unconformitv on which conWiner 1980:Fraser 1980:Frishman et al. i985: Fuzikawa tinentalsedimentsrest. An exceptionappearsto 1982; Giblin and Sneiling 1983;Goulevitch 1980;Gustaf- be Gunnar, which has many characteristics of a son and Curtis. 1983:Hegge and Rowntree 19781Hills and deposit. merasomatite-type Richards1972,1973.1976:Hochman 1980:Ingram 1974: The Beaverlodgedepositsare to some extent Johnston19841Koui et al. 1984,1987;Ludwig et al. 1987: Maas 1989; Mclennan and Taylor 1980: Vumme et al. similar to the depositsof the Allieator Rivers 1979: Nash and Frishman 1982. 1983. 1985: Needham distnctin northernAustralia(Chap.5.2.1)except 1979. 1984. 1985: Needham et ai. 1979. 1980. 1988: Needham and Roarr.v 1980; Needham and Stuan-Smith that the latter display a distinct strata control district 1976. 1980. 198,4.1985: Noakes 19-191 Nutt et ai. 1984: (e.g..Jabiluka),while in the Beaverlodge Ojakangas1986;Page 1976;Pageet al. 19801Pageiet al. the structurecontrol is more prominentand strata 1984; Paterson et al. 1984; Plumb and Derrick 19751 controlappearsto be subordinate.In addition,at Richards et al. 1977; Riley et al. 1980: Rossiter and the basementhost rocks underwent Ferguson 1980: Ryan 1977; Sman et al. 1975; Stewart Beaverlodge (aibitization) rvhereasat the 1965; Stuart-Smith et al. 1980;Wall et al. 1985; Walpole Na-metasomatism and Crohn 1965: Waloole et al. 1968: Wilde and Wall Alligator Rivers both basementhost rocks and 1987;Wilde et al. 198i, 1988;Ypma and Fuzikawa 1980. overlyingsediments(Kombolgie Formation) are Mg-metasomatized. indiThe followingdescriptionfocuseson the major papers on the published Other authors have vidual depositsof the Alligator Rivers Uranium depositsand is largely based on Beck (1969,
r92
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5 Selected Examples of Economically Significant Types of Uranium Deposits
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1986),Kalliokoskiet al. (1978).Smith(1986)and Tremblay(1972,1978). there are discrepancies In severalinstances in stratigraphic positionof rock rock nomenclature. inits, sequenceof geologicalevents,and other amongthe variousauthors. :eologicalparameters Reconciliationof conflictinsdata has not been possible.This mustbe left to further research.
GeologicalSettingof Nlineralization
193
Tabte5.17. UraniumCity region,Beaverlodgeoperarions of Eldorado ResourcesLtd., stratigraphicsection and lithology. (After Ward 1984) (stars refer to the rock age dated) Age Stratigraphy/Lithology (m.v.1 Paleo-Helikian Martin Formation l'190* 1630. 1799'.
Diabase*,siltstone,shale Andesite.basalt*,porphyry"* Arkose,sandstone. conslomerate
Aphebian Tremblay (1982) includesthe geology of the Tazin Group I,"raniumCity regionin the Tazin Belt which is :art of the Western Craton of the Churchill (MurmacBay Formationand Fav Complexof StructuralProvincein northernSaskatchewan. Tremblay1978) The major depositsin the Beaverlodgearea 1975* Pegmatite".pegmatitic (Fay-Ace-Verna)(Fig.5.23,Table5.17)occurin alaskite Orangemvlonite, feldMylonitic mica the Fay Compiex of the Tazin Group. These rock. spar mylonitic schist/paraeneiss were rocks originally deposited as pelitic to alaskite meta-argillite. psammitic, partly carbonaceoussedimentsof Argillite.paraschist migmarite Middle to Upper Aphebianage (or late Archean Epidotic argrllite. Siliceousargillite, greenschist.phyllonitic ultra-mvlonite accordingto Tremblay 1972,1978)over a baseamphibolite ment of Archean age.During the Hudsonianand SiLica,siiiceous Quartzite earlier orogenies,the sedimentswere metamorDiopside-calc-silicate mylonite phosed to an interbeddedsequenceof graphiterock Dolomite bearing quartzo-feldspathic gneisses,feldspathic DonaldsonLake Gneiss quartzites, argillites, amphibolites,hornblende. 2100' Alaskite.,leucocratic Mylonite(<1Oo1' and biotite schists. The metamorphic grade gneiss,(mafics<15%\ silica) rangesfrom amphiboliteto granulitefacieswith amphibolite localized anatexis. The metasedimentswere Archean tightly folded. and subsequentlyretrogressivelv Foot Bav Cneiss metamorphosedto greenschist faciesand affected Granitic crush rock Ultra-mylonite bv various alterations.Granites and pegmatites ?300* Cataclasite*,augenAmphrbolite intrudeddurine severalstages. J200'* gneiss'*, (mafics The crystailine basement is unconformably >15%). kakirite overlainby the Paleo-HelikianMarrin Formation. rvhich consistsof red bed sedimentsand locailv interbeddedvoicanicflows.Depositionoccurred phaseof deformationproducedfaultswhich trend in fauit-boundedgrabens.The Martin sediments NE-SW. E-W, NNW-SSE. and NW-SE. This rvereintruded by diabasedikes and sillsand were systemcreatedsuch major stmcturesas the St. slightlyfoldedalongNE-SW-trending axesduring Louis, Black Bay, and ABC faults.Along some linal Hudsoniandeformation. of the NE-SW-trending faults. major disThe basementpaleosurfacebelow the Martin placementstook placeproducingeither basinsor Formation displays effects of mainly physical grabenswhich were later filled with the Martin weathering. Formation. Two major episodesof. faulting have been recognized.The earlier phase occurredtowards the end of or after the main period (D2) of Host RockAlterations Hudsonian folding and granitization.It is representedby E-W and NE-SW-strikingmyloniteand Various alterationsof metasomaticand hydrobreccia zones a few meters to severalhundred thermal nature affectedthe crystallinerocks of meters wide and up to 10km long. The second the Uranium City region during pre-, syn- and
194
5 Selected Examples of Economically Significant Types of Uranium Deposits
post-ore episodes.Most prominent are albitiza- mineralbut dolomite,quartz,chlorite, and locally tion, epidotization,silicification,carbonatization, albite are alsopresent(Fig. 5.2a). Polymeullic mineralization consists of pitchchloritization, and hematitization. At Gunnar, desilicification/episyenitization and carbonatiza- blende accompaniedby Co-Ni-arsenidesand and native Pt. tion markedly altered the ore hostingzone within sulfides, Co-Ni-Pb-Cu-selenides the albitized country rocks (albitic granite- Au, Ag, and Cu. Associatedganguemineralsare albitite). Albite occurs in some veins intimately calcite, dolomite, ankerite, siderite, quartz, with pitchblende(Robinson1955)sug- chlorite, and abundant hematite. Polymetallic associated gestinga partialoverlappingof Na-metasomatism dvpositsare smallin size(<60mtU:Oe). The basementhostedmineralization occurs in and pitchblendedeposition. subsidiarystructureson both sidesand within 200 to 300m of major faults in the form of massive veins and veinlets (cm to m thick), and as Principal Characteristicsof Mineralization disseminationsin breccia zones (<15 m wide) Epigeneticmineralizationscanbe subdividedinto Ore bodiesmay suboutcropat the Paleo-Helilaan those of simple (or monometallic)and compiex unconformity or may be blind. The length of (polymetallic) mineralogl'. Over 90% of the individual veins rangesfrom meters to tens of deposits are monometallic, including all major meters,rarely hundredsof meters.Depth extenore bodiessuchas the Ace-Fay and Verna. Their sion. which is often intermittent, varies comdistribution is restricted to the Tazin Group, monly between100to 300m but persistsfor up to whereas polymetalltc deposits have been found 1660min the Ace-Faymine. Large ore bodies are of two principal conboth in the Tazin and the overlying Martin Formation. Monometallic deposis have pitch- figuratioru characterized by different internal blende as the principal ore mineral and locally structural preparation and distinct host rocks as brannerite and coffinite. Associated metallic exemplifiedby the "01" and "09" ore bodiesof by Smith(1986)(Figs.5.25,5.26). minerals include hematite, pyrite, and minor Fay described "01", is a highly elongated and first one, amounts of chalcopyrite, bornite, clausthalite, The plunging galena. gangue breccia stockwork cemented is steeply and Calcite the dominant
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Bornite 6oleno Pitchblende Cloustholite
Fig. 5.A. Beaverlodgedistrict, paragenetlc sequence of simple (monometallic) mineralization ( After Sassanoet al. 7972)
195
Uranium Deposits Examplesof Subunconformitv-Epimetamorphic-Type
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196
5 Seleeed Examples of Economically Significant Types of Uranium Deposits Ace -Foy
Verno
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[29] Pitcnutendeore body Fig. 5.26. Beaverlodge district, Ace-Fay and Verna deposits, generalized cross-sectionsdisplaying the attitude of ore lenses.tbeir position in the hanging wall (a) and footwall (b) of the St. l,ouis Fault. and host rock litbologies. Smith (1986)showsManin sedimentsdown-faulted on the St. Louis Fault (a). Tremblay (1978)attributes these rocks to the Lower Proterozoic unis- (After Smith 1986) (reprinted with permission from The Canadian Institute of Mining and Metallurgy)
by pitchblende and assosiatedminerals. It is restricted to the immediate footwall of the St. Louis Fault in mylonitized quartz-feldspar metasomaticgranite (quartz albitite or albitite accordingto Hoeve 1982) and parallel to that rock's contaa with the underlying Tazin strata hostingthe "09" ore body. The secondvariety. "09" is a highly elongated, steeply plunging networkof veinsemplacedin micaschist,argillite and amphiboliteof the Tazin Group. At leastfour generationsof pitchblendehave with agesdatedat about1780beenestablished, 1740,1110,270, and 100m.y. and less(Koppel 1968:Bell 1981.Table5.18).
U:Oe. Within tie metasediments,Beck found gneissicbands in zones of metamorphic and migmatitic rocks which contain uraninite, most with biotite. Someuraninitecommonlyassociated bearinggranitesand gneissescontain local concentrationsof up to 0.07"hU:Os. Trembla,v (1978)reportscontentsof 0.3 to 6.8ppm U for the and up to 13ppm U (average main metasediments 5.4ppm)ir granites.
PotentialSourcesof Uranium
Host Environment
Ore Controlsand RecognitionCriteria Significantore controlling parametersor recognition criteriaof the maior depositsinclude:
Although rrc distina uranium source has been establishedvarious rock types have anomalous uranium background values indicating the presenceof a uranium province and the availability of uraniumfor ore formation. Beck (1969) noticed three varieties of uraninite-bearing pegmatiteswhich have contentsof up to 0.08%
Deposits are emplaced in Archean-Lower Proterozoiccrystalline rocks affected by regional metamorphism, retrograde metamorphism,and strongalteration. The basementis coveredb1'redbed sediments and volcanicsof Paleo-Helikianage (Martin Formation).
Examplesof Subunconformity-Epimetamorphic-Type Uranium Deposits
197
Table 5.lE. Uranium City Region,selectedgeochronological data Rock/Ore
Agein m.y
Reference
(U-Pb zircon) 2510 F o o tB a y ' G n e i s s (U-Pb zircon) 2179 DonaldsonLake Gneiss 2155+ 40 (Rb-Sr*'hole rock) Box, Frontier granites 1975+ 20 (Rb-Sr whole rock) Pegmatite(Ace-Fayarea) 1835+ 50 (K-Ar wholerock) Gabbrodike tn5+q Gabbrodike t721 + 5l Gabbrodike, chloritized (K-Ar mica) gneiss 1795 Quartz-biotite-feldspar (8 km E of Uranium City) i630 + i80 (K-Ar whole rock) Martin Fm. basalt 1410+ 100(K-Ar wholerock) llartin Fm. gabbrosill (U-Pb) Uraninitein pegmatite(1) 1930+ ,10,(2) 1860 Pitchblende
+ 20.(2)L1q ( 1 )1 7 8 0
(u-Pb)
Pitchblende
( 1 )1 1 1 0+ 5 0 .( 2 ) r 1 0 0
(u-Pb)
Pitchblende
( r ) 2 7 0 +r s . ( z ) 2 1 0
(u-Pb)
Pitchblende
100to recent 270
Tremblayet al. 1981 Tremblayet al. 1981 Bell & McDonaidi982 Sassano et al. 1972a Wanlesset al. 1967 Wanlessct. Tremblay1978 Tremblay i972 Lowdon et al. 1963 Wanlesset al. 1966 Wanlesset al. 1966 Kdppel1968( 1), recaiculated b y B e l l 1 9 8 1( 2 ) Koppet1968( 1), recalculated by Bell 1981(2) Kdppel 1968(l), recalculated b y B e l l 1 9 8 1( 2 ) Koppel 1968(l ). recalculated by Bell 1981(2) Kdppel 1968(1), recalculated by Bell 1981(2)
Paleoweathering of the basementat the Paleo- - Uranium mineralization commonly occurs in a Helikian unconfonnitywas mainly of physical zone proximal to a major fault extending to a nature, which is in contrastto the chemically maximum of 200 to 300m away from that fault. weathered Meso-Helikian paleosurface as preservedunder the AthabascaGroup further Akerarion south (see AthabascaBasin Region, Chap.
s.1). Crystalline rocks, particuiarly in mineralized areas. exhibit an intense mylonitization of early age being rehealed prior to uranium empiacement. Most depositsoccur at a specificstratigraphic level within the TazinGroup. Major depositsare hostedby or occurproximal to mainly mafic rocks (amphibolite,chloriteepidote rock. chlorite schists,argillite). Few depositsare in granitoidrocks(Dubyna: quartz-feidspargraniteadjacentto mafic Foot Bay Gneiss. Gunnar: albitized and carbonatized,sranite). Some small deposits,in particular complextype mineralization, are in sediments and diabaseof the Martin Formation. Secondaryfaults, fracturesand brecciazones in proximitv to regionalor major faults(e.g., St. Louis, Black Bay, and Boom Lake faults) host most deposits. Structuresof NW-SE, NE-SW, and E-W strike appear to be equally favorable for uranium emDlacement.
-
-
-
Ore related wall rock alteration includes hematitization (dusty red hematite), chloritization (Mg and Fe chlorites) and carbonatization (mainiy calcite) commonly extending for oniy a few centimeters and rarely up to 4 m into the walls of mineraiized structures. Pre-ore alteration includes widespread hematitization reflected by disseminated hematite in breccia and mylonite zones and pink to red colored feldspars. Silicification. chloritization, epidotization. and carbonatization may be in part pnor to or contemporaneouswith early stagesof mineralization. Widespread Na-metasomatism resulted in albitization, locally leading to albitite. Hoeve (1982) suggests an intimate connection between Na-metasomatism and pitchblende formation.
Mineralization -
Ore bearing structures may be simple veins, networks of interconnected veins and veinlets, and breccia zones.
198
5 Selected Examples of Economically Significant Types of Uranium Deposits
- Tabular or planar ore shoots are elongated parallelto subparallelalongthe plungeof fold axesin host rocks. - Host structuresare onlv oartiallv filled with mineralization. - Ore zonesmay or may not outcropon surface, e.9., at Verna all larger ore bodiesare blind. - Individual ore shoots preferentially follow specifichorizons.In the Verna mine individual mineralized structuresare arrangedin a series of irregular lensesand tabular veins stacked above each other. Thel' follow drag folds and contortionsof the stratawithin the limits of the overturnedsyncline.For example,ore shoots of the "79" zone follo*' in cummulationthe fold axis of the Tazin svnclinestayingwithin a horizon of argillaceousmica schist. - Grade of the ore bodiesmay vary depending on the dip of the enclosinsstrate.For exampie. in the Verna deposit,at a steepto verticaldip in the plane of shearing.ore bodiesquite commoniy have higher gradesand are narrower compared to those which are horizontaliy flattened by folds. Here the ore bodies are thicker but of lower grade. - Two kinds of mineralassociations exist,one of simple (monometallic) and one of complex (polymetallic)mineralogy. - Depositswith simple mineralogyare restricted to the basementand containseveralphasesof pitchblende associatedwith calcite, chlorire, and quartz as parageneticgangue minerals, and minor sulfides.Age of the first generation of pitchblendeof this type is 1780to 1740m.y. (Kdppel 1968;Bell 1981). - Depositsof complexmineralogyhavea similar mineral assemblage but have additionallyCoNi-arsenides,sulfidesand selenides,and native preciousmetals. Pitchbiendeof the complex ore assembiageis dated 1110 to 1125m.),. (Koppel1968;Bell 1981)whichcorresponds to the secondgenerationof the simpletype. and typicallyis hosted in srrucrurescutting both Tazrn metamorphics and overlying Martin sedimentsand volcanics. - Initial temperatureof ore formation is about 440 + 30'C (Sassanoet al.1972b).
thermal origin, although some considered uraniferous Aphebian sediments as the source for the uranium. Sullivan (1957), Smith (1974), Langford (1977), and Tortosa and Langford (1986) have argued in favor of a supergene origin. Tremblay (1968, 1972) and Beck (1969) have proposed a metamorphic-hydrothermal model which was supported by Sassanoet al. (1972b). Ore formation in sensu stricto and the composition of the ore forming solutions have been discussedby Beck (1969, 1986). Based on mineraiogicai studies, Beck concludes that the paragenetic mineral sequencesindicate a considerabie change in the chemistry of solutions throughout a long and complex evolution of the mineralization. Sulfide minerals, in a general way, increased in variety and amount with time which suggeststhat the early uraniferous fluids were reiativelv free of other metal ions while later fluids must have been more enriched in metals. On a local scale, however, there are manv diversions from or reversals of this sequence. In spite of the variations in content and quantity of various metals, the mineralizing fluids must have contained both calcium and CO2 ions, as reflected by ubiquitous calcite associated with all pitchblende generations. Consequently, it may be deduced that uranium has been transported as carbonate complex. Initially, uranium may have been precipitated from a colloidal solution, as suggested by the colloform habit of early pitchblende. The later pitchblende, which is chiefly massive, has probably deposited from noncolloidal solutions. With respect to the mechanismsof ore mineral deposition, Beck states that loss of pressure and catalytic reaction with iron were the crucial factors in causing precipitation and that the structural environment has determined which of the two factors was dominant in the formation of a specific ore zone. Breccia zones and other open-space fractures provided the site for most of the large botryoidal massesand massiveveins of pitchblende. Because these structures develop open spaces it has to be assumed that a loss in pressure occurred. On the other hand hematite is here not ubiquitous and therefore iron involvement cannot be considered significant for the precipitation of colloform and massive pitchblende. Beck therefore concludes MetallogeneticConcepts that a drop in pressure has been the domrnant factor in precipitating this pitchblende. Most of the early investigators of the Beaverlodge In contrast, fine-grained pitchblende disuranium depositspostulateda magmatichydro- seminations are restricted to stronglv hematitic
Uranium Deposits Examplesof Subunconformity-Epimetamorphic-Type
rnylonite, and shear zones. In this environment confining pressureswere probabty high and the oimary mechanism of pitchblende precipitation \r'as most likely catalytic reaction of the uraniferous solutions with ferrous iron in the wall ;ocks. Since their initial emplacement, the pitchblende accumulationshave had a long history of reworking and rejuvenation. Rejuvenation of pitchblende is believed to have been caused by localized tectonic-thermal .\,ents, including downwarping of the basement pnor to deposition of the cover rocks, subsequent '-roliftand folding of the cover rocks, intrusion of ::ibase dikes and gabbro sills, and intermittent :tovements along the major fauits related to :peirogenic movements in the Canadian Shield. Beck (1986) addresses the problematics of inrerpreting relevant geologicaiparameters with respect to their impact on ore formation. He points in particular to the following three criteria. a) Does there exist a spatial relationship of the deposits to (pre-Martin) faults only or to the Paleo-Helikian unconformity as well on which the Martin Formation was deposited? b) Is a chemical paleosol developed along the the Martin basement surface below Formation? c) Is initial pitchblende formation related in time to a Hudsonian process,for example granitization, metasomatism (albitization) or perhaps to iate orogenic retrograde metamorphism? Beck (1986) notes to point a: that the deposits are more closeiy related to major faults and their subsidiary structures than to the pre-Marrin unconformitv although there appears to be a spatial relationship to both structures and unconformity. Smith (1974) argues for an uncontormity relationship. He contends that all deposits are rvithin 200m of the unconformitv. Smith explains the deep extension of the AceFay veins (>1700m), which is by far in excess of that of unconformity-contact deposits by suggesting that the St. Louis Fault closely follows the plane of the unconformity. The basis for this contention are slices and lenses of rocks defined by Smith as Martin Formation found down to a depth in excess of 2100m aiong the trace of the St. Louis Fault. In contrast. Tremblay (1978) considers these rocks to be brecciated Tazin GrouD strata.
199
To point b: There is no chemical weathering profile deveioped on the basementsurface. The Martin Formation rests directly on almost fresh Archean-Aphebian rocks except for a thin layer of. detritall physical weathering products. To point c: Initial pitchblende deposition can most likely be attributed to a Hudsonian event as suggested by the coinciding parameters of geochronologygiving an age of 1780to 1740m.y. for primary prtchblende, a K-Ar age for mica in metasedimentsof 1795m.y. (Lowdon et al. 1963) and the structural relationshipof depositsto Hudsonian faults which have been reactivatedin later penods. The influence of the Marrin Formation could theoretically have been twofold. Its thickness, composition, and associateddiageneticprocesses may have had an impact on the original ore formation similar to that assumedfor unconformitycontact type deposits in the Athabasca Basin. There is, however, no visible evidenceof such an involvement. Also in order to exert any inffuence, the Martin sediments would have to be at least 1 7 8 0t o 1 7 4 0 m . y .o l d . Another, more valid aspect is the role of the Formation as a protective cover Martin prohibiting leaching and destruction of near surface deposits by weathering. The postulated intense wet tropicai period preceding the deposition of the Meso-Helikian Athabasca Group (see Chap. 5.1, Athabasca Basin Region) must also have been active in the Uranium City Region. Beiow the Athabasca Group lateritic r,vpe weathering extends to depths exceeding 100m along structurally weakened zones. If not protected, near surface deposits would have been destroyed by these processes. In conclusion. presently known data, which are rather limited particularly in the minerochemicai and geochemical field (fluid inclusions,stable isotopes, geochemistrv of metasomatites, U-Clarke figures), suggest with certain reservations a metamorphic-hydrothermal origin for the first pitchblende generation of the Beaverlodge deposits. Tremblay (1978) presented a comprehensive model for the geological evolution of the Uranium City region which appears in principle still valid, and within this frame a metamorphic' hydrothermal origin of the Beaverlodge uranium deposits. His concept is as follows. 1. The accumulation of sedimens and volcanics of the Tazin rocks started in Late Archean
2m
5 Selected Examples of Economically Significant Types of Uranium Deposits
time (this, however,is rejectedby other authors, who proposea Lower Proterozoicage).The Fay Complex, regarded as the possible uranium source for the various uranium deposits of the Beaverlodgearea, is near the baseof the known and is composedof interbedded Tazinsuccession, and metavolcanics. metasediments Someof theserockshave relativelyhigh background uranium concentrationswhich are regardedas syngeneticaccumulations. 2. The sedimentsand volcanicsof the Tazin Group were folded, metamorphosedto the amphibolitefacies,and locallygranitrzedbetween 2300and 2000m.y.. leading to the formation of granite, granite gneiss, and pegmatite dikes (approximately 2000m.y. ago). This orogenic activity mobilized the uranium in the Tazin rocks into specializedgranites and pegmatiteswhiie syngeneticuranium accumulationsin the metasedimentsare now preservedin xenolithswithin granite and gneiss. Some epigenetic (vein) depositsmay have formed during theseevents, but no vein deposits of that age have been identified. 3. Toward the end of the Hudsonianorogenic activity (about 1800m.y.),the Tazin rocks were cataclasticallydeformed. Wide mylonite and breccia zones developed accompaniedby lowtemperature retrograde metamorphism and the circulation of metamorphic-hydrothermal solutions. Tremblay suggeststhat uranium became mobilizedduring one of theselast stagesof metamorphismthat affectedthe Tazin rocks and that the original formation of the vein deposis of the Uranium City Region occurred at this time by metamorphic-hydrothermal processes. 4. The second main period of faulting and formationof epigenetic(vein) depositstook place at about1110+ 50m.y.The secondgeneration of vein development correspondsto a period of uranium mobilization and concentrationin the Beaverlodgearea and in all of northern Saskatchewan. Faulting and uranium redistribution continued after this event, as evidenced by younger pitchblende ages. However, Tremblav (1978)notes,."it is possiblethat there was only one period of uranium mineralizationat 1780+ 20m.y., and that all the other pitchblendedates representmerely periodsof uranium remobilization and not new mineralizationperiods." 5. At a much later time, due to weathering and groundwatertransport, uranium may have movedagainwith precipitationin breccias,along
fractures, and at the unconformity to form supergenedeposits.The Bolger deposit may be of this type. This model is supported by Sassanoet al. (1972b), who propose that the deposits were generatedby metamorphic-hydrothermalfluids with somepossiblecontributionof surfacewaters during the final stagesof mineralizslion to an otherwise"closed" system.This hypothesiswas suggestedas a resultof fluid inclusionand stable isotope(613C,618O)studieson dolomite,calcite. and quartzfrom ore of the Fav and Bolger mines. The studies identified five generations of carbonatesand a cooling history from the initial phaseof mineralizationat M0 + 30"Cdown to the final stagesat about 80 + 10'C. Objectionsto any kind of ore-forming event related to granitizationare expressedby Beck (1986).Rb-Sr datesfor the pegmatitesfrom the Box and Frontier granites (1975m.y. and 2i55 m.y.) are too old to link this episodeto that of initial pitchblende emplacement (1780 to 1740m.y.).This, of course,doesnot contradict ore formation related to a late metamorphic/ orogenic event. By comparison with the Hercynianterranein Europe, which also evolved through several orogenic episodes, all major uranium deposits formed during the latest orogenicevents.This tendsto be consistentwith the Beaverlodgedeposits,becauseinitial pitchblendeprecipitationcoincidesrougbly with a late metamorphiceventdated about 1790m.y. which may have beenthe motor for the generationand circulationof the ore solutions. The secondproblemaddressedby Beck (1986) is the dilemma of the spatial coexistenceof the two diverse types of ore, monometallic and polymetallic. Hoeve and Sibbald (1978) and Hoeve (1978. 1982)contend, however, that the two parageneticassemblages are not cogenetic: instead they have formed during subsequent stages. This is indicatedby differentradiometricages, stratigraphicsetting, and wall rock alterations. Deposits of simple mineralogyare restricted to Tazin lithologies,and yield a pitchblende age of 1780 to 1740m.y. Deposits of complex mineralogy are hosted by fractures in Tazin metamorphicsand overlying unmetamorphosed Martin strata as well, yield an age of about lI25m.y., and host rock alterationis similar to that of unconformity-contact type depositsin the AthabascaBasin. Hoeve and Sibbaldconclude
Examplesof Vein-Type Uranium Deposits
rhat monometallic (simple) deposits are of Hudsonian metamorphic-hydrothermal origin and polymetallic deposits belong to a second generation of mineralizationcorrespondingto the i r s t r e m o b i l i z a t i o ne p i s o d ea t a b o u t 1 1 0 0 m . y . :stabiished by Koppel (1968). They also compare the polymetallic deposits with the unconformity-contact type deposits of the .{thabasca Basin and consider both to have formed as a result of the same metallosenetic event.
Referencesand Further Reading for Chapter 5.2.2 ,for details of publicationsseeBibliography) .{lcock1936;Beck1969,1970,1986;ChristieandKesten 1949;Dawson19561Eldorado1964:Evoy 1961,1986: Gandhi1983:Griffith 1967;Hoevei982;Kalliokoskiet al. 1978; Krupicka and Sassano1972;Lang et al. 1962; \lurphy 1948; Robinson1955a,1955b1Sassano7972; et al. 1972a,1972b;Smith 1974.1986:Thomas Sassano 1982;Tortosaand Langford1986;Tremblay1958,1970, 1972,1978;TruemanandFortuna1976:Turek 1965;Ward 1984.
5.3 Examples of Vein-Type Uranium Deposits(Type 3: Chap. 4) 5.3.1 IntragraniticVein UraniumDeposits: Limousin/LaCrouzilleDistrict, France The Limousin region is located in the northwestern Massif Central and includes the La Crouzille uranium district. the most important uranium district of France. Although most of the deposits are smail, (some tens to few thousand tonnes UrOs) the entire district cumulatively contained estimated reserves including production in excess of 40000mtu3os. Ore grades range in average between0.1 and 0.6% U3O8. Most depositsare centeredin an area, 15km tn N-S length and 5 km in E-W width, within the Saint Sylvestre granitic massif (Fig. 5.27). The deposits are structurally controlled within a distinct leucogranite considered the parent rock of the uranium and are therefore classified as granite-related. intragranitic vein uranium deposits (class3.1.1., Chap. a). Two varietiesare distinguished, classical veins and disseminations in eoisvenite bodies.
201
The following descnption is largely based on Leroy (I918a,b) who presents a comprehensive study on the Limousin deposits (Margnac and Fanay). Additional information is taken from Geffroy 1971,Moreau and Ranchin (1971), Ranc h i n ( 1 9 7 1 ) ,S a r c i aa n d S a r c i a( 1 9 5 6 ) , a n d o t h e r geoscientistscited later under References.
GeologicalSetting of Mineralization The Limousin uranium region is within the Moldanubian Zone, i.e., the core zone of the Hercynian orogenic belt (380-3t){)m.y.) and located at the junction of two regionai structural belts. The region is underlain by Proterozoic to Lower Paleozoic metamorphics which were intruded by granites and regionaily metamorphosed, locally attaining anatexis, during the early Paleozoic.During the Hercvnian Orogeny, a variety of granites invaded the ancient metamorphic basement. In the Crouzille uranium district (Fig. 5.27) the Saint-S.vlvestre leucogranitic complex intruded ca. 325m.y. ago (Holliger et al. i986). It includes wo principal genetically related granitic facies, the Brdme and the St. Sylvestre (-St. Goussaud) facies. Both are transected by dikes and stocks of younger finegrained granite and other magmatites (lamprophyre, pegmatite, aplite, quartz veins). Intrusion of the lamprophyres occurred ca. 285 m.y. ago (Leroy and Sonet 1976). The Brdme facies occupies the western part of the massif and constitutes the basal part of the granitic complex. It consists of migmatites and foliated porphvritic to medium-grained granites, is less differentiated than the St. Sylvestre facies and contains large xenoliths of partly transformed gneisses.Only minor U deposits(e.g.. Montulat) are present. The Sr. Sylvestrefacies, which hosts all major U deposits is highly differentiated and contains no xenoliths of gneiss except at the periphery adjacent to surrounding metamorphics. It occupies the central and eastern section of the massif. Major rock constituents of the St. Sylvestre facies are in average some 369'" quarlz. 27o/o orthoclase. 277o albite, and 10"/" muscovitebiotite (Ziegler and Dardel 1984). The average chemical composition is given by Dardel/ Peinador Fernandes et al. (1979\ as 7L-74"h SiO2, 14-15"/" Al2O3,1.3-1.6% FeO + Fe2O3,
202
5 Selected Examples of Economically Significant Types of Uranium Deposis
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Fig. 5.27' Limousin, La Cro'rzjlie district, generalizedmap of petrographicfaciesdisrribution and location of principal uraniumdeposits.(After Barbier and Ranchin1969a;Marquaireand Moreau 1969;Ranchin l97l:Ziegler and Dardel 1984)
: 0.2-0.5% MgO, 0.6-7.0% CaO, 3.3-3.8% Na2O,4.8-5.8% K2O, 0.2-0.25% TiO2, 0.30.4o/oPzOs,1%lossof ignition. Mineralogicaland chemicalcomposition.however, vary strongly but follow a coherent and continuousevolutionof the massifas a whole. The dominant facies is coarse- to mediumgrained,oftenporphyritic granite characterizedby
an abundance of Al-K phases, mainly Al-rich biotite, sillimanite, andalusite, cordierite, and muscovite over quarLz, an acidic plagioclase with commonly less than An26 and anomalous concentrationsof lithophile elements such as U, Th, Be, Li, F, Sn, and W (Friedrich and Cunev. 1986).
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5 Selected Examples of Economically Significanr Types of Uranium Deposits
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Highly evolved phasesare sodic with decreased K, Ca, Mg, Fe, Ti contents, and enrichmentsln incompatibleelementssuch as Sn, Li, Bi. As' Cu, Cs, and F. They typically contain albite, muscovite phenocrysts, Li, Fe and Al rich (XFe = 0.9, 416* = 2.2) biotite, highly perthitic orthoclase, Th-poor uraninite, anatase,apatite. rare zircon, and monazite. The least differentiated facies are potassicwith enrichments of Ca, Mg, Fe, Ti, and either Cr,
+
I +
Co, Ni, Ba, Sr or Zr, Hf. Th. LREE, typical compatible elements in peraluminous melts' Characteristicrock constituentsare biotite (XFe : 0.6, 416* = 0.8), less common muscovite phenocrysts, prismatic sillimanite' abundant ilmenite, monazite. zircon, apatite' and rare uraninite. Migmatites occur localiy in the lesser evolved phase (Cuney et ai. 1985; Friedrich 1984).
Fxamples of Vein-Type Uranium Deposrts M i n e t t eC o u r t y \ M i n e t t eL e t o r t = I M in e t t e C o n t
d
205
Gronite Pitchblende M o rc o s r te / p y r i te
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+++ 'l r
cm
mine. a Block diagram 1 7 0 Fig. 5.30. La Crouzille.Henriette of deposit showing relationshipof lamprophyre (minette) dike. to fault plane and vein formation at their intersection. (After Roubault 1958) b Schematic NW-SE 2 0 0 section perpendicular to strike of lamprophyre dike. (After Sarcia1958:Rouba,rlt1958).c Planviewsof intersection of lamprophyre dike and uranium vein at various inetteCoutty levels. (After Sarcia et al. 1958) d Parageneticmineral assemblagein uranium vein. (After Geffroy and Sarcia Minette Contiont 1954)
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206
5 Selected Examples of Economically Significant Types of Uranium Deposits
Numerousdikes and stocks of peraluminous tion in lamprophyre. This first stage alteration fine-grained granites geochemically of highly chemically corresponds to potassium metavariable composition intruded the coarse- to somatism associatedwith decrease of Si, Na, and medium-grained facies prior to its final con- Ca in granitic wall rock, and increase in SiO2 and solidation. A particular group of fine-grained decrease of Na, Ca, Mg and occasionally Fe in granites intruded near or into regional synmag- lamprophyres. Trace elements in pitchblende matic mylonite zones. These intrusions have reflect these chemical processes (Cathelineau higher concentrationsof both incompatible(U, et al. 1979). In a second stage, contemporaneous Sn, Li, F) and compatibleelements(Th, Ce, Zr. with development of coffinite from pitchblende Ba, Sr, etc.) than the surroundingcoarse-grained occurred hematitization, montmorilionitization, granites(Friedrich et al. 1987). Trace eiement formation of some adularia. and silicification of patterns within the mylonite zones correspond wall rocks. to those of associated fine-grained granites (Friedrichand Cuney 1983). Duthou (L977) reports eievated initial Sr Principal Characteristics of Mineralization isotoperatios(0.706to 0.711)and Turpin (1984) high d18Ovalues(+11%) for the peraiuminous Two settings of mineral2ation are recognized leucogranitessuggesting an anatectic, partial within the host granite, veins cutting the granite melting origin of. the granites from metasedi- ( F i g s . 5 . 2 9 , 5 . 3 0 . 5 . 3 1 ) a n d d i s s e m i n a t i o n si n mentarysialic cmstal material, very probablyof episyenite (Fig. 5.31). The mineralogy is verl' Cambro-Ordovicianage as proposed by Cuney similar in both settings. Primary mineralization is characterized by a and Peyrel(1984). trend N-S, NW-SE, NNE-SSW simple mineral association. Pitchblende is the Principalfau/as and E-W to ESE-WNW (Fig. 5.28). dominant uranium mineral and is mainly accompanied by iron sulfides. Other sulfides of Cu, Pb, Zn, and Mo are scarce. Gangue is present in Host Rock Alterations minor amounts and consistsof chalcedonic silica. fluorite, and locally baryte and calcite. Pre-ore granite aherations include albitization, The following principal paragenetic sequence muscovitization, greisenization, dequartzifica- f.or vein-rype mineralization is established by tion, chloritization, and argillitization. Pre-ore Geffroy 797L,L,eroy 1978a,b,Poty et al. 1986:
alterationsof hydrothermal nature resulted in 1. Nonhematitic, transparent quartz deposited localized qvartz dissolution associatedwith decomblike on vein walls, structionand neoformationof minerals,a process 2. spherulitic pitchblende associatedwith pyrite, termed epbyenitizationby French geoscientists. 3. microcrystalline quartz, at first chalcedonic Two types.ofepisyenitesare distinguished, earlier passing ilto combs and into large zoned feldspardominant episyenites(commonlybarren crystals containing pyrite and hematite: of uranium) and later formed mica (muscovite) hematitization immediately foliows pyrite dominant episyenites(which often - but not during quartz deposition; locally ankerite and necessarily- contain uranium deposits).Turpin hematite occurs instead of hematitic quartz, (1985)givesan ageof episyeniteformationat ca. 4. brecciation, 300m.y.ago. 5. transparent quartz associated with pyrite, Ore-relatedwall rock aherationsextend into marcasite; coffinite by alteration of stage 2 countryrocks for commonll'lessthan 0.5m from pitchblende, a pitchblende bearing vein. The characterof 6. banded purple fluorite, white calcite, and pink alteration dependsto some extent on the host baryte. rock. Leroy (1978a,b)notes for the La Crouzille district, a first stage, synchronouswith pitch- Disseminated type mineralization is mainly blendedeposition,of more or lesstotal musco- located in mica episvenitic pipes. some is in vitization (commonlyphengitization)of potassic brecciated leucogranite and rarely in feldspar feldsparsin leucogranitesand mica episyenites episyenite. Pitchblende and pyrite are the first and of the feldspathicmatrix, and chloritization minerals to form. Pitchblende coats rock faces of pyroxenesand biotite, associated with pyritiza- and penetrates into fractures and/or cleavages.
Examplesof \':in-Type Uranium Deposits
?0'7
-+,l-'ru" E-^
-;
-
\
l-
-
+
T
q'$.2
--
-
-+
Ra
+i,
-1-f 0
_:
15
_ \..
9^
T*o-m,co leucogronite
Fl
i ? i F l F e t d s p o r e o i s - v e n i t eu. n m i n e r o l i z e c i fIil
v i c o e p r s y e n i r e .m r n e r o l i z e d
f,
t " t i c oe p r s y e n i i e ,h i g h l y m i n e r o l i z e d
b
f]l
2z5m-
Frocture, iqult
@.
r r:-:rized vein
SE
NW Q
tcvet
-
EOm
- 1',t5m - 1 4 5n -185 m - 225m :
\i.:
rir
\:
.\ \
N
-305m :l :02 ---..---::':-----_l
0l
'/. Uh.i
. n\\ \
o fa r g n a c - l v e i n s w i t h M a r g n a c 2 F i g . 5 . 3 l . L a C r o u z i l l e , M a r g n a c m i n e . a P l a n v i e w omf i n e r a l i z a t i o n a t i n t e r s e c : ( r nM mrca-episyenite.(After de Fraipont et al. i982). b Cross-sectionrhrough miner.r-ir::cimicaepisyeniteof iVlargnac2 cut bv the uranium vein svstem of Margnac -1 (After Dardel/Peinador Fernandesel j. 1979: Leroy 1978b)c Zonation of uranium grades aiong Margnac .l veins. (After Chapot 1979)
Quartz. more rare than in veins,growsas very dip (Figs. 5.lS :o 5.31). The veins have dimentiny crystals with marcasiteand rare prismatic sions from a tew meters to several hundred coffinite at its base.This quartz is overgrownby meters but mav *xtend for up to a kilometer long. concretionarycoffinite. Fluoriteand subsequentlv Thickness of mineralized veins is frequentlv (1 to 2m but can range from few centimeters up to caiciteare the last mineralsto form. Mineralizedveinscommonlvtrend aboutNW- 15 m in highlv !:taclastic zones. Vertical extenSE and to a minor degreeE-W. and havea steep sion of the ore shoots varies between some meters
208
5 Selected Examples of Economically Significant Types of Uranium Deposits
(feldspar-and mica-episyenite), chloritization, and >300m. The gradeis highlyvariable,ranging and argillitLation. betweenseveralhundred ppm to >0.25y"U3O8 (Fig. 5.31c) but may be as much as several - Ore-related a.lteration includes muscovitization, chloritization, and locally pyritization. percentU3Osparticularlywhen veinsintersector A second-stagealteration associated with follow mafic dikes such as lamprophyres(Figs. uranium redistribution includes hematitiza5.29,5.30). tion, montmorillonitization,silicification,and Episyenite-hostedmineralization, may have adulariaformation. dimensions between several tens to several hundreds of square meters in plan-view and vertical extensionsbetween about 10 and 300m Mineralization (Fig. 5.31). The averagegrade is generallyhigh, ranging up to 1%U3O8 and more. At some - The primary ore control is by structure, locations, where mineralized veins enter into secondl_v by lithology. episyenite,the ore grade may range from 1 to - Two principal ore settings are recognized. veinswhich predominantlvtrend NW-SE and 10%u3o8. processes causedremobiliWeathering-related disseminationsin mica-episyenite.Micaepisyenitesare commonly only' mineralized zationof original mineralizationand redeposition of uraniumas"black products" (neo-pitchblende, when iltersected by or adjacent to pitchpara-pitchblende,sooty pitchblende)below the blendeveins. water table and in the cementationzone, and - Ore is monometallicand of simplemineralogy. consisting of pitchblende and alteration as colored hexavalent uranium mineralsin the productsthereof. oxidizednear surfacezone commonlyextending to a depth of 20 to 30 m and locally to 80m in - Associated minerals are dominantly pyrite. most of the Hercyniangranitic massifsin France. marcasiteand traces of Cu, Pb, Zn, and Mo sulfides. Gangue minerals include quartz, Strong radiometric disequilibriumis typical for this phreatic zone of meteoric water circuiachalcedony,fluorite, baryte,and calcite. tion and mineral redistribution. Where the remobilizationoccurred in complexelyfractured zones,as at Brugeaud, it resultedin an enlarge- MetallogeneticConcepts ment of ore bodiesbut also in dilution of the ore grades. The following synopsisreviewsthe evolution of Isotope agesof primary pitchblende are ca.275 uranium-fertile leucogranitesand the metallogenetic principles of related vein uranium deto 270m.y. (Leroy and Holliger 1984). positson a broader scale.The documentationis not restrictedto the Limousin uranium region. Ore Controlsand RecognitionCriteria althoughthis region servesas the main exampie. contributedfundamentaldata Many geoscientists Significantore controlling or recognitioncriteria to the understanding of thesevein depositsand of vein uranium depositsin the Limousindistrict their parent granites.The most recent and cominclude: prehensive researchis publishedby Cathelineau. Cuney,Friedrich. Poty and their coworkers.In Host Environment essence,their results,amendedby data of the - Depositsoccur structurally'controlledperiph- other authors listed, may be summarizedas eral within a highly differentiatedperalumin- follows. ous leucogranitetransectedby synmagmatic mylonitezonesand dikesand stocksof variable Choracterbticsof IJraniferousLeucogranites lithologies (details of petrologl' and geoAs defined b1' French geologists, a fertile chemistrysee later). leucogranite is a highly differentiated acid peraluminous (A2O3/(CaO* Na2O+ K2O) > 1 Aheration in molecularproportions)graniteof crustalorigin - Pre-orealterationincludesalbitization, musco- which evolvedby multistagemagmaticprocesses vitization. dequartzification/episvenitizationand wassubsequentlyaffectedby deutericandior
Examplesof Vein-Type Uranium Deposits
209
It containsuranium in hydrothermalprocesses. valuesabove the Clarke figure and in form of The Th/U ratios uraniniteasprimaryconstituent. arelow rangingfrom <3 to ca.0.1(Clarke= ca.
aplitic veinlets and dikes of fine-grained granite and pegmatite (Friedrich et al. 1987). Mollier et al. (i985) figured out that the mylonite zones are dominantly confined to proximities of fine- 1) . grained granitic intrusions and when they occur also termed within coarse-grainedgranite thev are within a Severaltypes of leucogranites, r*'o-mica granites, exist. Some have reiated few hundred meters of those. Petrographic-geochemicalfearures of the main uranium deposits (fertile granites)others are facies of the St. Svlvestre granite have been devoidof them (sterilegranites). Stussi(1985)and La Rocheet al. (1980)divide presented earlier in the section "Geological Hercyniangranitesinto petrochemical facies,viz. Setting. . .". In essence, the petrochemical caic-alkaline,subalkalineand alumino-potassic,composition of a fertile granite is characterized by rhe latter leucogranitesfurther into those of an abundance of Al-K phases and anomalous and alumino-silico-sodicconcentrationsof lithophile elements such as U, ,iumino-silico-potassic metaldeposits T h , B e , L i , F , S n . a n d W b u t i n v a n ' i n g q u a n t i t i e s and attributeconsanguine :endenc-uand mineralogical distribution in the varies various facies: io the granitic facies. U and Th values are given in Table a) U depositsand W, Be, and Au mineralizations 5 . 1 9 .
are typically associatedwith alumino-silicopotassictwo-micaleucogranites: b) Sn, W, Li. Nb, Ta mineralizationswith Table 5.19.St. SylvestreMassif,U and Th vaiuesof variousleucogranitefacies.(Ziegler and Dardel 1984; alumino-siiico-sodic two-micaleucogranites. Ranchin1971) Distribution of type a and b depositsand Th/U provincescoincidesregionally,but is separated kucogranite facies U (ppm) Th (ppm) on a local scale. The depositsformed from ?-0.86 ?-12.67 8 . 7- 1 4 . 8 La Brdme solutions of different chemical and physical Chateauponsacu 0.47 16../. 8.71 properties(Dubessyet al. 1987). t.t3-2.17 t 6 . 0- 2 2 . 0 / . 5 . J l - X . O t St. Sylvestre nonhern pan with are associated c) Some U deposits/domains ? (Brugeauds) subalkaline granites locally in relation with westernpart late alumino-potassicmagmatism.This type (Margnac) 20.r-22.0 23.37-30.99 producedalso Mo and Cu mineralizations. easternpart 31.67 16.0 (St. Ldger) d) Calc-alkalinegranite appearsto be sterile for -21.2 L8.rr-26.27 0 . 8 5- 1 . 7 5 14.9 St. Goussaud any specificmetalpotentiai. 1-
I
"Fine-grainedfaciesintrusive in La Brdme leucogtantte.
Geotectonically,favorable leucogranitesappear to have intruded preferentially along regional lineamentsnarrow in width but severalhundred kilometers long, geophysically marked by U and Th occur predominantlv in rwo mineiongated gravimetric lows. (Dardel/Peinador erals, uraninite and monazite. Both have charactenstic chemical compositions and are typically Fernandeset al. 1979). Uranium deposits are restricted to leuco- associatedwith other accessorymilerals in the granitesof a slab-shaped intru- rwo principal stages of the St. Svh'estre granite. or laccoiith-type sion (St. Sylvestregranite:2-.1km thick, 50km The main stage is characterized by ubiquitous wide) emplacedduringthe laststageof an intense distnbution of both minerals. whereas uraninite regionai thrusting event and at a structurallevel prevails in the late stage, with particular concentrations along synmagmatic shear zones. correspondingto the sillimaniteisograde. Distribution of U and Th in the main stage is as Major svnmagmaticductile detormationzones transectthe batholith.They are metersto some follows. Uranium is predominantlv (50 to 79Y", hundred meters wide and persist for tens of average 60"/" of the whole rock U endowment) kilometerslong and apparentlytbrmedin an early incorporated in minute crystals of Th-poor magmatic stage. The zones display foliation uraninite. Zircon, monazite. and apatite account reflectedby onentation of K-feldsparmegacrysts for 25-35%, and 5-6% of the whole rock U is and biotite laths and are occupiedby numerous attributed to U adsorbed on mineral faces and
214
5 Selected Examples of Economically Significant Types of Uranium Deposits
microfractures. Uraninite in its divene disposi- values of U and Ca in monazite attest to its tions is commonly associatedwith apatite,zircon, equilibrium crystallizationwith uraninite, which and monazite of specificcomposition.(Ranchin contraststo the early magmaticmonazite. 1971;Pagel1981,1982a) Dikes of fine-grained granites transecting Thorium is bound in refractory accessory porphyriticbiotite gTanite(av. 6ppmU) conrain minerals, predominantly in monazite. Early up to 40ppmU (Fig. 5.32b). The dikes contain magmatic monazite in coarse-grainedgranite disseminateduraninite associatedwith apatite, containsa variefy of trace elements,but lacksthe zircon. and lJ, Th. Ca, Y-enrichedmonazite.In elementalenrichmentsnoticed in late magmatic gneissosesections,thesemineralsare preferenmonazites (see later). Principally all vaneties tially emplacedaiongshearplanes. of monazite of the St. Sylvestregranites are The late magmatic monazite associatedwith unusuallyhigh in ThO2 (6-14%) and alsoin UO2 uraninite in mylonites display characteristic (0.4-7%). Early monazite typically associates crystal-chemicalfeatures.It contains high conwith apatite and zircon (Cuney and Friedrich tentsof U. Y, Ca. Th, and LREE (Table 5.20). 1987;Pagel 1982a). These elementsare incorporatedinto monazite Late magmaric smge U and Zh occur in two by progressivezonal recrystallization.Monazites hostvarietiesswrmagmaticshearzonesand dikes of fine-_qrained granitesare likewiseenrichedwith of fine-grainedgranites. theseelementsbut lack zonation.Consequently, Synmagmatic sheqr zones display increased monazitesenrichedin the abovelisted elements background values of up to 100ppmU, with and displavingmarked zonation are suggestive an averageof 35ppmU and 40ppmTh, vs. an for uraninite bearing, highly leucocratic and averageof 8 ppm U and 10ppmTh in surrounding hygromagmaphile element-rich(Li. Sn, Rb. F. undeformedporphyritic granite(Fig. 5.32a).U is Be, etc.) graniteswhich constitutethe source predominantlypresentin form of uraninitewhich rocks for later formed pitchblendeveins (Cuney typically associateswith monazite, elongated and Friedrich1987;Friedrichet al. 1987;Ranchin zircon, apatite, anatase,and ilmenite. The high 1968.i971).
ljronium contcnt ppm8 12 16 20
2,+6E10
0m
2.t6E10
1m 2m 3m 4m 5m 6m /m
n n
W
peroluminous leucogronite incipicnt gneissiflcotion intcnsc dcformotion
E3
dike of fine-groincd leucocrotic Aronite
Fig. 5'32. St. SylvestreMassif, uranium content and uraninite abundanceassociatedwith a a severalmeters wide shear zon-ewithin peraluminous gTanite, b dikes of leucocraticfine-grained granite transectingbiotiric porphyritic granite. (After Friedrichet al. 1987,basedon Ranchin1968.1971)
Examplesof Vein-TypeUranium Deposits
ztl
of main magmaric(1) and late magmatic(2) monazites(in Table 5.20. St. SvlvestreMassif.chemicalcharacteristics wt.%). (Friedrichet al. 1987)
Yror
CelO3
{
2.30 2.01
["'
t.o) 2.82
25.38 25.1,1 26.08 24.48 22.70
Sampte
SiOr
CaO
UO.
ThO:
PrOs
a
1.13 1.76 0.39 2.01 3.85
1?..15 1i . 6 1 10.]9 10.99 10.6+
29.28 28.38 24.6{) 24.63 ?6.11
6.36 '7.09 6.00 5.89
i.39 7.38 9.61 6.1i
30.82 32.19 28.54 30..15
'l(rarse-grained granire Th-rich(1) Th-poor zoned singlecrystal
I *r'
U-rich (Th-poor) (2) granite(2) Fine-grained
2.52
{
i
: ete-magmatic ductileshear zone(2) granite Coarse-grained zoned singlecrystal(vicinity of a shearzone)( l. - 2)
t-Jt
core rim
2.37 J .:+ J.11
1.88
21.96 25.11 23.46 26.95
0.12 0.01 0.14 0.0,4
1.25 1.80
?1.79 26.05
0.2r 0.r12
i .-r7 J.i+
4.i1 3.03
13.-r1 i0.35
18.08 27.15
0.55 1.81 0.36
31.5 26.83 25.86
008 0.00 0.17
l.66 2 . 0i 3.99
0.65 l.0l 3.62
6.32 5.i6 1-r.15
31.53 31.95 31.90
J.r.rl ;.:>
and Related Granite Petrogenesis Uranium-Concentating Pr ocesses A number of differing hypotheseson the evolution of fertile leucogranitesand associated modesof uranium concentration,particularlyof those of the St. SylvestreMassif/Limousin,have been forwardedin the past.The most recentand well-documentedconcepthas been publishedin various papers by Cuney, Friedrich, Poty and their coworkers. Friedrichet ai. (1987)distinguishfive principal modes of uranium mobilization and related processesassociatedwith uranium-concentrating the evolution of a peraluminousgranite: -
1l
/. /o
protoliths, partial meltingof uraniferous magmaticdifferentiation, late magmaticactivity. and hydrothermalprocesses supergenemeteoricprocesses.
The five modes may be separatedinto two groups. The first three account for continuous and selectiveuranium accumulations in course of the petrogenesisof the parent granite to generate a highly fertile U source rock with uraninite the dominant U captor. The last two stages,particuiarly hydrothermalprocesses.are instrumentalin liberation. transport, and redepositionof the uranium in the form of pitchblende veins. Adequate structural source and host rock preparationsare mandatoryinterludes for the latter.
J.I.J J,J+ 1
3 19
Parnal melting achieves concentration of leachable U in peraluminous silicate liquids only under two conditions, anomalous. above Clarke value uranium contents in the protolith and sufficient in quantity to form uraninite. Consequently, Friedrich et al. conclude that a highly peraluminous uraniferous leucogranite is the product of variable intense partial melting of upper crustal rocks and strong differentiation during its evolution. High initial o18O and 87Sr/ ffiSr ratios suggest shales variably enriched in organic matter and uranium to be the precursor rock. Such shales would provide the adequate composition for the peraluminous nature, low oxygen fugacity. and high U content of the granite. During magmaric differentiation U is first incorporated into common accessorv minerals oi usually refractory nature such as monazite, zircon, apatite,xenotime etc. Onlv if excessiveU is available will it fractionate as a pronounced incompatible element in favor of the anatectic melt. from where it can crystallize as uraninite. The proportion of U precipitating as uraninite and the quantity of U in the silicate liquids increases with the differentiation of the melt. Such an incompatible attitude of U occurs in magmasof reducing nature. which mostly derived by partial melting of metasediments (Ishihara 1977), as distinct to magmas of oxidizing capacity, where U will strongly fractionate into the fluids when an oversaturation of the melts exists.
212
5 Selected Examples of Economically Significant Types of Uranium Deposits
In the St. Sylvestremassif, progressivemagPost-magmatic hydrothermal processes rematic differentiation produced petrologic facies flected by their alterationimprints in the source with two distinct end membersand a subsequent rocks constitute the critical prerequisite for member, as describedby Friedrich et al. (1987). mobilization and redistribution of the primary The least differentiatedor front-end member is magmaticuranium into pitchblendeveins. cafemic and enriched in elements compatible Friedrich et al. (1987) discuss the various in peraluminous melts and has an accessorial modesof hydrothermalsourcerock alteration in mineral suite dominated by Th-rich (ca. 72"h the light of its efficiency in mobilization of ThO2, Table 5.20) monazite.zircon, apatite,and uranium. ilmpnite.In contrast.highlyevolvedmembersare Solution-controlled uranium liberation takes highly leucocratic and enriched in hygromag- place essentiallyalong zones of fracturing and maphile,incompatibleelements(U, Sn. Li, Rb. cataclasisthat provide the transmissivitvneeded Be, F, etc.) and havean accessory suitetypically aspathwaysfor the fluids.The natureof the fluids dominated by Th-poor uraninite and minor may vary betweenhypogenehvdrothermal and U-enriched. zonal monazite and zircon and supergenemeteoric.Favorablesites of adequate as characteristicrock constituents albite and permeabilit.vare restrictedto brittle structures, muscovitephenocrysts. transcurrentductilemylonitezones,and gneissose Late magmaticactivi4'prior to the final con' segmentsmarginal of granitic facies as docusolidationof the main granite body, may expell mented by oriented trails of fluid inclusions, highly uraniferousfluids and melts from syntec- microthermometry, and structural patterns tonic oversaturatedspecializedintrusions into (Lespinasseand Pecher1985). synmagmaticductile shearzoneswhere anomalThe liberationof uraniumfrom most accessory ous quantities of uraninite crystallize. In the mineralsby alterationfluidsis very limited except are refractoryand St. Sylvestre granite. the ductile deformation for uraninite.Most accessories reflected by mylonite zones was followed or demand either intense metamictization or exaccompaniedby intrusion of dikes and stocks tremely aggressive fluidsto releasetheir uranium. of fine-grained peraluminous granites. These The dissolution of uraninite is a function of the granites are of higlrly variable geochemical physico-chemical nature of the solutions, in the composition.A particular group of them, em- first instanceoxidationand complexingcapacity placed along major synmagmaticdeformation (Dubessyet al. 1987).Easy destructionof uranizones, is characterizedby marked enrichments nite may result from oxidizing and CO2-rich of compatible(Th, Ce, Zr, Ba, Sr, etc.) and solutionseven without any significanthost rock incompatible(U. Sn, Li, F, Rb, Cs. etc.) el- alterationif only minor quantitiesof solutionsare ements compared to the surrounding coarse- involved. In contrast, reducing fluids, even in grained granite. Concentrationof U and other large quantities, may induce intense alteration incompatibleelementsis mostiy restrictedto the but without liberatingany significantquantity of margins of the dikes and stocks althoughsome uranium. fine-grainedgranitesrich in SiO2 and Na2O are Alteration effecs with respectto the behavior ubiquitouslyennched. In this group of fine- and mobilization of uranium bi' the common grained granitesU displaysthe best correlation types of alteration observed in peralumrnous coefficientwith Sn, Cs, and Rb and is distinctll' leucogranites are as follows: independentof Th. Zr. and Ce and as such inAlbitization, a common.ubiquitousfeature in dicatesU concentrationprocessesdistinct from manyperaiuminous granites,is considereda late the main magmatic phase.The other elements magmatic processby Friedrich et ai. (1987), thaLcorrelatewith U are of highly mobile nature whereasother authorsfavor a hydrous metasoand typically representa late magmaticphaseof matic origin, €.9., Ranchin (1971). Uranium, coexisting crystallizedminerals, silicate liquid, together with other incompatibleelementssuch and a fluid phase. as F, Li, Be, and Sn, becameenrichedduring Subsequentto these magmaticprocesses, a albitization(Cuneyet al. 1985). variety of dikes (microgranite, lamprophyre, Muscovitizqtion, reflected by authigenic pegmatite) intruded and, after cooling below growth of muscovitephenocrystsis a typical and 400"C,the leucograniteunderwent,cataclasis and pervasivephenomenonfor leucogranites(therehydrothermalmodifications. fore also referred to as two-mica sranite).
Examplesof Vein-Type Uranium Deposits
213
Ranchin (1971) suggeststhat uraninite crystal- in the La Crouzilleand adjacentMarchedistricts. lization occurredin a subsolidus(hydrous)phase (Moreau and Ranchin I97l; Leroy I978a, 1984; This view is opposed Leroy and Cathelineau 1982) relatedto muscovitization. by Friedrichet al. (1987),who provideevidence Feldspar-dominant episyenitization (Table by gradualdissolutionof rhat uraninite is a magmaticproduct formed 5.2IA) is characterized .ndependentlyof this type of muscovitization. quartz, neoformationof albite, microciine and .{nother type of muscovitizationformed by adularia, alteration of muscovite to feldspar, hydrothermal alteration is of only very limited biotite to chlorite (ripidolite) and potassicfeidextent. spar,and intenseincreaseof perthitic intergrowth as documentedby Pagel (>50 vol. %) of original orthociasewhile plaDuring greisenization (198i), the original uranium concentrationin gioclasebecomesNa-enriched.Sites of feldspar variousieucogranites in Bretagne,France,and episyenitization have apparentlybeen prepared Britain. remainedstable.This by microfracturingnow iargely overprinted by Great Cornwall. iould be in agreement with the reducingnature recrystailization. ,'i the solutionsinvolvedin Sn-W greisenformMica-dominant epbyenitization (Table arion (Dubessyet al. 1987).In contrast,Simpson 5.21B,C)is recognized by quartzdissolution,and of partial to total muscovitizationreplacingalmost et al. (1979)reporta lossof U in the greisens CliggaHead, Cornwail. all pre-existingrock constituents of the host Chlorinzadon affectedto various extent and granite except orthoclase,which commonly is intensity peraluminous granites. Some leuco- incompletelytransformed.Some hematite crysgranite massifs such as Gudrande, Bretagne tallizedin suchwhite micasformed after biotite. (Ouddou 1984), and Grandrieu, Margeride dis- Leroy (1984) notes for the Bernardan deposit/ trict (Peyrou 1981), display aimost pervasive Marche district that the newly formed muscovite chloritization as distinct from the St. Sylvestre is of phengite type. Mica episyenitizationis Massif,where intensechloritealterationis limited clearly related to intersectingfracture systems, to restrictedareas.Uranium removalin response whereit formsirregularlyshapedpipe-likebodies to chloritizationappearsto be relateddirectly to of up to 30-40m in diameter and 30-300m water/rockratios.Smallratiosare associated with vertical extension.Intensity and dimension of minor U loss. as demonstratedby Turpin (1984). episyenitization are direct functions of local His oxygen isotope data indicate only small fracturing and microcataclasis. Formation amountsof meteoricwaters and low water/rock appears to be contemporaneouswith lamproratios respectivelyto be involved in pervasive phyre emplacementbut may also be related to generation of chlorite in rocks which suffered the late fine-grainedgranite intrusions. Leroy only insignificanturaniumlosses.For an efficient (1984),basedon fluid inclusionstudiescalculated U mobiiization one would expect higher waterl for the Bernardandepositfluid temperaturesof rock ratios and solutionsof different chemistry. 260 to 370'C and reports for that deposite a On the other hand, this kind of chloritization chemical changeby loss of SiO2 ?329/,) and resultsin a more or lessin situ redistributionof NaO2 (-1.2 to -2.5%) and increaseof K2O uranium. Le (1975)and Renard (1974)estab- (+0.5 to +3.2o/o)and changesin Rb (positive) lished that chlontesmay containmuch higher U and Sr (negative). concentrationsthan unalteredbiotites.This is due Kaolininzation affected widespread the to redepositionof remobilizeduraniumin micro- peraluminousgranitesbut is most intensewithin fracturesand on grainboundariesand by adsorp- the most evolved parts of the massifs. This tion on Fe- and Ti-oxideswhich derivedfrom the phenomenon suggests that eariy subsolidus chloritization process. Presence of hematite (hydrous)activitiesmust have occurredin large attests to orygenatedsolutionsinvolved in this volumesof granite.Comparedto the freshparent granite,kaolinizedgranitealwayshasvery low U process. (Allman-Ward 1985; Simpson et al. contents Dequartzification with progress into episyenitization is a common feature in many pera- 1979). Supergenealterationsby meteoric orygenated luminous granites and markedly efficient in destruction of uraninite and mobilization of solutions are very effective in dissolving uraniuranium. Two varietiesof episyeniteare distin- nite. Barbier and his coworkersnoticed a U loss guished.They revealthe followingcharacteristics of 30 to 50% from the leucosranite.The re-
214
5 Selected ft1:mples of Economically Significant Types of Uranium Deposis
Table 5.21. St. Sylvestre Massif, changes in mineral composition (in % volume) during alteration of leucogranite into (A) feldspar epis),enite, (B) mica episyenite and (C) highly mineralized mica episyenite A Feldsparepisyenite Facies Minerals in vol 7o
2 fine-grained
l normal K-feldspar Na-Ca feldspar I Feldspars
7
Quartz, granitic Quartz, secondary t Quartz
25
Voids Total in volume
23 65
5 (ch)
7 (ch) 00
8.5 6.5
1
00
25
\1
00 21 21
0
0
43
+t
100
100
)J
100
6 (ch)
7 (ch) 00
20 0 20
r00
40 49 89
55 32 87
40 49 89
50 34 84
AA
4l 20 61
Biotite-chlorite (ch) Muscovite
Fissurein F. episyenite
F.-Episyenite
Surrounding granite
100
100
B Mica Episyenite Facies Minerals in vol "ri' Orthoclase Plagiociase I Feldspars
Surrounding Srantte
23.5 37.0 65.5
Muscovite Mes Biotite
6.0 6.0
Quartz granitic Quartz secondary E Quartz
22.5 0 22.5
Wall rock granite
Compact episyenite
28.5
28.5 25.0 53.5
J-.U
60.5 ( 1( 1 5 . 0
( I
[
a^ < U aA <
Vesicular mica episyenite
28.5 11.0 39.5
,.)
10.0 8.0
0 25.0 25.0
0 5.0 5.0
--
Carbonates
0
0
1.5
4.0
Voids
0
0
12.5
33.5
100
100
Total in volume
100
100
C Hiehlv mineralizedmica episvenite Facies Minerals in vol 9o
Surrounding granite
Vesicular mica episyenite
Mineralized mica episyenite
46.5 0 46.5
28.s 3't.0 65.5
28.5 11.0 39.5
Micas.{YuToute I rtloute
4.5 6.5
12.0 6.0
27.5 5.0 (include chlorite)
Apatrte
i.0
1.0
2.0
22.5 0 22.5
0' 5.0 5.0
0 10.4 10.4
Orthoclase Plagioclase I Feldspars
Quartz granitic Quartz secondary I Quartz Carbonates Voids Pitchblende Total in volume
0
9 100
4.0 '?' i00
0 o.l
8.9 100
Data from Margnac 1(A), Vincou (B), Vincou and Column 125, Margnac (C). (Moreau and Ranchin 1971)
Examplesof Vein-Type Uranium Deposits
215
maining U is partly bound in uraniniterelicts Roubault and Coppens(1955, 19,s8)suggesta containedmainlyin quartz,monazite,andzircon, lateralsecretionprocessbut do not elaborateon and partly occursredistributedaiong microfrac- the natureof fluids,whetherhypogeneor superIntensityof uraninite gene. For Geffroy (197I), an ascendanthydroturesand grain boundaries. decompositionis a function of the rock per- thermal lithogeneticorigin is the least unlikely neability and the rateof upiift of the area.In the comparedto other hypotheses.lforeau et al. irst casemicrofracturingis the criticalfactor.It is (1966) and Barbier (1974) favor supergene In their modei, during certain climatic commonlybetter developedin the most evolved processes. parts of a granitic complex,where fluid over- conditionsdescribedas biorhexistasis,leachable has been dissolved saturationprevails.There sectionsprovide the uraniumof the leucogranites sites for easy leachingof the uranium. Also, from the granite and redepositedin favorable coarse-grainedgranites are commonly more structural traps forming pitchblende veins. easily attacked than the denser fine-grained Favorable periods for these processes have presumably prevailed during Permian and acies. Tertiary time. that complexes such as of St. Sylvestre, Granitic Leroy and Poty (1969), Leroy (1978a,b), rvhichhavebeenupliftedand exposedto erosion et ai. (1986) in recenttimes,still containfairly freshuraninite ZiegLerand Dardel(1984),and Por."grains in near-surface sections. In contrast, suggesta kind of hypogeneconvective hydrogranitic massifswith a smooth relief and sub- thermal ore genesis.In their model. a fluid of jected to only limited erosion such as those of mixed connate and meteoric w'aters staned Gu6randeand Grandrieu, France. display deep circulation in responseto tectonic movements penetrating weathering effects. Practically no and leached uranium for primary pitchblende uraninite can be found to a depth of 100m and formation from uraninite of enclosing leucomore. All uranium presentoccursin microfrac- granites.Leroy, Potv and their coworkers dotures and adsorbed on Fe- and Ti-oxides and cument their reasoning with extensive data, clayey material. It supposedly derived from particularly from minero-chemical. fluid inclusion, and mineralogical research work, data dissolutionof original uraninite. In summary, Friedrich et al. (1987)conclude which contradict a supergeneprimary ore forthat a superimpositionof uraniniteconcentrations mation. Later weatheringprocessesonly overprobably along synmagmaticmylonite zones associated printedthe originalmineralassemblage, rvith emplacementof fluid-oversaturatedhighly in Permian but definitely in Tertiary to recent evolved endograniticintrusionsenhancesthe U times. Poty et al. (1986)provide the following genetic fertility of peraluminous granites. Pervasive alterationssuch as albitization,muscovitization, model for pitchblendevein depositsassociated or chloritization do not provoke significantU with leucogranitesin the Limousin and other mobilizationat leastas long as water/rockratios districts in the French Hercynian massifs.(see are low. Of prime importancefor U redistribution alsoTable5.22) are, instead, severaloveriappingcnteria, viz. brittle deformationof zoneswith late magmatic a) The evolution of vein-tvpe uranium deposits startedduring the final stageof the Hercynian uraninite enrichments permitting subsequently Orogeny, 325 to 300m.y. ago which was intensesourcerock alterationby oxygenatedand marked by general upiift associated with CO2-enrichedhydrothermalsolutionsof sufficient intensivemagmaticactivitv in form of granite, quantity, i.e., the processof efficientU mobiimicrogranite and lamprophvre dike emization is restricted to very limited favorable placement. zones. b) In distinct crusta.lly derived peraluminous leucogranitesuraninite crystallizsalas a priProposedModelsfor the Evolution of mary magmaticrock constituent(Cuney et al. Limo usin-typ e Vein Uranium D eposis 1979',Pagel 1981,1982a;Friedrichet al. 1987). c) in the Saint Sylvestregranitic comhave been Uraninite, models A number of metallogenetic plex, two modesof distribution (Cuney shows of Hercynian formation forwarded for the final 1984; Friedrich et al. and et al. 1985; Friedrich Geffroy veins in leucogranite. pitchblende origin. 1987): epithermal (1955) propose an Sarcia
216
5 Selected Examples of Economically Significant Types of Uranium Deposits
Table 5.22, Limousin, summary of evolution and principal geological events of the Saint-Sylvestre Massif since Devonian time. (After Lerov 1978a amended by more recent data as referenced) Age m.y.
Period
_20 _30
Epoch
Principal geological events m.y.
Miocene Oligocene
30
u F
_44
Cementation U-redistribution,lossof radiogenicPb
Eocene
Saxonian
_zw
'; Autuilan
270-280
Stephanian
285t 10 ca.295 300+ 11 ca.300
L
_270 J80
Westphalian
?90
3 1 5+ 1 8
Apparent ageobtained for the Saint Svlvestre'superstructure'in courseof incompleteisotopichomogenizationb1'late magmatic processes(muscovitization. albitizaiion)
Visean
324+4 325+ 14 336+ 6
Emplacementof Br6me granite (Holliger 1984) EmplacementSt. Sylvestregranite (Holliger et al. 1986) granite Emplacementof ChAteauponsac
Tournaisian
ca. 10 fiim
Namurian
-300 _310
U
Minimum apparent age for primary pitchblende-pyrite mlneralEauon Lntrusionof lamprophwe. microgranitemica episvenitization. reacuvatronot Dnttle detonnatron Lamprophlre (Leroy & Sonet 1976) Closureof biotite cell (Rb-Sr) Feldsparepisyenitization(rectonicphaseP4) (Turpin 1985)
_320 _330 _340 Upper _350 Middle
_360 -
360-380
Culminationof reeionalmetamomhism
that caused local episyenitization. Lerov Homogeneous distribution with abundance (1978a.b)relatesthis or an additionalevent of uraninite correiated to magmatic differentiation, to a period of lamprophyre and/or micro- Uraninite accumulations aiong synmagmatic granite intrusion during the P 3 and P 4 tectonic phasesin the Limousin district. (Lamshear zones, derived from fluids expelled prophyre: ca. 295m.y.; Leroy and Sonet from highly uraniferous fine-grained granitic 1976) The fluids, heated to 370 to 260"C, bodies. The fine-grained facies contains - locally up to 100ppmu. The fine-grained migrated aiong earlv faults. trending N-S granites were emplaced along the synmagand E-W in Limousin, and formed mica 'fault matic zones almost contemporaneepisyenite. ously with the enclosing coarse-grained e) Mixtures of thesehot meteoric solutionswith connate "heavy" fluids chemicallyof C-H-O leucogranites (Mollier 1984). d) Subsequent to this magmatic stage tectonic character (CO2, hydrocarbons,H2S enrichmovements created zones of intense faulting ments:Turpin 1984)derivedfrom diagenetic and high heat flow permitting deep reaching or metamorphic rocks collected uranium by convective circulations of meteoric waters leachinguraninite and transportedit as uranyl
of Vein-TypeUraniumDeposits Examples carbonate complexes to the outer, marginal zone of the pluton. f) Deposition of pitchblendepresumablyresulted in response to boiling of fluids triggered by pressure drop, andior by reactions with mafic rocks such as iamprophyres. It was associated bv K-metasomatismand muscovitizarion of the wall rocks. The pitchblende crystallization occurred about 275m.y. ago, as indicated bv apparent U/Pb ages (Leroy and Holliger 1984). e) Sites of pitchblende deposition were either active dilational fractures(vein mineralization) or vesicular, vuggy mica episyenite bodies (disseminated mineralization). The latter experienced strong and extensive alteration, whereas fissures and breccia zones trending mostly about NW-SE in Limousin, underwent less extensivewall rock alteration. h) Primary pitchblende accompaniedor pre-dated by small amounts of quartz is associated exclusively with suifides of iron (pynte, marcasite) if iron is present. Hematite or other ferric minerals alwavs post-date primary pitchblende. a criterion excluding reduction of hexavalent uranium by ferrous iron oxidation. This raises the question of a valid reductant, since reduction of U6* to U4* is required for pitchblende deposition. Several reducing agents are proposed, such as hydrocarbons found in fluid inclusions or sulfur species,since pitchblende co-precipitated with pynte. The iron of lamprophyres has been suggested as reductant to explain the enrichment of pitchblende near lamprophvres. but this is contradicted by hematite formation well after pitchblende. as stated by Poty et al. In addition, lamprophyres in contact with uranium mineralization show changesin mineralogy and chemistry, but Leroy (1978b) points out that their iron content does not vary and occurs more in a reduced state in form oi pvnte instead of being in the mafic rock constituents (biotite, pyroxene). Ca and Mg decreasein the lamprophyres near pitchblende veins. This may be explained by reactions of the CO2-rich fluids with dike minerals. resulting in the formation of Ca-Mg-HCO3 complexes. The related lowering of the HCOI content, may causethe releaseof (UO2)2*, and as such may add to the boiling factor and to increased accumulation of pitchblende at the intersection of veins with lamprophyre dikes.
217
i) Fluid inclusion chemistry and isotope data suggestpitchblende deposition at 340 to 350"C from fluids containing CO2 enrichments. N2, H2S, and traces of hydrocarbons. Isotopic values of 613C: -17.67* contradict a mantel origin of this CO2, whereashigh valuesof 6r8O : +8 to +15Y6 and 6D : -45 to -307m may indicate involvement of connate. perhaps metamorphic waters in the ore depositing hydrothermal systems. j) During a second stage of still earl."-hydrothermal activity but subsequentto pitchblende deposition.the CO2 content of fluids decreased to a level that solutionsbecamepurelv aqueous and temperatures dropped from 350" (pitchblende) to 140'C. At 130 to 150'C coffinite formed from pitchblende associatedwith Fesulfidesand rare galena and quanz. lv{uscovites of first stage alteration were widely replaced by montmoriilonite and adularia. k) In a last hydrothermal stage,at about 100'C or less, calcite, baryte, and fluorite crystallized. l) The mixed hypogene convective hydrothermal activity lastedover a period of 20 to 30 m.y., as indicated by age dating. m)In Jurassic time, local remobilization of primary mineralization occurred in some veins of La Crouzille, or new mineralization from an unknown source may have been introduced, as indicated by data from Pierre Plantdes, Margeride district (Respaut 1984). n) Latest uranium remobilization is more or less recent, triggered bv supergene processes. Although Poty and his coworkers provide convincing evidencefor their metallogeneticconcept, other French geologsts argue against at least part of it. In some deposits, e.g.. Margnac in Limousin, pitchblende veins have been discovered in E-W trending structures in the bottom part of the deposit. A. Coste and I. Lozach (pers. commun. 1989) consider these veins to be of hypogene origin. Redistribution of uranium from these E-W veins, perhaps together rvith indigenous uranium from the granite. may have contributed to the formation of the NW-SE oriented veins, possibly by the processes suggestedby Poty et al. (1986).
5 Selected Examples of Economically Significant Types of Uranium Deposits
218
Refercncesand Further Readingfor Chapter5.3.1 (for detailsof publicationsseeBibliography) Autran and Guillot 1974; Autran and l:meyre 1980; Barbier 1968. 1970;Barbier et al. 1967,1970;Barbier and Ranchin 1969a, 1969b;Bernard-Griffith et al. 1985;Calas 1979;Carrat 1974; Cathelineau 1981,1983a,1983b,1985. 1987; Cathelineau and lrroy 1981; Cathelineau et al. 1982; Chenevoy 1958; Coppens and Bernard 1978; Coppens et al. 1969; Coste, A., pers. commun.; Cuney 1978, 1981. 198?; Cuney and Friedrich 1987; Cuney and Pewel 1984: Cuney et al. 1979. 1984. 1985: Dardeli Peinador Fernandes et al. 1979: de Fraipont et al. 1982; Drin et al. 1985; Dubessv et al. 1987: Duthou 1977: Duthou et al. 1984: Friedrich 1981, 1984; Friedrich and Cuney 1983, 1989; Friedrich et al. 1987, 1989: Geffroy 1971, 7973'. Geffroy and Sarcia 1958; Holliger and Cathelineau1986; Holliger et al. 1986;Jiashuand Z,ehong 1984;Jurain and Renard 1970; Lameyre 1966:La Roche et al. 1980: I*roy 1977,1978a. 1978b,1982. 1984:Lero-v and Cathelineau 1982: Leroy and Poty 19691krov and Holliger 1984: kroy and Sonet 1976; Lespinasse1984; Marignac and Lerol' 19791Marquaire and Moreau 1969; Martin 1981: Michel 1983: Mollier 1984: Mollier and
Bouchez 1982; Moreau 1977;Moreau and Ranchin 1971; Moreau et al. 1966: Ouddou l9&1; Pagel 1981. 1982a, 1982b;Pecheret al. 1985;Poty et al. 1974, 1986:Ranchin 1968, 1971; Renard 1974; Respaut 1984; Sarcia 1958; Sarcia et al. 1958; Sarcia and Sarcia 1956, 1962: Stussi 1985;Turpin 1984, 1985; Vidal et al. 1984: Weber et al. 1985; Ziegler and Dardel 1984.
5.3.2 Perigranitic Monometallic Vein Uranium Deposits: Piibram District, CSFR Pifbram is located about 60kmSW of Prague. The district containsperigraniticvein uranium depositsin a NE-SW elongatedstrip about 25 km long and 7 to 2km wide (Fig. 5.33). 66 shafts. asmuchas 1800mdeep,had beensunkto exploit the various vein uranium deposits. Pb-Zn-Ae r Piibrom
z P5
1
0
+++
+
7+ \+ +q+t+ + + t
Komenn6
+i+ + +/ + + +,+ +l+ + +/ +\+\+ +l+ +/+ L e i e t i c e- B r o d + 4l +
sw
**t*)\
++ + ++
rBr + P. +?++ Jeruzoldm
3rm
.-;
i : : :: :::s,). :i+a { ; ;
PT J
2
+ +
+
+\ + +
+
q +
+
Skotko -oboii;t;
Bytiz L
L
Cambrian l" : : :l Sandstone,greywackeetc. Upper Proterozoic. Upper or Post-SpiliteSeries
tE
Sandstone
f6-l
l-4
l-Fil
| Shale'siltstone
sirtrt-esandstone
J-FzI
congtomerale-sandstone
tE
l-p-ll sittsrone-claystone/shale series i-T-l t',lioote
E
Hercynian
E
ffi
tocks Granitic
w uE'r
P-A: axis of Piibram Anticline Faults Veins Veinswith U ore CentralBohemianPluton
Fig. 5.33. Piibram district. a Geological map with the distribution of the most important vein svstems/deposits:b curvilinear SW-NE section off the Central Bohemian Pluton (CBP) contact; c NW-SE section afioss the regional structural gtain. a and b show the restrictionof U veins to a nanow zone along the pluton contact and the attitude of principal veins. c illustrates in a schematicmode the location of. Pb-Zn-Ag, U and Au veins with respecl to the overhanging pluton contact. Structures of the Central Bohemian Lineament trend NE-SW, those of Jdchymov LineamentNW-SE. (After: Petroi et al. 1986(a and b); Ruzicka 1971(c) basedon Kattner 1926)
Examplesof Vein-Type Uranium Deposits
NW
P i ib r o m
219
SE AU
+ I
+
0
++ ++ ++ + ++ +
1km
Combrion l 'F l
Snoe. sondstone conqtornerote
4sptttt"
f
Svoto
I
't-
Hercynion
Sondstone, greywocke etc " {aip of stroto indicoted)
r . J p p e rP r o t e r o z o i c
.77V A
N
+ C.B,P
t --J-J
C t o y F.
;
CBP: rntrusives of Centrol Bohemion Pluton
l7l
volo. toutt
Diobose dike ond Pb-Zn-Ag verns
fTt
u '";nt
P;ibrom Anticline Piibrom Syncline
Fig. 5.33c
vein deposits, locally containing uranium, occur 35" to 85oSE to a depth in excessof 2CI10m(Fig. in the separate Piibram-Biezov6 Hory and 5.33c, 5.34b). Rock faciesborderine the uranium Bohutin districts, ca. 2-4kmNW of rhe uranium distnct to the SE include a border facies condistrict. sisting of dominantly adamellite which is followed The deposits are structurally controlled and ilwards by biotite and two-mica granodiorites. located immediately adjacent to a differentiated Granitic apophyses extending for up to 300 m granitic complex. The ore composition is prac- along bedding into the sediments are relatively tically monometallic. The deposits are therefore frequent. Isotope dating yield agesof 1L7m.y. for classified as perigranitic monometallic vein the oldest facies (amphibole-biotite granodiorite) uranium deposits (class3.2.2.1. Chap. 4). to 285m.y. (amphibole-biotite granite) for the Total reserves of the Pifbram district are es- various magmatic rocks of the pluton. timated in the order of 65 000 mt U3Os, at ore The district is at the junction of the NE-SW grades ranging from 0.1 to I.8o/oU:Oe. trending Central Bohemian Lineament and the The following geological description is based NW-SE oriented Jachymov Lineament. on the authors listed, particularly on Kolektiv Displacement structures within the district are {1984) and Petros et al., (1986). AII figures on srouped by Petros et al. (1986) inro two main reserves and grades are best estimates denved svstems with respect to their imponance and trom a variety of other sources. position to the Pifbram anticline viz. (a) regional and (b) local longitudinal + NE-SW. diagonai + N-S, and ca. NW-SE-oriented cross faults. Geological Setting of Mineralization Host rocks are predominantly Upper Proterozoic tlvschoid schists of the Post-Spilite Series and locally Cambnan clastic sediments which rest unconformably on the Proterozoic. The sediments are mildlv regionallv and contact-metamorphosed. complexely faulted, and folded into an asymmetric anticline and westeriy adjacent svncline (Piibram anticline and syncline Fig. 5.33). A great abundance and variety of dikes, sills, apophyses.and stockscut the sequences. The sediments are intruded by the heterogenous granitic Central Bohemian Pluton.Its NW contact to the sediments is overturned and dios
Host Rock Alteration Wail rocks along veins are modified bv a variety of alterations. Kolektiv (i984) arrribures the aiterations to the various stagesof mineralization (see below) as follows; -
Siderite-suifide stage: Sericitization of plagioclase and biotite. Early calcite stage: Illitization and sericitization as above but less intense associated with chloritization, carbonatization. and hemaritization. Hematite and goethite replace sulfides
220
5 Selected Examples of Economically Significant Types of Uranium Deposits
P2
200m
,dl-
80" Pt
v' FouIt DE d o
z ./.
+
+
'+ + ?qo t
+
+ ++ +++ + +800 ?+
500
tt
+ +
w
Pr ,i
and impose a pink hue on the wall rocks for up to 2m from veins. - Calcite-pitchblende stage: Continued sericitization and hematitization at the initial phase of this stage followed by chloritization (without hematite) in up to 5-cm-wide rims -during pitchblende deposition. - Calcite-sulfidestage:Similar to previousstage. Alteration aureoles in sediments commonlv persist for 0.1 to 3m from veins. Alteration is more extensive in intrusive rocks. In segments of multiple veins where the zones of cataclasisand vein infllling overlap, alteration may spread over inten'als to several tens of meters width.
Fig. 5.3. Pifbram district. S part (Le3etice-Brod area). a Plan of underground level; b W-E section approximately perpendicular to the Central Bohemian Pluton contact and fold trend. The figure illustrates the interrelationship of U mineralized vein clusters. N-S and NE-SW smrcrures joirung witb the Dddovsky Fault and the overhanging piuton contact. (Legendsee Fig. 5.33a.b; wr' = diabase) (After Petro5 et al. 1986:Kotektir 1984)
Principal Characteristicsof Mineralization Principal uranium minerals are pitchblende and minor coffinite and uraniferous anthraxolite. Sulfides are present but in minor quantities. Dominant gangue mineral is calcite. Others are quartz. baryte, chlorite and Fe-, Mg-carbonates. In total, more than 50 minerals including 35 ore minerals have been identified in the Pifbram uranium ores (Fig. 5.35). Four principal mineral parageneses/stages are distinguishedby Komfnek and Proke5 (in Petroi e t a l . 1 9 8 6 ) a s p r e s e n t e di n T a b l e 5 . 2 3 . T h e s e mineral assemblages may form veins of its own or hybrid veins.
Examplesof Vein-Type Uranium Deposits P o r o g e n e t i co s s e m b l o g e s , / s t o g e s
Minerols
S u b so 9 e s S i de r r te s u l fi d e
C o i c i et 4
2
1
0uortz S i ce r i t e Arsenopyrite S k ui t e r u d i t e Lcllingite Sohoterite G o le n o Cholcopyrite T et r o h e d r i t e 3 o r yt e A n ke r i t e Allemontite Chlorite H e m o t i t e .g o e t h i t e Colciie Dolomiie Pitchblende Anthroxolite C of f i n i t e Montroseite Pyrite Morcosite Notive Ag Nickeline Bornit Cholcocite S o ffl o r i t e Rommelsbergite G e r s d o r ff i t e Milleriie Erovoite Pyrorgyriie Cinnobor Py r r h o t i t e Potygorskite
221
tcr5{ende Ccrcite-or i6
C o l c i t e - s u il d f e 7 B
-
r
I
I
---
I
i
II .
I
--]
:i
JI
E
-l
I
-l
=l .:
-.1 I
I I
I
Fig, 5.35. Piibram district, parageneticschemeof uraniferous veins. (After Kolekdv 1984)
Kolektiv (1981) recognizesfour rypes of vein infillings based on the quantitative presenceof the respectiveassemblages :
The various mineral associations. as demonstrated by Kolektiv ( 1984) display preterencesfor different structure systems and some :onation in lateral and venical directions.
siderite-sulfide veins containing 60-90 vol oh a) Diagonal and cross veins of N-S orientation of the stage 1 siderite-sulfideassemblage, associatedwith major faults are of mixed type. - calcite-pitchblendeveins containing 90 vo| o/" Thickness of the mineralized section of these of minerals of stages2 to 4. veins averagesca. 150cm. Siderite-sulfideand - caicite veins containing 100 vol % of minerals calcite-sulfide assemblages are almost ubiof the stage .l calcite-sulfideassemblage, quitously distributed throughout a vein - mixed veins containing minerals of all stages. whereas the calcite-pitchblende association Three tvpes of recoverable ore composition are occupies only vein intervals of greater thickdistinguished based on the predominance of ness. The thickness of these veins decreases gradually with depth. distinct ore minerals; Pitchblende ore, uraniferous anthra-xolite ore and sphalerite-galena-Ag b) Longitudinal veins of NE-SW direction are of the calcite-pitchblende type. Average thickore. -
222
5 Selecred Examples of Economically Significant Types of Uranium Deposits
Table 5.23. Piibram district, paragenetic mineral assemblagesigenerationsin uranium-bearing veins (from oldest to youngest) (see also Fig. 5.35) (After Komfnek and Prokei in Petroi et al. 1986and Kolektiv 1984) Stage
Mineral association
Remarks
1. Siderite-sulfide
Siderite,dolomite-ankerite,quartz, baryre. chlorite; galena,sphalerite,some chalcopyrite,tetrahedriteand probably allemontite,skutterudite,I6llingrteand others
Mineralsmostlv:rsmarginal bands ofvein; ca. 5-10vol. % ofvein fill, locallyup to 90%, then forming minable'polymetallicformation'.
2. Earlv calcite
Calcite DK and K 1 with elevatedMn contents,chloritel hematireand goethite, somegalena,sphalerite.chalcopyrite, sometimesnative Ag
Constitute25-30 vol. % of vein fill: Fe-oxidesderived from decompositionof siderite-ankerite,
3. Calcite-pitchblende
CalciteK 2, K 3, and K 4. dolomite.chlorite; pitchblende1. hematite
4. Caicite-sulfide
Calcite K 5 in severalvarieties/generations. minor quartz,chlorite and palygorskite;some uraniferousantbraxolite".coffinite, pitchblende3, commonll,restnctedto zones with pitchblende1; severalgenerationsof pvrrhotite. pyrite:minor marcasite. sphalente,montroseite.chalcopyrite,bornite. chalcocite.tetrahedrite.pwargvrite. native Ag. millerite. nickeline. rammelsbergite. safflorite.allemontite,native As and Sb, and others
Constitute2-5-30 vol. "uI' of vein fill: pitchblende1 formed between K 3 andK 4. Constituteup to 40 vol. ozoof vein fill. formed bv repeated replacement,dissolution and recrystallizationof earlier minerals
"Uraniferous anthraxolite is a highlypolymeric bitumen containing pitchblende,coffinite, calcite and sulfides
ness of mineral fill is 20-25cm. The average thickness of all mineral associationsincreases to a depth of 700m from where it decreases. c) NW-SE and more rarely N-S veins associated with the NW-SE tectonic zones and the Piibram anticline preferentially belong to the calcite-pitchblende type. A few veins of this system can be attributed to the mixed or siderite-sulfide type. Thickness of the sideritesulfide association does not much change with increasingdepth. but the thicknessof the other assemblagesincreases to a certain depth and then decreases. Mineralized veins commonly aggregatein groups or vein clusters or vein knots by multiple veins closely paralleling, approaching, or intersecting each other. -Mineralized veins can be separated into three groups based on their dimensions: Large veins extend laterally and vertically for 500- 1000m and some even more (some veins persist to >2000 m deep). Thickness is 5-100cm, in extreme cases up to 12m. These veins constitute the principal elements of vein clusters and account for ca. 57o of all veins of the district. Medium veins are 100 to 500m in extension. Thicknessesare 1-50cm
and only exceptionally more. They commonly occur as subsidiary veins to the large veins and account for 45o/oof all veins. Small veins are up to 100m in extension and less than 50 cm wide. They occur subsidiary to the larger veins and constitute ca. 50"/" of all veins. Oldest mineralization (pitchblende 1) yield a UlPb age of 265 + 15m.y. (Legierski in Kolektiv 1984).
Ore Controls and RecognitionCriteria Principal ore controlling or recognition criteria include: Host Environment -
-
I-ocalization of the distrrct is (a) at the intersection of two lineaments;(b) proximal to and between the Central Bohemian Pluton and major regional faults, a zo,ne1 to 2km wide. and (c) along the axial zone of the Piibram anticline (Fig. 5.33a,c). Lodes are most numerous in the apex part of the Pifbram anticline and in its SE limb close to the pluton contact. NW-SE cross
Examplesoi Vein-Type Uranium Deposits
veins in the NE and central section of the district are richest near the anticlinal axis. They developed particularly where undulations, local brachyanticlinal folds, or closures and virgation modify the main axis. Here, tensional strain has generateda large width of cataclasisalong open fractures and wide subsidiary fissures. - Remarkabie concentrations of ore pods and lenses within the veins, often of high grade occur at sites where the pluton contact forms a roof overhanging the (meta-) sedimentsfor as much as 2km wide (Fig. 5.3ab). - Host rocks are pelitic and psammitic sediments regionaily metamorphosedup to greenschist facies. folded into major anticlines and svnclines, and overprinted by contactmetamorphism. - Lithoiogy imposed a twofold control on the deposits by physical-mechanical properties influencing location and development of strucrllres and by chemical composition affecting the chemistry of mineralizing fluids. - Most of the uranium mineralization is emplaced within the Post-Spilite Series (97%) whereas Cambrian sediments, tuffites of the Spilite Series and granites only contain 3oh of. the iodes. fuchest intervals developed where host rocks corresponded to tectonic stress by more bnttle deformation. Intraformational conglomerate beds created a barrier effect in their footwall reflected by deviation of the dip of veins and. associated with movements during mineralization, a thickening and enrichment of veins, particularly in longitudinal veins. Simiiar barrier effects are found below Cambrian sediments. granites. and thicker intrusive dikes. ,llteration -
Ore-related alteration includes illitization/ sericitization. chloritization. carbonatization. and hematitization which penetrated the wall rocks for commonly less than 3 m from a vein.
Mineralization -
-
The ore is essentially monometallic but composed of a great variety of minerals formed during several stages. Principal ore minerals are pitchblende, coffinite. and uraniferous anthraxolite.
223
-
Associated minerals are mainly pyrite and minor sulfidesand arsenidesof Fe, Cu. Pb. Zn. A g , N i , C o , a n d o t h e r e l e m e n t s .H e m a t i t e c a n be abundant. Calcite is the dominant gangue mineral; others are other carbonates,quartz, baryte, and chlorite. - Mineralogically the amount of ore-reiated calcite correlateswith the amount of uranium in any given lode (coefficientfactor 0.68), and individual ore lodes are principally associated w i t h t h e o l d e r c a l c i t eg e n e r a t i o n . - Younger post-ore solutions caused replacement of pitchblende and older calcite and dilution of the ore. - Major ore bodies are restricted to large verns but the bulk of ore shoots is in subsidiary structures. - Rich lodes, lenses.pods occur preferentiallyat a distinct bending of veins, ramification of veins, junction of subsidiary and horsetaii fractures with major faults, intersection of major veins with intrusive dikes, intervals immediately below shallow dipping pluton contact, or below Cambnan sediments and intraformational conglomerates associated with a complication of the vein morphology, and above a levei of600-700m under surface. - Below a level of 600-700 m begins a reduction of size and intensiry of veins (more barren ground between veins), length of veins (averaging 450-500m above this level), decrease of bifurcation and ramification of veins and payload material.
Metallogenetic Concepts PetroSet al. (1986)propose a closerelationshipof ore formation and the granitesigranitoids of the Central Bohemian Pluton. Transport of uranium occurred in the form of carbonate complexes in alkaline solutions of medium to low temperatures. Precipitation of pitchblende resulted from degassing and decarbonation of the fluids and changesin pH and Eh. The uranium is postulated to have originated from sediments with subsequent enrichment during magma differentiation. Leaching of uranium from the granitic rocks occurred by fluids migrating along NW-SE-oriented faults. Isotope studies suggest that a certain supply of uranium may have derived from residual magmatic solutions. Contact-metamomhism mav also have
5 Selected Examples of Economically Significant Types of Uranium Deposits
a1A
A mobilized uranium within the metasediments. direct relationshipof ore formation and granite intrusion is disputedby the large time gap of at least 20m.y. separatingboth (for referencesof the variousaspectsseePetro5et al. 1986). Strnad (1986) refusesthe (meta-) sediments and obviously also the granites as a uranium sourcebecausetheir uranium content is only in the order of Clarke valuesor less.Strnadargues that the district is located at the intersectionof deep-seated[neaments and that this situation conditioned the developmentof the Hercynian uraniferousvein accumulationsfrom Proterozoic precursors.
Referencesand Further Readingfor Chapter5.3.2 (for detailsof publicationsseeBibliography) Hruby, J., pers. commun.; Kolektiv 1984(German translation b-".-J. Hruby); Petro5 et al. 1986; Ruzicka 19711 Ruzicka V, pers. commun.l
5.3.3 PerigraniticPolymetallicVein Uranium Deposits:St. Joachimsthal/Jdchymov District, esfn St. Joachimsthal(German)or Jdchymov(Czech) is locatedin the westernErzgebirge,20kmN of Karlovy Vary (Karlsbad).More than 200 mineralizedveins are known from the district about 45km? in size. Sevenmajor vein svstems(Fig. 5.36) were subjectto uranium mining exploited by shaftsbetween300 and 1000m deep. Mining ceasedin 1963.Total uraniumreservesamountto at ore grades(mill feed) about 10000mtU_1Os averagingbetween0.1 and 1% U3O8in addition to other metals. The depositsare structurally controlied and locatedin the vicinrtl,of a differentiatedgranite. Jv{:neraiizationis characteristicallvpolymetaliic composedof Ag, Co, Ni, Bi. and U. The deposits are thereforeclassifiedas perigraniticpolymetallic vein uraniumdeposits(class3.1.2.2,Chap.4).
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mapprojectedintotheDaniel horizonlevel(630-610ma.s.1.).The Fig.5.36. J6chymovdisrrict,generalizedstructural map showsmajor faults4ineaments,the clustersof the NE to NW oriented U-Co-Ni-Bi-Ag "Midnight veins" and the EW trending "Moming veins," and the position of the district with respectto the autometamorphic"Erzgebirgsgranit." Vein clusters:.40 Abertamy; Br Bratrstvi; Ev-Ba Eva-Barbora; Pa Panorana; P/ Plavno: Ro-El Rovnost-Eli63; Sv Svornost. (After Kominek and Vesel! 1986;Kolektiv 1984: Bernard et al. 1967)
Examplesof Vein-Type Uranium Deposits
The discoveryof the district goesback to 1516. ; It was one of the leading siiver producers of = :!o^ central Europe in its early days. Later, cobalt, 7 = Y ' a nickel, and bismuth were recovered. Uranrum a 1 * Z !: mining started at the end of the last century. It c o 1 : = :: paints was used for and the extraction of radium. z z { <: J ! Two railroadcars of concentrate of residuesfrom T- t; Tn i: T- iTll I il rl li\ V'I l +!l-']-l \i\irrl paint production were shipped to Madame Curie | in Paris, who discovered radium in these ores. Another historical aspect of Joachimsthalis the l+ I naming of currency. Its medieval silver mint I produced coins called Thaler or Taler which, in l-ir later centuries. provided in the USA the root for i3t ;t+ :he dollar. -' ! Kolektiv (1981), Komfnek and Veselli (1986) I r ^ o o ol 091Ia0gF0z and Ruzicka (197I) published concisereviewson I -Tthe Jiichymov deposits. Their data provided the base for the following compilation. Figures on grades and reserves are best estimates derived from other sources. a
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Geological Setting of Mineralization The Jiichymov district is located at the southeastern margin of the Eibenstock Massif in the western Erzgebirge, a segment of the SaxoThuringian metallotectonic zone of the Hercynian orogenic belt. Host rocks are metasediments of the Barbora and Jiichymov series of the Mica Schist Formation. The metasedimentary compiex was intruded by early Hercynian dionte and gabbrodiorite stocks and late Hercynian granitic massifs followed by dikes of variable composition. Tertiary alkaline volcanics are the youngest igneous rocks (Ruzicka 1971,). The large Eibenstock Masstf (ca. 600 km2 in outcrop) is composed of late Hercynian granites. They outcrop on the southwestern side and in cupolas on the NW and SE margins of the Jrichymov district. The granites underlie the metasediments of the district at a depth of 300 to 1000m. The granites generated a contact-metamorphic aureole 10 to 60m wide (Figs. 5.36,
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Two principal typesof late HercynianEanites are recognized(agesfrom Smeykalin Kolektiv 1984).The o\der "Gebirgsgran[t" Qa0420m.y.) (or "normal granite") predominantlyconsistsof ; i porphyriticbiotite granite. I The younger " Erzgebirgsgranit" (310300m.y.) (or "autometamorphicgranite") is
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226
5 Selected Examplcs of Economically Significant Types of Uranium Deposits
leucocraticand typically containshigher amounts of accessory minerals and elevated Clarke uranium values. (Gebirgsgranit/biotitegranodiorite 6.8ppmU, 17-24ppmTb, Erzgebirgsgranit/autometamorphicbiotite granite i0.411.6ppmU, L?.2-12.5ppmTh, Stubat and Vachu5ka1973 and Absolonov6 and Matoulek 1975in Kolektiv 1984).Uranium occurs,among others,in form of minute uraninitecrystals(Kohl 1954). The Erzgebirgsgranitexhibits strong autometamorphicmodificationscomparableto a large extent with the late magmatic phenomenadescribedby Poty and coworkersfor graniterelated uranium districtsin France. (see Chap. 5.3.1) Modifications include albitization, muscovitiza(Sn-Wgreisens 280m.y.). tion, andgreisenization A variety of dikes including granite porphyry (290-280m.y.),quarz porphyry,pegmatite,and of to the emplacement apliteintrudedsubsequent the pluton. Brittle deformation produced four fault systemsoriented ENE-WSW, NW-SE. N-S, and N-S E-W (Fig.5.36).The NW-SEwith associated and the E-W structuresare the principalhostsfor mineralization.
Table 5.2. Erzgebirge, alteration features in metasediments and granitic rocks hosting vein-type uranium deposits. (After Dymkoff 1960 and Ruzicka 1971) In metasediments Pre-uranium alterations Initial stage: formation of skarns and biotitization Scapolitization of gneisses Pyritization, chloridzation of amphibolites, tuff s, greissesetc. Graphitization along shear zones Syn ? to post-uraniumalterations Silicification, carbonatization, hematitization Post-uraniumalterations Sericitization,silicification, kaolinirization ln granitic rocks Pre-uraniumalteration Greisenization,albitization,muscovirization, silicification Syn-uraniumalteration Sericitization Post-uraniumalteralion Sericrtization.kaolinitization
Principal Characteristicsof Mineralization Kolektiv (1984)and Kominek and Vesely (1986) separatethe Jiichymovmineralization into seven (in sequential order) Generalalterationfeaturesin rocks surrounding mineral sssociationslstages uranium depositsin the Erzgebirgeare given in (Fig. 5.38): Table5-24.Kolektiv (1984)recognizestwo stages 1. Garnet-pyroxene-magnetite. of wall rock alteration,a pre- and a syn-uranium 2. Quartz-wolframite-cassiterite. stage. 3. Quartz-sulfide. The pre-uranium stagealteration affected wall 4. Carbonate-pitchblende. rocks0.5 to 3m wide from veins.An earlyalter5. Carbonate-arsenides. ation phase begins with disintegrationof mafic 6. Sulfarsenides. minerals and recrystallizationof chlorite, phlo7. Quartz-hematite-manganite. gopite, rutile and, in immediate proximity to veins.pyrite and largerquantitiesof calcite.In a The first two associations are of more regional phase, replaced manl' last final intense silicification distributionand the five constitutethe actual produced wide and a vein district. including calcite, of the minerals fillings zone of quartzification. Ruzicka (1971) provides the following eleof metallic The syn-uranium stage alteration extends for mentalsuiteand rangesof percentages only centimetersto several tens of centimeters elementsin lodes of the Jiichymovveins: 1.007o away from the vein contactwhere it overprinted U, Fe; 1.00-0.10%V, Y, Cu, Pb,Zn, Sb,As, Se; the older alteration zone. Principal recrystal- 1 . 0 0 - 0 . 0 1 %M n , B i ; 0 . 1 0 - 0 . 0 1 %T i , T h , A g . lization productsare albite and adularia. Pyrite A u , W , Y , S c ,P ; < 0 . 0 1 %M o . S n , L i , B e , C o . wasreplacedby hematite,andcalciteby dolomite. Ni, Ba, Sr, T1,Ce. Primary uranium in form of colloform pitchrestrictedto the carbonateblendeis essentially pitchblendestage.Redistributeduranium occurs Host Rock Alteration
227
Examplesof Vein-Type Uranium Deposits
P o r o q e n e t i co s s e m b l o g e s / s t o g e s
Minerois 6orel pyr0! ene moqEIte
Pyroxene rl Gornet II Amphibole E Mico Fetdspor lEpidoie LMognetite I M o l y b d e ne it W o l fr o m i t e Cossiterite Tourmoline Apotiie 0uortz Topoz Corbonote Fluorite Scheelite Chlorite Arsenopyrite Py r it e P y r r h ot i t e Golenite Spholerite Cholcopyrite C o ff i n i t e Pitchblende Hemotite Boryte Skutterudite Notive Bi Notive Ag Nickeline Rommelsbergite S o ff l o r i t e Chloontiie Zeolite Lcillingite Notive As A g , C o . N i , F e - s uol fr s e n i d e Reolgor Argentite Bismuthinite T e t r o h e d r iet - i e n n o n t i t e Pyrolusite Mongonite Kooliniteqrouo min.
0uorlz
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sdlde
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!.Jlqsrde
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lrle
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I
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I
r
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-J
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Fig. 5.38. Jy'chmov district, paraseneticschemeof mineralizationof U, Co, Ni. Ag-bearingveins.(After Kolektiv 1984)
mostly in the form of sooty pitchblende and coffinite which are the main uranium minerals in the arsenide and sulfarsenide stages, i.e., the original uranium phase is of monometallic nature and it only became polymetailic by a younger, uranium-unreiated. introduction of other metals.
Younger hydrothermal solutions caused ore modifications which in part occurred after the extrusionof the Tertiary volcanics. Two varietiesof veinscontaininguranium, the simpleand the complexveins,are recognizedby Kolektiv (1984)and Kominek and Veself (1986).
228
5 Selected Examples of Economically Significant Types of Uranium Deposits
Veins of sbnple composition contain the car- with a young generation of Co-Ni minerals bonate-pitchblendeassemblageemplacedwithin formed during Tertiary tectonism. gouge and brecciated wall rock material. Fractures hosting simple mineralization are mostly those of fifth and sixth order. Veins are 150 to Ore Control and RecognitionCriteria 400m long and 3 to 25, rarely 50cm wide. Two substagesare present. Principal minerals Principal ore controlling parametersor recogof the older substageare pitchblende1 and cof- nition criteria include: finite 1 associatedwith comb-texturedaggregates of quartz,albite or adulariaand fluorite.Minerals Host Environment of the younger substage are pitchblende 2, - On a regional basis,most of the polymetallic dolomite and hematite. Veins containingprimary pitchblendeare only Ag, Co, Ni, Bi. U vein depositshave been preserved iD structures that had been sealed found in the Saxo-Thuringianmetallotectonic early. zoneor southerlyadjacentin the northern part Veins of complex composition contain addiof the Moldanubian Zone of the Hercynian tional to minerals of the carbonate-pitchblende belt. Whether this reflects geochemicalassemblagethose of the younger carbonatesedimentary zoning in the pre-Hercynian arsenide and quartz-sulfide stages. Fractures geosynclinealong the northern marginsof the hostingcomplex veins predominantlyare fourth now Moldanubian Zone or distinct geotecorderstructuresthat had beenreactivatedand are tonicihydrothermal processes different to now characterued by breccias and deformed those forming the Bohemian-typeveins, or gouge.Veins are more than 1000mlong and 10to other causesremainsopen to debate. - The district is associatedwith a highly differ60cm wide. Characteristicmineral componentsof complex entiated uraniferous leucogTanitelocated at veins are tri- and di-arsenides of Co and Ni the junction of two lineaments. together with native Ag, Bi and As. Gangue - Mineralization is essentially restricted to minerals include dolomite, ankerite, quartz, metasediments. minor baryte and fluorite. U is presentin several - Uranium distribution exhibits a pronounced generationsof pitchblende and coffinite largely spatial relationship to the intrusive surface of derivedby remobilizationof pitchblende1. the granite in depth. Uranium mineralization Figure 5.38 shows some examples of the prevails in a zone between (2&) 150m and compositionand texture of different veins hosted 400m aboveand parallelingthe granitecontact by mica schist. whichis 300to 1000mdeep(Fig. 5.39). The compositionof the veins showsa vertical - Uranium pinchesout with very few exceptions zonation(Fig. 5.37). within 20 to 150m abovethe granite,i.e., at the marginwithin the hornfelszone of contact- Upper levels, more than 600 to 800m above metamorphism. the granite basementor from surfaceto 400m deep respectively are dominated by native Alteration silver,arsenides and redistributed pitchblende. - Lower levels extending200 to 300m upwards - Pre-uranium stage alteration reflected b1' from the granite basement are dominated growth of chlorite, phlogopite, rutile. and by native bismuth, arsenidesand primary abundant calcite penetrateswall rocks for up to pitchblende. 3m. Subsequent pervasive silicification generated a wide silicified zone.
Smeykalin Kolektiv (Ig84) establishedisotope - Syn-uranium stage alteration reflected by ages of 270-230m.y. (247 + 7m.y.) for 1st albite, adularia, dolomite, and hematite generation pitchblendeand 150-100m.y.for 2nd development overprinted in a narrower halo generationpitchblende, that is associatedwith the former alteration zone. quartz,dolomite and the Co, Ni, Bi, As stage. Ruzicka (I97I) reports 30-5 m.y. for a 3rd generationof pitchblendesupposedlyassociated
Examplesof Vein-Type Uranium Deposits
229
U U
)U CM
C u + _ A g+ q u
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I F i g . 5 . 3 9 . J d c h v m o vd i s t r i c t .a G e i s t e r v e i n . 3 2 6 m b e l o w s u r f a c e .b a n d c Hildebrandvein ( b 214m belowsurface).Schemesof compositionof mineralized veins within mica schistwall rock. a Associationof massivepirchblende(Ul and chalcopyrite (Ca). b Main vein composed of pitch-blende (Lf associatedwith silver (,49), gangue of dominant dolomite (dol) and quanz (qu) and mica schist iragments: laterai veins consistof quartz. chalcopyrite(Crz)and silver (Cu + Ag - qu). c Detail of a vein displayingmica schistfrqgmentsencrustedwith quartz (qu) and pitchblende (Q embeddedin a ganguematrix of quartz and dolomite (qu + doQ. [After de Launay in Roubault 1958(a and b) and Step and Becke
ie0a(c)l
Mineralization -
The ore is polymetallic, consisting of U, Co, Ni, Ag, and Bi, but uranium is not paragenetically related to the other metals. - Various mineral assemblageshave formed during several stages. - Veins of simple composition contain pitchblende and coffinite associated with quartz. albite or adularia. fluorite. and hematite. - Veins of complex composition contain arsenides.sulfarsenides.and native forms of the other metals and redistributed uranium often as coffinite associatedwith dolomrte, ankerite. quartz. minor baryte, and fluorite. - The mineralized veins of the Joachimsthal/ J5chymov distnct group in clusters. Seven ot the clusters contained most of the uranium mined (Fig. 5.36). - Vein clusters often consist of tree-like asymmetric geometry, the variably trending branches or fault systems filled with different mineral associations. - Emplacement of the carbonate-pitchblende association occurred in structures of a distinct system trending about NW-SE to N-S with richest ore concentrations at sites of structural
complication of veins such as (a) marked changesin strike and dip direction of veins, (b) sitesof interjunction. branching or ramification of veins, (c) sites of splaying or horse-tailing, (d) intersections with faults or dikes providing structurel barriers (particularly E-W faults and granite porphyry dikes), (e) thickening of veins, and (f) moderately inclined sections. All intervals with above listed structural featuresdisplayintensecataciasisof wall rocks, development of a series of microjoints and relatively wide aureoles of host rock aiteration. Uraniferous intervals in veins are generally restricted to vein transections with mafic or semi-mafic schists. Best grades and the bulk of the ore occurs at intersections with strongly pyritic biotite and biotite-phlogopite schistsof the Ji{chymov Series. Similarly well minerallzed are sections cutting through lithologic boundaries between chlorite-sericite phyllites and pyritic amphibolites and biotite schists. Other favorable sites are proximal to dikes and zones of abundant dikes. At intersections of veins with red granite porphyry dikes ore
230
5 Selected Examples of Economically Significant Types of Uranium Deposits
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x
xx x XX x xx x
ZUUM
X X x
Xz
++
++
+ E
[U
J d c h y m o vS e r i e s 2nd horizon
t=-lr+l
Erzgebirgsgronit
a
rE
G r o n i t eP o r P h Y r Y
r'l
t--=
+-i-+
++
++-f
+f
-f
Junction line of s p l o y- v e r n s E-W foults ond lsopleth of relotive ore grodes
T.---;--:] l""l
Lowest grode
l:::::j
F,--41Hlgn"ttgrode
Fig. 5.,t0. Jiichymov district, A2 vein of the Abertamv vein cluster, (Ab in Fig. 5.36) longitudinal section (vertical projecrion) with isopleth of relative U ore grades.Mineratzation of variable grades and extension forms a lode between and controlled by granite porphyry dikes. (After Kominek and Veseli 1986)
shoots are often accumulated below the dikes
Metallogenetic Concepts
(Fig.s.a0). The carbonate-pitchblende stage occurs in variably shaped and extensive concentrations, in form of thin stringers, streaks, and patches. Thick veins commonly contain the richest lodes. The uranium accumulations are irregularll' distributed throughout the vein system and separated by barren ground (Fig.5.41). Uranium ore occuoies onlv 4 to 6"/o of.the total plane of a vein. l.odes are several square meters to some hundred. rarely more than thousand square meters in tabular size composed of sections of different grade (Fig.5.40). Richestlodes attain dimensions of 100 x 300m. Ore lodes most frequently occur in moderately dipping and only occasionally in flat or vertical dipping sections of the host structure.
The various mineral associationsof the Jachymov veins are thought to have formed by discontinuous multistage hydrothermal processes spanning over a time interval from Permian to Cretaceous/Tertiary (Mrna 1967). Geochemically different fluids introduced the various metals to form five or six mineral associationsof monometallic and polymetallic composition. Two principal periods of ore formation have been deciphered for the Erzgebirge in general by Baumann (1967). The first, which carbonate-pitchblende stage. includes the occurred during late Hercynian time and the second, with formation of Co-Ni arsenides/sulfarsenidesetc., some time berween late Triassic and Tertiary and probably is associated with the Saxonian/Alpidic tectonic event.
Examplesof Vein-TypeUranium Deposits fi ll rl - l lJ -
Jcichymov Serres |
^
lio
ioftzon
772
Corc-stl'cole rocks
Fj
Jrncl,on wrth Geschiebervern
l_-/v)
7
N
S
Z - w - t , e n d i n o d i s p l o c e m e n t fo u l l i. t / i/4.-. ",.. +
//
.^,'
.i*,t''.-l o 7-i+;"=T'
IIJ
minerolizoiron i -ilu;l U -ll -
I' A
/ --l
23r
/
II J
,l
/l
Mrn,ng evels erth tetol,vgTe:c. Jrades
'L I
,t.ti,"
,' r
-; !
ss-'
!: ( r
.9
f-/+t t: I
"r,
\t L/ /
I
_..
.4* .i. ,.. " -,...#,i,*_='=*' iI i7. ,^., ;;?.s"+<'ii,l"r+' /^
,/ Fig. 5.41. J6chvmov district. longirudinal section of the 4th splay vein of the Geschieber vein, Svornost vein cluster (Sv in Fig. 5.36). Relatively small ore shoots form cumulatively minable lodes. Section A-A' displays the attitude of the 4th splay vein and values of relative metal gradesof the various mining ievels. (After Kominek
f
/
-.,
.r!€9
/f
tl//
n i a. r- r /t .-, r''.ii,z 4 lii _ i'i *' ,,.,'."-.1 rarrf. t .: :' ."i *'i !,/ ir.'1.:.i!' 't:ii'
r.. .s",::-. :iz ':. +*. ,,
'.t!' :ij' +
[>'i ; fi;i f;i ,. ,r.;>'i -; ,/ i!!j!::,n
" "t gi"F= ,/'nn|8>
t. t
and Veself 1986)
The carbonate-pitchblende association is the oldest ore generation introduced by mesothermal fluids. Decrepitation tests by Mrna and Pavlu (1967) indicate temperatures of 370 to 470'C. The uranium-bearing solutions supposedly carried larger amounts of U. Fe, Mg, Ca. and CO2, minor amounts of SiO? and F and tracesof V and REE (Ruzicka 1971). Dvmkov (1960) proposes that most of the pitchblende precipitated together with carbonates within a geochemical environment of pH 8 to 9. Kolektiv (1984) remarks that rocks with high Ca and Fe contents in form of pyrite and calcite influenced the precipitation of ore minerals and therefore served as host rocks. Pyrite acted as reductant and calcite maintained the alkalinity required for the redox reactions between Fe't of pyrite and U6* to deposit pitchblende. The time gap between Erzgebirgsgranit crystallization. 310-300m.y. ago and pitchblende deposition, 270-230m.y. ago puts restrictions on magmatic hydrothermal models. Ruzicka (1971) favors the idea that lateral secretion was instrumental in the formation of the
Jiichymov deposits and suggeststhat both metasediments and autometamorphic granites played significant roles. Both have anomalous contents of leachable uranium and hence may be considered potential source rocks. Since the Erzgebirgsgranit resembles in composition and evolution, as far as is known, the St. Sylvestre granite in Limousin, France, (see Sect. 5.3.1) it would be of much interest to see whether the processes suggested for the formation of the French vein uranium deposits are also applicable for the Jdchymov and other pitchblende veins of the Erzgebirge. The various processesand results of redistnbution (150 to 100m.y. and 30 to 5m.y. ago) are addressed by Kolektiv (1984). Carbonatepitchblende of the early stage only survived in subsidiary structures of the fifth and sixth order. These structures were activated only initially and for only a short time interval and then remained sealed against attack by younger solutions. In mineralized structures of fourth order, which were reactivated and opened to circulation of younger solutions, pitchblende and carbonate
?32
5 Selected Examples of Economically Significant Types of Uranium Deposits
were partiauy replaced by quartz associatedwith impovenshmentof the ore and changesin the shapeof lodes. This dissolutionand subsequent redeposition of pitchblende 2 and coffinite occurred coeval with the precipitation of arsenidesand locally baryte. In third order structures that experienced relative long periods of activation,the uranium mineralization was attacked more heavily, and down-graded. The structures were reopened simultaneouslywith those of secondorder and prior to the introduction of sulfarsenidesand sulfides. This provided time for infiltration of oxygenatedfluids that leachedthe uranium and reacted contemporaneouslywith arsenides to form sulfarsenides. In first order structures which were after empiacementof earlv pitchblenderepeatedlyand for long periods reactivatedsolutionsdestroved pitchblende. carbonate, baryte, fluorite, and other mineralsand replacedthem by mineralsof the young quartz-hematitestageand alsoformed veins. magnetite-hematite-bearing
-,r'+\\-
+ + + + *.
References and Further Readingfor Chapter 5.3.3 (for detailsof publicationsseeBibliography) Bernard et al. 1967; Dymkov li)61: Kolektiv Komfnek and Veselj 1986; Kraus 1916; Leutwein Mria 1963a, 1963b, 1967; Oelsner 1958: Ruzicka Ruzicka V, pers. commun.; Schumacher1933;Vogl Zijckert 1926
5.3.4 Perigranitic Uranium Depositsin Contact-MetamorphicRocks: Iberian Meseta,Portugal-Spain The Iberian Meseta. locatedin the westernpart of the Iberian Peninsula.hostsuranium deoosits which are globallysimilarto vein depositJelsewhere in the Hercynian chain of Europe. But there are also vein-type depositswith differenr features,viz. the "Iberian type" depositspositioned in the contact-metamorphicaureole of piutons(class3.1.2.3,Chap.4),
* -fLf:
'g'.'K j'i'iti}F,""f,,''**;ti{j+D ijii rrI'F'P'iF7ffi",',.^,..:'+= ,o'J--\.**-
IA
-*"i-*i* * + +a.villovreio ,4-\Nl-LkY+ + + +an/,87' \ - +.t-'J!a;.o,.o* * * *p-tEsccionzo , * ' F r c r x r o s o , ul"l'...') A",^no, Aprnno, * . aromcaol){:^\FO/z ;4"1 . * *r oo{
PI
{.:: :';:;:"ll;*k m i':[i]i!.i**fultf+ f,RGErRrcail^r!d3lll"t;,i'',,; -^1'' : -X ir0oeo-.4oREt-co* ---r:a,iu: : : %frt-fri
ai -"':1':"i I Azere
-:-"_:V
[--l
BETRAa/-gu
i:H*/*il":,"':,.si \ 1:-:' :*---i ^qV -f,- -:- -:- -:' ,,^ --\
C o s- i t l o s .t
1984; 19-57; 1971, 1856;
Pre-Ordovician Schist-Greyivacke Complex (A: Precambrian, P: earty Paleozoic) Silurian€rdovician
Cenozoic-Quatemay
lr*1 Graniticterrane A U deposit Malorlautt -
Fig. 5.42. Iberian Peninsuia, generalizedgeology of the centrallberian Meseta and location of the main uranium districts of Portugal (Compiled from Dardel/ PeinadorFemandeset al. 1979:Cameron 1982a,b; Bashamand Matos Dias 1986)
of Vein-TypeUraniumDeposits Examples Seven districts with vein-type mineralization have been establishedin the Iberian Meseta. five in Spain and tr*'o in Portugal (Fig. 5.a2). They c o n t a i n a t o t a l o f a p p r o x i m a t e l y5 0 0 0 0 m t U 3 O g a t c o m m o n l y l o w ' g r a d e so f < 0 . 1 % U 3 O 6 . The following synopsisis essentiallybased on papers by the authors listed in the References uniessotherwise stated.
GeologicalSetting of Mineralization
233
morphic aureole of Hercynian granites, which often host pitchblende veinlets, and in schists distant from the granite. Principal ore minerals at depth are pitchblende and coffinite. Hexavalent uranium minerals. predominantly autunite. occur above the water table in the oxidation zone, which extends from surface 15 to 40 m deep. The uranium minerals impregnate fracture zones, schistosityplanes, or other heterogeneities of the host shale. Some pitchblende veinlets have been drill intersected up to 200 m deep. Associated minerals are sulfides. mainlv pyrite, marcasire, qalena. sphalente, and chalcopvrite. Gangue minerals may include carbonates,fluorite, quartz. chlorite. and adularia. Associatedand ganguemineral are present in only minor amounts. Three characteristic distributions of perigranitic mineralization are recognized: (Dardel/ Peinador Fernandes et al. 1979'. Zieeler and Dardel 198.1)
The Iberian lVIeseta.a southwesternextensionof :he European Hercynian orogenic belt, consisrs of a Precambrianto Hercynian block. surrounded or covered bv Phanerozoicsediments.Prevailing rocks of the Meseta are quartzites, schists, gneisses,and granites of Precambrian to lower Paieozoic age forming the Schist-Greywacke Complex. The complex was intruded by granites including uraniferous leucogranitesand folded during the Hercvnian Orogeny, but only slightly mineralization within a 1000regionally metamorphosed to mostly greenschist a) Disspminated contact-metamorphed zone 1500-m-wide facies grade, and contact-metamorphosedaround granite contact. In along the immediately the granite intrusions. zones of mineralization, the contact is often The Meseta can be divided into five structural offset by orthogonal faulting causing embayzones (Capdevila et al. 1973)comparable to those ments of shales displaced into the granite. of the Hercynian belt elsewhere in central and deposits of this setting include Typical western Europe. All imponant uranium minAlameda, Esperanza. and Caridad, Ciudad eralizations occur in the Centrai Iberian Zone Rodrigo district. Spain; Nisa, Tarabau. (comparable to the Moldanubian Zone) and some Palheiros de Tolosa, Alto Alentejo district, in the adjacent zones, but always ciose to the and Azere, Beiras district, Portugai (.econtact with the Central Zone. serves commonlv some ten to a few thousand
mru3os). b) Veiniike mineralization within roof pendants Host Rock Alteration of shalesin granite. (e.g., Senhoradas Fontes, Beiras distnct. Portugal, 100mtU3O6, some Two time-related alterations can be distin100m deep). guished. Pre-ore alteration of regional extent c) Disseminated. fracture and joint filling minincludes sericitization. chlontization. and toureralization in basins bordered by granites and malinization of variable intensity. Ore-related filled with clastic sediments of the Schistalteration is of limited extent around mineralized Gregvacke Complex. Depositscan occur up to zones and consists of intense chloritization. several kiiometers from the nearest granite argillitization. silicification. and hematitization. outcrop. A typical deposit of this setting is The latter is particularly noticeable around disMina Fe in the Ciuded Rodrigo district, Spain seminated type mineraiization. (Figs. 5..13.5..l.l. Table 5.25). Age datings on perigranitic pitchblende yield 100 to 60m.y. (Stieff and Stern 1960; Horne 1960) and 57 to 37m.y. (Ludwig in Arribas 1986). (pengranitic) Peribatholitic disseminations. Beside disseminated Iberian type mineraltermed lberian rype deposis, are emplaced in ization, intragraniric veins associated with less rocks within the up to 2 km wide contact-meta- differentiated sranites occur in the lberian Principal Characteristics of Mineralization
5 Selected Examples of Economically Significant Types of Uranium DePosits Qlsnite
Fl
+++
++++
Pre-OrdovicianScfristGreywacke Complex
++++ +++
T
z riNol \
tf*\\
TT-l - srewacke, quartzite,
EA
'
inglrte,marute
- with abundant carbonaceous shale
l---l
Terliary
F
Y";o,.t"r*
axs [=l Antform ) +
I,
+ ++
Ei,Ht! Zones with mineralization
\,:r t
+
/
l-.r--l Synform axis
r+\
+
U dePosit
A
eU
rl
t ino Fe Ciudod . Rodilqo N lsxm
i
I 5
tlr \
r C o rp i o 0
I
2
3km
Fig. 5.43. Ciudad Rodrigo district, Salamanca. geological map showing geoiogical setting of uraniferous zones and principal U deposits uithin the preOrdovician Schist-Grepvacke Complex. (After Arribas 1986)
Table 5.25. Ciudad Rodrigo district, petrographyof host rocks and ore-formingmineralsof major deposia (localitiessee Figs. 5.42 and 5.43). (After Dardel/PeinadorFernandeset al. 1979) Deposit
Host rocks
Uranium minerals Primarl
Gangue
Secondan'
Fe, Zone D Rio Agueda south of Saeiices
Argillaceousschist Micaschist Hornfels
Pitchblende Coracite.Gummites Coffinite Uranotile Aurunite. Torbernite Uranopilite, Ianthinite Black oxides
Esperanza Villar de la Yegua
Micaschist Hornfels Microgranitedike Quartz vein
Pitchblende Coffinite
Caridad Villa Vieja de Yeltes
Micaschist Hornfels
Coffinite
Znnes2l,23.24 Alameda de Gardon
Micaschist,feldspathic Hornfels Granodioritedike
Associated minerals
Quanz Carbonate Fluorite Adularia (?.1 Chlorire
Fe-sulfides Sphalerite Galena Chalcopvrite
Coracite,Gummites Uranotile, Renardite Kasolite,Torbernite Saleeite,Ianthinite
Quaru
Galena Marcasite Cerussite Pyrite
Autunite Torbernite Saleeite
Quartz
Pyrite
Autunite, Renardite Uranotile Sabugalite Phosphuranylite
Pyrite
Examplesof Vein-Type Uranium Deposits
235
0 r o g e n i e s , / T e c t o n rP c hoses H e rc y n i o n Rocks Minerols
U r o n i u mp h o s e s Ecrly
o
Supergene phoses
rs Metosedtmen
G r o n i t ei n t r u s i o n 0 u o r tz A p o t l te To u r m o l i n e Muscovtte Adulorio
P y r i t e{ m o r c o s r t e ) Chlorite(biotiie) S p h o i e r r t e ,g o l e n o
c h o l co p y r i t e Corbonote Hemotite Piichblende
Gummrte U5'minerols Limonite
Fig. 5.,14. Ciudad Rodn_eodistrict. Mina Fe. tectonic events and metallogenericsuccessionof U mineralization..S1 plastic deformation; 52 crenulation: a brittle deformation. (After Arribas 1986t
Meseta. both often in the same district. Typical deposits are Urgeinqa and Cunha Baixa in the Beiras district. Portugal (order of reserves:about 1 0 0 0 m tU : O a ) . Principal ore minerals in the veins are pitchblendc and coffinite rvhich are often. particularly near the surface. altered to hexavalenturanium minerals. Associated minerab include pyrite. galena. sphalerite, chalcopyrite, fluorite, carbonates. and a variety of quartz-silica species. (Dardel/Peinador Fernandes 1979)
Potential Sourcesof Uranium Trvo favorabe uranium sources are proposed, differentiated leucogranites and metasediments. "Younger" granites of the Sao Pedro do Sui piuton. northwestern Beiras district, Portugal, average 8 to 20ppmu, 50 to 7lo/o of which is contained in easily leachable, low-thorian uraninite and submicroscopic, intergranular form. Thorium content averages38 ppm. (Basham et ai. 1982a). Host granites to the Urgeiriqa, Bica and Cunha Baixa deposits, Beiras district, have tenors of 4 to 17ppm U and 20 to 37 ppmTh in surface samples (Pagel 1982c). The "younger" granites
236
5 SelectedExamples of Economically Significant Types of Uranium Deposits
characteristically contain a suite of accessory Alteration minerals comprisingapatite, monazite, zircon, - Contact-metamorphism transformedthe metarare xenotime,very rare sphene,and low-thorian sediments granite along contacts into andalusite (lessthan 2%),low REE uraninite.This suite of speckled rocks and hornfels. accessoriesexhibits a strong contrast to urani- Pre-orealterationis reflectedby sericitization, ferous but infertiie hornblende granites of chloritization, and tourmalinization. Hercynian and Caledonianage, which typically - Ore-relatedalteration includes chloritization, contain a suiteof high-thorianuraninite,thorite, argillitization,silicification-and often intense sphene.allanite.and zircon. hematitization. -Dekkers et al. (1983) note that present-day streamwatersdraining the fertile granitesof the de Vide batholith contain Mineralization Nisa-Tolosa-Castelo elevated amounts of uranium. whereas those Ore mineralsimpregnateand coat schistositv streamsdraininggranitescontainingan accessorJ' and fracture planes and as such appear primineral suitedominatedby high thorian uraninite marily structuraliycontrolled. A certain lithoand thorite havelow uranium concentrations. logiccontrolmay be envisionedby the position Sedimenn-metasedimens: Bashamand Matos of most depositsin the contact-metamorphic Dias (1986)report uranium concentrationsfrom aureolesurroundinguraniferousIeucogranires 1 to 13ppm for unweatheredhost metasediments (except Mina Fe in the Ciudad Rodrigo of the Azere and Nisa depositsarea, Portugal. district). Thesevaluesare typical averagesfor shales.No - Ore mineralogy is commonly monometallic evidenceof synsedimentary enrichmenthas been consisting predominantl,vof hexavalent U found. Only minor redistribution and concenminerals (autunite etc.) and locally sootv tration of uranium occurredduring regionaland pitchblende.Solid pitchblendeis rare. thermalcontact-metamorphism, reflectedby local - Associated minerals are present in minor redistribution of uranium into quartz veinlets, amountsand may include sulfides of Fe, Pb and cordierite knots or recrystallized chloritic and Cu. carbonates,fluorite, and adularia. bandswithin the contacthaloes.No indicationof The distribution of mineralization is of limited a marked change of the whole-rock uranium depth commonlylessthan 4Om deeP. could be detectedby content by theseprocesses Nearby granites often contain intragranitic the authors. vein deposits. pitchblende Arribas et al. (1984)report that carbonaceous of the Schist-Greywacke shales/phyllites Complex Iocated W and N of Ciudad Rodrigo, Spain, MetallogeneticConcepts contain from few ppm to 30ppmU, with some valuesup to almost200ppmU. The uraniferous Genetic models for peribatholitic Iberian type shales constitute ca. 40To of the stratigraphic uraniumdepositsare contentious,perhapsdue to sequencein the complex. restricted exploration in depth and limited research work. Hypotheses include supergene models,which contemplateleaching of uranium Ore Controlsand RecognitionCriteria b1'weatheringand erosionof uraniferousgranites Significantore controllingor recognitioncriteria and superficialredepositionof uranium in topographic lows and fractured or brecciatedzones of Iberian type mineralizationinclude: adjacentto the sourcegranites(FernandesPolo 1970;Cameron1982b). Host Environment Ziegler and Dardel (1984) propose a lateral - Localization of districts is within the Central secretionaryprocessprovoked by high-temperaIberian Zone. ture isothermsaccompanying granite intrusions. - Country rocks are low-grademetamorphosed Temperature increasewould create convective partly carbonaceouspelites and psammites cellsof heatedsurfacewatersextractinguranium intruded by a variety of granitesincludinga from sedimentarysource rocks such as black late stageuraniferousleucogranite. shalesand reprecipitatingit in favorable en- Host rocks are intenselyfractured. (structures, vironments heterogeneities).
Exampiesof Vein-Type Uranium Deposits
Pagel (1982c) presentssupporting evidencefor in the formation of veins and Iberian type split a mineralization. He suegests that (hypogene?) hvdrothermal activity and earliest pitchblende crystallization can be linked probably to late Hercynian events whereas supergene processes involved in the formation of disseminatedtype mineralization have been stronglv active from Tertiary to recent times. Basham and Matos Dias (1986) propose an interrelated metallogeneticevolution for the broad spectrum of vein-tvpe uranium deposits in Portugal. Their eenetic scheme spans from postgranite hydrothermai formation of moderatelv high temperature jasper-pitchblende veins through lower temperature quartz veins and peribatholitic Iberian type uranium dissemtnations. to supergeneore formations in near surface structures (15 to 40m below surface) by redistribution of earlier mineraiization, and uranium leached from granites. The authors, argue in favor of a continuum between the various primary mineralizations. in principle intragranitic and perigranitic pitchblende ores. They suggest that initial uraniferous. oxidizing solution which introduced uranium and minor quantities of other metals to form intragranitic veins may have continued to move outwards and upwards by convective circulation into the contact-metamorphic aureole. Here. the uraniferous solutions encountered a strongiy reducing environment in pyrite-bearing schists causing precipitation of uranium in the network of microfractures to form the Iberian tvpe deposits. Basham and Matos Dias (1986) ruie out a metasedimentaryuranlum source at least for the Azere. Beiras and Nisa, Alto Alentejo regions, Portugal, which contrasts to the presentation by Arribas (1985) on the nearby Ciudad Rodrigo distnct in Spain. Arribas (1986) proposes for the Ciudad Rodrigo district, Spain, leachine of uranium from uraniferous carbonaceousphyllites of the Schist-Greywacke Complex. and uranium transport as uranyl carbonate in hydrothermal fluids generated by seismic pumping. This model implies a hydrothermal fluid transport mechanism triggered bv tangential deformation. Deposition of pitchblende occurred in near-surface dilation zones created by hydraulic fracturing in the course of shallow tectonic activity during Tertiary time. Strains developed within the Hercvnian basement around shear zones in response to Alpine orogenic activity during lower to middle Tertiary
237
time could have initiated episodic remobilization of uranium by hydrothermal solutions. These U mobilizations are documented by the apparent U/Pb agesof pitchblende in the Ciudad Rodrigo district, 57 to 37 m.y.. which correlate with the Pyreneanphase of the Alpine Orogeny.
Referencesand Further Reading for Chapter 5.3.4 (for detailsof publicationssee Bibliography) ArndizJ. pers.commun.:Arribas1962.19i0.1975. 1978, 1986:Arribaset al. 1983.198,1: Barros1966:Basham and MatosDias1986;Bashamet al. 1982a.1982b;Cameron 1982b: Cameron 1959.1982a. et al.rIUREP1980: Cerveira 1956;Coma1983;CorretgeandLopezPlaza1916:' Dardel/ PeinadorFernandes et al. 1979:Dekkerset al. 1983: Poto 1970:G6mezJadnet al. 7977;Limpode Fernandes Faria1966:llangas and Arribas 198,1: Manin Izardand Arribasi984; MatosDias and Soaresde Andrade1970; Pilar 1966:PortugalFerreiraandMatosDias Pagel1982c; 1982
Not 5.3.5 Metasediment-Hosted, Vein Deposits: Uranium Granite-Related Mine, Front Range,USA Schwartzwalder The Schwartzwaidervein deposit is located in the Front Range. approximately2OkmNW of Goiden,Colorado(Fig. ,i.a5;. Productionfrom the 750m deep mine has been almost 9000mt U:Oe. Remaining reserves are unknown but thought to be at least in the order of 3000 to 5000mt U:Os, occurring in a drill-indicated depthdown to 900m. Ore mined until 1979averaged0.5 to 0.6'/"U:Os, then droppedto 0.2 to 0.25% U3Osat a cut-off grade of 0.09",/"(Wright 1980). The depositconsistsof veins hostedby metasedimentsseveral kilometers distant from the nearestknown granite.It is thereforeclassifiedas not granite-relatedvein metasediment-hosted uraniumdeposit.(class3.2.1,Chap.4) The most recent publications on the Schwartzwalderdeposit are by Wallace and coworkers,who comprehensivelyand thoroughiy researchedboth mineralization and alteration. Thesepublicationsprovidedthe prime sourcefor the following descriptionamendedby data from the other listed investigators,particularlyby De Voto and Paschis(1980). Wright (1980),and Young (1979a).
5 Selected Examples of Economically Significant Types of Uranium Deposits
238
/2/ /.. 44i
Tertiory ++++++ +++++
'/r4
/ ,,' Bovlder.,/ , .
iTT' Jlt 1:
ITll
votconics
C r e t o c e o u sT-e r l i o r y IiJ':lI Loromide intrusives -stocks -di*es. sitls lz-] Pennsylvonioo nnd Younger l--l
sediments
P r o te r o z o t c
r?l/r !
/
FJI
+ C e n U o tC i t y G o td e n
',f,tiriNrr ++++++ Y+ "t (+ / ++ ++++++++ +++++++\ N+/ d I / l ++++ + + + + +++++++++++++ F r ++++++++++++++++++++++++\ Y + ', -' ,atl+F+ + ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ +A++ + - ! r r - + { t l 3 J + + t++ + + ++ + + + + + l + + +i + \? ? ?++ + r f+ + + + \ + + - \ 7 - - - - - - +- + + + + + + + + + \-: ' : ( + + ? + + +++ ++l\++ :rY\. \ + ++ ++ ++ ++ ++ ++ + + '3m+\ - r \ r ' \ \ \\ -. '\\+{ + ++ ++ ++ + ZA-'-
za4\.\:\\\\-s*
--r-\-_\r:r\ lll- ) :--\ \ \--l\,*)) - \ ' )- -
:
;
o. Old Leydencool mine b. Four Corners. Monn
f:l:.:9lT_",i!' [-Al u ueins
metovolconlcs
Fl
M o j o rt o u l t
GV
uoio, sheor zone
[El
u in sediments
U vein deposiis/occurrences
++++++ +\q+ +; + + \ + +++ ++++ I +++ 301m + + t;.;
U in sediments
-isl -
Gronitrcrocks
sll
{+}+++t
R o l s t o nB u t t e s o r e o 1.Schwortzwolder 2 . M e n oB . o n k e r sL o d e . N o r t h S t o r G o l d e nG o t e C o n y o no r e o 3 . A s c e n s i o nA. u b r e y L o d w i g .O h m o n n , Union Pocific Shoft M o r r i s o no r e o 4 .G r o p e v i n eF. o o t h i t l s C r i t c h e lo l reo 5 . B l o c kK n i g h t .B i l l i k e nS , even Devits J o m e s t o w nq r e o 6.Foir Doy. Elue Joy C e n t r o lC i t y o r e o 7 . W o o d .E o s t C o l h o u n C . o r o l l .K i r k L Coribou
mapoftheeast-centralpanwithlocationoftheSchwanzwalder Fig,5.45. ColoradoFrontRange,generalizedgeological Mine and other uranium deposirsand maior occurrences.(After Sims and Sheridan1964;Tweto 1979;Wallace 1983a)
matite of SilverPlume age (tt+00m.1'.) cut the metamorphic rocksPhanerozoic sedimentary rocks occur to the The Schu'artzwalderuranium deposit is Iocalized in an area with a compiex arrav of structural east of the deposit. The nearest outcrop is about dilation zones, fault branchings, deflections, or 500m from the mine. Red bed facies of Pennjunctions between the E and W branchesof the svlvanian age form the basal sedimentary unit. NW-SE-trending Rogers fault system, and is It is overlain by marine and continental sedihosted by specific metamorphic lithologic units of ments of Permian to Cretaceous age. RegolithizaLovier Protbrozoic age, formerly named the tion of the crystalline rocks extends from the Idaho Springs Formation. The country rocks were Pennsylvanian unconformity for about 30 m subjected to regional amphibolite-grade meta- downward. morphism at about I.7 to 1.75b.y. ago (Peterman The petrographic-stratigraphic position of the and Hedge 1968) and to tight isoclinal folding. Schwartzwalderdeposit (Fig. 5.a6) is dominated Retrograde metamorphism is verv minor. Dikes by Lower Proterozoic foliated and layered of aplite and tourmaline-garnet-bearing peg- metasedimentarvand metavolcanic rocks which Geological Setting of Mineralization
Examplesof Vein-Type Uranium Deposits
LJ'
Mico schist
G o r n e t - b i o i j tgen e i s s \ I1:-il guqpi7li. ) I
Hnrnhlon.lo
=T]
E
tronsition zon"
^nar<<
Synform oxis Mojor foult wiih dip direction
7--.,1 Minerolized vein wrth dio direction
Fig. 5.46. SchwartzwalderMine. generalized eeologicalmap of the mine area displavingthe surfacedistribution of the four major lithologic units and the structural disposition. (After Wright 1980.In: Wallace1983a)
can be grouped into four major units: Mica schist, garnet-biotite gneiss, quartzite and hornblende gneiss. Hornblende gneiss and mica schist are thick and regionally extensive. Garnet-biotite gneiss and quartzite form a narrow 50 to 100m thick transitional sequencebetween hornblende gneiss and mica schist. The structural position of the Schwartzwalder deposit is dominated by two distinct elementsof folding and faulting (Fig. 5.a6). a) A tightly isoclinally foided svnform with a nearly verticai NNE-SSW-striking axial piane and a steepiy SSW-plungingfoid axis. In the nose of the svnform the original thickness of the ore-hosting garnet-biotite gneiss and quartzite units of the transition zone is almost doubled to more than 100m. whereas elsewhere mica schistsoccupy the core of the fold. bounded successivelvby garnet-biotite gneiss. quartzite, and hornblende gneiss. b) A set of two major approximatelyparallel NWSE-strikine, 60 to 70'NE-dipping faults (Eand W-Rogers faults). The faults are ca. 1000m apart and interconnectedbv diagonal cymoid faults trending NNW to WNW (Illinois Fault) with a stackedseriesof tensionalhorsetail-type fractures off-branching into the hanging wall of the Illinois Fault (Fig. 5..17).
The lllinois Fault system, which hosts the Schwartzwalder U veins, evolved through multiple repetitions of tectonic activiw. As a result, a number of pre-ore and subparallel postore faults trendins between NNW and NNE developed. Pre-ore faults dip 70 to 75" W and post-ore faults 60'W (Figs. 5.a7, 5.18). Horsetail fractures developed in a stacked fashion in competent rocks on the hanging wail side of, and apparently root in the pre-ore Illinois Fault. Wright (1980) considers the horsetail fractures as of Laramide age. A characteristic feature of the Schwartzwalder deposit are dikes of altered clastic material a few centimetersto a meter wide. rvhich invaded many of the structures and veins exceDt the oost-ore Illinois Fauit.
Host Rock Alterations Altered wall rocks tvpically displav bleached and overprinted reddish coloration along velns. regardless of the presence or grade of mineralization. The alteration is mineralogically uniform throughout the 900 m vertical mine interval. but it nowhere had an extensive effect. or invaded far into the wall rock. It may locally even be absent. even adjacent to pitchblende-bearingveins.
240
5 Selected Examples of Economically Significant Types of Uranium Deposits
WSW
ENE
fA lVt
Pegmotite Fourt
l-Fl
v"in. tminerotized
l={l
Horsetoil froctures. :mrnerolrzed
HG Hornblende oneiss MS Mico schist fZ Tronsition zone
5500' -\--
\ \
\
/
I
{/ 0
*
50
+ t
100
1 5 0m
Fig. 5.47. Schwartzwalder Mine. geological WSW-ENE cross-sectionshowing principal veins, stnrctures, and horsetail fractures, and their lithologic position. (After De Voto and Paschis 1980;Paschis1979:Waliace 1983a)
Two successive assemblages of wall rock alteration (Fig. 5.a9) are distinguished (Wallace 1983a).
Todd I I
i
t*t
Woshington
a) Early carbonat2ation and sericitization pseudomorphicalii' replacing all mafic minerals of the host rocks within 2m of the verns. indicating a large influx of CO2 and concomitant loss of SiO2; b) subsequent hematitization and potassium feldspathization (adularia) replacing pre-existing alteration minerals immediatelv adiacent to the veins.
I
\
\+
0
30m
Fig.5.48. SchwanzwalderMine, planview of the first level with distribution of major veins and mine workings. (After Paschis1979)
Examplesof Vein-Type Uranium Deposits
241
which formed I a ) hematite-chalcedony-carbonate contemporaneouslvwith the second stage of alteration; r b) c a r b o n a t e sm, a i n l y d o l o m i t e , n o n - t o s l i g h t l y hematitic adularia and base metal sulfides including zoned yellow sphalerite. pyrite, g a l e n a ,c h a l c o p v r i t e .a n d c h a l c o c i r e .
l--l
Metosediment
llllll
p--^r;ra-aA,tan1
L:II
Corbonote-sericite
ffi
U mrnerorizotion
Fig. 5.49. SchwartzwalderMine, schematicillustration of the relative distribution of the early carbonate-sericiteand later hematite-adularia alteration zones along a vein. Vertical foliation is shown by verrical /ines. Some of the foliation exhibits drag along the vein hosting fault. (After Wallace 1983a)
Principal Characteristics of Mineralization Schwanzwalder vein mineralization occurs predominantly in the lllinois. West Rogers. and associatedhorsetail structures. Tlte ore essentially consistsof pitchbiende accompaniedby a number of trace eiementsof which As. Ba. Cu. Sb. Sr. V, a n d Z n c a n b e g r e a t e rt h a n 1 0 0 p p m .a n d M o . P b . and Th severai 1000ppm. (Wallace 1933a). Wallace distinguishes three main successive paragenetic srages including several substages of hypogene uranium and base metal mineralization: I: II:
Suifide-carbonate-adularia pitchblende-coffinite-carbonate-adulariasulfide iII: calcite-sulfide Stage I mineralization is sparsely represented. It includes two assemblages:
Stage II is the principal uranium phase and produced the bulk of the vein fillings. Earlier workers (Heyse 1972; Rich and Barabas L9761describe o n l y o n e m a j o r u r a n i u m g e n e r a t i o n .I n c o n t r a s t , W a l l a c e ( 1 9 8 3 a ) s u b d i v i d e ss t a e e I I i n t o t h r e e s u b s t a g e sa, s p r e s e n t e di n F i e . 5 . 5 0 . Pitchblendeand coffinite are the onlr uranium minerals. Pitchblende is parageneticailv intergrown with an unnamed Fe-Mo-.\s-sulfide minerai with vanable Mo/As ratios as determined by Wallace (1983a). The mineral. formerly referred to as jordisite or molybdenite. is characteristic for horsetaii ore, whereas it is markedly rare in fractured ores along the Illinois and West Rogers faults. Stage III mineralization comprises a coarsegrained assemblage of calcite-sulfide. predominantly pyrite and marcasite with minor chalcopyrite. The minerals fill vugs and fissures in uranium veins .and in post-ore structures such as the post-ore Illinois Fauit. Chemical investigations show distinct /rcce elementassociationsfor the horsetail vein svstem. and different and less distinct correlations for the Illnois and West Rogers veins. Mineralization of horsetail veins exhibits two associations of elementsa : ) U, Mo. As, Hg and Sb. b) Ag, Co, and Ni. Uranium is also closeiy associated*'ith Pb. S, and Zr. The strong U, Mo, and As correlation reflects the paragenetic relationship of pitchblende and the Fe-Mo-As sulfide mineral in the horsetail veins which survived fault movements. (Wallace 1983a) Distriburion of uranium ore is intermittent, mainly in the steeplv SW-dipping Illinois vein and in associated horsetail structures in its hanging wail (Fig. 5.-17).Some ore is in the Washington vein which parailels on the footwall side and joins in depth the Illinois vein (Figs. 5.-17.5.'{8). The mineralized vein system extends laterally for approximately 150 to 200 m in NNW-SSE strike direction along the Illinois vein. and tor 50 to 150m in width perpendicular to and mainly on the western horsetail side of the Illinois vein. Depth
aA1
5 Selected Examples of Economically Signifrcant Types of Uranium Deposirs
M;;a----=Jitole
IIa
trb
IIc
Adulorio {not hemotiticl Ankeriie-dolomite P i t c h b l e n d eC o ff i n i t e Fe-Mo-As suliide (unnomed minerol) 7 Pyrrter ChotcopyriteJ Goleno-
F
Tennontine Spholerite R o m m e l s b e r o i i eN i c c o l i t e_ P b _ M o s u t fi d e ArsenoPYrtte Q u o r t z/ o m e t h y s t C o l c i t eM o r c o s t t echtorite-Fluoriie
Fig. 5.50. SchwartzwalderMine, parageneric diagram of mineralization of stage II. The thickness of lines indicates the relative abundance of minerals. (After Wallace 1983a)
extensionis more than 700m alongthe dip of the StableIsotopesand Fluid Inclusions Illinois vein. Wallace attributes this deep penetration to down-faultingof somevein sections. Stable isotope and fuid inclusion studies by Drilling has intersected additional mineral- Wallaceand Whelan (1986)permit the following izationin a depth of about 900m within the West geneticinterpretations. Carbon of vein dolomite (613C = -2.4 to RogersFault approximately200to 250m W of the -7.3%) and oxygenof mineralsof early alterIllinois vein. The width of mineralized veins varies from ation and mineral stage I (6180 : *4.6 to millimeters to several meters. Horsetail veins +8.6%) yield inconclusiveinformation because averageabout 0.5m and commonlvcontainhigh they can be interpretedas of both magmaticand grade ore. The Illinois vein has a width up to metamorphicorigin. Oxygenin stageII mineralshave 618Ovalues 15m, but ore, althoughof largeminabletonnage, is of relativelylow grade.The lengthof ore shoots ranging largely from 0 to -5%". This lighter may be as much as 100 to 200m, as within the compositionmav suggestan influx of meteoric cymoidloopsof the Illinoisvein. Horsetailveins water mixing with the original fluids or less which root on the cymoidcanhavecorresponding evolvedconnatewater from the overlyingsedieconomicstrike lengths,and dip extensionsof ments.6180valuesof stageIII average-5.69;. implying an entirely meteoric origin which is 150m and more (Wright 1980). UlPb agesof 69.3 + f .im.y. are established compatiblewith the supposedlysupergeneproprecipitatedsulfides. for ore from the Titan vein, a structurein the venanceof paragenetically deposit. Sulfur isotopes (WallaceAVhelanand Hevse horsetailsectionof the Schwartzwalder The U/Pb isochron intercepts suggestthat the 1972)of stageII sulfidesyield 51S = +1.6 to derivedfrom a source1.6to -I7.7% but are scanty.These valuescompare mineralcomponents which is equivalentto the ageof the with thoseof metamorphicsulfides.ThusWallace i.9b.y. old. (Ludwiget al. 1985). rocks metamorphichost and Whelan(1986)considerthesevein sulfidesto be derived from metamorphic progenitors. Sulfidesof stageIII have 635 : -18 to -47y*, whichsuggests that they possiblyoriginatedfrom
t
I
t
of Vein-TypeUraniumDeposits Examples late iron sulfides deposited from a sulfate-rich solution. ratios reflecting an Vein galena has?o1Pbl206Pb t h e s o u r c e o f 1 . 9 t o 1 . 6 b .y. old, i.e., timeo f age to the metamorphism of the country quivalent e rocks. This result excludesany magmatic source vounger than the Boulder Creek granodionte dated at 1750 to 1700m.y. (ages from Peterman and Hedge 1968), thereby refuting models using Silver Plume (+1400m.y.) or younger intrusions as sources for the ore-forming metals including uranium. Research on aiteration mineralogy. fluid in;lusion micro-thermometry and sulfur traction.rtion by Waliace (1983a)suggestthat temperatures of mineral formation were approximately 175 to 125'C during the alteration, 225'C in mineral stage I, decreasing to 100-125'C in stage Ii c (fluid inclusions in stage I sphalerite, trapping temperature: 225"C,in stageII c (?) amethystand dolomite: 168-205'C; sulfur isotope fractionation of coexisting sphaleriteand gaiena of stage II c: 68 and 120'C). The bulk of pitchblende formed between stages I and II c together with cenain sulfides (pyrite, chalcopyrite, Fe-Mo-As-sulfide) and large quantities of carbonate, mainly ankerite, and adularia, whereas bornite, iron oxides, sericite, and graphite are notably absent. The assemblage is suggestive for a solurion of a pH of 5 to 8, a dissolved CO2 concentrationof 4 to 10%. and a likely uranium transport as a uranyl carbonate or bicarbonate complex.
Potential Sources of Uranium
743
Ore Controls and Recognition Criteria The Schwartzwalder veins show a disr.inct struct u r a l a n d l i t h o l o g i c a lc o n t r o l a s r e f l e c t e db y t h e following ore controis and recognition criteria.
HostEnvironment -
Ore veins occupv tensionai structures such as the large Illinois and Rogers faults and associated horsetail fractures. The distribution of ore veins is restricted to distinct lithologies, particularlv to garnetbiotite gneiss,a former iron (and sulfur?)-rich peiitic sediment, and quartizite. These rocks reacted to faulting by dilational fractures creating adequate transmissivitvand spacefor minable ore shoots. Hornblende gneiss, a former mafic volcanic rock, and mica schist are unfavorable hosts, probably due to more ductile reaction to faulting that resulted in discontinuous openings and impermeable gouge fillings. Folding tectonically increased the thickness of the favorable host rocks. particularly in the nose of the Schwartzwalder synform. where the section of the transitional rock unit is approximately doubled in width. The structural combination of steeply dipping horizons of the brittle transition zone rocks and their disrupture bv the steeplv dipping Illinois and Rogers fault systemsgenerated the favorable laterally narrow but verv deep (at least 10CIm) system of continuous and interconnected permeable fauits and breccias for ore empiacement.
Uranium and the other metals are thought to have derived from the ore hosting metamorphics. , lteration Uranium background in the hornblende gneiss a v e r a g e s1 t o 4 p p m U ( m a x . 5 2 p p m ) , g a r n e t - - E a r l y a l t e r a t i o n o f w a l l r o c k s i n c l u d e s c a r bonatizationand sericitization.and later aiterbiotite gneiss 5 to 10ppm (max. 88ppm), quartation K-metasomatismand hematitization. zite unit 1.7 to 26.8ppm and the mica schistunit 2.2 to 5.8ppm U (Wallace 1983a).Wallace favors the extensive honblende gneiss unit as the most Mineralization likelv source of uranium. Mineralization includes three stages.StageII is Indirect evidence such as isotope composition the principal U phase. Stages I and III consist and no detected magmatic source also support the essentially of suifides associated with mainly assumption that the vein-hosting metavolcanics carbonates. and metasediments were the source of uranium U minerals are pitchblende and coffinite and other ore-related elements. paragenetically associated with a Fe-Mo-As sulfide mineral. and minor sulfides and ar-
244
5 Selected Examples of Economically Signifcant Types of Uranium Deposits
senides. Gangue minerals are Ca-, Fe-, Mgcarbonates and late chlorite and fluorite. - Mineralization is intermittently distributed in few, steep-dipping major veins and in a great number of tributary horsetail fractures. - Preferential sites for ore accumulation are at pronounced changes in the width of structures commonly derived by branchings of veins. and abrupt shiftings in strike and dip of horsetail fractures and major structures such as the Illinois and West Rogers faults. - Repeated reactivation of major structures such as the Illinois vein probably caused dilution of ore grade in heavill' affected zones, as indicated by trace element disparities compared to horsetail veins.
Metallogenetic Concepts Models on the formation of the Schwartzwalder pitchblende veins have been put forward bv a number of authors, e.g.. Fisher (1976). Maslyn (1978). De Voto and Paschis(1980), Nelson and Gallagher (1982), Rich and Barabas (1982). These models range from ore emplacement in Proterozoic/Silver Plume time to Mesozoici Laramide time, and from ore-forming processes initiated by magmatic centers such as the Silver Plume and Laramide intrusives of the nearby Colorado Mineral Belt, to hypogene processes that generated convective cells leaching uranium and the other metals from the Proterozoic country rocks, and to supergeneprocessesactive along the Phanerozoic unconformity and deriving U from perhaps the red beds of the Pennsylvanian Fountain Formation or other sources. Thorough research work by Wallace and coworkers narrowed the conceivableframe of ore formation to convective hypogene hydrotherms extracting the ore elements from surrounding Proterozoic rocks and depositingthe elementsin distinct structurally prepared rock units during early Laramide time. Wallace and his coworkers' concept may be summarized as follows: 1. Intense faulting along the frontal zone of the developing eastern Front Range in early Laramide time by interaction of NW-SE-trending major faults and the reverse faults along the eastern uplift boundary resulted in a dilation of the frontal zone.
2. Sites for optimal and extensive ore emplacement in the Schwaruwalder area were prepared in pre- and early l-aramide time by tensional tectonism in a segment of structural coincidenceof steeply dipping layers of brinle garnet-biotitegneissand quartzite of the lithologic transition zone and its transection by a steeply dipping fault system. As a result. a laterallynarrow but extremelydeep stockworkof continuousand interconnectedbreccias,faults, and fractures was established(Illinois, Rogers faults. horsetail fractures).Repeated cataclastic reactivationr*'asinstrumentalin the formation of the various successivestagesof alteration and mineralization by producing abrupt releases of confining pressure and related increase in permeabiiity. 3. Uranium mineralizationbegan69-70m.y. ago ouring the incipientstagesof Laramide upiift of the easternFront Rangeand beneatha cover of Phanerozoicsedimentsapproximately3000m thick. Fission track investigations of apatire (Naeser1978;Marvin and Dobson 7979)indicate a relativelybtgh heatflow at this time. The heat was supplied through an increased regional thermogradientdue to igneous activity in the ColoradoMineral Belt. 4. Initial hydrothermal solutions originated from connate/metamorphicwater that possibly residedin deepstoragereservoirsalongthe major fault zonesof Proterozoicancestrywhich possibly became reactivated during the Pennsylvanian uplift. These connatefluids contained abundant CO2 and were in isotopic equilibrium with the enclosingmetamorphicrocks, but becamemixed with somemeteoricwaterfrom Mineral StageII c onward.Rich and Barabas(1982)suggestthat the mineralizingfluids were of organic or organoaqueouscharacterduring StageI (early) adularia and StageII (late) ankerite formation and thus probabl,valsoduringthe pitchbiendedeposition. The fluids probablystarteda convectivecirculationin responseto early Laramide tectonic movements.On their structurallypreparedpaths they collectedthe elementsneededfor the ore formation from the metamorphiccountry rocks. This waspossiblebecause manyof the major preore faults consistedof relative wide cataclastic zones composed of abundant anastomosing fractureswhich providedlarge accessand surface areasfor elementleachingin relationto the rock volumewithin the zones.As a resultof the interaction with the rocks, the fluids could acquire
of Vein-Type Deposits 245 Uranium Examples whichstrongly proportionatelylarge quantitiesof CO3, Ca. K. orestogetherwith volatileelements andothermetalsincludinguranium.The uranium correlatewith uranium. Other mineralsinclude was very probably mobilizedas a solublecar- base metal sulfidesand sulfosalts,as well as bonatecomplexsimilarto reactionsas described ankeriteand adularia.Uraniumtransportwas in and Yermolayev(1971)and Dicksonet al. (1979). soiutionas uranylcarbonateor bicarbonate, o;,2 when release of pregnant which abrupt 1 lie solutions migratedinto the its depositionoccurred to permeable pressure due revived tectonic movesections of the zones fault and confining most of CO2, increaseof sitesof ore deposition,had at the start of the ments causedeffervescence of the uranyl phase pH pH, a corresponding break-up and ore formation a of 5 to and 8. alteration reductionof hexavalentto 5. Two successivewall rock alterationassem- complex.Subsequent blageswere producedby the hydrothermalfluids tetravalenturanium by aqueoussulfur species in virtuallyall existingfaultsand fracturesexcept then precipitatedpitchblende. The original suifur compounds were prein late Laramide faults such as the post-ore ';linoisfault, at a temperature of about225"Cand dominantly sulfates, but may have included .; pressureof 7,s0to 1000bar. Early alteration also significantamountsof intermediatesulfur. :ormed sericite and carbonateby CO2 metaso- Through complex mechanismsand intermediate matismand SiO2removalat relativelyslowflow sulfate-thiosulfateand sulfate steps by sulfate rates of the solutions.Intermediatestagesof reduction,as outlinedby Spirakis(1981),a probrecciation increased the transmissivity and gressivelymore reducingfluid can evolve with CO2 an associatedtemperatureand pressuredecrease thereby the flow rate at contemporaneous effervescenceand pressure release. In con- causedby tectonicbrecciation.This processmay sequence, the pH rose and oxygen fugacity produce and allow H2S or HS- to reduce the of hexavalenturanium and precipitateit together iropped, and the secondalterationassemblage irematite and adularia replacedmineralsof the with sulfidesin a rapid process,asdocumentedby and textures assemblages the pitchblende-sulfide early alteration stage. 6. Ore emplacementoccurred in three suc- in the Schwarzwalderore. Wallaceexcludesany involvementof external cessive stages from hypogene hydrothermal fluids, except in stageIII when supergenefluids reductants such as ferrous iron, methane, or entered the system. Early vein mineralization organicmatterassuggested by otherauthors(e.g., formed at about 2?5"Cand750to 1000bar similar Adams and Stugard 1956; Rich and Barabas to the P-T conditions of the alteration stage. i982). Wallace'sargumentsagainstthesekindsof During stage II c the temperaturedecreasedto reductantsare: (a) Hematiteis locallyabsent,(b) about 125"Cand the pressuredroppedsomewhat. uraniumalsoprecipitatedin fractureswithin ironUranium was emplacedin virtually all accessible deficient quartzites,(c) hematite formed as an faults and honetail fractures over a vertical alteration product prior to pitchblende from interval of ca. 700m below surfacein the pre-ore which it wasseparatedby unhematitizedadularia, Illinois Fault svstem,and in the bottom part of (d) iron sulfidesand ankerite,not hematite,are the western strand of the Rogers Fault, where the iron-bearing minerais parageneticallydemineralization occurred at a depth extending posited with pitchblende, (e) carbonaceous below 900m under the presentsurface.Depth material associatedwith pitchblendeis not obiimitation of the mineraiizationmay have been servedand chemicalanalysesindicateonly small controlledby the thresholdlevelof local pressure amounts(0.06 to 0.73% in weight) of organic gradientswhich extendeddeeperin larger and carbon and no correlationof the carbonwith the more open faults and at which ascendingfluids uranium. started to effervesceCOz and consequentlypre8. During the later part of stageII c and III. cipitated the ore minerals.Uranium distribution supergenefluids intermixed with the hypogene and emplacementmusthavebeenratheruniform, solutionsprecipitatedpredominantlycarbonate, as indicatedby the paragenesis and ore textures. pyrite. and marcasite. 9. Supergeneoverprintingof primary uranium 7. The mineralization eventsstarted with stage I basemetals.adulariaand carbonate.This event minerals is restricted to within approximately was only of minor magnitude.StageII was the 100m below the presentsurface. principal ore-forming period. It included three substages.The first two producedthe uranium
246
5 Selected Examples of Economically Significant Types of Uranium Deposits
(PrecambrianIII) overlain by the Upper Proterozoic Katanga Group (PrecambrianIV).The sedimentswere deposited1300to 620m.y. ago, Voto and and folded and faultedduring three phasesof the Adams and Stugard 1956;Bird 1958, 1979;De Paschis1980;Downs and Bird 1965;Heyse 1972:Karlson Lufilian Orogenydatedat 840 to 770 + 40,670 + and Krokosz 1983; Maslyn 1978; Nelson and Gallagher 20 and 670 to 620 + 20m.y. (Cahen 1970).The 1982:Paschis1979;Rich and Barabas1982;Sheridanet al. 1967; Walker et al. 19631Wallace AR, pen. @mmun.; Katanga Group includes, in descendingorder, Wallace 1983a, 1983b, 1986; Waliace and Karlson 1982, the Kundelungu, Grand Conglomerate and 1985; WaIIace and Whelan 1986; Wallace et ai. 1983; Shale-Dolomite 1= Roan Group) Systems.The Wright 1980; Wright et al. 1981; Young 197, 1979a. latter includes the MwashvaSeriesand the ore 1979b. 1985 hosting Mines Seies (S6rie des Mines) (600 to 1000mthick). Principal host rocks at Shinkolobwe are siliceous dolomites, and dolomitic and carbonaceous shalesof the RSF. RSC. and SD units 5.3.6 Sediment-Hosted, Not Granite-Related of the Mines Series.(Derriks and Oosterbosch Referencesand Further Readingfor Chapter5.3.5 (for detailsof publicationsseeBibliography)
Vein UraniumDeposits:Shinkolobwe, KatangaCopperProvince,Zaire
Shinkolobweis situatednear Likasi, in the Shaba part of the Katanga Copper Belt. The deposit. discoveredin 1915,was mined for radium in an open pit down to almost60m between192i and 1936,producinga total of ca. 500000m3of ore yielding about 100000mt radioactive material (Derriks and Vaes 1956).It is guessedthat this material graded severalpercent of uranium. In 1944 mining was resumed by underground methods,reachinga depth of 255m. Operations were suspended in 1960 after production of annually 1200to 2200mtUsOs,or cumulatively ca. 22500mtU3O8 (CEA 1969). Adding production and remainingreserves,the depositcontained a total of at least 30000mtU3Osat an estimatedore gradeof 0.1 to 1% U3O8andmore. The deposit consistsof a vein stockwork in sedimentsdistant from any known granite and is therefore classifiedas sediment-hosted.not granite-related vein uraniumdeposit(ciass3.2.2. Chap.4). The subsequentsummaryis largely from the two comprehensive contributionson the geologl, of the uranium depositby Derriks and coworkers (1956,1958)updatedby informationfrom more recentpublications.
GeologicalSettingof Mineralization Shinkolobweis in the medianpart of the Katanga Synclinorium, an arcuate fold belt ca. 300km long extendingfrom Zambia into Za\re. Regional stratigraphic units are the Kibara Group
1es8) Shinkolobweis locatedin a structurally complex deformedarea due to folding, overturning. faulting and overthrusting.A dominant structure is here the Shinkolobwe Fauh Zone trending betweenE and NE, dippingca. 60"S. Thrusting pushed Kundelungu strata over each other. sqeezingbeds of the Mines Series into an irregular NE-SW-orientedfold-fault wedge 150 to 250m wide and severalkilometers long on the surface. NE-SW-trendinglongitudinal faults and commonly NW-SE and NNE-SSW cross faults segment the Mines Serieswedgeinto three displaced blocks or spurs (Fig. 5.51). A fourth spur (IV) existsin depth, eastof the mine, under a 350m thick cover of Kundelungusediments.Two pronounced cross-faults,the eastern and western boundaryfaults,delimitthe main depositlaterally. The three main spurs persist from the surface downward to 100 to 150m where thel' abut againstthe shallowinclined contact of the RAT nappe (Fig. 5.52). The RAT nappe consistsof overturnedthick bedsof a varietv of shalesand dolomitesof the RAT unit, the lowestunit of the Mines Series.The boundan'betweenthe three blocksis markedby brecciazones.(Derriks and Vaes1956)Ore lodesoccurin the MinesSeriesof the MassifPrincipal,and in the lower blockIV in a crushedzone of the Mines Seriespinched between the downdroppedKundelunguwedge and the MassifPrincipal. De Magndeand Frangois(1985)interpretthe geologicalsettingof Shinkolobweas beingon the flankof an anticlinal,breccia-fiiled diapir (termed "extrusionfault"). The brecciais composedof numerous,partlyverylarge,fragmentsderivedto
Examplesof Vein-Type Uranium Deposits
F
uiool" Kundetungu
liJ]
PetitConglomerate
7),
to*"rKundelungu
114
Grand Conglomerate,Mwashya - ' I Series,upper Roan strata, (partlyheavily brecciated)
_, '-
MinesSedes,C = CMN, I s=sD,R=RSC,RSF and RATunits
--;r Strikeand dio oi strata i,/'{ i ovefturned strata
Fautt
7, \r.
vein ] Pitchblenoe;uraninite
Fig, 5.51. Shinkolobwe. generalizedgeologicalmap showingthe complexstrucruralsening of the depositwhich is hosted inrheovenurned!{inesSeries. 1, Il,llltectonicblocksof theMassifPrincrpal.(.{fterDerriksandVaes1956;NgongoKashisha1975)
W Mrqore Aunqerungu slrqrq
-:
I lnar ruoppe
M i n e s S e r i e s / R o o nG r o u p F=5-{
Sondv colomitrc shole iRAT unii)
ffi
(RsFl{RSc) s i l i c e o u sd o t o m i t e
tE
D o l o m i i r c . c c r b o n o c e o ussh o l e ( S D )
'-r!rY ' - r' y ' w - t o ' i t
Moss,r
Prrncrpot
Jtock u
ffi
s t r o n g l yf r o c t u er d / b r e c c i o t erdo c K
Fiitr u minerolizotion
Fig. 5.52. Shinkolobwe. schematicW-E longitudinalsectiondisplayingthe distribution of uranium veinsassociatedwith caiaclasticzonesin the RAT naooe. Host rocksare overturnedstrataof the Mines Series.(After Derriks and Vaes i956)
a large extent lrom Mines Series facies. Many of these clasts contain "rich Cu-Co mineraiization of the svndiasenetic Kupferschiefer type". Dolomitic mari with a variable component of silt, authigenic quartz. and hyper-magnesianchlorite (corresponding to Derriks's and Oosterbosch's (1958) RAT unit) provided the matrix material. Gypsum, anhyddte, and pyrolcastic constituents are common in this RAT unit.
Host Rock Alterations Dominant host rock alterations are pre-ore sllici' fication and Mg-metasomatism, resulting in siliceous dolomite etc., and magnesite replacing dolomite and forming veins, particularly in deeper sections of the deposit. Ore-related aherarion inciudes chioritization, dolomitization, and weak silicification.
28
5 Selected Examples of Economically Significant Types of Uranium Deposis
chieflyCo-Ni-sulfides andselenides,minor pynte, molybdenite,galena,Cu-sulfides,monazite, and Principal ore minerals (Table 5.26) arc uraninite traces of Au, Ag, Pd, and other elements. present in cubes up to 1cm, occasionally4cm Gangue minerab include abundant magnesite, long, and, in oxidized sections,a large variety of chlorite, quartz, dolomite, and rare aragonite. Uraninite forms massiveaggregatesand assohexavalentU minerals. Remarkably, no pitchblende has been formed. Associ,atedminerals are ciations with Co-Ni minerals. Co-Ni mineralization extends beyond uranium zones but Nivaluesdrop rapidly, as reflectedby Ni/Co ratios Tbble 5.26. Shinkolobwe, primary ore and ganguemine- being 3: 1 in uraniferousore dropping to 1 :3 rals. (After Derriks and Vaes 1956;"K--., t945 rn Derriks outsidethereof. and Vaes 1956) The ore and associatedmineralsoccur as disveins, veinlets, or stringers along continuous Uraniwn joints,beddingplanes,and minor faults. fissures, Uraninite Urano-molybdenite(molybdate?) as breccia matrix, replacement masses and nodules, and as disseminatedparticles and agTh-rare earth Monazite gregatesin the hostrocks(Fig. 5.53).Major faults are never mineralized but filled with clay-talc Sulfi des-selen ides-telluri des Cattierite" Chalcopyrite Molybdenite breccias. Vaesite" Bornite Se-molybdenite A successionof su mineral srages is disNi-cattierite Digenite Pyrite tinguished(Derriks and Vaes 1956): Co-vaesite Coveliite Galena Principal Characteristics of Mineralization
Se-vaesite Se-siegenite Millerite Melonite
Pd-sulfide(?) Pentiandite (rePoned)
Umangite
Naive elements: Gold Copper (very rare) Ganguc: Silicates Quartz (not common) Chlorite Tourmaline (minor)
Carbonates Calcite Dolomite Magnesite
1. Mg-metasomatism transformingthe dolomites of the Mines Series into highly magnesian lithologies,mainly magnesite, 2. uraninite and pynte deposition in open stnrctures.
Apatite (minor)
lrl
ffi ffi i
Ta
t
.r
)
Siliceousdolomite, often with Collenias (RSC) Siliceousfoliateddolomite (RSR
m
Sandy dolomiticshale (RAT)
ffi
Dolomitic breccia
l-Zl
Fig. 5.53. Shinkolobwe, cross-sectionat the 114m level showing distribution of uranium veins and pockets emplaced in heavily fractured dolomitic and siliceous and carbonaceousshale. (After Derriks and Vaes 1956)
Dolomitic carbonaceous shale/scfrist (SD unit)
uineralized vein,pod
0 L---J-J
2
tm
Examplesof Vein-Type Uranium Deposits
249
i. crystallization of molybdenite, monazite, and chlorite; chlorite is coevai with monazite but later than molvbdenite, -1.weak quartzification succeededby deposition of sulfidesof Ni and Co and selenides, :. weak brecciation and doiomitization, 6. formation of chalcopyrite.
the Katanga Copper Belt elevatedClarke values of the ore-forming metals although of locally variable amounts. Maucher (1962)reports values of up to 50 and 100ppmU. Ngongo-Kashisha(i975) establisheda regional N-S-oriented mineral zoning in the Katanga beit in Shaba. U, Cir, Ni, Co, Mo associatedwith magnesite and monazite prevail in the southern Subsequent modification resulted in formation of segment of the arc where also Shinkolobwe and a iarge varietv of hexavalent U minerals and other uranium deposits are located. Further to secondaryminerals of other metals. In a general the north U, Ni. magnesite and monazite way, the altered U minerals developed through decreasewhereas Cu and Co increase. icur stages:(I) h,"drousstage: (a) ianthinite, (b) -:cquerelite. curite, schoepite: (2) silicarcstage: iasolite, soddvite. sklodowskite,uranophane;(3) Ore Controls and Recognition Criteria phosphate stage: (a) parsonsite. (b) dewindtite, (c) torbernite; (l) minor recrvstallization stage'. Principal ore controlling or recognitioncriteria include: hydrous oxides, especiallycurite. Uranium ore rvithin the main ore zone extends for more than 200m in E-W direction, 100m Host Environment wide and at least 255m deep. More ore is drillindicated at depth in an easterly extension 300 to - Regional structural setting is (a) within a fold -1-<0mdeep (block IV). beit, (b) in or near a hinge zone along which Dimensions of individual veins vary between a the tectonic style and direction of the Katanga few centimeters to a meter thick. They commonly fold belt change. - Local structural setting is very complex due lack continuity, being only a meter or two, less often as much as 10 m long. Locally, veins to complicated rock dislocations which lead, and veinlets are numerous enough to form however, to favorable structural ground stockworks. preparation. Tectonism generated fissures, Mineralization occurs in the three blocks breccias, stockworks etc. in more brittle rocks rvithin the nappe mentioned earlier (Fig. 5.52). (siliceous dolomite) and displacementsof more Ore has accumuiated particularly in the numerous ductile lithologies (shales etc.) moving them fissures,shears.and joints developedin dolomitic into positions (RAT nappe) to act as impershale beneath the domelike structure formed by meable barriers for migrating ore-forming the impermeable front of the RAT nappe. Below fluids. about the 180m level, uranium mineraiization - Position of host and potential source rocks (lower Mines Series) is at the base of a several apparently shifts eastwards. whereas Co-Ni minerals still display ubiquitous persistance. 1000m thick marine sequence (Katanga Beneath the 220 m level, uranium tenors diminish Group) deposited in a large, though restricted, in the central segment of the wedge, whereas intracratonic basin. along the western boundary fault and on both - The ore-hosting strata. predominantly dolosides proximal to the eastern boundary fault, mitic rocks and shales, occur below the horizon uranium is stili present in appreciable amounts. of the Mines Series, which contains a variety (Derriks and Oosterbosch i958). of synsedimentarv metals including anomalous .,{ge dating of uraninite yicld agesof >706, 670 uranium tenors and the stratiform copper + 20, and 620 -t 10m.y. (Cahen et al. t97I). deposits. - Granitic or other intrusions are apparently absent. Potential Sources of Uranium Aheration
The most likely source of uranium and other metals appearsto be the cupriferoushorizonof - Pre-ore alteration includes Mq-metasomatism and silicification. the lower Mines Series. It containsthroughout
250
5 Seleaed Examoles of Economicallv Sienificant Tvpes of Uranium Deposits
- Ore-related alteration includeschloritization, considercirculatingpressurizedhot brines comparable to those known from certain active dolomitization.and weak silicification. geothermalfieldsasinstrumentalin the formation of the Shinkolobweore and for other depositsas Mineralization well, e.g.,the Cu, Zn depositKipushi. - The ore is polymetallic, composedof U. Ni, Isotopeagesof uraninite(ca.706and 670m.1'.. Co. Cu, and rninor other metalsbut U is only Cahenet al. 1971)correlatewith the first three tectonicphasesof the Lufilian Orogeny (ca. 840 with Ni in apparent paragenesis. - Gangueminerals include magnesite.chlorite. to 710m.y.and670m.y.,Cahen1970).This mav support a convective hydrothermal model in quartz, dolomite and aragonite. - U is present as euhedral uraninite. a habit which tectonic activitv generatedcirculation of connate waters/brinesand the associatedprotypical for high temperaturecrystallization. - Ore and ganguemineral assemblages are com- cessesmentionedabove. parablewith thoseof hlpogene deposits. - The mineralizationis structurally controlled, forming an irregular stockwork of fracture. References and Further Readingfor Chapter 5.3.6 fissure. and joint fillings and breccia (for detailsof publicationsseeBibliography) impregnations.
MetallogeneticConcepts
Bell 19871Cahen 1970: Caben et al. 1961. 19711Cluzel 1986;Deiiens et al. 1981: de Magnee and FranEois1985: Derriks and Oosterbosch 1958; Derriks and Vaes 1956: Dibobol and Malu 1979; Franqois 7974: Gerasimovskv 1956; Lrfebwe and Tsauka 1986; Meneghel 1979, 198i. 1984; Ngongo-Kashisha1975; Oosterbosch 1962, 1970: Roubault 1958; Schoep 1930; Thoreau and du Trieu de Terdonck 1933, 1.936;Unrug. 1988
Mineralization at Shinkolobwe is epigenetic hydrothermal.The actualprocesses involvedare. however,still enigmatic.A graniticsourcecan be discarded. Hypotheseson ore formation at Shinkolobwe. and of vein depositsin general in the Katanga Group of 7,aire and northern Zambia range over 5.4 Examplesof Sandstone-Type a wide spectrumfrom magmatichypogenehydro- UraniumDeposits(Type4, Chap. 4) thermal to synsedimentarywith subsequentredistribution. Most early workers, including 5.4.1ExtrinsicCarbon/Humate-Uranium Derriks and co-authors (1956/1958),favor a Deposits in Phanerozoic Sandstones: Grants magmatichydrothermal origin involving several pulses.Meneghel (1979)suggestsa synsedimen- Uranium Region, USA tary origin of the uranium which was later redistributed into structuresto form veins by various The Grants Uranium Region is located on the processesrelated to tectonism, metamorphism. southeasternColorado Plateau in northwestern thermal events associated with post-tectonic New Mexico.It includesfive major uraniumdismetamorphism, and supergene enrichments. tricts distributedover an area of ca. 150km in Stipanicic(in Meneghel1981)arrivesat the con- ESE-WSWlength and up to ca. 30Km in N-S clusion that Shinkolobwe.Swambo,Kalongwe width (Fig.5.54). in Shaba. and Nkana. Nchanga etc. in the The depositsoccur at variabledepthsranging Copperbelt"show a dominantstratiformpattern, from outcrop at the southernboundarl' to more to which pseudo-vein sectors are sometimes than 1200mdowndipto the north. " connected. The Grants Uranium Region which includes Most intriguing appearsa conceptmobilizing the Ambrosia Lake district, the singie iargest U and associatedmetals from synsedimentary uranium-producing areain the USA. hasat least or syndiagenetic protore inherentin the Mines 280000mtU3Osestimatedtotal resourcesin rhe Seriesstrataand redistributionof the metalsinto $80/kgU productioncategory.Productionuntil favorablestructuresby convectivecirculationof 1990was almost 160000mtUrOs.The average fluids correspondingto the model proposed by grade of ore mined ranged from 0.1 to de Magndeand FranEois(1985).Theseauthors 0.25%UrOr.
Examplesof Sandstone-Type Uranium Deposits
sz,.
251
\ -4 t, IUp/. r uA)
-
UTE MT
\
6
::t'. i -,1:':\
d
, . . : : : .: \
coLa9aoo
c 7 v I
Farmington
:: l :l I
C E N T R A LE A S I N
I tu
.::.:
','
:i i< r ' : . : : : : : : ' (.::::.::r::: LJ.:..f.::: Q :.':::
Gr l l u p
URAN/Ulvl
ffi
CALDERA
i::r: . l:!:
::,lira :::ri:::
{4
Ol
Q Albuoucroue :-I
ao ',1:i,,'\
, '... ,,,'.rrl,t\1
: I
tuq
!r
Lrgunt
I
l.a::;:::;1::::::.il::
\
q
s,0
1 0 0k m
Fig. 5.54. SoutheasternColorado Plateau. San Juan Basin- The fieure showsmajor srructuralelemenrs.the extendof the basin and the siruattonof the Grants Uranrum Region beween the Zuni Uplift of Proterozoiccrvstallinerocks and the Chaco Slope. Individual districts of the uranium region are outlined. Pnncipal districts: C.R. Church Rock; Cpl. Crownpoint: S.L. Smith Lake - Mariano Lake: d.L. Ambrosia Lake; La. Laguna. 1After Santosand Turner-Peterson 1986basedon Kellev l95i) (reprinredbv permissionof AAPG)
Mineralization is primarily of tabular sand- as Phanerozoic tabular/peneconcordant extrinsic stone tvpe in which the uranium is associatedwith carbon/humate-uraniumdeposits. redistributed carbonaceous material such as The region has been the subject of numerous humate. The deoosits are thereibre classified geoscientific investiqations. Adams and Saucier
252
5 seleaed Examplesof EconomicallysignificantTypesof uranium Deposits
(1981)have compiled a geologicalmodel study on "uraniferous humate deposits," as they call the uranium mineralization in the Grants region. Adams and Saucier's (1981) documentation describesthe known geologyof the Grantsregion adequatelyand presentsa comprehensive synopsis of the regional setting and local characteristicsof the uranium deposits, their recognitioncriteria *. fundamental principles of ore formation. ":O 360
Moncos Shoie Ookoto SondstoneU
The subsequent descriptionhasdrawn extensively from Adams and Saucier's(1981)report, and in many casesspecific text has been quoted but with some modifications (therefore not set in quotationmarks). Updatesare taken from more recent publications,in particular from Granger and Santos(1982),Holen (1982),and Crawleyet al. (198a). Meanwhile a new memoir edited by TurnerPetersonet al. (1986)has beenpublishedproviding extensivemultidisciplinaryresearchdata. and the readeris referredto this excellentpublication for more information.
330 J o c k p r l es o n d s t o n eU Brushy BosinMember P o r s o nC o n y o ns o n d s t o n eU
300
.\
.\ 27A
E
...>-_i ')-+
,rE
?LO
="J\'\'j\' '
.' ,' .' .' .ti .' ti : \'. ':
210
o
}
o L
"K"shole W e s t w c t e r C o n y o nM e m b e r "K"shole
+
o 'i E
R e c o p t u r eM e m b e rU ffi
180
M o r i n es o n d s t o n e l':.t,:'.'1.1
t--t
t . ' . ' . 1F l u v i o ls o n d s t o n e M u d f t o to n d s o l i n e - o l k o l i n e F= I -l
Cow Springs Sondstone (Bluff Sondstonet)
r50
loke sediments
t.-.1
S o b K h o( ? ) s i l t s t o n e
f.\-TTl E o l e o ns o n d s t o n e tLY:IJ
r20 B e c l o b i t oM e m b e r ( S u m m e r v r lFem . t l
.:l
90
E tr-
c G y p s u mu n i t T o d i l t o L i m e s t o n eM e m b e r ]
-0p
o o
P*1
F l u v i o lo n d l o c u s t r i n es e d i m e n t s
6
m
G y p s i l e r o us e d i m e n t s
r---r--1
M o r i n el i m e s t o n e
o
F..:.1
F;==
rn
30
Entrodo Sondstone
:-.--.-.-.G
: .-.-. 4.
g.Ui.Iir
-
-,
Chinle Formotion
Fig.5.55. Grants Uranium Resion. columnar section showing the strarigrapiic position of U depositsand litbologies of the Middle and Upper Jurassic rocks. (After Adams anci Saucier 1981; Turner-Peterson and Fishman 1986; description after Hilpert 1963) (reprinted by permissionof AApG)
Examplesof Sandstone-TypeUranium Deposits
Settingof Mineralization Geological The GrantsUranium Regionextendsinsideand alongthe southernborderof the SanJuanBasin, nr,;rthof the Zuni Upiift (Fig. 5.54).During late Jurassictime, three broad alluvial fans were sedidepositedin the basinupon Middle Jurassic part fans constitute the major of the The ments. from Formation and correspond. oldest Morrison to youngest,to the Salt Wash Member in the north,the Recapture,andthe WestwaterCanyonBrushyBasinmembersin the south. time. southernoutcrops In Upper Cretaceous ,,; the Morrison Formationwere partly eroded by oeforethey were unconformablytransgressed the Dakota Sandstoneand MancosShale. Younger tectonic movementsof presumably Laramide age (Cretaceous/Tertiary)resultedin the formation of the presentZuni Mountainsin the southern part of the San Juan Basin and causeda slightly northward tilt of the Morrison strata.Fracturetectonicsat the sametime caused displacements of generallyminor magnitude. Tertiary voicanics of Mt. Taylor cover the Mesozoic sediments between Laguna and Ambrosia Lake. Most of the uranium depositsof the Grants region are in sandstonesof the Westwater Canyon-BrushyBasin members,Morrison Formation (Figs. 5.55, 5.56). The Morrison sediments have a shallow dip (<5") to the N and NE exceptwhere locally disruptedby faultingor folding. The formation is up to 180m thick and comprises.from top to bottom.the
253
Brushy Basin ,Vember,6 to 90m thick, which mainly consists of lacustrine greenish-gray mudstone with rvidespread, in part rather thick, intercalations of cross-bedded feldspathic sandstone. The predominant clay mineral is montmorillonite derived bv devitrification of volcanic ash. Turner-Peterson(1985) separatesthe member into four lateral depositionalfacies.Going from the southern margins of the former Brushy Basin lake, the facies are: Alluvial plain-mudflat-playamargin-centralplaya (Fig.
s.57) . Assignedto the BrushyBasinMemberare two mineralizedsandstone units: - Jackpilesandstone,up to 70m thick, located in the eastern and topmost sectionof the BrushyBasin Member. It hostsU deposits in the Lagunadistrict. - Pobon Canyonsandstone.up to 25m thick hostingdeposirsin the Mt. Taylor and some in the Ambrosia Lake districts. WestwaterCanyon Member is the host for ore bodiesin the Ambrosia Lake and in most of the other districts of the Grants region. The WestwaterCanyon Member is 15m thick in the Lagunadistrict and thickensto 80m in the AmbrosiaLake district. It is composedof buff to gray, badly sorted, cross-bedded.fine- to very coarse-grained arkoseto feldspathicsandstone with local intercalationsof gray, often montmorillonitic mudstone. It containsvariable amounts of humate and oreanic plant material.Minor amountsof heaw mineralsare
NW
SE
Gallup
Grant3
Laguna
m r50 r20 on OU
30 n
lZl
pr;*qrt uroniumore
* of economic use
Fig. 5.56. Grants Uranium Region. schematicNW-SE stratigraphicsection of the Morrison Formarion along the southern part of the mineral belt displayingthe distribution of the uranium mineralizationwithin and the interrelationship of the host sandstone units. (After Turner-Petersonand Frishman i986, based on Hilpert 1963) (reprinted by permissionof AAPC)
254
5 Selected Examples of Economically Significant Types of Uranium Deposits
NW
SE BRUSHYEASIN FACIES
ALLUVIAL PLAIN
PLAYA
MUDFLAT
=
U
; z J
Goat Mountsln lBlue Peak Mlner
Laguna I
z ,-.t.:iilii....*
;
:tr,iit,i ::l.a,,'f.i'#
t. ..1 --t
Zeolitelocies
bJ
MOST FAVORABLE FOR PRIMARY URANIUM M I N E RA L I Z A T I ON
L---l
G r e e n i s h - g r e ys m e c r i t i c m u d s t o n ef o c i e s
ffi
n e Om u d s i o n ef o c , e s
-
[.'-:--,-],.:-jlFlsoncsione [ E l P r i m o r ul t r e n d )o r e
corrmon (Cadigan 1967; Adams et al. 7974). The Westwater interfingers with the Brushy Basin sediments. A thin pelitic horizon. designated K-Shale, is locally intercalated in the Westwater Canvon Member. Recapture Member, 10 to 60m thick, consists of alternating beds of gray sandstoneand grav to maroon silt- and mudstones which inrerfinger with the Westwater Canyon sandstones.
Fig. 5.57. Grants Uranium Region.diagrammaticsection extendingfrom the Laguna district in the SE to the southernpan of the Ambrosia Lake district in the NW and displayingthe relationshipof U mineralizedsandstonein the Westwater Canyon Member and Jackpilesandstoneto sedimentaryfaciesin the Brushy Basin Member and "K" shales.The zoneoverlain bv greenish-gravmudstone,' mudflat faciesis supposedll.' the most favorable for U mineralizationin the Mornson Formation sandstone. Similarly. facieschangesin the "K" shaleappearto be also important as suggestedby the positionof mineralizationin the Goat Mountain-BluePeak mines area. (Turner-Peterson 1985)(reprintedbv permission of AAPG)
part of the fan near the SW margin of the San Juan Basin. Their source was very probably the ancient Zurlu or Mogollon highlands, which provided the igneous, and metamorphic fraction complemented by widespread volcanic ash-fall material. The more proximal part of the fan has been truncated by late Jurassic to late (?) Cretaceous pre-Dakota erosion. The proximal to mid-section of the fan consists of braided bedload channel facies which grades down-fan into straight bed-load, sinuous mixed-ioad, and finaliy' distributary mixed-load channel facies at the distal front svstem. All major uranium deposits occur in the mid-fan facies.
The Westwater Canvon and Brushl, Basin sandstones contain trace to 0.5% heavy minerais and u'idespread but irregrrlarly distributed plant remains. Humate impregnates the sandstonesin large. blanket-like massesand averagesabout 0.1 to 0.25"/" in weight (Adams and Saucier 1981). The sandstoneshave variable amounts(up to 5%) o{ p1'rite when reduced. and hematite and/or Host Rock Alterations lifrronite when oxidized (Knox and Gruner 1957). The Westwater Canyon sandstone contains 5 to The bulk of the Westwater Canyon sandstones 15ppm uranium when reduced and 1 to 2ppm and the Brushy Basin sandstone lenses are uranium when oxidized (Brookins 1979). reduced due to diagenesisunder reducing conGaUoway (1980) interprets the Westwater ditions. Geological evidence suggests that the Canyon depositional environmenl as a wet alluvial sandstones,at least some of them, experienced fan derived by rivers flowing between NE and SE oxidation and re-reduction in pre-Dakota time. from a source in the SW. This is indicated by the The reduction or re-reduction presumably occurrence of coarsestsandstone and the thickest occurred in coniunction with humate formation
UraniumDeposits of Sandstone-Type Examples from formerly water-solubleorganic substances, v e r v p r o b a b l Yo f P l a n t o r i g i n . Early alteration of volcaniclastic material produced montmorillonite, smectite, illite, and mired layer clavs. The processoverlappedin part ,.rirhthe reduction of portions of the ore hosts, in particular of the upper part of the Westwater Canyon sandstones and of the entire Jackpile sandstone.The reduction destroyed the detrital magnetite and ilmenite with local replacements bv pyrite. In a subsequentstage, but partly overlapping :he ilmenite-magnetite destruction. humic ma::-.-laltogether rvith uranium precipitated in the ,'\estwater Canvon sands, where they are overiain by the above-mentioned Brushy Basin mudflat facies. Alteration and replacement of feldspars, including albitization and probably replacementof sodium by potassiumin an outer shell of sanidine erains (Austin 1980), developed locally within the
255
Jackpile sandstone.and in a wider range within the WestwaterCanyon sandstones.Clay coatings of various compositions formed within the sandstones in proximity to the uraniferous carbonaceousmaterial. Subsequentto the formation of the primary ore lenses,baryte and calcite precipitated and kaolinite crystailizedas nests within the sands. Figure 5.58 shows the paragenetic sequenceof the varies aiteration events. In a later post-Laramide alteration stage, the Morrison sandstones were affected b1' a regional oxidation front which advancedfrom the southern outcrop zone of the Vlorrison Formation into the San Juan Basin. The oxidation zone extendsfrom the outcrop for a ferv to 25 km and perhaps more downdip, as indicated by red hematitzation of the sandstone,but left unoxidized isiandsbehind. At its distal front it grades into a zone of brown limonitic sandstones. less than a kilometer to several kilometers wide, which border downdipreduced gray sandstones.
P o st - p r i m o yr Alterolion type
Urontum Loromide PrimoryU redistribution Orogeny minerotizotion - 1 3 0m . y -. 1 0 0m . y . 8 4m . y . 3 7m . y . 1 0m - y . 0m . y .
Host rock deposition - 1 3 2m . y . FeS2 grecrprtoiion
l-
Smectite precipitotion P l o g i o c l o s e - s o n i d i n ed i s s o l u t i o n fuortz
precipitotion
I
-
Potossium fetdspor precipitotion Fe-Ti oxide dissolution Orgonic moteriol PreciPitotion Uronium odsorption Chlorite precipitotion Albitizotion Ferroon colcite preciPttotton Gornet etching Anhydrite precipitotion Boryte precioiiotion Colcite precipitotion Koolinrte preciPitotion Uronium minerol Prectpltotlon Ferric oxide PreciPitotion
Fig. 5.5E. Grants Uranium Region. parageneticsequenceof diageneticalteration and mineralization events in sandstonesof the Morrison Formati6n. (furner-Peterson and Fishman 1986)(reprinted by permissionof AAPG)
5 Selected Examples of Fronomically Significant Types of Uranium Deposis
256
CentroI ployo focies -/
.$."'
(n
.r-1 \
\
Grllupt
Grtntrl Albuquarqu.t
?l ;;2 1"" [E
Morrison Fm. outcrop \ lsopochs of Fe-Ti oxide desiruclion I in Westwoter Conyon Mbr. sondstone (in m)
5 0k m
u mine./prospeci
Fig. 5.59. Grants Uranjum Region, compository map of isopachs of Fe-Ti-oxide destruction in sandstone beds of the Wesrwater Canyon Member and main faciesof the Brushy Basin Member. Most of the U ore bodies are positioned in the zone of greatesr Fe-Ti-oxide destruction which largely extends below the mudflat facies. (After Turner-Peterson and Fishman 1986) (reprinted by permission of AAPG)
SW -
Eiurhy Brrln
Manb ar
Warltrll C r n y oo rn
Zone of comPlete destructionof Fe-Tr oxides U ore bodres
Fig. 5.60. Grants Uranium Region. block diagram illustrating the disrribution and relationship of diagenetic alteration minerals/patterns in the Westwater Canyon Member (colluvial plain sandstone) and depositional facies and alteration minerals in the overlying Brushy Basin Member. (After Hansley 1986;Turner-Peterson and Fishman 1986) (reprinted by permissionof AAPG)
UraniumDeposits of Sandstone-Type Examples The variousalteration-derived mineralsdisplay certain lateral and vertical distibution paftens zonation(Bell 1986; includingrock facies-related and Fishman 1986; Whitnei Turner-Peterson Rieseet al. 1980). 11186; Distinct alteration patterns of feldsparsand Fe-Ti-oxidesare noticedin WestwaterCanyon Strongestalterationof theseminerals sandstones. is proximal to BrushyBasinpelitesand limited to those of the mudflat facies.The mudflat facies characteristicallycontains smectite formed in to alterationin an environmentof pH 7 response ro 8.5 (Bell 1986)(Figs.5.59.5.60).
257
of theseelementsoccur rarely in form of ore minerals in the pnmary mineralization.They appearto be dominantlyfixedby humate.Mo and commonly Se form halos around primary ore zones.V prevailsin the westernsectionof the mineralbelt. Pnmary mineralization(Figs. 5.62 to 5.65) occursas peneconcordant blanket and channel ore bodies, roll-type ore bodies and partly in brecciapipes.Blanketandchanneloresgeneraily form relatively thin peneconcordant lenses suspendedsubparallelto the sandstonestratification. Boundaries between ore and barren ground are sharp. Channelore is more tabular and elongated,sometimesfollowing individual fluvial sand channelsfor as much as 2000m. Principal Characteristicsof Mineralization Blanket ore displavsmore an undulating sheetThe bulk of the uraniumore in the Grantsregion like shapewith WNW-ESE trending "roll"-like is in unoxidized deposits. Some minor occur- thickenings. These "rolls" occur where the rencesconsistof oxidizedore. Reduceddeposits mineralizedblanketcutssharplyacrosssandstone are differentiatedinto primary and redistributed beddingto a different elevation.In cross-section, mineralization.In principal,all ore of pre-Dakota the step up resemblesa flattened S, where the age,which is offset by Laramrdefaults and asso- mineralizationin the sandstone is not only thicker ciateduniformly with humate,hasbeenattributed but is often higher in grade. Other roll-rype ore similarto to primary mineralization(also termed prefault, bodiesexhibit C-shapesin cross-section blanket, trend, black band, and uraniferous the Wyoming rollfront deposits.But, the upper humatemineralization).All ore of post-Laramide and lower limits are generallywide and the host age,which is randomlyassociated with humate,is rock on the convexupdip side has been reduced termed redistributedmineralization(also named again. post-faultand stack ore, explanationssee later). In many cases,both the tabular and roll-type Figure 5.61 providesa summaryof the character- mineralizationis WNW-ESE-trendingmore or isticsof the various typesof ore and their litho- lessparallelto the orientationof the sedimentary stratigraphic transportdirection(Fig. 5.66). distribution. Dimensiorsof individual ore lensesrange in Adams and Saucier (1981) summarizethe characteristics of mineralizationas follows.Their length from severaitens of meters up to about data are updated by information mainly from 2000m, and in width from several meters up Grangerand Santos(1982,1986),Holen (1982), to some hundredsof meters.Their thicknessis and Crawleyet al. (1984). usually less than 2.5m but ranges from a few Reduced Mineralization: Principal uranium centimetersto over 5 m. Exceptionalthicknesses minerals are coffinite, pitchblende.sooty pitch- of up to 18m are found where severalchannels biende. and black amorphous urano-organic convergeas at Mount Taylor (Fig. 5.67). Redistibutedreducedminerqlizarioncontains complexes/uraniferous humate. The ore minas the dominentU mineral. Pitchblende coffinite erals impregnatethe host sandstoneby coating and being distnbutedinterstitiallyto the sand is very rare. Associatedmineralsinciudemontrohiiggite,ferroselite,native grains. seite,paramontroseite, Primary reduced mineralization: Much of the selenium.pyrite, marcasiteand calcite (Granger uraniumis presenfin urano-organic compoundsi and Santos1986).Mo is practicallymissing. uraniferoushumate.Associatedmineralsinclude In contrastto the primary ore, where humate pyrite, marcasite, jordisite, ferroselite, chal- is an essentialconstituentand coextensivewith copyrite, galena, wurtzite, calcite. baryte, and uraniumat a ratio of approximatelyI : I by weight kaolinite. Spirakis and Pierson (1986) report (Grangeret al. 196i), the ratio is highly variable elementalenrichments of Cu. Fe. Mn, Mo, Se,V, in redistributedunoxidizedmineralization,probY, As, S and organiccarbonalongwith U. Most ably due to discriminative separation of the
5 Selected gyamples of Economically Sipificant Types of Uranium Deposits
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Examplesof Sandstone-TypeUranium Deposits
259
N Erushy Basin Llembec Reduced
?xidlzad sandston€
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Prlmary oro
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loc k ore earnnant :rrmary
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Fig. 5.62. Grants Uranium Region, diagrammaticS-N section illustrating the strucnrral position and configurationof pnmary and redistributedU mineraiizationin Morrison Formation sandstones.Primary mineralizationforms elongated tabular ore lenses suspendedwithin reduced sandstone,whi.le redistributed mineralization has accumulated along faults and at the contact between oxidized and reduced sandstone.Remnant pnmary mineralizationoccurs as islandswithin oxidized sandstone.(After Turner-Petenon and Fishman 1986)(reprinted b;r permissionof AAPG)
uranium from humate. Someredistributedore is virtually barren of organiccomplexes. Redistributedmineralization,also rtamed stack ore and post-fault ore, is essentiallytectonically and lithologicallycontrolled,but, in somecases. mav be also of rollfront character (Figs. 5.62, 5.64, 5.65). The ore is consideredto have been formed bv redistribution of pnmary mineralization. The uranium in stack ore bodies commonly concentratesin or aiong steeplydipping faultsof Tertiarv ageor younger,primarily where oxidized sandstones are in contactwith unoxidized. The redistributedore usuallyhas a relativeiy sharp updip contactagainstthick oxidized sandstone.It engulfsthe primaryblanketore in the reducedstrataby impregnatingthe sandstone above and beiow the primary blankets. Redistributed ore is 'darker and richer close to the oxidation front and gradually fades into the reducedground acrossthe fracture zone. Where several ore blankets with primary uranium existed. the redistributedore can mineralizein' tervening barren ground, creatingthick sections of fairly continuousore. Stackore is surroundedby a zone of limonitic and locally bleached rock. which evidently is transitional into red, hematitic sandstones(Fig.
5.64).The width of the limonitezonevariesfrom a few meters(e._e.,at AmbrosiaLake) to many tensof meters(e.g., at ChurchRock). Stack depositsare variablein shapeand may resemblea tree in cross-section. Dimensionsof individual ore bodiesrange in length to 100m and more, and in width and thicknessfrom a t'ew metersto more than 40m. Averagewidth is approximately3 m. The grades are generallylower than in primary ore, averaging berween0.1 and 0.2%U3Os. Brecciapipe hostedmineralizationis restricted to thosepipesthat cut primaryore. Onlv threeof such ore-bearingpipes have been found. The pipes are chimney-iikecollapsestructureswhich cut WestwaterCanvon and Brushv Basin sediments.They range in diametertiom a t'ewdecimetersto more than 25m. Their verticalextension can be up to 70m (Woodrow pipe. Laguna district). Mineralizationwith gradeslocallyin excess of 1% U3Osis predominantlyconcentratedin the upper portion of the pipe. It consistsof downfaulted blocks of strata-controlledprimary ore or redistributed ore as in the Woodrow pipe. Remobilized uranium impregnatesthe breccia matrix.
zffi
5 Selected Examples of Economically Significant Types of Uranium Deposia
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Fig. 5.63. Ambrosia Lake district, Ann Lee and Section 27 mines, composite diagram of fearures associatedwith mineralization. (After Adams and Saucier 1981based on Squyres i970, Kendall 1971)
McCammon et al. (1986)calculatedstatistical averagesof reservesfor the variousconfiguration types of primary deposits. At a cutoff grade of 0.1,%U3O6 trend-type depositsaveragean ore grade of 0.22"/"U3Osand a reserve of 7250mt UsOe.Correspondingfiguresfor roll-type deposits are 0.16%U3Os and 9400mtU:Oe, and for remnant ore bodies 0.2% U3Os and 1350mtUrOa. The authors also calculatedan amount of 422272mt U3O8discoveredresources for the whole region basedon a cutoff grade of 0 . 1 %u 3 o 8 . Age datingof uraniumoresand associated clay minerals from various districts indicate several
generationsof mineralizationand uranium redistribution respectively.Primarymineralizationapparentlyis at least140to 130m.1'.oid (Brookins i980b: Rosenbergerand Harper 1981, 1982). Apparent agesof ca. 115to 110m.y. (Brookins 1980b; Lee and Brookins 1978), supposedly reflect a remobilizationeventperhapsassociated with a first roll-typeore formation. Redistributed with hematiticand limonitic alterore associated ation zones yield apparent ages of ca. 13 to (Ludwiget 10m.y. andca.4 to 3 m.y. respectively al. 1984).RelativelyyoungU/Pb agesof <1m.y. have been establishedfor some rollfront ore in the Church Rock district (Ludwig et al. 1982).
Uranium Deposits Examplesof Sandstone-Type
[Til
141
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n"a. oxidized sondstone [i:---T]l
Outllo. ot 0ln. rorking.
Fig. 5.64. Ambrosia Lake district, Section23 mine, generaiized cross-sectionthrough a variably oxidized sandstonezone illustrating the relationship of post-fault roll- or stack-rype mineralization and remnant pre-fault trend mine:alization. (After Granger and Santos 1986) (reprinted by permission of AAPG)
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Fig. 5.65. Ambrosia [.ake district, Section 23 mine, N-S cross-sectionshowing post-fault roll-type U mineralization associated with the conract zone of oxidized sandstone. (After Grangerand Santos 1986)(reprinted by permission of AAPG)
262
5 Selected Examples of Economically Significant Types of Uranium Deposits
to
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Fig. 5.66. Ambrosia l-ake district, map of distribution of uranium ore bodies. Two ore zonesoccur in Westwater Canyon Member sandstonesin the Ambrosia Lake trend (,4.L.) to the nonh and a third zone in Poison Canyon sandstonel Brushy Basin Member in the Poison Canyon rrend(P.C.) to the south. (After Santos 1963)
and Saucier 1981) report a ThAJ ratio of about 1.5 for the member and interpret this as one point The source of the uranium is still in dispute. of evidence that uranium has not been released lntraformational acid volcqnic material either from the member. Another point may be given by within the host sands themselves or in the ad- Della Valle's findings that REE have apparently jacent Brushy Basin mudstones are favored by been depleted in the Brushy Basin pelites. Such a many workers as a probable source of uranium. process requires a reducing, probably organicThis postulation, however. is derived mainly from bearing groundwater capable of complexing or circumstantial evidence provided by the frequent chelating the REE (Della Valie in Adams and association of uranium deposits with tuffaceous Saucier 1981; Mclennan and Tavlor 1979) while sedimentarvsequencs. depressingthe solubility of uranium. On the other Much of the uranium and associatedelements hand. ThAJ ratios are higher in the Westwater may also have derived from altered igneous Canyon Member, than in the Brushy Basin ma'terial within the sandstones at some distance Member, particularly in the lower part of the up the hydrologic gradient from where the member as reported by Brookins (1979), suggestdeposits are presently located. Especially the ing that the Wesfwater Canyon has most likely coarse facies proximal to the sedimentarv source provided the uranium for the ore formation. This that was later removed bv erosion is considered a is compatible with the generallv reduced state of favorable source for the ore elements. the Brushy Basin sedimentsimplying low uranium The Brushy Basin Member can probably be solubility, whereas pervasive mild oxidation prediscarded as a valid source of uranium. Brookins vailed in the lower Westwater Canyon sandstone (1979)and Della Valle (pers. commun. in Adams as reflected by the state of ilmenite and magnetite Potential Sources of Uranium
Examplesof Sandstone-TypeUranium Deposis
Formotion
Thick-
Rodiomernc rog
i
263
Relotive U endcwment of sond units
5r0
a Brushy Bosin Member
W e st w o t e r C o n yo n
Lower We,stwo t er Conyon
R e co pt u re Member
? 0 0 -3 0 0m
b
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,rr;i':..i :li>t : 1:: : : :::::::i:i:
:
E
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o O
Fig. 5.67. Eastern extensionof Ambrosia Lake distnct, Mount Taylor mine a Srratigraphic column with distriburion of uranium (radiometric log) and relative endowment of uranium in the recoqrized sand units {,4 to F) of the Wesrwater Canyon Member. b. Planview and sections of mineraiization in the "C" sandstone unit showing the marked increaseof ore at the confluence of two channels. (After a Burgess et al. 1987; b. -{lieff. oers. commun. 1987)
F
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chonnei edge
Fi-Tfrlu minerotizotion
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cro @ro
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2&
5 Selected Examples of Economically Significant Types of Uranium Deposits
which are not destroyedby reductionand carry hematiterims.
-
Favorable host units have a high sandstone to mudstone ratio of 4: 1 to 1 : 1..a gross thickness of the host sandstones above average, and a good continuity of the sandstone beds. - Thicknesses of Westwater Canyon sandstone Uraniferous Humate units of up to 100m are especiall.vfavorable. as are Jackpile sandstone thicknesses of as much Humate is a term definedby Grangeret al. (1961) as 70m. substance to describethe particuiarcarbonaceous associatedwith the U mineralization of the - Sandstoneswith poor sorting and a wide range in grain size appear to make better hosts than Grants region. sandstones of moderate to good sorting and Humate constitutes0.05 to 0.5% by weight of finer-grained constituents. the host sandstone.Its dominant characteristic is its ubiquitous and coextensive association - The Brushy Basin shale, which is in depositional continuitv with the two principal with uranium in primary trend and channelores. host sands. the Westwater Canyon and JackAlthough unmineralized humic material has pile sandstoneis generallynot oxidued. reportedly been found. no completelyuraniumbarren humate has been confirmed in the - Tuff-derived bentonitic clavs are abundanr in the sediments,particuiarlv in the interbedded WestwaterCanyonsandstone. mudstones. Two principal hypothesesare forwarded on
the origin of the humatein the Grants Uranium Region, an extrinsic and an intrinsic one with Aheration respectto the uranium mineralizedsandstones. Both modelsenvisagea Morrison-internalsource. - The host rocks along mineralzed trends have been strongly altered. Reduction is indicated The extrinsic model derives the humate from by destruction of detrital magnetite and plant debris in the Brushy Basin pelites, the ilmenite, and local formation of authigenic intrinsic model from that in the Wesrwater pyrite. Quartz is corroded and silica has been Canyon sandstones.A third conceptenvisaging redeposited. Detrital feldspar is partly in a an external, non-Morrison source in form of stage of dissolution. partly coated b1' an albite surface vegetation or swamps, has been disreplacement rim. Ferric iron has been removed cussedby Grangeret al. (1961)and, in modified from mudstones. In the eastern central part of form, by Turner-Petersonand Fishman (1986). the San Juan Basin authigenic K-feldspar has The two internal source models complv best crystallized, and various clay minerals have with the establishedgeochemicaland geological derived from tuffaceous material. parametersof the region and will be discussed - Authigenic clav rims precipitated on detrital later.
Ore Controls and RecognitionCriteria Significantore controlling or rdcognitioncrireria of the carbon-uraniumdepositsin the Grants Uranium Regionaslistedby Adamsand Saucier (1981)and othersare as follows. Host Environment - The largestore bodiesoccurin reduced,pyrite. plant debris and humate-bearingcontinental fluvial sandstones of bed-load-dominated facies depositedin straight to sinuousstream systemsof the mid- to distal sectionof the alluvial fans of the Morrison Formation as pointedout by Galloway(1980).
clasts within and immediately adjacent to uraniferous humate lensesare in the Jackpile sandstone predominantly illite-montmorillonite. in Westwater Canyon sands at Ambrosia Lake thev are predominantly chlorite or a mixedlaver phase. Montmorillonite clay rims and clastsare altered to Mg-chlonte. Compositional variations exist within and adjacent to uranium mineralization: montmorillonite is the dominant clay mineral formed downdip of an ore body, chlorite is enriched in the ore zone. and kaolinite and altered montmorillonite are enriched updip of the ore. (Riese et ai. 1980) Characteristically the most intense decomposition of Fe-Ti-oxides and feldspars in Westwater Canyon and Jackpile sandstones is restricted to the proximity of the mudflat facies
!
Examplesof Sandstone-TypeUranium Deposits
{ of intervening Brushy Basin pelites where also the bulk of mineralization occurs (Figs. 5.59, 5 . 6 0 ) . ( T u r n e r - P e t e r s o ne t a l . 1 9 8 6 ) ."!!ileralization -
Primary peneconcordantore in blankets, pods, and channels is controlled by lithology-stratigraphy and by minero-geochemistry. The principal characteristics and controls of mineralization include: - uranium minerals are in most deposits coextensivervith the humate. - uranium is associatedwith enrichmentsof Cu, Mo. Se,V. Y, Fe, Mn, As, and S, - tabular ore generally occurs at several stratigraphic levels along linear trends of thickened s a n d s t o n e( F i g s .5 . 5 6 ,5 . 6 2 , 5 . 6 6 )w h i c h a p p e a r to be controlled by paleo-morphological sedimentary conditions, - blanket ore is more sheet-like, is coextensive with humate, and contains very little detrital vegetal material, - channel ore has a stronger facies control than blanket ore, sometimes following individual channels (Fig. 5.67b). While containing humates, it is also associated with more carbonized or coalified and silicified detrital vegetal matter than blanket ore. - Early roll-rype mineraiization associated with peneconcordant ore horizons are similar in character to Wyoming rolls except that the oxidized sandstone on the convex, updip side of the roll has been re-reduced. The roll-type ore is apparently caused by an early oxidation event resulting in the remodeling of pnmary peneconcordant uranium into C-shaped bodies. - Redistribured post-fauh ore is typically found in stack deposits (Fig. 5.62) and controlled by structure. Iithology,' and redox boundaries including: - presence of faults and fractures (of LaramideTertiary aee) cutting primary mineralized rock; - presence of reduced and oxidized sandstones along an oxidation front at or near the structures. - humate and detrital plant material may or may not be associated with the uranium, - probable chemical separation of Cu, Mo. Se, V, Pb, Y. Fe. Mn, Ca, and Sr, from uranium, - Redistributed type, late rollfronr mineralization is simiiar to that in Wyoming. The ore
265
may be localizedat nose and limb zonesof verticallystackedrolls, marking the contact betweenoxidizedand reducedsandstone. The oxidized sandstonegenerally seems to be limonitic near the redox-frontbut becomes hematiticwithin tens to hundredsof meters updip, as describedby Ludwig et al. (1982) from the UNC ChurchRock Mine.
Metallogenetic Concepts The genesisof the primary ore in the Grants UraniumRegionis stiil controversial. The peneconcordant. humate-associated trendandchannel uraniummineralizationsmostobviouslyrepresent the first accumuiationof ore grade uranium.All other types of mineralization,rangingfrom preDakota roll-type to Tertiary and recent redistributedore bodies,appearto have beenformed to and probablyfrom the tabularore. subsequent The primary peneconcordant ore is considered processes to have formed by diagenesis-related immediatelyafter sedimentationin late JurassicMorrison time. Isotope studies indicate that the ore is at least about 140 to 130m.y. old. Geologicalevidencedocumentsthat the ore formation occurred prior to the Laramide Orogeny because faultsof Laramideageoffsetore blankets, and prior to the depositionof the late Cretaceous Dakota Sandstonebecausean ore pod or roll in the Jackpile sandstonein the PaguateMine, Lagunadistrict.was tmncatedby the pre-Dakota erosionsurface. The peneconcordantprimary ore is characterized by an intimarc associarionof uranium with humate.This suggeststhat humic acidsexerteda cntical influenceon the formation and the siteof depositionof the primary ore. Theretbre, the identificationof the sourceof the humicacidsand the processes involved in their formation, mobilization, and finally precipitation as humate are criticalfor any metallogenicmodeiing. The humic acids are consideredto have derived either from the ore-hosting sandstones themselves or tiom the Brushy Basin pelitesand probably from a particular facies of the latter, namelythe mudflatfaciesasproposedby TurnerPetersonet al. (1986).Accordingiy,Adams and Saucier(1981) and later Turner-Petersonet al. (1986) have elaborated two alternativegenetic modelsfor the primary ore which are presented further below.
26
5 Selected Examples of Economically Significant Types of Uranium Deposits
Additional parameters and mechanisms possibly instrumental in the accumulation of the large quantity of humate as found, for example, in the Ambrosia Lake district, and consequently in the metallogenesisof the uraniferous humate type of uranium deposits are given by Adams and Saucier (1981) as follows. A concentration of transported organic debris appears to be located adjacent to the Zuni Uplift area in sedimentary trends that possibly are down-warps controlled by pre- to syn-Morrison basement displacements which channeled and caused the superposition of high energy fluvial systems. These shallow elongated depressions also funneled the groundwaters which transported dissolved humic substances and uranium along the same trends. In result. the very subtle structural influences controlled the accumulation of unusually large quantities of uraniferous humate and its deposition in relatively thin, sheet-iike lenses elongated in the down-stream direction along the axes of the down-warps. Assuming that the humate precipitated as tabular bodies along a shallow, horizontal chemical interface below the water table, it can be hypothesized that the earlier described distribution of uranium, selenium, and molybdenum may suggest that the humate body developed at an oxidation-reduction front separating surfacederived oxidizing waters from deeper perhaps H2S-rich, reducing groundwaters (Saucier 1980) or from reducing fluids expelled from the Brushy Basin Member. The chemical system may have been in principle similar to that producing the redox fronts in the Tertiarv Wyoming Basins. The marked difference is in the time of introduction of the oxidizing solutions. Oxvgenated water migrated in the Wvoming Basins long after the sediment deposition and progressed as large tongues downdip in the slightly iilted sandstone horizons (see Chap. 5.4.4). In contrast, as suggested by Adams and Saucier (198i), the redox fronts in the Morrison Formation were active penesvnsedimentarymigrating as a horizontal interface verticall)' down into the fresh sediments from the depositional surface. Mobilization and transport of uraniun in the sedimentological environment outlined above was strongly influenced by the hydrolysis of the volcanic components in the Morrison sediments. It may be deduced that due to this hydrolysisthe surface and shallow groundwaters in late Jurassic time were alkaline (pH of about 8). oxidizing, and
had a concentration of dissolved substancesin the range of 1000ppm or less with sodium in excess over calcium (Squires 1970). It seems further probable that if uranium and associated elements were derived mainly from acid volcanic tuff either in the area of provenance or from the volcanics deposited within the sediment or both, then the uranium and its associated eiements should also have been present in the late Jurassic groundwaters perhapsas uranvl carbonate complexes,as suggestedbv Gruner (1958) and/or fulvic acid complexes, as suggested by Schmidt-Collerus (1979\. Once the peneconcordant primary deposits had been estabiished. remobilizotion destroved part of the depositsand resulted in the formation of the younger tvpes of uranium deposits. This may have occurred during at least three periods when oxygenated meteoric waters could have entered the Morrison aquifer: 1. In late Jurassic time. during and soon after the sedimentation of the Morrison host sandstones, 2. in late Jurassic to late Cretaceous time during the erosional interval prior to the deposition of the Dakota Sandstone, 3. in Miocene to recent time. i.e., during the present erosional period. An early remobilization of primarv ore in probably late Jurassic to mid-Cretaceous time is considered for some of the breccia pipe ore, e.g., in the Cliffside mine, Ambrosia L-ake. where ore within the pipe is controlled by NE-striking joints and is thick and of higher grade than correiative ore outside the pipe. Roll-type ore as found at the Mariano Lake and Ruby mines. Smith Lake district. may also have formed during this remobilization event because geoloeical evidence suggests a post-trend, post-Jurassic but preDakota formation. This event must have been followed by subsequent alteration processes. since the ore rolls are enclosed in reduced sandstone. Stack and post-Laramide rollfront ore bodies are assumedto have formed bv later redistribution of uranium in Tertiary to recent time. This mineralization is tied to a broad tonsue of hematitic oxidation believed to be Mioc*ene or younger which migated down the north slope of the Zuni Uplift. The oxygenated solutions thar generatedthis tongue probably removed uranium in its path through the Grants Uranium Regron
Uranium Deposits Examplesof Sandstone-Type
except locally rvhere the oxidation was encountered or has been retarded in its basinward advance by organic-rich primary trend deposits. Betrveen such ore deposits, the oxidation has aclvancedfarther north leaving some of these J e p o s i t sa s r e m n a n t o r e b o d i e s b e h i n d , e . g . . t h e Blackjack No.1 in the Smith Lake district. which is more than 10km behind the resional redox tront. There are generallv no uranium accumulations along these interfaces,as found. for instance.on similar oxidation fronts in the Wvoming Basins. .An exception is the Church Rock district from '..here younger rollfront ore bodies are described. Age dates on redistributed ores of the Church Rock district as of the Ambrosia Lake district give apparent 4gesaround 8 to 10 m.y. Ore from a rollfront in the Northeast Church Rock mine and from other mines in the Church Rock distnct rs 1m.y. and younger (Ludwig et ai. 1982). The authors interpret the <1m.y. ages as being reiated to a rollfront type distribution of earlier tormed ores by Pleistocenegroundwater, perhaps to those that created the presentday limonitic zone within the Westwater Canyon sandstones. They attribute the 10m.y. age to an earlier phase of redistributed ore formation. perhaps related to the Tertiary oxidation front that formed the hematitic zone in the Westwater Canyon sandstones. .llternative Models The previousiv mentioned two alternative models on the formation of the humate-uraniumdeposits of the Grants region presented by Adams and Saucier read in abbreviatedform as follows. Model I of Adams and Saucier (1981). Essential criterion for this modei is the assumption that the regional. earlv alteration symptoms, reflected by the ilmenite-magnetitereduction and destruction. and the feidspar alteration, resulted from organic-rich reducing solutions derived from a Brushy Basin Member sottrce. The proposed ore-forming process then relies on the juxtaposition of the Brushy Basin Member and the underlying and overlying sandstonesof the Westwater Canvon and Jackpile units. Sedimentation: The thick sands of the Westwater Canvon below and the Jackpile sandstone above were laid down as a continuous sedimentary sequence with the intervening Brushy
267
Basin Member under an arid or semi-aridclimate. The thicknessof the sands(50 to 100m) and the s h a l e s( 6 0 t o 1 2 0 m ) p e r m i t t e d t h e d e v e l o p m e n t of a major hydrologic system with high transmissivityand an integratedhydrochemicalsystem that was active from the moment of sedimentation and earlv diasenesis. The sediment pile provided an extraordinary "chemical potential" due to the juxtaposition of two inherently unstable components. organic material in the form of plant debris and volcanic slass within the tuffaceous intercalations. The diagenesisand alteration of these constituents are known to independently produce rvidespread alteration assemblages. Both the sands and the muds received substantial amounts of organic debns during their deposition. In the sands, a larger component of logs and smaller fragments accumulated and in the muds fine carbonaceousmaterial accumulated. Silicified logs and occasional carbonized logs, or trash pockets. attest to the organic debris deposited with the sands. whereas fine carbonaceous matter is essentially absent in the sands, presumably due to its destruction. No organic debris is found in the shales. Therefore, its former presence is speculative. Strong circumstantial evidence suggests. however, that it was originally present. In conclusion, the pile of argillaceous and arenaceous sediments of the Brushy Basin and Westwater Canyon members contained all the ingredients necessarv for the formation of the uranium humate mineralization. andalso provided the hydrologic conditions durins diasenesis and compaction for the dewatering of the pelites, and the development of chemical interfaces in groundwater for the formation of the ore deposits. Hvdrologic sysrcms and diagenesr: With the accumulation of the pelites of the Brushy Basin Member over the Westwater Canyon sandstone. a regime of two juxtaposed groundwaters of different chemistrv evolved. The waters of the Westwater Canyon sandstone. particularly the deeper portions. were locally oxidizing and locally reducing, depending upon the distribution and influence of indigenous organic debris. In general. however, the groundwater flowing through the Wesfwater Canyon was more of oxidizing than of reducing nature but it was not strongly oxidizing becauseilmenite and magnetite have only thin hematite coatings. These waters destroyed most of the fine organic debris in the
26
5 Selected E:ramples of Economically Significant Types of Uranium Deposits
Ore formation: The hypothesized mixing of the sands,Ieavingonly silicified and carbonizedlarger fragments.This concept assumesa continuous two solutions outlined above provided a fairly flow of groundwater within the Westwater simple mechanism for the formation of the peneCanyonbeginningwith the time of its deposition. concordant uraniferous humate deposits. Where the Brushy Basin waters mixed with the With the sedimentation of the overlying groundwater within Westwater Canyon groundwaters, a decrease in Brushy Basin Member the pH leading to the precipitation of the occurred, Member was confronted the WestwaterCanyon which is notably less soluble in material, comorganic different with solutions of a markedly position derived from the pelitic Brushy Basin more acid solutions. Depending upon the hydrostrata. These sedimentsconsistedof 40 to 60"/' dynamics, the interface between the two water solidsby weigttt,the remainderbeingporewaters. regimes supposedly has fluctuated vertically, With increasingthicknessesof the Brushy Basin leading to the deposition of a vertical series of sediments,the containedwater beganto be ex- stacked organic-rich Iensesand to numerous local pelled. The chemistryof the mobilizedsolutions complexities in alteration, ore distribution, and was strongly influenced by their contact with mineral paragenesis. The fixation of the humate alteringvolcanicmaterial. The solutionsbecame in tabular lenses that locally transform into alkalineand hence capableof dissolvingthe fine S-shaped rolls ma,v have been the simplest organicdebrisdispersedwithin the BrushyBasin. minerochemical response to the redox interface The organic-enrichedalkaline and reducing between the two water regimes. Once presolutions imposed the rypical grayish-greento cipitated, the humic matter may have allowed the green reduction colors upon the Brushy Basin initiation of other alteration processes. Silica, pelitesbeforethey were releasedfrom the Brushy alumina, and alkali and alkali-earth cations Basin down into the Westwater Canyon sand- originally complexed on the organic material stone. Here they altered the detrital ilmenite, were released. The liberation of these ions partly magnetite, and feldspar. The alterations are resulted in the formation of authigenic clay rims strongestat the top of the WestwaterCanyon, on detrital clasts through changes in the chemical which is compatiblewith such an interpretation. environment around ,the organic material and Sincethe reducing Brushy Basin fluids were in partly throug! its degradation. Furthermore, the distinctchemical@ntrastto the oxidizingor verv alteration and replacement of feldspars within the mildly reducing groundwaters flowing in the reduced zones must also have been associated Westwater Canyon, a chemical interface and with the removal and mobilization of considergradient must have developed.It is postulated able amounts of alumina and silica a process that along this interface the primary uranium- which was probably also influenced b.v the dishumate mineralization of the Grants Uranium solved organic substance. Regionmost probably precipitated. It may also be postulated that once the humic The configuration and position of the front material had been precipitated as gel-like masses betweenthe two groundwaterregimeswasprob- in the form of tabular zones in the sandstone. ably controlled by the rate of flow of the West- the continued percolation of groundwater and the water Canyongroundwaterand the volumeand hydrodynamic setting would have modified the lateral changesin flou, rates acioss the Brushy organic distribution into those shapes that have Basin-Westu'ater Canyoninterface.Stratigraphic- been recognized in the deposits of the Ambrosia lithologicrelationssuggestthat in the vicinitv of Lake district (Squyres 1970). The solutions presentdayore districts of the Grants Uranium migrating through and past the humic zones, Regionthe WestwaterCanyonMember thickens furthermore, are thought to have introduced unusuallyat the expenseof the overlyingBrushy small amounts of uranium in solution, particularly BasinMember, and upper sandsof the Westwater as the solutions were mildly oxidizing Westwater Canyon laterally interfinger with the Brushy Canyon groundwaters proper. Basinsediments.This lithologic interrelationship With time, the expulsion of Brushy Basin providesa unique geometryfor the dischargeand waters into the Westwater Canyon sands probconcentrationof greater volumes of reducing ably diminished and ultimately ceased.This persolutionfrom the Brushy Basin Member laterally mitted the deeper more oxidizing water system to and verticallyinto thesezonesof overthickened increasingly encroach upon the shallower reduced WestwaterCanyon sands. groundwater regime. Such a groundwater flow
Examplesof Sandstone-TypeUranium Deposits
269
!
i
I
i
tation of uranyl humate complexesat a pH of 6.5 are reported by Schmidt-Collerus(1969). The humate will be distributed in the form of broad, tabular. semi-horizontalmassesin the channel sands, perhaps as shallow as 5 m below the stream bed. As soon as the channel deposit is buned, the humate lense is protected by the development of an acidic reducing environment, and by the continued adsorptionof cations. Once adequate younger sediments cover the channel deposit. the sequencecan be repeatedover again at a higher stratisraphic level, as in the Jackpile sandstone,if organic materiai is contained in the overlving sediments. Ore formation: The surface and shallow groundwatersmay have contained in the order of 50 to 300ppbU. The uranium was probably transportedby both uranyl carbonatecomplexes, as suggestedby Gruner (1958), and organic acid ,Vlodel II of Adams and Saucier (1981). Prerequisites to this hypothesis are: (a) Ail the complexes, as proposed by Schmidt-Collerus humic materiai and most of the uranium derived (1979). Precipitation of the uranyl humate comfrom a Westwater Canyon sandstone source and plexes could have occurred directly as a result of not from the Brushy Basin mudstones. (b) Suf- the drop in pH across the subhorizontal redox :icient organic debris was buried with the sedi- interface. In some contrast, the uranyl carbonate ments creating an anaerobic environment below complexes were probably broken up by the the static water table (Jensen 1958). (c) Bactero- deeper acid solutions, and the liberated uranium genic H2S and carbon dioxide, methane and became available for the enrichment of the urahydrogen gas from fermentation of the vegetal niferous humate. Hydrogen sulfide generated in material cause a drop in pH, as proposed by the humate gel by bacteria is assumed by Adams Rackley (1976). (d) Iron in the groundwater and and Saucier (1981) to be the cause for the prein iron-bearing detrital grains may react with the cipitation of Fe. Mo, Se, and As as sulfides. H2S to form monosulfides, and eventually, stable The moiybdenum apparently precipitated in a pyrite. negative colloidai sulfide which was repelled by This organic-chemical reduction of the sands the negative humic colloids. This is considered to can develop very early and at shallow depths. be the reason whv the jordisite mineralization is Hydrologic systetns and diagenesis: The sur- almost never in direct contact with humate. With ongoing time the tabular humate imface and shallow groundwaters in late Jurassic time are assumed to have been oxidizing and pregnation matured and hardened into a brittle alkaline with a probable pH of about 7.5 to 8 due material coating srains and filling interstices in to the hydrolysis of the voicanic ash in the sedi- the sandstone.During the maturation, which was ments. Humic acids leached from plant debris by accompanied by oxidation. microbial attack, the alkaline solutions in the upper oxidizing aging, and radiation. the organic molecules broke portions of the channel sediments could have down and much of the complexed uranium and vanadium was released. The uranium associated migrated down to a more static water table. At this position they encountered a pH environment with siiicate to form coffinite, and the vanadium sufficiently low for flocculation of the humic sub- entered the lattice of authigenic chlorite. The stances to occur. Granger (1968) hypothesizes grade and magnitude of any ore bodv formed by that the precipitation took piace aiong the inter- Adams and Saucier's (1981) Model II processes face between the stream underflow and the require as salient parameters: (a) an initial condeeper, slower-moving groundwaters. Floccula- centration of humic substances. (b) a uraniumtion of the humic material will definitely occur at bearing water, and (c) a permeability of the host a pH of 5, but may begin at a higher pH value sands which steered the rate of exposure of the if the humic acids contain metallic ions. Precipi- uranium to the organic particles. All of these could continuously contribute uranium to the deposits, ultimately producing the high grades now characteristic of the ore in the Grants Uranium Region. The soiutions are required to have been not sufficiently oxidizing, however, to ci,'npletely destroy the mineralization. Once the groundwater regime became more stagnant at the end of the Jurassicdepositional period, it changed to miidly reducing due to the influence of the residual detrital and epigenetic carbonaceous material. This last process created the widespread characteristic drab color of the sandstonesnow seen in the subsurface.A similar r.tailogenetic evolution may be envisioned to .-,veoperated between the Brushy Basin pelites ,:nd the Jackpile sandstones following their deposition.
270
5 Seleaed Examples of Economically Significant Types of Uranium Deposits
parameters had a maximum coincidenceimmediately after the deposition of the host sandstones.
Ludwig et d. 1977. 1982, 1984; MacRae 1963; Martinez 1979; Mathewson 1953; McCammon et al. 1986; Mclemore 1983; Megrue and Kerr 1965; Melvin 1976; Miller and Kulp 1963; Moench 196?. 1963: Moench and Schlee 1967: Moore and Lavery 1980; Nash 1967, 1968: Nash and Kerr 196,6;Peterson 1980: Pierson and Green 19801Rapaport 1952. 1963;Rautman 1980;Rawson 1980; Reimer 1969: Reynoldset al. 1986;Ridglev 1980;Ridglev et al. 1978:fuese 1977.1979:Riese and Brookins 1977, 1980, 1984: Ristorcelli 1980;Roeber 1972: Sachdev 1980; Santos 1963. 1970, 1975: Santos and Turner-Peterson 1986: Saucier 7971. 1976. 1980: Saucier AE oerson. commun.: Sayalaand Ward 1983;Schlee 196.i:Schleeand Moench 1961: Schmidt-Collerus1979: Sears et al. 1974: Sharp1955:Smith and Peterson1980;Spirakisand Pierson 1986;Squrres 1963.1970,1974.1980;Steele1984;Thaden 1981.1985.1986:TurnerandZech 1986:Turner-Peterson Petersonet al. 1980.1986;US AEC 19-59:Webster 1983: Weege 1963: Wentworth et al. 1980; Whitney 19t36: Whitne_v.andNorthrop 1984;Wvlie 1963: Young 1960:
Turner-Peterson and coworkers (1986), based on their studies particularly on the distribution of altered Fe-Ti-oxides. feldspars, and authigenic clav minerals. relate the favorable site for ore formation to a distinct sedimentary facies, the mudflar facies of the Brushy Basin Member. Diagenetic processeswithin this facies produced the solutions which were directly or indirectlv essentialfor the formation of the peneconcordant primary ore. In their Lacustrine-Humate Model. which corresponds in pnnciple with Adams and Saucier's (1981) Model I, Turner-Peterson and Fishman YoungandEal11956:Zittinger (1986) denve both the solubilizing fluids and the YoungandDeiicate1965; )'957 ' al' the humic acids from the mudffat facies of Brushv Basin Member. In their Internal Source Model. which corresponds to some extent with Adams and Saucier's Model II. the solubilizing fluids also originated from pore $'aters of the mudflat facies. But it was in the adjacent sandstones into which the fluids were expelied that they dissolved the organic debris to form the humic acids required for the formation of the uraniferous humate.
References and Further Reading for Chapter 5.4.1 (for detaiis of publication see Bibliography) Adams SS, pen. commun.: Adams et al. 1974. 1978: Adams and Saucier 1981: Austin 1963. 1980:Baird et al. 1980; Bell 1986: Birdseye 1951; Brookins 1975, 1979. 1980a, 1980b; Brookins et aI. 7977. Cadigan 1967: Chapman et al. 1979;Chenoweth 1953.1977;Chenoweth WL person. commun.: Chenoweth and Holen 1980: Chenoweth and Stehle 1957: Clark 1980: Clark and Havenstrite 1963:Coffin 1921:Condon and Peterson1986: Corbett 1964:Craig et al. i955: Crau'le! 1983:Crawler'. Holen and Chenoweth 1985;Cronk 1963:Doolel' et ai. 1966:Ethridge et al. 1980:Falkowski1980:Fishmanet al. 1984; Fishman and Revnolds 1986: Fitch 1980:Forhan 1977;Foster and Quintanar 1980:Gabelmanet al. 1956; Gallowar' 1976. 1980;Gould et al. 1963:Granger 1962. 1963,1968;Granseret al. 1961:Grangerand Santos19E?. 1986; Green 19801Hafen et al. 19'16:Haji-Vassiliou and Kerr 1973:Hanshawand Dahl 19-56; Hanslel'1984.1986a. 1986b; Harmon and Tar-lor 1963; Hatcber et al. 1986; Hazlett and Kreek 1963;Hicks et al. 1980;Hilpen 1963. 1969;Hilpen and Moench 1960:Holen 1962:Huffman and Lupe1971;'.Jacobsen1980:Jenkins and Cunningham1980; Jensen1963;Jobin 1962:Kellev 1955,1963:Kelly et al. 1968:Kendall 19711Kirk and Condon 1986tKirk et al. 1986: Kittel 19631Kittel er al, 7967:.Knox and Gruner 1957:Kozusko and Saucier19801Lease 1980;Lee 1976; Lee and Brookins 1978. 1980; Leventhal 1980;Leventhal and Threlkeld 1978; Livineston 1980; Ludwig 1981t:
5.4.2 Y anadium-Uranium Depositsin Phanerozoic Sandstones,Colorado Plateau, USA Eleven mining districts distributed through the north central part of the Colorado Plateauhave produced uranium and vanadium from the Salt Wash Member of the JurassicMorrison Formation. Total production amounts to about 42000mtU3Osof 55000mtlJ:Oe, approximately which and aboul five times as much vanadium have been delivered from the Uravan Minerai Belt. the oldest uranium-miningdistrict in the USA. The averagegrade of ore mined in most districtsrangedfrom ca. 0.15to 0.25%U3O6and ca. I.25 ro L.57"V2O5.(ChenowethW.L. pers. commun.) Due to the vanadium-uranium compositionof the ore emplacedasmoreor lesspeneconcordant lensesin Jurassicchannelsandstones thismineralizationhasbeenciassified asPhanerozoic tabular peneconcordant vanadium-uranium deposits (class4.1.2Chap.a). Abundant informationon Salt Wash Member uraniumdepositshasbeenpubiishedby numerous authors.Thamm et al.'s(1981)presenta comprehensives)'nopsis of the regionalsettingand local characteristics of the deposits,as well as a hypothesison ore formation.The following description has drawn extensively from Thamm et al.'s (1981)report and in many casestheir text has been used in the form of ouotations abbre-
Examplesof Sandstone-TvpeUranium Deposits
viated and modified and therefore not set in o u o t a t i o nm a r k s . Data of more recent research rvork, particularlv by Northrop (1982) rvho studied V-U n i n e r a l i z a t i o n si n t h e H e n r y N l o u n t a i n s ,U t a h . amendthe text.
GeologicalSetting of Mineralization The V-U hosting Salt Wash .VIemberof the Late Jurassic Morrison Formation consists of con:inental fluvial sediments in composition very
271
similar to the host rocks of the other major sandstone uranium districts in the western United States. The Salt Wash Member is overlain by the Brushy Basin Member and underlain by the Tidwell Member of the Upper and Lower Morrison Formation respectivelv.Thamm et al. (1981) classif_v the Salt Wash Member sandstones as orthoquartzites to feldspathic orthoquartzites. Thev were deposited as braided and meandering stream and floodplain deposits in an alluvial fan system during a semi-arid climate which supponed abundant vegetation aiong the water
t09.o0
%;
Thompson .o -f
38oi5'
f.--l 'r Intrusive rocks of I ^ La Sai Mountalns |
Monrson Formation | and post-Mornson formations
7777vPre-Mom*n Y///a lormattons
re/ Fr;i ( q.fu"lo-"/
iff+ . Krng Sotl .'-- Mine
v.t \> 'j^ir,.'
xE;;ie
l....rl
Edge of alluvial fan and area of subsidence
lJf
6gng13tflow direction
l7l
raun
t-::-l Limit of uravan 1 Mineral Belf | LoSolA
|i
rcsdl
.///^\r
I
\
x//////\o^
I
\
SignificantU-V T-:-l ' depositor group ' of deposits
It
M%"\',
25km
Fig. 5.6E, Uravan Mineral Belt. generalizedgeologicalmap displayingthe outline of the belt with position of significant U-V depositsor groups of deposirsat rhe toe of an alluvial fan of the Salt Wash Member of the Morrison Formation. Deposits often cluster in "cross-trends" (numberedI to l1). Mineralized crosstrends: -1Polar Mesa: I Beaver Mesa; J Outlaw Mesa: J Long Mesa: 5 Atkinson Mesa; 6 Club Mesa: 7 Long Park; 8 Monogram Channel: 9 Radium MT; 10 Slick Rock-Burro Canvon: .1,1Deremo. (After Butler and Fisher 1978;Motica 1968)
272
5 Selected Examples of Economically Significant Types of Uranium Deposits
courses. Clay lenses rich in volcanic debris were deposited locally as basal muds in shallow ponds or lakes. Proximal facies of the Salt Wash alluvial fan system are characterued by high sandstone to mudstone ratios and braided stream deposits.The proximal facies consist of thick, massive sandstone units containing oniy a few thin interbeds of clay. ln the intermediate distal facies. e.g., farther to the east, in the Uravan area (Fig. 5.68), a much lower sandstone to mudstone ratio prevails in the meandering stream deposits of that area. A great part of the channel sediments of this alluvial fan section subsided early below the water table, preserving accumulations of detrital plant debris which subsequently contributed to the formation of the carbon-faciessandstones. Adjacent overbank muds were oxidized above the water tabie and are now represented by the hematitic sediments that bound many ore zones. The distal facies of the alluvial fan system were deposited under conditions of low energy flow as discrete channel sandstonesand floodplain clays. Still farther to the east, the sedimentology changes to lacustrine facies. T\e Brushy Basin Member consists of predominantly oxidized fluvial sedimentspresumably deposited in a floodplain environment. Large volumes of acid volcanic ash are incorporated in these sediments. Brushy Basin deposition was followed by a cycle of erosion. then by deposition of the Burro Canyon and Dakota sandstones and the thick black shales of the Mancos Shale.
ter) and a reduced aheredfacies (the main ore host). Mineralogicallyand chemically,the rocks of the tlree faciesare similar exceptfor the redox state, the relative abundanceof heavy minerals and the black opaque mineralsfraction thereof and the abundanceof pyrite, which apparently reflectthe kind and degreeof alteration. The heavy mineral contents and the black opaque fractions thereof are in the oxidized. reducedcarbonand reducedalteredfacies0.31% (59% thereofblackopaques),0.76% (42"/"),and 0.72% (2"io) respectively.The amount of black opaquespresent in carbon faciesrocks is therefore about two-thirds of that in red bed facies rocks,and almostzero in alteredfaciesrocks due to their almost complete destruction. Pyrite is absentin red bed faciesrocks, sparsein carbon faciesrocks, and moderatelyabundantin altered faciesrocks. Small, but perhapsimportant, differencesin the amountsof trace elementsin the three facieshave been noticed.
Principal Characteristicsof Mineralization Mineralizationoccursin variousdegreesof oxidation, dependinglargely upon their proximity to the surfaceand their positionwith respectto the watertable.Three typesof mineralizationmay be distinguished(Thamm et al. 1981): - Unoxidized mineralizationreferred to as primary or black ore, - partially oxidizedmineralizationor blue-black ore, - oxidized mineralization or yellow carnotite ore.
Unoxidizedmineralizationcontainsas main ore mineralspitchblendeand coffinite,montroseite. Oxidation and reduction are the most prominent and vanadium alumino-silicates.Associateci pyriteand marcasite. alteration features observed in the ore-hosting mineralsare predominantly Salt Wash sediments. Associated processeswhich In minor to trace amountsoccurvariousCu, Fe. may or may not be related to mineraiization Pb,Zn, Mo, Se,Ni, Co, Ag, Cr, and As-bearing include silicification, calcitization, pyritization, minerals. destruction . of opaque minerals particularly Pitchblende and coffinite are commonly magnetite and ilmenite, and hematitization of intimately associatedwith carbonaceousdebris. certain facies. Pitchblendemay replacepiant material, in parShawe (I976a) distinguishes three redox facies ticular the cell walls of fossilwood. iron sulfides. of the Salt Wash sediments based on studiesin the and quartz grainsnear vegetaldebris. Vanadium of low-valentstate occurs in the Slick Rock district, ljravan Mineral Belt, which are more or less typical also for other Salt Wash form of oxide minerals,mainlymontroseite,and districts: an oxidized (hematitic) red bed facies, a in a suiteof alumino-silicates includingvanadium reduced carbon facies (rich in carbonaceousmat- bearingchlorites and hydromicas.The minerals Host Rock Alterations
Examplesof Sandstone-TypeUranium Deposits
273
occupv pore sPacesand replacequartz grains and and a series of vanadates including hewettite, pascoite, hummerite and, rarely, navajoite. The fossil wood in the sandstone. Calcite, dolomite, and baryte are present color of the ore can be brown, red, orange, and within and close to mineralization as cement in yellow. There appears virtually to be no loss of rh: sandstone.Total carbonatecontained in most Wash ore is less than The unoxidized 6o/o. and change in the uranium-vanadium uranium Sait mineralization constitutes the majority of the ratios of the ore during the oxidation processes, deposits. The preservation of this dark gray to only a change of mineralogy. black mineralization is due to its position below rhe water table. Table 5.27. Deremo mine, concentrations of selected elements in samples collected across anoxidation-reduction (Thamm boundary(seeFig. 5.69for samplelocations). Partially oxidized blue-black ore is intermediate et al. 1981.basedon UnionCarbideCom. data) te:ween the unoxidized black ore and the fully Samplenumber ,.ldized yellow carnotite ore. It contains as prin- Element (concentration ;lpal uranium mineral the uranyl vanadate ) rn ppm) rauvite, and as predominant vanadium (IV) and 803 878 10 1 3 1 1 3 3 7 1 1V) minerais doloresite and hewettite. The blue- I I l7 80't 38037 52939 280 250 black mineralization shows a strong affiliation 365 520 4i0 255 1i5 with accumulations of carbonaceous matter, as Se Mo 20 25 5<5<5 't8 does the carnotite ore, presumably reflecting Cu 170 40 502 302 <5 <5 areas of greatest resistance to intense oxidation Pb 15 190 10 Zn 19 38 62139 and destruction. d5 63 38 68 20 25 S 635 1012 736 2?0 230 814 1355 Completely oxidized yellow ore contains uranyl Fe3*/lFe2* 2.5 1.9 2.8 4.1 9.8
vanadates,primarily tyuyamuniteand carnotite
et€acH€o\ ZON€ H€HATIflC SANOSTONE
-
ZO
^"uAr
7e
N E
.G
e'z
l-;l Fig. 5.69. Uravan Mineral Belt, Deremo mine. diagrammaticcross-sectionof a redox boundary entirely within a Salt Wash sandstonehorizon and distribution of U-V mineralization.(Numben refer to samplesgiven in Table 5.27) (Thamm et al. 1981,based on Union Carbide Corporation)
274
5 Selected Examples of Economically Significant Types of Uranium Deposits
The partial oxidation alters the primary uranium and vanadium oxides, pitchblende, coffinite. montroseite, first, for example, to rauvite, and subsequently by complete oxidation to carnotite. Vanadium fixes all of the available hexavalent uranium in the form of uranyl vanadates thereby protecting it against removal. Excess vanadium crystallizes to the vanadates mentioned above. In contrast, the vanadium aluminosilicates of the unoxidized ore remain reiatively stable during the oxidation processes. Zoning of ore and ore-related elements similar to that of the Wyoming rollfronts. in particular of Se. V. U. and Mo is evident across roll and tabular deposirs of the Salt Wash Member (Figs. 5.69. 5.70. Tables 5.27, 5.28). The orientation and sequence of the Se-V-U-Mo zoning differs, however. between ore shapes and between deposits. Rolls or C-shaped configurations almost alwavs displal- zones with selenium on the concave side of the uranium-vanadium zone. whereas caicite has commonly crvstaliized at the convex side. Tabular deposits tend to show more variation, with selenium concentrated at the top or bottom, or at the top and bottom of the ore lenses, althougfi in general. selenium was most commonly precipitated at the top of a tabular ore zone. In contrast to this scheme, Brooks and Campbell (1976) describe from the La Sal Mine, Utah, a systematic zoning of Se-V-U-Mo from the bottom to the top of ore lenses. An inverted ore-forming svstem or irregularities in the ore horizons are thought to be the reason for the inverted element zonlng. Mineralization forms two geometric configuratiorc of ore bodies which can be transitional into each other: Tabular ore bodies concordantto the bedding and eiongated to the sedimentarvtrends (Fig. 5.71) and roll-shaped ore bodies that discordantlv transect the bedding (Figs. 5.69, 5.70.
Table 5.2E. Virgin No. 3. mine, concentration of selected elements in samplescollected across vanadium-uranrum rolls. (Thamm et al. 1981,basedon Shawe 1966) (see Fig. 5.70 for sample locations) Element
eUUV o/ /o
o/ /o
0.3 0.006 0 . 2 0.3
5 6 1 8 9 10
0.004 0.005 0.003 0.003 0.065 0.11 0.07-3 0.016 0.041 0.13
0.068 0.12 0.72 0.021 0.051 0.15
1._s t.5 2.-5 2.5 2.5 2.5
1 2 3 4 5 6
0.009 0.008 0.030 0.22 0.?6 0.39
0.011 0.009 0.021 0.34 0.40 0.61
0.09 0.09 0.2 1.2
Sampleno.
Figure a
7 2 3 A
Figure b
Figure
c
o/ /o
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l --)
1.2
0.008 0.012 0.09 0.10 0.15 0.1 0.17 0.17 >6.0
As ppm
Se ppm
20 -50 40 10 40 40 60 20 40 90
10 20 20 2N 70 1-s ,: 1[r 10 1-5
40 10 20 i00 1-50 150
20 10 50 150 20 1500
10 20 40
40 150(1 30
bution of this style of ore body and the relativelv small size of individual ore shoots. The roll-shaped ore bodies in plan view are generally narrow. not more than a few decimeters to about 1m wide. They are sinuous and elongated parallel to local sedimentological features, major channels,or axesof good permeability. Most rolls are C- or S-shapedin cross-section. The upper and lower surfaces are commonlv bounded against clayey beds. The concave boundary of rolls is usually sharp whereas the convex side is diffuse. Ore bodies tend to be clustered within elongatedfavorable trends a feu'kilometers long s.71). by several hundred to thousand meters wide The tabular ore bodies tirpicallv are elongated (Fig. 5.68). But in these trends, the distributiort parallel to sedimentary trends and concordant to of vanadium-uranium mineralization is rather bedding (Fig. 5.12a). Their horzontal extensions erratic and unpredictable(Fig. 5.73) as compared generalll' are small. in the order of some meters with other types of sandstoneuranium deposits. to tens of meters. The ore averagesabout 1 to Although the mineralization is widespread. it is 1.5m thick, but in a feu' piaces Jre thicknesses generally thin. Only locally does it become sufapproach 10m. Although individual ore bodies ficientlv thick and high grade to be economic. ma1'locally be connectedby weak mineralization, Even within deposits. the shapes, orientatrons. more commonly the ore terminates abruptly and dimensions of ore pods are often highh against barren rock. Ore lensesgenerally have a unpredictable. sharp upper and a more diffuse lover boundary. Average production from these elongated Figure 5.72 illustrates the typically erratic distri- favorable areas has ranged from a few hundred to
Uranium Deposits Examplesof Sandstone-Type
?75
a Scrdstone wiih ,reok U-V minerolizotion
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n
Scndstone with ore grode U-V mrneroljzotion
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Muostone minerotized where shown in block
toyers of U-v mineroiizotion
W
mh
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c o r o o n o c e o um s oteriol
of U-V rolls and contentof U. V, As. and Se in Fig. 5.70. Central Uravan Mineral Belt, Virgin No. 3 mine. cross-sections the various parts of rhe rolls. (rVzrnbersrefer to samplesgiven in Table 5.28) (Thamm et al. 1981,basedon Shawe,1966)
5 Selected Examples of Economically Significant Types of Uranium Deposits
276
s|
G r e e n i s h - g r e ym u d s t o n eo n d c l o y s t o n e
ffi
R " o d i r n - b r o w n s i l t s l o n e ,m u d s t o n eo n d c l o y s t o n e
J-Z
S o n d s t o n ew i t h U - V m i n e r o l i z o t i o (nb l o c k )
b
Fig. 5.71. Uravan Mineral Belt. Sl-rck Rock disrrict. Cougar mrne. cross sections (a) and cutaway block diagram (b) of the west central edge of the ore zone showing the tabular to roll-type configuration of the mineralization bosted in Salt Wash Member sandstone.Note that at point .l the ore surface is against mudstone and at point 2 against sandstone. (After Shawe et al. 1959) (repro' duced from Economic Geology, 1959,v. 5a, p. 410)
-.$€
:0xidized + ilooo ptotnroctes
-)
uppe, sondstone
! r-----rReduced chonnel t :"il :l U sondslones , 5oll wosh rHSt
,,
,,
l t ; i 1 ; : lU: :-:V il ore body {generolized)
Fig.5.72a
Examplesof Sandstone-TypeUranium Deposits
277
sw
NE 'l co. 200 m
it-
--.1
t.az
Sase of hloin Lo sot Chcnn.l
ffi
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Minerolizotion lqroo€ over mrneroltzecl iritervoi in equivolent7o U.O")
rrrl
Fig. 5.72. La Sal district. a Generalized map of distribution of reduced channel sand facies and oxidized flood plain sediments of the upper sandstoneunit of the Salt Wash Member. All ore is in the reduced channel facieswhere it occurs at several levels as shown in the cross-sections.b Western pan of La Sal district. c [.a Sal mine. (After a Thamm et al. 1981;b, c Kovschak and Nylund 1981)
zt6
5 Selected Examples of Economically Significant Types of Uranium DePosits
Fig. 5.73. UravanMineralBelt a King Salomonmine. b Deremo mine, plan view showing the small size and inegular distribution of ore pods and complexitvof the mine workingsin Salt WashMember sandstone(Thamm et al. 1981basedon Union CarbideCorporation)
a few thousand mtU3O3. Individual ore bodies range in size from less than 1mt to more than 1000mtU3Os.
Potential Sources ofUranium and Vanadium The origin of the uraniunt is still open to debate. Salt Wash deposits and other uranium deposits of the Colorado Plateau, as for example, those in Chinle sediments (Chap. 5.4.3) occur within a province of uraniferous Precambrian basement. Thamm et al. (1981) suggestthat the uraniumenriched Precambrian rocks, or younger uraniferous igneous or volcanic rocks constitute potentiai sources for much of the uranium nou' found in the sandstone deposits of the Colorado Plateau. Particularll' the tuffaceous material incorporated within the Salt Wash host sandstones and the overlying Brushy Basin pelites are considered to have contained anomalous amounts of uranium which provided an adequate source for the Salt Wash mineralization. This idea, however. is based more on the presence of tuffaceous constituents in the various V-U districts than on any geochemicaldocumentation. The source of the vanadium is likewise unknown. Favorite hypotheses consider vanadium to be derived from either decomoosition of
detrital magnetite and ilmenite within the host sediments, diagenetic introduction from the overlying Cretaceous sediments, and/or leaching and erosion of Paleozoic sedimentslocated to the west of the Colorado Plateau. All of these hypotheses are, to some extent, plausible, but remain as yet speculations.
Ore Controls and RecognitionCriteria Essential ore controlling or recognition criteria of Salt Wash type vanadium-uranium deposits as listed by Thamm et al. (1981) include: Host Environment -
-
A sequence of continental clastic sediments interbedded with tuffaceous layers contains the ore deposits. The dominant host rock is an altered-facies sandstone of fluvial provenance characterized by megascopically buff to gray, highly crossbeddedchannelsands,relative high permeabil-
Examples of Sandstone-Type Uranium Deposia
-
-
-
-
-
ity, some combinationof detritalplant debris with minor redistributed humates and iron sulfides,detrital ilmenite,and magnetitegrains largelyor completelyaltered,and interbedded gray clay layers. The ore hostingSalt Wash Memberis overlain by mainly pelitic sedimentsof the BrushyBasin Member, Morrison Formation, which are largely, if not dominantly,oxidized in the generalregion of the Salt Wash deposits. Sedimentologicalpeculiaritiesinfluencedthe localizationof ore districtsand ore bodieson a regional and local scale. Regionalscaleparametersare reflectedby the restriction of larger depositsto major transmissivesandstoneswhich can be either major sandstone channels deposited in the intermediatedistal faciesof an alluvial fan system, as in the lJravan Mineral Belt, or thicker alluvial sand accumulations,as in the Henry Mountains. Local scaleparameterscontrolling the position of ore include a relatively thick reducedsandstonebed interbeddedwith gray mudstonesor gallzones,crossmudstoneconglomeratesiclay materialand bedding,abundantcarbonaceous scoursurfaces. A control by sedimentaryfaciesis indicatedin the Uravan Mineral Belt by the concentration of highestgrade ore predominantlyin point bar sediments,a kind of transitionalfaciesbetween meandering and braided stream deposited sandstones (Tyler and Ethridge 1981). A similarrelationshipbetweencertainsandfacies and the V-U grade also characterizesthe deposits in the Henry Mountains (Northrop 1982).
Alteration - Significant aiterations are related to redox processes. - Two different types of lithologically controlled oxidation-reduction boundaries are distinguished: - The first type is entirely within sandstoneand seemsto occur typically within major channel systems. In this setting, oxidized sands are generally located in the upstream direction, i.e., from where the sedimentswere derived. Minor redistribution and enrichmentof the mineralizationappear to have taken place at this redox interface.
279
- The second type is between predominantly sandstone and predominantly mudstone sections. Processesgenerating this redox boundary apparently influenced also the mineralizationas reflectedby the positionand concentration of ore bodies. Mineralization - Mineralization of the Salt Wash Member sandstone type is of vanadium-uranium composition. - Within ore lenses,the V-U mineralsare conplanes.adjacent centratedalongcross-bedding to scoursurfacesand in clay gall zones. - Ore bodiesare generallyof smallsizeoccurring within broaderzonesor trendsin the form of commonly thin, discontinuous,and rather erratic mineralization. - Ore bodies are predominantly tabular, elongate, and lenticular, mostly peneconcordant with the bedding of the host rocks. Occasionally, mineralization crosses the beddingat a sharp anglein the form of a roll. No apparent genetic difference, however, appears to have caused the tabular and roll shapeconfigurations,as indicated by a similar zonal distributionof U, V and Se acrossboth types(Shawe1966,1916a).Consequently,the - Salt Wash rolls are not true roll-type deposits comparableto those in the Wyoming Basins. : Ore lensesoccursuspended within a particular level in the host sandstonerarely touching either the upper or the lower contact of the hostsandstone unit (Fig. 5.72b,c).No properry of the host beds has been establishedas a control for this tendency. - Although the long axesof manyindividualore bodiesis subparallelto the major trend axes. those of other ore bodies strike at oblique anglesto them. - Individual depositsor lensesof miner:lization commonly terminate against shale horizons, channel margins, and any other sedimentologicalfeature that createschangesin permeability. - Clustersor trends of depositsemplacedwithin major sedimentarychannelstend to be aligned aiong their margins(Fig. 5.72a).Most of the ore trends are oriented parallel to the paleocurrent directions,suggestiveof an affinity to a broaderhydrologicsystem.
5 Seleaed Examples of Economically Significant Types of Uranium Deposits
- Ore bodies (a) occur in reduced channelsands adjacentto oxidizedoverbankdepositsand (b) are commonlylargerand more numerousnear the redox interfacethan in reducedsandsmore distant from such contacts. - The highest grade ore in any deposit occurs proximal to the oxidation-reductionboundary (Fig. 5.69). Where narrow zones of gray, reduced sandstoneextend into red, oxidized sands,both the grade and the continuity of the ore substantiallyincrease.These zones, bounded above and below by red sediments, do not make major ore bodies in themselves,but constitute high-gradepods within larger deposits. - Although all major SaltWashdepositsoccurin someproximity to oxidizedsediments(red bed overbankdepositsor oxidizedsandstone),the mineralization itself is generally emplaced entirely within reduced sandstone,without adjacenttonguesof oxidized sandstone.This suggeststhat the ore did not originateby the mechanismthat forms roll-type depositsof the Wyoming Basinstype. The mineralizationis, in this respect,more like that of the depositsof the Grants Uranium Region, which similarly occurwithin reducedsandstones.
systemswere abandoned,the rapped porewater in the sandsbecamestagnantand reducing from the decay of buried vegetal organic material. Reducedzonescould have been small to large, dependingon the sizeof the abandonedchannels and the amount of organic debris within them. Larger channelswith abundant carbonaceous debrisunderwentmore intensereduction,leading to the destructionof magnetiteand ilmenite. This type of alteration, representedby the altered faciesdescribedby Shawe (7976a),corresponds to the alterationassociatedwith the depositsof the GrantsUraniumRegion(seeChap. 5.4.1). The more intenselyreduced portions of the channelsands tend to occur near the base of thicker sandstoneunits along one margin of a major channelsystem. The predominantly red Sak Wash floodplain sedimentsachievedpresumabll'their color as a result of oxidation of magnetiteand ilmenite to hematiteunderalternatingwet and dry conditions of deposition.This oxidarionalso destroyedany carbonaceousdebris which had accumuiated in the floodplain environment. These red bed and mudstonesdo not contain faciessandstones uranium deposits and rarely contain uranium occulTences. systemsof the After the channsl-sandstone Wash were covered by the predomiSalt Member pelites of tuff-interbedded nantly oxidized, MetallogeneticConcepts diagenesis Brushy caused the Basin Member, porewaters vanadiumoxidizing the the the expulsion of from Genetic models for Salt Wash sediments into the Wash vary widely from synsediBrushy Basin Salt uranium deposits aquifers. These waters are mentary to hydrothermalore-formingprocesses. channel-sandstone All salientfeatures,however,are moresuggestive consideredto have cnrntnined significantamounts for a formation from groundwatersat low tem- of uraniumleachedfrom the tuffaceousmaterial. peratures than for either a hypogene hydroThe chemicalcondinonsfor the ore-forming thermal or synsedimentaryorigin. More recent processes, as suggested by'Thamm et al. (1981), conceptsby Nash et al. (1981) and Northrop includea partial decompositionof plant trash in (1982) (details see later), which were alreadl' the deeper channelsands, contributinghumic proposedas early as 1947by Fischer,consider acidsto the groundwater.As a result, a reducing epigeneticore formation along a solution inter- environmentdeveloped,forcine the decompositfaceexistingwithin the host sandstonebetweena ion of ilmenite and magnetite, with associated salinefluid and meteoricwater. liberationof iron and vanadium. The hydrolysis Thamm et al. (1981)visualizethe ore-forming of volcanicmaterial liberated silica and alumina process,in summary,as follows: and produceda rise in pH, which further proThe gray Sah Washsandstonesand mudstones moted the decompositionof plant material. The filling channelswere depositedunder conditions contact of these reducing solutions with the permitting the formation of a reducingenviron- moreoxygenated, uranium(VI)-bearing solutions. ment, as reflectedby the presenceof pyrite and expelledfrom adjacentSalt Wash red beds,overcarbonaceousmaterial. Otherwise these two lying BrushyBasin sedimentsand rechargeareas componentswould have been destroyed.When up the hydrologicgradient, was the site of ore individual small channelswithin maior channel formation.
Examples of Sandstone-TypeUranium Deposits
281
The dominant groundwater movement fol- ever, that sucha redoxregimerequireda groundgradient.The oxidiz- water compositionother than that derived from lowed the sedimentological ing waterwhich wasunderhydrologicheadfrom the Morrison sediments.They propose that the the compactingpelitesaboveand marginalto the solution possibly originated from underlying channelaxesmigratedtangentialto and into the stratacontainingevaporites,and that the solution reducedsands.In more uraniferous,thicker sand carried uranium and sufficient magnesiumto units as found in the Henry Mountainsregion, displace complexedvanadium and aluminum the redox contact assumeda simpletabular con- containedin solublehumates.The coupledprefiguration and produced the more consistent, cipitation of uranium, vanadium, and aluminum tabular orebodies of the Tony M/Shootering as hydroxide gels co-precipitatedMg2* and K*, agedto form the clay-bearing Canyonand adjacentdeposits.Where the sedi- which subsequently characteristicof the mineralization. ments consistedof complex sand-shalealterna- assemblages Breit and Goldhaber(1983)studiedauthigenic tions, the solution interface became contorted betweenshaleinterbedsleadingto mineralization mineralsin the SaltWashMember in the Dolores anticline,GypsumValleyanticline.and the interof mixedtabular-rollpatterns. The precipitation of ore elementsat the inter- vening Disappointment Valley syncline. Their face between an overlying oxidizing and an resultsindicatea multi-stagegeochemicalhistory underlying reducing water systemproduced an with implicationsfor the genesisof the vanadiumelement zoning that proceededfrom selenium uranium and the copper depositsof the region. and vanadium at the top, through uranium to The authigenic minerals consist of dolomite, molybdenum at the bottom of the mineralized calcite, kaolinite, chlorite, baryte, pyrite, and section.Although this is the elementpatternmost albite. Someof them, in particular dolomite and commonlyobservedore rolls alsodisplayinverted chlorite, imply the presenceof salinefluids. These salinefluids are believedby the authorsto have elementdistribution on their overturnedlimbs. The vanadium-uranium mineralization pre- influenced the formation of the vanadiumdominantly occurs within altered-facies sand- uranium deposits. Potential sources for these stones commonly adjacent to carbon-facies brines includethe ParadoxFormation evaporites sandstones,both of which contain carbonaceous (Pennsylvanian),which occupy the cores of the material. There is, however, no evidence of anticlines,and the paleoformationwaters of the the type of oxidized sandstonetonguesthat are Morrison Formation. typically associatedwith Wyoming type rollfront Northrop (1982) studied Salt Wash mineraluranium deposits. It appears. therefore, that uation in the Henry structural basin (Henry the uraniferoussolutionswere balancedin their Mountains). The author proposes a model in oxidationcapaciry,namelystrongenoughto carry which the uranium precipitation occurred at an uranium in the hexavalentstate but sufficiently interface of bine and meteoic waters. This low to leave the majority of the carbonaceous concept is based on the chemical and isotopic debris unaltered. The absenceof redistributed compositionsand the stratigraphicdistributionof humateswithin the vanadium-uraniummineral- authigenic/diageneticminerai phases, in parization suggeststhat the pH of the fwo mixing ticular those phases which are more or less solutionswas sufficientlysimilar to prevent the cogeneticwith the vanadium-uraniummineralprecipitationof humates. ization. Northrop'sargumentsare as follows: Once deposited, the V-U mineralization The ore-related tabular-planar layers of remained essentiallyat the site of its precipita- authigenic dolomite cement, the 6r8O, 613C tion. The majoriry of the U of the origrnalore valuesof thesedolomites,and the 6yS valuesof minerals,especiallyin those ores of high vana- the disulfidephasessuggesta higb Mg: Ca ratio of dium content. that becameoxidizedremainedin greaterthan 1:1, enrichmentin dissolvedsulfate situ due to the fixationof the uraniumin insoluble and carbonate,and probably also an abundance secondaryV-U minerals(Thamm et al. 1981). of Na for brines derived from the Tidwell Unit The mechanismsof ore mineral precipitation which underlies the Salt Wash Member. The include the hypothesisof Granger and Warren brines or other fluids did not deplete any ore and (1981),who suggestthat the reductionof U6* by ore-relatedelements(Se, Mo, Co, Ni, Cr) of the V3* led to the precipitationof insolubleUa+ and Tidwell Unit and the author concludesthat thes€ Va* minerals. The two authors point out, how- brines. unlike the ore fluids. were confinedwithin
282
5 Seleaed Examples of Economically Significant Types of Uranium Deposits
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UraniumDeposis Examplesof Sandstone-Type the Henry Basin. In contrast,uranium and vanadium have been mobilized from the two higher lv{orrison members, Salt Wash, and Brushy Basin, and were transportedby meteoric fluids to the sites of ore formation in the Henry Basin. Northrop (1982) identified four stratigraphic oositions of a stable brine-meteoricwater interiace in the Henry Basin. Dolomite maxima that are superjacent to each interface position identify the general position of an interface, whereas the mineralogy and composition of clay minerals esrablish the exact position of an interface. Chlorite andlor chlorite-smectite, enriched in vanadium and magnesium, occur only at the brine-meteoricwater interfacepositions,which in turn also control the position of the vanadiumuranium ore. The mineralization is restricted to intrabasinal sltnclines in which it is confined to those sites rvhere the brine-meteoric water interface intersects sandstone horizons containing anomalous concentrations of organic matter. Where the sandstone does not contain detrital organic debris, Mg- and V-enriched chlorites still occur. These lateral extensions of ore-related gangue (?) minerals also contain anomalous uranium above the normal background values of the Salt Wash sandstone. The authigenic clay mineral in the upper part of the mineralized interval in the Tony M ore body is a vanadium-bearing chlorite. It derived apparently from a precursor smectite and detrital illite-smectite. The clays in the mineralized zones and in their unmineralized lateral extensions show an asymmetric distribution of magnesium and vanadium into the interlayer octahedral sheets in the ore-clays, with Mg generally more abundant than Fe in the chlorites. The destruction of pre-mineralization clay minerals and also of quartz generated a siliceousrich environment which probably caused the silicification as observed in zones of quartz overgrowths positioned both above and below the vanadium-uranium ore horizons. The prevalence of coffinite may be likewise the result of abundant availability of silica. Provision of silica is further indicated by the close spatial relationship between coffinite and vanadium chlorite (Goldhaber, unpublished data in Northrop 1982). Northrop (1982) concludes from his data that the formation of the Salt Wash Member hosted vanadium-uranium mineralization in the Henry structural basin/Henry Mountains is the result
283
of distinct geological features associated with integrated processes(Fig. 5.7a) namely: 1. The appropriate setting and the kind of sedimentation within the Henry Basin provided (a) an evaporite hoizon in the Tidwell Member of the Morrison Formation, (b) sandstonescontaining abundant, preserved, detital organic matter and (c) an available source of uranium and vanadiun in the overlying Salt Wash and Brushy Basin members. 2. Diagenesis generated a saline fluid from the Tidwell Member evaporitesbut unlike the vanadium-uranium transporting fluids the brines remained confined within the Henrv Basin. 3. Oxygenatedmeteoric watersmoving downward into the basin leached uranium and vanadium from the overlying Salt Wash and Brushy Basin members. 4. Deposition of the ore elements occurred at a stable interface between the reducing saline and oxidizing meteoric fluids.
Referencesand Further Reading for Chapter 5.4.2 (for details of publications see Bibliography Botinellyand Weeks1957;Boutwell 1905:Boyer 1956; Bowen andShawe1961;Breit 1986;Breit andGoldhaber 1989;Brooks and Campbell1976;Brooks et al. 1978; Burwell1920;ButlerandFischer1978;Butleret aI.1920; Carterand Gualtieri1965;Chenoweth1975.1978,1980, WL, pers.cornmun.:Chenoweth 1981,1983;Chenoweth and Malan 1969, 1973; Coffin 1921; Crawley 1983; Doeiling1969;Ethridgeet al. 1980;Finch 1967;Fischer 1947.1950.1957.1959.1960.1968.1970.1974:Fischer and Hilpert 1952;Fischerand Stewart196O.196l: Fleck and Haldane1907;Galloway1978;Garrels1953.1955, 1957;Garrelsand Larsen1959;Goldhaberet al. 1983; Grangerand Finch 1988;Grangeret al. 1985;Gruner 1954;Hess 1914;Heyl 1957;Hillebrandand Ransome 1900.1$5; Hintze et al. 1967;Huff and Lesure1962. 1965;Huffrnanand Lupe 1977;Huffrnan et al. 1980; Isachsen1955;Jobin 1962;Johnson1959:Johnsonand Koeberlin1938; Thordarson1966;Keller 1959;Kerr 19581 Kovschakand Nylund 1981;LaPointand Markos 1977; Mclemore 1983;Meunier1984,1989;ltsrrnisl and Breit 1987:Meunieret al. 1987;Miesch1962.1963:Miller and Kuip 1963;Motica 1968;Nestlerand Chenoweth1958; Newman1962;NewmanandElston1959;Nashet al. 1981; Noble1960;Nonhrop1982;Nonhropetal. 1982;Peirceet al. 1970:Peterson1977.1978.1980a.l980b: Peterson 1980;Phoenix1958;Pitman 1958; and Turner-Peterson PlateauResources Ltd. 1983;Rackley1976,1980;fudgley 1981;Scott196l; Shawe1956a, et al. 1978;Scarborough 1956b,1962,1966,1976a,1976b; Shaweet al. 1959;Silver et al. 1980;Spirakis1977;Stokes1953a.1953b,1953c, 1954a,1954b,1954c,1954d, 1967a,1967b:Stokes and Mobley 1954;Thamm JK, pen. commun.; Thamm,
2U
5 Selected Examples of Economically Sipificant Types of Uranium Deposits
5.4.3 ChannellBasalUranium Depositsin PhanerozoicSandstones:Monument Valley-WhiteCanyonDistricts, USA
Kovschakand Adams 1981;Trimble and Doelling 1978; Tyler 1983;Tyler and Ethridge 1981; US-AEC 1959; Wanry1986;Weeks1951;Weekset al. 1957,1959;Wright 1955;Young198; Younget al. 1957
Eightprincipalminingdistrictsdistributedthrough the Colorado Plateau have oroduced in excessof
a' ,/
S h r n o r u m DM e m b e r c h o n n e l slippled where not eroded
El
e"rt. contoiningU minerotizotion
Rlool"
[ .ludeposit
\ Ctinta
||a6ora,
at i
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M o n um e n t V o l l e y
ulla rltlox^
Acrcl
+ 20km
t7 Chinla
I
0.a,
Cotat
110000,
Fig. 5,75. Monument Valley and White Canvon districts, map of the Shinarump channel systems and location of uranium deposits.Numbers refer to former mines 1 Monument-Mitten 2; 2 Moon-tight;3 Daylight; 4 Starlight; 5 Tract 11; 6 Tract 14;7 Traa 11; 8 Tract 2a; 9 Suniight; 10 Big Four; 11 Big Chief; 12 Boot Jack; 1-i Joe Rock; .14Naschoy; 15 Alma-Seggin;/6 Black Rock; /7 Sally; /8 Fern; 19 Harvey Black 2; 20 Radium Hill. (After Malan 1968)
Examples of Sandstone-Type Uranium Deposits
50000mtU3O8from the TriassicChinle Formation. The most typical channel/basaluranium depositsare found in the MonumentValley-White borderlineof Canyondistrictsat the southeastern Utah. (Fig. 5.75). They produced ca. 3900mt U3O6 and 4500mtV2O5at averagegradesof 0.32 and 0.26o/.U3Osand 0.23 and 0.94"/"VzOs respectively(Chenoweth,WL, pers. commun.) All major deposits consist of more or less lenticularore pods locatedwithin distinctTriassic sandstonechannelsand generally in the lower
Formotron Memoer
Peflod
part thereof. They are therefore classifiedas Phanerozoic tabular/peneconcordantchannel/ basaluraniumdeposits(class4.1.3,Chap.4). Malan (1968),Wood (1968),Stewart et al. (1972) Chenoweth (1975) and Chenoweth and Malan (1973)publishedcomprehensive studiesof the various districts with Chinle mineralization including the Monument Valley-White Canyon districts.They are used as the main basefor the following descriptionsamendedby data of the other authorsmentioned.
ihrck Lrlhology m
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90200
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Whtie Rim Mem
Mossive white sondstone
H o s k r n t n iT o n g u e
Reddish-brown silty sondstone / /
#l
De Chelly Sondstone Mem Permron
E u
orgon Rock To n g u e
0-135
zz5
Mossrve. lon fine-o.orneo eolion sondston. Reddish-brown siitsione ond fine-grorned sondstone
g 3 Cesor Meso Sondstcne Mem.
Holgorlo Tongue
360
I J / / /
M o s s a v e .b u f f - w h i t e . e o l i o n sondstone wrth thrn red srltslone porlrngs neor lop
Reddish-brown siltstone 0- 11,0 ond iine-groined sondstone
Fig. 5.76. Monumenr Valley and White Canyon area, generalized litho-stratigraphic column showing the stratigraphic position of U deposits in Shinarump Member channels and the lithologies of the Permian-Triassic-Jwassicunits. (After Malan 1968)
286
5 Selected Examples of Economically Significant Types of Uranium Deposits
GeologicalSetting of Mineralization Ore host is the 50 to 600m thick Chinle Formation of. late Triassic age. It consists of. fluvial sedimentsdepositedon an extensivealluvial plain in braided and meanderingchannels,temporary lakes.and mud flats.The dominantsedimentsare red to vari-colored mudstonesand siltstones. and conand generally light-coloredsandstones was material Volcanic glomeratic sandstones. was sediments, as the fluvial incorporated in abundant carbonaceous debris. The Chinle Formation is subdividedinto sevenmembers. Three of the lower members (Fig. 5.76). from bottomto top. tbe Shinarump,MonitorButte and Moss Back members,contain uranium deposits whereeverany one of them formslocallythe basal unit of tne Chinle Formation (Fig. 5.77a). The following description focusseson the Shinarump Member which is in the Monument Valley-White Canyon districts the basal Chinle unit and the host to the uraniumdeposits. The ShinarumpMember restsunconformably on the Middle TriassicMoenkopiFormation(Fig. 5.76) composedof continentalsandyand silty red beds interfingering with minor marine calcareous sediments. The Shinarump Member consistsof light gray feldspathic sandstonesand conglomerateswith rare lensesof mudstone.The clasticdepositscontain volcanic componentsand locally abundant carbonizedand silicifiedplant fragmentsand logs. The sedimentsare cementedvariablybv calcite and silica. Thicknessis generallybetween10 and 30m. but locallyattains100mandmore. The Shinarump Member forms commonly a fairly extensive and uniform conglomeratic blanket but locally also distinctnarrow channels incised into the underlyingred Moenkopi siltstoneas in the MonumentValleywhichare filled with mainly cross-bedded coarsepsammitesand psephites,partll' of granitic provenance.and somepelites(Fie. 5.77c). Local subsidencecausedthe abandonmentof segmentsof the Shinarumpchannels(Fig. 5.75) and their subsequentpreservationas valley-fi.Il deposits.Reducingconditionsprevailedin certain areas where groundwatertables were probably high. The Shinarump sedimentsgrade upward and intertongue laterally with the youngermembers of the Chinle Formation. The most pronouncedstructuralfeaturein the regionis the MonumentUpwarp.The Monument
Valley is located on the southern flank and the White Canyon district on the western flank of the uplift.
Host Rock Alterations Oxidation and reduction affected the Shinarump sediments apparently repeatedly. Most prominent reduction feature is bieaching around uranium mineral2ation. The bleaching extends locally into the underiying red Moenkopi sediments for a quarter of a meter or more particularly under mineraiized scour and fill of paleochannels. Calcitization and silicification cemented to some extent the host rocks.
Principal Characteristics of Mineralization Principal ore minerals in the reduced zone include pitchblende, coffinite, montroseite, localll' doloresite, vanadium-bearing mica, -chlorite and -clay. Associated minerals may include sulfides of Cu, Fe, Mo, Pb, Zn. and trace minerals containing Se, Cr, Ni, Co, Ag, Cd, Sr. The oxidized zone contains hexavalent uranium minerals. uranyl vanadates, highervalent vanadium oxides, and hydro-mica; copper carbonates, native copper. etc. The ore minerals impregnate sandstone voids, replace quartz grains, clay particles, and abundant fossil plant debris, and fill vertical fractures which extend beneath the scour base. Calcium carbonate (I-10%) is present in ore mostly as cementing material of the sandstone host rock. Meander loops and channels with deep scours appear to be the most favorable sites for the location of mineralization. In the Monument Valley ore bodies are primarilv restricted to scours in the basal part of the Shinarump Member. Commonly there is only one ore-bearing scour, but not all scours of the paleochannels contain uranium mineralization. In a few mines. ore extended downward as much as 5 m from the Shinarump channel into the underlying beds. Most of the deposits are linear in plan vieu with some deposits having a curviiinear to nonlinear shape, e.9., the Happy Jack deposit of the White Canyon district. The ore bodies are usualll' composed of lenticular mineralized pods also referred to as rods (Fig. 5.77c,d) that are subparallel to the bedding and closely spaced.
Examplesof Sandstone-TypeUranium Deposits
?87
S
N
MONUMENT VALLEY A RE A
DIRTY DEVIL RIVER AREA
MOAB AREA
am 300
200
t00
l_l -
Monrtor Butte Member
ChinleFm, not differentiateo ShinarumpMember
li.|? l4...
Coarse-qrained sandstoie
F1
sittstone
deposit M o e n k o p rF o r m o tr o n
E.
^
!-_-l urey man, snare T::....:-I ^ 1 . . .r S andstone
Moss Eack Member
Shinarumo Membcr
i ---l F{eoman, snare F..:=
Chinle Formation
Rcs
b
MOenKOptFornanon
Fn,'
ffiIil u mineralization T
Flil
RC
congtomerate
Sandstone, r::.r : :::l [::
gJg5g[g6lf,g6l
E= t*
shale
E*
Silt-,mudstone,
zone ie-El Bteached l-Ol
Petrifiedtogs
l+l
U mineratization
I
HM
; E" Chinle Fm. undifferentiated; tr"sShinarumpMember; tr. MoenkopiFm.; ecCutlerFm. (Permian)
Fig. 5.77. Nonh central Colorado Plateau, diagrammatic secrionsillustrating the position and lithologicenvironmentof uranium deposits hosted in basal fluvial channelsof the Chinle Formarion. a Regionai geological S-N section from Monument Valley to the south and Moab to the nonh showing the distribution of the basalmembersof the Chinle Formation and the position of uranium deposits. b.c.d Details of mineralization and ore distribution in Shinarumo Member channels.b White Canyon districi. c Monument Valley distict. d Rod ore bodies in the Monuntent No. 2 mine. (After a Johnson and Thordarson 1966 based on Stewartet al. 1959:b Miller 1955;c Mitcham and Evensen 1955: d Witkind 1956b) (b and c reproduced &om Economic Geology, 1955,v. 50. p. 156tr, 170ff resp.)
circutor pottern of froctures limonite
tyuyomunite_
greI frioble
Simplerod tyuyomunite.limonite, grey sondstone core
frioble sondstone
Complexrod B silicified wood core
288
5 Selected Examples of Economically Significant Types of Uranium Deposits
Dimerciors of individual ore pods range from 0.3 to 3 m high, 0.3 to 3 m wide, and a few meters to 100m in length. Depositsrangein sizefrom a few to rarely >2000mtU3Os. About half of the depositsare smaller than 5 mt U3Os and all but a few are smallerthan 150mtUsOs. The largestdepositin the Monument Valley, Monument No. 2, (2400mtU:Oa, 0.34%UsOa, 1.42%VzOs) is emplacedin a Shinarumppaleochannel scour extendingfor at least 3km in a north-south direction within a wider depression or scour about 15m deep cut into underlving Moenkopiand DeChellyunits(Figs.5.78,5.77c). A narrow, inner scouris another 10m deep and 200m wide. The ore-bearingscour is eroded to the north and south.Best oresare in typicalcigar or rod-shapedconcentrations as much as2.5m in diameterand 30m long. (Chenoweth,WL, pers. commun.;Malan 1968).
od\
'/ L
\ \ t a8 2
I
\
riYAY4
,ss..w2I+ \a\
l\t9\
rI\a I
l\e--
|.^ |
\
-q,
PotentialSourcesofUrani 'm No valid sourceof the uranium has been establishedto date. Potentialsourcesare speculatedto have been uraniferous volcanic ash preservedin the form of bentonitic clays in the lower Chinle sediments or uranium-enriched granites which supplied the arkosic ore-hostingsandsor these clasticsthemselves,or a combination of these sources.(seealso Chap.5.4.2)
0
Fl
S t r u c t u r o lc o n t o u r sd r o w n on MonumentNo-2chonnel lin meterso.s.t.) gr11in"of open pit mine
61 .-
Blockenedoreos ore upper level workings whit€ oreos ore lower levei workings
Ff
Ore Controls and RecognitionCriteria Essentialore controlling or recognitioncriteria of the uranium depositshosted in channelsof the Shinarump Member, Chinle Formation, in the Monument Vallev-White Canyon districts include: Host Environment
2o0m
Fig. 5.7E. Monument Vallev district. Monument No. 2 deposit,geologicalmap with outline of uranium ore bodies and mines in a Shrnarump channel s€gment. Numbers refer to former mines. For a generalizedcross-sectionsee F)g.5.77c. (After Witkind and Thaden 1963; US AEC
19s9)
- Host rocks are reduced, buff to gray feldspathic sandstones and conglomerates of continentalfluvialoriginfilling channelsystems Aheration scouredinto pelitic-psammitic sediments. - The arenaceoussedimentshave a relatively - Significant alterations are related to redox high permeability. processesreflected by color changes of the host - Tuffacous material is incorporated in the and country rocks. Most prominent is bleachsediments. ing around uranium mineralization. - Carbonaceous debris is abundant in the - Calcitization and silicification cemented the channeland overbankfacies. host rocks to some extent.
Examples of Sandstone-Type UraniumDeposits Mineralization
289
found in the channelsandsand associatedoverbank deposits. - Principal ore minerals are pitchblende and were probablyinitiated Mineralizingprocesses coffinite associatedwith vanadium minerals, soon after the sedimentation of the fluvial and sulfidesof a numberof othermetals,most sequence. Surfaceand groundwaterscontaining of them in trace amounts. small amountsof U, V, and Cu migrated down - Uranium mineralizationcommonlyoccursasthe hydrologic gradient along the Shinarump sociatedwith or near accumulations of detrital channelsystemsduring or soonafter the depositpiant debris, redistributedhumate, and iron ion of the channelsands.Circumstantialevidence sulfides. suggeststhat the metalsmay have been leached - Uranium occursdisseminated in reducedsand- from granitic rocks, arkosicsandstones, and tuff stone but also replaces quartz grains, clay bedsaffectedby activeerosionin the source areas particles, and particularly plant fragments. of the sediments. Locally it extends as fissure filling into the Wherever the dissolveduranium encountered underlyingmud/siitstones. accumulationsof organicdebrisalong the course - Depositsconsistof a numberof closelyspaced of the river bed. it wasextractedfrom the waters small ore pods of medium grade commonly and fixed by reductionascolloidor adsorptionon locatedin the lower part of a channei. organic fragmentsforming minor and dispersed - Ore pods are'mostly lenticularin shapemore concentrations,a kind of protore, of U, V, and or lessconcordantwith the bedding. Cu. The reducingagent was either the organic - Uranium occursin a wide distributionof small. debris itself or H2S producedfrom the carbonlow-grade occurrenceswithin reduced sedi- aceousmatter by activity of anaerobicbacteria. ments but larger economic depositsare reSubsequentto the Shinarumpsedimentation, stricted to distinct sandstonechannels. renewed upwarp of large areastook place north - Preferential sites of deposits are relatively and southeastof the Monument Valley and east narrow shennels,where these channelscon- of the White Canyon district. The dispersed verge, and where the permeable channel protore uranium occurrences depositedinitially in sands gradually change into impervious the Shinarumpsandswere remobilizedduingthe carbonaceousmudstone. erosion of channelsfrom the higher parts of the uplifted areas and the partial destruction of numerouslargechannelsin the adjacentlowland. Groundwaterstransportedthe liberateduranium MetallogeneticConcepts into reducing environmentsin the channelsthat survivedalongthe lower flanksof the rejuvenated Although a magmatic origin by hypogenefluids uplifu and formed the ore bodies as they are was formerly proposed for the Chinle uranium found today. In the Monument Valley, the most deposits, the most convincingmodel envisages favorable sitesfor uranium precipintion were the supergeneore forming processesthrough multiple heterogenouschannelsandsthat were laid down migration-accrerion as presentedby Malan (1968) where anastomosingstreams converged (Fig. for the Monument Valley-WhiteCanyondistricts 5.75). In the White Canyon area. preferential and later in modified form for the Lisbon Valley sitesfor ore formationwerein the transitionzone districtby Huber (1980). betweenthe uplandand lowlandwhich is characFluvial deposition of the arkosicChinle sedi- terized by a gradualchangefrom permeablesands ments occurred into channel systems within in the channels on the flanks of the uplifts to a basin adjacent to a granitic highland. The imperviouscarbonaceousmudstonein the same exposedgranitic rocksprovidedthe detritusand a channels in the lowlands (Fig. 5.7b) (Malan possiblesourceof uranium as well. 1968). which Tlte river systems depositedthe clastics of the Shinarump Member in the Monument Referencesand Further Readingfor Chapter 5.4.3 Vailey-White Canyon area were of degrading (for detailsof publicationsseeBibliogaphy) nafure and scoured into the bedrock. Volcanic material was incorporated in the fluvial sedi- Bohn 197; Butler and Fsher 198; Clhenoweth 1975; ments, as was abundant carbonaceousdebris as Chenoweth WL pers. commun.' Chenoweth and Magleby
290
5 Selected Examples of Economically Significant Types of Uranium Deposits
districts are in the Maybell-Baggsarea, and the Washaki-Sand Wash Basin, Wyoming-Colorado. The Wyoming Basins constitute a prominent uranium province, second in magnitude in the USA after the ColoradoPlateau.Total resources including ca. 75000mtu3os production until 1990amountto more than 250000mt U3O6in the $80/kgU productioncategory.The averagegrade of ore mined rangesfrom 0.05 to 0.25"/"UsOe. Individual depositsrange in size from few 100 to several1000mtU3O6. Uranium occursin the form of rollfront or rolltype mineralizationemplaced at a redox front in continental sandstone containing detrital carbonaceousdebris. As such the Wyoming deposits represent a distinctive class of the Phanerozoicsandstone-typeuranium deposits 5.4.4 RoU-TypeDetrital Carbon-Uranium classified as roll-type detrital carbon-uranium Depositsin PhanerozoicContinentalBasin deposits(class4.2.1,Chap.a.). Wyoming Basins,USA Sandstones: The following compilationis essentiallybased on Harshmanand Adams (1981), who have Major uranium districtsoccur in four Tertiary elaboratedin great detail on the Wyoming urabasinsin Wyoming,in the Windriver,Shirley, nium deposits,amendedby data of other authors PowderRiver, and GreatDividebasins.Minor listed. 1971; Chenowcth and Malan 1969, 1973l, Corey L959; Crawley 1983; Davidson 1967; Dix 1953; Doelling 1969; Droullard and Jones 19551Dubiel 1983; Elevatorski 1978; Finch 1959; Finnell et al. 1963; Fischer 1968; Gross 1956; Gruner et al. 1954; Hawley et al. 1968; Hinze et al. 1967: Huber 1980; Isachsen 1954; Isachsen and Evensen 1956;Jennings 1976; Johnson and Thordarson 1966; Koch et al. 1964; I-ekas and Dahl 1956; kwis and Campbell 1!}65;L,ewis and Trimble 1959; Loring 1958; Lupe 1976; McRae and Grubaugh 1957; Malan 1968; Miesch 1963; Miller and Kulp 1963; Pilmore pers. commun.; Purvance 1980; Rackley 1976; Scarborougb 1981; Schmitt 1968; Spirakis 19801Stewart er al. 1972;Stokes 1967a, 1967b; Thaden et al. 1964; Thomson 1967;US-AEC 1959;Weir 1960;Witkind and Thaden 1963;Wood 1968;Young 1964. 1978
E E
POWDER ;flar
ffi
fi rir,,.'l
RTVER E ^a -ef
Boundary of Tertiary basin Thrust fault Precambrian rocks (incl. granite) U deposits
-
sBN
'(i'
E E
G
Tabular, in Miocene sediments Surficial,in brecciated granite (Copper Mountain) Rollfront,in E = Eocene, P: Paleocene sediments
SANO WASH
Fig. 5.79. Wyoming, generalized structural map of Tertiary Basins, Precambrian uplifts and location of uranium distrias. The Precambrian rocks include uraniferous granites of Archean to Lower Proterozoic age. Districts with rollfront uranium deposits include 1 Kayceet 2 Pumpkin Buftes; 3 Turnercrest; 4 Monument Hill; 5 Highland-Box Creek; 6 Crooks Gap; 7 Green Mountain; 8 Sweetwater-RedDesert; 9 Shirley Basin; .10Gas Hilis. (After Harshman 19701Bailev and Childers 1974)
Examplesof Sandstone-TypeUranium Deposits
Settingof Mineralization Geological
291
compact but somewhatfriable siltstones.They are brown to tan, greenish-gray,dark gray, and almostblack when high in organicmaterial. Theselithologies, favorablefor ore deposition, are found principally in the central part of the fluvial systemin the basins, in large fans that rangein lengthfrom a few km or tensof km as in the Wind River and Shirleybasins,to more than 100km in the Powder River Basin'(Figs.5.80,
The major rollfront uranium deposits occur in intermontaneTertiary basinsranging in size from a few hundred (ShirleyBasin) to several thousand square kilometers (Powder River Basin). Crystalline rocks including uraniferous granitesof Archean to Proterozoicage surround mostof the basins(Fig. 5.79). The basinsare predominantlyfilled with con- s.81) . Individualsandstonebedswithin the fanshave dnental sedimens of Paleoceneto Quaternary age.The younger formations contain acid only a limited lateral continuit-v. Thicknesses volcanicinterbeds with elevateduranium back- range from a few to several tens of meters. uplift, and displacegroundvalues.Subsidence, ment along faults at the marginsof some basins continued through most of the Tertiary, but in most caseslater tectonismdid not greatly affect the basinfill and its uranium deposits. Significant rollfront uranium deposits have formed in the early EoceneWasatch,Wind River, and Battle Spnng formations, and to a lesser extent in the PaleoceneFort Union Formation. The most favorable host rocks are friable fineto coarse-grainedor pebbly, arkosic sandstones containingconsiderablepyrite and carbonaceous substance,and occasionaliron-stainedmudstone clastsand tabular mudstonesplits. Sand or siltfi.lledchannelswith cross-beddingare universally present. Calcite cement or concretionsare sparingiy present in, but not characteristicof host sandstones.Calcite content of favorablesandstones, unaffected by the mineralizationprocesses,is generally less than 1,"/o.Calcite accumulations, however.occur associatedwith ore bodies. Carbonaceousmaterial is dispersedthrough the host sandstoneswith somewhatgreaterconcentration on cross-stratificationbeds. This Mixene sediments ^organic debris is generally fragmentsof leaves ,VrZ Ewne and twigs. but occasionallyalso humic material L-J Wnd River Formation coatingsand grains.A few large sectionsof tree N Mesozoicsediments trunks and branches,some silicifiedand others fl? Pt*"tO an granite carbonized,are presentin the ShirleyBasin host F EI Conglomerateand sand channel of Wind River Formation rocks. The organic carbon content of the host Wind River Formation (in m) rocks rangeswidely from placeto placebut prob- ltei 1*O""nr of buried front alteration of E General ably averages 0.5% or less. ldl u oeposit Detrital heavy mineralsconstitutebetween 1 l-?l U occunence and 5oh of the ore-hostingsandstones. Below the water table, and where not altered t-lC. 5.E0. Wind River Basin, Gas Hills, sedimentary by the ore-formingsolutions,the host sandstones parterns and distribution of alteration front in the Wind fuver Formation moving northward from the Precambrian are generallylight gray or greenishgray. Granite Mountains. Uranium deposits are dominantly The fine-grainedrocks interbeddedwith the localized along the main solution front. (After Galloway sandstones rangefrom mudstonesor claystones to 1979b; Hanhman and Adams l98l)
5 Selected Examples of Economically Significant Types of Uranium Deposits
292 U K P T M H
districts Koycee Pumpkin Buttes Turnercrest Monument Hill H i g h l o n d - B o xC r e e k 0
Fl
Principol fluviol oxes
l=*l
Mixed lood chonnel locies
N
Suspended lood chonnel focaes
t M N F l o o d p l o i n - t r i b u t o r yl o c i e s
50km
@
BockswomP locustrine locies
f-l-l Mojor U deposit f-R-l U districl
+ T Ft
r(oFrt.
ffie" Pi vlj
/
Fig. 5.81. Southern Powder River Basin, regional flow pattems and facies distribution of the upper Fort Unionlower Wasatch (?) fluvial systemsand position of uranium deposits. (After Galloway 1979b)
- a seleniumcontentconsiderablyhigher than rn unaltered sand. - an eU/U ratio generally higher than in unaltered sand. - calciumcarbonate,organiccarbon,and sulfate contentsmuch lower than in unaltered sand, - partial or complete destruction of some or most of the heavyminerals,particularlypyrite and magnetite. Figure5.82a,bshowsthe different alterationand mineralizationzones,relatedauthigenicminerals and (Fig. 5.83) chemicalreactionsinvolved. The oxi-dativeaheration penetrates the host formationsin a tongue-likefashion.Tonguesmay range considerablyin size and shape.They can have lateral extensionsof several hundreds of of asmuch asa squarekilometersand thicknesses few tens of meters. In most districts there are mineralizaseveralalteredtonguesand associated lisns wilhin a generally favorable sequenceof beds,eachseparatedfrom the other by an interval of fine-grainedimpervioussediment.In some instances superimposed tongues of oxidized sandstoneare connectedby altered sandfilling a depositionalbreachin the bounding impermeable sediments.Although altered tongues frequently overlap, thet edges are rarely superimposed. Tonguesof oxidized sandstonemay or may not completelyfill the sandyintewal in which they are confined.Incompletely filled intervals occur most frequentlyat the endsof the oxidation tongues.
Although widespread, favorable sandstoneunits contribute only a limited fraction of the total volume of the sedimentsin the basins. Principal Characteristicsof Mineralization According to Galloway(1979b),the sediments were depositedunder coarsebed-loadconditions Uranium has accumulatedin a roll-shapedform or on the distal parts of. wet alluvial /aru in two at the edge of oxidizedsandstonetongues(Figs. Eocenedrainagesystpms. major Paleocene-early 5.80, 5.84)the featuresof which are displayedin Figs.5.82a,band 5.83. The principalore mineralsare pitchblendeand Host Rock Alterations coffrnite.They occur as coatingson sand grains, as void fillings in the sandstonesand possiblyas Diagenetic and particularly oxidation processes replacements of organicmatter. caused distinct alterations of the ore-hosting Uranium is accompaniedby a number of clastic strata. Although there are certain difelementswhich have been depositedin or adferencesin characterand extensionof alteration jacent to the alteredtonguesforming the rollfront of the host rocks in the variousWvomineBasins. (Figs. 5.82b, 5.83, 5.85). Selenium occurs as there are some distinct similariiies ciused by native selenium and ferroselite on the concave oxidationof the sandstones in the major districts. side of the roll-front, and as native Se in the Harshman and Adams (1981) list for altered unaltered mineralized rocks. Molybdenum sandstones: (iordisite, MoS2) and calcite are presenton the - a distinctivechangein color from the normal convex side. Further on exist arsenic, phosgray of unalteredsand, phorus,and copper.
Examplesof Sandstone-TypeUranium Deposits
293
Silty claystone
'
".'
'ia
:,'
Probable directionof mineralizingsolutionmovement (pH?)
T
------_ouT:n"e
I
I
b
E
o (\l
Altarcd Zone (ground through whictl mineralizing sdutions hav€ pass€d) characlorized by: C/a)4mostly nontronite (h!lh iron montnpdllonite) Pyrita: rcmc^ted Umonite: abundant Coalified wood d€compos€d or destroyed Pitchblende: rernoved Calcite: rcnoved Hematite: remored Usual @lon rusty to greenish
-o\ C
v)l
Mineralized htd (ch€mical interface wfpre mineralizingsoludors w€r6 anested) characlerized by: Clal montmorillonite Pynb: abi.xf,arl'', also marcasite Limonite: abs€,nl quite abundant, nuclei Coalifred M: for uraniumdeposition Pitchblende'. $e*nt (EC Calcite: quld abuMant, mostly in aix distinct concretions Hematte: otlen present, usually with calcite [email protected]
Unaftercd Zone: (unatfected by mineralizirEsolutjons) characterized by: C/a,4 rnontrnorillonite Pyite:lcrl,lly Limonite: absefit Coalifrd wod. quile abundant has hesh aoo€afanca Pitd1blende: absent Calcite: str2tce Herratrte: absent Usual @lor lightgtey
Present (pre-mine)direction of ground water movement(pH 7.8) Fig. 5.82. Wyoming Basins, cross-sectionswith characteristics of roll-type uranium deposits. a Typical position and shape. b Schematic presenration of characteristic authigenic and allogenic minerals related to alteration zones (Shirley Basin). (After a Harsbman 197.1;b Bailev 1965)
Ludwig (1978),for his geochronologic studies, subdividedthe uranium ore in the ShirleyBasin into three types: disseminatedpitchblendeore, calcite-cementedore, and massivepitchblende ore, a divisionthat canbe, somewhatgenerrlized, applied to most of the depositsin the Wyoming Basins.
Disseminatedpitchblendeore is found in watersaturated,essentiallyunconsolidated,texturally and mineralogically immature, coarse-grained arkose. One of the sampleswith 9% U3Os contained abundant charcoal-like organic material. Such fossil carbonaceoustrash is moderately common in the unalteredarkose,in many cases
294
5 Selected Examples of Economically Significant Types of Uranium Deposits
Symbol Zone Significont componenrs in solution S i g n i fi c o n t minerols
coaa
altarolr on anvalopr
u". 0., Hcos-
u'! r.{qor)* Hco;
Hamolrltc
O.a - 3toga utontum
Ora-tloga ptrrla
t nollarad londrlonr
Fr++. sror=
Fc++,sror=
HCq-, U+.
HCO!-
pitchblende pynte hemotite siderite pyrite pyrite m o g n e t i t e s u l fu r FeS ferroselite F"! L-\-J . selenlum j o r d i s i l e .c o l c i t e
C h e m i c orl e o c t i o n s 2FrCOs+f,Oz+ zH.O -4
r + + + 7 S . O s =+ 3 x t o +
r t F . s r +6 s o . = + 6 H +
FrrOr+ 2xCO!- r ?H+ Samrga.naobl.
2 F r C O ! +4 S o r 2 * 0 2 + H z O +
? F c ( S . O r ) +r 2 H C O i
Sanrgrrhrobla
? t ( s 2 o ! ) + + F . s z+ H c o l - +
roctr
Frco. + zso + aFrrr+zsf!:+Hr
r06'07'30" I
6
I o
*"
I (
I \-^, .--€ /
1"\
toct!
Fig. 5.83. Wvoming Basins, idealizedschemeof a rollfront system with alterationzones.related mineral components. solution componentsand summaryreactionsin the Fe-S-O-CO2aqueous systemduring formation of a rollfront deoosit. (After Granger and W anen 1974)
n
Upper ollerotton tongue
N
Lower olterotion t o n g u e.
E
Edge of oltered sondslone longue
@
C o n t o u ro f p r e - W i n d R i v e r F m . e r o s i o n s u r t o c e { d o t u m :m e o n s e o l e v e l ) U ore body
+ T 2km
Fig. 5.84a
Examplesof Sandstone-TypeUranium Deposits
295
C.l cll.
Uaallaraa
I o-a
raaaalaaa
-:aE6.ici-o1.---
-
uffi-.trffihjriet|
dlEl. %-
6-i
Fig' 5.E4..Shirley Basin. a Distribution of an upper and lower altered sandstonetongue in the Wind fuver Formation and position of mineralization. b, c. and d Croii-se-ctionsat the westernedge of thjupper altered sandstoneroDgue. e Cross-sectionof small ore-podsalong the bottom of rhe altered sandstonetoigrr" .u. 36bm backwards from the edgeof the tongue in Perrotomics Section 9 pit. (After a Harshman and Adams 19gl; b, c, d, and e Harshman 1972)
with well-preserved log- or limb-like forms. Pitchblende, pyrite-marcasite,and rare baryte were the only nondetrital mineralsrecognizedin the ores. Pyrite occursin three varietiesIncluding framboid habits which perhaps formed dunng early diagenesis. The pitchblende appeared quite pure with pyrite being the only visible contaminant. Calcite-cemented ore is unusualboth in its high grade(12.3ohU:Or) and its texture.Calciteforms a pervasivematrix for the detritalgrainsof mostly quartz and feldspar, with a few heavy minerals. Calcite also corrodesthe feldspar.pvrite-
marcasite (ratio: 20-5011) and pitchblende are intimately associated.These minerals occur both as incomplete replacements of the detrital grains and as rims around them. Massive pitchblende ore (65-72"/" U3Os) consists almost entirely of pitchblende. with very minor quartz and pyrite. No oxidation products of pitchblende were observed. Harshman (1974) investigated in much detail the distribution of the ore and associated. elements and minerab in ore bodies within and adjacent to the redox fronts in the various Wyoming Basins. One of his striking results is the great similarity of
296
5 Selected Examples of Economically Significant Types of Uranium Deposits
the chemicaldistributionin most of the deposits. The essentialelements and minerals reveal in surnmary,the following patterns(Fig. 5.85). Uraniwn has been addedto reducedsandstone in zones close to or in contact with the edses of
altered sandstonetongues.The redox-interface for uranium coincideswith that for iron in some of the deposits;in othersthe uranium interfaceis separatedfrom the iron interface by as much as 5m of pyrite-bearingreduced sandstone.The
IIOLYBOE NUI
sE!Exrur VANAOIUY
tooo
FIRITE
2500
URlNlU|l
o.
UJOS
2000 P a l r o t o m i eS r rc.9 Plr S H I R L E Y S A S I I I .W Y O UN I G
t500 tooo 500
4000 3500
,
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?ooo
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-
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oo
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Butl.r Pll KARNES CO. TEXAS
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rooo 500
scxes^r,c
R o L L- F R O X T C i O S S S € C r t O N
-SOlltlOr.fLOr+
Fig.5.E5. Roll-type uranium deposits,graphic summarvof the distributionof U. Se. V. Mo and pvrite acrossa rollfront in the Wy oming B asins (Shirley' Basin, Gas HilblWindiver Basin),andSouthTexas Coastal Plain (Karnes County, Live Oak County). (Harshman
r974)
Examples of Sandstone-Type Uranium Deposits
29'7
uranium content of altered sandstoneis slightly uranium contents. Uranium/phosphateratios greater (6 ppm) than that of unmineralized range from 0.02 to ca. 30, so the phosphatereducedsandstone(2 to 4ppm), at leastwithin uraniumrelationshipis not a directone. T-he quantitative distribution of. the above300mof the rollfront. Iron, pincipally as pyrite and to a lesserex- mentionedelementson both sidesof the rollfront tent as marcasite,has been added to reduced showsthat contentsof Se, V, and As are greater sandstoneat the edgesof the alteredsandstone in aitered oxidizedsandstoneswhereasFe, Mo, tongues. Pyrite gradually decreasesin amount and organic carbon are higher in unaltered towardunalteredsandstone. reducedsandstone. awayfrom the ed_ees ln most depositsit extendsbeyondthe uranium In its simple form, the shape of.an ore body mineralization.\rite has been destroyed,gen- resemblesa crescentin cross-section, in plan view erally but not always totally, in most of the it is like that of an irregularlylaid pipe following altered oxidized tongues. Marcasite occurs the edgesof the alteredsandstonetongues(Figs. iogether with pyrite and is most abundant in 5.80,5.84).Ore boundaries generallytransectthe at andadjacentto stratification of the host sandstonesat sharp mineralizedreducedsandstone the edgesof the alteredtongues. angles,althoughthe tails of the crescentor roll Seleniumhas been depositedin narrow zones may be peneconcordant with the bedding(Figs. at the edgesof the altered tongues,astridethe 5.82a,5.84). The innercontacts of ore andaltered edges,or in reducedmineralizedsandstoneclose sandstones. i.e., the concavesideor trailingedge to the edges.Seleniumin the alteredsandstones of a roll, are generallysharp,whereasthe convex may be present as ferroselite(FeSe2)or native side is gradational.Simple crescent-shaped ore selenium, but in the reduced sandstonesit is bodiesare rare. Most of the ore bodiesare comgenerallypresentas native selenium. plex and consist of several interconnected Molybdenum contentsrangefrom a few tens of rolls. Ore is alsonot confinuousin their lateraldirecppm to severalpercent. It is concentratedat the distal, downdip edge of the mineralued zone. tion alongthe edgesof the alteredtongues.Small Although molybdenum is present in many de- ore bodies are found on the top and bottom posits,the amountsvary stronglyin the different surfacesof the altered tonguesand are associated with smallbodiesof residualunalteredsandstone basins. Vanadium occursin amountsof a few hundred envelopedby the tongues(Figs.5.80,5.84). Dimeruiottsof rollfront type ore bodiesare up ppm in most of the deposits.It hasbeendeposited on the convexside of the rollfront where it over- to 15m (typically a few tens of centimetersto lapsthe zonesof ferroselite,pyrite, and uranium. 10m) thick in the apex zone. Their widths perArsenic is present in amountsof <50 ppm to pendicularto the redoxfront rangebetweena few centimetersand several hundred meters. The ca. lo/", varying from basinto basin. Beryllium ranges from 1.5 to 5.5ppm. It is strike lengthsextendup to severalkilometers. Most uraniumore in the rollfrontsis in radioassociatedwith uranium in the ore and is below metic dbequilibriwn Chemical U values are 1.5ppmin alteredand unalteredsandstone Coppertenorsare 10to 20ppm. It hasnot been higher on the convex side of the roll and radiometric U valuesare higheron the concaveside. depositedwith any of the other ore minerals. Although the radiometric disequilibrium Organic carbon has generallya rather erratic <0.05 to 2"h. hampersa preciseU/Pb datingof the ore, Dooley distribution. Contentsrange from Similar rangesare obtainedfrom mineralcarbon. et al. (1974)and Ludwig (1978,1979)couldestabprincipally calcite. No consistentdirect correla- lish apparent ages scattering between 22 to that mineralizationstartedat tion between the organic carbon and uranium 35m.y., suggesting in Oligocene. Mineralizingor uranium could be least the contents of mineralized sandstones processes continued over a large redistribution established. probably present.This may be to the time span of 17" or more in amounts Sulfatesulfur occurs in mineralized sandstone.It has been removed inferred from studiesby Dooley et al. (19&1)and from the altered sandstonewhere it wasoriginally Rosholtet al. (1964,1965a,b).Their dataindicate that significantmigration within the ore bodiesof present as gypsum. a4U and total U occurred within the last Phosphateis present in many deposits. High both amounts(up to 1% PzOs)can be relatedto high 100000years.
298
5 Selected Examples of Economically Significant Types of Uranium Deposits
Potential Sourcesof Uranrum
Ore Controls and Recognition Criteria
The two most probable sources for the ura- Significant ore controlling or recognition criteria nium are uraniferous granitic rocks of Lower of the rollfront uranium deposits in the Wyoming Proterozoic to Archean age that crop out in the Basins as listed by Harshman and Adams (1981) ranges which surround the uranium bearing are as follows. basins in Wyoming, and uraniferous tuffs and bentoniticvolcaniclasticsedimentsof Eoceneand Host Environment younger age that overlie or once overlaid the - Most favorable host sediments are permeable basins. medium- to coarse-grained carbonaceous, Granitic rocl<sas found in the Lararnie and pyrite-bearing (<1 to 37"), arkosic sandstones Shirley Mountains, and consideredto be the of fluvial provenance interbedded with imsourcesof the arkose of the Shirley Basin, and permeable pelitic sediments of late Paleocene partly of the Powder River Basin, contain0.5 to to earh'Eocene age. Tppm of leachabieuranium (Harshman 1972). Granitesof the SweetwaterUplift, that furnished - Favorable lithoiogic settings are found along Paleocene-early Miocene drainage systems much of the arkosichost sedimentfor the Shirley in Wyoming, and principally in the central part Basin, Gas Hills, and Crooks Gap-GreatDivide of the fluvial systems. where stream gradients Basin contain up to 30ppmU and locally more. are moderate and permeable carbonaceous These graniteslost an estimated70 to 75% of channel sands with considerable iongitudinal their original uranium endowment over the past 40m.y., as deducedfrom the existingradiocontinuity, interbedded with fine-grained genicleadamounts(Rosholtet aI.1973;Stuckless sediments, are deposited in shallow but wide stream channels. and Nkomo 1978). Tuffaceoussed.imentsare anomalous in their - The rock constituents derived largely from granitic higNands that surrounded the downuranium content and experienceda loss of the warping basins. uranium as documented by Zielinski (1980). Zielinski repofts 250ppmU in chalcedonycol- - Detrital organic debris present in the sediments was supplied by abundant vegetation. lected from and directly beneath accumulations of rhyolite ash in the White River Formation in Reducing conditions reflected by diagenetic pyrite prevailed in the lower parts of the basins the Shirley Basin. He postulatesthat uranium were leached by downwhere water tables were high. and silica from the ash ward percolating groundwater and were pre- - The early Eocene host rocks are covered by cipitatedas a uraniferoussilicagel directly above fine-grained clastic sediments admrxed with volcanic debris which were deposited durthe relativelyimperviousclaystoneunderlyingthe silicifiedmaterial.A minimum ageof 20m.v. was ing the middle Eocene (White River Fm.) to obtained for the uraniferoussilica and 32.4 + Pliocene intemrpted by penods of erosion. 2.6m.y. for the overlying rhyolite tuff. These This burial protected the early Eocene investigationsshow that uranium was being arkosic rocks and most of their uranium leachedfrom ash and carried by groundwaterat deposits from Pieistocene and recent erosion the time the ShirleyBasindepositsarethoughtto and oxidation. haveformed between20 and35m.y. ago. Spring watersin both granitic and tuffaceous (White River Fm.) terrainscontain anomalous M inerali z atio n an d A lter atio n uraniumcontentsof up to 8 and 52ppb respectively (Harshman 1972), supporting any hypo- - Uranium occurs in roll-shaped ore bodies in thesis deriving the uranium for ore formation the marginal zone of oxidized sandstone from either one of thesesources. tongues,i.e., in or immediately adjacent to the redox front that separates oxidized from unoxidized sandstones and that is regarded as the furthest downdip or outer penetration front of oxid2ing groundwater.
Examplesof Sandstone-Type UraniumDeposits -
A characteristic elemental zoning of U, Fe, Mo, Se, V, and As has developed acrossthe redox interface (Fig. 5.85). - Uranium accretion apparently is highest - at the convex side of the redox-intert'ace ar the outer edge of an altered tongue, - in gently. generallv <5' dipping beds of arkosic sandstone which are interbedded with continuous impermeable beds, - where zones of mineralization narrow down. - where larger amounts of carbonaceous debris and/or pyrite is presenr in the rollfront. - at changesof permeability partly causedby gradation of coarse arkosic sandstoneinto more pelitic beds, - at changes'instrike of the rollfront. - Deposits range from narrow and high-grade (Shirley Basin, Gas Hills) to wide and relatively low-grade (Great Divide Basin), and there appears to be an inverse relation of grade to width. The lower-grade, wider deposits frequently occur in sandstones with small amounts of carbonaceousplant remains and/or small amounts of pyrite in the unaltered host
Olractlon
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oa
ol
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of
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299
rock. This contrasts with larger amounts of carbonaceous debris and/or pyrite associated with the higher-grade relatively narrow deposits. - The edge of the oxidized tongue is not everywhere uniformly mineralized, and ore bodies are dispersed at irregular intervals along it. The best ore bodies seem to be where there are changes in strike of the tongue's edge and where the direction of flow of the solutionswas nearly perpendicuiar to the edge of the rongue. Local concentrationsof carbonaceousmaterial and/or pyrite mav additionally produce very reducing conditions in local areas and thus causeincreaseddeposition in those areas.
Metallogenetic Concepts A variety of models have been put forward to explain the formation of the Wyoming roll-rype uranium deposits. Harshman and Adams (1981) have comprehensively analyzed all information on the formation of this type of uranium deposits in Wyoming and elsewhere. The following is an
Fludo.r
*
d.toalllon
\ Un.ft.7ad !aadatorlrl
I
Po,O,
_
Fig. 5.E6. Shirley Basin, graphic summary of epigenetic mineral deposition. Length of arrows indicates relative positions and widths of zones through which minerals were deposited; dashed lines show intervals of possible deviarions from normal conditions. In a general way, the taik of the anows aa the left side of the figure indicate the points at which the various elements or compounds began to deposit as a unit of ore-bearing solution flowed toward the right from the altered sandstone tongue to the left. Tlte anow /recds show the poina at which deposition stopped for the particular position of the rollfront shown. As tbe rollfront advanced to the right, so also did the zone of precipitation for each elemen( or compound. (Harshman 1972;Hanhman and Adams l98l)
300
5 Selected Examples of Economicalry significant Types of uranium Deposits
excerpt largel_vbasedon their findingsand conUranium ore formation began in Wyoming clusions,unlessotherwisecited. after the deposition of the White River and Uranium deposits as found in the Wyoming youngerformations,i.e., at _ about Oligocenetime Basins are related to roll or redoxfronts that are when oxygenatedgroundwarer stanedio enter the the frontal edgesof an altered sandstonetongue exposedor truncated edgesof the early Eocene in permeablearkosicsandstoneinterbeddedwith sedimentsin the Wyoming Basins. Thi, *ur", lessor impemreablestrata. originated in the granitic, metamorphic, and A rollfoont is a dynamic feature migrating sedimentaryrocks of the mountainsupdip from down a hydrologicgradient,generallybasinward, the host rock outcrops and moved down the by oxidation and solution on its updip side and flanksof the basins. reduction and depositionon its downd'ipside.In There is generalagreementthat this ground_ sensu stricto, the redox front is an oxidation- water flow was the mechanismfor the alieratron reduction interfacefor iron. This interfacemav, in the hostrocks and the formation of the deposrrs but mostly does not, coincide with the redox associated with them interfacesof other elemenrs(Fig. 5.g6). Never_ Furthermore, all of the elements in the de_ theless.there is a striking similanty in the distri_ posits and the character of the alteration associ_ bution of elementsand mineralsalongthe redox atedwith them canbe relatedto the geochemistry fronts irr all of the roll-type uranium-deposits in of groundwaterunder the environmentalcondi_ the Wvoming Basinsand in other districtJaswell, tions extantin the WyomingBasinsat the trme e.g.,Biack Hills or TexasCoastalpiains.Neither the depositsare thought to have formed. the sizeof the depositnor the geologicalenviron_ The physico-chemicalmechanism for dissoiu_ ment in which it is found seemto havehad much tion, migration, and precipitation of the various effecton this distribution,a fact that suggests that elementsin rollfront type depositsis to a major similar geneticprocesses were responsiblefor all extent a function of changes of the oxidation rollfront uranium deposits. potential (Eh) and of the gradeof aciditv (pH) of the mineralizinefluids.
_::=__
JL.*!I:___
_ --ffi-
- - - -ffi
Fig' 5'87' wyoming Basins, general schemeof flow paths for mineralizing fluids. (Harshman 1972)
Examplesof Sandstone-TypeUranium Deposits
Olr.ctlon
of llo
of o.>b.rdna
JUI
r€{sdo.t
>7 pH
<7 Fig. 5.EE. Wyoming Basins,postulatedEh and pH conditions for groundwatersduring transportationand deposition of '.rraniumand other elements.(Harshman 1970)
Uranium was probably transported in the hexavalentstate as a uranyl-ion.Somewhatgeneralized,it can be said with respectto alkaliniry that, in the presenceof phosphorous, uranyl-ions form predominantly(more than 50%) complexes with phosphatebetween pH 4.5-7.5. In the absenceof phosphate, uranyl carbonate complexes predominate above pH 4.5, but in the presenceof UO2 (HPO4)2,theypredominateonly abovepH 7.5. Vanadium,selenium,arsenic,andmolybdenum all form oxygenatedanioniccomplexesand can be carried in those forms by solutionsof pH near 8 and Eh above -200 mV, conditionsextant in ground waters of the Wyoming Basins. Other
elementsin the ore are probablycamed in solution as simplecations, Figure 5.87 shows the postulated paths of alteration and ore-forming solutioru and Figure 5.88 the presumedpath of a unit of solution, within an Eh-pH diagram,as the solution moves from the oxidizing environmentof the altered sandstonethrough the rollfront and into the unalteredsandstone.The postulateddecreasein Eh downdip away from the redox front for iron would causeelementsrequiringthe lowestEh for this reduction to be depositedthe furthest advanced from the edge of the iron redox interface. Figure 5.89 is a compositeEh-pH diagram showing the equilibrium boundaries befween
l* I r
I
Pottt of decreosing Eh os solution posses roll front
5e s:
FeS, +
t=-
Fig. 5.E9. Roll-r_vpe uranium deposits, composite Eh-pH diagram for the mobility and stabil.ity of the principal elements. Solid phases are underlined. (Harshman and Adams l98l based on Harshman 1974, with data from Garrels 1960; Garrels and Christ 1965; Hansuld 1966; Hostetler and Garrels l!b2; Lakin 1961;Lisitsin 1!)69)
5eul-
FerOa
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UOz.- UO2(CO 3)2-2H2O2-
MoS,e Mo0!-
302
5 Selected Examples of Economically Significant Types of Uranium Deposits
the relatively soluble and insoluble forms of selenium,vanadium,uranium, and molybdenum and between pyrite and two ferric iron compounds, expressedfor conditions of temperature, pressure, and solution composition, and concentrationthat approximatethe likely but somewhatsimplified conditionsin the mineralbearingsolutions.If one assumesa startingsoiuits Eh from +300 tion pH of 7.5. then decreases to -300mV near the rollfront. at constantpH. the diagrampredictsthe path of solutionEh will intersect the element boundariesin the same order that the-v are actually found in rolltype deposits(Fig. 5.85).The diagramalsoshowsthat in pH concurrentwith a one-or two-unit decrease decreasingEh will not changethe sequenceof deposition. Investigations by Lisitsin and Kuznetsova (1967) on groundwater collected below the permanentwater table alongtraversesextending from altered sandstonethrouglr ore and into unalteredsandstonein a sedimentarybasinin the USSR confirmed Eh and pH conditionsalmost identical with those of the Wvomins Basinsand shownin Fie. 5.88.
Although there is wide agreement on the concept of a migrating redox front responsible for the ore formation, there remains some disagreement on the processesinvolved. Two ahernative geochemical systems are paramount, biogenicbiochemical processes, and chemical processes. Perhaps a combination of both might have accomplished the oxidation, migration and reduction environments necessary for the ore formation as summarized by Harshman and Adams (1981) in the following. Biochemical system: Racklel' Q972) states that an all-biogenic system involves two species of bacteria functioning in two ven' restricted environments of different Eh and pH zones, sharply separated fiom one another bv the redox front. On the reduced side of the front (Fig. 5.90) anaerobic bacteria of the genus Desulfovibrio predominate. They are sulfate reducers, derive their energy from organic matter in the sandstone or in solution. and produce H2S. Other bacteria may assist in breaking down cellulose into products usable by Desulfovibrio in its life process. These bacteria are strict anaerobes and create an environment of pH 7.8 to 8.4 and an Eh of -200mV or less.
Fe" colcite
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Fig. 5.90. Wyoming Basins, possiblechemical reactionsin a host sand bed (white) involved in the formation of and acrossa rollfront due to activity of bacteria. H2O arron,indicates flow direction of solution. (AJrer Rackley 1972) (reprinted by permissionof AAPG)
Examplesof Sandstone-TypeUranium Deposits
On the oxidized or altered side of the front. Thiobacillts ferrooxidans and related bacteria predominate. They are strictlv aerobic, derive their carbon from COu, their energy from nitrogen and sulfur compounds, hydrogen, and iron, and they are capableof producing pH as low as 1.8, although the optimum pH for maximum activity is from 2 to 4. In addition to producing a low pH, thev are capableof producing Eh values as high as +760 mV although such high valuesare unlikely in a natural environment. Thiobacillus is thought to act as an intermediate agent in the conversion of pyrite to iron hydroxide and eventually to goethite, hematite, or high iron montmorillonite. The first step is the reaction of pyrite, ferric sulfate and water to produce ferrous suifate and sulfuric acid; the second step, the biochemical oxidation (by Thiobacillus) of. ferrous sulfate and sulfuric acid to ferric sulfate and water, and finaily the hydrolysis of ferric sulfate to ferric hydroxide and sulfuric acid. Excess sulfate in the system is carried across the front into the reducing environment of Desulfovibrio, where it is reduced to H2S. The biogenic system is almost self-perpetuating and needs only orygen, pyrite, CO2, and organic matter to complete the oxidation, migration, and reduction cycle required by a dynamic system of deposition. The needs of the svstem can be met by constituents of the host sandstones (pyrite. organic matter, CO2) and the ore-bearing solution (oxygen and CO2). With respect to the need of pyrite, Austin (written commun. in Harshman and Adams 1981) points out that there is no need for sulfide to initiate the processas long as there is a supply of sulfate in the groundwater. However, it is likely that once the process is started- at least part of the sulfate will be provided by oxidation of pyrite or other sulfides, and if the sulfide is pyrite, oxidizing bacteria can. and most likely will, speed up the process and provide the continuous system described. If the sulfide is a less stable species, such as marcasite, or other metal sulfides, the process has much less need of oxidizing bacteria. The biogenic system requires no sulfur more reduced than sulfate for initiarion, whereas the inorganic system requires more reduced phases such as sulfide. Studies by Thode et al. (1951) on the role played by sulfate-reducing bacteria reveal that the bacteria selectively reduce the lighter 32S.Cheney and Jensen (1966) report that sulfur isotope
303
analysesfrom the Gas Hills, from both diagenetic pyrite in unaltered sandstone and pyrite in the ore bodies, have the wide isotopic spread in the 32Si34S ratios and the relative enrichment in 3?S that is characteristicof H2S of biogenic origin. Reynolds and Goldhaber (1983) arrive at a similar conclusion, based on their sulfur isotope studies,for pre-ore-stagepynte. Some support is given the biogeniCsystem by the investigations of Lisitsyn and Kuznetsova (1967). They sampled waters from wells and mine openings in and near roll-type deposits in an artesian basin at an undisclosedlocation in the USSR. From the limited geological data given, one can conclude that the deposit (or deposits)is similar in all major aspects to those found in the Wyoming Basins, and that it was forming at the time of their investigation. The host rocks are permeable Cretaceous sandstones,and the waters flowing in the mineralized zone have a pH of about 7.5. and an Eh of about -200mV. The authors conclude that (a) the negative Eh results from the anaerobic activiry of microflora that produce H2S and H2, and (b) the character and distribution of elements within the deposit depend on the relative amounts of the various products of microbiological activity. Lisitsyn and Kuznetsova's (1967) data are difficult to analyze, but they seem to confirm the presence of H,- and H2S-forming bacteria in the water on the reduced and mineralized side of the oxidation-reduction interface, but the presence of Thio-bacteria in the oxidized sandstone near the interface is open to serious question. The chemical-biochemical concept postulates that mineral-bearing solutions are alkaline and oxyenated when they approach the redox interface. Without the aid of bacteria. the solutions oxidize pyrite to sulfuric acid and ferrous sulfate. The oxidation process will be slow if pyrite is the only sulfide. but much faster if marcasite or other sulfur-deficient species are present. These products are carried downdip into the reduced mineralized zone. where sulfate-reducing bacteria convert the sulfate to H2S and the sulfuric acid is converted to gypsum by the alkaline environment of the slightly calcareous arkose. Chemical system; Granger and Warren (1969, 1974,Fig.5.83) have shown that solution, migration, and redeposition of the elements along a rollfront can be accomplished by inorganic chemical reactions and without the intervention
3M
5 Seleaed Examples of Economically Significant Types of Uranium Deposits
of bacteria. They recognize the probability that Cretaceous formationssuggests a possiblegenetic the widely disseminatedpyrite in the unaltered relationshipbefweenuraniumdepositionand H2S groundandthat destroyedin the alteredtongueis from sour gas. Grutt (1957)concludedthat H2S of biogenic origin and formed shortly after de- derived from sour gas w:ls the precipitant for the position of the host sediments.Their thesisis that ore mineralsin the GasHills area.He pointed out oxidation of pyrite at the rollfront is accomplished that producing oil and gas wells are adjacent to by the ore-bearingsolution, but that the amount the area, that the gas from the wells containsas of oxygenin that solution is limited. This results much as 2'/"}{25, that there are gas seepsin the in the formation of soluble, metastable,partly area, and that exploration drilling encountered oxidizedsulfur specieswhich are carried downdip H2S in the Wind River Formation. He proposed by the ore-bearingsolution until they sponta- that sour gas ascendedalong the faults that neouslyundergodisproportionation;that is, they border the district on the south, entered the divide into equivalentamountsof more reduced unconformiw at the base of the Wind River speciessuch as H2S, and more oxidizedspecies Formation,and enteredthe host sandstonewhere suchassulfate(SOo)'-. Sulfateis kineticallyinert it restedon the unconformity. to further reduction or oxidation reactionsand The abovepresenteddocumentationsmake it this leavesthe more reactivereducedmetastable difficultto decidewhat kind of chemicalprocesses sulfur speciesand H2S to control the environ- were ultimatelv responsiblefor the ore-forming ment. Chemical theory and laboratory experi- chemical environment in and adjacent to rollmentssupportthe aboveconsiderations and show fronts. More recent work by' Goldhaber and that isotopicfractionationof sulfur, similarto that Reynolds(1979 and 1983) may shed some light causedby biochemicalreactions,can resultfrom on this problem. Their experimentalwork and inorganic chemical reactions. It appearsthere- studies on marcasite of Texas and Wyoming fore, that a pyrite-bearing sandstoneis com- uranium deposits,and on geochemicalenvironpletely capableof establishingand maintaininga mentsanalagous to thosegoverningthe formation rollfront, onceoxygenatedwatersare introduced, of rollfront deposits, indicate that relatively low without the participation of any organic com- pH and the presenceof elemental sulfur favor ponentswhatsoever. marcasite,whereashigherpH and the presenceof Two other reductants have been proposed polysulfideions favor pynte, i.e., a dominant becauseof their spatial relation to the deposits, factor in the formation of marcasite or pyrite as vegetalmaterialand H2S. ore-stagemineralsis the complex interrelationCoaffied woody material: Coalified woody ship of pH and sulfur speciesthat are the prematerial, or humates derived from such mate- cursorsof iron-disulide minerals. rial, are presentin all of the host rocks of the Conditionsthat favor marcasiteas the domiWyomingdeposits.For this reason,aswell asfor nant ore-stageiron disulfide are most likely to the fact that someore containsas much as 0.5 to arise in noncarbonaceous rocks. In rocks with 1% organic carbon, some investigatorssuggest considerableorganic matter, the presence of that organic carbon is the direct reductant of polysulfideions and pH buffering by anaerobic uraniumand associatedelements. bacterial metabolic processesapparently led to However, the correlation between uranium the formation of ore-stagepyrite. Since in the and organiccarbonwithin the roll-type systemis Wyoming Basins most of the ore-stage iron neithersimplenor consistent, hencetheprecipita- sulfideis in pyrite, this can be regardedas a clear tion of 'uranium by organic material may not implicationthat biochemicalactivity was responbe the only important mechanism.Studies by siblefor the reductionalongthe redox front. Schmidt-Collerus(1969, 7979),Kochenovet al. In summary,Harshman and Adams (1981) (1965), and others, suggestthat the organic view the actualore-formingproces.resas follows: matter may initially complexuranium from soluOxygenatedgroundwaterthat carrieduranium tion to subsequently be reducedby the oxidation and associated elementswasprobablyderivedin of somecombinationof carbon and sulfide,and part from the basinflankingPrecambriangranitic perhapsother elements. massifsand in part from the tuffaceousWhite Hydrogen sulfide: The close spatial rela- River Formation.Where thesesolutionsreached tionship between the uranium depositsin the equilibriumwith the host rocksby the oxidation Wyoming Basins and oil- and gas-producingof the rock components,particularly pyrite and
Examples of Sandstone-TypeUranium Deposits
carbonaceousdebris, an interface. the rollfront, was established between oxidizing conditions in the updip part of the host and reducing conditions in the downdip part. Across this interface, deposition of U, Fe, Se, Mo, V, and other elements carried in solution has been most pronounced at the respective redox boundary for each element, and decreased rapidly as the solution moved beyond that interface into the reduced zone. Any mineralized zone established at any time in the cycle would migrate generally in the downdip direction of the groundwater flow bv oxidation and solution on the updip side of the nineralized zone and reduction and redeposition cn the downdip side. A continuous extrinsic supply of uranium and associatedelements in the mrneralizing solution passingthrough the zone of deposition would cause the mineralized zone to increase in both grade and magnitude and eventually to reach, particularly in gently dipping beds, the size of the presentday deposits. The processes described above continued downdip in the host rocks for distances of 8 to 20 km and to depths of several tens to several hundreds of meters below the ground surface. As long as oxygen and slightly mineral-bearing water is supplied to the redox interface, mineralization will accumulate at a rate governed by the supply of oxygen, amount of pyrite and/or carbonaceous material, biological or abiological reduction of sulfur, rate of flow of the mineral-bearing solution, and possibly the length of the altered tongue. The process stops when either the hydrologic system changes, the reduced zone is destroyed by oxidation, or the oxygenated water entering the host sandstone at or near the outcrop or paleo-outcrop becomes diluted by reducing water entering the host from overlving sediments. Some of the deposits,particularly those associated with limonite-bearingaltered tongues (e.g.. in the Powder River Basin) may have undergone recent remobilization, migration, and redeposition of elements in the original deposits. In case of a reversal of the groundwater flow, for example by tilting of the area, as indicated by Harshman (1972) for the Shirley Basin, previous deposits could even be turned into a stage of destruction.
305
References and Further Reading for Chapter 5.4.4 (for details of publications see Bibliography) Adler 1974; Adler and Sharp 1967; Anderson 1969; Armstrong 1970; Austin 1970; Baily 1965, 1969, i980; Cheney and Jensen1966;Chenoweth 1980; Childers 1970, 1974;Crawley1983;Curry 1976:Dahl and Hagmaier 1976; Davis 1969;Densonand Chisolm i971 ; Files 1970:Fischer 190; Gabelman 1970; Gabelman and Krusiewski 1961; Galloway 1979a,1979b: Goldhaber and Reynolds 1979; Granger and Warren 1969,1978,1979;Gruner 1956;Grutt 1957, 1972; Harris 1984; Harshman 1962. l9(fi. 1968, 1970. 19'12, 1974; Harshman EN pers. commun.; Harshman and Adams 1981;Houston 1969: Jensen 1958; Klingmtiler 1989; Keefer 1970; King and Austin 1966; Lange and Kidwell 1974;Ludwig 1978. 1979;Ludwig and Grauch 1980:Masursky 1962:Melin 196{. 1969;Rackley 1968. 1972,1976;Reade 1976;Santos 1981: Santosand Ludwig 1983; Seeland 1976; Sharp and Gibbons 1964; Sharp et al. 1964;Sherborneet al. 1980: Sheridan et al. 1961; Stephens i964; Van Houten 1961: Vickers 1957; Vine and Tourteiot 1970; Warren l9'71, 1972
5.4.5 Roll-Type Extrinsic Sulfi de-Uranium Deposits in Phanerozoic Coast Plain Sandstones:South Texas Coastal Ptain. USA The Texas CoastalPlain, also referred to as the South Texas Uranium Region or South Texas Mineral Belt, contains sandstone f-ype uranium deposits in a 20 to 45km wide curvilinear belt which approximatelyparallels the coast of the Gulf of Mexicoabout130km inland. The mineral belt is almost400km long in a NE to SSW direction, extending from eastern-centralTexas to the Mexican border, and probablv continues into Mexico. Some hundred properties have been mined principally in Karnes. Live Oak, McMullen, and Duval counties,but also in some other counties(Fig.5.91a). Total reservesin the south Texas region are estimatedat between45000and 80000mt U3Os, with averageore grades running around 0.1% Uroa. The southTexasuranium depositsare roll-type mineralizationsto someextent similar to those in the WyomingBasins.But in contrastto the latter, the reduction of the host rocks resulted from the influx of reductantssuchas H2S, and the deposits occur in a marginalmarine environment. Therefore, the south Texasuranium deposits are classified as roll-type extrinsic sulfide-uranium depositsin coastplains(class4.2-2, Chap. 4).
306
5 Selected Examples of Economically Signifcant Types of Uranium Deposis 980
a
I
gAL
29': . i : . : :. . r j
9i ,l
a
*lr
H . E .H o u s t o n E m b o y m e n t R . G . ER , io 6rond Emboyment oreos with U deposits in the
IJ
l-Z-l ffi 3
Cotohoulo tutf resting on older Tertiory uni'ts
P t i o " " n " G o l i o dF o r m o t i o n u l o c e n e O o k v i l l eF o r m o t i o n
[---l
O t i g o c e n eC o t o h o u l oF o r m o t i o n
F
Eoce^e Jockson Group
A p p r o x r m o t ep o s i t i o n o i present ooy oulcrop 0 o k v i l l eF m
SE
G o t i o c iF m o n d y o u n g e r s e d i m e n i s Seo level
e r rr o r y
Fig. 5.91. South Texas, generalizedgeologicalmap (a) and NW-SE cross-section(b) of the coastalplain and location of areascontainins major uranium deposits.Host units for depositsare fluvial-marginalmarine sedimentsof the Eocene Jackson Group, Oligocene Catahoula, Miocene Oakville and Pliocene Goliad formations. Counties containing U d e p o s i r s1: \ l ' a s h i n g t o n 2 ; F a v e t t e ;3 G o n z a l e s 4 ; K a r n e s ; 5 A t a s c o s a : 6 M c M u l l e nT : LiveOak:8Webb; gDuyal 10 Z,apata;// Jrm Hogg; 12 Stan. Uranium districts:K.C. Karnes Counn'; R.P. Ray Point; C.W. Clay West-Burns;R.R. Rhodes Ranch area; S.D.C. SouthDuval Countv Mineral Trend. (After a Crawleyet al. 1983basedon Eargle et al. 1975;b Adams and Smith 1981)(a reprintedby permissionof AAPG)
Uranium Deposits Examplesof Sandstone-T;"pe
The following description is largely based on A d a m s a n d S m i t h ( 1 9 8 1 ) ,w h o h a v e p u b l i s h e da concisecompilation of the south Texas uranium deposits which incorporates information from man)' authors, particularly from Eargle. Fisher, G;lloway. Weeks, and their coworkers.
-307
portant geological features, such as the strike of the formations and positions of the favorable lithologic facies in some of the host sediments, some geologicalfeatures cross the trend. Of particular interest are persistent fluvial depositional zones forming mega-channels. These megachannels appear to provide environments important to the location of ore bodies. In principle, the ore-hostingsequencesare flatGeologicalSetting of Mineralization lying, dipping about 1" SE and strike around NEThe Texas Coastal Plain is underiain bv more SW almost parallel to the Gulf coast.They are of than 15000 m of flat-lying interbedded manne continental fluviai and marginal marine deltaic. and nonmarine sedimentsof Tertiary age which lagoonal,and littoral provenance. The prominent uranium host rocks are peri3ji on Jurassicand Cretaceoussediments(Figs. The were deposited Tertiary sediments meable, hne-grained. tutfaceous. pvrite-bearing, : rla,b). p l a i n s a n d arkosic sandstones. containing only locally. as c o a s t a l b y m a j o r n v e r s y s t e m s i n on carbonaceousplant mamarine transgressions. Instabiiities in the Jackson Group, due rervening ro the constant loading of sediments into the terial. The sandsare interbeddedrvith tuffaceous. subsidinggulf led to local fauiting particularly to zeolitic, and bentonitic mudstones. and iignite growth faults that become )'ounger toward the seams.The volcanics are of rhyoiitic, trachytic, coast. The principal faults and fault systemsform and trachyandesiticcomposition. a general arcuate pattern around the basin of sedimentation.Acid volcankm. synchronouswith part of the subsidence, provided volcaniclastic- Host Rock Alterations rich sediments to a portion of the Tertiary sedimentary sequence. Principal alterations of the host rocks resulted Associated with the sediments and the con- from diagenetic and repeated pre-, syn- and temporaneous growth faults are occurrences of post-ore oxidation and reduction processes. oil and gas, iignite, geothermal resources and Alterations related directly or indirectly to ore uranium. Oil and gas occulTencesare controlled formation resuited in the situation that some regionally by depositional facies and locally deposits are associated with a ciassicai geobv structures which in many cases place sands chemical interface at the edge of an oxidized and shales in juxtaposition. forming ideal sandstone tongue whereas other deposits are traps for hydrocarbon accumulation. Galloway emplacedin entirely reduced sands.For example, (1977) has estimated that one third of the depositsin the Jackson Group are associatedwith South Texas Coastal Plain is underlain by closely geochemical rollfronts located at the edges of spaced hydrocarbon reservoirs that are largely altered sandstone tongues. Their host rocks exhibit the generai mineralogic and geochemicai fault-controlled. Host sedimenrs of the known uranium occur- characteristics of the unaltered downdip and rences are sandstonesdeposited between middle altered, oxidized updip sands similar to those E o c e n e a n d e a r l v P l i o c e n e( F i g s . 5 . 9 1 a . b .5 . 9 2 ) . of the rollfront deposits in the W1'oming Basins Stratigraphic units containing major depositsare (see Sect. 5.,1..1for more details on alteration. mineralogy,and chemistry). particularly the late Eocene Jackson Group,late In addition to the normal alteration characOligocene Camhoula Formaion, early Miocene Oakville Formarion, and the late Miocene-early teristics of a rollfront. some of the south Texas Pliocene Goliad Formation. Deposits of modest roils exhibit pecuiiarities. Adams and Smith size also occur in other formations. Very young (1981) descnbe, for example. the setting and mineralogy of a roll-type deposit in the South uranium was discovered in the Pleistocene Lrsie Formation. In spite of this wide stratigraphic Duval County Mineral Trend. Here, two alterarange, the uranium deposits are restricted to a tion zones exist within the oxidized tongue (Fig. belt (Fig. 5.91a), the boundaries of which do 5.93). The first is well updip from the rollfront not entirely correspond with regional geological and contains Fe-Ti-oxides that are in vanous boundaries. Althoueh the trend does reflect im- states of oxidation. The second extends for a
5 Selected Examples of Economically Sipificant Types of Uranium Deposits
308 E
3
Lithostrotigrophic unit
Lithology
Flood-ploin olluvium lluviol terroce deoosits
Sond, grovel. silt. cloy
P l e i s t o c e n e , D e w e y v i l l eF m . . B e o u m o n t F m . , Montgomery Fm.. Bentley Fm.. L i s s i e F m . . P l i o c e n e { ? ) .W i t l i s F m .
Sond, grovel, silt, cloy
F i n e t o c o o r s e s o n d o n d c o n g l o m e r o t e ;c o l c o r e o u s c l o y ; b o s o l m e d i u mt o c o o r s e s o n d stone. Stronoly colichified
Colcoreous cloy ond sond
C o l c o r e o u s ,c r o s s b e d d e d ,c o o r s e s o n d . S o m e cloy ond silt ond reworked sond ond cloy pebbles neor bose
Ookville Sondstone
c
Chuso Tuff .9 F
E ii r=
o o o
Soledod Conglomerote
q:,,
U
6:t +
>o
Colcoreous tufi; bentonitic cloy; some grovel ond voricolored sond neor bose. Soledod in Duvol County. grodes into sond lenses in northern Duvol ond odjocent counties
Font Tufl
Light-groy to green cloy; locol sond-filled chonnels
Frio Cloy lor Formolion) U
C h i e f l yc l o y ; s o m e l i g n i t e .s o ^ d . C o r b i c u l o c o q u t n o .o y s t e r s g
=
T o r d i l l o S o n d s t o n e .C o l l i h o mS o n d stone west of Kornes County lDubose e e s v i l l eS o n d s t o n e
M o s i l y f r n e s o n d ; s o m e c o r b o n o c e o u ss i l t o n d c l o y
C o n q u i s t oC l o y l D i l w o r t hS o n d s t o n e
C o r b o n o c e o u sc l o y Fine sond, obundonl OPhiomorPho
ery fine sond
Fig. 5.92. South Texas, litho-stratigraphic coiumn showing the principal uranium bearing unjts in the fuo Grande Embaymenr. (After Gallowav 1979b)
variable distance back updip from the geochemical interface and contains vestiges of ilmenite and magnetite. In the reduced sands downdip from the rollfront, Fe-Ti-oxideshave also been completelydestroyed,in part through replacementby pyrite. A subsequentintroduction of oxidizine solutions into these sulfide-bearine
sands oxidized the pyrite and produced the geometricrelationsas they are observedtoday. At other locations,someroll-shapeddeposits occur entirely within reduced, sulfide-bearing sanfutone. Researchby Goldhaber and coworkers supports earlier suggestions that the distribution and
Examples of Sandstone-TypeUranium Deposits
309
Oxygenoted meteoricwoters
.\i\ .on l:.:.r'
\\\\\\
iir-o.,{i-da t ion
'n'|erclon,>:R:rra -ocs e"")t*\\\\\\\ -ronum-disseminored eos ix?'"q".n oommoroy-none secondoryoxidotion yeltow Cotor-brownish
misrotion
{-is; G
\iirl.xsad
'%r;\\\N* ,"
tt,N\\\\*
tr
t'"*N
"oro 'o :_\\\\\\ s"^\\::ii115 '.r,o^\\\\:::\'
,ron-limonite Uronium-concentrotedin iimbs G o m m or q y - k i c k o t t o p o n d b o t t o m o f s o n d , very low to none in center Rottfront lolor-block ron-pyrite U r o n i u m - c o n c e n t r o t eodt o x i d o t i o n - r e d u c t i oinn t e r f o c e G o m m or o y - v e r y h i g h , p e o k n e o r c e n t e r o f s o n d Protore/Reduced Co l o u r - g r e y lron-ovrite U ro n i um - d i sp e r s ed G o m m or o y - m o d e r o t et o w e o k , p e o k n e o r c e n t e r o f s o n d
"N
\\\l S
N\ FN\=l\Nt6\-j,
, I I | | lll ll I HZS
J:.N *j;,,.r} d,l:l;:i;;" t i o n ,i"::ui',]
,r::: \ i r "-Jgo\ :\ -,8:^, ; ',, . ,. \<4 )\{ / \do)\l:r I \-SSta \\\\ / \ Extent or d o w n d i pg o s m'grotlon
Fig. 5.93. South Texas, diagrammaticsectioo acrossa rollfront iilustrating the relations for fault-conrolled influx of reducing media (H2S, hydrocarbon) leading to secondaryreduction followed by partiai re-oxidation of the alteration tonque. (Adams and Smith 1981)
abundanceof pyrite within the host sandsof these Principal Characteristicsof Mineralization depositsprobably reflectsthe introductionof H2S up along faults from various hydrocarbon re- In unoxidizedore, the dominanturanium minerals servoirs within the Mesozoic-Tertiarysediment are pitchblendeisootypitchblendeand coffinite. pile. Such pre-ore introductions prepareC the Associatedmineralsand elementsincludepyrite, sands for rollfront development,whereaspost- marcasite,calcite; Se. Mo, and locally clinopore H2S influx producedre-reductionof portions tilolite, authigenic ieldspar, opal. and montof the altered tongue, leaving the deposit sus- monilonite. Pyrite and marcasiteoccur in severai pended in reducedsandstone.The depositsare, pre-, syn-,and post-orestages. Oxidized ore contains hexavalent uranium therefore, only a variant of the normai roll-type chieflyuranyl phosphates(autunite). Oakville minerals, and the Catahoula deposit. Depositsin (tyuyamunite,more rareiy carnotite) vanadates zone broad in a marcasite formations contain -silicates. and marcasite This rollfronts. downdip from the The south Texas depositsoccur in four prinhas been interpretedto have formed during ore pre-ore geologicalsettings:(1) In beach sandstones cipal of destruction the oxydative by formation alongthe and related sediments;(2) in sandstones carbonaceous-poor pyrite in sulfide-rich, stage marginsof major fluvial channelsystems;(3) in svstems(Adamsand Smith1981). sandstonesclose to faults along which hydrogen suifidecould be introducedinto the aquifer: and
310
5 Selected f,1:mples of Economically Significant Types of Uranium Deposits
(4) in sandstones abovesalt domes.The various deposit settingsmay overlap or be superimposed uPon one another. Within these environments, uranium has formed ore bodiesof roll-type and nonroli-type shape(Figs.5.93,5.94,5.95). Many of the depositsare of the classicalrol/front type emplacedat the marginsof alteration tonguesin sandstone.They display the characteristic uranium disequilibriumpatterns,elemental zoning, and mineral distributions of the Wyoming roll-typedeposits(Fig. 5.85). The shape of these ore bodies is commonlv cuspateor C-shapedwhen they occurin uniform sandsthat are boundedbf imperviousshalesor siltstones. Many deposits exhibit a complex geometry, however, due to the often complex interrelations between sand units of variabie permeabiliryand pelitic units of very low permeability. Indigenousand introducedreductants exertedan additionalinfluenceon the distribution and habit of uraniumrolls. Other depositsdo not resembleclassicalrolltype mineraliz3liep in that their geochemicai
environmentis strikinglydissimilarfrom the rollfronts of the Wyoming Basins. The important differencesof the nonroll-rypedepositsinclude: (1) the mineralizationdoes not occur at the margin of altered sandstonetongues but rather occurs entirely within reduced. pyrite-bearing practicallyhematite-and limonite-freesandstoneI and (2) the host sands contain essentiallv no plant material. onlv abundant discarbonaceous pyrite. seminated Although anomalousuranium concentrations are widespreadin the uranium belt of the Texas CoastalPlain,Adams and Smith (1981)suggesr that all noteworthyuranium occurrencesappear to be grouped in fluvial mega-charutelsvstems which formed and were maintained practicallv over the entire time spanfrom the earlv Eocene Wilcox Group to the PleistoceneLissie Formation. Most of the depositsoccur in ciustersmore or lessalong the fluvial trends in either one of these mega-channels. An example. the South Duval County Mineral Trend. is given in Fig. 5.96.The exceptionto this are thosedepositsin the JacksonGroup which formed in littoral environmentssuchas beachsands.
:::: '..:i
:..:
l':,!r:l Syndeposilionol oxidotion zone
Ii:...:-Epigenetic sulfidic zone FII Presulfidizedottered zone ffi
L o t e - s t o g ep e r v o s i v ec o l c i t e p o r e - f i l l i n gc e m e n t
[:-l
Epig"n"tic oxidotionzone
5
Selenium concentrotion
U
Uroniumconcentrotion
m
Molybdenumconcentrotion
Fig. 5.94. South Texas,diagrammaticseclionacrossa roll-shapedore body illustratingthe typical associationwith a faulr zone, and metal and alteration zonation characteristicof Catahoula and younger uranium host aquifers. Multiple epigeneticalterationstagesinclude pre- and post-mineralizationsulfidization,ore-stageepigeneticoxidation and modern oxidation. U, Se, and Mo minerals precipitatedand are zoned acrossthe front of mineralization. (Gallou'a_vi985)
Uranium Deposits Examplesof Sandstone-Type
311
I
\
+
1u
) l
2km
U d e p o si t l-
Vcld^ ")t
U n
SE'
l
z o F
z
; o
NW
z F
z
J J
;
Fj,--Lll sondston.
IiE
|-
ciovsrone
N ffi?:[#]:';l';;""'
-
,,0,, l.o"6i*
Fig, 5.95. Live Oak County, Ray Point district, Felder-Zamzow-Lamprechtdeposit. a Map of outline of the Oakville Sandstonehosteddeposit. b and i NIV-SE sectionsthrough the Felder mine with U gradedistribution. (After a Galloway 1979a:b and c Klohn and Pickens1970,modified by Eargie et al. 1975)(see also Fig. 5.97)
312
5 Selected Examples of Economically Signfficant Types of Urznium DePosits
A number of mining districtsare known within the mega-channelsystems(see Table 5.29 arrd Fig. 5.91a). Of the listed stratigraphicunits, the Catahoula and Oakville formations furnish the most productive uranium hosts. They combine more than 60"/" of the known uranium reserves in south Texas. In general, the dimensionsof south Texas ore bodiestend to be smallerand of lower gradethan the roll-type deposits in the Wyoming Basi:rs. They are generallythin, seldomexceeding5 m in thickness,and rarelyoccurasstackedor multiplefront deposits, such as are common in some depositsin Wyoming. The averagedeposit contains only about 5000mt U3Os, and the largest depositsknown to date are estimatedto contain The averagegradeis i-nthe about 14000mt U_rO6. order of 0.1% U3O8.
Reductants Two kinds of reducingagentsappearto be critical for the south Texas deposits:Hydrocarbons-H2S in the pyritic fluvial sands of the Catahoula, Oakville, and Goliad formations, and detrital vegetal organic matter il the littoral sandsof the JacksonGroup. Hydrocarbons-H2S:Most of the non-Jackson deposits contain none or negligible quantities of carbonaceousmatter. which mav be due to \=\ Sontonrno_\----\ Co. _WebbCo.iDuvot
L/ ,4-t',i)/,r'e
V
=-l
i \-=
'I
lI3 L:
Table 5.29. South Texas, the most irnponant districts in the meea-channel svstems
Main ore-hosting formation
KarnesCounryDistrict RayPointDistrict,Uve Oak Coun!v ClayWest-Burns Dist., Live Oak Couoty RhodeRanchDistrict,McMuIIen Count-v SouthDuvalCounryMineral Trend
Jackson Group Oakville Fm. Oakvilie Fm. Oakville Fm. Catahoula. Oakville, Goliad Fms.
oxidation at or shortly after burial. essentially destroying any indigenous carbonaceousmaterial. If this was the case,then the introduction must have been here a of hydrocarbons-H2S critical factor in the ore-forming process. The assumptionthat H2Sintroductionoccurred into the aquifers probably along faults and presumablyfrom hydrocarbonreservoirsat depth is largelybasedon the followingobservations:(a) the spatialdistributionof sandscontainingfins1t dispersedpyrite with respectto faults, (b) sulfur isotopedata, (c) the occurrenceof some reduced sandstonesentirely within oxidized sandstone, and (d) the virtual absence of carbonaceous material. Goldhaber et al. (1979) and Reynolds and at leastfour periods Goldhaber(1983)established
Boundory of South Duvol County MinerolTrend
+ ry
lZl U oeposit
r
mining
'-\-
H e b b r o n v i t t e\
e a ; , r. - ^ ( \
\
--J'
\:.\
v\
\
\
\
Fig. 5.96. South Duval County Mineral Trend. a Map with outline of uranium deposits,b NW-SE section. c N-S section through the mineral trend showing the stratigraphicposition of the U depositshosted in the Soledad Member of the OligoceneCatahoula Formation in the northwesternpart, and of minor depositsin tbe Miocene Oakville Sandstonein the southeasternpart of the belt. Sandstoneunits are indicated by patternsin Sections.(After Adams and Smith 1981)
Examolesof Sandstone-Tvpe Uranium Deposits
313
SE
=:::J
G O LI A D
>
<\ F
(-)
Solrdod
E*ffi,.:iliiii. ili;:ll:li*I:I'inEjIIE o ffi
Solcdod
F
314
5 Selected Examples of Economically Significant Types of Uranium Deposits
of probableH2Sintroduction,and the presenceof two different sulfur isotopes in the sulfides of Oakville host sandstones:Isotopically heavy sulfur which has been proposed as being derived from the deep Edwards Limestoneoil and gas fields and isotopically light sulfur as being derived from shallower horizons.The light sulfur isotopes may have originated by bacterialsulfatereduction in shallow aquifers promoted by the seepage of organic matter from Tertiary hydrocarbon deposits. Detrital corbonaceousmatter: Only sediments of the JacksonGroup and possiblyof the Carrizo Sand contain abundant plant material which visibly exerted a strong influence on a roltlront formation. Comparedwith the host rocks of depositsin other Tertiary sequences in southTexas. those of the JacksonGroup are of beach sand provenance.
by geochemicaland geologicaldata provided by Dickinson (1976b), Eargle and Weeks (1973), Galloway(197'7),Adams and Smith (L981), etc. The depositsmay have derived their uranium either intraformationallyfrom interbedded volcaniclastics, as found in the Catahoula,Jackson, and Oakville formations, or extraformationally from the (former) superpositionof the Catahoula or similarfertile tuffs abovean unconformity.
Ore Controlsand RecognitionCriteria Significantore controlling or recognitioncriteria of the southTexasuranium depositsas listed by Adamsand Smith (1981)are as follows. Host Environment
- The uranium deposits occur within or in proximity to permeable sandstones which range in composition from littoral quartz PotentialSourcesof Uraniun arenitesto fluvial arkoses.Metallogenetically. the sandstones appearto be of primary impormost volcaniclastics are the favored Uraniferous tance as aquifers for migrating uraniferous is supported of This hypothesis source uranium.
N
Ookville Sondstone.
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U deposit
50km
Cloy Wesl
F eellddeerr
Fig. 5.97. Live Oak County, Ray Point and Clay West districts. a Generalizedgeologicalmap. b SSW-NNE section showing location of major uranium deposits in, and their relation to depositional facies of the Miocene Oakville Sandstone.(After Eargle et al. 195 basedon Klohn and Pickens 190)
Uranium Deposits Examplesof Sandstone-Tvpe
i15
+" 7a2
jA
o " p o t t t L o n om l orphorogy Net sono isolrth {rn m) Uronium deposit
5km
Fig. 5.9E. SouthwesternKames Counry, schematicmap of relations befweenuranium depositsand littoral depositional environmentsand ner-sandisoliths for the Tordilla SandstoneMember of the upper JacksonGroup- (After Bonner et al. 1982based on Galloway 1979a)
groundwaters, whereas the composition and provenanceof the sandstonesseem to be of no or only minor importance for the emplacement of ore provided most clasts are resistant to alteration. Favorabie sand facies susceptible for mineralization include point bars. lateral bars, and crevasse spiays associated rvith fluvial channel systems (Fig. 5.97), and barrier bars and offshore bars deposited in littoral domains (Fig. 5.98). Relations between these permeabieand adjacent nonpermeable sediments vary both laterally and verticallv. The positions of the fluvial channels in the various formations tend to be superimposed and stacked in the form ot mega-channels. Growth faults cut the multi-stratigraphic sediment pile. thus interconneci more or less the various permeable sand units. These faults acted as pathways for oxydizing groundwaters and mierating reductants such as H2S and therebv controlled the deveiopment of reduced, pynt-beanng sandstones. and finally the uranium emplacement (Figs. 5.93, 5.99). Broad-scale sedimentary features such as mega-channei systems combined with the above-listed parameters control the distnbution and configuration of major uranium deposits and ore trends. For exampie. most of the deposits occur in mesa-channelsvstems.Ore
trends are dip-onentated in these systemsas, for example, in the South Duval County Mineral Trend, and deposits are developed in successivelyyounger formations in a downdip direction. The.strike-oriented rollfronts in the beach sands of the Jackson Group reflect a different broad-scale sedimentary feature. They do not occur in continental fluvial channel sandsbut rather in littoral sands.
M inera li zatio n an d .4 I terario n -
Principai ore minerals are pitchblendelsooty pitchblende and coffi,niteassociatedwith mainly Fe-suifidesand minor Mo. Se and V minerals. - The major south Texas uranium depositsoccur in two pnncipal configurations related to two different geochemical environments: - At the redox boundar-v on the downdip margin of tongues of oxidized sandstone in the form of ciassicalrollfront deposits.e.g., in the Jackson Group and the Catahoula Formation. - Entirely within reduced sandstone as nonrollfront associated deposits. e.g., in the Catahoula. Oakville and Goliad Formations. - Rollfront deposits exhibit in cross-section in many cases the t-vpical C-shaped form and element zoning (Fig. 5.85) that reflects the
316
5 Selected E:
Non-rollfront deposits occur entirely within pyrite-bearingsandstone.Some of these deposits display almost the classic crescentic shapesuch as many of the Oakville deposits, €.9., Felder, Clay West, Rhode Ranch, etc. They are classicrollfronts in shape but are enveloped in later stages of reduction (Figs.5.94,5.95b,c). Other deposits,for example the Holiday-El Mesquitedeposit in the South Duval County Mineral Trend, are elongateparallelto the axis of groundwatermovement and occur at the lateral boundary between oxidized sandstone and H2S-reducedsandstone (Figs. 5.96). The orientarion of the deposit parallel ro. rather than perpendicularto. the direction of groundwaterflow suggeststhat large volumes of uraniferouswater flowed tangentiallypast reducedsandstonerather than directly through the rollfront leadingto the depositionof considerableuraniumwithin and alongthe contact with pyrite-rich sandstone.The reduced and subsequentlyoxidized sands in proximity to these deposits do not contain either Fe-Ti oxidesor their oxidationproducts. Most depositsare of low grade (+0.t"t" UgOs) and low tonnage(<5000mt UrOg).
direction of groundwater flow and the propagationof the redox front. The oxidizedsands show the typical mineralogic effects of exposureto oxidizingsolutions,in particularthe oxidationof pyrite (Fig. 5.93).At other places theseore bodiesdo not displaythe crescentic roll shape, and this seemsto be due to the presenceof interbeddedargillaceousmaterial which disrupts the groundwater flow and prevents the developmentof the classicaldeposit shape.
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Models on ore formation have been presented by a number of geoscientistsincluding Eargle, Galloway, Goldhaber, Revnolds, and their coworkers. Adams and Smith (1981) provide a concise concept in which the Texas uranium province appears to be unique among welldescribedrollfront regions becausethe deposits are developedwithin a complex heterogeneow sedimentologicsequenceranging from littoralmarine to fluvial-continentalprovenance. The geometrvof the sand bodies and the resulting groundwaterregimesare vastlvmore complicated
Ora!c6t
F.ig. 5,99. South Texas, diagrammatic sections illustrating the sequenceof geological events involved in the genesis of roll-type uranium deposits related to fault-derived H2S.The ingressof H2S probably preceded and followed the formation of the rollfront but was not svnchronous. (Adams and Smith 1981)
Examplesof Sandstone-TypeUranium Deposits
317
than those of the braided ffuvial systems,such as in the Wyoming Basins. The distribution and characteristicsof the uranium deposits depend, therefore, on the local mode of sedimentation, .rnd the relations between the sediments of the iiverse depositional environments,in addition to the normal geochemical and physico-chemical conditions required for uranium transport and precipitation. Adams and Smith (1981) summarize the sequence of metallogeneticallyessential processes which led to the formation of the south Texas uranium ore bodies as follows (Fig. 5.99):
The roll-rype ore bodies in the carbonaceousrich beach sands of the Jackson Group are the examplesof depositsof this type. Their uranium was probably infiltrated dip-oriented along fluvial channels which extended from the Catahoula unconformity down to the Jackson beach sand environment. With minor exceptionsrelated to rhe nature of the sediments,the depositsin the JacksonGroup, therefore, are similar in terms of ore-forming processesto some of the classical rollfront districts, particularly in the Black Hiils. WyomingSouth Dakota, and Weid County, Colorado.
Sedimentation
Formation of TexasRoll-Type Uranium Deposiu
1. The ore-hosting Terriary sediments of south Texas were deposited in highly variable continental fluvial to marginal marine environments. As a resuit, the stratigraphic l-ithologic situation and the related hydrologic regimes are rather variable and complex. The transmissivity and hydrology of the sediments can range from simple in the mega-channeis to more contorted and complicated in crevassesplays and barrier bars. 2. Early oxidation affected the mega-channels either during the sedimentation or during early diagenesis. This inferred earlv oxidation does not seem to have been related to. and therefore is probably not essentiai for the formation of the uranium deposits.
5. With continued sedimentation from the Catahoulainto Oakviile time, many of the sands subsequently invoived in ore formation were buned. Connate waters derived from the compaction of adjacent and overlying shales probably moved into these permeable sand systems. This was the period of greatest uranium availability, and it is likely that all important deposits in the Catahoula and Oakville formations gained their initial uranium endowment during this very early diagenetic period. 6. Eariy after the deposition of the sediments, hydrogen sulfidelhydrocarbon was introduced locally along structures into the sands of the Catahoula and Oakville formations and caused the crystallization of finely dispersed authigenic pyrite in the potential ore sands. The H2S apFormation of Wyoming-Type Rollfront Uranium parently emanated into the sands along growth Deposits faults (Fig. 5.93) that formed contemporaneously 3. Uranium mobilizadon occurred in an early with and intermittently after sedimentation. stage from interbedded uraniferous tuffaceous Reduction of the sands may have occurred locally lavers within the Catahoula and adjacent Jackson even during sedimentationwhere the H2Sreached Group sequences.This can be deduced from the the surfaceand permeated the sandstoneadjacent findings of Gallowav et al. (1979). rvhich indicate to the structures. The local presen'ation of caran eifective release of uranium directly associated bonaceous trash and highly sulfidic sediments with pedogenic processes.Hence. the uranrum tends to confirm this. The source of some of the reiease must have occurred immediately after the H2S is believed to have been. on the basis of geological relations and sulfur isotope data, the sedimentation of the respectiveformation. .1. The uranium was transported by groundCretaceousEdwards Formation (Goidhaber et al. w a t e r s , f o r e x a m p l e ( F i g . - i . 1 0 0 ) , f r o m t h e 1979), whereas other sulfur sources are in the Catahoula source down into the underlying Tertiary sediments. Reynolds and Goldhaber's permeable horizons. Where the penetrated (1983) investigations suggest that H2S introhorizons contained reductants. e.g., in the form duction was predominantly pre- and post-ore and of plant debris. the oxygenated uranium-bearing apparently only limited during the ore formation. 7. In consequence of the above described waters lost their oxidation potential. As a result, redox fronts developed along which uranium and lithologic and physico-chemical circumstances, rollfrons within the Catahoula and Oakville other elements were precipitated.
318
5 Seleoed Examples of Economically Sipificart Types of Uranium Deposits o
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Uranium Deposits Examplesof Sandstone-Tvpe
319
formations verv probably formed by the intro- 1957;Eargle and Weeks 1961,1973; Fisher et al.1970; duction of oxygenateduraniferouswaters into the Galloway 1979, 1982, 1985; Calloway and Kaiser 1979, 1980; Galloway et a1.1979a,1979b. 1982'.Garner et al. sulfidized, pyrite-bearing sandstones. The ore- 1979;Gotdhaberand Re1'nolds1979;Goldhaber et al. forming process was similar to the formation of 1978. 19'19;Harshman 1971; Henry et al. 1982; Hoel :oll-type deposits elsewhere,except that in the 1982rHuang 1978; Johnston 1977; Klohn and Pickens ,.irtual absenceof carbonaceousmatter in the ore- 19?0;Ludwig et al. i982; \lcBride et al. 1968;McKnight 1972; Quick et al. 1977: Renick 1926: Reynolds and bearing sands, H2S-derivedpyrite was essentially Goldhaber1978,1979:Re1'noldset al.1980.1982;Ricoy rhe only reductant available to establish the and Brown 1977; Smith 1977, 1979 Smith R.B., pers. commun.; Walton et al. 1981; Weeks and Eargle 1960, oxidation-reduction boundarv. 1963:Wilbert and Temolain 1978
.Vodification Events 3. Subsequent to the ore formation in some of :he sandstones. several phases of re-reduction and partial re-oxidation (Figs. 5.93. 5.94, 5.100) affected the ore-hosting sediments. Renewed introduction, probably of H2S, resuited in a rereduction of portions of the altered sandstone tongues and arrested the ore formation and the propagation of the rollfront. Oxidizing groundwaters may have again moved down the sand horizons to form a second rollfront at a new updip redox boundary. But they may also have proceeded to and joined the original front or may have gone beyond the first front, leaving only relict or ghost deposits with the main ore formed as a downdip rollfront deposit. The progress of the younger front, however, might also have been interrupted by the interference of renewed introduction of H2S. 9. The formation of uranium deposits in vounger formarions, such as the Goliad, may be the results of continued movement of oxygenated groundwaters. particularly in the mega-channels. The oxidizing fluids probably destroyed some early mineralization in older formations and moved the uranium into younger formations. The apparent presence of both oxidized sandstones and uranium mineralization in younger formations, e.g., in the Goliad sands.rvell down some mega-channel systems suggeststhat the two are related.
5.4.6 Uranium Depositsin Proterozoic Sandstones:Franceville Basin, Gabon
The Franceville Basinis locatedabout500km SE of Libreviile,Gabon. All depositsfound to date clusterin two groups. 30km apan. aiong the southwesternedge of the basin adjacent to the Massif du Chaillu. The northwestern group includesall major deposits-Boyindzi, Mounana, Oklo, and Okelobondo; the southwesterngroup containsMikouloungouand Kaya-Kaya. Total reservesincludingproductionamountto ca. 42000mt U3Os (NEA/IAEA- 1986). The gradeis about0.2 to 0.4"/"U:Os. The uraniumdepositsof the FrancevilleBasin (type 4b, Chap. a) occurring as they do in unmetamorphosedsandstone,are similar in many respectsto the tabular and tectonic-lithologic uranium depositsof Phanerozoic sandstone-type ageexceptthat they occurin Proterozoicarenites and that, therefore, the organic material is supposedlyderivedfrom marine flora (e.g., algae) deposits,of land and not, as in the Phanerozoic plant origin. The depositsmay representa precursortype of uranium mineraiization which mav have been presentin the middle to upper Lower Proterozoic sedimentsin Austraiia and Canadaprior to their metamorphism.The depositsof Gabon may provide, therefore.a link in the evoiutionof subunconformity-epimetamorphict!?e uranium (type2, Chap. 4). deposits Referencesand Further Reading for Chapter 5.4.5 descriptionis largelybasedon The subsequent (for detailsof publicationsseeBibliography) the papersby Chauvet(1975),Diouly-Ossoand Adams S.S. pen. commun.iAdams and Smith 1981; Chauvet(1979)and Pffielmann (1975)amended 1986; Bonner by dataof the other authorslisted. Baker1979; Boenigi970;Bombereta1.1980. et ai.l982;Bowmanet a1.1981: Brewton1970;Bunker and MacKallor 1973; Buscheet ai.198l: Craig 1980; Dickinson 1976a.1976b,1976c:Dickinson and Duval 1977;Dickinsonand Sullivan1976:Duex 1971;Eargle 1968,1972;Eargle et a1.1971. 1975:Eargle and Snider
320
5 Selected Examples of Economically Significant Types of Uranium Deposits
a
F r o n c e v i l l eB o s i n Legend for figures
X Boy indzi
5 . 1 0 15-. 1 0 5
XX Okto I O k e t o-
I
bondo
F Mn-l FB1 Monqonese l==-=:l beoringhorizon I -
m
FA
C o n g l o m e r o i i cs o n d s t o n e
FA
Sondstone
F=
r
FTi x l lx
C r y s t o l l i n eb o s e m e n t( M o u n o n oh o r s t / C h o i l l uM o s s i f )
l".l
Doterite
t----l
F B 1 S h o l e , u p e l i t eb.l o c k s h o l e
l-
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I
ffi
U d e p o s i t .t e c t o - l i t h o l o g i ct Y P e U ore
Fig. 5.f0f . Franceville Basin Mounana-Oklo area. a Simplified geological map dispiaying the position of the sandstone hsslgd ulrnium deposits of (from N to S) Boyindzi, Mounana. Oklo and Okelobondo. The first two are tectoniclitbologically controlied while the laner two exhibit a tabular stratabound nature. b Simplified geological W-E crosssection illustrating diagrammatically the localization of the different deposits. (After Chauvet 1975)
GeologicalSetting of Mineralization The Franceville Basin was formed as an inrracratonic structural depression during the late Lower to early Middle Proterozoic.It is surrounded by Archean-Lower Proterozoiccrystalline complexes.The contact betweensediments and crvstallinehorstsis commonlyfaulted. The crystalline basement consists predominantiy of granites and gneisses,which were dated about 2600m.y. in the Massifdu Chaillu. The age of the Francevillesedimentsis dated at 1740+ 20m.y. (Weber and Bonhomme1975), brif U/Pb datesfor the Oklo ore yield asesof up to 2000m.y. The sediments transgressedthe crystalline basement in fwo major psammitepelitesedimentary cycles. T);.elower cycle comprisesthe FA Formation and the overlying FB 1 Formation. The FA consistsof fluvial and fluvio-deltaic (arkosic)sandstones. Its thicknessis about 50m
alongthe edgeof the basinandincreasesto 1000m towards the center. The basal part consistsof quartz pebble conglomerates and coarse-grained arkosic (microcline) sandstones,which grade progressivelyupwards into medium- to coarsegrainedsandstones with dolomiticmatrix. They in turn are succeededby a succession of alternating fine-, medium-, and coarse-grained sandstones, with occasionalintercalationsof conglomeratic lenses.The matrix of theseyounger. feldsparpoor sandstonesconsistsof silica or organic material.The individual sandstoneiayers exhibit cross-bedding and rapid lateralfacieschange.All uraniumdepositsoccurin this upper sandstoneof the FA Formation. The FA Formationis transgressed by the FB / Formation which consistsof up to 600m thick pelitic sedimentscontainingorganicmaterial. The upper cycleis composedof the FB 2a to FE formations consistingof psammitesand organic bearing pelites intercalatedwith minor cherty beds,dolomites.and tuffs.
of Sandstone-Tvpe UraniumDeposits Examples
321
Deformation resulted in slight folding along Mineralization, Ore Controls and Recognition trendingN-S and Criteria )IW-SE axes.and fault systems NW-SE. Along the latter are the NE-blocks eenerallydownfaultedrelativeto the SW-blocks. In the reduced ores, the principal minerals -\ third set of faults trendsE to NE. Dolerite are pitchblendeand cotfinite. In the area of the and545to 530m.y. natural reactor at Oklo, pitchblende is recrystalJrkesdatedca.870,730to715 (Weber and Bonhomme1975)follow this E ro lizedto uraninite. Associated and gangue minerals NE system. include karelianite, montroseite, roscoelite and The northwestern group of deposits which minor pyrite, galena. sphalente, chalcopyrite, containsall major ore bodiesis immediatelyad- marcasite, melnikovite, baryte, and calcite as jacent to the granite-gneiss horsrof Mounana,a fissure fillings. ln the oxidized ores, the principal tectonicoutlierof the ChailluMassif.The eastern mineralsare francevillite,vanuralite,uranocircite, edge of the horst comprisesa zone of predo- autunite, torbernite and renardite. Associated ::inantly N-S-trending70 to 80'E dipping frac- minerals include vanadinite. chervetite and :ures. They separatethe granitesand gneisses brackebuschite. All uranium deposits display a lithological lrom the FA and FB 1 sediments.The faults produceddisplacements control and most show some structural control. of up to severaihundred various The development Based on local geologic features, Pfiffelmann meters. of the segments causedtilting of the sedimentarystrata towards (I97 5) identified three rypesof deposis: stratiform the east,forming a monoclinelikestructurewhich (e.g., Oklo), veinlike (e.g., Mounana, Boyindzi), dips45'E (Fig. 5.101). and mixed stratiform-tectonic deposits (".g., Mikouloungou). Characteristicsof mineralization and geologic setting of these deposits are briefly reviewed below based on descriptions particularly by Chauvet (1975), Diouly-Osso and Chauvet (1979), and Pfiffetmann (1975).
0
50m
F B c h o n n c l Sqndslone
[Ffl
- b c o r i n g s o n d s t o n cb e d" C 1 " uronium nucleor reocfort
Fig.5.f02. Oklo deposit. schematicblock diagram showing the folded natureof the U mineralizedCl horizon and the location of natural reacrorswhich were active between 2000 and 1700m.y.aso. (After Gauthier-Lafayeet al. 1980)
5 Selected Examples of Economically Significant Types of Uranium Deposits
322 a
clev. m 4 5 0-
w thote btock shote 90nd
-T
l
I
cD IL
II
r 5 0-
0
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Fig. 5.f03. Oklo deposit. a Geological-lithologic W-E section. b Lithologic details of the mineralized Cl horizon. I FB 1 shale;2-4 Cl; 2 sandstone:J intercalatedconglomeratel4 intercalatedfine-grainedsandstone;5 footwall fine-grained sandstone;6 foorwall conglomerate.(After Chauvet 1975: Diouly-Osso and Chauvet 1979)
The natural reactors of OkIo (Bover et al. Oklo (resewesincludingproductionca. 18000mt 7975)are a uniquephenomenonamong uranium U:Or, gradeca. 0.5%U?O8) Mineralizatiorzis concentratedin the upper 5 to depositsof the world. They are characterizedby a 10m of the FA Formation which is subdivided depletionof rsU from 0.7?"hto between0.62% into two units. The lower n + 3 unit (<30m thick) znd 0.296%.Six reactorshave been found which containsthe weakly and irregularly minerali'sd occupieda total areaof 3500to 4000m2 (i.5% of Q. horuon. The overlying n * 4 unit (4 to 10m the total mineralizedarea). Individual reactors thick) containsthe main ore horizon (C1) (Figs. extend laterally from a few meters up to 20m. of up to 1m. At each locality, 5.1.02,5.103). Both unitsgradefrom fine-grained, at ore thicknesses in part pelitic, sandstoneupwards into coarse- uranium was enrichedto more than 50 times the grained sandstonesand conglomerates.Green- normal concentrationwith extremevalues up to and biack-bandedpelitesof the FB 1 Formation 70y"U3O8. The reactorsare located at places where the overlyingpeiitesare cut by sand-filled rest slightly unconformableon the n * 4 rrni1. (Fig. 5.102).About 800t of natural (Ci) consistsof a series paleochannels The main ore horizon of lenses of cross-beddedfluvial sandstones uranium was destroyedby the reactors. Alterawith a conglomeraticinterbed. The Cl was de- tion mineralsgeneratedby the reactor processes posited immediately above the upper con- are presentedin Table5.30.Ages of ore from the glomerateand a fine-grainedpel-iticbed of the nuclearreactionzonesare dated at 2000m.y. by unit n * 3. Uranium is concentratedpredo- Devillerset al. (1975)and 1700to 1900m.y.by minantly in sandy zonesin the form of micro- Cowan et al. (1975),which predatesthe isotopic crvstalline pitchblende within organic matrix. age of l740m.y. of the host sediments. The Mineralizationappearsto avoid pelitic horizons. period of criticality lastedat least 200000 years. In contrastto Mounana(seebeiow),no vanadium At the time the reactors were active, 23sg is associatedwith uranium at Oklo. The ore amounted to almost 3% of the total uranium horizonplungesbetween15 and 60'E and flanens (comparedwith 0.7% toda,v). at depth.Maximumdepthis 300m.N-S lengthof The Oklo mineralizationshoq'sa strong lithothe deposit is 900m and maximum E-W width logic ore control. Within the fluvial-deltaicsand600m. stones of the Cl zone. uranium seems to be ':_ Table 5.30. Oklo deposit, alteration producs of the natural reactor zone (Gauthier-Lafayeet al. 1980) Sandstone
Clay of transition zone
Chlorite fringe
Clay of reactorzone
Dominant clay minerals
Illite 1M
Illite 2 M
Mg-chlorite
Iliite 1Md + chlorite
Other constituents
-
Reactor zonefacies
Qu3rtz Heaw minerals
Pitchblende
Hematite. leucoxene Uraninite
Uranium Deposits Examplesof Sandstone-Type
)!)
bsw etev m Jf,U _
X 'Y .
x X
50m
Fig. 5.f04. Mounana deposit.a Geologicalplan map for the 350m level. b GeologicalSW-NE section(gris Mounana = lfounana sandstone:FN. F/ etc. denote faults). (Legendsee Fig. 5.101) (After Pfiffelmann 1975)
concentrated on the flanks or rims of paleochannelsor where intermediatethicknesses of sandstoneare combinedwith high concentrations of organics and moderate interbedding. The mineralizationdecreases in zonesof thick sandstone and/or low concentrations of organicmaterial. There also seemsto be somerelationshipto the dip. Best ore occurs in sedimentsdipping about 30". A regionalore control may be related to the western boundingfault of the Franceville Basin and the crystallineMassif du Chaillu less than 1000mto the westof the deposit.
The ore control is structural as well as lithological. Mineralizationis restricted to the FA sandstoneswhere organic material is abundant. An assumedcontrolis the apparentoccurrenceof the ore body on the westflank of a paleochannel that is scouredinto the upper FA Formation. Later tectonics,. however, have strongly overprinted this feature.Structuralcontrol is reflected by marginalfaultsand fracturesthat separateore from nonmineralizedrocks and by structures within the depositthat separaterich from poor mineraiization.
Mounana (reserves [depleted] 6800mt UrOa, grade0.58% U:Os) .Vineralizationoccursin the sameC1 horizonand regionalstructuraisettingas Oklo. Mounanalies immediatelyeast of the fault separatingthe crystalline wedgeof the Massifdu Chaiilu from the Francevillesediments(Fig.5.10a).Anotherfault zone bordersthe ore body to the east. dip steeply,possThe ore-hostinssandstones ibly due to drag on the faults and are heavily fractured. Quartz and microclineconstitute90% of the sandstone.The matnx containsorganic material and is silicified.The ore body wasabout 100m long, .10m wide, andextendedto a depthof 120m. In the oxidized upper 40m of the deposit. the ore consistedof uranyl vanadates.Primary mineralizationthen occurreddown to a depth of 120m.
Mikouloungou (estimated reserves: 6000 to 12000mtU3Os,grade:0.2 to 0.25% UeOs) Mikouloungou and the nearby Kaya-Kaya deposit are within the Franceville Basin, several kilometersdistantfrom a crystallinehorst. The Mikouloungoumineralizationextendsfor more than 1500m along an E-W-striking overthrust that dips 45'N and persistsfor more than 250m deep.N-dippingFB 1 pelitesin the upper plate of the thrust overlie horizontal to gently SW-dippingFA sandstonesin the lower plate (Fig.s.105). The FA Formation (>500m thick) is subdivided into four sequences of fine- and coarsegrained sandstones.Uranium is restricted to sequence2 whichis i75 m thick and comprises21 bedseachof alternatingfine- and coarse-grained, feldspathicsandstones. Near the overthrust,the host rocks are shatteredby micro-fractures,and
324
5 Selected Examples of Economically Significant Types of Uranium Deposits
S r
Referencesand Further Readingfor Chapter 5.4.6 (for detailsof publicationseeBibliography) Baud 1967; Bernazeaud 1959; Bonhomme et al. 1965; Bonhomme and Weber 1969; Bourrel and Pfiffelmann 1972; Boyer et al. 195; Branche et al. 1975; Chauvet 1975r Cortial 1985; Cowan et al. 1975: Devillers et al. 1975;Diouly-Ossoand Chauvet 1979;Donnot and Weber 1969; Drozd et al. 1974; Faure-Mercuret 1965; Gangloff 1970; Gauthier-LaFaye 1977; Gauthier-l,aFaye et al^ 1975, 1980; Geffroy 1975; Geffioy et al. 1964; Lecoq et al. 1958; Molina and Besombes 1975; Morin and Tardieu 1972;Naudet et al. 1975; Openshaw et al. 1978; Pfiffelmann 1975;Rouzaud 1979; Ruhlend and Gauthierl-afaye 1978;Weber and Bonhomme and 1975.
zone 4
Fig. 5.f05. Mikouloungou deposit, schematicN-S crosssection displaying the tectonic-lithologic control of the uranium minerelization. Uranium penetratestongue-like from the Mikouloungou Fault along coarser sandstone beds into the FA sandstoneunit. (Legendsee Fig. 5.101) (After Pfiffelmann 195)
5.5 Examplesof CollapseBreccia Pipe-TypeUranium Deposits(Type 5, Chap. 4): ArizonaStrip Area, USA
The uranium district is located in the Arizona Strip area, to the north and south of the Grand Canyon, northwestern Arizona. Nurnerous solution collapsebrecciapipes crop out on the feldspars are kaolinized. The sandstones zre plateaus and in the canyons. Some contain variably silicified.Sequence/ i5 evsllain by 75m economicCu, Pb, Zn, Ag, andior U mineralization. Minable deposits, classified as collapse of sequenc€1. of fine-grainedsandstones Mineralization occurs(a) along the footwall of breccia pipe-type uranium deposits (Type 5, the thrust zone independentof the lithology and Chap. 4), contain uranium reservesin a range (b) extending as tonguelike projections for 5 to from a few hundreds to about 2500mtUrOs. sand- Total reservesdiscoveredso far are in the order 20m from the thrust into the coarse-grained stone horizons. In plan, the ore tonguesshow of about 14000mtU3Oscontainedin at least 11 pipes (Fig. 5.106). Production grades are high irregular, amoeba-likeshapes. The ore @nsistsof a mixture of pitchblende averagingfrom 0.4 to 0.8% U3O6 and locally and clay mineralsaccompaniedby pyrite and, in more. Ag, Cu, and V are recoverablefrom some of the lodes. fissuresand joints, calcite. The ore controb are stmctural and lithologic. Mineralizationis sharplyseparatedfrom the adjacent pelitesby the overthrust.It occursonly on GeologicalSettingof Mineralization the footwall side of the fault and within coarsegrained kaoiinized sandstoneswhere joints with The regionalgeologicalsettingis the classicalone gouge partly control the mineralization. A of the Grand Canyonregionwith its well estabchangein alteration is observedwithin the host lished stratigraphy and evolution. (Billingsley rocks from a barren, fresh feldspathicsandstone 1978;Breedand Roat 1978;Four CornersGeol. to a sandstoneimpregnatedwith pitchbiendeand Soc. 1969;Karlstrom et aI., 1974).The rock sequencerangesin agefrom Precambrian(>2 b.y.) clay mineralscloseto the fault. A lithochemicalcontrol was possiblyprovided to Permian(approx.200m.y.)andlocallyTriassic. by the overthrustedpelites. Their organic ma- Theserocksare intrudedand cappedin placesbv terial may haveproduceda reducingenvironment Miocene to late Pleistocenevolcanics. Figure the stratigraphyand supportingthe precipitationof uranium.Silicifica- 5.107displaysschematically tion also appearsto have influencedthe distri- lithology of the variousformationsin the Grand bution of the uranium. Canyonregion.
Examplesof Collapse Breccia Pipe-Type Uranium Deposits I
Il-.ts{rl ll44" "
?t
! St. George
Poge
Konob
1l30
UTAH
?s
AR IZONA
325
U
-:_-]
U
z
v ts |npc
.7
I i i
a-'
\
Q
i--"3 .\.;3 l't-
.c' q o-. ^\ .\c
i
*'b-
3.< a-'*S
I
\*o
1,. Q. . ^Cott z\ c"
." r2 /
.
Jb"
'd
\o" o)
\o F
.d 1O \r \o
./ Fig. 5.f06. Arizona Strip area, inferred distribution and thicknessof MississippianRedwall to PermianQs66n1;roand Triassic Moenkopi and ChinlelShinarumpformations and location of principal uranium ore bearing breccra 2ipes. Arrows at isoliths indicate directionof increasingthickness.i"c-sChinle Fm/Shinarump:lrn Moenkopi Fm: Pc C-'t,onino Sandstone;P/r Hermit Fm; Ps SupaiFm/ Esplanade:ore bearingpipes1 Canyon;2 EZ-2: J Hack Canyon1,2 iz:td3; I Kanab North: 5 Pigeon; 6 Pinenut; 7 Union Pacific;8 Orphan Lode: 9 fuverview. (Karlstrom er al. 1974,McC;c and Gutschick 1969: Mallory et al. 7972and other unpublishedsources)
The breccia pipe hosting Mbsissippian to Lower Triassic stratigraphic units consist of aimost horizontallv bedded marine to marginal continentai limestones, mudstones.and sandy to siity sediments (McGee 1974). Major tectonic features of the Arizona Strip area include block-faulting rvith associated warping (e.g., Kaibab Uplift). N- to NNEtrending lineaments (e.g., Toroweap Fault), and NNW- and NE-stnking fault systems. Numerous collapse stmctures called "collapse or solution breccia pipes" or simply "breccia pipes" have been found in the Arizona Strip area in formations younger than the Mississippian. The pipes are of combined solution and tectonic origin and should not be confused with diatreme pipes generated by volcanicgaseousexplosion. A
pipe may crop out on the surface or may be rlind
(Fig.s.107). Roof collapse of caverns(paleokarst) .n the carbonate strata of the Mississippian Re'Jwail Limestone initiated the formation of a pipe. The generallv circular collapse transgressedchi;eneylike upwards into the overlying, essentiail-.flatbedded sediments, trom the Redwall Lime:tone as much as about 1200m into the Permian Kaibab Limestone, and locailv into the overlving Lower Tnassic Moenkopi Formation or Upper Taassic Chinle Formation. A breccia pipe consisc of a throat rangingin diameter trom about 15 to -00 m and more. Pipes stoped upward to surface ezhibit a surface expression in form of a collapse cone that mav have a diameter of up to 750m- The cone results from dissolution of gypsum anci other
326
5 Selected Examples of Economically Significant Types of Uranium Deposits Breccio pipe
lj TypeI
TypeII
Zoneof Mojor Uronium OreDistribution in BreccioPioes
Fig. 5.107. Arizona Strip area. diaprammatic stratigraphic section and position of collapse breccia pipes. Type 1 out-cropping, Type II blind. (After drawing by R.F. Holland unpubl. : stratigraphv after Breed and Roat 1976)
leachabie material of the Kaibab and Toroweap formations, causing a gradual thinning of the affected strata towards the pipe throat. In contrast, a pipe which does not outcrop has no solution cone. A breccia pipe is filled with clasts of highly variable size up to several meters in diameter which are embedded in matrix. Fragments and matrix are made up of downward displaced material of the stoped sediments.
Host Rock Alterations A variety of alterations affected either both or separately the pipe infill and the strata peripheral
to the pipe. Principal alterations inciude pyritization, dolomitization, calcitzation. silicification, desilicification,Mg-depletion (dedolomitization). gypsumianhydriteformation. and bieaching. It is not established.however, which of these processeswere related to mineralization and which developed during the pipe formation itself. Most alteration processestend to have been very mild although some produced striking color changes. Figure 5.108 displays the distribution of the various alteration features within and outside a pipe.
Examplesof CollapseBrecciaPipe-TypeUranium Deposits Allerolron Mrnerclr2ctron Ppe exle.nol
3trotigrophy Collopse cone ( . 7 5 0m ) +
-oroweop
1 Collopse cone/surfcce 1/presson c o n c e n t i c c o l l o p s e . 3 r e c c r o ,c . c ! o l e irociures,onomolous,oc^rte P s e u d o m o r ? n r co f t e r t / r j t e
Pipe throot {co.30m to '100m)
\\\ \\
/oibob Fm 30-i50 m
327
2 Kotbob Fm. - d r s s o l u t r o nc f e v o c c r : r e s c o r a o n o l e -removol of SO. Fe - thrn^rng -deioTmcf,on -sircto-5e{eciive (eatc:ian/ s u r r i d r 2 o t r o n4 r t h g . : : c t i o n c r I n C : e O S el O . r O r d p r 9 e
\
3 -oroweca Fm -os n Korcc: Fm
Fm
:5-250m
.'ref9vOl
a' ^'/drcCa'aa-S SurfidiZ n ::5cl -(cclrnizotrcn -CiS3em
: ara iletrc
tn atce. :mCSSlve SU'lide CCo | ^ ^r'l.r^^.r^^.'^. ll
SoconrnoSs . 1 2 0m
L aoconno Ss. m o t f l x d o m r n o t e d S r e c c r o - d r s c g g r e g o t r o nw r l h : i r ^ n r n 9 -feouction porticulcfil/ n upper clost domrnoted breccro ond lower portrons - l o c o l l y s u l fi d i z o t i o n -locolly U. or Pb.Zn.As onomotret prpe rn prpe coilopse - l o c c l l y s l r o n g h e m o r r t .i e o r u p p e r c o r , t -kootnrzotron onnulor frocture zone or olternotively i?) 5. Hermit Fm -bleochrng : en echelon frocture -locJl. Fe-removol DOUnOOTy -loccl.onomol U ot :ca
v-
1 1
tlermrt Fm 9 U - Z v Um
a Clina proe (?)
\
-----5-Fe3'-tre 2' -onomolies
l o p e r S u p o iF m e s p l o n o d eS s . 5 0 - i 2 0m
\,,
A D
-ower Supor Fr
zone oi reduction. bleoching of red beds loc .emovol of Fe
T=-
3 0 -2 0 0m
. ,. ledwoll Limestone :50-200m
6. Suoar.cm - b !e o c h r n g -loccl. Fe cnd COr -enovcr - i o c c l .o n o m o l u
a;\ L/
/."
4
Pipe internol olteration/mnerolizo(ton -sulfide coo - : i o t o l r e d u c i , / b l e o c h i n go f r e d f r c A m e n r s In well mrneroltzed gipes - c o l c r l r z o l r o nd o i o m I r z o t r o n s I l c t r c o l t o n - m r n e r o l i z o t t o ns e e i o b t e . . . . .
7 aedwotl !-s. -
Fig. 5.10E. Arizona Strip area. diagrammarrc section of an outcroppinq uranium ore-bearing breccia pipe with tvpes of alteration and mineralizationwhich mav or mav not be present.
328
5 Selected Examples of Economically Significant Types of Uranium Deposits
Principal Characteristics of Mineralization The ore mineralogy of the collapse breccia pipes is quite heterogenous.The most common uranium mineral is pirchblende, locally coffinite. A seriesof associatedmineralsbut not necessarily parageneticonesare presentin variableamounts (Table 5.31, Fig. 5.109).They inciudesulfides, arsenides, sulfoarsenides,locally oxides, carbonates,and sulfatesof Fe, Cu, Ni, Co, Mo, Pb, Zn, and Ag. Tracesof Au. Cd, Mo, Sb, Se, and V are also present. Gangue minerals which may or may not be related to mineralizationare calcite, dolomite, siderite, baryte, collophane. chalcedony,quartz, anhydrite, and gypsum. A biack glassybitumen is locally abundantbut only in somepipes. The mineral assemblagesvary from pipe to pipe. The Orphan l-ode and Hack pipes for example(Table 5.3i), display a large variety of
Tabh 5.31, Arizona Strip area. selected ore minerals of breccia pipes Mineral
Hack Pigeon Orphan Canyon
Arsenopynre Bornite Bravoite x Chalcocite x Chalcopyrite x Cinnabar Clausthalite Cobalrite Coffinite Covellite T Digenite Enargite (Luzonite) Galena x T Gersdorffite Hematite x Ilsemannite x Jordisite Marcasite x Millerite x Pitchblende x Proustite Pyrite Rammelsbergite Siegenite x Skutterudite Sphalerite x Stibnite Violarite Tennantite-tetrahedrite x
x x
? x x
x x x
x x x x x x
metallicmineralswhereasthe Pigeon pipe has a more simple mineralogy (RasmussenJ.D. and Gautier A., pers. commun.).In general,three principal paragenetic stages of mineralization may be distinguishedsucceededby a fourth, supergenestage(Fig. 5.109). Uranium forms irregularore shoos distributed intermittentlyover a vertical pipe interval of as much as 200m from the Coconino stratigraphic level downward. Ore grade mineraiization is generally concentratedin sections filled with not or partially cementedsand and silt. These particular lithologic sections are Iocated predominantlv(a) at and just belou' the CoconinoHermit contact in the stratigraphic *'all rock pile. (b) within the Hermit interval, and/or (c) immediatelybeiow the Hermit-Supai boundary (Fig.5.108). The pitchblendedistribution within theseintervals is in form of (a) veinletsand stringersboth within the pipe and in an annular nng closeiy surrounding the pipe, (b) pseudo-stratiform mineralizationin permeable sandstoneblocks, and predominantly(c) disseminationin irregular masses,disseminatedin mainly sandy-siltymatrix within the reducedbrecciatedinterior of the pipe. Sulfides occur disseminated in variable amountswithin the whole pipe. Particular concentrationsof sulfidecontainingup to 80% pyrite and marcasiteform a 3 to 15m thick sulfide cap above the uranium ore, at an elevation corresponding to the stratigraphic ToroweapCoconinocontact(Fig. 5.108). Uranium- lead isotopedatingof pitchblende from variouspipes yield four groups of apparent ages:ca.260m.y.(Ludwig,K., pers. commun.). ca.220to 200m.y.(Ludwiget al. 1986),ca. 184to 165m.y. (Krewedl and Carisey 1986) and ca. 141m.y.(minimumage,Miller and Kulp 1963).
StableIsotopesand Fluid Inclusions x x x x x
(Ref. Orphan [,ode: Gorniu and Kerr 1970;Kofford 1969; Hack, Pigeon, Canyon: Rasmussen J.D. and Gautier A.M., pers. commun.)
Sulfidesin brecciapipesgive 6vS valuesof -3 to -20%" and in nonmineralized Kaibab and Toroweapsedimentsof +12 to +74o/o(Adamek P., pers.commun.). Galenawith nonradiosenicleadhasz%PblzBPb ratiosof >19 and zoep6fioep6 ratios of >38 comparableto Mississippi Valleytype mineralization. Assumedsourceof the lead is ca. 1.8b.y. old (LudwigK.R.. pers.common.). basement
Examplesof CollapseBrecciaPipe-TypeUranium Deposits
Time
Stoge Colcite Dolomite 1 Boryte Siderite Koolinite
J29
I'--
2-7
Siegenite B r o v o iet Pyrite Millerite 2 Gersdorffite N i c c o l iet Rommelsbergite P o r o r o m m e i sebr g i t e A r s e n o p y r iet -Mqrcosite Pitchblende Q u o r zt E o r n iet Goleno C h o t c oy r i t e a Di u r l e i i e rJ ^: ulgenrre Cove{lite Tennontite Spholerite
---I
Fnnrniio
I___o____
_)_7
I
\_ --l-_ \-____, I
Hemotite 4 S u p e r g e n em i n e r o l s
I
Fig. 5.f09. Arizona Strip area, Orphan Lode, parageneticsequencefor minerals in the breccia pipes. (hatched vertical /rne indicatesfracturing of pyrite). (After Wenrich and Sutphin 1987;J.D. Rasmussenand A. Gautier pers. commun.)
Fluid inclusionstudiesfrom variouspipesyieid in summary the following homogenizationtemperatures and salinities. Gypsum, anhydrite. caicite, sphalente:54 to I25"C (average90"C) (Landaisin Krewedl and Carisey1986);calcite, doiomite, sphaierite: 80 ro ll3"C (salinities >9wt. '/" eq.NaCl with most common values >i8wt. % eq. NaCl),primaryinclusions of which in sphaleritemeasurefilling temperaturesof 80 to 100"C and secondaryinclusions 103 to 173"C (Wenrich and Sutphin 1989); sphaleritefrom Hack 1 and 2: 93 to 115"C(salinity9.9 to 16.4wt. 9/. eq. NaCl) (Rasmussen et al. 1986).
crystallineProterozoicbasementunderlying the Arizona Strip area, crystallineProterozoicrocks that were alreadyexposedin Triassictime in the MogollonHighlandat the southernmarginof the Colorado Plateau.fluvial channel sedimentsof the lower ChinleFormationof Triassicage.which coveredas a NW-SE belt part of the Arizona Strip area, fluvial sediments of the Jurassic MorrisonFormationdistributedto the E and NE of the Arizona Strip area. and altered volcanics forming the bentonitesin the Petrified Forest Memberof the Chinle Formation.
Ore Controlsand Recopition Criteria PotentialSourcesof Uranium Ore bearingbrecciapipes display the following No definite uranium source could be established ore controlsand recognitioncriteria (Figs.5.107, to date. Speculative uranium sources include the 5.108).
330
5 Selected Examples of Economically Significant Types of Uranium Deposits
Host Environment -
-
-
-
-
-
-
-
-
Breccia pipes occur in flat lying sediments extending over a s$atigraphic inten'al from the Mississippian Redwall Limestone through Pennsylvanian and Permian sediments into the Permian Kaibab Formation, locally into the Lower Triassic Moenkopi and rarell, into the Upper Triassic Chinle Formation. A breccia pipe is a vertical to steeply inclined sometimes slightly curved structure with a pipe or hourglas shape, circular to oval in planview, and as much as 1200 deep. Surface expression of pipes persistant up into the Moenkopi Formation, is a circular depression up to 750m in diameter. and as much as 100m deep filied by downdropped Moenkopi sediments. Strata above the Coconino Sandstonedip pipeward due to concentric removal of evaporitic and calcareous constituents in the Toroweao and Kaibab formations. Pipes are bordered by concentric fractures (annular ring fractures) (e.g., Orphan Lode) or by steeply outward inclined sets of fractures interconnected by flat inward dipping fractures (e.g., Pigeon). Intrapipe breccia consists of angular to rounded rock fragments ranging in size from a lgvv millimeters to several meters, embedded in a matrix derived both from the host strata and overlying formations. Clasts in the mineralized sectionsof most pipes consist of more or less silicified or carbonate cemented and reduced Coconino, Hermit. and Esplanade/Supai minor Toroweap, material (e.g.. Hack 7. 2, 3, EZ-Z, Orphan. Canyon). Those in the northern segment of the Arizona Strip, where the Coconino Sandstone pinches out. are chieffy derived from the Hermit, Toroweap and Fossil Mountain Member/Kaibab formations with verv minor Coconino Sandstone (Pigeon, Kanab North). Mineralized matrix ranges from reduced Coconino or Espianade derived sand and Hermit or Toroweap derived silt/sand, loosely to strongJvcemented by clay, sericite, calcite. dolomite, gypsum and/or anhydrite. Altered hvdrocarbons (pyrobitumen etc.) occur in all ore-bearing pipes, although in highly variable quantities. The highest concentrations commonlv are found at the lower
-
Toroweap to upper Hermit stratigraphic invervals. Pipe infill may be a structurally simple though heterogenous assemblage of downfaulted material or it may display a more complex structural zoning of material repeatedly displaced by selected internal subsidence forming "pipe in pipes" (Fig. 5.108).
Aheration Alteration features include bleaching (common at all pipes). pyntization (common). dolomitization (common but variable intense), caicitization (variable), gypsum/anhydrite formation (variable. locally none), siiicification (minor), desilicification (common), Mg-depietion (dedolomitzation) (locally in ore zones).
Mineralization - Principal uranium minerals are pitchblende and minor coffinite. - Associated minerals include sulfides, arseDides,sulfoarsenides, Iocallyoxidesand sulfates of Fe, Cu, Ni, Co, Mo, Pb, and locallyAg, and tracesof Au, As, Sb and V. - Principal gangueminerals are Ca-, Mg-, Fecarbonates,Ba- and Ca-sulfates,phosphates and Si-oxides. - Distribution and quantity of above elementsi mineralsis highly variable from pipe to pipe (Table5.31). - An almost characteristicphenomenonin ore bearingpipes is a massivesulfide (pyrite) cap locatedat and below the Toroweap-Coconlno formationalcontact. - Uranium ore persistsintermittentlvin irregular ore bodiesovera verticalintervalof asmuchas 200m from the Coconinostratigraphiclevel downwardinto the middleSupailevel. - Ore grade mineralizationoccursin lithoiogic zones of favorable permeabilit.v,that are a function of pipe fill material and structure. Favorablehosts are loosely packed matrix material of dominantly sandv composition. porous sandy or vuggy siltv-sand1',rarelv calcareousclasts.fractures.joints or fissures within the pipe brecciaor along the annular ring. Near such structures, uranium may impregnatetongue-likeinto permeablewall rock.
Examplesof CollapseBrecciaPipe-TypeUranium Deposits
lv{ost favorable host for disseminated mineralization is non- or partially cemented, medium- to fine-grained sand derived from the Coconino Sandstone,the EsplanadeSandstone, and, to a minor degree, sand-siltstone from the Hermit and lower Toroweap formations. In the latter case, however, mineralization is more fracture-hosted than in purer sands. Uranium ore concentrationswithin the pipe are particularly at and below the stratigraphic Coconino-Hermit contact (e.g.. Hack, Kanab North. EZ-2. and Canyon), within the Hermit interval (Pigeon. Kanab North, Pinenut, Hack, EZ-2, Orphan), at and below the HermitEsplanade/Supai contact (Orphan, Pigeon, Kanab North) (Fig. 5.108).
Metallogenetic Concepts A collapse breccia pipe containing economic uranium ore bodies is a unique feature generated by the superimposition of multiple geologic and metallotectic processes. Critical parameters required for these processesand hence constrain any genetic modeling include -
a thick sequence of flat-lying sediments which include interbedded arenaceousunits, - presence of a basal limestone which underwent extensive karst development to form large caverns. - coilapse of the caverns associated with discriminative stoping through the superjacent sediments to create a chimney-like breccra pipe. Certain structural systemssuch as sets of joints or shearsare very probably required for the karst and pipe development, - arenaceous lithologies providing a porous and transmissive pipe infill in volume and physicochemicalpropertiesadeqateto host ore bodies. - presence of chemical constituents in the svstem to supply the essential elements for the creation of a reducing environment, like H2S, SO2, Fe-*, hydrocarbons, etc., - transmissive svstems to permit mineralizing and other chemical fluids needed for ore formation to migrate to a pipe and within a pipe to optimum sites for ore emplacement, - a uranium source capable of supplying the quantities required for economic ore grades and reserves.
-
-
331
distinct epeirogenic. geohydrologic, and climate-related conditions to generate the necessaryprocessesfor leaching, transport, and concentrationof the elements invoived in ore formation. a long-lastingcratonic stability that continued until the present and prevented the deposits from destruction.
Impact of Lithologic-Stratigraphic Units on Formation of Breccia Pipes and Mineralization The individual stratigraphic units of the Arizona Strip area may have contributed in a positive or negative wav to the above criteria. and as such delineatedan area favorable for ore formation as follows(Fig. 5.106): Redwall Limestone: - Sufficient thickness of carbonatic unit is required for formation of karst caverns large enough to initiate stoping of pipes of adequate size: estimated minimum thickness of karst unit in Redwall: ca. 25m, which is present throughout Arizona Strip area; - zones affected by strong karst development [according to Wenrich and Sutphin (1989), controlled by pre-Supai fracture sets oriented
NW-SEand NE-SWI; - Redwall sedimentscontain Pb, Zn and other metal mineralizations.and may have provided theseelementsto the pipe mineralization. Upper Supai Formation/EsplanadeSandstone: - Arenaceousred bedstratawith minor siitstonemudstonebeds mav provide both favorable pipe infilling, and host rocks for annular ring ore; - red beds constitutea potential source of Fe requiredfor Fe-suifideformation in the pipe; - distributionof favorablelithology:to the E of a line trending about NNW-SSE on the N side of Grand Canyon and running somewhere between Kanab Creek and the Hurricane Fault: S of Grand Canyon the line turns into SW direction along abour Peach Springs Canyon.Westwardsof this line. the Esplanade gradesinto the marine PakoonLimestone. Hermit Formarion: - The dominantly ferrugenous siltstones are commonly unfavorable hosts for disseminated
5 Selected plamples of Economically Significant Types of Uranium Deposits
eastward.may have been the source for ore mineral2ation unless they become more hostingsandand silt pipe infill (e.g., in Pigeon sandy,which appearsto be the casein someof and Kanab North), the northern segments of the Arizona Strip evaporitic gypsiferousbeds are a potential area, or where they are jointed and fiactured. sourcefor sulfur (all sulfidecapsin pipesare at The annular ring and other pipe-bounding the lower Toroweapinterval), faults within the Hermit interval are good ore hydrocarbonsare abundantin the two lower hosts, - where too thick and too fine-grained.the silts Toroweap membersand are consideredcanparpipes, didates for reducing agentsin breccia pipes; may more or less seal breccia also fwo oilfields in southwesternUtah proticularly those with small diameters,hence duce from the Kaibab Formation. hamperingcirculationof mineralizingfluids, - the unit representsa potential sourceof Fe, distribution:both formationsoccurthroughout - distribution: the Hermit thickensfrom an 0 the Arizona Strip area, with typical marine isopacb near Flagstaff to the N and W with a sedimentsto the W and somelittoral influence thicknessof 120m at the south rim of Grand to the E. Canyon,ca. 200m at PigeonandEZ-Zand ca. 300m at Andrus Canvon. Moenkopi Formation: - The pelitic to semi-psammitic,partly calcareous sediments, 100 to 500m thick from Coconino Sandstone: - The fine- and localll,medium-grainedquartzthe E to the W of the Arizona Strip area, probably formed a rather impermeablecover ose sandstoneprovides in many pipes the over most of the Permianand older rocks of most important host for disseminated the Arizona Strip, particularly in its western minerelization, - minimum thickness for providing sufficient section, hampering downward migration of solutions; host material for ore-bearing pipe fill is esti- distribution: originally (no*' largely eroded) matedto be 5 to 10m, - zonesof limited consolidationat time of pipe throughoutthe Arizona Strip area, thickening from about 120m nearMarble Canyonin the E formation would have permitted the flow of larger amounts of sands into the pipe (inand Red Butte in the SE to 300-350m in the Kanab area to the N and in excessof 500m dicated sometimes by concentric thinning near Grand Wash Cliffs to the W. around pipes), - noncemented zones could have served as conduit for rnineralizing fluids, Chinle Formaion: - distribution:northern limit of the Coconinois - The channel-filling fluvial arenites of the along a more or less E-W-trending0 isopach Shinarump Member are host to minable running about 10 to 30km S of the Aruonauranium deposits(e.g., Cameron at the eastern Utah state line, turning SE on the Paria edge of the Arizona Strip area). The uranium Plateau.The Coconinothickenssouthwardto has formed coeval with the 220 to 200m.y. about 25m in the Hack Canvonarea,75m at uranium generation in breccia pipes. This the OrphanLode and 120to 150min the Red opensthe possibilitythat uranium of both hosts Butte area.The grain sizeapparentlydecreases derived from the same source and/or uranium in the westernpart of the Arizona Strip alonga has been transportedfrom the Chinle channels transitionzone which is more indicativefor a through structural pathways into the pipes, perhaps via a permeable intermediary conduit marginalmarine than continentalenvironment of deposition. like Coconino or Supai sandstones; Toroweapand Kaibab Formations: - Much of the solublematerialof the two formations has been leachedconcentricallyaround breccia pipes and pafts thereof certainly enteredthe pipe, - interbeddedarenaceousfacies,which become more abundant from about Kanab Creek
- distribution: originally (now largely eroded) in a NW-SE-oriented belt, 80 to 130km wide, coming from the Petrified Forest National Park area in the far SE through Cameron to KanabColorado City in the NW and further beyond. Flow direction was from SE to NW.
Examplesof CollapseBreccia Pipe-Type Uranium Deposits
333
commonly >19 wt."/" eq.NaCl (Wenrich and Sutphin1989). Sulfur isotopestudiesyield 6vS valuesof -3 Ore formationwas apparentlynot a singleevent but the resultof severalpulsesof mineralization to -20"/. (Adamek P., pers.commun.)a spread uraniumde,rs indicated by the parageneticinterrelation- similar to common sandstone-type ship of ore and gangueminerals.The time frame positson the ColoradoPlateaubut in contrastto is given by the following the narrow rangeof isotopicratiosin magmatic ior the metallogenesis criteria. The time of formation of the breccia hydrothermaldeposits. Nonradioactivelead isotope systematicsof pipes in the Arizona Strip area started in late galenas give ratios comparableto Mississippi (Billingsley 1986) and conMississippiantime Valley type mineralization(Ludwig K.R., pers. tinued intermittently into late TriassicChinle the commun.). time. Time constraintson the episodeof In summary,fluid inclusionand stable isooriginal mineraiizationare providedby the stratirraphic sequenceinvolvedin the collapseand tope data compareto someextentwith thoseof the radiometric pitchblende ages. The strati- MississippiValley type mineralization.hence graphicallybracketedperiod of ore formation is they supporta metallogenetic synthesis involving thereby comprisedwithin the late Permianand brinesdenvedfrom connateor supergenesources the Triassic. and that of.earliesturanium introduc- in ore formationbut reject a magmaticorigin of tion by the oldest host rock, viz. the Permian the mineraiizingfluids. A not magmaticorigin is Esplanade Sandstone. Consequently,the first also consistentwith the geologicalevolution of uranium introduction cannot be older than the the Grand Canyon region becausethere are no Esplanade what appears to be consistentwith igneous intrusions contemporaneouswith the the 260m.y. U-Pb pitchblendeage, and almost presumedtime of ore formation.Conduis for the certainly not younger than Triassic.Further on, uranium mineralizing fluids are speculatedto initial ore introduction into the pipes must have be either sandy horizons or structures.A conoccurredprior to the final cementationof the ore duit horizon favored by many geologistsis the hostingsand pipe frll. Coconino Sandstone.But any metallogenetic Mineralogical. isotope, and fluid inclusion model involving this lithologicallyfairly uniform data provide certain evidenceon the processes horizon of eolean clasticsfacesthe problem of involved in ore formation. TIte presenceof calcite explaining the discriminativeore formation in suggeststhat uranium was probablv transported different pipes that occur adjacentto eachother as a uranyl carbonate complex. Deposition of in the samearea and in the samefaciesof strata pitchblendeand other mineralsas well may have insteadof a mutuai ore emplacementin all these occurredwhen the mineralizingfluidsenteredthe pipes.A more selectivepathwayfor the solutions moreopenspacesin a solutionpipe.Effervescence could more easilyexplain this phenomenon,for of CO2 with associatedPressurereleasein breccia example,more restrictedconduitssuch as faults zones,fissures(annularring structuresetc.) and or cataclasticzones through which fluids could porous sandsare considereda significantfactor migrate from uranium mineralizedChinle sandin the break-upof uranyl compounds.Invading stone channelslocally into the Coconino horhydrocarbons or H2S derived from the pipe izon and then downdip along the unit into a surroundingsedimentsmay havebeenthe agent nearby brecciapipe if not directly into a pipe. for the required reductionof the Uo* ions but Other potential conduits includechannel-hosted sedimentsas found in the Esplanade dissolution of early sulfidessuch as pyrite may arenaceous have likewise created a reducingenvironment. Sandstoneor perhapssilty-sandychannelsin the A coeval oxidadon of some pwite to hematite ToroweapFormation. The sourceof the uranium is still enigmatic. associatedwith the pitchblendedepositioncould possibly explain the local coexistenceof these Consideringthe different agedeterminationsfor mineralsand may supportthe latter hypothesis. pitchblende,a speculatedsourcecan includeuraof Permian,Triassic,andeven Fluid inclusion studies suggestthat the ore niferoussandstones deposited which which Jurassic age. elsewhereon the Colorado forming solutions.at leastthose gave necessPlateau to but not rise extensiveuranium mining. dolomite calcite, and sphalerite from ranging These formations pitchblende are now largelyeroded in the temperanrres arily had >9 and always Grand but remnantstestify the of Canyon region to 170"C salinities about 80 and ProposedModelfor Ore Formadon
334
5 Selected Examples of Economically Significant Types of Uranium Deposits
former presence at least of the Triassic Chinle Formation. Last but not least the Proterozoic basementmay be considereda potential source either in the Mogollon Highlandexposedto the S and SE, or immediatelyunderlyingthe Arizona Strip area.
Referencesand Further Reading (for detaiisof publicationseeBibliography) Adamek P pers. commun.; Barrington and Kerr 1963: Baillieul and Zollinger 1982; Billingsley et al. 1986; Bowles 1965, 1977: Boyden 1978: Chenoweth 1986; Chenoweth and Malan 19691Energy Fuels Personnel. pers. commun.; Finch 1967; Gautier A pers. commun.; Gornitz 1969t Gornia and Kerr 1970;Granger and Raup 1962; Hoffman 197'7:Holiand R pers. commun.; Jensen et al. 1960; Kofford 1969; Krewedl and Carisel' 1986: Landais et al. 19871Ludwig et al. 1986: Magleb1 1961: Mathews 1978b; Mathisen IW, pers. commun.; Miller 7954a, 1954b1'Miller and Kulp 1963;O'Neill et al. 19811 Peirce et al. 1970; Rasmussenet al. 1986:RasmussenID pers. commun.; Scarborough1981; Sutphin1986; Wenrich 1985, 1986, 1989; Wenrich and Sutphin 1989; Watkins
r976
5.6 Examplesof Surficial-TypeUranium Deposits(Type6, Chap.4): Surficial Uranium DePositsin Duricrusted Sediments:Yilgarn Block, Australia The Archean Yilgarn Block in Western Australia hosts in its northern part surficiai-type uranium occurrences(Fig. 5.110). Almost all occurrences are of small size (few tens to <5000mt U3O6) and low grade (0.025 to 0.09% U:Oa) exceptYeelirrie which contains 52500mtUjOs at a grade of 0.1,5%U:Oa. Total known resourcesof the dist r i c t a r e 6 8 3 0 0 m t U - ? O 8( B a t t e - "e- t a l . 1 9 8 7 ) . Mineralization is confined to surficial duricrusted vallev and playa sediments. The occurrences are therefore classified as surficialtype uranium deposits in duricrusted sediments (subtype 6.1, Chap. 4). They are sometimesalso referred to ds calcrete U deposits. The following synopsis is essentially based on publications by the authors listed in secrion References unless otherwise stated.
Geological Setting of Mineralization Noteworthy surficial uranium occurrences are associated with nonpedogenic calcrete or dolocrete within intracratonic Tertiary to Quaternary drainage sysrcms. The shallow (dry) lakes and valleys are incised into Archean granites and greenstones, and filled with clastic material from the basement and evaporitic products. Nonpedogenic calcretes, also referred to as valley caicretes developed only north of the Menzies Line (roughli' parallel to latitude 29'S, Fig. 5.110) whereassouth of it pedogenic calcreteshave formed. Butt et al. (1977, 1984) refer to the Menzies Line as a boundary separatingdifferent surface related geochemical environments. North of the line neutral to acid soils prevail composed of red, noncaicareous earths, sands and lithosols, more or less compacted by extensive hardpans. Calcretes are common in vallev fills and locall-v in playas. The hydrochemistry of groundwaters is neutral to alkaline and less saiine, particularly in calcrete terrane. South of the Menzies Line, groundwaters are neutral to acid and tend to be saline. Soils are predominantly neutral to acid and are comprised of orange to pink earthy sands, noncalcareous earths and calcareous and saline loams with kankar development. Butt et al. (1984) note the importance of another morphologic-geochemical line, the Meckering Line. The line trends curvilinear in N-S direction and is the eastern boundary to all active streams dewatering to the Indian Ocean. No uranium occurrencesare found in the Yilgarn Biock west of this line. Host Rock Alterations Principal alteration of the host rocks is surface to subsurface hydrology related Ca-, Mg-, St-. and CO2-metasomatism leading selectively to the formation of calcrete, dolocrete, and other duricrust facies by gradual replacement of claysand material within the upper 10 m of the alluvial host sediments. Valley sedimentsconsist of varving amounts of qvartz and kaolinite. with minor illite, montmorillonite. and feldsoar. Calcrete and dolocrete formation resulted by the partial to complete replacement of kaolinite and quartz by carbonates (dolomite, calcite) and montmorillonite. Vugs within the calcrete may be filled with sepioiite or other Mg-clays.
Examplesof Surficial-TypeUranium Deposits l l60 I
0
1200 I
'ai
_)_.
335
,1lo
I
+
T
G.P
\ Meekothorr
/\-
Wrtu
-{
: o o_ - 2 8 "
08.
G e r o t dt o n
fV1
ntona droinoge
a *
with ployo l-
-l
Meckering Line
Fllf
u"n.ies Line
Fi
ContinentolDivide
I r
lMoior surficiol u occurrence
M e n z r e 5o
o -a
: z
a A
-52-
Perth
o o rD q
l9lMrnorsurllctol U occurrence
f,\l
2
5 0 0k m
Fouo.oble oreo for surficiol occurrences
Fig. 5.ff0. Yilgarn Block, distribution of surficial uranium occurrencesin relation to drainage systemsand geomorphological features. .A,llsignificantU occurrenceswithin the Yilgarn Block are restricted to the seclion nonh of the Menzies Line and east of the Meckering Line. Minor U occurrencesare found in the northerlv adjacent Gascoyne Province.U occurrences: 1 Yeelirrie:2Lake Way;J Hinkler-Centipede; J Lake Maitland:i Mount Joel: 6 Lake Mason: 7 Lake Austin; 8 Lake Raesidel 9 Minindi Creek. Geotectonicunits: B.B. Bangemall Basin; C.B. Carnarvon Basin; G.P. GascovneProvince;N.B. Nabberu Basin: O.^8.OfficerBasin: P.8. Penh Basin. Major nvers:,{.R. Ashbunon River; G.R. GascovneRiver; Gr.R. GreenoughRiver: M.R. Murchisonfuver. (After Butt et al. 198+)
Gypsum and celestite may occur locally in larger amounts. Gypsum was preferentiallv precipitated in the upper part of a channel profile and is concentrated particularly in overburden. Celestite is found mainly in the clav-sand unit both as crystals and as fine bands. The replacement processes were associated with an increase in rock volume provoking smallscale folds, fauits. ubiquitous slickensidesand, locally, diapiric structures or mounds bounded by faults. Karst development. another feature common to caicretes. was accompanied by caving and siumping (Mann and Horwitz 1979).
Host rock alteration in calcreted plaltas is in principle simiiar to that in valleys but often the caicrete development is not as well pronounced.
Principal Characteristics of Mineralization Pnncipal uranium mineral is carnorite. It occurs disseminated throughout earthv calcrete and in quartz-clay facies. fills small fissures and voids, replaces the argillaceous matrix of the host rock, and coats calcite, doiomite, silica and/or sepiolite, pore spaces in porcellaneous calcrete, and sand
336
5 Selected Examples of Economically Significant Tlpes of Uranium Deposits Gronitic source lerrone V o l l e y fiI
Pedogenic
b
I
++
+++
I
+++
I I
+
++ ?-
Volley fill
deposrt
+ ++
Flood oloin deoosit Flood ploin ++
/\
++++
,l
/\ Deltc ++
+++ +
Ployo deposit, low
:/ +++++++++++ +++++++++
grainsof clay-sandunits. Someuranium in concentrationsof up to 400ppm occurswithin thin veinletsof highly fluorescentopalinesilica. Two principal types of geomorphologiclithologic uranium distribution, valley-fill and playahostedmineral2ationprevailin the Yilgarn Block (Fig. 5.111).Butt et al. (1984)turnishthe following characteristics of the two types. Vallev-fill depositsare hosted by calcrete or dolocreteand associated underlyingsedimentsin the central channelsof major drainagesvstems Ifor exampleYeelirne(Fig.5.112),Hinkler Well] and in deltaic platforms transitionalwith these channelsat playamargins[e.g., Lake Way (Fig. 5.113),Lake Raeside, Centipede].
Fig.5.111. Schemeof distriburion of surfi cial uranium mineralizations located in valley-fiIl, flood piain. delta. and playa environments within a drainagesystem. lnterspersed pedogenic formations representing periods of quiescence occur, asindicated, throughout tbe sediments.(After Toens and Hambieton-Jones1984)
less than 2m thick. They frequently rest on sandy and silty clays that may contain calcareous nodules. Uraniferous playas :tre interconnected with valley calcrete drainages which themselves contain at least some uranium either in the main channelor the chemicaldelta (e.g., Lake Austin). The dimension of. uranium accumulations is often very large, in valleys up to some hundred meters wide, several kilometers iong and 1 to 5 m thick in average. Pla,vamineralization can extend over a few square kilometers at a thickness of <1 to 5 m. Grades are mostly low, in the order of some 100ppmU (Table 5.32). Ore grade pods (above ca. O.l'hU:Oa) are rare exhibiting a patchy distribution within the anomalous zone. Playa deposiu [e.g., Lake Maitland (Fig Best uranium concentrations formed along the 5.114),Lake Austin] are hostedby near surface water table in calcrete which constitutes the prin(a few meters deep), locally developed thin cipal aquifer. At several localities, however. calcrete horizons and/or alluvial and evaporitic as in the chemical delta type North Lake Way' composedof orangeto red and brown occurrence,uranium concentratedpredominantly sediments salineand gypsiferousclaysand mudscommoniy in subcalcrete clays and clay-quartz layers.
Examples of Sur6cial-TypeUranium Deposits
331
r Wituno
Iry l--
Alluvium. coiluvium Prolerozoic 'ocks
F i---il Archeon grcnire ond gneiss lTl F
Arcneon greenstone gq16rs1s
l-l
U .in"rolizotion
l-ll
Boundqry of YeelirrieChonnel Cotchment areo
-l
Bore hole
Fig. 5.112. Yilgarn Block, Yeelirrie area. generalized geological map showing semi-continuousdistribution of calcretelensesand position of the Yeelirrie depositwithin the Yeelirrie Channel and the catchment area of the channel. (After Cameron 1984)
Potential Sources of Uraniunr
The spatial relationshipof uranium occurrences with granitic rocks, anomalousin U content, strongly suggeststhat theserocks also servedas the source for U, K and probably V to form carnotite. Fresh granites commonly contain 3 to 8ppmU but range up to SOppmU (Gamble 1984).Sincethev are deeplyweathereddown to 250m, the uranium has becomeaccessibleto leaching. Vanadium is probably derived from mafic minerais in the granitic rocks or perhaps (Butt et al. 1984). from greenstones.
Ore Controlsand Recognition Criteria Criteria that mav have had a direct or indirect influenceon the metallosenesis include: Host Environment and Alteration - Hot arid climate with occasionalsummer-only rainfall. and evaporation (+3500mm/y) exceedingprecipitation(+200mmiy) by more than 15 to 1.
Former humid climate reflected by intense lateritic weatheringprofilesin granite extending about 250m deep(Oversby1975). Very stableancientblock. Mature morphologywith low relief. Drainage systemscomposedof linear depressions discharginginto closedbasinsless than 100mdeep. Verv low drainagegradient(ca. 0.001). Large catchmentarea in a terranedominated by deeply weathered granitic rocks, many appreciabiy anomalousin uranium (av. 810ppm with maxima of 8OppmU, Gamble 198.1) and greenstone belts considered a potentiaisourcefor vanadiumadditionalto the mafic mineraisin granite. Absenceof soilscapableof fixing liberatedU (pedogeniccalcrete.t'erricreteetc.). Depression frll of predominantlvfine-clastic (clay-silt) fluvial, alluvial and lacustrineevaporiticmaterial. Absenceof organicmaterialin soil and depression fills. Epigenetic replacementof near surfacesediments (within upper 10m) by nonpedogenic calcrete, dolocrete, gypcrete and silcrete causingvariableindurarion.
5 Selected Examples of Economically Significant Types of Uranium Depo,aits
338
v.F.c. l--_l Attu,rium JIII Ftuviotctostics Loke Oepostts
F--{
+
FEI cotcr"t" [i!ij
Minerotrzotion > l t r u p p mu
r! I a os.t N m 495
E
69U
t85
480
FEI l....E^
i---l
atluuiut
fT---:l l--l
l::: : :Juypsrlerous sond
b
Jl:: :l Sona
A 2.7km
os.l.. m
+-
B
Ppm
c
=-
135
_:-_=.-=-
Woter toble M i n e r o lo i zt i o n ' 2 5 0 p p mU
D
't550
8r5 525
5Ia
510
--I
l-I-
I-Ii
_.___
l=:
r:l
ppm
50
C o r b o n toe d sediment
Ppm
::; -
/.86
+
::.e
490
488
1 , 1k m
Colcrete uorcrere. cloyey
83( 80(
ff
t5(
3r0
I
u
r570
-:-=
73A
bU
Fig. 5.113. Yilgarn Block. Lake Wa1'.a Map of surficial geologyu'ith distribution of uranium mineralization in valler'fill-deltaenvironmenr. b Lithological N-S section through the uranium mineralized zone. c Lithological columns with distribution and grades of uranium (bulk samples)from three locationsshown in a. Calcrete rypes: V.F.C. vallel' fill calcrete, iight to dark gral'l C.D.C. chemical delta calcrete (r'ounger than V.F.C.). more massive,dolomitic. distinct brown .1 carbonatezone; ll transitionzone. (After French and Allen 1984)
E.xamplesof Surficial-TvpeUranium Deposits
o-.1
339
t:
L'o w
*[ ,
2km
u ppm 250-500
L e g e n df o r : r o l o g i cs e c t i o n( b l lEll
3 y p s u m : r y s r o l s ,s o n d c n d r e d b r o w n rrvr,,c. --r
2km
,-^c
5ilt
F
coi.r"r", ninor siit
*[ l l
D o m l n o r r l yd o r k t o b l o c k s i t t some pcre grey silt
l .l l
3 r o * n c n o b u ff l o k e s i i t
Fli -
Red brown loke mud ond silt wiih 20% sond
Fig. 5.f 14. Yilgam Block. Lake Maitland. a Map of distribution and grade of uranium mineralizarionin plava environment.b Lithological W-E section. c Distribution and _eradeof uranium mineralizaiion within the given lithological section. (Afrer Cavanay 1984)
Authigenic formation of silica hardpan cementlns permeable. reddish acidic soils devoid of calcrete/dolocrete in headwater areas and vailev flanks by opaline or chalcedonic siijca: distribution of this type of hardpan coincides with the zone of nonpedogenic calcrere (Carlisle 1984). Distribution of calcrete/dolocrete mainly in axial segmentsof large channelsbecomingless well developed at plava margins(deltas)where the -cretes occur in irregular thin layers (up to 2m thick). Thickest and most continuous calcrete/ dolocrete deveiopment in the vadose and rppermost phreatic section above a shallow groundwater rable in valleys. Distinct morphologies, structures, textures of calcreteidolocrete (fault bound mounds, collapse structures, karst-solution cavities) with large-scale downdrainage variations of evaporative authigenic minerals, e.g., increasing dolomitization.
Calcreteidolocrete being the main and excellent aquifer. Verv slow lateral movement of groundwater. Constrictions of channels by bedrock barriers resulting in a decreased flow rate and upward migration of groundwarer to rhe evaporative surtace. Similar effects as above by argillaceous horizons and hrpersaline waters in salt lakes. Hvdrochemistrv of vailev groundwater enn c h e d i n d i s s o l v e dU . V . C a . M g . K . N a . C O 3 , SOa, and Cl. with U largely present as uranyl carbonate complex ions. Disproponionate down-drainage increase of ionic species. pH and TDS (total dissolved solids) except CO.t-. by approximately an order of magnitude between catchment area and mineralized zones supposedly due to evaporitic enrichment. Between lower va-lleysand salt lakes further increaseof Ca. Na- K, Cl, SOr, and TDS by about another order of maenitude or more at
344
5 Selected Examples of EconomicaUy Sieoifcant Types of Uranium Deposits
Table 5.32. Yilgarn Block, summary of characteristicsand dimensions of surficial uranium occutreDces.(VF = valley fill, P = playa, D = delta/chemical delta, T = terrace type). (Data from Baney et al. 1987;Toens (ed.) (IAEA 1984) (for localities see Fig. 5.110) Type of Ininer:|i".
Geological sening of mineralization
Dimension m
Grade % U3O8
Resource mt U3Os
l. Yeelirrie
VF
Ca.lcretein Tertiary drainage, intense dolomitization and development of sepiolite ; mineral, parallel to channel
l:9000 w:750 th: 3-7
0.15
525m
2. Lake Way
VF
Calcrete in drainage channel entering Lake Way. in carbonadzed fluvial clastics and in chemical delta carbonatic sed.
4 areas th: fewcm to5m
0.09
33m
3. Hinkler WellCenripede
VF
Calcretein drainagechannel, partly eroded, composed of calcite, chaicedonywith minor dolomite, gypsum,sepioiite;chemicaldelta sediments:few bener grade podsin very low grade U zone
l:15000 w: <2 000 t h :1 - 5 (vervlow gradezone) podsin _ 1x 3kmr
0.0001 -0.2
?
4. Lake Maitland
P
Discontinuous caicrete rich in dolomite, sand, sandvclays.silt; ? zones
a) l: 6000 w: 300-500 th:1-2
a) <0.07 b) <0.06
a) 3 500 b) 500
\f(?) D(?)
Channel calcrete borderine salt lake system
l: >5200 w:550
??
6 . Iake Mason
\T(?)
Calcrete zone
l: 4 900 w: 250-750 th: <1
0.035
27m
7. Lake Austin
P
Calcrete (calcite, dolomite) with chalcedony in silici6ed zones. sandy clays, carbonate-rich, with kaolinite/smectite
l: I 500 w: 50 th:1-5
>0.045
?
8. Lake Raeside
D
Calcareousclay, clayey grit in lowlying peninsula
l: 5 600 w: 100-800 th:1-2
0.0-5
I 700
9. Minindi Creek
T (vF?)
Calcrete and calcareousaliuvial. colluvial material on river valley terraces
?
0.014
470
10. ThatcherSoak
D(?) vF(?)
Calcrete bordering salt lake system
l: 7 500 w:100-200 t h :< 2 m
0.03
4 100
U occurrence
MountJoel
indifferent behavior of U, V, and Mg partlv due to ore precipitation and its remobilization (Carlisle 1982).
-
Commonly low grade and small tonnage ore bodies (<0.05%UrOs, rarely up to 5000mt UaOs) in form of thin lenses (<1 to <5m thick), Yeelirrie being the only exception.
Mineralization -
-
Principal ore mineral is carnotite precipitated in patchy ore grade accumulation within both the main calcrete/dolocrete lenses and immediately below it in + carbonate-rich alluvium. Irregular distribution of mineralization both in grade and configuration.
Metallogenetic Concepts Models for the formation of calcrete-related carnotite mineralization have been developed by a number of workers. The uranium source does not create a seriousproblem. More dubious is the
Examplesof Surficial-TypeUranium Deposits
341
aclualsourceof vanadium.The principalpoints In channelsystems,such as the Yeelirrie channel, are, however,(a) the composition limited down-drainage recharge of the groundof controversy and down-streamevolutionof fertile solutions, water initiated slightly oxidizing conditions that rvhichon the one hand require an oxygenated acted as a geochemical barrier where dissolved iuid to transport hexavalenturanium, on the Va* was oxidized to V5-. thereby providing the ,.'therhand a reduced fluid to carry tetravalent conditions required for carnotite precipitation by vanadium, and (b) mechanismsto precipirare reaction of V5* with already available (UOr)'* carnotiteand to prevent it from remobilizationin and K* in solution. Concomitantly, Mg activity order to form ore. provoked partiai doiomitization of the calcitic Precipitationof carnotite may have theoretically rock. This led to an increase in volume and occurred under a variety of conditions.Mann related mound uplifting followed by sepiolite (1974)and Mann and Deutscher(1978a)consider deposition in voids and fractures. In a subsevenpossibilities sequent interval of greater aridity, the epigenetic - separateU and V groundwateraquifers, dolomitization process was interrupted. It was - variation of carnotitesolubilitywith changeof succeededby silicification expressedby the develpH, opment of opaline silica. The silica partly covered - dissociation of uranyl carbonatecomplexes, the carnotite. thereby protecting it against sig- local rise in K activity, nificant leaching. - changein partial pressureof CO2, Carlisle et al. (1978) believe that silica hard- redox-controUed deposition. pan, valley calcrete and carnotite formation are - evaporationof groundwater. closely related to the same arid climate prevailing in Western Australia for the past 25 000
Butt et al. (1984) arrive at the conclusionthat years. All three phenomena are restricted to carnotite precipitateswhere concentrationsof U the same region north of the Menzies Line and and K have been increasedbv evaporationand east of the Meckering Line. The soil-moisture where V4* is oxidized to the pentavalentstate. regime, resulting from evaporation significantly This may be at a site where V has migrated exceeding precipitation, is a critical factor in upwardsalong a redox gradientfrom a saturated this scheme. In this climatic environment, opaline zone at depth or where a subsurfacebedrock or chalcedonic hardpans devoid of calcrete barrier has forced upwellingof suchvanadiferous are common in headwater areas and on valley groundwatersinto a relativelvoxidizingenviron- flanks. ment, placesalso characterized Within depressions, calcrete developed best by moundingand lateral spreadof calcrete. and is most likely mineralized where chemiBriot and Fuchs (1984) suggestthe follow- cally fertile groundwaters migrate laterally at ing evolution of carnotite deposits. Alluvial a very low rate and approach or are forced and calcareous lacustrine sedimentswere de- toward the surface where thev are exposed to posited during earlier wet periods. Subsequent evaporatlon. drier. semi-arid conditions led to epigenetic The evaporation promotes an increase in the alterations accompanied by mineralization. dissolved calcrete and ore-forming elements by Limited precipitation and surface water flow about an order of magnitude between the headcontributed oniv minor rechargeto the ground- waters and the site of calcrete and carnotite water. As a result. semi-stagnantgroundwater deposition. conditionsevolved particularlyin regimeswhere Carlisle (1980. 1983) addresses the hydrothe hydraulic eradient was low (ca. 0.001 in chemical aspectsof calcrete and carnotite precipithe Yeelirrie channel) and/or where the waters tation and conservation. Groundwaters in most ponded in front of a natural barrier such as a of the drainage svstems, except in the headbasementrise cuning a channel.Retardmentof waters, are neutral to alkaline, carbonated, and the waterswas accompaniedby a hydrochemical oxygenated. This composition makes a transport changeto weaklv oxidizingconditionsand over- of uranium as uranyl carbonate complex ions saturationof dissolvedelements.Thesehydrologic most likely. Precipitation of calcrete from these and hydrochemicalconditionsimpiementedthe solutions can be initiated by epigeneticmodificationof lacustrinelimestoneby - evaporative concentration of Ca2* and Mg2* doiomitizationand carnotitedepositionin playas. down the drainaqe svstem,
342
5 Selected Examples of Economically Significant Types of Uranium Deposits
- common-ion precipitation where waters enriched in Caltfg-carbonate intersed. Ca/Mgchloride or sulfate brines as in salt lakes, - removal of CO2 and concurrentincreasein pH and the ratio CO32-/ HCO3-. The first two mechanismsare prerequisitesfor the carnotiteprecipitationand preservationsince they provoke crystallizationof calciteor dolomite with continuousdecreaseof COI-. Other essential factors required for carnotite crystellization include the availabilityand concentrationof U, V, and K. Simpleevaporativeenrichmentof U, V, and K ions improvesthe conditionsof carnotiteformation which precipitates*'here U- and V-beanng watersreact with K-enrichedhypersalinewaters as in playas. Another critical factor for localizing carnodteis the oxidationof V to the pentavalent proposedbv Mann andDeutscher state.Processes
(1978) and mentioned earlier may be instrumental here. The influenceof CO]- activiry is as follows. Simple extraction of CO2 and associatedcarbonateprecipitationis insufficientfor the fixation of carnotite. COz extraction in fact decreases the solubility of CaCOr, but it simultaneously increasesthe pH and the CO3-/HCO! ratio. In this environmentand influencedb1'other factors, the solubilityof carnotiteis likely to increaseand not to drop. Therefore, calcrete formation by simpleremovalof CO2from groundwateris not a satisfactory mechanism for fixing carnotite. On the other hand, combinedevaporationand common-ionextraction associatedwith the deposition of carbonate removes CO]- directli' from the groundwater and thereby it makes camotite lesssoluble. of U minerali"ationsuchas Other mechanisms adsorptionon clays, deposition of carnotite in
Table 5.33. Yilgarn Block, summary of geological criteria and processesinvolved in the formation of surficial carnotite deposirsin inland drainagesystems,(Arakel 1988) Control
Influence
Tectonic stabiliry
Development of subdued weathering topography and subsurface morphology
Arid to semi-arid climatic conditions
Development of phreatic carbonate and gypsite facies in axial sectionsof paleodrainagechannels Developmentof vadosecarbonateand gypsitefaciesoverprinting the phreaticlithofacies Evaporativeconcentrationof ionic species(including Ca, Mg, U, V, and K ions) in groundwater flowing down the drainages
Developmentof internal drainagesystems
Relativelv largecatchmentareas Sourceterrainsvariably anomalousin U content and rich in V, K. and Mg Relativelv low drainagegTadients Subsurfaceflow of groundwaterin aquifen with different hydraulic characteristicsand storagecapacities
Carnotite precipitation
Destabilizationof uranvl carbonatecomplexesbv low,ereda.o:Evaporative concentrationof U, V. and/or K Oxidation processes Common ion precipitationof carbonates Absorption and surfacecatalysis
Ponding
Physicalconstriction Chemicalbarrier development
Postdepositionalchanges
Reconcentrationdue to continuousdissolutionand reorecioitation of carnotite Removal of carnotitedue to rejuvenatedground*'ater flow Relocationof carnotitedue to the redistributionof hydrologic zones Redistributionof carnotitedue to localized*obili"ution of vadose calcretes Complete preservationof carnotite due to a complete shift in groundwaterffow patterns
Examplesof Quanz-PebbleConglomerate-TypeUranium Deposits
response to a pH change toward neutral or by introduction of V from independentsourceshave been discussedbut not substantiatedas significant contributions to calcretetype ore formation. Finally, preservation of carnotite mineralization demands tectonic and climatic stabilitv to protect it from dissoiutionand/or erosion. Table 5.33 provides in summary the criteria and processesinvolved in the formation of surficial carnotite deposits.
Referencesand Further Reading for Chapter 5.6 (for detailsof publication seeBibliography) Arakel 1988;Arriens1971:Banevet al. 1987;Bettenayet al. 19'16:Briot 1978,i982. 1983;Butt 1988;Butt et ai. 1977,1984;Cameron1976,1984:Cameronet al. 1980; CameronE, pers.commun.;Carlisle1984;Carlisleet al. 1978;Carter1981:Deutscher et al. 1980;Dickson1984; Gaskinet ai. 1981;Gee 19'76:Heath et al. 1980,1984; IAEA 1984;Johnstoneet al. 1973:Jutson19501Mann andDeutscher1978a.1978b;Morgan1965;Stuck.less et al. 1981; Thomas WN. pers. commun.; Vels B, pers. commun.;WesternMiningCorp 1976,1978
r
5.7 Examplesof Quartz-Pebble Conglomerate-TypeUranium Deposits (Type 7, Chap.4) 5.7.1 Uranium-RareEarth Elements Conglomerate: Depositsin Quartz-Pebble Blind River- Elliot Lake. Canada The Blind River-ElliotLakedistrictis locatedin southernOntarioto the north of Lake Huron (Fig.5.i 15).The districtencloses threemajorore trends.They are from N to S: 1. The Quirke trend in the Quirke Lake area with the active mines (1991) of Quirke, Denison and Panel. and abandonedmines Spanish-American.Stanrock and Can-Met ( F i g .5 . 1 1 6 ) . 2. The Nordic trend in the Elliot Lake areawith the Stanleighmine and the former mines of Milliken Lake, Lacnor, Nordic, and Buckles. 3. The Pronto trend in the Blind River area. A small deposit.Agnew Lake, is locatedca. 100km E of the district. Production to 1990 amounts to more than 150000mt U:Oa. Minor amountsof Th and Y were additionallyproduced. The mining grade varied between 0.06 and 0.13% U3Os averaging0.108% U:Oa. More recentlymined ore had an averagegrade of less areabout than0.095%U3Os.Remainingreserves at costs 300000to 350000mtU3Os,recoverable (Robertson 1987). of up to $120/lb.U3Os
Pro
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Fig. 5.f f 5. Blind River-Elliot Lake-Agnew Lake area. distribution of the Huronian Supergroup(hatched)and position of uranium production centers(black circle\. (After D.S. Robertson 1974)
5 Selected Examples of Economically Significant Types of Uranium Deposits
Y4
uranium in the area to the north and northwest of Elliot Lake (Richardsonet al. 1975). T\e Huronian Supergroup is a thick clastic sequencewith minor tholeiitic basalts.Rocks of the Archean craton located to the northwest of Elliot Lake providedthe sediments.Four groups constitutethe supergroup.They are from bottom to top: T\e Elliot Lake Group which consists of initial eugeosynclinalpsammites,psephites,and volcanicsand includes the uraniferous quartzpebble conglomerate-arkose bearing Matinenda Formation (formerly lamed I-ower Mississagi/ GeologicalSettingof Mineralization Bruce Group). The Hough Lake and Quirke Lake groups. The Blind River-Elliot Lake district lies at the Each displaysa sedimentarvcycle starting with southernmarginof the CanadianShield.The area para-conglomeratesinterpreted as polymictic principal stratigraphicis underlain by three and followed by clasticscoarseningupward, age: tillite petrographic units of early Precambrian in the Lake Group with interveningcalcathe Lower Proterozoic The Archean basement, Quirke dolomitic beds. reous and post-Huronian intruand Huronian Supergroup, The Cobalt Group consistsof three glacial to sives. Paleozoic sediments and Pleistoceneto Recentglacialdepositsrest on the Precambrian. fluvial cyclesendingwith an upper coastalbeach The Archeanincludes(a) older metavolcanics, facies. The evolution of the Huroni-an Supergroup metasediments, and granitic and gabbroic intrusionsand (b) late Archean/I(enoranquartz- stratais reflectedin the lower Huronian section monzonite (Algoman Granite). The quartz- by the presenceof sulfides,dominantly pyrite, with uranium in the near-shoreclastic monzonite contains anomalous contents of associated units such as the ore-bearing greenish-gray colored Matinenda Formation. In contrast, the upper Huronian containsthorium and abundant Minable mineraiization consists of uranlum and REE minerals emplaced in a distinct oligomicticpaleoconglomerate facies.The deposits are thereforeclassifiedas U-REE quartz-pebble conglomerate-typedepositsof Lower Proterozoic age(subtype7.1, Chap.4). The subsequentpresentationis largely based on the comprehensivepapersby J.A. Robertson (1978to 1989),Roscoe(1969),and Theis (1979) unlessotherwisestated.
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Zones of insufficient informotion
Fig.5.f16. Blind fuver-Elliot [.ake, Quirke Syncline, distribution of uraniferous and other radioactive conglomerates within the MatinendaFormation.Mineralizedtrends and mines:I Quirke trend; / Quirke 1;2 Quirke 2;3 Denison;4 SpanishAmerican;5 Panel; 6 Stanrock; 7 Can-Met; II Nordic trend: 8 Stanleigh;9 Milliken; 10 L.acnor;// Buckles; 12 Nordic; I3 PardeelPardeetrend. [After Roscoe in Douglas (ed.) 1970]
Examplesof Quanz-PebbleConglomerate-Type Uranium Deposits
N m
345
S
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Fig.5.ll7. Blind River-Elliot Lake. Quirke Syncline.restoredstratigraphicsectionprior to Hudsonian folding. 1 \'latinenda Formation. Ryan Member: greenishsubarkosewith radioactivequanz-pebble conglomerate.2 Matinenda Formation, Stinson Member: various sorted conglomeratesincluding beds rich in quanz pebbles.J Matinenda Formation. \lanfred Member: whire subarkoseand greenish subarkosewith beds of radioactive quanz-pebble conglomerate. 4 VcKim Formation: fine-grainedsubgrepvacke,subarkose.siltstoneand argillite, grading norrhward into white and -ereenishsubarkose.5 Ramsay Lake Formation: conglomeraticgreywacke. 6 Pecors Formarion: argillite, siltstoneand subgrevwacke.7 MississagiFormation: medium-grained,well-soned white subarkoseat base and coarsegrained, poorlv soned greenishsubarkoseabove. [After Roscoein Douglas (ed.) 1970]
hematitethat imposesa pink hue on certainrocks 5.115),the Huronian rocks are developedrelafrom the Gowganda Formation upwards. The tively uniform over a width of about 25km and variation is considereddue to an increaseof a thicknessof 1200m. All economic uranium atmospheric oxvgen dunng the period of the depositsare locatedin this northern section.To Huronian Supergroupdeposition. the south of the Murray Fault, the thickness The Matinenda Formation, the basalunit of the of the Huronian suite increasesassociatedwith early Lower ProterozoicElliot Lake Group is the facieschanges. host to the uranium deposits.The psephiticand In the Blind River-Elliot Lake district, i.e., psammiticsedimentsof the MatinendaFormation on the northern side of the Murray Fault, the were deposited in regressive cycles, result- Huronian sedimentsare folded into a WNWing in several superimposed quartz-pebble ESE-orientedsyncline,the Quirke Syncline(Fig. conglomeratehorizons interbeddedwith arkose 5.116),paralleledto the south by the Chiblow and sandstonewithin depressions of the Archean Anticline, which is formed in its easternpart by (Fig. 5.117). basement Archean graniticrocks. Pienaar(1963) and Fralick and Miall (1982) documenta high energy,braidedfluvial environment with a S to SE-directedstreamflow for the Host Rock Alterations depositionof the MatinendaFormation. During the Blezardian(ca. 2100 m.y.) and No major alteration related to the initial Penokean (Hudsonian) (ca. 1900-1750m.y.) uranium emplacement has affected the host orogenies the sediments were mildly meta- rocks. Post-deposirional modifi.cations as remorphosed and eently folded. The main fold ported by Robinsonand Spooner(i98a) include axes trend WNW-ESE in the western part of (a) pressuresolutionof quartz reflectedby slight the belt and E-W to ENE-WSW in the eastern impressionof adjacentquartz-pebblesinto each part. other and tormation of irregular quartz masses, Diabase (NipissingDiabase) intruded about (b) growth of secondaryquartz. (c) sericitiza115m.y. ago (Stockwell1982). tion in different degreesof K-feldspan and (d) Structuresconsist of steepiv dipping, mainly pyntization rvith more than about 90% of the NW-SE-strikingfaults with minor displacements. pyrite presentbeingof post-deposirional origin. Remobilizationof the uraninite was accomA regionalstructurein the uranium regionis the Murralt Fault. a high angle reverse fault which panied by certain mineral transformationsand trendsabout E-W. parallelto the main axesof the recrystallization of someauthigenicminerals,parfold belt. To the north of the Murray Fault (Fig. ticularly involvingTi-mineralsand hydrocarbons.
346
5 Selected Examples of Economically Significant Types of Uranium Deposis
Intrwion of NipissingDiabase dikes and sills caused locally albitization, chloritization, and carbonatization of the wall rocks but affected generallyonly on a very minor scalepre-existing uranium mineral2ation (Robertson1968,1978). An exceptionto this is at the Pronto mine, where a larger part of the deposit has been modified partly into albitic-chloritic-carbonatic host rocks (Heinrich1958,1982). Alteration related to paleoweatheringof the Archean basementis best preservedon granitic rocks. where it produced a paleosolcomposed of quartz, microciine, and sericite with a conservedgranitic texture gradinginto fresh granite (Robertson1978,1986;Pienaar1963). The chemical characterof the paleosol,particularly the presenceof uranium and ferrous operated iron, suggeststhe weatheringprocesses under reducing conditions. (for more details on the paleosol see Gay and Grandstaff 1980; Goddard1987;Kimberlevet al. 1984).
Principal Characteristics of Mineralization Principal ore minerals are uraninite, U-Ti-phases ("brannerite"), and monazite.constitutingup to 5wt. o/"of the ore mined. Coffinite, uraniferous kerogen (formerly listed as thucholite), uranothorianite, uranothorite, xenotime, and gummite occurin subordinateto trace quantities.The in-situ U/Th ratios range within the variousore bodiesfrom 5 to 0.3 and in ore milled from2to 3. Dominant associated mineral is pyrite (5 to 20wt. %). In minor to trace amountsoccur zircon, ilmenite, chromite, cassiterite.ilmenomagnetite. alianite. rutiie, garnet, spinel, tourmaline,titanite, apatite,pyroxene,and perthitic microcline.Gold is very rare. Uraninite is the important ore mineralin some ore zones. whereasin others U-Ti-phasespredominateu'ith uraninitesecond. Uraninitegrains are up to 0.2 rarely 1mm in diameterand contain 48-69% (av. 65%) UO2. 4-10% (av. 6.5%) ThO2, I.5-4.5o/,(av. 2.5%) Y2O3,Ca.0,9Yo Ce2O3 and ca. 18% PbO.Further rare earth elements are in order of relative abundanceNd, Dy, Gd, Sm, Er, Pr. and La, totalling 3-8% rare earth oxides (Grandstaff 1980;Roscoe1969;Robertson1978;Theis1979). Uraninite occurs(a) as small clustersof grains betweenpebbles,(b) in 0.5 to L cm thick, almost monomineralicsubparallelbands of well-sorted
angulargrains, (c) concentratedproximal to the base of small-scaledepositional units within conglomerates and (d) in a notable positive correlation with maximum apparent quartz-pebble diameter(r : 0.47) (Theis 1979;Roscoe1969). Ruzickaand Steacy(I976) describeribbons0.3 to lcm thick composedof uraninite fragments cemented by sulfur-rich filamentous hydrowith subroundedgrainsof carbonand associated brannerite,monazite,and fracruredpvrite below the bands.Small quartz pebblesdepressthe top of the band. The feature is interpreted by the authors as reflecting a hvdraulic separationof the ore constituentsand that the hydrocarbons originally accumulatedas an oily material on the top of that depositionalsequence.possibly capableof actingas a collectorfor the uraninite. "Thucholite". also referred to as uraniferous hydrocarbonor kerogen,is presentin only minor quantities.It occurs(a) as ribbons on the top of ore reefs as mentionedabove, (b) as dissemrnationswithin ore bedsand (c) in post-orefractures associatedwith post-oresulfides.Wiliingham et al. (1985)identifiedthe hydrocarbonas kerogen probablyderivedfrom algal mats. "Branneite" (amorphous U-Ti-oxide mixtures) is consideredto be the reaction product of uranium with titanium oxides within the conglomeratesafter their deposition. It occurs as minutegrainsfilling poreswith sizesrangingup to 1.5mm. Theis (1979)noticed three varieties of "brannerite" which may contain 8-47% U:Oa, 0-2% T\O2, up to >5% CaO and20-37'h TiO2 with wide variations in the U3O8/Ti02 and U3O8/Th02ratios. Pyriteis the most commonassociatedmineral. It occursas 0.5 to 3mm grainscif variousshapes. composition.and _eenerations dispersedthroughout the matrix of the uraniferouscongiomerates and. to a lesserextent. the inten'ening quartzite beds. Individual grains range from rounded ("buckshot"pyrite) to euhedral.and some are spongy. For more detailed mineralogicaldescriptions the readeris referredto Roscoe(1969)and Theis (1979). Economic uranium mineralization has been found in three ore trends corresponding to three NW-SE-oriented fluvial systems of the Matinenda Formation: (1) the Quirke trend on the northern and (2) the Nordic trend on the southernlimb of the Quirke Synclinelocatedto the north of the ChiblowAnticline, and (3) the
Uranium Deposits Examplesof Quartz-Pebble Conglomerate-Type
Pronto trend located on the southern slope of the C h i b l o w A n t i c l i n e ( F i g s . 5 . 1 1 5 ,5 . 1 1 6 ) . Uranium mineralization is confined to several horizons of oligomictic quartz-pebble conglomerate (Figs. 5.117, 5.118) rich in pyrite which were deposited in braided and interfingering stream channels or as lenticular, sheetlike beds. The conglomeratesare interbedded with weakly mineralized or unmineralized, poorly sorted and coarse-grainedquartzitesor arkoseswith greenish colored sericitic matrix. The uraniferotts conglomerdlesconsist of wellrounded and well-sorted quartz (>95% of the pebble fraction) and chert pebbles (diameter 2l0cm) which are often smokev. and a matrix of quartz, feldspar, sericite and pyrite, the latter constituting 5-20wt. o/o.Heav.'tminerals including the ore specimen occur in the matrix. Clayand silt-size particles are almost absent. The conglomeratesdisplay a gradationalsedimentary, and mineralogic and geochemical zoning in the direction of transport. (Fralick and Miall 1982: Pienaar 1958,1963; Robertson 1968; Roscoe 1969; Theis 1979). Characteristics of this zoning are listed in section Ore Control and Recoqnition Criteria. o. 350.
N
The major and presently (1991) mined ore bodies occur in the Quirke Syncline. Dimensions of the syncline are up to 15 km wide, more than 50 km long, and slightly more than 1000m thick in its mineralized portion. The axis of the Quirke Svncline trends WNW-ESE and plunges to the W N W a t a l o w a n g l e ( F i g . 5 . 1 1 6 ) .U r a n i u m m i n eralization occurs in a basal stratigraphic zone which is up to 150m thick but does not necessarily l i e d i r e c t l yo n t h e A r c h e a n b a s e m e n (t F i g s .5 . 1 1 7 , 5.118). It crops out locally at surfaceand extends downdipto a depth of 1000m. Two economicore trends are recognizedin the Quirke Syncline(Fig. 5.116) but it should be noted that uranium mineralization is not restricted to these two. The Quirke rend, the main ore producer, has a length of aimost 10km in WNW-ESE direction and a width of 1.8 to 2.7 km. The Quirke trend coversan area of about 20 km2. Up to eight mineralized conglomeratic horizons, referred to as reefs. are developed within a 50m thick sequence of the Manfred Member near the base of the Matinenda Formation. The strata dip to the south at angles of 15 to 2 5 " ( T a b l e 5 . 3 4 ,F i g . 5 . 1 i 8 ) .
s
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Fig. S.llE. Blind fuver-Elliot Lake. Denison mine. geological N-S section showing position and anitude of the horizons(A to F zones)Sery. B S"rp.nt Quartzitet Es. fu EspanolaLimestone: Br. Ls Bruce uriniferous conglomerate -Cg Mrss. Q MisiissagiQu_anzite:Pec. ArgPecorsArgillite;.R. !.-.95 RamsayLake Bnrce Congtomeiate; Limestone: ^8r. Conglomerate: lilat. g Matiienda Quarzite: k. D, Keyes Dike (diabase);N. Di Negri Dike (diabase)' Archean" undifferentiated, essentiallygreensrone.(After J.A. Robertson 1989,basedon Denison Mines Ltd.)
348
5 Selected Examples of Economically Signifcant Types of Uranium Deposits
Table 5.3. Blind River-Elliot Lake, Quirke Trend, stratigraphic position and correlation and thickness of uraniferous quartz-pebble conglomerates in the Denbon and Quirke mining areas. Minable conglomerate bedslzones are printed in italics. (Based on data from Gunning in RobertsonJ.A. i966 and Little et al. 1972)(for location see Fig. 5.116) Denison Mine
New Quirke Mine
Post-Matinenda cover
300to 900m
115to 700m
Matinenda Formation Dip of strata: 20"s +E-W Strike of strara: Plungeof ore zones:5" SE Quartzite: 15m
Dip of strata:0 to 50'S, av. 23'S Strike of strata: ESE-WNl*' Quaruite + thin conglomerates:0 to 9 m
F-zone:1.2 to2.7 m, av. 1.8m Quartzite:6m
U p p e ru p p e rz o n e :a r . 1 . 5m Quartzite + thin congiomerates:7.5 m Upper zone: ar'. 1.5m Quartzite + thin conglomerates:2.4to 3.6m
E-zone: 1.5to 2.7m, a'r'.2.1m Quanzite:6m
A (Quirke1)-zone:av.2.7m Quanzite3 : . 6 t o 5 . 1m
D - z o n e :1 . 5t o 2 . 4 m , a v . 1 . 8 m Quartzite:33m
B-zone:av.?.25m Quartzite + thin conglomerates:19.-5to 25.5 m Cl-zone: a'r'.3 m Quanzite:2.4 to 6 m
A l + A 2 - z o n e : 1 . 8 t o 3 . 6 m ,a v . 2 . 4 m lntercalated quartzites:I to 2m B-zone:,1.2 to 3m, av. 2.4m Quartzite: 0 ro 55 m
C (Quirke2)-zone:av.3.3m C-Footwallzone: 0 to 12m, av. 9 m Quartzite. balal conglomerate:0 to 27.6m
Archean basement
Individual mineralized conglomeratesheets 1987) and grade of the conglomerates from top to vary in thicknessfrom 1.5 to 3.6m. They are bottom are !.2-3.9 m with 0.115% U:Oe in the separatedbv quartzitic arenites6 to 33m thick. upper Pardee horizon, which extends from the Robertson (1962)notes the conglomeratebanks Nordic mine into the Pardee and Pecors zones. have reiatively abrupt up-channelterminations 1.5-5.4m with 0.725% U3Os in the middle main and long fingering down-channelends. They ( N o r d i c ) h o r i z o n , a n d 1 . 5 - 7 . 5 m w i t h 0 . 1 % U 3 O s wedgeout againstArchean highsto the NW and in the lower Lacnor horizon. All three horrzons W and are truncated to the N bv unconform- extend downdip into the Stanleigh mine. The ably transgressed RamsayLake sediments. The conglomeratesare truncated in up-channel direceastern and southernboundariesare associated tion b1' the vounger Stinson conglomerate or bv with the thinning of the congiomerates.the facies changes within the Matinenda Formation thickeningof interveningquartzitesand decrease or againstArchean basement. in the uraniumgrade. The Pronto trend.located some 25 km S of the The Nordic trend is located near Elliot Lake, Quirke Syncline, has an extension of about 1 x about 12km'to the southof the Quirke trend. It 1 km. It contains one mineralized conglomeratic hasa NW-SE lengh of almost6 km anda width of bed which has a thickness of 2.0 to 2.5 m in the 1 . 3t o 1 . 8 k m . average and which dips 15 to 20" S. The Pronro The Nordic trend contains three uraniferous ore trend is truncated at its southern end at a conglomeratehorizonsin the lower 50m of the depth of about 300m by the Pronto overthrust, Ryan Member of the Matinenda Formation, a structure associated with the Murray Fault. which dips about L5oN. Thickness(Robertson The northern end of the Pronto trend is eroded.
Examplesof Quartz-PebbleConglomerate-TypeUranium Deposits
Uranium mineralizationoccursnot only in the but alsoin structures. conglomerate Dating of heav.vmineralsin the lvlatinenda congiomeratesyields ages of. 2550 + 50m.y. for uraninite (Meddaugh and Holland 1981), 2500m.y.for monaziteand 2450m.y.for zircon (Mair et al. 1960).Thesedataarecomparible wirh a primaryageof an Archeansource.
349
Ore Control and Recognition Criteria Essential ore controllinq or recognition criteria include: Host Environment -
Host rock is a pyritiferous oligomictic quartzpebble conglomerate deposited in Lower Proterozoic time prior to the oxyatmoversion PotentialSourcesof Uranium of the earth atmosphere. - The uraniferousconglomerateswere laid down The primary ore-forming mineral assemblages in alternation with arenaceoussediments by attestto a detrital origin of graniticprovenance. rivers flowing from NW to SE as reflected by The uraninitecontainsamountsof ThOl (up to regronai variations in pebble size and relaabout 9%) and rare earth oxides (up to 8%) tive pebble packing and heavy mineral zonal rvhich are typical for pyrogeneticuraninitesof distnbution. granitesand particularlyof pegmatites.e.g., as - Provenanceof the conglomerateswas largely found in the Bancroft district and a number of from granitic terrane. other occurrencesof uraninite in Archean rocks - Deposition occurred immediately above the as outlinedby J.A. Robertson.The apparentage Archean-Lower Proterozoic unconformity in of ca. 2550+ 50m.y. of detritaluraninitegrains paleovalleysor braided fan deltas (Fig. 5.117). (Meddaugh and Holland 1981) attesr a pre- - Heavy minerals are particularly concentrated Huronian origin. Similar ageswere obtainedfor on the lee-side of Archean basement highs
monaziteand zircon. Thesedata reflecta source (Fig.5.1le). of Archean age. Depositionai trends indicate a With respect to the Quirke trend, the following sourceto the N and NW of the ore district,where features are listed by Theis (1979) (Fig. 5.120): predominantlygranitesand minor greenstones of - Conglomerate development within the ore Archean age outcrop. horizon is more variable to the NW and more persistantand continuous to the SE.
__.\* Fig. 5.119. Blind River-Elliot Lake. Denisonnrine. stratigraphic, spatial and fluvial dvnamic relationship of ore reefs and a loca.l basement high. //U Hanging wall (upper reef); O Thin quartziteunitl 8U Basal upper reef: IQ Interstitial quartzite: LR Lower reef: Bl'Basement. :5
gU restson BT:
tR 6l
.--*
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rests onBT:LR/BT LR cut off bv BT; w ff "" onBr. (After J.A. Robertson 1987. t* rests kl based on Goddard 1987and Theis 1979)
YO
+5i! l:5e
350
5 Selected Examples of Economically Signfficant Types of Uranium Deposits
- A downslopeincreaseof the Th/U ratio, over a distance of about 8km from 0.27 in the Old Quirke mine to 0.4-0.5 in the Denison mine and to 0.8 in the Panel and Stanrock mines. Chemical data of handspecimen (Theis 1979) attest to a corresponding quantitative distribution of U. Samplescontain ca. 7T" in the NW part of the Quirke trend decreasingto ca. 300ppm Alteration U3O6 in the SE part. In contrast, Ce, - There is no alteration associated with the La, Th, Ti, Y, and Zr increasedown the paleoflow direction. Th shows a less clear deposition of the detrital ore-forming minerals. - Postdepositional modifications include intense pattern than the othersdue to its incorporapyritization, some silicification, minor serition (a) in uraninite, which is dominant in citization of feldspar, destruction and neothe coarser upstream fraction. and (b) in formation of Ti-minerals. and hydrocarbons. thoriferousminerals,which prevail further - Intrusion of diabase dikes and sills resulted downstream. - A certaincorreiation.but not as a rule. of locally in albitization. chloritization. and carbonatization of the wall rocks. high content of pyrite (5-20wt. %) with - Paleoweathering of the Archean paleosurface uranium accumulations. - The mineral assemblages was essentially of physical nature. and lithology attest to a sedimentarydifferentiation of the heaq' minerals during the fluvial transport due to Mineralization different specificgravities: - Original ore minerals are detrital uraninite, - Trappingof the more denseuranium minerals uranothorianite, uranothorite, xenotime, and occurredin a high energyenvironment,while monazite deposited synsedimentary with the the lessdensethorium,zirconiumetc. minerals were transportedfurther into a distal zone of enclosing conglomerate. - Post-depositional modification of the ore prolower energy. duced U-Ti-phases (brannerite), coffinite, and - Heavy minerals of a restrictedrange of grain uraniferous kerogen (thucholite). sizesuchasuraninite(0.05-0.2mm), monazite - A sedimentological control of the mineraliza(0.2-0.4mm) and zircon (0.075-0.2mm) tion is indicated by lithologic, mineralogical, reflect variations in the depositional energy and geochemical zonation. The zoning is conditions by variations in their concentraexpressedby changesin the relative abundance tions,while mineralswith a largerangein grain of specific heavy minerals related to variations size, such as pyrite (0.5 to 2.9mm). reflect in the depositional facies as reflected by the changesin the depositionalenergyspectrumbv size and packing densitv of quartz pebbles. correspondingchanges in their grain size. Saiient criteria of this sort in the Quirke trend Pyrite grain size and quartz pebblesize give a are as foliows (Fig. 5.120) (similar characcorrelationcoefficientof 0.93.suggesting thal teristics are known from the Nordic trend the control of depositionof most of the pvrite except that here the U/Th ratios are lower): wasexertedby the samemechanismas that of - Association of ore minerals with well-sorted the quartzpebbles(Theis1979). -
Well-packed conglomerate is commonly more abundant to the NW and less abundant to the SE. Poorly packed conglomerate and quartzite within the ore horizon increase to the SE. Pebbles decrease in sLe from NW to SE and within a vertical section from bottom to toD.
-
oligomictic quartz and minor chert pebbles. Downslope decrease of pebble diameter from ca. 10cm near the source to about 2 cm several kilometers away, correlates with a transition from uranium-rich to thorium-rich and finally to titaniumzirconium-enriched zones. (uraninite is associated with coarse pebbles while it is scarceor absent in facies with pebbles of less than 3cm in size).
MetallogeneticConcepts The most favored metallogenetichypothesisis the modified placer model that includes a svnsedimentaryplacer deposit modified by postgeneratedby diagenesisor depositionalprocesses mild metamorphism(Table5.35).
Examplesof Quartz-PebbleConglomerate-T;-pe Uranium Deposits Proximol:ccres
351
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Poorly pocxed conglomerote ond suborkose
of a Fig. 5.120. Blind River-Elliot Lake, lithologic-mineralogicalcharacteriscics mineralized Matinendaconglomeratesheet a Planview of a tyrrical shape of a mineralizedconglomeratesheet with relativelv blunt up
D. ll/ot
There can be little doubt that uraninite of the conglomerates of the Matinenda Formation was derived from an igneous parent rock as outlined eariier. The uraninite grains are rounded in a way that is best explained by fluvial transport and abrasion. Hence it is assumed that they were liberated by ph1'sical weathering under oxygendeficient conditions from granites and pegmatites. During the time of deposition in the early Proterozoic. there was still a sufficiently oxygendeficient atmosphere, as well documented world-
wide by Schidlowski and others. During this episode, physical weathering with verv limited chemical disintegration was predominant, as reflected by the composition of the paleosol deveioped on the Archean surtace. This environment prohibited or at least minimized meteoric chemical dissoiution of uranium from its igneous source rocks. Instead. uraninite and other minerais could be transported as detital grains over distancesof several kilometers from their place of origrn into the conglomeratic host rock. Th€
352
5 Selected Examples of Economically Sigaificant Types of uranium Deposits
Table 5.35. Blind River-Elliot l-ake, metallogenetic model presented by Robertson J.A' (1989) based on various papen by Ruzicka | 1. Source I Radioactive anomalies, U sbowings in pegmatite
Uraniferous gFanitic rocks + pegmatite 2600m'y. I
| 2. Weathering I Disintegration& weathering,formation of uraninite-bearing detritus. little or no oxidation or solution
Regolith at Archean-Proterozoicconlact
I
| 3. Transponation I Rapid transponation by SE-flowingstreamsand rivers of quanz pebbles.feldspar,uraninite and resistates.little or no oxidation or solution
Huronian sedimentationdirections(SE), no red beds
I | 4. Deoosition I Rapid deposition and burial of guartz-pebble conglomerate with uraninite, iron oxide (?), pwite, monaziteand zircon tn shallowfluvialAittoral environment.Little or no exposureto oxygen. Algal mats in quiescent areastrap uraninite grains
Scourand fill structures.Cross-beddrng,thick arkose sequences,fining upwardscycles.basementvallevs (magneticexpression).Embaymena in Archean contact.Radiometric anomalies.QPC lensesand beds + radioactivity, pyrite, kerogen
5. Diagenesisand modification Compaction and consolidation, migration of pore fluids. Clay minerals form from feldspar debris, "brannerite" b-v migration of U from uraninite to titaniferous aggregates' sulfur of authigenic pyrite possibly derived from volcanism
Yellow greencolor. No magnedte
Hydrothermal/contact-metamorphic alteration near Nipissing diabase (2115m.y.), Iinle U remobilization
Albite (red), chlorite (green), carbnate near diabasecontacts. Post-ore veins and joint coatingsof sulfides
Lack of exposure to oxygen, little or no post-depositional solution. no erosionof unconsolidatedmaterials,no destructionb;' later metamorphism,erosion
ore minerals were transportedby streams,and redepositedsyngeneticallf in placers within a braidedfluvialsystem. Deposition of uraninite and other heavv mineralswas governedby the hydraulic energ)' gradientof the transportingstreamsystemand b1' paleomorphology.The result is a zonal distribution'of the heav-vminerals,uraninitebeing depositedmost proximal to the sourceterrane. After the primary sedimentationthere has been somerecyclingof uraninite.At siteswhere conglomerateof the Matinenda uraninite-bearing Formationis truncatedand eroded by a younger as for examplethe RamsayLake conglomerate, the latteris uraniferous(Robertson conglomerate, 1987).
A different post-depositional modification occurred by mobilizationof uranium and other elements and redepositionas authigenic and replacementmineralsdue to diagenesis-reiated processes. Robinsonand Spooner(1984a)estabthree stagesof alteration: lished lst stage:Completefluid-mediatedleachingof Fe from detrital ilmeno-magnetiteand mobilization of U, Th, REE, Y, PO?-, and SiOz.During this stage(a) uraninite was replacedby coffinite and quartz, (b) monazite was corroded and slightlyaltered,(c) U reactedwith TiO2 iiberated from titaniferous heavy minerals to form comof branneriteand other U and positeaggregates Ti-bearing alteration phases and (d) coffinite formed togetherwith a Y-REE-U phosphate.
Examplesof Quarz-Pebble Conglomerate-TypeUranium Deposits
Znd stage:Precipitation of authigenicpynte. 3rd stage: Sealing of the conglomerate by pressure solution associated with additional alteration of microcline; impressionof marginsof the quartz pebbles and fiiling of intersticeswith quartz and sericite. The most likely Eh-pH rangesfor the solutions governing the post-depositional aiterations are given by the two authors as (a) a low to moderate Eh of ca. -1 to +0.2 V (field of HzSOr - SOipredominance) and slightly acid pH of ca. 3 to 6 for the ilmeno-magnetite leaching and (b) a low Eh of ca. < -0. I V (field of H2S dominance)and a slightly acid to neutral pH of ca. 5 to 8 for the precipitation of authigenic pvrite. Both fields of inferred Eh-pH values fall within the range of thermodynamic stability of uraninite and coffinite where solubiliry of U is low to extremely low. Phosphate leached from apatite and monazite may have improved the uranium soiubility at the higher inferred Eh values. Ferris and Ruud (1971) address the lack of magnetite and hematite and state that these minerals and also ilmenite were converted to pyrite by sulfurization Sulfur isotope studies indicate sulfur of magmatic (volcanic ?) origin rather than of biogenic fractionation. Therefore, sulfurization of ilmenite and magnetite produced a reduced environment, stabilizing detrital uraninite which had been previously affected by partial leaching. A different qvpe of alteration is represented by a pronounce d p o st-N ip issi ng me taso mansm which affected the western part of the Pronto mine, the southernmost deposit of the Blind River-Elliot Lake distnct. Similar but less intense alteration is seen locally at Elliot Lake mines, where it is more clearly linked to diabase intrusives (Robertson
353
-2.7 to +3.1, which indicates a magmatic origin. These data suggest that the altered zone of the Pronto deposit resulted from a magmatic-derived metasomatlc overprint of the original Lower Proterozoic uraniferous conglomerates and as such the altered deposit represents a special type of an albitite deposit.
References and Further Reading for Chapter 5.7.1 (for detaiis of pubiication see Bibliography) Armstrong (ed.) 1981:Bottrill 1971;Bo-vle1979:Button and Adams 1981; Derry 1960; Ellswonh 1928: Fairbaim 1965; Ferris and Ruud 19711Frarey and Roscoe 1970: Fralick and Miall 1982:Free B. oers. commun.: Gnffith and Roscoe 1964; Gnffith 1967; Heinnch 1958. 1982; Holmes i957; Joubin 1960: Kimberley et al. 1980.1984; Lang et al. 1976;Little et al. 1972',Mossman and Harron 1983; Nuffield 1954: Patchett 1960: Pienaar 1958, 1963; Pretonus 1981; Ramdohr 1957, 1980: Rice 1958; Robertson.D.S. 1962.197-l:Robertson,D.S. et ai. 1978: Robertson, D.S. and Steeniand 1960; Robertson, J.A. i978. 1981,1986,1987,1989;RobensonJ.A. and Gould 1981, 1983; Robertson J.A. pers. commun.; Robinson and Spooner 1982.1984a:Roscoe 1959, 1%0, 1969,L973, 1981;Roscoeand Steacy 1958; Ross 1981; Ruzicka 1979, 1981; Ruzicka and Le Cheminant 1984; Ruzicka and Steacy 196; Saagerand Srupp 1983; Sarnt Martin 1983; Sims et ai. 1980, 1981; Theis 1973, lm6, 978, 1%9: Thorpe 1963;Traill 1954:Willingham et al. 1985
5.7.2 Gold-UraniumDepositsin : Conglomerates Quartz-Pebble WitwatersrandBasin. SouthAfrica
The WitwatersrandBasin is located along the border between the Orange Free State and Transvaal.The variousore horizonscrop out in a discontinuousband approximately-150km long 1968.r986). on the westernand northern sidesof the basin. (1958, Heinrich 1982) documentsfor the The band includesthe Virginia-WelkomrOrange Pronto mine intenseand extensiveNa-, Ca-, and Freestate, Klerksdorp, Carletonville, Westwith Rand, Central-Rand.East-Rand.and Evander Mg-, andminor Ba-metasomatism associated detrital modification of the originally Gold Fields(Fig.5.121). marked The WitwatersrandBasih is the largestgoldmineralization resulting in the redistributionof about 25% of ore grade uranium. The general uraniumprovincein the world. aiteration sequenceis albitization-chlontization Since the uranium grade of in-situ ore is (+biotite)-doiomitizationicaicitization (locally generally very low, ranging from 0.016% to with abundant baryte)-sericitization.Sulfidiza- 0.07% U:Os, more than 80% of the uranium tion occurred in a postmetasomatic stage. reservesare economically recoverableonly as a Sulfur isotope ratios (M.L. Jensenin Heinrich by-productof gold nining. Former production 1982) give for authigenicand detrital pyrite a amounts to ca. 150000m1UrOs. Remaining range of 63aS7- of -1.3 to +0.4 and for reserves,recoverableat lessthan $304bU3Osare chalcopyrite, pyrrhotite, galena, and sphalerite ca. 320000mtU3O8(Brynard et al. 1988).
5 Selected Examples of Economically Significant Types of Uranium Deposis
354
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Fig. 5.Df. Wifwatersrand Basin, generalized geological map showing the distribution of the Lower Proterozoic Dominion, Wesr Rand and Cenrral Rand groups.granite domes adjacentto the basin. and the location of the major U and Au mining districts/goldfieldswhich are identical with the major fluvial fans. Mining districts/Goldfieldsfrom E to SW: E Evander: ER East Rand: CR Central Rand: WR West Rand; CV Carletonvilleor Far West Rand: K Klerksdorp: OFS Orange Free State or Welkom Goldfield. Granite domes. clockwisefrom E: CD Cedarmont: ED Edenvillel SD Steynsrus;ZD Theunissen VeD Yermaasl WD Westerdam VrD Vrevsrus;.ID Johannesbwg;DD Devon Dome. (After Andreoli et al. 1988;Brvnard et al. 1988;IAEA 1986a;Buck and Minter 1987)
..Minable mineralizationconsistsprimarill' of gold and uranium emplaced in a distinct oligomicticpaleoconglomerate facies.The deposits are therefore classifiedas gold-uraniumquartzpebble congiomerate-t)?e deposits of Lower Proterozoicage (subtype7.2, Chap.4). Geology, lithology, mineralogy,and origin of the gold-uranium conglomerates of the Witwatersrandhave attracteda sreat number of
geoscientists.Comprehensivesummaries have been publishedby Pretorius(1974, 1975, 1976), Featherand Koen (7975),Liebenberg(1955)and Whiteside (1970).Their data, supplementedby more recent information on selected subjects, publishedby the authorslisted,providedthe base for the subsequentcompilation.(see also papers on individualdepositsin Anhaeusserand Maske, eds.1986).
Examplesof euanz-pebble conglomerate-Type Uranium Deposits
GeologicalSettingof Mineralization
J))
mining areasand in a numberof conglomerate horizonscontainAu-U ore. The WitwatersrandBasin (Fig. 5.I2I) coversan Althoughthe main Au-U producer,the perarea of almost 50000kmr,of which approxi- centageof conglomeratein the Witwatersrand mately 30000km2are underlainby more than Supergroup is relativelysmall.Pretorius(1976a) 8000m thick, slightly meramorphosedstrara estimatesthat only 8% of. the clastic column of the Dominion Group and Witwatersrand is composedof conglomerateon the NW or Supergroup,which host the ore-bearingquartz- proximal to sourceside of the basin and this pebble conglomerates,and the Ventersdorp amount decreasesto ca. 1o/oon the distal SE Supergroup,which is oniy minor mineralized. side. All three units combinedare referredto as the The VentersdorpSupergroup (age approxiWitwatersrandTriad. The Transvaal Seouence mately 2.6b.y.) unconformablyoverlies the restsunconformablyon the Triad. Witwatersrand sediments.Mainlv andesiticand The Lower Proterozoicsedimentsrest un- someacidporphyriticlavaswith thin interbedded conformabiyupon graniticgneisses, ultramafics, quartzitesand conglomerates are the constitand metasediments of the SwaziiandSupergroup, uents. The supergroupwas originallv at least a unit of the Archean KaapvaalCraton.Pnor to 5000mthick. the depositionof the arenites,a paleosolhad The TransvaalSequence(age approximately developedbv predominantlyphysicalweathering 2.2b.y.) transgressedover a distinct un(Grandstaffet al. 1986). conformityover the Ventersdorpstrata. It conThe lowermostsedimentunit is the Dominion tains slates,quartzites,conglomerates, dolomitic Group (age approximately2.8 b.y.). The basal limestones,and diabasesills. Someuranium and sequenceis 40 to 100m thick and consistsof gold is found in the basalpart of the Black Reef conglomerates,arkoses, and quartzites capped conglomerate,which fills erosion channels,cut by lavas,tuffs, and shalewith a compositethick- into the oldersystems. nessof almost2700m.Uranium occurswithin the All sedimentaryunits have experiencedstrucbasal section in two oligomictic quartz-pebble tural disturbances.A considerablenumber of congiomerates,the Lower Reef and the Upper faults cut and displacethe stratawith offsetsin the Reef. The Lower Reefformsnarrow lenseswithin mineralizedhorizonsfrom severalmeters up to paleo-valleysincisedinto the Archean basement. tens of meters. The Upper Reef, in contrast, is laterally more persistent. It was depositedon an intraformaIlost RockAlterations tionai erosionsurface. The overlying WitwatersrandSupergroup(Fig. 5.I22) (age approximately2.8 to 2.6b.y.) is No major alteration related to the initial syndivided into a lower division. the West Rand sedimentaryuranium depositionhas affectedthe Group. about 4500m thick. and an upper divi- host rocks.Diageneticand/or mild metamorphic and locally intrusion of sills and dikes sion. the Central Rand Group, about 2500m processes a certain remobilization of the orecaused thick. elements forming under reducing conditions as T\e West Rand Group includesshalesislates, reflected by crystallization of abundantpyrite. quartzites,occasionalconglomerates.and andesitic lavas. Notable quantitiesof uranium occur in oniy one zone of congiomerateswithin the Principal Characteristicsof Mineralization GovernmentSubgroup. The Central Rand Group contains predomi- Minable mineral assemblages, which can vary quartzitesand from reef to reef and within the same horizon, nantly medium to coarse-grained conglomerates with minor amounts of inter- commonly contain as principal ore minerab bedded slatesand andesiticlavas depositedin a uraninite, native gold, osmiridium. locally fluvial fan environment. uranothorite, brannerite or other U-Tlphases Pretorius (1975) identified six major fluvial (leucoxene), thucholite, coffinite. and minor fans along the northern, northwestern and amountsof hexavalenturaniumminerals. southern margin of the Witwatersrand Basin Parageneticor associatedminerab. some of (Fig. 5.121).They are identicalwith the major them in several generations, include pyrite,
356
5 Selected Examples of Economically Significant Types of Uraninm Deposits
euxenite,xenotime,monzvite,zircon,columbite, chromite, ilmenite, magnetite, cassiterite, arsenopyrite,pyrrhotite, cobaltite, gersdorffite, galena, corundum, diamond, rutile, tourmaline, apatite, and garnet. Monazite,euxenite,zircr:.n,allanite,xenotime, and rarely kerogen contain minor amounts of uranium. Phyllosiiicatesmav have significantU contents (>500ppm, Feather and Glatthaar 1987). Remobilization by erosional redistribution (Minter et al. 1987)and diageneticor mild metamorphic processes(Thiel et al. 7979)resultedin modificationand neo-formationof uranium ore and associatedminerals.The diageneticallymost affectedmineralswere sulfides,uraninite.native gold, and titaniferousminerals. Mobiiized uranium and titanium led to the formation of brannerite and other U-Ti-phases such as uraniferous leucoxene (Smits 1981: Saagerand Strupp1983).Other authigenicalteration products include uranothorite, coffinite, and orangite (Feather and Glatthaar 1987)and cobaltite, gersdorffite, millgrilg, pentlandite, tucekite, and breithauptite. These authigenic arsenosulfides have Co/l.ii ratios <1 comparedto detrital arsenosulfideswhich have a Co,t{i ratio of >1 (Saagerand Oberthtir 1984). Pyrite is the most abundantsulfide.It is present in severalvarietiesand generations. Uraninite grains contain an averageof 62.7"/" UO2,3.9o/oThO2 and 23.6"/oPbO2.The ThOzl UO2 ratio is highly variablerangingfrom ca. 6 to 47 (Feather7976,7981).Individual grainsare up to ca.0.Zmmin diameter. Feather and Koen (1976) have describedthe mineral assemblages in much detail and established a mineralparagenesis as presentedin Fig. 5.123. Button and Adams (1981) give a comprehensivesynopsison the various detrital and authigenicminerals of the Witwatersrandreefs and compare them with those found in other Lower Proterozic conglomerate-typedeposits (Blind River-Elliot Lake, Serra de Jacobina, etc.). The readeris referred to thesepapersand to more recent works by Saagerand coworkers, Schidlowski(1981), Hallbauer (1986),Feather and Glatthaar (1987)and the classicalworks by Ramdohr (1955)and Liebenberg(1955),which also provide extensivecoverageof referencesfor detailed information on the ore and associated minerals.
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lirb
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alo
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Examplesof Quanz-PebbleConglomerate-Type Uranium Deposits
XLERKSOORP t0tcaIt0l
O F SG O L O F I E L D
c0ur/ur ilur rtttt
:to
fb.tdqt
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ata I
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357
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Fig. 5.122. Wirwatersrand Basin, tithostrarigraphy of the Witwatenrand Supergroup and Dominian Group in the Centrat Rand, Klerksdorp and Orange Free State goldfieids. Principal uraniferous reefs are listed. The West Rand Group correlates approximatelv with the former Lower Division and the Central Rand Group with the Upper Division of the Witwatersrand Group. (Camisani-Calzolari et al. 1986)
358
5 Selected Examples of Economically Significant Types of Uranium Deposits
Economic Minerals Diamond Gold Silver Pyrite Dyscrasire (Os,li.Ru.Pt) attoys lsoferroplatinum
Oxides: Cassiterite Chromite Chrome-spinel Magnetite Columbite Corundum llmenite Magnetite Hematite Goethite Leocoxene, rutiie llmeno-rutile Anatase Brookrte
Rhs-' (Hh.-Pr) alloys
Michenerite Moncheire (Pd,Ag,Te) mineral Geversite Ru (As,S).' Uraninite Uranothorite Carbon Sperrylite Hollingwonhite Laurite Stromeyerite Proustite Gold telluride Braggite* Cooperile* Brannerite
Arsenides,etc. Arsenopyrite, danaite Glaucodot Marcasite MolyMenite Cobaltite Galena Pyrrhotite Niccolire Millerire Leucopyrite Loellingite Satllorire Tennanlite Pentlandire Chalcopyrite Garsdorffite Sphalerire Cubanite S k u n e r u d i t er Chalcopyrrhotite * Linnaeite Bravo'rte Tetrahedrite Mackinawite Ni,sb2s: Bornite Chalcocire Covellite Neo-digenire Stibnire Troilire
-mineral has no name
7 ? ( ?-
r - 6 l o u b t fu l i d e n t i l i c a r i o n .
Four principal modesof uranium mineralization, based on U ore mineralogy, may be distinguished consisting of (a) detrital grains of dominantly uraninite associated with minor monazite.euxenite. amountsof uranium-bearing
Fig. 5.123. Wirwatersrand Basin, mineralogl, and paragenesisof ore and associated minerals contained in the matrix of conglomerare reefs. Heavt rypefaceshows the relativelv most abundant and economicallv imponant phases. Stage I derrital minerals: stage2 main period of pyrite mineralization; stage3 main period of gold remobilization and secondarv sulide mineraiization. (Feather and Koen 1975)
zircon,allanite,and/orxenotime.(b) uraniferous phyllosilicates in concretionarl'pyrite nodules and in lenticles of clay minerals (Simpsonand Bowles 1977),(c) authigenicmineralsof dominantly complex U-Ti-phases ranging from
Examplesof Quartz-PebbleConglomerate-TypeUranium Deposits
brannerite to uraniferous leucoxene,and locally some uranothorite, and coffinite, (d) uraniferous carbonaceous matter composed of either polymenzed hydrocarbons or columnar "thucholite" containing entrapped uraninite grains or of granular ("fly-speck") carbon buttons associated with pitchblendeiuraniniteand pyrite (Robb and lvfeyer 1985). Lithology-related uranium distribution is differentiated into five types by Pretorius (1974). Uranium accumulated(a) as matrix constituentin conglomerates composed of dominantly quartz pebbles rvith a matrix of mainly quartz, sericite, chlorite, pyrophyllite and chioritoid. (b) in pyritic sands that filled shallow erosion channels during successive cycles of sedimentation, (c) in quartzites along unconformity surfaces, (d) in shales along unconformity surfaces. and (e) in carbonaceous bands on or adjacent to unconformirv surfaces often present as continuous mineralized seams, ca. 1mm to 2cm thick. The last three ore types were formed in the terminal stages of one cycle of sedimentation and the two first tyoes in the initial stages of a sedimentation cycle. Tlte svatigraphic distribution of the Au-U reefs in the Witwatersrand Supergroup is shown in Fig. 5.722. The Central Rand Group hosrs the mosr important uraniferous reefs particularly within the Johannesburg Subgroup. More than 80% of the uranium production of the Witwatersrand has come from this subgroup. Other urani,ferous reefs are within the basal section of the Dominion Group (Lower and Upper Reef), the Government SubgroupAVest Rand Group. the Ventersdorp Contact Reefi Ventersdorp Supergroup, and the Black Reef/ Transvaal Sequence. Most of these mineralized conglomerates are subeconomicwith respect to uranlum.
359
Typical features of a mineralized reef in the more than 800m thick JohannesburgSubgroup may be demonstratedby the Vaal Reef as found in the Stilfontain Mine, Klerksdorp district (Hahn 197,1). In general. the Vaal Reef is defined by numerous transport channels and local deltas which cover an area of approximately 260km2 (Winter 196a) (Table 5.36). The Vaal Reef is considered to be the product of a meandering stream system on a broad coalescentflood plain. The direction of transportwas to the southeast.In total view, the Vaal Reef dips to the southeast from 12 to 60" and has a maximum thickness of 100cm. It overlies a carbonaceous(coaly) and frequently thucholite-bearinghorizon. Fauiting caused downdip displacementsand occasionally overthrusts, which resulted locally in tectonic superposition (doubling) of the ore horizon. The Vaal Reef is composed of well-sorted and well-rounded pebbles of milky to dark quanz and occasionally chert in a matrix of silica and carbonate. Larger pebbles are normally concentrated at the baseof the reef. The matrix contains gold (average 13ppm), the uranium minerals (500ppm U:Oa), pyrite (3%) and accessory minerals of silver, copper, lead. nickel. and cobalt. Fossil hydrocarbon occurs disseminated as fine specks, in a seam approximately 5cm thick at the base of the reef. Uraninite is the main uranium mineral. Its degree of roundness is equivalent to that of monazite and other heav.v minerais. Uranium is distributed over the whole reef. About 20"h of the uraninite. however. has accumulated at the base of the reef with 80% of the gold. The total uranium content increases with reef thickness. In the Stilfontain Mine itself the Vaal Reef consists mainlv of one bed. which bifurcates occasionallv into rwo. The thickness ranses from
Table 5.36. Witwatersrand Basin, Central Rand Group, dimensionsof selectedAu-U-bearing reefs. [Aner Bunon and Adams 1981. based on (1) Minter t978; (2) areasgiven bv Minter 1978,for the placer sheetas a whole. incluciingsome unpay intervals. (3) approximatefigurescalculatedby Button and Adams 1981,(4) Prerorius1976b.(5) Whireside et ai.
r9761
Dimension Reef Vaal Basal/Steyn Carbon Leader
Long dimension("' (km)
Short dimension (km)
Approxrmate area(km-)
Thickness Ore tonnage (mt x ld) range(m)
Grade U3O3 (ppm)
Grade Au (ppm)
:0'
3-gt 4-gr ca. )-
z# 4N 1253
o-1r 0-.1' 0-0.3
259{ 1964 2494
13' 15' 21'
1a I
uindicates long axis perpendicular to mean sediment transpon direction
2od 50d 45r
3ffi
5 Selected gyamples of Economically Significant Types of tlranium Deposits
1 to 90crnand averages24cm. The averagemetal contentis 510ppmU:Os, 10ppm Au,2"h pyrite. The mineralized conglomerate overlies a layer with a matrix containingcarbonaceousmatter and uranium, the latter often as thucholite. In the past, uranium has also been recovered from other reefs of.the Central Rand Group. The minesat Carletonville (Far West Rand) exploited of the Main ConglomerateFormaconglomerates tion (grade: 0.015 to 0.077o/oUsOe). In the Kinross area, uranium \\'as extracted from the Kimberlev ConglomerateFormation (grade:0.02 to 0.025p/' U:Oe). Conglomerate beds of the Elsburg Quartzite Formation containonly minor amountsof uranium, which have not yet been recovered. On a regional basis, Au-U-bearing beds are found within six major fluvial /ans constituting from SW to NE the gold fieldsof the OrangeFree Stateor Welkom, Klerksdorp, Far West Rand or Carletonville. West Rand. East Rand, and Evander(Fig. 5.121). The original Central Rand represents the coalescence of the easternpart of the West Rand fan and the westernpart of the East Rand fan. The fans have lateral dimersioru of up to several tens of kilometers (Fig.5.124). The largestof the fans is that which containsthe East Rand gold and uranium field. It measures40km in length along the fan center, 50km in width acrossthe midfan section and 90km in width acrossthe fan base. The western margin of this fan is 45 km long and the easternflank 60km. A fan may contain several oligomictic conglomerate beds (reefs) with uranium and gold
contentsof economicinterest. The thicknessof individual reefs rangesfrom about 5 to 200cm and locallyto about 400cm. Lateral extensionsof pay-streakscan reachseveralhundredsto several thousandsof metersparallelto the paleo-stream. Across simple paleo-channelsthe deposits are severalmetersto tensof meterswide, and across coalescentchannelsup to several hundreds of meters. The uranium gradesare variablebut generallv very low. During the peak period of U production, the in-situgradeof ore mined rn 7982ranged from 0.016(Harmony)to 0.077"/o U3Os (Beisa). Approximately50 to 70% of the in-situ contentis recoverable. For comparison, dimensions.grades, U/Au ratios and pebble compositionof selectedreefs are given in Tabies5.36 and 5.37. Figure 5.125 showsthe iithologicfeaturesand positionand Fig. 5.126the distribution and spatial relationshipof U and Au in a seiectedreef. Uraninite grains from Witwatersrand ores yield apparent U/Pb ages of 3065 + 100m.y., which is compatiblewith that of granitesin the surroundingof WitwatersrandBasin and a rejuvenationageof 2M0m.y. that is coevalwith the emplacementof the BushveldComplex (Allsopp and Welke 1986).
PotentialSourcesof Uranium Uraninite grainshave up to ca. 10o/oThO2 which is typical for uraninites from granites and pegmatites.The apparent age of ca. 3065m.y. of
Table 5.37. Wirwatersrand Basin. Central Rand Group. thickness,composition and U/Au ratios of selected reef horizons. (After Pretorius 1974) Reef group
Reef horizon
Gold-field
Bird Bird Bird Bird Bird
Znne2 Monarch White Vaal Basal
West Rand West Rand West Rand Klerksdorp Welkom/OFS
Main Main Main Main
Uvingstone South Main North
West Rand West Rand West Rand West Rand
TN
sz
QU CT
Average thickness(cm) Average sizeof l0largest pebbles(mm) Percentage vein quartz pebbles Percentage chert pebbles
TN
SZ
QU
7'/ 62 21 991882611-435 35128583440 152285t23-11 3620602351218 50 35 i0l't8677-14 1103784610-9 503083611
CT
QT
OS
7
16
-
U/Au 769
11
Q T : Percentagequaruite pebbles O S : Percentageother types of pebbles U/Au : Ratio of uranium content to sold content
Examplesof Quartz-Pebble Conglomerate-Type Uranium Deposits
E + ++
+ ++ +
6 55: h-^-o:
ffi
=^l
361
u c r i t e cd o m e s Fcnneod focies
Miifcniocies
[=f::Tl] F o n b c s e f o c i e s Lr'tij::::iia I
-.o')
Ecrlier low-energy foc:es Zone of wover a w c r ki n g -
6"6,
N
-->
TT
L c n g r r u d i n o lf c u l t
P:ysrreok chonnels F , r , v l oSt y s i e m
L c c u s t r i n el o n g s h o r e Fig. 5.124. WitwatersrandBasin.conceprualmodelof the configuration, sedimentologi-= cal facresdistnbutron and localizacionof mineralizedchannelsin a progradingfluvial fan. The ian developed at the mouth of a flulial systemdewateringfrom betweenuptifted granite domes. Clockwise flow direction of long-shore currents in the lake causedthe asymmetricalshapeof the lobe. (After Pretorius 1974. 1981) (in part reproducedfrom EconomicGeology,1981.Spec.Vol., p. 134)
uraninite (Allsopp and Welke 1986) atteststo a source of Archean age. Granitic domes of Archean age are mapped in the vicinity of the WitwatersrandBasin.Thev includecraniticfacies cm 900
800
600
enrichedin uranium.For example,the Vredefort dome containsrelicsof a "HHP-tvpe" (high heat production) granite affected by hydrothermal alteration.The alteredgranite has uranium contentsof up to 30ppm and 70ppm Th. Part of the uranium is bound in uraninite. (Andreoli et al. 1988;Robb and Meyer 1985).These data suggest that these kinds of granites provided a potential sourcefor the uraninite found in the conglomerates.
Ore Control and RecognitionCriteria Essentialore controllineor recognitioncriteria include:
400
Host Environment Au-tJ Plc,cer Z o ne
Fig. 5.f25. Witwatersrand Basin. Carletonville Goldfield, lithologic section of the Carbon Leader Formation. JohannesburgSubgroup, showing the posirion of mineralized conglomerate beds. (Buck and Minter 1987)
- Host rock is a pyritiferousoiigomicticquartzpebble conglomerate deposited in Lower Proterozoictime prior to the oxvatmoversion of the earth atmosphere. - Favorable conglomerates alternating with dominantly arenaceous horizons were deposited in severalcyclesseparatedby major and intraformationalunconformities. - The conglomerateswere laid down in fluvialdeltaicfansin an intracontinentalbasin.
'362
5 Selected Examples of Economically Signifcant Types of Uranium Deposits
+ r ffiuign",
g r o d e( o ) U o n d ( b )A u t r e n d s
F-.J nreo of beiter {o) U ond lb) Au minerolizotion [T]
nreo of poorer {o) U ond {b) Au minerolizotion
G
m
u/Au rotio
'12
rr=l 'o L-:--J Fig. 5.12.6.Wit*'atersrand Basin, CarletonvilleGoldfield, surfaceplan of the Carbon Leader reef with distribution of a relative uranium contents, b relative gold contents, and c UiAu ratios sbowing the downstream oriented relative enrichment of uranium. (Buck and Minter 1987)
-
-
Provenance of the congiomerates was from a variety of Archean rocks including uraniferous granites. Younger conglomerates derived partly by reworking of those deposited earlier.
-
remobilization of the ore forming minerals (see below). Paleoweathering of the Archean surface and the intraformational erosion planes in the Witwatersrand Basin fill was predominantly of physical nature.
Aheration -
There is no alteration associated with the deposition of the detrital ore forming minerals. Post-depositional modifications resulted particularly in intense pyritization and partial
Mineralization -
Original uranium minerals are detrital uraninite and uranothorite associated with gold and other hearryminerals (Fig. 5.123).
Examplesof Quartz-Pebble Conglomerate-T,v-pe Uranium Deposits
363
Ore minerals produced by postdepositional- In shallow depressionswith gentle slopes, processes include U-Ti-phases, coffinite. normally well-developedcong.lomerates with arsenosulfides, and sulfides. high metalgradesprevail.In deepdepressions The distributionof the Au-U ore reflectsa with steepslopesthe conglomerates are poorly predominantsedimentological conrrol and is developedin the center but the peripheral characterized by the following features(von zones consist of well-developed,densely BackstromI975, 1976;Pretorius1974,I975. packed conglomerateswith eenerally high 19't6b): valuesof uraniumand gold. Uranium and gold are concentratedin a - The uranium and gold mineralizationsrarely quantitative ratio of 5->500U to 1Au (Fig. extend into the adjacent arenaceoussedi5.126, Tables 5.36. 5.37) in conslomerates ments.exceptwhereredistributionby erosion adjacentto the edgeof a continentalstructural and redeposition of older ore bedsoccurred. - Most reefscontaincarbon/kerogen basin. often formRecoverable uraniumand gold mineralization ing continuousmineralizedseamsas much as occursparticularlyin fluvial-deltaic fansin the 5 cm thick. Centrai Rand Group of the Wirwatersrand Supergroupwhich are distributedin a belt running parallel to the original western and Metallogenetic Concepts northerncoastline(Fig. 5.121). Important concentrations of ore are found only The most favored metallogenetichypothesisis in distinctnarrow stratigraphic-lithologic zones the modifiedplacermodel that includesa synsediwhich occupy approximately2o/oof the total mentarydetritalore mineraldepositionsucceeded CentralRand Group. by post-depositional modificationsthrough both A tight correlationexistsbefweenconcentra- erosional redistribution into younger conglomtion of ore and sedimentaryfeatures.Enrich- erates and diagenetic-mildmetamorphic proments are commonlyfound cesses.Other geneticmodelsput forward range - in the midfan faciesof the fluvial fans (Fis. from hypogene hydrothermal (e.g. Davidson s.124), 1957) to supergenerollfront (Clemmy 1981) - near the base of congiomeratesalong dis- forming processes. In the early days of uranium tinct argillaceous/shalv footwall boundaries. miningin the 1950'sDavidson(1957)vehemently - in conglomeratic beds less than 30cm defended a hypogene hydrothermal genesis thick. against the "placerists" Ramdohr (1955) and - in pay-streaksrunninggenerallysubparallel Liebenberg(1955). to eachother (Figs.5.124.5.126)and that From the areal geometryof the different oreare characterizedby denselypacked well- bearinghorizonsthe pattern of faciesvariations, roundedand well-sortedpebblesof predom- the trends in the grain-sizechangeof the host inantly quartz and by larger accumulations rock, the patternof the paleo-currentdirections, of heavyminerals. the nature of the depositionalenvironment,and Uraniumand goidtendto be enrichedimmedi- the distributionof heaw minerals.it appearsthat ately abovestratigraphicand intraformational, gold and uraniummineralsaccumulatedwithin a local unconformities, in particular where fluvial fan systemor a fan delta where the river conglomerates fill depressions or excavations systemwas dischargedinto a large lake. which in the footwall rocks. or where conglomerates meansthat the uranium and gold accumulations abut a slight rise or swell in the underlying constituted an integrai part of a short. highformations (Fig. 5.I25). (Some important energyfluvial transfersystemfrom the sourceto uranium- and gold-bearing conglomerates, the depositorv. however, also occur within the stratigraphic Pretorius i1974) elaboratesin detail on this sequenceindependentof unconformities). hypothesisand arrives. in summary,at tbe folThe distributionof uranium and gold within a lowing conclusion. The older Witwatersrand transgressivereef, suprajacent to an un- sedimentswere depositedin a fluvial-deltaicenconformity, is clearlyinfluencedby the degree vironmentwithin an intermontaneiintracratonic, of denudationor depthof erosionof the under- yokedbasin.The basinwas fault-boundedon the northwestern edge and downwarped on the lying sediments.
3&
5 Selected Examples of Economically Signifcant Types of Uranium Deposits
southeastern boundary. Transport of the sediments was by high-energy transfer systems from the source in the northwest to the depository. Fluvial fan systems or fan deltas were formed where the river systems were discharged into a large lake via a canyon. After emerging from the canyons, the rivers flowed short distances over a piedmont plain and were then dispersed through a braided-stream system into the basin (Fig. 5.124). The fluvial fans were restricted to the northwestern margin of the depository. Some of them coalescedin their more distal parts, creating extensive sheets of uniform gravel (Fig. 5.L27). A rypical fluvial fan of this kind had two main Iobes in which a large number of braided-stream channels developed (Fig. 5.124), thicker and coarser clastic sediments deposited and higher concentrations of detrital gold and uranium accumulated. The material laid down between the lobes was predominantly psammites and pelites, similar to the sediments accumulated on the fan margins and on its base. Conditions under these lower-energy regimes between the lobes, which existed also at the end of cenail cycles of sedimentation on the fans of the major rivers, at times provided a favorable environment for the growth of thin algal or lichen colonies. Trarcport of gold and uranium was as detrital af1g1als, and the gold also in solution perhaps as chloride and cyanide complexes. Concentration of Au and U took place (a) physically through gravity settling and subsequent winnowing by wave and current action and (b) biochemically through interaction between Au and {J, and algal or lichen mats. The gold was supposedly of too small grain size to precipitate in the fanhead facies. Instead, the highest gold settiement took place in the midfan lobes, whereas the peak of the uranium deposition occurred a little farther down-slope (Figs. 5.124, 5.126). Further downstream, the fluvial energv level dropped rapidli' below the transport capability' for detntal gold and uranium, hence these heav-vminerals could not be transported to the fanbase section. Only the gold and uranium in solution were transported beyond the energy threshold until they encountered and interacted with the biogenic material in the low-energy environment. End-of-cycle winnowing by the waters of the streams and the lake led to a greater concentration of residual heavy minerals on the erosion surface.
At the start of the sedimentation of the next cycle, reworking of. this depositional pediment destroyed the thin streaks of lag gold and uranium at the unconformity plane. These minerals were then redistributed into the younger gravels by pickup from the footwall rocks and downward infiltration during the sand deposition that followed the deposition of the pebble phase. Dunng this stage and after burial, uraninite was destabil2ed to various degrees by physical and chemical action including break-up by lichen growth, as documented by Pretorius (1974), and Smits(1981). Minter et aI. (1987) establishedthe mineralogical changes at the Eldorado paleosurface at the Welkom Goldfield. where the pediment representsthe iast major period of erosionat the end of the deposition of the Witwatersrand sequence. The suite of reworked ore minerak identified in fluvial bedioad concentrates within shallow paleochannels and conglomeratic diamictites resembles that present in the suboutcropping conglomerates except for the virtual lack of uraninite. The dominant uranium minerals are brannerite and uraniferous leucoxene, which occur associated with detrital pyrite and other heavy minerals. Pyrite is present in three varieties, all of which display abrasional rounding. One type of pyrite correspondsto that originally formed as authigenic mineral in the older placers and therefore suggests that it represents a reworked accumulate. This interpretation is supported by the rounded nature of kerogen particles which appear to have originated by erosion from fragmented columnar kerogen. The kerogen granules contain uraninite largely altered to brannerite. The alteration of uraninite to brannerite is attributed by the authors to weathering in both the source and depositional area rather than to selectivemodification processesafter burial. The almost complete removal of uraninite may reflect an increasing content of oxygen in the atmosphere at the end of the Witwatersrand period. Hallbauer and von Gehlen (1983) addressthe impact of regional and thermal metamorphism on the Witu'atersrand mineralization. Regional metamorphism was weak (lower greenschist facies) and essentialiy isochemical,i.e., without extrinsic admissions. Temperatures ranged between 157 and 250'C, as deduced from inversion of marcasite into pyrite and decrepitation temperatures of secondary fluid inclusions in quartz pebblesand ore mineral compositions.The upper
Examplesof euartz-pebbleconglomerate-Type Uranium Deposits
limit of 250'C is further supported by as much as 17% volatiles contained in the carbonaceous matter, which would have been expelled at temperatures in excessof 250'C. These findings are supported by Saager and Oberthiir (1984). Based on their studies on authigenic Ni-Co minerals. the two authors assume a low-grade metamorphic overprint at temperatures commoniy beiow 250"C. Intrusion of. volcanic dikes and slils caused a certain mineral redistribution within a spatially restricted halo. Particularly large quantities of authigenic pyrite recrystallized. and pvrite. galena, sphalerite, and chalcopvrite. but rarely
365
gold were redepositedin veinlets (Hallbauer and von Gehlen 1983). In summary,the above-listedsedimentological, mineralogical,and minerochemicalparametersof the auriferous and uraniferous conglomerate reefs of the Dominion Group and Witwatersrand Supergroup are largely consistent with the modified placer modei favored by most investigators. though not unanimouslv. The suggestedsuccessionof events leading to the formation of uraniferous reefs beeins with -
anomalous uranium concentrations in form of uraninite and other ore-forming heavv
Table 5.3E. South Africa. late Archean-early Lower Proterozoiceventsin the KaapvaalCraton. (Andreoli er al. 1988. basedon referenceslistedthere) Cvcle
Geologicalevent
U-relatedprocess
VIII
VentersdorpSupergroupdeposition: rifting; basalticand bimodal volcanism;arenaceoussediments
Minor brannerite/uraninitein goldbearingbasaiplacer
\/II
CentralRandGroup deposirion: repeateduptifting of the hinterland causesreworkingof older strata, intensifiedbasementerosion and developmentof quaru-pebble conglomeratesin intra-group unconformities
Depositionof economically imponant uraninite- kerogen-U-Tioxides- gold-bearingcongiomerates. Rare monazite and other Th-bearins phases
\/I
West Rand Group deposition: prevalenceof shalein marine shelf and tidal environment
Minor developmentof conglomerates with minor U (andTh)-beanng phases
)
Widespreadhydrothermalactivity affectsbasementand Domrnron Group cover in the hinterland of the WitwatersrandBasin.Tectonic activitv, hydraulicfracturing. brecciation
H2O. CO?,F, B, S. -bearingfluids introducepyrite, uraniumand gold
Wide rangeof aqesmay applv
iV
Dominion Croup deposition:minor clasticsedimentationand voicanism
(Th-) Uraninite, monazite-richgoldbearingbasalconglomerate:kerogen rale
2 . i 2+ 0 . 0 7
III
Late Archaeangranite intrusionsin - RenekeDome. the Schweizer Major tectonothermalevent results in the formation of the Vredefon discontinuitv : granulitefacies metamorphismat depth
Migrationof uraniumat hieher crustallevel (?) Depletion of uraniumfrom rocks belowthe Vredefortdiscontinuirv
Mozaan Group depositionin the EasternTransyaal
Brannenteand minor urarunitein Mozaanconglomerates
+2.9
Metamorphismand extensiveU. Th. fluxrngthrough a Barbenon Mountain Land-typeterrarn as evident in the verticai Vredefon profiie. High level HHP granite in the Vredefon structure
Distribution of U and Th accordingto an exponentia.lvertical profile
+3.05
II
PreferentialU enrichmentat high crustal level
Age (b.y.)
<2.74 >-2.74 =-. t6
366
5 Selected Examples of Economically Significant Tlpes of Uraniun Deposits
minerals in certain hydrothermally altered granitic and/or pegmatitic-$anitic complexes integrated in the Archean granitegreenstonebelt, located northwesterlyof the WitwatersrandBasin. - Liberation of the detritalmineralsby predominantly physicalweatheringas reflectedby the paleosol topping the crystalline basement (Grandstaffet al. 1986). - Transport and redepositiongovernedby fluvial systemsas documentedby Pretorius (1974). Schidlowskiet al. (1975)provided convincing evidence that the atmospherewas orygendeficient during this time interval (see Chap. - Destabilizationof uraniniteto variousdegrees b-v physical and chemical processesdunng depositionand after burial. Post-burialmobilization of uraniumtogetherwith alterationand recrystallizqtionof other minerals occurred during and/or subsequentto regional metamorphism. locally associatedwith volcanic intrusions. - Preservationof the ore by a thick cover of sedimentsand cratonicstabilization,i.e., lack of major intrusiveand tectonicevents. A summary of eventsactive in the Kaapvaal Craton and leadingto the Au-U mineralizationin the WitwatersrandBasin has been compiled by Andreoli et al. (1988)and is presentedin Table 5.38.
Saager1983;Hutchison 1975;Jacob 1966; Knowles 1966; Koen 1961, 1962;Koppel and Saager1974; Krapez 1980; Liebenberg1955,1973;Malan !.959;McKinney eral. 1964; Mclachlan 1968; Meyer et al. 1987: Minter 1970, 1972, 1976, 19'78,1979; Minter et al. 1986. 1987: Nicolaysen et al. 1962; Oberthiir 1983, 1985. 1987; Onlepp 1962: Papentus1964;Pretorius 1964a.1964b.1974, 1975.1976a. 1976b, 1981, 1986a,1986b;Ramdohr 1955. 1958, 1980; Reimer 1975; Reinecke 1927;Rundle and Snelling 1977; Saager1968,1969,1970,193: Saageret al. 1982, 1983; Saager and Esselaar 1969; Saager and Mih:iiik 1967; Saager and Muff 1978; Saager and Oberthrir 1984; Schidlowski 1966a.b,c.d,e,f,1967b. 1969. 1970, 1981: Shepherd1977; Simpsonand Bowles 1977. 1981; Sims 1969; Smith and Minter 1980;Smits 1981. 1984, 1987: Snyman 1965;Steyn 1977;Taylor et al. 1962: Thiel et ai. 1979;Toens and Griffiths 1964;Toens and le Roux 197E; Toens et al. 1980;Tweedie1968.197E.1986:Tucker and Viljoen 1986; Utter 1977:Viljoen 1961. 1961. 1968; von von Rahden 1970:von Rahden and Backstr6m 19'15.1976l. Hiemstra 1967; Whiteside 1970;Winter 196-11 Zumberse e t a l . 1 9 7 8 .1 9 8 1 .
5.8 Examplesof Intrusive-TypeUranium Deposits(Type9, Chap.4)Alaskite Uranium Deposits:Rtissing,Damara OrogenicBelt, Namibia
R6ssingis located in the Namib desert approximately 65km NE of Swakopmundin western central Namibia. Total reservesin the $30nb. U3O8 RAR categoryare estimatedat approximately 130000mtU3O8(OECD/NEA 1986)of which about 55000mt U3Oshave been recovered Referencesand Further Readingfor Chapter5.7.2 from a large open pit until 1991. The average gradeis approximately 0.03to 0.0491' U:Oa. (for detailsof publicationseeBibliography) The uranium mineralizationis disseminated, Andreoli et al. 1988; Anhaeusser 1973. 1975. 1976: indigenousin an intrusive leucogranite/alaskite Anhaeusser et al. 19691Antrobus 1986; Anrrobus er al. and is thereforeclassified as intrusiveaiaskite1986; Antrobus and Wlriteside 1964: Armstrone 1968: tvpe uraniumdeposit(subtype9.1. Chap. 4). Bounet 1975:Bowles 197?:Brvnard. HJ. pers.coit-un.: The subsequentdepositdescriptionis largely Brynard et al. 19881Buck 1983;Buck and Minter 1987; Burger and Coertze1973;Burger et al. 1962;Burke et al. basedon Berning et al. (1976).Berning (1986). 1986;Button 1968,7973,1976-l Button and Adams 1981; and Toens and Corner (1980)suppiemented by Button and Tyler 1979: Camisani-Calzolarier al. 1985; data of the other authors cited. The regional Chamber of Mines of S.A. and A.E.B. of S.A. 1985; Clemmev 1981:Coetzee1965;Corner et al. 1986;Cousins geology has been summarized mainly from 1960.1965,1973;Davidson1953.1957:De Kock 1964;De Brynardand Andreoli (1988).
Waal and Herzberg 1969; Dimroth 1979; Du Toit 1954; Feather 1976,'1981:'Feather and Glanhaar 1987;Feather and Koen 1975,198i; Featherand Snegg1978;Frel' 1981; Frey et al. 1987;Fuller 1958;Glanhaarand Feather1984; Grandstaff1974b1980:Hahn 1974:Hallbauer19'75,1977, 1980, 1983, 1986; Hallbauer et al. 1977:Hallbauer and Joughin 1973; Hallbauer and Kable 1982;Hallbauer and Urter 1971; Hallbauer and van Warmelo 1974;Hallbauer and von Gehlen 1983; Herzberg W, pers. commun.; Hiemstra 1968a, 1968b; Hirdes 1979, 1984; Hirdes and
Geological Settingof Mineralization The Upper ProterozoicDamara orogenic belt is 400 to 500km wide and extendsin northeasterly' direction from the Atlantic Ocean in the SW acrosssouthwesternAfrica before submerging
Examplesof Intrusive-TvpeUranium Deposits,Alaskite Uranium Deposits
tot
beneaththe post-Paleozoic KalahariBasin.Fur- sediments.The geometric forms are generally of ther north it eventuallylinksup with the Lufilian irrezular shape and extension. They occur as belt of similar ase in Zambia-Zaire. simple pegmatitic appearing dikes. apophyses, The Damarabranchin Namibiahas beensub- or lenses.and batholithic bodies of considerable dividedinto four mainstructuralzones(from NW size. The batholithic bodies contain numerous to SE): The Northern. Transition. Central or xenolithsof metasedimentsseveraltens of meters Core. and the SouthernZone. to more than 100m in diameter. Country rocks All known uraniumalaskiteoccurrences occur intruded by alaskitic granites are biotite gneisses within the CentralZone.Thiszone is constituted of the Etusis Formation. pyroxene-amphibolebv a broadly uplifted block of metasedimenrsbiotite gneissesand schists. and marbles of the and intrusions.and displaysdomal antiforms, Khan and Rossing formations. Quartzites and commonlvNE-SW elongated, and synforms.The similar rock tvpes were mostly avoided. metasedimentswere subjectedto amphibolite Where alaskitesrvere emplaced into Rossing grade metamorphism(estimatedpeak T -555to metasediments,metablastesisof feldspar affected 645'C, P 2.6 to 3.4kbar)with polvphasedefor- adjacent biotite-cordierite schists. Contactmationaccompanied by theintrusionof pre-,syn- metamorphism is aiso noted as skarn developed and post-tectonic granites. s e v e r a lm e t e r si n t o m a r b l e s . Five pnncipal varieties/generations Dolerite dikes of post-Karoo (Triassic) age cut of granitic rocks intruded into the metasediments of the all the Precambriansuites. Damara belt (Table 5.39) includin_ealaskites. The regional tectonic grain is characterized by The intruded metasedimentsinciude severai dense, almost vertical to slightly ovenurned, NEstratigraphicunits thosein the Rossingarea are SW-trending folds. Steeply inclined cleavagesand presentedin Table 5.40with their corresponding joints within alaskitic bodies and metasediments lithologies. exhibit a pronounced NNW-SSE strike and a less Certain alaskitic granites (for petrographic distinct NE-SW strike. descriptionseenext paragraph)are of interestfor Rdssing is located on the southern flank of a uranium mineralization.Although granitic intru- regional oval NE-SW trending dome. Fracturing sivesare widespreadand may contain anomalous created in the zone of the deposit transverse, concentrationsof uranium.they rarelv carrv ore steeply dipping faults with small scale vertical grade concentrations. The alaskitic granites, dispiacements.but with strike-slipmovements of sterile as well as mineralized, intruded con- m o r e t h a n 5 0 m . cordantlv and disconcordantlv along beddingand schistositvplanes into the steeplydipping meta-
Table 5,39. Damara Orogen. tvpes, averagecontentsof U, Th. K. Th/U ratios and agesof the main graruregenerations (Bnnard and Andreoli 1988,basedon Haack et al. 1983and (l) Toens er aI. 19'79', agesfrom (2) lvlarlow 1983(3) Kroner and Hawkesworth 1977)n = nwnber of samples.
U (ppm) Th (ppmt K (96) Th/U .\gem.y. min-max min-max min-max min-max
Rock type
Saiem-tvpegranite (granodiorite,granire. adamellite emplacedwell below level of Karibib Fm.)
78 5.8 1-14
Red granite (domes. dikes. lit-par-lit intrusions emplacedbelow level of Karibib Fm.)
26
Leucogranite (diapin, plugs)
36 3.7 l- 15
r-.6
36.8 12-104
5 .i 3-7
79.0
5.7
tJ- zt J
L-t
la
n
0.1-34
|
1
7.5 2-27
|ill
12.8
i16 + a? /r1)
:-
+ 7q 1?\
LO
J. I
2-7
0 . 1- 1 3
18-l +-25 (2)
Mineralized alaskite' (domes,dikes, anastomosingveins emplacedat Khan and R6ssingFm. level)
12 319.0 39.0 38- I 120 2 - 9 8
5.1 2-8
0.3 0.02-2
{68 t 25(3) (Rossing)
Unmineralized alaskitel
106
5.2 t-7
,1.3 I-20
512+ 33(2) (SwakopRiver)
I
t?
30 1-l0l
368
5 Selected Examples of Economically Significant Types of Uranium Deposits
Table 5.40. Rossing area, lithostratigraphy of metasediments. (After Berning 1986,basedon South African Committee on Stratigraphic Nomenclature) Sequence
Group
Subgroup Formation l,ocal lithostratigraphic units
I(homas
3000
Kuiseb
Quaru-biotite schist
Karibib
Marble and ouartz-biotite schist
350
Cbuos
Tillite
300
Swakop Ugab
Thickness (m)
R6ssing
Damara (Upper Proterozoic) Khan
Feldspathic quaruite Upper biotite-cordieritegneiss
50 50
Upper marble Conglomerate l-ower biotite-cordierite gnerss Lower marble
60 5 40 40
Biotite-amphiboleschist Upper pyroxene-hornblendegneiss
15 90
Lower pyroxene-hornblende gneiss Upper biotite gneiss Etusis Marker quartzite L-owerbiorite gneiss Feldspathic quaruite Unconformitv Abbabis Complex (early Precambrian) Nosib
Principal Characteristicsof Mineralization
Strarigraphicposition of Rossinguraniferous pegnatitic granite/ alaskite (late to oost Damaran tectogenesis)
110 2ffi 5 180 zffi
Mineralogicaldistribution of uranium in crude ore is approximately55% in uraninite, 5"r" in Principal ore mineralsare uraninite, betafite, and betafite,and40Yoin hexavalentU minerals. hexavalent uranium minerals, primarily betaThe uranium-bearinghost rock is generally uranophane.Associatedminerals are monazite, describedas alaskiteor alaskitic-granite(Berning zircon, apatite, titanite, occasionally pyrite, et al. 1976)intruded into the Damara Sequence chalcopyrite,bornite, molybdenite,arsenopyrite, (Table 5.40, Figs. 5.I27, 5.128). The main magnetite,hematite,ilmenite.and fluorite. rock constituentsare quartz, microcline. and The ore mineralsare of minute size.Uraninite mirocline-perthite.Biotite occursin subordinate hasa diameterof a few micronsto 0.3mm, with a quantity, although local enrichments exist. frequencyin the 0.05to 0.1mm fraction.It occurs Fluorite is present in accessorialamount. Acas inclusionsin quartz.feldspar,and biotite, in cording to other investigations(Adloff pers. intergranularspacesor intersticesand in micro- commun.), there are rock facies with more fractures within the mentioned minerals. and than 20% plagioclase,which would classif' revealsa particularaffinity to biotite and zircon. the investigatedsamplesas aplite-graniteacIt encloses or is interglown with the latter. cording to Troger (1969).The grain size averBetafite occurs predominantly as inclusions in ages1 to 5 mm, but can be as much as several quartz and feldspar. centimeters.Textures are variable, correspondThe hexavalent uranium minerals are ap- ing to those of aplite, granite, and pegmatite. parently of secondaryorigin. Beta-uranophane, The latter is prevailing. Graphic fabric occurs and others, replace the primary uraninite and locally. partly the betafitein situ. They also occur as fine Chemistryand mineralcompositionare rather films or rarely ascrystalsin cleavages and fissures. uniform and constantboth in pian and vertical Their presenceis not restrictedto alaskite, but view. Primary uraniummineralsoccur dissemiextendslocallyinto adjacentmetasediments. nated throughout the alaskitic body', but are
Examplesof Intrusive-TypeUranium Deposits,Alaskite Uranium Deposits
locally concentratedin bandsor clustersof biotite causing an irregular distribution of the ore. The Rdssing deposit has dimensions of a NWSE width of approximately 500 to 600m and a slightly longer NE-SW length. Minable ore is proven to a depth of approximately300 m (lowest level of open pit) but drilling intersected ore grades to a depth of at least 700m. The average ore grade is 0.03 to 0.04% U3Os but there are strong variations with local grades of up to 1% U3O8. The ore occurs in two main zones, a northern and central zone, separatedby a wide wedge of of the barren upper pyroxene-hornblendegneiss Khan Formation. Toward the east, the barren track pinches out and the rwo mineralized zones. although thinned. conjugate. All the metasedimentsstrike about NE-SW and dip steeplySE ( F i g . 5 . L 2 7 , 5 . 1 2 8 ) . A t t h e w e s t e r ne n d o f b o t h ore zones exist a near surface accumulation of ore that lacks depth. The enrichment plunges progressively deeper towards the east unti-l it is lost in blind ore shoots in depth. Tlte age of the Rossing alaskite is given as 468 + Sm.y. (Kroner and Hawkesworth 1977 in Berning 1986).
-
-
-
-
-
369
Syn- and post-tectonicemplacement. Larger and better mineralized alaskites are post-tectonic. Petrographic composition corresponds to potassium alkali-feldspar granites with dominance. Rapidly changinggrain size between 1 mm and 20cm. No textural or mineral zonation. Considerable variations in U content in lndividual bodies and among adjacent bodies. Completely barren and economically mineralized intrusives of apparently similar mineralogy and texture are positioned close together. Mineralized intrusives are usually biotitebearing and microcline rich, contain smoky quartz and weather reddish-brown. Intrusives characterized by presence of muscovite,garnet or major amounts of plagioclase are less likely to be mineralized. Thru ratios in mineralized alaskites range from 0.03 to 0.5 but commonly fall within the lower end of this range for better mineralized varieties.
Berning (1986) hints at the complex nature of the R6ssing ore zone reflected by: Ore Control and Recognition Criteria Principal ore controls and recognition criteria include:
-
H ost Environment and Mineralization
Mixture of uraniferous alaskite and barren metasediments. Size of alaskite bodies ranges from large masses to narrow bodies interbanded with barren metasediments.
Jacob et al. (1986) report the following recog- Berning et al. (1976) list the following parameters nition criteria for uraniferous alaskites in the controlling uranium concentrations within the Damara mobile belt in Namibia includingRossing: Rossingalaskite: -
-
Occurrence restricted to the Central Zone of. the belt. Confinement to areas of highest metamorphic grade. Situation along the Abbabis Swell. Preferential occurrence in and around anticlinal and dome structures. Association with the older red granite-gneiss suite. Intrusion levels in basement: Nosib Group and lower Swakop Group, mainly below the prominent marbles of the Karibib Formation and with apparent preference to the Khan and Rdssing formations. Intrusion after earlv migmatization.
-
Alaskite bodies intruded into metasediments along axial planes. - Alaskites replacing amphibolite. - Locations where wide bodies of alaskite become abruptly constricted and merge into apophysesor dikes. - Zones in alaskite rich in biotite. - The bulk of mineralization is contained at the level of the pyroxene-garnet gneissiamphibolebiotite schist/lower marble, lower cordieritebiotite gneiss units. Because the position, size, and uranium content of the alaskitic intrusions are spatially close to the lower marble band in the Swakop Group,
370
5 Selected Example.sof Emnomically Signifcant Types of Uranium DePosits
l-__l
Scre" ond olluvium
I
uroniterous oloskite
[[T[l[Tln6.ri.'q Formotion [.-Tlll xnon Formotion
,.o? ,e\'
I f
Fig. 5.L27. Rtissing, generaLized geological map of the deposit (for details of lithology, see Table 5.40) (Berning 1986)
Smith (1965) interprets the composition of, and the close spatial relationship between the uraniferous alaskites and their emplacement level at the disconformitybetweenthe Khan and Rossing formations as being suggestiveof an alaskiteorigin during amphibolite grade metamorphism associatedwith pegmatrre(alaskite) formation from uraniferous protosediments. Intrusionof the pegmatitesoccurredwhen declinMetallogenetic Concepts ing metamorphismpermittedbrittle deformation. Von Backstromand Jacobs(1979)and Toens A number of theories on the origin and evolution of the, uraniferous alaskites have been put et al. (1980) propose that the generation of forward. Brynard and Andreoli (1988) present ROssing-typeuranium mineralization involves the most recent review and discussmost models. partialmebingof uraniferousrocksof the Abbabis In essence they state that none of the various Complex and overlying metasedimentsof the models proposed to date adequateiy explains the Nosib Group. Enrichment of uranium in the observed petrological features. Some concepts anatecticmelt resultedfrom fractionalcrystallizapresented arrive at the following metallogenetic tion. The final intrusiveproduct was a fluid-rich pegmatiticgranite. conclusions.
some authors think that the marble band has exerted a control on this setting. Jacob (1974b) suggests that the band acred as a trap for the intruding alaskitic granites. while Cunev (1980) considers it a geochemical reaction barrier (see below).
Examplesof Intrusive-TypeUranium Deposits,Alaskite Uranium Deposits
o.9 :e 6e
ffi li::l
0
Con-ciomerote q.A,-1
brottre gnetss
LILJ Lcwer morble horizon c
YE
F
200m
\
fiTtr G^",r, J
oo
)t I
f--l
S.."" ond otluvium
I
U r o n i t e r o ucsl o s k i t e
g i o t l t " o m p h i b o l es c h i s t
[I''--]:lupper-towerpyroxene-hornblendegneiss
Fig- 5.12E. Rcissing,YW-SE through the uranium mineralizedzone showingthe unpredictabtedistribution -cross-section and irregular shapesof uraniferousalaskite.The openpit outline (dotted) indicatesthe literal eitent of minable mineraiization.(Berning et ai. 196) (reproducedtrom EconomicGeoiogy,1976,v.71.p. 35a)
Winkler (1983) applies phase relationshipsin the system Qz-Ab-Or-An-H2O, and concludes that no origin other than anatexis of metasediments tends to be feasible. A singular longlasting episode of magma generarion by anatexis for a varietv of metasediments provided the source to a varietv of dioritic and granitic magmas rvhich then intruded at different levels of the overlving stratigraphicsuccession. Cuney (1980) studied geochemistry and fluid inclusions in the uraniferous alaskites and presents in summary this hypothesis. The intruding alaskitic magma reacted with the marbles of the Rossing Formation which acted as a geochemical reaction barrier. Reacrion with the marble caused boiling of the magma by an increase in CO2 partial pressure and crystallization. An immiscibility befween a CO2-rich and a dense saline fluid supposedlyexisted contemporaneously or immediately after the fluid saturarion of the magma. Uranium mineralization possibly originated during deuteic alteration of the
aiaskite. Part of the uraninite crvstallized at the magmatic stage and another part from magmatic fluids in the biotite-rich selvagesof the alaskite or in the uraninite-fluorite veins. Crysrailization of uraninite is predominanrly controiled by the oxygen fugacity that prevailed in the magma and the surroundingrocks. Crirical paramercrs that contradict the above concepts and other models mentioned below are given by Brynard and Andreoli (1988) as follows: a) Any concept deriving uraniferous alaskite by anatexis of sediments has to consider the fact that the minerals constituting the alaskite are those with the lowest melting points and should therefore be the first to melt. This would imply the alaskite to be the oldest intrusives and not the youngest. as is mostly the case. b) Isotope age dating shows that crystallization of diorites and granites predates that of the alaskites and intrusion of uraniferous alaskites occurred at a late to posttectonic stage. These are features which lack explanation in the models
372
\ 5 Seleaed Examples of Economically Significant Tlpes of Urani 'm Deposits
proposedby Smith (1965)and von Backst0mand the order of 10"Cn(m higher than in adjacent Jacob (1979)/Toenset al. (1980). The latter zones. f) Another possibility, addressedby Brynard model assumesthat uranium and volatiles remained in the parent metasedimentsduring the and Andreoli (1988),considersa derivationof the main tectonicevents.Their escapeoccurredonly alaskite by remelting of high heat-generating during the posttectonicanatecticepisode.This granites(e.g., red granites)which were intruded would require a storagesystemfor uranium and during an earlier stageof the Damara Orogeny. volatiles in an environment with high pressure The remeltingmay haveoccurredduring the peak and temperatureconditions.But, givena suiteof of the orogeny when temperatureswere highest uraniferous hydrous (evaporitic) sediments, in the Central Znne. Since the alaskiteshave a in the one would expect that volatiles and uranium muchlower densirythan the metasediments should have been depleted already during the stratigraphiccoiumn, they could haveinvadedthe early stagesof anatexis under prograde meta- presentlevel by diapiric uprise in a suitabiefield of temperature,accompaniedbv a high volatile morphism. c) Although mineralzation patterns in componentas noted by Barnes and Hambletonthe Rossing deposit show that the bulk of the Jones(1978).Upward migrationof the alaskite uraniumconcentrarionoccursat certainlithologic may have been associatedby fractionationof the levelsnear the lower marble band somemineral- volatile phase to produce a residual potassiumization occurs well below this level particuiarly rich faciesfrom which uranium crystallized. g) An alternative source rock was provided in the easternpart of the deposit where better grades occur at greater depth. Brynard and by Barnesand Hambleton-Jones(1978).They reAndreoli (1988)therefore concludethat most of port orbicular-iikegray granite schlierensin the the uranium must have been presentwithin the alaskitewhich appearto have been derivedfrom magmaat the time of intrusion at the ievelsand melting of the Khan Formation. The schlierens that the effects of the marbles could not have arefrequentlymineralizedevenif the hostalaskite affected the magma as suggestedby Cuney is barren. Otherwise, nonuraniferousschlierens (1e80). occur in mineralized alaskite.The gray granite d) Ifuranium is consideredto be indigenousto schlierens display a remanent paleomagnetic the parentmetasediments of the alaskite,the lack orientationdi-fferentfrom that of the enveloping of mineralization in the early alaskiteleucosomes granite, which permits the assumptionthat the were mineralizedin piace. requiresan explanation.The occurrenceof these Khan metasediments In summary, Brynard and Andreoli (1988) alaskitesat stratigraphiclevels from the older Nosib Group to the younger RossingFormation concludethat availabledata do not satisfyeither may indicate a source at a level of the Khan ofthe abovepresentedconcepts.The authorshint at the possibilify that other processesmay have Formationor the underlyingunit. e) Uraniferousalaskitesdo not occurthrough- beeninvolvedin the formation of the uraniferous out the Central Zone but are confined to the alaskites.The position of most of the significant wrder R6ssingarea.This raisesthe questionwhv occurrencesaiong the Welwitschia Lineament adequate P-T conditions for alaskite-forming may possiblyprovideadditionalclues. anatexisprevailedin this restrictedregionand not elsewhereand what was their nature?Haack et al. (1983)infer a high potentialof heatgeneration References and Further Readingfor Chapter5.8 in the Damaran granites(9.5-9.8HGU) which (for detaiisof publicationseebibliography) may be partl),due to the high crustalradioactivity in the high grade zones.They calculatedthat at Barnesand Hambleton-Jones1978;Brynard and Andreoli the level of depth exposedat the presentsurface, 1988:Berning 1986;Berning et al.1976;Brynard HJ, pers. commun.; Cunel' 1980;Jacob et a1.1986;Mouillac et al. metamorphictemperatureshaveincreasedby 100 1986;Robb 1986;Robb et a1.1986;Robb and Schoch1985; to 150'C per 5 HGU of heat generationin the Toens and Corner 1980;Toens et aI.1980;von Backstr6m CentralZ-ane,which impliesa thermalgradientof 1970;von Backstrom and Jacob 1979.
Appendix Uranium Minerals(basedon Smith 1984),for referencelistsseeDahlkamp1979andSmith 1984
llineral
Formula
Svstem
(U.Ca,CeXTi,Fe)206
Mono.
u(sio4)r_,(oH)4, (U.Fe,Y,CaXNb,Ta)Or (U,Ca,Ce)PO1(OH)'H?O UMosOrz(OH)ro (U. Ca,Ce)z(PO)2' L- 2H2O UFe(Nb,Ta)20g U(MoOo)t (U,Ca,Ce)z(Nb,Ta)206(OH.F) (U, Ca,Ce)z(Ta,Nb)206(OH,F)
Tetra.
UO2*,(X < ca.0.3)(+a6,ggE,Pb a.o.) UO2*,(X < ca.0.67X+Cb.Si,Ti,Pba.o.) UOr-,(X < ca.0.3)(+gtg ".o;
Cubic Cubic Amorphous
u(uo2)5(oH):H'3H2o U(Nb,Ta)2O3 H4U(U02L(MoOa)7' 18H2O
Orth. Hex. Amorphous Orth. Tetr. Orth.
Ua* minerals BranneriteiU-Ti phases Coffinite Ishikawaite Lermontovite llourite ),lingyoite Petscheckite Sedovite Uranmicrolite Uranpyrochlore
Mono. Orth. Hex. Orth. Cubic Cubic
[J4+ - IJ6+ minerais
Uraninite Pitchblende Sooty/earthy pitchbl. Ianthinite Liandratite Moluranite Onhobrannerite Unnamed Wyartite
uri4ou(oH)2 o,-U307 3-5HzO Ca3U(UO2)5(CO3)2(OH)18'
Uranyl oxide hydrates(gummite minerais) Ianthinite Metaschoepite Paraschoepite Schoepite Studtite Metastudtite
uo2.5uo3.10H2o u03.2H2o uo3.2H2o uo3'2H2o uo4.4H2o uo4.2H2o
Onh. Onh. Orth. Orth. Mono. Orth. Sobrv Formula
Alkali and alkaline-earthuranvI oxide hvdrates Agrinierite Bauranoite Becquerelite Billietite Calciouranoite Clarkeite Compreignacite Curite Fourmarierite Masuyite Metacalciouranoite Metavandendriesscheite Rameauite
(K2Ca.Sr)U3O16 BaU2O7.4-5H2O Ca(UOj6Oa(OH)6 H2O Ba(UO2)60n(OI{)6'8H2O (Ca,Ba,Pb)U2O7'5H2O (Na,Ca,Pb)2U?(O OH)? K2(UO2)601(OH)6.8H2O Pb?uso17.4H2O Pbu4o13.6H2o Pb3uso27.10H2o (Ca.Na,Ba)U2ol'2H2O PbU7O22nH2O(n<12) K2CaU6O2s'9H2O
Orth. Orth. Orth. Metamict
x(H3o)5[(uo:)6o?( o H)5j 3H2o x(H3ox(uor)2o3(oH)1.
Orth. Orth. Orth. Onh. Metamict
Pb4(H3O)6[(UO2) loOrl(OH)6j Pb(H3O)3[(UOt4O.(OH)3] Ph(H3O)5[(UOt8O.( OH)5j
x(H3ox(uor)r03(oH)l
Pb(H3O)6[(UOrhos(OH)61 Mono.
374
Appeodix (Table of U-Minerals)
Formula
System
SobryFormula
Alkali and alkaline-earth uranvl oxide hvdrates Richetite Roubaultite Sayrite Uranospbaerite Vandenbrandeite Vandendriesscheite Wolsendorfite
Pb-Uoxide Cu:(UOJr(OH)ro'5H?O Ph(u02)506(oH)2.4H2o Bi2u2oe.3H2o Cu(UOzXOH)o Pbu1022.22H?o (Pb.Ca)U2O?'2H2O
TricL
Tricl. Orth. Orth.
Uranyl carbonates Wyartite Joliotite Rutherfordine Sharpite Metazellerite Tnllerite Andersonite Bavleyite Grimselite Liebigite Rabbittite Schroeckingerite Swartzite Widenmannite Voglite Mckelveyite Bijvoerite l-epersonnite
CarU(UO2)6(C03)2(OH)!8'3-5H?O
(uo2)(co3).1.5-2H2o (uo1xco3) (uo2xco3).H2o
Ca(UOz)(CO:)2 3H2O Ca(UOrXCO3h'5H2O 6HrO Na2Ca(UO2)(CO3)3' Mgr(UOr)(CO3)3 18H2O K.Na(UOz)(CO3)1'H2O Ca2(U02)(CO3)i'11H2O ca,qMg3(U02)z(co:)r,(oH)r'l8Hzo 10H?O NaCa3(U02)(CO3)3SO4F' CaMg(UOr)(CO3)3'12H2O
Pbr(uorxco3)3 Ca:Cu(UOz)(CO3)4'6H?O Ca3Na(Ca.U)Y(CO3)63HrO (RE)r(UOr)1(CO3)1(OH)6 60H?O Ca(RE)2U24(CO3)esi4.2O?5'
Orth. Orth. Orth. Orth. Orth. Orth. Hex. Mono. Hex. Orth. Mono. Tricl. Mono. Orth. Mono. Hex.
Uranyl molybdates Calcurmolite Cousinite Irigrnite Moluranite Mourite Sedovite Umohoite
Ca(UOJ:(Mo04)3(OH)2' I 1H2O Mg(UO)2(Mo01)2(OH)2' 5H2C) (UO2)Mo2O7'3H2O HoU(U02)3(MoOa)7 UMo5Ot2(OH)10 U(MoOr)z (UO:)(Mooz)(OH)4' 2H2O
Orth. Mono. Mono.
Uranyl phospbarcs and zrsenates
Dumontite Huegelite
6H2O possibly Ca(UO2)a(AsO4)2(OH)4' 6H2O Ca2(UO2)3(As04)2(OH){' Ba(UOz)r(PO4)r(OH)4 8H2Opossibly Ba2(UOj3(PO4)2(OH)48H2O (Th.Ca,PbXH3O)2(UO2)4(PO4)z(OH)s' 5H2O possibly (Th,Ca,PbXH3O)r(UOz)3(PO4)2(OH)6' 5H2O 7H2Opossibll' Pb(UO2)4(P04)2(OH)4 7H20 Pb2(UO2)3(PO4)2(OH){. Pb2(uot3(Po4)2(oH)4 3H2o Pb2(UO2)3(AsO4)2(OH)4 3H2O
Phosphuranul-ite Phurcalite
(H3O)?Ca(U 02 )3(PO4)z(OH) 4' 4H?O 4H2O Ca2(UO2)3(PO4)2(OH)4
Arsenuranylite Bergenite Kiwite
Renardite
Orth. Onh. Orth. Onh. Mono. Simiiarto dumontite Orth. Orth-
Pb(HjO)6[(U02)7Os(OH)6]. 1oH2O (O H )] Pb(Hl O ) [ (U O?)2O-j
Appendix (Table of U-Minerals)
Mineral
Formula
System
Uranyl phosphatesand arsenates Phuralumite Upalite Vanmeersscheite Metavanmeersscheite Autunite familv
Al2(u02)3(Po4)2(oH)610H2O Ar(uor)r(Po1)z(oH)r u(uo2)3(Po4)2(oH)6 4H2o u(uo2)3(Po1)2(oH)4 2H2o
Mono. Onh. Orth. Orth.
Rr_2(UO2)2(TO4)2 8- 12HzO
Dewindtite Meta-autunite I famiiy Coconinoite Furongite
Pb(u02)2(Po,)2.3H?o Rr_2(U02)2(TO1)?. 6- 8H?O Fe2Al2(UO2)2(PO4)2(SO4)(OH)2 20H2O Alr(uo?xPo4)2(oH)2 8H2o
seeextra table = renardite? seeextra table Mono. Tncl.
Hallimondite
Pb(UO2)(AsOo)2.nH2O
Tncl.
Parsonsite
Pb(UO2)(PO.)2.nH2O (H3O)oCa2(UO2)2(POr).' 5H2O (BiO)4(UOr)r(AsOa)a6H2O
Tricl.
Pseudo-autunite Walpurgite Walpurgrte-(P)
(Bio)4(uot2(Po4)4. 6H2o
Tricl.
Orth. Tricl.
Uranyl phosphatesof the autunitefamily Arsenuranospathite Autunite Fritzcheite Heinrichite Kahlerite
HAI(UO2)4(AsO*)a.40H2O Ca(UOj2(PO )2.8-12H2O Mn(UOJz(VO1)2.i0H2O Ba(UO)2(AsO )2.10-l2H2O Fe(UO)2(AsO)2'10-I2H2O
Tetr. Tetr.
Novacekite Sabugaiite Saieeite Threadgoidite Torbernite Uranocircite Uranospathite Uranospinite Xiangjiangite Zeunerite
Mg(UOJz(AsOd)2.12H2O FIAI(UO2)4(PO4)4.16H2O Mg(UOj2(PO4)2'10H2O AI(UO)2(PO4)2(OH)'8HzO Cu(UO2)2(PO)2'8-12H2O Ba(UOj2(POi2.l2H2O HAI(UOJ4(PO)4.4AH2O Ca(UO)2(AsO4)2.10H?O (Fe,AI)(UO:)r(PO4)2(SO4)2(OH).22H2O Cu(UO2)2@O.)2'40H2O
Tetr. Tetr. Tetr. Mono Tetr. Tetr. Tetr. Tetr. Orth. Tetr.
Tetr. Tetr.
Uranyl phosphatesof the meta-autunitefamily Abernathyrte Bassettite Meta-ankoleite Meta-autunrte Meta-autunite II Metaheinrichite Metakahlerite Metakirchheimerite Metaiodevrte Metanovacekite Metatorbernite Meta-uranocircite Meta-uranocircite II Meta-uranospinite Metazeunerite Przhevalskite Ranunculite Sodium metaautunite
K2(UOj2(AsO.)2.8H:O Fe(UO2)2(POa)2 8H2O K2(UOJ2(POr)2.6H?O Ca(UO3)2(PO{)2'6H?O Ca(UO:)z(POi2'4-6H2O Ba(UO)2(AsOl)2'8H2O Fe(UO)2(AsO4)2'8H2O Co(UOj2(AsO4)2'8HzO
Tetr. Mono. Tetr. Tetr. Orth. Tetr. Tetr. Tetr.
Zn(UOJ(AsOl)2.10H:O Mg(UOz)z(AsOi)2'4- 8H2O Cu(UOr)r(POd)2'8H?O Ba(UOJz(PO{)2'8H?O Ba(UO:)z(PO4)2'6HzO
Tetr. Tetr. Tetr. Tetr. Mono.
Ca(UO:)z(AsO4)2'8H2O Cu(UOr)2(AsO1)2'8HzO Pb(uot2(Po4)2.2H2o (H3O)Al(UO?XPO4XOH)3.3H2O (Na2,Ca)(U02)2(PO1)2' 8H2O
Tetr. Tetr. Orth. Mono. Tetr.
375
376
Appcndix (Iable of U-Minerals)
Mineral
Formula
System
Uranyl phospbates of the meta-autunite family g64lirrm uranospinite Triangulite Tr6gerite Trogerite-(P) Uramphite
(Naz,CaXUOz)2(AsOa)2' 5H2O
Tetr.
AI3(U02)1(PO4)1(OH)5. 5H2O UO2(UOt2(AsOa)2.8H2O uo2(uo2)2(Po4)2.8H2o (M{4)rQOr)r(PO4)r.4-6H?O
Tetr. Tetr. Tetr.
Uranyl selenatesand tellurates Ba(U02[(SeO3)2(OH)4. 3H]O Cu(UO2)j(SeO3)3(OH)z'7H?O UO2TeO3 Cua(U02)(SeO3)2(OH)6. H2O Pb(U02)(TeO3)2 UO2Te307 Pb2Cu5(UO2)2(SeO3)6(OH)u' 2H2O
Orth. Orth. Orth. Orth. Mono Cub. Tricl.
Soddyite Oursinite Beta-uranophane Boltwoodite Cuprosklodowskite
(uo2)2sio4.2H2o (CO,Mg)(UO2)2Si2O?' 6H?O (H3O)?Ca(UO2)2(SiO4)2. 3HzO K?(UOr2(SiO3OH)2.5H2O (H3O)2Cu(UOr2(SiO4)?'4HzO
Orth. Mono. Mono. Tricl.
Kasolite Skiodowskite gsdirrm boltwoodite Uranophane Swamlrei1g Haiweeite Weeksite Haiweeite-(Mg) Uranosilite
Pb2(uor)2(sio )2.2H2o (H3O)rMg(UO2)2(SiO4)2' 4H?O (H3O)2(Na,K)2(UOt2(SiOl)2.2H2O
Mono. Mono. Orth.
(HsO)zCa(UO2)2(SiO4)2. 3H2O VogFIz(UOz)z(SiO4)2. I 0H2O Ca(UOr)rSi6O15'5H2O K2(UO2)2SkO15'4H2O Mg(UOt)2SaO15.9HzO u6*si7o17
Mono.
Guilleminite Marthozite Schminerite Derriksite Moctezumite Cliffordite Demesmaekerite Uranyl silicates
Mono. Orth.
Uranyl sulfates Meta-uranopilite Uranopilite Cobalt zippeire Magnesium zippeite Nickel zippeite Sodium zippeire Zincappeite Tippeite Johannite Coconinoite Schroeckingerite
(u0r)6(s04)(oH)10. 5H?o (uor)6(so4xoH)ro. 12H2o Co2(U02)6(SO4)3(OH)ro. i6H2O Mg2(U02)u(S04)3(OH)ro. 16H2O
Mono. Orth. Orth.
Ni2(uoJ6(s04)3(oH)ro.l6H2o Naa(U02)5(S04)3(OH)ro. 16H2O Znr(U02)o(S04)3(OH)ro.I 6H2O Iq(uot6(so4)3(oH)10. 16H2o Cu(UO)2(S04)2(OH)?.8H2O FqAl2(UO2)2(PO4)zS04(OHh.20H2O NaCa3(UO2)2(CO3)3SO4F. l0H20
Orth. Onh. Orth. Orth. Tricl. Mono. Tricl.
K2(UOr2V208.3H2O Pb(uor2v208.5H2o
Mono. Orth. Orth. Orth. Mono. Orth. Tricl. Mono. Orth.
Uranyl vanadates Carnotite group Camotite Curienite Francevillite Fritzcheite Margaritasite Metaty.ryamunite Metavanuralite Sengierite Strelkinite
Ba(UOr)rV2Os.5H?O Mn(UOj2V2Os.l0H?O Cs(UOr)rVrO8.l.5H2O Ca(UOt)tVrOs.3-5H2O Al(uo2)2v2o8(oH). 8H2O Cu2(UOr)rV2OB(OH)2. 6H2O Na2(UOt)rV2Os.6H2O
Appendix (Table of U-Minerals)
Mineral
Formula
System
Ca(UO2)2V2Oe 8H2O Al(uo?)2v2os(oH). 11H2o (H3O)2(UOr)2V:O8..1H2O
Orth. Mono Mono
377
Uranyl vanadates Tvuyamunite Vanuralite Vanuranylite Unclassified Ferehanite Rauvite Unnamed Unnamed Uvanite
(u02)3vro3 6Hro Ca(UOj2V16Or'16HrO Ca-U-V-O-H2O
Pb-u-v-o-H:o (uo:)2v6o17.i5H2o
Uranium niobates.tantalatesand titanates{U substitutional but not dominantion)
Ashanite Berafite Davidite Euxenite Kobeite Pisekite Plumbobetafite Plumbomicrolite Plumbopyrochlore Polycrase Samarskite Tanteuxenite Thorutite Yttrobetafite Yttrocolumbite Yttrocrasite Ynromicroljte (hj eknite) Yttropyrochlore
Formula
Structuretype
Svstem
Nb.Ta.U,Fe.Mn)rOs Ca.Na,U)(Ti,Nb,Ta)zOo(OH) Fe.La,U.Ca)6(Ti.Fe)15(O,OH)3u Y, Ca.Ce.U,Th)(Nb,Ta.Ti)u Oe Y,UXTi.Nb)r(O,OH)6 As.Ca.UXNb,Ta.Ti)O1 Pb,U,Ca)(Nb,Ti)205(OH,F) Pb.Ca,U)Ta2O6(OH) Pb,Y,U,Ca)2-,Nb2O5(OH) Y,Ca,Ce,U,Th)(Ti,Nb,Ta)2O5 Y,Ce.U.Ca,Pb)(Nb,Ta,Ti,Sn)206 U.Fe.V)(Ti.Sn)206 'Th,U,Ca)Tir(O,OH)6
Lriolite Pvrochlore Crichtonite Columbite Columbite
Orth. Cubic Hex. Onh.
Y,U,Ce)2(Ti,Nb,Ta)206(OH) Y,U,Fe)(Nb,Ta)Oo Y,Th.Ca.U)(Ti,Fe)z(O.OH) Y,Ca.U)2(Ta,Nb)?O6(OH) Y,Na,Ca.U)1-lNb,Ta,Ti)zOc(OH)
Pvrochlore Rrochlore Prrochlore Columbite Columbite Columbite Brannerite hrochlore Stannocolumbite? Columbite Prrochlore Prrochlore
Minerals with tracesof uranrum Aeschynite Allanite Belovite Brirholite Cerianite Cheralite Ekanite Ervaldite Fergusonite Formanite Iimoriite Iraqite Melanocerite Ivlonazite Niobo-aeschynite Rhabdophane Thorianite Thonte Umbozerite
(Ce.Ca. . . XTi.Nb)z(O,OH)6 (Ce.Ca.Y,UXAl,Fe)3(SiOa)3(OH) (Sr.Ce.Na.Ca)5(PO4)3(OH) (Ce.Ca)s([Si.PlO4)r(OH,F) (Ce.U)O: (Ca.Ce,Th)(P,Si)Oa (Th.U)(Ca.Fe,Pb)25i8O20 Ba(Ca.RE)(CO:)z YNbOI YTaO" (Y, Ca.Zr)15(Mg,Fe.AlXSi.Al,P)eO14(OH)16 (La.Ce.Th.U)r(K,Y)r(Ca,La.Ce.Na)a(Si.Al)16O4 (Ce.Ca)s(Si.B)rOrz(OH,F) nH:O (Ce.Th.Ca.U)POa (Ce.Ca.ThXNb.Ti)r(O,OH)6 (Y.. . )PO4.HzO
(Th.u)or (Th.u)siol
(Na.K)3(Sr.Ba)a(Th,U.Fe)3O2a
.{eschynite Epidote .{.patlte .\patite Fluorite \{onazite Ekanite Ewaldite Fergusonite Fergusonite .{patlte ELanite ADatlte \{onazite .A,eschynite Rbabdophane Fluorite Zircon Umbozerite
Cubic
Cubic
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